adsorption and desorption of aliphatic amines, amino acids and acetate by clay minerals and marine...

Marine Chemistry, 44 (1993) 1-23 1 0304-4203/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

Adsorption and desorption of aliphatic amines, amino acids and acetate by clay minerals and marine sediments

Xu-Chen Wang, Cindy Lee Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, N Y 11794-5000, USA

(Received August 11, 1992; revision accepted March 3, 1993)

Abstract

The adsorption and desorption behavior of three aliphatic amines, three amino acids, and acetate by organic-free clay minerals (kaolinite and montmorillonite) and by marine sediment was studied in laboratory experiments using 14C-labeled compounds. These compounds were chosen to represent basic, neutral, and acidic functional groups. Adsorption partition coefficients ranged from < 0.5 to 4.2 for acetate, < 1 to 15 for monomethyl-, dimethyl- and trimethyl amine, and < 1 to 128 for alanine, glutamic acid and lysine. Single desorption experiments were conducted using these compounds to calculate desorption partition coefficients. Repetitive desorption experiments and a consecutive desorption model were used to calculate partition coefficients for the reversible and resistant components of desorption. Adsorption of the three positively-charged amines and lysine was mostly reversible, but glutamic acid, alanine and acetate exhibited irreversible association. This is most likely related to the different adsorption mechanisms caused by the different functional groups on these compounds. Ion exchange and electrostatic interactions are more important for the positively-charged compounds while chemical interactions with surface organic functional groups may be more important for the neutral and negatively-charged compounds. Adsorption of amines, amino acids and acetate is greater in sediments with a higher organic content although the acidic and neutral compounds were affected more than the basic compounds. Oxidation state of the sediments may also be an important factor in adsorption behavior.

1. Introduction

Adsorption at particle surfaces plays an important role in biogeochemical cycles of elements in natural aquatic systems (Parks, 1975; Stumm and Morgan, 1985). For example, the distribution and concentrations of many trace metals in seawater are largely controlled by adsorption processes (Whitfield and Turner, 1985). Adsorption also affects the distributions in marine sediments of many biogenic organic compounds produced during organic matter decomposition, such as amines (Wang, 1989; Wang and Lee, 1990), amino acids (Henrichs and Farrington, 1987; Burdige and Martens, 1990; Henrichs and Sugai, 1993) and short- chain fatty acids (Shaw et al., 1984; Sansone

et al., 1987). Adsorption processes not only influence the distributions of these compounds but also their bioavailability (Christensen and Blackburn, 1980; Sugai and Henrichs, 1992). Adsorption behavior of different organic compounds may explain, in part, the differences in remineralization rates and in preservation of sedimentary organic carbon in various environments (Henrichs and Sugai, 1993). These studies and others have demonstrated that adsorption influences the diagenesis of amino acids in marine sediments and should be considered as an important factor in models of amino acid decomposition.

Previous studies on the theoretical basis for adsorption of amines and amino acids have more commonly used well-defined surfaces

2 Xu-Chen Wang, C. Lee~Marine Chemistry 44 (1993) 1-23

such as clay minerals (Theng, 1974; Dashman and Stotzky, 1982; Hedges and Hare, 1987) rather than more chemically complex natural sediments. These studies have indicated that adsorption of amines and amino acids by clay minerals is largely affected by the electrostatic interaction between clay surfaces and various functional groups on the organic compounds. Positively-charged compounds tend to be adsorbed more strongly on negatively-charged clay surfaces. These studies on amines and amino acids leave unanswered how functional groups of different compounds affect the mechanism of adsorption, the reversibility of adsorption, and interactions with organic matter present in sediments.

An understanding of the reversibility of adsorption of dissolved organic compounds on particle surfaces is a necessary first step toward understanding the adsorption mechanism of organic compounds in sediments. Frequently, diagenesis models for organic matter decomposition in sediments treat adsorption as a distribution process and assume that adsorption is rapid and reversible, as is the case, for example, of ammonium (Berner, 1980). However, several studies have found that a large fraction of dissolved amino acids adsorbed onto marine sediments is irreversibly associated or desorbed extremely slowly (Rosenfeld, 1979a; Henrichs and Sugai, 1993). Desorption behavior can be even more important than adsorption as a control on the remineralization rates and preservation of organic compounds, particularly if it is slow or only partial. Very few studies have addressed this question in a quantitative fashion for organic compounds in marine sediments. However, DiToro and Horzempa (1982) pre- sented a model that allows a quantitative analy- sis of reversible and irreversible adsorption behavior.

In our previous studies (Wang, 1989; Wang and Lee, 1990), we investigated the distribution and adsorption behavior of three aliphatic amines in marine sediments of different environments. Although amines are highly soluble in

water, much (30-45%) of the total amine found in sediments was adsorbed onto the solid phase rather than in the porewaters. Adsorption was enhanced by higher clay and organic carbon contents of the sediments. In this paper, we report laboratory studies of the adsorption- desorption behavior of aliphatic amines, lysine, alanine, and glutamic and acetic acids by clay minerals and marine sediment. In seawater, the aliphatic amines and lysine are positively charged; alanine exists as the zwitter ion; and glutamic and acetic acids are negatively charged. The purpose of this study was to com- pare functional group effects on the adsorption and desorption of different organic compounds by clay minerals and marine sediment and gain a better understanding of the adsorption mechanism. The use of the consecutive desorption model of DiToro and Horzempa (1982) allowed us to quantitatively investigate the effects of sedimentary organic matter and particle size on adsorption. For both adsorption and desorption, some chemical and physical processes other than adsorption may be involved and these processes may affect the reversibility of adsorption of organic compounds. The use of the consecutive desorption model enables an investigation of the distinction between irreversible chemical and physical processes and adsorption.

2. Methods

2.1 Sediment sampling

Sediment from Flax Pond (FP), a small marsh located on the north side of Long Island, NY, and connected to Long Island Sound (LIS), was used for the adsorption experiments. FP sediment at this site is sandy with a clay content of 7% by weight; the annual average C content for surface sediment is 2.8% and the average porosity of the surface sediment is 0.80 (Wang, 1989; Wang and Lee, 1990). Sediment was collected from the top 10 cm at low tide using a

Xu-Chen Wang, C. Lee~Marine Chemistry 44 (1993) 1 23 3

plastic core liner. The sediment was manually homogenized and centrifuged before use. Sea- water (salinity = 28%o) was also collected from Flax Pond and filtered through 0.4 #m Nucle- pore filters for use in the adsorption experiments.

2.2 Clay minerals

Two clay minerals, American Petroleum Insti- tute standard kaolinite #9 (Mesa Alta, NM) and montmorillonite #26 (Clay Spur, WY), were used in this study. Both clays were ground with a mortar and pestle and sieved (USA Standard Testing Sieve, No. 230, 65 #m) before use. The < 65 #m size fraction was used for the adsorption experiments. The two clays were predomi- nantly in the Na form. The surface areas and organic carbon contents for these two clays are 8.6 m2/g and < 0.05% for kaolinite, and 37.8 m2/ g and 0.4% for montmorillonite, respectively (Mackin and Swider, 1987).

2.3 Radiolabeled organic compounds

The adsorption experiments were conducted using uniformly 14C-labeled compounds. Specific activities of compounds were: mono- methylamine (MMA), 50 mCi/mmol; dimethyl- amine (DMA), 55 mCi/mmol; trimethylamine (TMA), 3 mCi/mmol; alanine (ALA), 170 mCi/ mmol; glutamic acid (GLU), 293 mCi/mmol; lysine (LYS), 309 mCi/mmol and acetate (ACET), 58 mCi/mmol, respectively. All labeled compounds were obtained from New England Nuclear except DMA which was obtained from American Radiolabeled Chemicals, Inc. All standards were assayed for 14CO2 to check for possible microbial decomposition in shipping or storage; none was found.

Stock solutions of the compounds were diluted with distilled water to activities of 5-10 IzCi/ml for use in the adsorption experiments. About 10 #1 of aqueous solution was generally added in all experiments. Radioactivity was measured by liquid scintillation (Packard 1600 CA) using Opti-Fluor (Packard Inst) as

scintillant. All samples were counted for 5 minutes and corrected for quench by the trans- formed spectral index of the internal standard spectrum with automatic efficiency control (tSIE/AEC) method.

2.4 Sample preservation

Bacteria can rapidly decompose both aliphatic amines and amino acids and thus compete with adsorption as a removal process. In our experiments, HgC12 was used as a poison to prevent microbial degradation. Our previous study (Wang, 1989) showed that adding 0.5 g/1 HgC12 stopped any bacterial uptake of amines in fresh FP sediment and caused no significant effects on adsorption coefficients. For adsorption experiments conducted with clay minerals and fresh seawater, 0.1 g/l HgC12 was added to prevent microbial degradation; this concentration was based on the amount needed to prevent glutamic acid uptake in FP seawater. We also tested the possible effect of HgC12 on adsorption coefficients calculated for amino acids adsorbed by clay minerals in FP sea water. No significant effect on adsorption was found when up to 2g/1 HgC12 was added to seawater (data not shown).

