Fe-Modified Sewage Sludge Biochar for Efficient Removal of Nanoplastics from Water: Mechanistic Insights and Multi-Pathway Adsorption Analysis

TL;DR

This study shows how iron-modified biochar can efficiently remove nanoplastics from water by understanding the key mechanisms involved.

Contribution

The study quantifies the relative roles of multiple adsorption mechanisms using structural equation modeling for NP removal by biochar.

Findings

Fe-modified biochar achieved 96.09% nanoplastic removal efficiency.

Electrostatic interactions were the dominant adsorption mechanism (52.6%).

Surface protonation and pH strongly influence NP adsorption efficiency.

Abstract

Nanoplastics (NPs) have emerged as pervasive aquatic pollutants due to their small size, high surface activity, and potential ecological and health risks. Although sludge-derived biochar is a sustainable adsorbent for NP removal, the relative importance of coexisting adsorption mechanisms remains poorly quantified. Here, iron-modified sludge biochar (FeBC) was synthesized and evaluated for NP removal from water. Batch experiments showed that FeBC significantly outperformed pristine biochar, achieving a maximum removal efficiency of 96.09%. Adsorption was strongly pH-dependent, with enhanced removal under acidic conditions due to surface protonation and strengthened electrostatic attraction toward negatively charged NPs. SEM, BET, FTIR, and XPS analyses indicated that electrostatic interactions, hydrogen bonding, π–π interactions, and pore adsorption jointly contributed to NP capture. Importantly, structural equation modeling quantitatively disentangled these mechanisms, revealing electrostatic interactions as the dominant driver (52.6%), followed by hydrogen bonding (23%), pore adsorption (16.6%), and π–π interactions (7.9%), and further identified synergistic and antagonistic relationships among them. These results demonstrate that surface charge regulation governs NP adsorption efficiency, providing a quantitative mechanistic basis for the rational design of biochar-based adsorbents. This study advances a multi-mechanistic framework for understanding and optimizing NP removal while promoting sludge resource valorization.

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1. Introduction

In recent years, micro- and nanoplastics (MNPs), as emerging pollutants, have attracted significant attention [1]. The large-scale production and widespread use of plastic products have led to their persistent accumulation and fragmentation in the environment, ultimately generating microplastics (MPs) (>100 nm and <5 mm) and NPs (1 to <1000 nm) [2,3]. Compared with MPs, NPs have smaller particle sizes, larger specific surface areas, and higher surface activity, making them more capable of penetrating biological membranes and exhibiting potential toxicity [4,5]. Consequently, they can readily transport and disperse in aquatic environments, posing severe threats to ecosystems and human health [6,7]. Recent studies have even detected and quantified plastic particles in human blood [8]. Therefore, developing effective strategies to remediate NP pollution in water is of great importance.

To mitigate the environmental and health risks associated with NPs, various removal technologies have been explored, including coagulation–flocculation, membrane filtration, advanced oxidation, and biological treatment [9]. Among these approaches, adsorption has emerged as a particularly attractive strategy due to its operational simplicity, high removal efficiency, and minimal secondary pollution [10]. The effectiveness of adsorption-based processes largely depends on the physicochemical properties of the adsorbent, prompting extensive research into the development of efficient, low-cost, and sustainable adsorption materials [11]. In this context, biochar has attracted increasing attention because of its porous structure, surface functional groups, and tunable properties, making it a promising candidate for the removal of emerging contaminants, including NPs [2].

Notably, biochar derived from sewage sludge offers additional advantages by enabling waste valorization and resource recovery, thereby aligning NP remediation with circular economy principles [12]. Unmodified sludge biochar generally suffers from insufficient active sites, which constrain its performance in NP removal [13]. Previous studies have shown that metal-loading modifications can effectively improve the surface structure and functional properties of sludge biochar [14]. In particular, iron-based modification not only endows the material with good magnetic separation properties, enabling rapid separation from aqueous solutions, but also provides additional metal active sites, thereby enhancing the adsorption process [15,16]. Previous studies have shown that sludge-derived biochar modified through approaches such as Mg-Al layered double hydroxide loading [17], (3-aminopropyl)triethoxysilane grafting and magnetization can efficiently remove NPs from aqueous environments [18]; spontaneous magnetic biochar prepared from the pyrolysis of municipal sludge and industrial red mud achieved a removal efficiency of up to 97.87% for NPs within 30 min [19]. However, its removal behavior and underlying mechanisms have not yet been systematically and thoroughly elucidated. Current studies on NP adsorption mainly focus on electrostatic interactions, hydrogen bonding, hydrophobic interactions, pore filling, and π–π interactions. Such analyses are often based on single characterization techniques or empirical assumptions, making it difficult to simultaneously quantify the contribution of different mechanisms and reveal their potential synergistic or antagonistic relationships [20].

