The Study Of Adsorption Of Cd²⁺from Aqueous Solution On Wheat Stalk And Peanut Shell

On the basis of the study of adsorption of heavy metals on agricultural wastes at domestic and abroad, wheat stalk and peanut shell were picked as raw materials, and were carbonized and chemically modified. Then, raw materials, carbonized adsorbents and chemically modified adsorbents were used for studying adsorption of cadmium ion. Firstly, it was discussed that the influences of time, pH, dosage, initial concentration on adsorption with one-factor method. Under the most optimistic conditions, the experiments of adsorption kinetics and adsorption isotherm were conducted and described the process of adsorption and adsorption mechanism by parameters of isotherm models.1. Under the best conditions (adsorption time is12h, pH is7, dosage is0.8g), wheat stalk could remove90%cadmium from100mL of100mg/L of cadmium ions. Under the best conditions (adsorption time is12h, pH is7, dosage is0.6g), peanut shell could remove97%cadmium from100mL of100mg/L of cadmium ions. With modeling of Langmuir equation, in313K, the theoretical maximum adsorption capacity of wheat stalk is35.971mg/g; peanut shell is30.395mg/g. The adsorption process of two types of adsorbents can be fitted by Langmuir and Freundlich models. Inserting experimental data into pseudo-first kinetic and pseudo-second kinetic models, it can be concluded that the two types of adsorbents are fitted better with pseudo-second kinetic models. Thermodynamic parameters can be calculated by Gibbs equation, both of△G are less than zero and△H are both more than zero, it can be concluded that their adsorption process are spontaneous and endothermic.2. By controlling the carbonization temperature (150°C,300°C,450°C,600°C,750°C),the best of carbonization temperature can be fended according to the assay results. The experiments show that the carbonized wheat straw and peanut shells under750℃have a maximum adsorption capacity. The optimum adsorption conditions of carbonized wheat straw are those, the adsorption time is12h, pH is7, and the dosage is0.1g. According to the Langmuir model, the theoretical maximum adsorption capacity of carbonized wheat straw to Cd2+under303K in aqueous solution is114.980mg/g, while the carbonized peanut shell is156.250mg/g. Adsorption of the two kinds of carbonized material is fitted better with Langmuir and Freundlich models and the adsorption kinetics is fitted better with pseudo-first order kinetic model. Thermodynamic parameters can be calculated by Gibbs equation, both of△G are less than zero and△H are both more than zero, it can be concluded that their adsorption process are spontaneous and endothermic.3. The adsorption materials are modified by soaking in50%of ZnCl2solution with1:1ratio. The modification time is24h. And then the modified adsorption materials are irradiated for4minutes under the microwave of640W. The modified adsorbents have a good adsorption capacity of Cd2+in the aqueous solution. The modified wheat straw and peanut shell can achieve the best adsorption effect when the adsorption time is12h, pH is7and the dosage is0.20g. The adsorption rates are95%and99%. According to the Langmuir model, the theoretical maximum adsorption capacity of the modified wheat straw and peanut shells to Cd2+are respectively133.333mg/g and145.515mg/g. Adsorption of the two kinds of modified materials can be fitted well with Langmuir and Freundlich models and the adsorption kinetics is fitted better with pseudo-second order kinetic model. Thermodynamic parameters can be calculated by Gibbs equation, both of△G are less than zero and△H are both more than zero, it can be concluded that their adsorption process are spontaneous and endothermic.