In this paper, Fe2+ , Pb2+ , Zn2+ and Cr6+ ions removal from contaminated water with natural Nigerian kaolinite clay (AK-clay), and that removed with kaolinite clay modified with either ammonium oxalate (AK-AO) or sodium hydroxide (AK-S) were presented. The clay was characterized using X-Ray Diffractometry (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Brunanuer-EmmettTeller (BET) method. The parameters investigated include pH, adsorbent particle size, shaking speed, metal ion concentration and temperature. The optimum conditions were applied to the modified samples. Design-expert software was used to design the experimental conditions using Response Surface Methodology (RSM). The mineralogical characterization showed the purified 63 µm fraction of the clay as kaolin. Analysis of Variance shows the adsorption of the metal ions was statistically significant with p-valves > 0.0001 at 95 % confidence limit. Pseudo second-order kinetic model was found to fit the adsorption data for all the metal ions. The  adsorption of Fe2+ , Pb2+ and Cr6+ ions was best described by the Langmuir isotherm model, while Freundlich  isotherm model best fitted the adsorption data for Zn2+ ion. Thermodynamic analysis of adsorption data shows the metal ions adsorption was spontaneous, endothermic and accompanied by positive entropy change.   Compared to unmodified natural clay, AK-S clay increased adsorption capacity for Fe2+ , Pb2+ , Zn 2+ and Cr 6+ ions from 14.1 to 18.425 mg/g, 18.4 to 19.8 mg/g, 16.875 to 19.9 mg/g and 7.65 to 8.15 mg/g respectively. The AK2+  AO2+ clay increased the uptake of Fe ion from 14.1 to 17.35 mg/g with a slight increase for Zn ion (16.875 to 16.95 mg/g). The adsorption capacity of AK-clay for all the metal ions was enhanced by NaOH modification 2+ while the ammonium oxalate modification significantly enhanced the adsorption capacity only for Fe ion. The results show that NaOH and ammonium oxalate modified kaolinite clay are effective for remediation of heavy metal-laden wastewater. Keywords: Kaolinite, metal ions, adsorption, kinetics, adsorption isotherm

Natural kaolinite obtained from Share, Nigeria was treated with ammonium carbonate to enhance the adsorption capacity. The optimum operating conditions for the adsorption of metal ions on the natural clay (SK clay) were employed on the ammonium carbonate-treated clay (SK-AC clay). The adsorption data were analysed using adsorption isotherm and kinetic models. The statistical p-values were less than 0.05, indicating that the model terms are significant for the adsorption of the metal ions. The pseudo-second-order kinetic model best describes the metal ions adsorption mechanism. The activation energy values less than 40 kJ mol-1 obtained indicated physisorption. Langmuir adsorption isotherm model fitted the adsorption data for Fe2+, Zn2+ and Cr6+ ions while the Freundlich model fitted Pb2+ ions adsorption. Analysis of thermodynamic parameters shows the adsorption is spontaneous and endothermic. The ammonium carbonate-modified clay exhibited an increase in adsorption capacity from 10.74 to 13.60 mg g-1, 16.48 to 17.38 mg g-1 and 11. 96 to 13.8 mg g-1 for Fe2+, Pb2+ and Zn2+ ions while uptake of Cr6+ decreased from 8.75 to 7.25 mg g-1. The results showed that the adsorption of Fe2+, Pb2+ and Zn2+ ions onto kaolin-containing cowlesite can be enhanced by treating it with ammonium carbonate.