Synergistic iron enhanced aerogel and peracetic acid for degradation of emerging organic contaminants

Introduction

Over recent decades, the continuous discharge of organic substances with low biodegradability, such as antibiotics, pharmaceuticals, personal care products, and pesticides, into aquatic environments has led to a significant increase in emerging organic contaminants in water^1,2,3. These emerging organic contaminants, characterized by their small molecular weight, high toxicity, and strong persistence, pose a significant threat to ecological safety and human health, even at low concentrations^4,5,6. Traditional wastewater treatment methods, including physical adsorption and the conventional activated sludge process, often struggle to effectively address these stable contaminants^7,8. Physical adsorption, while effective for initial capture, typically suffers from limited capacity and potential desorption issues. Biological processes like activated sludge can be inhibited by compounds toxic to microbial communities, such as certain antibiotics and pesticides. This is particularly true for those antibiotics that inhibit microbial growth, like sulfonamide antibiotics^9,10. Consequently, there is a pressing need for advanced treatment strategies that can effectively degrade emerging organic contaminants across diverse environmental conditions and mitigate the environmental risks posed by these persistent pollutants to ensure water quality safety and sustainability.

This demand has intensified research into advanced oxidation processes, which leverage highly reactive species (e.g., hydroxyl and sulfate radicals) to target and decompose stable organic contaminants at a molecular level. Among advanced oxidation processes, peracetic acid (PAA)-based processes have emerged as a promising alternative due to the dual advantages of producing various reactive oxygen species and functioning under a broader pH range^11,12,13. Nevertheless, achieving optimal PAA activation remains a critical challenge, especially in real water matrices, where factors such as pH variability and coexisting substances can significantly impact process efficacy and selectivity. Current PAA activation methods, including ultraviolet (UV)¹⁴, thermal¹⁵, carbon materials¹⁶, and transition metals¹⁷, each come with inherent limitations. The UV/PAA process, for instance, is effective at generating free radicals but has high energy consumption, and its catalytic performance is notably influenced by the pH and PAA dosage, as demonstrated in previous research^18,19. Activation using carbon materials is environmentally friendly and broadens the pH application range, but it tends to have lower efficiency. Conversely, iron (Fe) as a transition metal shows higher efficiency in PAA activation, for example, 80.0% of diclofenac removal was achieved using Fe²⁺ activation²⁰. However, it often suffers from significant metal leaching, which not only reduces the catalyst’s longevity but also introduces additional environmental risks.

In response to these challenges, recent research has turned to hybrid catalysts, particularly metal-loaded carbon materials, which seek to combine the reactivity of metal species with the stability and adsorption capacity of carbon-based structures^21,22. Metal-loaded carbon materials have shown superior performance in PAA activation by facilitating electron transfer and reducing metal leaching^23,24. The combination of metals with carbon frameworks can stabilize active metal ions within the porous structure, effectively improving stability, which is a key advantage in tackling persistent pollutants. In line with these advancements, recent research introduced a g-C₃N₄ carbon aerogel, which degraded heterocyclic drugs of 133–216% higher than g-C₃N₄, underscoring the effectiveness of the porous structure of carbon aerogel and the potential for metal loading²⁵. Despite these advances, application challenges remain, particularly regarding the limited catalytic active sites, narrow pH applicability, and performance instability. Additionally, many of these materials have yet to demonstrate practical efficacy in real water matrices, limiting their immediate applicability in water treatment^21,26.

To address these gaps, this study focuses on synthesizing an original Fe-doped aerogel catalyst (FeCAS), utilizing the unique properties of Fe activation and the expansive surface area and porosity of aerogels^27,28. As ultralight and highly porous materials, aerogels offer unique structural advantages for enhancing contaminant diffusion and catalyst–substrate interactions. By incorporating Fe within the aerogel matrix, FeCAS is expected to exhibit stable and efficient PAA activation without the drawback of excessive metal leaching. Moreover, the potential of calcination in improving metal surface agglomeration on Fe-doped aerogel materials, reducing metal leaching, increasing reactive sites, and promoting electron transfer has been innovatively explored. The FeCAS and its carbonization-enhanced variant (FeCAS-400)/PAA processes are further tested for sulfamethoxazole (SMX) degradation in real water matrices (tap water, secondary effluent, river water, and lake water) and extended to four widely detected emerging organic contaminants to assess the applicability and selectivity of this process. Additionally, the interfacial interaction behavior between PAA and SMX on the catalyst site, as well as the degradation mechanisms of both systems, are thoroughly elucidated at the atomic level to offer a new paradigm for enhancing electron transfer between metal–carbon substrates through carbonization effects, thereby providing vital solutions for effectively removing emerging organic contaminants from contaminated water.

