ABSTRACT

Arsenic Removal and Stabilization with Synthesized Pyrite

Arsenic is the second most commonly found contaminant of concern both at sites on the National Priority List and at sites under DOE control. Furthermore, the recent lowering of the MCL for arsenic in drinking water will result in additional production of arsenic-contaminated residuals. Lowering of other regulatory standards for arsenic to maintain similar risk levels, would result in higher levels of treatment required for arsenic-contaminated ground water, soils, sediments and other wastes found at contaminated sites. The primary method that is currently used for removing arsenic from water produces residuals that can be unstable when disposed under anoxic conditions, such as typically found in a landfills. An alternative approach would be to remove arsenic with compounds that are have a high affinity for arsenic and are stable under anoxic conditions. Pyrite is such a compound and recent research has demonstrated how to produce micro- and nano-sized pyrite crystals, which have high specific surface areas that lead to high capacities to adsorb arsenic. Although the initial removal mechanism would be adsorption, chemical reactions would be expected to occur on the surface between arsenic and pyrite producing new solid phases such as arsenian pyrite (Fe(S,As)₂) and arsenopyrite (FeAsS). These compounds are stable for geologic time periods and are very insoluble under reducing conditions. Pyrite could also be applied to stabilize arsenic-contaminated media such as soils, sediments and sludges. Therefore, the goal of this project is to develop a novel treatment method for removal of arsenic from water and stabilization of arsenic contaminated soils, sediments and sludges based on application of micro- and nano-sized synthesized pyrite. This goal will be achieved by pursuing five specific objectives. The first task would characterize and optimize the synthesis procedure for micro- and nano-sized pyrite. Effects on the synthesis of pyrite particles of iron concentration, sulfide or polysulfide concentration, pH, reaction time, and method of reagent addition will be investigated in batch reactor systems. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrometry will be used to size and characterize the pyrite particles. The second task would be to measure the effects of operational variables on removal of arsenic from solution. The effects of synthesized pyrite size and dose, pH, and competing ions on adsorption of arsenic (III and V) will be measured in a series of batch equilibrium adsorption experiments. The third task would develop a chemical equilibrium model to describe behavior observed in Task 2. The fourth task would measure the stability of residuals from pyrite-treated water. Arsenic-pyrite residuals will be produced with both valence states of arsenic (III, V) using conditions identified by Task 2 as being optimal for arsenic removal. The leachability of arsenic from these residuals will be measured under conditions of the TCLP test and over a range of pH . The fifth task would measure the stability of arsenic-contaminated soils treated with pyrite. The effects on stability of arsenic contaminated soils of pyrite size and dose, arsenic type (III, V), arsenic concentration and soil type will be investigated. Stability of the treated soils will be evaluated by the TCLP and equilibrium extraction test over a range of pH. Costs of pyrite-based treatment of soils/sediments/wastes would be similar to those for solidification/stabilization ($60 - $290/ton). However these costs could be as much as $50/ton lower due to reduced reagent costs. Costs for removing arsenic from water with pyrite would be similar to existing technologies, but would have the advantage of producing inherently stable residuals.