Relative Importance of Solid-PhasePhosphorus and Iron on theSorption Behavior of SedimentsJ I A - Z H O N G Z H A N G * , † A N DX I A O - L A N H U A N G † , ‡

Ocean Chemistry Division, Atlantic Oceanographic andMeteorological Laboratory, National Oceanic and AtmosphericAdministration, Miami, Florida 33149, and CIMAS,Rosenstiel School of Marine and Atmospheric Science,University of Miami, Miami, Florida 33149

Of all the metal oxide particles, amorphous iron oxideshave the greatest adsorption capacity for phosphate. Coastalsediments are often coated with terrigenous amorphousiron oxides, and those containing high iron are thought tohave a high adsorption capacity. However, this conventionalwisdom is based largely upon studies of phosphate adsorptionon laboratory-synthesized minerals themselves containingno phosphorus. Using natural sediments that containvariable phosphorus and iron, our results demonstratethat the exchangeable phosphate rather than the iron oxidesof sediments governs the overall sorption behavior. Theiron oxide content becomes important only in sediments thatare poor in phosphorus. A total of 40 sampling sitesacross the Florida Bay provide detailed spatial distributionsboth of the sediment’s zero equilibrium phosphateconcentration (EPC0) and of the distribution coefficient(Kd) that are consistent with the distribution of theexchangeable phosphate content of the sediment. Thisstudy provides the first quantitative relationships betweensorption characteristics (EPC0 and Kd) and the exchangeablephosphate content of natural sediments.

IntroductionAlthough phosphorus (P) is an essential macronutrient toliving organisms, its biological availability in aquatic envi-ronments is limited by the low solubility of P-bearing mineralsand the strong affinity of dissolved phosphate to solidsurfaces. For example, more than 90% of the riverine flux ofP to the oceans is associated with suspended particulatematter (1). Most riverborne suspended particulate matter isaggregated and settles out, becoming bottom sediments inestuarine and coastal regions (2). Not surprisingly, sedimenthas been identified as the dominant P reservoir in a varietyof aquatic ecosystems (3-6). As a result, P exchange acrossthe sediment-water interface, through adsorption-desorp-tion and coprecipitation-dissolution processes, plays acritical role in governing dissolved phosphate concentrationsin the overlying waters (7, 8). Such exchange processes arelargely controlled by the chemical composition and physicalproperties of particles. Among the different metal oxideparticles, amorphous iron oxides have been shown to havethe greatest adsorption capacity for phosphate (9-14).

Because iron is ubiquitous in the environment, the contentof amorphous iron oxides in the sediments has beenconsidered to be the most important factor in regulatingsediment’s adsorption capacity. Coastal sediments are oftencoated with terrigenous amorphous iron oxides, and thosecontaining high iron are thought to have a high adsorptioncapacity. However, this conventional wisdom is based largelyupon studies of phosphate adsorption on laboratory-synthesized minerals themselves containing no P (9-12).Natural particles, such as sediments, soils, and dust, alwayscontain P, an abundance of which depends upon their originand sedimentary environments. Although aquatic scienceliterature is replete with phosphate sorption studies, studieson sediment sorption in conjunction with the analysis ofsolid-phase P speciation have been rare. Moreover, due toa heavy workload involved in sediment sampling, processing,and analysis, most studies are often limited to a few samplingsites, in many cases, sampling only two end-members. Ameaningful spatial distribution pattern cannot be obtained,and a statistically sound relationship, particularly a nonlinearfunction, cannot be derived from such studies. To ourknowledge, the influence of solid-phase P on phosphatesorption behavior of sediments has not been systematicallystudied.