In real adsorption systems, NP removal is typically driven by multiple physicochemical mechanisms that may mutually enhance or constrain one another, and traditional analysis methods cannot provide an integrated understanding of this complex process [21]. Structural equation modeling, as a multivariate statistical tool, can simultaneously handle multiple causal pathways, allowing the quantification of the relative contribution of different adsorption mechanisms and the elucidation of correlations and synergistic or competitive effects among them [22]. Therefore, applying structural equation modeling to the study of NP adsorption by sludge biochar helps overcome the limitations of single-mechanism analyses and provides a more systematic and quantitative understanding of the adsorption process [23].

Based on this, the present study prepared iron-modified sludge biochar from sewage treatment plant sludge and systematically evaluated its removal performance toward NPs in water. Adsorption behavior was analyzed through kinetic and isotherm models. By integrating SEM, BET, FTIR, and XPS characterization techniques, the adsorption mechanisms of NPs were elucidated in terms of material structure, surface chemical properties, and interactions. Furthermore, structural equation modeling was employed to quantitatively assess the relative contributions and coupling relationships of multiple adsorption mechanisms. This study aims to deepen the understanding of NP removal by iron-modified sludge biochar from a multi-mechanism perspective and provide theoretical guidance for the rational design of sludge-derived adsorbents for efficient NP removal.

2. Results and Discussion

2.1. Removal of NPs from Aqueous Solution

The adsorption performance of sludge biochar can be influenced by feedstock characteristics, pyrolysis conditions, and the application of modifiers [24]. Preliminary experiments were conducted to determine the optimal adsorption conditions. After systematically investigating the effects of pyrolysis temperature, modification, NP concentration, adsorbent dosage, and pH on removal efficiency, the results are shown in Figure 1a. Compared with the BC group, the FeBC group exhibited higher removal efficiencies for NPs (p < 0.001). Iron modification of sludge biochar significantly enhanced microplastic removal, with FeBC500 achieving the highest removal efficiency of 65.94% (Figure 1a).

Moreover, the NP concentration significantly affected the removal efficiency (Figure 1b). As the concentration increased, the removal rate first rose and then declined, reaching a maximum of 96.09% at 10 mg·L^−1^. The adsorbent dosage also influenced removal (Figure 1c); as the dosage increased, the NP removal rate rapidly increased and stabilized at 10 mg, indicating maximal utilization of adsorption sites. The optimal pH was found to be 4 (Figure 1d), with a 10 mg dosage yielding the best removal performance.

Under the optimal adsorption conditions, sludge biochar prepared at 300 °C, whether modified with iron or not, showed the lowest removal rates, while FeBC500 achieved the highest, and FeBC700 slightly lower. This phenomenon is mainly attributed to changes in iron content and pyrolysis temperature, which affect the surface functional groups, porosity, and specific surface area of the adsorbent materials [25,26]. At 700 °C, the removal efficiency decreased slightly, likely due to the development of a more mature pore structure at higher pyrolysis temperatures but simultaneous degradation of many oxygen-containing functional groups. The reduction of these functional groups leads to a decline in removal efficiency [27]. These results indicate that iron modification of sludge biochar can significantly enhance its adsorption capacity for NPs [15].

Adsorption capacity is also strongly affected by solution pH and the presence of coexisting anions [28,29]. Therefore, the effects of pH (3–10) and different anions (1 mmol·L^−1^, NO_3_^−^, HCO_3_^−^, and SO_4_^2−^) on NP adsorption were investigated.

