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!