Adsorption Of Au³⁺, Cu²⁺ And Pb²⁺ By Bio-adsorbent Prepared From Persimmon Powder

A new adsorption resin (PPFR) has been developed by immobilizing persimmon powder with formaldehyde, and its adsorption properties of precious metal ion Au+and heavy metal ions Cu2+and Pb2+were investigated in detail. PPFR was chemically modified by NaOH to prepare a NaOH modified persimmon powder-formaldehyde resin (NPPFR), and its adsorption properties of heavy metal ions Cu+and Pb+were evaluated. The characterization of these two adsorbents was investigated by SEM, microscope, FT-IR spectra, XRD, Zeta potentials, thermal denaturation temperature and BET specific surface area. The effects of solution pH, adsorption time, temperature, metal ion concentration, adsorbent dose and coexisting metal ions on adsorption process were investigated by conventional batch adsorption method, and the adsorption kinetics and adsorption isotherms of these three metal ions were analyzed systematically. The column studies were conducted to evaluate the regeneration and reusability of both adsorbents. The adsorption mechanism of Au3+, Cu2+and Pb2+on PPFR and NPPFR were also discussed, respectively. The main results of this study are as following:1. The FT-IR spectra, Zeta potentials, thermal denaturation temperature and BET specific surface area could characterize the biosorbents. The FT-IR spectra of PPFR suggested that persimmon tannin was highly crosslinked with formaldehyde. The zeta potentials of PPFR exhibited high negative values within the pH range of5.0-8.0, and the isoelectric point of PPFR occurred at pH2.21, however, the zeta potentials of NPPFR exhibited all low negative values within the pH range2.0-10.0. The bulk density, BET specific surface area and thermal denaturation temperature of PPFR were measured as0.81g/cm3,0.23m2/g and367.6K, respectively, and these three characterization parameters of NPPFR were determined as0.55g/cm3,0.47m2/g and411.7K, respectively. 2. PPFR exhibited outstanding selective adsorption capacity towards Au3+from acidic aqueous chloride solutions (pH≤2.0), which the equilibrium adsorption capacity was high up to3025.20mg/g at323K. The PPFR could100%recover Au3+from an actual industrial sample, meanwhile the adsorption of other metal ions was negligible although their concentration was much higher than Au3+. The experimental data were well fitted with the pseudo-second-order rate model (R2>0.99). The adsorption of Au3+was an endothermic process, and the apparent activation energy (Ea) was evaluated as53.19kJ/mol. The adsorption isotherms could be well described by Freundlich equation. The column studies suggested that PPFR was effective for the adsorption of Au3+from aqueous solutions, and the loaded Au+could be easily desorbed by acidic thiourea solution. The adsorption of Au3+on PPFR was a reductive adsorption process, in which Au3+was reduced to its elemental form and aggregated to fine gold particles.3. The adsorption of Cu2+and Pb2+on PPFR were also effective from aqueous solutions. The adsorption process depended on the solution pH, and the optimal pH range was5.0-6.0. The adsorption of Cu2+and Pb2+on PPFR was rapid which could reach adsorption equilibrium around180min, and the adsorption kinetics data were well fitted with the pseudo-second-order rate model. The adsorption rate was slightly increased with increasing temperature from303K to323K, however, there was no obvious influence of temperature on the adsorption capacity. Higher concentration of solutions led to higher equilibrium adsorption capacity, and the adsorption isotherms of Cu+and Pb+on PPFR could be well described by Langmuir model. The resin exhibited high adsorption capacity towards Cu2+and Pb2+from coexisting metal ions solution, and the pollutant removal reached83.73%and98.63%when the adsorbent dose was2.0g/L, respectively. The regeneration and reusability tests result indicated the PPFR was suitable for repeated use for more than four cycles. The adsorption mechanism of Cu2+and Pb2+on PPFR might be the combination of electrostatic interaction and coordination reaction.4. The adsorption capacity of Cu+and Pb+on NPPFR was much higher than that of PPFR, which increased by90.50%and62.17%, respectively. The solution pH had great effect on the adsorption of Cu2+and Pb2+by NPPFR, and the optimal pH range was5.0-5.5. The pseudo-second-order rate model could be used to satisfactorily describe the adsorption kinetics of Cu2+and Pb2+on NPPFR (R2≥0.9998), and the adsorption capacities calculated by the model were close to those determined by experiments. The adsorption isotherms of Cu2+and Pb2+on NPPFR could be well described by Langmuir equation (R2>0.99), and the equilibrium adsorption capacities of Cu2+and Pb2+at293K and313K calculated by Langmuir equation were much higher than that of PPFR. Coexisting metal ions (Fe3+, Pb2+, Cu2+, Cd2+, Zn2+, Mn2+, Mg2+and Ca2+) had no obvious influence on the adsorption of Cu2+and Pb2+on PPFR and NPPFR. Moreover, most of metal ions coexisting in solutions could simultaneity removed by NPPFR when the adsorbent dose was1.0g/L, while the adsorption of Mg2+and Ca2+on PPFR were not satisfactory at the same adsorbent dose. The column studies suggested that NPPFR was effective for the adsorption of Cu2+and Pb2+from aqueous solutions, and the loaded Cu2+and Pb2+could be easily desorbed by dilute HNO3solution, which the maximum concentration of Cu2+and Pb+in eluates were20.97and33.94times in comparison with that in original solutions, respectively.