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Article

Photocatalytic Degradation of Polycyclic Aromatic Hydrocarbons in Simulated Offshore Oil Spill Wastewater by Magnesium Phthalocyanine Derivatives Supported on Coconut Shell Activated Carbon

by
Dejun Wang
1,
Hui Li
2,
Lihua Liang
1,
Juan Su
1,
Jifang Wei
3,*,
Dong Wang
1,*,
Changjiang Zuo
1 and
Qiyou Liu
4,*
1
School of Biological & Chemical Engineering (Blue Engineering), Qingdao Technical College, Qingdao 266555, China
2
Human Resource Department, Qingdao Technical College, Qingdao 266555, China
3
North China Sea Ecological Center of the Ministry of Natural Resources, Qingdao 266000, China
4
College of Chemistry and Chemical Engineering, China University of Petroleum, Qingdao 266580, China
*
Authors to whom correspondence should be addressed.
Catalysts 2026, 16(5), 470; https://doi.org/10.3390/catal16050470
Submission received: 14 April 2026 / Revised: 2 May 2026 / Accepted: 15 May 2026 / Published: 19 May 2026
(This article belongs to the Special Issue Catalytic Materials for Hazardous Wastewater Treatment)
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Abstract

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic pollutants in marine ecosystems, necessitating efficient remediation. This study synthesized magnesium phthalocyanine (MgPc) and its modified derivatives, magnesium azaphthalocyanine (NMgPc) and methyl-substituted magnesium azaphthalocyanine (MeNMgPc), as visible-light-driven photocatalysts for PAH degradation. To enhance efficiency and recoverability, these photosensitizers were immobilized onto coconut shell activated carbon (AC) via multiple ultrasonic impregnation. Characterizations (UV-Vis, SEM, EDAX, BET) confirmed successful active component deposition; nitrogen substitution and peripheral methyl groups synergistically tuned the electronic structure and suppressed aggregation. Under xenon lamp irradiation, the MeNMgPc/C composite exhibited superior activity, degrading 90.55% of naphthalene. Box-Behnken response surface optimization identified optimal conditions (13.18 g/L dosage, 20 A, 2.28 h), yielding 96.67% experimental removal and adhering to pseudo-first-order kinetics. Mechanistic studies via electron spin resonance identified hydroxyl (•OH) and superoxide radicals (O2•−) as primary reactive species. GC-MS analysis elucidated a sequential phenanthrene ring-opening pathway, progressing to ultimate mineralization into CO2. Consequently, MeNMgPc/C presents a highly efficient, recoverable photocatalytic platform for marine PAH remediation.

Graphical Abstract

1. Introduction

Offshore oil spills severely threaten marine ecosystems by introducing persistent and highly toxic polycyclic aromatic hydrocarbons (PAHs) [1]. Characterized by fused-ring structures, PAHs are highly lipophilic, recalcitrant to biodegradation, and prone to bioaccumulation, prompting the US Environmental Protection Agency (USEPA) to classify 16 PAHs as priority pollutants [2]. Conventional remediation strategies—such as physical containment, chemical dispersants, and biological treatments—are frequently hindered by low efficiency, strict environmental prerequisites, and the risk of secondary pollution [3,4].
Advanced oxidation processes (AOPs), particularly heterogeneous photocatalysis, offer a robust alternative by generating highly reactive oxygen species (ROS) capable of mineralizing recalcitrant organic pollutants [5]. While inorganic semiconductors like TiO2 are widely used, their wide bandgaps restrict them to ultraviolet activation. Phthalocyanines (Pcs), possessing a highly conjugated 18π-electron system, exhibit intense Q-band absorption in the visible region, making them exceptionally effective visible-light photosensitizers [6,7].
However, unsubstituted metallophthalocyanines exhibit a pronounced tendency to self-aggregate via intermolecular π–π stacking. This aggregation quenches excited-state reactivity and severely diminishes catalytic efficiency [8,9]. Peripheral chemical modifications, such as aza-substitution (nitrogen doping into the isoindole units) and the introduction of bulky alkyl groups (e.g., methyl substituents), effectively disrupt this cofacial stacking, tuning the HOMO-LUMO gap and enhancing photocatalytic activity [10,11]. Furthermore, to facilitate catalyst recovery and prevent secondary marine pollution, it is essential to immobilize these homogeneous photosensitizers onto porous, high-surface-area supports, such as biomass-derived coconut shell activated carbon (AC), via ultrasonic impregnation [12,13].
This study reports the synthesis, characterization, and immobilization of magnesium phthalocyanine (MgPc) and its modified derivatives (NMgPc, MeNMgPc) onto coconut shell AC. We systematically evaluate their photocatalytic efficacy against four priority PAHs (naphthalene, phenanthrene, fluorene, and chrysene) under visible-light irradiation. Key operational parameters were optimized utilizing a Box-Behnken response surface methodology (RSM). Finally, the dominant ROS and the photodegradation pathway of phenanthrene were elucidated using electron spin resonance (ESR) and GC-MS analyses.

