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Titanium dioxide (TiO 2 ) nanoparticles have been widely studied for water treatment applications; however, natural organic matter (NOM) is often reported to hamper the efficiency of the nanoparticles toward the degradation of target pollutants. Phosphate treatment has been proposed as a potentially facile solution to this problem, as phosphate competes for TiO 2 surface sites to diminish the NOM adsorption. However, the potential importance of the conditions of the NOM exposure and the residual NOM remaining after phosphate treatment have not been fully explored. Here, we investigate the reactivity of phosphate-treated TiO 2 nanoparticles with NOM coatings adsorbed from two background water chemistries, deionized water (TiO 2 –NOM DIW ) and moderately hard water (TiO 2 –NOM MHW ). Thorough characterization by size exclusion chromatography revealed that the adsorbed NOM was only partially displaced after phosphate treatment, with a higher adsorbed mass and wider variety of NOM species persisting on TiO 2 –NOM MHW compared to TiO 2 –NOM DIW . Although the remaining adsorbed NOM did not significantly influence the degradation rate of phenol as a model pollutant, remarkably distinct effects were observed in the degradation of catechol as an oxidative byproduct of phenol, with TiO 2 –NOM MHW hindering catechol degradation and TiO 2 –NOM DIW accelerating catechol degradation. The suppressed reactivity for TiO 2 –NOM MHW was attributed to hindrance of the physical adsorption of catechol to the TiO 2 surface by the NOM MHW layer as well as changes in the reactive oxygen species profile as measured by electron paramagnetic resonance (EPR) spectroscopy, whereas the enhanced reactivity for TiO 2 –NOM DIW was attributed to higher hole formation, suggesting participation of the NOM DIW layer in electron transfer processes. This research highlights the critical importance of the NOM surface coating in directing the mechanisms for pollutant degradation in photocatalytic nano-enabled water treatment applications. 

Use of visible light photocatalytic nanomaterials in water treatment can be promising in treating contaminants. However, little research has been conducted examining the effects of more complex chemistries in the nanomaterial's performance. In this work, the effects of inorganic salts (NaCl and CaCl 2 ) and natural organic matter (NOM) such as humic acid (HA) and extracellular polymeric substances (EPS) on nanoparticle aggregation, dissolution, and ultimately on the photocatalytic properties of molybdenum trioxide (MoO 3 ), i.e. nanorods, nanowires, and nanoplates were examined. In the presence of NaCl, nanorod, nanowire, and nanoplate MoO 3 had similar critical coagulation concentrations, while the nanorods showed higher instability in CaCl 2 . Overall, the presence of inorganic salts caused high colloidal instability in the MoO 3 nanostructures in terms of aggregation behavior, but greatly aided in the reduction of MoO 3 dissolution. NOM presence decreased aggregation rates, albeit dissolution was not similarly affected in all three structures. Only the dissolution of the nanowire structures was reduced in the presence of HA or EPS. Furthermore, the photocatalytic activity of the nanowires and nanoplates was overall reduced when inorganic salts or natural organic matter were present. While the presence of natural organic matter alone did reduce photocatalytic effectiveness of the nanorod MoO 3 , the presence of salts seemed to negate the effects from the organic compounds. Furthermore, the presence of CaCl 2 resulted in a highly enhanced photocatalytic activity regardless of the presence of natural organic matter. The structural and chemical differences of the nanomaterials played a significant role in their aggregation, dissolution, and ability to photocatalytically degrade methylene blue in solution. This study demonstrates that a better understanding of water chemistry effects on nanomaterials is essential prior to their intended applications.