2.5 Adsorption partition coefficients

Isotherms of amine, amino acid and acetate adsorption by kaolinite, montmorillonite and FP sediments in seawater slurries were determined at room temperature (25°C). Usually, cores were used within 2 h of returning them to the laboratory. In each case, either 200 mg dry clay (kaolinite or montmorillonite) or 2 g wet sediment was placed in a series of plastic scintillation vials and allowed to equilibrate for 1 h (sediment) or 4 h (clays) with filtered oxygenated seawater (10 ml) to which HgCIE (0.1 g/1 for clay and 0.5 g/1 for sediment) had been added. Placing the clays in seawater before the experiment allowed pre-equilibration of the major seawater ions with the clays (Mackin and Swider, 1987). However, no significant


differences in adsorption of the amines were found on clay with or without pre-equilibration with seawater (data not shown). Increasing concentrations of 14C-labeled MMA (0-4 #M), DMA (0-3.5 #M), TMA (0-2.5 #M), ALA (0-1 #M), GLU (0-0.3 #M), LYS ((0-0.5 #M) and ACET (0-0.15 #M) were added separately to a series of vials. These added label concentrations are in the range of natural concentrations commonly found in marine sediments and seawater (Shaw et al., 1984; Henrichs and Farrington, 1987; Lee, 1988; Michelson et al., 1989; Burdige and Martens, 1990; Wang and Lee, 1990). All vials were capped and shaken for 2 h. After centrifugation (2000 × g, 10 min), 0.5 ml of supernatant was taken from each vial to measure 14C activity in solution. The pH of the supernatant was in the range of 6.3-7.2 for the two clays and 7.6-7.8 for FP sediment. Controls without particles were run simultaneously for each compound to check possible adsorption of label by the container wall. The amount of label adsorbed by the particles was determined as the difference between 14C activity in solution initially and after 2 h. Adsorption partition coefficients for the compounds were then determined from the slope of the isotherms and relate the amount adsorbed (nmol/gdw) to the amount still dissolved (#M) at equilibrium. Kads values thus have units of ml/gdw.

2.6 Desorption partition coefficients

The reversibility of amine, amino acid and acetate adsorption by clay minerals and FP sediments in seawater slurries was determined using both a single-stage desorption and a consecutive desorption model (DiToro and Horzempa, 1982; Gschwend and Wu, 1985). Isotherms from batch desorption experiments were constructed to calculate desorption partition coefficients. Briefly, adsorption of each compound was determined as described above. After adsorption reached equilibrium, the slurries containing different concentrations of labeled compound were centrifuged at 2000 × g for 10 min; then, half of the

seawater (with remaining label) was removed and replaced with the same amount of seawater (with HgC12 but without added label). This solution replacement diluted the label concentration without changing the solid to solution ratio. The vials were then capped and shaken for 2 h. After centrifugation, 0.5 ml of supernatant was taken from each vial to count 14C activity in solution. The desorption partition coefficient, Kdes, was then determined from the slope of the isotherm and relates the amount of label remaining on the solid phase (nmol/gdw) to the amount of label in the solution phase (#M) at the new equilibrium. Kdes values thus also have units of ml/gdw.

Partition coefficients for the reversible component (Krev) and the resistant component (Kres) of desorption were also calculated based on the consecutive desorption model described by DiToro and Horzempa (1982). The Kads, Kdes, Krev, and Kres terms correspond to the partition coefficients 7ra, 7rd, 7rx, and 7r o used by DiToro and Horzempa (1982) and DiToro et al. (1986). For consecutive desorption, sequential desorption experiments were conducted repeti- tively (or until the 14C activity in solution reached zero). Krev is the ratio of the total adsorbed label that is desorbed (nmol/gdw) at the end of the desorption cycles (when label in solution reaches zero) to the dissolved label concentration (#M) before desorption begins (or at adsorption equilibrium); Kres is the ratio of the label remaining on the solid phase (nmol/gdw) at the end of the desorption cycles to the dissolved label concentration (#M) before desorption begins. Thus Kre v + Kre s is approximately equal to Kads. Selected samples were also analyzed for 14C02 production to check for possible microbial degradation of the compounds during desorption.

2.7 Adsorption and desorption kinetics

Before adsorption and desorption partition coefficients could be calculated, it was necessary to first determine the adsorption equilibration

Xu-Chen Wang, C. Lee~Marine Chemistry 44 (1993) 1-23 5

time for each compound. Our previous work suggested that adsorption equilibrium for the amines was reached in less than 2 h (Wang and Lee, 1989). To determine equilibration times for the other compounds, we measured in duplicate the rate of adsorption and desorption of each compound at a single concentration. For the clays, 300 mg montmorillonite was placed in each of two plastic vials with 15 ml filtered FP seawater (with HgC12). The clay and seawater were allowed to pre-equilibrate for 4 h before adding the label. For the sediments, 3 g wet sediment (after centrifugation) was added to each of two vials with 15 ml filtered seawater (with HgC12). Then, lnC-TMA (1.7 #M) and 14C-LYS (75 nM) were added separately to vials containing montmorillonite, and 14C- GLU (80 nM) and 14C-LYS (75 nM) were added separately to vials containing sediment. All vials were capped and shaken on a mech- anical shaker throughout a 120 h incubation period. At various times, the vials were centrifuged and 200 #1 of the supernatant counted for laC activity.

The rate of desorption was determined using clay and sediments from the adsorption rate experiments described above. After the 120 h adsorption period, the vials were centrifuged and 10 ml supernatant was withdrawn and discarded. Then, 10 ml filtered unlabeled seawater was added to each vial. The vials were capped and shaken for a second 120 h incubation period. At various times, the vials were centrifuged and the 14C in solution was measured.

We also tested the effect of centrifugation on the measurement of the adsorption partition coefficient of 14C-TMA and laC-LYS using montmorillonite and FP sediment. The adsorption experiment was conducted as described above but samples were centrifuged at different speeds (100 x g, 1000 x g, 2000 x g, 3000 x g, 5000 x g and 6000 × g). No significant change in Kaas was found for adsorption of TMA by either montmorillonite or sediment. Adsorption of LYS by montmorillonite was not affected

while a 10% increase in Kaas was found for adsorption of LYS by FP sediment at the highest speed used (6000 × g; data not shown).

2.8 Effects of sedimentary organic matter

To determine the effects of sedimentary organic matter on the adsorption of acetate, amines and amino acids, two experiments were conducted. Treatment of sediments with peroxide (after Rosenfeld, 1979a) or with NaOH (Hayes, 1985) both result in at least partial removal of organic matter. Peroxide treatment was used in our previous studies and is repeated here for comparison. Briefly, after removal of porewater, the sediment is treated with 30% H202 to remove organic matter; the efficiency of organic matter removal by this treatment was 55 to 77% for FP sediment (Wang and Lee, 1990). Adsorption experiments as described above were repeated using the sediment treated with H202, and resulting isotherms were compared to those of untreated sediment.

In a second experiment, fresh FP sediment was extracted with seawater or with 0.1 N NaOH. NaOH extracts humic substances and other organic matter from sediment (Hayes, 1985). In the seawater extraction, about 60 g uncentri- fuged fresh sediment was placed in each of four plastic bottles with 150 ml seawater. All bottles were then capped and shaken for 2 h. After centrifugation (3000 x g, 10 min), the seawater was removed. Each of the four sediment samples was extracted with seawater, one sample once, a second sample twice, etc. up to four times. In the NaOH extraction, 120 ml 0.1 N NaOH was used instead of seawater and the sediment extracted only once. The sediment was shaken for 24 h, centrifuged (3000 × g, 10 min), the NaOH extract removed, and the sediment washed with filtered seawater until pH 8. Adsorption experiments were repeated using the extracted sediments. Only TMA, ALA, GLU and LYS were tested with NaOH- extracted sediments.


g " 0 M

m o

.12

. J

100'

80 A

m o n t m o r l l l o n i t e

60

LYS

0 20 40 60 80 100 120 140

Time (hour)

O ~

E

_m

" O ( p

0 W " 0

1"i _ . _ m o n t m o r l l l o n l t e

eo, ~ , . _ :

40 ̧

2O

LYS

TMA

. , . , . , . . - , . , -

20 40 60 80 100 120 140

Time (hour)

100

" o

£

m

LYS

GLU

100 ~ r . _ GLU A A A

c _=

E o

. 1 3

" O O

£ O

" O <

80"

6 0 '

40

20

LYS

C FP sediment D FP sediment

0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140

Time (hour) Time (hour)

Fig. 1. Time-course adsorption and desorption. (A) Adsorption and (B) desorption of 14C-TMA and 14C-LYS by montmor- ilionite (20 gdw/1). (C) Adsorption and (D) desorption of 14C-GLU and ]4C-LYS by FP sediment (100 gdw/1). Adsorption is shown as the percentage of label added; desorption is shown as the percentage of the label adsorbed during the initial adsorption experiment.

