Transformation of Chrome Leather Waste into Nanofunctional Composite for the Remediation of Tannery Dye Wastewater

The leather industry faces inevitable challenges in producing large volumes of chrome-containing solid waste as well as dye-containing colored wastewater. In this study, chrome-tanned leather shaving (CTLS), one of the major hazardous tannery solid wastes, was upcycled into a chromic oxide nanoparticles composite (Cr₂O₃@CTLS) through a thermochemical process and utilized to treat toxic tannery dye wastewater. The material exhibited a high fixed carbon content (69.56 ± 0.03%), indicating efficient carbonization. Fourier transform infrared (FTIR) analysis revealed several functional groups in Cr₂O₃@CTLS, while TGA showed minimal weight loss up to 450 °C (12%), reflecting the good thermal stability of the nanocomposite. Scanning electron microscopy (SEM) imaging revealed a highly mesoporous structure, attributed to the effective activation of KOH. The Brunauer-Emmett-Teller (BET) surface area and mean pore size of the composite are 57.98 m² g^-1 and 4.83 nm (mesopore), respectively, thereby enhancing its capacity to adsorb complex dye molecules. The dye removal process (acid yellow dye-17) was optimized using response surface methodology with a central composite design (RSM-CCD) using the latest version of software. The optimal conditions for achieving 98% dye removal were determined within a contact time of only 120 min, using 2 g L^-1 of adsorbent. Furthermore, the pseudo-second-order kinetics (<i>R</i> ² = 0.998) and the Temkin isotherm (<i>R</i> ² = 0.990) models provide the best mathematical descriptions, with a maximum adsorption capacity (<i>q</i> _max) of 386.26 ± 16.04 mg g^-1 derived from the Langmuir isotherm. The thermodynamic analysis revealed (Δ<i>G</i> < 0, Δ<i>H</i> > 0, and Δ<i>S</i> > 0) that the uptake process was feasible, exothermic, and spontaneous. The dye sorption mechanism was explored using X-ray photoelectron spectroscopy (XPS), SEM, energy-dispersive X-ray spectroscopy (EDS), FTIR, etc. and suggests that pore filling, diffusion, and hydrophobic and electrostatic interactions are involved in sorption. Moreover, Cr₂O₃@CTLS exhibited impressive reusability, maintaining a removal efficiency of 65.76% at the sixth cycle of dye adsorption, showcasing its economic viability in dye wastewater treatment. After six consecutive cycles, the material exhibited excellent structural stability, as evidenced by the similarity of X-ray diffraction (XRD) patterns between the pre- and postadsorbed Cr₂O₃@CTLS. The nonexistence of Cr (VI) in both solid and liquid phases was confirmed by XPS and ultraviolet-visible (UV-vis) analyses, respectively, ensuring its safe use in real-world water treatment.