Manganese(II) Enhanced Ferrate(VI) Pretreatment: Effects on Membrane Fouling and Pollutants Interception

Abstract

To mitigate membrane fouling in the ultrafiltration process of surface water, this study focused on the source water from the Songhua River, systematically investigating the efficacy and mechanism of combined ferrate(VI) (Fe(VI)) and manganese(II) (Mn(II)) pretreatment in controlling ultrafiltration membrane fouling. Emphasis was placed on analyzing the impacts of pretreatment on membrane fouling performance, physicochemical properties of influent and effluent, membrane surface characteristics, and interfacial interactions. The results showed that the combined pretreatment with Fe(VI) and Mn(II) outperformed individual pretreatments and the untreated group significantly. When Fe(VI)/Mn(II) was 2/3, the normalized flux reached 0.66, a 35% increase compared to the untreated group; meanwhile, the pollutants retention was enhanced to 41.5%, with reversible and irreversible fouling resistances reduced by 75% and 77%, respectively. At this optimal ratio, the reaction products of Fe(VI) and Mn(II) coagulation acted as the core mechanism. It enhances pollutant particle repulsion, reduces particle size to form a loose structure, leading to a porous, hydrophilic membrane surface fouling layer with low roughness, thus minimizing membrane pore blockage. The combined pretreatment maintained a repulsive total interaction energy between pollutants and the membrane throughout the process, significantly reducing irreversible adsorption, which further verified the effectiveness of fouling mitigation. This study demonstrated that combined Fe(VI)/Mn(II) pretreatment at a molar ratio of 2:3 could efficiently control ultrafiltration membrane fouling by regulating pollutant characteristics and interfacial interactions, providing a theoretical basis and technical support for advanced treatment of surface water.

1. Introduction

Ultrafiltration (UF) membrane technology has found extensive application in drinking water treatment and wastewater recycling, thanks to its remarkable ability to trap microorganisms and macromolecular pollutants [1]. Ferrates are a class of compounds centered on the ferrate ion (${\mathrm{F}\mathrm{e}\mathrm{O}}_4^{2-}$), in which iron exists in the +6 oxidation state-a relatively rare high oxidation state of iron in compounds [2]. However, membrane fouling is one of the critical issues that restrict the development and application of ultrafiltration membrane technology [3,4]. To alleviate fouling in ultrafiltration membranes, pretreatment techniques including coagulation, adsorption, and oxidation have been commonly utilized [5,6,7]. Natural organic matter (NOM) is widely regarded as a primary cause of intensified membrane fouling and shortened service life of ultrafiltration membranes [8,9,10]. In the ultrafiltration of natural surface water, NOM inevitably builds up on the membrane surface or within its pores, and this accumulation leads to pore blockage, reduced membrane flux, and truncated operational lifespan [11,12].

Ferrate(VI), as an emerging green agent integrating oxidation and coagulation, has been widely studied and applied in water treatment [13]. Existing studies have shown that it exhibits excellent performance in removing organic micro-pollutants [14,15], heavy metals [16,17,18], microorganisms [19], and so on, which is due to its selective oxidizing property and the excellent coagulation and adsorption performance of its reduction products. Therefore, the role of Fe(VI) as a pretreatment technology for ultrafiltration in controlling membrane fouling is worthy of exploration. Of course, there have already been some studies reporting the positive effects of Fe(VI) applied as a pretreatment technology of ultrafiltration. When Fe(VI)-coupled ceramic membrane technology was applied to reclaimed water treatment, it greatly improved the quality of reclaimed water and significantly alleviated membrane fouling [20]. When Fe(VI) pretreatment was applied to the ultrafiltration process of secondary effluent, it significantly mitigated membrane fouling by removing macromolecular substances that block membrane pores and reducing hydrophobic adsorption between pollutants and the membrane [21]. A fundamentally similar fouling mitigation mechanism was also identified in studies on Fe(VI)-coupled ultrafiltration technology for shale gas wastewater treatment [22].

Given that Fe(VI) is relatively difficult to prepare [23] and its price remains high [24] at present, it is therefore quite necessary to strengthen research and application of Fe(VI)-related technologies. There have been reports on the application of Fe(VI) enhancement technologies in the treatment of micro-pollutants [25,26]. Of course, Fe(VI) enhancement technologies can also be extended to the pretreatment of ultrafiltration. The cheap NaClO reagent coupled with Fe(VI) improved 52.01% of membrane flux and 40.34% of DOC rejection ratio by forming more dispersive nanoparticles with more positive charge [24]. Fe(VI) and calcium sulfite can activate each other, which improved oxidation efficiency, eliminated intracellular organic matter in water, reduced the deposition of algal organic matter, and prevented the formation of filter cake layers in the ultrafiltration of algae-laden water [27]. It had reported that, reactive manganese species generated through the interaction between Fe(VI) and Mn(II) enhanced the utilization efficiency of Fe(VI)’s oxidative capacity and strengthened the removal of micro-pollutants [26]. Similar studies have pointed out that Mn ions promote the removal of diclofenac (DCF) by Fe(VI) [28,29]. Fe(VI) oxidized Mn(II) at a 2:3 stoichiometry, and the stoichiometry was not impacted by the presence of NOM [30], the reaction equation is shown in Equation (1).

