Chromium removal by biochar/nanoparticulate iron oxide mineral composites: mechanistic insights and performance under batch and column systems

The overarching aim of the current study was to synthesize nanoparticulate iron oxides (NP-FeOx; nano-magnetite (n-Mg), nano-goethite (n-Gh), nano-ferrihydrite (n-Fh), nano-hematite (n-Ht)) and develop rice husk biochar/NP-FeOx-based composites (BC/NP-FeOx) to remove hexavalent chromium (Cr(VI)) from contaminated water. Batch sorption experiments revealed that Cr removal efficiency was in the order: n-Gh (97.5%) > n-Mg (95.8%) > n-Fh (94.1%) > n-Ht (79.5%) at pH 7.0, sorbent dose of 0.5 g L−1 and Co = 12 mg L−1. Desorption studies showed the reusability of n-Gh and n-Mg over three cycles (65%–70%). Hence, both the most promising NP-FeOx were integrated with rice husk biochar to fabricate novel BC/n-Gh and BC/n-Mg composites and examined in column experiments for Cr(VI) removal at low and high Cr concentrations (3 and 10 mg L−1). High Cr(VI) removal efficiencies (up to 97%–99%) were obtained by both BC/NP-FeX composites across different time intervals (0–72 h). FTIR spectroscopy showed that Cr(VI) sorption was mainly governed by –OH anion exchange and interactions with –C–H, –C = O, and –Fe–O functional groups. This study highlights the significance of BC/n-Gh and BC/n-Mg composites in removing Cr(VI) from contaminated water, providing a suitable and sustainable solution for Cr treatment in wastewater.