Magnetic Metal–Organic Framework Enhanced Inorganic Coagulation for Water Purification

Abstract

Green water treatment technologies are widely popular, and magnetic coagulation is one of the most popular methods and has been successfully applied in industry. Among them, magnetic seeds are crucial for the flocculation of contaminants. The objective of this work was to investigate the potential of magnetic metal–organic frameworks (MMOFs) as a seed in assisting polymeric ferric sulfate (PFS) flocculant, specifically exploring their applicability in algal-contaminated water. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, ferrite timing spectroscopy, and flocculation tests were used to characterize the structure and flocculation properties of MMOFs and PFS (PFS-MMOFs) composites, highlighting the stability of magnetic seed MMOFs and the flocculation effect of the composites. The results show that MMOFs have good dispersion and stability in acidic PFS solutions, which are favorable for engineering applications. MMOFs and PFS are bonded by hydrogen bonds, which enhance the polarity and dispersion of MMOFs, as well as the molecular chains of PFS. In the presence of MMOFs, it affected the distribution of iron species in the PFS, which means that the performance of coagulation may be changed. Coagulation with PFS-MMOFs was effective under different hydraulic conditions. It also showed better results than PFS in terms of dissolved organic carbon (DOC) removal and ultraviolet absorption value at 254 nm (UV₂₅₄). In addition, the PFS-MMOFs in algal-infested waters were superior to the PFS. Overall, the findings tested in this study indicated that MMOFs are good magnetic seeds for remediation of water pollution in conjunction with PFS, potentially enhancing conventional coagulation.

1. Introduction

Water is a scarce resource and a necessary ingredient for human survival [1]. Some countries are experiencing rapid economic development but still lack water resources, mainly due to large populations and a shortage of available freshwater resources. For example, China’s per capita water consumption is about 2100 m³, which is one-quarter of the global average [2]. However, it is still the most severely water-scarce country [3]. As the water-using population and economy increase, the demand for clear water is also increasing rapidly [4,5], while the destruction of ecosystems due to human activities has caused serious water quality degradation and poses a potential threat to human health [6,7]. Among them, the explosive growth of algae caused by eutrophication is also a serious problem of water pollution [8,9]. Because it can lead to taste and odor, oxygen depletion, and toxin production [10]. Remediation of water pollution is one of the methods to increase the amount of water resources [11,12,13]. Therefore, it requires advanced water or wastewater treatment technologies, and research on cost-effective and eco-friendly pathways has become a hot topic [14,15,16,17].

Coagulation is a cost-effective technology that is widely used in water and wastewater treatment [18,19], where coagulants play a key role [20]. Meanwhile, flocculants are also often used to remove algae [9]. Coagulants for algae removal are efficient and versatile, suitable for diverse water bodies [21]. They swiftly eliminate algae and harmful substances with simple operation [22]. In recent years, magnetic materials have received a lot of attention [23,24,25], and the development of new magnetic coagulants, hoping to achieve fantastic coagulation effects, requires the selection of a new magnetic feed. Magnetic particles, namely Fe₃O₄ nanoparticles, are the most commonly used magnetic seeds. When combined with conventional coagulants, they are able to produce some exceptional purification results. For example, only a short time is required to precipitate flocs [26,27,28]. However, magnetic seeds are significantly affected by the external environment, and it is crucial to make a magnetic seed to make it stable in an acidic solution environment and avoid the destruction of the strongly acidic flocculant solution environment. Moreover, it is important to extend the function of flocculants beyond just the acceleration of floc settling time. Metal–organic backbones (MOFs), characterized by porous structure, low density, high adsorption capacity, and high specific surface [29,30], have been widely used as drug carriers [31,32], sensors [33,34], catalysts [34,35], and hydrogen storage materials [36,37]. Despite their wide range of applications, many MOFs are still unstable in water, and few applications in water treatment have been reported [38]. Fortunately, there are some water-stable MOFs such as ZIFs (Zeolitic imidazolate framework), UiO-66 (Universitetet i Oslo) and MIL (Materials of Institute of Lavoisier) that have been rapidly developed [26,39]. These MOFs are readily modified by -NH₂, -SH₂, and magnetization [40]. In our previous study, a magnetic coagulant was developed using magnetic metal–organic frameworks (MMOFs) as seeds for the modification of Fe-based coagulants where the magnetic particles were encapsulated by a shell layer of SiO₂ and MOFs, which may prevent the magnetic nanoparticles from being attacked by acids, as reported in other literature [41,42]. In-depth studies still need to discuss the mechanism of their stable existence, necessitating a systematic characterization analysis. Research on the development of such magnetic seeds—magnetic MOFs—will demonstrate the scalable potential of the coagulation function.

