Chemical Optimization of In Situ Emplacement of Nano-Particulate Iron Sulfide in Porous Media.

Abstract

Chemical optimization of an alternative construction method for permeable reactive barriers (PRBs) was investigated. Conventional trench-and-fill construction methods of PRBs are limited to shallow and thin aquifers, and hindered by the presence of a highly consolidated aquitard, subsurface utilities, or aboveground structures. In situ emplacement of reactive materials by direct injection technique offers a potential cost effective method to overcome these obstacles. The feasibility is largely expected to depend on whether sufficient coverage of the sand by FeS can be obtained without plugging the inlet region. Optimal deposition rates of FeS nanoparticle were established by modulating the chemistry of influent FeS suspensions. Such optimal conditions can be obtained when electrostatic interactions between FeS particles are sufficiently unfavorable while interaction between FeS particles and the quartz sand are sufficiently favorable. These optimum conditions were obtained at neutral pH, pH 6.5 to 8.3, and relatively low ionic strength, 0.025 M. At these conditions 3.4 × 10-6 mole FeS on average was deposited per gram of sand. To better understand the surface charge characteristics of FeS, aggregation rates studies were performed as a function of solution chemistry. The stability of FeS suspension gradually increased as pH increased in the neutral pH region, suggesting that the FeS surface becomes more negatively charged as pH increases. The stability sharply increased between pH 8.3 and 9.0, which can be explained by the hypothesis that FeS has multiple surface functional groups. Loss rates of FeS coating were investigated to evaluate the longevity of FeS-type PRBs as a function of solution chemistry. Over the pH range tested, pH 5.5 to 10.0, particulate iron detachment due to repulsive interactions was not observed on FeS pre-coated sands. When FeS was deposited by direct injection, however, detachment by repulsion appeared to explain greater FeS loss at more alkaline pH. In the range of pH 5.5 to 7.5, the dissolution rates of FeS increased as pH decreases, achieving steady-state effluent concentrations of 6.1 mg L-1 as Fe at pH 5.5. Excessive iron loss rates at relatively acidic pH might significantly shorten the long term operation of FeS-type PRBs.