$$2\mathrm{Fe}\left(\mathrm{VI}\right)+3\mathrm{Mn}\left(\mathrm{II}\right) \to 2\mathrm{Fe}\left(\mathrm{III}\right)+3\mathrm{Mn}\left(\mathrm{IV}\right)$$

(1)

This paper explores the efficacy of Mn(II)-enhanced potassium Fe(VI) as a pretreatment for ultrafiltration in fouling control, along with the underlying mechanisms involved.

This study has chosen manganese chloride (MnCl₂) as an intensifier to accelerate the reduction process of Fe(VI), promote the formation of nascent ferric particles and manganese oxide particles, and investigate the effects of this pretreatment method on membrane fouling control and pollutant retention during the ultrafiltration of Songhua River water. Subsequently, this study used membrane fouling model fitting and XDLVO theory analysis to explore the mechanisms underlying the formation and evolution of membrane fouling as well as the pollutant retention process.

2. Materials and Methods

2.1. Materials and Feed Water Sampling

The commercial flat-sheet ultrafiltration membrane used in this paper was purchased from Mosu Corporation, Shanghai, China. It is a Polyethersulfone (PES) ultrafiltration membrane with a molecular weight cutoff of 100 KDa and an effective membrane area of 26.4 cm². Potassium ferrate (K₂FeO₄) was synthesized in the laboratory using a previously reported method [23]. Fe(VI) solution was freshly prepared as needed, formulated quickly before each dosing; its exact concentration was determined by measuring absorbance at 510 nm via ultraviolet-visible (UV-Vis) spectrophotometer. Natural water sample for the experiment was sampled from the Harbin section of the Songhua River in May. Upon arrival at the lab, it was suction-filtered through quantitative filter paper (22 μm pore size) to remove suspended particles like sediment, then stored at 4 °C. The parameters of the stored Songhua River water samples were presented in Table S1. The Milli-Q water used in this experiment was obtained from an ultra-pure water system (Milli-Q Advantage A10, produced by Millipore Corporation, Billerica, MA, USA). MnCl₂, NaOH, HCl, NaHCO₃, methanol, ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Formamide and diiodomethane were purchased from Sigma-Aldrich, St. Louis, MO, USA. All reagents were at least analytical grade.

2.2. Filtration Experiment

The experiment adopted the water of the Songhua River as the target pollutant. The experimental samples main divided into raw sample (the Songhua River sample without pretreatment), Mn(II) (45 µM) treated sample, Fe(VI) (30 µM) treated sample, and Fe(VI)/Mn(II) treated sample abbreviating to FM 2:1 [30 µM Fe(VI)/15 µM Mn(II)], FM 2:3 [30 µM Fe(VI)/45 µM Mn(II)], and FM 2:5 [30 µM Fe(VI)/75 µM Mn(II)]. The specific pretreatment process before ultrafiltration was as follows: 1 L of Songhua River sample was placed on a program-controlled coagulation stirrer. Different concentrations of Mn(II) solution were added according to the above experimental requirements, followed by the fresh prepared Fe(VI) solution to initiate the coagulation process. The coagulation program was set as: rapid stirring at 200 r/min for 1 min, then slow stirring at 50 r/min for 20 min.

After the pretreatment process, the liquid was transferred to an ultrafiltration cell and a 1 L feed bottle connected to it, followed by the ultrafiltration experiment. The ultrafiltration experiment was performed using a flat-sheet ultrafiltration membrane filtration device under constant-pressure dead-end filtration conditions with a transmembrane pressure of 100 kPa. The ultrafiltration cell employed was Model 8200 from Millipore (USA), with a volume of 200 mL. A 1 L feed bottle was connected upstream of the ultrafiltration cell, and the volume of the contaminant solution used in the experiment was 1 L. Before filtration of sample, 1 L Milli-Q was filtered under the same conditions to ensure membrane compaction and record the pure water flux. After the filtration process, the membrane was backwashed with 200 mL Milli-Q, and then pure water flux was measured again. The schematic diagram of experimental setup and procedure was shown in Figure S1.

2.3. Membrane Fouling Analysis

The membrane’s normalized flux has its calculation formula as follows:

$$\mathrm{N}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d} \mathrm{f}\mathrm{l}\mathrm{u}\mathrm{x}=J/J_0$$

(2)

Here, J is the membrane permeate flux. J₀ is the pure water flux.

To assess how Fe(VI) pretreatment influences membrane fouling, fouling resistances were computed using the resistance-in-series model [31]. This model centers on intrinsic membrane resistance (m⁻¹) and reversible fouling resistance (m⁻¹), with step-by-step calculation procedures available in Text S1.

