Arsenic removal using oxidative media and nanofiltration

Abstract

Nanofiltration (NF) is a promising drinking water treatment technology for arsenic removal; however, most of the research on NF treatment of arsenic has used synthetic water. In this investigation, a pilot membrane system treated groundwater naturally contaminated with arsenic to test the performance of two NF membranes and one reverse osmosis (RO) membrane, both with and without oxidizing pretreatment using manganese dioxide (MnO2). The arsenic concentration in the groundwater was ~ 40 μg/L, mostly present as arsenite, a neutral species. Without the oxidizing pretreatment, the two NF membranes provided almost no removal whereas the RO membrane provided ~ 25 to 50% arsenic removal, depending on operating conditions. Following pretreatment with MnO2, the treated arsenic concentration dropped to < 4 μg/L (90% removal) for all three membranes. The substantially improved performance for these negatively charged membranes was attributed to the oxidation of the neutrally charged arsenite to negatively charged arsenate. These results indicate that where arsenite is present, facilities with RO or NF processes can dramatically enhance their arsenic removal by adding a membrane‐compatible oxidation step such as MnO2 filtration.

Evaluating salinity sources of groundwater and implications for sustainable reverse osmosis desalination in coastal North Carolina, USA

“…The production wells contain As concentrations from 8 to 54 μg/L and the rejection in the RO process is only ∼54-75% (Table 3). Low rejection of As has been reported previously in RO systems (Oreskovich and Watson 2003;Pawlak et al 2006;George et al 2006;Moore et al 2008;Walker et al 2008;Geucke et al 2009) and depends on the prevalence of charged As species. Arsenic speciation in water depends on oxidation state and pH.…”

“…Under oxic conditions, arsenate (As(V)) species are dominant, particularly the monovalent species H 2 AsO 4 -(pH< 6.5) and the divalent species HAsO 4 2-(pH>6.5). Consequently, RO rejection for the charged As(V) in oxygenated water is more effective (Oreskovich and Watson 2003;Pawlak et al 2006;George et al 2006;Moore et al 2008;Walker et al 2008;Geucke et al 2009). In the production wells, dissolved oxygen concentration is typically below 1 mg/L (Table 1).…”

“…1), the approach in Dare County to meet increasing demand was the installation of several reverse osmosis (RO) desalination plants in which local saline groundwater is used as the water source (Oreskovich and Watson 2003;Dare County Water Department 2006). The hydrogeologic properties of the Atlantic coast (Meisler 1989;Barlow Desalination of brackish and/or saline groundwater presents other challenges such as the presence of naturally occurring contaminants in groundwater that are not removed effectively by RO desalination, including boron (US Environmental Protection Agency (EPA) 2008; Kloppmann et al 2008) and uncharged forms of arsenic (Oreskovich and Watson 2003;Pawlak et al 2006;George et al 2006;Moore et al 2008;Walker et al 2008;Geucke et al 2009). High boron concentrations are commonly observed in coastal aquifers due to boron desorption from marine sediments (e.g., D'Avino and Spandre 1995; Vengosh and Spivack 1999) and at near-neutral pH, boron occurs primarily as uncharged B(OH) 3 0 , which is not efficiently adsorbed and can accumulate in groundwater.…”

Tetravalent Manganese Feroxyhyte: A Novel Nanoadsorbent Equally Selective for As(III) and As(V) Removal from Drinking Water

“…Nevertheless, the above-mentioned processes remove effectively only the As(V) oxy-anions (H 2 AsO 4 – /HAsO 4 2– ) and not the uncharged H 3 AsO 3 of As(III), which appear as the dominant forms at natural water pH . Therefore, a preoxidation step, activated by a chemical reagent, a granular solid (e.g., MnO 2 ) or biological oxidation is usually required for effective As(III) removal. The preoxidation significantly increases the capital cost of the arsenic removal process.…”