Maximizing the adsorption capacity of iron oxide nanocatalysts for the degradation of organic dyes

Abstract

Advanced oxidation processes can counteract the hazardous effects of polluted effluents in a highly efficient way, in many cases limited by the adsorption capacity of the nanocatalyst that depends on their size, internal structure and coating. Here, magnetic iron oxide nanocatalysts consisting on single core (SC), multicore (MC) and core-shell (CS) structures, stabilized with citrate and silica, have been evaluated for the degradation of anionic acid orange 8 (AO8) and cationic methylene blue (MB). It was observed that the adsorption is a limiting parameter, as expected in a mainly heterogeneous process involving molecular adsorption, reaction, and desorption at the catalyst surface. Thus, for the anionic dye, AO8, no degradation is observed by any of the nanocatalysts considering their negative surface charge. However, for MB loaded SC or CS nanocatalysts, highest degradation yields (almost 100% after 180 min at 90 °C) were achieved through a homogeneous and heterogeneous catalysis in the case of SC and a pure heterogeneous process in the case of CS. MC presents the lager aggregate size due to the lack of coating and low surface charge, leading to poor capacity of adsorption and degradation. On the other hand, magnetic induction heating promotes the degradation of MB (up to ≈50%, respect to room temperature). The results show that iron oxide nanocatalysts through Fenton reactions are an interesting alternative for wastewater treatment considering also that iron is non-toxic and one of the most abundant elements on Earth and can be recovered simply by applying a magnetic field.