Laboratory Tests on the Fate of Arsenic in Landfills

The new arsenic in drinking water regulation will require many utilities to implement technologies for arsenic removal. Of the EPA identified treatment options, adsorption onto solid media seems to be the most attractive for small treatment facilities, especially in the arid Southwest. The arsenic bearing solid residuals from these adsorption processes are to be disposed in non-hazardous landfills. The Toxicity Characteristic Leaching Procedure (TCLP) test defines whether a waste is hazardous or non-hazardous (with respect to landfill disposal) and most solid residuals pass the TCLP. However, the TCLP may not correctly estimate the leaching potential of arsenic residuals. The TCLP poorly simulates the alkaline pH, anaerobic microbial activity, mineralogical aging, high phosphate, and concentrated organic characteristics of landfills. These same conditions are expected to favor mobilization of arsenic from metal oxide adsorbents. This study quantifies leaching of arsenic from solid residuals under landfill conditions. Activated Alumina (AA) and Granular Ferric Hydroxide (GFH) have been identified as the adsorbents that will be most widely used for arsenic removal. Consequently, this research focused on leaching from these two residuals. It examines the effect of pH and arsenic loading and the impact of anaerobic microbial activity on the mobilization and biotransformation of arsenate. In addition, alternative leaching tests, including two specifically developed to better simulate landfill conditions, are utilized and the leaching observed is compared to that from the TCLP. Small volume batch experiments indicated arsenic partitions significantly more strongly to the aqueous phase in the pH range of landfill conditions than that of the TCLP. Results from batch assays showed that soluble As(V) was readily reduced to trivalent arsenite (As(III)) by an unadapted methanogenic consortium when a mixture of volatile fatty acids was used as electron donating substrate. Dissimilatory reduction of As(V) sorbed onto AA was also observed in continuous column experiments. Microbial activity increased As mobility, and the total arsenic concentration in the reactor effluent ranged from 130-365 μg/L. On the average, 63% of the total As mobilized consisted of As(III). The work indicates that significantly greater arsenic leachate concentrations can be expected than predicted by the TCLP for both activated alumina and GFH. It further indicates that actual landfill leachate provides a significantly more aggressive leaching condition than either the TCLP or Waste Extraction Test (WET).