Selective capture of ions from wastewater with layered metal sulfide

It is important to develop chemically selective scavengers for ions such as Ba₂⁺, Ni₂⁺, and Co₂⁺ from complex wastewater for environmental remediation and human health. The current natural and synthetic absorbent materials, however, share the problems of low capacity and poor selectivity. In particular, the selective removal and separation of radionuclides from industrial nuclear wastes with high salt concentrations is a challenge.

In a recent study published in Chemistry of Materials., Prof. FENG Meiling from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, cooperating with Prof. Mercouri G. Kanatzidis from Northwestern University, U.S., reported the efficient and selective capture of Ba₂⁺, Ni₂⁺, and Co₂⁺ with an acid- and radiation-resistant layered metal sulfide [Me₂NH₂]_4/3[Me₃NH]_2/3Sn₃S₇·^1.25H₂O (FJSM-SnS).

133Ba, as one of the byproducts of nuclear fuel fissions, emits γ ray at the energy of 356 keV. Ba₂⁺ is also employed as the simulant for highly radiotoxic 226Ra₂⁺ because of their comparable ionic radii and similar adsorption behaviors. ⁶⁰Co isotope and ⁶³Ni emit strong γ and pure β radiation, respectively, both of them can be derived from neutron activation of reactor materials. Non-radioactive barium, nickel and cobalt ions are harmful to the ecosystem and human health.

The advantages of FJSM-SnS include rapid kinetics, strong exchange capacity, high selectivity and recovery rates, excellent acid and alkali resistance and efficient elution.

FJSM-SnS exhibits a faster adsorption ability with short equilibrium times, which is within five minutes. It has high adsorption capacity (289 mg/g for Ba, 83 mg/g for Ni, 52 mg/g for Co). It shows extremely high selectivity for Ba₂⁺ even in the presence of excess Na⁺, K⁺, Cs⁺, Ca₂⁺, Mg₂⁺ and Sr₂⁺. It also presents outstanding selectivities for Ni₂⁺ and Co₂⁺ (KdNi = 8.92 × 10⁴ mL/g; KdCo = 3.75 × 10⁵ mL/g) which surpass most of the reported ones.

Besides, FJSM-SnS retains its robust framework in wide pH range of 0.6-12.7.

Moreover, the captured barium, nickel, and cobalt in the exchanged products can be easily recovered by eluting them with 0.5 M KCl solution which highlights the recycling of resource. The recovery rates can be maintained 99.74, 99.30, and 99.65% of Ba₂⁺, Ni₂⁺, and Co₂⁺ after processing 1100 bed volumes in low concentration (Ba₂⁺, Ni₂⁺, and Co₂⁺: 704.9 ~ 928.6 μg/L), respectively.

These advantages, combined with the easy synthesis and excellent resistances β and γ irradiation make FJSM-SnS promising candidate as the radioactive Ba₂⁺, Ni₂⁺, and Co₂⁺ ion-exchanger from nuclear waste solutions.

This study underscores the enormous potential of metal sulfides as new and low-cost materials for highly efficient and highly selective uptake of 133Ba, 63Ni, and 60Co and their nonradioactive isotopes, as well as 226Ra from complex wastewater.