Technologies and Costs for Removal of Arsenic From Drinking Water

2.6.5 Photo-OxidationResearchers at the Australian Nuclear Science and Technology Organisation (ANSTO) havefound that in the presence of light and naturally occurring light-absorbing materials, the oxidation rateof As(III) by oxygen can be increased ten-thousandfold (Cooperative Research Centres for WasteManagement and Pollution Control Limited, 1999). The oxidized arsenic, now As(V), can then beeffectively removed by co-precipitation.ANSTO evaluated both UV lamp reactors and sunlight-assisted-photo-oxidation using acidic,metal-bearing water from an abandoned gold, silver, and lead mine. Air sparging was required forsunlight-assisted oxidation due to the high initial As(III) concentration (12 mg/L). Tests demonstratedthat near complete oxidation of As(III) can be achieved using the photochemical process. Analysisof process waters showed 97% of the arsenic in the process stream was present as As(V).Researchers also concluded that As(III) was preferentially oxidized in the presence of excessdissolved Fe(II) (22:1 iron to arsenic mole ratio). This is a contrast to conventional plants wheredissolved Fe(II) represents an extra chemical oxidant demand which has to be satisfied duringoxidation of As(III) (CRC-WMPC, 1999).Photo-oxidation of the mine water followed by co-precipitation was able to reduce arsenicconcentrations to as low as 17 Fg/L, which meets the current MCL for arsenic. Initial total arsenicconcentrations were unknown, though the As(III) concentration was given as approximately 12 mg/L,which is considerably higher than typical raw water arsenic concentrations. ANSTO reportedresiduals from this process are environmentally stable and passed the Toxicity Characteristic LeachingProcedure (TCLP) test necessary to declare waste non-hazardous and suitable for landfill disposal.Based on the removals achieved and residuals characteristics, it is expected that photooxidationfollowed by co-precipitation would be an effective arsenic removal technology. However,this technology is still largely experimental and should be further evaluated before recommendationas an approved arsenic removal technology for drinking water.2-48