As shown in Figure 1d, FeBC500 exhibited high removal efficiencies for NPs under strongly acidic conditions, ranging from 82.45% to 97.84%. This is attributed to the electrostatic attraction between the positively charged adsorbent and negatively charged NPs, which enhances adsorption [30]. When pH reached 6, the zeta potential of the adsorbent shifted from positive to negative, resulting in a sharp decrease in NP removal from 82.45% to 30.03%. At this point, the interaction between the negatively charged adsorbent and NPs changed from electrostatic attraction to repulsion, leading to a rapid decline in removal efficiency. As the solution pH further increased, the negative charge on the adsorbent continued to rise, generating stronger electrostatic repulsion and further reducing removal. Additionally, at high pH, elevated concentrations of OH^−^ compete with NPs for metal active sites on FeBC500, hindering the formation of metal-O-NP bonds and decreasing adsorption [29].

Different anions had distinct effects on adsorption capacity. The influence of competitive anions is shown in Figure 1e, where NO_3_^−^ and SO_4_^2−^ exhibited the most significant inhibitory effect on NP removal (p < 0.001). In the presence of HCO_3_^−^, the NP removal efficiency by FeBC500 decreased to 69%, which can be attributed to the alkalinity of NO_3_^−^, HCO_3_^−^, and SO_4_^2−^ in deionized water or their specific affinity for FeBC500 [28,29].

Regeneration of FeBC500 was achieved by thermal treatment. To evaluate its regeneration performance, adsorption experiments were conducted under the same conditions as those for the pristine FeBC500 (FeBC500: 10 mg; NP solution: 10 mL at 10 mg·L^−1^; pH = 4). Figure 1f shows the removal efficiency of regenerated FeBC500 toward NPs after five adsorption–regeneration cycles. With increasing cycle number, no significant decrease in removal efficiency was observed, and the removal efficiency remained above 92% throughout all cycles. These results indicate that the easily prepared magnetic sludge biochar is a novel, efficient, and stable adsorbent with great potential for NP removal.

Under the optimized experimental conditions, adsorption kinetics were investigated. The experimental data were fitted using pseudo-first-order and pseudo-second-order kinetic models. The adsorption kinetics of NPs on FeBC500 are shown in Figure 2c,d. The adsorption capacity of FeBC500 for NPs (qₜ) increased with reaction time and reached a maximum at 300 min. The adsorption process could be divided into two stages. In the first stage (0–100 min), the adsorption rate was rapid, accounting for approximately 75% of the equilibrium adsorption capacity. This can be attributed to the high concentration difference in terms of NPs between the aqueous phase and the FeBC500 surface at the initial stage and the abundance of available adsorption sites on FeBC500. Consequently, NPs in water were rapidly adsorbed onto FeBC500 through electrostatic interactions.

In the subsequent stage (100–300 min), as the concentration difference between the aqueous and solid phases decreased and the adsorption sites on FeBC500 were partially occupied, the adsorption rate slowed down and became primarily controlled by chemisorption [31]. As listed in Table 1, the adsorption of NPs on FeBC500 was well described by the pseudo-second-order kinetic model (R^2^ = 0.981), indicating that the adsorption process mainly depends on time and the availability of binding sites. The pseudo-first-order model was found to be suitable for the initial 0-100 min. The formation of Fe-O-PS bonds between the FeBC500 surface and NPs contributed to adsorption [32]. Ultimately, the synergistic effect of electrostatic interactions and chemical bonding led to adsorption saturation [31].

In contrast, the adsorption kinetics of NPs on BC500 were better fitted by the pseudo-first-order model (R^2^ = 0.976), suggesting that physical adsorption dominated the removal process. This difference indicates that iron modification altered the adsorption mechanism from primarily physisorption on pristine sludge biochar to chemisorption-dominated adsorption on FeBC500, thereby enhancing the interaction strength between the adsorbent and NPs.

The adsorption isotherms of NPs on BC500 and FeBC500 at different initial concentrations are shown in Figure 3. With increasing initial concentration, the adsorption capacities of both BC500 and FeBC500 gradually increased and then tended to level off at higher concentrations, indicating the gradual saturation of surface adsorption sites.

The equilibrium data were fitted using the Langmuir and Freundlich isotherm models (Table 2). The results showed that the Langmuir model provided a better fit for both BC500 and FeBC500 (R^2^ = 0.954 and R^2^ = 0.991, respectively), suggesting that NP adsorption mainly occurred as monolayer adsorption on homogeneous surfaces. Moreover, the higher correlation coefficient obtained for FeBC500 indicates that iron modification resulted in more uniform surface properties and stronger affinity toward NPs.

