Study on the effect of modified Fe₃O₄@SiO₂@PAC magnetic seeds on magnetic flocculation separation of ballast water sediments

Since the beginning of the 21st century, economic globalisation has flourished and the volume of global trade has been increasing, with maritime transport accounting for more than 90% of the total, and the environmental problems caused by ship transport have gradually emerged (Liu et al. 2019; Ivče et al. 2021; Sayinli et al. 2022). Currently, 12 billion tonnes of ballast water are transferred around the world by ship transport every year, and the algae, bacteria, and pathogens contained in ballast water and its sediments can have a huge impact on local biodiversity, causing a global economic loss of more than $100 billion per year (Apetroaei et al. 2018; Bradie et al. 2021). Therefore, the International Maritime Organisation convened an International Conference on Ballast Water Management in London in February 2004, at which the International Convention for the Control and Management of Ships' Ballast Water and Sediments (Ballast Water Convention) was adopted, aiming to reduce the environmental problems caused by the transfer of ballast water and sediments (Gerhard et al. 2019; Zhang et al. 2021).

Ballast water sediment pre-treatment is a prerequisite to ensure the effective inactivation of subsequent microorganisms (Hyun et al. 2021), and currently, the main method used is the mechanical method (Kumar et al. 2021; Guney. 2022). Among them, the mechanical methods are subdivided into filtration, cyclone separation, and magnetic separation (Chai et al. 2018). The filtration method suffers from inefficiency, easy clogging, and high demand for space and technology (He et al. 2021; Stang et al. 2022). Hydrocyclone has an extremely limited ability to remove phytoplankton such as algae at low density in ballast water and cannot be used alone for ballast water pre-treatment (Lakshmi et al. 2021). Magnetic separation technology mainly uses the flocculation and adsorption abilities of magnetic seeds to remove particles and phytoplankton from ballast water (Ringler et al. 2018; Cui et al. 2020). Currently, high-gradient magnetic separation (HGMS) has been playing an important role in ballast water treatment (Ren et al. 2017). Using a combination of HGMS and ultraviolet (UV) technology, Ren et al. demonstrated that the inactivation of microorganisms by the HGMS-UV composite process was more than 99% at a ballast water turbidity of 70 nephelometric turbidity units (NTU) (Ye 2012; Ren et al. 2016). Magnetic seed is an important component in HGMS and plays an important role in ballast water pre-treatment. Lu et al. prepared magnetic seed using wheat straw, and the removal rates were 96, 60, 98, and 96% for algae, organic matter, bacteria, and coliforms, respectively, at a ballast water turbidity of 50 NTU (Lu et al. 2018). Lv et al. combined polyaluminum chloride (PACl) and magnetic particles composite to prepare magnetic polyaluminium chloride (MPACl), which was then mixed with polyacrylamide (PAM) and showed 97% particle removal for low turbidity (20 NTU) ballast water (Lv et al. 2019). Although HGMS has been studied in the field of ballast water pre-treatment, the following problems still exist. First, the magnetic separation efficiency of the existing magnetic seeds for pollutants in high-turbidity ballast water is limited, and there are relatively few studies on the treatment effect (Wang et al. 2011; Yuan et al. 2021), and at the same time, there is a lack of a combination of material characterisation and adsorption kinetics to analyse the adsorption mechanism of the magnetic seeds, which results in an unclear removal mechanism for pollutants in high-turbidity ballast water (Elcicek & Guzel 2020; Liu & Lu 2022). On the other hand, the low reuse rate of the existing magnetic seeds does not meet the economic requirements, limiting the practical application of magnetic seeds (Li et al. 2020; Proskurina et al. 2023).

Therefore, in this study, modified Fe₃O₄@SiO₂@PAC magnetic seeds were prepared for high-turbidity ballast water using sol–gel and other methods, and the effects of magnetic seed dosing, stirring time, and stirring speed on the magnetic separation efficiency of algae and particulate matter in ballast water were investigated. By re-coating the magnetic seeds after ultrasonic separation, the reuse rate of the magnetic seeds was improved to meet the requirements of economic and practical type. On this basis, scanning electron microscope (SEM), transmission electron microscope (TEM), and adsorption kinetics were used to analyse the reasons for the increased removal rate of pollutants by the modified magnetic seeds, and the removal mechanism of algae and particulate matter by the modified magnetic seeds in ballast water pre-treatment was revealed, which realised the high-efficiency treatment of high-turbidity ballast water.

