Effect of different climate zone’s humic and fulvic acid on aggregation of UV irradiated graphene oxide

Highlights

	● GO converted to rGO with less hydrophilicity after 2 h UV irradiation.
	● UV irradiated GO aggregated more with different climate zone’s FA than HA.
	● Physicochemical properties of HA/FA had obvious effect on UV-aged GO aggregation.
	● C–C/C=C and C–O functional groups involved in GO’s stability/aggregation.

Abstract

UV light absorption by aquatic systems affect the physicochemical characteristics of graphene oxide (GO) nanoparticles which ultimately influence its aggregation behavior in water. Regarding this research, various humic and fulvic acids (HA/FA), extracted from China’s different climate zones, were treated with 2 h UV irradiated large (~500 nm) and (~200 nm) GO in 200 mmol/L NaCl. UV irradiated GO particles displayed aggregation even at low humic acid/fulvic acid (HA/FA) concentrations ranging from 0.2 to 1.0 mgC/L, whereas pristine GO particles did not exhibit such behavior. Reduction of functional groups, containing Oxygen (C=O/C–O), via UV irradiation is responsible for this aggregation phenomenon and conversion of GO to reduced graphene oxide (rGO). Consequently, rGO exhibits lower dispersibility, facilitating its agglomeration. Moreover, both small and large-sized GO particles exhibited less aggregation in HAs compared to FAs due to large molecular weight and high polarity of HAs. Aggregation of GO was more obvious with Makou FA and Maqin HA from Plateau and Mountain climate zone and Subtropical Monsoon climate zone, respectively, owing to DOM’s lower molecular weight and aromaticity that reduced their adsorption. The application of the Derjaguin-landau-verwey-overbeek (DLVO) theory did not reveal any significant interaction energy barrier between the 2 h UV irradiated GO particles even in the presence of DOM, indicating that aggregation prevailed despite the addition of DOM. These findings highlight that UV irradiation poses a significant threat to the GO stability in aquatic environments, particularly in the presence of DOM.

Graphical abstract

Keywords

Aggregation / Fulvic acid / Humic acid / Molecular weight / Reduced graphene oxide / Ultraviolet irradiation

1 Introduction

Graphene, with its unique properties, has been widely used in fields of academics, research and various industries (Qu et al., 2013; Ali et al., 2023a; Hu et al., 2024). Graphene oxide (GO), a functionalized derivative of graphene, comprises functional groups such as carboxyl (O–C=O) and carbonyl (C=O) at its edges and hydroxyl (C–OH) and epoxide (C–O–C) groups on its basal planes (Hou et al., 2015; Du et al., 2018). The high hydrophilicity of GO, enabling its easy dispersion in water is attributed to these oxygen containing functional groups (Adeleye et al., 2016; Gao et al., 2019; Wang et al., 2023). The photoactive nature of GO due to its light absorption capacity through conjugated π-bond reduces oxygen containing functional groups, destruct carbon sheets, and eventually decrease the stability of GO in aquatic systems (Du et al., 2018; Gao et al., 2019). A study by Yuan et al. (2018) observed the removal of oxygen containing functional groups from GO’s nanosheets surface upon exposure to UV light, resulted in increased hydrophobicity and enhanced aggregation of reduced GO (rGO). Generation of h⁺ and e⁻ by π–π* band due to UV irradiation may excite GO to GO^•− and generate reactive oxygen species (ROS) like superoxide (O₂^•−), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH) (Zhang et al., 2014; Hou et al., 2015; Mozaffarpour et al., 2023). Consequently, UV irradiation of GO can significantly alter its fate, mobility, toxicity, and behavior toward different pollutants in aquatic environments.

Dissolved organic matter (DOM), ubiquitous in soil and water, can exert a significant influence on aggregation of GO in aquatic environment (Shen et al., 2019). Studies have shown that GO becomes more stable by adsorbing DOM through the Lewis acid-base reaction and π–π interaction, even in presence of cationic ions like NaCl, CaCl₂, and MgCl₂ (Lanphere et al., 2014; Qi et al., 2019). Our previous study, conducted under dark conditions, also noticed that the addition of DOM significantly reduced the aggregation of GO in NaCl as well (Ali et al., 2020). The composition, structure and concentration of DOM largely depend on environmental factors like temperature, humidity, land use, and light irradiation (Mattsson et al., 2005). For example, the content of organic carbon in cold/wet conditions is found to be higher as compared to dry/hot climate (Hartley et al., 2021). The difference is attributed to reduced bacterial decomposition of DOM at low temperature in cold regions, whereas frequent wildfires that consume organic matter are more common in hot and dry climates (Hu et al., 2022). In addition, light irradiation can reduce the humification, molecular weight, and aromaticity of DOM (Minor and Stephens, 2008). Based on these observations, a hypothesis can be formulated that the impact of DOM from diverse climate zones on the aggregation of GO may vary due to its unique characteristics.

