Characteristics of soil pore structure response to electric field strength and their effects on Cr(VI) removal from a historically chromium-contaminated soil

Abstract

The characteristics of soil pore structure response to electric field strength and their effects on the migration of hexavalent chromium during electrokinetic remediation remain unclear. This paper investigates the effects of soil actual voltage and pore redistribution on hexavalent chromium removal under various voltage gradients during electrokinetic remediation. The electric field across the soil was the dominant factor that affected the soil properties, pore structure, and transportation of hexavalent chromium. The removal efficiency of dissolved hexavalent chromium was significantly enhanced from 36.1 % to 80.5 % as the theoretical voltage gradients increased from 0.5 V·cm−1 to 3 V·cm−1 due to the highest voltage proportion (52.7 %) of the soil chamber at 3 V·cm−1, while it only increased to 82.0 % at 4 V·cm−1. The optimal actual electric field strength of 1.00–1.50 V·cm−1 across the soil matrix facilitated the efficient electromigration of hexavalent chromium. The electric field force exerted a direct (0.70) or indirect action intensity (1.55) on soil pore structure by altering soil chemical properties. Soil mesopores and ultra-micropores transformed into micropores through collapsing, shrinking, ion migration, weakening electrostatic repulsion, and enhancing adhesive bonding among particles with increasing electric field. The distribution and structure of soil pores were more homogenous under optimal electric field conditions, thereby improving the migration path of hexavalent chromium. Soil mesopores facilitated hexavalent chromium removal and the formed micropores were opposite. These findings have significant scientific and practical implications for applying the in-situ electrokinetic techniques in the remediation of metal-contaminated soils.