New remediation technology for metals contaminated soil and sediment

Kurzfassung

Slovene SME, university spin-out, developed new technologies for remediation of metals contaminated soil and sediment. Technology includes »on sight« washing of soil or sediment, recycling of processed water and recycling of EDTA. The company also offers also laboratory and pilot scale feasibility tests to find the optimal remedial solution for a given contaminated site. They are looking for partners from industry/municipalities for implementation of their technology.

Details

Extraction with chelating agent ethylenediamine tetraacetate (EDTA) is the most efficient remediation technique for toxic metal removal for most types of contaminated soils and sediments. EDTA desorbs toxic metals from soil solid phases by forming strong and water-soluble metal-chelant complexes. Following an initial screening of the excavated soil to remove the surface debris, the soil is vigorously mixed with the EDTA solution, separated by filtration and then returned to the ground. Spent washing solution is innovatively electrochemically treated for EDTA and process water recovery and reused in a closed cycle. The technology is developed as a prototype plant.

The current existing prototype plant enables treatment of 75-120 kg soil batches (112-180 L soil slurry). Using the prototype remediation plant extraction of soil with 10 - 75 mM of EDTA (fresh and recycled, given in 1-3 dosages) removes from 55 to 90% of Pb from soils containing 1350 – 1800 mg kg-1 Pb.

Total estimated material and energy cost: 61.5 € per ton of remediated soil (not including capital investment, work etc.).

Innovative Aspects:
The innovation aspects of technology relate to the treatment of a large volume of waste solutions generated during soil extraction. These toxic wastewaters containing complexed EDTA cannot be treated using conventional methods (filtration, flocculation, precipitation). Other approaches have been demonstrated, but they are not practical and commercially available: reverse osmosis (the soil particles tend to clog the membranes); separation of EDTA anion using an anion exchange resin (expensive resin, recycling not practical); zero-valent bimetallic mixtures (Mg0-Pd0, Mg0-Ag0) precipitate Pb from EDTA in alkaline pH (economically prohibitive); separation of Pb from EDTA with Na2S recovers metals through precipitation as sulphides (the sludge produced is toxic); substitution of Pb with Fe3+ followed by Pb precipitation with phosphate and (at high pH) Fe3+ as hydroxides liberates EDTA (high reagent consumption); electrolytic recovery of metals and EDTA in a two-chamber cell separated with a cation exchange membrane (membrane fouling and degradation).

Electrochemical recovery and reuse of EDTA and process water from the spent solutions, as novelties in soil remediation technology, were extensively tested for process robustness. Once again electrochemical methods were proven as a simple and efficient, characterized by a compact size of the equipment, simplicity of operation, and low capital and operating costs. Other parts of our remediation technology use conventional equipment.

Unlike waters and air, soils are very heterogeneous and versatile media. Existing prototype remediation plant is designed to allow adjustment and optimisation of the key process parameters to a range of conditions given by soil/contamination properties, and to obtain necessary scale-up data for the design of the commercial remediation plant for specific contaminated site.

Slovene company is the only company offering chelating agent (EDTA) based soil washing. The technology enables Pb and other metal removal from fertile and organically rich garden soils, with the final goal of the soil reuse as a plant substrate.

Economic risks could arise as a consequence of still emerging land reclamation/soil remediation market, social acceptance of remediated soil as plant substrate, and relatively high cost of soil remediation and access to external financial sources. Although the need for soil remediation was already clearly established there are only few examples of successful remediation actions (this is especially the case for metal contaminated sites).