Coupled reduction of chlorinated hydrocarbons and heavy metals by zerovalent silicon

The feasibility of using zerovalent silicon (Si⁰) as a novel reductant to remove chlorinated ompounds and heavy metals in contaminated sites was investigated. The kinetics and degradation mechanism of carbon tetrachloride (CT) by Si⁰ were also examined. Results showed that zerovalent silicon could effectively dechlorinate the chlorinated compounds. A nearly complete dechlorination of CT by Si⁰ was obtained within 14 h. The produced concentrations of chloroform (CF) accounted for 71-88% loss of CT, showing that reductive dechlorination is the major degradation pathway for the degradation of chlorinated hydrocarbons by Si⁰. The degradation followed pseudo first-order kinetics and the normalized surface reaction rate constant (ksa) for CT dechlorination ranged between 0.0342 and 0.0454 L m^-2 h^-1 when CT concentrations were in the range of 3-20 μM. A linear relationship between the k_sa and pH value was also established. In addition, zerovalent silicon has a high capability in the removal of heavy metals. 83% of Cr(VI) was removed by 0.5g Si⁰ within 5 h, which is higher than that by Fe⁰. The removal efficiency of divalent metal ions by Si⁰ followed the order of Cu(II) > Pb(II) > Ni(II). This indicates that zerovalent silicon is an alternative reductant and can undergo coupled reduction of heavy metals and chlorinated hydrocarbons in contaminated groundwater.