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

Layered Fe-II-Fe-III in 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 pseudofirst-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.