1
Role of Dissolved Organic Matter (DOM)
in Copper Mobilisation in Soils
Fien Amery, Fien Degryse, Wim Degeling, Thomas Noë and Erik Smolders
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Overview1. Introduction2. Cu Mobilising Potential (CuMP): a new method to
quantify Cu affinity of DOM3. Experiment 1: Variation of CuMP and DOM quality
among soils, incubation and extraction methods4. Experiment 2: Variation of CuMP and DOM quality
with soil depth5. Experiment 3: Incubation simulation of the field
experiment6. Conclusions7. New methods and experiments: preliminary results
and problems
Overview
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3
1. Introduction
Dissolved Organic Matter (DOM) = organic matter in solution that passes 0.45 µm membrane
In soils, DOM is a complex mixture of components with varying molecular weight, functional groups,…
Humic acid Fulvic acid Sugars,
small aliphatic
and aromatic acids
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Introduction
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DOM mobilises hydrophobic contaminants and trace metals in soils
liquid solid
Cu2+
Cu-DOM
By complexing Cu, DOM brings Cu in solution
→ higher Cu mobility in soils
Cu mobility is directly proportional to DOC concentration
(= quantity), provided that the DOM quality is constant
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Introduction
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Field experiment:
Introduction
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How variable is DOM quality and quantity:• before/after drying-wetting cycles
• at varying duration of air-dry soil storage
• at various incubation times
• with different extraction procedures
• with different soils
• at different soil depths
• after addition of soil amendments (manure, plant materials, waste water)
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Introduction
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2. Copper Mobilising Potential (CuMP)
Limited research on the importance of DOM quality for Cu mobilisation
Most studies compare Cu complexation capacities of DOM: not relevant for (uncontaminated) soils
Need for a new method to measure Cu affinity of DOM at relevant conditions
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CuMP
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Use of a resin to buffer the free Cu2+ activity to a constant, relevant value: 10-11.3 M (checked with EDTA)
Solution with DOM in equilibrium with Cu/Ca resin
Constant pH 7.0 and [Ca]eq= 1.5 mM
Equilibrium after 6 days end-over-end shaking
Cu Ca
resin
Cu2+ + DOM
Cu-DOM
solution
DOC
Cu .compl
≈ DOC
Cu .tot
All conditions ((Cu2+), pH and [Ca]) kept constant and normalisation for DOC-concentration
→ CuMP only a measure for the Cu affinity of DOM
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CuMP
CuMP =
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3. Experiment 1: Variation of CuMP among soils, incubation and extraction procedures
13 uncontaminated soils– varying soil characteristics– varying duration of air-dry storage (10 years air-dry - freshly from the field) – varying length of moist incubation: short (4 days) and long (> 1 month)– with and without 2 drying-wetting cycles before incubation– different soil solution extraction procedures:
• Pore water (pw) by centrifugation• Water extraction (solid/liquid ≈ 1/5)• 0.01 M CaCl2 extraction (solid/liquid ≈ 1/5)
Analysis on soil solution: – DOC-concentration– CuMP– Specific UV-absorbance SUVA =
SUVA is a measure for the aromaticity of DOM– DOM acidity measured by potentiometric titration between pH 3 and 11
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Exp 1: M&M
DOC.