Results

Morphology characteristics and chemical state of Fe-doped aerogel

Numerous small particles and a considerable amount of rough Fe aggregates on the aerogel surface were observed in the SEM of FeCAS, indicating the successful preparation of Fe-doped aerogel material (Fig. 1a). In contrast, FeCAS-400 exhibited alleviated aggregation, increased surface roughness, and the emergence of a more abundant pore structure, attributed to the decomposition of more organic components and the release of CO₂ during carbonization. Concurrently, the smaller particles on the catalyst’s surface became more uniformly dense, providing additional active sites for catalysis²⁹. Specifically, the specific surface area of FeCAS-400 increased from 70.6 to 100 m²/g, representing a 41.6% improvement (Supplementary Table 1). Furthermore, both FeCAS and FeCAS-400 surfaces displayed adsorption/desorption isotherm of type IV with an H3 hysteresis loop, revealing the presence of mesoporous structures (Fig. 1b). This enhancement in structural properties contributed to an increased pollutant adsorption capability and catalytic activity²³. Supplementary Fig. 1 shows the elemental mapping images of FeCAS and FeCAS-400. All elements, including Fe, were clearly visible, indicating more uniform Fe doping in FeCAS-400 compared to FeCAS.

Synergistic iron enhanced aerogel and peracetic acid for degradation of emerging organic contaminants — **Fig. 1: Characterization of FeCAS and FeCAS-400.**

XRD results depicted that FeCAS-400 retained the crystallographic structure of FeCAS but experienced disruption in the graphite carbon’s conjugated structure³⁰ (Fig. 1c). Additionally, the intensity of FeCAS-400’s diffraction peaks at 2θ of 44.7° (400) and 26.1° (211) was diminished compared to FeCAS, suggesting a decrease in material crystallinity post-carbonization³¹. Notably, sharp diffraction peaks at 2θ of 35.4° (311), 57.0° (511), and 62.5° (440) in FeCAS-400 corresponded to the crystal patterns of Fe oxides³². Further XPS results disclosed the presence of C, O, N, Fe, and S elements, confirming the successful loading of Fe on the aerogel surface. The high-resolution C 1s spectrum of FeCAS (Fig. 1e) exhibited three peaks corresponding to C=O/C–N, C=N/C–OH, and C=C/C–C³³, while the Fe 2p spectrum (Fig. 1f) indicated the presence of surface Fe(II) and Fe(III). Compared with FeCAS, the proportion of Fe 2p in FeCAS-400 increased after calcination (Fig. 1d and g), while the C 1s content decreased from 11.3% to 1.27% (Supplementary Table 2). FeCAS-400 contained three carbon-based functional groups including C=C/C–C, epoxy C–O–C, and C=O/C–N (Fig. 1h), and the Fe 2p spectrum featured peaks at 711 and 724 eV (Fig. 1i), corresponding to high-valent iron-oxo species (>Fe(III)) and Fe(III)^34,35. This suggested that N-based ligands, formed from nitrogenous substances under high temperatures, interact with Fe to form >Fe(III). Post-carbonization, Fe(II) on the surface of FeCAS appeared to be oxidized to Fe(III) and >Fe(III), aligning with XRD characterizations.

Performance evaluation toward removal of SMX

SMX was used as the typical pollutant to evaluate the activation of PAA by FeCAS and FeCAS-400 under neutral conditions (Fig. 2a). In the FeCAS/PAA and FeCAS-400/PAA systems, almost 100% of SMX was successfully removed within 30 min. The oxidation of SMX by Fe³⁺/PAA and the adsorption of SMX by FeCAS and FeCAS-400 were both negligible. In the H₂O₂, peroxydisulfate, and peroxymonosulfate systems, the catalytic efficiency of FeCAS was relatively low, with SMX removal rates of 1.68%, 6.45%, and 51.9%, respectively. FeCAS-400 demonstrated a 100% removal efficiency of SMX in the H₂O₂ system, but lower removal rates in the peroxydisulfate and peroxymonosulfate systems. Notably, the Fe-doped aerogel in the PAA system had a higher removal rate and fairly faster catalytic efficiency compared with other catalysts, facilitating effective degradation of SMX (>95%) within 5 min (Supplementary Table 3). In particular, FeCAS-400/PAA showcased remarkable properties with an extremely high rate of emerging organic contaminants degradation, marking a significant increase of 32.9% compared to ordinary PAA catalysts. In comparison to similar systems, FeCAS-400/PAA achieved a faster reaction rate at lower catalyst and PAA concentrations³⁶. For instance, the metallic Fe-modified sludge biochar/PAA system required 0.6 g/L catalyst and 0.6 mM PAA to reach 92% SMX degradation in 30 min³⁷. This may be due to the lower activation energy of PAA compared with other peroxides and the better selectivity of PAA with Fe-doped aerogel¹⁸.

