Coupling electrokinetic with a cork-based permeable reactive barrier to prevent groundwater pollution: A case study on hexavalent chromium-contaminated soil

Abstract

This work proposes an eco-efficient treatment technology for the remediation of a kaolinite-based clay soil artificially contaminated with hexavalent chromium (50 mg Cr(VI) kg–1 soil), combining electrokinetics (EK) with permeable reactive barriers (PRB) composed of cork granules, the major by-product of cork stoppers production. This 100% natural and sustainable material can act as (i) an electron donor in the Cr(VI) reduction into trivalent chromium [Cr(III)], the less toxic state, and as (ii) a binder for the reduced Cr(III) on its pre-oxidized surface. The EK and Cr(VI) reduction efficiencies were assessed over 15 days as a function of the: (i) supporting electrolyte solution (demineralized water – DW, tap water, citric acid – CA, and sodium chloride – NaCl); and (ii) cork-PRB inclusion and position (near the anodic compartment, using direct current, or in the soil middle section, applying reversal polarity). Results showed that DW was the best supporting electrolyte solution, removing about 33% of total chromium (CrT) from the soil towards the anode, mainly under the Cr(VI) form, even though CA and NaCl presented higher electrical conductivity. Besides, nearly 67% Cr(VI) was reduced into less mobile Cr(III) only by soil-borne electron donor constituents, especially iron (> 6 g kg–1), which impaired the overall Cr migration due to the Cr(III) precipitation/adsorption over/onto the soil. Such reaction was boosted by CA and NaCl electrolytes, which increased H+ ions availability, reaching reduction efficiencies higher than 98%. When the cork-PRB was incorporated into the DW-driven EK process near the anode, the best position owing to the low pH, the Cr(VI) reduction and CrT removal efficiencies improved to about 97% and 42%, respectively. Furthermore, virtually no Cr(VI) migrated to the anolyte/catholyte, and less than 2% Cr(III) was found in the anodic chamber, being c.a. 40% of CrT retained in the cork-PRB as Cr(III) and c.a. 3%/55% of Cr(VI)/Cr(III) into the soil. Notwithstanding, the EK-PRB process can render polluted soil somewhat less dangerous and prevent the spreading of contamination to natural aquifers.