Simulated Experimental Study on the Removal of Methylene Blue-Cu(II) Composite Pollution by Magnetized Vaterite

Abstract

The combined pollution of organics and heavy metals represents a significant environ-mental problem that has attracted widespread attention. This explores the treatment of methylene blue (MB) and Cu(II), which are common pollutants in dye wastewater, and the recycling of Cu. A magnetized vaterite (V-M) was synthesized using Bacillus velezensis, and its structure and magnetic performance were investigated. The effects and mechanisms of removing MB-Cu(II) composite pollution using V-M and H₂O₂ in combination were estimated. The results indicated that V-M is a combination of organic and inorganic substances, with 21.5 wt% organic matter and multiple organic functional groups, including O-H, -SH, and others. The combination of V-M and H₂O₂ can achieve a maximum removal percentage of 90% for MB-Cu(II) pollution. The analysis showed that MB was oxidized by the ·OH generated from the H₂O₂-based Fenton-like reaction, and was catalyzed by the Fe₃O₄ in V-M. The immobilization of Cu(II) by V-M was mostly realized through the binding of the organic substances on the surface of the V-M, multilayer adsorption, and a replacement reaction with Ca(II). Magnetic separation and the addition of diluted HCl were used for the recycling of the Cu(II) enriched by V-M, with a recycling percentage reaching 85%. This study introduced a novel approach to the remediation of MB-Cu(II) composite pollution, and the recycling of Cu(II).

1. Introduction

With the increasing impact of human activities and the acceleration of global industrialization, the diversity and quantity of pollutants continue to rise, making the combined pollution of organics and heavy metals a common phenomenon [1]. The heavy metals present in water and soil, which are mainly derived from industrial sources, can not only be absorbed or accumulated in living organisms, damaging the ecosystem and human health, but also interact with other organo-pollutants, resulting in a more harmful form of combined pollution [2]. It is thus a key focus among researchers to reduce such combined contamination of organics and heavy metals.

Dyes are among the most common organo-pollutants. Among all dyes, methylene blue (MB) is the most challenging organo-pollutant in wastewater treatment. It accounts for approximately 50% of all dye production [3,4]. MB has a complex structure with high chroma, which can reduce the light transmittance of the solution [5], and is difficult to biodegrade [6]. MB exerts a long-term detrimental influence on the growth of animals and plants, as well as on human health. Copper salts, a major component in the production of dyes, were employed as a moderant to optimize the performance of printing and dyeing products, resulting in the generation of considerable quantities of Cu(II) in dye wastewater. An excess of Cu(II) can damage the structure and functionality of proteins, enzymes, and DNA. This can lead to adverse effects on organ function and even result in metabolic disorders and, in extreme cases, death [7]. Furthermore, the presence of Cu(II) in polluted dye water can interact with organic substances, thereby amplifying the toxicity and enhancing the damaging effect on cells [8]. Therefore, common heavy metals and organic substances can be represented by Cu(II) and MB, respectively, to explore a novel approach to the treatment of organic substance–heavy-metal-contaminated water.

Calcium carbonate is one of the most economical porous materials. Its carbonate group has been shown to enhance the replacement reaction between metal ions, making it a widely used material for the immobilization of heavy metal ions [9,10,11,12]; however, a multitude of adsorbents, including calcium carbonate, can prove challenging to recycle and may lead to secondary pollution. Moreover, the adsorption method may not achieve the chroma standard for wastewater containing a significant quantity of organic dyes and exhibiting a high chroma. A Fenton-like reaction has been assessed as effective in the degradation of organic dyes. It has attracted considerable interest due to its broad pH range and ease of application, and it is widely applied in the treatment of organic pollutants in water [13,14,15]. Fe₃O₄ is an environmentally friendly, cost-effective material that can be easily separated. It has been shown to be able to catalyze the production of ·OH from H₂O₂, and the ·OH can decompose MB. While this method has been demonstrated to facilitate the eventual oxidation of MB to CO₂ and H₂O, the underlying degradation process remains to be elucidated [16,17]. Some studies combined Fe₃O₄ and calcium carbonate for the removal of heavy metals and organic pollutants. Islam et al. [11] synthesized a carbonate-based mesoporous magnetic adsorbent (IO@CaCO₃), which comprised acicular iron oxide and calcite, and exhibited remarkable capabilities for the removal of As(V), Cr(VI), and Pb(II). Wang et al. [18] synthesized magnetic mesoporous calcium carbonate-based nanocomposites that could immobilize Pb(II) and Cd(II), and demonstrated an ion-exchange-based heavy metal adsorbent process. In addition to the use of chemically synthesized CaCO₃, biogenic CaCO₃, synthesized by microorganisms, presents a unique organic–inorganic composite structure and often exhibits superior heavy metal adsorption capabilities [19,20], making it a popular material for the remediation of such pollution. Paul et al. [21] combined Fe₃O₄ with biomineralized vaterite and calcite, respectively, to assess their capability to degrade the MB and Rhodamine B. Their findings implied that the presence of CaCO₃ can improve the catalytic activity and that vaterite-supported Fe₃O₄ shows a superior result. Despite the fact that the aforementioned studies broadened the scope of application of calcium carbonate, it remains costly and requires intricate processing. Furthermore, the studies did not investigate removal methods of composite pollution involving both organics and heavy metals.

