New insight into quinones triggered ferrate in-situ synthesized polynuclear Fe-hydroxyl complex for enhancing interfacial adsorption in highly efficient removal of natural organic matter

Highlights: • BQ significantly enhanced the formation of PnFe-H for flocculation by Fe(VI) activation. • PnFe-H responsible for the flocculation was derived from Fe(V)/Fe(IV) rather than Fe(VI). • Moderate BQ promoted mononuclear converted to polynuclear to improve adsorption tendency between PnFe-H-contaminant compounds. • The cooperation and antagonism mechanisms of oxidation, self-decay and flocculation caused by quinone were revealed. In this study, the effects of quinone on the formation of in-situ synthesized polynuclear Fe-hydroxide (PnFe-H) from ferrate activation and enhanced degradation of organics were investigated by in-situ UV linear differential absorbance spectra for the first time. Results indicated benzoquinone (BQ) efficiently activated ferrate for the flocculation of humic acid (HA) that the flocculation reactions rate constants in Fe(VI)-0.1 mM BQ was 3.3 times as much as the blank. Interestingly, quenching studies suggested PnFe-H derived from the high-valence iron species which were the active components by BQ activation, was proved the vital factor for removing of HA. According to the analysis of interaction energy, BQ promoted FeOH²⁺ converted to Fe(OH)₂⁺ and Fe₂(OH)₂⁴⁺ which weakened the polar property and increased hydrophobicity of compounds, further benefited for adsorption with lower Lifshitz-van del Waals (LW) and Lewis acid-base (AB) interfacial energy between PnFe-H-contaminant compounds. However, excessive BQ reduced freshly particulate Fe(III) to Fe(II), weakened the PnFe-H flocculation performance which retarded the transformation of iron species. In addition, the effects of HA concentration were also studied due to the existent of functional quinone-like moieties. The contribution of PnFe-H flocculation removal on the total removal (Re_flocculation/Re_total) improved from 2.6% to 17.09% with Fe(VI)/HA from 0.1 to 1.12. Fe(VI) sufficient oxidized electron-rich moieties and decreased the aromaticity due to π bond was broken, further cooperated with PnFe-H captured small fragment particles by sweep flocculation that Fe(VI) self-accelerating decay produced more Fe(III). The research elucidated a new insight into of ferrate activation by quinone which could expand our knowledge of activation pathway, further regulate the relationship between oxidation and flocculation for enhancing organic and colloidal particle removal in practical application.