For exploring the mechanisms of pollutant deposition, the present study adopted the combined pore blockage-cake filtration model, which is applicable to constant dead-end filtration processes. This model integrates three types of fouling mechanisms: pore blocking (encompassing complete and intermediate blocking), pore constriction (referred to as standard blocking), and cake formation (known as cake fouling). These mechanisms are mathematically expressed by the equation below [32,33,34]:

$${\displaystyle \frac{d^{2}t}{{dV}^2}}=k{({\displaystyle \frac{dt}{dV}})}^n$$

(3)

Here, t denotes filtration duration and V stands for permeate volume. The filtration number n can take values of 0, 1, 1.5, and 2, corresponding to cake fouling, intermediate blocking, standard blocking, and complete blocking, respectively. A diagram illustrating these fouling mechanisms is presented in Figure S2, and the other details was shown in Text S2.

2.4. XDLVO Theory

The Extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory served to analyze the physicochemical interactions between foulants and the membrane, categorizing them into Lifshitz-van der Waals (LW), acid-base (AB), and electrostatic (EL) interactions [35,36,37,38]. As per this theory, the interfacial energy balance equals the sum of LW, AB, and EL energies, with detailed calculation formulas and descriptions provided in Text S3. Within this theoretical framework, Milli-Q water, formamide, and diiodomethane functioned as probing agents, and their surface tension values are listed in Table S3.

2.5. Analytical Methods

Total organic carbon (TOC) content in samples was determined using a Carbon/Nitrogen analyzer (model multi N/C 3100, manufactured by Analytic-Jena, Jena, Germany). A UV-visible spectrophotometer (Puxi T6, Beijing, China) was employed for multiple measurements: the absorbance of Fe(VI) solutions at 510 nm, the UV254 value of humic acid (HA) at 254 nm, and the concentration of bovine serum albumin (BSA) samples via the Lowry assay at 750 nm. Zeta potentials of solutions and particle sizes of natural organic matter (NOM) foulants were analyzed with a Zetasizer instrument (Nano-ZS90 from the Zetasizer Nano series, produced by Malvern Panalytical, Malvern, Worcs, UK & Almelo, The Netherlands). Solution pH was tracked using a PHS-3C pH meter (Leici, Shanghai, China). Contact angles of both foulants and membranes were measured using a contact angle goniometer (QSPJ-360, Jinshengxin, Beijing, China). Fluorescence excitation-emission matrix (FEEM) analysis was conducted to examine changes in fluorescent components, utilizing a fluorescence spectrophotometer (F7000, Hitachi, Tokyo, Japan). The surface morphology of membranes was characterized by a scanning electron microscope (SEM, model TM4000, Hitachi, Tokyo, Japan).

3. Results and Discussion

3.1. Performance of Membrane Fouling

The variation in ultrafiltration permeate flux following pretreatment with different Fe(VI)/Mn(II) systems was presented in Figure 1a. For 1 L of raw sample without pretreatment, the normalized flux dropped to 0.31 at the end of filtration, corresponding to a 69% reduction in flux. When pretreated with 45 μM Mn(II) alone, the flux decline curve was observed to almost overlap with that of ultrafiltration of raw sample, confirming that Mn(II) exerted negligible influence on membrane fouling. In the case of pretreatment with 30 μM Fe(VI), normalized flux decreased to 0.57 at the end of filtration, which demonstrates that Fe(VI) pretreatment significantly mitigated membrane fouling. The flux of raw sample filtration pretreated with the Fe(VI)/Mn(II) system was further enhanced compared to that pretreated with Fe(VI) alone. At a Mn(II) concentration of 15 μM (FM 2:1), the normalized flux after filtration was measured at 0.60, demonstrating that the presence of Mn(II) potentiated the membrane fouling mitigation efficacy of Fe(VI) pretreatment. When the Mn(II) concentration was increased to 45 μM (FM 2:3) and 75 μM (FM 2:5), the normalized flux after filtration increased to 0.66 in both scenarios relative to the control group. These results indicated that the optimal fouling control efficiency was achieved at the 2:3 molar ratio of Fe(VI) to Mn(II), which corresponds to the stoichiometric ratio of their oxidation-reduction reaction [30]. Additionally, excess Mn(II) did not exert a negative impact on the membrane fouling control performance of the Fe(VI)/Mn(II) pretreatment system. In conclusion, Mn(II) enhanced the ability of Fe(VI) to mitigate membrane fouling, with the optimal Fe(VI)/Mn(II) ratio being 2:3, consistent with the stoichiometry of their redox reaction.