In this study, magnetic Fe₃O₄ was encapsulated by SiO₂ shells, and stable and uniformly dispersed magnetic nanoparticles Fe₃O₄@ SiO₂ (MNPs) were synthesized. UiO-66-2COOH was loaded on the surface of MNPs to prepare magnetic metal–organic frameworks (MMOFs). Subsequently, the MMOFs were introduced into polyferric sulfate (PFS), referred to as PFS-MMOFs. They were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The distribution of iron species in PFS-MMOFs was examined using ferron-timed spectrophotometry. The formation mechanism was further discussed. Coagulation efficiency was evaluated by measuring turbidity, ultraviolet absorbance at 254 nm (UV₂₅₄), and dissolved organic carbon (DOC), while the flocculation effect was also tested symmetrically under different hydrodynamic conditions. In addition, we found it effective for the removal of algae, and a comparison with the PFS was made in this study.

2. Materials and Methods

2.1. Materials

Ferric chloride hexahydrate (FeCl₃·6H₂O, AR) was purchased from Taishan Chemical Factory Co., Ltd. (Taishan, China). Anhydrous sodium acetate (CH₃COONa, AR), ammonium hydroxide (NH₃·H₂O, 30%wt), ferrous sulfate (FeSO₄·7H₂O, AR), 1, 10-phenanthroline monohydrate (C₁₂H₈N_2, AR), acetic acid (CH₃COOH, AR), ferron reagent (8-hydroxy-7-iodoquinoline-5-sulfuric acid, AR), and iron dust (Fe, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pyromellitic acid (BTEC, AR), tetraethyl orthosilicate (TEOS, AR), and zirconium (IV) chloride (ZrCl₄, AR) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Hydrochloric acid (HCl, AR) and concentrated sulfuric acid (H₂SO₄, AR) were purchased from Zhuzhou Xingkong Chemical Co., Ltd. (Zhuzhou, China). Sodium chlorate (NaClO₃, AR), Ascorbic Acid (C₆H₈O₆, AR), and potassium bromide (KBr, AR) were purchased from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Ethylene glycol (EG, AR) was obtained from Guangdong Polytron Technologies Inc. (Shantou, China). Sodium carbonate anhydrous (NaCO₃, GR) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Ethanol (C₂H₆O, AR) and sodium hydroxide (NaOH, AR) were obtained from Huihong Reagent Co., Ltd. (Changsha, China) and Yongda Chemical Reagent Co., Ltd. (Tianjin, China), respectively.

2.2. Preparation of PFS-MMOFs

In this study, Fe₃O₄ nanoparticles (Nano-Fe₃O₄), Fe₃O₄@SiO₂, UiO-66-2COOH (MOF), and MOF-modified MPNs (denoted as MMOFs) were prepared according to our previous publication [26]. Briefly, Fe₃O₄ nanoparticles were prepared according to the solvothermal method [42]. The Stober method was used to obtain silica-shell nanoparticles, Fe₃O₄@SiO₂ (MNPs). The UiO-66-2COOH was prepared using the same method. PFS was prepared from concentrated sulfuric acid and sodium chlorate using a typical method [26]. Combined solutions of PFS and MMOFs were prepared by mixing PFS and MMOFs in a beaker for 24 h of aging to produce a stable mixed coagulant (PFS-MMOFs). Figure 1 shows a simple scheme for the synthesis of PFS-MMOFs. Figure 1 shows a simple scheme for synthesizing PFS-MMOF.