Humic and fulvic acids (HA/FA), key constituents of DOM, show diverse physicochemical properties. i.e., hydrophilicity/hydrophobicity, molecular weight, chemical composition, surface energy and surface charge (Ren et al., 2018). These characteristics may affect directly or indirectly the interaction of DOM with other pollutants, such as GO in aquatic system (Li et al., 2015). Various factors related to HA/FA characteristics including adsorption/desorption, degradation, and reactivity, significantly influence the aggregation of GO in aquatic systems. The adsorption of HA/FA can either stabilize or destabilize GO particles, depending on the nature of surface interactions and the molecular weight of HA/FA (Tang et al., 2021). Degradation of HA/FA through UV irradiation alters their stabilizing capacity, ultimately affecting the aggregation behavior of nanoparticles (Wu et al., 2021). Additionally, the reactivity of HA/FA, such as their ability to complex with metal ions or participate in redox reactions, is highly dependent on specific environmental conditions (Adegboyega et al., 2013). A recent study by Xu et al. (2022) reported that chromophores in HA and FA could reduce the photodegradation rates of nanoparticles by acting as optical light filters and ROS scavengers. Previously, our group investigated the pristine GO aggregation behavior under dark conditions using HA and FA samples from distinct climate zones in China (Ali et al., 2020). However, it remains unclear how the unique characteristics of HA and FA from different climate zones respond to the aggregation behavior of UV irradiated GO.

In addition to environmental conditions, the inherent properties and attachment efficiency of GO, several other factors such as ionic strength, pH, type of electrolytes, UV irradiation dose and wavelength, as well as GO particle size significantly influence the aggregation behavior of GO (Chowdhury et al., 2015b). Among these factors, particle size plays a crucial role in influencing GO aggregation across various ecosystems (Amaro-Gahete et al., 2019). Some researchers have noted a negative relation between aggregation rates and GO particle sizes, suggesting that smaller particles exhibit higher surface energy, which increases their instability in aqueous solutions (Waychunas et al., 2005; He et al., 2008). On the contrary, Ding et al. (2018) observed a positive relation between GO particle size and aggregation, indicating more stability due to forceful Brownian motion among smaller-sized GO particles which keep them in collision. These contrasting findings highlight the importance of GO particle size in determining how colloidal DOM interacts with GO. This study will concentrate on how the particle size of UV irradiated GO influences its interaction with HA/FA collected from sediments of different climate zones.

Some recent studies have highlighted the interactions between GO and various organic and inorganic substances, showcasing both its potential and challenges in environmental applications (Park et al., 2018; Kim et al., 2020; Gao et al., 2022). This study investigates the influence of sediment-derived DOM from different climatic conditions on the stability and aggregation of UV transformed GO with varying particle sizes. This work is continuity of our previous research (Ali et al., 2020), which explored GO aggregation behavior in presence of HA/FA form different climate zones in NaCl under dark conditions. The novel focus on the interaction between UV irradiated GO and climate-specific DOM highlights fills a critical gap in the existing literature and emphasizes the need to consider regional climatic variations in DOM composition when examining the environmental impact of GO. The influence of GO aggregation and stability on fate and transport as well as its interaction with other contaminants in natural aquatic environment is significant. Aggregated GO particles are more likely to settle and accumulate in sediments compared to dispersed GO particles, which remain more mobile and are difficult to settle. Furthermore, GO aggregates can bind with various contaminants, including heavy metals, organic pollutants, microplastics, nutrients, and pathogens, thereby influencing their mobility, bioavailability, and toxicity. These interactions can potentially change their environmental impact and bioaccumulation of both GO and associated pollutants in aquatic systems. Understanding the aggregation behavior of GO is therefore critical for assessing its environmental impacts and ensuring its safe use, protecting ecosystems from potential risks.

2 Materials and methods

2.1 Chemicals

Two different mean sizes of GO particles with approximately 200 and 500 nm were received from a Chinese company of XFNANO Materials Tech Co., Ltd., and the Institute of Coal Chemistry, Chinese Academy of Sciences, respectively. These sizes were categorized as small (200 nm) and large (500 nm) due to their frequent use in nanomaterial research and their distinctly different aggregation behavior (Szabo et al., 2020; Gacka et al., 2021). Details of all chemicals purchased and used in this study are discussed in details in S1 section of Supplementary material.

