Understanding the positive effects of dissolved Fe2+ and limited aeration on selenate removal from water by nano zero-valent iron supported on biochar

Batch test was conducted to investigate Se(VI) removal by nano zero-valent iron supported on biochar (BC@nZVI) prepared by a one-step carbothermal synthesis in the presence of ferrous ions and limited aeration. The presence of ZVI and biochar was confirmed in the synthesised material by X-ray diffraction and Raman spectroscopy. TEM analysis revealed that nZVI with a diameter of about 50 nm was dispersed well on the biochar. The Effects of BC@nZVI dosage, initial solution pH, Fe2+ concentration, limited aeration, and coexisting anions on Se(Ⅵ) removal were investigated. The role of Fe2+ on Se(VI) removal have been examined by sequential Fe2+ and Se(VI) dosing tests. More than 98% of Se(VI) (10 mg/L) was removed with BC@nZVI dosage of 1.0 g/L at pH 4.0. The removal percentage of Se(VI) decreased with an increasing in pH from 4.0 to 9.0. The strong electrostatic attraction between Se(VI) and nZVI and the high nZVI activity at low pH are responsible for the significant selenium removal. The removal efficiency of Se(VI) increased with increasing Fe2+ concentration from 0 to 0.5 mmol/L. Fe2+ promotes a shift in the passive layer on nZVI surface towards a medium that favours electron transfer. Se(VI) reduction by ZVI is thought to be accelerated under limited aeration. The positive effect of O2 on selenate reduction can be attributed either to mechanical removal of the oxide layer on the ZVI particles, or to a difference in electron transfer rates, or to a reactive Fe(II)-bearing solid formed when ZVI was oxidised by O2. Cl−, NO3−, SO42− and HA can inhibit the removal of Se(VI) on BC@nZVI in different degrees. The results of XRD, Raman, and XPS characterisation showed that the removal mechanism of Se(VI) include the adsorption and reduction ed of Se(VI) on BC@nZVI. Fe2+ and limited aeration would play a critical role in Se(VI) removal by BC@nZVI.