2.3. Coagulation Test

The water samples were collected from the artificial lake, which is located at Hunan University of Science and Technology, China. The initial pH value was 7.5–8.5, and the turbidity was 4–20 NTU. A coagulation experiment was carried out by a coagulant stirrer (ZR4-6, Zhongrun Water Industry Technology Development Co., Ltd., Shenzhen, China). Coagulation–flocculation of a one-liter sample was rapidly stirred for a pre-determined time with a pre-determined stirring speed, followed by a slow stirring for a fixed time at a pre-determined stirring speed, and then a 30 min settling time. The dosage was expressed in terms of the effective iron content of the coagulant as determined by the Chinese national standard. The supernatant sample was extracted from a beaker 2 cm below the water surface and used to analyze the water characteristics.

2.4. Analytical Methods

The MMOFs samples were observed using a COXEM-30 Plus SEM, TEM (FEI Tecnai G2 F20, FEI company, Hillsboro, OR, USA), and EDS (COXEM Co., Ltd., Daejeon, Republic of Korea). The possible functional groups present in the PFS-MMOFs were scanned with FTIR (Nicolet 380, Thermoelectric, Madison, WI, USA) in the range of 400–4000 cm⁻¹; the crystal structure of the PFS-MMOFs was measured with XRD (D8 ADVANCE, Bruker, Bremen, Germany). The Ferron-timed spectroscopy method was used to determine the distribution of iron species in coagulant [43]. Iron species were classified as Fe_a, Fe_b, and Fe_c, which correspond to monomeric species, intermediate complex species, and colloidal species, respectively. The Fe_a species are defined as the reaction products of the iron species and ferron reagent in the first 1 min. The Fe_b species are defined as the reaction products in 120 min. The Fe_c species are compounds that do not react with the iron reagent. The species content was calculated using a standard curve (see Figure 2a).

The turbidity was determined by a turbidimeter (2100Q, Hash Company, Vancouver, WA, USA). The DOC was determined using a TOC meter (Vario TOC, Elementa Corp., Cardiff, UK). The UV₂₅₄ of the water sample was measured by a double-beam UV spectrophotometer (TU-1901, Beijing Puxi-Analysis General Instrument Co., Ltd., Beijing, China). Prior to the analysis, water samples were pre-filtered through a 0.45μm filter (Tianjin Zinteng Experimental Equipment Co., Tianjin, China) for measurement of the UV₂₅₄ and the DOC.

2.5. Cultivation and Determination of Algae

The Microcystis aeruginosa (FACHB-315) was purchased from the Institute of Hydrobiology, Chinese Academy of Sciences (Wuhan, China), and cultured in blue-green medium (BG11) within an algal incubator (BSG-400; Shanghai Boxun, Shanghai, China) for further use. Cultivation was conducted under 2000 l× incandescent light at 25 °C with 12-h light/dark cycles. Algal density was determined at a wavelength of 680 nm using a UV–visible spectrophotometer (DR6000; Hach, Loveland, CO, USA) [44,45]. And the standard curve between ABS₆₈₀ and algal density is shown in Figure 2b.

3. Results and Discussion

3.1. SEM, TEM and EDS Analysis

The electron micrographs of MMOFs were obtained through SEM, and Figure 3a–c shows the appearance and structural characteristics magnified at 200×, 1000×, and 5000×, respectively. The transmission electron microscopy results are shown in Figure 3e,f, with sample results obtained at 200 nm, 100 nm, and 50 nm, respectively. It can be seen that the material is distributed in blocks, which is consistent with the previously reported results [46,47]. In the TEM image, the spherical structure of MMOFs can be seen. Figure 3g–i shows the EDS elemental spectrum scanning results. The mass percentages of the four elements, O, Fe, C, and Zr, together account for nearly 99%, with individual proportions of 37.06%, 29.73%, 29.72%, and 2.46%, respectively. They are closely intertwined, forming the primary constituents of the magnetic MOFs, which are consistent with the expected structure of the framework material.

3.2. FT-IR Spectra Analysis

In order to analyze the possible functional groups in the nanoparticles as well as coagulants, the FT-IR spectra were scanned, and the results are shown in Figure 4.

