Desorption kinetics and mechanisms of Cr, V, As, Se, Ni, Co, Cd from spent keratin-based biosorbent matrices

The disposal of metal-laden adsorbents poses environmental concerns, necessitating the desorption of toxic metal ions from adsorbents for their regeneration or safe disposal. Hence, this study investigated the effectiveness of NaOH, HCl, NaNO 3 and EDTA for simultaneous desorption of various metals adsorbed on recoverable solvent-treated keratin-based biosorbent (STKB), having high surface area (86.09 m 2 /g). Desorption from metal-loaded STKB was carried out in a batch mode at room temperature and pH 7.5, with initial adsorption capacities of 31.3–114.2 mg/kg for the investigated metal ions (Cr(VI),V(V),As(III),Se(VI),Co(II),Ni(II),Cd(II)). Among all eluents tested, 1 M HCl exhibited the highest desorption capacities (De) for multiple metal ions with De values of about 253.1, 91.8, 18.4, 18.1, 115.5, 259.0, and 270.9 mg/kg for Cr(VI),V(V),As(III),Se(VI),Co(II),Ni(II), and Cd(II), respectively. Under all tested conditions, FTIR spectra showed noticeable shifts and intensity changes in –OH, –NH, C=O, and C–N bands, indicating metal desorption and recovery of adsorbent functionality. Only 1 M HCl restored the STKB spectrum close to its original state, demonstrating strong regeneration potential. Further, the STKB was regenerated up to three cycles and the metal uptake capacities of Cr(VI),V(V),As(III),Se(VI),Co(II),Ni(II),Cd(II) ions on the recycled STKB were maintained at 48, 64, 66, 19, 23, 59, and 54 mg/kg, respectively. To evaluate the sorption mechanism and its three mass transfer steps (external diffusion, internal diffusion, and binding to the active sites), several diffusion and kinetic models were implemented. Analysis of R 2 and χ² values revealed that sorption on STKB involves multiple rate-controlling steps, dominated by pseudo-second order chemisorption.