Evaluation of the coagulation properties of magnesium hydroxide for removal combined contamination of reactive dyes and microfibers

Microfibers are a new type of pollutants that are widely distributed in water bodies. And the simultaneous removal of pollutants in water is popular research in the field of water treatment. In this study, magnesium hydroxide was used as coagulant to investigate the performance and mechanism of coagulation and removal of dyes (reactive orange) and microfibers (MFs). The presence of dyestuff in the composite system promoted the removal of microfibers, and the maximum removal efficiency of both could reach 95.55% and 95.35%. The coagulation mechanism was explored by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and zeta potential. The removal of reactive orange and microfibers relied on electrical neutralization, sweep flocculation, and adsorption mechanisms. Turbidity can enhance the removal efficiency of both. Boosting the rotational speed can increase the removal efficiency of microfibers. This study provides an important theoretical support for an in-depth understanding of the characteristics and mechanisms of coagulation for the removal of complex pollutants from printing and dyeing wastewater.


Introduction
The concept of microplastics (MPs) was first introduced in 2004 (Thompson et al. 2004) and is defined as plastic particles less than 5 mm in diameter (Koelmans et al. 2015), which are widely found in freshwater systems (Stanton et al. 2020) and have even been detected in soil (Huang et al. 2021;Rillig 2012), drinking water (Pivokonsky et al. 2018), and polar environments (Lusher et al. 2015).Microplastics have also been found in various shapes and structures, mainly classified as fragments, particles, fibers, and films (Ngo et al. 2019).Microfibers (MFs), one of the most prevalent particulate pollutants (Carr 2017;Ryan et al. 2020), include natural and synthetic textile fibers such as cotton, wool, linen, and acrylics, polyesters, and polyamides (Barrows et al. 2018;Miller et al. 2017;Stanton et al. 2019).They are small, numerous, and wide in distribution, which can absorb heavy metals (Ashton et al. 2010;Holmes et al. 2012) and persistent pollutants (Trevisan et al. 2019) in the environment and increase their toxicity.
Many studies have been attempted to remove MPs from water, including coagulation (Zhang et al. 2021b), adsorption (Skaf et al. 2020), and photocatalysis (Wang et al. 2019).Among them, coagulation is an important process in water treatment and has excellent performance in MPs removal.For example, Ma et al. (2019) studied the removal of microplastics from drinking water using iron-based coagulants, and the results showed that the smaller the particle size of PE, the higher the removal efficiency.Similar to Ma, Zhou et al. (2021) also investigated the coagulation performance and mechanism of removal of polystyrene (PS) and polyethylene (PE) microplastics by PAC and FeCl 3 , and the results revealed that the removal of microplastics by PAC was better than that by FeCl 3 in both cases, and the PAC system could reach 77.83% and 29.70% of the removal efficiencies of PS and PE.It was also found that the selection of polymeric aluminum chloride (PAC) and anionic polyacrylamide (PAM), Responsible Editor: Tito Roberto Cadaval Jr sodium alginate (SA), and activated silicic acid (ASA) for the removal of polyethylene terephthalate (PET) resulted in the highest removal efficiency by ASA at conventional dose (54.70%) and PAM at high dose (91.45%) (Zhang et al. 2021a).
China is the world's largest textile producer and exporter; its textile industry in the global textile trade occupies an important position.The textile industry is a highly polluting industry (Xu et al. 2018), with a large amount of waste dyes directly discharged during the production process.Moreover, due to the application of strong mechanical force, microfibers are released during processing (Stone et al. 2020;Zhou et al. 2020).The terminal discharge of textile printing and dyeing wastewater is one of the important sources of microfiber in natural water, and microfibers should therefore be treated as a separate contaminant (Ryan et al. 2020).
Table 1 shows the summary of removal efficiency and the abundance of influent and effluent water of MFs and MPs from different wastewater treatment plants (WWTPs).The effectiveness of a typical textile industry wastewater treatment plant in removing microfibers from wastewater at various stages was investigated by Xu et al.They found that the wastewater treatment plant was capable of removing pollutants from wastewater, including microfibers, and the removal process of microfibers occurred primarily during the sedimentation period (Xu et al. 2018).Zhou et al. studied fibers in conventional textile mills and centralized wastewater treatment plants in the park; the main finding showed that the density of ultrafine fibers is low; ultrafine fiber removal efficiency for existing processes can reach 85%, but most of these processes are expensive, and there will be unremoved fibers with high adsorption capacity carrying dyes and other substances into the water (Zhou et al. 2020).Several investigations have also shown that microfibers are present throughout the entire wastewater treatment process (Hou et al. 2019;Lares et al. 2018;Pedrotti et al. 2021).Although most of the treated wastewater from these WWTPs has high removal efficiencies and low MFs and MPs contents in the effluent, large amounts of MFs and MPs still enter natural water bodies.
Textile printing and dyeing wastewater contains many harmful chemicals, such as dyes, heavy metals, and aniline (Yang et al. 2021), which were adsorbed on microfibers.Due to their large aspect ratio, microfibers may escape longitudinally through the pores of filters or membranes when passing through wastewater treatment plants.Microfibers are more prone to fragmentation under various environmental conditions, leading to the release of large amounts of secondary microplastics, which further aggravates pollution.When PET and tetracycline (TC) were removed using composite coagulation, Lu et al. (2021) evaluated the removal process.The results revealed that TC's complexation competition increased TC's removal efficiency by 13% while decreasing PET's removal efficiency by 9%.He et al. (2022) investigated the removal effect of PAC and APAM on composite PE and NOR, and the removal efficiency of PE could reach more than 99% and NOR would be reduced to 42%.At present, the simultaneous removal of MP-dye is not well studied, and the mutual influencing factors and mechanisms between them have not been investigated.Therefore, there is still much room to study the characteristics and mechanism of coagulation for the removal of composite pollutants.
This study aimed to investigate and compare the removal performance of reactive orange and microfibers by coagulation with magnesium hydroxide under both single and composite systems.PET was selected as one typical microfiber in this study.The coagulation process was first investigated for the decolorization performance of textile dyeing wastewater, as well as for the removal of microfibers.The changes in the physical properties such as surface charge and morphology of floc in aqueous environment were investigated by zeta potential, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) analysis, and the main mechanisms of coagulation were studied for a deeper the understanding of coagulation behavior.Finally, the effects of turbidity mixing speed on coagulation effectiveness were investigated.

