Layered potassium calcium phosphate with multiple exchangeable cations for Sr(II) and Co(II) removal from water

Abstract

Developing adsorbents with fast uptake rates and high adsorption capacities for both Sr2+ and Co2+ is a major challenge for treating radioactive wastewater. Traditional calcium phosphates (CaPs) can adsorb Sr2+ and Co2+ but their adsorption performance is limited by their compact structures. Because of the looser structure and the presence of more exchangeable cations than other CaPs, layered potassium calcium phosphate (K3HCa(PO4)2, KCaP) would be appropriate for removing Sr2+ and Co2+ from wastewater. KCaP demonstrated fast uptake (94.1 % Sr2+ in 120 min and 96.6 % Co2+ in 10 min) and remarkable adsorption capacity (384 mg g−1 for Sr2+ and 382 mg g−1 for Co2+). It also exhibited high distribution coefficients for Sr2+ (Kd = 2.3 × 104 mLg−1) and Co2+ (Kd = 5.6 × 104 mLg−1) at pH 7. Although KCaP prefers to adsorb Co2+, it can efficiently remove Sr2+ and Co2+ in the presence of multiple competing cations. Adsorption on KCaP is believed to occur as a two-stage ion-exchange process: fast exchange with K+ and subsequent progressive exchange with Ca2+. As a result of the uptake of Sr2+ and Co2+, KCaP was transformed into chemically more stable Sr- and Co-containing CaPs. The nonreactivity of the used adsorbent makes the disposal of radioactive wastes manageable and relieves the environmental burden.