Hydraulic oil leaks during mechanical maintenance, resulting in flushing wastewater contaminated with dispersed nano-oil droplets. In this study, 75 mg L−1 of polysilicate aluminum ferric (PSAF) was stirred at 350 rpm and the optimal chemical oxygen demand (COD) removal was 71%. The increase of PSAF led to more hydrolysis of Fe, and 1,175 cm−1 hydroxyl bridged with negative oil droplets. At the same molar concentration, PSAF hydrolyzes cationic metals more rapidly than polymeric aluminum chloride (PAC). PSAF forms flocs of smaller complex structures with greater bridging. The Al–O and Si–O peaks occurred at 611 and 1,138 cm−1, indicating the formation of Si–O–Fe and Si–O–Al bonds on the flocs surface. Higher stirring speeds did not change the free energy of the flocs surface γTot, mainly because the decrease in the van der Waals force (γLW) offset the increase of Lewis acid–base force (γAB). Preserving the non-polar surface, in summary, owing to its bridging abilities and affinity for non-polar surfaces, PSAF demonstrates superior efficiency over PAC in capturing and removing oil droplets.

  • 75 mg L−1 of PSAF was stirred at 350 rpm and the optimal COD removal was 71%.

  • PSAF bridging was superior to PAC in terms of Si–O–Fe and Si–O–Al bonds.

  • PSAF forms flocs of smaller complex structures with greater bridging.

  • Higher stirring speeds did not change the non-polar surface of PSAF flocs to have good adsorption.

The flushing machinery produces wastewater containing a large amount of dispersed oil droplets. Improper treatment may result in severe environmental pollution, necessitating urgent resolution (Chen et al. 2020; Yalcinkaya et al. 2020). The treated water must conform to the GB1576-2008 standard, crucial for water resource recycling (Zhao et al. 2021). Conventional treatment methods mainly include physical and chemical methods, of which the former mainly include flotation, flocculation, and neutralization, and the latter mainly include oxidation and AC adsorption (Qin et al. 2018). Even after treatment in the oil floating separator, dispersed and emulsified oil residues remain (Alquraish et al. 2021). A solid interfacial membrane was formed on the oil–water interface, which reduces the surface tension and surface Gibbs energy of the interface, thus preventing the mutual contact between oil–water droplets and making it difficult to break the oil droplets (Diraki et al. 2019). During industrial production, significant quantities of oil wastewater are produced. In this wastewater, emulsified oil and nano-sized dissolved oil prove more challenging to remove than floating and dispersed oil (Hu et al. 2021). Nano-oil droplets are laden with substantial amounts of toxic hydrocarbons and heavy metals, which are lethal to higher organisms, microbes, and plants upon entering the environment, thus causing severe pollution (Ngoc et al. 2021). Additionally, some toxic hydrocarbons within these droplets can be absorbed and accumulated by plants. As they ascend the food chain, they ultimately threaten human health.

Flocculation is an economical, effective, and simple method for realizing oil–liquid separation (Qin et al. 2021). Multivalent metal inorganic salts are widely used as flocculants because of their low costs (Perez-Calderon et al. 2018). Consequently, polysilicate aluminum ferric (PSAF) has garnered increased attention. Oil colloids adsorb on both ends of the PSAF. A bridge forms between the colloids, and these bridges connect to settle (Cen et al. 2018). The addition of Fe and polysilicic acid can improve the degree of polymerization of PSAF and the bridging ability of the flocculant, but a high degree of polymerization affects the stability of the product. The morphology of PSAF is a branching aggregation state with fractal characteristics, which is completely different from the spherical or ellipsoidal shape of polysilicic acid. The morphology of PSAF provides more adsorption sites and promotes charge neutralization, bridging ability, and friction between flocs. The charge of PSAF is opposite to that of oil colloids. Attributable to the dual effects of concentration diffusion and opposite charge attraction, colloids and Al/Fe ions undergo exchange within the diffusion layer. This results in a reduced diffusion layer thickness, consequently diminishing the ζ-potential (Cen et al. 2018). Electrostatic repulsion among pollutant colloidal droplets diminishes, leading to aggregation. PSAF achieved a 77.7% chemical oxygen demand (COD) removal rate in oil wastewater, offering efficiency and cost-effectiveness (Huang et al. 2016). When the dosage was increased to 50 mg L−1, the optimal COD and turbidity removal rates were 65.3 and 97.2%, respectively (Yang et al. 2019; Zhang et al. 2021). The greater the dosage, the better the effect. However, if the dosage was too high, the surface activity of the PSAF particles was reduced, which led to re-stabilization, while flocculation worsened (Haixia et al. 2019). Due to the strong π-π effect, the PSAF dispersant undergoes standing adsorption instead of horizontal adsorption of SAF. This standing adsorption behavior leads to the change of water state and the steric hindrance effect. The aromatic ring and aliphatic groups in the molecular chain of PSAF dispersant will compete and interact with (carboxyl, hydroxyl, amino, etc.) during adsorption (Shuai et al. 2022), so the use of PSAF can generate chemical bonding linkages more efficiently and thus get better degreasing effect. The optimal dosage and method of PSAF for removing dispersed oil droplets have not been reported.