The processes of P exchange across the sediment-waterinterface become particularly important in shallow coastalsystems such as the Florida Bay where dissolved phosphatein the water column is present at nanomolar concentrations(15, 16) and P has been identified as a limiting nutrient toseagrass, phytoplankton, and bacteria (17-19). Sedimentsin the Florida Bay consist mainly of biogenic carbonate(81-96% by weight (20)), which strongly adsorbs phosphate(21-23). The amount of adsorption is known to be propor-tional to surface area, which increases exponentially with adecreasing particle size. It has been shown that fine particlesaccounted for only 16% of weight but represented 76% oftotal surface area in bulk sediments (24). In addition, fineparticles are easy to resuspend into the water column bydisturbances. Once in the water column, their residence timesare much longer than coarser particles since their settlingvelocity is proportional to the square of the particle diameter(25). In the shallow waters of the Florida Bay (average waterdepth 1 m), fine-grained carbonate muds were readilyresuspended to the surface by wind and tidal mixing andsupply P to euphotic zone for biological assimilation (26).Wind conditions sufficient to resuspend fine-grained sedi-ments throughout the Florida Bay occur at an ca. weeklyinterval of frontal passages during the dry season, whilelocalized strong convective events are common during thewet season (27).

Accurately quantifying the P exchange across the sedi-ment-water interface during sediment resuspension isessential for estimating benthic P flux and understandingthe biogeochemistry and ecology of the bay. Our previousstudy documented a strong spatial gradient in both sedi-mentary P and iron across the bay (20). It is reasonable tohypothesize that sorption behavior of sediments will sys-tematically vary due to the gradients in these physicochemicalproperties. The aim of the present study is to document thespatial variability of sediment characteristics with respect tothe sediment-water exchange of P and to identify the factorsgoverning such variability.

Experimental ProceduresStudy Region. Located at the southern end of the Floridapeninsula, the Florida Bay is one of the world’s largest (2200

* Corresponding author phone: (305) 361-4512; fax: (305) 361-4447; e-mail: jia-zhong.zhang@noaa.gov.

† AOML, NOAA.‡ CIMAS, University of Miami.

Environ. Sci. Technol. 2007, 41, 2789-2795

10.1021/es061836q CCC: $37.00 2007 American Chemical Society VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2789Published on Web 03/16/2007

km2) coastal lagoons (Figure 1). Its triangular-shaped area isbordered to the north by the Everglades, the world’s largestwetlands. A chain of islands known as the Florida Keysseparates the shallow bay from the Atlantic Ocean, formingits eastern and southern boundaries. Its westerly margin isopen to the Gulf of Mexico, although water exchange is highlyrestricted by extensive shallow western margin mudbanks.Dotted mangrove islands and a complex network of carbonatemudbanks divide the interior bay into numerous isolatedsub-basins with maximal water depths of 2-3 meters. Waterexchange between sub-basins occurs through narrow cutsand overbank wash. Our previous study revealed that theexternal sources of two limiting nutrients, P and iron, to theFlorida Bay were spatially separated with P introduced bycoastal waters across its western margin and iron introducedfrom freshwater canal flows into the eastern bay (20).

Sediment Samples. Surface sediments were collectedfrom 40 stations across Florida Bay (Figure 1). Details ofsampling and sample processing were given elsewhere (20).To simulate resuspension events, easily resuspended fine-grained sediments (<124 µm) were obtained at each stationfor use in sorption experiments. The chemical forms of P inthese same sediments had been previously determined usingan improved sequential extraction technique (20). Theexchangeable phosphate of the sediment, Pexch, was opera-tionally defined as inorganic P extracted by MgCl2 solution,and the surface-reactive iron oxide content of sediments,Fe(III)O, was determined from reductive dissolution usinga bicarbonate-dithionate solution (20).

Adsorption-Desorption Isotherm Experiments. Foreach experiment, 100 mg of sediment was mixed with 50 mLof low-nutrient seawater in 60 mL high-density polypropylenebottles. The low-nutrient seawater had been collected fromthe surface of the Gulf Stream in the Florida Straits and hada typical salinity of 36 and a pHT of 7.95 at 25 °C (28). Onemilliliter of chloroform was added to the mixture to inhibitmicrobial activity. The mixture was agitated by a temperature-controlled orbital shaking incubator at 25 ( 0.1 °C for 4 daysto ensure complete hydration and to reach the steady-state.At the end of the conditioning period, an appropriatephosphate standard solution was added to each bottle. Eightinitial phosphate concentrations, [P]i, were used with eachsediment sample, and maximum concentrations ranged from10 to 60 µM, depending on sedimentary P levels. After another24 h shaking, the suspension was filtered through a 0.45 µmpore size filter. Phosphate in the filtrate was determined bythe phosphomolybdenum blue method with a Hewlett-Packard 8453 spectrophotometer (29). The precision ofreplicate experiments was 1-2%.