The effect of catalyst loading and PAA concentration on the degradation of SMX in the Fe-doped aerogel/PAA system was investigated (Fig. 2b and c). Specifically, complete degradation of SMX was achieved with a catalyst loading of 0.3 g/L for both FeCAS and FeCAS-400, where the observed first-order rate constants (k_obs) reached their peak at 0.616 and 0.721 min⁻¹, respectively. However, when the catalyst loading exceeded 0.6 g/L, the degradation rate of SMX declined, likely due to catalyst agglomeration and competitive reactions between the excess Fe-based catalyst and SMX. In the FeCAS-400/PAA system, the addition of 20 mg/L of PAA led to a relatively large removal of SMX, achieving a 98.4% efficiency, with k_obs recorded as 0.326 min⁻¹. No significant improvement in SMX removal was observed when the PAA concentration exceeded 20 mg/L. This could be attributed to the excess PAA competing with the radicals, affecting the degradation of SMX. Considering that the commercial PAA solution is a mixture of PAA, H₂O₂, acetic acid, and water, further examination revealed the influence of H₂O₂ and acetic acid on the reaction rate within the system (Fig. 2d). Both systems exhibited greater sensitivity to H₂O₂, extending the reaction time from 5 to 30 min, while the high concentration of acetic acid resulted in a decreased degradation rate.

Solution pH is a crucial factor influencing the performance of PAA-based advanced oxidation processes^23,38. The FeCAS/PAA system effectively removed over 90% of SMX within a pH range of 3–9, with the highest removal rate reaching 96.1% at pH 7, while the FeCAS-400/PAA system consistently maintained over 95% SMX removal across a broader pH range of 3–11 (Fig. 2e). The reduced efficiency at pH 11 in the FeCAS/PAA system was due to the complexation of Fe and the electrostatic repulsion from the negative charge on FeCAS surface (Fig. 2f). During SMX degradation at pH 7 and 11, the system’s pH gradually shifted towards acidic (Supplementary Fig. 2), addressing the technical challenge of low efficiency in PAA-based advanced oxidation processes under strongly alkaline conditions³⁹. An exploration into the effects of natural anions (Cl⁻, SO₄²⁻, HCO₃⁻, HPO₄²⁻, NO₃⁻) and humic acid (HA) on two systems revealed that Cl⁻ enhanced degradation of SMX through increased electrostatic interaction⁴⁰ (Fig. 2g). HCO₃⁻ and HPO₄²⁻ reduced k_obs in the FeCAS/PAA system to 0.004 and 0.028 min⁻¹, respectively, due to Fe-based complex formation and active site occupation^41,42,43,44. SO₄²⁻ slightly inhibited SMX removal in both systems, while NO₃⁻ and HA had minimal impact, with the FeCAS-400/PAA system showing greater interference resistance than the FeCAS/PAA system.

Identification of reactive species in the Fe-doped aerogel/PAA system

According to previous research, the degradation of SMX in the FeCAS and FeCAS-400 systems was originally driven by various reactive oxygen species, particularly hydroxyl (•OH), organic radicals (R–O•), superoxide radical (•O₂⁻), and singlet oxygen (¹O₂)⁴⁵. Quenching experiments were conducted to identify the dominant reactive oxygen species responsible for SMZ degradation. Tert-butanol (TBA) was selected as the quencher for •OH, while 2,4-hexadiene (2,4-HD) was chosen to quench both •OH and R–O•. P-benzoquinone (PBQ) was used to quench •O₂⁻, while furfuryl alcohol (FFA) was selected to quench ¹O₂ and •OH^46,47. Notably, an increase in TBA concentrations led to a rise in SMX degradation inhibition rates from 29.6% to 62.8%, underscoring the significant role of •OH (Fig. 3a). Concurrently, •O₂⁻ also significantly influenced SMX degradation, as evidenced by the observed inhibition rates when PBQ was added, which was 46.7%. Relative to the FeCAS/PAA system, the addition of the four quenchers in the FeCAS-400/PAA system resulted in varying degrees of inhibition on SMX degradation (Fig. 3b). The contribution rates of various reactive oxygen species to SMX degradation were calculated using the methods detailed in Supplementary Text S1, with results presented in Fig. 3c. In the FeCAS/PAA system, •OH, •O₂⁻, R–O•, and ¹O₂ contributed 44.8%, 45.1%, 3.71%, and 6.37%, respectively. For the FeCAS-400/PAA system, the contributions of •OH and •O₂⁻ were comparable to those in the FeCAS/PAA system, at 42.2% and 45.5%, while R–O• increased by nearly half. Overall, while •OH played a significant role in both systems, •O₂⁻ had a more dominant effect.