The MPN was created through silicon-coated Fe₃O₄, as indicated by the FT-IR results. As shown in Figure 4a, in the spectrum of Fe₃O₄, the strong broad peak at 3420.96 cm⁻¹ and the peak at 1417.21 cm⁻¹ are assigned to the antisymmetric stretching vibration peaks of structural water O–H and bending vibration absorption of alcoholic hydroxyl O–H, respectively [48]. Those at 1648.16 cm⁻¹ and 577.30 cm⁻¹ were the characteristic absorption peaks of Fe–O–Fe [49]. The strong weak absorption peaks at 1087.90 cm⁻¹ and 465.07 cm⁻¹ were assigned to the antisymmetric stretching vibration and bending vibration absorption peaks of Si–O–Si, respectively [50]. They indicated the successful introduction of silica into the surface of the MNPs. A shifting of the characteristic peaks of Fe–O–Fe appeared in MNPs from 1648.16 cm⁻¹ and 577.30 cm⁻¹ to 1634.14 cm⁻¹ and 579.19 cm⁻¹, respectively. A new chemical group, Fe–O–Si, occurred, and silicon was coated onto the surface of the Fe₃O₄.

Some characteristic adsorption peaks were identified in the MOFs, which were different from MMOFs. The peaks at 1715.63 cm⁻¹ and 1423.93 cm⁻¹ were attributed to the stretching vibration of C=O and C–O in the carboxylic acid of the organic ligand of UiO-66-2COOH, respectively. Moreover, the characteristic peaks at 498.47 cm⁻¹ and 1577.78 cm⁻¹ [51,52,53] were assigned to the C=C vibration of aromatic rings of UiO-66-2COOH. After a new MMOF formation, the antisymmetric stretching vibrational peak of structural water O–H shifted from 3422.74 cm⁻¹ to 3478.21 cm⁻¹; the antisymmetric stretching vibrational peak of Si–O–Si had a shift from 1087.90 cm⁻¹ to 1084.65 cm⁻¹; the C=C vibrational peak of the aromatic ring in MMOFs had a shift from 1498.47 cm⁻¹ and 1577.78 cm⁻¹ to 1496.07 cm⁻¹ and 1548.59 cm⁻¹, respectively. These results indicated that the introduction of MOFs into MNP altered the positions of each other’s chemical groups, forming a new complex and obtaining a new magnetic MOF composite.

PFS-MMOFs were a new combination composite of the PFS and the MMOFs, as indicated by the FT-IR results. Figure 4b shows the results of the FT-IR spectra of the PFS, MMOFs, and PFS-MMOFs. In the PFS, a strong broad absorption peak at 3453.72 cm⁻¹ is assigned to the characteristic frequency for Fe–OH groups [54]. The symmetrical stretching vibration peak at 1197.64 cm⁻¹ is attributed to the chemical group SO₄²⁻ [55]. The strong, sharp absorption peak at 1082.66 cm⁻¹ is due to the lattice vibration of SO₄²⁻ and HSO₄⁻ [56]. The strong absorption peak at 1635.42 cm⁻¹ is due to the bending vibration of the H–O–H group in adsorbed or structured water molecules [57]. Two strong characteristic absorption peaks occurring at 670–550 cm⁻¹ were due to the winding vibration of Fe–O and the bending vibration of Fe–OH groups [58]. The absorption peak at 469.26 cm⁻¹ was due to the absorption peak of HSO₄⁻ [59]. The presence of the MMOFs affected some adsorption peaks in PFS, indicating their effective combination. In the PFS-MMOFs, the stretching vibration peak of the iron hydroxyl group appearing at 3406.32 cm⁻¹ was different from that at 3453.72 cm⁻¹ for PFS. The H–O–H group adsorption peak position at 1635.42 cm⁻¹ in PFS-MMOFs was the same as the PFS. The symmetric stretching vibration peak of SO₄²⁻ in PFS-MMOFs had a shift from 1197.64 cm⁻¹ to 1217.85 cm⁻¹. The strong, sharp absorption peaks corresponding to the lattice vibration of SO₄²⁻ and HSO₄²⁻ shifted from 1082.66 cm⁻¹ to 1110.87 cm⁻¹. The symmetric stretching vibration peak of SO₄²⁻ in PFS-MMOFs had a shift from 1197.64 cm⁻¹ to 1217.85 cm⁻¹. These results showed that the MMOFs affected the structure of PFS, and a new combination of PFS and MMOFs was created.

