Superhydrophilic and Underwater Superoleophobic Cotton Fabric for Oil–Water Separation and Removal of Heavy-Metal Ion

Superhydrophilic and underwater superoleophobic cotton fabric (named CS-CF-PDA, or m-CF) was prepared by modifying the cotton fabric (CF) with dopamine (DA) and chitosan (CS). The oil–water separation and heavy-metal ion (e.g., Cu(II)) adsorption performances of m-CF were investigated, and m-CF was characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FT-IR), and thermogravimetric analysis (TGA). The results showed that the underwater oil contact angle (UWOCA) of m-CF was more than 156°. The m-CF was used to treat artificial oily wastewater containing Cu(II) under room temperature and atmospheric pressure and gravity, by which the separation efficiency, water flux, and Cu(II) removal rate could reach 99%, 17 400 L·m–2·h–1, and 89%, respectively. Additionally, in the process of continuous treatment of oily wastewater, the water flux slightly decreased; on the contrary, the Cu(II) removal rate decreased significantly to 67% within 120 s. Cu(II) was one of the reasons for the decrease of water flux. The m-CF of adsorbed Cu(II) could be leached with HCl (0.1 mol·L–1) solution, and the Cu(II) desorption rate could reach over 95% within 120 s. After strong acid, strong alkali, high salt, and abrasion treatment, the UWOCAs of m-CF were still higher than 150°. In a word, in terms of oil–water separation, m-CF exhibited good acid, alkali, salt, and abrasion resistances. Also, it is an underwater superoleophobic material involving simple preparation, low cost, and environmental friendliness, which could remove the floating oil and heavy-metal ions from wastewater and has good industrial application prospects.

INTRODUCTION
With the rapid development of the petroleum and chemical industries, the discharge of oily wastewater and the risk of oil spill accidents increased; 1−3 it was the goal of people's continuous efforts to develop efficient and environmentally friendly methods for treating oily wastewater. So far, many technologies have been reported for the disposal of oily wastewater, including flotation, centrifuges, oil skimmers, membrane separation, etc. 4 The membrane separation method based on a superwetting surface has been attracting extensive attention in the field of oily wastewater treatment because of its simple operation and high separation efficiency. 5−8 Notably, oily wastewater contains other pollutants, such as organic dyes and heavy-metal ions, among which heavy-metal ions are one of the most common and harmful pollutants in water pollution. 9,10 Adsorption is one of the most commonly used methods to remove heavy metals because of its simple operation, easy recycling, and low cost. 9,11−13 A superhydrophilic membrane with functional groups for heavymetal ion adsorption could simultaneously remove oil and heavy-metal ions from wastewater, which simplifies the purification process of oily wastewater containing heavymetal ions and reduces the cost of wastewater treatment. 14−20 Wang et al. 14 prepared an organic-inorganic composite membrane by the electrospinning technique modified with 3-[(trimethoxysilyl) propyl]-diethylenetriamine to obtain a superhydrophilic and underwater superhydrophobic membrane. The membrane was used to treat artificial emulsified oily wastewater with the desired concentrations of heavy-metal ions Pb(II), Cr(III), and Ni(II); the water flux could reach 1517 ± 53 L·m −2 ·h −1 , and the removal efficiencies of both oil and heavy-metal ions were larger than 99%.
Cotton fabric (CF), which is abundant in nature, easily biodegradable, and environmentally friendly, is widely used as a substrate for the preparation of oil−water separation membranes. 3,8,21 Chitosan (CS) is a natural polymeric alkaline polysaccharide with abundant sources, degradable, and ecofriendly; it contains a large number of functional groups (such as −NH 2 and −OH, etc.) that can interact with heavy-metal ions, 9,22,23 resulting in its being widely used to treat heavy-metal wastewater by adsorption. 8,11,24 Therefore, environmentally friendly membrane materials for oil−water separation and heavy-metal ion removal can be obtained by using the CSmodified CF. 25 Krishnamoorthi et al. 25 prepared a superhydrophilic CA-CS-CF membrane by using laccase to initiate the oxidation of catechol groups in caffeic acid (CA) to generate reactive quinoids, which were subsequently crosslinked with the amino groups of CS to coat CS on CF. They found that the membrane after multilayer compaction was used to treat artificial emulsified oily wastewater containing heavy-metal ions (Cu(II), Pb(II), and Hg(II)), which yielded good water flux, oil−water separation efficiency, and heavymetal removal rate.
In general, there are a few reports on superhydrophilic and underwater superhydrophobic cotton fabric materials for simultaneous removal of heavy-metal ions and oils from wastewater. 25 Dopamine is an established mussel-inspired molecule that forms a hydrophilic surface when selfpolymerized to form polydopamine (PDA) on various substrates. 25,26 CS can be coated on the substrate by crosslinking the amino group on it with the quinone structure in the polydopamine molecule. 25,27−29 Therefore, we propose to prepare a modified CF by coating the CS on CF with PDA to treat oily wastewater to study its performances of oil−water separation and removal of heavy-metal ions, which can provide technical support for the industrial application of environmentally friendly and low-cost cotton fabric superwetting membranes.

EXPERIMENTAL SECTION
2.1. Materials. Chitosan (CS, deacetylation degree ≥ 95%) was purchased from Aladdin industrial Corporation. Dopamine hydrochloride (DA, 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Tri-(hydroxymethyl) aminomethane hydrochloride (Tris-HCl, AR) was purchased from Chengdu Aikeda Chemical Reagent Co., Ltd. All of the reagents (hexane, toluene, glacial acetic acid, absolute ethyl alcohol, HCl, NaOH, NaCl, Sudan red III, CuCl 2 ·2H 2 O (AR)) were used without further purification. Deionized water was self-made through a laboratory water purifier. The fabric used was commercially available cotton fabric (CF, thickness 0.4 mm). Figure 1 shows the preparation process of the modified CF (named m-CF). The CF was ultrasonically cleaned with ethanol for 20 min and deionized water three times, and then dried in the oven at 60°C before use. Preparation of modified CF was done according to the literature: 30 4 mg·mL −1 dopamine hydrochloride (DA) was dissolved in 100 mL of Tris-HCl buffered aqueous solution (pH 8.5); the cleaned CF was put into the prepared DA buffer solution. Oxidative polymerization of DA generated polydopamine (PDA) after 24 h of reaction with constant shaking and at room temperature, which was deposited on the surface of CF. The obtained hydrophilic modified CF (named CF-PDA) was rinsed several times with deionized water, then dried in vacuum at 60°C for 6 h. 2 wt % of chitosan (CS) solution was prepared by dissolving CS in 1% glacial acetic acid solution under magnetic stirring condition. The CF-PDA was immersed in CS solution for 10 h, then washed with 0.1% NaOH and a large amount of deionized water, and then dried in vacuum at 60°C for 6 h to prepare the m-CF.

Preparation of m-CF.
2.3. Characterization Analysis. The surface morphologies and element mappings of the samples were observed by field emission scanning electron microscopy (FE-SEM, Tescan Mira 3XH) at an accelerating voltage of 3 kV and by energydispersive spectroscopy (EDS, Aztec X-MaxN80), respectively. Thermogravimetric analysis (TGA) was performed under air atmosphere with 10°C·min −1 heating rate from 25 to 600°C, and the weight of the samples was about 8−10 mg. Fourier transform infrared (FT-IR) spectrum was recorded to investigate the modified effect and chemical compatibility between pristine CF and modified CF, and the testing wavenumber range was 4000−400 cm −1 . Determination of oil in water was done with an infrared spectrophotometry oil measuring instrument (JLBG-126). The wettability of the samples was characterized by a contact angle measuring instrument. The infiltration in air and underwater oil contact angle were measured with 10 μL of water and n-hexane, respectively. The time required for the water droplet (10 μL) from just touching the sample surface to spread (i.e., the water contact angle was 0°) was named the water drop infiltration time (WIT).