2.2 Study area and DOM’s extraction details

Eight different sites located in distinct climate zones of China were selected for sediment collection in 2017. The details of these climate zones, rivers and rationale for their selection are provided in our previous study (Ali et al., 2020). Briefly, the sediments were collected from Liaohe River, Yellow River, Yangtze River, and Pearl River which traverse the temperate monsoon climate zone, temperate continental climate zone, plateau and mountain climate zone, and subtropical monsoon climate zone, respectively. Following the sediments (0–10 cm) collection, the samples were dried in open air, crushed with mortar and pestle, separated via a 2-mm mesh, and stored for subsequent analysis. HA/FA obtained from these sediments exhibited significant difference in terms of aromaticity, functional groups, and molecular weight. These differences, along with extraction, purification, and characterization techniques, are discussed in details in our previous work (Ali et al., 2020) and S2 of Supplementary material.

2.3 GO aging by UV irradiation and characterization experiments

The effects of UV irradiation on small/large GO (10 mg/L) in DI water were studied by adjusting them 15 cm away from a UV light source (UVGL-21, UVP Co., San Gabriel, USA) for two hours. GO concentrations near certain locations around the wastewater discharge outlets can exceed 10 mg/L, making this concentration suitable for generating high-quality data and strong dynamic light scattering signals (Ko et al., 2019). The light intensity (≈ 0.4 mW/cm²) at solution surfaces was comparable to sunlight’s intensity at the Earth’s surface (ranging from 0.3 to 9.80 mW/cm²) as mentioned earlier (Song et al., 2017; Liu et al., 2019). To ensure continuous circulation of GO suspension, two peristaltic pumps were employed. One pump directed the suspension from a beaker into the cuvette of a dynamic light scattering (DLS) machine (Brookhaven Instruments Corporation, New York, USA), while the other pump returned the outflow from the cuvette back into the beaker. Tygon tubes with 1 mm diameter were employed to transport the solution between beaker and DLS instrument, maintaining a flow rate ranging from 0.9 to 1.5 mL/min. Detailed method for experimental procedure and setup is provided elsewhere (Wang et al., 2020c). Various characterization techniques were used to analyze the properties of small and large GO before and after the 2 h UV irradiation. These included X-ray photoelectron spectrometer (XPS), Field emission transmission electron microscopy (TEM), Fourier Transform infrared spectrometer (FTIR), and X-ray diffraction (XRD) techniques. Further details regarding sample preparation and the specific techniques used for analysis can be found in S3 of Supplementary material.

2.4 Aggregation experiment of UV irradiated GO with DOM

Influence of varying electrolyte and concentrations of DOM on the aggregation behavior of 2 h UV irradiated GO was analyzed. The 2 h UV irradiation was chosen to effectively reduce GO by removing oxygen-containing functional groups (C=O and C–O) (Spilarewicz-Stanek et al., 2021). Briefly, various mixed suspensions of small or large 2 h UV irradiated GO (10 mg/L) in 0.2/1.0 mgC/L of HA/FA and NaCl solutions (200 mmol/L) were prepared. The concentration of 0.2 and 1.0 mgC/L correspond to 1.7–14.5 mg/L HA or 1.0–8.5 mg/L FA, which fall within the typical DOM concentration range (0.1–20 mg/L) observed in groundwater and surface waters (Crittenden et al., 2012; Lanphere et al., 2014; Ren et al., 2018). For analysis, one mL of GO aqueous solution was collected in a glass cuvette and stirred for one second. The D_h of GO suspensions were then measured using DLS during time periods of 120 min. During the aggregation investigation, the evaluation of scattering light intensity, and the auto correlation function was acquired for 20 s.

2.5 Electron spin resonance spectrometry (ESR) for reactive oxygen species

Reactive oxygen species (ROS) generation due to UV light irradiation was analyzed in pristine GO (small and large) in presence of 200 mmol/L NaCl using electron ESR (JES-FA100, JEOL Ltd., Tokyo, Japan) as discussed earlier by (Wang et al., 2020b). For all ESR measurements, the center field was adjusted to 336.0 mT, microwave power at 4 mW, modulation width at 0.1 mT, scan width at 5 mT, amplitude at 500, sweep time for 1 min, and illumination time for 10 min. Trapping agents such as 2,2,6,6-tetramethyl-4-hydroxy-piperidinyloxy (TEMP) and 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) and 5-Tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) were applied to detect ¹O₂, and •OH, and O₂^•−, respectively. To solve the overlapping of ESR spectra for BMPO-O₂^•− and BMPO-•OH adducts, superoxide dismutase (SOD, 250 U/mL) was added to quench the O₂^•− adducts, allowing differentiation between •OH and O₂^•− generation. The details about preparation of stock solutions and different concentrations used for trapping agents were explained elsewhere (Ali et al., 2023b).

