Research Collection. Extraction of heavy metals from soil with selected biodegradable complexing agents diploma thesis. Master Thesis.

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1 Research Collection Master Thesis Extraction of heavy metals from soil with selected biodegradable complexing agents diploma thesis Author(s): Ritschel, Jens Publication Date: 2003 Permanent Link: Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library

2 F A C H H O C H SCHULE J E N A UNIVERSITY OF APPLIED SCIENCES Extraction of heavy metals from soil with selected biodegradable complexing agents Diploma thesis by Jens Ritschel Study course Environmental Engineering, FH Jena Jena, May 2003

3 Table of contents 1 Introduction Importance of metals Treatment of contaminated soils Results of other researches Objective of thesis Material and methods Characterisation of soils Used complexing agents EDTA EDDS NTA Other chemicals used Analytical methods Atomic absorption spectrometry (AAS) X-ray fluorescence analysis (XRF) Experimental methods Kinetic experiment ph variation Methods of sequential extraction Consideration of complex stability Results Kinetics of extraction (Rafz soil) Calcium Magnesium Iron Zinc Lead Influence of detergents... 22

4 3.2 Variation of ph value Calcium Magnesium Manganese Iron Copper Zinc Lead Humic acid Sequential extraction Fe and Mn Copper Zinc Lead Discussion Extraction with EDDS compared to EDTA Conditional formation constants of Me-EDTA and Me-EDDS Effectiveness of extraction with EDDS compared to EDTA Extraction with NTA Conditional formation constants and speciation of Me-NTA Effectiveness of extraction with NTA Error discussion Summary Acknowledgments References... 55

5 Table of figures Figure 2-1: Structural formula EDTA... 7 Figure 2-2: Structural formula EDDS... 7 Figure 2-3: Structural formula NTA... 8 Figure 2-4: Dilution of reference soil with glucose Figure 3-1: Kinetics of the Ca extraction with different complexing agents at ph 4 and Figure 3-2: Kinetics of the Mg extraction with different complexing agents at ph 4 and Figure 3-3: Kinetics of the Fe extraction with different complexing agents at ph 4 and Figure 3-4: Kinetics of the Zn extraction with different complexing agents at ph 4 and Figure 3-5: Kinetics of the Pb extraction with different complexing agents at ph 4 and Figure 3-6: Extracted Ca from Kirschgarten soil as a function of ph Figure 3-7: Extracted Ca from Rafz soil as a function of ph Figure 3-8: Extracted Mg from Kirschgarten soil as a function of ph Figure 3-9: Extracted Mg from Rafz soil as a function of ph Figure 3-10: Extracted Mn from Kirschgarten soil as a function of ph Figure 3-11: Extracted Mn from Rafz soil as a function of ph Figure 3-12: Extracted Fe from Kirschgarten soil as a function ph Figure 3-13: Extracted Fe from Rafz soil as a function of ph Figure 3-14: Extracted Cu from Kirschgarten soil as a function of ph Figure 3-15: Extracted Zn from Kirschgarten soil as a function of ph Figure 3-16: Extracted Zn from Rafz soil as a function of ph Figure 3-17: Extracted Pb from Rafz soil as a function of ph Figure 3-18: UV/Vis extinction as function of humic acid concentration Figure 3-19: Extracted humic acid as function of ph Figure 3-20: Binding forms of Fe in Kirschgarten soil Figure 3-21: Binding forms of Mn in Rafz soil Figure 3-22: Binding forms of Cu in Kirschgarten soil Figure 3-23: Binding forms of Cu in Mattenweg soil Figure 3-24: Binding forms of Zn in Kirschgarten soil Figure 3-25: Binding forms of Zn in Mattenweg soil Figure 3-26: Binding forms of Zn in Rafz soil Figure 3-27: Binding forms of Pb in Rafz soil Figure 4-1: Conditional formation constant lg K eff of Me-EDTA as function of ph Figure 4-2: Conditional formation constant lg K eff of Me-EDDS as function of ph Figure 4-3: Conditional formation constant lgk eff of Me-NTA as function of ph Figure 4-4: Molar concentrations in extraction solution at ph Figure 4-5: Molar concentrations in extraction solution at ph Figure 4-6: Molar concentrations in extraction solution at ph Figure 4-7: Calculated speciation of NTA concentration 1 (ChemEQL) Figure 4-8: Calculated speciation of NTA concentration 2 (ChemEQL)... 50

6 Table of abbreviations Complexing agents EDTA EDDS NTA IDSA MGDA Ethylenediamine tetra acetic acid N,N -Ethylenediamine disuccinic acid Nitriliotriacetic acid Iminodisuccinic acid Methylglycinediacetic acid Analytical methods AAS UV/Vis XRF Atomic absorption spectrometry Ultraviolet / visible absorption spectrometry X-ray fluorescence analysis

7 1 Introduction 1.1 Importance of metals Metals can be found in all parts of the lithosphere. Most of them are dispersely distributed, only small portions are concentrated in ores. Beside these geogenic concentrations, human use of metals has led to significant changes in the circulation of metals. Many metals are of great importance for biological processes. These metals needed as nutrients by organisms are called essential metals. Some of them are needed in higher amounts (macro-elements), others in smaller amounts (trace-elements). Shortage, but also surplus of essential metals have negative influence on biomass growth. Non-essential metals however can cause toxic effects at low concentrations. This thesis is focused on the metals Cu, Pb and Zn as examples for relevant anthropogenic contaminants of soil. While Pb is a non-essential metal, Cu and Zn are trace-elements. High concentrations of these metals, typical for anthropogenic contamination, can have serious effects on growth of organisms especially plants. Another danger is the accumulation in upper parts of the food chain. Additionally the essential metals Ca, Fe, Mg and Mn are observed with respect to negative side effects of extraction. 1.2 Treatment of contaminated soils Anthropogenic heavy metal contamination cannot be degraded chemically or biologically because metals are elements. This limits the possibilities for treatment: Securing the contaminated area: These are actions taken to lower the risk caused by the contamination, including immobilisation and local binding, without actually removing the heavy metals from soil. 1