3.3. XRD Analysis

The effective combination of PFS and MMOFs can also be investigated based on their XRD results. The crystal structures of the Fe₃O₄, MNPs, MMOFs, PFS, MMOFs, and PFS-MMOFs were investigated using XRD. The XRD diffraction patterns are shown in Figure 5.

As shown in Figure 5a, a series of characteristic peaks of Fe₃O₄, MNPs, and MMOFs at 2θ = 18.335°, 30.097°, 35.514°, 43.122°, 53.542°, 57.032°, 62.576°, 74.137°, and 79.113° were assigned to the (111), (220), (311), (400), (422), (511), (440), (533) and (444) planes of cubic structure Fe₃O₄ (PDF card number: 99-0073) [60], respectively. The main characteristic peaks (planes) of the SiO₂ at 35.421° (220), 35.924° (200), 38.548° (201), 43.094° (113), 57.050° (301), and 66.999° (223) were attributed to the cubic structure of SiO₂ (PDF card number: 99-0038) [61]. The characteristic peaks of the Fe₃O₄ and the SiO₂ appeared in pattern of MNPs and MMOFs, implying that the SiO₂ shell layer was successfully coated on the surface of the Fe₃O₄, and their crystal structures of Fe₃O₄ and SiO₂ were not changed. The diffraction peak intensity of the MMOFs was lower than that of the MNPs, which may be due to the dissolution of the Fe₃O₄ by an extreme external environment such as strong acid and high temperature [62]. It was therefore not completely coated by the silica shell. While Fe₃O₄ was coated by SiO₂ and MOF shells, it could still maintain its original crystal structure.

Figure 5b shows that the crystal structure of MMOF disappeared in the PFS-MMOFs. The MMOF and PFS were likely to form a new structure. At 2θ = 25–35°, there was a narrow peak in the PFS that changed to a widespread peak in the PFS-MMOFs. In general, it is an indicator of increased degree of aggregation [63]. Because their molecular structure did not change significantly in the combination of the PFS and the MMOFs, as indicated by the FT-IR results, the presence of the MMOFs suggested an increase in their molecular chains in the PFS.

3.4. Iron Species Analysis

A typical PFS is prepared by the oxidation of Fe²⁺ by NaClO₃, and the reaction proceeds as follows:

$$\mathrm{C}\mathrm{l}{\mathrm{O}}_3{}^{-}+6\mathrm{F}{\mathrm{e}}^{2+}+3{\mathrm{H}}_2\mathrm{O}\to 6\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{C}{\mathrm{l}}^{-}+6\mathrm{O}{\mathrm{H}}^{-}$$

(1)

$$\mathrm{m}\mathrm{F}\mathrm{e}{(\mathrm{S}{\mathrm{O}}_4)}_3+\mathrm{m}\cdot \mathrm{n}\mathrm{O}{\mathrm{H}}^{-}\to \left[\mathrm{F}{\mathrm{e}}_2{(\mathrm{O}\mathrm{H})}_{\mathrm{n}}{(\mathrm{S}{\mathrm{O}}_4)}_{3-\frac{2}{\mathrm{n}}}\right]\mathrm{m}+\frac{1}{2}\mathrm{m}\cdot \mathrm{n}\mathrm{S}{\mathrm{O}}_4{}^{2-}$$

(2)

Further studies on the influence of MMOFs on the structure of PFS and its coagulation properties are needed. Obviously, the OH content determined by alkaline concentration in PFS has a crucial influence on its structure, and any change in alkaline concentration will affect the distribution of its iron species and subsequently the coagulation properties [64]. Usually, there are three types of iron species, including Fe_a, Feb, and Fe_c. Fe_a is responsible for charge neutralization, Fe_b is responsible for charge neutralization and bridging adsorption, and Fe_c is responsible for bridging adsorption [63]. The molecular sizes of Fe_c and Fe_b species are larger compared to Fe_a. The XRD results show an increase in molecular size but cannot ensure an increase in the level of Fe_c and Fe_b species because the chemistry property of the combination product is not clear. The species results tested in this study are shown in Figure 6.