2.9 Effect of redox condition of sediment

We tested the potential effects of sediment oxidation-reduction status on adsorption of LYS. To prepare sediment for oxic adsorption experiments, 5.0 g wet FP sediment was placed in each of five 15 ml centrifuge tubes. Then, 10 ml filtered (0.4 #m) FP seawater was added to each tube. The tubes were uncapped and shaken several times a day for four days to let the sediment oxidize completely. For anoxic adsorption, the same amount of sediment was placed in each of five 15 ml serum bottles in an N2-filled glove bag. Then, 10 ml filtered, deoxygenated FP seawater was added to each bottle. The bottles were sealed with teflon-coated rubber septa and aluminum crimp seals. Each bottle was bubbled

with N 2 for 1 min. A concentrated HgC12 solution (dissolved in either oxic or deoxygenated FP seawater) was added to each tube or bottle to a final concentration of 0.5 g/1 HgC12. Then, various concentrations of 14C-LYS (0-0.25 #M) were added. The label was dissolved in oxic distilled water. Less than 200 #1 was added to each sample. After shaking 2 h, adsorbed laC-LYS was determined as described above. 14CO2 production was also measured in both the oxic and anoxic experiments.

2.10 Particle size effect

The effect of particle size on the adsorption of amines and amino acids was investigated using different size classes of sediment. This does not

Xu-Chen Wang, C. Lee/Marine Chemistry 44 (1993) 1-23 7

measure the effect of size alone since the sediment particles of different size undoubtedly have different compositions as well. In the experiments, fresh surface sediment was first wet sieved with FP seawater using a 1 mm plastic sieve to remove large particles and organic detritus. Then, the sediment was wet fractionated using a series of sieves (USA Stan- dard Testing Sieve, No. 50, 120 and 230) into four fractions (1 ram-300 #m, 300-125 #m, 125-65 #m, and < 65 #m). Particles in each fraction were concentrated by centrifugation at 3000 x g f o r 2 0 min. For adsorption experiments, 1 g wet sediment from each fraction was placed in each of a series of vials with 5 ml filtered seawater and 0.5 g/1 HgC12. 14C-labeled MMA (20 nM), DMA (20 nM), TMA (270 nM), ALA (20 nM), GLU (12 nM) and LYS (14 nM)

were added to each of a series of vials containing the different size particles. After shaking 2 h, the amount of the label adsorbed by each particle fraction was determined as described above.

3. Results

3.1 Adsorption and desorption rates

The results of the time-course adsorption and desorption of TMA and LYS by montmorillonite and GLU and LYS by FP sediments are shown in Fig. 1. Both adsorption and desorption appeared to be rapid processes. The adsorption of TMA and LYS by montmorillonite reached apparent equilibrium in < 1 h while GLU and LYS equilibrium with sediment was reached in < 2 h. At equilibrium under these

Table 1 Isotherm parameters for amines, amino acids and acetate adsorption and desorption

Compound Adsorbent Adsorbent Partition coefficients b (ml/g) concentration a

(gdw/l) Kad s Kde s Krev Kr~

M M A Montmoril lonite 20 7.0 9.3 4.7 1.6 Kaolinite 20 < 1 FP sediment 100 3.5 5.4 2.2 1.4

D M A Montmoril lonite 20 7.4 7.0 6.6 0.8 Kaolinite 20 1.0 FP sediment 100 5.9 7.0 4.5 1.3

T M A Montmoril lonite 20 15.2 13.4 14.6 0.1 Kaolinite 20 1.8 2.2 1.7 0.1 FP sediment 100 6.7 8.2 6.0 1.0

A L A Montmoril lonite 20 < 1 Kaolinite 20 < 1 FP sediment 100 9.1 21.2 2.6 6.3

G L U Montmoril lonite 20 2.1 2.3 1.6 0.4 Kaolinite 20 5.6 11.7 1.0 5.0 FP sediment 100 11.0 20.6 4.1 7.3

LYS Montmoril lonite 20 35.3 40.5 30.3 5.8 Kaolinite 20 33.0 42.2 24.3 8.6 FP sediment 100 128 76.4 50.8

ACET Montmoril lonite 20 < 0.5 Kaolinite 20 2.7 5.6 0.4 2.6 FP sediment 100 4.2 9.2 < 0.5 4.2

a The aqueous phase is Flax Pond sea~vater.

b The partition coefficients are for adsorption (Kaas); desorption (Kdes); and the reversible component (Krev) and the resistant component (K~s) o f desorption. When the adsorption partition coefficient was < 1, the partition coefficients for desorption were not determined.

25 ¸

20'

s<

15'

,0 0

. ,

&

0 1

2 3

Dis

solv

ed

MM

A

(p.M

)

25"

15

K

=

o:.~

- -

~ A

0 1

2 3

Dis

solv

ed D

MA

(p

M)

25

A

20

.2

/ jK

=07

10

"~

~ ~

•

OI

v .

, -

, .

0 1

2 3

Dis

solv

ed

TM

A

(~M

)

lii ,'!I

/ A

A

A

1 =

128

K=3

5.3

4"

.~

~ ~

4 ,,~

2"

o

o

~r~

f ~

- ,

°,.t,

,

- ,

- ,

- 0

. ,

. 0

.0

0.1

0.2

0.3

0.4

0

.5

0.6

0,0

0.1

0.2

0.3

0

.0

0.1

012

0.3

Dls

solv

ed

ALA

(p

.M)

Dls

solv

ed

GLU

(~

M)

Dls

solv

ed

LYS

(p

.M)

Fig.

2.

Ads

orpt

ion

isot

herm

s fo

r th

e am

ines

(M

MA

, D

MA

and

TM

A)

and

the

amin

o ac

ids

(AL

A,

GL

U a

nd L

YS

) by

kao

lini

te (

A),

mon

tmor

illo

nite

(O

) an

d F

lax

Pon

d se

dim

ent

(0).

Lin

es a

re l

inea

r re

gres

sion

fit

s to

the

dat

a.

n~

r~

~v

Xu-Chen Wang, C. Lee/Marine Chemistry 44 (1993) 1-23 9

conditions, 30 + 3% of the TMA and 38 ± 3% of the LYS added were adsorbed by the clay, and 43 -4- 5% of the G L U and 87 4- 5% of the LYS added were adsorbed by the sediment. Since the solid to seawater ratio was different for the clay and sediment adsorption, the percentages of these compounds adsorbed by montmorillonite and FP sediment could not be compared directly. For the desorption experiment where 70% of the seawater with label was replaced by seawater without label, 45% of the adsorbed TMA and 40% of the adsorbed LYS were desorbed from montmorillonite although desorption appeared complete after less than 2 h. Only 10% of the adsorbed G L U and 15% of the LYS were desorbed from the FP sediment. After 2 h, both adsorption and desorption remained constant over the 120 h incubation period. Rates of acetate adsorption and desorption were not tested but previous work indicated that adsorption of acetate in FP sediment reached equilibrium in less than 1 h (Michelson et al., 1989). A 2 h equilibration time was chosen for all adsorption and desorption experiments in this study.

3.2 Adsorption partition coefficients

Adsorption isotherms for the amines and amino acids are illustrated in Fig. 2. In the concentration range studied for each compound, isotherms were linear except when the compound was not significantly adsorbed. The partition coefficients (Kads) were calculated from the slopes of the regression lines as the ratio of the label adsorbed (nmol/g) to the label dissolved (#M) at equilibrium. The values of Ka,l+ for all compounds tested are summarized in Table 1.

MMA, DMA and TMA were adsorbed much more by montmorillonite than by kaolinite. At the same concentrations of amines, FP sediment adsorbed more amines than kaolinite, but less than montmorillonite. The adsorption of the amines followed the trend TMA > DM A > MMA. In contrast to the three amines, the three amino acids and acetate

10 , , y "~,. 8 t montmorlllon

I J 1(--7.0

Z

0 1 2 Dissolved MMA (p.M)

A

m o E

< :E a

0

m

25" ,

20"

15"

10"

5'

0 0

montmorlllonlts

4 i 3 Dissolved DMA (~.M)

25

20" E =

< 15" I'--

~ 10"

" 0

0 0 2

m o n t m o r l l l o n l t ~ , /

K=15.2

1 Dissolved TMA (pM)

Fig. 3. Desorption of MMA, DMA and TMA from montmorillonite. The solid lines (O) are adsorption isotherms; dotted lines and open symbols represent consecutive desorption from individual samples represented on the adsorption isotherm.


1° t f A FP sediment

s t ~ ..--~K=s.s

0 ~ o ; i 3 Dissolved MMA (lsM)

12 A

-- 10' O E =~ 8' ,¢ =S 6' r~

"O @

J : h. 0 M "0

4

2 ̧

0 - 0

FP sediment . . J

. o j ~ y K=5,9 ~o ~ :o.- : : . . .

; Dissolved DMA (rtM)

A

"6 E C

0

<

20" FP sediment

15-

K=6.7

10 , ~ J /

s - o ~ : ? ............