For this, the effective dosage, stirring speeds, and stirring times of PSAF on the removal of nano-oil droplets in wastewater were investigated. The oil content in the flocs was tested by stained and paraffin-embedded sections, the flocs structure and Fe distribution were observed by scanning electron microscopy in combination with X-ray spectroscopy (SEM-EDS), the functional groups on the flocs were studied by Fourier transform infrared spectroscopy (FTIR), and the surface free energy of flocs generated by PSAF and PAC was calculated by the contact angle. The findings demonstrate that PSAF can effectively remove dispersed nano-oil droplets from wastewater.

Materials

Wastewater was collected from a maintenance plant and stored at 4 °C before use. Analytically pure sodium hydroxide, PAC, PSAF, petroleum ether, o-phenanthroline, sulfuric acid, silver sulfate, mercury sulfate, and reagents were purchased from Sigma-Aldrich (MO, USA). Milli-Q water (Millipore, USA) was used to prepare the solutions.

Methods

Flocculant concentration effects on COD removal

In order to obtain better COD treatment results, we designed controlled experiments for three factors, namely, the amount of flocculant, the rotation speed of agitation, and the time between mixing and agitation, respectively:

Different concentrations of PSAF and PAC (0, 25, 50, 75, 100, and 125 mg L−1) were added to 200 mL of wastewater. The solution was mixed at 200 rpm for 3 min, then agitated at 50 rpm for 15 min, and after mixing, the solution was allowed to stand for 15 min to remove 10 mL of the supernatant for determination of COD using the potassium dichromate method.

The mixture stirring speed effects on COD removal

PSAF (50 mg L−1) and PAC (75 mg L−1) were added to 200 mL of wastewater taking the most effective parameter from the previous set of experiments. The mixture was stirred rapidly at 200, 250, 300, 350, and 400 rpm for 3 min and then agitated at 50 rpm for 15 min and after stirring 10 mL of supernatant was left for 15 min for determination of COD by the potassium dichromate method.

The mixing time and aging time effects on COD removal

PSAF (50 mg L−1) was added to 200 mL of wastewater and mixed at 350 rpm and PAC (75 mg L−1) was added to 200 mL of wastewater and mixed at 300 rpm for the parameters that gave the best results in the previous experiments. The mixing time was set at 1, 3, 5, 7, and 9 min. The aging time was set at 25, 23, 21, 19, and 17 min. At the end of mixing, 10 mL of supernatant was allowed to stand for 15 min and COD was determined by the potassium dichromate method.

Microscopic observation and analysis of PSAF flocs

The PSAF (50 mg L−1) was added to 200 ml of wastewater and the mixture was stirred for 3 min at 60, 100, 300, and 400 rpm. The flocs (100 μL) were taken, embedded in paraffin and sectioned (Leica, Germany). Its flocs structure and fragmentation were observed with a UV fluorescence microscope (Olympus BX61, Japan).

Analysis of flocs microstructure by SEM-EDS

Different concentrations of PSAF and PAC (0, 25, 50, 75, 100, and 125 mg L−1) were added to 200 mL of wastewater. The mixed solution was stirred at 60, 100, 300, and 400 rpm for 3 min. The 100 μL of flocs were removed and embedded in paraffin. A scanning electron microscope SEM (Zeiss Sigma 300, Germany) was used to observe topographic characteristics. The distribution of Fe after gold spraying was observed using X-ray spectroscopy EDS.

FTIR characteristics of flocs surface

After the flocs precipitated, they were dried, ground, and pressed. The chemical functional groups on the surfaces were detected using an FTIR spectrometer (Nicolet6700 Thermo, USA) (Table 1).

