Study On Preparation Of The Biosorbents By Wheat Straw And Their Biosorption Of Cadmium(Ⅱ)

In this work, wheat straw was chosen as raw material to prepare three kinds of heavymetal biosorbents: original wheat straw （WS）, ZnCl^2-microwave modified wheat straw （ZWS）and ethanol-NaOH modified wheat straw （AWS）. These three biosorbents were used toremove aquatic Cd²⁺. Scanning electron microscope and infrared spectrum analysis wereemployed to characterize the three biosorbents. The Effects of sorbent dosage, initial pH ofsolution, contact time, temperature and Cd²⁺concentration on adsorption process wereinvestigated by adsorption experitment in batch mode. Asorption kinetics, adsorptionisotherms and adsorption thermodynamic parameters of the absorptions of Cd²⁺by the threebiosorbents were investigated. In addition, desorption and reutilization rates of the threebiosorbents were studied by using hydrochloric acid as desorbent.The characterization results of the three biosorbents showed that the original wheatstraw had the potential to absorb heavy metals, because the surface of WS was coarse andmany functional groups which can be used in adsorption were found in WS. The results alsoshowed that ZWS and AWS had larger specific surface area compared to WS and functionalgroups such as–OH and C=O were changed to a certain degree after modifcation. Thesechanges after modification all result in better adsorption ability.The Cd²⁺removal rates of WS, ZWS and AWS increased with the reduction of sorbentdosage, on the contrary, the adsorbed amount of Cd²⁺decreased. The best sorbent dosages forWS, ZWS and AWS were6g/L,4g/L and6g/L, respectively. Initial pH of solution had greatinfluences on the biosorption of Cd²⁺by using WS, ZWS and AWS. The best biosorption pHvalues for WS, ZWS and AWS were7.0,6.0and5.0⁷.0, separately. The equilibriumadsorbed amounts onto the three biosorbents rose with temperature increasing from293K to313K. The thermodynamic parameters indicated that the biosorption processes of the threebiosorbents were all spontaneous, endothermic and entropy-increased. Within theexperitmental-designed conenctration, the equilibrium adsorbed amounts of the threebiosorbents increased with the rise of initial concentration of Cd²⁺, until the adsorptioncapacities reached saturation.It was also found that the biosorption of Cd²⁺by WS followed Pseudo-second-order kinetics well. The whole process reached equilibrium after60min adsorption. Thebiosorption of Cd²⁺by ZWS could be described by Pseudo-first-order kinetics andPseudo-second-order kinetics. The biosorption equilibrium could be achieved afterapproximately120min. Furthermore, the biosorption of Cd²⁺by AWS also followed bothPseudo-first-order kinetics and Pseudo-second-order kinetics, and Pseudo-second-orderkinetics was better matched with it. The biosorption equilibrium could be reached within40min.The Langmuir and Freudlich adsorption isotherms were used to describe partitioningbehavior for the systems at three temperatures, namely293,303, and313K. The biosorptionof Cd²⁺by WS fitted both Langmuir and Freudlich isotherm model, and Langmuir model wasfitted better. The saturation adsorption capacities from Langmuir model were16.393,17.544and17.857mg/g at the temperatures of293,303and313K, respectively. The biosorption ofCd²⁺by ZWS fitted only Langmuir isotherm model and the saturation adsorption capacitiesfrom Langmuir model were62.500、64.935and67.568mg/g at the temperatures of293,303and313K, separately, substantially higher than that of WS （Increased by281.3%,270.1%and278.4%, respectively）. Besides, the biosorption of Cd²⁺by AWS also fitted onlyLangmuir isotherm model and the saturation adsorption capacities from Langmuir modelwere20.000,20.833and21.739mg/g at the temperatures of293,303and313K, separately,higher than that of WS （Increased by22.0%,18.7%and21.7%, respectively）.All loaded biorsobents including WS, ZWS and AWS could be regenerated by using0.1mol/L HCl, and the desorption rates were all above95%. The adsorption capacities of thethree biosorbents decreased after desorption, the decrease extent followed the order: WS＞ZWS＞AWS.