Chromium (VI) removal kinetics by magnetite-coated sand: Small-scale flow-through column experiments.

Magnetite nanoparticles are promising materials for treating toxic Cr(VI), but safe handling is challenging due to their small size. We prepared flow-through columns containing 10% or 100% (v/v) magnetite-coated sand. Cr(VI) removal efficiency was determined for different Cr(VI) concentrations (0.1 or 1.0 mM), neutral or alkaline pH, and oxic/anoxic conditions. We formulated a reactive-transport model that accurately predicted total Cr removal, accounting for reversible and irreversible (chemi)sorption reactions. Our results show that the material removes and irreversibly sequesters Cr(VI). For the concentration range used 10% and 100% (v/v) -packed columns removed > 99% and 72% of influent Cr(VI), respectively. Two distinct parameter sets were necessary to fit the identical model formulation to the 10 or 100% (v/v) columns (e.g., maximum sorption capacities (q) of 1.37 µmol Cr/g sand and 2.48 µmol Cr/g, respectively), which we attributed to abrasion-driven magnetite micro-particle detachment during packing yielding an increase in reactive surface area. Furthermore, experiments under oxic conditions showed that, even when handled in the presence of O, the magnetite-coated sand maintained a high removal capacity (47%). Our coupled experimental and modelling analyses indicates that magnetite-coated sand is a promising and suitable medium for treating Cr(VI)-contaminated water in fixed-bed reactors or permeable reactive barriers.