Fe₃O₄ and PAC were compounded using the sol–gel method (Soares et al. 2016). 0.5 g of nano-Fe₃O₄ was weighed in a beaker, 50 mL of deionised water dispersed in an ultrasonic disperser was added, and set aside to obtain solution A. 0.5 g of sodium carboxymethylcellulose thickener in 50 mL of deionised water was dissolved and stirred for 30 min in a water-bath shaker at 38 °C at 300 r/min to obtain solution B. 1.5 g of PAC was dissolved in 30 mL of the solution C, which was dispersed in an ultrasonic disperser for 15 min, then the solution A was slowly poured in, stirred with a glass rod, and ultrasonicated in an ultrasonic disperser for 20 min. The gelatinous material was poured into a petri dish and dried in a blow drying oven at 60 °C for 24 h. After removal, it was fully milled to obtain nano-Fe₃O₄@PAC magnetic seeds.

Fe₃O₄@SiO₂ composite was prepared by the Stober method, and then Fe₃O₄@SiO₂ was combined with PAC by the sol–gel method to prepare modified Fe₃O₄@SiO₂@PAC magnetic seeds (Gong & Tang 2020). 1 g nano-Fe₃O₄ was weighed and put it into a conical bottle, 500 mL anhydrous ethanol and 300 mL deionised water was added, ultrasonic dispersed for 40 min, 100 mL ammonia water was added, and then 50 mL ethyl orthosilicate (TEOS) was added after ultrasonic dispersal for 10 min. It was then stirred in a water-bath oscillating chamber at 30 °C at a speed of 300 r/min for 8 h. The resulting product was cleaned with deionised water and anhydrous ethanol three times each using a magnet, then poured into a petri dish and dried in a blast drying oven at 60 °C for 12 h. After taking it out, the product was fully ground to obtain nano-Fe₃O₄@SiO₂ composite material.

1 g Fe₃O₄@SiO₂ was weighed and put into a conical bottle, 500 mL anhydrous ethanol was added, and dispersed by ultrasonic for 10 min. 5 mL silane coupling agent was added and stirred in a water-bath oscillating chamber at 30 °C for 12 h. It was then washed with deionised water and anhydrous ethanol three times each. 500 mL deionised water and 300 mL anhydrous ethanol were added to the treated Fe₃O₄@SiO₂ and ultrasonicated for 5 min. 5 g polyvinylpyrrolidone was added, ultrasonic dispersed for 20 min, a certain amount of PAC was added, and stirred in a water-bath oscillating tank at 50 °C for 12 h. The obtained product was cleaned with deionised water and anhydrous ethanol three times each using a magnet, then poured into a petri dish and dried in a blast drying oven at 60 °C for 12 h. After taking it out, it was ground well to get Fe₃O₄@SiO₂@PAC magnetic seed.

Fe₃O₄@SiO₂ composites were prepared using the Stöber method, and then modified Fe₃O₄@SiO₂@PAC magnetic seeds were produced by compositing Fe₃O₄@SiO₂ with PAC using the sol–gel method (Gong & Tang 2020). 1 g of nano-Fe₃O₄ was weighed into a conical flask, 500 mL of anhydrous ethanol and 300 mL of deionised water was added, ultrasonically dispersed for 40 min adding 100 mL of ammonia, ultrasonically dispersed for 10 min and then 50 mL of ethyl orthosilicate (TEOS) was added, and placed it into a water-bath oscillator at 30 °C at a rotational speed of 300 r/min for 8 h. The obtained products were washed three times each with deionised water and anhydrous ethanol using a magnet, poured into Petri dishes, and dried in a blast oven at 60 °C for 12 h. After removal, they were fully milled to obtain the nano-Fe₃O₄@SiO₂ composites.

1 g of Fe₃O₄@SiO₂ was weighed into a conical flask, 500 mL of anhydrous ethanol was added, and ultrasonically dispersed it for 10 min. 5 mL of silane coupling agent was added, it was stirred in a water-bath shaking box at 30 °C for 12 h, and it was washed with deionised water and anhydrous ethanol three times each. 500 mL of deionised water and 300 mL of anhydrous ethanol was added to the treated Fe₃O₄@SiO₂, sonicated for 5 min, 5 g of polyvinylpyrrolidone was added, sonicated, and dispersed for 20 min, a certain amount of PAC was added, it was put into a water-bath oscillation box at 50 °C, and stirred for 12 h. The products obtained were washed three times each with deionised water and anhydrous ethanol using a magnet, poured into a Petri dish, and dried in a blower-type drying oven at 60 °C for 12 h. After removal, they were ground sufficiently to obtain Fe₃O₄@SiO₂@PAC magnetic seeds.

After the magnetic flocculation separation was completed, the device was cleaned with backwash water, the flocs in the backwash water were collected with a magnet, and the deionised water was reintroduced and placed in a beaker. The beaker was placed in an ultrasonic cleaner for different lengths of time, and immediately after ultrasonication, the bottom of the beaker was sucked with a magnet, the supernatant was poured off, washed with deionised water for three times, and placed in an oven for drying and grinding (Shen et al. 2013).

The experimental steps of 2.1.2 were repeated and the recovered magnetic seeds were re-coated with PAC.

