Effects of pH value and calcium hardness on the removal of 1,1,1-trichloroethane by immobilized nanoscale zero-valent iron on silica based supports.

Immobilizing nanoscale zero-valent iron (NZVI) particles on silica-based supports is an effective way to overcome the NZVI aggregation. The pH value and calcium hardness can change the aggregation kinetics and alter the stability of the suspensions of NZVI-silica based materials, thus change the reactivity of these NZVI-silica based materials to remove chlorinated aliphatic hydrocarbons (CAHs). The removal of CAHs by these NZVI-silica based materials includes adsorption by silica based supports and degradation by NZVI particles. Using 1,1,1-TCA and mesoporous hydrated silica (mHS) as model chlorinated aliphatic hydrocarbon (CAH) and silica based support, the effects of pH value and Ca concentration on both the adsorption and adsorption-degradation processes of CAHs by NZVI-silica based materials were studied. The structural and textural features, suspension stability, particle size distribution, and Zeta potential of the materials under various conditions were characterized by different techniques. Both decreasing initial pH value and increasing Ca concentration can reduce the Zeta potential of mHS and lead to the aggregation of mHS particles, thus inhibiting the removal of 1,1,1-TCA via adsorption by mHS through decreasing the number of sites for adsorption. Low initial pH value can accelerate the corrosion of NZVI core and remove the passivation layer, thus promoting the removal of 1,1,1-TCA via adsorption-degradation by NZVI@mHS. Ca can decrease the sites for adsorption and form precipitates which can block mesoporous channels, thus hinder the 1,1,1-TCA removal via adsorption-degradation by NZVI@mHS.