**Fig. 3: Dominant reactive oxygen species are responsible for SMX degradation in the Fe-doped aerogel/PAA system.**

EPR techniques were employed to confirm the reliability of quenching experiments and the presence of reactive oxygen species, utilizing DMPO as the spin-trapping agent for •OH, •O₂⁻, and R–O•, and TEMP for ¹O₂^48,49,50. The EPR results revealed characteristic DMPO–•OH signals (with the intensity ratio of 1:2:2:1) in both the FeCAS/PAA and FeCAS-400/PAA systems, confirming the formation of •OH (Fig. 3d). Additionally, both systems exhibited six-line signal of DMPO–•O₂⁻, indicating the production of •O₂⁻ (Fig. 3e). The FeCAS-400/PAA system displayed a more pronounced DMPO–•O₂⁻ signal, suggesting enhanced catalytic activity due to calcination. However, no TEMP–¹O₂ signal was detected in either system, which could be attributed to ¹O₂ not being the dominant reactive oxygen species responsible for SMX degradation, or that high-valent iron-oxo species and electron transfer were involved in the FFA quenching reaction^51,52,53 (Supplementary Fig. 3). The characteristic signals of R–O• were cluttered, likely due to their low concentrations and the susceptibility of EPR to interference from other coexisting non-radical species^54,55. Furthermore, the spin concentrations of •OH and •O₂⁻ in the FeCAS-400/PAA system were 4.89 and 1.92 times higher, respectively, than in the FeCAS/PAA system, ascribed to enhanced electron transfer capabilities between Fe and carbon matrix⁵⁶ (Fig. 3f). In both systems, the concentration of •O₂⁻ was higher than that of •OH, indicating a dominant role of •O₂⁻ in the degradation of pollutants by the Fe-doped aerogel/PAA system, which aligned with the results of quenching experiments.

Theoretical insights into interfacial interaction in the system

Density Functional Theory (DFT) calculations were performed to gain insights into the electron transfer in the catalysts, aiming to deeply investigate site-PAA interactions at the atomic level⁵⁷. The optimized theoretical models of FeCAS and FeCAS-400 were constructed based on the characterization, as shown in Supplementary Fig. 4. The PAA adsorption energies (E_ads) at FeCAS and FeCAS-400 sites were −1.80 and -4.60 eV, respectively, indicating that the calcined Fe-doped aerogel material sites could bind PAA more firmly, thus leading to a higher concentration of PAA on the catalyst surface and accelerating the catalysis process (Fig. 4a). Compared to FeCAS, the smaller E_ads and longer O–O bond length of PAA on FeCAS-400 suggested that the PAA was more readily adsorbed, thus triggering spontaneous dissociation. Furthermore, when SMX adsorbed onto the two materials, an electron cloud formed between SMX’s O and Fe, but no chemical bond was created (Supplementary Figs. 5 and 6). The adsorption energy of SMX on FeCAS-400 (−1.54 eV) was lower than on FeCAS (−0.582 eV), implying that calcination significantly enhanced the material’s adsorption capacity for pollutants.

**Fig. 4: Theoretical calculation of interfacial interaction in the Fe-doped aerogel/PAA system.**

Besides, the bonding patterns, dominant interactions, and electron donors involved in PAA activation were further identified using the three-dimensional charge density difference map⁵⁸. As shown in Fig. 4b, a certain region of interaction could be observed between the PAA molecule and the surfaces of both materials, causing obvious electron transfer around the adsorption site. Fe-doped aerogel exhibited stronger electron depletion, thus donating electrons to the adsorbed PAA. Fe on the surface of the material produced electron transfer with the O of PAA, and FeCAS-400 caused a larger amount of electron transfer compared to FeCAS. Meanwhile, two-dimensional charge density difference slice maps revealed that the highest charge densities of FeCAS and FeCAS-400 were 0.005 and 0.02, respectively, suggesting that calcined carbonization significantly promoted electron transfer (Fig. 4c).

The topological analysis of the electron localization function was applied to describe the local electronic states of a reaction intermediate in two systems. As depicted in Fig. 4d, the electron density between the O of PAA and FeCAS exhibited a high distribution, confirming the strong coupling interactions between them. Additionally, the presence of Fe around the O–O bond of PAA significantly altered its charge distribution. In the FeCAS-400/PAA system, there was strong electron localization in the O–O interaction due to the covalent nature of the paired electrons. For Fe and O at the interface, an overlap of charge density was observed, but the valence electrons of Fe were highly delocalized, implying that the Fe–O bond was ionic. Moreover, the electron accumulation on the O–O bond of PAA suggested that PAA could act as an electron acceptor, facilitating the transfer of electrons from Fe to PAA to generate reactive species⁵⁹. The density of states (DOS) and crystal orbital Hamilton population (COHP) were calculated to investigate the PAA activation characteristic (Fig. 4e and f). The DOS analysis revealed no significant electronic states at the Fermi energy level for PAA, but there were pronounced peaks near it, indicating that electrons could be easily excited to these states, enhancing reactivity. The negative –pCOHP values close to the Fermi energy level suggested that the O–O bond was in an anti-bonding state, making it unstable and prone to dissociation or chemical reactions⁶⁰.

