Enhanced Peroxydisulfate (PDS) Activation for Sulfamethoxazole (SMX) Degradation by Modified Sludge Biochar: Focusing on the Role of Functional Groups

Enhanced Peroxydisulfate (PDS) Activation for Sulfamethoxazole (SMX) Degradation by Modified Sludge Biochar: Focusing on the Role of Functional Groups

1. Introduction

Sulfamethoxazole (SMX), a sulfonamide antibiotic, is widely used in a multitude of therapeutic contexts in human and veterinary medicine, highlighting its pivotal role in modern medical practice [1]. However, the recalcitrant and strong antibacterial nature of SMX makes conventional physical and biological wastewater treatment methods inadequate to remove it [2,3]. Moreover, existing wastewater treatment systems lack effective antibiotic monitoring, leading to substantial concentrations of SMX remaining in treated wastewater [4]. As a result, the residual SMX accumulates in the environment, evidenced by its detection in various matrices, including surface water (especially rivers and lakes), groundwater, sediment, and even soil [5,6,7]. The accumulation of SMX in the environment not only increases the risk of bacterial resistance, but also stimulates the production and spread of antibiotic resistance genes (ARGs) through the food chain, presenting a significant threat to ecological integrity and human health [8,9].
Recent studies have conclusively demonstrated the efficacy of peroxydisulfate (PDS)-based advanced oxidation processes (AOPs) in the degradation of SMX. The growing interest in these processes stemmed from their exceptional chemical stability and adaptability across diverse environmental applications [10,11]. It is imperative to acknowledge that PDS remains inert in its native state and necessitates external energy activation. Consequently, the role of catalysts becomes paramount in improving the efficiency of PDS-based AOPs [12]. Recent evidence has illustrated that metal-free carbonaceous materials, such as graphene and biochar, exhibit effective activation of PDS, providing sustainable and environmentally advantageous alternatives [13,14]. As of recent, biochar derived from sewage sludge presents a viable choice for PDS-AOPs, attributed to its cost efficiency, biocompatibility, and sustainability [15].
In recent decades, rapid urbanization has led to a significant increase in wastewater volumes, resulting in substantial quantities of difficult-to-treat sludge that pose new environmental challenges [16]. Traditional sludge treatment methods, such as ocean dumping, incineration, and landfills, pose significant environmental risks due to the potential presence of toxic substances and heavy metals in the sludge [17]. Therefore, the advancing sludge resource recovery technology is now a critical necessity. Pyrolysis, which transforms sludge into bio-oil, biochar, and other valuable by-products, has emerged as a promising and high-performing solution in recent research [18]. Sludge-derived biochar, with its distinctive s p 2 and s p 3 carbon configuration, effectively activates PDS in both radical and non-radical ways, which enhances the versatility of the system [19]. Yin et al. [20] demonstrated that the incorporation of sludge biochar into PDS-based degradation systems significantly improved the catalytic efficiency via both radical and non-radical pathways, resulting in a substantial 48.3-fold acceleration in reaction rate compared to the separate use of PDS and sludge biochar. Nevertheless, the catalytic efficiency of sludge biochar is potentially constrained by its limited surface area, which restricts interactions with catalytic targets [21]. In addition, the potential leaching of heavy metals from sludge biochar raises concerns about its applicability in environmental pollution remediation, rendering its use controversial.
In response to these challenges, extensive research has focused on the co-pyrolysis of sludge with biomass, demonstrating considerable promise in improving the properties of sludge biochar [22,23]. It has been shown to improve the immobilization of heavy metals in sludge and facilitate the use of synthetic biochar for environmental purposes [24]. Co-pyrolysis also facilitates the integration of foreign atoms, offering the sludge biochar specific properties, which increases the interaction between PDS and organic pollutants [25]. Yin et al. [26] reported a notable increase in biochar surface area from 31.35 m 2 g 1 to 122.89 m 2 g 1 by incorporating walnut shells into sludge pyrolysis. Our previous study revealed that biochar produced from co-pyrolysis of sludge and tannin extract exhibited a highly porous structure and increased surface area, making it an effective PDS activator [27]. Notably, X-ray photoelectron spectroscopy (XPS) analysis of the synthesized biochar revealed that the heavy metal signals, which can be associated with catalytic efficiency, were weakened. Instead, it contained predominantly the heteroatoms of nitrogen (N) and oxygen (O) that promote catalysis. Nevertheless, the specific mechanisms underlying the PDS activation process remain to be fully elucidated.
Besides the role of surface area in determining the catalytic efficiency of biochar, the functional groups on its surface are also critical in influencing its properties. Further detailed analyses of the catalytic mechanism have revealed a notable influence of N- and O-containing functional groups on the catalytic efficiency of sludge biochar. Tan et al. [28] identified graphitic C, pyridine N, and graphitic N as the primary active sites in biochar, which are crucial for facilitating electron transfer and thus promoting PDS activation. Contrarily, Mian et al. [29] showed that pyridine N was the predominant active species in their research, facilitating the degradation of pollutants via non-radical pathways. Additionally, Yang et al. [30] emphasized the importance of O-functional containing groups, specifically hydroxyl (-OH), carbonyl (C=O), and ether (C-O) groups, in enhancing catalytic performance. However, in the current discourse on PDS activation by sludge biochar, the specific roles of different N and O species remain unclear and controversial, highlighting the need for further investigation. Therefore, identifying the precise forms of N and O doping that enhance catalytic efficiency is critical to advancing the development of sludge biochar as an effective catalyst.

