Stability of solid form Cr(VI) after being loaded onto hydrochar under different soil pH conditions

Abstract

Heavy metal ions such as Cr(VI) leaching from agricultural soil is one of the significant environmental pollution threats. Hydrochar from hydrothermal co‑carbonization (HtcoC) of digested sewage sludge (DSS) and rice straw (RS) shows excellent adsorption potential of Cr(VI). Still, its stability at soil pH change is not well understood. This study examined solid form Cr(VI) stability at soil pH 3.5 and 8.0 (i.e., the average arable land soil pH range) in soil, hydrochar (HtcoC1:1) and soil + HtcoC1:1 mixture systems. Results indicate that HtcoC1:1 and soil + HtcoC1:1 mixture systems exhibited 100 % Cr(VI) adsorption at pH 3.5 while with comparative decline at pH 8.0. Moreover, Cr fractionation reflected the increased environmental risk in the HtcoC1:1 system at pH 8.0, suggesting its relatively high Cr leaching risk. This work implies that soil application of hydrochar produced from HtcoC of DSS and RS is relatively safe as most of the loaded Cr(VI) is stable within soil pH 3.5–8.0.