2. Results

2.1. Optical Characterization

UV-Vis absorption spectroscopy was employed using N,N-dimethylformamide (DMF) as the solvent to confirm the successful synthesis of the macrocyclic catalysts and to evaluate their aggregation behavior. Typical phthalocyanine derivatives exhibit two prominent absorption features: the B-band (Soret band) in the near-ultraviolet region (250–350 nm), assigned to deeper π→π* transitions to the lowest unoccupied molecular orbital (LUMO), and the Q-band in the visible region (600–700 nm), corresponding to the doubly degenerate a1u→eg transition characteristic of the highly conjugated 18π-electron macrocycle [14,15].
As shown in Figure 1, the UV-Vis spectrum of pristine MgPc displayed a characteristic B-band at 273 nm and a sharp Q-band at 670.5 nm. Subsidiary absorption peaks observed at 492 nm and 606 nm indicated the formation of dimeric or oligomeric aggregates driven by intermolecular cofacial π–π stacking [16]. Upon aza-substitution, the spectrum of NMgPc exhibited a B-band at 275 nm and a distinctly blue-shifted Q-band at 635 nm, accompanied by a significant attenuation of the aggregation-related peaks. For MeNMgPc, the B-band shifted to 290 nm and the Q-band emerged at 638 nm. Notably, the subsidiary aggregation peaks were virtually eliminated, confirming that the steric hindrance introduced by peripheral methyl substitution effectively disrupts cofacial stacking [17,18]. Collectively, these spectral evolutions corroborate that nitrogen substitution and the introduction of bulky peripheral groups synergistically modulate the delocalized π-electron system and inherently suppress self-aggregation, thereby preserving the monomeric state crucial for high photocatalytic activity.

2.2. Textural Properties and Pore Structure Evolution

Nitrogen physisorption isotherms were recorded to evaluate the textural evolution of the carbonaceous support following catalyst immobilization. As detailed in Table 1, the pristine coconut shell activated carbon (AC) exhibited a Brunauer–Emmett–Teller (BET) specific surface area of 377.66 m2/g. Subsequent to the multiple ultrasonic impregnation process, the specific surface areas of the MgPc/C, NMgPc/C, and MeNMgPc/C composites decreased systematically to 184.40, 130.93, and 88.09 m2/g, respectively. This progressive attenuation in both surface area and cumulative pore volume provides compelling evidence for the successful deposition and internal distribution of the macrocyclic photosensitizers within the porous network [19].
Importantly, despite this partial pore occupation, the average pore diameters of all synthesized composites were rigidly maintained within the narrow mesoporous regime (2.27–2.31 nm). The retention of this mesoporosity indicates that the applied ultrasonic impregnation strategy efficiently mitigated severe pore blockage. This structural preservation is crucial for environmental catalysis, as it ensures that the active catalytic sites remain sterically accessible, thereby facilitating the rapid internal mass transfer and diffusion of bulky PAH molecules during subsequent photocatalytic degradation processes [20,21].

2.3. Morphological and Elemental Characterization

SEM images (Figure 2) illustrate the surface morphologies of the materials before and after catalyst loading. The pristine coconut shell-derived activated carbon support is initially characterized by numerous spherical protrusions of varying sizes. Following repeated ultrasonic impregnation, these native features are largely obscured, indicating successful deposition and extensive coverage by the active components.
Morphological comparisons reveal that all three catalysts form granular crystals on the support. Notably, the particle size of MgPc is significantly smaller than that of both NMgPc and MeNMgPc. Although smaller particle sizes typically facilitate higher loading capacities, residual spherical protrusions remain visible on the MgPc-loaded support (Figure 2d), suggesting a comparatively thinner catalyst layer. The implications of this variation on photocatalytic performance will be discussed in Chapter 3. In contrast, NMgPc/C and MeNMgPc/C exhibit similar morphologies with uniform surface distribution and comparable particle sizes. The complete concealment of the support’s original protrusions and trenches further confirms the excellent loading efficacy of these two aza-catalysts.
Energy-dispersive X-ray spectroscopy (EDAX) analyses (Figure 3) further verify the successful immobilization of the synthesized magnesium phthalocyanine complexes on the support, as evidenced by the distinct Mg signals. Furthermore, the implementation of the multiple ultrasonic impregnation technique resulted in a substantial enhancement in Mg content—increasing by over fivefold compared to single ultrasonic impregnation. This significantly elevated Mg content corresponds to a higher loading of the active catalyst, which is anticipated to effectively promote degradation during subsequent photocatalytic evaluations. To quantitatively assess the catalyst loading, the theoretical weight percentage of MeNMgPc on the AC support was calculated based on the EDAX results. Given that the mass fraction of the central Mg atom in the MeNMgPc/C composite is 1.93 wt% (Figure 3), and considering the molecular weights of Mg (24.31 g/mol) and the MeNMgPc macrocycle (656.96 g/mol), the actual loading amount of MeNMgPc is estimated to be approximately 52.16 wt%. This substantial loading capacity further validates the efficiency of the multiple ultrasonic impregnation method and provides a solid material foundation for the observed superior photocatalytic performance.