Simultaneous Removal of Oil and Cu(II) by m-CF.
The separation experiment of 100 mL of artificial oily wastewater containing oil and Cu(II) was carried out using m-CF (effective area was 9.6 cm 2 ) under gravity and prewetted by water prior to use. Toluene (dyed by Sudan Red III) was used as the oil phase (30 mL); deionized water and CuCl 2 were used to prepare the water solution, wherein the concentration of Cu(II) was 5 mg·L −1 (70 mL, pH 5.6). For clearly, the artificial oily wastewater was denoted as "oil− water", and the Cu(II) containing artificial oily wastewater was denoted as "Cu(II)−oil−water". The concentration of Cu(II) was determined via an inductively coupled plasma atomic emission spectrophotometer (ICP-AES). The oil−water separation efficiency and water flux are calculated using eqs (1) and (2), respectively. The Cu(II) removal rate and adsorption capacity are calculated using eqs (3) and (4), respectively.
where E and F are the oil−water separation efficiency (%) and water flux (L·m −2 ·h −1 ), respectively. R and Q are the removal rate (%) and adsorption capacity of Cu(II) (mg/g), respectively. M 0 and M 1 represent the weight of water or water solution in the original mixture and the collected water after separation (g), respectively. S is the effective area of m-CF (m 2 ), V 1 is the volume of water (L) passing through the m-CF, and t is the permeate time (h). C 0 and C are the initial and final concentrations of copper ions in water (mg/L), respectively; V is the volume (L) and M is the weight of the m-CF (g).
2.6. Abrasion Treatment of m-CF. The mechanical stability and adsorption capacity of m-CF were evaluated by rubbing on a sandpaper, 32 in which 500# sandpaper served as an abrasion surface, m-CF was placed and moved on the sandpaper under a 200 g weight, and m-CF was dragged back and forth (40 cm as a cycle) in one direction with a speed of 4 cm·s −1 , and then the wetting behavior was tested after 300 cycles of sandpaper abrasion. The sample was named abrasionm-CF after abrasion treatment.

RESULTS AND DISCUSSION
3.1. Characterizations. The surface morphologies of the samples were investigated by SEM and EDS. According to Figure 2A,B, the surface of CF was smooth. 30 PDA particles with different sizes and irregular shapes were distributed on the surface of CF-PDA, which contained 4.9 wt % of N element. Compared with CF-PDA, the particle size on the surface of m-CF was more unifrom, and the content of N element was slightly reduced (4.2 wt %). DA was oxidized and polymerized on the surface of CF to form PDA particles, and long-chain CS were "wound" around the PDA particles when grafted on the CF-PDA surface, which made the depth of the "gully" formed between PDA particles shallower. 30 The chemical formulas of DA and CS were C 8 H 11 NO 2 and (C 6 H 11 NO 4 ) n , respectively. The content of N in CS was 8.7% by weight, which was slightly less than that of DA (9.2% by weight). Therefore, the N content of CS grafted on CF-PDA was slightly low.
The DTG curves (differential form of TGA) are shown in Figure 2D. The weight loss peak of CF ( Figure 2D(a)) at 346°C can be attributed to the decomposition of the cellulose chain of the cotton fabric. 24,33,34 The weight loss peak of CF-PDA ( Figure 2D(b)) appeared at 325 and 442°C, respectively. Besides, the weight loss was lower than that of CF. The weight loss peak at 442°C could be attributed to PDA, which may have degraded some substances on the fiber under alkaline environment (pH 8.5), leading to the easy oxidation and decomposition of the fiber and decrease of the weight loss at low temperature. The weight loss peaks of m-CF ( Figure 2D(c)) appeared at 325 and 465°C, respectively. The peak value of high-temperature weight loss increased and its shape widened compared with CF-PDA. The high-temperature peak could be attributed to the decomposition of CS and PDA, and the grafting of CS may also have changed the thermal decomposition performance of PDA, resulting in the increase of the width and peak value of the high-temperature weight loss peak of m-CF.
The above analyses showed that both PDA and CS were successfully coated on CF and the distribution of C, O, and N elements on m-CF was consistent ( Figure 2C).

Wettability of the Sample.
For the porous material that is hydrophilic and rich in capillaries, the capillary pressure is one of the main driving forces of water penetration into it. According to the Young−Laplace equation (eq (5)), the smaller the contact angle of water in the inner wall of the capillary (i.e., the stronger the hydrophilicity), the greater the capillary pressure.
where Δp is the pressure difference between the upper and lower liquid surfaces of the capillary, θ WA is the water contact angle of the water on the capillary wall, R is the capillary radius, and γ WA is the surface tension of water. The water drop infiltration time (WIT) on porous materials can be calculated by Washburn equation (as shown in eq (6)). Equation (6) shows that the longer the WIT on the porous material, the larger the contact angle of water on the capillary wall, and the weaker the hydrophilicity of the material.
where t is the infiltration time of the water droplet on porous materials, l is the capillary length, and η is the viscosity of water.
Obviously, the longer the WIT on the porous material, the lesser the capillary force that pushes the water through the membrane.
According to the Wenzel equation (as shown in eq (7)), the surface roughness reduces the water contact angle and enhances the hydrophilicity of the material.
where θ WA ′ is the water contact angle of the rough surface, and r is the surface roughness factor (i.e., the roughness factor is the ratio of actual surface area to the apparent surface area; r is always larger than 1). Figure 3 shows the experimental results of wettability of the samples. According to Figure 3, the WIT values of CF, CF-PDA, and m-CF were 855, 354, and 117 s, respectively, and the underwater oil contact angles (UWOCA) were 139.5, 147.3, and 156.1°, respectively. CF was mainly composed of cellulose, which was rich in hydrophilic groups (−OH). The surface or capillary wall of CF was hydrophilic, and the water contact angle was less than 90°(θ WA < 90°). According to the Young− Laplace equation (eq (5)), the capillary force generated on the CF can absorb water into the capillary. DA had polar groups (such as −OH and −NH 2 ) as well as oxidative polymerization on the surface of CF to form PDA particles, making the surface of the material rough ( Figure 2B(b)). Compared with CF, therefore, the hydrophilicity of the CF-PDA surface or capillary inner wall increased and the WIT of the material decreased; at the same time, the UWOCA increased. We knew that the content of non-hydrocarbon elements (O and N) in CS ((C 6 H 11 NO 4 ) n ) was higher than DA (C 8 H 11 NO 2 ), which enhanced the hydrophilicity of m-CF, while UWOCA increased and showed the superoleophobic state (θ OWS > 150°). Figure 4A shows the experimental phenomenon of oil−water separation. It can be seen from the figure that both oil (red) and water can pass through CF; in contrast, for CF-PDA and m-CF, water can completely permeate through, while oil cannot. 30 Among them, it took a shorter time (only 15 s) for water to pass thoroughly through m-CF. Figure 4B shows the photographs of the critical heights of the oil column of CF, CF-PDA, and m-CF, which were 2.2, 9.0, and 11.5 cm, respectively. Besides, the loading of PDA and CS on CF (i.e., m-CF), compared with unmodified CF, brought about a decrease in R and an increase in θ OWS , resulting in the increase in the H OC (see eq 8). Therefore, m-CF had a higher oil repellent ability than CF-PDA.

