A Silicate/Glycine Switch To Control the Reactivity of Layered Iron(II)-Iron(III) Hydroxides for Dechlorination of Carbon Tetrachloride

Abstract

Layered Fe^II-Fe^III hydroxide chloride (chloride green rust, GR_Cl) has high reactivity toward reducible pollutants such as chlorinated solvents. However, this reactive solid is prone to dissolution, and hence loss of reactivity, during storage and handling. In this study, adsorption of silicate (Si) to GR_Cl was tested for its ability to minimize GR_Cl dissolution and to inhibit reduction of carbon tetrachloride (CT). Silicate adsorbed with high affinity to GR_Cl yielding a sorption maximum of 0.026 g of Si/g of GR_Cl. In the absence of Si, the pseudo-first-order rate constant for CT dehalogenation by GR_Cl was 2.1 h^-1, demonstrating very high reactivity of GR_Cl but with substantial Fe^II dissolution up to 2.5 mM. When Si was adsorbed to GR_Cl, CT dehalogenation was blocked and Fe^II dissolution extent was reduced by a factor of 28. The addition of glycine (Gly) was tested for reactivation of the Si-blocked GR_Cl for CT dehalogenation. At 30 mM Gly, partial reactivation of the GR_Cl was observed with pseudo-first-order rate constant for CT reduction of 0.075 h^-1. This blockage and reactivation of GR_Cl reactivity demonstrates that it is possible to design a switch for GR_Cl to control its stability and reactivity under anoxic conditions.