Nano iron oxides impregnated chitosan beads towards aqueous Cr(VI) elimination: Components optimization and performance evaluation

Nano iron oxide based sorbents are very promising candidates for Cr(VI) removal from water. However, powdery nano iron oxides are difficult to separate from water, limiting their application in water treatment process. Herein, granular adsorbents (nano iron oxides impregnated chitosan beads, NIOC) were developed via green synthesis route using Fe(III) salts and chitosan for Cr(VI) removal. The NIOC with millimeter size displayed bifunction (sorption combined with group reduction) towards Cr(VI). Key synthesis parameter i.e. the molar ratio of Fe(III)/chitosan monomer (denoted as R) was optimized and the sorption performance of adsorbents with different R values (R = 0-4.8) was evaluated. Results indicate that all the sorbents displayed excellent mechanical strength (>10.428 N). NIOC-2 (R = 1.6) was found to show the maximum adsorption capacity towards Cr(VI) (92.04 mg/g) and total Cr (86.16 mg/g) at pH 5, 20 degrees C. Characterization analyses indicate that the main crystalline phases in NIOC-2 were iron oxide nanoparticles (beta-FeOOH and alpha-Fe2O3). Coexistent PO43- and SO42- had a significant competitive effect on Cr(VI) adsorption. Cr(VI)-loaded NIOC-2 could be effectively regenerated with NaOH solution and used repeatedly. FTIR and XPS analyses show that Cr(VI) removal mechanisms included electrostatic attraction, ligand exchange, reduction of Cr(VI) into Cr(III) and re-sorption of Cr(III) via amino complexation. Long-term column experiment shows that the number of treated bed volume in flow-through system was large (similar to 4800) and long run time could be maintained (similar to 75 d) before breakthrough. The synthesis procedure of granular NIOC-2 is simple, green and cheap, thereby it is a potential sorbent to be utilized in fixed-bed system for Cr(VI) removal from drinking water.

Automated summary

Nano iron oxide based granular adsorbents (NIOC) were developed for Cr(VI) removal, showing bifunctional sorption abilities and high adsorption capacity. NIOC-2 with optimized Fe(III)/chitosan monomer ratio demonstrated the highest adsorption capacity and could be effectively regenerated for repeated use.