Chromium (III) Removal from Synthetic Tannery Wastewater Using Chlorella vulgaris Biomass as a Sustainable Biosorbent: Kinetics, Isotherms and Mechanistic Insights

Chromium-bearing effluents from the tanning industry pose significant environmental concerns owing to high metal loading and ecological risks. Cr(III) can accumulate in aquatic systems and may oxidize to more hazardous species under certain conditions. This study investigated Cr(III) removal from synthetic tanning wastewater using Chlorella vulgaris in a bubble column photobioreactor. The influence of key operational parameters, including pH, initial chromium concentration (250–1000 mg·L−1), and microalgal dry weight (DW), was systematically assessed using response surface methodology, and biomass-free controls were used to separate biomass-associated removal from chemical precipitation. Cr(III) removal was rapid, with 72.7% biomass-associated removal achieved within 15 min and equilibrium reached within 20 min at 75.54% precipitation-corrected biomass-associated Cr(III) removal. Equilibrium data fitted the Langmuir isotherm model, indicating monolayer adsorption and a maximum capacity (qmax) of 256.41 mg·g−1. Kinetic analysis showed that the pseudo-second-order model provided the best statistical fit to the biosorption kinetics data (R2=0.9976, RMSE=1.45 mg·g−1). Also, FTIR, SEM, and EDX analyses revealed changes in the biomass surface and vibrational environment of functional groups, as well as the presence of chromium after exposure. Furthermore, the biomass retained high removal performance (>68%) over five successive adsorption–desorption cycles, demonstrating its reusability. These findings highlight Chlorella vulgaris as a promising option for treating high-strength tannery wastewater.