Supplementary Material for:
Chelant soil washing technology for metal contaminated soil
(Environmental Technology)
David Voglar1 and Domen Lestan1,2*
1
Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101,
1000 Ljubljana; Slovenia
2
Envit Ltd., Vojkova 63, 1000 Ljubljana; Slovenia
*
E-mail address: domen.lestan@bf.uni-lj.si
(10 pages, 5 Figures)
Chemical background of the EDTA and process water recycling
Fig. SM1
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Chemical background of the EDTA and process water recycling
The recycling method relies on a combination of substitution/precipitation reactions in an
imposed pH gradient for EDTA recovery, and on electrochemical polishing of process waters
for total water recycle in a closed loop. By using Ca(OH)2 and H2SO4 as the base/acid pair,
the accumulation of added reagents in the process water after multiple soil remediation
batches/recycles is prevented by precipitation and removal of poorly-soluble CaSO4 during
the process.
Alkaline metal precipitation:
At low pH [1] Fe interferes with acidic EDTA precipitation from the used washing solution
[2.3] and must be removed. Alkaline conditions make the formation of Fe-EDTA complexes
less favorable [1], leading to Fe hydrolysis and Fe(OH)3 precipitation [2]. Alkalization with
Ca(OH)2 efficiently removed not only Fe but also Pb, Zn and Cd [3], since Ca replaces PTMs
in the EDTA complex due to a higher stability constant of Ca-EDTA at high pH [1].
However, the hydroxides of most PTMs are amphoteric. They are increasingly soluble at both
low and high pHs, and the optimum pH for precipitation depends on the metal: Pb (pH 9.5),
Zn (pH 9.2) and Cd (pH 11.2) [4]. Since the pH must be raised to 12 and above before the
stability constant for Ca-EDTA exceeds that of Pb-EDTA [5], part of the PTMs remains in the
solution after alkaline precipitation. Using NaOH did not remove any of the Pb, Zn or Cd
from the solution [2] due to the low affinity of Na to form complexes with EDTA.
Ca(OH)2 has fewer free electrons than NaOH, making it less likely to change its structure
when exposed to water and is therefore less water-soluble (0.173 g L-1 at 20oC) and normally
decreases with pH. However, Ca replacement of Fe and PTMs in the EDTA complex at high
pH pushes the chemical equilibriums towards Ca(OH)2 dissolution. Released metals (Me)
proteolitically react with waters, releasing H+, which additionally enhances Ca(OH)2
dissolution:
Me2+ + 2H2O → Me(OH)2 +2H+
(1)
The product of alkaline precipitation is process water with an active Ca-EDTA form of
chelant. Fe and PTMs substituted from the chelant precipitate as hydroxides.
Acidic precipitation of EDTA:
As mentioned above, removal of Fe in the alkaline phase prior to acidification significantly
improves the yield of recovered EDTA [2,3], since the stability of Fe-EDTA complexes is
relatively high in acidic conditions between pH 2-4 and higher than the stability of EDTA
complexes with other metals [1]. Protonated EDTA (H4EDTA) is poorly soluble in acidic
media below pH 3 [6] and, after acid (H2SO4) addition, precipitates from the solution. PbEDTA complexes are increasingly less stable in more acidic conditions [1], leading to sharp
PbSO4 precipitation at pH < 1.8 [2.31]. To avoid contamination of recycled EDTA with
precipitated Pb, the process water was not acidified below pH 2. In addition to EDTA, excess
Ca in the process water is removed as poorly-soluble CaSO4 (solubility 2.98 g L-1 at 20 °C,
[7]). This prevented the accumulation of added reagents through consecutive soil
remediation/EDTA recycling batches
Electrochemical cleansing of process waters:
Naumczyk et al. [8] demonstrated that several anode materials, such as graphite and noble
metal anodes, successfully mediate the oxidation of organic compounds. According to the
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generally accepted mechanism of EAOP, water is first discharged (at potentials above 1.23V
vs standard hydrogen electrode, standard conditions) at anode active sites (M), producing
adsorbed hydroxyl radicals, M(•OH)ads, which are involved in the mineralization of an
organic compound (R) in aqueous solutions:
Raq + (n/2)M(•OH)ads → (n/2)M + mineralization products + (n/2)H+ + (n/2)e−
(2)
where n is the number of electrons involved in the oxidation reaction of organics [9]. Reaction
(2) is in competition with the anodic discharge of these radicals to form dioxygen:
M(•OH)ads → M + (1/2)O2 + H+ + e−
(3)
The activity of these electrogenerated hydroxyl radicals is therefore strongly linked to their
interaction with the anode surface; the weaker the interaction, the lower the electrochemical
activity towards oxygen evolution (higher O2 overpotential). In relation to the EDTA
degradation pathway in the electrolytic cell, EDTA is electro-oxidized through sequential
removal of the acetate groups, until small size hydrocarbon products are formed [10].