8 Introduction Removing the contaminated soil: Removing all of the contaminated soil is probably the safest method, but only suited for locally concentrated contamination. Otherwise the expenses of removing and disposing the soil are too high. Additionally this method has a big impact on the ecosystem around the contamination site and moves the problem to another site rather than solves it. Soil washing / chemical extraction With suitable extraction agents most metals can be brought into solution and so washed out from soil. Problems are caused by non-specific solving of essential material, such as essential elements and organic matter and possible side effects of extraction solution itself. In this thesis chemical extractions with bio-degradable complexing agents are to be investigated. Phytoextraction Phytoextraction is a very soft method which causes the least negative changes to soil. The principle is the absorption of bio-available metals by selected plants followed by their harvest and removal. The main disadvantage is the long treatment time. 2

9 Introduction 1.3 Results of other researches The basis of this work is the thesis by Bossart/Müller (4). They compared different complexing agents like EDTA, EDDS, IDSA and MGDA with regard to their properties for extraction of Zn and Cu from Dornach soils. Pb and Cd were not examined due to low contaminations with these metals. The result showed EDDS as most promising biodegradable alternative to EDTA. With concentrations equal to the molar sum of heavy metals in soil up to 72 % of Cu and 36 % of Zn could be extracted from a non-calcareous soil. Higher concentrations of complexing agent led to better extraction results, but also to an increase in the extraction of Ca, Fe and humic matter. Additionally better results were achieved by the use of detergents together with EDDS. 1.4 Objective of thesis The suitability of EDDS and NTA as biodegradable substitutes for EDTA in extraction of heavy metal contaminated soils shall be investigated. This thesis is part of a research project at the ETH Zurich. The experiments done here complement the results of former experiments within this project. In this thesis EDDS shall be investigated further. One experiment examines the kinetics of EDTA and EDDS extractions for a soil with significant Pb contamination at two different ph values. Extraction of Pb with EDDS has not been examined in the previous experiments by Bossart/Müller (4). Additionally sequential extractions are done for original soil and already extracted soil. This helps to understand how the binding forms of metals and the extraction affect each other. Finally, NTA shall be used in two concentrations for the extraction of Cu, Zn and Pb at different ph values. NTA was not considered in the previous experiments, but is known as strong and biodegradable chelating agent. In all experiments negative side effects of the extraction have to be observed. These effects include extraction of essential metals and organic material. 3

10 Material and methods 2 Material and methods 2.1 Characterisation of soils For the experiments soils from three different Swiss locations were used. Two of them, Kirschgarten and Mattenweg, are situated in Dornach (south of Basel), which is one of the largest areas of high contamination with heavy metals in Switzerland. This pollution is caused by air emissions from the non-ferrous metal industry. The third sample is from an agriculturally used soil near Rafz (15 km north of Zurich) which was contaminated with heavy metals by sludge fertilizer. Samples from all soils were already dried at 40 C, ground and sieved to a particle size of 2 mm. General characteristics of the soils, except dry residue, were determined according to VBBo. (16). Dry residue was measured according to DIN EN (7). The results are shown in Table 2-1. Mattenweg soil is a calcareous soil which results in a high ph value, it also has a the highest organic matter content of all the soils. Kirschgarten is a silty soil which is non-calcareous. Sandy Rafz soil has the lowest ph value and organic matter. Dry residue was determined for testing purposes. Table 2-1: Characteristics of soils used ph value in Organic Dry Carbonate Particle-size distribution 0.01 M matter residue content (Pipett method) CaCl 2 Sand (50 µm Silt (2 µm Clay (< 2 µm) 2 mm) 50 µm) [%] [%] [% CaCO 3 ] [%] [%] [%] Kirschgarten Mattenweg Rafz

11 Material and methods Table 2-2: Total metal concentrations [µg/g] measured with XRF Na Mg Al Ca Mn Fe Ni Cu Zn Cd Pb Kirschgarten Mattenweg Rafz Reference soil measured Reference soil Median Difference in % Total metal concentrations were analysed using XRF. Table 2-2 shows the results. For validation a reference soil (inter-laboratory test /921) was also measured. The largest difference between measured concentration and Median 1 was determined for Na and metals with small concentrations. However, analyses for Ca, Fe, Mg, Mn, Pb and Zn are reliable with XRF. Obviously anthropogenic heavy metal contamination is found in all soils. The most significant contamination is Cu in Kirschgarten and Mattenweg soils, Pb in Rafz soil and Zn in all three soils. The XRF analysis is not yet a standardised method. Therefore in addition an open digestion with 2 M HNO 3 was done. Extracts were analysed using AAS. Although not all metals can be brought into solution with HNO 3, the results represent total metals according to VBBo. (16). The soluble metals were determined by extraction with 0.1 M NaNO 3. The extraction solutions were analysed using AAS. The results of both extractions are shown in Table

