Removal of heavy metals from wastewater using raw diatomite and manganese oxide-diatomite composite

Abstract

Access to clean water is a basic human right, however about 43% of the population in Kenya do not have access to clean water mainly due to water pollution. Most industries and wastewater treatment plants in the country discharge their effluents laden with heavy metals such as Cd and Pb into the environment at levels exceeding the maximum limits setup by NEMA. Pb (II) and Cd (II) ions are toxic to humans, therefore, there is a great need to remove them from wastewater before discharge into the environment. Adsorption is an effective method in remediating wastewater laden with heavy metals, but the high cost of adsorbents limits its application in developing countries such as Kenya. This study aimed at synthesizing MnO2−diatomite composite, characterizing and using both raw diatomite and MnO2−diatomite composite as low-cost adsorbents for the removal of Pb (II) and Cd (II) ions from aqueous solutions. The raw diatomite obtained from the African Diatomite Industries Limited (ADIL) in Kariandusi, Gilgil, Nakuru County, Kenya was used to prepare MnO2−diatomite composite by acid reduction of KMnO4−diatomite mixture at room temperature using 6M HCl acid. These adsorbents were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) techniques. Both raw diatomite and MnO2−diatomite composite were found to be mainly composed of silica and alumina with hydroxyl (-OH), silanol (Si-OH), siloxane (Si-O-Si) and aluminosilicate (Al-O-Si) functional groups. In addition, the adsorbents were amorphous in nature with BET surface area of 32.29m2/g for raw diatomite and 30.48m2/g for MnO2-diatomite composite. The surface area and porosity of the MnO2-diatomite composite was lower compared to raw diatomite due to the blocking of pores by MnO2 particles during the modification process. Batch adsorption experiments were carried out to optimize the effects of pH, initial metal ion concentration, contact time, adsorbent dosage and temperature on the percentage removal of Pb (II) and Cd (II) ions from single ion and binary ion solutions. For single ion studies, the optimal conditions for Pb (II) adsorption were initial metal concentration 10mg/L, contact time 60 minutes, temperature 25 oC, pH of 4 (raw diatomite) and pH of 6 (MnO2-diatomite composite) and dosage of 4g/L (raw diatomite) and 6g/L (MnO2-diatomite composite). Optimal adsorption conditions for Cd (II) were initial metal concentration 10mg/L, pH of 6, contact time 80 minutes, temperature 65 oC, adsorbent dose 6g/L (raw diatomite) and 8g/L (MnO2diatomite composite) resulting in 100% removal. Raw diatomite had higher removal efficiency of Pb (II) and Cd (II) ions compared to MnO2-diatomite composite since it had a larger surface area and porosity, therefore, it was chosen for adsorption of heavy metal ions from Pb-Cd binary mixture. For binary ion studies, optimal conditions were pH of 6, initial metal ion concentration of 10mg/L each, contact time 80 minutes, adsorbent dosage 10g/L and temperature of 65 oC. The removal efficiency of the metal ions from binary solutions was lower compared to that of single ion solutions. The order of removal of the heavy metal ions by both adsorbents was Pb (II)>Cd (II) ions. Adsorption kinetics and isotherm analysis of experimental data showed that the adsorption of Pb (II) and Cd (II) ions by the adsorbents was well described by pseudo-second order kinetic model and Langmuir isotherm model respectively. Thermodynamic parameters namely Gibbs free energy (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) were evaluated and revealed that the adsorption process was feasible and spontaneous. ΔH° indicated that Pb (II) ions adsorption was exothermic process while Cd (II) adsorption was an endothermic process. Equilibrium studies demonstrated that both raw diatomite and MnO2-diatomite composite were efficient in the removal of Pb (II) and Cd (II) ions from aqueous solutions. The results of this study indicate that both raw diatomite and MnO2-diatomite composite are effective in heavy metal remediation, therefore, they should be employed as low-cost adsorbents in the treatment of industrial effluents laden with heavy metals.