Chemicals and materials
Reagents used in this experiment were analytical reagents (AR) unless otherwise mentioned.MgCl 2

Coagulation experiments
Coagulation and sedimentation experiments were carried out by programmable hexagonal stirrer (ZR4-6, Zhongrun Water Industry 3 Technology Development Co. China).A certain amount of reactive orange powder and microfibers were weighed to simulate textile dyeing wastewater.The pH value was adjusted with 2 mol/L NaOH.Coagulation experiments were carried out at 20 ± 1 °C.The volume of each water sample was 1 L. The simulated wastewater was stirred at a rapid speed of 300 rpm for 2 min, and then the stirring speed was reduced to a slow stirring speed of 60 rpm for 3 min, followed by 30 min of sedimentation.Mg 2+ and non-ionic polyacrylamide were added at 10 s of rapid stirring and 30 s before slow stirring, respectively.).There is a correlation between the initial absorbance and reactive orange concentration.As the reactive orange concentration increases, the absorbance value increases linearly.Therefore, there is a linear relationship between the absorbance and concentration of the dye wastewater supernatant.

Removal of reactive orange
where C 0 (mg/L) is the initial concentration of reactive orange and C 1 (mg/L) is the final concentration.To ensure the reproducibility and reliability of the experiments, each experiment was repeated 3 times.