2.2. Characterization of Sludge Biochar

The surface morphologies of BC (300, 500, and 700 °C), FeBC (300, 500, and 700 °C), and FeBC500-NPs (FeBC500 after adsorption of NPs) were observed using SEM (Figure 4), and the elemental compositions of the samples were analyzed by elemental analysis (EA) and energy-dispersive X-ray spectroscopy (EDS),and the specific surface area (SA) and total pore volume (PV) of the samples were determined by nitrogen adsorption–desorption isotherms (Table 3). The nitrogen adsorption–desorption isotherms and pore size distributions of the samples are shown in Supplementary Figures S1 and S2. SEM images revealed that the surface of BC300 was dense with only a few cracks; BC500 displayed distinct cavities and pores, and the pores of BC700 were further enlarged. After iron modification, FeBC300 exhibited a smooth and dense surface with almost invisible pores; FeBC500 and FeBC700 developed pores progressively, with FeBC500 showing a noticeable honeycomb-like pore structure. SEM images after adsorption experiments indicated that NPs adhered to the sludge biochar surface, while metal oxide particles aggregated around them, suggesting that the microplastics were captured on the sludge biochar surface during the adsorption process and that Fe active sites enhanced their attachment. EDS confirmed the successful loading of Fe onto the sludge biochar surface, with FeBC500 exhibiting the highest Fe content (Table 3). The presence of Fe introduced additional active sites, which facilitated the attachment of NPs onto the sludge biochar during the adsorption process, and also endowed the material with magnetic properties, enabling rapid separation of the adsorbent from water after treatment. SA increased significantly with increasing pyrolysis temperature. Compared with the corresponding unmodified sludge biochar, Fe modification slightly reduced SA and PV (Table 3), but the introduction of iron provided new active sites on the material surface, thereby enhancing the interactions between the sludge biochar and NPs [33].

FTIR was used to analyze the surface functional groups of the sludge biochars, as shown in Figure 5. Both carbonization temperature and iron modification significantly influenced the types and structural features of surface functional groups. Sludge biochars prepared at 300 °C exhibited a broad peak around 3421 cm^−1^, corresponding to -OH stretching vibrations, indicating the presence of abundant hydroxyl, phenolic groups, and adsorbed water molecules [34]. Additionally, stretching vibrations of aliphatic C-H were observed at 2925 and 2840 cm^−1^ [35], and the characteristic C-O peaks in the 1100–1000 cm^−1^ region further confirmed that low-temperature sludge biochars retained rich oxygen-containing functional groups, such as carboxyl, ester, and ether groups [36]. With increasing carbonization temperature to 500 °C and 700 °C, the peaks associated with oxygen-containing functional groups gradually weakened or disappeared, whereas the aromatic C=C peak around 1600 cm^−1^ intensified, indicating decarboxylation, dehydroxylation, and condensation reactions at high temperatures, leading to a highly aromatic and stabilized carbon framework [37].

Compared with unmodified samples, Fe-modified sludge biochars showed relatively enhanced C-H symmetric stretching vibrations in the 1400–1450 cm^−1^ range, suggesting coordination between Fe and surface carboxyl groups to form Fe-carboxylate complexes. Moreover, Fe-modified sludge biochars exhibited stronger absorption in the 500–600 cm^−1^ region, typically associated with Fe-O stretching vibrations, indicating the presence of iron oxides or Fe-O-C structures on the surface [30]. These results suggest that the introduction of Fe not only accelerates the cleavage and condensation of oxygen-containing functional groups but also introduces new coordination structures and potential active sites through interactions with residual carboxyl or phenolic hydroxyl groups, significantly altering surface chemical properties.

Comparing FTIR spectra of FeBC500 before and after adsorption of NPs, enhanced C-H vibration peaks were observed at 2925 cm^−1^ and 2840 cm^−1^, consistent with aliphatic C-H stretching vibrations of polystyrene NPs. Additionally, C-H vibrations in the 1400–1450 cm^−1^ region were further strengthened after adsorption, and changes in the 500–600 cm^−1^ Fe-O peak suggest that NPs may interact with Fe-O-NP sites during adsorption [29]. These findings indicate that oxygen-containing functional groups and hydrogen bonding play critical roles in improving NP removal efficiency [19].