2.4. Photocatalytic Performance

The photocatalytic efficacy of the synthesized composites was systematically evaluated through the degradation of four representative PAHs (naphthalene, phenanthrene, fluorene, and chrysene) under visible-light irradiation (300 W xenon lamp, λ > 400 nm). Across all tested aromatic substrates, the MeNMgPc/C composite exhibited significantly superior photocatalytic performance compared to both the unmodified MgPc/C and the aza-substituted NMgPc/C. Notably, under optimized conditions, MeNMgPc/C achieved an exceptional degradation efficiency of 90.55% for naphthalene, substantially surpassing its counterparts. As shown in Figure 4, the blank control experiment without any catalyst exhibited negligible degradation under visible-light irradiation, confirming that the removal of PAHs is strictly driven by the photocatalysts.
Furthermore, to rigorously distinguish the contribution of physical adsorption from true photocatalysis, the initial 30-min dark adsorption phase was explicitly monitored and is now displayed in Figure 4. During this dark equilibrium period, the physical adsorption removal rates for the four PAH pollutants by the composites ranged from 10.5% to 15.3%. This quantitative separation clearly demonstrates that while porous adsorption plays a preliminary role, the subsequent and substantial decline in PAH concentrations is predominantly driven by the genuine photocatalytic degradation process. This pronounced enhancement in photoactivity is in excellent agreement with the optical characterizations (e.g., UV-Vis spectroscopy). The structure-activity relationship elucidated here demonstrates that the introduction of peripheral methyl groups effectively sterically hinders the intermolecular π-π stacking of the macrocycles. By efficiently suppressing self-aggregation and preserving the monomeric state of the photosensitizer on the carbon support, this targeted structural modification maximizes the density of accessible photoactive sites, thereby facilitating a more efficient interfacial transfer of photo-induced charge carriers and the subsequent generation of reactive oxygen species (ROS) [22,23].

2.5. Optimization of Operational Parameters: Single-Factor Analysis

To systematically optimize the photocatalytic degradation of naphthalene and elucidate the influence of critical operational parameters, single-factor experiments were conducted utilizing the superior MeNMgPc/C composite (Figure 5).
  • Effect of Reaction Time: Temporal monitoring over a 4-h period revealed a rapid initial surge in degradation efficiency, which peaked between 120 and 140 min before reaching a plateau. To balance maximal pollutant removal with optimal energy consumption, 120 min was established as the standard reaction duration (Figure 5a).
  • Effect of Catalyst Dosage: Varying the catalyst loading (1–25 g/L) in a 50 μg/L naphthalene solution identified 10 g/L as the optimal dosage, achieving a peak degradation efficiency of 90.55%. Insufficient loading (1 g/L) yielded a marginal removal rate of 15.7% due to a severe scarcity of accessible catalytic active sites. Conversely, dosages exceeding 10 g/L induced detrimental light scattering and inner-filter effects; excessive suspended particles hindered photon penetration into the suspension bulk [24]. Furthermore, excessive carrier amounts introduced competitive physical adsorption, convoluting the true photocatalytic contribution (Figure 5b).
  • Effect of Light Intensity (Operating Current): The applied current, which directly dictates the photon flux of the light source, exhibited a positive correlation with degradation efficiency up to 16 A. Beyond this threshold, the efficiency plateaued, indicating that the generation and separation of electron-hole pairs had reached thermodynamic saturation [25]. Consequently, 16 A was selected to prevent superfluous energy expenditure without compromising kinetic benefits (Figure 5c).
  • Effect of Initial Concentration: An inversely proportional relationship was observed between the initial naphthalene concentration and degradation efficiency. At highly dilute concentrations (<50 μg/L), rapid removal was predominantly governed by the profound physical adsorption capacity of the porous carbonaceous support rather than genuine photocatalysis. Conversely, at concentrations exceeding 50 μg/L, the fixed density of photoactive sites and constrained interfacial mass transfer restricted the overall efficiency [26]. Thus, 50 μg/L was standardized for subsequent evaluations (Figure 5d).

2.6. Process Optimization via Box-Behnken Response Surface Methodology

To systematically evaluate the interactive effects of critical independent variables on the photocatalytic degradation of naphthalene, a Box-Behnken design (BBD) coupled with response surface methodology (RSM) was employed (Table 2). The optimization process focused on three key operational parameters: catalyst dosage (X1, g/L), operating current (X2, A), and reaction time (X3, h). Center-point replicates (runs 13–15) yielded a consistent actual efficiency of 90.55% with negligible variance, verifying the exceptional reproducibility of the experimental setup [27].
The empirical relationship between the target response (Y, representing naphthalene removal efficiency) and the independent variables was mathematically expressed by the following second-order polynomial equation:
Y = 90.55 + 8.85X1 + 5.69X2 + 4.63X3 − 2.34X1X2 + 1.18X1X3 + 0.89X2X3 − 3.12X12 − 2.76X22 − 1.55X32
Analysis of variance (ANOVA) was utilized to robustly assess the statistical validity of the proposed regression model. The highly significant model F-value (p < 0.01) and an insignificant lack-of-fit firmly established the model’s reliability and its adequate fitting to the experimental data [28]. Numerical optimization based on the established model was executed to maximize degradation performance. The theoretical optimum conditions were identified as a catalyst dosage of 13.18 g/L, an operating current of 20 A, and a reaction time of 2.28 h, yielding a predicted maximum removal efficiency of 96.48%. A subsequent independent validation experiment conducted under these precise optimal conditions achieved an actual degradation efficiency of 96.67%. This remarkable agreement between the predicted and experimental values unequivocally validates the precision of the BBD model for optimizing the proposed photocatalytic remediation process [29].