Measurement method of microfibers
For the quantification of microfibers, the microscopic counting method is commonly used (Pedrotti et al. 2021;Zhou et al. 2020), but this method is error-prone and time-consuming (Dekiff et al. 2014).The raw material of microfibers used in this experiment was polyethylene terephthalate (PET) with a density of 1.38 g/cm 3 (Yang et al. 2015), so the removal efficiency of microfibers could also be calculated by the weighing method.Firstly, a known weight of microfibers was weighed and added to the coagulation mixing tank for the experiment.After coagulation and sedimentation, a 4-cm-deep sample of treated water (with flocs) at the bottom was retained and acid washed with 1 mol/L hydrochloric acid to remove flocs.The 0.45 microfilter membrane was weighed before and after the filtration.The extracted membrane-microfibers were dried in an oven at 100 °C for 3 h.After cooling to room temperature, the weights were measured hourly until the difference between each two measurements was less than 0.001 g.
(1) where M 0 is the weight of microfibers; M 1 is the weight of 0.45-µm filter membrane; and M is the weight of filter membrane-microfibers at the end of coagulation.To ensure the reproducibility and reliability of the experiments, each experiment was repeated 3 times.

Determination of hardness
The hardness of water was determined by EDTA titration (Czekala et al. 2011).In the buffer solution with pH = 10, the total amount of Ca 2+ and Mg 2+ in water was titrated with EDTA standard solution using chromium black T as indicator, and the solution changed from purple-red to sky blue to reach the end point of titration.Ammonia buffer solution with pH = 10 and 0.01 mol/L EDTA standard solution was configured separately.After coagulation, 25 mL of supernatant was taken and passed through 0.45µm filter membrane, and ammonia-based buffer solution and 30 mg of chromium black T were added, shaken well, and titrated with EDTA standard solution.
where C EDTA (mol/L) is the concentration of EDTA standard solution, V EDTA (mL) is the volume of EDTA standard solution required to reach the titration end point, M CaO is the relative molecular mass of CaO, which is 56 g/mol, and V (mL) is the volume of supernatant. (2)

Analysis methods
The supernatant was extracted before and after the coagulation reaction, and the zeta potential of the supernatant and floc was measured with a Zetasizer Nano ZS to analyze the coagulation mechanism.The flocs were collected and dried, and the floc microscopic morphology was photographed by microphotography and scanning electron microscopy (TES-CAN MIRA4).The floc composition was characterized using Fourier transform infrared spectroscopy (Nicolet iS 10 FT-IR spectrometer, Thermo Scientific, USA) to analyze the mechanism.

Discussion of experimental conditions for coagulation
PET microfibers were added to the jar test.Due to a density of 1.38 g/cm 3 of PET (Yang et al. 2015), natural settling experiments were first performed.The rotation program was set to 300 rpm for 2 min, 60 rpm for 3 min, and settling for 40 min (consistent with the coagulation process, but without any added coagulants).The results demonstrated that at 40 min, 70.05% of the MFs could be removed by gravitational sedimentation (Fig. 1a).On the surface of the water, only part of the fiber surface has contact with water, and fibers cannot be completely soaked by the surface tension of water (Duan et al. 2022).The microfibers involved in natural sedimentation had a smooth surface, were cylindrical in shape, and were uniformly sized.
Figure 1b shows the removal efficiency of different types of treatment methods for reactive orange (K-GN) and Magnesium hydroxide coagulant can contribute up to 14% to the elimination of MFs; however, the settling process takes a while, and it takes more than 30 min to reach the bottom 4 cm without obvious flocs.At this time, magnesium hydroxide is accumulated on the surface of MFs in a shallow layer with a few bumps on the floc surface and a loose shape (Fig. 2b).According to an analysis of its morphological characteristics by Liu et al. (2019), this accumulation is the result of fast stirring stage formed by magnesium hydroxide adsorption dye.The removal efficiency of K-GN and MFs could be enhanced, and both were close to 100% when 1 mg/L PAM was added.Additionally, the settling time was greatly decreased to only 6.8 min.The floc surface was now rougher, the quantity of projection had grown further, some larger lamellar structures had emerged, and the agglomeration had improved due to the addition of a little amount of PAM (Fig. 2c).This outcome is mostly attributed to the fact that the addition of PAM boosts the coagulation process by increasing the floc size and density through adsorption and bridging action (Li et al. 2022).After the addition of 3 mg/L PAM, the precipitation time is significantly shorter, taking only 1 min to complete.While K-GN removal efficiency is no longer improved, the removal efficiency of MFs will decrease.It is presumed that the higher PAM dose causes the flocs to aggregate too quickly, preventing some fibers from Figure 2d shows the morphological characteristics of the generated flocs when using higher doses of PAM; it can be seen that the agglomeration effect is very obvious, the length of the flake flocs can reach about 200 µm, and the adsorption bridging ability of PAM itself makes the floc products closely combined (Ma et al. 2019).
The results showed that the coagulation removal of K-GN was less influenced by PAM, but the settling time and PAM dosing had a greater effect on MFs.The main objective of conventional coagulation is to investigate the coagulation and flocculation processes (Santos Nunes et al. 2022).Subsequent experiments were carried out without the addition of PAM, and the settling time was set to 30 min to assess the impact of magnesium hydroxide on the simultaneous removal of reactive orange and microfibers.