Performance of m-CF Experiments.
When using a hydrophilic-oleophobic material for oil−water separation, the combined action of three forces determined whether the oil droplets could permeate through the filter membrane (as shown in Figure 5): (1) The porous material surface is superoleophobic under water; the oil droplets on the surface of the modified CF are subjected to an upward capillary force.
(2) If the density of oil is lighter than that of water, it will be subjected to upward net buoyancy. (3) Neglecting the net buoyancy, the oil droplets will permeate through the filter membrane when the capillary force is less than the static pressure of the liquid. According to Young−Laplace equation and hydrostatics equation, we can know the critical liquid height when the oil droplet can pass through the filter membrane (as shown in eqs (8) and (9)); when the actual  liquid height is larger than the critical liquid height, the oil droplet will penetrate the filter membrane. where H OC is the critical height of oil, H WC is the critical height of water, ρ O and ρ W are the densities of oil and water, respectively, γ ow is the oil−water interfacial tension, and θ OWS is the contact angle of the underwater oil on the solid surface (i.e., UWOCA). Commonly if ρ O < ρ W , then H OC > H WC . Therefore, when the total heights of liquid and oil in the oil−water separation equipment are less than H WC , it can ensure that the oil droplets do not pass through the filter membrane and successfully separate oil from water. Obviously, the larger the θ OWS , the higher the H WC , or the smaller the particle size of the oil droplets that can just pass through the filter membrane under certain conditions, the higher the oil−water separation efficiency.
The loading of PDA and CS on CF (i.e., m-CF), compared with unmodified CF, led to a decrease in R and an increase in θ OWS , resulting in the increase in H WC . Therefore, when the oil−water separation conditions are the same, the m-CF material can carry out oil−water separation successfully while CF cannot. These results are consistent with those in Figure 4. Figure 6 shows the continuous treatment of the solution containing oil and Cu(II) wastewater (i.e., Cu(II)−oil−water), which was divided ten times to investigate the performances of m-CF. As shown in Figure 6A,B, respectively, the E value of m-CF was larger than 99%, which was almost unchanged with the increasing time of the experiments, and the F values of the first time and the tenth time were about 17 400 and 15 400 L·m −2 · h −1 , respectively, which was decreased by 11%. The R value is shown in Figure 6C, which was 89% in the first experiment and decreased by 57% in the tenth time. Meanwhile, in a similar study by Krishnamoorthi et al., 25 the water flux, oil−water separation efficiency, and heavy-metal ion removal rate were 50 050 L·m −2 ·h −1 ·bar −1 , 99, and 99%, respectively. Compared with our study, the reason for the higher water flux and heavymetal ion removal rate of the membrane may be that it was allowed under pressure and was treated by multilayer compaction. Figure 6D shows that the Cu(II) cumulative adsorption capacity on the membrane after ten times of continuous treatment of wastewater by m-CF (named used-I (10) -m-CF) was about 11.0 mg·g −1 . The above results indicated that (1) the m-CF had good oil−water separation efficiency and water flux, as well as removed trace amounts of Cu(II) from water solution.
(2) With the increasing times of the experiments, the cumulative adsorption capacity of Cu(II) on m-CF increased, while the strength of the adsorption site or non-adsorption site decreased, resulting in the reduction in the removal rate of Cu(II). 35,36 (3) With the increasing times of the experiments, the water flux decreased gradually, while the oil−water separation efficiency remained almost unchanged. What was the reason for the decrease of water flux of the m-CF? Therefore, the used-I (10) -m-CF was analyzed by SEM, EDS, FT-IR, and DTG, as shown in Figure 7. As shown in Figure 7A,C, the WIT of used-I (10) -m-CF was 185 s and UWOCA was 155.3°. Compared with m-CF, the surface roughness of used-I (10) -m-CF decreased slightly and the Cu element appeared with a content of 1.5 wt %, which was close to the result in Figure 6D. The oil was slightly soluble in water (under the conditions of room temperature and atmospheric pressure, the solubility of toluene in water is about 0.5 g·L −1 ). It may be that when the aqueous phase containing trace amounts of oil permeated m-CF, the oil was absorbed and filled in the "gully" on the surface of the membrane, resulting in the slightly decreased roughness of used-I (10) -m-CF. Figure 7D shows that the two thermal degradation peaks of used-I (10) -m-CF were higher than those of m-CF, which were 334 and 491°C, respectively. It was possible that the adsorption of Cu(II) increased the difficulty of pyrolysis of used-I (10) -m-CF.
As shown in Figure 7E, the absorption peak of used-I (10) -m-CF at 2995−2800 cm −1 (methyl and methylene) was slightly stronger than that of m-CF. The methyl or methylene content per unit mass of toluene was higher than that of cellulose, DA, or CS. It may be that used-I (10) -m-CF adsorbed trace amounts of toluene and slightly enhanced its adsorption peak at 2995− 2800 cm −1 .
CF and modified CF are porous materials rich in capillaries, 37 where the inner surface area is much greater than the outer surface area. Pollutants of toluene and Cu(II) were mainly adsorbed on the inner surface (i.e., the wall of the capillary) and affected the WIT of modified CF, while they hardly affected the UWOCA. Therefore, WIT increased with the time of the experiments, the capillary force of driving water through m-CF decreased, and the F value also decreased. Even so, the values of E and UWOCA were almost unchanged.

Adsorption−Desorption Cu(II) Performances of m-CF.
The results of the studies mentioned in Section 3.3 showed that m-CF had a certain ability of simultaneous removal of Cu(II) and oil from oily wastewater. On continuous feeding of the oily wastewater, within 120 s (that is, the total time of seven consecutive feeds of oily wastewater) the Cu(II) removal rate decreased from 89 to 67%, and less than 38% in the tenth second. Thus, the following questions were mentioned: (1) Can the removal rate of Cu(II) be increased by changing the operating conditions? (2) Can m-CF regenerate quickly and recover the ability of Cu(II) adsorption? For these reasons, the Cu(II) static adsorption and dynamic desorption experiments of m-CF were carried out, as shown in Figure 8.
The static adsorption experiment of m-CF was carried out with artificial wastewater containing 50 mg·L −1 Cu(II) (i.e., Cu(II)−water). Fifty milliliters of wastewater and about 0.40 ± 0.08 g of the m-CF sample were added to a 250 mL conical flask at a constant shaking for 6 h, and then the liquid phase was taken for further analysis. The artificial wastewater containing Cu(II) was prepared with CuCl 2 and deionized water, and the initial pH of the wastewater was adjusted using 0.1 mol·L −1 NaOH or HCl. The adsorption capacity of Cu(II) is calculated by eq (4). used-I (10) -m-CF was fixed on the sandcore apparatus, and the desorption experiment was leached with the HCl solution of 0.1 mol·L −1 at 20 mL·min −1 . The desorption rate (named D) of Cu(II) is calculated according to eq (10).
where D is the desorption rate (%) of Cu(II), C 1 and V 2 are the concentration of Cu(II) in the desorption solution (mg· L −1 ) and the volume of desorption solution (L), and M Cu(II) is the initial Cu(II) content on used-I (10) -m-CF (mg). It can be seen from Figure 8A that the Cu(II) equilibrium adsorption capacities of CF, CF-PDA, and m-CF were 2.59, 3.60, and 4.90 mg·g −1 , respectively. Compared with CF, the Cu(II) adsorption capacity of CF-PDA and m-CF increased by 28.1 and 47.1%, respectively. The above results showed that DA and CS loaded with heavy-metal adsorption functional groups (such as −NH 2 and −OH, etc.) could improve the modified CF adsorption capacity. 9,38 As shown in Figure  8A(a), the adsorption capacity of Cu(II) could reach 50% of the equilibrium adsorption capacity of m-CF within 1 min, indicating that m-CF can remove Cu(II) from wastewater in a short time. Figure 8B,C shows that the effects of pH and temperature on the adsorption of Cu(II) by m-CF were consistent with the results of other studies with CS as the main heavy-metal adsorption composition. 12,39 As the pH increased, the adsorption capacity of Cu(II) decreased slowly, 12,40 and the adsorption capacity decreased with increasing temperature. 41,42 Figure 8A,C shows that the equilibrium adsorption capacity of m-CF was about 4.90 mg·g −1 under the experimental conditions of pH 5.6 and Cu(II) concentration of 50 mg·L −1 . The literature showed that −OH and −NH 2 have a strong binding ability to heavy-metal ions, and −NH 2 is the main heavy-metal adsorption site under the condition of weak acid. 9,12,39 Sahebjamee et al. 42 prepared the CS/PVA/ PEI membrane for Cu(II) removal, for which the equilibrium adsorption capacity of Cu(II) was 125 mg·g −1 under the conditions of 25°C, pH 6, and Cu(II) concentration of 30 mg· L −1 . It was possible that there was either a small amount of PDA and CS on the modified CF or the density of the exposing adsorption groups of the heavy metal was low. The Cu(II) adsorption capacity of m-CF was less than that of other adsorbents based on CS modification, resulting in the low removal rate of the simultaneous removal of Cu(II) from wastewater separation. Figure 8D shows that when the m-CF containing 11.0 mg· g −1 Cu(II) (i.e., used-I (10) -m-CF, see Figure 6D) was leached with the HCl solution of 0.1 mol·L −1 at 20 mL·min −1 , the desorption rate of Cu(II) was more than 95% within 120 s. According to Figures 6D and 8D, we therefore considered that when using multiple oil−water separation devices in parallel, one of the devices simultaneously carried out oil−water separation and adsorption of Cu(II), while another performed the regeneration process, which could be contributing to the simultaneous removal of oil and Cu(II) by m-CF. However, in order to make m-CF suitable for practical application in industry, it is necessary to improve the membrane preparation and desorption processes to increase the adsorption capacity and desorption rate.

