The effect of silica on the degradation of organohalides in granular iron columns

Abstract

Dissolved silica species are naturally occurring, ubiquitous groundwater constituents with corrosioninhibiting properties. Their influence on the performance and longevity of iron-based permeable reactive barriers for treatment of organohalides was investigated through long-term column studies using Connelly iron as the reactive medium. Addition of dissolved silica (0.5 mM) to the column feed solution led to a reduction in iron reactivity of 65% for trichloroethylene (TCE), 74% for 1,1,2-trichloroethane (1,1,2-TCA), and 93% for 1,1,1-trichloroethane (1,1,1-TCA), compared to columns operated under silica-free conditions. Even though silica adsorption was a gradual process, the inhibitory effect was evident within the first week, with subsequent decreases in reactivity over 288 days being relatively minor. Lower concentrations of dissolved silica species (0.2 mM) led to a lesser decrease (70%) in iron reactivity toward 1,1,1-TCA. The presence of dissolved silica species produced a shift in TCE product distribution toward the more highly chlorinated product cis-dichloroethylene (cis-DCE), although it did not appear to alter products originating from the trichloroethanes. The major corrosion products identified were magnetite (Fe 3 O 4 ) ) was only found in the silica-free column, indicating that silica may hinder its formation. A comparison with columns operated under the same conditions, but using Master Builder iron as the reactive matrix, showed that Connelly iron is initially less reactive, but performs better than Master Builder iron over 288 days. D

Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers

“…Despite the smaller amount of ZVI present in the mixture, the performance of this material and of pure ZVI at the same sampling port (3 cm) are comparable. It is interesting to note that the poorest performance was obtained using pumiceeZVI in series, probably since the presence of pumice upstream of the ZVI results in dissolution of small quantities of silica that reduce the performance of the iron (Kohn and Roberts, 2006). Moreover, Fig.…”

Solid phase studies and geochemical modelling of low-cost permeable reactive barriers

“…Although some researchers argue that dissolved silica promotes Fe 0 corrosion reaction by releasing protons in the solution through interaction with iron oxyhydroxide surface [45,46], recent studies have revealed its inhibitory effect on zero-valent iron reactivity over time [42,43]. In this study, three main distinct regions are observed by using SEM on polished sections of reacted iron-sand grains (Fig.…”

“…The decrease in iron reactivity could be also explained by the tendency of silica to serve either as an anodic corrosion inhibitor that prevents iron from releasing electrons to the metal contaminants or as chemical converter that changes the types of iron (hydr)oxide phases that may be formed. A previous iron-sand column study conducted for the treatment of organohalides showed that the inhibitory effect of silica sand is not only dependent on the concentration of dissolved silica species in the feed solution, but also on the type of iron used in the reactive matrix [42].…”

“…This cost-effective design helps groundwater flow distribution through the PRB unit, minimizes both mineral precipitation and fouling at reactive surface sites, and therefore enhances PRB reactivity and lifetime. Although little is yet known about the spatial distribution of silica species within an iron-sand reactive zone and with regard to iron corro- sion, recent studies have shown that dissolution of silica may limits system efficiency [42,43].…”

Predicting longevity of iron permeable reactive barriers using multiple iron deactivation models

“…Furthermore, the role that silica plays in reduced iron reactivity may also be compound-specific (Kohn and Lynn Roberts, 2006).…”