Bacillus velezensis (B. velezensis) is widely distributed in the soil and has a clear genetic background and excellent biosafety record [22]. It is a powerful microbe that can induce biogenetic CaCO₃ precipitation [23], as elaborated in our previous work [24]. In this study, a magnetized vaterite (V-M) is synthesized under the induction of bacteria, and its magnetic performance and mineral structure are characterized. The mechanism of using V-M as an adsorbent and a catalyst for a Fenton-like reaction to remove composite pollution (MB-Cu) is expounded. Subsequently, the magnetic V-M with immobilized Cu could be recycled by magnetic separation, and the copper could then be mobilized by the addition of diluted HCl. This study presents a novel approach to the removal of composite contamination from MB and Cu(II).

2. Materials and Methods

2.1. Materials

The strain used in the experiment was isolated from biofertilizer, identified as Bacillus velezensis (Gene Bank No. CP037417.1), and was kept in our laboratory. The medium used in this study was lysogeny broth (LB) liquid medium (100 mL of ultra-pure water, 1 g of tryptone, 0.5 g of yeast extract, and 1 g of NaCl, 6.5 ≤ pH ≤ 7.5). The Fe₃O₄, CaCl₂, CuSO₄·5H₂O, NaOH, MB, 30% H₂O₂ solution, 36% hydrochloric acid (HCl), and tertiary butanol (TB) were of analytical grade and sourced from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, and Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Ultra-pure water was used to prepare all the solutions. Then, 0.1 mol/L NaOH and 0.1 mol/L HCl were used to adjust the pH values.

2.2. Experimental Methods

2.2.1. Preparation of V-M and Structure Analysis

Here, 0.05% (m/v) Fe₃O₄, 2% B. velezensis inoculum, and 0.8% CaCl₂ were added into a 100 mL sterilized LB liquid medium, and then cultured at 30 °C, 180 rpm for seven days. V-M was obtained from the centrifuged medium (5804R, Eppendorf, Hamburg, Germany, 9000 rpm, 8 min) and was dried at 65 °C and ground. Meanwhile, the biogenetic vaterite without Fe₃O₄ was synthesized by using the same method for structural comparison.

The X-ray diffraction (XRD) data were collected on an BTX-III diffractometer (Olympus, Tokyo, Japan) equipped with CoKα radiation. The Fourier transform infrared spectroscopy (FTIR) was performed with a Nicolet Nexus 670 FTIR Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The FTIR spectra were recorded with 32 scans per spectrum at a resolution of 4 cm⁻¹. The thermal gravimetric–derivative thermal gravimetric (TG-DTG) curve was obtained from a STA7300 thermal analysis system (PerkinElmer, Waltham, MA, USA). The purge gas was N₂, with a flow rate of 100 mL/min and a heating rate of 10 °C/min. The scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM-EDS) data were collected on a Supra 55 (Carl Zeiss, Oberkochen, Germany). The magnetic performance of V-M was obtained using a 7404 vibrating sample magnetometer (Lake Shore Cryotronics, Westerville, OH, USA).

