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 (CO 3 2−, NO3−, S 5 2− and MoS 4 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−, NO 3 − and SO 4 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 MoS 4 (Fe MoS 4 LDH) as the most selective and promising material. FeMgAl S 5 (Fe S 5 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 CO 3 and FeMgAl NO 3 (Fe CO 3 LDH and Fe NO 3 LDH) showed very poor selectivity, lower capacities and very small K d values. Furthermore, Fe MoS 4 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 MoS 4 or Fe S 5 LDHs. In contrast, a dominant precipitation mechanism as metal-hydroxide or metal-carbonate was suggested for Fe CO 3 and Fe NO 3 LDHs. Graphical abstract Image 1 Highlights • Adsorption by Fe-LDHs with different intercalated anions was compared. • Intercalating with sulfides functional groups offered excellent selectivity. • Adsorption performance was consistent in diverse types of wastewater. • Distinguished adsorption mechanism was proposed. [ABSTRACT FROM AUTHOR]