Removal of heavy metal ions from aqueous solutions by chemically modified biosorbents

Present study deals with the evaluation of single and binary adsorption of lead and cadmium ions from their aqueous solutions using olive pomace as a biosorbent. Olive pomace was modified with several chemicals and best results were obtained by nitric acid modification. Batch-wise experiments were conducted with raw (R-OP) and nitric acid modified olive pomace (N-OP). Process parameters were investigated. Binary adsorption of lead and cadmium ions onto N-OP was studied varying the initial heavy metal ion concentration ratios. Regenerative and reusable properties of N-OP were confirmed by consecutive adsorption-desorption cycles. Temperature study indicated that the adsorption process is feasible and spontaneous. Adsorbent characterization was accomplished by ATR-FTIR, SEM, and BET analyses. Surface charge and chemical content of adsorbents were determined by point of zero charge measurement and Van Soest method. Surface functional groups were characterized by Boehm titration method. Kinetic study revealed that lead and cadmium adsorption followed a pseudo-second-order model. Lead and cadmium uptake were enhanced by nitric acid modification from 33 mg/g to 64 mg/g and 8 mg/g to 20.5 mg/g, respectively, using 1 g/L N-OP at the optimum pH of 5.5 for lead ions and that of 6 for cadmium ions. Redlich-Peterson and Langmuir isotherms were the best fitting models for lead adsorption onto R-OP and N-OP, respectively. However, Langmuir-Freundlich and Radke-Prausnitz models were superior in cadmium adsorption onto R-OP and N-OP, respectively. The prevalence of lead adsorption was specified by the binary system investigation. A modified version of Radke-Prausnitz and Freundlich models showed the best fitting for concomitant presence of lead and cadmium ions in binary mixture, whereas a modified version of Dubinin-Raduskevich and Freundlich models showed the best fitting for constant background concentration of cadmium ion systems.