0= - ~ • - 0 1

Dissolved TMA (gM)

Fig. 4. Desorption of MMA, DMA and TMA from Flax Pond sediment. The solid lines (Q) are adsorption isotherms; dotted lines and open symbols represent consecutive desorption from individual samples represented on the adsorption isotherm.

exhibited quite different adsorption behavior. LYS, a basic amino acid which carries a net positive charge was adsorbed more than any other compound tested by both clays and sediment. Neutrally-charged ALA and negatively- charged GLU and acetate were adsorbed by FP sediment but less or not at all by the two clays.

3.3 Desorption partition coefficients

Calculated desorption partition coefficients (/':des) and partition coefficients for the reversible component (Krev) and resistant component

12

_~ 10 o E = 8" ¢/1 > - -I 6"

"0 e

0 m "0

4"

2"

0= o.o 0:1 o:=

k a o l i n i t e

. . . . . .

0 . 3

Dissolved LYS (v.M)

A

"6 E

>. .J

O

<

12

10

8

6

4

2

0 0.1

m o n t m o r l l l o n l t a

j - 0 . 1 0 . 2 0 . 3

Dissolved LYS (P.M)

Fig. 5. Desorption of LYS from kaolinite and montmorillonite. The solid lines (O) are adsorption isotherms; dotted lines and open symbols represent consecutive desorption from individual samples represented on the adsorption isotherm.


0 E =_

,< ...J <

" 0 0

J= k . 0 M

" 0 <[

2"

0 0,0

FP sediment

/ ............... 7 / i " " g ' ° "

0 .2 0 . 4 0 . 6

Dissolved ALA (p,M)

A

m O E t -

v

..J (9

"O @

O M

"O <[

3'

2

0 1.0

FP sediment

0 . . 0 0 0 ~ 0 . . . . . . . . . d "0 . . . . . . . . . . . . . . . . . . . . .

c ~ : P a f "e

0.1 0 . 2

Dissolved GLU (p,M)

E e-

v

I - - UJ 0 <

"O O

,O i . O m

"O

1.2 FP sediment

o.o..o....o....~o ............... ~ ~ 0.8

1 1 _ ~ K=4.2

o,4 r.~=--o---° . . . . . 7 . ' Z

o . 0 0.1 0 . 2

Dissolved ACET (p,M)

Fig. 6. Desorption of ALA, GLU and ACET from Flax Pond sediment. The solid lines ( 0 ) are adsorption isotherms; dotted lines and open symbols represent consecutive desorption from individual samples represented on the adsorption isotherm.

(Kres) of desorption are summarized in Table 1. Desorption isotherms for all three amines on montmorillonite and FP sediment were roughly linear (Figs. 3 and 4). Adsorption of MMA, DMA and TMA by montmorillonite was essentially reversible (Kdes ~ Kads), although at concentrations above 4 nmol/g, about 20 + 5% of the M M A did not desorb. Desorption of the amines from kaolinite could not be tested since so little adsorbed. Desorption of MMA, DMA and TMA from FP sediment also showed linear

'3 E e-

o') >. ,,,,J "o o

0

" 0

10" =128

8"

4 ' . o " = " " ° ....... :,o-~': ......... ",'.'::: o

.o- ~o

2

0 o.0 011 012 013

FP s e d i m e n t

A

0 . 4

Dissolved LYS (pM)

0.4

A :E 0 . 3

>. "J 0.2 "13 0 >

0 m _~ 0.1

FP sediment

0.0 0

Dilut ion cyc le

Fig. 7. Consecutive desorption of LYS from Flax Pond sediment. (A) The solid line ( 0 ) is the adsorption isotherm; the dotted lines and open symbols represent consecutive desorption from individual samples represented on the adsorption isotherm. (B) Aqueous concentration change with consecutive desorption dilution cycles.


and mostly reversible behavior (Fig. 4). As in the case of montmorillonite, TMA adsorption was more reversible than that of MMA and DMA from FP sediment.

In contrast to the amines, the three amino acids and acetate showed different desorption behavior. Figs. 5 and 6 show the desorption of LYS from the clays and ALA, GLU and ACET from FP sediment. Only LYS adsorbed by the two clays showed reversible behavior, while desorption of ALA, GLU and ACET from the sediment showed an irreversible association. The desorption of LYS from FP sediment also showed largely reversible behavior. However, the dissolved label concentrations unexpectedly increased instead of decreasing during the first three dilution cycles resulting in an initial peak before decreasing to low values (Fig. 7). This experiment was repeated twice with different FP sediment and gave similar results. During the desorption cycles, 14CO2 production was measured for GLU and LYS to check for possible microbial degradation of the compounds; no significant 14CO 2 production was observed (data not shown).

3.4 Sedimentary organic matter effects

Adsorption partition coefficients for compounds before and after peroxide treatment are summarized in Table 2. After the sediment was treated with H202, adsorption of the three amines decreased by 50-60%. Adsorption of the three amino acids and acetate decreased

Table 2 Adsorption partition coefficients (/fads) of amines, amino acids and acetate by Flax Pond sediments before and after H202 treatment

Compound Before treatment After treatment

MMA 3.5 1.2 DMA 5.9 2.6 TMA 6.7 3.2 ALA 9.1 < 0,5 GLU 11.0 < 0.5 LYS 128 4,4 ACET 4.2 < 0.5

more substantially, with only LYS being adsorbed after the treatment; adsorption of ALA, GLU and ACET after peroxide treatment was negligible.

Adsorption of TMA by sediments was not affected by pre-extraction of the sediments with seawater, while adsorption of ALA, GLU and LYS decreased after this treatment (Fig. 8). After sediment was extracted with 0.1 N NaOH, adsorption of all compounds was less than adsorption by unextracted sediment, or, except for LYS, by sediment extracted four times with seawater.

3.5 Effect of redox condition of sediment

The results of LYS adsorption in oxic and anoxic FP sediments are shown in Fig. 9. When the sediment was oxidized, adsorption of LYS was much higher than that in anoxic sediment. The calculated partition coefficient Kads was 510 for oxic adsorption and 88 for anoxic adsorption. Kads for FP sediment measured by the usual method was 128 (Table 1), intermediate between the oxidized and reduced sediments in this experiment. 14CO2 production of added label was ,-~ 1% for oxic and 1.1% for anoxic adsorption (data not shown). This was much higher than in previous experiments, perhaps due to the longer time the sediment sat at room

log K 1

3-

2 !

----o---- LYS GLU

TMA

" , ~ ...........

-1 0 "1 2 3 ~, 0.1N NaOH

Sequential seawater extraction

Fig. 8. Adsorption partit ion coefficients of compounds adsorbed by Flax Pond sediments that were previously extracted with seawater or 0.1 N NaOH. Log Kads is shown to scale the Kads values for comparison purposes.


o m o E

to

23 @ £ g <

3 ~ - - ~ o x l c

o o.oo o.oi 0.02 0 .03

Dissolved LYS (pM)

Fig. 9. Oxic and anoxic adsorption isotherms for ]4C-LYS in FP sediment. Lines are linear regression fits to the data.

temperature (4 days vs. 2 h). Therefore, dissolved 14C-LYS concentrations "used to calculate the partition d0efficient were corrected for loss due to 14CO2 production.

3.6 Particle'size effects

The adsorption of each of the compounds investigated varied with particle size (Fig. 10). For all compounds tested, the smallest particles (< 65 #m) adsorbed the most. Particles in the size range 300/zm-1 mm adsorbed more labeled MMA, DMA, TMA, LYS and GLU than particles in the size range of 125-300 /zm and 65-125 #m. The largest size range contained visible organic detritus from Spartina sp. LYS was adsorbed more than any other compound in all size fractions.

100

[ ] 30011m-lmm [ ] 125-3001~m

80 [ ] 65-125~.m • < 65~m

60'

~ 40"

20"

0 MMA DMA TMA ALA GLU LYS

Compounds

Fig. 10. Adsorption of the radiolabeled compounds by particles of different size.

4. Discussion

4.1 Clay minerals

(1) Adsorption The adsorption of small positively-charged

organic compounds onto clays is essentially a cation exchange process, due to the negatively- charged surfaces of clays (Theng, 1974; van Olphen, 1977), and involves electrostatic and van der Waals forces although hydrophobic interactions may also play a role. Montmorillo- nite is a 2:1 layer clay (2 tetrahedral sheets:l octahedral sheet) whose chemical structure causes its cation exchange capacity (CEC) to be more than 40 times higher than that of kaolinite (1:1 layer clay); so the former is expected to adsorb more organic cations. The adsorption coefficients (Table 1) indicate that the three positively-charged amines were more strongly adsorbed by montmorillonite than by kaolinite. LYS, a basic amino acid, was adsorbed even more than the amines by montmorillonite. The observed adsorption affinity of the positively- charged compounds (LYS > TMA > DMA > MMA) by montmorillonite decreased generally with decreasing basicity (amine PKb values are 5.05 for the a-amine in LYS, 4.28, 3.29 and 3.36 for TMA, DMA and MMA, respectively). Adsorption affinity increased with increasing van der Waals attractive forces caused by higher molecular weight. Because the order of adsorption affinity is not perfectly correlated with basicity since all four compounds are almost completely protonated in seawater, van der Waals forces or hydrophobic effects likely play a predominant role in adsorption behavior. Molecular shape and structure may also influence adsorption of the amines (Theng, 1974; Huheey, 1983).