2.7. Identification of Degradation Intermediates

Gas chromatography-mass spectrometry (GC-MS) was employed to systematically identify the intermediate products formed during the photocatalytic degradation of phenanthrene over the MeNMgPc/C composite. Temporal analysis revealed a dynamic evolution of intermediate species (Table 3). During the initial phase of irradiation (0–30 min), 9,10-phenanthrenequinone and phthalic anhydride were detected as the predominant primary intermediates, demonstrating that the initial oxidative attack preferentially targets the highly reactive 9,10-bond of the phenanthrene molecule [30]. As the photocatalytic reaction progressed, continuous oxidation yielded lower-molecular-weight aliphatic compounds, primarily including various phthalate esters and aliphatic dicarboxylic acids. Upon prolonged irradiation (120 min), the complex aromatic structures were extensively degraded, leaving only trace concentrations of short-chain carboxylic acids (C2–C4) in the solution [31]. These short-chain acids served as the final observable aqueous intermediates prior to ultimate mineralization into CO2 and H2O.

3. Discussion

3.1. Role of Chemical Modification in Catalytic Activity

The photocatalytic activity follows the order MeNMgPc/C > NMgPc/C > MgPc/C. This trend is driven by reduced self-aggregation and electronic tuning achieved through structural modifications. Pristine MgPc possesses a highly planar, deeply conjugated π-system that inherently drives cofacial self-assembly via strong intermolecular π–π interactions, a phenomenon directly corroborated by the distinct aggregation bands at 492 nm and 606 nm in its UV-Vis spectrum. Aggregation is detrimental to photocatalysis because excitonic coupling dissipates energy and quenches excited states, especially long-lived triplets. This prevents the electron or energy transfer required to generate reactive oxygen species (ROS) [32,33].
The incorporation of aza-nitrogen atoms into the isoindole moieties (yielding NMgPc) creates a more electron-deficient macrocyclic ring. Rather than significantly distorting the molecular planarity, this heteroatom substitution fundamentally alters the surface electrostatic potential landscape of the macrocycle, effectively diminishing the thermodynamic driving force for intermolecular π–π stacking [34]. Subsequent functionalization with peripheral methyl groups (MeNMgPc) introduces substantial steric hindrance. This bulky spatial shielding physically obstructs close cofacial intermolecular contact, successfully locking the catalyst in its highly photoactive monomeric state on the carbon support [35,36]. Consequently, the density of sterically accessible active sites and the effective lifespan of photo-induced charge carriers are maximized, comprehensively accounting for the superior PAH degradation performance exhibited by the MeNMgPc/C composite.

3.2. Photocatalytic Degradation Kinetics

To quantitatively evaluate the catalytic efficiency of the synthesized composites, the photocatalytic degradation kinetics were modeled using the classic Langmuir-Hinshelwood (L-H) mechanism. Given the highly dilute initial concentration of the PAH substrates (e.g., 50 μg/L), the L-H expression can be robustly simplified to a pseudo-first-order kinetic model, described by the integrated rate equation: ln(C0/Ct) = kt [37].
Linear regression analysis of ln(C0/Ct) against irradiation time (t) yielded excellent linear correlations for all three catalyst formulations, with correlation coefficients (R2) consistently exceeding 0.988 (Figure 6 and Table 4). This high degree of linearity unequivocally confirms that the degradation process strictly adheres to pseudo-first-order kinetics over the entire irradiation period [38]. From the slopes of the linear plots, the apparent first-order rate constant (k) for the superior MeNMgPc/C composite was calculated to be 23.1 × 10−3 min−1. Remarkably, this kinetic rate is approximately 5.1 times and 1.4 times greater than those exhibited by the unmodified MgPc/C (4.5 × 10−3 min−1) and the aza-substituted NMgPc/C (16.5 × 10−3 min−1), respectively. This pronounced kinetic enhancement further corroborates that the synergistic structural modifications—specifically the minimization of self-aggregation—profoundly accelerate the interfacial catalytic reaction rate. Furthermore, within the context of the Langmuir-Hinshelwood mechanism, this superior kinetic performance suggests that the sterically hindered, monomeric dispersion of MeNMgPc/C significantly enhances both the apparent adsorption constant for PAH molecules and the intrinsic catalytic reaction rate at the interface, effectively overcoming the mass-transfer and thermodynamic limitations suffered by the heavily aggregated pristine MgPc.