2.6 Attachment efficiency and DLVO calculation

The attachment efficiency (α) of small and large GO was calculated to determine their aggregation behavior. Moreover, forces with surface interaction between GO-GO particles was observed by Derjaguin-landau-verwey-overbeek (DLVO) theory. Our previous work described the detailed methodology for the calculation of α and DLVO (Ali et al., 2020).

3 Results and discussion

3.1 Aggregation behavior of UV transformed GO with different parameters

3.1.1 NaCl effect on UV irradiated GO aggregation

The reduction in size of small and large GO in DI water during UV irradiation is shown in Fig. S1. After UV irradiation for 2 h, the average D_h of large GO decreased to ~355 from ~500 nm, while that of small GO decreased from ~200 to ~120 nm. Decrease in size is likely due to the breakup of GO into smaller fragments upon UV irradiation (Andryushina et al., 2014; Chowdhury et al., 2015a). After 2 h UV irradiation in DI, both small and large GO were treated with different concentration (0–200 mmol/L) of NaCl to investigate the aggregation behavior and critical coagulation concentrations (CCC) (Fig.1(a) and Fig.1(b)). No significant aggregation was found even at 20 and 40 mmol/L NaCl of UV irradiated large and small GO, respectively. However, both small and large GO showed an enhancement in aggregation rates with increasing the concentration of NaCl from 80 to 200 mmol/L. The negative surface charges of both small and large GO declined from −33.14 mV in DI water to −18.11 mV in 200 mmol/L NaCl and from −41.22 mV in DI water to −20.61 mV in 200 mmol/L NaCl, respectively (Table S1 of SI). Decrease in surface charges of GO (small and large) at higher concentrations of NaCl electrolyte compressed the electrostatic double layer and neutralized the surface charge. These effects weakened the electrostatic repulsion forces between GO particles, thereby enhancing their aggregation rates (He et al., 2017). Similar increase in aggregation behavior and D_h of GO with rising NaCl concentration have been reported in previous studies (Chowdhury et al., 2013; Lanphere et al., 2013; Tang et al., 2020).

Fig.1 Aggregation behavior of 10 mg/L small (a) and large (b) GO in 0–200 mmol/L NaCl solutions after 2 h UV irradiation. CCC value (c) of 2 h UV irradiated small and large GO.

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Our previous study observed different values of CCC for large (≈ 49 mmol/L NaCl) and small (≈ 150 mmol/L NaCl) GO under dark conditions (Ali et al., 2020). On the contrary, the CCC values observed in this study for 2 h UV irradiated small and large GO increased to 190 and 120 mmol/L in NaCl solution, respectively (Fig.1(c)). It might be attributed to the fragmentation of GO particles into smaller fragments during UV irradiation, resulting in more intense Brownian motion. The enhanced motion increased collision among GO particles and ultimately improved their stability (Ding et al., 2018), thus more electrolyte (NaCl) concentration was needed to aggregate GO particles. Further increase in NaCl concentration had negligible effect on small and large GO aggregation rates, indicating that diffusion-limited regime has been attained for GO.

3.1.2 HA’s effect on aggregation of UV irradiated GO

Fig.2 depicts the effect of HA’s different concentrations (0.2 and 1.0 mgC/L) and sources on aggregation kinetics of UV irradiated GO (small and large) in 200 mmol/L NaCl solution. As shown in Fig.2(a) and 2(b), small GO reduced aggregation slightly more than large GO (Fig.2(c) and Fig.2(d)) at both 0.2 and 1.0 mgC/L concentrations of HAs. In our earlier research, we found that HA’s both concentrations (0.2 and 1.0 mgC/L) completely hindered the aggregation of pristine small GO as compared to large GO under dark conditions, indicating that steric forces between GO particles were enhanced by the full coating of HAs on smaller GO particle (Ali et al., 2020). However, in this work, 2 h UV irradiated small and large GO demonstrated apparent aggregation with HA’s both concentration of 0.2 and 1.0 mgC/L (Fig.2). A plausible explanation for this behavior is that reduction of GO to rGO via 2 h UV irradiation significantly decreased the concentration of functional groups that contained oxygen. rGO with less functional groups of oxygen decreased the hydrophilicity and steric forces between GO particles, ultimately reduced its dispersibility in water and increased aggregation (Tada and Baptista, 2015; Shams et al., 2019).

Fig.2 Effect of HAs with 0.2 (a, c) and 1.0 (b, d) mgC/L on aggregation behavior of small (a, b) and large (c, d) GO (10 mg/L) after 2 h UV light irradiation in 200 mmol/L NaCl.

