Varying growth behavior of redox-sensitive nanoparticles on 1:1 and 2:1 clay surfaces: Mechanistic insights on preferential toxic ions removal in mono, co, and multi-metal contaminated waters.

[Display omitted] • Dominant interlayer growth of Fe0 NPs on 2:1 bentonite form B-nZVI (−ve charge). • Non-swelling 1:1 kaolinite with surface coverage of Fe0 NPs led to K-nZVI (+ve). • Preferential sorption of toxic cations on B-nZVI (63 mg/g) than K-nZVI (33 mg/g). • Higher removal of oxy-anions with K-nZVI (248 mg/g) than B-nZVI (149 mg/g). • Removal mechanisms were redox transformation, complexation and co-precipitation. In the current study, contrasting growth behaviour of redox sensitive Fe0 nanoparticles (nZVI) was observed on different clay surfaces i.e., 1:1 non-swelling kaolinite (K-nZVI) and 2:1 swelling bentonite (B-nZVI). Osmotic swelling of bentonite led to Fe0 nucleation and growth of 5–7 nm size particles in the broadened interlayer spaces. B-nZVI had negative zeta potential due to the domination of the surface charge of bentonite clay. In contrast, kaolinite has shown dominant surface growth of nZVI particles (>24.8 ± 7.4 nm) and positive zeta potential, suggesting domination of Fe0 nanoparticles (nZVI) characteristics. This surface-dependent variation led to higher and faster removal of oxy-anions with K-nZVI, i.e., chromium and arsenic (87.5 and 157.35 mg/g) than B-nZVI (18.4 and 86.9 mg/g). In comparison, B-nZVI has shown higher sorption of cations i.e., nickel and cadmium (36 mg/g and 46 mg/g) than K-nZVI (25 and 27 mg/g). XPS and pXRD analysis of reaction precipitates confirmed reductive sorption of chromium, co-precipitation/ complexation of arsenic, electrostatic attraction and complexation of nickel and cadmium as major removal mechanisms. Drastically higher total contaminant sorption capacities of B-nZVI (327 mg/g) and K-nZVI (372 mg/g) in multi-contaminant (Cr + As + Ni + Cd) solutions than individual capacities in mono-ionic solutions was due to co-operative effects and newer sites induced via sorption and redox-transformation of other ionic species. K-nZVI removed chromium and arsenic to below drinking water permissible limits whereas B-nZVI succeeded in separating nickel and cadmium to drinkable levels in groundwater, freshwater, river water, and wastewater samples, emphasizing their applicability in high cationic—low anionic and low cationic-higher anionic species contaminated waters, respectively. [ABSTRACT FROM AUTHOR]