Table 1

Flocculation scheme for detecting functional groups and free energy of flocs

FlocculentCon. (mg L−1)Stirring speed (rpm)
PAC 75 300 
PAC 25 100 
PSAF 50 350 
PSAF 50 100 
FlocculentCon. (mg L−1)Stirring speed (rpm)
PAC 75 300 
PAC 25 100 
PSAF 50 350 
PSAF 50 100 

Measurement of contact angle on flocculent surfaces

Contact angles were measured using a contact angle microscope (Zhongchen JC2000D1, China) with Milli-Q water, diiodomethane, and formamide via the three-liquid method to evaluate surface free energy changes. All measurements were performed at 25 °C, and the data was listed in Supplementary Table S1.

The surface Gibbs free energy () of the flocculant (mJ·m−2) were calculated based on the use of water, formamide, and diiodomethane as probe liquids, determination of the contact angle (θ), and Young's equation according to the following equation. The total surface Gibbs free energy of the solid () can be expressed as the sum of the Lifshitz-van der Waals component () and the Lewis acid–base component, which in turn can be expressed as the Lewis acid component (γ+) and the Lewis base component (γ).
formula
formula
formula

All experiments were carried out triplicatedly.

Regulation of PSAF flocculation for COD derived from nano-oil droplet removal

For oil wastewater from flushing machinery, COD results from dispersed nano-oil droplets (Qin et al. 2023). Figure 1 shows the ability of the PSAF and PAC molecules to remove COD. It was observed that with the increase of the flocculent, the removal of COD first increased and then decreased. When it was between 0.2 and 0.7 mM (mmol L−1), the COD removal efficiencies of PSAF and PAC were comparable. Insufficient PSAF results in weak bridging and inadequate flocculation of oil colloidal droplets (Ma et al. 2018). The optimal addition of PAC and PSAF was 75 mg L−1, and COD removal reached 47%. When the concentration was higher than 0.7 mM, the COD removal started to decline. The removal effect of PAC on COD was little affected by the molar concentration, but every 0.1 mM increase in PSAF reduced the COD removal rate by 46%. The influence of concentration on COD removal by PSAF was 3.5 times that of PAC. By the way, excess PSAF is filled with anionic SiO2, which reduces the charge neutralization ability of the coagulant. By the way, to address the issue of the concentrates flocs generated during the treatment operation, the optimal concentration of PSAF should be 0.7 mM. After flocculation treatment, we produce PSAF flocs formed by mechanical oil accumulation, so the oil can be pyrolyzed and used as fuel. In addition, the adsorption capacity of SS on PSAF is too large, which may lead to the re-stabilization of the colloid, or PSAF wraps the SS, causing ‘colloidal protection’ and the emergence of its isoelectric point, which will decrease the flocculation capacity (Decio et al. 2021). Therefore, the most appropriate dosage of flocculant should not only ensure the rapid aggregation of SS but also prevent them from desorption.
Figure 1

Effect of PSAF and PAC concentration (a), stirring speed (b), and time (c) on COD removal efficiency.

Figure 1

Effect of PSAF and PAC concentration (a), stirring speed (b), and time (c) on COD removal efficiency.

Close modal

As shown in Figure 1(b), the influence of stirring speed on the removal efficiency of PSAF and PAC exhibited the same trend, first increasing and then decreasing. The optimal speeds for PAC and PSAF were 300 and 350 rpm, respectively, and the corresponding COD removal rates were 57 and 71%, respectively. Excessively high stirring speeds disrupt flocs formation, hindering the stability of flocs and precipitates. Different stirring speeds affected the stable structure of the flocs, thus affecting the removal effect of sedimentation. Should COD removal by PSAF rely solely on charge neutralization, an increase in stirring speed leading to enhanced cation release would result in a unidirectional effect on COD removal. If PSAF's COD removal operates exclusively via adsorption, accelerated stirring enhances the molecular sieve effect and COD removal efficiency. However, with increasing stirring speed, COD removal initially rises then declines. Thus, PSAF's removal of COD from dispersed oil droplets predominantly relies on bridging.