Comparison of coagulation between single system and composite system
The removal effects of 100 mg/L Mg 2+ dosage on different concentrations of K-GN and MFs were studied in single system.The removal effects of different Mg 2+ dosage on K-GN and MFs were investigated in single and composite systems.
The coagulation removal efficiency of the single reactive orange system is shown in Fig. 3a.It can be seen that as the initial K-GN concentration increased from 50 to 300 mg/L, the amount of K-GN removed by the same Mg 2+ dosage was increasing, but the overall removal efficiency showed a decreasing trend.The removal efficiency at low concentration (50 mg/L) was only improved by 5% relative to that at 150 mg/L.The removal efficiency decreases at 100 mg/L instead, but the removal amounts of both are 44.17mg/L and 80.58 mg/L, respectively.It is presumed that the low concentration of reactive orange wastewater is one of the reasons, the low concentration of pollutants in the wastewater contains few colloidal particles, and it is difficult to form large particles of magnesium hydroxide to adsorb the dye during the coagulation process (Li et al. 2016).Due to the high concentration (300 mg/L), the removal amount was large and relatively stable with 189.36 mg/L, which was only enhanced by 5 mg/L relative to 250 mg/L K-GN concentration, which was since the high concentration wastewater contained a large number of colloidal particles, which helped the formation and adsorption of magnesium hydroxide (Wei et al. 2014), while the equivalent amount of magnesium hydroxide had saturated the adsorption of reactive orange, so the enhancement was not obvious.
The coagulation and removal efficiency of the single microfiber system is shown in Fig. 3b.The removal efficiency improved from 34.44 to 96.47% as the initial concentration of MFs increased from 6 to 300 mg/L, indicating that magnesium hydroxide has stable removal efficiency in various concentrations of microfibers.The larger the concentration of MFs was, the better the removal efficiency would be acquired.In the water which was no other pollutants, the magnesium hydroxide could generate, wrap, and attach to the microfiber, making the floc density increased and easily to settle.It was difficult for magnesium hydroxide to capture the fiber when the concentration of MFs was lower.Conversely, when the concentration of MFs became higher, the floc could be more uniform.And thus, the removal efficiency could improve as the fiber concentration increasing.
The composite system's removal efficiency for reactive orange was noticeable lower than that of the single system.Figure 4 shows that with an increase in Mg 2+ dosage, the removal efficiency of reactive orange by both single and composite systems exhibited an increasing trend.Compared with the composite systems (150 mg/L K-GN), the removal efficiency was reduced by a maximum of 12% except for the highest magnesium ion dosage (140 mg/L).The maximum reduction of 6% was observed for the composite system with 250 mg/L reactive orange concentration.It is assumed that this is because the magnesium hydroxide generated during the coagulation process partially binds to the fibers and hinders the removal of reactive orange, and this hindrance is inhibited when the concentration of reactive orange in the composite system is high.It was demonstrated that the microfibers had an inhibitory effect on the removal of reactive orange.
For microfibers, the removal efficiency increased from 83.74 to 91.41% in the 150 mg/L composite system and from 86.22 to 97.35% in the 250 mg/L composite system as the amount of Mg 2+ increased from 20 to 140 mg/L.Comparing the removal efficiency to natural settling, it was increased by more than 20%.The removal of MFs was enhanced in the composite system, and compared with the single system, the removal of microfibers was improved by about 4% at the concentration of 150 mg/L of reactive orange and could be improved by 7% at the concentration of 250 mg/L.In the addition of reactive orange to the wastewater, the larger the amount, the more it can promote the removal of microfibers.This is due to the fact that a large number of colloidal particles in the water promotes the production of magnesium hydroxide (Li et al. 2016), which binds to the microfibers and adsorbs the dye, increasing its density and settling better.These findings support the enhanced effect of reactive orange on microfiber removal.

