Adsorption Characteristics And Mechanism Of Heavy Metal Ions Onto Forestry And Agricultural Residues-tolerant Fungus Composite Adsorbents

Biosorption is an effective method using the sorptive nature of biological substances to remove pollutants.Preparation of biosorbents using either forestry and agricultural residues or microbial strains as raw materials for removal of heavy metals is high significance accounted for by their economical,ecological and practical values.The shortcomings of using forestry and agricultural residues as biosorbents,however,are that most of the residual materials are loose in texture with low mass density and thus low adsorption capacity,and they are easily drifting in aqueous solutions.Accordingly,use of microbial strains as sorbent materials also has difficulties in prilling and solid-liquid separation.The disadvantages of using either forestry and agricultural residues or bacterium strains along as biosorbents have thus limited their potential applications to large-scale wastewater treatment.In the past few years,there are numbers of reports on modification methods applied to improve the adsorption capacity and performance of various types of biological materials.There have been,however,very few reports on preparation of composite adsorbents using forestry and agricultural residues inoculated with isolated metal tolerant fungus.In the present study,maize straw（MS）,wheat stalk（WS）,peanut shell（PS）and millet chaff（MC）were chosen as raw materials.The ground powders of the raw materials were inoculated with a metal-tolerant fungus（Aspergillus oryzae,HA）isolated from a lead-zinc tailing contaminated site to form composite biosorbents.The particle structure and characteristics of the composite biosorbents were analyzed by FTIR and SEM.Equilibrium and kinetic experiments were conducted to investigate the effect of pH,sorbent concentration,initial sorbate concentration and contact time on sorption of Pb²⁺ ions onto different biosorbents.Langmuir,Freundlich,and Dubinin-Radushkevich isotherm models were applied to analyze the data obtained from equilibrium tests.Pseudo-first-order,pseudo-second-order,and intra-particle diffusion models were used to describe the kinetic sorption processes.The sorption mechanisms and characteristics of the fixed biosorbent bed were analyzed using Thomas and BDST models.The main findings are summarized as follows:① Composition of MS,WS,PS and MC,respectively,with HA greatly enhanced the sorption capacities of the biosorbents.There were significant differences in sorption capacities among the composite bisorbents,maize straw-HA（MSH）,wheat stalk-HA（WSH）,peanut shell-HA（PSH）and millet chaff-HA（MCH）,which followed the order:MCH>PSH>WSH>MSH.② The data obtained from equilibrium Pb²⁺ sorption experiments indicated that PSH and MCH performed much better than PS and MC with respect to enhanced sorption capacities,raised adsorption rates and extended adequate pH range.③ Results from the conducted tests showed that Langmuir isotherm fit the experimental data better than both Freundlich and D-R equations.The estimated maximum Pb²⁺ sorption capacities of PS,PSH,MC and MCH were 14.62,20.92,11.89 and 21.79 mg/g,respectively.The pseudo-second-order equation was shown to be the best model for description of the kinetic Pb²⁺ sorption process,indicating that chemical adsorption was the main process that limited the sorption rates.④ The FTIR and SEM spectra analysis indicated that surface morphology of millet chaff were apparently different after composition and Pb²⁺ sorption.The functional groups responsible for Pb²⁺ binding were hydroxyl,carboxyl,amino,and alcoholic hydroxyl.Compared with PSH,MCH possessed more functional groups and the change in its surface morphology was more obvious.⑤ The kinetic column flow experiments showed that the breakthrough time tb and saturation point te of the fixed sorption beds of PSH and MCH were apparently longer than those of PS and MC.After composition,the sorption capacities of the biosorbents,PSH and MCH,were increased from 4.62 and 7.13 mg/g to 6.49 and 8.30 mg/g,respectively,which led thus to reduction in the height of sorbate mass transfer zone duo to enhanced total system sorption capacity.⑥ Both the breakthrough time and the height of mass transfer zone increased with increase in bed height and decrease in both flow rate and initial Pb²⁺ concentration.In comparison,the effect of flow rates was significant at higher levels than those of bed column height and initial concentration.⑦ Thomas model gave satisfactory descriptions of the kinetic Pb²⁺ion adsorption process in the fixed sorbent beds for both PSH and MCH（R2[PSH]=0.9532～0.9828,R2[MCH]=0.9410～0.9681）,indicating that neither external nor internal diffusions were the factors limiting adsorption rates.BDST model could accurately predict the relationship among bed height,ct/c0（the ratio of concentration at a given time ct to initial concentration c0）and breakthrough time under different operation conditions.The prediction accuracy level of BDST model was higher for PSH than MCH.⑧The removal rates of PSH and MCH for different heavy metal ions were significantly higher than those of PS and MC.Compared to those of PS,the removal rates of PSH for Pb²⁺,Zn²⁺,Cd²⁺ and Cu²⁺ were increased by 75.50%,123.56%,119.39%and 95.15%,respectively.Compared to those of MC,the removal rates of MCH for Pb²⁺,Zn²⁺,Cd²⁺and Cu²⁺ were increased by 188.26%,135.50%,139.49%and 173.92%,respectively.The increment in heavy metal removal rates of MCH was significantly higher than that of PSH.