As shown in Figure 1(c), the COD removal rate of PSAF in 1–5 min was 3% CODmin−1, while that of PAC was 7.9% CODmin−1. There was an obvious difference in their flocculation mechanisms. When PAC is added to water, it will produce flocculent as hydroxide precipitation occurs. However, owing to their short branches, many parts of the mesh are still disconnected, and their adsorption and bridging capabilities are weak (Lin et al. 2020). In contrast, PSAF mainly relies on electric neutralization generated by the hydrolysis of metal ions. The anions of pollutants can be absorbed into the Fe polymer, and a small amount of micro-flocs is rapidly generated. The charge neutralization ability of PAC with equal molar concentrations is equal to that of PSAF, but the metal ions in PSAF undergo a strong hydrolysis reaction. Metal ions quickly play an electric neutralization role, accompanied by hydroxyl bridging polymerization, so that they can bridge faster.

Field experimental data

Parameters obtained after the experiment we carried out industrial tests in the maintenance fee water treatment plant at the base of specialization, due to the COD and Fe concentration of the raw water and the concentration of oil content are high, we adopted a composite process for the treatment of industrial wastewater, and on-site for a period of 15 days of maintenance and tracking tests, high concentration of oily wastewater through the aeration of air flotation tanks through the activated sludge reactors and then after the carrier. The COD concentration of the effluent was finally obtained by the flocculation process. The measured influent concentration and effluent concentration are the effluent from the activated sludge tank and the effluent from the flocculation reaction tank, respectively.

As shown in Figure 2, the removal rate of COD concentration in the first 4 days of the process is not satisfactory, only 33.3% on the first day, but with the stable operation of the equipment, the COD removal rate also rises gradually, the process is gradually stabilized after one week's operation and the removal rate of COD can be maintained at more than 80%, and the COD concentration in the effluent water is lower than 15 mg/L in the following 10 days, which is sufficient to show that the use of PSAF for oil removal is a good indication that the use of PSAF for oil removal is very effective.
Figure 2

Tracking data of COD concentration and removal rate.

Figure 2

Tracking data of COD concentration and removal rate.

Close modal

Effect of concentration on the morphology of PSAF flocs

It can be seen from the SEM-EDS (Figure 3(a)–3(c)) that increasing the dosage of the flocculant and the concentration of Fe can promote the polymerization of flocs, which is conducive to the formation of a flocs structure. Rapid hydrolysis of Fe in PSAF facilitates the formation of large, highly charged cations. Considering the charge neutralization mechanism, anionic oil droplets among suspended solids (SS) can be incorporated into the Fe polymer, resulting in the formation of small flocs (Decio et al. 2021). The concentration of Fe in Figure 3(a) and 3(b) was low; therefore, low-magnification SEM was chosen to capture the distribution of Fe in the flocs. The flocs formed by 25 mg L−1 of PSAF were a spreading flat layer, which tended to agglomerate but did not form a dense mesh structure; therefore, coagulation in local areas led to a poor treatment effect. When the concentration was increased to 50 mg L−1, the EDS showed that the Fe content increased and the flocs were more three-dimensional in their spatial distribution, but the layers could not adhere. Figure 5 shows that as the concentration of PSAF increases, the red points in the SEM image become denser. These red points represent Fe ions absorbed by the flocculant, and their increasing number suggests a higher degree of polymerization in the flocculant. The enhanced polymerization of PSAF contributes to a more robust flocculation structure. The improvement in PSAF polymerization helps improve the flocculation structure (Zhao et al. 2021).
Figure 3

The flocs morphology in water at 25 mg L−1 (a), 50 mg L−1 (b), 75 mg L−1 (c), and 100 mg L−1 (d) PSFA dosage.

Figure 3

The flocs morphology in water at 25 mg L−1 (a), 50 mg L−1 (b), 75 mg L−1 (c), and 100 mg L−1 (d) PSFA dosage.

Close modal

The observed structural changes in the SEM images occur due to the increased dosage of PSAF, leading to the generation of more chemical bonds such as Si–O–Al and Fe–O–Si bonds (refer to Section 3.5). This phenomenon results in flocs adopting a more stable laminar and reticulated structure, enhancing their efficiency in capturing oil bead molecules. Consequently, the floc structure transitions into a three-dimensional and stabilized form, allowing small flocs to grow larger and better fulfill their bridging role. This three-dimensional and stable floc structure enhances the adsorption and bridging capabilities of the flocs.