Effect of Cu(II) on Oil−Water Separation.
In order to investigate the effect of Cu(II) on the oil−water separation performance, experiments similar to the ones described above in Section 3.3, were carried out with oily wastewater (i.e., oil− water system, 30 mL of toluene, and 70 mL of deionized water) and Cu(II)-containing oily wastewater (i.e., Cu(II)− oil−water system) using m-CF. To make it clear that the m-CF adsorption of Cu(II) (which contained 4.90 mg·g −1 Cu(II) and was named Cu (4.90 mg/g) -m-CF) was carried out on an oil− water system, see Figure 9. Figure 9 shows the m-CF-treated Cu(II)−oil−water and oil−water system, respectively. The F values of the first experiment were both 17 400 L·m −2 ·h −1 . But the F value of Cu(II)−oil−water was 14 500 L·m −2 ·h −1 at the 20th treatment, which decreased by 16.7%, and the F value of oil−water was 16 400 L·m −2 ·h −1 , which only decreased by 5.7%. Obviously, the Cu(II) in wastewater had negatively affected the F value of m-CF. Figure 9 shows that the F value of Cu (4.90 mg/g) -m-CF was 16 400 L·m −2 ·h −1 for the first time, which decreased by 6.1% at the 20th treatment. However, the decrease rate of the F value of Cu (4.90 mg/g) -m-CF was much smaller than that of m-CFtreated Cu(II)−oil−water (16.7%). In addition, the Cu(II) contents of the samples treated with oil−water by Cu (4.90 mg/g)m-CF and Cu(II)−oil−water treated by m-CF for 20 times were analyzed, which were 4.20 and 13.8 mg·g −1 , respectively. The above phenomena showed that the increase of the content of Cu(II) in m-CF will lead to a significant decrease in F value.
It can be seen from Figure 9 that Cu(II) had no influence on the E value, which was more than 98%.

Effect of Dissolved Oil on the Adsorption of Cu(II).
In this study, the effect of trace amounts of dissolved oil in water on Cu(II) removal was studied. Taking 200 mL of deionized water and 30 mL of toluene, the mixture was stirred magnetically at room temperature for 24 h and subjected to stratification. Then, the water phase was taken to further prepare the artificial wastewater with Cu(II), which contained 252 mg·L −1 dissolved toluene (determined with an infrared spectrophotometry oil measuring instrument) and 50 mg·L −1 Cu(II) (i.e., Cu(II)−water-dissolved oil). Subsequently, an adsorption experiment was carried out. The results are shown in Figure 8A(d); the equilibrium adsorption capacity of m-CF was 4.50 mg·g −1 , which was 8.2% lower than that for the treatment with "Cu(II)-water" wastewater ( Figure 8A(a)). The large π bond in the molecule of toluene endowed it with a certain polarity, and it could interact with polar groups (such as −NH 2 and −OH) on m-CF, which slightly reduced the adsorption capacity of Cu(II).

Stability of m-CF.
In this work, we investigated the wetting behavior and static adsorption of Cu(II) by m-CF after acid, alkali, and salt immersion treatments and after sandpaper abrasion, as shown in Figure 10.
From Figure 10B, compared with m-CF, the equilibrium adsorption of Cu(II) by HCl-m-CF, NaCl (36%) -m-CF, NaOHm-CF, and abrasion-m-CF were all decreased to 4.04, 4.64, 3.18, and 4.36 mg·g −1 , respectively, i.e., the equilibrium adsorption capacity of Cu(II) decreased by 17.6, 5.3, 35.1, and 11.0%, respectively. What was the reason for the decrease of the equilibrium adsorption of Cu(II) on the m-CF? In order to illustrate the effects of m-CF after treatment in harsh environment on the wettability and Cu(II) adsorption, SEM and EDS analyses were performed for HCl-m-CF, NaOH-m-CF, NaCl (36%) -m-CF, and abrasion-m-CF, respectively, as shown in Figure 11.
It can be seen from Figure 11a that there were etching holes on the surface of HCl-m-CF, and the content of N element (4.1 wt %) was close to that of m-CF. The H + of HCl entered the internal structure of CF, and, adhering to the β-1,4 glycosidic bond of CF, could hydrolyze cellulose, which led to the partial breakage of the cellulose macromolecules and decrease of its polymerization. 43 CS was similar to the cellulose structure (β-1,4 glycosidic bonds), which could be hydrolyzed in acidic aqueous solution, resulting in partial hydrolysis shedding or molecular chain breaking of CS, and forming etching holes. It may also be that the shedding amounts of CS were not much, so that the content of N element did not change obviously. However, the hydrophilicity and the content of polar functional groups (−NH 3 , −OH) chelating with Cu(II) were decreased, resulting in decrease of the equilibrium adsorption of Cu(II).
As shown in Figure 11b, compared with m-CF, the surface roughness of NaOH-m-CF and the content of N element (3.3 wt %) decreased obviously. The loss of PDA deposited on m-CF and the CS adhered on PDA were attributed to the dissolution of PDA by NaOH; 44 thus, the roughness, the content of N element, and the hydrophilicity of the NaOH-m-CF decreased. What's worse, the equilibrium adsorption capacity of Cu(II) was greatly reduced.
From Figure 11c, it can be seen that the surface of NaCl (36%) -m-CF was still rough, the content of N was 4.3 wt %, and there were Na and Cl elements, which may be because NaCl did not dissolve or decompose PDA and CS on m-CF, so that the morphology of NaCl-m-CF was similar to that of m-CF. Additionally, it may be that Na + and Cl − adsorbed on the surface of m-CF and increased the hydrophilicity of the capillary wall, 45 so the WIT of NaCl-m-CF decreased and UWOCA was almost unchanged. However, the residual Na(I) on m-CF competed with Cu(II) for the active site, 40,46 resulting in the decrease of the equilibrium adsorption of Cu(II). It can be seen from Figure 11d that abrasion-m-CF was partially worn, resulting in the decrease of N content (3.3 wt %), and indicating that parts of PDA and CS were lost, compared with m-CF, causing the WIT to be increased. It was possible that m-CF immersed in acid, alkali, or salt solution, which had a uniform effect on the inner and outer surfaces of the porous materials. In contrast, the abrasion treatment had a great influence on the outer surface of the porous m-CF and the pores changed. Immersing in acid, alkali, or salt solution changed the WIT of m-CF but UWOCA was almost unchanged, while abrasion treatment changed both WIT and UWOCA (UWOCA decreased, but was still larger than 150°).
In a word, although the F value of the oil−water separation of m-CF decreased to some extent under the condition of strong acid or strong alkali or abrasion, as well as the removal ability of Cu(II), its E value remained almost unchanged. It showed that m-CF still had good oil−water separation ability under harsh environments (strong acid, strong alkali, high salt and scouring, etc.).