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Fig.2(a) and Fig.2(c) show a minor effect with lower concentration (0.2 mgC/L) of HA on the both small and large GO’s aggregation behavior. While higher concentration (1.0 mg/L) of HAs inhibits aggregation significantly for both small and large GO (Fig.2(b) and Fig.2(d)). The honeycomb pattern of GO provides an ideal environment for the π–π interaction-mediated adsorption of aromatic organic compounds (Lee et al., 2015). The adsorption of HA at higher concentration on GO surface likely enhances steric hindrance between particles, thus inhibiting aggregation of small and large GO. Furthermore, higher negative Z-potential values (Table S1) observed for small and large GO after addition of HAs with 1.0 mg/L indicate more electrostatic repulsion between GO particles that further inhibits GO aggregation. Chowdhury et al. (2015b) also applied commercial HA and examined the aggregation of GO and rGO by providing steric stabilization, though its effectiveness is highly dependent on the HA’s characteristics and concentration.

Fig.2 shows the comparative aggregation levels of UV irradiated small and large GO with HAs sourced from different sampling sites. The results indicate that the highest degree of aggregation was observed for small and large GO with HAs derived from the Maqin site, while the lowest degree of aggregation was observed for samples obtained from the Makou site. The primary adsorption mechanism is attributed to π–π attraction forces between HAs and GO, which are believed to be enhanced by the higher molecular weight and aromatic content of DOM, such as Makou HA (Ren et al., 2018; Engel and Chefetz, 2019). The E4/E6 ratio was the lowest for Maqin HA (6.8), followed by Tangke (5.3), Panjin (5.2), and Makou (5.0) (Ali et al., 2020). Above scenario reflects a varying adsorption behavior of HAs from different climate zones on UV irradiated small and large GO, explaining the observed aggregation patterns. Furthermore, the Makou’s negative Z-potential values of small and large UV irradiated GO were higher than other HAs. Specifically, the Z-potential values for small GO were −30.94 and −33.03 mV at HA concentrations of 0.2 and 1.0 mgC/L, respectively, while for large GO, they were −29.52 and −31.44 mV at the same concentrations (Table S1). This finding supports the observed trend in aggregation, indicating increased stability of UV irradiated GO with Makou HAs when contrasted with the samples from other sampling sites.

3.1.3 FA’s effect on aggregation of UV irradiated GO

The effect of FAs with different concentrations of 0.2 and 1.0 mgC/L and sources on aggregation behavior of 2 h UV irradiated GO (small and large) in 200 mmol/L NaCl solution was analyzed (Fig.3). Significant aggregation of UV irradiated small and large GO was observed at both concentrations of 0.2 and 1.0 mgC/L for FAs (Fig.3(a)–3(d)). Following UV exposure, GO lost its hydrophilic oxygen-containing functional and developed nanopores which ultimately destruct the carbon sheets of graphene (Gao et al., 2019). This phenomenon might be responsible for conversion of GO to rGO with fewer hydrophilic functional groups, thereby decreased its dispersibility in water and increased aggregation, even after addition of FAs. A higher FA concentration of 1.0 mgC/L inhibited the aggregation of both UV irradiated small and large GO more effectively than 0.2 mgC/L. The reason might be the adsorption of comparatively higher concentration of FAs on GO surface, which provided stronger steric force between particles by inhibiting aggregation of both GO sizes. Moreover, higher negative Z-potential values (Table S1) of small and large GO after addition of 1.0 mgC/L FAs with increased electrostatic repulsion forces among GO particles further inhibited aggregation.

Fig.3 Effect of FAs with 0.2 (a, c) and 1.0 (b, d) mgC/L on aggregation behavior of small (a, b) and large (c, d) GO (10 mg/L) after 2 h UV irradiation in 200 mmol/L NaCl.

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The aggregation behavior for UV irradiated small and large GO particles was the highest in the presence of the Makou sampling site, while the lowest aggregation was observed at the Maqin sampling site. The inverse trend of GO aggregation with FAs compared to HAs might be due to the enhanced aromatic content and molecular weight of FAs. The E4/E6 content of FAs from various climate zones followed the order: Tangke FA 13.0 ≈ Makou FA 13.0 > Panjin FA 8.5 > Maqin FA 5.0 (Ali et al., 2020). The inverse relation of E4/E6 ratio with molecular weight and aromatic content suggests that Maqin and Makou sampling sites exhibited high and low adsorption capacity, respectively. Therefore, Maqin FA with higher molecular weight showed a stronger adsorption affinity toward the GO surface, which inhibited GO aggregation more effectively than other FAs. Low molecular weight FA provided less steric stabilization, while high molecular weight FA were more effective in decreasing aggregation (Xia et al., 2023). The higher negative Z-potential values (−26.13 to −28.32) of GO with Maqin FA in Table S1 further confirms higher stability of UV irradiated small and large GO with Maqin FA.