XPS was employed to analyze the elemental composition and chemical states of the FeBC500 adsorbent. Figure 6 presents the XPS spectra of FeBC500 before and after adsorption of NPs. In the high-resolution C1s spectra, four deconvoluted peaks were observed, corresponding to C=C bonds at 284.8 eV (42.68%), C-O bonds at 286 eV (45.30%), C=O bonds at 288.5 eV (8.12%), and π–π* excitation at 3.90% [17]. In the O1s spectra, the binding energies of oxygen-containing groups were observed at 530.3 eV (C=O/Fe-O), 532 eV (COOH), and 534 eV (OH) [38]. After adsorption of NPs, the C1s peak intensities increased, with the proportion of C=C bonds rising to 75.56%, C-O bonds decreasing to 9.99%, and C=O bonds increasing to 10.10%. Meanwhile, the π–π* excitation peak weakened. A similar trend was observed in the O1s spectra (Figure 6), with a marked decrease in peak intensities, indicating successful adsorption of NPs by the sludge biochar. Oxygen-containing functional groups played a crucial role in the adsorption process, likely due to the activation of carbon-oxygen double bonds and the formation of hydrogen bonds with NPs. Additionally, the adsorption performance of FeBC500 toward NPs was further enhanced through π–π interactions with the adsorbent [19].

2.3. Mechanism of NP Adsorption by Sludge Biochar

The experimental results indicate that electrostatic adsorption, hydrogen bonding, pore filling, and π–π interactions all contribute to the adsorption of NPs by FeBC500, with electrostatic adsorption and hydrogen bonding likely serving as the dominant mechanisms, and pore filling and π–π interactions playing supportive roles. Although these findings highlight the overall influence of multiple mechanisms, experimental data alone cannot quantify their individual contributions and interactions. Therefore, structural equation modeling was employed to quantitatively assess the relative contributions and synergistic or antagonistic effects of each mechanism [39,40], providing a systematic understanding of the multi-mechanism adsorption behavior of FeBC500.

The results indicated (Figure 7a) that electrostatic adsorption was the dominant mechanism, accounting for 52.6% of the total adsorption, followed by hydrogen bonding (23%), pore filling (16.6%), and π–π interactions (7.9%). These mechanisms are consistent with the main forces summarized in previous NP adsorption studies, including electrostatic attraction, hydrogen bonding, pore capture, and π–π stacking, forming the essential physicochemical basis for adsorbents to capture NPs [41].

Electrostatic adsorption is the core factor driving the migration and fixation of NPs onto the adsorbent surface [41]. Under acidic conditions, the FeBC500 surface is protonated, becoming positively charged and generating a significant potential difference with negatively charged polystyrene NPs, thereby enhancing adsorption. This charge interaction is considered a key driving force at the interface between NPs and charged adsorbents, strongly influenced by pH and surface potential. Hydrogen bonding, as an important synergistic mechanism, occurs between the hydroxyl, carboxyl, and carbonyl groups on the sludge biochar surface and the oxygen-containing functional groups on NPs, further stabilizing the particles on the surface and enhancing retention [21].

Pore filling mainly relies on the mesoporous structures to physically trap NPs and facilitate diffusion and contact, although its contribution is limited by pore size distribution and site accessibility. Previous studies have shown that in adsorbents with well-developed pore structures, pore capture is an important component of physical adsorption, but its contribution is often restricted by spatial hindrance and mass transfer limitations [42]. Although the contribution of π–π interactions is relatively minor, they provide local stability at specific aromatic sites. π–π interactions generally occur between aromatic polymer NPs and adsorbent aromatic structures, enhancing local binding strength [41].

Further correlation analysis (Figure 7b) revealed complex couplings among material elemental composition, surface chemistry, and adsorption mechanisms. Iron content was positively correlated with electrostatic adsorption (r = 0.90) and hydrogen bonding (r = 0.49), and negatively correlated with C and H content, indicating that iron modification provides charged active sites while altering the distribution of organic components. Nitrogen content was highly correlated with H, C, and H/C ratio, highlighting its role in modulating surface chemistry and indirectly influencing hydrogen bonding and pore adsorption. BET surface area and pore volume promoted pore filling but could slightly limit electrostatic adsorption and hydrogen bonding; hydroxyl groups enhanced both electrostatic and hydrogen bonding, while carbonyl groups mainly supported hydrogen bonding. These correlations between surface chemistry and NP adsorption mechanisms align with previously summarized mechanistic insights [41].

