Waste valorization of starch obtained from hawthorn yam as bioadsorbent on Chromium (VI) and Nickel (II) removal

Introduction- Among the heavy metals discharged on water sources the Chromium (VI) and Nickel (II) cause dangerous health effects. Objective- Optimize the effect of the initial concentration of pollutant, temperature and adsorbent dose using residues of the starch obtaining process from hawthorn yam (D. rotundata) in the removal of Chromium (VI) and Nickel (II). Method- The optimization was carried out by applying the Response Surface Methodology (RSM), performing the kinetic and equilibrium study to the optimal condition found, evaluating the fit of the kinetic data to the pseudo-first order, pseudo-second order and Elovich models; those of isotherms to the models of Langmuir and Freundlich. The thermodynamic parameters-Gibbs free Energy (Delta G degrees), Enthalpy (Delta H degrees) and Entropy (Delta S degrees), were calculated using the Van't Hoff graphical method. Results- From the MSR it was found that the optimal conditions for Cr(VI) were 76.6 degrees C, 0.14 g and 368.18 ppm, and for Ni (II) 70 degrees C, 1.19 g and 31.82 ppm. The maximum adsorption capacity was 66.25 mg/g Cr(VI) and 17.67 mg/g Ni(II). The pseudo-second order kinetic model adjusts the adsorption data of Cr(VI) and that of Elovich adjusts the Ni(II) data; on the other hand, the Freundlich isotherm model showed the best adjustment of the adsorption data of the ions under study. The values of Delta G degrees, Delta S degrees, and Delta H degrees established that the process for Cr(VI) is endothermic, not spontaneous, not favorable and reversible; for Ni (II) which is non-spontaneous, exothermic and controlled by physisorption. Conclusions- The residues of the hawthorn yam starch extraction process are an effective adsorbent for the removal of Cr (VI) and Ni (II) present in aqueous solution and the MSR yielded optimal values that serve as the basis for the scalability of the process.