Metal oxide enhanced microfiltration for the selective removal of Co and Sr ions from nuclear laundry wastewater

• A combined adsorption-microfiltration system for treating nuclear wastewater was studied.
• CST had better adsorption capacities and higher selectivities for Co and Sr ions than IOPs.
• CST and ferrihydrite were well regenerated without any significant loss in adsorption capacity.
• Adsorbents augmented membrane permeability while preventing cobalt precipitation.

The present study investigated the potential use of a hybridized adsorption/microfiltration system for the selective removal of hazardous cobalt and strontium ions from nuclear power plant laundry wastewater, which is essential for managing low-level radioactive wastes. A crystalline silicotitanate (CST) and four different iron oxides were fabricated and used as adsorbents, which had different crystal structures and adsorptive mechanisms, such as ion exchange and surface coordination. CST showed the greatest affinity for Co and Sr ions (e.g., the Freundlich isotherm constant, KF=23.5 mg/g and 33.9 mg/g, respectively, at pH 7) and its adsorption capacity was independent of solution pH (pH 5–9), whereas the iron oxides removed substantive amounts of Co ions at neutral and alkaline pH levels, but only marginal amounts of strontium. Background species (e.g., K+, Na+, Ca2+, and Mg2+) had no significant impact on the affinities of the adsorbents for Co and Sr ions. The selectivity coefficients of CST for both Co and Sr ions ranged from 3 to 130. CST and ferrihydrite adsorbents were successfully regenerated using NaCl and HCl solutions, respectively, and their post-regeneration adsorption capacities were unchanged without any sign of significant inactivation after multiple adsorption and regeneration cycles. The addition of adsorbents to the microfiltration system enhanced the membrane permeability (>30%), probably because the cobalt species were removed prior to precipitation at the membrane surface. Substantial and stable metal removals (>90% for both Co and Sr) were achieved during 50 h of continuous system operations.

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