Quantitative evaluation of the contributions of individual adsorption mechanisms is critical for moving beyond qualitative descriptions and enabling the rational design of adsorbents. By delineating the relative significance of different interaction pathways, such analysis identifies the key surface properties that govern NP removal and provides a theoretical foundation for targeted material optimization. The dominance of electrostatic adsorption indicates that surface charge characteristics, particularly charge density and surface potential, are the primary determinants of NP capture efficiency. Accordingly, enhancing electrostatic interactions constitutes the most effective strategy for improving adsorption performance. This can be achieved by increasing the density of positively charged active sites via metal modification, regulating surface protonation under relevant pH conditions, or tailoring surface functional groups to elevate the zeta potential.

In summary, the adsorption of NPs by iron-modified sewage sludge biochar results from multi-mechanism coupling. Electrostatic adsorption and hydrogen bonding act as primary driving forces, rapidly fixing and stabilizing particles via surface charge differences and functional group interactions. Pore filling provides physical trapping through mesopores, assisting diffusion and contact, while π–π interactions offer local stabilization at specific aromatic sites. Synergistic and antagonistic effects exist among different mechanisms: electrostatic adsorption and hydrogen bonding significantly cooperate to enhance adsorption, pore filling is constrained by spatial hindrance and site competition, and π–π interactions contribute local stability. Overall adsorption efficiency is determined by the combined action of these mechanisms, providing theoretical guidance for optimizing sludge biochar surface functional groups, charge distribution, and pore structure to improve NP removal performance.

3. Materials and Methods

3.1. Materials

Green-labeled monodisperse polystyrene fluorescent nanobeads were used as NP models (diameter: 100 nm; excitation: 488 nm; emission: 520 nm). Sludge was collected from a dewatered sludge outlet of a municipal wastewater treatment plant in Hangzhou, China.

3.2. Preparation of Adsorbents

Preparation of sludge biochar: Dewatered sludge was first air-dried, then ground using a high-speed grinder and sieved through a 20-mesh screen. 100 g of the dried sludge was loaded into a tubular furnace and pyrolyzed at 300, 500, and 700 °C for 2 h under a heating rate of 10 °C·min^−1^. After cooling to room temperature, the sludge biochar was ground and sieved through a 100-mesh screen, yielding BC300, BC500, and BC700 powders.

Preparation of iron-modified sludge biochar: Iron-modified magnetic sludge biochar was prepared by the impregnation method. 25 g of sludge particles were soaked in 500 mL of 0.124 mol·L^−1^ Fe(NO_3_)3·9H_2_O solution and stirred magnetically for 24 h. The precipitate was separated from the solution by centrifugation, dried at 105 °C for 8 h to obtain the original dried sludge sediment, and then pyrolyzed at 300, 500, and 700 °C for 2 h. After grinding and sieving through a 100-mesh screen, the resulting powders were labeled FeBC300, FeBC500, and FeBC700. The mass fraction of Fe in the biochar samples was determined by surface elemental analysis and is presented in Table 3.

3.3. NP Removal Experiments

Deionized water was used to prepare the NP stock solution. The NP concentration was determined using a fluorescence spectrophotometer (Hitachi F-4700, Tokyo, Japan) with a 10 × 10 mm quartz cuvette. The optimal excitation wavelength was set to 488 nm, with both excitation and emission slit widths set to 5 nm. A linear relationship between NP concentration and fluorescence intensity was confirmed (R^2^ = 0.999).

Batch adsorption experiments were conducted by adding 10 mg of sludge biochar to 10 mL of NP solution (10 mg·L^−1^) in a 50 mL conical flask, resulting in a solid-to-liquid ratio of 1 g·L^−1^, and the mixture was shaken at 180 rpm for 5 h. Magnetic sludge biochar was separated using a magnet. The residual NP concentration in the solution was then measured to calculate the removal efficiency. The adsorption capacity of the sludge biochar samples was calculated using the following formula.

$$[eqn]$$ $$[eqn]$$

In the equation, R represents the removal efficiency, while C_0_ and C_t_ are the concentrations of microplastics in the solution at 0 h and t h, respectively. Q_e_ (mg·g^−1^) denotes the mass of microplastics adsorbed per gram of biochar adsorbent. V and W are the solution volume and the mass of adsorbent used in each test, respectively.

