Copper, chromium, and arsenic adsorption and equilibrium modeling in an iron-oxide-coated sand, background electrolyte system

The adsorption of copper, chromate, and arsenate (CCA) onto iron-oxide-coated sand (IOCS) was examined in single-metal and mixed-metal systems. Copper and arsenate were strongly adsorbed or formed inner-sphere surface complexes with the IOCS surface while chromate was weakly adsorbed or formed an outer-sphere surface complex with the IOCS surface. Copper adsorption by IOCS slightly increased in the presence of arsenate but was not affected by the presence of chromate. Arsenate adsorption was not affected by the presence of copper and/or chromate. Chromate adsorption increased in the presence of copper by the combination of electrostatic effects and possible surface-copper-chromate ternary complex formation. The presence of arsenate significantly decreased chromate adsorption due to competition for adsorption sites and electrostatic effects. Using inner-sphere surface complexation constants for copper and arsenate and outer-sphere surface complexation constants for chromate, the triple-layer model (TLM) was successful in describing adsorption of copper, chromate, and arsenate in single-solute systems. The model indicated that each metal uses a different number of adsorption sites. For CCA metals-IOCS 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.