Multiple roles of dissolved organic matter released from decomposing rice straw at different times in organic pollutant photodegradation

Highlights

• Rice straw-derived DOM in short decomposition term included three main components.

• Impacts of the DOM on DIU and E2 photodegradation were opposite to that on SMX.

• Photochemically produced HO was responsible for DIU and E2 degradation.

• Promoted SMX photolysis was attributed to tryptophan-like components of the DOM.

• Photosensitization of DOM was determined by its aromaticity and molecular weight.

Abstract

Rice straw returning causes a considerable amount of dissolved organic matter (DOM) release into aquatic croplands in a relatively short-term. The presence of rice straw-derived DOM in cropland waters may alter the photochemical behaviors of organic pollutants. However, the photochemical activity and photosensitization role of the DOMs are poorly understood. Here, eight DOM samples were extracted from decomposing rice straw at different times in 49 days to explore their photosensitizing capacities toward diuron (DIU), 17β-estradiol (E2), and sulfamethoxazole (SMX). All of the DOMs were photosensitive and mainly composed of tryptophan-, tyrosine- and fulvic-like substances. Over the decomposition period, the amount of photochemically produced reactive intermediates (PPRIs) by the DOMs peaked on days 7 and 14. The evolution of the DOM photosensitizing capacity towards DIU and E2 was consistent with the variations of PPRIs, and HO· was confirmed as a critical factor. However, the influence of the DOMs on SMX photodegradation was opposite to that on DIU and E2. The positive role of the DOMs in SMX photodegradation was attributed to the tryptophan-like components. The results suggest that straw-derived DOM is an important photosensitizer and that its photosensitization towards organic pollutants is dependent on straw decomposing time and pollutant type.

Introduction

The incorporation of crop straw into farmland is a widely used method for facilitating sustainable agricultural production in many countries. Although a complete decay of straw in crop fields is rather time-consuming, the vulnerable components of straw are usually decomposed within a few days (Chen et al., 2010; Wang et al., 2019). Besides enhancing soil fertility, the returning straw can also introduce a mass of dissolved organic matter (DOM) into croplands, e.g., soil DOM can be enhanced by 37 % after seven years of biowaste amendment (Musadji et al., 2020). As a component of the mobile and reactive organic carbon pool in aquatic cropland such as paddy fields, straw-derived DOM may play critical roles in regulating the environmental behaviors of organic pollutants, e.g., pesticides, endocrine disrupting chemicals and antibiotics. These organic chemicals in farmland are usually introduced by pesticide spraying, animal manure application, drug discarding, and sewage irrigation, and have aroused worldwide focus on the food security and environmental health (Carvalho, 2017; Pan and Chu, 2017).

Photodegradation has been identified as one of the most important elimination ways in the natural aquatic environment of organic pollutants (Remucal, 2014; Zuo et al., 2013). Prior studies have already underlined that humic and fulvic acids, two types of DOM, are critical factors determining the lifetime of pharmaceutically active compounds in sunlit waters (Zhang et al., 2020b; Zhou et al., 2020). Generally, the two DOMs can act as accelerators, inhibitors, or both in inducing organic pollutant photodegradation (Adachi et al., 2018; Janssen et al., 2014; Zhou et al., 2020). The apparent effect is thus governed by the combination results of the reactive intermediate formation and elimination, light screening, and antioxidant properties. Specifically, the photochemically produced reactive intermediates (PPRIs) such as hydroxyl radical (HO·), singlet oxygen (¹O₂), superoxide anion (O₂⁻), and triplet-state excited DOM (³DOM*) are responsible for the promotion effect (Adachi et al., 2018; Apell et al., 2019; Filipe et al., 2020; Ren et al., 2019), while light screening, PPRIs quenching and antioxidant properties are the reasons for the inhibitory effect of DOM (Niu et al., 2018; Zhang et al., 2020b; Zhou et al., 2020). It is well known that dissolved humic substances (humic and fulvic acids) are very different from non-humic DOM (proteinaceous, amino, and phenolic substances) in physicochemical properties, e.g., oxygen-containing groups, antioxidant properties, and molecular size. Nevertheless, some non-humic substances such as effluent DOM, algal organic matter, and macrophyte litter-derived DOM are also confirmed to possess certain photosensitivity and to promote organic chemical transformation by generating reactive species (Li et al., 2015; Nayak et al., 2016; Niu and Croué, 2019). From this viewpoint, it is necessary to explore the influence of straw-derived non-humic DOM on the photochemical behaviors and fate of organic pollutants.

In a recent study, DOM derived from crop straw decomposed for 30 days in aquatic systems was found to inhibit algae growth by producing PPRIs under sunlight irradiation (Ma et al., 2018). However, information on the photosensitization evolution of this type of DOM is unclear. In aquatic cropland, DOM is periodically released from straw, and the DOM derived from different decomposition stages is highly heterogeneous in physicochemical and optical properties (Chen et al., 2010). At the early straw decomposition stage (usually less than 15 days), a sharp increase in dissolved organic carbon (DOC) concentration usually occurs, and the DOM produced in this stage is typically composed of carbohydrates, amino acids, and aliphatic acids (Wang et al., 2019). As decomposition proceeds, the aromatic components are enriched due to their recalcitrant biochemical degradation characteristics (Chen et al., 2010). The component and physicochemical characteristics of DOM have been demonstrated as the key factors determining its photosensitizing capacity towards organic pollutants (Berg et al., 2019; Maizel and Remucal, 2017; McKay et al., 2016; Zhang et al., 2020b). On this basis, we hypothesize that the DOMs derived from decomposing straw at different times play distinct roles in regulating the photochemical behaviors of organic pollutants in aquatic croplands such as paddy fields. However, to date, straw-derived DOMs have not been characterized the reactivity in generating PPRIs, and their influence on organic pollutant photodegradation is also unclear.

This study aims to determine the photochemical reactivity of rice straw-derived DOMs in HO·, ¹O₂, and ³DOM* formation and organic pollutant photodegradation. The DOMs were extracted at different decomposition times in a relatively short-term of 49 days, and determined the optical and component characteristics. Diuron (DIU), 17β-estradiol (E2), and sulfamethoxazole (SMX) were selected as the representatives of pesticides, endocrine disruptors, and antibiotics to explore the photosensitizing capacity of the DOM extracts. Finally, the probable mechanisms responsible for the DOMs regulating the organic pollutants photolysis were identified by PPRIs quenching assays and correlation analysis.