COMPARATIVE SORPTION STUDIES OF DIVALENT METAL IONS ON NICKEL OXIDE (NiO)

Abstract

The present work focuses on the characterization, potentiometric titration, kinetics and adsorption studies of a novel and efficient adsorbent (NiO). The surface structure of the NiO was identified and characterized by X-ray diffractometry (XRD), FTIR spectroscopy, surface area, point of zero charge (PZC), transmission electron microscopy (TEM), thermogravimetric and differential thermal analyses (TGA/DTA), scanning electron microscopy (SEM) coupled with energy dispersive X-ray analyses (EDX). The dissolution study of NiO was undertaken as a function of pH (2.00-11.00) and temperature (283-323K), which was observed to increase substantially by decreasing the initial pH of the system. The point of zero charge (PZC) of NiO in the presence of background electrolyte was determined by the salt addition, mass and fast titration techniques in the temperature range 303-333 ± 1K. The PZC of NiO determined by salt addition method was comparable in magnitude to the one obtained by mass titration technique. The coincidence between the PZC and CIP indicated that no specific adsorption of the electrolyte occurs at pHPZC of NiO and the surface carries a net zero charge at CIP. However, a shift in the PZC values with concentration and nature of divalent metal cations indicated their specific adsorption onto NiO surface. The PZC and the concentration of NiOH2+ groups were decreased whereas the concentration of NiO- was increased with the increase in temperature of the medium. A comprehensive study of the potentiometric titrations of NiO in the presence of Na+, Cd2+, Zn2+, Ni2+, Pb2+, Cu2+, Ca2+, and Mg2+ was conducted under different experimental conditions of temperature, concentration and pH. The affinity of metal cations evaluated from the potentiometric titration data was found to be in order: divalent transition metals > alkaline earth metals > alkali metal. The deprotonation of NiO was observed to be dependent upon the concentration, temperature of metal cations present in the system. The changes in enthalpy (ΔH) and entropy (ΔS) connected with the surface deprotonation of the NiO/electrolyte interface were measured. The loss in entropy indicated that potential determining ions were less hydrated at the interfacial region. Thermodynamic parameters showed that the positive value of enthalpy was the driving force for the deprotonation of the nickel oxide surface. The kinetics of metal cations sorption onto NiO were studied at different temperatures (303–323K). The applicability of the various kinetic models was checked to determine the mechanism of adsorption. The pseudo first order model was best fitted among the kinetic models to describe the kinetic data. The values of activation energy (Ea) determined from Arrhenius equation were observed to be 29.40, 43.74, 32.66, 3.77 and 12.96 kJ mol-1 for Cd2+, Zn2+, Co2+, Pb2+ and Ca2+ respectively. The cation exchange sorption of Cd2+, Zn2+, Pb2+, Co2+, Ca2+, and Mg2+ from aqueous solution on NiO was studied under batch process as a function of concentration of metals, amount of adsorbent and temperature of the suspension. The effect of initial solution pH on divalent metals removal from aqueous solution was examined to assess the sorption behaviour of NiO at different pH values. The solution pH was found to play a decisive role in the metal ions precipitation, surface dissolution and adsorption of metal ions onto the NiO. The preferential uptake of divalent metals from their co–ions was observed in the order: NO3- > Cl- > SO42- which reflected that the presence of nitrate ions was more effective in exchanging the adsorption of divalent metals as compared to chloride and sulfate anions. Desorption of divalent metals from impregnated NiO was checked with protons by varying the initial pH of the suspension at 303 ± 1 K. The adsorption experiments showed that the selectivity of NiO towards different divalent metals followed the trend: Pb2+ > Zn2+ > Co2+ > Cd2+ > Ca2+ > Mg2+ which was in a line with the first hydrolysis constant values of these metal cations. Korbatov equation was used successfully to derive the H+/M2+ stiochiometry of the ion exchange reaction. The exchange between the proton from the NiO surface and the metal from solution was responsible for the adsorption. The uptake of divalent metals by NiO was achieved neither by the replacement of Ni from NiO nor due to precipitation of metal at pH 7.50. The cation exchange data was explained with the help of Freundlich and Langmuir models. The isosteric heat of adsorption (ΔΉ) was also calculated between 303-323 K. The Langmuir constants were used to compute the apparent thermodynamic parameters H, S and G. The positive S values demonstrated that the adsorption reaction was spontaneous. The spontaneity of the metals adsorption onto the NiO was justified thermodynamically by the decrease in Gibbs free energy. The increase in the entropy (S) of the system followed the trend: Co2+ > Ca2+ > Zn2+ > Pb2+ > Cd2+ > Mg2+ which was almost parallel to their corresponding enthalpic values. The cation exchange sorption of divalent metals by the NiO was endothermic driven by entropy increase in the system. The spectroscopic analyses of the solid media also give a strong support to the conclusion that divalent metal ions were chemisorbed onto the surface of nickel oxide.