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We demonstrate here, in a laboratory scale experiment, the feasibility of using the base/acid pair Ca(OH)(2)/H(2)SO(4) to impose a pH gradient for EDTA recycling and, coupled with an electrochemical advanced oxidation process using a graphite anode, of recycling process water as part of a novel remediation technology for multi-metal contaminated soils. In the first batch, 60 mmol EDTA kg(-1) of soil removed 72, 27, and 71% of Pb, Zn, and Cd, respectively, from soil contaminated with 5329±685, 3401±193, and 35±6 mg kg(-1) of Pb, Zn, and Cd, respectively. In the subsequent four batches, we demonstrated that up to 88% of EDTA was recycled from each batch, with the potential to extract up to 98, 94, and 109% of Pb, Zn, and Cd, respectively, that the fresh EDTA extracted. Accumulation of salts in the process water through multiple remediation batches/recycles was prevented by CaSO(4) precipitation. Recycled process water toxicity testing indicated no significant effect on plant seed germination but some inhibition of root elongation.

The development of EDTA-based soil washing technologies is hampered by the lack of treatment methods of the spent solution, particularly when multi-metal contaminated soils have to be remediated. Extraction of Pb (5329 mg kg −1 ), Zn (3401 mg kg −1 ), Cd (35 mg kg −1 ) and As (279 mg kg −1 ) contaminated soil with 60 mmol EDTA kg −1 of soil removed 72%, 27%, 71%, and 80% of contaminants, respectively. We demonstrate here, on a laboratory scale experiment, the feasibility of using acid precipitation with HCl and H 2 SO 4 , coupled to initial alkaline Fe removal, to recover up to 88% of EDTA from a spent soil washing solution containing 11,578 mg L −1 of EDTA and 1109, 267, 7.1 and 64 mg L −1 of Pb, Zn, Cd and As, respectively. An electrochemical advanced oxidation process with a graphite anode was subsequently used to degrade 99.9% of the remaining EDTA in the spent washing solution and remove 99.7% Pb, 100% Zn, 96.6% Cd and 100% of As as (electro)precipitate. The cleansed process water obtained after electrochemical treatment was then used to prepare recycled washing solution by re-dissolving the recovered/recycled part of the EDTA. Washing solutions prepared from recycled EDTA had the same potential to extract Pb, Zn, Cd and As from soil as washing solution prepared from fresh EDTA of the same molarity. The novel recycling method is simple and robust and enables reuse of both EDTA and process water in a closed process loop.

Recycling of chelant decreases the cost of EDTA-based soil washing. Current methods, however, are not effective when the spent soil washing solution contains more than one contaminating metal. In this study, we applied electrochemical treatment of the washing solution obtained after EDTA extraction of Pb, Zn and Cd contaminated soil. A sacrificial Al anode and stainless steel cathode in a conventional electrolytic cell at pH 10 efficiently removed Pb from the solution. The method efficiency, specific electricity and Al consumption were significantly higher for solutions with a higher initial metal concentration. Partial replacement of NaCl with KNO3 as an electrolyte (aggressive Cl− are required to prevent passivisation of the Al anode) prevented EDTA degradation during the electrolysis. The addition of FeCl3 to the acidified washing solution prior to electrolysis improved Zn removal. Using the novel method 98, 73 and 66% of Pb, Zn and Cd, respectively, were removed, while 88% of EDTA was preserved in the treated washing solution. The recycled EDTA retained 86, 84 and 85% of Pb, Zn and Cd extraction potential from contaminated soil, respectively.

Electrocoagulation with an Al sacrificial anode was tested for the separation of chelant and heavy metals from a washing solution obtained after leaching Pb (3200 mg kg(-1)), Zn (1100 mg kg(-1)), and Cd (21 mg kg(-1)) contaminated soil with EDTA. In the electrochemical process, the sacrificial anode corroded to release Al(3+) which served as coagulant for precipitation of chelant and metals. A constant current density of 16-128 mAc m(-2) applied between the Al anode and the stainless-steel cathode removed up to 95% Pb, 68% Zn and 66% Cd from the soil washing solution. Approximately half of the initial EDTA remained in the washing solution after treatment, up to 16.3% of the EDTA was adsorbed on Al coagulant and precipitated, the rest of the EDTA was degraded by anodic oxidation. In a separate laboratory-scale remediation experiment, we leached a soil with 40 mmol EDTA per kg of soil and reused the washing solution (after electrocoagulation) in a closed loop. It removed 53% of Pb, 26% of Zn and 52% of Cd from the soil. The discharge solution was clear and colourless, with pH 7.52 and 170 mg L(-1) Pb, 50 mg L(-1) Zn, 1.5 mg L(-1) Cd and 11 mM EDTA.