The final phosphate concentration in seawater, [P]f, wasoperationally defined as an equilibrium phosphate concen-

tration, EPC. The difference between [P]i, and [P]f was usedto calculate the amount of phosphate adsorbed (if positive)to or desorbed (if negative) from the sediment, ∆[Psed]

A plot of ∆[Psed] versus [P]f at a given temperature is calledan adsorption isotherm. A modified Freundlich equation wasused to parameterize the adsorption isotherm data

where NAP is the native adsorbed phosphate, Kf is theFreundlich coefficient, and n is the exponential factor. NAPwas introduced in eq 2 to account for desorption fromsediments at low EPC. The value of [P]f at ∆[Psed] ) 0 is thezero equilibrium phosphate concentration, EPC0, and rep-resents the crossover concentration at which sediment is inequilibrium with seawater with respect to the exchange ofphosphate across their interface. Sediment at EPC0 displaysits maximum intensity for buffering phosphate. In this regionof the diagram, the true equilibrium is rapidly re-attained ifthe system is perturbed slightly, and a final phosphateconcentration will be re-established very near the EPC0 aftera small amount of phosphate is added to or removed fromthe sediment-water system (8). Because sediment is alwaysin close contact with a confined, small volume of porewater(high solid to water ratios), it reaches quasi-equilibrium overtime and establishes an EPC close to EPC0 in its porewater.Measured EPC0 can serve as a proxy for porewater phosphateconcentrations in sediments under steady-state conditions(i.e., in the absence of large sources and sinks). The slope ofthe isotherm curve at ∆[Psed] ) 0 is the linear adsorptioncoefficient Kd (the distribution coefficient or the bufferintensity at the EPC0). It is defined as the number of mol ofphosphate adsorbed to (or desorbed from) the sedimentsrequired to change the phosphate concentration in water by1 mol/L near the EPC0. It is calculated by taking the derivativeof the modified Freundlich equation with respect to [P]f atthe EPC0.

Results and DiscussionSediments from different regions of the bay exhibited distinctcharacteristics (Figure 2). Sediments from the eastern bay(stations 1-7) released little phosphate even at very low EPCand showed a strong tendency to adsorb phosphate fromseawater when the EPC slightly increased. On the other hand,sediments from the western bay (stations 9-13) releasedlarge amounts of phosphate until the EPC reached a highconcentration (10-20 µM). Moreover, the slopes of theisotherm curves in Figure 2 indicate that the buffer intensityof sediments decreases rapidly from east to west along thenorthern boundary of the bay.

The modified Freundlich equation (eq 2) provided anadequate model to describe the adsorption-desorptionisotherm data (mean correlation coefficient 0.984 ( 0.014,p < 0.001) and quantitatively characterizes sedimentarybehaviors with respect to the sediment-water exchange ofphosphate (Table S1). NAP (in µmol g-1) ranged from 0.33at station 4 in the eastern bay to 2.04 at station 15 on thewestern margin of the bay, with an average value of 1.09 (0.50. Kf (in L g-1) ranged from 0.13 at station 13 in thenorthwest corner of the bay to 1.41 at station 3 in the easternbay, with an average value of 0.76 ( 0.35. The n ranged from0.21 at station 3 in the eastern bay to 0.78 at stations 13 and30 in the western bay, with an average value of 0.47 ( 0.16.Sediment behavior with respect to phosphate exchange innatural systems is best characterized by EPC0 and Kd. EPC0

FIGURE 1. Location of the 40 sediment sampling sites across theFlorida Bay.