References
[1] C. Kim, Y Lee, and S.K. Ong, Factors effecting EDTA extraction of lead from leadcontaminated soils, Chemosphere 51 (2003), pp. 845-853.
[2] M. Pociecha, and D. Lestan, Novel EDTA and process water recycling method after soil
washing of multi-metal contaminated soil, J. Hazard. Mater. 201-202 (2012), pp. 273-279.
[3] M. Pociecha, and D. Lestan, Soil washing of metal contaminated soil with EDTA and
process water recycling, J. Hazard. Mater. 235-236 (2012), pp. 384-387.
[4] G.C. Cushnie, Pollution Prevention and Control Technologies for Plating Operation, 2nd
ed., National Center for Manufacturing Sciences: Ann Arbor, MI, USA, 2009.
[5] G.A. Brown, and H.A. Elliot, Influence of electrolytes on EDTA extraction of Pb from
contaminated soil, Water, Air and Soil Pollut. 62 (1991), pp. 157-165.
[6] L. Di Palma, P. Ferrantelli, C. Merli, and F. Biancifiori, Recovery of EDTA and metal
precipitation from soil washing solutions, J. Hazard. Mater. 103 (2003), pp. 153-168.
[7] P. Theodoratos, N. Papassiopi, T. Georgoudis, and A. Kontopoulos, Selective removal of
read from calcareous polluted soils using the Ca-EDTA salt, Water, Air and Soil Pollut. 122
(2000), pp. 351-368.
[8] J. Naumczyk, L. Szpyrkowicz, M. De Faveri, and F. Zilio Grandi, Electrochemical
treatment of tannery wastewater containing high strength pollutants, Trans. I ChemE B. 74
(1996), pp. 58-59.
[9] C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, and D. Mantzavinos,
Advanced oxidation processes for water treatment, J. Chem. Technol. Biotechnol. 83 (2008),
pp. 769-776.
[10] Y. Yamaguchi, Y. Yamanaka, M. Miyamoto, A. Fujishima, and K. Honda, Hybrid
electrochemical treatment for persistent metal complexes at conductive diamond electrodes
and clarification of its reaction rute, J. Electrochem. Soc. 153 (2003), pp. 1123-1132.
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Fig. SM1. Pilot-scale soil washing plant. Step 1: soil washing. Step 2: separation and rinsing
of the process oversize material. Step 3: phase separation and soil rinsing. Step 4: treatment of
remediated soil. Step 5: alkaline substitution and metal precipitation. Step 6: acid precipitation
and EDTA recovery. Step 7: electrolytic degradation of EDTA remaining in the process
solution. Step 8: preparation of the recycled washing solution.
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Fig. SM2. Coloration of process waters from different steps of novel soil remediation
technology (step 1: soil washing solution; step 5: solution after alkaline substitution and metal
precipitation; step 6: solution after acid precipitation and EDTA recovery; step 7: solution
after electrolytic cleansing).
Fig. SM3. Solid wastes stabilized with bitumen (0.9 kg of bitumen per 1 kg of solid waste).
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Fig. SM4. Experimental plots with original and remediated soils and vegetables produced.
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