Use of hydrochar from hydrothermal co‑carbonization of rice straw and sewage sludge for Cr(VI) bioremediation in soil

Cr(VI) is among the most hazardous heavy metals with high toxicity in acidic soil. This study for the first time examined the solid form Cr(VI) remediation by hydrochar from hydrothermal co‑carbonization (HTcoC) of rice straw (RS) and digested sewage sludge (DSS) and evaluated its soil conditioner potentials. Results indicated that hydrochar produced at RS: DSS mass ratio of 1:1 (HTcoC1:1) had a C/N ratio of 29.01 that may ensure better microbial activity. When solid form Cr(VI) was applied, the maximum Cr release was noticed after 6 h operation in the Cr(VI)-loaded HTcoC1:1 (32.5%) and soil+HTcoC1:1 (39.5%) systems. Then re-adsorption of Cr(VI) occurred from the 8 h and reached to 100% in the HTcoC1:1 system after 48 h, in which 100% of the loaded Cr(VI) was reduced into less toxic Cr(III). This work implies that the co-hydrochar from RS and DSS can be potentially applied for Cr(VI) remediation in agricultural soils.