Investigation into the synergy at interfaces and recyclability of functional magnetic reed biochar-based demulsifying agents

3.1.1. Scanning electron microscopy

To elucidate the microstructural and elemental attributes of magnetic polyamine-modified biochar (MPRB), a comprehensive characterization of its morphology and chemical composition was conducted using SEM coupled with EDS, as depicted in Figure 2. The SEM image of the pristine biochar (Figure 2a) highlights its characteristic honeycomb-like porous framework. This intricately porous architecture offers a substantial specific surface area, which is advantageous for further modification steps. Figure 2(b) displays evenly dispersed nanometer-sized particles, albeit with some agglomeration present. Notably, the surface of the biochar after modification (Figure 2c) is significantly coated with Fe₃O₄ nanoparticles and a polyamine composite, resulting in a notable enhancement in surface roughness. This multi-scale composite structure is advantageous for augmenting the material’s surface active sites, thus boosting its adsorption capabilities. The quantitative EDS analysis (Figure 2d) reveals that carbon (58.8 wt%) and oxygen (22.8 wt%) constitute the main elements of MPRB. The detected trace silicon (Si) stems from both the KH-570 coupling agent and the reed straw biochar matrix, offering direct proof of the coupling agent’s successful involvement in the grafting reaction. Moreover, the iron content of 15.6 wt% verifies the efficient loading of Fe₃O₄ nanoparticles, imparting the material with notable magnetic responsiveness. Elemental mapping demonstrates a uniform distribution of constituent elements throughout the material, signifying robust interfacial bonding among the various components and substantiating the successful fabrication of the composite material.

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Figure 2.

(a) SEM of Biochar, (b) SEM of Fe₃O₄, (c) SEM of MPRB, (d) EDS spectrum of MPRB; (e-i) XPS spectra of MPRB; (j) FT-IR of RBC, Fe₃O₄, polyamine demulsifier, MPRB-1, MPRB-4.

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3.1.2. X-ray photoelectron spectroscopy

For a deeper analysis of the material’s elemental and chemical properties, XPS was performed on MPRB, as illustrated in Figure 2. The full spectrum of MPRB is depicted in Figure 2(e). As depicted in Figure 2(f), the C1s spectrum exhibits multiple distinct peaks: those at 284.8 eV and 283.96 eV are associated with C-C/C-H bonds, predominantly stemming from the carbon framework of the biochar and the organic segments within the polyamine demulsifier. The peak at 287.9 eV is indicative of the C=O bond, originating from the carbonyl groups within the polyamine demulsifier; the peak at 285.61 eV is ascribed to the C-O bond, resulting from the cross-linking product of epichlorohydrin and the hydroxyl groups on the biochar surface. The XPS spectrum for iron (Fe) in the material can be seen in Figure 2(g), revealing the presence of both Fe³⁺ and Fe²⁺ valence states. The peak at 711.88 eV corresponds to the Fe 2p_3/2 of Fe³⁺, linked to the γ-Fe₂O₃ structure; the peak at 723.99 eV signifies Fe 2p_1/2, and the satellite peak at 714.03 eV arises from Fe³⁺ charge transfer; the peak at 709.36 eV is attributed to Fe²⁺. The outcomes solidly validate the successful synthesis of magnetic particles. The O1s spectrum in Figure 2(h) presents two main peaks: one at 530.5 eV corresponding to Fe-O, signifying the presence of iron oxide; and one at 532.0 eV attributed to Si-O, due to the hydrolysis product of KH-570, affirming the effective grafting of the silane coupling agent. Additionally, the Si signal observed in the full-spectrum graph is also associated with the natural silicon content in the biochar. In the N1s spectrum of Figure 2(i), the peak at 399.7 eV is ascribed to the -NH₂ group, while the peak at 401.3 eV corresponds to the -CONH- group; these characteristic peaks are both attributable to the polyamine demulsifier.