2.2.2. Factors Influencing the Removal of MB-Cu Composite Pollution

A 50 mL centrifuge tube was used for the study, and the reaction system was controlled to a volume of 20 mL. Unless otherwise stated, 0.05 g of V-M was added, and the concentrations of H₂O₂, Cu(II), and MB were maintained at 50 mmol/L, 200 mg/L [25], and 200 mg/L [26], respectively. Considering that wastewater containing Cu(II) is typically acidic [27,28], and in light of relevant research findings [26,29], the pH was adjusted to 3. The solution was kept at 25 °C at 100 rpm for 24 h. Thereafter, it was subjected to centrifugation at 9000 rpm for 8 min. Subsequently, the supernatant was collected for determining the concentrations of Cu(II) and MB using a flame atomic absorption spectrophotometer (AAS, AA-6300C, Shimadzu, Kyoto, Japan) and spectrophotometer (SpectraMax M2, Molecular Devices, Sunnyvale, CA, USA, λ = 665 nm), respectively. Three replicates were set for each experiment.

The following methods were used to assess the related influencing factors:

A.

Effects of the initial quantity of V-M and concentration of H₂O₂

The quantity of V-M was set to 0.005, 0.01, 0.03, 0.05, and 0.1 g. Following the experiment, the concentrations of Cu(II) and MB were measured. The removal percentages of both Cu(II) and MB were then calculated.

The concentration of H₂O₂ was set to 5, 10, 20, 40, 50, 100, 150, and 200 mmol/L. Following the experiment, the concentrations of Cu(II) and MB were measured. The removal percentages of both Cu(II) and MB were then calculated.

Equations (1) and (2) illustrate the methodology for calculating the removal percentages of Cu(II) and MB:

$$A_e={\displaystyle \frac{C_i-C_e}{C_i}}\times 100$$

(1)

$$D_e={\displaystyle \frac{C_i-C_e}{C_i}}\times 100$$

(2)

where A_e and D_e represent the removal percentage of Cu(II) (%) and degradation percentage of MB (%), respectively, while C_i and C_e represent the initial concentration and equilibrium concentration of Cu(II) or MB (mg/L), respectively.

B.

Effect of the initial pH

The initial pH was set to 2.0, 3.0, 4.0, 5.0, and 6.0 using HCl (12 mol/L) or NaOH (12.5 mol/L). Following the experiment, the equilibrium pH (pHe) was measured, and the removal percentages of Cu(II) and MB were calculated using Equations (1) and (2).

C.

Effect of the initial concentrations of pollutants

The concentration of Cu(II) was kept at 200 mg/L and the concentration of MB was set at 5, 10, 20, 50, 100, 150, and 200 to estimate the effect of the concentration of MB. Similarly, the concentration of MB was kept at 200 mg/L and the concentration of Cu(II) was set at 5, 10, 20, 50, 100, 150, and 200 mg/L to assess the effect of the concentration of Cu(II). The removal percentages of MB and Cu(II) were calculated using Equations (1) and (2), and the unit adsorption capacity (Q_e, mg/g) of Cu(II) by V-M was calculated using Equation (3):

$$Q_e={\displaystyle \frac{\left(C_i-C_e\right)\times V}{M}}$$

(3)

where C_i and C_e represent the initial concentration and equilibrium concentration of Cu(II) (mg/L), respectively, V represents the volume of the adsorbent solution (L), and M represents the weight of V-M (g).

D.

The recycling condition of the Cu(II) immobilized by V-M

After the removal reaction was finished, the V-M was separated by a magnet. Different concentrations of HCl (0.001, 0.01, 0.05, and 0.1 mol/L) were used to dissolve V-M. The supernatant was collected after 30 min, the concentrations of Cu(II) and MB in the supernatant were tested, and the recycling percentage (R_e, %) was calculated using Equation (4):

$$R_e={\displaystyle \frac{C_R}{C_i}}\times 100$$

(4)

where C_i and C_R represent the initial concentration and recycled concentration (mg/L), respectively.

2.2.3. Analysis of the Pollutant Removal Mechanism

A.

The role of OH

Different concentrations of tertiary butanol (0, 20, and 40 mol/L) were added to the reaction system, with the concentrations of H₂O₂, Cu(II), and MB maintained at 50 mmol/L, 200 mg/L, and 200 mg/L, respectively. The samples were collected at different times (10, 30, 60, and 150 min), and the removal percentages of Cu(II) and MB were calculated.

B.