Coagulation mechanism of single system and composite system
There is no single mechanism in the coagulation process, and adsorption, electrical neutralization, and sweep flocculation are the main mechanisms (Zhao et al. 2022) when using magnesium hydroxide as coagulant to treat wastewater, and different mechanisms may play a dominant role in different systems.

Electrical neutralization
In order to explain the coagulation mechanism, it is necessary to explore the electrical neutralization in the coagulation process (Lu et al. 2021).According to the change of zeta potential, the formation process of flocs is related to the electrical neutralization.
Mg 2+ , when added into the water containing a large amount of OH − , generates positively charged magnesium hydroxide (Duan et al. 2022;Zhang et al. 2021b), which can easily adsorb on the surface of negatively charged particles and neutralize the initial charge on the surface of the particles.Figure 5a shows the zeta potential of MFs and the zeta potential before and after coagulation of the single and composite systems.The results show that the zeta potential on the surface of MFs is − 6.17 mV (Zhang et al. 2021a), the initial zeta potential of single reactive orange system is − 7.77 mV, the single microfiber system is − 8.16 mV, the composite system is − 8.66 mV, and the zeta potential of the supernatant after coagulation is − 2.31 mV, 25.85 mV, and − 1.74 mV.The potentials were all elevated.The results show that the charge neutralization exists in single and composite systems.After low a dosage of Mg 2+ was added, the zeta potential was significantly reduced when the system exists reactive orange.Due to reactive orange being an anionic dye carrying a negative charge, the concentration of reactive orange in the floc is high, leading to the decrease of zeta potential of the floc.The zeta potential after coagulation was closer to zero potential in the composite system, indicating that the electrical neutralization was stronger in this system.In the single reactive orange system, the zeta potential after coagulation is also close to zero potential because of the presence of a large number of negatively charged reactive orange particles that will combine with magnesium hydroxide, but the strength is not as great as the composite system.However, in the single microfiber system, there is an excess of positively charged magnesium hydroxide, so the zeta potential after coagulation is positive, but the electrical neutralization reaction also occurs in the system.

Sweep flocculation
Sweep flocculation is commonly understood to occur when flocs rap the water's suspended solids during the sedimentation process (He et al. 2022).When the coagulant dosage is large enough, the coagulant will produce a large number of hydroxide precipitation, relying on the net to sweep the water particles in order to generate precipitation separation.Due to the electrical neutralization of the agent, the system particle destabilization gradually reaches coagulation.The charge of the particles will be reversed with an increase in dosage, allowing the sweep flocculation effect to fully manifest itself (Nan et al. 2016).As can be seen from Fig. 5b, the flocs of the single microfiber system are all positive electrically after coagulation, indicating that there are not enough negatively charged particles for neutralization, and the sweep flocculation is dominant at this time.The electrical properties of the flocs after coagulation in both the single reactive orange system and the composite system were negative and gradually approached zero potential with the increase of the Mg 2+ dosage.

