title: Phosphate Changes Effect of Humic Acids on TiO2Photocatalysis: From Inhibition to Mitigation of Electron-Hole Recombination.
abstract: A major challenge for photocatalytic water purification with TiO2is the strong inhibitory effect of natural organic matter (NOM), which can scavenge photogenerated holes and radicals and occlude ROS generation sites upon adsorption. This study shows that phosphate counteracts the inhibitory effect of humic acids (HA) by decreasing HA adsorption and mitigating electron-hole recombination. As a measure of the inhibitory effect of HA, the ratios of first-order reaction rate constants between photocatalytic phenol degradation in the absence versus presence of HA were calculated. This ratio was very high, up to 5.72 at 30 mg/L HA and pH 4.8 without phosphate, but was decreased to 0.76 (5 mg/L HA, pH 8.4) with 2 mM phosphate. The latter ratio indicates a surprising favorable effect of HA on TiO2photocatalysis. FTIR analyses suggest that this favorable effect is likely due to a change in the conformation of adsorbed HA, from a multiligand exchange arrangement to a complexation predominantly between COOH groups in HA and the TiO2surface in the presence of phosphate. This configuration can reduce hole consumption and facilitate electron transfer to O2by the adsorbed HA (indicated by linear sweep voltammetry), which mitigates electron-hole recombination and enhances contaminant degradation. A decrease in HA surface adsorption and hole scavenging (the predominant inhibitory mechanisms of HA) by phosphate (2 mM) was indicated by a 50% decrease in the photocatalytic degradation rate of HA and 80% decrease in the decay rate coefficient of interfacial-related photooxidation in photocurrent transients. These results, which were validated with other compounds (FFA and cimetidine), indicate that anchoring phosphate - or anions that exert similar effects on the TiO2surface - might be a feasible strategy to counteract the inhibitory effect of NOM during photocatalytic water treatment.
authors.affiliation: School of Environmental Science and Engineering, Shanghai Jiao Tong University , Shanghai 200240, China.|Civil and Environmental Engineering, Rice University , Houston, Texas 77005, United States.
authors.forename: Mingce
authors.initials: M
authors.lastname: Long
authors.affiliation: U.S. Army Engineer Research & Development Center, Vicksburg, Mississippi 39180, United States.
authors.forename: Jonathon
authors.identifier: 0000-0002-0398-8240
authors.initials: J
authors.lastname: Brame
authors.affiliation: Department of Electrical & Computer Engineering, University of Houston , Houston, Texas 77204, United States.
authors.forename: Fan
authors.initials: F
authors.lastname: Qin
authors.affiliation: Department of Electrical & Computer Engineering, University of Houston , Houston, Texas 77204, United States.
authors.forename: Jiming
authors.identifier: 0000-0002-6819-0117
authors.initials: J
authors.lastname: Bao
authors.affiliation: Civil and Environmental Engineering, Rice University , Houston, Texas 77005, United States.
authors.forename: Qilin
authors.initials: Q
authors.lastname: Li
authors.affiliation: Civil and Environmental Engineering, Rice University , Houston, Texas 77005, United States.
authors.forename: Pedro J J
authors.identifier: 0000-0002-6725-7199
authors.initials: PJ
authors.lastname: Alvarez
mesh_terms: D000327:Adsorption; D004583:Electrons; D006812:Humic Substances; D010710:Phosphates; D011995:Recombination, Genetic; D014025:Titanium / Q000737:chemistry
chemical_list: D006812:Humic Substances; D010710:Phosphates; D014025:Titanium