Response surface optimization of hexavalent chromium removal from real tannery wastewater using carbonized eggshells

Abstract

Hexavalent chromium (Cr(VI)) is a highly toxic and carcinogenic pollutant widely discharged from tannery industries and poses serious environmental and public health risks when released into aquatic ecosystems. In this study, a sustainable and low-cost approach was developed for the efficient removal of Cr(VI) from real tannery wastewater using carbonized chicken eggshell, a readily available biowaste material. Eggshells were carbonized at 800 °C for 2 h, yielding a material with improved physicochemical properties and a surface area of 68.01 m 2 /g. The material was characterized using FTIR, XRD, proximate analysis, and BET surface area analysis. Optimization of adsorption parameters was conducted using Response Surface Methodology with a Central Composite Design employed to statistically evaluate the individual and interactive effects of key adsorption parameters. The developed quadratic model showed high statistical significance (P < 0.0001, R 2 = 0.9776). The effects of pH, adsorbent dosage, initial Cr(VI) concentration, and contact time were systematically evaluated. The optimal conditions for maximum Cr(VI) removal (99.63%) were pH 7, adsorbent dose 1.5 g, initial concentration 80 mg/L, and contact time 120 min. Adsorption data were best described by the Langmuir isotherm model (R 2 = 0.990), indicating monolayer adsorption, while kinetic studies followed the pseudo-second-order model (R 2 = 0.993), suggesting chemisorption. This study demonstrates the effective application of carbonized eggshells for Cr 6+ removal from tannery wastewater, offering a cost-effective, eco-friendly solution for industrial effluent treatment.