12 Material and methods Table 2-3: Soluble portion (NaNO 3 extractable) and total (HNO 3 extractable) metals [µg/g] measured with AAS Extractable with Cu Pb Zn NaNO 3 HNO 3 NaNO 3 HNO 3 NaNO 3 HNO 3 ( soluble ) ( total ) ( soluble ) ( total ) ( soluble ) ( total ) Kirschgarten < Mattenweg < Rafz < Results for total metals analysed with AAS are slightly different to the XRF results mainly for Zn. Tests with reference soil showed that results for Zn were generally too high (cp. appendix). After a maintenance of the AAS this problem was solved. As expected Pb is immobile in all soils. A strikingly high amount of mobile Zn is found in Rafz soil. Beside all differences the molar sum of heavy metal contamination is similar in all three soils (Table 2-4). Therefore the same concentrations of extraction agents could be used. Table 2-4: Molar concentrations of heavy metals Cu [µmol/g] Zn [µmol/g] Cd [µmol/g] Pb [µmol/g] Ni [µmol/g] [µmol/g] Kirschgarten Mattenweg Rafz

13 Material and methods 2.2 Used complexing agents EDTA EDTA (Ethylenediamine tetra acetic acid) is the most widely used complexing agent. It forms strong complexes with many metals. Disadvantages are its unselective nature and the poor biodegradability (5). For the experiments Na 2 -EDTA was used (company Merck, M = g/mol). Figure 2-1: Structural formula EDTA EDDS EDDS (N,N -Ethylenediamine disuccinic acid) is a structural isomer of EDTA. Its ability to form stable complexes is similar to EDTA. It has been reported that both EDDS and its metal complexes are readily biodegradable (14). This only relates to the stereoisomer SS-EDDS. In the last years many investigations have tried to test its suitability as a substitute for EDTA for many purposes, e.g. in laundry detergents. EDDS was used as Na 3 -N,N -EDDS (company Procter & Gamble, M = g/mol). Figure 2-2: Structural formula EDDS NTA NTA (Nitrilotriacetic acid) is also a strong complexing agent. It is widely used in many countries, but restricted or banned in others because NTA is rated as a possible carcinogen. Newer examinations query a risk for humans at usual concentrations (17). 7

14 Material and methods NTA is reported to be readily biodegradable in some sources, but this seems to depend on adaptation of micro-organisms (5) (13). The chemical form used was Na 3 -NTA salt (company Fluka, purity purum, M = g/mol). Figure 2-3: Structural formula NTA 2.3 Other chemicals used NaNO 3 Sodium nitrate (company Fluka, purity pa) was used to simulate the ionic strength of tap water which would be used in a big scale operation, without introducing metal ions present in tap water. It was basis for all extraction solutions. HNO 3 Nitric acid (company Merck, purity pa) of different concentrations was used to set ph value and to acidify samples for AAS. NaOH Sodium hydroxide (company Fluka, purity pa) of different concentrations was also used to set the ph value. Glucopon 650 EC Glucopon (company Henkel) is an aqueous, non-ionic solution of alkyl polyclycosides based on natural fatty alcohols. Schinkel solution Schinkel solution contains 10 g/l CsCl (caesium chloride) and 100 g/l LaCl (lanthanum chloride). It is used to eliminate chemical interferences which depress absorbance of Ca and Mg during measurement with AAS. 8

15 Material and methods 2.4 Analytical methods Atomic absorption spectrometry (AAS) AAS is the standard method for determination of metals in solutions. Per analysis only one element can be determined. Samples are centrifuged for 15 minutes at 2500 rpm and then vacuum filtrated with pore size 0,45 µm. The used Flame-AAS (type Varian) has got a slot burner, an auto sampler and computer based evaluation. All measurements were done with an oxidising air-acetylene flame (T 2200 C). Table 2-5 shows the other working conditions. Table 2-5: Working conditions for AAS measurements (based on Recommended instrument parameters (15)) Element Wavelength [nm] Slit width [nm] Optimum working range [µg/ml] Remarks Ca Schinkel solution added as releasing agent (cp. 2.3) Cu Fe Mg Schinkel solution added as releasing agent (cp. 2.3) Mn Pb Zn

16 Material and methods X-ray fluorescence analysis (XRF) XRF is a physical measuring method which allows non-destructive analysis of solid samples. With one analysis all elements measurable with the used detector can be captured. 4 g of ground soil are mixed with 0.9 g wax C micro powder and then shaken for 8 minutes at 17 Hz. The mixed powder will be pressed to a pellet at 150 kn. After that the pellet is analysed with XRF. The used energy-dispersive XRF with a Si (Li) solid-state detector allows analysis for elements from atomic number 11 (Na) to 92 (U). Samples of sequential extraction residue were not sufficient for XRF. Therefore these samples were diluted with glucose. A test with reference soil in different relations soil / glucose was done to confirm that glucose does not affect the results significantly. Figure 2-4 shows the measured results compared to the reference values. Due to this results a dilution of approximately 1:1 was chosen for the residue. 600 metal concentration [ug/g] weight of soil in 3 g sample [g] Pb measured Zn measured Cu measured Reference Pb Reference Zn Reference Cu Figure 2-4: Dilution of reference soil with glucose 10

