Formation of Fe nanoparticles on water-washed coal fly ash for enhanced reduction of p-nitrophenol

The catalytic reduction of p-nitrophenol (p-NP) by coal fly ash (FA) washed with water was investigated in this study. A significant increase in pH (from 7.0 to 10.1) was observed in the suspension of raw fly ash (RFA), while water-washed fly ash (WFA) showed a relatively lower increase in pH (7.2), which was caused by the dissolution of Ca species during the water-washing process. Almost 33.4% of p-NP reduction was observed in the RFA suspension with NaBH₄ in 1 h, while the enhanced reduction of p-NP (87.2%) was observed in the WFA suspension. The catalytic reduction of p-NP was inhibited by addition of CaO and Ca(OH)₂, indicating that higher amount of CaO dissolved from RFA resulted in the inhibition effect. Similar experiments using different oxides (i.e., Al₂O₃, SiO₂, CaO and MgO) revealed no significant reduction of p-NP, which was comparable with Fe₂O₃ (75.8%). Results from various surface analyses revealed that iron oxides on the surface of WFA can be reduced to elemental Fe nanoparticles, which can effectively reduce p-NP with NaBH₄. No significant leaching of heavy metals such as Cr, Pb, and As was observed during the catalytic reduction of p-NP and in the suspension of WFA after reaction at pH 3, 5, 7, and 9 for 24 h, which can solve the toxic effect when the FA is used for environmental applications. We also observed a good reusability of WFA during the recycling test, indicating the potential use of WFA for the treatment of wastewater containing reductively degradable pollutants.