Activated Biochar from Sewage Sludge: A Sustainable Solution for Effective Removal of Emerging Water Contaminants

Academic Editor: M. Gilles Mailhot Received: 27 June 2025 Revised: 28 July 2025 Accepted: 28 July 2025 Published: 28 August 2025 Citation: Anastasiou, M.; Sakkas, V.; Sleiman, M. Activated Biochar from Sewage Sludge: A Sustainable Solution for Effective Removal of Emerging Water Contaminants. Molecules 2025,30, 3514. https:// doi.org/10.3390/molecules30173514 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Activated Biochar from Sewage Sludge: A Sustainable Solution for Effective Removal of Emerging Water Contaminants Marina Anastasiou 1,2, Vasilios Sakkas 2and Mohamad Sleiman 1,* 1 Institute of Chemistry of Clermont Ferrand, Université Clermont Auvergne, Clermont Auvergne INP, CNRS, ICCF, F-63000 Clermont-Ferrand, France; [email protected] 2Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece; [email protected] *Correspondence: [email protected] Abstract Sewage sludge, a byproduct of wastewater treatment, can be converted into biochar, offering a sustainable solution for waste management and water treatment. Although biochars from biomass have been widely studied, sewage sludge-derived biochars remain underexplored. This study investigated the use of alkaline-treated sewage sludge-derived biochar (AlBC) as an adsorbent for three water pollutants: caffeine (CAF), carbamazepine (CBZ), and 17 α -ethinyl estradiol (EE2). A comprehensive analysis was conducted to explore the kinetic and thermodynamic behaviors of these pollutants under varying conditions, such as different adsorbent dosage, temperature, and water matrix values. The AlBCSS showed enhanced surface area and improved adsorption capacity, with EE2 being preferentially adsorbed (q e : 9.51 mg g −1 ), followed by CAF (6.12 mg g −1 ) and CBZ (4.58 mg g −1 ). Adsorption followed the Langmuir isotherm for CAF and CBZ, and the Freundlich isotherm for EE2, while kinetics were best described by the pseudo-second-order and Elovich models. Thermodynamic analysis revealed that the adsorption process was spontaneous, primarily driven by physical interactions. Factors such as dosage, temperature, and pollutant concentration influenced adsorption, with no saturation observed at higher concentrations. The natural water matrix had a minimal effect on removal efficiency (40–100%), whereas AlBC exhibited promising results after four adsorption cycles. These results highlight the potential of sewage sludge-derived biochar as a sustainable adsorbent for emerging water pollutants, supporting circular economy practices in wastewater management. Keywords: sewage sludge; biochar; adsorption; caffeine; carbamazepine; 17 α -ethinyl estradiol 1. Introduction Sewage sludge, the primary solid waste generated by wastewater treatment plants (WWTPs), amounts to millions of tons. In 2022 alone, European Union members produced approximately 4 million tons of sludge (dry weight) [ 1 ], with its disposal and processing costs representing nearly 50% of WWTP operational expenses [ 2 ]. Consequently, there is growing interest in adopting alternative sludge disposal strategies. Currently, common sludge management methods include incineration, landfilling, and agricultural reuse [ 3 ], while, in many cases, only half of the generated sludge is eventually recycled, with the remainder classified as waste [ 1 ]. Thermal transformation processes such as gasification, hydrothermal carbonization, and pyrolysis offer promising avenues for converting this waste into biochar, a carbon-rich material with significant potential for environmental applications [4,5]. Molecules 2025,30, 3514 https://doi.org/10.3390/molecules30173514 Molecules 2025,30, 3514 2 of 21 Biochar, characterized by its high porosity and functional groups, shares similarities with activated carbon but is distinguished by its waste-derived origins, making it an attractive alternative [ 6 , 7 ]. While much of the research on biochar has focused on agricultural and food waste as biomass sources (e.g., wood chips, bagasse, coffee husks) [ 8 – 10 ], sewage sludge and manure could be viable sources [ 11 , 12 ]. Compared to biochars from other biomass, sewage sludge-derived biochar tends