**1. Introduction**

As a kind of two-dimensional layered nanomaterial, graphene oxide (GO) possesses good mechanical, electrical, and thermal properties, and is widely applied in various fields, including biology, medicine, chemistry, and electronic engineering [1]. The global production of GO is expected to reach 3800 metric tons in 2027 [2]. Due to the presence of a large number of oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups, GO has excellent hydrophilicity and a high probability of being present in natural aquatic environments, thus having uncertain environmental impacts and ecological risks. It has been reported that GO and its derivatives exhibited cytotoxicity to bacteria, biofilms, and algae [3,4]. Moreover, GO could cause developmental genotoxicity in aquatic animals such as zebrafish at trace concentrations [5], and could even accumulate in humans through the food chain [6]. Therefore, an increasing number of studies on the environmental behaviors of GO have received attention.

Once released into the environment, GO can interact with other pollutants mainly through π bonds, hydrophobic interactions, hydrogen bonds, and electrostatic interactions [7–10], thus affecting the transport and fate of coexisting compounds. For example, GO exhibited a high affinity for heavy-metal ions, which improved the transport ability of Pb2+ and Cd2+ in saturated porous media [11]. GO also facilitated the transport of antibiotics (levofloxacin, ciprofloxacin, and tetracycline) in saturated or unsaturated porous

**Citation:** Liu, F.-F.; Li, M.-R.; Wang, S.-C.; Zhang, Y.-X.; Liu, G.-Z.; Fan, J.-L. Phototransformation of Graphene Oxide on the Removal of Sulfamethazine in a Water Environment. *Nanomaterials* **2021**, *11*, 2134. https://doi.org/10.3390/ nano11082134

Academic Editor: Vivian Hsiu-Chuan Liao

Received: 15 July 2021 Accepted: 19 August 2021 Published: 22 August 2021

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media because of the high sorption capacity of antibiotics by GO [12,13]. Furthermore, highly hydrophilic and mobile GO could serve as a carrier and promote the transport of nano-TiO2 in porous media [14]. In addition, the interaction between GO and other pollutants would change their combined toxicity to organisms. GO enhanced Cd toxicity on photosynthesis, biomass, and cell membrane lipids in wheat seedlings [15]. GO also promoted lipotoxicity and hepatic function deficits caused by *cis*-bifenthrin exposure in tadpoles [16]. Cao et al. revealed that environmentally relevant concentrations of GO (1 mg/L) significantly increased the phytotoxicity of As (III) and As (V) in plants, which resulted in more severe oxidative stress and a significant reduction in nutrient content [17].

However, it should be noted that GO may be subjected to the phototransformation process in the environment because its special sp<sup>2</sup> domains can effectively adsorb sunlight, especially UV light [18–20]. GO was structurally degraded and chemically formed reduced GO under UV or sunlight irradiation [21]. After phototransformation, the toxicity of GO to bacteria (such as Gram-negative *Escherichia coli* and Gram-positive *Staphylococcus aureus*) and algal cells (*Chlorella pyrenoidosa*) was enhanced [22,23]. Meanwhile, GO can be regarded as a semiconductor with a zero energy gap to generate electrons, holes, and a series of reactive oxygen species (ROS) [24,25], which can mediate the transformation of the coexisting pollutants in the environment. For example, Cao et al. reported that silver nanoparticles could be formed from aqueous Ag2+ in the presence of GO under light [26]. Cu2+ on the surface of GO sheets could also trap e<sup>−</sup> generated by GO and be reduced to Cu(I) and then form Cu2O nanoparticles with the assistance of ROS, which suppressed the joint toxicity of GO and Cu2+ to freshwater algae after phototransformation [22]. In addition, GO could oxidize 42% of the adsorbed As (III) to As (V) under light irradiation, which was induced by electron-hole pairs on the surface of GO. However, coexposure to GO greatly enhanced the toxicity of As (III, V) to algae [27]. Therefore, it is of grea<sup>t</sup> significance to explore the photochemical transformation of GO on coexisting contaminants, especially when evaluating their environmental fate and possible toxicity and risks.

Antibiotics, as emerging contaminants, have gained increasing attention in recent years due to their widespread application and large production amounts [28,29]. As a result, antibiotics will inevitably find their way into the environment. Sulfamethazine (SMZ), one of the most common broad-spectrum antibiotics, is widely used in aquaculture, animal husbandry, hospitals, pharmaceutical factories, and other processes. Previous studies revealed that SMZ was frequently detected in wastewater, surface water, and even groundwater at concentrations ranging from ng L−<sup>1</sup> to μg L−<sup>1</sup> [30,31]. An increasing number of studies have focused on the environmental behaviors of SMZ, including its adsorption, migration, photooxidation, and so on [32–35].

Therefore, in this study, SMZ was selected as the model compound to reveal the effect of phototransformation of GO on coexisting contaminants. We systematically investigated the interaction between GO and SMZ under UV light, considering the influence of different environmental factors, including pH values, ionic strength and species, and natural organic matter (NOM). The phototransformation of GO together with the generation mechanisms of ROS were further explored to reveal the possible cotransformation pathways of antibiotics and GO.
