Large-scale demonstration of novel, entirely sustainable chelator-based washing of heavy metals contaminated soils

Domen Lestana, S. Gluhara, N. Finzgarb and M. Gerlc

a Agronomy Dept., University of Ljubljana, Slovenia

b Envit Ltd., Slovenia

c Arhel Ltd., Slovenia

Fertile soil is a valuable, limited resource, but often contaminated with a number of toxic metals i.e. Pb, Cd, Cu, As etc., representing pervasive and persistent risk to human health. For example: any exposure to Pb is considered to be potentially harmful to human health; no threshold for adverse effects has been identified. However, only a fraction of heavy metals contaminated sites are treated today due to the lack of efficient and environmentally sustainable technologies. We recently developed unique soil washing technology - ReSoil, which efficiently removes heavy metals from soil by complexation with any of strong (biodegradable and environmental persistent) amino-polycarboxylic chelators, for example EDTA. The patented technology features a novel reaction of alkaline substitution, precipitation and adsorption of toxic metals on polysaccharides, and chelator acidic precipitation for process waters and chelator recycling in a closed loop. No wastewaters and max. 1% of solid wastes are generated (US Patent 9108233 B2; US Patent 10124378 B2). The post-remedial toxic emissions from soil are mitigated to the levels close or bellow limits of quantification by thorough soil rinsing and addition of zero-valent Fe (ZVI) into the soil slurry (GB Patent Appl. 1720126.0) which enables for fast and permanent adsorption of small residual quantities of chelator and toxic metals chelates. The process is abiotic; for example the poor EDTA biodegradability is not an issue even if exceedingly high concentration of this extremely efficient chelator is used in soil washing. Furthermore, ZVI slurry addition simultaneously immobilize oxy-anion forming contaminants such as As. ReSoil technology is cost-efficient (140-190 $ / ton without profit, economy of scale) due to the reagent recycling and using inexpensive / waste auxiliary materials (lime, sulphuric acid, waste paper, scrap iron) and common process machinery. Realistically tended vegetable gardens with EDTA-remediated soil (chelator was selected as the most efficient) have supported the growth of vegetables, grasses and horticultural plants. No significant leaching of toxic chelates from remediated soil was measured in beds constructed as lysimeters. The uptake of contaminating Pb, Zn and Cd into the edible parts of vegetables was greatly reduced bellow values stipulated by legislature for safe food. Remediation with high EDTA doses initially to some extend affected the soil C and N cycles, soil enzyme activities and the structure and abundance of soil microbial populations, especially arbuscular mycorrhizae (AM). However, using simple and inexpensive revitalisation measures, e.g., addition of compost, healthy un-polluted soil and commercial or indigenous microbial AM inoculum, completely restored microbial life. ReSoil technology is evaluated in recently constructed demonstrational remediation plant with capacity of up to 6 ton of soil per day ( and demonstrational gardens with remediated soil which are open for local community and other public.

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