MMA, DMA and TMA were not significantly adsorbed by kaolinite. However, at the same concentration, LYS was adsorbed by kaolinite to the same extent as by montmorillonite. This great adsorption of LYS by kaolinite is surpris- ing considering the much smaller cation


exchange capacity of kaolinite compared to montmorillonite. As mentioned earlier, Hedges and Hare (1987) also found that basic amino acids (LYS and ARG) were adsorbed to the same degree by montmorillonite and kaolinite in distilled water (the adsorption coefficients were 340 and 320 for LYS, and 370 and 240 for ARG, respectively). They suggested that this similar adsorption was because the maxi- mum adsorption capacity of the two clays was not approached in the dilute amino acid solution (10 #M) used in their experiments. However, since concentrations of lysine were similar to that of the amines in our experiments, the fact that lysine did not show preferential adsorption on montmorillonite while the amines did requires another explanation.

A more likely explanation for similar adsorption by both clays is that both amino and carboxyl functional groups affect LYS adsorption. When the amino group is strongly attached to clay surfaces, zwitterion association of LYS molecules with other adsorbed amino acids through the free carboxyl and amino groups either by hydrogen bonding, van der Waals attraction, or ionic interactions may occur (Theng, 1974). In an early study of adsorption of basic amino acids by montmorillonite, Cloos et al. (1966) hypothesized that two different types of adsorption were involved. Strong adsorption occurred through ionic attraction of the cationic amino group to the negative clay surfaces, while weaker adsorption of the amino group occurred through the carboxyl group of another amino acid already strongly adsorbed on the clay surface. In their recent study of adsorption of amino acids by Alaskan Resurrection Bay sediments, Henrichs and Sugai (1993) suggested that LYS-LYS binding might explain the large adsorption capacities they found, similar to the "weaker" adsorption suggested by Cloos et al. (1966). LYS-LYS binding may explain in part the large Kads values for LYS in our study. How- ever, there may be other explanations for the similar adsorption of LYS by the two clays.

The calculated adsorption partition coef-

ficients (Table 1) indicated that (gads)kaol > (Kads)mont for negatively-charged compounds while the reverse is true for positively-charged compounds. Van Olphen (1977) proposed that organic anion adsorption by kaolinite occurred at the edges of the clay particles since the exposed edges of alumina sheets carry a positive charge. Usually, 10-20% of the total surface of kaolinite carries a positive charge which has a strong tendency to adsorb negatively-charged compounds (Theng, 1974). Large partition coefficients for the acidic amino acids, GLU and ASP, on kaolinite (/(ads = 51 and 27, respectively) were measured in distilled water by Hedges and Hare (1987).

(2) Desorption Calculated desorption partition coefficients

indicated that desorption of amines from montmorillonite was essentially reversible (/(des is similar to Kads and Krev > Kres). However, the calculated Kres showed that about 20% of the MMA and 10% of the DMA initially adsorbed onto montmorillonite at high concentrations was not desorbed. This difference between the amines may be due to the different interlayer complex formation of the two compounds. The interlayer complex is much more stable than the complex formed on clay surfaces and can not be easily desorbed by ion-exchange (Theng, 1974). Palmer and Bauer (1961) and Rowland and Weiss (1963) reported that methylamines could be adsorbed into the interlayer of montmorillonite by exchanging with interlayer cations. They found that interlayer adsorption of methylamines decreased in the order of Mel > Me2 > Me3 due to steric hindrance. The smaller MMA can most easily penetrate into the clay interlayer. This interlayer adsorption may explain in part the observed difference in desorption of the amines from montmorillonite.

Amino acids and acetate exhibited different desorption behavior. Like the amines, positively-charged LYS showed mostly reversible desorption from both kaolinite and montmorillonite. Removal was greater from


montmorillonite (86%) than from kaolinite (74%). Negatively-charged GLU and ACET showed a more irreversible association with kaolinite. Most of the label adsorbed was not released to solution. This different desorption behavior between positively- and negatively- charged compounds suggests that the mechanism of adsorption of negatively-charged compounds at the edges of kaolinite may be different from the surface adsorption that occurs on montmorillonite.

Whether irreversible adsorption really occurs or not is subject to some controversy (Gschwend and Wu, 1985; Morel and Gschwend, 1987). Gschwend and Wu (1985) suggested that observations of irreversible adsorption were artifactual and likely due to washing out of compounds adsorbed to colloidal material. However, their experiments were conducted with hydrophobic compounds as were the original studies by DiToro and Horzempa (1982). The amines are very soluble, charged molecules and are thus subject to very different chemical and elec- trical interactions. Perhaps, in fact, the "adsorption" processes we and others are observing on clay minerals and in marine sediments are not due solely to chemical adsorption but to other surface chemical reactions as well. The irreversible association observed may be simply due to the rate of desorption being much slower than the time we allowed for our experiments (> 120 h), but we think this unlikely. Loss of colloidal material cannot explain our results since in identical experiments some of the compounds tested were completely reversibly desorbed.

4.2 Flax Pond sediment

(1) Adsorption The adsorption partition coefficients of MMA

(3.5), DMA (5.9) and TMA (6.7) indicated that these amines were significantly adsorbed by FP sediments, although less so than by montmorillonite. A clear-cut quantitative direct comparison of sediment and clay adsorption

cannot be made for these experiments. The solid to solution ratio and particle size were different in the two systems and the composition of the sediments much more complex. However, a relative comparison could be useful. Values of K are similar to those found two years prior to this study using sediments collected from the same location (Wang and Lee, 1990). Like the clays, the adsorption affinity on sediments followed the order TMA > DMA > MMA, suggesting that electrostatic attraction, hydrophobicity, and van der Waals forces are also the dominant processes controlling adsorption of amines in sediment. In contrast with the amines, adsorption of the three amino acids and acetate by FP sediment was much greater than by the clays. The amino acids were adsorbed more by the sediment than were the three amines while acetate was adsorbed roughly to the same extent.

If ion-exchange were the dominant process for adsorption of amino acids, as it appears to be for the amines, we might have expected the negatively-charged GLU and ACET to be less strongly adsorbed by the negatively-charged sediment particles compared to the amines. Clearly, the adsorption of these compounds in sediments can not be explained simply by ion- exchange processes. Additional adsorption processes and associated chemical reactions were likely involved in the adsorption of amino acids and acetate in the sediments. In their recent study of amino acid adsorption, Henrichs and Sugai (1993) suggested that, in addition to the adsorption of amino functional groups to negatively-charged sediment particle surfaces, the negatively-charged carboxyl functional group also played a role in adsorption, either by reacting with sedimentary organic matter or by binding via a bridging cation such as Ca 2+. We noted that amine adsorption was also enhanced by organic matter present in FP sediments in our earlier study, most likely either through additional ion-exchange capacity or through chemical reactions.

In general, the adsorption of these naturally occurring biogenic compounds in marine


sediments is not as great as adsorption of hydrophobic compounds such as large molecular weight lipids and hydrocarbons. For example, Zullig and Morse (1988) determined partition coefficients in the range of 1.3× 104 to 2.5 × 104 for the fatty acids palmitate and stearate adsorbed by carbonate minerals. Sansone et al. (1987) measured a Kaas of 210 for stearate adsorbed by Cape Lookout Bight anoxic sediments. Brownawell (1986) reported partition coefficients in the range of 103 to 104 for polychlorinated biphenyl compounds adsorbed in marine sediments. However, adsorption of amines and amino acids in marine sediments is much greater than for other small soluble compounds such as glucose. Henrichs and Sugai (1993) measured Kads for glucose of 0.58 at 10 nM concentration and < 0.1 at 1/zM concentration in marine sediment.

(2) Effect of sedimentary organic matter Adsorption of organic compounds on sedi-

mentary organic matter is not as well under- stood as adsorption on clay minerals. Several studies have indicated that organic material plays a major role in the surface chemistry of particulate matter in natural aquatic systems (Hunter and Liss, 1979; Tipping and Cooke, 1982; Loder and Liss, 1985). Since natural organic matter is composed of a mixture of various compounds, the organic coating on the surfaces of sediment particles provides a variety of functional groups on the surfaces. However, most natural particles are negatively charged since negatively-charged functional groups, such as carboxylate ( -CO0- ) and phenol (tO- O-), are dominant in the organic coatings (Hunter, 1980; Davis, 1982). Thus, the larger- than-expected adsorption of negatively-charged amino acids and acetate in sediment is more likely due to chemical binding via surface functional groups than to ion exchange with the organic coating or inorganic matrix. This conclusion is consistent with studies showing that sedimentary organic matter plays an important role in the adsorption of amino acids and acetate

in different marine sediments (Rosenfeld, 1979a; Sansone et al., 1987; Henrichs and Sugai, 1993). This conclusion is also strongly supported by the results of peroxide treatment experiments where no adsorption of ALA, GLU or ACET and very little adsorption of LYS was measured after sediment was treated with H202 (Table 2). In contrast, after H202 treatment, significant adsorption of the three amines was still measured. This suggests that the adsorption of amino acids and acetate with their carboxyl functional groups is more affected by organic matter in sediment than adsorption of the amines. How- ever, since peroxide treatment can also affect cation exchange capacity of clay minerals by oxidizing metals in their structure (Rashid, 1969), it is difficult to quantitatively determine the effect of organic matter alone on the adsorption of these compounds based only on the peroxide treatments.

