Fe(II)-sodium alginate hydrogel with magnetic biochar-confined for enhanced Cr(VI) reduction and immobilization

The aggregation and separation of magnetic biochar (MBC) are the bottlenecks in its application for the removal of hexavalent chromium (Cr(VI)) from aquatic media. To solve this problem, MBC and Fe(II)–sodium alginate (SA) hydrogel were combined to form SA–Fe(II)–MBC beads (SA–Fe–C), which were used for Cr(VI) removal and immobilization from aqueous solutions. SEM and XPS revealed that the 3D network structure of SA–Fe(II) hydrogel effectively prevented the aggregation of MBC. The experimental maximum Cr(VI) removal capacity of SA–Fe–C beads was 30.03 mg/g at pH 4. The spontaneous process of Cr(VI) removal by SA–Fe–C beads was best described by a pseudo-second-order model and dominated by chemical adsorption. Toxicity characteristic leaching procedure extraction showed that SA–Fe–C beads exhibited excellent immobilization capacity for Cr. The mechanism whereby SA–Fe–C beads removed and immobilized Cr(VI) from solution was found to involve: (1) adsorption, (2) reduction of Cr(VI) to trivalent chromium (Cr(III)) by Fe(II) and hydroxyl groups, and (3) co-precipitation of Cr(III) with Fe(II) to form chromite. In addition, the beads demonstrated excellent regeneration capability and reusability, showing promising potential for heavy metal removal from contaminated water.