Photocatalytic degradation of DOM in urban stormwater runoff with TiO₂ nanoparticles under UV light irradiation: EEM-PARAFAC analysis and influence of co-existing inorganic ions

Highlights

• DOM in stormwater runoff can be photocatalytically removed by UV/TiO₂ system.

• EEM-PARAFAC is firstly applied to photocatalytic DOM removal in stormwater runoff.

• Excessive Cu²⁺ leads to declined removal activity due to Cu-protein complexes.

Abstract

In situ photocatalytic degradation of dissolved organic matter (DOM) of stormwater runoff can efficiently improve the aquatic environment quality and relieve the wastewater treatment pressure. In this work, photocatalytic degradation of DOM in TiO₂ (AEROXIDE^® P-25) photocatalyst under illumination of ultraviolet (UV) light was carried out, considering the influence of various factors like TiO₂ dosage, solution pH along with the existence of co-existing ions (Cu²⁺ and H₂PO₄⁻). Generally, the variations of dissolved organic carbon (DOC), UV-based parameters and peak intensities of fluorescent constituents with UV exposure time fitted perfectly with the pseudo-first-order kinetics model. The total DOM removal efficiency was affected by diversiform factors like adsorption capacity of TiO₂, UV light utilization efficiency, reactive free radicals produced and the influence of co-existing ions. The results of fluorescence excitation-emission matrix (EEM) coupled with parallel factor analysis (PARAFAC) modeling demonstrated that all the photodegradation rates for three identified fluorescent constituents (protein-like constituent 1 and 3, humic-like constituent 2) were faster than UV-absorbing chromophores, suggesting the DOM molecules in urban stormwater runoff contained much more π*-π transition structures. In addition, H₂PO₄⁻ ions affected the photodegradation of DOM by capturing positive holes (h⁺) and hydroxyl radical (·OH), whereas Cu²⁺ ions were inclined to generate Cu-protein complexes that were more difficult to degrade than the other Cu-DOM complexes. This study supplied novel insights into the photocatalytic degradation mechanism of individual organic constituent in urban stormwater runoff and explored the influences of co-existing contaminants on their adsorption-photocatalysis processes.

Introduction

With the acceleration of urbanization, the problem of urban stormwater runoff pollution has become a hot topic discussed by researchers and engineers (Goonetilleke et al., 2005; Hong et al., 2017; Mcelmurry et al., 2014). In stormwater system, dissolved organic matter (DOM) is mainly composed of organic substances with multiple functional groups and molecular sizes (Chen et al., 2002; Santos et al., 2012; Zhao et al., 2015; Zhao et al., 2016). DOM compositions and structures are influenced by their sources and biogeochemical processes. (Mcelmurry et al., 2014; Schulten and Gleixner, 1999). As a ubiquitous and reactive fraction, DOM can forcefully interact with the co-existing contaminants and nutrient substances, thus altering their migration, transformation, bioavailability, toxicity and fate (Kieber et al., 2005; Kieber et al., 2004). Furthermore, considering that current methods to protect aquatic environments are massively inoperative and even guided by the “end-of-pipe” operations, the inefficient DOM treatment by former stormwater facilities will significantly increase the difficulty of sewage treatment plants. Previous researches had proved that DOM could lead to the formation of disinfection by products (DBP) and membrane fouling (Zhang et al., 2008).

Among various treatment technologies, heterogeneous photocatalysis employing UV light and TiO₂ catalyst has proved its high-efficiency in photodegrading a large number of ambiguous recalcitrant organic substances into unsteadily biodegradable substances. Moreover, under the appropriate conditions, it is possible to ultimately mineralize organic molecules to form pollution-free CO₂ and H₂O (Cai et al., 2018; Li et al., 2018). The dominating reaction mechanism of photocatalysis degradation is based on the generation of clean and environmentally friendly free radicals with strong oxidizing ability (Wang et al., 2017; Zou et al., 2015). As well, the strong stability (including chemical-, thermal- and photo-stability), the undemanding experimental conditions (ambient temperature and pressure), the efficient recyclability and low operating costs promote wide application of UV/TiO₂ technique in wastewater treatment (Wang et al., 2016; Wang et al., 2014).

In former studies of photocatalytic DOM degradation, dissolved organic carbon (DOC) concentrations and some parameters fitted from UV–visible spectral data were mainly applied as indicators for estimating the total removal efficiency (Rajca and Bodzek, 2013; Zanardi-Lamardo et al., 2004). However, these indexes are insufficient to fully clarify the behaviors of the extremely nonhomogeneous DOM. As a credible and hypersensitized optical instrument, fluorescence spectroscopy can be used to efficiently identify the different types of organic compounds (Baker et al., 2004; Coble, 1996; Her et al., 2003). Furthermore, the photocatalytic DOM decomposition performances have been explored applying fluorescence-based instruments in a range of aquatic environments (Kavurmaci and Bekbolet, 2014; Patel-Sorrentino et al., 2004; Zanardi-Lamardo et al., 2004). However, the previous studies assessed the photodegradation tendency of DOM simply via the analysis of changes in intrinsic fluorescence intensity or peak position, neglecting the possible error caused by the spectral overlaps from a complicated mixture of various fluorescent compounds.

As the most popular stoichiometry technique, parallel factor analysis (PARAFAC) can deconvolute the complicated EEMs spectra into independent fluorescent organic substances that represent different types of fluorophores with similar physicochemical properties and structures (Ishii and Boyer, 2012; Stedmon and Bro, 2008). The EEM-PARAFAC technique has been widely utilized to monitor the activities of various DOM substances in water environments (Hudson et al., 2010; Seredyn'Skasobecka et al., 2011; Stedmon et al., 2011; Yang et al., 2015). The peak locations and intensities of the separate PARAFAC constituent reflect the water quality and treatment performance. Therefore, the photocatalytic degradation behaviors of different constituents can be effectively identified by utilizing EEM-PARAFAC technique, and the overlaps between various fluorescents will be effectively avoided. It is noteworthy that Phong and Hur (2015) explored the photodegradation behaviors of different fluorescents by EEM-PARAFAC, in which the litter-derived DOM, Elliott soil humic acid and Pony lake fulvic acid were selected as DOM references. But up to now, few researchers applied EEM-PARAFAC technique to investigate the adsorption-photocatalytic processes and mechanism of urban stormwater runoff DOM in UV/TiO₂ system.

More importantly, dissolved inorganic ions are quite ubiquitous in DOM-containing urban stormwater runoff (Brown and Peake, 2006; Gammons et al., 2005; Zhao et al., 2017). They may appreciably affect the photocatalytic reactions for DOM disposal. Therefore, it is important to explore the influences of different inorganic species on photocatalytic DOM decomposition in UV/TiO₂ system, which will be beneficial to clarify the photocatalytic degradation mechanism of DOM in real stormwater runoff samples. This study will aim (1) to demonstrate the practicability of the environmentally friendly and cost effective UV/TiO₂ system for the elimination of DOM in urban stormwater runoff, (2) to compare the variations in DOC concentrations, UV parameters and fluorescents identified by EEM-PARAFAC of urban stormwater runoff DOM in UV/TiO₂ system under different experimental conditions, including TiO₂ doses and pH values, and (3) to examine the effects of co-existing inorganic ions (Cu²⁺ and H₂PO₄⁻) that are common in stormwater runoff on DOM degradation with TiO₂ as catalyst under UV light illumination.