Adsorption isotherm and adsorption kinetic analysis

Based on the data collected (Section 2.2.2, C), a Langmuir isotherm equation (Equation (5)) and Freundlich isotherm equation (Equation (6)) were applied to fit the data [30,31]:

$$Langmuir: {\displaystyle \frac{C_e}{Q_e}}={\displaystyle \frac{1}{Q_{max}\times K_L}}+{\displaystyle \frac{C_e}{Q_{max}}}$$

(5)

$$Freunclich: \ln Q_e=\ln K_F+{\displaystyle \frac{1}{n}}\times \ln C_e$$

(6)

where C_e and Q_e represent the equilibrium concentration (mg/L) and equilibrium adsorption capacity (mg/g) of Cu(II), respectively, K_L represents the adsorption coefficient of the Langmuir isotherm equation, K_F represents the relative adsorption capacity of the adsorbent, and n is a constant representing the adsorption strength.

The experiment was conducted in accordance with the methodology outlined in Section 2.2.2. The samples were collected at different times (5 to 1440 min). The adsorption capacity (Q_t, mg/g) was calculated using Equation (7):

$$Q_t={\displaystyle \frac{\left(C_i-C_t\right)\times V}{M}}$$

(7)

where C_i and C_t represent the initial concentration and concentration at time t of Cu(II), V represents the volume of the adsorbent solution (L), and M represents the weight of V-M (g).

The linear equations of pseudo-first-order dynamics (Equation (8)) and pseudo-second-order dynamics (Equation (9)) were used to fit the data [32]:

$${\displaystyle \frac{1}{Q_t}}={\displaystyle \frac{K_1}{t\times Q_e}}+{\displaystyle \frac{1}{Q_e}}$$

(8)

$${\displaystyle \frac{1}{Q_t}}={\displaystyle \frac{1}{K_2\times {Q_e}^2}}+{\displaystyle \frac{t}{Q_e}}$$

(9)

where Q_t represents the unit adsorption capacity at time t of Cu(II) by V-M (mg/g), Q_e represents the unit adsorption capacity at equilibrium (mg/g), while K₁ and K₂ are the pseudo-first-order kinetic model constant (1/min) and pseudo-second-order kinetic model constant (g/mg·min), respectively.

3. Results and Discussion

3.1. Characterization of V-M

3.1.1. Structural Analysis and Characterization

The structural analysis and characterization results are shown in Figure 1.

The XRD results showed that in the absence of Fe₃O₄ in the medium, the precipitate synthesized by B. velezensis was vaterite; however, upon addition of Fe₃O₄, the precipitate exhibited diffraction peaks characteristic of both vaterite and Fe₃O₄ (Figure 1a), indicating that the mineral synthesized by B. velezensis was an Fe₃O₄-containing vaterite, designated V-M. Except for those vibration peaks of vaterite (1426, 1087, 879, and 743 cm⁻¹) [33] and Fe₃O₄ (585 cm⁻¹) [34], the FTIR pattern (Figure 1b) suggested that V-M possesses certain organic groups, including O-H (3407 cm⁻¹), C-H (2964 and 2933 cm⁻¹), C-O (1688 cm⁻¹), and -SH (569 cm⁻¹) [35], implying that V-M also contains many organic groups other than vaterite and Fe₃O₄. Previous research conducted in our laboratory identified these organic compounds as the primary metabolites of B. velezensis [36], which is consistent with the findings of other studies exploring the mechanism of action and factors influencing microbial participation in the precipitation of CaCO₃ [37,38]. These metabolites serve as nucleation sites and are incorporated into the lattice of vaterite, forming a distinctive organic-inorganic complex structure that enhances the stability of the vaterite. Additionally, organic compounds play a role in the adsorption of pollutants [39,40], contributing to the favorable Cu(II) fixation properties of V-M (see below). The TG/DTG patterns are shown in Figure 1c. There were three mass-loss steps with increasing temperature: the mass loss between 27.6 and 282.1 °C was attributed to the evaporation of water (12.4 wt%), the mass loss between 282.1 and 631.6 °C was caused by the degradation of organic matter (21.5 wt%), and CaCO₃ was degraded at 631.6–992.5 °C, with a 14.6% loss of mass [38,41].

SEM was used to characterize the B. velezensis-synthesized vaterite, Fe₃O₄, and V-M, and the results are shown in Figure 2.

The mineral synthesized by B. velezensis was vaterite without the presence of Fe₃O₄. According to previous research in our laboratory [42], it has a porous surface with a diameter of approximately 4.88 µm. Upon the introduction of Fe₃O₄ into the synthesis system, B. velezensis produced a spherical or irregular porous structure with a diameter of approximately 1 µm (Figure 2g). EDS indicated that there are C, O, Ca, Fe, and other elements in V-M (Figure 2i).

