Mechanistic Understanding of Cadmium Sorption by Sulfonated and Esterified Spent Black Tea

The effective removal of heavy metals from aqueous wastes is among the most vital issues for many industrialized countries. During recent years Biosorption has gained imperative credibility because of its good performance and low cost. Present study is the adsorption behavior of cadmium (Cd II) on spent black tea in aqueous solution, using batch technique. It was carried out as a function of biosorbent particle size, contact time, dosage, initial metal ion concentration and pH. The equilibrium time was achieved within 20min, with 52% removal at an adsorbent dosage of 0.2g/20ml metal ion solution of 50mg/L concentration. The equilibrium metal uptake decreased 17% with an increase in the initial concentration (25-75mg/L). The maximum biosorption occurred at pH7. Experimental data was also tested in terms of biosorption kinetics using pseudo-first-order and pseudo-second-order kinetic models. The results showed good correlation coefficient (R2)0.98 for pseudo-second-order kinetic model. Four isotherms; Langmuir, Freundlich, Dubbin-Radushkevich (D-R) and Temkin were used to model the equilibrium sorption experimental data. The equilibrium process was described well by the D-R isotherm with R2 =0.98. Biosorbent has been specifically modified by sulfonation and methanol esterification to understand the mechanism of cadmium adsorption from synthetic solution. The origin of the enhancement was investigated by observing the sorption behavior and the change in surface functional group concentrations by FTIR and active sites by Boehm method. Sulfonated spent tea revealed 31% increase and esterified 51% decrease in cadmium sorption compared to pristine biomass. The present study recommended that spent tea raw as well as modified can serve as a cost effective and easily available substitute for conventional carbon based adsorbents.