1 Role of Dissolved Organic Matter (DOM) in Copper Mobilisation in Soils Fien Amery, Fien Degryse, Wim Degeling, Thomas Noë and Erik Smolders 1

2 Overview 1.Introduction 2.Cu Mobilising Potential (CuMP): a new method to quantify Cu affinity of DOM 3.Experiment 1: Variation of CuMP and DOM quality among soils, incubation and extraction methods 4.Experiment 2: Variation of CuMP and DOM quality with soil depth 5.Experiment 3: Incubation simulation of the field experiment 6.Conclusions 7.New methods and experiments: preliminary results and problems Overview 2

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 3 Introduction

4 DOM mobilises hydrophobic contaminants and trace metals in soils liquidsolid Cu 2+ 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 4 Introduction

5 Field experiment: Introduction

6 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) 6 Introduction

7 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 7 CuMP

8 Use of a resin to buffer the free Cu 2+ activity to a constant, relevant value: 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 Cu 2+ + DOM Cu-DOM solution ≈ All conditions ((Cu 2+ ), pH and [Ca]) kept constant and normalisation for DOC-concentration → CuMP only a measure for the Cu affinity of DOM 8 CuMP CuMP =

9 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 CaCl 2 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 9 Exp 1: M&M

10 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

11 DOCpwCa Ewater E (mg/L) (mg/kg) DOCpwCa Ewater E (mg/L) (mg/kg) CuMPpwCa Ewater E mmol Cu/ kg DOC) CuMPpwCa Ewater E mmol Cu/ kg DOC) Results Effect of incubation time and extraction procedure, soil 7: Short incubation time: Long incubation time 11 Exp 1: results

12 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 > CaCl 2 extract > pore water 12 Exp 1: results long incubation > short incubation fresh from field > dry storage prior to incubation

13 Effect of drying-wetting cycles and incubation period: Ter Munck CuMP-DW+DW short179 long1917 DOC-DW+DW short71272 long3265 DOC-DW+DW short long CuMP-DW+DW short78 long77 < >≥>≥ ≈ ≈ 13 Exp 1: results BS 1

14 SUVA SUVA: same trends as CuMP 14 Exp 1: results Positive significant correlation between SUVA and CuMP

15 Total DOM acidity No clear relationships between total DOM acidity and CuMP Q 7-11 (phenolic acidity) increased during incubation 15 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 soil

16 Discussion Long 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 ↑ Q 7-11 ↑ 16 Exp 1: discussion Hypothesis: DOM from microbial cellysis (non-humified components, low Cu affinity) As CuMP, SUVA and Q 7-11 (phenolic acidity) increases during incubation: low quality DOM is preferentially degraded by micro-organisms

17 Effect of extraction procedure OM in pore water centrifugation extraction OM in pore water solid organic matter soil with low quality DOM soil with high quality DOM 17 Exp 1: discussion

18 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

19 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 19 Exp 2: M&M

20 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

21 Results 21 Exp 2: results 0-20 cm20-40 cm40-60 cm DOC (mg/L) CuMP (mmol Cu/ kg C) SUVA (L/(g.cm)

22 Discussion Layer 0-20 cm not significantly different from 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 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) 22 Exp 2: discussion

23 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) – 14 C-labeled maize roots (1 g/kg soil) Pore water sampled after 3, 20 and 64 days incubation (20°C) Analyses: DOC, SUVA, CuMP, 14 C, respiration between day 38(34) and Exp 3: M&M

24 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

25 Results Exp 3: results anaerobic? high Ca-conc

26 CuMP Exp 3: results CuMP vs SUVA CuMP pig manure: 79 mmol/kg C

27 14 C Exp 3: results Respiration from day 38(34) till C: 3.1% of respired C 14 C: 3.3% of DOC

28 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

29 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 29 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

30 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

31 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 Hydrophobic Hydrophilic Hydrophobic Low quality DOMHigh quality DOM Fractionation

32 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

33 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

34 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: SUVAlongshort hydrophilic165 hydrophobic3116 Hydrophilic Hydrophobic Fractions not homogeneous

35 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

36 Gel permeation chromatography Fractionation based on molecular size Which molecular size ranges? Hypothesis: high quality DOM consists of relatively larger molecules Fractionation

37 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 37 Future

38 Thank you for your attention!