Adsorption of Cr (VI) onto micro- and nanoparticles of palygorskite in aqueous solutions: Effects of pH and humic acid.

Palygorskite is a mineral widely applied for the removal of potentially toxic trace elements from the environment. This study aimed to identify the influence of pH (3, 4, 5, and 6) and humic acid (200 mg L) addition on the adsorption of hexavalent chromium (Cr (VI)) ions by the microparticles and nanoparticles of palygorskite. Therefore, the mineral was prepared as the micron- and nano-sized particles saturated with Ba ions, and finally used in adsorption experiments. The results indicated that regardless of the mineral size, Cr(VI) adsorption was enhanced by increasing the contact time from 5 to 2880 min and the equilibrium conditions achieved after 1440 min from the beginning of experiments. With increasing the pH values from 3 to 6, the adsorption efficiency of Cr(VI) decreased significantly, and the highest rate of removal (64%) was found at pH = 3. The pseudo-second-order model best described the kinetics of Cr(VI) adsorption onto both micro- and nanoparticles of palygorskite. Also, the experimental data showed maximum consistency with the data calculated by the Freundlich isotherm model. The two processes of film and pore diffusion were recognized as the main mechanisms that controlled the adsorption of Cr(VI) by palygorskite microparticles and nanoparticles. A comparison of the maximum adsorption capacity of Cr(VI) in different treatments followed the order of nanoparticles > humic acid > humic acid + nanoparticles ≈ humic acid + microparticles > microparticles. Accordingly, the adsorption capacities of "palygorskite nanoparticles" and "humic acid" for Cr(VI) ions were 3.7 and 3.2 times higher than that of palygorskite microparticles, respectively. Moreover, the adsorption capacities observed in the simultaneous application of humic acid with palygorskite microparticles and nanoparticles were 20% and 45% lower than those without humic acid, respectively. To conclude, palygorskite nanoparticles were found to have a significant adsorption capacity for Cr(VI) ions, which is negatively affected by rise in the pH of the aqueous matrix.