Arsenic in Water: Residuals Disposal and Stabilization

 Wendell P. Ela, A. Eduardo Sáez, Brian Zelinski, Amlan Ghosh, Mohammed Mukiibi
Department of Chemical and Environmental Engineering
University of Arizona , Tucson AZ

 Abstract

Arsenic is one of the five most common inorganic contaminants at Superfund sites. In addition, water providers, solid waste disposal entities, and regulatory agencies are gearing up to comply with a new standard for arsenic in drinking water. Whatever technology for treatment is selected, a residual stream containing the removed arsenic will be generated. Consequently, appropriate methods must be developed to assess the hazard posed by the arsenic-bearing residuals. Furthermore, to the degree that arsenic-bearing residuals are shown to pose an unacceptable risk, technologies must be developed to stabilize these residuals and mitigate the risk to an acceptable level. If this latter need is left unaddressed, it is foreseeable that disposed residuals will become the focus of future Superfund clean-ups. This work focuses on two critical areas: residuals assessment and residuals stabilization. The leaching of arsenic and iron from an arsenic-bearing solid residual, granular ferric hydroxide, was studied in a laboratory-scale column that approximates the conditions of a mixed solid waste landfill. Results show that both iron and arsenic are likely to be reduced and mobilized under the anaerobic, reducing conditions expected in a landfill. During the early stages of operation, the majority of arsenic and iron leaching occurs in the form of particulates, and the leaching rates of the two elements correlate. At later stages, depletion of the iron inside the column leads to a sizable increase in arsenic leaching rates. To stabilize the arsenic-bearing residuals, a process based on the encapsulation of the residuals in a polyceram matrix has been developed. The process is based on the synthesis of a polymeric matrix that contains both a polystyrene butadiene rubber and an epoxy resin. Results show that mechanically stable waste forms containing up to 65 wt% of spent residuals can be produced, while the capacity of conventional cement encapsulation does not exceed 15 wt%. Preliminary results show that arsenic leaching from polyceram waste forms is substantially slower than leaching from conventional cement matrices and untreated residuals.