Evidence for the influence of organic matter on the adsorption of amino acids can also be seen from the experiment using FP sediments extracted with seawater or 0.1 N NaOH (Fig. 8). Adsorption of the three amino acids decreased with each seawater extraction and when extracted with NaOH, whereas TMA was not as strongly affected. However, Gschwend and Wu (1985) pointed out that this type of sediment extraction might be subject to a centrifugation artifact. They suggested that the decreased adsorption of PCBs by sediment with each wash cycle could be due to nonsettling fine particles being lost from the sediment since centrifugation could not remove these fine particles. However, in our experiment, this does not appear to be a problem. If the fine particles (clay) were lost during each wash, we would have expected a decrease in TMA adsorption as was observed with the amino acids; but none was observed. Perhaps the extraction of fresh sediment removed organic matter that was either dissolved in porewater or coated on the sediment particles and that increased the adsorption capacity of the sediment.

The seawater and NaOH extraction treat-


ments clearly changed the adsorption behavior of the sediments. In marine sediments, a large fraction of dissolved organic matter is present as humic substances or colloidal material (Krom and Sholkovitz, 1977). The reaction of amino acids with sedimentary humic substances may be responsible, in part, for the observed differences in amino acid adsorption by sediments before and after treatment with base. After humic substances and other organic matter were removed from sediment by 0.1N NaOH extraction, adsorption of ALA and GLU was negligible. However, positively-charged TMA and LYS were still greatly adsorbed. Boatman and Murray (1982) studied the effects of humic matter on the adsorption of ammonium in marine sediments. They found that after humic matter was removed from sediment by extraction with base, the adsorption of NH~- was essentially the same as that of untreated sediment, similar to our results for TMA. This suggests that adsorption of amines is less dependent on sedimentary humic matter than is adsorption of amino acids. Abelson and Hare (1970, 1971) reported that in the presence of humic acids, amino acids are removed from solution although at elevated temperature. Perhaps interactions between amino acids (but not ammonium or amines) and humic or other organic substances in the sediments increase their apparent adsorption.

Adsorption of organic compounds could greatly affect their distributions in marine sediments. In our adsorption study, the concentration ranges used covered the natural range measured in different marine sediment porewaters. The adsorption coefficients determined in laboratory experiments suggest that adsorption could be an important removal process of free amines and amino acids from sediment porewaters. Low dissolved concentrations compared to higher solid-phase concentrations measured in many sedimentary environments for both amines (King et al., 1983; Lee and Olson, 1984; Glob and Sorensen, 1987; Lee, 1988; Wang and Lee, 1990) and amino acids (Rosenfeld, 1979a; Hen-

richs and Farrington, 1987; Burdige and Mar- tens, 1988, 1990) could be partly due to adsorption of these compounds by sediments, although the particulate input of the compounds to sediments is more likely the major factor influencing their distributions. From a model of the cycling of dissolved free amino acids (DFAA) in the anoxic marine sediments of Cape Lookout Bight, NC, Burdige and Martens (1990) estimated that during organic matter decomposition, a large fraction of DFAA produced was reincorporated back into the sediments (incorporation rate of 15.3 mol N/m2yr compared to the annual production rate of 18.3 mol N/m2yr) rather than being remineralized. They suggested this incorporation was most likely due to bacterial uptake. However, abiotic processes such as irreversible association and geopolymerization may also play an important role. The incorporation of DFAA into the sediments might play a role in the preservation of amino acids and perhaps total organic matter in anoxic marine sediments.

(3) Desorption The adsorption of MMA, DMA and TMA by

FP sediment was mostly reversible (the percentages of Krev to Kads are 63, 76, and 90% for MMA, DMA and TMA, respectively). Simi- larly, 60% of the positively-charged LYS adsorbed onto FP sediment was also easily desorbed. This reversible behavior was consistent with the idea that ion-exchange or other electrostatic interactions are the dominant processes controlling adsorption of these positively- charged compounds in marine sediments as is the case for ammonium adsorption in marine sediments (Rosenfeld, 1979b; Mackin and Aller, 1984). Adsorption of ALA and GLU by FP sediment resulted in a much larger resistant fraction and 69% of ALA and 66% of GLU remained in the solid phase after the consecutive desorption experiment. For ACET, with only a carboxyl functional group, more than 90% remained on the kaolinite and FP sediment after consecutive desorption. This strong irrever-

18 Xu-Chen Wang, C. Lee/Marine Chemistry 44 (1993) 1-23

sible adsorption behavior has also been reported for amino acids (Rosenfeld, 1979a; Christensen and Blackburn, 1980; Burdige and Martens, 1990; Henrichs and Sugai, 1993) and acetate (Shaw et al., 1984) in other marine sediments.

This different desorption behavior between the amines, amino acids and acetate again illustrates the different mechanisms controlling the adsorption of these organic compounds onto sediments as discussed above. Amino acids can react with sugars to form humic-like melanoidins, whose chemical properties are strongly influenced by the type of amino acid precursor. Hedges (1978) found that production of melanoidins from the basic amino acid, LYS, was faster than from acidic and neutral amino acids. He also suggested that in marine sediments, large nitrogen-rich molecules, such as lysine melanoidins, should have a great affinity for clay mineral surfaces. Thus, the reaction of amino acids with other organic compounds in porewater or on sedimentary particles may be partially responsible for the irreversible uptake of amino acids or their reaction products onto marine sediments.

Adsorption of amino acids and acetate may greatly influence the diagenesis of these compounds in marine sediments. Sugai and Hen- richs (1992) have reported that amino acids adsorbed onto sediment decompose less rapidly than dissolved free amino acids in the porewaters. Christensen and Blackburn (1980, 1982) also reported that significant fractions of alanine and acetate adsorbed irreversibly to marine sediments were not decomposed during 4 week incubation periods. Therefore, the reversibility of adsorption of different organic compounds can greatly influence their decomposition rates and preservation in marine sediments.

(4) Effect of redox condition of sediment The reason for unusual desorption behavior of

LYS from FP sediment as shown in Fig. 7 was not clear. Instead of decreasing, the concentration of dissolved LYS increased during the first three consecutive desorption cycles. We thought

that release of LYS during the desorptlon cycles might be due to a change in redox condition of the sediment after replacing the solution phase with fresh oxic seawater. Thus, oxic and anoxic experiments were conducted to see how sediment redox condition affects LYS adsorption. As shown in Fig. 9, the redox status greatly influenced adsorption of LYS. The partition coefficient, Kads, of LYS was about six times higher in oxidized sediment than in anoxic sediment, and was almost four times higher than the Kads value in Table 1. The normal adsorption experiments were always conducted within 2 h after collecting the sediment and the entire adsorption experiment was conducted within 3 h. This may not have been enough time to allow complete oxidation of the sediment. In comparison the "oxidized" sediments used to obtain the KaOs shown in Fig. 9 were allowed to react for four days.

The greater adsorption of LYS by oxidized sediment was inconsistent with the results of the seawater extraction experiment in which adsorption of LYS and other amino acids decreased when the sediment was extracted with seawater more than one time (Fig. 8). As discussed earlier, seawater extraction of the sediment may have removed organic matter dissolved in porewater or coated on sediment particles that influenced adsorption of LYS and other amino acids. However, in the oxic adsorption experiment, although the sediment was oxidized for four days, organic matter dissolved during this process was not removed. Also, during the 4 day oxidation of the sediment, reduced chemical species were likely oxidized. For example, metal oxides such as FeOOH (from FeS) and MnOOH (from MnS) likely form and provide more adsorption surfaces in the sediment. Many studies have shown that metal oxides and other minerals commonly found in oxic marine sediments have a great potential to adsorb organic compounds (Stumm et al., 1980; Sigg and Stumm, 1981; Tipping, 1981; Davis, 1982). This may explain the increased adsorption of LYS observed in oxic compared to


anoxic FP sediments. The effect of redox condition of sediment on adsorption is not well under- stood; its potential influence on adsorption needs to be further studied.

Unfortunately, increased adsorption of LYS in oxidized sediment did not explain the behavior observed in Fig. 7. We are at a loss to explain this phenomenon of LYS release. Per- haps the adsorption capability of the sediment decreased during the dilution desorption cycles; or, competition for sedimentary adsorption sites from other materials, either in the fresh seawater or released from the sediment, resulted in the replacement of 14C-LYS, thus increasing its concentration in solution. Another possibility is that if LYS-LYS bonding is occurring as discussed earlier, this type of bonding may be more susceptible to release from the

sediments than LYS bound by other adsorption mechanisms.

