Comparison of Physicochemical Methods to Remove Arsenic from Landfill Leachate and Gas Condensate

Abstract

Arsenic is an important contaminant widely found in municipal solid waste (MSW). It has been accumulating in landfills and its concentrations in the landfill leachate have been observed to increase in several sites, notably at the Cedar Hills Regional Landfill Facility (CHRLF) located in the State of Washington. The chemistry and mobility of arsenic are unusual since it involves an array of solutes, gaseous arsines, and solids whose formation is greatly affected by landfill conditions that are dependent on MSW composition, hydrology, and other site-specific factors. The reducing conditions in a landfill result in intense microbial activity that generates landfill gas (LFG), LFG condensates, and leachate all of which can contain arsenic. The biological activity also results in the formation of methylated and sulfur-containing arsenic complexes that tend to be resistant to the conventional methods of removal. This thesis evaluated the effectiveness of conventional and emerging technology particularly micro-electrolysis (ME) in the removal of arsenic from landfill leachate and LFG condensate. ME uses a combination of adsorption and zero-valent iron (ZVI) driven reduction that results in the immobilization of arsenic. While this treatment has been researched in the context of COD removal, its usage in the removal of arsenic from landfill systems has not been investigated. In this study, landfill leachate and LFG condensate were collected over multiple sampling rounds from various locations in CHRLF. Conventional methods including ferric chloride coagulation, coagulation with dispersed gas flotation, and permanganate oxidation resulted in inadequate removal of arsenic. Pre-treatment by ozonation and electrochemical oxidation also did not improve treatment efficiency. In contrast, ME resulted in >90% removal of arsenic from both leachate and LFG condensate at high doses (>20 g/L) of activated carbon and ZVI. This process was enhanced by nitrogen or carbon dioxide purging. Adsorption on activated carbon removed 70-75% of arsenic from landfill leachate at a dose of 20 g/L, but it did not remove arsenic from LFG condensate. Simultaneous removal of organic matter and co-occurring contaminants such as chromium and nickel were also observed. While ME and activated carbon were successful in arsenic removal from leachate and LFG condensate samples from certain sampling rounds, they were relatively ineffective for others. This highlighted two major concerns: (i) the speciation of arsenic in leachate and LFG condensate appears to vary within the site and, (ii) the chemistry of arsenic in the leachate and its response to treatment may fluctuate seasonally. Therefore, it is essential to elucidate further the speciation of arsenic and other characteristics of the landfill leachate and LFG condensate to establish an efficient and reliable treatment train.