In this study, biochar (TSBC) prepared by co-pyrolysis of sludge and tannin extract was investigated as a catalyst for PDS to effectively degrade SMX in aquatic environments. The active species within the TSBC/PDS system and the catalytic sites on TSBC were systematically investigated by quenching experiments and advanced characterization techniques. Additionally, the enhancement of PDS activation by various N- and O-containing functional groups on the TSBC surface was analyzed by Density Functional Theory (DFT) calculations, providing an in-depth investigation of the adsorption dynamics of PDS on TSBC and revealing the role of these functional groups. Moreover, the identification of SMX degradation intermediates, coupled with theoretical calculations, shed light on the degradation pathways of SMX. This study aims to clarify the role of functional groups on sludge biochar in facilitating PDS activation. It can contribute to a deeper understanding of the mechanical aspects and provide guidance for targeted modification of sludge biochar to improve its effectiveness as a catalyst.

4. Conclusions

In summary, we focused on exploring the impact of O- and N-containing functional groups on augmenting the catalytic efficiency of modified sludge biochar (TSBC) to activate PDS for SMX degradation. The TSBC/PDS system exhibited exceptional efficiency in degrading SMX, achieving a removal rate of 96.83% within 120 min. Notably, the reaction rate of SMX degradation via TSBC-catalyzed PDS was seven times higher than that achieved with conventional sludge biochar (SDBC). In addition, this system demonstrated remarkable stability and efficiency, maintaining over 90% degradation efficiency across a broad pH range (3–10), making it a promising solution for varied applications. The quenching experiment and characterization results indicate that the C=O and C-N groups in TSBC primarily served as catalytic sites, mainly facilitating the non-radical pathway degradation of SMX by activating PDS to produce O 2 1 . Further theoretical analysis demonstrated that, among various N- and O-containing functional groups, graphitic N predominantly enhanced the integration of biochar with PDS, consequently facilitating the activation of PDS. SMX exhibited pronounced electrophilic properties, facilitating its reactivity with O 2 1 . Theoretical calculations and experimental evidence suggested that SMX degraded by two mechanisms: S-N cleavage and direct oxidation by O 2 1 . This research investigated the influence of various N and O species on PDS activation, emphasizing the significant role of graphitic N in enhancing PDS adsorption by biochar. These findings provide a fundamental framework for subsequent advances in improving the catalytic efficiency of sludge-derived biochar through specific modifications. The proposed improvements will streamline the application of sludge biochar in AOPs, thereby reducing operating costs. Furthermore, considering the similarities between certain industrial and sewage sludges, this approach can potentially extend to treating industrial sludges, particularly in the tannery and paper industries with low heavy metal concentrations, pending further validation.

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