The variations in reversible and irreversible fouling resistances during ultrafiltration following pretreatment with different Fe(VI)/Mn(II) systems were illustrated in Figure 1b. For untreated raw sample, reversible and irreversible fouling resistances were 2.98 × 10⁻¹² m⁻¹ and 1.61 × 10⁻¹¹ m⁻¹ after filtering 1 L of sample, respectively. Notably, reversible fouling resistance was substantially higher than irreversible fouling resistance—a phenomenon attributed to the large filtration volume (1 L) employed in the experiments, which also indicated that Songhua River water is less prone to inducing irreversible fouling during ultrafiltration. Pretreatment with Mn(II) alone yielded reversible and irreversible fouling resistances of 2.90 × 10⁻¹² m⁻¹ and 1.55 × 10⁻¹¹ m⁻¹, respectively, values that were comparable to those observed for ultrafiltration of untreated sample. Fe(VI) pretreatment resulted in a 63% reduction in reversible fouling resistance (1.10 × 10⁻¹² m⁻¹) relative to untreated sample, whereas irreversible fouling resistance (1.23 × 10⁻¹¹ m⁻¹) was only reduced by 24%. These findings indicated that Fe(VI) pretreatment effectively mitigates reversible fouling but exerted limited influence on irreversible fouling. Within the Fe(VI)/Mn(II) system, both reversible and irreversible fouling resistances continued to decline at a Mn(II) concentration of 15 μM (FM 2:1). Minimum values were attained at a Mn(II) concentration of 45 μM (FM 2:3), with reversible and irreversible fouling resistances decreasing to 7.56 × 10⁻¹¹ m⁻¹ and 3.67 × 10⁻¹⁰ m⁻¹, respectively—representing a 75% reduction in reversible fouling and a 77% reduction in irreversible fouling compared to untreated water. Importantly, Fe(VI)/Mn(II) pretreatment demonstrated a more pronounced mitigation effect on irreversible fouling than Fe(VI) pretreatment alone. In conclusion, Fe(VI) pretreatment significantly alleviates reversible fouling during ultrafiltration but exhibits limited efficacy in reducing irreversible fouling. Conversely, the Fe(VI)/Mn(II) system effectively mitigates both reversible and irreversible fouling during ultrafiltration.

3.2. Effects of Pretreatment on Physicochemical Properties of Influent and Effluent

3.2.1. Pollutants Removal Efficiency

The pollutant removal performance of ultrafiltration systems treating Songhua River water following pretreatment with various Fe(VI)/Mn(II) systems was depicted in Figure 2. The raw Songhua River water utilized in this study exhibited a TOC concentration of 5.9 ± 0.3 mg/L, with ultrafiltration alone achieving a TOC removal efficiency of 30.2%. Pretreatment with Mn(II) alone resulted in a comparable TOC removal efficiency of 29.7%, confirming its negligible impact on TOC reduction. Similarly, Fe(VI) pretreatment alone provided only a marginal improvement in TOC removal, increasing efficiency to 31.1%. Within the Fe(VI)/Mn(II) pretreatment-ultrafiltration system, the addition of 15 μM Mn(II) (FM 2:1) significantly enhanced TOC removal to 34.4%. At a Mn(II) dosage of 45 μM, TOC removal efficiency reached 41.5%—an 11.3% improvement over filtration without pretreatment. Further increasing Mn(II) dosage to 75 μM produced no appreciable change in TOC removal, indicating that optimal pollutant removal in the Fe(VI)/Mn(II) pretreatment-ultrafiltration hybrid system occurred at a Fe(VI)/Mn(II) ratio of 2:3. In the Fe(VI)/Mn(II) pretreatment-ultrafiltration coupling system, the TOC removal efficiency was probably related to oxidation and coagulation. In the above phenomenon, the TOC removal efficiency was the highest when Fe(VI)/Mn(II) = 2/3. At this ratio, the oxidizing property of Fe(VI) was completely consumed in the reaction of oxidizing Mn(II) rather than oxidizing organic matter in the river water. In other words, when Fe(VI)/Mn(II) = 2/3, Fe(VI) did not react with organic matter in the river water. Based on this, a preliminary conclusion was drawn: the excellent control effect of Fe(VI)/Mn(II) pretreatment on membrane fouling when Fe(VI)/Mn(II) = 2/3 was based on the coagulation of the reaction products of Fe(VI) and Mn(II), rather than the oxidation of Fe(VI).

3.2.2. Analysis of Fluorescence Characteristics

The FEEM spectra of ultrafiltration effluents derived from raw sample of Songhua River pretreated with various Fe(VI)/Mn(II) systems was presented in Figure 3. Natural water bodies are complex mixtures containing diverse substances, among which natural organic matter could be categorized into distinct classes based on the unique excitation and emission wavelengths of characteristic groups in different fluorescent components. Typically, fluorescent substances in natural waters were distributed across five regions: Regions “A” and “B” correspond to aromatic protein; Region “C” to fulvic acid-like substances; Region “D” to SMP (soluble microbial product)-like substances; and Region “E” to humic-like substances [20,39]. As indicated in Figure 3, SMP-like substances dominated the composition of Songhua River water sampled in this study. Thus, this section focuses on analyzing changes in fluorescent substances in the water after ultrafiltration (following pretreatment with different Fe(VI)/Mn(II) systems) by assessing the content of this specific class—specifically, the peak intensity in Region “D”. For individual fluorescent components, regional peak volume served as a valid quantitative indicator of their content. Calculations revealed that, compared to raw water, fluorescent substances in the ultrafiltration effluent without pretreatment was reduced by 31.2%, while pretreatment with Fe(VI) yields a removal rate of 33.7% for such substances. In the Fe(VI)/Mn(II) pretreatment system, when Mn(II) concentrations are 15, 45, and 75 μM, the rejection rates of fluorescent substances were 35.4%, 42.3%, and 37.8%, respectively. A comparison with TOC removal data showed a high degree of consistency in pollutant rejection efficiency between the two datasets. This indicated that pretreatment with different Fe(VI)/Mn(II) systems could not alter the structure of fluorescent substances in Songhua River water, and the pretreatment-coupled ultrafiltration system exhibited selective rejection of fluorescent substances in the river water.

