Synthesis of WS₄²– Intercalated NiZnAl LDHs as Effective Adsorbents to Remove Copper Ions from Water

A series of WS42– 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 WS42– 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.