Thermal and chemical pretreatment of Cassia sieberiana seed as biosorbent for Pb2+ removal from aqueous solution

Activated carbons obtained from Cassia sieberiana were studied for the adsorption of Pb 2+ from an aqueous solution. Characterization of the activated carbon by Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller, point of zero charge, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were performed on the unmodified Cassia sieberiana (UMCS), thermally modified Cassia sieberiana (TMCS) and base modified Cassia sieberiana (BMCS) adsorbents. The well-known batch sorption isotherm models such as Langmuir, Freundlich and Temkin were tested with experimental data for the adsorption of Pb 2+ to estimate the adsorption equilibrium parameters (rate constant and, adsorption capacity). The model with the best fit was identified from extensive statistical analysis of the results of nonlinear fitting of the experimental data. The FTIR analysis before and after adsorption studies confirmed the presence of different functional groups as well as shifting of functional groups after the adsorption of Pb 2+ . SEM analysis showed the surface morphology of the different adsorbents being relatively rough and irregular indicating higher pores on the surface of the modified adsorbents. The modified C. sieberiana showed a large surface area (471.521, 624.772 and 628.585 m 2 /g) and a point of zero charge at 4.42, 7.22 and 7.23 for UMCS, TMCS and BMCS, respectively. TGA analysis showed that the thermal decomposition of the adsorbents took place in three or less well-evidenced stages, with significant mass losses. The Langmuir isotherm gave the best fit with determination coefficients (R 2 ) of 1. The adsorption of Pb 2+ ion onto UMCS, TMCS and BMCS best fitted into pseudo-second-order model with R 2 0.999 suggesting chemical adsorption mechanism. The overall adsorption process was controlled by one or more steps such as external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface or a combination of more than one step as predicted by the intraparticle diffusion model. In general, modified C. sieberiana seed as an adsorbent can be used as an effective material for the removal of Pb 2+ from an aqueous solution.