3.2.3. Transformation of the Characteristics of Pollutant Particles

Coagulation performance of ferric particles and manganese dioxide—reaction products of Fe(VI) and Mn(II)—by analyzing changes in the zeta potential and contaminant particle size of Songhua River water after pretreatment with different Fe(VI)/Mn(II) systems, and the relevant data are presented in Figure 4 and Table S2. As shown in the figure, the zeta potential of raw Songhua River water is −18.1 mV, indicating that the water sample used in this experiment exhibited electronegativity under natural conditions. Treatment with 45 μM Mn(II) alone had almost no impact on the zeta potential or contaminant particle size of the river water. After pretreatment with 30 μM Fe(VI), the zeta potential of the mixed system decreased to −25.4 mV. According to Figure S3, the zeta potential of products from a 30 μM Fe(VI) solution after 30 min of self-decay reaction was −24.5 mV at pH 7, suggesting that ferric particles generated by Fe(VI) pretreatment introduced additional negative charges into the mixed system during coagulation. When Fe(VI) and Mn(II) were used in combination, varying concentrations of Mn(II) all led to a further reduction in zeta potential. Specifically, at a Fe(VI)/Mn(II) molar ratio of 2:3, the zeta potential reaches −37.03 mV. This indicates that the co-reaction products (mixture of ferric particles and MnO₂) of Fe(VI) and Mn(II) exhibited a more pronounced negative charge (lower zeta potential). A decrease in the negative zeta potential indicated a stronger repulsive force between pollutant particles, meaning that pollutant particles were less likely to aggregate [24]. During coagulation and ultrafiltration, pollutants inevitably approached each other under the action of stirring and water pressure. The increased zeta potential after Fe(VI)/Mn(II) pretreatment ensured that there were still pores between aggregated pollutant particles instead of tight binding. In addition, the surface potential of the PES flat-sheet ultrafiltration membrane was −14.02 mV, which also exerted a repulsive effect on negatively charged pollutant particles. The reduced zeta potential after Fe(VI) or Fe(VI)/Mn(II) pretreatment increased this repulsive effect, preventing pollutants from tightly adhering to the membrane surface and thus making them easier to remove through hydraulic cleaning. This could also explain why both reversible and irreversible fouling during ultrafiltration after pretreatment were significantly reduced, as discussed in Section 3.1.

The average particle size of contaminants in the river water was 3798 nm. These were residual components after the removal of suspended solids and sediment via filtration through a 22 μm quantitative filter paper, so their particle size was slightly smaller than the pore size of the filter paper. The sole addition of Mn(II) exerted no significant effect on the particle size of contaminants in the raw water; although the value decreased to 3573 nm, it remained within the measurement error range of the instrument. After pretreatment with Fe(VI), the particle size of contaminants decreased significantly to 2531 nm. This could be attributed to the increased zeta potential after coagulation, which enhanced the repulsive force between particles, compressed the volume of large-sized particles, inhibited the agglomeration of small particles, and additionally, the shaking operation during sample measurement caused the already agglomerated particles to redisperse. Pretreatment with the Fe(VI)/Mn(II) system further reduced the particle size of contaminants, reaching a minimum of 1612 nm when the ratio of the two was 2:3. When contaminants accumulate on the membrane surface, it is possible that small-sized particles form porous channels under the action of strong electrostatic repulsion, which may in turn contribute to the construction of a loose fouling layer. In conclusion, Fe(VI)/Mn(II) pretreatment could reduce the zeta potential and particle size of contaminants; this reduction might further lead to the possibility that contaminants form a loose and porous fouling layer on the membrane surface after ultrafiltration.

