Cr(VI) adsorption by a green adsorbent walnut shell: Adsorption studies, regeneration studies, scale-up design and economic feasibility

Natural water resources are polluted due to human activities like discharge from municipal and industrial wastewater. In general industrial wastewater contains toxic chemicals like heavy metals, dyes, acids etc. To save the water bodies it is necessary for industries to treat their wastewater before disposal. Agricultural wastes like green adsorbents may be considered as an alternative to minimize the problems regarding heavy metal pollution particularly for small and medium sized industries. In this study walnut shell, an agricultural waste is used for Cr(VI) removal as green adsorbent. Brunauer-Emmett-Teller (BET), Scanning electron microscopy (SEM), Point of zero charge (pH(pzc)) and Fourier transform infrared spectroscopy (FTIR) were used to characterise the walnut shell. Column studies were performed for different operating conditions and low influent flow rate, low influent Cr(VI) concentration and higher bed depth give better adsorption. Langmuir isotherm model fitted well for Cr(VI) adsorption equilibrium (K-L = 0.6754 L/mg, R-2 = 0.9996). Different kinetic models are applied on experimental data to evaluate model parameters and their applicability. Yan et al. model fitted well for the specified operating range (K-gamma = 5903.63 mL/(mg min), R-2 = 0.9785) and applied for scale-up designing. Regeneration studies are conducted by using different concentration of NaOH to investigate the reusability characteristics of walnut shell. Scale-up design for Cr(VI) removal using walnut shell as an adsorbent and its economic feasibility are done to find out its applicability in real life. This study reveals that walnut shell is an efficient and cost effective adsorbent for Cr(VI) removal and it can be a solution for Cr(VI) discharging industries. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.