3.3. Identification of Reactive Oxygen Species and Photocatalytic Mechanism

To definitively identify the reactive oxygen species (ROS) responsible for the oxidative degradation, electron spin resonance (ESR) spin-trapping assays employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were conducted. Specifically, the detection of hydroxyl radicals (•OH) was performed in a pure water dispersion, while superoxide radicals (O2•−) were trapped in a methanol system under visible-light irradiation (300 W xenon lamp).
As illustrated in Figure 7, the ESR spectra under dark conditions exhibited chaotic noise with no distinct signals, unequivocally confirming the absence of radical generation without photon excitation. However, upon visible-light irradiation, prominent characteristic signals emerged. In the aqueous system, a classic four-line spectrum with a relative intensity ratio of 1:2:2:1 was observed, which is the standard fingerprint of the DMPO-•OH adduct. Concurrently, the methanol system displayed the characteristic four-line spectrum of the DMPO-O2•− adduct (typical for neutral solutions). These results verify that the supported photocatalysts simultaneously generate highly oxidative •OH and O2•− radicals under visible light [39].
While ESR successfully identified these primary radicals, a supplementary radical scavenging experiment was designed to rigorously ascertain whether other transient active species significantly contribute to the photocatalytic process. Methylene blue (MB) was used as a model dye substrate, with benzoquinone and methanol introduced as specific scavengers to quench all photogenerated O2•− and •OH radicals, respectively. Remarkably, upon the addition of these scavengers, the photocatalytic degradation efficiency of MB plummeted to below 1.5% across all three catalytic systems. This near-complete suppression of photoactivity indicates that the contributions from any other potential active species are negligible. Therefore, it is conclusively established that •OH and O2•− are the overwhelmingly dominant ROS driving the subsequent degradation mechanisms.

3.4. Proposed Photodegradation Pathway and Comprehensive Mechanism

Based on the temporal evolution of the intermediates elucidated via GC-MS and the confirmed generation of reactive oxygen species (ROS), a comprehensive, multi-step photodegradation pathway for phenanthrene over the MeNMgPc/C composite is proposed (Figure 8).
The degradation cascade is initiated by the electrophilic attack of the highly reactive hydroxyl (•OH) and superoxide (O2•−) radicals. According to molecular orbital theory, oxidation begins at the electron-rich 9,10-double bond (the ‘K-region’) of phenanthrene, quickly forming 9,10-phenanthrenequinone [40,41]. Subsequently, sustained oxidative bombardment induces the cleavage of the central aromatic ring, transforming the quinone intermediate into phthalic anhydride and corresponding phthalate derivatives.
As the photocatalytic process advances, these mononuclear aromatic intermediates undergo further rigorous oxidation and continuous decarboxylation. This leads to the complete breakdown of the aromaticity, producing various aliphatic dicarboxylic acids and esters. Upon prolonged irradiation, these aliphatic carbon chains are sequentially shortened into transient short-chain carboxylic acids (C2–C4, e.g., acetic and oxalic acids) [4], indicating the extensive structural dismantling and deep oxidation of the PAH molecules. The MeNMgPc/C system shows high capacity for remediating PAH-contaminated marine environments. This effectiveness stems from optimized kinetics, a stable monomeric catalyst state, and systematic pollutant degradation.
Drawing upon the empirical ESR evidence and the intrinsic photophysical properties of aza-phthalocyanines, a dual-pathway photocatalytic mechanism is proposed. The catalytic cycle initiates when the ground-state MeNMgPc macrocycle absorbs visible-light photons, rapidly exciting it to its short-lived singlet excited state (1Pc*). Facilitated by the central magnesium atom and the highly conjugated framework, this species undergoes highly efficient intersystem crossing (ISC) to form a relatively long-lived triplet excited state (3Pc*). This crucial intermediate subsequently interacts with dissolved molecular oxygen (3O2) via two parallel photosensitization routes. In the Type II pathway, direct energy transfer from 3Pc* to 3O2 yields highly reactive singlet oxygen (1O2). Concurrently, the Type I pathway proceeds via successive single-electron transfer reactions, generating superoxide radicals (O2•−) and, subsequently, hydroxyl radicals (•OH) [23,42]. Ultimately, it is the synergistic oxidative attack by this diverse pool of photogenerated ROS that drives the ring-opening and complete mineralization of the recalcitrant PAH substrates. While the current ESR and scavenging data definitively confirm the coexistence and synergistic contribution of both the Type I and Type II pathways to the overall degradation, precisely quantifying their relative functional ratios remains challenging without advanced transient absorption spectroscopy, which will be the subject of our future targeted investigations.

3.5. Practical Implications and Future Perspectives

The optimized MeNMgPc/C composite demonstrates exceptional potential for marine environmental remediation, achieving near-complete (>96%) elimination of priority PAHs. Immobilizing the macrocyclic photosensitizer onto biomass-derived activated carbon provides a cost-effective, sustainable platform that strategically circumvents the recovery challenges inherent to homogeneous catalysis [43,44]. Its robust heterogeneous nature facilitates facile post-treatment separation via straightforward sedimentation or filtration.
To bridge the gap from laboratory-scale conceptualization to practical marine deployment, future investigations must prioritize evaluating the catalyst’s long-term operational stability over extended cycles. Furthermore, assessing its catalytic efficacy in complex real-seawater matrices—which contain high concentrations of competing inorganic anions—alongside the engineering of scalable, pilot-scale photoreactor configurations, remains strictly necessary for the industrial translation of this promising technology [45].

4. Materials and Methods

4.1. Reagents and Materials

Analytical standards of four polycyclic aromatic hydrocarbons (naphthalene, phenanthrene, fluorene, and chrysene) were purchased from AccuStandard, Inc. (New Haven, CT, USA) as stock solutions. All other chemicals and reagents (e.g., phthalic anhydride, pyrazine-2,3-dicarboxylic acid, diaminomaleonitrile, 2,3-butanedione, MgCl2·6H2O) were of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Industrial-grade coconut shell activated carbon (AC) purchased from Taobao (Hangzhou, China), with an initial BET surface area of 377.66 m2/g, was utilized as the catalyst support.