Reaction mechanism and degradation pathway

The degradation mechanism for the Fe-doped aerogel/PAA system was proposed by determining reactive oxygen species, identifying active sites, analyzing interfacial interactions, and evaluating electron transfer capabilities (Fig. 5a). Notably, Fe(II) maintained a dominant presence in the FeCAS, constituting over 70% of the Fe content. After the reaction in the FeCAS/PAA system, the proportion of Fe(III) decreased by 7.04%, indicating that Fe(III) played a significant role as an active site (Supplementary Fig. 7 and Supplementary Table 4). On the FeCAS-400 surface, Fe primarily existed in the forms of Fe(III) and >Fe(III) before the reaction. Furthermore, the changes in the Fe valence state were more pronounced in the FeCAS-400/PAA system after the reaction, and the content of >Fe(III) significantly reduced from 69.9% to 19.5%, suggesting that high-valent iron-oxo species were the principal active sites. Methyl phenyl sulfoxide (PMSO) was commonly used as a probe agent, reacting with high-valent metal species like Fe(V) and Fe(IV) to form methyl phenyl sulfone (PMSO₂) through oxygen transfer^61,62. Therefore, PMSO was employed as a probe substrate to understand the mechanism of action of high-valent iron-oxo species in pollutant removal. The FeCAS-400/PAA system was markedly efficient, achieving 84.9% PMSO conversion (Supplementary Fig. 8). Contrastingly, the FeCAS/PAA system exhibited a lower conversion rate of 66.2%, highlighting the preferential formation of high-valent iron-oxo species (Fe(IV)/Fe(V)) in the FeCAS-400/PAA system.

**Fig. 5: Reaction mechanism of the Fe-doped aerogel/PAA system, degradation pathway, and toxicity evaluation of SMX and its possible degradation intermediate products.**

Based on these observations, catalytic mechanisms for both systems were proposed (Fig. 5b). In the FeCAS/PAA system, the graphitic carbon in FeCAS interacted with the benzene rings of SMX through π–π interactions, thereby enhancing the enrichment of SMX on the catalyst surface. Concurrently, the mesoporous structure facilitated the mass transfer between SMX and the active sites. During the reaction, Fe(II) was converted to Fe(III), generating •OH and R–O•, and was regenerated by reacting with PAA (Eqs. (1)–(6) in Supplementary Text 2)^63,64. In the FeCAS-400/PAA system, carbonization increased both the specific surface area and the reactive sites of the catalyst. The graphitic carbon in FeCAS-400 similarly interacted with SMX’s benzene rings, with the electron transfer between Fe and the carbon matrix being amplified by the carbonization effect. This led to the formation of Fe(IV) and Fe(V), which reacted with PAA to rapidly generate reactive oxygen species and lower-valence Fe species. These species further catalyzed the formation of •OH radicals, accelerating the conversion between Fe(II) and Fe(III) (Eqs. (1)–(10) in Supplementary Text 2). Therefore, carbonization significantly improved metal surface agglomeration on carbon materials, reduced metal leaching, and promoted electron transfer, leading to an increased catalytic efficiency.

The aromatic ring, –NH₂ group, and S–N bond of SMX were found to have deeper highest occupied molecular orbital, suggesting these sites were prone to lose electrons⁶⁵. In the FeCAS/PAA system, seven transformation products were identified (Supplementary Fig. 9 and Supplementary Table 5), involving hydroxylation oxidation, addition, substitution, and N-centered radical coupling⁶⁶. For instance, •OH attacked the S–N bond, forming intermediates TP-3 (m/z = 156) and TP-5 (m/z = 99), and initiated hydroxylation on the aniline ring to form TP-1 (m/z = 270), which further underwent addition to producing TP-2 (m/z = 288)⁶⁷. The aromatic ring of SMX was also attacked by •OH, replacing the –NH₂ by –OH to form TP-4 (m/z = 254), which could further undergo S–N bond breakage to produce TP-5⁶⁸. Additionally, the CH₃C(O)OO• played a significant role in degradation through electron transfer and hydrogen atom abstraction. In the FeCAS-400/PAA system, eight transformation products of SMX were revealed (Supplementary Fig. 10 and Supplementary Table 6). The primary mechanisms included hydroxylation oxidation, addition, and substitution. Furthermore, CH₃C(O)OO• reacted with the –NH₂ of SMX, forming N-centered radicals and coupling reactions generated dimeric products such as TP-6 (m/z = 347), TP-7 (m/z = 503), and TP-8 (m/z = 520), confirming the coexistence of R–O• and •OH in the FeCAS-400/PAA system⁶⁹. Overall, the FeCAS-400/PAA system demonstrated a similar but more efficient degradation mechanism compared to the FeCAS/PAA system, particularly in forming hydroxylated and substituted products.