The Fe content in Figure 3(c) and 3(d) was higher, so higher magnification was used for the observations. Based on the results shown in Figure 3(a), the positive charge of Fe generated by the hydrolysis of 75 mg L−1 PSAF increased. With the assistance of polysilicic acid molecular chains, negatively charged oil droplets bridge hydroxyl with the Fe polymer to form layered or reticular flocs for sedimentation (Zhao et al. 2008). However, the higher concentration of Fe does not mean the flocs will be more stable. At a concentration of 100 mg L−1, electrons were generated and the flocs easier to sink; therefore, they could not be suspended in water for a long time to fully capture the oil droplets. Thus, an appropriate concentration of PSAF can play the role of medium-polymerized Fe (Decio et al. 2021). Only by ensuring that the flocs are suspended in water for an appropriately long time can they effectively absorb and capture nano-oil droplets and improve the treatment effectiveness (Uzunova & Mikosch 2004).

Effect of stirring speed on the PSAF flocs structure

As shown in Figure 4(a), the flocs formed at 60 rpm were a block that was unevenly distributed in the water, resulting in less adsorption and capture of oil droplets with blue fluorescence. With an increase in the stirring speed to 100 rpm, the dispersion of flocs increased. The bridging effect of the PSAF between oil droplets became prominent, and the flocs covered a larger area in the water. Upon increasing the stirring speed to 300 rpm, the flocs distribution became more dispersed, leading to partial disintegration. However, flocs had more contact with pollutants, and an obvious bridging effect occurred between the flocs, which was conducive to the maturation and precipitation of the flocs (Chen et al. 2020). Compared with the former two, the flocs generated at 300 rpm can absorb and remove more fluorescent oil droplets. Upon stirring at 400 rpm, most of the PSAF flocs were broken and dispersed, which was not conducive to flocs re-aggregation. Thus, the sorption of the oil drops was reduced (Suzaimi et al. 2021).
Figure 4

Effects of 60 rpm (a), 100 rpm (b), 300 rpm (c), and 400 rpm (d) on the PSAF flocs structure.

Figure 4

Effects of 60 rpm (a), 100 rpm (b), 300 rpm (c), and 400 rpm (d) on the PSAF flocs structure.

Close modal
Figure 5

The FITR characteristics of PAC flocs (at 300 and 100 rpm) and PSAF flocs (at 350 and 100 rpm), respectively.

Figure 5

The FITR characteristics of PAC flocs (at 300 and 100 rpm) and PSAF flocs (at 350 and 100 rpm), respectively.

Close modal

The removal of oil droplets and the COD in water by the PSAF mainly depended on the hydrolysis of Fe and Al. These can generate metal cations to adsorb negatively charged nano-oil droplets, forming small complex structures under the bridging effect of silicic acid and hydroxyl groups. This finally formed micro-flocs with colloidal oil droplets. Proper stirring at 300–350 rpm promoted the continuous growth of flocs. If the stirring speed was higher than 350 rpm, the flocs mesh was generally destructured into a small and irregular distribution, and the removal of soluble COD also declined sharply. Therefore, ensuring full contact between the flocculant and pollutants is crucial for aggregating dispersed flocs at the mature stage into larger clusters (Suzaimi et al. 2021). Adjusting the speed to 300 rpm is essential for effective oil removal.

Characteristics of infrared functional groups on the surface of PSAF flocs

As shown in Figure 5, the functional groups formed on the flocs of PAC300, PAC100, PSAF350, and PSAF100 were detected using FTIR. The Al–O appeared at 611 cm−1, while 1,138 cm−1 represented the vibration absorption peak of Si–O (Lao et al. 2018). The appearance of the Si–O–Al vibration peak and the change in peak strength showed that aluminum sulfate (Al2(SO4)3) and polysilicic acid are not simple composites, and Al and Si interact by chemical bonds. The PSAF increased the two absorption peaks between 700 and 780 cm−1, which were caused by the vibration or rotation of the Al–O–Si bonds and Fe–O–Si bonds (Zheng et al. 2014). The removal effect of PSAF350 was the best, followed by that of PAC300. The wave at 1,175 cm−1 corresponded to the absorption of the O–H hydroxyl, and the peak values were in the order of PAC300 > PSAF350 > PAC100 > PSAF100. The wave at 1,403 cm−1 was the C = O bond in the carbonyl or carboxyl group. The peak value of PAC300 was the highest, followed by PSAF350, PSAF100, and PAC100. It was speculated that Fe3+ and Al3+ were hydrolyzed in weakly acidic water and reacted with the nano-oil droplets to form carboxyl groups. The stronger the wave crest, the higher the oil content in the flocs, that is, the cleaner the sorption. The strong absorption peak at 1,450 cm−1 corresponded to the stretching vibration of the oil alkane CH. The flocs had obvious peaks at 2,852 and 2,925 cm−1, which corresponded to methyl and methylene, respectively, proving that flocculation captured the nano-oil droplets. In this band, the wave crest of PAC300 was the highest, followed by that of the PSAF350 > PAC100> PSAF100 flocs. Many hydrophobic hydrocarbon bonds can be observed because many oil contaminants are absorbed by flocs (Feng et al. 2021).