to have a smaller surface area and lower porosity, which can limit its practical applications [ 13 ]. Nonetheless, biochar properties can be optimized through careful selection of feedstock and precise control of pyrolysis conditions [14,15]. Although less extensively studied than other biochar sources, sewage sludge-derived biochar has been explored for various applications, including sorption of nutrients, heavy metals, and organic pollutants, as well as soil amendment and catalysis for pollutant degradation [ 16 – 18 ]. The effectiveness of these applications depends largely on the physicochemical properties of biochar, such as its surface area, porosity, chemical stability, and ion exchange capacity [ 7 ]. Recent studies have demonstrated that chemical and physical treatments can enhance biochar’s properties for specific environmental applications, particularly in the removal of organic pollutants. Among the most favored treatments, alkaline and acidic activation are used for the chemical modification of biochar, while ball milling is preferred in some studies for achieving a physically modified material [19–22]. In recent decades, the growing presence of organic micropollutants, including psychotropic substances, pharmaceutical compounds (PhACs), and endocrine-disrupting chemicals (EDCs), in water bodies and wastewater has led to a significant environmental and health challenge considering that, even at low concentrations (<1 µ g L −1 ), these pollutants can disrupt biochemical processes and adversely affect aquatic ecosystems [ 23 , 24 ]. For example, caffeine (CAF), a widely consumed compound found in food, drinks, and medicines, is difficult to metabolize and has been shown to cause biochemical alterations in marine species [ 25 , 26 ]. Similarly, studies of endocrine disruptors such as 17 α -ethinyl estradiol (EE2) and carbamazepine (CBZ) exhibited that they can also be acutely toxic to aquatic life [ 27 ]. Among others, CAF, CBZ, and EE2 are persistent compounds that cannot be removed by conventional wastewater treatments, such as sedimentation and filtration [28], highlighting the need for more efficient treatment technologies. Among the most promising methods for micropollutant removal, adsorption stands out due to its cost-effectiveness, simplicity, and time efficiency [ 29 ]. Materials such as activated carbon, clay, and metal–organic frameworks are commonly used as adsorbents [ 30 , 31 ]. However, activated carbon remains expensive and energy-intensive, underscoring the need for more sustainable and affordable alternatives [ 32 ]. While biomass-derived biochars have been extensively studied for their adsorption capabilities, the use of biochar derived from sewage sludge remains underexplored. This study investigated the potential of alkaline-treated sewage sludge-derived biochar (AlBC) as an adsorbent for three organic pollutants—CAF, CBZ, and EE2—with distinct chemical properties. This research systematically examined the kinetic and thermodynamic behaviors of these pollutants under varying conditions, such as different adsorbent dosage and temperature values. The findings provide valuable insights into the potential of sewage sludge-derived biochar as a sustainable, low-cost solution for wastewater treatment, thereby contributing to the principles of a circular economy. 2. Results and Discussion 2.1. Characterization of the Biochar Samples Surface functionality, specific surface area, pore volume, and morphology are physicochemical parameters that significantly affect the adsorption capacity of materials. Both Molecules 2025,30, 3514 3 of 21 untreated (BC) and alkaline-treated biochar (AlBC) were characterized to better understand their properties and confirm the success of the treatment. The functional groups present on the surfaces of BC and AlBC were explored by ATR-FTIR. As shown in Figure 1a, the surface of the biochar contained various functional moieties. A vibrational band observed at 3327 cm −1 , although quite weakened due to high-temperature pyrolysis [ 33 ], is representative of R-O-H stretching of alcohols. Other oxygenated groups are also present, with bands at 1595 cm −1 and 1376 cm −1 assigned to C=O stretching of carboxylic acids and phenolic-OH, respectively [ 34 , 35 ]. The enhancement of the bands at 3327 cm −1 and 1370 cm −1 (shaded area in Figure 1a) is an indication of further functionalization of the initial samples and enrichment with oxygen-containing groups, confirming the successful chemical modification of BC. Moreover, the band observed at 1000 cm −1 can be attributed to C-OH stretching vibration. As reported by Yin et al. [ 34 ], Si-O vibrations were observed at the same wavenumbers, and their presence was a result of the rich content of minerals and metals preserved from the sewage sludge. Following this observation, bands at 780 cm −1 and below 600 cm −1 can be considered a result of M-X stretching vibrations (M: metal, X: halogen) [ 36 , 37 ]. Energy-Dispersive Spectroscopy (EDS) conducted on BC confirmed the high carbon (55%) and oxygen content (35%) of the material, but it also revealed the presence of small amounts of Al, Si, P, S, Ca, and Fe, sourced from the selected feedstock, sewage sludge, as shown in Table S1. Figure 1. Characterization of biochar samples with ATR-FTIR (a) and SEM before (b) and after (c) alkaline treatment. The specific surface area (S BET ) and pore size are critical factors that influence the performance of an adsorbent. Table S2 presents the N 2 adsorption and desorption isotherms for the BC and AlBC. As shown, the BC exhibited a relatively low S BET of 28.9 m 2 g −1 , consistent with previously reported values [ 38 , 39 ]. For instance, Kalderis et al. [ 40 ] reported an S BET of 25.6 m 2 g −1 for sewage sludge biochar in their study on the adsorption of Molecules 2025,30, 3514 4 of 21 2,4-dichlorophenol. These findings suggest that biochar derived from sewage sludge generally has a low specific surface area, which may limit its adsorptive capacity. To address this limitation, alkaline treatment has been shown to significantly improve S BET and enhance the material’s porosity [ 41 ]. In this study, NaOH treatment resulted in a threefold increase in S BET . The NaOH-treated biochar, AlBC, also exhibited enhanced N 2 adsorption and increased pore volume, as illustrated in Figure S1, supporting the hypothesis that the treatment created a more porous structure. As shown in Figure 1b,c, both BC and AlBC exhibited considerable morphological heterogeneity, consisting of irregular, non-spherical particles. Notably, AlBC particles had a rougher surface, which may enhance its adsorptive capacity, and a larger particle size compared to BC. This observation is further supported by a particle size analysis, which revealed DX50 values of 8.19 µ m and 24.1 µ m for BC and AlBC, respectively. The increase in size was likely due to the agglomeration of smaller particles during the activation process. Although alkaline treatment of biochar has been widely used as a modification method [42–45], there are very few studies on its effect on particle size. Liu et al. [ 46 ] and Yang et al. [ 47 ] utilized alkaline agents to improve their material’s adsorptive capacities for the removal of methylene blue and ammonium, respectively. As reported, an increase in the particle size of biochar was noted after treatment at the scale of a few nanometers in the first case and in a range of micrometers in the second case, as is evident from the size distribution results. These reports support the findings of this study regarding the increased particle size of biochar after treatment. Another set of factors with critical influence in the performance of biochar is pH and zeta potential. Suspensions of BC and AlBC in deionized (DI) water exhibited pH values of 7.10 and 7.69, respectively. The zeta potential increased from − 26.18 ± 1.08 mV for BC to − 32.69 ± 0.9 mV for AlBC, as shown in Table S2. These values confirm the negatively charged surfaces of the biochars and their high stability in suspension, while also suggesting an increased concentration of oxygenated functional groups in the treated biochar. 