Deep reducing Cr(VI) and simultaneously removing total Cr from Cr(VI)-contaminated soil via thermally activated oxalic acid reduction: A novel soil remediation strategy inspired by nature

An oxalic acid-based remediation method enables deep Cr(VI) reduction and Cr removal. • Oxalic acid dissolves refractory Cr(VI) species and removes Fe Cr associated Cr(III). • Cr(VI) speciation-guided processes achieve efficient remediation of diverse soils. • One-step and two-step treatments address different Cr(VI) contamination scenarios. • Plant-derived oxalic acid provides an environmentally benign reagent. The complex distribution of Cr(VI) species in contaminated soils limits the effectiveness of conventional reduction methods, which often fail to achieve regulatory targets and result in the accumulation of substantial Cr(III) residues. In this study, a thermally activated oxalic acid reduction strategy was developed based on the molecular characteristics of Cr(VI) and Cr(III) species in soils, enabling deep Cr(VI) reduction and simultaneous total chromium removal. For soils with high proportions of adsorbed Cr(VI) but limited water-soluble Cr(VI), a one-step thermally activated oxalic acid treatment reduced Cr(VI) concentrations below the regulatory limit (5.7 mg kg −1 ) and removed approximately 80% of total chromium. In contrast, soils dominated by water-soluble Cr(VI) were effectively remediated through a two-step process involving water washing followed by thermally activated oxalic acid reduction, achieving regulatory compliance and substantial total chromium removal. Mechanistically, oxalic acid enables near-complete reduction of stably coordinated Cr(VI) via a dissolution-release-reduction pathway, while other adsorbed Cr(VI) species are reduced through dissolution-reduction or direct reduction pathways. Meanwhile, the predominant Cr(III) species Fe Cr coprecipitate in the soil is simultaneously dissolved by oxalic acid. After remediation, residual Cr is mainly retained as highly stable residual Cr(III) species with minimal environmental risk. The oxalic acid filtrate can be regenerated via ion exchange treatment and reused for subsequent reduction, and oxalic acid itself, as a common plant root exudate, serves as an environmentally benign reagent. Moreover, this method is applicable to different soils with varying textures and Cr(VI) speciation, providing a potentially sustainable approach for the remediation of Cr(VI)-contaminated soils. Download: Download high-res image (265KB) Download: Download full-size image