17 Material and methods 2.5 Experimental methods Kinetic experiment 8 g of fine ground Rafz soil were extracted for 48 hours with constant shaking. The extraction solution was 400 ml 0.01 M NaNO 3 containing 400 µmol/l complexing agent. This concentration equals the molar sum of Cd, Cu, Ni, Pb, Zn (about 20 µmol/g soil, cp. Table 2-4). EDDS and EDTA (reference) were used as complexing agents, additionally one sample was extracted without complexing agent. The soil was extracted at ph 4 and ph 7 by both chelating agents. The ph value was set and kept constant during extraction by addition of HNO 3 or NaOH. Correction of ph was done before every sample taking with a tolerance of ± 0.1. An additional sample containing EDDS and the detergent Glucopon (20 g/l) was extracted at ph 7. After 1, 2, 4, 8, 24, 32, 48 hours samples of 30 ml were taken. Here an attempt was made to keep the soil / solution ratio constant by taking the sample from the shaken solution. These samples were analysed for Ca, Cu, Fe, Mg, Pb and Zn by Flame-AAS (cp ). It has to be noted that the solution containing soil and NaNO 3 was made 2.5 days before the beginning of the extraction. This was necessary to set a stable ph value. Therefore the solutions could already contain dissolved metals at the beginning of extraction, mainly at ph ph variation Always 0.8 g of fine ground soil from Kirschgarten, Mattenweg and Rafz were used for extraction. Extraction time was 24 hours with constant shaking. The extraction solution was 40 ml 0.01 M NaNO 3 to get the same soil / solution ratio as in the kinetics experiment. Two different concentrations of NTA were added as complexing agent: concentration 1 (400 µmol/l) which equals the molar sum of Cd, Cu, Ni, Pb, Zn and concentration 2 (4000 µmol/l) which is ten times this concentration (cp. Table 2-4). Additionally one extraction was done without complexing agent. 11

18 Material and methods For each concentration of NTA with each combination soil / extraction solution, ph values were set between 3 and 8 in steps of 0.5 by adding HNO 3 or NaOH. Twice a day the ph value was controlled and set with a tolerance of ± 0.1. After the extraction all samples were analysed with Flame-AAS for Ca, Cu, Fe, Mg, Mn, Pb and Zn (cp ). During experiments it could be observed that the colours of the extraction solutions differed. With increasing ph value it changed from clear over yellow to brown. Generally the colour was stronger for higher concentrations of NTA. To find a quantification for this effect exemplary all solutions from Kirschgarten soil were analysed with UV/Vis at a wavelength of 432 nm. It could be expected that solved humic acids are the main cause for the colour change. Therefore the UV/Vis was calibrated with humic-acid-na-salt Methods of sequential extraction The sequential extraction was accomplished according to the process developed by Zeien (18). The principle is a sequence of different extraction steps with increasing strength of extraction agent and decreasing ph value of extraction solution. Metals found in each extracted fraction can be assigned to specific binding forms in soil. All 7 extraction steps are shown in Table 2-6. However, the residual fraction was not determined by digestion but with XRF. The samples from all other extraction steps were analysed with Flame-AAS for Cu, Fe, Mn, Pb and Zn. Before the start of sequential extraction 5 g from Kirschgarten, Mattenweg and Rafz soil were extracted with 400 µmol/l EDDS in 250 ml 0.01 M NaNO 3. From each soil and a blank extractions were done for 24 hours and for 48 hours in duplicate analysis. The ph value was observed but not changed. Extraction solution was then analysed with Flame-AAS. Residue was rinsed with 50 ml of 0.01 M NaNO 3 twice, each time including centrifugation and filtration. After that 2 g of each dried residue and 2 g from each fresh soil were taken for sequential extraction. This was done also including a blank and in duplicate analysis. 12

19 Material and methods Table 2-6: Sequential extraction (18) Scheme of sequential extraction Fraction 1. Mobile 2. Easy available 3. Occluded in Mn-Oxides 4. Organic bound 5. Occluded in amorphous Fe-oxides 6. Occluded in crystalline Fe-oxides 7. Residue (bound in silicates) Extraction agent used 1 M NH 4 NO 3 1 M NH 4 OAc. (ph 6.0) 0.1 M NH 2 OH-HCl + 1 M NH 4 OAc. (ph 5.5) M NH 4 EDTA (ph 4.6) 0.2 M NH 4 Oxalate (ph 3.25) 0.1 M Ascorbic acid in 0.2 M NH 4 Oxalate (ph 3.25) HNO 3 + H 2 O 2 + HF conc. 2.6 Consideration of complex stability To understand the reactions during extraction with complexing agents it is important to know the metal ion ligand interactions. The complexation can be regarded as equilibrium reaction between the ligand and the competing metal ions: M i+ + L j- ML (i-j) (1) with M Metal ion (e - pair acceptor) i charge of M L Ligand (e - pair donor) j charge of L 13

20 Material and methods According to the principle of mass action the activities of M, L and ML relate as follows: ( i j) + [ ML ] K ML = (2) i+ j [ M ][ L ] with [..] activity equilibrium constant K ML For complexations K ML is also called formation constant or stability constant K St. This constant describes the strength of a complex with this specific metal. However, it does not take into account the effects of ph value. For this reason a conditional stability constant K cond can be defined: lg K cond (ph) = lg K St lg α HL lg α M (3) with K cond conditional stability constant K St stability constant, equals K ML from equation (2) α HL α M coefficient of ligand protonation coefficient of side reactions competing with the ligand for the metal ions Note: lg always means logarithm to the base 10 Ligand protonation L j- is a reasonable strong base. Therefore the amount of free L j- increases with increasing ph value. The single steps of protonation are described by the equilibrium constants K 1, K 2,.., K m. α HL is defined as: α HL = 1 + [H + ] K 1 + [H + ] 2 K 1 K [H + ] m K m! (4) Note: K m! means factorial of K m 14

