Paper waste sludge derived-hydrochar modified by iron (III) chloride for effective removal of Cr(VI) from aqueous solution: Kinetic and isotherm studies

This work investigated Cr(VI) adsorption onto pristine and iron modified hydrochar (PSH and PSH-Fe15, respectively) which were generated from paper waste sludge. The results indicate that the hydrothermal carbonization temperature of 200 degrees C was the most suitable for hydrochar production. The batch experiments and the adsorption isotherm and kinetic of Cr(VI) onto PSH and PSH-Fe15 were conducted and used to elucidate the adsorption mechanisms. The results demonstrate that the Cr(VI) adsorption capacity and efficiency onto both PSH and PSH-Fe15 reached equilibrium after 120 min at pH 3 and an initial Cr(VI) concentration of 60 mg/L. In this condition, the adsorption capacities were 5.94 mg/g and 8.67 mg/g and the removal efficiencies were 39.62 % and 57.80 % for PSH and PSH-Fe15, respectively. The adsorption processes were well described by the Sips and Redlich-Peterson isotherm models (with R-2 of 0.9895, 0.9987 and 0.9947 and 0.9981 for PSH and PSHFe15, respectively) and pseudo-second-order kinetic model (with R-2 of 0.98412 and 0.99681, respectively, for PSH and PSH-Fe15). The analysis of characteristics of PSH and PSH-Fe15 indicates the rough and heterogamous surface structures. Moreover, there was presence of iron ions on PSH-Fe15 based on EDS analysis and appearance of oxygen-containing surface functional groups recorded through FTIR results. Finally, it was observed that the reduction of Cr(VI) to Cr(III) caused by iron ions on PSM-Fe15's surface and the presence of O-H groups resulting in the precipitation of Cr(OH)(3) on the hydrochar's surface was the main mechanism of Cr(VI) adsorption onto PSH-Fe15.

Automated summary

This study investigated the adsorption of Cr(VI) onto pristine and iron modified hydrochar generated from paper waste sludge. The results showed that the hydrothermal carbonization temperature of 200°C was most suitable for hydrochar production. Both PSH and PSH-Fe15 exhibited equilibrium adsorption of Cr(VI) after 120 minutes, with removal efficiencies of 39.62% and 57.80% respectively. The adsorption process was well described by the Sips and Redlich-Peterson isotherm models, and followed a pseudo-second-order kinetic model.