3.3. Membrane Surface Characteristics Analysis

3.3.1. Membrane Surface Morphology Analysis

The microscopic morphology of foulants on the membrane surface after ultrafiltration of Songhua River water, and the effects of various Fe(VI)/Mn(II) pretreatment systems on such morphology, are illustrated in Figure 5a. After 1 L of Songhua River water was ultrafiltered through a PES membrane, massive foulants formed large agglomerates on the membrane surface. These densely stacked agglomerates fully covered the membrane, obscuring its pores and causing severe flux decline and membrane fouling. Pretreatment with 45 μML Mn(II) showed negligible impact on foulant morphology, as the fouling layer remained composed of large, dense block-like accumulations. In contrast, 30 μM Fe(VI) pretreatment significantly reduced foulant agglomerate size, resulting in a loose and porous fouling layer that minimized resistance to filtration and enhanced permeate flux. For the Fe(VI)/Mn(II) system, FM 2:3 and FM 2:5 pretreatments yielded a more uniform, porous fouling layer with smaller pores than those observed after sole Fe(VI) pretreatment, and no obvious agglomerates. However, FM 2:1 pretreatment led to relatively large agglomerates. These comparisons indicated that Fe(VI) pretreatment reduced foulant agglomeration, while the Fe(VI)/Mn(II) system coupled system further diminished foulant size, yielding a more uniform and porous fouling layer—consistent with the observed significant flux improvement over non-pretreated conditions.

Figure 5b presented macroscopic images of membrane fouling after ultrafiltration with different pretreatments, showing uniform foulant distribution across the membrane surface. The fouling layer of raw Songhua River water appeared light brown, with Mn(II) addition failing to alter this color. Fe(VI) pretreatment resulted in a distinct yellowish-brown fouling layer, characteristic of ferric particles. Fe(VI)/Mn(II) pretreatment yielded a reddish-brown layer—attributed to mixed ferric particles and manganese oxides. FM 2:3 and FM 2:5 pretreatments produced nearly identical colors, darker than that of FM 2:1; this is because the FM 2:1 group contained less Mn(II) (yielding less manganese oxide), while excess Mn(II) in FM 2:5 remained unreacted. Notably, Fe(VI) was fully converted to equivalent ferric particles via reduction or self-decomposition in all cases.

3.3.2. Analysis of Membrane Surface Roughness

AFM images and surface roughness parameters of the virgin PES membrane and the membrane surfaces after ultrafiltration with different Fe(VI)/Mn(II) pretreatment systems were showed in Figure 6. Since the mean square value of roughness (Rq) and average roughness (Ra) of each group follow the same relative trend, only the average roughness is discussed below. From the roughness data, the average roughness of the virgin PES membrane is 4.63 nm, while that of the membrane fouled by Songhua River water increases to 154 nm—consistent with the large pollutant agglomerates observed in the SEM images. After Fe(VI) pretreatment, the average surface roughness decreased to 79.2 nm, which is attributed to the coagulation of Fe(VI) decomposition products with pollutants in the river water, reducing pollutant particle size. As indicated by particle size data, the deposition of more homogeneous pollutant particles on the membrane surface leads to lower average roughness. In the Fe(VI)/Mn(II) system, the addition of Mn(II) further reduced membrane surface roughness, with a continuous decrease as Mn(II) dosage increases—indicating enhanced retention of natural organic matter in the fouling layer. The minimum average roughness of 39.3 nm was achieved with FM 2:3 pretreatment. From the membrane surface characteristics, after ultrafiltration of Songhua River water, a dense fouling layer composed of large blocky pollutants formed on the membrane surface. This fouling layer would block membrane pores, leading to a severe decline in flux. After pretreatment with Fe(VI), the membrane surface roughness decreased, indicating a reduction in the size of protrusions or depressions in the membrane surface fouling layer, and the fouling layer showed a loose and porous state, which corresponded to the increase in membrane flux after Fe(VI) pretreatment. Similarly, after pretreatment with Fe(VI)/Mn(II), the membrane surface roughness further decreased, which corresponded to a further increase in membrane flux and an improvement in pollutant rejection.

3.3.3. Analysis of Hydrophilicity/Hydrophobicity of Membrane Surface

The hydrophilicity/hydrophobicity of the membrane surface fouling layer after different pretreatments was characterized by contact angle data measured using pure water as the probe reagent, as shown in Figure 7. The contact angle of the fouling layer after ultrafiltration of Songhua River water was 69°, which is more hydrophobic compared to the uncontaminated membrane (54°), thus making it more prone to membrane fouling. After Fe(VI) pretreatment, the contact angle of the fouling layer was 48°, showing better hydrophilicity compared to Songhua River water without pretreatment. This indicated that the ferric particles, a decomposition product of Fe(VI), could reduce the hydrophobicity after coagulating with organic pollutants in the river water. After ultrafiltration with Fe(VI)/Mn(II) pretreatment, the contact angle of the fouling layer further decreased. When the Fe(VI)/Mn(II) ratio was 2/3, the contact angle was 24°, indicating that the mixture of ferric particles and manganese oxide mixture produced by the reaction of Fe(VI) and Mn(II) has hydrophilicity. Moreover, by comparing the data of FM 2:3 group and the FM 2:1 group, it can be seen that the more MnO₂ present, the stronger the hydrophilicity. In this experiment, Fe(VI)/Mn(II) pretreatment showed the best hydrophilicity when the Fe(VI)/Mn(II) ratio was 2/3. The stronger the hydrophilicity of the membrane surface fouling layer, the easier it is for water molecules to pass through the pores of the fouling layer and the membrane pores into the effluent. Therefore, Fe(VI)/Mn(II) pretreatment improved the hydrophilicity of the membrane surface fouling layer, enhanced the water permeability of the fouling layer, and alleviated the flux decline during membrane fouling, that is, mitigated membrane fouling.

