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.