Based on the aforementioned findings, it can be posited that V-M is an Fe₃O₄-containing vaterite.

3.1.2. Magnetic Performance of V-M

The hysteresis loop can reflect the magnetization properties of ferromagnetic materials, which is the relationship curve of magnetization with the magnetic field strength. The hysteresis loops of Fe₃O₄ and V-M are shown in Figure 3. The saturation magnetization of Fe₃O₄ (Figure 3a) and V-M (Figure 3b) was 20.174 and 2.986 emu/g. The coercivity of V-M was 0.047 kOe, which made it almost free of hysteresis, and it could be re-dispersed after being separated by an external magnetic field, exhibiting paramagnetism [43]. In comparison to the coercivity of Fe₃O₄, the coercivity of V-M remained largely unaltered, suggesting that the synthesis of V-M by B. velezensis in this study had no discernible effect on the magnetic properties of Fe₃O₄.

V-M was fully mixed in water to create a suspension. When an external magnetic field was applied, V-M could be completely sucked to one side of the beaker within 10 min (Figure 3b). The absence of solid particles in the water indicated that V-M could be collected through the application of a magnetic field, exhibiting effective magnetic separation performance and a notable magnetic field response. However, the liquid after the magnet separation was not clear (Figure 3b). This was due to the presence of residual vaterite and bacteria within the solid precipitate, which remained uncombined with Fe₃O₄ and was, therefore, unable to be attracted by a magnet. Nevertheless, the majority of V-M could be recycled through the use of supplementary magnets.

3.2. Factors Influencing the Removal of MB-Cu Composite Contamination

3.2.1. Effects of the Initial Quantity of V-M and Concentration of H₂O₂

The Fenton-like reaction has a broad range of potential applications for the degradation of organic pollutants. The addition of H₂O₂ and adsorbents has a direct impact on the removal of pollutants, with a substantial body of research examining the effects of these additions on the removal of pollutants [26,44,45]. To identify the optimal quantity and ratio of H₂O₂ and adsorbent materials, the removal effects of varying quantities of V-M, acting as both a Fenton-like reaction catalyst and an adsorbent, and of different concentrations of H₂O₂ on the combined pollution of organics and heavy metals were analyzed, as illustrated in Figure 4.

As illustrated in Figure 4, the degradation percentage of MB gradually increased with an increase in the H₂O₂ concentration in the system upon addition of 0.05 g of V-M. Conversely, the removal percentage of Cu(II) displayed an inverse trend. At a concentration of 50 mmol/L, the degradation percentage of MB was observed to reach a high level, without a notable decline in the removal percentage of Cu(II); however, at concentrations exceeding 100 mmol/L, the removal percentage of Cu(II) showed a considerable reduction (Figure 4a). This phenomenon may be attributed to the elevated generation of ·OH when the H₂O₂ content was excessive, which accelerated the degradation of MB while disrupting the organic–inorganic composite structure of the V-M. At a concentration of 50 mmol/L, the removal percentages of MB and Cu(II) initially increased with the addition of V-M, subsequently reaching equilibrium. This indicates that the addition of V-M had a beneficial effect on the removal of MB-Cu composite pollution. Upon reaching a V-M addition of 0.05 g, the removal percentages of MB and Cu(II) exhibited a near-equilibrium state (Figure 4b). This can be attributed to the insufficient availability of adsorption sites and iron donors provided by the low V-M addition, which consequently resulted in a diminished production of ·OH radicals during the Fenton-like reaction process. This ultimately led to a reduced percentage of removal. In light of the economic cost and pollution control considerations, the subsequent experiments were conducted with a quantity of V-M and a concentration of H₂O₂ of 0.05 g and 50 mmol/L, respectively. However, further investigation is required to assess the effects of the addition sequence of the two on the removal percentage.

3.2.2. Effect of the Initial pH

The pH of wastewater can affect the stability of V-M and the formation of ·OH, which exerts a significant influence on the efficacy of the treatment process. Figure 5 illustrates the effect of varying initial pH values on the treatment of MB-Cu composite pollution.