CONCLUSIONS
Superhydrophilic and underwater superoleophobic cotton fabric (i.e., m-CF) was prepared using DA and CS, whereby m-CF simultaneously removed the oil and Cu(II) from oily wastewater under gravity, and the oil−water separation efficiency, the water flux, and removal rate of Cu(II) were more than 99%, 17 400 L·m −2 ·h −1 , and 89%, respectively. The oil−water separation efficiency was not affected during the continuous treatment of oily wastewater containing Cu(II), while the water flux of m-CF decreased slowly. Besides, in about 120 s, the Cu(II) removal rate decreased to 67%. The reason we proposed for the decrease of water flux was that both the dissolved oil and Cu(II) on the m-CF and, in turn, trace amounts of dissolved toluene in the water had a slight effect on the removal of Cu(II).
After immersing in strong acid, strong alkali, and high salt solution of m-CF for 20 days at room temperature, and abrasion with 500# sandpaper, the equilibrium adsorption capacity of Cu(II) decreased by 17.6, 35.1, 5.3, and 11.0%, respectively. Although the water droplet infiltration time was prolonged by different degrees (except for the case of high salt), the UWOCAs were still in superoleophobic state. In terms of oil−water separation, m-CF had good acid resistance, alkali resistance, salt resistance, and wear resistance.
The m-CF on which Cu(II) was adsorbed was leached with 0.1 mol·L −1 HCl solution at 20 mL·min −1 , and the desorption rate within 120 s was more than 95%. The decline rate of the Cu(II) adsorption capacity of m-CF was approximately equal to its desorption rate. Thus, the use of multiple oil−water separation devices in parallel would be conducive to the industrial applications of m-CF.


Insight into the hetero-interactions of 4-nonylphenol with dissolved organic matter: multiple spectroscopic methods, 1H NMR study and principal component analysis

Understanding the interactions between heterogeneous dissolved organic matter (DOM) and nonylphenols (NPs) is essential for predicting their behavior and fate in the environment. Herein, we firstly obtained different MW-fractionated humic acids (HAs) using the ultrafiltration method. Afterward, the molecular weight (MW)-dependent interactions of HAs with 4-nonylphenol (4-NP) were analysed by excitation emission matrix (EEM) fluorescence spectroscopy, fluorescence quenching, UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and principal component analysis (PCA). EEM spectra indicated that the quenching mechanism was static. In the binding process, the higher MW fractions showed stronger interaction with 4-NP than the lower MW counterparts, exhibiting a clear MW-dependent interaction heterogeneity. The interaction constants for the 4-NP–HAs system were suppressed as the ionic strength decreased and pH increased, which was especially obvious in the binding of 4-NP to the lower MW-fractionated HAs. The FTIR spectra revealed that hydroxyl and aromatics were involved in the interaction process of HA fractions with 4-NP. It was also found from 1H NMR that π–π interactions between aromatic rings of 4-NP and MW-fractionated HAs were responsible for the complexation. The correlation analysis and PCA results indicated that aromaticity and MW play important roles in the interaction process and confirmed an obvious interaction heterogeneity among MW-fractionated HAs samples. This work highlighted MW-dependent interaction heterogeneities of HA, which suggested that heterogeneity in MW distribution should be taken into consideration when exploring the fate and biogeochemistry cycling of 4-NP from contaminated environments.

Introduction
Nonylphenol ethoxylates (NPEOs), as one of the crucial classes of nonionic surfactants, are widely used in agricultural, domestic, and industrial elds. 1 As a result of chemical and biological processes, NPEOs can be degraded stepwise by losing ethoxylates to form more toxic nonylphenols (NPs). 2 Nowadays, NPs have been broadly detected in various environmental matrices, including water, soil, and atmosphere. 1,3-5 They were found to mimic the natural hormone oestrogen, resulting in disrupting the normal functioning of the hormonal system. Due to these disturbing results, NPs were designated as priority hazardous substances in European Union. 6 Currently, investigating the occurrence, fate, and potential ecological risks of NPs in the aquatic, terrestrial, and atmospheric environments is urgent. 1,[3][4][5] Due to the nonyl chain, NPs have hydrophobic properties. 4 The highly hydrophobic NPs pose a strong intendancy to interaction with soil/sediment organic matter, especially the ubiquitous dissolved organic matter (DOM), which signicantly inuence the transportation, transformation, and bioavailability of NPs in the environments. 7-10 By using dialysis equilibrium experiments, Li et al. 10 found DOM had high affinity to adsorb NPs. Liao et al. 8 investigated the kinetics and thermodynamics of NP sorption to the DOM, and the results indicated that the pseudo rst order model and Freundlich model could better describe the sorption process. Because of the interactions between DOM and NPs, the adsorption of NPs was inhibited by magnetic reduced graphene oxides. Unfortunately, DOM is a well-known heterogeneous mixture in the environment. However, till now, the previous works about 4-NP-HA interaction mainly focused on the bulk HA samples, ignoring the effects of the heterogeneities within the bulk HA on the environmental behaviors.
DOM is a complex and heterogeneous mixture of organic macromolecules consisting of different sizes, structures, and chemical properties. 11,12 The interactions between pollutants with DOM molecules were signicantly related to the organic compound species and MW distribution. For example, some studies revealed that high molecular weight MW-fractionated HAs which exhibited higher aromaticity and hydrophobicity had superior sorption ability on the other hydrophobic organic compounds (HOCs). [12][13][14][15] Nevertheless, recent researches observed that lower molecular weight MW-fractionated DOM (<1 kDa) had strong adsorption affinities on bisphenol A, octylphenol and pentachlorobenzene. 16,17 This discrepancy may be attributed to the composition heterogeneity of DOM and non-uniformity of HOCs binding functionalities in DOM. Therefore, understanding the interactions between different MW DOM and NPs under different environmental conditions, particularly the underlying interaction mechanisms, is imperative.
The interaction of HOCs with DOM has been investigated using various techniques, such as dialysis equilibrium, passive dosing, solubility enhancement, and uorescence quenching. [18][19][20][21] Among them, uorescence quenching is a popular and powerful method due to its sensitivity, non-destructivity, and simplicity. 22 It has been reported as an efficient method to investigate the interaction mechanism between HOCs and uorescent substances. 11,16,20 However, the binding sites are easily overlooked when exploring interactions. 20 Nuclear magnetic resonance (NMR) can be used to provide specic, molecular-level information involved in the interaction, and it is also applied to investigate those occurring between environmental pollutants and DOM. 23,24 Additionally, principal component analysis (PCA) is a multivariate statistical technique that can obtain the main comprehensive inuencing factors of interactions. However, limited information has been probed into the interaction mechanism between DOM and multiantibiotic situations. Therefore, combining these techniques can better understand the interaction mechanism between DOM and NPs deeply regarding binding capacities and binding sites.
The aim of this paper was to provide insights into the heterointeractions of NP with DOM in the natural environment. To achieve this purpose, 4-nonylphenol (4-NP) and a soil HA, were selected as the representative NP and DOM, respectively. We utilized the ultraltration technique was applied to fractionate the HA samples into different MW-fractionated HAs. Furthermore, the interaction properties between MW-fractionated HAs and 4-NP were investigated at different temperatures/pHs/ionic strengths by uorescence quenching method. UV-visible spectroscopy and FTIR spectroscopy were introduced to further explore the internal mechanism of the interaction of 4-NP with MW-fractionated HAs. The possible interaction sites were discussed by 1 H NMR analysis. The correlation analysis and principal component analysis were performed to understand the hetero-interaction. The results provide new insights into the binding interactions of the MW-dependent interaction heterogeneities of HA with 4-NP, which can improve our understanding of the fate and bioavailability of HOCs in aquatic environments.

Materials
4-NP (>99.5%), NaOD, DMSO-D 6 and D 2 O were purchased from Sigma-Aldrich. Physicochemical properties of 4-NP were listed in Table S1. † According to the method of the International Humic Substances Society (IHSS), HA was extracted from soil samples, collected from an undisturbed area at Dunhua, Jilin Province, China (43 18 0 36 00 N, 128 23 0 26 00 E). Amicon Ultra-15 centrifugal lters with nominal molecular weight cut-off of 3 kDa, 10 kDa, 30 kDa, and 100 kDa were obtained from Millipore (Darmstadt, Germany). The other reagents were purchased from Guangzhou Chemical Reagent (Guangzhou, China). If not specied, all the reagents were used as received without further purication. Ultrapure water (18.2 U cm À1 , purication by a Millipore system) was used throughout the experiments.