21 Material and methods Competing reactions Side reactions include formation of metal hydroxides, effects of buffers and forming of MLH (metal ion ligand proton) or MLOH (metal ion ligand hydroxide) species. For simplification this calculation considers only formation of metal hydroxides as most important effect. Formation of insoluble metal hydroxides prevents these metal ions from being complexed. The concentration of metal hydroxides increases with increasing ph value. The steps of OH - acceptance by the metal ion are described by the equilibrium constants K I, K II,.., K n. However, only the species which are formed in the system may be considered. α M is defined as: α M = 1 + s I [OH - ] K I + s II [OH - ] 2 K I K II + + s n [OH - ] n K n! (5) with s factor which determines if species n exists (s = 1) or not (s = 0) The calculation and the definitions are based on Davidge et al (6). Selected results calculated with these equations will be used in chapter 4. 15

22 Results 3 Results 3.1 Kinetics of extraction (Rafz soil) The values from the AAS were used to calculate concentrations of metals extracted from soil in µg metal per g dry soil. These results can be found in the appendix. With these values and the total concentrations measured with XRF it was possible to calculate the extracted metal in % of the total concentration in soil. These results were chosen for graphical presentation except for the extraction of Fe. Here the concentration in µg/g soil was used because the percentage portion is very small but the total concentration is relevant as Fe complexes. The diagrams show the development of the concentration of the specific metal in the extraction solution during extraction time. The results for the different extraction solutions (EDTA, EDDS and without complexing agent) at ph values 4 and 7 are shown in one diagram. 16

23 Results Calcium The extraction of Ca occurs very fast. The maximum for most of the extraction solutions is reached within the first hour. Then the curves stay nearly constant. 45 to 50 % was extracted at ph 4 and about 15 % at ph 7. There is no significant influence of the extracting agent on the amount of Ca in the extraction solution. extracted metal in % of total EDTA ph4 EDTA ph7 EDDS ph4 EDDS ph7 w/o complexing agent ph extraction time [h] w/o complexing agent ph7 Figure 3-1: Kinetics of the Ca extraction with different complexing agents at ph 4 and 7 17

24 Results Magnesium The kinetic of the Mg extraction is very similar to that of Ca, but at a lower level. At ph 4 about 10 % was extracted and at ph 7 about 2 %. Again the only significant increase in extraction is during the first hour and the presence of a complexing agent had no effect on the Mg concentration measured with AAS. 15 EDTA ph4 extracted metal in % of total 10 5 EDTA ph7 EDDS ph4 EDDS ph7 w/o complexing agent ph extraction time [h] w/o complexing agent ph7 Figure 3-2: Kinetics of the Mg extraction with different complexing agents at ph 4 and 7 18

25 Results Iron The kinetics of the Fe extraction show a different trend to Ca and Mg. There are no significant amounts of Fe extracted without a complexing agent or with EDTA at ph 7, but with EDDS at ph 7 there is a steadily increasing amount of Fe extracted over time. After 48 hours about 180 µg/g soil was extracted and the extraction has not reached a steady state. At ph 4 the value is even higher with a maximum of 450 µg/g soil. Also EDTA was able to extract up to 300 µg/g soil at this ph value during the 48 hours. EDDS at ph 4 seems to reach a maximum extraction at 32 hours, but to ensure this is correct it would be necessary to study a longer extraction time. The observed amounts of extracted iron have no influence on plant growth. However, the importance of Fe lies in its properties as competitive ion of complexation. extracted metal in ug/g EDTA ph4 EDTA ph7 EDDS ph4 EDDS ph7 w/o complexing agent ph extraction time [h] w/o complexing agent ph7 Figure 3-3: Kinetics of the Fe extraction with different complexing agents at ph 4 and 7 19

26 Results Zinc Zn is not mobile at ph 7 due to forming of insoluble Zn-hydoxides, so practically nothing could be extracted without a complexing agent at this ph value. With EDTA and EDDS it was possible to extract about 50 % of the Zn from soil. There is no significant difference between the two complexing agents at this ph value. The extraction shows an exponential rise within the first hour and then the increase slows considerably. At ph 4 Zn in Rafz soil is very mobile, about 60 % is extracted without adding any complexing agent. With EDTA this can be increased up to 90 %. EDDS performs worse than EDTA extracting up to 80 % of total Zn. Again the largest increase is within the first hour. EDTA extraction then stays constant around 80 % and EDDS drops down to the level reached without complexing agent by 32 hours. 100 EDTA ph4 extracted metal in % of total EDTA ph7 EDDS ph4 EDDS ph7 w/o complexing agent ph extraction time [h] w/o complexing agent ph7 Figure 3-4: Kinetics of the Zn extraction with different complexing agents at ph 4 and 7 20

27 Results Lead Pb is mobile neither at ph 4 nor at ph 7. Without complexing agent the extracted amount was below 2 %. At ph 7 the addition of EDTA or EDDS leads to an extraction of about 25 %. In the first 24 hours both performed identically, then the extraction with EDTA rose further but EDDS dropped slightly. At ph 4 EDTA was able to extract nearly 60 % of the Pb from soil. The extraction reaches its maximum after 8 hours which is slower than most of the kinetics examined. EDDS showed at this ph value no significant effect compared to the extraction solution without complexing agent. 70 extracted metal in % of total EDTA ph4 EDTA ph7 EDDS ph4 EDDS ph7 w/o complexing agent ph extraction time [h] w/o complexing agent ph7 Figure 3-5: Kinetics of the Pb extraction with different complexing agents at ph 4 and 7 21

