- 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.