Abstract:
The adsorption of heavy metals using materials that are readily available at a low cost is the core of this research. Different studies raveled high quality limestone as an excellent adsorbent for heavy metals in wastewater. Limestone taken from two sites in Rwanda (Gishyita and Mpenge limestone deposits) was characterized by determining its physical and chemical composition; thereafter, it was used to remove the selected heavy metals (Pb, Cd, Zn, and Cu) from synthetic wastewater. To determine the impact of adsorbent particle size and dosage, contact time, varying heavy metal concentrations, pH, and temperature, batch adsorption studies were conducted. This research included the study of adsorption isotherms, kinetics, and thermodynamics.
The elemental characterization of the adsorbent was done by XRF. Following the adsorption procedures, the content of these chosen heavy metals in wastewater was assessed using MPAES. The adsorption studies revealed that Pb, Cd, Cu and Zn were removed to an extent greater than 78.3%, 75%, 65% and 57% respectively, with a dose of 3.5 g while the particle size ranged between 0.1 and 3.5 mm. The optimum contact time was 15 minutes for Pb, 120 minutes for Cd and Cu and greater than 180 minutes for Zn. Moreover, the optimum pH for adsorption was 5, while that for Cd, Cu and Zn was 6. The outcome from isotherm experiments demonstrated that the Pb adsorption was highly suitable for Freundlich, Cd was fitted with the Langmuir adsorption model, and the adsorption of Zn showed no significant difference between the models for Freundlich and Langmuir isotherms. According to the adsorption kinetic results, the pseudo-first-order model greatly promoted the adsorption of Pb, Cd, Cu and Zn. In the thermodynamic study, the standard enthalpy change (-ΔHo) was negative, impressing the exothermic reaction, while the negative standard free energy (-ΔGo) indicated a favorable and spontaneous process. According to the results, the Gishyita and Mpenge limestone might be utilized as alternatives for removing Pb,Cd, Cu and Z from wastewater.