3.4. Fouling Models Fitting

The pore blocking-cake filtration model was used to investigate the fouling formation mechanism and control effect of membrane fouling under different Fe(VI)/Mn(II) pretreatment systems, as shown in Figure S4, all the data fitted well with this model (R² > 0.99). To further analyze the transition of membrane fouling modes, the n value was calculated by fitting the data of d²t/dV² versus dt/dV curves in Figure 8. The model identified two primary fouling stages for raw river water: a transition stage (n < 0) [21,40] and a cake layer stage (n = 0), with the transition occurring at ~59.6 mL filtration volume. All pretreatment systems follow this two-stage pattern, though Fe(VI) and Fe(VI)/Mn(II) pretreatments slightly reduced the transition volume. Given that river water pollutants (Figure 4) are far larger than ultrafiltration membrane pores (1–100 nm), pore blocking is irrelevant. Thus, the initial phase (prior to cake layer formation) corresponds to the transition stage (n < 0). While Fe(VI) or Fe(VI)/Mn(II) may reduce pollutant size, Fe(VI) pretreatment only converted pore blocking (n > 0) to the transition stage without altering the primary fouling mode. Conclusively, cake layer formation dominated Songhua River water ultrafiltration fouling, and this primary mechanism remained unaltered by Fe(VI) or Fe(VI)/Mn(II) pretreatments. Further, it was indicated that the mitigation of membrane fouling by Fe(VI)/Mn(II) system pretreatment was related to the properties of the fouling layer. As discussed earlier, the properties of the membrane surface fouling layer, i.e., the cake layer, after pretreatment with Fe(VI) and Fe(VI)/Mn(II) system tended to be more loose, porous, and hydrophilic. Therefore, the principle underlying the mitigation of membrane fouling by Fe(VI)/Mn(II) system pretreatment was the rapid formation of a loose, porous, and hydrophilic cake layer.

3.5. Analysis of Interaction Between Membrane and Foulant During Filtration

The interfacial interactions between the membrane and pollutants during the ultrafiltration and ultrafiltration with different Fe(VI)/Mn(II) systems pretreatment were analyzed using the XDLVO theory. The contact angles of membrane surfaces after ultrafiltration with different pretreatment systems, measured using various probe reagents, and the surface free energies between the membrane surfaces and pollutants calculated therefrom are shown in Table S4. The curves of the interfacial interaction energy between the membrane and pollutants as a function of separation distance, calculated based on these data, are presented in Figure 9. During the ultrafiltration of Songhua River water, the LW and AB interaction energies remained attractive within the separation distance and increased as the separation distance decreases. In contrast, the EL interaction energy exhibited repulsive behavior throughout the separation process and also increased with decreasing separation distance. As a result, the total interaction energy first showed a gradually increasing repulsive effect with decreasing separation distance, then the repulsive effect decreased to a critical point and transformed into an attractive effect. From the pollutant-membrane perspective, during ultrafiltration, pollutants in the Songhua River water experienced increasing repulsion as they approached the membrane, hindering their proximity and passage through the membrane surface—aiding pollutant interception. Under pressure, the pollutant-membrane distance continued to decrease. Once repulsion diminished to zero, pollutants were attracted to the membrane surface, adhering to it and causing fouling. Additionally, such adhered pollutants were more likely to permeate membrane pores into the effluent under pressure, degrading water quality. This process indicated that pollutants only needed to overcome a relatively weak repulsive force to adhere to the membrane and form fouling. Pretreatment with Mn(II) had little effect on the interfacial interaction between the membrane and pollutants. After pretreatment with Fe(VI), it could be observed that at the same separation distance, the LW and AB interaction energies, which exhibit attractive effects, decreased, while the EL interaction energy increased. Therefore, although the total interaction energy still showed the phenomenon of first repulsion and then attraction with decreasing separation distance, the maximum repulsive effect was significantly greater than that during the filtration process. It was shown that after Fe(VI) pretreatment, pollutants needed to overcome a greater repulsive force when approaching the membrane surface. Compared with the scenario without pretreatment, the membrane surface was less likely to be approached and adhered to by pollutants, thus reducing fouling, and pollutants were less likely to penetrate membrane pores and contaminate the effluent. Therefore, after Fe(VI) pretreatment, the membrane flux was significantly higher than that in the case of Songhua River water fouling, and the pollutant rejection rate was also improved. After pretreatment with the Fe(VI)/Mn(II) system, the AB interaction energy in the interaction energy between the membrane and pollutants during ultrafiltration transformed into a repulsive effect. The LW interaction energy at the same separation distance continued to decrease compared to other groups. The total interaction energy showed repulsive behavior throughout the entire separation process, and the repulsive effect increased rapidly as the separation distance decreased. Among them, the total interaction energies of FM 2:3 and FM 2:5 still exhibited stronger repulsive effects than that of FM 2:1. Although Fe(VI) pretreatment increased the resistance for pollutants to approach the membrane, it ultimately still exhibited an attractive effect. Pollutants adsorbed on the membrane surface were not easily removed by hydraulic cleaning, resulting in irreversible fouling. In contrast, after pretreatment with the Fe(VI)/Mn(II) system, a repulsive effect was observed when pollutants approached the membrane surface, meaning that pollutants would not adsorb on the membrane due to interfacial interactions. Consequently, it caused less irreversible fouling, which was consistent with the phenomenon that irreversible fouling was significantly reduced after pretreatment with the Fe(VI)/Mn(II) system.

