Elucidation of physicochemical properties of granular activated carbon for monochloramine destruction in natural waters
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Monochloramine reduction in fixed-bed reactors (FBRs) was quantified using five types of granular activated carbon (GAC) and two background waters (Lake Austin water, LAW, and synthetic organic-free nutrient water, NW). For a given set of influent conditions, steady-state was reached following a period of higher removal. Steady-state monochloramine reduction varied with GAC type and source water characteristics. Steady-state monochloramine reduction was simulated for GAC particle sizes used in practice for each condition studied in the FBR studies. A previously developed numerical model was used for this purpose. For typical drinking water conditions, steadystate simulations indicated that less than 7 minutes of empty-bed contact time was required to meet the monochloramine standard for kidney dialysis water (0.1 mg/L as Cl2) with Calgon Centaur, the most effective GAC in the LAW studies. The use of more traditional GACs like Filtrasorb 400 may be feasible despite a required EBCT as large as 20 minutes, provided that benefits gained by steady-state operation (i.e., never having to replace filter media) outweigh filter size and portability considerations. While more monochloramine was reduced at steady-state using NW compared to LAW for each GAC and empty-bed contact time studied, the differences in removal varied considerably among the GACs tested. The degree of interference caused by natural organic matter (NOM) increased with increasing GAC surface area contained within pores greater than 2 nm in width. Acid/base and electrostatic properties of the GACs were not found to be significant in terms of NOM uptake, which indicated that size exclusion effects of the GAC pores overwhelmed the impact of GAC surface chemistry. Therefore, selection of GAC to limit the impact of NOM should be based on pore size distribution alone, with the impact of NOM decreasing with decreasing meso- and macroporosity. While all GACs showed intrinsic catalytic capability for monochloramine destruction, GACs with low oxygen to carbon ratios and high nitrogen to carbon ratios generally performed better. The presence of lactams and imides were found on all the virgin GACs and shown to have formed as a result of monochloramine reduction, indicating the importance of these functional groups in this process.