Modeling and optimizing parameters affecting hexavalent chromium adsorption from aqueous solutions using Ti-XAD7 nanocomposite: RSM-CCD approach, kinetic, and isotherm studies

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Abstract
Background Due to the high toxicity of chromium, particularly as Hexavalent chromium Cr (VI), it is removed from industrial effluents before their discharge into the environment by a variety of methods, including loading catalysts onto the polymeric supports. This study focused on the removal of Cr(VI) from aqueous solutions using Amberlite XAD7 resin loaded titanium dioxide (Ti-XAD7). Methods Ti-XAD7 was synthesized using Amberlite XAD-7 impregnated with titanium tetraethoxide. The prepared Ti-XAD7 was characterized by using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Isotherms and kinetic studies were carried out to describe the adsorption behavior of adsorbent for the removal of Cr(VI) ions. Quadratic models considering independent variables, i.e. the initial Cr(VI) concentration, adsorbent dosage, time, and pH, were evaluated and optimized to describe the behavior of Cr(VI) adsorption onto the Ti-XAD7 using RSM based on a Five-level-four-factor CCD approach. Results The accuracy and the fitting of the model were evaluated by ANOVA with R2 > 0.725 and P value = 5.221 × 10−5. The optimum conditions for the adsorption process were an initial Cr(VI) concentration 2750 ppb, contact time of 51.53 min, pH of 8.7, and Ti-XAD7 dosage of 5.05 g/L. The results revealed that the Langmuir and Sips isotherm models with R2 = 0.998 and 0.999 were the best models fitting the experimental data. The adsorption capacity of Ti-XAD7 and RL constant were 2.73 mg/g and 0.063–0.076 based on the Langmuir isotherm, respectively. Kinetic studies also indicated that the adsorption behavior of Cr(VI) was acceptably explained by the Elovich kinetic model with a good fitting (R2 = 0.97). Conclusions Comparison of the Ti-XAD7 and XAD7 yield in chromium adsorption showed that modified XAD7 had higher removal efficiency (about 98%) compared to XAD7 alone.

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