The hydroxyl group in polysilicic acid acted as a bridge for polymerization, forming highly charged poly nuclear Al/Fe hydroxyl complex ions and Al/Fe Si polymers with a certain degree of polymerization (Feng et al. 2021). From the peak of the functional groups, it was observed that the metal ions of PAC and PSAF at high stirring speeds can fully hydrolyze contacted nano-oil droplets and play a better role in chemical adsorption to remove pollutants. The PSAF molecule is not a simple combination of SiO2, Fe, and Al, but a co-polymerized form of complexation to generate a Si–Al–Fe polymer, which has strong capabilities of electric neutralization, adsorption, and bridging (Xu et al. 2017). This showed that the addition of Al and Fe helped to form a new bond bridge between polysilicic acid, which was conducive to the adhesion and bridging of the nano-oil droplets. Therefore, PSAF was superior to PAC in terms of mesh trapping.

Characteristics of non-polar surface free energy of PSAF

Compared with PAC300 and PAC100, the electron donor γ+ was higher than the electron acceptor γ in the surface tension (Table 2). The former was approximately three times that of the latter, indicating that the proportion of electron donors was more dominant. This proved that when PAC was added, flocs surface with a high positive charge and strong electrostatic adhesion was produced, especially for anionic oil colloids. The γAB of PAC300 was 20.62 mJ m−2, while the γAB of PAC100 was only 5.72 mJ m−2. This proved that the surface tension of a Lewis acid and base will decrease with the decrease of the stirring speed. When the stirring speed was 100 rpm, many negatively charged nano-oil droplets were swept by the PAC mesh. With an increase in the stirring speed, the PAC molecules were significantly dispersed, and the electric neutralization produced by hydrolysis played a major role. SS adhered to cationic Al to form a flocculent mesh that captured anionic oil droplets. However, the of 30.52 mJ m−2 of PAC100 was significantly lower than the 53.43 mJ m−2 of PAC300, indicating that the surface polarity of PAC flocs increased at 300 rpm, which was not conducive to the adsorption of non-polar oil droplets (Feng et al. 2021).

Table 2

The surface energy parameter of PAC and PSAF (mJ·m−2)

Contact angle (°)
Surface energy parameter (mJ·m−2)
(mJ·m−2)
H2OCH3NOCH2I2
PAC300 83.77 68.35 51.82 32.81 24.77 4.29 20.62 53.43 
PAC100 80.94 66.58 54.69 24.8 7.1 1.15 5.72 30.52 
PSAF350 88.88 74.7 39.53 20.29 0.43 6.4 6.31 26.6 
PSAF100 94.72 68.57 62.69 23.68 1.09 1.15 2.23 25.91 
Contact angle (°)
Surface energy parameter (mJ·m−2)
(mJ·m−2)
H2OCH3NOCH2I2
PAC300 83.77 68.35 51.82 32.81 24.77 4.29 20.62 53.43 
PAC100 80.94 66.58 54.69 24.8 7.1 1.15 5.72 30.52 
PSAF350 88.88 74.7 39.53 20.29 0.43 6.4 6.31 26.6 
PSAF100 94.72 68.57 62.69 23.68 1.09 1.15 2.23 25.91 

Compared with PSAF350 and PSAF100, the fragmentation of PSAF molecules had little effect on oil removal, which was because PSAF relied on Al–O–Fe-based complexation bridging with oil droplets. When the stirring speed was increased from 100 to 350 rpm, the surface free energy was always about 26 mJ m−2, indicating that changes in the stirring speed had little influence. Rutemman et al. obtained SFE values of about 30 mJ m−2 before mastication simulation, and of the materials tested, only PET-G, which was used for retention, had a surface free energy ranging from 19.5 ± 12.6 to 33.1 ± 3.1 mJ m−2, with good surface properties of low surface energy and a good range of bioconformity (Liber-Kneć & Łagan 2021).