The algae used in this experiment are: Karenia mikimotoi, Chaetoceros gracilis, and Prorocentrum donghaiense. Algae species were all wet algae, purchased from the algae species bank of the First Institute of Oceanography, Ministry of Natural Resources, and the algae were used for experiments through activation and expansion. The seawater was pumped through a 0.22 μm aqueous filter membrane, and three types of algae were added to the pumped seawater in equal proportions, resulting in a final algal concentration of 5,000 cells/mL. Since the density of quartz sand is similar to the density of particulate matter such as mud and sand in ballast water and sediment, quartz sand is used instead of solid particles in ballast water and sediment. 15, 40, and 75 μm quartz sands were added in the ratio of 5:4:1, resulting in a final mixture with a turbidity of 500 NTU.

Finally, the single-factor experiment was carried out. By controlling the single variable method, the effects of magnetic seed dosage (50, 100, 150, 200, 250, and 300 mg/L), stirring time (30, 60, 90, 120, 150, and 180 s), and stirring speed (50, 100, 150, 200, 250, and 300 r/min) on the removal rate of algae and turbidity were investigated. Based on 150 mg/L, 120 s, and 250 r/min, respectively. The effluent was sampled and tested when the hydraulic retention time was 30, 60, 90, 120, 150, and 180 s.

A Box–Behnken design experiment (Adachi et al. 2023) was used to obtain the best combination of parameters for algae and turbidity removal using response surface analysis of magnetic seed dosage, stirring speed, and stirring time affecting algae and turbidity removal in magnetic flocculation separation. The level values of magnetic seed dosage, stirring speed, and stirring time were selected as the best values of each factor in the single-factor experiment, and the specific level values are shown in Table 1.

In Equations (1)–(3), q_e (mg/g), q_t (mg/g), k (min⁻¹), k₂ (g/mg·min), α (mg/g·min), and β (g/mg) represent, respectively, the absorption capacity at equilibrium, the absorption capacity at time t, the absorption rate constant of the pseudo-first-order kinetic model, the absorption rate constant of the pseudo-second-order kinetic model, the initial adsorption rate, and the desorption constant during any experiment. Where, q_e = (C₀₋C_f)V/M, C₀ represents the initial concentration of the mixture of particles and algae (mg/L), C_f represents the final concentration of the mixture of particles and algae (mg/L), V represents the volume of the solution (L), and M represents the amount of magnetic seeds (g).

In the study, the difference between the degree of PAC and Fe₃O₄@SiO₂ composite for algae and turbidity removal is huge, which indicates that PAC plays an important role in algae and turbidity removal. The surfaces of Fe₃O₄@SiO₂ and algae or particles are negatively charged, which cannot produce flocculation and sedimentation through electrostatic adsorption. The PAC composite on Fe₃O₄@SiO₂ can make the surface of the magnetic seeds have a positive charge, through electro-neutralisation and adsorption to achieve the removal of algae and turbidity. When the proportion of PAC composite is small, it fails to reach the threshold of flocculation, so the removal rate of algae and turbidity is low. As the proportion of PAC composite increased, the removal rate of algae and turbidity gradually increased. When the proportion of PAC was too large, it resulted in a thicker PAC layer of the magnetic seed, and some of the flocs might not be adsorbed by the magnetic medium. Due to the algae and turbidity detection method, the unadsorbed flocs flowed out of the outlet, resulting in a larger decrease in turbidity removal rate, while the algae on the flocs were not counted, resulting in a small change in algae removal rate. Therefore, in this study, the mass ratio of Fe₃O₄@SiO₂ and PAC of 1:3 were taken as the magnetic seeds for subsequent investigations.

Some studies have shown that there exists an optimal dosage of magnetic seeds, and the magnetic flocculation effect will be enhanced only within a certain range as the dosage of magnetic seeds increases, and when too much is dosed, it will cause the re-stabilisation phenomenon of the system, which will lead to a decrease in the flocculation effect (Li & Wang 2016). When the magnetic seed dosage was 50 mg/L, the PAC content in the solution was the lowest, resulting in some algae and particles failing to be adsorbed by electrostatic adsorption. With the increase of magnetic seed, PAC in the solution increased, and more algae and particles were removed by adsorption; while when there was too much PAC in the solution, it would make the surface of algae and particles change from negative to positive charge, which led to a decrease in the electro-neutralisation and adsorption, and thus a decrease in the removal rate of algae and turbidity.

