Application of FeCl3 and TiO2-coated algae as innovative biophotocatalysts for Cr(VI) removal from aqueous solution: A process intensification strategy

Chromium is a metal that could be both toxic and essential dependent on its oxidation state. While Cr(VI) is toxic to plants, animals and microorganisms, Cr(III) at low levels is an essential dietary nutrient for human beings. In this study, novel biophotocatalysts were developed and applied for Cr(VI) reduction and removal from aqueous solution. The biophotocatalysts were successfully prepared by the direct coating of the raw and protonated (immersion into 0.2 M HNO3) brown algae Laminaria hyperborea with FeCl3, AlCl3, and TiO2 solutions. The products of each treatment were characterized and utilized in an equilibrium study for Cr(VI) removal, which revealed a low efficiency for algae coated with aluminum. The Cr(VI) removal via the synergic effect of photocatalysis and adsorption was investigated through kinetic studies in the presence and in the absence of UV radiation using protonated algae, protonated algae coated with iron and protonated algae coated with titanium as biophotocatalysts. The results showed that the material previously protonated and coated with iron presented the best Cr(VI) removal results, reaching an efficiency removal of 100% after 90 min. The UV radiation improves the removal efficiency of the chromium species for the experiments carried out with protonated algae coated with titanium. The scale-up of the combined process was successfully performed on a radial photoreactor with a concentric UV radiation using the protonated algae coated with iron and titanium as biophotocatalysts. Protonated algae coated with iron, the Cr(VI) species disappeared completely after 120 min, while for protonated algae coated with titanium, the Cr(VI) was still in the solution even after 210 min. The reuse of the biophotocatalysts in consecutive Cr(VI) treatment cycles without regeneration was also performed, indicating that the protonated algae coated with iron have the potential to be used in as biophotocatalysts in the treatment of wastewaters contaminated with Cr(VI), reaching the restricted discharge limit of this metal.