Fractionation and characterization of HA
The stock solution of HA was stirred overnight, followed by ltering through a 0.22 mm cellulose acetate lter paper (Millipore, Billerica, MA, USA). The ltrate was referred to as pristine HA. The HA fractions were obtained by using the Amicon Ultra-15 centrifugal lters and the detailed fractionation procedures were given elsewhere. 25 The fractions achieved were named >100 kDa HA, 30-100 kDa HA, 10-30 kDa HA, 3-10 kDa HA, and <3 kDa HA. Concentrations of pristine and MW-fraction HAs were quantied using a total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan). The pristine-and MWfractionated HAs were stored in the dark at 4 C until use. The MW distribution, elemental compositions, UV-visible absorption spectra, size exclusion chromatography (SEC), and excitation-emission matrices (EEM) uorescence spectra of pristine-and MW-fractionated HAs were characterized and the results were are given in the ESI. †

Fluorescence quenching experiments
The interaction properties of pristine and MW-fractionated HAs with 4-NP were examined by using the uorescence quenching method. Batch interaction experiments were conducted using several glass vials lled with MW-fractionated HA, then the 4-NP stock solution was spiked into the HA solutions. The corresponding electrolyte solution was added to each vial to maintain a nal volume of 10 mL, HAs at 0.5-10 mg C L À1 and 4-NP at 1 mg L À1 . The pH was adjusted using HNO 3 or NaOH. The IS was modied by adding NaNO 3 . Aer mixing, the vials were gently shaken by an oscillator for 4 h at 288 K, 298 K, and 308 K. This time period was adequate for reaction equilibrium based on preliminary experiments. Aer the equilibrium period, UVvisible absorbance spectra and EEM uorescence spectra were measured.

Spectral measurement
The UV-visible absorbance spectra were measured between 200 and 800 nm using a spectrophotometer (UV-2550, Shimadzu, Japan) with 1 cm quartz cells. If not specied, the concentration of HAs was 5 mg C L À1 . To get better FTIR spectra of 4-NP, HAs, and their complexes, the high concentration solutions of samples were prepared and freeze-dried. And all the obtained particles were homogenized and pressed under the irradiation of an infrared lamp to eliminate the effect of moisture in the samples. The FTIR spectra were recorded in a PerkinElmer frontier spectrometer equipped with a Germanium attenuated total reection accessory and a KBr beam splitter with 32 scans at a resolution of 4 cm À1 . All 1 H NMR spectra data were collected on a Bruker Avance 400 MHz instrument at 400. 13 MHz with a 5 mm Bruker inverse broadband probe. The temperature was set as 298 AE 1 K. Typically, 1 mg 4-NP was rstly added with 50 mL DMSO-D 6 to ensure 4-NP dissolution, and then diluted in 0.1 mol L À1 NaOD solution with the nal concentration of 120 mg mL À1 , and then the solid pristine and MW-fractionated HAs were added, respectively. The concentration ratio of HA with 4-NP in all samples was 0.5. The alkaline conditions were favorable for the dissolution of 4-NP and HAs. The resulting solution was sonicated for 120 s and transferred to NMR tubes. High concentrations of HAs and 4-NP were used to enhance the intensity of 1 H NMR and thus improve the signal-to-noise ratio of the analysis. 1 H NMR spectra were referenced to the chemical shi of the solvent being 4.709 ppm. All spectra were processed by MestReNova soware. All samples were performed in triplicate and the results in average were reported.

Data analysis
All uorescence spectra were corrected for the inner lter effect as follows: 26 where F obs and F were the uorescence intensities before and aer correction, and A ex and A em were the absorbance of the test solution at excitation and emission wavelengths, respectively. The interaction parameters of 4-NP to the pristine and MWfractionated HAs were calculated using the Stern-Volmer equation: 20 where F 0 and F were the corrected uorescence intensities of 4-NP without and with pristine and MW-fractionated HAs. k q was the energy transfer rate (L mol À1 s À1 ), s 0 was the lifetime of uorescence (3.63 ns). 27 [HA] was the added concentration of HA and K HA was the sorption coefficient. Because the HA fraction possessed intrinsic uorescence at excitation wavelength of 4-NP. In order to remove background effects, a blank was made for each concentration of HA fraction. The blank spectrum was subtracted from the emission spectrum of the corresponding 4-NP-HA solution. 20 In addition, the interaction constant (K) and the number of interaction sites (n) according to uorescence titration experiments were calculated from the site-binding equation for an interaction process involving a static quenching mechanism: 28 where F 0 and F were the corrected uorescence intensities of 4-NP in the absence and presence of pristine and MWfractionated HAs.
[HA] was the added concentration of HA (mg C L À1 ). The thermodynamic parameters for uorescence quenching of 4-NP by HAs, including the enthalpy change (DH), the entropy change (DS), and the Gibbs free energy change (DG), were used to interpret the interaction forces. The values of DH and DS can be estimated from eqn (4): where R was the ideal gas constant (8.314 J mol À1 K À1 ), T was the absolute temperature and K was the interaction constant at temperature T. Then the DG could be determined using the following eq: Details of the data analysis for the interaction of pristine and MW-fractionated HAs with 4-NP were given in the ESI. †