3.1.3. Infrared spectrum analysis

The functional groups on the surface of RBC, Fe₃O₄, polyamine demulsifier, MPRB-1, and MPRB-4 samples were analyzed by FTIR spectroscopy. The specific results have been shown in Figure 2(j), a comparable absorption peak is present at 3422 cm^-1 across all samples, which is indicative of the broad stretching vibration band of the -OH groups. Notably, the polyamine demulsifier displays an absorption peak resulting from the stretching vibrations of N-H bonds. In the polyamine demulsifier’s spectrum, the peak at 2815 cm^-1 is associated with the symmetric stretching of -CH₂- groups within long-chain alkanes, while the peak at 1019 cm^-1 is indicative of the vibrational motion of C-O-C bonds that arise from the ring-opening reaction involving epichlorohydrin. For the Fe₃O₄, absorption bands at 573 cm^-1 and 439 cm^-1 correspond to the stretching vibrations of Fe-O bonds and the lattice vibrations of the Fe-O-Fe groups, respectively. Within the infrared spectrum of RBC, a characteristic absorption peak is observed at 1348 cm^-1, which is attributable to the bending vibrations of C-H bonds within the aromatic carbon structure. In the case of MPRB-1 and MPRB-4 composites, the presence of absorption peaks at 573 cm^-1 and 439 cm^-1 signifies the successful integration of magnetic properties. Additionally, the peaks located at 2815 cm^-1 and 1019 cm^-1 confirm the successful attachment of polyamine to the material’s surface. These findings collectively validate the successful modification of the composites with magnetic and polyamine features. Furthermore, as the quantity of biochar added increases, there is a noticeable reduction in the intensity of the absorption peaks associated with magnetic particles and polyamine demulsifiers in the spectrum of MPRB-4 compared to MPRB-1. This indicates that an overabundance of biochar may obscure the distinctive absorption peaks of the magnetic components. It becomes clear that the increment in biochar proportion significantly alters the structural and chemical attributes of the composite materials. The correlation between higher biochar content and the subsequent effects on the material’s properties highlights the necessity for careful consideration of the composition ratio when formulating these composites.

3.2.1. Demulsification performance of different materials

The demulsification performance of different materials was tested at 70°C, 150 min, and 300 mg∙L^-1. The experimental results have been shown in Figures 3(a-c). As depicted in Figures 3(b,c), the demulsification outcomes of various materials on crude oil and diesel emulsions are compared. It is apparent that the introduction of these demulsifiers leads to a marked demulsification effect. Notably, the untreated RBC shows less effective demulsification, with the diesel emulsion retaining its opaque, milky appearance. However, upon the application of demulsifiers from MP through to MPRB-4, there is a clear delineation at the oil-water interface, rendering the oil phase of the diesel emulsion considerably clearer. When compared with MP, MPRB exhibits enhanced water removal, indicating that the high surface area of RBC promotes a more even dispersion and increased presence of polyamine and magnetic particles within the composite MPRB. This configuration offers a multitude of reactive sites. Additionally, the adsorptive nature of RBC bolsters the separation efficiency of oil from water. Consequently, these findings underscore the composite’s design advantages in enhancing demulsification performance and improving the clarity of the oil phase after treatment. Figure 3(a) illustrates the demulsification results of different materials on emulsions, highlighting MPRB-3 as the most effective, with demulsification rates of 91.47% for crude oil emulsions and 99.23% for diesel emulsions. The biochar content at this level is found to be most suitable, enhancing the combined performance of the polyamine demulsifier and magnetic particles. This optimal blend results in a more even distribution of components, thereby maximizing their individual demulsifying capabilities. Conversely, a deficiency in biochar leads to inadequate adsorption and contact points, which diminishes the synergistic effects among the components and consequently, the demulsification effectiveness. Excessive biochar, meanwhile, causes the composite material to clump, adversely impacting its demulsification efficiency.

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Figure 3.

Effect of different materials on demulsification of emulsion ( (a) demulsification data; (b) Demulsification effect diagram of crude oil emulsion; (c) demulsification effect of diesel emulsion); the effect of different addition amount of demulsifier on demulsification of water-in-oil emulsion ( (d) demulsification data; (e) demulsifying effect diagram of crude oil emulsion; (f) demulsification effect diagram of diesel emulsion); effect of temperature and action time on demulsification of water-in-oil emulsion ( (g) demulsification data of crude oil emulsion; (h) demulsification data of diesel emulsion).

