Adsorption of Fe , Pb , Zn and Cr ions from aqueous solutions using natural, ammonium oxalate and sodium hydroxide modified Kaolinite clay

Abstract

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