Multi-metal equilibrium sorption and transport modeling for copper, chromium, and arsenic in an iron oxide-coated sand, synthetic groundwater system

The mixed metal compound, Chromated Copper Arsenate, or CCA, has been widely used as a wood preservative. The metal ions in CCA, CrO2− ₄, Cu2+, and AsO3− ₄, have been found in contaminated surface and subsurface soils and groundwater nearby some wood preservative facilities and nearby wood structures. Iron oxides are a ubiquitous soil-coating constituent and are believed to be a main factor in controlling the transport and fate of many metals in the soil solution. In this research, iron-oxide-coated sand (IOCS) is used as a surrogate soil to investigate the adsorption and transport behavior of the mixed metals solution, copper, chromate, and arsenate, in the subsurface environment.; Copper adsorption increases with increasing pH. The presence of arsenate in the solution slightly increases, while chromate has minimal effect, on the amount of copper adsorbed. Chromate adsorption decreases with increasing pH. With arsenate present in solution, chromate adsorption is significantly suppressed over the pH range studied. In contrast, the presence of copper slightly increases chromate adsorption. Similar to chromate, arsenate adsorption decreases with increasing pH. The presence of chromate or copper does not affect the amount of arsenate adsorbed over the range of concentrations studied.; Two surface complexation models, the triple layer model (TLM) and the electrostatic implicit model (EIM), were used to simulate equilibrium adsorption in both single-metal and multi-metal systems. Simulations using the specific surface complexation equilibrium constants derived from either the single-metal or the multi-metal systems with both the TLM and the EIM were successful in fitting the adsorption data in that respective single or multi-metal system.; The local equilibrium assumption using batch-derived sorption isotherm parameters from the EIM failed to predict the copper and arsenate transport, while it adequately described chromate transport. The breakthrough curves of all three metals were asymmetrical and showed long-tailing behavior. This nonideal behavior is caused by nonlinear sorption and/or non-equilibrium conditions during transport. The two-site chemical non-equilibrium model, which accounts for the kinetically controlled adsorption sites, was able to fit the observed breakthrough curves for all three metals in single-metal systems. However, the model was partially successful in predicting transport in multi-metal systems.