As ultrafiltration proceeds, pollutants gradually covered the membrane surface, and the interaction exerted when pollutants approached the membrane surface transformed into the interfacial interaction between pollutants. The surface free energies between pollutants are presented in Table S5, and the curves of the interfacial interaction energy between pollutants as a function of separation distance, calculated therefrom, are shown in Figure S5. During the filtration of Songhua River water, the variation pattern of the total interfacial energy between pollutants was almost the same as that between the membrane and pollutants. The difference was that at the same separation distance, both the LW and EL interaction energies decreased, and the critical separation distance at which the total interaction energy transformed from repulsive to attractive increases slightly. After pretreatment with Fe(VI) alone, the LW interaction decreased while the EL interaction increased. Consequently, the total interaction energy exhibited a larger repulsive peak and a smaller critical separation distance. Thus, as the fouling layer (composed of ferric particles and pollutants) formed, pollutants required greater repulsive force to approach the membrane. Strong inter-pollutant repulsion within this layer prevented dense accumulation, favoring a loose, porous structure—consistent with zeta potential analysis and membrane surface morphology. Fouling layer formation enhanced membrane fouling control and pollutant interception. Similarly, after pretreatment with the Fe(VI)/Mn(II) system, the AB interaction energy transformed into a repulsive effect, and the EL interaction energy also increased significantly. The total interaction energies for Fe(VI)/Mn(II) ratios of 2/3 and 2/5 showed further increased repulsion. In summary, Fe(VI) pretreatment makes the total interaction energy exhibited greater repulsion by reducing the LW interaction energy and increasing the electrical double layer interaction energy. Thus, as filtration proceeded, the repulsive effect of the fouling layer on pollutants increased, making it difficult for pollutants to approach the fouling layer. The formed fouling layer became looser and more porous, and pollutants were more easily intercepted on the membrane surface side. In contrast, after pretreatment with the Fe(VI)/Mn(II) system, the AB interaction energy transformed into a repulsive effect, causing the total interaction energy to remain repulsive throughout the entire separation distance. In addition, the interaction between pollutants had basically the same variation pattern as that between the membrane and pollutants. However, the fouling layers after pretreatment with different systems exhibited better fouling repulsion and pollutant interception effects than the membrane surface.

4. Conclusions

This study explored the control effect and mechanism of Fe (VI) and Mn (II) pretreatment on ultrafiltration membrane pollution in the source water of the Songhua River, and reached the following conclusions:

(1)

The combined pretreatment with Fe(VI) and Mn(II) exhibits significantly better performance in alleviating ultrafiltration membrane fouling compared to individual Fe(VI) pretreatment, individual Mn(II) pretreatment, or no pretreatment. The optimal effect is achieved at a molar ratio of 2:3 with 30 μM Fe(VI) and 45 μM Mn(II), which increases the normalized flux to 0.66, representing a 35% improvement over the untreated group. Additionally, this ratio reduces reversible and irreversible fouling resistances by 75% and 77% compared with the filtration alone, respectively.

(2)

At the optimal ratio, Fe(VI) and Mn(II) react completely, and the coagulation of their reaction products serves as the core mechanism. This not only enhances the total organic carbon removal efficiency to 41.5%, an 11.3% increase compared to the untreated group, but also effectively removes the main fluorescent substances, soluble microbial products, in water without altering their structure.

(3)

The combined pretreatment significantly modifies pollutant properties, reducing the zeta potential to −37.03 mV to enhance inter-particle repulsion and decreasing the average particle size to 1612 nm to facilitate the formation of a loose structure. This results in a porous, hydrophilic fouling layer with a contact angle of 24° and low roughness of 39.3 nm on the membrane surface. And from the model fitting results, the main fouling pattern was cake layer fouling, therefore the formation of such a fouling layer helped alleviate membrane fouling and enhance pollutant rejection.

(4)

Based on the XDLVO theory, the combined pretreatment at the optimal ratio maintains a repulsive total interaction energy of pollutants-membrane and pollutants-pollutants in the filtration process, significantly reducing irreversible fouling. This further confirms the effectiveness of fouling mitigation.