∆[Psed] ) [P]i - [P]f (1)

∆[Psed] + NAP ) Kf([P]f)n (2)

Kd ) d{Kf([P]f)n}/d([P]f)EPC0 ) nKf([P]f)

n-1 (3)

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ranged from 0.4 to 27.8 µM with an average value of 4.47 (6.92 µM. Kd (in L g-1) ranged from 0.033 to 0.716 with anaverage value of 0.289 ( 0.184 (Table S1). The wide rangesof EPC0 (70-fold) and Kd (20-fold) observed reflect largevariations in the sedimentary characteristics with respect tothe exchange of phosphate across the Florida Bay.

Observed sorption parameters in the Florida Bay sedi-ments are comparable with measurements made in theCooper Estuary on the North Atlantic coast where EPC0 andKf ranged from 0.1 to 15.1 µM and 0.025 to 0.858 L g-1,respectively (30). Our values are also within the rangemeasured in North Sea sediments where Kf ranged from 0.03to 1.81 L g-1 and n ranged from 0.23 to 0.54 (14). Our measuredEPC0 values (ranging from 0.16 to 27.78 µM) are within therange of porewater phosphate (0.05-33.8 µM) measured inthe Florida Bay (31). However, a rigorous comparison of therespective spatial distributions cannot be made becausestation locations are not available for the earlier porewaterstudy.

The grid distribution and number of sampling sites acrossthe bay afford a relatively high-resolution spatial distributionof EPC0 and Kd for the Florida Bay. EPC0 showed a strongwest-east spatial gradient (Figure 3a), with the highest value(27.8 µM) at station 13 in the northwest corner of the bay andthe lowest value (0.4 µM) at stations 2 and 40 in the easternbay. The strong west-east spatial gradient is in goodagreement with the distribution of Pexch (Figure 3b) deter-mined in our previous study (20). In contrast, the spatialdistribution of Kd (Figure 4a) showed a strong east-westgradient, with the highest values (about 0.7 L g-1) at stations2 and 21 in the eastern bay and the lowest value (0.033 L g-1)at station 13. In the eastern bay, an area of maximum Kd

overlapped with that of the lowest Pexch (Figure 3b) and thehighest Fe(III)O (Figure 4b). In the northwest corner of bay,an area of minimum Kd overlapped with that of the highestPexch (Figure 3b). Sediments in this area also containedrelatively high Fe(III)O (Figure 4b) (20).

The high Kd and the low EPC0 in the eastern bay indicatethat sediments in this region have a strong potential to

maintain the low water column phosphate concentrationsas previously observed (16) and a large buffer intensity withrespect to any external loading of dissolved phosphate. Onthe other hand, the high EPC0 and low Kd in the northwestcorner of the bay indicate that sediments in that region serveas a sustained source of phosphate to the overlying watersbecause observed phosphate concentrations in those waters(16) never reached the high EPC0 predicted from theunderlying sediments in this study.

In addition to the consistent spatial distributions describedpreviously, there is a strong correlation of Pexch with bothEPC0 and Kd (Figure 5). A quantitative relationship can beestablished between EPC0 and Pexch

with a correlation coefficient of 0.9598 (n ) 40, p < 0.0001).The relationship predicts that EPC0 should increase rapidlywith increasing Pexch. It is important to note that the Fe(III)Ocontent has little apparent influence on the EPC0 of sedi-ment. For example, sediments from station 2 in the easternbay contained the largest Fe(III)O (4.21 µmol g-1) and yethad an EPC0 value (0.484 µM) similar to that of station39 (0.552 µM), whose Fe(III)O was among the lowest (1.1µmol g-1) measured within the bay. Low values of EPC0

observed at these two stations are solely related to their lowPexch (0.031 and 0.056 µmol g-1 for stations 2 and 39,respectively) despite the extreme Fe(III)O variability noted.

