Rapid Simultaneous Removal of Toxic Anions [HSeO₃]⁻, [SeO₃]^2–, and [SeO₄]^2–, and Metals Hg²⁺, Cu²⁺, and Cd²⁺ by MoS₄^2– Intercalated Layered Double Hydroxide

Abstract

We demonstrate fast, highly efficient concurrent removal of toxic oxoanions of Se(VI) (SeO₄^2-) and Se(IV) (SeO₃^2-/HSeO₃^-) and heavy metal ions of Hg²⁺, Cu²⁺, and Cd²⁺ by the MoS₄^2- intercalated Mg/Al layered double hydroxide (MgAl-MoS₄-LDH, abbr. MoS₄-LDH). Using the MoS₄-LDH as a sorbent, we observe that the presence of Hg²⁺ ions greatly promotes the capture of SeO₄^2-, while the three metal ions (Hg²⁺, Cu²⁺, Cd²⁺) enable a remarkable improvement in the removal of SeO₃^2-/HSeO₃^-. For the pair Se(VI)+Hg²⁺, the MoS₄-LDH exhibits outstanding removal rates (>99.9%) for both Hg²⁺ and Se(VI), compared to 81% removal for SeO₄^2- alone. For individual SeO₃^2- (without metal ions), 99.1% Se(IV) removal is achieved, while ≥99.9% removals are reached in the presence of Hg²⁺, Cu²⁺, and Cd²⁺. Simultaneously, the removal rates for these metal ions are also >99.9%, and nearly all concentrations of the elements can be reduced to <10 ppb, a limit acceptable for drinking water. The maximum sorption capacities for individual Se(VI) and Se(IV) are 85 and 294 mg/g, respectively. The 294 mg/g capacity for Se(IV) reaches a record value, placing the MoS₄-LDH among the highest-capacity selenite adsorbing materials described to date. More interestingly, the presence of metal ions extremely accelerates the capture of the selenium oxoanions because of the reactions of the metal ions with the interlayer MoS₄^2- anions. The sorptions of Se(VI)+Hg and Se(IV)+M (M = Hg²⁺, Cu²⁺, Cd²⁺) are exceptionally rapid, showing >99.5% removals for Hg²⁺ within 1 min and ∼99.0% removal for Se(VI) within 30 min, as well as >99.5% removals for pairs Cu²⁺ and Se(IV) within 10 min, and Cd²⁺ and Se(IV) within 30 min. During the sorption of SeO₃^2-/HSeO₃^-, reduction of Se(IV) occurs to Se⁰ caused by the S^2- sites in MoS₄^2-. Sorption kinetics for the oxoanions follows a pseudo-second-order model consistent with chemisorption. The intercalated material of MoS₄-LDH is very promising as a highly effective filter for decontamination of water with toxic Se(IV)/Se(VI) oxoanions along with heavy metals such as Hg²⁺, Cd²⁺, and Cu²⁺.