Effective removal of radioactive cobalt from aqueous solution by a layered metal sulfide adsorbent: Mechanism, adsorption performance, and practical application

The removal of radioactive Co2+ from aqueous solutions is a critical issue because it is highly toxic and hazardous to humans. Metal sulfides are promising materials for the removal of toxic ions; however, studies on the adsorption of Co2+ by this class of adsorbents as well as their adsorption mechanisms and practical application in complex natural water matrixes are limited. To fill this knowledge gap, in this study, a layered K/Zn/Sn/S metal sulfide nanosheet (KZTS-NS) was employed as an adsorbent for Co2+ removal from aqueous solutions. Besides ion exchange, the mechanism of surface adsorption by forming a Zn0.76Co0.24S composite was found when initial Co2+ concentration was higher than 20 mg/L, which was reported for the first time in layered metal sulfides. The Co2+ adsorption performance of KZTS-NS was systematically studied using batch methods in deionized water, including isotherms, kinetics, influential factors, and selectivity. The results indicated that KZTS-NS exhibited a high adsorption capacity, rapid kinetics, and strong selectivity in a wide pH range. Furthermore, tap water was used as a complex natural water matrix, and the practical application of KZTS-NS in a bench-scale countercurrent two-stage adsorption-flocculation-microfiltration (CTA-F-MF) process for Co2+ removal from tap water matrix was demonstrated. The CTA-F-MF process significantly enhanced the performance of the adsorbent, thereby achieving ultrahigh Co2+ removal (decontamination factor of up to 1176). This study not only provides a new insight into the adsorption of Co2+ by layered metal sulfides, but also forms the basis to enable the practical application of metal sulfides in the field of radioactive wastewater treatment.