Selective removal of heavy metals by hydrotalcites as adsorbents in diverse wastewater: Different intercalated anions with different mechanisms

The designing of selective and efficient adsorbent for removal of heavy metals from wastewater of diverse chemistry is a challenging task for researchers. Herein, FeMgAl derived from layered double hydroxides (LDH) with different intercalated anions (CO₃ ²⁻, NO³⁻, S₅ ²⁻ and MoS₄ ²⁻) as selective adsorbents were evaluated by considering capturing heavy metals from a complex aqueous environment such as: 1) adsorption of single metals ions from a mixture of Ag⁺, Hg²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Co²⁺, Ni²⁺, Zn²⁺, 2) adsorption of heavy metals in the presence of huge amount of Ca²⁺, Mg⁺, Na⁺, Cl⁻, NO₃ ⁻ and SO₄ ²⁻ ions, 3) adsorption under different solution pHs, 4) solutions with high salinity etc. Out of the studied materials, fast kinetics, record capacities (582 for Hg²⁺ and 565 mg/g for Ag⁺) with enormous K_d values (1.0 × 10⁸–2.5 × 10⁷) placed FeMgAl–MoS₄ (Fe–MoS₄ LDH) as the most selective and promising material. FeMgAl–S₅ (Fe–S₅ LDH) was also impressive due to its good capacities (279 mg/g for Hg²⁺ and 341 mg/g for Ag⁺ ions) with K_d values (8.3 × 10⁵–1.0 × 10³). In contrast, FeMgAl–CO₃ and FeMgAl–NO₃ (Fe–CO₃ LDH and Fe–NO₃ LDH) showed very poor selectivity, lower capacities and very small K_d values. Furthermore, Fe–MoS₄ LDH and Fe–S₅ demonstrated negligible efficiency loss in the presence of competitive cations/anions or with change in solution pH. As confirmed by X-ray Diffraction (XRD) and Fourier Transformed Infrared (FTIR) analysis, a mechanism for adsorbed metals in the form of various coordinated complexes inside LDH layers was proposed for Fe–MoS₄ or Fe–S₅ LDHs. In contrast, a dominant precipitation mechanism as metal-hydroxide or metal-carbonate was suggested for Fe–CO₃ and Fe–NO₃ LDHs.