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

Abstract

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 (CO32−, NO3−, S52− and MoS42−) 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 SO42− 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 Kd values (1.0 × 108–2.5 × 107) placed FeMgAlMoS4 (FeMoS4 LDH) as the most selective and promising material. FeMgAlS5 (FeS5 LDH) was also impressive due to its good capacities (279 mg/g for Hg2+ and 341 mg/g for Ag+ ions) with Kd values (8.3 × 105–1.0 × 103). In contrast, FeMgAlCO3 and FeMgAlNO3 (FeCO3 LDH and FeNO3 LDH) showed very poor selectivity, lower capacities and very small Kd values. Furthermore, FeMoS4 LDH and FeS5 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 FeMoS4 or FeS5 LDHs. In contrast, a dominant precipitation mechanism as metal-hydroxide or metal-carbonate was suggested for FeCO3 and FeNO3 LDHs.