Physicochemical properties of the pristine and MWfractionated HAs
Separated by ultraltration, the MW-fractionated HAs showed apparent increases in molecular weights ranging from 2.30 to 20.9 kDa (Fig. S1 and Table S2 †). Elemental compositions and atomic ratios of the pristine and MW-fractionated HAs were listed in Table S2. † With the enhancement of molecular weights of the MW-fractionated HAs, an increasing and a decreasing trend was observed for the H/C ratios and the O/C ratios, respectively, indicated that the high molecular weight MWfractionated HAs contained more unsaturated hydrocarbons and less oxygen-containing functional groups. [29][30][31] Fig. 1a shows that UV-visible absorbance of the pristine and MW-fractionated HAs decreased monotonously as the wavelength increased and no absorption peak was observed. Except for 30-100 kDa HA, SUVA 254 values of molecular weight MW-fractionated HAs which were calculated via the division of the absorbance at 254 nm with TOC concentration, decreased from 9.24 to 6.99 with decreasing molecular weight of the MW-fractionated HAs. Polarity which dened as the elemental ratio of (O + N)/C increased from 0.51 for >100 kDa HA to 0.65 for <3 kDa HA. These results indicated that the higher molecular weight MWfractionated HAs (>30 kDa HA) contained more aromatic compounds and hydrophobic structures. 29,32,33 As presented in Fig. 1b, with decreasing molecular weight of the MW-fractionated HA, the absorbances of peaks at 3405, 1714 and 1240 cm À1 were increased slightly, corresponding to the O-H stretching of carboxyl acids, phenols and alcohol, C]O stretching of carboxylic acid, aldehyde and ketone, and C-O stretching of ester, ethers and phenols, respectively, 30 suggesting that the lower molecular weight MW-fractionated HA contained more carboxyl groups, consistent with the elemental analysis.
One uorescence peak (Ex/Em, Ex: 280-305 nm; Em: 435-474 nm) in each sample was observed (Fig. 2), indicating that the uorophore was the humic acid-like structure. 21 Generally, the greater abundance of aromatic chromophore possessed, the more red-shi of Em maxima was observed. 29,34 As the molecular weight of the MW-fractionated HAs increased, the peak Em wavelengths shied towards the longer wavelengths, which suggested more condensed and polymerized structures in higher molecular weight counterparts. Furthermore, the uorescence intensity of HA was positively related with the content of electron donating groups, such as hydroxyl and methoxyl groups. [35][36][37] These groups could improve the uorescence of DOM by increasing the transition probability between the singlet and the ground state. In our study, the uorescence intensity of the MW-fractionated HAs decreased with the increased molecular weight. The result may contribute to the low content of the oxygen-containing functional groups in HA fractions of higher molecular weight. [35][36][37] Similar ndings have also been reported in the MW-fractionation of DOM. 29,32,33,37 Humication index (HIX) was positively related to the MW of the fractions (except for >100 kDa HA), indicating that the extent of humication decreased with decreasing molecular weight. 33 3.2. Hetero-interaction of 4-NP with MW-fractionated HAs via an EEM uorescence spectral analysis 3.2.1. Fluorescence quenching spectra. Fluorescence quenching is a popular and powerful method due to its sensitivity, non-destructivity, and simplicity. 22 It occurs when a uorescent component interacts with a quencher, the uorescence intensity of the component weakens or disappears. Fig. 3 and S2 † showed the uorescence quenching of 4-NP by different concentrations of pristine and MW-fractionated HAs. The 4-NP presented a high uorescence intensity at Ex/Em ¼ 280/308 nm. The uorescence intensity of the peak was gradually reduced with increasing HAs concentrations due to the suppression of energy emission. 38 At an identical HA concentration, the reduction of the uorescence intensity was correlated with the molecular weight of the HAs, indicating a strong uorescence quenching effect of the high molecular weight of MWfractionated HAs on 4-NP (Fig. S3 †).
Generally, uorescence quenching includes dynamic and static quenching. In order to conrm the quenching mechanism, the Stern-Volmer model was utilized and the exponential curves were presented in Fig. S4. † It can be seen that all the titration data were well tted with the Stern-Volmer equation (R 2 ¼ 0.995-0.999), and the highly linear plot indicated a sole dynamic or static quenching process was responsible for the uorescence quenching of 4-NP by the HAs. 38,39 The quenching constant of K q for pristine and MW-fractionated HAs were signicantly larger than the maximum scatter collision quenching constant reported for dynamic quenching (2.0 Â 10 10 L mol À1 s À1 ), 27 indicating that static quenching dominated the quenching process and a ground-state complex formed Fig. 2 The fluorescence EEM spectra of the 5 mg C L À1 pristine-and MW-fractionated HAs. Fig. 3 The fluorescence EEM spectra of 4-NP in the absence and presence of 5 mg C L À1 pristine-and MW-fractionated HAs; T ¼ 298 K; pH ¼ 6.0, 1 mol L À1 NaNO 3 .
between HAs and 4-NP. For an interaction involving static quenching mechanism, the site-binding equation was applied to determine the K and n. As listed in Table 1, the log K HA of the MW-fractionated HAs decreased as the molecular weight of the MW-fractionated HAs decreased, displaying a clear MWdependent interaction heterogeneity. The results indicated the higher interaction affinities for the high molecular weight MWfractionated HA. [12][13][14][15]40 The values of n were approximately 1, suggesting a single binding site in 4-NP for MW-fractionated HAs, which was similar to the previous studies of disinfection byproducts-human serum albumin binary systems 41 and antibiotics-HA. 20 3.2.2. Thermodynamic parameters. In order to interpret the possible interaction forces between 4-NP and HAs, the interaction of HAs to 4-NP at different temperatures were performed and the corresponding thermodynamic parameters, including DH, DS, and DG were calculated. As listed in Table 1, the K and K HA values MW-fractionated HAs also exhibited similar trends, which were decreased with the molecular weight of HAs decreasing. K and K HA increased with increasing temperature for all HAs, suggesting that the interaction properties of 4-NP for MW-fractionated HAs became stronger with increasing temperature. As we know, dynamic quenching is enhanced by increasing temperature, whereas decreasing temperature most likely causes higher static quenching constants. The temperature-dependent results further demonstrated that the interactions were static quenching.
The negative DG values indicated that the interaction processes were thermodynamically spontaneous ( Table 1). The highest negative DG value in the 4-NP > 100 kDa HA system was observed at the same temperature, manifesting that >100 kDa HA possessed the largest interaction potential for 4-NP. 20 The negative DH values implied that the interaction processes were exothermic, and the positive values of DS suggested the disorder of the reaction systems increased aer 4-NP interaction with MW-fractionated HAs. Previous studies 41,42 reported that the positive DH and DS values were frequently considered as hydrophobic forces, whereas negative DH and DS values occurred for van der Waals forces and hydrogen bonding; besides, if DH < 0 and DS > 0, the interaction process was mainly driven by electrostatic forces. Therefore, the positive values of DH and DS observed in this study indicated that hydrophobic forces were responsible for the interactions between MWfractionated HAs and 4-NP under the experimental condition (pH 6, 1 mM NaNO 3 ).

Interactions between 4-NP and MW-fractionated HAs at different pH values and ionic strengths.
Ionizable organic pollutants binding to HA exhibits a strong pHdependence, reecting the speciation of both the sorbent and sorbate. 41,43 The binding parameters at various pH values were listed in Table S3. † The uorescence emission spectra of the 4-NP-HAs systems at pH 3.0 and pH 11.0 were provided in Fig. S5 and S6. † The relationship of K HA vs. pH was presented in Fig. 4a. At pH 3.0, the carboxyl groups of HAs were protonated, resulting Table 1 Binding parameters and thermodynamic parameters of 4-NP-HAs systems based on the fluorescence titration experiments (pH 6.0, 1 mmol L À1 NaNO 3 )

HAs
T (K)  in a reduction of polarity. At the same time, protonation of HAs facilitated intra-and intermolecular interactions, leading to the formation of additional hydrophobic cavities in which more hydrophobic compounds were adsorbed. 44 With the increasing pH, the molecular would extend and uncoil due to the deprotonation of phenolic and carboxyl groups. Consequently, MWfractionated HAs become less hydrophobic and more polar, which was unfavorable for binding of 4-NP. 45 Furthermore, with further increasing to pH 11.0, the 4-NP mainly existed as foremost ionic forms when pH was higher than pK a (10.7). As the electronegativity of HAs also enhanced with the elevating pH, electrostatic repulsion between negative charge of HAs and 4-NP would weaken the binding strength. 28 Generally, a signicant reduction of K HA for low molecular weight MW-fractionated HAs was observed with increasing pH (Fig. S7a †). The different extent of decrease among HA fractions was probably attributed to their respective compositional and structural characteristics. 46 As shown in Table S2, † low molecular weight MW-fractionated HAs were more enriched in polar groups and have much higher polarity compare with other fractions. Consequently, <3 kDa HA could be easily bound via molecular interactions at lower pH, resulting in more sensitivity to pH. 46 In addition, IS would affect the interactions by destroying the HA structure and compressing the double electric layers on molecule surface. 20,47 The uorescence emission spectra of the 4-NP-HAs systems at 10 mM and 100 mM NaNO 3 were provided in Fig. S8 and S9. † Fig. 4b showed the binding constants of MWfractionated HAs dramatically decreased with NaNO 3 concentration from 1 mmol L À1 to 100 mmol L À1 at neutral pH solution. All MW-fractionated HAs exhibited a similar trend. The high concentration ions compressed the thickness of the electric double layer of HA colloids effectively, inducing HA aggregation and even precipitation. 45,48 This tightly packed molecular shape of HA would reduce the capability of 4-NP to interact with HA. 49 Furthermore, the decrease of the K HA in high molecular weight MW-fractionated HAs was smaller than that in low molecular weight MW-fractionated HAs (Fig. S7b †). The reduction of K HA was mainly deemed to the change of HAs molecular size, which was caused by the associations between cations and acidic groups. Thus, the contents of negatively charged acidic functional groups of HAs were probably responsible for the different results among MW-fractionated HAs, which was consistent with the experimental results for hydrophobic fractions and different origins of DOM. 49,50

