Hexavalent chromium (Cr6+) in potable groundwater sources above regulatory thresholds, such as the California limit of 10 µg/L, will require treatment. Strong base anion exchange is effective for Cr6+ removal, but efficient resin regeneration and waste minimization is important for economic and environmental considerations. This study evaluates two regeneration methods on identical pilot-scale columns comparing regeneration efficiency, waste production and salt usage. A conventional 1-Stage regeneration using 3 bed volumes (BV) of 2 N sodium chloride (NaCl) was compared to a 2-Stage process with 7 BV of 0.2 N NaCl followed by 2 BV of 2 N NaCl. Significant differences in regeneration efficiency were observed for chromium and uranium. The 2-Stage regeneration approach eluted 17% more total chromium, but the difference was attributed to elution during the 0.2 N stage. Typically this fraction is recycled to the anion exchange plant headworks, reducing throughput of the subsequent treatment cycle. Therefore, removing less selective anions (e.g., sulfate and bicarbonate) during the 0.2 N stage regeneration did not improve chromium elution during the subsequent 2 N stage. Limiting factors impacting regeneration, such as intra-particle mass transfer, are explored. The 1-Stage regeneration removed 100x more uranium from the resin than the 2-Stage approach. Regeneration without co-eluting sulfate and bicarbonate may be promoting the formation of insoluble uranium complexes that could lead to long-term resin fouling, decreased capacity, and resin with increased radioactivity. Absorbance at 375 nm (UV375) was demonstrated as a monitoring surrogate for chromate elution, providing an online tool to optimize regeneration processes. Waste production and salt usage was comparable for both regeneration approaches operated to similar cut-off points with respect to chromium elution, but there was a trade-off with respect to chromium recycling and uranium fouling that must be balanced for appropriate process selection.

Regeneration of strong base ion exchange resin for hexavalent chromium removal

253rd National Meeting (2017)

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