Fixed-Bed Column Cr(VI) Removal by Acid-Modified Cordia africana Sawdust from Synthetic and Industrial Wastewater

Abstract

The removal of hexavalent chromium [Cr(VI)] from industrial wastewater remains a significant environmental challenge. This study evaluates the dynamic performance and reusability of acid-modified Cordia africana sawdust in a fixed-bed column system. Continuous-flow experiments were conducted with varying bed depths (6-12 cm) and flow rates (5-10 mL min⁻¹) at pH 3 and 47 mg L⁻¹ Cr(VI). The optimal configuration (12 cm, 5 mL min⁻¹) achieved an exhaustion time of 1045 min (17.4 h) and uptake capacity of 1.94 mg·g⁻¹ for synthetic Cr(VI) solution (47 mg L⁻¹). Three adsorption-desorption cycles using 0.1 M NaOH demonstrated 75% capacity retention after three cycles. When validated with real tannery wastewater (35 mg L⁻¹ Cr(VI), pH 8.55), the column achieved breakthrough at 180 min (3 h) and exhaustion at 760 min (12.7 h), with 0.99 mg g⁻¹ capacity (64% of synthetic). The adsorbent remained mechanically stable throughout. Acid-modified Cordia africana sawdust is a locally available, low-cost (USD 0.10-0.30/kg), reusable (75% after 3 cycles), and effective biosorbent for continuous Cr(VI) removal, representing an appropriate technology suitable for low-resource settings.