.1000A254
b
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Hypothesis:• DOM flush after drying-rewetting of the soil
• DOM released after drying-rewetting is derived from decaying biomass and has a low reactivity (not humified); this DOM is readily biodegradable
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DOC pw Ca E water E
(mg/L) 540 60 170
(mg/kg) 290 300 870
DOC pw Ca E water E
(mg/L) 370 40 108
(mg/kg) 200 200 540
CuMP pw Ca E water E
mmol Cu/ kg DOC)
6 12 22
CuMP pw Ca E water E
mmol Cu/ kg DOC)
11 14 29
Results
Effect of incubation time and extraction procedure, soil 7:• Short incubation time:
• Long incubation time
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Exp 1: results
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Effect of extraction procedure and incubation time on CuMP of soil 1 to 9
CuMP of soil 1 to 9 with different extraction procedures and incubation times
CuMP: water extract > CaCl2 extract > pore water
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Exp 1: results
long incubation > short incubation
fresh from field > dry storage prior to incubation
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Effect of drying-wetting cycles and incubation period:
• Ter Munck
CuMP -DW +DW
short 17 9
long 19 17
DOC -DW +DW
short 71 272
long 32 65
DOC -DW +DW
short 2013 2526
long 694 789
CuMP -DW +DW
short 7 8
long 7 7
< >
≥
≈ ≈
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Exp 1: results
• BS 1
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SUVA
SUVA: same trends as CuMP
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Exp 1: results
Positive significant correlation between SUVA and CuMP
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Total DOM acidity
No clear relationships between total DOM acidity and CuMP
Q7-11 (phenolic acidity) increased during incubation
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Exp 1: results
Note: soil samples kept 10 years air-dry:
Extremely large DOC, low quality
DOC (mg/L) CuMP (mmol Cu/kg C)
soil 1 1702 4.2
soil 2 1740 3.9
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DiscussionLong dry storage and/or drying-wetting cycles cause a flush of low quality DOM: low CuMP and low aromaticity
dry storage
dry-wet cycles
incubation
CuMP ↓
SUVA ↓
CuMP ↑
SUVA ↑
Q7-11 ↑ 16
Exp 1: discussion
Hypothesis: DOM from microbial cellysis (non-humified components, low Cu affinity)
As CuMP, SUVA and Q7-11 (phenolic acidity) increases during incubation: low quality DOM is preferentially degraded by micro-organisms
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Effect of extraction procedure
OM in pore water centrifugation
extraction
OM in pore water
solid organic matter solid organic matter
soil with low quality DOM soil with high quality DOM
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Exp 1: discussion
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Conclusions experiment 1
• Quality (CuMP) varies up to 10-fold with soil treatment and extraction procedure
• Cu affinity of DOM is related with aromaticity of DOM
• DOM from air-dried soils differs in quality from soils not recently subjected to drying
• Extracted DOM is different from true pore water DOM
Exp 1: conclusions
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4. Experiment 2: Variation of CuMP with soil depth
Materials and methods• 3 soil columns from Ter Munck divided in 3 dephts:
– 0-20 cm– 20-40 cm– 40-60 cm
• Wetted until same water content• Moist incubation at 20°C during 4 days• Pore water extraction by centrifugation• Analysis on pore water:
– DOC-concentration– CuMP– SUVA
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Exp 2: M&M
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Hypothesis:• DOM in deeper layers more humified and of higher
quality because easily degradable, low quality DOM is degraded in upper soil layers
OR• DOM in deeper layers less humified and of lower quality
because higher quality DOM (with higher aromaticity and hydrophobicity) is preferentially adsorbed by soil particles
Exp 2: hypothesis
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Results
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Exp 2: results
0-20 cm 20-40 cm 40-60 cm
DOC(mg/L)
20.3 17.3 24.9
CuMP(mmol Cu/ kg C)
28.1 25.2 17.4
SUVA(L/(g.cm)
28.3 26.1 14.7
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Discussion
• Layer 0-20 cm not significantly different from 20-40 cm→ field was ploughed 1 month prior to soil sampling
• Good positive correlation between SUVA and CuMP (R² = 0.80)
• CuMP and SUVA was lowest in 40-60 cm → preferential adsorption of hydrophobic and aromatic DOM on soil particles in higher soil layers
(low quality DOM migrates easier through the soil)
New depth experiment in February-March: same trends?Additional parameters will be measured (total C, total Cu)
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Exp 2: discussion
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5. Exp 3: Variation of DOM quality and quantity with soil amendments
Materials and methods• Fresh top soil from the field experiment
• 6 treatments:– Blank– Pig manure (60 mL/kg soil)– Waste water (770 mL/kg soil, in 5 drying-wetting cycles)– Waste water without DOM (770 mL/kg soil, 5 d-w cycles)– Straw (3 g/kg soil)– 14C-labeled maize roots (1 g/kg soil)
• Pore water sampled after 3, 20 and 64 days incubation (20°C)
• Analyses: DOC, SUVA, CuMP, 14C, respiration between day 38(34) and 64
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Exp 3: M&M
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Hypothesis:• Day 3: higher DOC concentrations and lower DOM
quality in soils with amendments (except for amendments with high quality DOM)
• During incubation: DOC concentrations decrease and DOM quality increases due to humification
• Maize-derived DOM in soil solutions diminishes in time
Exp 3: hypothesis
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Results
Exp 3: resultsChanges in DOC concentrations in time
0.0
10.0
20.0
30.0
40.0
50.0
60.0
0 20 40 60 80
time (days)
DO
C c
on
c (m
gC
/L) blank
pig manure
waste water
ww without DOM
straw
maize
Changes in SUVA in time
0.0
10.0
20.0
30.0
40.0
50.0
0 20 40 60 80
time (days)
SU
VA
(L
/(g
.cm
)) blank
pig manure
waste water
ww without DOM
straw
maize
anaerobic?