An electrochemical advanced oxidation process (EAOP) was used for treatment of the washing solution obtained during leaching of Cu (364 +/- 2 mg kg(-1)) contaminated soil, with chelant S,S isomer of ethylenediamine disuccinate ([S,S]-EDDS). In the EAOP (constant current density 40 mA cm(-2)), a boron-doped diamond anode was used for the generation of hydroxyl radicals and oxidative decomposition of [S,S]-EDDS-metal complexes in the washing solution. The released Cu was mostly electro-deposited on the stainless-steel cathode. Three consecutive additions of 5 mmol kg(-1) [S,S]-EDDS removed 46% of the Cu from the soil, mostly from carbonate and oxide soil fractions (87 and 99% Cu reduction). The soil Cu oral availability in the simulated stomach and intestinal phases (in vitro physiologically based extraction test) was reduced by 5.5 and 4.6-times. Cu plant availability (in vitro diethylenetriamine pentaacetate test) was reduced by 3.6-times. The discharge solution was clear, almost colorless, with pH 8.4, 0.45 mg L(-1) Cu and 0.01 mM EDDS.

Soil washing with EDTA is known to be an effective means of removing toxic metals from contaminated soil. A practical way of recycling of used soil washing solution remains, however, an unsolved technical problem. We demonstrate here, in a laboratory scale experiment, the feasibility of using acid precipitation to recover up to 50% of EDTA from used soil washing solution obtained after extraction of Pb (5330 mg kg −1 ), Zn (3400 mg kg −1 ), Cd (35 mg kg −1 ) and As (279 mg kg −1 ) contaminated soil. Up to 100% of EDTA residual in the washing solution and 100%, 97%, 98% and 100% of initial Pb, Zn, Cd and As concentration in the solution, respectively, were removed in an electrolytic cell using a graphite anode. We employed the recovered EDTA and treated washing solution to prepare recycled soil washing solution with the same potential for extracting toxic metals from soil as the original. The efficiency of soil washing depends on the EDTA concentration. Using twice recycled 30 mmol EDTA kg −1 soil, we removed 44%, 20%, 53% and 61% of Pb, Zn, Cd and As, respectively, from contaminated soil.

Recycling chelant is a precondition for cost-effective EDTA-based soil remediation. Extraction with EDTA removed 67.5% of Pb from the contaminated soil and yielded washing solution with 1535 mg L −1 Pb and 33.4 mM EDTA. Electrochemical treatment of the washing solution using Al anode, current density 96 mA cm −2 and pH 10 removed 90% of Pb from the solution (by electrodeposition on the stainless steel cathode) while the concentration of EDTA in the treated solution remained the same. The obtained data indicate that the Pb in the EDTA complex was replaced by electro-corroded Al after electro-reduction of the EDTA and subsequently removed from the solution. Additional soil extraction with the treated washing solution resulted in total removal of 87% of Pb from the contaminated soil. The recycled EDTA retained the Pb extraction potential through several steps of soil extraction and washing solution treatment, although part of the EDTA was lost by soil absorption.

The feasibility of a two-phase method for remediation of Cu (364+/-2 mg kg(-1)) contaminated vineyard soil was evaluated. In the first phase we used ethylenediamine tetraacetae (EDTA) for Cu leaching, while in the second phase we used an electrochemical advanced oxidation process (EAOP) for the treatment and reuse of the washing solution for soil rinsing (removal of soil-retained, chelant-mobilized Cu complexes) in a closed loop. In the EAOP, a boron-doped diamond anode was used for the generation of hydroxyl radicals and oxidative decomposition of EDTA-metal complexes at a constant current density (40 mA cm(-2)). The released Cu was removed from the solution mostly as an electro-deposit on the cathode. Two consecutive additions of 10 mmol kg(-1) EDTA removed 26% of Cu from the soil, mostly from carbonate and oxide soil fractions (58% and 40% Cu reduction). The soil Cu oral availability (in vitro Physiologically Based Extraction Test) was reduced after remediation by 42% and 51% in the simulated stomach and intestinal phases. The discharge solution was clear, almost colorless, with pH 8.4 and 0.5 mg L(-1) Cu and 0.07 mM EDTA. The novel method enables soil Cu availability stripping using small volumes of process waters, and no wastewater generation or other emissions into the environment.