28 Results Influence of detergents Contrary to the expectation the use of Glucopon as detergent did not change the kinetics and total extracted amounts of metals. The results can be found in the appendix. Obviously the increased dispersion of soil particles does not increase the metal extraction. It has to be considered that for all experiments the soil was fine ground. If the grain size is much bigger, e.g. in big-scale extractions, detergents could have influence on the kinetics. Additionally the results of Bossart/Müller (4) show a significant increase in extracted Zn and Cu for different soils. Compared to the soils examined there, Rafz soil is very sandy. This suggests that the soil character is also of importance for the effect of detergents. 22

29 Results 3.2 Variation of ph value The results from the AAS were used to calculate metal concentrations in µg/g soil taking into account dilutions made for analysis. These results can be found in the appendix. With these results and the total concentrations measured with XRF it was possible to calculate the extracted metal in % of the total concentration in soil. These results were chosen for graphical presentation except for the extraction of Fe. Here the concentration in µg/g soil was used because the percentage portion is very small but the total concentration is relevant as Fe complexes. As for all experiments Pb for Kirschgarten and Cu for Rafz were not evaluated because little contamination was found there. The diagrams show the development of the concentration of the specific metal in the extraction solution as a function of ph. The results for the different extraction solutions (NTA 1, NTA 2 and without complexing agent) are shown in one diagram. NTA 1 concentration (400 µmol/l) equals the molar sum of Cd, Cu, Ni, Pb, Zn in soil used. NTA 2 concentration (4000 µmol/l) equals ten times this concentration (cp. Table 2-4) Calcium The curves of the extraction of Ca as a function of ph are not smooth for both soils. Additionally some results do not seem to be logical (higher amounts of extracted metal without complexing agent than with the use of NTA). The general trend is the decrease of extracted Ca with increasing ph value. For Rafz there is a abrupt drop from ph 7 with NTA 1 and without complexing agent, while the decrease for Kirschgarten is more gradual. For ph values from 6.5 to 8 there is an increasing effect of NTA 2 on extraction leading to a difference of about 30 % compared to NTA 1 at ph 8. 23

30 Results extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-6: Extracted Ca from Kirschgarten soil as a function of ph extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-7: Extracted Ca from Rafz soil as a function of ph 24

31 Results Magnesium Only small percentages of Mg were extracted. The amounts range from about 4 % (Kirschgarten) to 10 % (Rafz) at ph 3 to about 1 % at ph 8. There was no significant difference in the extraction between NTA 1 and without complexing agent (the value for Rafz at ph 3 can be considered as outlier). NTA 2 increased the extracted amount of Mg slightly at ph values higher than 7 for Kirschgarten compared to NTA 1 and without NTA. 4 extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-8: Extracted Mg from Kirschgarten soil as a function of ph 25

32 Results 20 extracted metal in % of total 10 NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-9: Extracted Mg from Rafz soil as a function of ph 26

33 Results Manganese As for most metals the amount of extracted Mn increases with lowering ph values. Without complexing agent it ranges from about 40 % (Kirschgarten) - 20 % (Rafz) at ph 3 to 1 % at ph 8. A significant increase in extraction due to NTA 1 can only be observed for Rafz at ph 3 to 4. The use of NTA 2 led to a higher extraction rate of Mn. The highest increase compared to the extraction with NTA 1 can be observed at ph values from 5 to 6 for Kirschgarten and 4 to 6 for Rafz. At higher ph values the difference decreases abruptly for Kirschgarten at ph 7.5 and more slowly for Rafz extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-10: Extracted Mn from Kirschgarten soil as a function of ph 27

34 Results extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-11: Extracted Mn from Rafz soil as a function of ph 28

35 Results Iron The results are similar for both soils. Without complexing agent there is no significant extraction of Fe. With NTA 1 the amount of extracted Fe decreases with increasing ph value steadily from about 300 µg/g to about 30 µg/g. The use of NTA 2 led to a greatly increasing extraction of Fe, about 1000 µg/g more at ph 3 compared to NTA 1. In the discussion it will be shown how this amount of extracted Fe influences the formation of complexes extracted metal in ug/g NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-12: Extracted Fe from Kirschgarten soil as a function ph 29

36 Results extracted metal in ug/g NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-13: Extracted Fe from Rafz soil as a function of ph 30

37 Results Copper With NTA 1 up to 58 % could be extracted at ph 4.5. For lower ph values this percentage decreases slightly to 50 % at ph 3. Also higher ph values lead to a decreasing extracted amount of Cu. The decrease is slight to 46 % at ph 7 and then steeper down to 25 % at ph 8. With NTA 2 the extraction curve has a different shape. The maximum lies here at ph 3 with 82 %. With higher ph values the extracted percentage decreases steadily to 56 % at ph 8. Without complexing agent Cu goes into solution only at low ph values extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-14: Extracted Cu from Kirschgarten soil as a function of ph 31

38 Results Zinc The shape of the extraction curves are similar for both soils but for Rafz generally more extraction occurs. Compared to Cu, Zn is more mobile, leading to higher extraction without the use of complexing agents especially at low ph values. With NTA 1 the amount of extracted Zn can be held nearly constant at 40 % for Kirschgarten. For Rafz it decreases slowly from 80 % to about 60 % by ph 7. After ph 7 there is a drop in extraction by about 20 %. The effect of NTA 2 is not as big as for Cu. The extracted amounts are in average 10 % higher than with concentration 1. At ph values below 4.5 and for Rafz at 7.5 and 8 the higher concentration has a bigger influence than in the intermediate ph ranges extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-15: Extracted Zn from Kirschgarten soil as a function of ph 32

