The Influence of Silica on the Reactivity of Iron Towards Chlorinated Hydrocarbons

Previous work has illustrated that dissolved silica present in groundwater can adsorb onto the iron media of a permeable reactive barrier. To date, little is known about the effect of adsorbed silica on the reactivity of iron towards chlorinated contaminants. Silica is a known corrosion inhibitor, and therefore silica sorption may affect the reactivity of cast iron towards redox-active species such as chlorinated ethanes. To determine the effect of silica sorption on iron reactivity towards chlorinated hydrocarbons, it is necessary to examine contaminant degradation rates under conditions where silica adsorption has been carefully accounted for. In this study we couple measurements of chlorinated hydrocarbon (CHC) (either trichloroethane (TCA) or trichloroethene) degradation rates with measurements of silica surface content. Both batch and columns studies were conducted as part of this effort. Batch studies were performed using electrolytic iron powder or iron coupons (1 cm²) in solutions containing sodium metasilicate and TCA. To evaluate the potential long-term effect of silica sorption, columns packed with untreated sieved Master Builder's iron were fed with simulated groundwater containing low levels of sodium metasilicate and chlorinated hydrocarbons (CHCs). Batch studies showed that at pH 8.5 the rate of TCA degradation decreased significantly with increasing silica concentration. Silica concentrations of 50 mg/L or more led to a two-fold decrease in the reaction rate, and a shift in the distribution of the reaction products towards less chlorinated compounds was observed. This shift could also be discerned at pH 7.5, even though the overall reaction rates were unaffected by the presence of silica. We ascribe the loss of reactivity at pH 8.5 but not at pH 7.5 to the fact that silica adsorbs onto iron more readily at higher pH. A decrease in iron reactivity towards CHCs was also observed in column experiments. Addition of silica to the feed resulted in depressed reaction rates relative to silica-free controls, even after silica was removed from the feed. We attribute this permanent reactivity loss to the partially irreversible buildup of a silica layer. Surface studies were conducted using iron coupons that had been exposed to solutions containing various amounts of sodium metasilicate for different periods of time. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES) were used to determine the relative amounts of and distribution of silica on the iron surface. Surface analyses of the iron coupons exposed to 100 mg/L silica solution indicated that the concentration of adsorbed silica continuously grew with increasing exposure time. After two weeks, scanning AES showed that silica completely covers the surface of the iron. Below a silica concentration of 50 mg/L, the amount of silica sorbed by the iron surface was observed to be a linear function of the silica concentration. Above 50 mg/L, however, silica uptake was unchanged with increasing silica loading. This corresponds well to the reactivity loss pattern observed in batch experiments.