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3.2.2. Effect of demulsifier dosage on demulsification performance of demulsifiers

In order to assess systematically how the dosage of a demulsifier affects its performance, this study tested a range of concentrations, 0, 100, 200, 300, 400, and 500 mg∙L^-1, at a constant temperature of 70°C, and evaluated the demulsification outcomes after 150 min. The findings, as shown in Figures 3(d-f), reveal that Figure 3(d) summarizes the comprehensive demulsification data, whereas Figures 3(e, f) focus specifically on the performance related to crude oil and diesel emulsions, respectively. It is evident that both types of emulsions respond to demulsification in a comparable manner. As the dosage of the demulsifier increases from 0 to 400 mg/L, there is a notable rise in the demulsification efficiency, with a corresponding increase in the separation of water and a clarification of the oil phase in diesel emulsions. The peak performance is attained at a dosage of 400 mg∙L^-1, where the demulsification rates for crude oil and diesel emulsions are 92.07% and 99.17%, respectively. Interestingly, raising the dosage to 500 mg∙L^-1 leads to a slight reduction in efficiency. This outcome suggests that an overabundance of demulsifier can saturate the molecules at the oil-water interface, diminishing their effectiveness and consequently, their interaction with the interface. Consequently, this saturation diminishes the demulsification effectiveness. To sum up, the quantity of demulsifier added significantly influences the outcome, with an optimal range identified between 0 and 400 mg∙L^-1 for the most effective demulsification. Exceeding this threshold appears to counterproductively reduce the demulsification capability due to the saturation of demulsifier molecules. These insights offer valuable guidance for determining the appropriate amount of demulsifier to use in real-world scenarios.

3.2.3. Effect of temperature and action time on the demulsification performance of the demulsifier

In an effort to evaluate the influence exerted by a demulsifier at a dosage of 400 mg∙L^-1 on both crude oil and diesel emulsions, a series of experiments was executed across a spectrum of temperatures and durations. The findings are graphically represented in Figures 3(g and h). Upon a demulsifier interaction period of 150 min, a noticeable escalation in demulsification efficacy was recorded for crude oil emulsions, escalating from 83.26% to 91.85%, and for diesel emulsions, from 96.36% to 99.28%, as the temperature was elevated from 50°C to 70°C. This trend underscores the substantial enhancement in demulsification efficiency and the acceleration of the demulsification process attributable to elevated temperatures, which diminishes oil viscosity and mitigates inter-droplet forces, thereby fostering droplet collision and coalescence to expedite oil-water demulsification. Concurrently, the increased temperature augments the diffusion of demulsifier molecules within the emulsion, facilitating their adsorption onto the oil-water interface. At a temperature of 70°C, following 60 min of demulsifier exposure, the demulsification rates for crude oil and diesel emulsions were recorded at 85.23% and 96.23%, respectively, indicating that the demulsifier molecules have had adequate time to migrate and adsorb onto the oil-water interface, yielding effective demulsification. Prolonging the demulsification duration to 120 min further elevated the rates to 90.72% for crude oil and 98.28% for diesel emulsions, nearing their peak performance levels. Further extensions in demulsification time yielded marginal improvements in demulsification rates, not surpassing 3%. To summarize, a synergistic interaction exists between temperature and the duration of demulsifier action. While higher temperatures permit a reduction in the required demulsifier action time, sufficient duration is still essential to ensure complete adsorption of the demulsifier. This interplay is pivotal for optimizing demulsification outcomes.

3.4.1. Interfacial activity analysis

In pursuit of elucidating the oil-water separation efficacy of demulsifiers, an assessment of interfacial activities across diverse materials was undertaken, with corresponding results presented in Figures 5(a-c). The pristine two-phase system, characterized by a clear demarcation between oil and water phases, is portrayed in Figure 5(a). Upon the introduction of various materials and subsequent vigorous agitation, a homogeneous mixture of each sample material with oil and water within the vials was achieved, as depicted in Figure 5(b). After a 20-min sedimentation period, the samples displayed distinct interfacial affinities, as evidenced in Figure 5(c). Sample bottle 1, devoid of any demulsifying agent, reverted entirely to a stratified oil-water configuration. Sample bottle 2, containing RBC particles, exhibited turbidity within the oil phase, alongside the presence of substantial particle aggregates, attributable to the particles’ pronounced hydrophobicity and diminished dispersibility. The amalgamation of a polyamine demulsifier with Fe₃O₄ endows the samples with interfacial activity. In sample bottle 3, the MP samples demonstrated a marked propensity to translocate towards the oil-water interface. Nonetheless, a considerable portion of the samples persisted in the oil phase. Conversely, sample bottle 4 housed a substantial amount of MPRB-3 samples that localized at the oil-water boundary, signifying the superior amphiphilicity of MPRB. This characteristic facilitates its rapid translocation to the oil-water interface, thereby effectively exerting its demulsifying function.

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Figure 5.

Interface activity of different materials (a) original state, (b) fully shaken, (c) standing for 20 min; 1. blank, 2. RBC, 3. MP, 4. MPRB-3); (d) the effects of different samples and (e) different amounts of MPRB on interfacial tension; (f) the contact angles of RBC, (g) Fe₃O₄ and (h) MPRB were measured.

