Inhibitory effect on the uptake and diffusion of Cd2+ onto soybean hull sorbent in Cd-Pb binary sorption systems

The uptake of Cd2+ and Pb2+ ions by a soybean hull (SH) biosorbent in single and binary systems has been investigated. Sorption tests regarding SH in natura and chemically treated were carried out testing a suitable value range of solution pH, sorption temperature and shaking velocity. Sorption capacity is improved at pH 4, 30 °C temperature and 100 rpm. When a strong base is applied, a related-to-untreated SH increasing of 20% in the sorption capacity of Pb2+ ions was observed, but with poor results for Cd2+ uptake. Additionally, a relatively strong decreasing in both sorption capacities of Pb2+ and Cd2+ ions was evidenced for all acidic treatments. Regarding untreated SH, kinetic sorption data of both metals were well-interpreted by a pseudo second-order model and a rate-limiting step on the basis of an intra-particle diffusion model was suggested to occur. An inhibitory effect of Pb2+ diffusion over Cd2+ one was observed, limiting to reach the obtained maximum sorption capacity in single system. Maximum adsorption capacities of 0.49 and 0.67 mequiv g−1 for Cd2+ and Pb2+, respectively, were predicted by the Langmuir isotherm model that reproduced well the equilibrium sorption data for single systems. The inhibitory effect of one metal over the other one was verified in equilibrium sorption data for binary systems interpreted on the basis of a modified extended Langmuir isotherm model, predicting changes in metal affinity onto the SH surface. Finally, SH is an alternative biosorbent with a great potential for the wastewater treatment containing cadmium and lead ions.