Figure 6a,b shows the distribution results of iron species in PFS and PFS MMOFs under different alkalinities (molar ratio of OH to Fe, denoted as R_nOH/nFe). For example, in PFS with an alkalinity of 0.16, the total content of Fe_a and Fe_b is 53.4%. After introducing MMOFs into PFS, the content of both Fe_a and Fe_b exceeded 84.47%. And compared with the introduction of MMOFs by PFS before and after, except for an increase in Fe_c species content at an alkalinity of 0.04, the Fe_c content decreased at all other alkalinity levels.

Figure 6c–e shows the removal efficiency of PFS-MMOFs for turbidity, UV254, and DOC in our study before testing Fe-ferron specifications. The results indicate that PFS-MMOFs exhibit optimal coagulation at an alkalinity of 0.1. Compared with PFS-MMOFs with similar Fea content and alkalinity of 0.16, higher Feb does not achieve higher coagulation performance; therefore, Fec is important for PFS-MMOFs in coagulation. However, when the alkalinity is 0.04, the proportion of Fec is higher, but the coagulation effect is not the best. Overall, the iron species affected the coagulation performance, while the MMOFs were able to change the iron species distribution, resulting in a good coagulation effect. This once again proves that the combination of PFS and MMOF is not a simple mixing process. A better iron species composition by varying basicity was achieved at 21.8% of Fe_c, 39.9% of Fe_b, and 38.3% of Fe_a.

3.5. Discussion on Dispersion of MMOFs in PFS

The appearance of PFS-MMOFs is shown in Figure 7. The PFS was characterized as brown, transparent, and oily, while PFS-MMOFs appeared black, opaque, and oily, which indicated that MMOFs were well dispersed in the PFS and could also be magnetically separated from the PFS by magnetic fields (Figure 7a,c). As shown in Figure 7d,e, there is a clear difference in appearance between the flocs coagulated with PFS and PFS-MMOFs. Flocs coagulated with PFS-MMOFs can be collected in the bottom center under the magnetic field, while the vacuum freeze-dried flocs retain their magnetic behavior after flocculation (Figure 7f).

The dispersion and stability of magnetically seeded MMOFs in acidic PFS solutions are crucial for the application of MMOFs. Good dispersibility depends on their chemical properties. PFS-MMOFs may be hydrogen bonds between PFS and MMOF. Polar carboxyl groups enhance their dispersion in water because PFS is a strongly polar polymeric coagulant with many polar hydroxyl groups, which should easily combine with some polar groups, such as carboxyl groups in MMOF, thus forming a weak hydrogen bond. Such a hydrogen bond increases the polarity and dispersibility of MMOFs in water. It has been proven that the weak hydrogen bond can be simply broken by an external magnetic field [65]. The experiment tested in this study also showed that the hydrogen bonds between PFS and MMOFs can be broken under a magnetic field, and when the magnet was removed, the hydrogen bonds were formed again, which still maintained a good and stable dispersion of MMOFs in solution. Figure 7b shows a simple scheme showing the bonding.

3.6. Resistance to Hydraulic Factors

To understand whether the combination of PFS-MMOFs can achieve good flocculation by varying the flow intensity in a real water environment, we examined the effect of the flow intensity of PFS-MMOFs on the coagulation–flocculation of contaminants such as turbidity, UV₂₅₄, and DOC. A coagulation–flocculation experiment of the artificial lake water was conducted with 22 mg/L of PFS-MMOFs for the treatment of the water sample at pH 9. Flocculation performance was evaluated by measuring their removal efficiency. The results are shown in Figure 8. Among them, Figure 8a–d shows the effect of rapid stirring time and slow stirring time on flocculation efficiency, which was investigated in the range of 30 s–180 s and 4 min–24 min, respectively. The initial pH, turbidity, UV₂₅₄, and DOC were 8.63, 14.83 NTU, 0.054 cm⁻¹ and 3.45 mg/L, respectively. Figure 8e–h shows the effect of rapid stirring speed and slow stirring speed on coagulation efficiency, which was investigated in the range of 120 r/min–270 r/min and 20 r/min–120 r/min, respectively. The initial pH, turbidity, UV₂₅₄, and DOC were 7.98, 12.33 NTU, 0.054 cm⁻¹, and 3.67 mg/L, respectively.

