Synthesis of WS₄^2– Intercalated NiZnAl LDHs as Effective Adsorbents to Remove Copper Ions from Water

A series of WS₄^2– intercalated NiZnAl ternary-layered double-hydroxides (LDHs) with various Ni/Zn ratios were synthesized by an ion-exchange method and used as adsorbents to remove Cu²⁺ from water. The introduction of Zn produced ZnS on the surface of LDHs. The LDH with the Ni/Zn/Al molar ratio of 0.1/1.9/1 showed the best adsorption ability. Cu²⁺ ions are removed via three routes: forming [Cu–WS₄]ⁿ⁻ complexes via soft acid–soft base interaction between WS₄^2– and Cu²⁺, isomorphic substitution of Zn²⁺ in sheets by Cu²⁺, and cation exchange of Cu²⁺, with ZnS on the surface of LDHs. With the increased Cu²⁺ concentration, the complexes dominated the adsorption because polynuclear [Cu–WS₄]ⁿ⁻ complexes with high Cu/W ratios (2–6) may be formed. Cu⁺ is present in such complexes, which is produced by the internal redox. Even at Cu²⁺ concentration up to 600 mg·L^–1, neither amorphous CuWS₄ nor decreased interlayer distance was observed. Contrarily, the interlayer distance was slightly enlarged due to forming bigger [Cu–WS₄]ⁿ⁻ complexes. The adsorption followed the pseudo-second-order kinetics and Langmuir isotherm model. The experimental maximum adsorption capacity reached 555.4 mg·g^–1.