Adsorption
Adsorption is also considered as one of the main mechanisms of coagulation (Zhao et al. 2017;Zhou et al. 2021).The flocs were analyzed using microphotography, SEM, and FTIR. Figure 6 shows the surface morphology of MFs and flocs in different systems.
It can be seen that the initial fiber surface is smooth and elongated, with a uniform aspect ratio and good light transmission (Fig. 6A, 6a).In the single reactive orange system, microphotography showed that the flocs were orange in color (Fig. 6C), and the flocs could reach 50 µm, with a smooth surface and some small particles or flaky bumps (Fig. 6c), which was caused by the adsorption of reactive orange dye by magnesium hydroxide.In the single microfiber system, it can be seen that there is a blocky light white substance on the fiber surface and the light transmission becomes poor (Fig. 6D), and it can also be seen by SEM that the white substance is wrapped on the fiber surface (Fig. 6d), which is due to the magnesium hydroxide generated during the coagulation process, which can be adsorbed by the negatively charged fibers to form flocs that neutralize the charge and then precipitate.The floc morphology of the composite pollutant system is shown in Fig. 6B and b.The flocculant material on the fiber surface is orange in color, and the block is bigger.SEM can see that the surface of the floc after coagulation becomes rougher, and many small particles appear, which are more tightly wrapped compared with that in the single system.
Reactive orange is a water-soluble anionic dye containing the − SO 3 Na group (sodium sulfonate group) (Table 2), which dissolves into − SO 3 − when dissolved in water (Zonoozi et al. 2009).When the pH of the solution is high, the addition of magnesium ion produces positively charged magnesium hydroxide, which can adsorb negatively charged dye molecules and agglomerate into small particle flocs.This adsorption is highly selective, not chemisorption and ion adsorption, and the driving force behind it is ionic adsorption.
To further elucidate the mechanism of floc combination, the flocs were analyzed using FTIR with contaminants and in different systems.As shown in Fig. 7, the peak 2360 cm −1 belongs to the interference peak, which is generated by the vibration of the neutral molecule CO 2 with polar bonds (Zhao et al. 2017).The peaks 1715 cm −1 , 1339 cm −1 , 1242 cm −1 , 1102 cm −1 , and 1015 cm −1 are present in PET MFs (Käppler et al. 2015).The peaks 1560 cm −1 and 1480 cm −1 are present in reactive orange, which are generated by the C = C double-bond stretching vibration of the benzene ring in reactive dyes, and the peak 1050 cm −1 is caused by the stretching vibration of alcohol C-OH (Zhao et al. 2017(Zhao et al. , 2018)).Comparing the single and composite systems, there are peaks 3700 cm −1 , which are generated by the free radical O-H stretching vibration; peaks 1610 cm −1 and 1420 cm −1 are attributed to the Mg-OH bending vibration (Zhao et al. 2017), and peak 450 cm −1 is the stretching vibration of Mg-O (Zhang et al. 2021b).1640 cm −1 is the bending vibration of water molecules, and the rest of the peaks can correspond to the contaminants themselves.In summary, 3700 cm −1 , 1610 cm −1 , 1420 cm −1 , and 450 cm −1 in the infrared spectrum indicate that the product is magnesium hydroxide, while the reactive orange and microfibers are only adsorbed on the surface of magnesium hydroxide, and no chemisorption occurred (Scheme 1).

Turbidity
Under the same experimental conditions, kaolin was added to simulate the turbidity of the wastewater, and the effect of different kaolin concentrations on the coagulation and the removal of pollutants were analyzed.With the increase of kaolin concentration, the turbidity in the simulated wastewater also increased.When the kaolin concentration increased from 10 to 30 mg/L, the turbidity in the water increased from 6.66 to 19.70 NTU (Fig. 8a).The turbidity of the water sample after coagulation was only about 1 NTU, and the higher the initial turbidity, the better the removal effect of turbidity (Zhao et al. 2015).
When the kaolin concentration reaches 20 mg/L, the removal efficiency of kaolin itself increases (Fig. 8a).The removals of reactive orange and microfibers were more effective (Fig. 8b).It does this by simulating increased turbidity in the wastewater through the addition of kaolin, which increases the likelihood of particle collision and will combine to form larger flocs, netting and sweeping more colloidal material.On the other hand, the added kaolin particles can act as crystal nuclei, which creates a facilitation effect on the rapid nucleation of magnesium hydroxide (Zhao et al. 2018).

