Performance and mechanisms of microwave-assisted zerovalent iron/pyrite for advance remediation of strongly alkaline high Cr(VI) contaminated soil

Strongly alkaline high Cr(VI) contaminated (SAHCR) soil poses a high risk to the environment and public health, yet lacks rapid and efficient remediation technology. In this study, a novel approach combining microwave irradiation with zerovalent iron/pyrite (FeS2/ZVI) was developed for the remediation of SAHCR soil. The results indicated that fast and efficient remediation of the SAHCR soil was achieved by microwave irradiation-assisted FeS2/ZVI, with 99.9% of removal rate of Cr(VI) within 10 min, and Cr(VI) concentration from 3900.8 plummeted to 2.38 mg kg-1. The data of Cr(VI) reduction kinetics at different temperatures indicated that the activation energies (Ea) for microwave-FeS2/ZVI system was 27.4 kJ mol-1, 28.5% lower than that without microwave irradiation, suggesting that in addition to the heating effect of microwave, the accelerated Cr(VI) reduction also comes from the catalytic effect of "hot spots" on FeS2/ZVI under microwave irradiation. Furthermore, it was demonstrated that microwave irradiation promoted the transformation of reduced Cr(III) into the stable FeCr2O4 mineral and the excellent long-term stability of the remediated SAHCR soil. These findings can provide a perspective for advanced remediation of the difficult-to-treat SAHCR soil by the synergism of microwave irradiation with the iron-sulfur based reducing materials.