Superior reduction and immobilization of Cr(VI) in soil utilizing sulfide nanoscale zero-valent iron supported by phosphoric acid-modified biochar: Efficiency and mechanism investigation

A novel strategy was proposed to remediate Cr(VI)-contaminated soil via phosphoric acid-modified biochar supported sulfide nanoscale zero-valent iron (SnZVI@PBC). Results of characterizations revealed that FeSX shell existed in outer layer of nZVI to prevent its oxidation after sulfidation modification, and SnZVI was effectively dispersed owing to the support of PBC, accelerating the electron transport for Cr(VI) reduction. The SnZVI@PBC presented pH-dependence and fast capture for Cr(VI) with outstanding binding amount of 335.55 mg/g. More importantly, the Cr(VI) content declined from 1300.75 to 223.30 mg/kg with conversion into stable Cr(III) in soil after 42 d of remediation with 2.0 % SnZVI@PBC under 60 % moisture content. Furthermore, leaching experiments showed that SnZVI@PBC could effectively immobilize Cr(VI), decreasing its migration and harmful risks to plants and human. Particularly, the fractions of exchangeable and carbonate-bound Cr decreased by 96.77 % and 83.60 %, which transformed to relatively stable fractions. Interestingly, the presence of humic acid, and the freezing-thawing/wetting-drying process promoted the immobilization performance of SnZVI@PBC for Cr(VI). SnZVI@PBC could alleviate the migration and poisonousness of Cr(VI) in soil primarily via reduction, co-precipitation, pore filling, and electrostatic attraction. Overall, SnZVI@PBC could be considered as a feasible amendment with superior reducing capacity and immobilization performance for Cr(VI)-contaminated soil.