In order to assess the potential risks of degradation products, the bioconcentration factor and developmental toxicity of SMX and its possible degradation intermediate products were evaluated (Fig. 5c and d). Except for the bioconcentration factor of TP-4 and TP-6, which were slightly higher than that of the original SMX, the bioconcentration factor of other intermediates was significantly decreased. The developmental toxicity of each degradation intermediate product was lower than that of SMX, and TP-3 was even classified as “developmental non-toxicant”. It was illustrated that the biotoxicity of SMX decreased with the progress of the reaction in the Fe-doped aerogel/PAA system.

Practical applicability and environmental impact

To further investigate the universality of Fe-doped aerogel catalytic systems, four widely detected emerging organic contaminants, including sulfadiazine, trimethoprim, carbamazepine, and atrazine were selected for evaluation (Fig. 6a). In the FeCAS/PAA system, the removal rates exceeded 90% for sulfadiazine, carbamazepine, and atrazine, and exceeded 70% for trimethoprim, all within 30 min. This variation in effectiveness might be due to trimethoprim’s larger molecular size, making it less readily adsorbed by the microporous structure of the Fe-doped aerogels. Importantly, the FeCAS-400/PAA system showed increased catalytic efficiency compared to the FeCAS/PAA system. To evaluate the practicability, four typical water matrices were tested: tap water, secondary effluent, river water, and lake water, with their properties detailed in Supplementary Table 7. The FeCAS/PAA system removed SMX with rates of 83.9% in tap water, 60.7% in secondary effluent, 17.3% in lake water, and 14.9% in river water (Fig. 6b). The FeCAS-400/PAA system outperformed it, boosting SMX degradation by 13.1% (tap water), 16.4% (secondary effluent), 29.8% (lake water), and 35.7% (river water). Overall, these Fe-doped aerogels, particularly FeCAS-400, showed significant potential in addressing recalcitrant pollutants and resisting inorganic anions and organic substances, indicating their viability for practical application in various water matrices.

**Fig. 6: Application potential of the Fe-doped aerogel/PAA system.**

The stability and reusability of Fe-doped aerogel catalysts were also verified. FeCAS demonstrated good performance over multiple uses, achieving nearly 80% degradation of SMX after the third cycle (Fig. 6c). However, its catalytic efficiency dropped to 66.6% in the fourth cycle, likely due to the leaching of Fe and the formation of non-reactive Fe-organic complexes, leading to a decrease in active sites⁶⁵. In comparison, the carbonized FeCAS-400 demonstrated superior performance, consistently maintaining an SMX removal rate exceeding 80% over four cycles. Additionally, Fe leaching in the Fe-doped aerogel/PAA system was investigated (Fig. 6d). In the FeCAS/PAA system, the Fe leaching concentration increased to 7.93 mg/L after reaction, which was below the Chinese discharge standard for Fe (10 mg/L). Compared to the FeCAS/PAA system, Fe leaching in the FeCAS-400/PAA system significantly decreased by 39.9% (4.77 mg/L), effectively reducing potential environmental risks. These findings underscore the advantages of the FeCAS-400/PAA system, particularly in terms of environmental safety and catalyst stability.

To emphasize the commercial viability of Fe-doped aerogels, the pollutant degradation performance and economic efficacy of the Fe-doped aerogel/PAA system were compared with various catalysts documented in the literature (Supplementary Table 8 and Supplementary Fig. 11). The FeCAS/PAA system displayed a high pollutant removal efficiency over 90% within a pH range of 3–9. Meanwhile, the FeCAS-400/PAA system sustained over 95% efficiency across a wider pH spectrum (3–11), showcasing superior pH adaptability and robust catalytic activity. Unlike some photocatalysts such as Fe single-atom@TiO₂, which show limited pH adaptability with a significant decrease in degradation rate at pH 7–9 (achieving only 80% degradation within 30 min), these catalysts maintain high efficiency across a broader pH range^70,71. Compared to other PAA-based advanced oxidation processes and peroxymonosulfate-based advanced oxidation processes, the Fe-doped aerogel/PAA system was notably cost-effective^72,73. The lower the effectiveness value, the more economically efficient the system. Remarkably, the FeCAS-400/PAA system in degrading SMX achieved an effectiveness value of ~0.140, which was 67.9% and 58.7% lower than that of CoFe-LDH and Fe-Zeolite, respectively, signifying a significant economic advantage over existing technologies. Consequently, the Fe-doped aerogel/PAA system has strong potential for practical application due to its high efficiency, sustainability, reusability, and recyclability.

Discussion

In this study, a novel approach is presented to enhance electron transfer among metal–carbon substrates by leveraging the effects of carbonization, and two active Fe-doped aerogel materials are constructed for the efficient degradation of emerging organic contaminants to tackle the challenge of sustainable water treatment. Specifically, three interfacial design strategies are employed to enhance electron transfer and pollutant transport:

(1) The high surface area and rich mesoporous structure of the aerogel are customized to maximize adsorption capacity, facilitating the effective transfer of pollutants to the catalytic centers. (2) The interface Fe boosts the generation of •OH, •O₂⁻ and R–O• radicals, increasing the active sites of the catalyst. (3) Calcination-induced carbonization not only promotes the formation of high-valence iron, enhancing catalytic activity under alkaline conditions, but also reduces the leaching of metal ions.

