Testing biodegradable chelators in realistic soil-washing process conditions: feasibility, efficiency and effect on soil properties

Simon Gluhara and D. Lestana

a Biotechnical faculty , University of Ljubljana, Slovenia


Some recent studies have indicated that the capacity of biodegradable aminopolycarboxylic chelators to reduce the labile fraction of Cd, Pb, Zn and other toxic metals in contaminated soil is similar to that of the conventional non-biodegradable chelator ethylenediamine tetraacetate (EDTA), with less impact on soil properties. Biodegradable chelators iminodisuccinic acid (IDS) and glutamate-N,N-diacetic acid (GLDA) appear to possess the greatest remediation potential. However, up to now the biodegradable chelators were tested in small laboratory scale and not in realistic process conditions of chelator and process water treatment and recycling. It is therefore the aim of our study to apply novel, patented ReSoil technology (US 9108233B2, US 10124378B2, GB patent appl. 201720126) currently the single available chelator-based soil washing technology which was developed into commercial scale. ReSoil uses alkaline conditions (lime, waste polysaccharide material) to recycle chelator in washing solution and acidic conditions (H2SO4) to precipitate chelator from diluted soil-rinsing solutions. Very small quantities of chelator, which remain in remediated soil are permanently absorbed on zero-valent Fe (added into soil slurry) to curb any potential emissions of toxic chelates. Acidic and calcareous soils historically polluted with Pb (main contaminant), Zn and Cd were used to represent two different soil types. Tests was performed in a pilot-scale ReSoil remediation facility (soil batches 10-100 kg, solid: liquid ratio 1:1). Initial results indicate that GLDA removes toxic metals almost as efficiently as EDTA: up to 60 and 70% of Pb was removed from calcareous and acid soil, respectively. GLDA is soluble under wide pH range and was not precipitated in acidic phase of ReSoil process. Close to complete chelator removal is essential for close-loop reuse of process waters and to prevent generation of waste-waters. Biodegradation of GLDA in process solutions or spry drying are presumable solutions, but could prove time-consuming and expensive. GLDA was also much more aggressive for soil organic matter (SOM) compared to EDTA, causing extensive depolymerisation and SOM lost. IDS exhibited less than 50% efficiency of toxic metal removal compared to EDTA. IDS was, however, easily recycled in ReSoil process. The process parameters (material balance, cost, potential toxic emissions from remediated soil), soil remediation efficiency (total toxic metal removal, metal bioavailability reduction, uptake into buckwheat (Fagopyrum esculentum) as a test plant), biological soil properties (dehydrogenase, urease and β-glucosidase activity) and pedological soil properties (SOM, available P and K, pH, KIK) of soils remediated using biodegradable GLDA and IDS and benchmark EDTA chelator will be reported.

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