4.2. Synthesis of MgPc, NMgPc, and MeNMgPc

The macrocyclic catalysts were synthesized via a solid-state melt method.
MgPc: A finely ground mixture of phthalic anhydride (0.05 mol), MgCl2·6H2O (0.01 mol), urea, and ammonium molybdate (catalyst, 2 wt%) was heated stepwise to 220 °C and held for 2 h. The crude product was washed sequentially with 2% HCl, distilled water, and 5% NaOH, then vacuum-dried to yield MgPc.
NMgPc: Synthesized similarly using pyrazine-2,3-dicarboxylic acid, MgCl2·6H2O, urea, and ammonium molybdate. The mixture was reacted at 230 °C for approximately 20 h. After acid-base washing, the product was further purified by dissolution in concentrated H2SO4 and reprecipitation in cold water.
MeNMgPc: Prepared via a one-pot reaction by mixing diaminomaleonitrile (0.05 mol), 2,3-butanedione (0.05 mol), MgCl2·6H2O (0.025 mol), and urea (0.2 mol). The mixture was heated to 230 °C. The crude solid was washed with water and boiling absolute ethanol, followed by recrystallization from a DMF/water mixture.

4.3. Preparation of Catalyst Composites

The coconut shell AC was pretreated by consecutive soaking in 10% NaOH and 10% HNO3 solutions to remove impurities. The supported catalysts were prepared via a multiple ultrasonic impregnation technique. Specifically, 0.5 g of the respective catalyst (MgPc, NMgPc, or MeNMgPc) was fully dissolved in 80 mL DMF under ultrasonication for 30 min. Subsequently, 5 g of the pretreated AC was added, and the suspension was ultrasonicated for an additional 1 h to facilitate deep impregnation. The resulting composite was filtered, thoroughly washed with DMF and distilled water, and vacuum-dried at 75 °C. This impregnation-drying cycle was repeated three times.

4.4. Characterization Methods

UV-Vis absorption spectra were recorded on a Shimadzu UV-1240 spectrophotometer (Kyoto, Japan) using DMF as the solvent. Surface morphology and elemental mapping were evaluated using a scanning electron microscope (Quanta 200, FEI, Hillsboro, OR, USA) equipped with an EDAX detector (operated at 20 kV after gold sputtering). Nitrogen physisorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010M analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA); all samples were degassed at 300 °C for 2 h prior to testing. Reactive oxygen species (ROS) were identified via electron spin resonance (ESR) spectroscopy (Bruker Elexsys E580, Billerica, MA, USA) using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent.

4.5. Photocatalytic Degradation Experiments

Photocatalytic evaluations were performed in a temperature-controlled (25 ± 1 °C) double-jacketed batch reactor (500 mL capacity). A 300 W xenon lamp (CEL-HXF300-T3, CEAuLight Co., Ltd., Beijing, China) with an adjustable operating current (10–22 A) served as the visible-light source and was positioned perpendicularly above the reactor. In a typical run, 250 mL of simulated wastewater containing a specific PAH (initial concentration: 50 μg/L, prepared by diluting the commercial stock solutions) was mixed with a predetermined dosage of the catalyst composite. The suspension was magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium. During irradiation, 5 mL aliquots were periodically sampled (e.g., every 20 min), filtered through 0.22 μm PTFE syringe filters, and subjected to liquid-liquid extraction prior to analysis.

4.6. GC-MS Analysis of Degradation Intermediates

Residual PAHs and degradation intermediates were identified using a gas chromatography-mass spectrometer (GC-MS, Agilent 7890A/5975C, Santa Clara, CA, USA) equipped with a DB-5MS capillary column (30 m × 0.25 mm × 0.25 μm). High-purity helium (99.999%) served as the carrier gas at a constant flow rate of 1.0 mL/min. A 1 μL sample was injected in splitless mode with the injector temperature set at 280 °C. The oven temperature program was initiated at 50 °C, ramped at 4 °C/min to 220 °C (held for 3 min), and subsequently increased at 10 °C/min to 300 °C (held for 9 min). Mass spectra were acquired in full-scan mode (m/z 35–500), and intermediates were identified by comparing their mass fragmentation patterns with standard reference libraries (e.g., NIST).

5. Conclusions

In summary, magnesium phthalocyanine (MgPc) and its chemically modified derivatives were successfully immobilized onto coconut shell activated carbon to develop robust visible-light photocatalysts. Among them, the MeNMgPc/C composite exhibited superior catalytic activity. This enhanced performance is fundamentally attributed to the synergistic steric and electronic effects of nitrogen substitution and peripheral methyl groups, which effectively suppress intermolecular aggregation. Under Box-Behnken optimized conditions (13.18 g/L dosage, 20 A, 2.28 h), MeNMgPc/C achieved an exceptional naphthalene removal efficiency of 96.67%, strictly following pseudo-first-order kinetics. Mechanistic investigations confirmed that photogenerated hydroxyl (•OH) and superoxide (O2•−) radicals drive the sequential ring-opening and ultimate mineralization of PAHs. These findings demonstrate that the rationally designed MeNMgPc/C composite is a highly efficient, recoverable, and promising catalytic platform for the advanced remediation of PAH-contaminated marine environments.

