Study Of The Preparation Of Red Mud Modified Pumice And Its Adsorption Characteristics Of Arsenate

Arsenic is a toxic metalloid, which exists mainly in the Earth’s core and in the crust of clay and sulfides. Due to increasing natural and human activities, such as industrial and mining, a lot of arsenic migrates into water, to cause arsenic contamination of ground and drinking water. To effectively remove arsenic from water, scientists have been developing the cost-effective adsorbents for the adsorption of arsenic.In this study, the industrial solid waste red mud was used to modify natural pumice, to find an effective method for the modification, we focused on the studies of modification treatments on pumice. Meanwhile, we analyze the effects of various factors on the removing aqueous arsenic by modified pumice, and study its kinetic process on arsenic removal, in order to further optimize the modification parameters for improving arsenic-removing capacity of pumice. Finally, we compare the modified and unmodified pumice, in order to characterize the mechanisms for pumice modification and for arsenic-removing. We believe that these studies will lay a theoretical foundation for the practical application of red mud in the field of environmental protection, and to provide technical support for engineering applications.(1) We first focused on the modification studies on pumice. Two modification methods were used. The method was used to treat pumice with the red mud hydrochloric acid leaching solution. The second method was used to calcine the impregnating treatment. Preliminary results suggest that the pumice after these treatments has an enhanced ability to absorb taqueous As(V). The pumice after impregnation has an improved adsorption ability in removing As(V) from water, and additional baking process further optimize this capacity.(2) Second, we have identified the factors affecting modification. Preliminary results indicate that the ratio between leaching solution volume and pumice mass, soaking time, and baking temperature all have noticeable effects on the modification, while the baking time has a minimal effect. The optimal conditions for the modification have been identified, with ratio of2.5ml/g for the leaching solution volume and pumice mass;30min for the soaking time;300℃for the calcination temperature; and2h for calcination time.(3) Third, we have analyzed the factors affecting adsorption of arsenic. We have found that the dosage of the modified pumice, initial aqueous As(V) concentration, adsorption time, and solution pH all have significant impacts on the arsenic adsorption by pumice. When the pumice dosage is less than7.5g/L, the increase in dosage can improve the removal yield for arsenic. When the initial aqueous As(V) concentration increases, the efficiency for removing As(V) by pumice decreases, however, the adsorption yield per unit of pumice increases significantly. In the first4h adsorption period, increasing the adsorption time could help improve adsorption yield. After that, the adsorption reaches an equilibrium, therefore, the additional adsorption time no longer improve the adsorption yield. The solution pH also has a noticeable impact on the adsorption yield of pumice. A weak acidic pH (pH=3～5) has a benefit effect on the arsenic adsorption by pumice, while alkaline pH (pH>7) has adverse effect.(4) Fourth, we have performed the kinetic studies on the pumice adsorption of As(V). The results show that the adsorption isotherm by the modified pumice for As(V) is more consistent with the Langmuir isotherm model instead of the Freundlich model. Using the Langmuir isotherm model,2.53mg/g for monolayer adsorption capacity for As(V) could be determined. Furthermore, the As(V) adsorption isotherm follows a Lagergren pseudo-second order kinetic model. The As(V) adsorption mediated by pumice is a chemical process, as it is associated with electronic gains and losses. In the first3h period, the adsorption process is controlled by the particle diffusion. After that, the adsorption rate is dictated by both the film diffusion and particle diffusion.(5) Fifth, we have characterized the molecular mechanism of the pumice modification using red mud. This modification enables the original surface of pumice coated with a large number of hydroxyl iron and aluminum oxides. The hydroxyl iron and aluminum oxides on the surface of the pumice undergo coordination reactions with arsenate anions to release the hydroxyl groups; consequently, the arsenic is adsorbed on the modified surfaces or pores of pumice to be removed from the solution. The entire process is controlled by both the arsenic adsorption and ion exchange.