Evaluation of Olive Stones for Cd(II), Cu(II), Pb(II) and Cr(VI) Biosorption from Aqueous Solution: Equilibrium and Kinetics

Milled olive stones are evaluated as a biosorbent for the removal of heavy metals such as Cd (II), Cu (II), Pb(II) and Cr(VI) from aqueous effluents. To this end, thermodynamic and kinetic studies for single and multimetal systems are performed through batch equilibrium isotherms. The biosorbent was characterized by elemental and FTIR analysis and scanning electron microscopy. The effect of the different parameters, such as contact time, pH, amount of adsorbent and initial metal concentration, on the sorption process is also investigated. The maximum removal percentage for 1 mg L−1 of cadmium, copper, and lead was 77.4%, 80.5%, and 94.5%, respectively, at pH 6 with 5 g L−1 of sorbent. In the case of Cr(VI), a removal percentage of 46% was obtained in 2 h at pH 2 using a larger amount of sorbent (10 g L−1) and an initial concentration of 2 mg L−1. Equilibrium data were analyzed by applying different adsorption isotherm models, resulting in—a good agreement with—the Langmuir model with maximum capacities of 0.557, 0.3 and 0.581 mg g−1 for Cu(II), Cd(II) and Pb(II), respectively, whereas for Cr(VI), the Temkin model provided the best fit with a maximum capacity of 2.34 mg g−1. The kinetic data fitted well into the pseudo-second-order model which allowed the adsorption rate constants to be calculated. Cd(II) resulted to have the highest kinetic constant, followed by Cu(II), Cr(VI) and Pb(II). The results showed that milled olive stones can be used as a biosorbent for the removal of these metals from aqueous solutions.