Author Contributions

Conceptualization, methodology, D.W. (Dejun Wang), J.W. and D.W. (Dong Wang); validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, D.W. (Dejun Wang), D.W. (Dong Wang) and H.L.; writing—review and editing, visualization, supervision, D.W. (Dejun Wang), L.L. and J.S.; project administration, funding acquisition, D.W. (Dong Wang), Q.L. and C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the 2024 Research Projects (General Projects, No.2024YB09) of Qingdao Technical College.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The author declares no conflicts of interest.

Abbreviations

ACactivated carbon
AOPsadvanced oxidation processes
BETBrunauer-Emmett-Teller
DMFN,N-dimethylformamide
DMPO5,5-dimethyl-1-pyrroline-N-oxide
EDAXenergy-dispersive X-ray spectroscopy
ESRelectron spin resonance
GC-MSgas chromatography-mass spectrometry
HOMOhighest occupied molecular orbital
LUMOlowest unoccupied molecular orbital
MeNMgPcmethyl-substituted magnesium azaphthalocyanine
MgPcmagnesium phthalocyanine
NMgPcmagnesium azaphthalocyanine
PAHspolycyclic aromatic hydrocarbons
Pcphthalocyanine
ROSreactive oxygen species
RSMresponse surface methodology
SEMscanning electron microscopy
USEPAUnited States Environmental Protection Agency
UV-Visultraviolet-visible
•OHhydroxyl radical
O2•−superoxide radical anion

