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, FeMgAI derived from layered double hydroxides (LDH) with different intercalated anions (CO32- NO3-, Si- 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 Ka values (1.0 x 10(8) -2.5 x 10(7)) placed FeMgAl-MoS4 (Fe-MoS4 LDH) as the most selective and promising material. FeMgAI-S5 (Fe-Ss 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 x 10(5)-1.0 x 10(3)). In contrast, FeMgAl-CO3 and FeMgAl-NO3 (Fe-CO3 LDH and Fe-NO3 LDH) showed very poor selectivity, lower capacities and very small Kd values. Furthermore, Fe-MoS4 LDH and Fe-S-5 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-S-5 LDHs. In contrast, a dominant precipitation mechanism as metal-hydroxide-ifir-nTetal:carbonate was suggested for Fe-CO3 and Fe-NO3 LDHs. (C) 2018 Published by Elsevier Ltd.