Process optimization of Cr(VI) removal from aqueous solution using activated orange peel for treatment of tannery wastewater

Abstract

The oxidation of trivalent chromium (Cr(III)) to more toxic hexavalent chromium (Cr(VI)) in wastewater causes several problems in aquatic environments and downstream users. The aim of this research was to optimize Cr(VI) removal from an aqueous solution using activated orange peel adsorbent for treating tannery wastewater by analyzing the effects of adsorbent dose, pH, and contact time. The design expert software was used. Raw and activated orange adsorbents were characterized by FT-IR spectroscopy, scanning electron microscope (SEM), and Brunauer-Emmett -Teller (BET). Batch adsorption experiments were carried out at room temperature and the residual concentration of Cr(VI) was analyzed by UV-VIS spectrometer. Bio-sorbent desorption was also conducted to regenerate the bio-adsorbent and recover the metal. The removal efficiency was maximum (94.74%) at pH of 2, dosage 2.5 g/L, and contact time of 90 minutes. FTIR results confirm that hydroxyl functional groups, which have high affinity towards heavy metals, are responsible for the removal of Cr(VI). Methoxy groups, which undergo demethylation to generate new hydroxyl groups after carbonization confirming oxidation, also play significant role in the adsorption process. SEM results indicate that activated orange peel adsorbent has highly porous surface created due to the removal of viscous compounds during activation. The Langmuir isotherm model shows a better fit and the reaction kinetics is described by a pseudo-second-order kinetic model (R²=0.99). The Cr(VI) removal efficiency in real effluents is lower than in stock solution due to the scavenging effects of competitors.  In summary, results of this research indicate that the use of activated orange peel adsorbent is promising to treat wastewater effluents for removal of Cr(VI).