Study On The Modified Adsorbents Of Tannin And Their Adsorption Properties On Heavy Metals

Tannin is an important biomass resource. Tannin has recently been investigated widely as a high-performance adsorbent for the removal of heavy metals from aqueous solution, because tannin has abundant adjacent phenolic hydroxyls chelating with metal ions.In this paper, bayberry tannin and acacia mangium tannin were chosen as the raw material to prepare five adsorbents FAMT, CBT, CAMT, PBT and PAMT through chemical modification. These five adsorbents were used to remove Pb(II) and Cu(II) from aqueous solution. SEM, FTIR and EDS were used to characterize the adsorbents and the adsorption of metal ions. The effects of various experimental parameters, such as contact time, initial metal concentration, temperature and initial pH on the adsorption capacity were investigated. The adsorption thermodynamic, adsorption kinetics and adsorption isotherms of these two individual metals were analyzed systematically. Moreover, the competitive adsorption behavior for binary metal ion solutions by FAMT, CBT and CAMT were also discussed.After tannin modified, FAMT had much porous structure and bigger particle size in comparison with AMT. It indicated that the tannin particles were successfully condensed through the cross-linking reaction to overcome the disadvantage of being dissolved by water.The adsorption behaviors of these five adsorbents for the removal of Pb(Ⅱ) and Cu(Ⅱ) were investigated systematically, and similar results were obtained. At low pH values, the adsorption capacity increased slowly during to the H+ ions could compete with metal ion for the exchange sites on the adsorbents. To avoid the Pb(II) and Cu(Ⅱ) be precipitated at high pH, the optimal pH range was chose from 5.0 to 5.5. Moreover, the adsorption capacity was decreased with the increase of temperature in the range 288-333 K, indicating that the adsorption process were exothermic and spontaneous. The adsorption rate of Cu (II) and Pb(Ⅱ) were extremely rapid in initial few minutes, and then slowly reached equilibrium in around 30 min. The adsorption kinetics was accordance with pseudo-second-order rate model. The adsorption capacity increased rapidly with the increase of initial concentration in low value. The adsorption isotherm could be well described with the Langmuir model. The calculated maximum adsorption capacities of five adsorbents were higher than that of original tannin, and the maximum adsorption capacities of 49.75 and 142.86 mg·g-1 were obtained for Cu(Ⅱ) and Pb(Ⅱ) on PBT, respectively. The maximum adsorption capacities of 34.16 and 115.61 mg·g-1 were obtained for Cu(Ⅱ) and Pb(Ⅱ) on PAMT, respectively. The corresponding relation between dimensionless constant seperation factor and type of isotherm were calculated, and the result showed that the adsorption of these four types of adsorption all corresponding to the preferential adsorption isotherm.The competitive adsorption behavior for binary metal ion solutions by FAMT, CBT and CAMT were discussed. The effects of existence of Cu(Ⅱ) on the adsorption of Pb(Ⅱ) was notable. The high concentrations of Cu(Ⅱ) will decrease the the adsorption capacity of Pb(Ⅱ), but the adsorption capacity of Pb(Ⅱ) was great than that of Cu(Ⅱ) anytime.