As shown in Figure 5, Kd can also be described as a simplefunction of Pexch

with a correlation coefficient of 0.8706 (n ) 40, p < 0.0001).This predicts that Kd should decrease exponentially withincreasing Pexch. At high Pexch, Kd were low and remainedalmost constant despite varying Fe(III)O. When Pexch de-creased from 0.6 to 0.2 µmol g-1, Kd increased gradually from

FIGURE 2. Typical sorption isotherms of sediments across sedimentary P gradient along the northern coast (stations 1-13) in the FloridaBay. Note that the slopes of isotherms decrease from east (stations 1-7) to west (stations 9-13), indicating a spatial gradient of sedimentbuffer intensity across the bay.

EPC0 ) 4.4494Pexch + 57.31(Pexch)2 (4)

Kd ) 0.0391(Pexch)-0.886 (5)

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less than 0.05 to 0.15 L g-1. However, a rapid increase in Kd

was observed when Pexch was depleted below 0.12 µmol g-1.The wide range of Kd observed at low levels of Pexch is attributedto varying Fe(III)O in these P-poor sediments.

A plot of Kd versus Fe(III)O from all 40 sediments acrossthe bay (Figure 6) shows no significant correlation (p ) 0.082).However, two distinctive trends were observed if data aregrouped into two categories based on the levels of Pexch. Inthe eastern bay where Pexch was low (<0.12 µmol g-1), apositive correlation (n ) 19, r2 ) 0.4665, p ) 0.0013) betweenKd and Fe(III)O was observed (the data and their linearregression line in blue are shown in Figure 6).

In other areas of the bay, where sediments contain moderateto high Pexch (data in red in Figure 6), there is no significantcorrelation between Kd and Fe(III)O (p ) 0.025). Among thesesamples, a cluster of data with the highest Fe(III)O has thelowest Kd value (six red data points circled in green in Figure6). These six sediments were from stations 8-13, and all ofthem were located in the western coastal region of the bay(Figure 1) and contained the highest Pexch (0.22-0.61 µmolg-1) measured within the bay. They also contained relativelyhigh Fe(III)O (2.6-3.5 µmol g-1) as a result of wetland surface

runoff that contained terrigenous iron (20). Although theycontained relatively high Fe(III)O, their Kd was among thelowest and remained almost constant despite varyingFe(III)O.

Our results clearly demonstrated the primary role of Pexch

in regulating both EPC0 and Kd in sediment-water exchangeof phosphate. On the other hand, Fe(III)O has no influenceon EPC0, and it plays only a secondary role in regulating Kd,becoming important only in sediments poor in Pexch. Theeffect of Pexch on sorption behavior of sediments may berelated to the different energy levels required for stepwiseadsorption. Forces involved in surface adsorption can rangefrom chemical bonding of solutes to surface functional groupsto electrostatic interaction between ions and charged surfaces(32). Sediment surfaces are known to be highly heterogeneousin their chemical composition and physical properties. Theiron oxide coating on carbonate sediments of the FloridaBay is a good example of such heterogeneity in chemicalcomposition in natural sediments. Recent studies usingadvanced surface spectroscopic and microscopic techniqueshave revealed the microtopography of the particle-waterinterface featuring steps, kinks, defect outcrops, microcracks,and pits on atomic and molecular scales (33). It is generallybelieved that there is a continuum of binding energy levelsamong reactive sites on the solid surface. During adsorption

FIGURE 3. Spatial distribution of (a) EPC0 and (b) Pexch in the Florida Bay (20). Note the very similar spatial distributions of EPC0 and Pexch.

Kd ) 0.2429 + 0.0809Fe(III)O (6)

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processes, absorbates will likely occupy the strongest affinitysites available, such as the amorphous iron oxides oncarbonate sediments. Once a site is occupied by an adsorbate,it becomes inactive. As a result, only the available sites onthe surface regulate the sorption behavior of sediment. Theindependency of Kd on Fe(III)O in stations 8-13 (Figure 6)

suggests that reactive sites provided by iron oxides in thesesediments were saturated as a result of their high Pexch. As

FIGURE 4. Spatial distribution of (a) Kd and (b) Fe(III)O in the Florida Bay (20). Note that both the lowest Kd (Figure 4a) and the highestPexch (Figure 3b) were observed in the northwest corner of the bay.