2.2. Adsorption Kinetics Study The kinetic profiles of CAF, CBZ, and EE2 adsorption onto AlBC were evaluated using three models: the Pseudo-First Order—PFO, Pseudo-Second Order—PSO, and Elovich, as depicted in Figure 2. Under the experimental conditions, all three contaminants exhibited rapid adsorption within the first 60 min, likely due to initial physical adsorption onto the biochar’s surface. After 60 min, the adsorption rate decreased, and equilibrium was reached at 360 min. This slower second phase of adsorption may be attributed to the filling of the inner pores of the biochar, as most of the available surface active sites were likely initially occupied by the contaminants. The adsorption of CAF, CBZ, and EE2 on AlBC was efficient, with EE2 showing the highest removal efficiency. The equilibrium adsorption capacities (q e,exp ) for the three contaminants were 4.58, 6.12, and 9.51 mg g −1 , respectively. The higher removal of EE2 can be attributed to its higher lipophilicity (LogK ow = 3.67), which likely enhances its affinity for biochar, a highly carbonaceous material. Similar adsorption capacities have been reported in the literature for biochars employed for the adsorptive removal of these three contaminants. More specifically, Anastopoulos et al. (2020) [ 48 ] reported an adsorption capacity of 5.35 mg g −1 (at 25 ◦ C) for the removal of CAF using oxidized pine needlederived biochar, a value that is in the same range as the results reported in this study. However, several studies have reported q e values in the range of hundredths for food waste-derived biochars that possess equally high surface area values [ 49 – 51 ]. In the case of CBZ, Zhang et al. [ 52 ] reported a maximum adsorption capacity of 118.4 mg g −1 when using wastewater algae-derived biochar, while, for EE2, Shin et al. [ 53 ] reported q e values Molecules 2025,30, 3514 5 of 21 equal to 18.73 mg g −1 for alkali-modified biochar from spent coffee grounds, a value that is almost double those reported in this study. Figure 2. Adsorption kinetics of CAF, CBZ, and EE2 on AlBC with Pseudo-First-Order (dashed line), Pseudo-Second-Order (continuous line), and Elovich fitting (dotted line) (C AlBC : 1 g L −1 ; Ccontaminant: 10 mg L−1; 400 rpm; 25 ◦C). Among the three kinetic models applied, the PSO and Elovich models provided the best fit to the experimental data, as evidenced by the high R 2 values (R 2 > 0.99), the statistical parameters (Adjusted R 2 , Residual Sum of Squares, Reduced Chi-Square) exhibited in Table S3, and the close agreement between the experimental and calculated q e , as shown in Table 1. Both the PSO and Elovich models are suitable for describing adsorption processes that involve chemisorption during the rate-determining step [ 54 ]. These findings are consistent with previous studies, which have also reported that the PSO model effectively describes the adsorption of CAF, CBZ, and EE2 on biomass-derived biochar [55,56]. Table 1. Kinetic parameters of CAF, CBZ, and EE2 adsorption on AlBCSS. Compound Pseudo-First Order Pseudo-Second Order Elovich qe,exp qe,cal K1R2qe,cal K2R2α β R2 mg g−1mg g−1 min −1mg g−1g mg−1min−1mg g−1min−1g mg−1 CAF 4.58 4.58 0.021 0.694 4.61 0.0306 0.998 51.936 2.278 0.996 CBZ 6.12 5.81 0.073 0.977 6.14 0.018 0.996 18.077 1.469 0.998 EE2 9.51 9.15 0.094 0.980 9.53 0.017 0.994 253.87 1.195 0.9987 2.3. Adsorption Isotherm Study The Langmuir and Freundlich models were applied to better understand the adsorption behavior and equilibrium characteristics of the contaminants, as shown in Figure 3. The Langmuir model assumes monolayer adsorption on a homogeneous surface, whereas the Freundlich model describes multilayer adsorption on a heterogeneous surface with active sites of varying intensities. In this study, the Langmuir model best described the adsorption behavior of CAF and CBZ on AlBC, according to the adsorption isotherm studies (Table S4). The monolayer adsorption capacities (q e ) were 17.05 mg g −1 for CAF and 16.56 mg g −1 for CBZ. The obtained values were higher than the adsorption capacities of white pine needle biochar for the adsorption of CAF, as reported by Oginni and Singh [ 57 ], and comparable to the adsorption capacity of biochar for the adsorption of CBZ, as reported by Całus-Makowska et al. [58]. Molecules 2025,30, 3514 6 of 21 These findings prove that the efficiency of AlBC is comparable, if not superior, to other biochars already reported in the literature [59,60]. Figure 3. Langmuir and Freundlich adsorption isotherms of CAF, CBZ, and EE2 on AlBCSS (CAlBC:1gL−1; Ccontaminant: 7–28 mg L−1; 400 rpm; 25 ◦C). Apart from the maximum adsorption capacity, the Langmuir model provides information regarding the affinity between the adsorbent and adsorbate through the Langmuir isotherm constant K L . High K L values indicate a stronger interaction between the adsorbent and adsorbate. As presented in Table 2, CBZ exhibited a higher K L (0.161) than CAF (0.071). This result is consistent with the fact that CBZ is a more lipophilic molecule (LogK ow : 2.45) than the hydrophilic CAF and thus shows a greater affinity for the hydrophobic AlBC [ 61 ]. Table 2. Parameters from the isotherms study of CAF, CBZ, and EE2 adsorption on AlBCSS. Compound Langmuir Freundlich qmax KLR2KF1/nFR2 K mg g−1L mg−1mg g−1(mg−1)n CAF 17.05 0.071 0.995 1.771 0.591 0.988 CBZ 16.59 0.161 0.973 3.485 0.455 0.951 EE2 23.61 1.196 0.685 13.251 0.262 0.930 The adsorption of EE2, on the other hand, adhered to the Freundlich model, as indicated by the higher R 2 values summarized in Table 2. EE2 has the most distinct properties among the studied contaminants, with a higher molecular weight and lipophilicity (high LogK ow ). As observed, its adsorption was higher than that of CAF and CBZ, with nearly double the q e values. These results are in agreement with those reported in the literature, where the Freundlich model best describes the experimental results compared to the Langmuir [62]. The key parameters derived from the Freundlich model include the heterogeneity factor ( nF ) and 1/ nF . An nF value between 1 and 10 suggests that the adsorption is favorable, with higher values of n indicating a strong interaction between the adsorbent and adsorbate. For EE2, the nF value was 3.82, indicating favorable adsorption. This value is higher than those reported by Ahmed et al. [ 63 ] for the removal of endocrine disruptors, including EE2, by Eucalyptus globulus wood biochar ( nF between 0.17 and 0.27) and by Vieira et al. [ 64 ], where the nF value was 1.107 for the adsorption of EE2 from fungiculture waste-derived biochar. Molecules 2025,30, 3514 7 of 21 2.4. Thermodynamic Adsorption Parameters While kinetic and isotherm parameters have been extensively studied in adsorption research, thermodynamics remain less explored. Thermodynamic analysis of the adsorption process provides insight into the nature of adsorption (whether physical or chemical) and validates the predictions made by isotherm models based on the values of enthalpy change ( ∆ H 0 ), Gibbs free energy ( ∆ G 0 ), and entropy change ( ∆ S 0 ). To calculate these parameters, the dimensionless thermodynamic equilibrium constant (K 0 ) is used in the Van ‘t Hoff equation, ensuring correct units for ∆ G 0 [ 65 ], in contrast to studies that directly use the Langmuir or Freundlich constants. Table 3presents the calculated thermodynamic parameters for the adsorption of CAF, CBZ, and EE2 onto AlBC. The ∆ H 0 values increased from CAF to EE2. ∆ H 0 values below 20 kJ mol −1 , such as those calculated for CAF ( ∆ H 0 = − 15.99 KJ mol −1 ), CBZ ( ∆ H 0 = 2.30 KJ mol −1 ), and EE2 ( ∆ H 0 = 3.09 KJ mol −1 ), indicate physisorption. The negative ∆ H 0 value for CAF indicates an exothermic process, whereas the positive ∆ H values for CBZ and EE2 suggest endothermic processes. For all three contaminants, the entropy change ( ∆ S 0 ) was positive, with values increasing from CAF (60.32 J mol −1 K −1 ) to EE2 (176.25 J mol −1 K −1 ), indicating greater disorder at the solid–liquid interface. The negative ∆ G values for all three contaminants confirmed the spontaneity of the adsorption process. Notably, as the temperature increased from 10 to 40 ◦ C, the magnitude of the negative ∆ G 0 values became more pronounced, further supporting the endothermic nature of the adsorption processes, particularly for CBZ and EE2. It is important to note that the obtained results are in agreement with previously reported thermodynamic values for the adsorption of CAF [ 66 ] and CBZ [ 67 ] on lignocellulosic biomass-derived biochar. As expected, owing to the variation in the chemical composition of the adsorbent, the compared values are not arithmetically identical; however, there is agreement on their range and the conclusions drawn based on them. In