(5) Particle size effect The greater adsorption by particles in the

smallest size range (< 65 #M) (Fig. 10) was expected due to their large surface areas (Mayer and Rossi, 1982). Clay minerals and organic matter are likely enriched in this smallest size range (Meyers and Quinn, 1973). Adsorption of positively-charged MMA, DMA, TMA and LYS in this fraction increased with molecular weight, similar to the sediment as a whole. Adsorption of all compounds except LYS by the particle fractions in the ranges of 65-125 #M and 125-300 #M was negligible. However, in the largest size fraction (300 # M - 1 mm), significant adsorption of DMA, TMA

Table 3 Dimensionless phase ratios of amines, amino acids and acetate in marine sediments

Compound Site Concentration K a Reference

MMA FP 0-2.3 #M 0.38 This study FP 0-4.0 #M 0.41 Wang and Lee, 1990 LIS 0-2.3 #M 0.71 Wang and Lee, 1990 Peru 0-2.3 #M 1.02 Wang and Lee, 1990

DMA FP 0-3.5 #M 0.65 This study FP 0-1.6 #M 0.58 Wang and Lee, 1990

TMA FP 0-2.5 #M 0.73 This study FP 0-1.7 #M 0.68 Wang and Lee, 1990 LIS 0-1.7 #M 0.95 Wang and Lee, 1990 Peru 0-1.7 #M 1.00 Wang and Lee, 1990

LYS FP 0-0.5 #M 13.9 This study Alaska 10 nM-1 M 2.7 7.4 Henrichs and Sugai, 1993

GLU FP 0-0.3 #M 1.2 This study Alaska 10 nM-0.1 mM 0.35-0.54 Henrichs and Sugai, 1993 L1S 0.3 mM 3.3 Rosenfeld, 1979a

ALA FP 0-1.0 #M 1.0 This study Alaska 10 nM-0.1 M 0.19-0.39 Henrichs and Sugai, 1993 Denmark 35 nM-0.2 #M 0.5-0.7 Christensen and Blackburn, 1980

ACET FP 0-0.2 #M 0.46 This study FP 0-0.012 #M 0.35 Michelson et al., 1989 CLB 5 #M-0.5 M 0.3 Sansone et al., 1987 Alaska 1 mM-1 M 0.3-1.6 Shaw et al., 1984

aKvalues are calculated based on sediment-seawater slurry porosity (see eq. in text). The K values from Wang and Lee (1990) are different from those published since K was calculated then using sediment rather than slurry porosity. K values from Rosenfeld (1979a), Christensen and Blackburn (1980), Michelson et al. (1989), Sansone et al. (1987) and Shaw et al. (1984) are estimated based on Kad~ values and slurry porosity given in the references.

20 Xu-Chen Wang, C. Lee/Marine Chemistry 44 (1993) 1-23

and LYS, as well as GLU was measured. This size fraction contained organic detritus from marsh grasses. Mackin and Swider (1989) have reported that most organic detritus in FP sediments is derived from surrounding stands of S. alterniflora. This organic detritus has a great influence on the adsorption behavior of amines (Wang and Lee, 1990) and ammonium (Mackin and Aller, 1984) in sediments.

4.3 Comparison with other studies

Several previous studies have investigated adsorption of amines, amino acids and acetate in different marine sediments. In order to com- pare our study with others, we converted Kads, the porosity-independent partition coefficient, to a dimensionless phase ratio, K, based on the equation:

l - K-- ~ q~ psKads

where Ps is the dry sediment density (assumed to be 2.6 g/m1), and ~b is the porosity of the sediment-seawater slurry. Comparison of porosity-corrected K rather than Kaas values was necessary since the data being compared were obtained on both slurried and non-slurried sediments. Table 3 summarizes the calculated K values for the compounds tested in this study and estimated K values for adsorption of these compounds from other studies in different marine environments.

The K values for the three amines adsorbed by FP sediment in these experiments are similar to values we measured earlier (Wang and Lee, 1990) and also to those of ammonium reported by Rosenfeld (1979a) for LIS sediment and by Mackin and Aller (1984) for other marine sediments. For the adsorption of the three amino acids, the dimensionless K values we obtained for FP sediments are somewhat higher than the values determined by Henrichs and Sugai (1993) for the same compounds adsorbed by Resurrec- tion Bay (AK) sediments. This difference may reflect the different adsorption capacity of the

sediments due to the much lower organic content in Resurrection Bay sediment (organic carbon content was 2.8% for FP sediment and 0.6% for RB sediment). The FP values are more similar to phase ratios reported by Henrichs and Farrington (1987) for nonprotein amino acids in more organic-rich (OC ~ 2%) Buzzards Bay, MA, sediments (1.1 to 2.2 for the acidic c~-ami- noadipic acid and 4.6 to 5.7 for the basic dia- minobutyric acid). In their study of decomposition of alanine in the coastal marine sediments of Denmark, Christensen and Black- burn (1982) also found K values for alanine in the 0.5-0.7 range. Rosenfeld (1979a) reported strong uptake of GLU by LIS sediment (OC ~ 2.7%) and concluded that most amino acid adsorption in the very anoxic LIS sediment he studied was by organic matter rather than by clay minerals. However, since no attention was given to microbial degradation in his study, bio- logical uptake may have affected the results. In spite of the differences between sediment types, the phase ratios of amino acids determined in these studies indicated that higher K values are generally associated with higher organic carbon content of the sediments, suggesting that sedimentary organic matter plays an important role in adsorption of amino acids in marine sediments.

The dimensionless K value for acetate in this study was similar to values reported for other marine sediments. Strong adsorption of acetate was also measured by Sansone et al. (1987) in the anoxic sediments of Cape Lookout Bight and by Shaw et al. (1984) in Skan Bay (AK) sediments. For the same site in Flax Pond as in this study, Michelson et al. (1989) determined an almost identical K of 0.35 for acetate adsorption. Sansone et al. (1987) also reported a linear relationship between the sediment organic matter content and Kads of acetate and several other short-chain fatty acids, such as butyrate and lactate, in marine sediments. However, to better understand the importance of sedimentary organic matter on the process of adsorption, further studies are needed on the


chemical reactions between dissolved organic matter and the naturally occurring organic compounds in particle coatings.

5. Conclusions

The results of this study lead to these conclusions:

(1) Adsorption of amines, amino acids and acetate by montmorillonite and kaolinite is consistent with control by both the electrostatic attraction between the functional group of the compounds and the clay particle surfaces and van der Waals forces between the compounds and the particle surfaces. Positively-charged compounds were adsorbed greatly by montmorillonite, while negatively-charged compounds were adsorbed much less by the two clays.

(2) This study suggests that adsorption of amines, amino acids and acetate in organic-rich marine sediments could be an important control on the distribution of these organic compounds in sediment porewaters. The smallest particles present in the sediment showed the highest adsorption capacity.

(3) Adsorption of amines by both clays and FP sediment was essentially a reversible process. The adsorption of the basic amino acid (LYS) showed mostly reversible behavior, while the acidic (GLU) and neutral (ALA) amino acids and acetate exhibited a major component of irreversible association. This is consistent with the adsorption of amines and LYS being controlled mainly by ion-exchange processes, while interactions with sedimentary organic matter are more likely the dominant processes in adsorption of ALA, GLU and ACET.

(4) Comparisons of K values from this study and from the literature with organic C values from the sediments also suggest the importance of organic matter in sediment adsorption processes.

(5) The redox condition of sediments may

have a great influence on adsorption of organic compounds in sediments.

Acknowledgments

We would like to thank B. Brownawell and S. Henrichs for their critical reviews of early ver- sions of the manuscript. A multitude of detailed comments from J. Hedges greatly improved the final manuscript. This research was supported by the National Science Foundation and the Office of Naval Research.

References

Abelson, P.H. and Hare, P.E., 1970. Uptake of amino acids by kerogen. Carnegie Inst. Washington Yearb., 68: 297-303.

Abelson, P.H. and Hare, P.E., 1971. Reaction of amino acids with natural and artificial humus and kerogens. Carnegie Inst. Washington Yearb., 69: 327-334.

Berner, R.A., 1980. Early Diagenesis: A Theoretical Approach. Princeton Univ. Press, Princeton, NJ, 241 pp.

Boatman, C.D. and Murray, J.W., 1982. Modeling exchangeable NH + adsorption in marine sediments: Process and controls of adsorption. Limnol. Oceanogr., 27: 99-100.

Brownaweli, B.J., 1986. The Role of Colloidal Organic Mat- ter in the Marine Geochemistry of PCBs. Ph.D. Thesis. Woods Hole Oceanogr. Inst./MIT Joint Progr., 318 pp.

Burdige, D.J. and Martens, C.S., 1988. Biogeochemical cycling in an organic-rich marine basin: 10. The role of amino acids in sedimentary carbon and nitrogen cycling. Geochim. Cosmochim. Acta, 52: 1571-1584.

Burdige, D.J. and Martens, C.S., 1990. Biogeochemical cycling in an organic-rich marine basin: 11. The sedimentary cycling of dissolved, free amino acids. Geochim. Cosmochim. Acta, 54: 3033-3052.

Christensen, D. and Blackburn, T.H., 1980. Turnover of tracer (t4C, aH labeled) alanine in inshore marine sediments. Mar. Biol., 58: 97-103.