Figure 5 shows that the removal percentages of Cu(II) and MB were markedly influenced by alterations in the initial pH value of the solution. As the pH increased, the removal percentages initially exhibited an upward trend, subsequently reaching a point of equilibrium (Figure 5a). At a pH of 2, the removal percentages of Cu(II) and MB were low due to the fact that V-M was unable to exist in a stable state under conditions of strong acidity. At a pH of 3, the removal percentages of Cu(II) and MB reached 91.2% and 95.5%, respectively. Thereafter, an increase in the pH value had a negligible effect on the removal of Cu(II). The percentage of degradation of MB slightly declined with an increase in pH, yet remained at approximately 80% (Figure 5a), suggesting that acidic conditions were more conducive to the degradation of MB. This is due to the superior performance of Fe₃O₄ in catalyzing the formation of ·OH under acidic conditions [46]. As the pH increased, the capacity to generate ·OH decreased, thereby reducing the percentage of degradation of MB. Furthermore, the combination of V-M with H₂O₂ treatment resulted in a notable elevation in the pH value following the reaction (p < 0.01; Figure 5b). This suggests that V-M, as a Fenton-like catalyst, not only showed a broad pH range of applicability but also demonstrated a robust capacity to regulate pH, addressing the issue of system acidity following the conventional Fenton-like reaction and the necessity for alkaline solution supplementation.

3.2.3. Effect of the Initial Concentrations of Pollutants

An investigation into the effects of the initial pollutant concentration on the efficacy of the treatment process can ascertain the applicable concentration range of pollutants under specific conditions and reveal the influence of the pollutant concentration on the reaction process. Figure 6 illustrates the effects of the initial concentration of pollutants on the combined V-M and H₂O₂ treatment of MB-Cu composite pollution.

As the concentration of MB in the system increased, the rates of removal of Cu(II) and MB slightly decreased, yet they remained at a relatively high level (Figure 6a). When the initial concentration of MB was less than 50 mg/L, the percentage of degradation was approximately 100%; however, as the initial concentration of MB increased, the percentage of degradation subsequently declined. This is attributable to the restricted concentration of H₂O₂ and the concomitant reduction in the production of ·OH, which were insufficient to facilitate the complete degradation of excess MB. On the other hand, the elevated Cu(II) concentration in the mixed system not only increased the removal percentage of Cu(II) but also facilitated the degradation of MB (Figure 6b). This is due to the fact that both Cu(II) and Fe(II) are capable of promoting the generation of H₂O₂ to produce OH. The specific principle is illustrated in Equations (10) and (11) [47,48]. Therefore, it can be reasonably deduced that the presence of an appropriate Cu(II) can improve the efficiency of production of ·OH, which in turn will increase the percentage of degradation of MB.

$${\mathrm{Cu}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Cu}}^{+}+{\mathrm{HO}}_2·+{\mathrm{H}}^{+}$$

(10)

$${\mathrm{Cu}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Cu}}^{2+}+·\mathrm{OH}+{\mathrm{OH}}^{-}$$

(11)

In conclusion, the percentage of degradation of MB can be affected by the concentration of Cu(II), while the removal percentage of Cu(II) was essentially uninfluenced by the initial concentration of MB when V-M was employed to treat MB-Cu composite pollution. In a mixed system where the concentration of MB is less than 50 mg/L and the concentration of Cu(II) is greater than 100 mg/L, the percentage of removal of both can exceed 90%.

3.2.4. The Recycling Condition of the Cu(II) Immobilized by V-M

The recycling of metal ions from magnetically separated adsorbed samples represents a significant economic opportunity. The dissolution of V-M by HCl releases those heavy metals that have been adsorbed. Figure 7 illustrates the effects of varying HCl concentrations on the recycling of Cu(II). The magnetically separated samples showed robust retention of Cu(II) at a 0.001 mol/L HCl concentration (pH = 3.0), indicating that V-M exhibited notable acid resistance. The recycling percentage of Cu(II) increased notably (p < 0.01) when the concentration of HCl was set to 0.01 mol/L, with the highest recycling (in percentage terms) of Cu(II) observed at an HCl concentration of 0.05 mol/L, reaching 85.82%. The capacity of V-M to recycle Cu(II) is mediocre in comparison to other Cu(II) recycling methods (

Table 1. The comparison of recycling percentages of Cu(II).

Methods or Adsorbents	Recycling Percentage	Reference
V-M	85.82%	This study
Immobilized Saccharomyces cerevisiae	80% to 98.23%	[49]
Silica-gel-immobilized ditopic Schiff base	82% to 98%	[50]
Leaching-biosorption circulation system	90%	[51]
Immobilized cells of Pseudomonas putida II-11	More than 90%	[52]
Magnetic polymer beads	98%	[53]

). The findings indicated that the V-M synthesized in this study exhibited a notable adsorption capacity for Cu(II). The utilization of V-M facilitated the separation and recycling of heavy metal ions in composite pollution.