39 Results extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-16: Extracted Zn from Rafz soil as a function of ph 33

40 Results Lead Pb is nearly immobile at ph values higher than 4. So only at very low ph values there is a significant extraction without complexing agents. With NTA 1 the extracted amount of Pb is generally higher. Surprisingly the maximum is not reached at very low ph values but at ph 5.5 with 31 %. The use of NTA 2 causes the largest increase in amount of extracted metal compared to NTA 1. At ph values below 5 over 90 % of Pb was extracted. And at ph 8 still over 50 % was found in the extraction solution extracted metal in % of total NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-17: Extracted Pb from Rafz soil as a function of ph Humic acid The calibration shows a linear correlation of extinction and humic acid concentration between 0 and 100 mg/l (Figure 3-18). Therefore it could be used for the determination of humic acid concentration in the extraction solutions. The results from the UV/Vis analyses were calculated as % of organic matter and are shown in Figure

41 Results Calibration of UV/Vis Extinction 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 R 2 = 0, c [mg/l] of humic acid Figure 3-18: UV/Vis extinction as function of humic acid concentration 7 6 extracted humic acid in % of organic matter NTA concentration 1 NTA concentration 2 w/o complexing agent ph Figure 3-19: Extracted humic acid as function of ph The results confirm the observations made with the naked eye. With increasing ph value the amount of extracted organic matter increases steadily, but for NTA 2 at ph > 7 the increase is exponential. However, it has to be considered that probably not only humic acids have been 35

42 Results measured. Despite that, the results can be seen as an indication for the extracted organic material dependent on ph value and NTA concentration. 3.3 Sequential extraction The results of all fractions measured with the AAS were calculated as µg per g soil. Then all fractions were added up to get the total metal concentration in the soil. With this value it was possible to calculate the percentage of metal in the single fractions in relation to the total concentration. The total concentrations from the characterisation with XRF (Table 2-2) were not used because of the large differences between the two results. These were caused by the sum up of errors during the single extraction steps (cp. 4.3). Beam diagrams were used for the graphical presentation of the results. Each beam equals 100% of the specific metal in the soil. The different shades represent the 7 fractions. Additionally the part which was extracted with EDDS is shown. So it is possible to see from which fractions the metal could be extracted Fe and Mn Fe and Mn were only measured to show the reliability of the extraction used. The results show that the distribution is similar for all soils. Fe and Mn are found in the expected fractions, Mn in Mn-oxides fraction, Fe in Fe-oxides fractions. Suspicious is the large amount of Fe in the residual fraction. As examples Figure 3-20 shows the distribution of Fe in Kirschgarten soil and Figure 3-21 shows the distribution of Mn in Rafz soil. The other results are given in the appendix. 36

43 Results Figure 3-20: Binding forms of Fe in Kirschgarten soil Figure 3-21: Binding forms of Mn in Rafz soil 37

44 Results Copper As in the other experiments Cu was only evaluated for Kirschgarten and Mattenweg. In both evaluated soils most of the copper (69 % for Kirschgarten and 60 % for Mattenweg) can be found in the organic fraction. This corresponds to data in literature (3). About 20 % in total are found in the Fe-oxides and the residue fractions. The remaining 10 % (Kirschgarten) respectively 19 % (Mattenweg) of Cu are in the first three fractions. From Kirschgarten 59 % of Cu could be extracted after 24 hours and 64 % after 48 hours with EDDS. The extraction results for Mattenweg are 42 % after 24 hours and 48 % after 48 hours. For both soils there is no significant change in Fe-oxides and residue fractions during extraction. Mobile, easily available and Mn-oxides fractions nearly disappear and the organic fraction decreases greatly. Figure 3-22: Binding forms of Cu in Kirschgarten soil 38

45 Results Figure 3-23: Binding forms of Cu in Mattenweg soil 39

46 Results Zinc The distribution of Zn was evaluated for all three soils. All soils have in common that a big portion of Zn is bound in Fe-oxides and residue fractions. These are 60 % for Kirschgarten, 69 % for Mattenweg and 44 % for Rafz soil. 16 % of Zn can be found in the organic fraction of all soils. 26 % of Zn in Rafz is mobile or easily available, 14 % of Kirschgarten s Zn and only 9 % of Zn in Mattenweg soil. The remaining Zn (8 to 14 %) is bound in the Mn-oxide fraction. The extraction results are disappointing. Only 12 % of Mattenwegs Zn could be extracted after 24 hours, 28 % of Kirschgarten and 33 % of Rafz with EDDS. After 48 hours the results are only slightly better. Again it is obviously that the Fe-oxides and residue fractions are nearly unchanged during extraction. Figure 3-24: Binding forms of Zn in Kirschgarten soil 40

47 Results Figure 3-25: Binding forms of Zn in Mattenweg soil Figure 3-26: Binding forms of Zn in Rafz soil 41

48 Results Lead The only significant contamination with Pb was found in Rafz soil, therefore only these results were evaluated. In the unextracted soil 4 % of Pb were bound in the Fe-oxides and residue fractions, 51 % in the organic fraction, 31 % occluded in Mn-oxides and 15 % were mobile or easy available. Only 22 % of Pb was extracted after 24 hours and this did not change after 48 hours. This result seems strange, because the extraction result for the other metals have indicated that the fractions up to the organic fraction can be well extracted with EDDS. The reasons for the poor extraction of Pb will be discussed later. The changes in Fe-oxides and residue fractions are within the error range (± 5%, cp. 4.3), all other fractions have decreased slightly. Figure 3-27: Binding forms of Pb in Rafz soil 42