The stirring time had an impact on the flocculation, and a good flocculation was achieved at an appropriate time. Figure 8a,b shows the effect of rapid mixing time. As shown in Figure 8a, the turbidity removal efficiency reached its maximum, 94.25%, at a rapid mixing time of 150 s. As shown in Figure 8b, the trend of the rapid stirring time effect on the removal of UV₂₅₄ and DOC was inconsistent with that of turbidity removal. At a rapid stirring time of 90 s, the best removal efficiencies of turbidity, UV₂₅₄, and DOC were 93.4%, 66.67%, and 43.28%, respectively. Figure 8c,d shows the effect of slow stirring time on the coagulation efficiencies of contaminants. As shown in Figure 8c, the turbidity removal efficiencies increased with an increase in the slow stirring time, and the turbidity removal efficiencies remained above 92%. The DOC and the UV₂₅₄ varied significantly and achieved good removal efficiencies like those illustrated in Figure 8b. The best DOC removal efficiencies were 47.27%, and the turbidity and UV₂₅₄ removal efficiencies were 95.37% and 64.81%, respectively.

Moreover, the stirring speed had an impact on the flocculation, and a good flocculation was achieved at an appropriate speed. Figure 8e,f shows the effect of rapidly stirring speed. As shown in the figure, with the increase in rapidly stirring speed, the removal efficiencies of turbidity, UV₂₅₄, and DOC gradually increased first and then decreased. When the rapid stirring speed was 210 r/min, the highest removal efficiencies were obtained, where the removal efficiencies of turbidity, UV₂₅₄, and DOC were 93.99%, 64.81%, and 48.07%, respectively. As shown in Figure 8g,h, with the increase in slow stirring speed, the removal efficiencies of turbidity, UV₂₅₄, and DOC gradually increased first and then decreased. When the slow stirring speed was 80 r/min, the highest removal efficiency was obtained, and the removal efficiencies of turbidity, UV₂₅₄, and DOC were 95.72%, 61.11%, and 47.36%, respectively.

From the above results, it is clear that the hydraulic conditions did not affect the PFS-MMOF flocculation for turbidity, DOC, and UV₂₅₄. The flocculation was proven to be effective under different hydraulic conditions, indicating that PFS-MMOFs have great potential to be used in a wider range of applications.

3.7. Comparison in Performance

In general, the use of a combination of PFS and MMOF results in a rapid separation of turbidity. It is difficult for MMOF to inhibit the flocculation behaviors of the PFS through weak hydrogen bonding. This was demonstrated in coagulation–flocculation experiments by varying the hydraulic factors. A study on the effectiveness of MMOFs in improving the removal of specific pollutants by PFS is one of the main purposes. Dissolved organic matter is widely distributed in the water environment which is regarded as one of the largest active pools of organic carbon on earth [66] and it is an important standard for evaluating water quality, representing the content of organic matter in water bodies. UV₂₅₄ reflects how much humus macromolecular organics, aromatic compounds containing C=C double bond and C=O double bond, exist in water [67]. Such pollutants have a significant impact on the aquatic ecological environment. Therefore, in this study, a simple flocculation comparison between the PFS and the PFS-MMOF was made by measuring the DOC and the UV₂₅₄ removal efficiency. The initial pH, UV₂₅₄, and DOC of the pond water were 8.2, 0.052 cm⁻¹, and 10.68 mg/L, respectively. The effect of the PFS and the PFS-MMOFs on the removal of the DOC and the UV₂₅₄ was investigated by varying the coagulant dosage. The results are shown in Figure 9.

