Removal of chromium, copper, and arsenic from contaminated groundwater using iron-oxide composite adsorbents

Groundwater contamination at wood preservative facilities may contain metals, including copper, chromium, and arsenic. In this study, laboratory batch experiments were conducted to study the removal of copper, chromium, and arsenic (CCA) from synthetic groundwater using composite iron-oxide adsorbents. Two types of composite adsorbents, iron-oxide-coated sand (IOCS) and magnetite-coated sand (MCS), were used. MCS provides both adsorptive and reductive capabilities for CCA metals removal. IOCS was used for studying single-solute adsorption for each metal and competitive adsorption in mixed-metals systems.; Copper and arsenate were strongly adsorbed or formed inner-sphere surface complexes, while chromate was weakly adsorbed or formed an outer-sphere surface complex, with the iron-oxide surface. The presence of arsenate in the solution slightly increased the amount of copper adsorbed while the presence of chromate did not affect copper adsorption by IOCS. The presence of copper and/or chromate did not affect arsenate adsorption. The presence of copper increased chromate adsorption which is mainly by electrostatic force. The presence of arsenate significantly decreased chromate adsorption due to electrostatic force and competitive effects.; Using inner-sphere surface adsorption constants for copper and arsenate and outer-sphere surface adsorption constants for chromate, the triple-layer model (TLM) was successful in describing adsorption of copper, chromate, and arsenate in single-solute systems. For mixed-metals systems, the equilibrium constants determined from single-solute systems were not able to predict adsorption from multi-solute systems. The TLM does not currently account for the heterogeneity of oxide surface sites and the formation of ternary complexes and/or solid phases that do not exist in single-solute systems.; Chromate was effectively removed by MCS mainly by reduction of Cr(VI) to a Cr(III) precipitated phase. Initial Cr(VI) concentration, solution pH, and MCS concentration are directly related to the reduction reaction, and a Cr(VI) reduction rate equation was developed based upon these three parameters, as follows:{dollar}{dollar}{lcub}dlbrack Cr(VI)rbrack over dt{rcub} = {lcub}-{rcub}klbrack Cr(VI)rbracklbrack Hrbracksp{lcub}0.2{rcub}{lcub}MCS{rcub}sp{lcub}1.8{rcub}{dollar}{dollar}Cumulative evidence from Fe(II) dissolution rate and amount, competition from inner-spherically bound {dollar}rm ASOsb4sp{lcub}3-{rcub},{dollar} lack of dependence on ionic strength, and the indication of surface site saturation suggests that the reduction process involves an interfacial reaction mechanism. In the long-term, Cr(VI) reduction may be limited by the precipitation of {dollar}rm(Cr,Fe)(OHsb3)(s){dollar} on the MCS surface.