It is noteworthy that the release of MB also occurred during the recycling of Cu(II), but only up to 5.83%. This finding suggests that the process of MB removal by V-M in combination with H₂O₂ was dominated by the oxidative decomposition of MB by ·OH produced by the Fenton-like reaction, and that a small amount of MB was also adsorbed by V-M.

3.3. Analysis of the Removal Mechanism

3.3.1. The Role of ·OH

The ability of ·OH to degrade organic matter efficiently is well documented. Tertiary butanol (TB), a widely used ·OH scavenger, has been found to trap ·OH and prevent its oxidation of organic matter with great efficiency [48]. Figure 8 illustrates the effects of varying TB concentrations on the removal efficiency of MB and Cu(II).

The findings indicated that, when a high concentration of ·OH inhibitor was present in the system, the degradation percentage of MB decreased significantly with an increase in the TB concentration (p < 0.01; Figure 8b). This phenomenon can be attributed to the fact that the TB can trap the OH produced by H₂O₂ and prevent its oxidation of MB, as illustrated by Equations (12)–(15). In these equations, the ≡ symbol denotes the iron surface site [54,55,56,57]:

$$\equiv {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \equiv {\mathrm{Fe}}^{3+}+{\mathrm{OH}}^{-}+·\mathrm{OH}$$

(12)

$$\equiv {\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \equiv {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2\mathrm{O}+·\mathrm{OH}/{\mathrm{O}}_2^{·-}$$

(13)

$$\mathrm{MB}+·\mathrm{OH}\to \mathrm{intermediates}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(14)

$$·\mathrm{OH}+{\left({\mathrm{CH}}_3\right)}_3\mathrm{COH}\to {\mathrm{H}}_2\mathrm{O}+·{\mathrm{CH}}_2\mathrm{C}{\left({\mathrm{CH}}_3\right)}_2\mathrm{OH}$$

(15)

Equations (12) and (13) reveal the mechanism by which Fe(II) and Fe(III), at the surface site of Fe₃O₄, react with H₂O₂ to form ·OH. Equations (14) and (15) illustrate the degradation of MB by ·OH and the capture of ·OH by TB to form alkyl radicals, respectively.

Furthermore, the adsorption efficiency of Cu(II) increased with the TB concentration. This phenomenon can be attributed to the ability of TB to capture the OH generated by H₂O₂, thereby impeding its oxidation of organic matter in V-M. This, in turn, facilitates the adsorption of Cu(II). In conclusion, the degradation of MB was primarily attributed to the action of OH radicals, which were predominantly generated through the reaction of H₂O₂ with Fe₃O₄ in V-M.

3.3.2. Analysis of Pollutant Removal Mechanism

To investigate the distribution of Cu(II) in the solid–liquid phase when V-M was combined with H₂O₂ to treat MB-Cu composite pollution, the Langmuir and Freundlich adsorption isotherms were employed to fit the experimental data. Figure 9 illustrates the adsorption isotherm of Cu(II) and the resulting change in the concentration of Ca(II) in solution. The findings indicated that the variation in the Ca(II) concentration in the system was largely consistent with that of the adsorption isotherm, exhibiting an increase alongside the equilibrium concentration of Cu(II). This suggested the possibility of a displacement phenomenon occurring between Cu(II) and Ca(II) in V-M during the adsorption of Cu(II). As the initial equilibrium concentration of Cu(II) increased, so too did the displacement of Ca(II). The results of the isotherm adsorption fitting (Figure S1) demonstrated that the determination coefficient of the Freundlich adsorption isotherm equation was higher (R² = 0.98627). This suggested that the Freundlich model was more appropriate for describing the adsorption process of Cu(II) by V-M. In other words, the adsorption of Cu(II) by V-M can be defined as a multimolecular layer adsorption process occurring on a heterogeneous surface [58,59].

Figure 10 illustrates the adsorption kinetics of Cu(II) by V-M. The findings indicated that the unit adsorption capacity of Cu(II) increased with the prolongation of the adsorption period. Additionally, the concentration of Ca(II) within the system exhibited a parallel trend to that of Cu(II), which was in accordance with the results of isothermal adsorption.