By varying their dose, PFS-MMOFs were more effective than PFS in the removal of DOC and UV₂₅₄. As shown in Figure 9a, at a lower coagulant dosage (6 mg/L), the effects of PFS-MMOFs and PFS on the removal of DOC were 63.01% and 30.32%, respectively, and their difference value was up to 32.69%. As can be seen from Figure 9b, the difference for UV₂₅₄ was small at low dosing levels, while the maximum value was up to 26% at the dosing level of 22 mg/L, where the effect of the PFS-MMOFs and PFS on the removal of UV₂₅₄ reached 86% and 60%, respectively. The MMOFs reduced the amount of PFS and produced a good flocculation. As seen from the above results, the MMOFs had a better effect on the removal of the DOC and the UV₂₅₄. It is a positive result of introducing MMOFs into PFS-MMOFs.

3.8. Application in Algae Water Treatment

In order to further investigate the performance of PFS-MMOFs, a comparative study was conducted against traditional coagulant PFS for algae removal. The investigation encompassed varying dosages and pH levels to assess the effectiveness of algae removal. Pond water was mixed with artificially cultured algal water, featuring Microcystis aeruginosa (cyanobacteria) as the source water. Based on the comprehensive analysis, control parameters were set at 90 s for rapid mixing time and 20 min for slow mixing time. Rapid mixing was executed at 210 rpm, while slow mixing was conducted at 80 rpm. Under different dosages, the outcomes of residual algal density and removal rates for PFS-MMOFs and PFS are illustrated in Figure 10a. Initial algal densities were 0.215 abs and 0.19 abs, with a pH of 7.08. As the dosage increased, the removal efficiency of both coagulants gradually improved. Initially, PFS exhibited a slightly superior performance, but it lacked effective algae removal. However, once the dosage exceeded 14 mg/L, PFS-MMOFs surpassed PFS in terms of removal efficacy, resulting in highly effective algae removal. This underscores how increasing coagulant dosage enhances physicochemical reactions with algae and suspended particles, leading to the formation of larger flocs that facilitate sedimentation. MMOFs demonstrated superior coagulation performance.

Figure 10b presents the results of residual algal density and removal rates with PFS-MMOFs and PFS under different pH conditions. Both coagulants worked well at a dosage of 26 mg/L for the treatment of an initial algal density of 0.261 abs. The results indicate the consistent superiority of PFS-MMOFs across different pH levels. Notably, at pH 5, both coagulants achieved optimal algae removal. Compared to that at pH 5, the removal efficiency diminished progressively from pH 6 to 10, reaching its lowest point at pH 8. This suggests that both coagulants perform better for algae removal under weak acidic conditions, possibly due to the looser distribution of algae and enhanced contact area resulting from cell disruption under acidic conditions [68,69].

Figure 10c,d shows the algae removal outcomes for PFS and PFS-MMOFs at a dosage of 26 mg/L. The PFS-MMOFs exhibited superior algae removal efficiency. As seen from the above results, the PFS-MMOFs offered better algae removal efficacy.

4. Conclusions

In this work, we mainly discussed the structural characteristics and flocculation tests of MMOFs with modified conventional flocculants (PFS). From the results, it can be seen that MMOFs can not only be stably dispersed in PFS but also have a good flocculation effect. This is very favorable for the application of magnetic flocculation. Because conventional magnetic seeds are mainly advantageous for rapid precipitation of flocs, there are few designs for the removal of target pollutants (e.g., algae). There is an urgent need to develop some effective magnetic seeds that are easy to design for the lack of removal of complex pollutants by traditional flocculation methods. MMOFs have been reported to be very effective in removing heavy metals, organic pollutants, and inorganic pollutants because of their unique and easily tunable structures. This provides an avenue for an innovative technology to extend the functionality of conventional flocculation. MMOFs in PFS show good stability and dispersion through possible hydrogen bonding. MMOFs affect the distribution of iron species in PF, as well as its coagulation. These results lay the foundation for an in-depth application of MMOFs as an effective magnetic seed in the field of flocculation.