Introduction

Arsenic pollution in surface water and groundwater is a worldwide problem originated by dissolution of arsenic from soil, mainly due to anthropogenic activities. Due to the possibility to form inner-sphere complexes, the high surface-to-volume ratio and, therefore, the high density of active sites (-OH groups), nanopowders of iron oxides and oxyhydroxides show a high affinity for arsenate and arsenite species in wide pH ranges and pollutant concentrations.

Methods

Akaganeite (β-FeOOH), ferrihydrite (Fe₅HO₈·4H₂O), and feroxyhyte (δ-FeOOH) were synthesized by simple precipitation methods and tested for the removal of arsenic from water by changing different parameters (initial concentration, contact time, ionic strength, presence of competing ions and solid to liquid ratio). The same experimental conditions for the batch tests allowed a direct comparison between the adsorbents. All sorbents were characterized by XRD, TEM and HRTEM, N₂-physisorption, ELS, TGA,ATR-FTIR.

Results

Thanks to its high and positive surface charge, akaganeite is found to be the most promising sorbent in the whole pH range for As(V) (adsorption capacity equal to 99% for C_As = 100 mg/L); it also shows good adsorption capacity for As(III) (81% and 83% for pH 3 and pH 8 respectively at 100 mg/L,). The best sorbent for As(III) is, however, ferrihydrite (98% at 100 mg/L for both pH 3 and pH 8), due to its high surface area. Preliminary results on feroxyhyte prove its suitability as a sorbent for As(V) (92% at 100 mg/L for pH 3).

Conclusions

Akaganeite is the most promising sorbent in the whole pH range for As(V), while ferrihydrite is the best sorbent for As(III). Results on feroxyhyte proved its suitability as a sorbent for As(V) as a promising alternative to akageneite, due to its straightforward and quick synthesis process.