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3.4.2. Interfacial tension analysis

In an effort to assess the efficacy of demulsifiers in disrupting oil-water interfaces, this study conducted measurements of dynamic interfacial tension for different samples, as illustrated in Figures 5(d,e). The oil phase is composed of crude oil from the Daqing Oilfield in China, while the water phase is laboratory-grade deionized water. All measurements were conducted under a temperature range of 25 ± 1°C. With a testing concentration set at 300 mg∙L^-1, the interfacial tension measurements for biochar, MPEB, and polyamine demulsifiers have been presented in Figure 5(d). In contrast to the blank experiment, the introduction of these three materials led to a marked decrease in oil-water interfacial tension. Among them, the polyamine demulsifier had the most significant impact, reducing the interfacial tension from 42.29 mN∙m^-1 to 23.5 mN∙m^-1; MPRB also showed a notable effect, lowering the tension to 26.02 mN∙m^-1. Additional tests were carried out by varying the amount of MPRB added, and the interfacial tension results have been depicted in Figure 5(e). As the amount of MPRB added increased, the reduction in interfacial tension became increasingly more pronounced. This demonstrates that MPRB has superior interfacial performance, capable of effectively diminishing the oil-water interfacial tension, thus promoting the demulsification process.

3.4.3. Wettability analysis

The material’s wettability significantly influences the demulsification efficacy and the underlying mechanisms of demulsifiers. Contact angle measurements were performed on RBC, Fe₃O₄, and MPRB, as depicted in Figures 5(f-h). Figures 5(f and g) demonstrate that RBC exhibits a contact angle of 112.2°, indicative of its pronounced hydrophobic nature. In contrast, unmodified Fe₃O₄ presents a contact angle of 75.15°, which suggests a certain level of hydrophilicity. Figure 5(h) shows that MPRB has a contact angle of 87.3°, situated between 85° and 95°, signifying its superior wettability. These shifts in contact angles imply that the successful combination of these materials has incorporated additional polar functional groups, thereby augmenting the hydrophilicity of the material’s surface. This enhancement is instrumental in bolstering MPRB’s capacity to migrate towards the oil-water interface and disrupts the emulsion’s stability.

3.4.4. Microscopic demulsification process and demulsification mechanism analysis

To further elucidate the corresponding demulsification mechanisms, the demulsification process under the microscope was observed and analyzed, with the results shown in Figures 6(a-d). Figure 6(a) presents the initial emulsion image, from which it can be seen that the oil phase contains a multitude of tiny water droplets, representing a stable oil-in-water emulsion. Figure 6(b) corresponds to the state of the emulsion 30 min after the addition of the demulsifier; as the Figure 6 illustrates, the number of water droplets has decreased. Under the influence of the demulsifier, the smaller droplets come closer together and gradually coalesce, ultimately forming larger water droplets. Figure 6(c) depicts the state of the oil-water emulsion during the later stages of demulsification; the water phase in the image is noticeably reduced. Due to the difference in density, the coalesced larger droplets, under the influence of gravity, sink and ultimately achieve complete separation from the oil phase.

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Figure 6.

(a-d) Microscopic demulsification process and demulsification process diagram of MPRB.

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Figure 6 illustrates a comprehensive diagram of the demulsification procedure. In essence, the honeycomb network structure of biochar imparts MPRB with a substantial specific surface area and an extensive porous framework. The incorporation of magnetic particles endows the demulsifier with robust magnetic responsiveness, allowing MPRB to exhibit commendable magnetic properties. When subjected to an external magnetic field, it induces a magnetophoretic effect, which propels the demulsifier to swiftly relocate to the oil-water interface and aggregate. Moreover, the addition of a polyamine demulsifier on the surface of the material augments the presence of hydrophilic groups such as amines and hydroxyls, enhancing the surface’s active interaction sites. By examining the interfacial characteristics of the composite and the microscale demulsification process, the collaborative action of these three materials renders MPRB an amphiphilic substance with excellent wettability, high interfacial activity, and reduced interfacial tension. This enables it to efficiently migrate to the oil-water interface, displace emulsifier molecules through interfacial substitution, destabilize the stability of the interface film, encourage the approach and coalescence of dispersed droplets, and hasten the demulsification process, culminating in the effective separation of oil and water phases. Furthermore, as diesel-in-water emulsions lack intricate natural active substances, resulting in lower interfacial film stability, MPRB can more readily accomplish demulsification.