UV-vis, FT-IR, 1 H NMR spectra of 4-NP-HAs complexations
The UV-vis absorption spectra of 4-NP in the absence and presence of MW-fractionated HAs at pH 6 and 1 mM NaNO 3 were measured and presented in Fig. 5a. With addition of equivalent MW-fractionated HAs, the absorption intensities and shapes of 4-NP changed. As the molecular weight of fractionated HAs decreased, a slight blue shi was observed for the absorption peak at around 275 nm. 20 The apparent variation indicated that steady 4-NP-HAs complexes were formed and the molecular weight of HA had a signicant effect on the micro-environmental changes of 4-NP. Besides, the distinct changes further conrmed that static quenching occurs in the interactions. This was because the dynamic quenching hardly affected absorption spectra of quenching matters.
To explore the changes of MW-fractionated HAs functional groups complexing with 4-NP, the FT-IR spectra of 4-NP exposed to HA at pH 6 and 1 mM NaNO 3 were collected and shown in Fig. 5b. Compared with the MW-fractionated HAs spectra, the signicantly increased absorption intensity at 1380, 1240, and 1124 cm À1 was attributed to C-O stretching vibration, phenolic O-H, and C-H bending vibration of 4-NP, respectively. 51 The peaks of the H-bonded OH vibration moved slightly to the low wavenumber, and the peaks were broadened when exposed to HAs, implying H-bonding involving the 4-NP in HA. 20 The C]C peak of MW-fractionated HAs aer binding shied from 1635 to 1596 cm À1 , which could be attributed to the p-p interactions between the benzene rings of 4-NP and MW-fractionated HAs. 39,52 These features of the FTIR spectra proved that the 4-NP was not mechanically mixed with the HAs but bound to the MW-fractionated HAs.
Nuclear magnetic resonance (NMR) was applied to provide molecular-level information involved in the reactions between 4-NP and HAs. As shown in Fig. 5c, the chemical shis above 6.2 ppm were attributed to the 4-NP protons of the aromatic region, while the signals resonating between 1 and 4.7 ppm corresponded to protons in the aliphatic regions. Due to the lower concentration of MW-fractionated HAs, almost no proton NMR signal was observed (Fig. S10 †). Upon addition of MWfractionated HAs, the proton signals of 4-NP were quenched and broadened, suggesting a more rapid relaxation rate due to dipolar interactions arising from restricted mobility of 4-NP molecules. 20,53,54 Simultaneously, the chemical shis of aromatic proton signals moved upeld due to the increased electron shielding around proton nuclei. 23 Though similar trends were observed in all 4-NP-HAs systems, the extent of signal changes (H3, H4 and H2, H5) was larger for high molecular weight MWfractionated HAs than that of low molecular weight MWfractionated HAs (Fig. S11 †). These observations might be attributed to the establishment of p-p interactions between 4-NP and the aromatic parts of HAs, where the center of magnetic anisotropy was shied out of the 4-NP aromatic plane toward the center of newly formed p-p complexes. 20,53 The strong p-p interactions would be formed between 4-NP and high molecular weight MW-fractionated HAs, leading to increasing the electron shielding of proton nuclei. In addition, the nuclei of 4-NP entrapped in higher hydrophobic cavities of >100 kDa HA might induce an improved magnetic shielding, causing a larger upeld shi. 55,56 In previous studies, the hydrogen bonding interaction was also used to explain the upeld shi between HA and aromatic organic pollutants. 20,57 However, the chemical shi variation was smaller for more hydrophilic and nonpolar MWfractionated HAs. Therefore, the interaction between 4-NP and HAs should not be primarily related to H-bonding at high pH. 53 Overall, p-p interactions between aromatic rings of 4-NP and MW-fractionated HAs and hydrophobic interactions played important roles in the interaction process.

Relationships of the physicochemical properties with interaction properties
The varied interaction properties of MW-fractionated HAs may be related to their distinct chemical compositions and structures. As shown in Fig. 6, the log K HA values are strongly positively correlated with SUVA 254 (R 2 > 0.872), implying that the aromatic structures of HAs have critical importance in the interaction process of 4-NP with MW-fractionated HAs. The results were consistent with the studies of HOCs interaction with MW-fractionated HAs. 13,14,58,59 HIX was also positively correlated with the log K HA . Higher values of HIX suggested higher degrees of aromaticity, lower rates of mineralization, and lower contents of oxygen-containing functional groups for DOM. 40 The importance of humication degree was highlighted in the interaction process between HOCs and DOM fractions. 14,15 Additionally, (O + N)/C atomic ratios, as an index of the polarity of HA, negatively correlated with log K HA values. Similarly, Jin et al. 58 observed that HA fractions with higher polarity had lower interaction properties for phenanthrene than other HA fractions. It has been proposed that the hydrophilic moieties of HA would reduce the accessibility of HOCs to sorption domains and compete for sorption sites, thus decreasing their interactions. 60 Taken together, the superior interaction property for the high molecular weight MW-fractionated HAs resulted from its higher aromaticity, humication degree, and lower polarity.
Principal component analysis (PCA) is a multivariate statistical technique that can be used to simplify the selected parameters and convert the data into graphical form. To further understand the hetero-interaction of 4-NP with HA, PCA was performed by using the selected parameters of the HA samples (e.g., C, H, O, N, MW, Mn, HIX, SUVA 254 , O/C, (N + O)/C, BIX (biological index), FI (uorescence index), K HA , K, n) that had relationships with formation of 4-NP-HAs complex. As seen in Fig. 7a, the percentage of variance of principal component 1 (PC1) and principal component 2 (PC2) was approximately 78.4% and 10.3% in the HA samples, respectively. The cumulative percentage of variance of PC1 and PC2 is 88.7%, indicating the interaction behavior mainly considered the two components of PC1 and PC2. The factors associated with aromaticity and MW were loaded mainly by PC1 and were positively related to PC1. The high statistically signicant correlation among MW, Mn, HIX, SUVA 254 , K HA suggest that these factors are similar. In addition, the loadings graph also exhibited that K HA has a strong relationship with SUVA 254 , MW and HIX, conrming that aromatic C in MW-fractionated HAs may signicantly take part in the interactions. These results are vital for elucidating the formation of 4-NP-HAs in the future. Furthermore, in the scores graph (Fig. 7b), the distribution of different types of HA was obvious distinct, suggesting a clear interaction heterogeneity among MW-fractionated HAs sample, and proving that the critical differences in structural characteristics resulted in a difference in the 4-NP-HA complex.

Environmental implication
4-NP has been broadly distributed in aquatic environments. Thus, highly hydrophobic NPs 4-NP has inevitably interacted with ubiquitous HA in the environments. In this study, the HA samples used were environmentally related concentrations, which indicated the feasibility of extrapolating these results to natural ecosystems. Additionally, previous work about 4-NP-HA interaction mainly focused on the bulk HA samples, ignoring the effects of the heterogeneities within the bulk HA on the interaction behaviors. 10 This work highlighted MW-dependent interaction heterogeneities of HA, which suggested that heterogeneity in MW distribution should be taken into consideration when exploring the 4-NP-HA interactions and assessing the fate and transport of 4-NP in aquatic environments.
4-NP could interact with MW-fractionated HAs to form a relative stable complex spontaneously. Generally, higher molecular weight MW-fractionated HAs have stronger sorption ability to 4-NP than lower higher molecular weight MWfractionated HAs. Via a correlation analysis and PCA, the results indicated that aromaticity and MW played vital roles in the interaction process. Higher MW of HA accompanied with higher aromatic moieties had prone to interaction with 4-NP, which might signicantly inuence the biodegradation of 4-NP. As Lin et al. 12 pointed out, the DOM concentration and structure, organism species, exposure regime. As well as water conditions could inuence the biodegradation of pollutants. Thus, the biodegradation of MW-fractionated HAs-associated 4-NP are needed to be precisely investigated in further researches. Moreover, the formation of 4-NP-HAs complex was mainly induced by hydrophobic interactions and p-p interactions, which could be interfered by different pHs and ISs. These results herein were helpful for understanding the effects of MW distribution of HA on the interaction with 4-NP under environmentally relevant conditions.

Conclusions
The interaction of the MW-dependent fractionated HAs with 4-NP were explored by EEM, UV-visible spectroscopy, FT-IR, and 1 H NMR analysis. The quenching processes of all samples were static, and the binding affinities calculated from the Stern-Volmer and site-binding equations reduced with the MW decreased, exhibiting a clear MW-dependent binding heterogeneity in aquatic environments. The binding constants for 4-NP-HAs system were suppressed as the ionic strength decreased and pH increased. The binding reactions occurred spontaneously, and the interactions were mainly driven by hydrophobic interaction. Additionally, p-p interactions between aromatic rings of 4-NP and MW-fractionated HAs might play crucial roles in forming the 4-NP-HAs complex, as indicated by FT-IR and 1 H NMR. This study highlighted the importance of MW in HA interaction with 4-NP, which can inuence the fate and behavior of 4-NP in aquatic environments. Besides heterogeneity, the regional and temporal variation in different sources of HA also has important impacts on the interaction, which should be further assessed for predicting the behavior of 4-NP in natural environments.