49 4 Discussion 4.1 Extraction with EDDS compared to EDTA Conditional formation constants of Me-EDTA and Me-EDDS For an interpretation of the results it is helpful to know the complex stabilities. These are described by the formation constants. In Figure 4-1 and Figure 4-2 lg K eff as function of ph value is shown. K eff was calculated according to the equations in chapter 2.6. The used constants can be found in the appendix. Suspicious is the decrease of complex strength below ph 6 due to ligand protonation. The effect of Fe hydroxilation at high ph values leads to a change in the order of complex formation ( ph 4: Fe > Cu > Pb > Zn > Mn > Ca; ph 7: Cu > Pb > Fe > Zn > Mn > Ca ). 25,00 20,00 15,00 log (Keff) 10,00 5,00 CuEDTA FeEDTA PbEDTA 0, ZnEDTA CaEDTA -5,00 MnEDTA -10,00 ph Figure 4-1: Conditional formation constant lg K eff of Me-EDTA as function of ph 43

50 Discussion 25,00 20,00 15,00 10,00 log (Keff) 5,00 0, ,00-10,00 CuEDDS FeEDDS PbEDDS ZnEDDS CaEDDS MnEDDS -15,00-20,00 ph Figure 4-2: Conditional formation constant lg K eff of Me-EDDS as function of ph Effectiveness of extraction with EDDS compared to EDTA At neutral ph values EDDS performed similarly to EDTA in the extraction of Pb and Zn. However, there are still significant concentrations of these two metals in soil after extraction. For example, the best extraction results for Rafz leave about 500 µg/g Pb and about 540 µg/g Zn in soil. This is far above the limits of AbfKlärV (1) for agricultural soils (ph 5-6) which are allowed to be fertilised with sludge: 100 µg/g Pb and 160 µg/g Zn. The sequential extraction results indicated that while Zn extraction is limited mainly by the binding forms, Pb may benefit from a higher concentration of EDDS. This assumption is supported by results of experiments with different concentrations of NTA. Generally it has been shown that strong Fe complexes compete with weaker Pb and Zn complexes. This effect is even more severe at low ph values. Here EDDS is nearly of no use for Pb and Zn extraction at the concentration used. Fe ions are stronger competitors in EDDS complexation than in EDTA complexation. This effect would have to be compensated with higher concentrations of EDDS. 44

51 Discussion Other potentially competitive ions are Ca and Mn. Although the total amount of these metals is not changed by the complexing agent, they form complexes if ligands are available. The smaller stability of these complexes is compensated by the higher concentration of the metal ions. The largest effect can be expected by Ca at high ph values. However, due to a very weak Ca-EDDS species compared to Ca-EDTA this problem is substantially greater for EDTA. Chapter shows that Cu forms very strong complexes over a wide ph range. Although no evaluation of Cu kinetics was done, sequential extraction results show that Cu can be well extracted by EDDS. Best extraction results are 65 % of Cu from Kirschgarten soil, leaving about 160 µg/g in soil. However this concentration still fails to comply to AbfKlärV (1) which gives a limit of 60 µg/g. Although Kirschgarten soil is not an agriculturally used soil, this value can be seen as a guideline. The kinetics show that most metals are extracted within the first 8 hours. The only exception is Fe which shows slow kinetics due to binding in amorphous and crystalline form. The increasing amount of Fe being dissolved means that a shorter extraction time may benefit the extraction of other metals due to less competition from Fe. Negative side effects with regard to the extraction of essential metals are not increased significantly by the use of EDTA or EDDS at the concentration used. However, chemical extraction is no soft process for decontamination of soils and does always change the soil characteristics considerably. If higher concentrations of complexing agent are used, these effects can be expected to increase similarly to the ones examined with NTA concentration 2. The results of the sequential extraction showed that EDDS is able to extract metals from Mnoxides and organic complexes, together with the mobile and easily available metals these fractions are the potential extractable portion of heavy metals in soil. On the other hand it is not possible to extract significant amounts of metals from the Fe oxides and the silicates. This must be considered when the suitability of a contaminated soil for chemical extraction has to be determined. 45

52 Discussion 4.2 Extraction with NTA Conditional formation constants and speciation of Me-NTA As for EDTA and EDDS a calculation of the conditional formation constants was done using equations from chapter 2.6. Resulting lg K eff as a function of ph value is shown in Figure 4-3: 20,00 15,00 10,00 CuNTA log (Keff) 5,00 0, FeNTA PbNTA ZnNTA CaNTA MnNTA -5,00-10,00 ph Figure 4-3: Conditional formation constant lgk eff of Me-NTA as function of ph The amount of formed complexes depends not only on the stability constants, but also on the concentration of free ions in solution. Figures 4-4 to 4-6 show the molar concentrations of metals at selected ph values for Rafz soil. Ca is the major fraction, however it has the lowest complex stability. A strange effect is the fact that the Ca and Mg concentrations at lower ph values are smaller with NTA 1 than they are without complexing agent. This was also observed for Kirschgarten soil, but cannot be explained. 46

53 Discussion c in umol/l w/o NTA NTA1 NTA2 Pb Mn Cu Fe Zn Mg Ca Figure 4-4: Molar concentrations in extraction solution at ph Pb Mn c in umol/l Cu Fe Zn Mg Ca 0 w/o NTA NTA1 NTA2 Figure 4-5: Molar concentrations in extraction solution at ph 6 47