Biosorption of cadmium (II) onto loquat leaves (Eriobotrya japonica) and their ash from aqueous solution, equilibrium, kinetics, and thermodynamic studies

Ammar H Al-Dujaili 0 Akl M Awwad 1 Nida M Salem 1 0 Department of Chemistry, College of Education , Ibn Al-Haitham, University of Baghdad , Baghdad, Iraq 1 Industrial Chemistry Center, Royal Scientific Society , P.O. Box 1438, Al-Jubaiha, Amman 11941, Jordan Biosorption of Cd(II) onto loquat leaves (LL) powder and loquat leaves ash (LA) from aqueous solution has been investigated. The extent of biosorption of Cd(II) ions was found to be dependent on the solution pH, biosorbent dose, initial cadmium ions concentration, contact time, and temperature. The experimental equilibrium biosorption data were analyzed by three widely used two-parameter Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm equations. The Langmuir isotherm model provided a better fit with the experimental data than Freundlich and DubininRadushkevich (D-R) isotherm models by high correlation coefficients R2. The biosorption capacity of LL and LA was found to be 29.240 mg g1 and 21.322 mg g1 for Cd(II) ions, respectively. The mean adsorption energies evaluated using the D-R model indicated that the adsorption of Cd(II) onto loquat leaves and their ash mainly proceeds through binding surface functional groups, i.e., were taken place by physisorption. The thermodynamic analysis indicated that the biosorption behavior of cadmium ions onto LL and LA biosorbents was an endothermic process. The negative sign values of Go and positive values of Ho revealed that the biosorption process was spontaneous and endothermic. Adsorption mechanisms were also investigated using the pseudo-first-order and pseudo-second-order kinetics models. The kinetic results showed that the adsorption of Cd(II) onto LL and LA followed well pseudo-second order kinetics. - Background Heavy metals are recognized as long-term hazardous contaminants because of their high toxicity, accumulation and retention in human body. Cadmium (II) is one of the toxic heavy metals responsible for causing kidney damage, renal disorder, high blood pressure, bone fraction, and destruction of red blood cells [1]. Major sources of cadmium (II) in environment are electroplating, industries of pigments, plastic, and metal finishing industries. The conventional methods for the removal of cadmium and other heavy metals from water and wastewater include chemical precipitation [2], ion exchange [3], electrochemical precipitation [4], membrane separation [5], and adsorption [6-9]. All these methods are, in this case, either economically unfavorable or technically complicated and thus used only in special cases. Each of these methods has some limitations in practice. The problems with the aforementioned methods make it necessary to develop easily available, inexpensive, eco-friendly, and equally effective alternatives for water and wastewater treatment. Biosorption of heavy metals by agricultural waste materials, which are produced in large quantities as a solid waste, is one of these alternative treatment methods [10-19]. The present work was carried out to show the potential of biosorption of cadmium ions from aqueous solutions by LL and LA. Results and discussion Effect of pH on adsorption The pH is one of the most important parameters that is effective on metal biosorption. It is directly related with the competition abilities of hydrogen ions with metal ions to bind with the active sites on the biosorbent surface. The effect of pH on biosorbtion of Cd(II) ions onto LL an LA was studied in the pH range from 1.0 to 8.0, keeping the concentration of metal ions constant at 30 mg L1 and at 30C. The percentage removal of the Cd(II) ions as a function of pH is shown in Figure 1. It can be seen from this figure that the removal of metal ions percent increases with the increasing pH of Cd(II) from 1.0 to 6.0 and then decreases to reach pH 8.0. The maximum adsorption was observed around pH 6.0. The biosorption of Cd(II) ions onto LL and LA surfaces reflected the nature of the physiochemical interactions of the solution. At highly acidic pH (pH < 2.0), the overall surface charge on the active sites of LL and LA became positive, and Cd(II) ions and protons compete for binding in the active sites on LL and LA surfaces, which results in a lower uptake of the metal ions. The LL and LA biosorbent surfaces became more negatively charged as the pH solution increased from 2.0 to 6.0. The functional groups became more deprotonated and thus available for the Cd(II) ions. Effect of contact time on adsorption The effects of contact time of Cd(II) on the biosorption process were studied in the time range from 10 to 120 min at pH 6.0 and 30C with a fixed LL and LA biosorbent dose. It can be seen from Figure 2 that the percentage removal of metal ions increases with contact time until equilibrium is attained between the amount of metal ions on LL and LA and the remaining metal ions in solution. Figure 2 shows that the percentage removal of Cd(II) increases with contact time from (0 to 60) min and then becomes almost constant up to the end of the experiment. It can be concluded that the binding of Figure 1 Effect of pH on the removal efficiency of Cd(II) by LL and LA (metal concentration, 30 mg L1; temperature, 30C). Figure 2 Effect of contact time on the percentage removal of Cd(II) by LL and LA (metal concentration, 30 mg L1; temperature, 30C; adsorbent dosage, 4 g L1; pH 6). Cd(II) with LL and LA is high at initial stages and becomes almost constant after an optimum contact time of 60 min. Effect of biosorbent dosage To determine the effect of biosorbent dose, different amounts 1 to 10 g L1 of biosorbent were suspended in 30 mL cadmium solution in which the concentration of cadmium is 20 mg L1 under optimized conditions of pH 6.0 and at contact time of 60 min. From experimental results obtained, we found that an optimum dose of 4 g L1 is appropriate to use for all the experiments. Effect of Cd(II) concentration The effect of initial Cd(II) concentration on the biosorption capacity of LL and LA was investigated under optimum conditions, pH = 6.0 and 30C (Figure 3). Biosorption of cadmium onto LL and LA increased with increasing initial concentration of cadmium ions. If the initial metal concentrations were found up to 400 mg L1, the amount of adsorbed Cd (II) ions per unit mass of the LL and LA increased. These data are reasonable, indicating the presence of many active sites on LL and LA surface, which are available to bind with Cd(II) ions at low concentration. Beyond the initial metal ion concentrations 400 mg/L, the adsorption capacity of LL and LA biosorbent remained relatively constant, indicating that the optimal concentration was 400 mg/L. This is probably due to saturation of cadmium ion binding sites at LL and LA surface at a concentration higher than 300 mg L1. These results may be explained by an increase in the number of cadmium ions competing for the available binding sites in the biosorbent for the complexation of cadmium ions at higher concentration l (...truncated)