Direct solid-state synthesis of maghemite as a magnetically recoverable adsorbent for the abatement of methylene blue

[Display omitted] •One-step solid state synthesis of maghemite through thermolysis of ferric citrate.•Mesoporous maghemite nanoparticles (300-2) have surface area of 163 m2/g.•Extent of γ-Fe2O3 surface oxidation is correlated with SAXS using core-shell model.•Maghemite nanoparticles synthesized at 300 °C showed adsorption capacity of 36 mg/g.•Pre-concentration of MB by 100 times than that of the original volume is achieved. Solid phase extraction of dyes using a magnetically recoverable and reusable adsorbent is highly desirable from a sustainable environmental perspective. A simple, direct and highly scalable solid-state thermal decomposition of ferric citrate in an oxygen deprived environment has been shown to yield magnetic maghemite phase in the temperature range of 300–350 °C. By increasing the calcination temperature, the formation of less-magnetic hematite phase has been found to be predominant. The synthesized iron oxides have been thoroughly characterized using various techniques. SAXS analyses have revealed that the less-magnetic hematite phase has been formed through the surface oxidation of maghemite domains, and thus transitioning from γ-Fe2O3 to α-Fe2O3 passing through γ-Fe2O3@α-Fe2O3 core-shell architecture. The thermal decomposition temperature and duration of the ferric citrate precursor have been optimized towards maximizing the saturation magnetization value and adsorption capacity of MB. The adsorption of MB over iron oxides has been thoroughly studied for its vital sorption parameters such as pH, adsorbent amount, isotherms, adsorption capacity, kinetics, and thermodynamics. The surface area of the adsorbent synthesized at 300 °C and 2 h heating duration is determined to be 163.1 m2/g. Under the optimal pH of 10 and 1 g/L loading of the adsorbent, the adsorption maximum was found to be in the range of 34 to 38 mg/g. With the magnetic iron oxide adsorbent synthesized at 350 °C and 2 h heating duration, we have demonstrated 100 times pre-concentration of MB in a magnetically recoverable and recyclable manner.