Christensen, D. and Blackburn, T.H., 1982. Turnover of 14C-labelled acetate in marine sediments. Mar. Biol., 71: 113-119.

Cloos, P., Calicis, B., Fripiat, J.J. and Makay, K., 1966. Adsorption of amino acids and peptides by mont- moriilonite. I. Chemical and X-ray diffraction studies. Proc. Int. Clay Conf., Jerusalem, Vol. 1. Clay Miner. Soc., pp. 223-232.

Dashman, T. and Stotzky, G., 1982. Adsorption and binding of amino acids on homoionic montmorillonite and kaolinite. Soil Biol. Biochem., 14: 447-456.


Davis, J.A., 1982. Adsorption of natural dissolved organic matter at the oxide/water interface. Geochim. Cosmochim. Acta, 46: 2381-2393.

DiToro, D.M. and Horzempa, L.M., 1982. Reversible and resistant components of PCB adsorption-desorption: isotherms. Environ. Sci. Technol., 16: 594-602.

DiToro, D.M., Mahony, J.D., Kirchgraber, P.R., O'Byrne, A.L., Pasquale, L.R., and Piccirilli, D.C., 1986. Effects of nonreversibility, particle concentration, and ionic strength on heavy metal sorption. Environ. Sci. Tech- nol., 20: 55-61.

Glob, E. and Sorensen, J., 1987. Determination of dissolved and exchangeable trimethylamine pool in sediments. J. Microbiol. Methods, 6: 347-355.

Gschwend, P.M. and Wu, S.C., 1985. On the constancy of sediment-water partition coefficients of hydrophobic organic pollutants. Environ. Sci. Technol., 19: 90-96.

Hayes, M., 1985. Extraction of humic substances from soil. In: G.R. Aiken, D.M. Mcknight, R.U Wershaw and P. MaCarthy (Editors), Humic Substances in Soil, Sedi- ment, and Water: Geochemistry, Isolation, and Charac- terization. Wiley, New York, NY, pp. 329-362.

Hedges, J.I., 1978. The formation and clay mineral reactions of melanoidins. Geochim. Cosmochim. Acta, 42: 69-76.

Hedges, J.I. and Hare, P.E., 1987. Amino acid adsorption by clay minerals in distilled water. Geochim. Cosmochim. Acta, 51: 255-259.

Henrichs, S.M. and Farrington, J.W., 1987. Early diagenesis of amino acids and organic matter in two coastal marine sediments. Geochim. Cosmochim. Acta, 51: 1-15.

Henrichs, S.M. and Sugai, S.F., 1993. Adsorption of amino acids and glucose by sediments of Resurrection Bay (Alaska): Functional group effects. Geochim. Cosmo- chim. Acta, 57: 823-835.

Huheey, J.E., 1983. Inorganic Chemistry: Principles of Structure and Reactivity. Harper and Row, New York, NY, 3rd ed., 936 pp.

Hunter, K.A., 1980. Microelectrophoretic properties of surface-active organic matter in coastal seawater. Limnol. Oceanogr., 25: 807-822.

Hunter, K.A. and Liss, P.S., 1979. The surface charge of suspended particles in estuarine and coastal waters. Nat- ure, 282: 823-825.

King, G.M., Klug, M.J. and Lovley, D.R., 1983. Metabo- lism of acetate, methanol, and methylated amines in intertidal sediments of Lowes Cove, Maine. Appl. Envir- on. Microbiol., 45: 1848-1853.

Krom, M.D. and Sholkovitz, E.R., 1977. Nature and reactions of dissolved organic matter in the interstitial waters of marine sediments. Geochim. Cosmochim. Acta, 41: 1565-1573.

Lee, C,, 1988. Amino acid and amine biogeochemistry in marine particulate material and sediments. In: T.H. Blackburn and J. Sorensen (Editors), Nitrogen Cycling in Coastal Marine Environments. ScoPE Ser., 33. Wiley, New York, NY, pp. 125-141.

Lee, C. and Olson, B.L., 1984. Dissolved, exchangeable and bound aliphatic amines in marine sediments: initial results. Org. Geochem., 6: 259-263.

Loder, T.C. and Liss, P.S., 1985. Control by organic coatings of the surface charge of estuarine suspended particles. Limnol. Oceanogr., 30: 418-421.

Mackin, J.E. and Aller, R.C., 1984. Ammonium adsorption in marine sediments. Limnol. Oceanogr., 29: 250-257.

Mackin, J.E. and Swider, K.T., 1987. Modeling the dissolu- tion behavior of standard clays in seawater. Geochim. Cosmochim. Acta, 51: 2947-2964.

Mayer, L.M. and Rossi, P.M., 1982. Specific surface areas in coastal sediments: relationships with other textural fac- tors. Mar. Geol., 45: 241-252.

Meyers, P.T. and Quinn, J.G., 1973. Organic matter on clay minerals and marine sediments. Chem. Geol., 13: 63-68.

Michelson, A.R., Jacobson, M.E., Scranton, M.I. and Mackin, J.E., 1989. Modeling the distribution of acetate in anoxic estuarine sediments. Limnol. Oceanogr., 34: 747-757.

Morel, F.M.M. and Gschwend, P.M., 1987. The role of colloids in the partitioning of solutes in natural waters. In: W. Stumm (Editor), Aquatic Surface Chemistry. Wi- ley, New York, NY, pp. 405-422.

Palmer, J. and Bauer, N., 1961. Sorption of amines by montmorillonite. J. Phys. Chem. 65: 894-895.

Parks, G.A., 1975. Adsorption in the marine environment. In: J.P. Riley and G. Skirrow (Editors), Chemical Ocea- nography, Voi. 1. Academic Press, New York, NY, pp. 241-308.

Rashid, M.A., 1969. Contribution of humic substances to the cation exchange capacity of different marine sediments. Marit. Sediments, 5: 44-50.

Rosenfeld, J.K., 1979a. Amino acid diagenesis and adsorption in nearshore anoxic sediments. Limnol. Oceanogr., 24: 1014-1021.

Rosenfeld, J.K., 1979b. Ammonium adsorption in nearshore anoxic sediments. Limnol. Oceanogr., 24: 356- 364.

Rowland, R.A. and Weiss, E.J., 1963. Bentonite-methyta- mine complexes. Clays Clay Miner., 10: 460-468.

Sansone, F.J., Andrews, C.C. and Okamoto, M.Y., 1987. Adsorption of short-chain organic acids onto nearshore marine sediments. Geochim. Cosmochim. Acta, 51: 1889-1896.

Shaw, D.W., Alperin, M.J., Reeburgh, W.S. and McIntosh, D.J., 1984. Biogeochemistry of acetate in the anoxic sediments of Skan Bay, Alaska. Geochim. Cosmochim. Acta, 48: 1819-1825.

Sigg, L. and Stumm, W., 1981. The interaction of anions and weak acids with the hydrous goethite (aFeOOH) surface. Colloids Surf., 2: 101-117.

Stumm, W. and Morgan, J.J., 1981. Aquatic Chemistry. Wiley, New York, NY, 2nd ed., 780 pp.

Stumm, W., Kummert, R. and Sigg, L., 1980. A ligand exchange model for the adsorption of inorganic and


organic ligands at hydrous oxide interfaces. Croat. Chem. Acta, 53: 291-312.

Sugai, S.F. and Henrichs, S.M., 1992. Rates of amino acid decomposition in Resurrection Bay (Alaska) sediments. Mar. Ecol. Prog. Ser., 88: 129-141.

Theng, B.K.G., 1974. The Chemistry of Clay--Organic Reactions. Wiley, New York, NY, 343 pp.

Tipping, E., 1981. The adsorption of aquatic humic substances by iron oxides. Geochim. Cosmochim. Acta, 45: 191-199.

Tipping, E. and Cooke, D., 1982. The effect of adsorbed humic substances on the surface charge of goethite (c~- FeOOH) in freshwaters. Geochim. Cosmochim. Acta, 46: 75-80.

Van Olphen, H., 1977. An Introduction to Clay Colloid Chemistry. Wiley, New York, NY, 2nd ed., 318 pp.

Wang, X.C., 1989. The Distribution and Adsorption Behavior of Aliphatic Amines in Coastal Marine Sedi- ments. M.S. thesis. State Univ. New York, Stony Brook, NY, 175 pp.

Wang, X.C. and Lee, C., 1990. The distribution and adsorption behavior of aliphatic amines in marine and lacus- trine sediments. Geochim. Cosmochim. Acta, 54: 2759-2774.

Whitfield, M. and Turner, D., 1985. The role of particles in regulating the composition of seawater. In: W. Stumm (Editor), Aquatic Surface Chemistry. Wiley, New York, NY, pp. 457-494.

Zullig, J.J. and Morse, J.W., 1988. Interaction of organic acids with carbonate mineral surfaces in seawater and related solutions: I. fatty acid adsorption. Geochim. Cosmochim. Acta, 52: 1667-1678.

adsorption and desorption of aliphatic amines, amino acids and acetate by clay minerals and marine...

Documents