Assessment of biochar and zero-valent iron for in-situ remediation of chromated copper arsenate contaminated soil

Chromated copper arsenates (CCA) have been extensively used as wood impregnation agents in Europe and North America. Today, CCA contaminated sites remain abundant and pose environmental risks that need to be properly managed. Using a TRIAD approach that combined chemical, ecotoxicological and ecological assessment of soil quality, we investigated the abilities of biochar and zero-valent iron (ZVI) to remediate CCA contaminated soil in a microcosm experiment. Soil samples from a highly contaminated CCA site (1364, 1662 and 540 μg g^-1 of As, Cu and Cr, respectively) were treated with two different biochars (fine and coarse particle size; 1% w w^-1) and ZVI (5% w w^-1), both as sole and as combined treatments, and incubated for 56 days at 15 °C. In general, bioavailable As (As_bio) and Cu (Cu_bio) determined by whole-cell bacterial bioreporters corresponded well to water-extractable As and Cu (As_water and Cu_water). However, in biochar treatments, only Cu_bio and not Cu_water was significantly reduced. In contrast, under ZVI treatments only Cu_water and not Cu_bio was reduced, demonstrating the value of complementing analytical with bacterial bioreporter measurements to infer bioavailability of elements to soil microorganisms. The combined fine particle size biochar and ZVI treatment effectively reduced water extractable concentrations of Cr, Cu, and As on site by 45%, 45% and 43% respectively, and led to the highest ecological recovery of the soil bacterial community, as measured using the [³H]leucine incorporation technique. We conclude that the combined application of biochar and ZVI as soil amendments holds promise for in-situ stabilization of CCA contaminated sites.