Adsorption Characteristic of Cr(VI) onto Different Activated Coal Fly Ashes: Kinetics, Thermodynamic, Application Feasibility, and Error Analysis

Raw coal fly ash (RCFA) was recycled as three kinds of adsorbents by hydroxyl anion (OH−), hydrogen ion (H+), and thermal activation, respectively, for the adsorption of Cr(VI) from water. The H+ activation can explore the adsorptive potential of RCFA more effectively than the other two methods. The specific surface areas of the adsorbents are 12.33, 16.32, and 13.89 m2 g−1 for OH−, H+, and thermal activation, respectively. The adsorption process follows the pseudo-second-order kinetic model and the Langmuir model better and exhibits exothermic property. The activation energy (20.65–31.88 kJ mol−1) and the negative Gibbs free energy reveal that the adsorption is a physical and spontaneous process. The adsorbents derived from OH−, H+, and thermal activation can be used at least 5, 7, and 4 times, respectively, while the one from H+ activation has the best adsorption capacity (6.41 μg g−1 for the first run). The adsorption process can introduce other metallic/toxic elements, but within the Chinese standard. The preparation cost of the H+ activation is $1103 ton−1 adsorbent, while the treatment cost is $1.6 ton−1 water. The more accurate parameters in the pseudo-second-order kinetic model and Langmuir model can be calculated by nonlinear method and provided by the error function of the sum of the squares of the errors.