Stirring speed
The stirring speed affects the distribution of flocculation molecules in the water and also enhances the contact surface between pollutant particles and coagulant the water, which has an impact on the treatment effect (Zhao et al. 2014).
As shown in Fig. 8c, with the increase of stirring speed, the removal efficiency of dyestuff under the condition of 100 to 300 rpm did not change much and could reach more than 86% and dropped to 83.43% at 400 rpm.The removal of dye still depends mainly on the coagulant dosage; when stirring speed increases from 100 to 400 rpm, the removal efficiency of PET can be increased by 7%, and the highest removal efficiency can be 93.25%.These results indicate that when the stirring speed is too slow, the shear intensity of the flow field surrounding the floc is too low, and the inter-particle collision rate is too low to promote the growth of the floc (Ding et al. 2019).When the stirring speed reaches 400 rpm, it will affect the formation of magnesium hydroxide and the adsorption of reactive orange during rapid stirring, and it will also break the already formed flocs (Zhao et al. 2014), so there will be a phenomenon that the removal efficiency of reactive orange decreases under this condition.The comixing probability of flocs and fibers becomes larger, flocs are more easily attached to microfibers, and the settling performance increases when the density increases, which may be the reason why the removal efficiency of microfibers increases with the increase of stirring speed.

Hardness
The hardness of the supernatant after coagulation with different dosage of Mg 2+ in different systems was measured, and the results were shown in Fig. 8d.The hardness of the supernatant of single system is higher than composite system.Due to fewer impurities in single system than there in composite system, there is less magnesium hydroxide formed throughout the reaction, resulting in a higher concentration of residual magnesium ions in the supernatant.Due to the water-soluble reactive orange's ability to facilitate the creation of magnesium hydroxide, the single reactive orange system consumes magnesium ions more thoroughly than the single microfiber.

Conclusion
In this study, we evaluated and compared the coagulation removal performance of reactive orange and microfibers in both single system and composite system by magnesium hydroxide.The coagulation mechanism for reactive orange and microfibers composite pollutants was explored by using the analysis of zeta potential, SEM, and FT-IR analysis.Magnesium hydroxide as coagulant has good ability to remove pollutants.Adding PAM as a coagulant aid can also improve the pollutant removal capacity and settling time, which can be added appropriately in actual work.
The main mechanisms of single reactive orange/microfibers system are adsorption, electrical neutralization, and sweep flocculation.The composite system has stronger electrical neutralization and adsorption ability and can significantly enhance the removal ability of microfibers, but microfibers will slightly reduce the removal efficiency of reactive orange through electrical neutralization competition.In addition, the increases of turbidity and stirring speed can increase the collision probability of particles in wastewater, which can improve the removal efficiency of reactive orange and microfibers.This work could provide important theoretical support for the characteristics and mechanisms of coagulation technology to remove compound pollutants and is of great significance for the treatment of compound pollution in wastewater.

Fig. 2
Fig. 2 SEM images with different types of treatment methods.a Natural settling; b 100 mg/L Mg 2+ ; c 100 mg/L Mg 2+ and 1 mg/L PAM; d 100 mg/L Mg 2+ and 3 mg/L PAM

Fig. 4
Fig.4Comparison of removal efficiency of K-GN and MFs between single and composite systems (S, single system; C, combined system)

Fig. 5
Fig. 5 Zeta potential of supernatant and flocs in single and composite systems.a Zeta potential of wastewater and coagulated supernatant; b zeta potential of the floc after coagulation

Fig
Fig. 7 FTIR spectrums of contaminants and single and composite system

Fig. 8
Fig. 8 Effects of experimental conditions on coagulation performance.a Effect of kaolin concentration and removal of turbidity; b effect of kaolin concentration on removal efficiency of reactive orange and microfibers; c effect of stirring speed on removal effi-

Table 1
Microfiber/microplastic abundance in influent and effluent from different printing and dyeing plants and wastewater treatment plants