the case of EE2, information on thermodynamic values was not found regarding the adsorption of the compound on carbon-rich materials with properties similar to those of biochar. Table 3. Thermodynamic parameters for the adsorption of CAF, CBZ, and EE2 on AlBCSS at 10, 25, and 40 ◦C. Temperature Caffeine Carbamazepine 17 α-Ethinylestradiol ∆G0∆H0∆S0∆G0∆H0∆S0∆G0∆H0∆S0 K KJ mol−1KJ mol−1J (mol K)−1KJ mol−1KJ mol−1J (mol K)−1KJ mol−1KJ mol−1J (mol K)−1 283 −33.07 −15.99 60.32 −34.24 2.30 131.48 −46.78 3.09 176.25 298 −33.97 −36.21 −49.43 313 −34.88 −38.19 −52.07 2.5. Effect of Various Parameters 2.5.1. Effect of Dosage Several experimental factors influence the efficiency of biochar in adsorption processes, with biochar concentration being one of the most significant factors. In this study, three concentrations of biochar (0.2, 0.6, and 1 g L −1 ) were tested for their ability to remove a fixed concentration of contaminants (10 mg L −1 ) from a solution. As shown in Figure 4a, an increase in biochar concentration led to a decrease in adsorption capacity. Specifically, this decrease was highly evident in the case of EE2, where q e dropped from 35.58 mg g −1 to 9.51 mg g −1 . For CAF and CBZ, the phenomenon was milder because the decrease in the adsorption capacity was below 35% of the maximum q e . Although a higher biochar concentration resulted in a reduced q e , it led to higher removal efficiencies. The %R for EE2 increased from 71% to 95%, whereas the %R for CAF and CBZ exhibited more pronounced improvements, rising from 13% to 46% for CAF and from 18% to 61% for CBZ. This suggests that optimizing biochar loading can lead to the near-total removal Molecules 2025,30, 3514 8 of 21 of contaminants such as EE2. At higher biochar concentrations, the availability of active sites is greater, resulting in an increase in %R. However, particle agglomeration may occur, limiting the accessibility of contaminant molecules to the internal pores of the biochar, leading to reduced q e values [ 68 ]. Similar results were observed by other researchers, where an increased concentration of sludge-derived biochar led to higher removal efficiencies for Methylene Blue but lowered the adsorption capacities [ 69 ]. Similar effects were reported by Zeghioud et al. [ 70 ] in their study on the removal of methylene blue and CBZ from aqueous solutions using biochars derived from beech and flax. Figure 4. The effect of (a) biochar dosage, (b) contaminant dosage, (c) temperature, (d) competitiveness on the adsorption of CAF, CBZ, and EE2 on AlBC. Conversely, increasing the contaminant concentration from 7 to 28 mg L −1 resulted in an increased adsorption capacity, which was observed for all of the contaminants, as depicted in Figure 4b. Even though a slight decrease was observed in the %R, removal efficiencies were similar, with the lowest values being 35–40% for CAF and CBZ and the highest being 75% for EE2. The same effect was reported by other studies [ 71 ], where higher concentrations in the solution led to higher q e values until saturation of the adsorbent was observed, as a result of the occupation of the available active sites. In our case, given that these concentrations were much higher than typical environmental concentrations, it can be concluded that the adsorbent did not reach saturation even under extreme conditions. Molecules 2025,30, 3514 9 of 21 2.5.2. Effect of Temperature Temperature plays a critical role in adsorption efficiency. To assess its effect, the removal percentages (%R) of the three contaminants were evaluated at an initial concentration of 7 mg L −1 under three different temperatures (10, 25, and 40 ◦ C), as presented in Figure 4c. For caffeine (CAF), adsorption decreased with increasing temperature, from 4.47 mg g −1 at 10 ◦ C to 2.92 mg g −1 at 40 ◦ C. This trend aligns with the weak and exothermic nature of the adsorption process observed in thermodynamic studies. These findings are consistent with previous research indicating that adsorption is exothermic, with higher temperatures leading to reduced adsorption capacities [ 48 , 72 ]. Similarly, Anastopoulos and Pashalidis [ 73 ] reported a decline in adsorption capacity for caffeine on oxidized carbon derived from Luffa