high Ca-conc
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• CuMP
Exp 3: results
Changes of CuMP in time
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0 5 10 15 20 25
time (days)
Cu
MP
(m
mo
l C
u/k
g C
)
blank
pig manure
waste water
ww without DOM
straw
maize
• CuMP vs SUVA
CuMP pig manure:
79 mmol/kg C
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14C
Exp 3: results
Respiration from day 38(34) till 64
y = 0.0535x + 0.6101
R2 = 0.8703
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0 10 20 30 40 50
DOC day 64 (mg/L)
res
pir
ati
on
d3
8(3
4)-
64
(m
gC
/(k
g
so
il.d
ay
)
14C: 3.1% of respired C
14C: 3.3% of DOC
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Exp 3: discussion
Discussion• Blank: DOC, SUVA and CuMP rather constant in time• Soil with waste water and waste water without DOM:
Flush of low quality DOM due to drying-wetting cycles, this flush is easily degradable
• Soil with pig manure: same trends as for the waste water
But CuMP and SUVA of pig manure very high?• Soil with straw and maize: DOC remains high
SUVA comparative to blank: extra DOM of plant material comparative aromaticity to basic DOM?
BUT: CuMP initially high, decreases during incubation
• Respiration ~ DOC concentration in soil solution
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6. Conclusions
• Large variation in Cu affinity of DOM
→ not only DOM quantity (= DOC-concentration) but also DOM quality is important in prediction
of Cu mobilisation in soils
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Conclusions
• DOM of soils kept dry in the lab ≠ DOM of soils freshly from the field
• Differences in DOM quality in depth, and with soil amendments
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7. New questions
• Other DOM characteristics?
→ Some tests on fluorescence measurements: no additional information
• Can the variation in DOM quality be explained by the relative presence of different DOM fractions with more homogeneous quality? – Hydrophobic/hydrophilic fractions– Low / high molecular mass fractions
→ Tests at this moment
New questions
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DAX-fractionation
• Fractionation based on hydrophobicity• Sample at pH 2 over DAX-resin (methylmethacrylate polymer):
hydrophobic components adsorb, hydrophilics don’t
Elution with 0.1 M NaOH: hydrophobic components desorb• Technique normally used for large volumes, over large DAX-columns• Goal: adapt the method for small soil solution volumes• Hypothesis:
– DOM with higher CuMP and SUVA: relatively more hydrophobics– SUVA and CuMP rather homogeneous in one fraction
Hydrophilic HydrophobicHydrophilic Hydrophobic
Low quality DOM High quality DOM
Fractionation
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DAX in column
• Some tests with one soil solution• SUVA (hydrophilic): +/- 7 L/(g.cm)
SUVA (hydrophobic): 40 → 20 L/(g.cm)
• Advantages: nice distribution, less dilution (concentration of the hydrophobic part), less resin bleeding
• Disadvantages: less reproducible (certainly when done manually)
→ Purchase of chromatographic apparatus?
Fractionation
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DAX in batch
Fractionation
• Some tests with one soil solution• Resin bleeding → apply blank correction• SUVA (hydrophilic): +/- 8.5 L/(g.cm)
SUVA (hydrophobic): 30 L/(g.cm)• Other tests: good reproducibility, also when using diluted
samples
• Advantages: more reproducible • Disadvantages: no fluent distribution, more dilution, more
resin bleeding
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DAX in batch
Fractionation
• Tests with short- and long-term incubated Zegveld soil
• SUVA (short) = 12.6 L/(g.cm); SUVA (long) = 22.0
• Hydrophilic/hydrophobic = – 3.1 +/- 0.1 for short-term incubated soil– 2.1 +/- 0.5 for long-term incubated soil
• BUT: SUVA long short
hydrophilic 16 5
hydrophobic 31 16
Hydrophilic Hydrophobic
Fractions not homogeneous
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What now?
• Purchase of chromatographic apparatus (also for gel permeation chromatography) for fractionation over columns
• OR: serial batch fractionation (but high dilution and resin bleeding!)
• Further fractionation in acids, neutrals, bases?
Fractionation
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Gel permeation chromatography
• Fractionation based on molecular size• Which molecular size ranges?
• Hypothesis: high quality DOM consists of relatively larger molecules
Fractionation
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8. More future research• New depth experiment• Field experiment: modelling Cu fluxes by means of DOM
quality measurements• Soil column experiments: adding DOM with different
quality – analyse Cu fluxes– measuring Cu speciation by Donnan dialyse– measuring Cu-DOM dissociation kinetics by the competitive
ligand method
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Future
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Thank you for your attention!