Immobilizing magnetite onto quartz sand for chromium remediation

Magnetite nanoparticles are often promoted as remediation agents for heavy metals such as chromium due to their reactivity and high surface area. However, their small size also makes them highly mobile increasing the risk that reacted pollutants will be transported to different locations rather than being safely controlled. Released to aquatic environments, aggregation leads to a loss of their nano-specific properties and contaminant-removal capacity. We immobilized magnetite onto sand to overcome these issues whilst maintaining reactivity. We compare biogenic magnetite and abiogenic magnetite coated sand against magnetite nanoparticles. Magnetite coatings mostly exhibited a Fe(II)/Fe(III) ratio close to stoichiometry (0.5). We tested the efficacy of the magnetite-coated sand to adsorb chromium, with respect to biogenic/abiogenic nanoparticles. Langmuir-type sorption of Cr(VI) onto magnetite (4.32 mM total Fe) was observed over the tested concentration range (10-1000 μM). Biogenic nanoparticles showed the highest potential for Cr(VI) removal with maximum adsorption capacity (Q_max) of 1250 μmol Cr/g Fe followed by abiogenic nanoparticles with 693 μmol Cr/g Fe. All magnetite coated sands exhibited similar sorption behavior with average Q_max ranging between 257-471 μmol Cr/g Fe. These results indicate coating magnetite onto sand may be more suitable than free nanoparticles for treating environmental pollutants such as chromium.