Activated carbon impregnated by zero-valent iron nanoparticles (AC/nZVI) optimized for simultaneous adsorption and reduction of aqueous hexavalent chromium: Material characterizations and kinetic studies

Nano-scale zero valent iron (nZVI) particles are one of the efficient materials for water treatment. However, their tendency for agglomeration is one of the major reported drawbacks. In this study, nZVI particles were immobilized onto activated carbon (AC/nZVI) using a two-step synthesis procedure and were applied for simultaneous adsorption and reduction of hexavalent chromium (Cr(VI)) from aqueous solutions. Synthesized AC/nZVI was characterized by SEM, EDS, XRD, XPS, and the Tafel corrosion test. The effects of varying annealing temperatures and times in the first synthesis step were investigated by examining crystal structure changes in the final AC/nZVI samples using XRD, and by running Cr(VI) removal batch experiments. Consequently, the best annealing conditions that provided the AC/nZVI composite with the greatest amount of Cr(VI) removal was determined. The Cr(VI) removal experiments showed that the optimized AC/nZVI was significantly superior than virgin AC applied under the same conditions. A pseudo-second-order model was found to be a good fit for both the AC and AC/nZVI data, showing that Cr(VI) removal rate was 2.2 times faster when using AC/nZVI compared to AC (k'(2AC/nZVI)= 0.363 g mg(-1) h(-1) vs. k'(2AC)= 0.164 g mg(-1) h(-1)). Weber and Boyd kinetics models were used to study adsorbate diffusion in AC and AC/nZVI. Isotherm studies revealed that both Freundlich and Langmuir isotherms could describe the AC data, while the Freundlich model was well suited for AC/nZVI. Comparing the constants of the Freundlich models, AC/nZVI showed 33-times higher adsorption capacity and a greater affinity for Cr(VI) than did AC. Reusability of AC/nZVI for Cr(VI) treatment was assessed by regenerating the adsorbent and indicated that the material maintained a 68% removal efficiency after five cycles.