Overall, the HAs from different sources with both concentrations of 0.2 and 1.0 mgC/L exhibited relatively higher inhibition of aggregation than their counterpart FAs for 2 h UV irradiated small and large GO. The average final size of UV irradiated small and large GO, from ≈3200 nm to ≈500 nm, resulted in more than 80% decrease with addition of different HAs (Fig.2). Molecular weight and compositional changes between HA and FA from different sources were dominant factors for aggregation behavior of GO (Ali et al., 2020). In other words, structure complexity, molecular weight, polarity, and carbon chain length of HAs and FAs influenced steric forces and stability toward GO (Li et al., 2015).

3.2 Effect of ROS generation on UV irradiated GO aggregation

Carbon-based nanomaterials in aquatic system under photo irradiation can generate free radicals (Wang et al., 2020b). To analyze UV induced mechanism of large GO aggregation in the presence of NaCl, ROS photo-generation was detected by ESR technique. Interestingly, pristine GO in the dark produced higher intensity peaks of TEMPO spin adducts compared to UV irradiated GO (Fig.4(a)). The lower production of ¹O₂, after UV irradiation might be due to the rapid reduction of GO to rGO, generating less GO* (excited state) and ultimately lower production of ¹O₂ (Guardia et al., 2012). Our aggregation experiments showed that aggregation of UV irradiated GO was still obvious even after the addition of HA and FA. So, from above discussion it can be speculated that ¹O₂ is not the determining factor for the aggregation of UV irradiated GO.

Fig.4 ESR spectra of TEMP adduct with 1O2 (a), DMPO adduct with •OH (b), and BMPO adduct with O2•−/•OH with or without SOD (c) of pristine and UV irradiated GO suspension in 200 mmol/L NaCl.

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As shown in Fig.4(b), the DMPO − •OH spin adduct peaks for pristine and UV irradiated GO were compared. After the addition of DMPO as trapping agent, no typical peaks of DMPO − •OH spin adducts were found for the dark and UV irradiated GO. Some previous studies revealed that GO alone has inadequate capacity to produce DMPO − •OH, likely due to its π–π structure, which might help in quenching the free radicals (Qiu et al., 2014; Wang et al., 2020b). A single centered peak around 336.0 mT might be due to intrinsic defects in GO or the presence of oxygen vacancies. UV irradiation may induce additional oxygen vacancies by breaking oxygen bonds, introducing unpaired electrons that can be detected by ESR (Wang et al., 2020a). Even under dark condition, intrinsic defects in GO may produce oxygen vacancies similar to other metal oxides, which may also produce an ESR signal, though it may be weaker than that observed under UV irradiation (Singh et al., 2009). Since GO aggregation in the mixtures of DOM/GO was obvious form our aggregation results, it can be inferred that generation of •OH has no significant effect on the aggregation behavior of GO.

As shown in Fig.4(c), the characteristic peaks of BMPO − •OH/O₂^•− spin adducts were not detected for pristine GO with or without SOD under UV irradiation, indicating that no O₂^•− was generated by pristine and UV irradiated GO. The reason might be the rapid reduction of GO upon exposure to UV irradiation. Likewise TEMPO and DMPO spin adducts, BMPO spin adducts also showed no prominent peaks with pristine and UV irradiated GO, indicating no O₂^•− generation after UV irradiation in presence or absence of SOD. BMPO spin adducts with and without SOD under dark and UV are separately presented in Fig. S2. From ESR analysis, it can be concluded that free radicals showed no contribution in affecting the aggregation behavior of UV irradiated small and large GO.

3.3 Physicochemical changes of GO before and after UV irradiation

3.3.1 Morphology transformations

The variations in GO physical structure in dark and after 2 h UV irradiation was observed by TEM images and presented in Fig.5(a)–Fig.5(d). The UV irradiation effects on both small and large GO is obvious and comparable. Before UV irradiation, GO exhibits crumbly texture with several wrinkles on its edges and planes indicating a large number of defects with high surface-to-volume ratio (Hu et al., 2015). After 2 h of UV exposure, small fragments/aggregates appeared on both (small and large) GO sheets, indicating the decomposition and destruction of GO sheets. Under UV light, destruction in structure and morphology leads to decrease or removal of oxygen containing functional groups and reduction of GO to rGO. With less content of oxygen containing functional groups and more hydrophobicity, rGO is more prone to aggregation than pristine GO as analyzed by GO aggregation behavior in Fig.2 and Fig.3 (Shams et al., 2019; Gallegos-Pérez et al., 2020). More physicochemical transformation results of the reduction of GO by UV light are provided in Sections 3.3.2–3.3.4.