However, the of PSAF100 and PSAF350 were 2.23 and 6.31 mJ m−2, respectively. This showed that increasing the stirring speed will increase the surface tension of the Lewis acid and base of PSAF (Suzaimi et al. 2021), mainly affecting its early electric neutralization, promoting the rapid hydrolysis of metals to form cations, facilitating the absorption of nano-oil droplets into metal polymers, and contributing to the formation of flocculent precipitates. As the stirring speed was increased from 100 to 350 rpm, the van der Waals force of the surface tension in PSAF flocs decreased from 23.68 to 20.29 mJ m−2. The decrease in compensated for the increase in (Uzunova & Mikosch 2004).

Consequently, an elevated stirring rate does not affect the free energy of the flocs surface . The preservation of non-polar surface tension in PSAF flocs is particularly advantageous for non-polar nano-oil droplets. In practical applications, an appropriate increase in the PSAF concentration at high stirring speeds will increase its effectiveness in water treatment.

The oil from the machinery used was replaced quarterly in the mining area, which made it difficult to remove the dispersed nano-oil droplets from the flushing machine wastewater. Concerning the PAC flocculant, the ability of PSAF to remove nano-oil droplets and reduce the COD was studied by investigating the concentration of the flocculant, stirring speed, and stirring time.

The optimal PSAF concentration was 75 mg L−1, and the corresponding COD removal was 47%. When the stirring speed was increased to 350 rpm, the COD removal increased to 71%, while the COD removal rates of PAC was 57%. The flocs grew continuously with an increase to 350 rpm. With an increase in the stirring speed, the COD removal first increased and then decreased. Fast stirring increased the contact probability between the flocs and oil droplets, but a higher speed broke the flocs, making it difficult for PSAF to bridge the precipitated flocs. At the same molar concentration, PSAF hydrolyzes cationic metals faster than PAC; therefore, short stirring for 1–5 min had a higher COD removal rate. The SEM observations showed that a high Fe content was conducive to the stable formation of flocs. The Fe in PSAF was rapidly hydrolyzed into a cationic polymer, which absorbed anionic SS to form flocs. PSAF forms flocs of smaller complex structures with greater bridging. The FTIR showed that, compared with PAC300, PSAF350 had a strong oil alkane CH absorption peak at 1,450 cm−1; that is, the adsorbed oil in the flocs was very high. The Al–O and Si–O peaks occurred at 611 and 1,138 cm−1, indicating the formation of Si–O–Fe and Si–O–Al bonds. Therefore, PSAF was superior to PAC in terms of bridging. It is worth noting that the increase in the stirring rate did not change the free energy of the flocs surface , mainly because the decrease in offset the increase in .

In practical application, 75 mg L−1 of PSAF was stirred at 350 rpm for 5 min, resulting in smaller, bridged flocs and the formation of Si–O–Fe and Si–O–Al bonds on the mesh surface. No significant change occurred on the non-polar surface. This stability enhances the flocculant's suspension time in water and facilitates the capture of nano-oil droplets.

It is difficult to remove nano-oil droplets by conventional flocculants. However, it can be well captured by using PSAF with strong bridging, which leads to a better COD removal effect. Nano-oil droplets are laden with substantial amounts of toxic hydrocarbons and heavy metals. The flocculant PSAF effectively targets and captures nano-oil droplets, significantly mitigating their environmental impact. Moreover, the release of cation from PSAF disrupts the stable state of the water-in-oil emulsion, causing the oil droplets to merge into larger beads. These larger beads can be easily extracted and purified from the wastewater back to crude oil. This study also provides ideas for the treatment and resource utilization for charged nano-pollutants, i.e. nano-metal. It provides examples and possibilities for the treatment of nano-pollutants using the PSAF adsorption and bridging.

This work was financially supported by the Natural Science Foundation of China (Grant No. 51808044) and Qinchuangyuan ‘Scientist + Engineer’ Team Construction Project of Shaanxi Province (2022KXJ-119), and Shendong Coal Branch Technology Innovation Project of China Shenhua Energy Co., Ltd (Grant No. CEZB210304069) and Key R&D Program of Shaanxi Province, China (2024SF-YBXM-535).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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