The experimental data were fitted using pseudo-first-order and pseudo-second-order kinetic models. The results indicated that the pseudo-second-order kinetic model exhibited a higher degree of correlation and a superior fitting coefficient (R² = 0.9773; Figure S2;

Table 2. Pseudo-first-order and pseudo-second-order adsorption kinetic model parameters.

Metal Cation	Equations of Pseudo-First-Order Dynamics			Equations of Pseudo-Second-Order Dynamics
	Qe	K1	R2	Qe	K2	R2
Cu(II)	58.2750	0.004	0.6398	58.2750	0.001	0.9773

). The findings suggested that ion exchange occurred during the adsorption of Cu(II) by V-M, with the percentage of adsorption dependent on the concentration of the activation site on the surface of V-M [59].

Our previous research found that, in comparison to chemically synthesized minerals, the organic matter present in biominerals not only contributed to the formation of mineral crystals but also enhanced their stability. Furthermore, this organic matter played an essential role in the fixation and adsorption of pollutants [39,40]. Further investigation is required to elucidate the adsorption and fixation of pollutants by organic matter in V-M.

For comparison, the maximum unit adsorption capacity (Q_max) of V-M for Cu(II) in an adsorption system devoid of MB and the Q_max of other materials can be found in

Table 3. The comparison of Q_max (mg/g) values for Cu(II).

Adsorbents	Qmax	References
Mesoporous chitin-blended MoO3-Montmorillonite nanocomposite	19.03	[60]
Biogenic iron compounds	25.42	[61]
Immobilized biomass of Penicillium citrinum	29.68	[62]
Bentonite	32.26	[63]
Chitosan/CaCO3-silane nanocomposites	33.90	[64]
Sesame straw biochar	55.00	[65]
Zirconium oxide immobilized alginate beads	69.90	[66]
Amorphous CaCO3	69.93	[67]
Graphene oxide	71.43	[68]
V-M	76.38	This study
Zwitterion-chitosan bed	123.50	[69]
Immobilized Saccharomyces cerevisiae	144.90	[70]
Biogenic vaterite	178.57	[42]
CaCO3/polyaniline-polypyrrole-modified GO/alginate	291.20	[71]

. Among the numerous materials that can adsorb Cu(II), the adsorption performance of V-M was intermediate, and the Q_max of V-M was lower than that of bioderived vaterite, as synthesized in our laboratory [42]. This suggested that the incorporation of Fe₃O₄ can exert a certain influence on the Cu(II) retention properties of biogenic vaterite. Nevertheless, the magnetic separation characteristics of V-M and its capacity to catalyze the oxidation of MB by H₂O₂, as a Fenton-like catalyst, were not exhibited by other materials. Consequently, V-M continues to demonstrate advantages in the treatment of complex polluted water bodies.

In conclusion, V-M is a type of porous organic–inorganic complex comprising Fe₃O₄, and the mechanism of its action in the removal of MB-Cu composite pollution in the presence of H₂O₂ is illustrated in Figure 11. The main elements are as follows: (1) Magnetic vaterite could be synthesized through the induction of B. velezensis. (2) The Fe₃O₄ component of V-M was capable of reacting with H₂O₂ to form ·OH and degrade MB to CO₂ and H₂O. The presence of Cu(II) was observed to facilitate this reaction. (3) In addition to providing adsorption sites for Cu(II) and MB, the organic functional groups in V-M also stabilized the structure of V-M, thereby extending the pH range of its applicability. (4) Cu(II) was primarily removed through V-M adsorption, which occurred through multilayer adsorption on heterogeneous surfaces. In addition, displacement with Ca(II) played a role in the removal process. (5) Following the adsorption of Cu(II), the V-M could be efficiently collected using an applied magnetic field. (6) The immobilized Cu(II) in the V-M could be efficiently recycled through a diluted HCl treatment.

4. Conclusions

The V-M was synthesized through the use of B. velezensis. V-M can catalyze the production of ·OH from H₂O₂ for the purpose of degrading MB. Additionally, it is capable of fixing Cu(II) through chemisorption, and Cu(II) can also participate in Fenton-like reactions to facilitate the degradation of MB. Following the treatment of the composite contamination, V-M was separated by magnetic separation. The Cu(II) fixed by V-M could then be recycled by diluted HCl. In conclusion, the combination of V-M and H₂O₂ was shown to be an effective method for the removal of MB-Cu pollution in water, as well as for the recycling of Cu(II). This provides a novel approach for addressing the combined pollution of organic substances and heavy metals in the environment and the recycling of heavy metals.