References

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Catalysts 16 00470 g001
Figure 1. UV-Vis absorption spectra of MgPc, NMgPc, and MeNMgPc in DMF solution (20 mg/L). Arrows indicate the Q-band position for each catalyst.
Figure 1. UV-Vis absorption spectra of MgPc, NMgPc, and MeNMgPc in DMF solution (20 mg/L). Arrows indicate the Q-band position for each catalyst.
Catalysts 16 00470 g001
Catalysts 16 00470 g002
Figure 2. SEM of catalyst before and after loaded. (a): Coconut shell AC 50 μm; (b): Coconut shell AC 10 μm; (c): Coconut shell AC 1 μm; (d): MgPc/C 50 μm; (e): MgPc/C 10 μm; (f): MgPc/C 1 μm; (g): NMgPc/C 50 μm; (h): NMgPc/C 10 μm; (i): NMgPc/C 1 μm; (j): MeNMgPc/C 50 μm; (k): MeNMgPc/C 10 μm; (l): MeNMgPc/C 1 μm.
Figure 2. SEM of catalyst before and after loaded. (a): Coconut shell AC 50 μm; (b): Coconut shell AC 10 μm; (c): Coconut shell AC 1 μm; (d): MgPc/C 50 μm; (e): MgPc/C 10 μm; (f): MgPc/C 1 μm; (g): NMgPc/C 50 μm; (h): NMgPc/C 10 μm; (i): NMgPc/C 1 μm; (j): MeNMgPc/C 50 μm; (k): MeNMgPc/C 10 μm; (l): MeNMgPc/C 1 μm.
Catalysts 16 00470 g002
Catalysts 16 00470 g003
Figure 3. EDAX of catalyst before and after loaded.
Figure 3. EDAX of catalyst before and after loaded.
Catalysts 16 00470 g003
Catalysts 16 00470 g004
Figure 4. Comparison of photocatalytic degradation of PAHs by different catalysts (C0 = 50 μg/L, I = 15 A, m = 10 g/L, 300 W xenon lamp).
Figure 4. Comparison of photocatalytic degradation of PAHs by different catalysts (C0 = 50 μg/L, I = 15 A, m = 10 g/L, 300 W xenon lamp).
Catalysts 16 00470 g004
Catalysts 16 00470 g005
Figure 5. Effect of single operating conditions on catalytic degradation of naphthalene. (a): C0 = 50 μg/L, I = 15 A, m = 10 g/L; (b): MeNMgPc/C, C0 = 50 μg/L, I = 15 A; (c): MeNMgPc/C, C0 = 50 μg/L, m = 10 g/L; (d): MeNMgPc/C, m = 10 g/L, I = 15 A.
Figure 5. Effect of single operating conditions on catalytic degradation of naphthalene. (a): C0 = 50 μg/L, I = 15 A, m = 10 g/L; (b): MeNMgPc/C, C0 = 50 μg/L, I = 15 A; (c): MeNMgPc/C, C0 = 50 μg/L, m = 10 g/L; (d): MeNMgPc/C, m = 10 g/L, I = 15 A.
Catalysts 16 00470 g005
Catalysts 16 00470 g006
Figure 6. Kinetic analysis of three catalytic processes. (C0 = 50 μg/L, m = 10 g/L, I = 15 A).
Figure 6. Kinetic analysis of three catalytic processes. (C0 = 50 μg/L, m = 10 g/L, I = 15 A).
Catalysts 16 00470 g006
Catalysts 16 00470 g007aCatalysts 16 00470 g007b
Figure 7. ESR detection spectra of radicals in photocatalytic process.
Figure 7. ESR detection spectra of radicals in photocatalytic process.
Catalysts 16 00470 g007aCatalysts 16 00470 g007b
Catalysts 16 00470 g008
Figure 8. Schematic diagram of the degradation path of phenanthrene.
Figure 8. Schematic diagram of the degradation path of phenanthrene.
Catalysts 16 00470 g008
Table 1. BET surface area (SBET), pore volume (Vp), and mean pore diameter (Dp) of the activated carbon carrier and composite catalysts.
Table 1. BET surface area (SBET), pore volume (Vp), and mean pore diameter (Dp) of the activated carbon carrier and composite catalysts.
MaterialSBET (m2/g)Vp (cm3/g)Dp (nm)
Unloaded AC377.660.151.92
MgPc/C184.400.072.27
NMgPc/C130.930.052.31
MeNMgPc/C88.090.032.29
Data from N2 adsorption–desorption analysis at 77 K.
Table 2. Box-Behnken design matrix and naphthalene removal efficiencies (MeNMgPc/C).
Table 2. Box-Behnken design matrix and naphthalene removal efficiencies (MeNMgPc/C).
RunX1 (g/L)X2 (A)X3 (h)Actual (%)Predicted (%)
15102.071.0070.81
215102.088.7086.08
35202.078.6781.29
415202.095.1095.29
55151.575.3374.42
615151.587.4488.96
75152.579.5077.98
815152.591.8092.71
910101.579.3080.40
1010201.592.2590.54
1110102.582.6384.34
1210202.595.0193.91
1310152.090.5590.55
1410152.090.5590.55
1510152.090.5590.55
X1: catalyst dosage; X2: operating current; X3: reaction time. Predicted values obtained from the second-order polynomial model.
Table 3. Analysis of components of phenanthrene solution sample in different reaction period.
Table 3. Analysis of components of phenanthrene solution sample in different reaction period.
No.Intermediate ProductRetention Time (min)Irradiation (15 min)Irradiation (45 min)Irradiation (90 min)
1Octanoic acid3.883–3.890DetectedDetectedDetected
23-Allyl-2-methoxyphenol6.893–6.900DetectedN.D.N.D.
32,6-Diisobutyl-4-methylphenol9.993–10.000DetectedDetectedN.D.
4Bis(2-methylpropyl) phthalate (DIBP)17.303–17.310DetectedDetectedN.D.
51-Heptadecanol17.573–17.580DetectedDetectedDetected
6Butyl isobutyl phthalate19.110–19.117DetectedN.D.N.D.
71-Octadecanol21.390–21.397DetectedDetectedDetected
8Bis(2-ethylhexyl) phthalate (DEHP)28.660–28.667DetectedDetectedN.D.
9Ethyl 4-(2-hydroxyethyl)benzoate38.540–38.547N.D.DetectedN.D.
109,10-Anthraquinone64.850–64.857DetectedDetectedN.D.
11Nonahydro-9H-xanthen-9-one65.910–65.917DetectedDetectedN.D.
Note: “N.D.” stands for Not Detected.
Table 4. Pseudo-first-order rate constants and correlation coefficients for naphthalene photodegradation.
Table 4. Pseudo-first-order rate constants and correlation coefficients for naphthalene photodegradation.
Catalystk (×10−3 min−1)R2
MgPc/C4.50.9969
NMgPc/C16.50.9884
MeNMgPc/C23.10.9921
k: apparent first-order rate constant; R2: coefficient of determination from linear regression of ln(C0/Ct) vs. irradiation time.
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Wang, D.; Li, H.; Liang, L.; Su, J.; Wei, J.; Wang, D.; Zuo, C.; Liu, Q. Photocatalytic Degradation of Polycyclic Aromatic Hydrocarbons in Simulated Offshore Oil Spill Wastewater by Magnesium Phthalocyanine Derivatives Supported on Coconut Shell Activated Carbon. Catalysts 2026, 16, 470. https://doi.org/10.3390/catal16050470

AMA Style

Wang D, Li H, Liang L, Su J, Wei J, Wang D, Zuo C, Liu Q. Photocatalytic Degradation of Polycyclic Aromatic Hydrocarbons in Simulated Offshore Oil Spill Wastewater by Magnesium Phthalocyanine Derivatives Supported on Coconut Shell Activated Carbon. Catalysts. 2026; 16(5):470. https://doi.org/10.3390/catal16050470

Chicago/Turabian Style

Wang, Dejun, Hui Li, Lihua Liang, Juan Su, Jifang Wei, Dong Wang, Changjiang Zuo, and Qiyou Liu. 2026. "Photocatalytic Degradation of Polycyclic Aromatic Hydrocarbons in Simulated Offshore Oil Spill Wastewater by Magnesium Phthalocyanine Derivatives Supported on Coconut Shell Activated Carbon" Catalysts 16, no. 5: 470. https://doi.org/10.3390/catal16050470

APA Style

Wang, D., Li, H., Liang, L., Su, J., Wei, J., Wang, D., Zuo, C., & Liu, Q. (2026). Photocatalytic Degradation of Polycyclic Aromatic Hydrocarbons in Simulated Offshore Oil Spill Wastewater by Magnesium Phthalocyanine Derivatives Supported on Coconut Shell Activated Carbon. Catalysts, 16(5), 470. https://doi.org/10.3390/catal16050470

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