Novel EDTA and process water recycling method after soil washing of multi-metal contaminated soil

Abstract

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 2SO 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.