The experimental results show that there is an optimal stirring time for algae and turbidity removal. When the stirring time is short, the magnetic seeds, algae, and particles are not evenly dispersed in the solution, and the magnetic seeds cannot make sufficient collisions with algae and particles in the solution, and only a small amount of algae and particles are adsorbed by the magnetic seeds to form flocs, which affects the removal rate; when the stirring time is too long, a large amount of algae and particles are adsorbed by the magnetic seeds to form flocs with larger diameters, which increases the shear force on them. When the stirring continues, the shear force on the flocs is greater than the electrostatic adsorption force between the flocs, resulting in the flocs breaking up and re-dispersing into the solution, causing the removal rate to decrease. In addition, substances with larger particle sizes are more likely to collide in an agitated solution. Since the diameter of algae is smaller than the diameter of particles, the flocculation of algae is more affected by the stirring time, resulting in a large difference in the algae removal rate between insufficient stirring and optimal stirring time.

The experimental results show that there exists an optimal stirring speed that allows the removal of algae and turbidity to be maximised. Lower stirring speeds do not allow the magnetic seeds, algae, and particles to be evenly dispersed in the solution, resulting in a lower chance of collision between the algae and particles and the magnetic seeds, which leads to a lower removal rate; while when the stirring speed is too high, the previously precipitated flocs will be re-coiled, which are broken and re-dispersed into the solution by the action of a larger shear force, resulting in a lower removal rate. Compared to particles, algae have smaller diameters and therefore require a larger stirring speed to better collide with the magnetic seeds and form flocs.

The pseudo-first-order kinetic model, based on the assumption that adsorption is controlled by a diffusion step, refers to an adsorption process in which there is a linear relationship between the rate of adsorption and the concentration of the magnetic seeds, i.e., as the concentration of the magnetic seeds decreases leading to a decrease in the rate of adsorption. The pseudo-second-order kinetic model, on the other hand, assumes that the adsorption rate is determined by the squared value of the number of unoccupied adsorption vacancies on the surface of the magnetic seeds, and that the adsorption process is governed by a chemisorption mechanism, which involves the sharing and transfer of electrons between the magnetic seeds and the adsorbed material. The Elovich kinetic model describes a series of processes by a series of reaction mechanisms, and is adapted to the reaction processes with a large change in the activation energy processes. Therefore, we can determine that the experimental mechanism of algae and turbidity removal is electrostatic adsorption due to electron transfer between algae and particles and magnetic seeds. This also verifies the previous conjecture that by compounding PAC on Fe₃O₄@SiO₂, we changed the surface of the magnetic seeds from negative charge to positive charge, and the surface of algae and particles in the solution were negatively charged, and electrostatic adsorption took place after they were fully contacted by stirring, which led to the removal of algae and particles.

The modified magnetic seeds Fe₃O₄@SiO₂@PAC had the highest removal rate of algae and turbidity when Fe₃O₄@SiO₂ and PAC were 1:3, and compared with the magnetic seeds Fe₃O₄@PAC before modification, the removal rate of algae increased by 20%, and the removal rate of turbidity increased by 15.7%. With the increase of magnetic seed dosage, stirring time and stirring speed, the removal rate of algae and turbidity showed a trend of increasing and then decreasing. In order to reduce the influence of turbidity on the inactivation efficiency of the subsequent inactivation unit, we should choose the operating parameters of the magnetic seed dosage of 217.5 mg/L, the stirring time of 129.2 s, and the stirring speed of 211.6 r/min, and at this time, the maximum removal rate of turbidity was 83.23%, and the removal rate of algae was 97.85%. After separating the flocs, the magnetic seeds were recovered by ultrasonication for 2 min, and after recovery, the magnetic seeds were re-coated with PAC, and the second reuse of the magnetic seeds achieved 97.8 and 96.57% removal of algae and turbidity for the first time. After the fourth reuse, more than 85% of the first time was still achieved. According to the SEM, TEM, and adsorption kinetic analysis, it can be known that the modified magnetic seed removes algae and turbidity through the electrostatic adsorption generated by PAC, in which SiO₂ acts as an intermediate layer to make the PAC better compounded on the magnetic seed, so as to allow it to play a maximum role.

We are deeply grateful to the editors and reviewers for their guiding suggestions and other efforts to improve this manuscript.

J.Y. and Z.L. conceptualized the whole article. J.Y. developed the methodology, H.S., J.Y., and J.Z. rendered support in formal analysis. C.Z. rendered support in investigation. Y.S. developed the resources. Q.L. and F.D. rendered support in data curation. J.Y. wrote the original draft preparation. H.S., J.Z., and J.Y. wrote the review and edited the article. C.L. developed the visualisation. Y.S. and Z.L. rendered support in funding acquisition. All authors have read and agreed to the published version of the manuscript.

This research was financially supported by the National Natural Science Foundation of China (52171347), the Natural Science Foundation of Shandong Province (ZR2023QE073), the Natural Science Foundation of Heilongjiang Province (LH2023E070) and the Qingdao Natural Science Foundation (23-2-1-97-zyyd-jch).

All relevant data are available from an online repository or repositories.

The authors declare there is no conflict.