FIGURE 5. EPC0 and Kd as a function of Pexch in the Florida Bay. FIGURE 6. Relationship between Kd and Fe(III)O in the Florida Bay.Sediments containing less than 0.12 µmol g-1 of Pexch are in blue,and greater than 0.12 µmol g-1 are in red. The blue line representsa linear regression of the blue data points. The six red data pointscircled in green are from the northwestern coastal stations 8-13.

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adsorption progressed, the number of occupied sites in-creased, and an average binding energy of remaining availablesites decreased. Further adsorption on the remaining sitesof weaker affinity would require therefore a higher concen-tration of absorbate (EPC0) in the aqueous phase to providethe dissolved absorbate with the requisite higher chemicalpotential. As the coverage of absorbate on the surface furtherincreases, buffer intensity (Kd) also decreases because ofthe diminishing number of sites available for additionaladsorption.

The role of sedimentary P in regulating a sediment’sbuffering capacity with respect to external loading of P hassignificant implications upon ecosystem functions andprocesses. Sediments are typically in quasi-equilibrium withporewaters that contain higher phosphate concentrationsthan the overlying waters. When sediments are suspendedinto the water column, it requires a much longer time toestablish a new equilibrium with ambient water given a lowsolid to water ratio in suspension, and disequilibrium is oftenobserved. When the water column phosphate concentration,[P]w, is different from EPC0, the extent of disequilibrium isgiven by the concentration difference, [P]w - EPC0. Inunpolluted environments, the difference is usually negative,indicating that resuspended sediment acts as a source andreleases P to ambient water. In waterbodies subject to Ploading from external sources, such as agricultural runoffsor sewage outfalls, the previous difference can be positive,indicating that the suspended sediment becomes an internalsink and will take up excess dissolved phosphate fromambient water. Total phosphate taken up on or released fromthe resuspended sediment under slightly disequilibriumconditions can be estimated from

where ∆[Psed]EPCo is the change of phosphate content ofresuspended sediment near EPC0. Continuous loading ofexternal P to an ecosystem will decline its buffering capacity,which predominantly resides in its sediments, as a result ofthe saturation of Pexch on sediment surfaces. The findingsfrom this study need to be verified in other geographiclocations to determine if the relationships hold in differentsedimentary environments. Future studies on phosphatesorption on sediments must include an analysis of the Pforms in sediment samples. Knowledge of sedimentary Pspeciation is essential to determine the status of the sedimentwith respect to its buffering capacity for changing phosphateconcentrations.

Although this study focused only on P, the regulating roleof Pexch demonstrated here may be extended to other particle-reactive, environmentally important species, such as arsenic,heavy metals, and some organic pollutants. Future studieson other particle-reactive species in sediments are essentialto determine if the degree of adsorbate saturation onadsorbent surface, in general, regulates the adsorbent’sbuffering intensity to changing absorbate concentrations inaquatic systems.

AcknowledgmentsWe thank C. J. Fischer for assistance in sample collectionand processing and P. B. Ortner for English editing. Theanonymous reviewers provided constructive comments thatsignificantly improved the manuscript. This study wassupported by NOAA’s Center for Sponsored Coastal OceanResearch and carried out, in part, under the auspices of theCooperative Institute of Marine and Atmospheric Studies(CIMAS), a joint institute of the University of Miami andthe National Oceanic and Atmospheric Administration,cooperative agreement NA67RJ0149. The statements, find-ings, conclusions, and recommendations are those of the

authors and do not necessarily reflect the views of the NOAAor the U.S. Department of Commerce.

Supporting Information AvailableTable S1. This material is available free of charge via theInternet at http://pubs.acs.org.

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Received for review August 1, 2006. Revised manuscriptreceived December 29, 2006. Accepted February 6, 2007.

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