In the FeCAS/PAA system, over 90% degradation of SMX is achieved within 30 min across a pH range of 3–9, demonstrating the system’s rapid reactivity and adaptability to pH variations typically encountered in water environments. The carbonization-enhanced FeCAS-400 shows even greater catalytic efficacy, achieving an outstanding 98.4% SMX removal rate within a pH range of 3–11, which indicates its enhanced structural stability and catalytic robustness. Notably, Fe leaching is reduced by 39.9% in the FeCAS-400, effectively minimizing the environmental risks associated with metal ion release. Moreover, the carbonized catalyst exhibits significant resistance to interference from co-existing water matrices, with SMX removal rates increasing by 13.1% to 35.7% in tap water, secondary tailwater, river water, and lake water. These findings underscore the potential for FeCAS-400 to maintain its catalytic performance and structural integrity under complex environmental conditions, a notable advancement over traditional catalytic systems that often exhibit diminished activity under similar conditions. In terms of the universality of the system in degrading various emerging organic contaminants, both systems sustain degradation rates of over 90% for most typical emerging organic contaminants, highlighting their potential as a broadly applicable purification technology. Overall, the Fe-doped aerogel/PAA system demonstrates strong catalytic activity in real water environments, offering a scalable, environmentally compatible strategy for sustainable water purification, with significant potential for practical application.

Methods

Chemicals and materials

Sulfamethoxazole (98% purity), Sulfadiazine (99%), Trimethoprim (99%), Atrazine (97%), and Carbamazepine (99%) were purchased from Macklin (Shanghai, China). Commercial peracetic acid (>15% PAA, ≤6% H₂O₂, ≤16% acetic acid, w/w) was obtained from Nanjing Chemical Reagent Company. Sodium thiosulfate (Na₂S₂O₃), sodium hydrogen carbonate (NaHCO₃), potassium nitrate (KNO₃), sodium chloride (NaCl), sodium hydroxide (NaOH), and hydrochloric acid (HCl) with their purity in the analytical grade were supplied by Sinopharm Chemical Reagent Co. Ltd. Sodium alginate, HPLC-grade acetonitrile, methanol, ethyl acetate, and analytical-grade tert-butyl alcohol were bought from Aladdin (Shanghai, China). Humic acid (HA, >99%) was purchased from Alfa Aesar (USA).

Preparation of Fe-doped aerogel

To fabricate Fe-doped aerogel catalysts, economical, mass-produced sodium alginate and urea were selected as the initial materials. A combination of mixing pretreatment and ultrasonic technique was employed to produce the hydrogel, which was then shaped into spheres, frozen, and immersed in Fe₂(SO₄)₃ solution for cross-linking. After washing and vacuum drying, the resultant FeCAS underwent a gradual heating process to be calcined, producing FeCAS-400. Specifically, sodium alginate (3 g) and urea (0.5 g) were dissolved in 500 mL of deionized water. This mixture was then heated to 90 °C in a water bath and mechanically stirred to form a uniform hydrogel solution. After heating, the solution was sonicated for 30 min to eliminate bubbles and then filled into a syringe. The hydrogel was injected into spherical molds using the syringe and frozen at −40 °C for 12 h to set. Subsequently, the formed spherical materials were immersed in a Fe₂(SO₄)₃ solution and allowed to stand for 24 h to ensure full cross-linking with Fe³⁺. After repeated washing with deionized water, the materials were vacuum-dried at 180 °C for 30 h to obtain FeCAS. The FeCAS was placed in a muffle furnace, where the temperature was increased at a rate of 5 °C/min to calcination temperature and held there for 4 h. After air calcination, the material was cooled to room temperature to obtain the calcined material (Fig. 7). Based on previous experiments, it was found that the material performed better at a calcination temperature of 400 °C and was named FeCAS-400 (Supplementary Fig. 12).

Characterization

The pore characteristics and specific surface area of the Fe-doped aerogel catalysts were analyzed using scanning electron microscope (SEM, ZEISS GeminiSEM 300, Germany) and Brunauer–Emmett–Teller (BET, Autosorb-IQ-MP, USA) method. The crystal structures of the materials were investigated using X-ray diffractometer (XRD, Ultimate IV, Japan) with Cu Kα radiation in the range of 10°–80°. Surface elemental composition of the materials was determined by X-ray photoelectron spectroscope (XPS, K-Alpha, USA) under Al Kα radiation at 1486.8 V. To confirm the reliability of quenching experiments and the presence of reactive species, Electron paramagnetic resonance (EPR, EMXplus-6/1, Germany) techniques were employed, utilizing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent for •OH and •O₂⁻, and 2,2,6,6-tetramethyl-4-piperidone (TEMP) for ¹O₂. The surface charge of the materials at different pH was determined by a Zeta potential analyzer (Zetasizer Nano ZS, UK).