Fig.5 TEM structures showing small/large GO before (a, c) and after (b, d) 2 h UV irradiation in DI water. FTIR spectra of dark and UV irradiated small and large GO (e). XPS spectra of C1s region of GO (small/large) before (f, g) and after (h, i) 2 h UV irradiation in DI water. XRD spectra of GO (small/large) before and after 2 h UV irradiation in DI water (j).

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3.3.2 FTIR analysis of functional groups

The functional groups of both small and large GO, before and after 2 h UV irradiation, were characterized by FTIR. The FTIR spectra of small and large GO in dark and UV irradiation conditions showed an apparent variation for carbon and oxygen containing functional groups (Fig.5(e)). In the dark, small and large GO showed a broad peak at 3430 cm⁻¹ attributed to O–H stretching vibration. A peak at 1625 cm⁻¹ associated to C=C aromatic stretching, while peaks at 1640 and 1400 cm⁻¹ associated to oxygen containing carbonyl (C = O) and epoxy (C–O) groups, respectively. The observed FTIR spectra closely matched the spectra of pristine GO in various previous literature (Gao et al., 2019; Yoo et al., 2023). After 2 h UV irradiation, an obvious increase in peak of C=C at 1625 cm⁻¹ was detected, while both functional groups of C=O at 1640 cm⁻¹ and C–O at1400 cm⁻¹ contained oxygen were either disappeared or decreased significantly. The reduction or removal of oxygen containing functional groups, along with the increase in C=C groups for UV irradiated small and large GO indicate the transformation of GO into rGO, leading to GO hydrophobicity and aggregation. Overall, the FTIR data align with aggregation analysis and TEM images explained earlier.

3.3.3 XPS analysis of surface elements

As shown in Fig.5(f)–5(i), the high resolution spectra of GO for deconvolution of C 1s can be distributed into three different regions of 284.8, 286.2, and 288.1 eV, attributed to different functional groups of C–C/C=C, C–O, and C=O, respectively (Wang et al., 2020c). After 2 h of UV irradiation, a significant decrease in C=O/C–O groups and an increase in C=C groups was observed, indicating the reduction of GO to rGO. The C–O, and C=O (oxygen containing groups) in small GO decreased from 47% to 36%, while the carbon containing functional groups (C=C/C–C) increased from 53% to 64% after 2 h UV irradiation (Table S2). Similarly, intensification in carbon containing and declination in oxygen containing functional groups of large GO was noticed around 53% to 61% and 47% to 39%, respectively (Table S2). Higher loss of oxygen containing functional groups, especially C–O, might be attributed to relative low bond energy required for detachment of functional group from C atom (Mulyana et al., 2014). The higher UV light energy (3.3–4.3 eV) was sufficient for easily detachment of 3.7 eV bond energy of C–O group (Pang et al., 2019). The oxygen to carbon (O/C) ratio represented in Fig. S3, indicating the decrease in O/C during UV irradiation from 0.51 to 0.42 for small and 0.40 to 0.33 for large GO. The decrease in oxygen containing functional groups of GO confirms the conversion of GO to rGO with low dispersibility and high hydrophobicity that promoted aggregation. The XPS analysis also well align with FTIR, TEM and aggregation analysis.

3.3.4 XRD analysis of crystal structure

Fig.5(j) shows the peaks obtained in XRD diffraction of small and large GO under dark and after 2 h of UV irradiation. Under dark conditions, sharp diffraction peaks at 11.46° (small GO) and 11.06° (large GO) with the Miller index (001) clearly validates the crystalline nature of the samples with 0.70 and 0.77 nm interlayer distance, respectively. These observations align well with findings reported in a recent study by Varodi et al. (2022). After the UV irradiation of 2 h, the shifting of both peaks to 23.03° with interlayer distance of (0.50 nm) confirms the removal of oxygen containing functional groups that reduced GO to rGO (Rana et al., 2019). The rGO generated by reduction of GO with low dispersibility and high hydrophobicity favored the aggregation of GO as depicted in our aggregation experiments.

3.4 DLVO calculation

The stability of charged particles depends on surface forces, which can be computed using the DLVO theory. (Tadros, 2007; Trefalt et al., 2016). Figsure S4a and S4b predict the forces involved between two interacting 2 h UV irradiated GO particles in various NaCl concentrations (0–200 mmol/L). In DI water, the high stability is observed due to the extremely strong repulsive forces between two interacting large GO (around 200 K_BT) and two interacting small GO (around 55 K_BT) particles. The DLVO modeling results are consistent with our aggregation analysis, indicating that the stability of UV irradiated GO depends on both its particle size and electrolyte concentration. Moreover, decrease in the energy barrier between two interacting GO particles with NaCl concentration, indicating the more aggregation of 2 h UV irradiated GO in higher NaCl concentrations. At the highest NaCl concentration (200 mmol/L), the electrostatic repulsive forces might be suppressed due to attractive van der Waals forces, thereby causing higher aggregation rate for both small and large GO, as analyzed by the attachment efficiency and aggregation behavior in Fig.1.

