Facile preparation of amidoxime-functionalized fiber by microwave-assisted method for the enhanced adsorption of chromium(VI) from aqueous solution

In this study, a facile and highly efficient approach, the microwave-assisted (MW-aid) method, was applied for the synthesis of amidoxime-functionalized fibrous adsorbent (PAN(MW)-AO fibers), which exhibited enhanced adsorption capacities for Cr(VI) in aqueous solution. The preparation condition was optimized under four independent variables including: MW power, mass of NH2OH center dot HCl, time and bath ratio, based on the Box-Behnken design with response surface methodology (RSM). A period of 1 x 5 min was determined to be the optimum microwave (MW) ageing time for the synthesis of PAN(MW)-AO, which is dramatically faster than using conventional heating methods. Effects of pH, contact time, initial concentration of Cr(VI) and temperature on adsorption were investigated systematically by bath adsorption experiments. Nonlinear solutions of a pseudo-second-order kinetic model and Langmuir isotherm model were found to provide the closest fit to the experimental data for the adsorption process of Cr(VI), which indicated that chemisorption is the controlling mechanism and monolayer adsorption is dominant. Thermodynamic parameters revealed the spontaneity of the adsorption process, and higher temperature favored adsorption. The formation of PAN(MW)-AO and Cr(VI)-adsorbed PAN(MW)-AO has been characterized by FTIR, SEM/EDX and XPS instrumentations, which demonstrate that Cr(VI) was adsorbed by the amidoxime group via a surface complexation mechanism. The adsorption of Cr(VI) ions was hardly affected by common coexisting ions such as SO42-, NO3- and Cl-. High desorption efficiency (>90%) of Cr(VI) was achieved using 0.1 M H2SO4 as effluent, by which the investigated adsorbent could be used repeatedly five times with a small decrease in sorption capacity. Rapidity of synthesis and low cost, coupled with highly efficient and rapid adsorption of Cr(VI) ions, make PAN(MW)-AO fibers an attractive adsorbent in the potential application for wastewater treatment.