Cadmium, mercury, and nickel adsorption by tetravalent manganese feroxyhyte: selectivity, kinetic modeling, and thermodynamic study

This study is aiming to investigate tetravalent manganese feroxyhyte (TMFx) adsorption efficiency in removing heavy metals. The motivation of this study was the fact that TMFx is a highly negatively charged nanostructure material and that the metals Cd, Hg, and Ni were characterized as priority pollutants for drinking water. TMFx was evaluated through batch and continuous flow experiments in National Sanitation Foundation (NSF) water matrix which simulated the physicochemical characteristics of natural water. Water's pH significantly influences Cd and Ni adsorption efficiency which gradually increases when pH value rises from 5 to 9, while the corresponding one for Hg remains almost constant. Thermodynamic data showed a spontaneous and an exothermic nature weak-chemisorption (Δ#ETA#° = −17.5 ± 2 kJ/mol) of Cd, Ni, and Hg by TMFx. The determined ranking of adsorption affinity and selectivity (Cd > Ni > Hg) seems to be governed by the metals' speciation, as well as by hydration free energy, which is influenced, however, by their atomic radius. The lower adsorption capacity and selectivity of TMFx for Hg should be attributed both to uncharged species and to higher atomic radius. The similar Cd and Ni speciation in the NSF water matrix leads to the conclusion that the better affinity, selectivity, and adsorption kinetic of Cd versus Ni should be attributed to the lower hydration free energy of Cd which is in turn related to its higher atomic radius. The faster adsorption kinetic (Hg > Cd > Ni) of Hg may be attributed to the lower radius of its anhydrate species. Furthermore, TMFx showed high removal efficiency under continuous flow application in an adsorption bed setup. The determined uptake capacity (q_RL) at equilibrium-breakthrough concentration equal to the drinking water regulation limit (RL) of each metal were q₁ = 2.5 μg Hg/mg TMFx, q₅ = 5.2 μg Cd/mg TMFx, and q₂₀ = 7.1 μg Ni/mg TMFx. Leaching tests of spent TMFx samples from the rapid small-scale column tests (RSSCTs) could be treated either as inert wastes after Cd and Ni adsorption or as non-hazardous waste after Hg adsorption.