Iron/Carbon Composites for Cr(VI) Removal Prepared from Harmful Algal Bloom Biomass via Metal Bioaccumulation or Biosorption

Iron/carbon (Fe/C) composites efficiently remove Cr(VI) because of synergistic adsorption and reduction effects. This study uses harmful algal bloom (HAB) biomass and ferric ammonium citrate (FAC) or ferric nitrate as precursors for preparing Fe/Cs with a one-pot synthesis. The investigation uniquely differentiates material and performance impacts associated with two iron loading approaches, bioaccumulation (metal uptake by living algae), and biosorption (metal deposition onto dry algae). As-prepared Fe/Cs are up to 70% mesoporous with iron loading reaching 8.3 wt %. Uniformly dispersed nanoparticles (20-50 nm) are observed in all Fe/Cs, and microscale particles are present on the surface of biosorption samples due to sintering. Fe₃O₄ is the dominant iron species in Fe (NO₃)₃ added samples, while Fe⁰ dominates samples prepared with FAC, attributed to the reducing atmosphere generated during FAC pyrolysis. Up to 4.0 wt % nitrogen doping is achieved, from nitrogen in HAB biomass and iron precursors. Fe/Cs remove up to 165 mg/g Cr(VI) at pH = 2 and 73 mg/g Cr(VI) at pH = 6, with rapid kinetics. Magnetic properties (>16 emu/g) from reduced iron nanoparticles facilitate Fe/C separation and reuse, and samples maintain 73-82% of their removal capacity after five removal/recovery cycles. This work is important because it converts HAB biomass waste into functional materials with value in environmental applications.