The Enhancement of Cd(II) Adsorption by Immobilsation of Tannic Acid onto Powdered Activated Carbon

Over the past several decades, trace heavy metal ion pollution, such as Cd(II), Cu(II), Pd(IV), etc., have attracted much attention because of their virulent and hard degradation properties. Recently, great efforts have been devoted to establish new methods and technologies to removal toxic metals. Among them, activated carbon adsorption, especially the modified activated carbon technology has been extensively studied due to its feasibility, such as, the low-cost of activated carbon, the recovery of the activated carbon. In this study, Cd(II) was chosen as the target heavy metal ion to evaluate the activity of modified powdered activated carbon(PAC) and commercial available tannic acid(TA), a kind of Chinese gallotannin with molecular weight 1701, was employed in the modification of the surface properties of PAC. Optimum experimental parameters such as pH, contact time, initial concentration of TA and adsorption capacity for tannic acid immobilisation onto PAC and Cd(II) adsorption on tannic acid-powdered activated carbon combination were determined , respectively. Adsorption of Cd(II) on PAC alone was also determined to compare the experimental results. The adsorption capacity for tannic acid immobilisation onto PAC was analysed by the solution absorbance variation of maximal ultraviolet absorption peak at 276 nm, and the concentration of Cd(II) in the solution was measured through AAS and ICP-AES. Specific surface area, pHpzc and surface functional groups of PAC and TA-PAC combination were determined by BET, powder addition method and Boehm titration method, respectively, to study on the adsorption mechanism of Cd(II) on modified surface. Results showed that TA was effective to enhance the metal adsorption capacity of PAC surfaces and the modified PAC got the maximal Cd(II) adsorption capacity with 25 gram tannic acid immobilised per gram PAC at the optimum immobilization pH value 3.5, at room temperature 18°C . The polyphenol structure of TA immobilised on PAC surfaces may contribute to the changes of adsorption capacity. Neither of the Langmuir isotherm equation and Freundlich isotherm equation was found to provide reasonable well fit for the TA-PAC and Cd(II)-TA-PAC combinations.