Experimental procedures

In the experiments, all degradation tests were conducted in 250 mL conical flasks, which were stirred constantly at 150 revolutions per minute. Typically, a 20 mg/L SMX solution was transferred into the reactor, followed by the addition of PAA at a predetermined concentration⁷⁴. The initial reactive pH was adjusted to 7.0 using diluted sodium hydroxide and hydrochloric acid solutions. Subsequently, the catalytic reaction was triggered by adding FeCAS or FeCAS-400 at room temperature. Additionally, to validate the universality of the Fe-doped aerogel/PAA system, sulfadiazine, carbamazepine, trimethoprim, and atrazine were included as representative recalcitrant pollutants, each at a concentration of 20 mg/L. Unless stated otherwise, all degradation experiments were conducted in the solution prepared with deionized water. Actual water samples for the experiments were collected from Lizhao Lake and Jiuxiang River (Nanjing, China), and from the Wuxi Reclaimed Water Plant (Wuxi, China).

Analytical methods

The concentrations of SMX and other emerging organic contaminants were measured using High-Performance Liquid Chromatography (HPLC, Waters e2695, USA) equipped with an ACQUITY UPLC BEH-C18 column (4.6 mm × 250 mm, 5 μm). SMX was detected at a wavelength of 275 nm using an isocratic mobile phase of 0.1% formic acid solution (50%, v/v) and methanol (50%, v/v). Sulfadiazine was quantified at 260 nm with the same mobile phase composition. Trimethoprim was monitored at 271 nm using a mobile phase of 0.1% formic acid solution (70%, v/v) and methanol (30%, v/v). Carbamazepine was quantified at 286 nm with a mobile phase of 0.1% formic acid solution (40%, v/v) and methanol (60%, v/v). Atrazine was detected at 221 nm using a mobile phase of 0.1% formic acid solution (30%, v/v) and methanol (70%, v/v). All contaminants were analyzed using a flow rate of 1.0 mL/min and an injection volume of 10 μL. The transformation products of SMX were identified by an ultra-performance liquid chromatography–mass spectrometry (UPLC–MS) system (Xevo TQ-S, USA) coupled with an Eclipse XDB-C18 column (2.1 mm × 100 mm, 1.7 μm). The concentration of peroxides was determined using a spectrophotometric method with KI as the indicator⁷⁵. The toxicity of SMX and its degradation intermediate products were divined via Toxicity Estimation Software Tool (TEST) with the quantitative structure–activity relationship method. The degradation, efficiency, applicability, economy, and sustainability of systems were systematically compared with those of similar types⁷⁶. The scoring criteria for the scale were listed as follows: (1) degradation: the degradation rate of emerging organic contaminants; (2) efficiency: the degradation efficiency constant of emerging organic contaminants; (3) applicability: the fields and pH range where the system is applicable; (4) economy: the cost-effectiveness of the system; (5) sustainability: the PAA and energy consumption. The cost-effectiveness of the systems characterized by cost-effectiveness value (CE) was calculated by equations as follows⁷⁷:

$$text{CE}=frac{{N}cdot {P}}{{R}cdot {{C}}_{0}}$$

(1)

where N represented the concentration of oxidants (PAA, H₂O₂) in mM; R was the removal rate of SMX; C₀ was the initial concentration of SMX (20 mg/L); and P was the unit price of the oxidizer in yuan per mole.

Theoretical calculations

DFT calculations were performed with the Materials Studio and the Vienna ab-initio simulation package (VASP)⁷⁸. Perdew–Burke–Ernzerhof (PBE) Generalized Gradient Approximation (GGA) functional was chosen to describe the electron exchange. The interaction between valence electrons and the ionic core was described by projector augmented wave (PAW) pseudo-potential. An energy cutoff of 400 eV and a Monkhorst–Pack k-point set of 2 × 2 × 1 were chosen for the calculations. The convergence criterion of the total energy and maximum force were set to be 1.0 × 10⁻⁵ eV/atom and 5.0 × 10^-−2 eV/Å, respectively. A vacuum layer of approximately 25 Å in the z direction was used to avoid the interaction between neighboring images under periodic boundary conditions. Considering that Fe is a transition metal, the interactions between Fe orbital electrons were corrected using the DFT + U method, with a U value of 2.50 eV. The COHP was determined for analyzing chemical bonding and quantifying interatomic bond strength. Additionally, quantum chemical calculations for SMX were conducted in Gaussian 16, using the B3LYP function for geometry optimization of all atoms with the 6-31G(d) basis set.