The remediation of arsenic in chromated copper arsenate contaminated soil using natural and modified zeolite material
Arsenic is a toxic metalloid that is present in the Earth’s crust in small amounts, over time becoming exposed due to both natural and manmade processes. The arsenic exposed in the ground can be subject to weathering such as rainfall, allowing the arsenic to become mobilized. A manmade source of arsenic is wood that has been pressure treated with chromated copper arsenate (CCA), a compound that protects the wood against insects and fungi. Over time, the wood can start to break down, allowing the arsenic to leach out into the surrounding soil. Once in the soil, plants can uptake the arsenic, which can lead to accumulation in the edible portions such as the leaves and berries, potentially harming humans and animals which consume them. In this study, the zeolite chabazite was used to sorb arsenic from the soil, reducing the amount available for uptake by the plant. Using radishes, with both non-modified and iron modified chabazite mixed in with the soil, the arsenic concentration in the radishes was reduced from 36.111±9.601 mg/kg in the leaves to 15.345±5.837 mg/kg and 14.116±2.403 mg/kg for the non-modified and the iron modified chabazite, respectively. It was also found that chabazite particle size had a significant effect on arsenic adsorption, further reducing the arsenic concentration in leaves from 15.345±5.837 to 7.713±0.569 mg/kg for the non-modified chabazite, and from 14.116±2.403 to 6.179±2.083 mg/kg in the iron modified chabazite with reduced particle size. Additionally, sawdust from CCA treated wood was incorporated into soil, as to model uptake of arsenic resulting from leaching. The data for the sawdust plantings suggests complex ion interactions between the chabazite and the CCA treated wood, which may make remediation of soils contaminated with CCA more difficult. Furthermore, the component ions of CCA, chromium, copper and arsenic leach from the wood at different rates. Chabazite may have different affinities for each of these ions, creating less adsorption capacity for arsenic, potentially leading to higher concentrations in the plants. In addition to the planting studies, leaching studies were performed, assessing the maximum amount of arsenic that the iron modified chabazite could adsorb from aqueous solution. The adsorption was modeled using Langmuir and Freundlich isotherms. Using data from the isotherms, it was found that the adsorption of arsenic by iron modified chabazite does not follow the Langmuir model, but fits the Freundlich model well.