Adsorption of Pb (II) Ions on to Raw Diatomite from Kenya: Equilibrium, Kinetic and Thermodynamics

Abstract

Lead (Pb) is a hazardous heavy metal that is routinely discharged into the environment by industries; hence, its removal from waste water is essential. In this study, raw diatomite mineral from Kenya was employed in the adsorptive removal of Pb (II) ions from aqueous solutions. The raw diatomite was purified by acid treatment using HNO₃, and then it was characterized by Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Fluorescence (XRF), Brunauer–Emmett–Teller (BET), and Barrett–Joyner–Halenda (BJH) techniques. XRF data showed that the diatomite was composed mainly of silica. The effect of contact time, initial metal ion concentration, adsorbent dose, temperature, and pH on the removal efficiency of Pb (II) ions from aqueous solutions by the raw diatomite was evaluated. The removal efficiency of Pb (II) ions was found to increase with an increase in contact time, adsorbent dose, and pH but decreased with increasing temperature and initial metal ion concentration. Optimum removal conditions obtained were pH of 4, 10 mg/L initial metal ion concentration, contact time of 60 minutes, adsorbent dose of 4 g/L, and temperature of 25±2 °C. The equilibrium was observed to be well described by adsorption isotherms in the order Langmuir > Freundlich > Temkin isotherm model. The adsorption kinetics of Pb (II) ions was described better by the pseudo-second-order kinetic model (R² = 1), which ascertained that chemisorption was the rate-limiting step in the sorption of Pb (II) ions onto raw diatomite. The thermodynamics of the adsorption process indicated that adsorption of Pb (II) ions onto raw diatomite was spontaneous and exothermic in nature. This study demonstrated that raw diatomite from Kenya is effective in heavy metal remediation even without chemical or thermal modifications; therefore, it acts as a promising adsorbent for heavy metal remediation.