Sulfuric acid activated water hyacinth root biosorbent for hexavalent chromium removal from synthetic aqueous solutions

Hexavalent chromium [Cr(VI)] discharged from tannery effluents poses severe environmental and human health risks. This study evaluates sulfuric-acid-treated water hyacinth root powder (TWHRP) as a low-cost biosorbent for Cr(VI) removal and recovery, using synthetic Cr(VI) solutions representative of the polishing/tertiary-treatment stage of tannery effluent. The biosorbent was characterised by proximate analysis, BET, point of zero charge, FTIR, and SEM before and after modification and adsorption. Batch experiments varying pH, contact time, adsorbent dosage, and initial Cr(VI) concentration yielded removal efficiencies of 16.98–97.11%. Response surface methodology with a face-centred central composite design identified the optimum as pH 2.01, dosage 0.59 g/100 mL, contact time 62.77 min, and 10.12 mg/L Cr(VI), giving 98.08% predicted removal (experimentally validated at 97.39 ± 0.14%). Equilibrium data fitted both Langmuir and Freundlich isotherms, and the pseudo-second-order model best described the rate data; the underlying mechanism is presented as proposed rather than experimentally demonstrated. Desorption with 0.5 M NaOH (30 mL) recovered 91.27% of the Cr(VI); subsequent reaction with BaCl 2 ·2H 2 O yielded a yellow precipitate consistent with, but not confirmed as, BaCrO 4 , pending XRD and SEM–EDS characterisation.