3.4. Characterization

The surface area and pore volume of the prepared samples were determined using an ASAP 2460 adsorption analyzer (Micromeritics, Norcross, GA, USA). The morphology of the samples was observed using a desktop scanning electron microscope (SEM, Zeiss Sigma 300, Carl Zeiss AG, Jena, Germany). Fourier-transform infrared (FTIR, iS50, Thermo Fisher Scientific, Waltham, MA, USA) spectroscopy was employed to analyze functional groups. X-ray photoelectron spectroscopy (XPS, Escalab 250, Thermo Fisher Scientific, Waltham, MA, USA) was applied to determine the binding energy and chemical states of elements. The zeta potential of the samples under different pH conditions was measured using a zeta potential analyzer (Zetasizer Nano ZS, Malvern Panalytical, Malvern, UK). Elemental composition was analyzed using an elemental analyzer (EA, Vario EL III, Elementar Analysensysteme GmbH, Langenselbold, Germany) and energy-dispersive spectroscopy (EDS, Oxford Instruments X-Max, High Wycombe, UK).

3.5. Data Processing

To eliminate the influence of differences in variable units and ranges on the statistical analysis, all variables used in the analysis were normalized prior to statistical processing.

NP removal efficiency was selected as the final response variable to investigate its relationships with material physicochemical properties and adsorption mechanism-related indicators. Based on the structural characteristics of the materials and potential adsorption mechanisms, the relevant parameters were classified into four latent adsorption mechanisms: electrostatic interactions (characterized by zeta potential), hydrogen bonding (characterized by surface oxygen-containing functional groups), pore structure adsorption (characterized by specific surface area and pore volume), and π–π interactions (characterized by π–π* bonding features).

All data processing and graphical visualizations were conducted in the R 4.3.3 software environment.

3.6. Statistical Analysis

All statistical analyses were performed using the R 4.3.3 software environment, including one-way analysis of variance (ANOVA) and Student’s t-test, as appropriate. The results of ANOVA and t-tests are reported by providing the corresponding F or t values, respectively, together with the associated p-values. The degrees of freedom for each statistical test are reported in parentheses. Differences were considered statistically significant at p < 0.05.

4. Conclusions

This study demonstrates that iron-modified sewage sludge biochar is an effective adsorbent for the removal of NPs from aqueous solutions. Compared with unmodified sludge biochar, FeBC exhibited significantly enhanced removal performance, achieving up to 96.09% under optimal conditions. The adsorption process was pH-dependent, with higher removal efficiency under acidic conditions, closely related to the protonation of the adsorbent surface and the enhanced electrostatic interactions with negatively charged NPs. Moreover, FeBC500 exhibited excellent regeneration performance, maintaining over 92% removal efficiency after five adsorption–regeneration cycles. Characterizations by FTIR, XPS, and SEM indicated that hydrogen bonding, π–π interactions, and pore structure adsorption all contributed to NP capture. Quantitative analysis using structural equation modeling revealed the contributions of different mechanisms: electrostatic interactions dominated (52.6%), followed by hydrogen bonding (23%), pore structure adsorption (16.6%), and π–π interactions (7.9%). The dominance of electrostatic adsorption highlights surface charge density and surface potential as the key determinants of NP capture efficiency, indicating that strengthening electrostatic interactions—through metal modification, pH-regulated protonation, or surface functionalization to increase zeta potential—is the most effective strategy for performance enhancement. The model further demonstrated significant correlations among mechanisms, with electrostatic interactions positively correlated with hydrogen bonding, suggesting a synergistic effect, whereas pore structure adsorption showed partial antagonism with surface chemical interaction. This work provides a quantitative mechanistic understanding of NP removal by FeBC and highlights its potential as a sustainable and efficient water treatment material. Although this study was conducted primarily under simulated water conditions, future investigations should validate the performance in complex natural waters, considering coexisting pollutants, ionic strength, and varying water quality, to promote the application of sludge biochar in large-scale water treatment and sludge resource recovery.