cylindrica, where the maximum adsorption capacity (q max ) decreased from 59.9 to 49.3 mg g−1as temperature increased from 298 K to 323 K. In contrast, carbamazepine (CBZ) removal was largely unaffected by temperature, with only a slight decrease in adsorption from 4.47 mg g −1 at 10 ◦ C to 4.22 mg g −1 at 40 ◦ C. This observation is in agreement with findings by Zhang et al. [ 52 ], who noted a decrease in CBZ adsorption by algae-derived biochar with increasing temperature. Among the three pollutants, the adsorption capacity for 17 α -ethinylestradiol (EE2) increased from 5.36 mg g −1 at 10 ◦ C to 6.98 mg g −1 at 25 ◦ C, followed by a slight decline to 6.72 mg g −1 at 40 ◦ C. This behavior supports the endothermic nature of EE2 adsorption ( ∆ H 0 > 0). Similar results were reported by Proki´c et al. [ 74 ], who observed increased adsorption of EE2 using activated carbonized hydrothermal carbon as the system temperature rose from 25 ◦ C to 55 ◦C. 2.5.3. Effect of Co-Presence of the Contaminants All the above experiments were performed under single-contaminant conditions. However, in real-world scenarios, multiple contaminants often coexist. To mimic this complexity, competitive adsorption experiments were conducted and compared with single-contaminant systems to evaluate the influence of coexisting contaminants on adsorption performance. As illustrated in Figure 4d, the adsorption capacities of CAF and CBZ were slightly reduced under competitive conditions, while EE2 removal remained largely unaffected. This suggests that EE2 exhibits a higher affinity for AlBC compared to CAF and CBZ. The observed reduction in CAF and CBZ adsorption may be attributed to the preferential occupation of active sites by EE2 molecules. Once adsorbed, EE2 may hinder further access to adsorption sites for CAF and CBZ, thereby decreasing their equilibrium adsorption capacities (q e ). Despite the presence of multiple contaminants, AlBC demonstrated effective adsorption performance, achieving removal efficiencies ranging from 40% to 100% across all pollutants under competitive conditions. 2.6. Application in Real Samples To evaluate the performance of AlBC under more realistic conditions, its adsorption efficiency for CAF, CBZ, and EE2 was tested using a natural water sample collected from the Artière River, located in the Puy-de-Dôme region of France. The chemical characteristics of the sample are detailed in Table S5. Total Organic Carbon (TOC) measurements revealed that most of the carbon content was inorganic in nature (23.83 out of 30.95 mg L −1 ). Additionally, fluorescence and UV–Vis spectra (Figure S2) confirmed the presence of only a small fraction of organic matter. Single-contaminant adsorption tests in river water showed comparable or even improved removal efficiencies relative to those in deionized (DI) water, as illustrated in Figure 5. Specifically, the adsorption capacity for CAF increased from 3.56 to 4.10 mg g −1 , Molecules 2025,30, 3514 16 of 21 For this experiment, the lowest contaminant concentration (7 mg L −1 ) was selected under the same conditions as in the kinetics study. Samples were collected at 0, 15, and 60 min, followed by additional sampling at 60 min intervals up to 360 min. 3.11. Determination and Quantification of the Contaminants Samples collected during the adsorption experiments were filtered through 0.45 µ m PTFE filters. Contaminant concentrations in the filtrate were determined by highperformance liquid chromatography with a diode array detector (HPLC-DAD). Specifically, analyses were performed using an Agilent 1290 series HPLC system equipped with a reverse-phase NUCLEODUR C18 ec column (150 × 4.6 mm) and a mobile phase consisting of acetonitrile (ACN) and 0.3% formic acid in water (50:50 v/v). The injection volume was 10 µ L, the column temperature was maintained at 40 ◦ C, and the flow rate was set at 1 mL min −1 . Detection wavelengths were 274 nm for caffeine, 286 nm for carbamazepine, and 280 nm for EE2. The limits of detection (LOD) and quantification (LOQ) are provided in Table S6. 