Fig.6(a₁)–Fig.6(a₄) show that the energy barrier between two interacting UV irradiated small and large GO in the presence of DOM/NaCl is less than zero, except for Makou HA in 200 mmol/L NaCl. The obtained outcomes closely correlated with the experimental data, where UV irradiated small and large GO formed aggregates in all HAs, with comparatively less aggregation observed in Makou HA (Fig.2). Aggregation of UV irradiated small and large GO revealed a clear distinction between HAs (Fig.6(a₁)–Fig.6(a₄)) and FAs (Fig.6(b₁)–Fig.6(b₄)), with FAs showing no clear effect on the interaction energy barrier, in contrast to HAs. This indicates that no perceptible inhibition of aggregation as observed in the presence of 0.2 mgC/L and 1.0 mgC/L FA (Fig.3). Moreover, the addition of 1.0 mgC/L HA from Makou climate zone slightly enhanced the repulsive energy barriers (Fig.6(a₂) and Fig.6(a₄)), thereby inhibition of aggregation was noticed for large GO (Fig.2(b) and 2(d)). Also, higher interaction energy values (Fig.6(a₂), Fig.6(a₄), Fig.6(b₂), and Fig.6(b₄)) were noticed with higher concentration of 1.0 mgC/L than lower concentration 0.2 mgC/L of DOM, indicating the stronger inhibitory effect on the aggregation process of irradiated GO.

Fig.6 DLVO interaction energy of 2 h UV irradiated small (a1, a2, b1, b2) and large (a3, a4, b3, b4) GO in 200 mmol/L NaCl and with/without 0.2 and 1.0 mgC/L DOM addition.

Full size|PPT slide

Although experimental aggregation kinetics analysis obtained by DLS (Fig.2 and Fig.3) generally align with modeling results obtained by DLVO theory. Still, there were differences between experimental observations and prediction of model in numerous conditions. Form our aggregation experiments, it is clear that small and large GO with Makou FA showed more aggregation among all other FAs (Fig.3), but DLVO modeling showed the equal K_BT values for Maqin and Makou (Fig.6(b₂)). This difference may be justified by the fact that DLVO model assumes GO to be hard spheres, whereas they are porous aggregates and non-spherical whose surface might have ions or adsorbed HA/FA (Hua et al., 2016).

4 Conclusions

GO aggregation is closely related to its fate and transport in aquatic system, which may be influenced by environmental conditions, including UV irradiation. The study findings demonstrated that aggregation kinetics of GO in water bodies are consequences of physicochemical characteristics of DOM as well as GO itself. UV irradiation influenced the GO aggregation kinetics in water considerably by changing the physicochemical nature of GO, which altered the interaction of GO with DOM. Contrary to the complete inhibition of aggregation observed with DOM on pristine GO, the UV irradiated GO in solution of NaCl and HA/FA from different climatic zones showed obvious aggregation kinetics. UV irradiation significantly decrease the size of GO by decreasing functional groups and its structural composition. The hydrophilicity of oxygen containing functional groups (C=O and C–O) contributes key role in controlling aggregation behavior of GO in aquatic environment. We found a direct relation between DOM's molecular weight, chromophores, and structure complexity and its ability to inhibit GO aggregation to a greater extent. Size and concentration of HA/FA was also a determining factor for aggregation behavior of UV irradiated GO in aquatic environment. ROS generation analysis showed no significant role of singlet oxygen, superoxide oxygen and hydroxyl radical in the aggregation behavior of GO. The findings suggested that UV light can pose a significant threat to the stability of GO in aquatic environments.

To address the limitation regarding specific climate zones, future studies including DOM samples from a broader range of environmental conditions and climate zones would provide a more comprehensive understanding of GO aggregation. Furthermore, conducting GO aggregation experiments in natural waters containing calcium, magnesium ions, and radical quenchers such as bicarbonate and ferrous ions, would offer a more comprehensive understanding of GO aggregation dynamics in natural environment. Nonetheless, the insights gained from our study contribute to the foundational knowledge necessary for assessing the environmental impact of graphene-based materials.

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Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Nos. 42250410319, 52170024 and 21677015), the Qingdao Municipal Bureau of Human Resources and Social Security, China (No. QDBSH20220201006).

Conflict Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Electronic Supplementary Material

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