Pilot-Testing an Ion Exchange Technology for Selective and Regenerative Arsenate Removal and an Engineered Approach for Stabilizing Arsenic-Laden Process Waste Residuals
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Pilot-scale tests of a regenerable and selective polymeric ligand exchanger (PLE) were conducted to remove As(V) at the well field of El Paso Water Utilities (EPWU) in El Paso, TX, USA. The PLE was prepared by immobilizing Cu(II) onto a commercially available chelating resin of a uniform particle size of ~0.41 mm. For a period of three months, we tested the breakthrough behaviors of fresh and regenerated PLE for removal of arsenate in a pilot-scale column configuration and under various operating conditions (empty bed contact time (EBCT) and column size) and raw water characteristics (pH). Column runs were also carried out to study the PLE’s performance under conditions of the NSF international standard 53 arsenic challenge water (NSF, 2006). The results confirmed that the PLE’s affinity for various anions follows the sequence: HAsO42- >>HCO3->SO42->Cl-. The PLE bed can be regenerated efficiently using 6-8% (w/w) NaCl at pH 9.0, and more than 90% of As(V) recovery can be repeatedly achieved within ~25 BV’s of regeneration with the brine, and the same spent brine can be reused for multiple runs with only pH adjustment. The regeneration was controlled by bicarbonate, which was completely eluted with ~10 BV’s of the regenerant, indicating the level of groundwater alkalinity can be an important factor for the process design. Lowering the influent water pH from 8.3 to 6.5 increased the treatment capacity by 30%, confirming that slightly acidic pH is more favorable for arsenic removal. When the influent As(V) was increased from 50 to 300 µg/L, the treatment capacity was nearly proportionally decreased from 3000 BVs to 500 BVs. Mass balance calculation of copper eluted from the PLE column indicated that the copper leakage per operation cycle (saturation and regeneration) was less than 0.077% of the total copper loaded in the resin. The selective and regenerable ligand exchange process may serve as a viable alternative to remove trace levels of arsenate from drinking water. The arsenic-laden process waste residues can be effectively stabilized using either ferric chloride or iron-based nanoparticles at Fe/As ratio of 10-20, pH in the range of 6-8. The stabilized sludge can pass leaching tests (either TCLP or WET) after sufficient air drying period (~2 weeks) and the resulted sludge would not be characterized as hazardous wastes.