4. Conclusions In this study, sewage sludge-derived biochar (BC) was successfully activated using NaOH, resulting in the incorporation of additional oxygen-containing functional groups and a threefold increase in specific surface area (AlBC). Adsorption behavior varied by contaminant: CAF and CBZ followed the Langmuir isotherm model, indicating monolayer adsorption, while EE2 was better described by the Freundlich model, suggesting heterogeneous surface interactions. Kinetic modeling showed that the Pseudo-Second-Order and Elovich models best fitted all three contaminants, and thermodynamic analysis confirmed that adsorption was spontaneous and primarily driven by physical interactions. AlBC’s adsorption performance was influenced by biochar dosage and temperature but not significantly limited by higher contaminant concentrations. In multicomponent systems, EE2 was preferentially adsorbed, consistent with its higher lipophilicity (LogK ow ), while CAF and CBZ were only slightly affected. Notably, AlBC maintained high adsorption efficiency in natural river water, comparable to its performance in deionized water, indicating its robustness under environmentally relevant conditions. Reusability tests demonstrated that AlBC retained considerable adsorption capacity over multiple cycles, with EE2 showing the least decline. Although its overall adsorption capacity is lower than that of some engineered biochars, AlBC’s consistent performance across diverse conditions, combined with its low-cost waste-derived origin, highlights its potential as a sustainable and effective adsorbent for the removal of emerging contaminants in water treatment applications. Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/molecules30173514/s1, Figure S1: Brunauer–Emmett–Teller (BET) measurements with N 2 adsorption and desorption isotherms of (a) BC and (b) AlBC; Figure S2: Fluorescence (a) and UV-Vis (b) spectra of the river sample; Figure S3: ATR-FTIR of AlBC before and after the adsorption of CAF, CBZ, and EE2; Table S1: EDS characterization of BC; Table S2: Characterization of BC and AlBC; Table S3: Comparison of the different applied kinetic models based on the adjusted R 2 , Residual Sum of Squares, and Reduced Chi-Square values; Table S4: Comparison of the different applied isotherm models based on the adjusted R 2 , Residual Sum of Squares, and Reduced ChiSquare values; Table S5: Physicochemical parameters of the natural water sample; Table S6: Validation parameters of the applied HPLC-DAD method for the analysis of the studied contaminants. Molecules 2025,30, 3514 17 of 21 Author Contributions: Conceptualization, M.A. and M.S.; methodology, M.A.; software, M.A.; validation, M.A., M.S. and V.S.; formal analysis, M.A. and M.S.; investigation, M.A.; resources, M.S.; data curation, M.A., M.S. and V.S.; writing—original draft preparation, M.A. and M.S.; writing—review and editing, M.A., M.S. and V.S.; visualization, M.A. and M.S.; supervision, M.S. and V.S.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript. Funding: This project has received funding from the European Union under the Horizon Europe Marie Skłodowska-Curie Actions (MSCA) Doctoral Network program (IN2AQUAS-grant agreement number 101119555). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available upon request. Acknowledgments: The authors sincerely thank Y. Faschinetti, L. Frezet, C. Forano, and S. Laid for their valuable technical assistance with the materials characterization. Conflicts of Interest: The authors declare no conflicts of interest. Abbreviations The following abbreviations are used in this manuscript: WWTPs Wastewater Treatment Plants BC Sewage Sludge-Derived Biochar AlBC Alkaline-Treated Sewage Sludge-Derived Biochar CAF Caffeine CBZ Carbamazepine EE2 17α-ethinyl estradiol PhACs Pharmaceutical compounds EDCs Endocrine-disrupting chemicals BET Brunauer–Emmett–Teller SEM Scanning electron microscopy ATR-FTIR Attenuated Total Reflection Fourier-Transform Infrared Spectroscopy LDE Laser Doppler Electrophoresis TOC Total Organic Carbon IC Inorganic Carbon UV-Vis Ultraviolet–Visible NOM Natural Organic Matter HPLC-DAD High-Performance Liquid Chromatography with a Diode Array Detector ACN Acetonitrile PFO Pseudo-First Order PSO Pseudo-Second Order DI water Deionized water References 1. Eurostat 2022, Sewage Sludge Production and Disposal. 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