Study On Adsorption Of Heavy Metal Ions By Tannin-based Adsoebents

Tannin,chitosan,bentonite and sodium alginate are abundant in nature with relatively low cost of utilization.In addition,they are potential resources used in the environmental field.Tannin has abundant adjacent phenolic hydroxyls which can chelate with metal ions.However,raw tannin cannot be used as an adsorbent because of its water solubility.Therefore,the tannin is needed to be modified before it is used to treat wastewater.In this paper,two adsorbents,the tannin/chitosan/bentonite composites（TCBC）and the bayberry tannin/alkaline Ca-bent/calcium alginate composites（BT/ACB/CAC）,were successfully synthesized.The effects of pH,TCBC dosage,adsorption temperature and initial Cr（Ⅵ）concentration on the adsorption capacity of TCBC were investigated.The experimental results showed that the adsorption of Cr（Ⅵ）by TCBC almost reached saturation within 100 min.The adsorption kinetics of Cr（Ⅵ）on TCBC followed the pseudo-second-order kinetics model.The isothermal data were well described by the model of Langmuir.The maximum adsorption capacity of TCBC for Cr（Ⅵ）was 264.73 mg/g at 333K and the initial pH level of 2.5.The adsorption capacity of TCBC for Cr（Ⅵ）could still reach 192.17mg/g after the fourth TCBC regenatation with sodium hydroxide as desorption agent.It showed that TCBC used as adsorbent has good reusability.XRD and FT-IR were used to analyze TCBC,and the results showed that there are compound interactions among the three components of TCBC.The Cr（Ⅵ）adsorption property of TCBC was greatly improved by synergistic effect from phenolic hydroxyl of tannin,and amino and hydroxyl of chitosan.The removal of Ni（Ⅱ）by BT/ACB/CAC was studied.The maximum adsorption capacity of BT/ACB/CAC for Ni（Ⅱ）was 75 mg/g at 303K and the initial pH level of 7.0.The adsorption almost reached equilibrium when the adsorption time was about 40min.It showed that the adsorption of Ni（Ⅱ）by adsorbent was a rapid adsorption process.The pseudo-second-order kinetic model and Langmuir adsorption isotherm could better describe the adsorption behaviors of BT/ACB/CAC.The adsorption capacity of BT/ACB/CAC could still maintain 66.84%of the maximum adsorption capacity after the fourth BT/ACB/CAC regenatation with the solution of 5%Ca Cl₂ as desorption agent.It showed that BT/ACB/CAC as adsorbent has good reusability.From FT-IR analysis the major conclusions were as follows:The three components of BT/ACB/CAC have compound action,which reduced the water solubility of tannin.The Ni（Ⅱ）adsorption capacity properties of BT/ACB/CAC was enhanced,which was attributed to the synergetic effect between the phenolic hydroxyl of tannin and the carboxyl group of sodium alginate.