Magnetic nitrogen-doped sludge-derived biochar catalysts for persulfate activation: Internal electron transfer mechanism
Introduction
Environmental deterioration by the over-discharged contaminations has been highly concerned in a global scale. Especially, the multitudinous and easy to bio-concentrated pharmaceutical and personal care products (PPCPs), such as antibiotics, clinical medicine, cosmetics and bactericide, have emerged as severe menace for water environment recently [1], [2]. For countermeasure, the development of efficient restoration technologies is necessary, where the advanced oxidation processes (AOP) are prominent due to the strong oxidative degradation ability. Most AOPs are dominated by catalytic process with the addition of homogeneous or heterogeneous catalysts, where the generated radicals or active species are responsible for organics’ degradation. Generally, hydroxyl radical (OH) is highly pH-dependent and exhibits no selectivity for targets along with short half-life (t1/2 ≤ 1 μs) [3]. Therefore, as compared to the OH-dominated AOPs such as ozonation and Fenton reaction, persulfate-based AOP with introduced SO4− (t1/2 = 30 ∼ 40 μs) would be preferable [4], which is not only effective in a wider pH range, exhibits a relatively higher selectivity for electron-donating groups (such as NH2, OH and -NHCOR) and aromatic compounds [5], but also presents a higher redox potential (2.5–3.1 V) than that of OH (1.9–2.7 V) under neutral condition, leading to a higher mineralization, especially for the refractory pollutants [6].
Although metals and their oxides catalysts have been proved to exhibit superior catalytic ability for persulfate (PS) activation [7], their extensive application is still limited by the serious toxic metal leaching and weak stability. Recently, carbon materials, typically represented by nanodiamond, graphene oxide, carbon nanotubes, ordered mesoporous carbon, and biochar [8], [9], [10], [11], have emerged as sustainable alternatives in PS catalysis, where biochar, obtained from the pyrolysis of waste biomass, exhibits low-cost preponderance for practical application [12].
Along with the emerging research hotspot of biochar, the resource exploration of municipal sewage sludge has attracted much concern, such as serving as adsorbents, electro-catalysts, capacitance materials and electrode materials for energy harvesting [13], [14]. In addition, recent investigation demonstrates that sludge biochar could also act as catalyst for H2O2, peroxymonosulfate (PMS) and persulfate (PDS) [11], [15].
However, different from the graphitic-based carbons, pristine biochar in bulk phase composed of amorphous carbon generally presents limited catalytic ability. Therefore, appropriate modification for the performance improvement of sludge biochar’s development is still needed. On one hand, the doped heteroatoms could change the electron distribution of carbon matrix which might be effective for the increase of possible catalytic sites for PDS activating [16], [17]. Particularly, nitrogen doping has been proved as the most potential one among the majority of heteroatoms [18], [19]. On the other hand, the metal-based matters, including Mn-, Co-, Fe-based compounds, have been proved as superior catalyst in PS-based AOP system [20], [21]. And due to the porous carbon structure, loading metal on biochar might be another effective selection, which could not only further improve the obtained composites’ catalytic activity but also effectively reduce the metal leakage, maintaining its stability under the protection by carbon matrix [22], [23]. Among the metal-based catalysts, Fe-based compounds emerge as a better choice, not only due to its hypotoxicity but also ascribed to its magnetism which might be helpful for catalysts’ magnetic recycling. Therefore, the combination of nitrogen-doping and iron loading might be favorable for a high-efficient biochar catalyst preparation. Tactfully, in the existing wastewater treatment plant process, the sludge generally needs to be flocculated by agents, where PAM (polyacrylamide) and PFS (polyferricsulfate) were two frequently-used auxiliary flocculants [24] which could simultaneously serve as nitrogen and iron resource, respectively. Then the obtained biochar pyrolyzed by PAM-PFS-flocculated sludge would be intrinsically modified with iron and nitrogen and serves as a potential high-efficient catalyst for PDS.
For verification, the PAM-PFS-flocculated municipal sewage sludge was obtained from Xingsha sewage treatment plant which was applied for the preparation of magnetic nitrogen-doped sludge biochar (MS-biochar). The catalytic property of prepared MS-biochar was compared to other typical graphitic carbons and biochars in PDS system. And tetracycline, a frequently detected antibiotic among PPCPs, was chosen as targeted contaminant. For activating mechanism investigation, due to the complexity of MS-biochar, it’s not suitable to figure out the contribution of its each part by a conventional wisdom. Then, similar to “top-down”, a research thought from the whole to specific part for contribution measurement was presented here. Specifically, MS-biochar was regarded as the combination of four parts, including dissolved organic matter (DOM), acid-soluble substance (ASS), carbon matrix (CM), and basal part (BSP), which was “separated” by specific physicochemical methods. Then the specific contribution by each part could be calculated by their difference values. And based on radicals quenching tests and electron spin resonance (ESR) conducted, three kinds of catalytic sites were proved along with their activating mechanism. Reusability, metal leaching detection and pharmaceutical wastewater application were also examined.
Section snippets
MS-biochar preparation
All used chemical reagents were introduced in Text S1.
For magnetic N-doped sludge biochar (MS-biochar) preparation, the municipal sewage sludge was obtained from sludge dewatering room in Xingsha sewage treatment plant which had been flocculated by PAM (2 mg/L polyacrylamide) and PFS (polyferric sulfate) with the mass ratio ([PFS/PAM]) of 44, where PAM could also serve as nitrogen resource and PFS served as iron source. After being dried at 80 °C, smashed and sifted through 100 mesh sieves, the
Characterizations and effects of HCl-treatment
As shown in Fig. 1a and h, the obtained MS-biochar was consisted of complex components dominated by the evenly distributed C, O, Si, Al, Ca and Fe elements (Fig. 1b–f), where four parts, namely DOM, ASS, CM and BSP, were included. The iron-oxide loading and nitrogen doping of MS-biochar were proved by XPS (Fig. 1i), demonstrating its successful preparation. In addition, the XPS (Fig. 1i) results of MS-800, MS-800-HCl (the effective removal of P, Fe, Na, K, Ca, Mg and portion of Al, namely the
Conclusion
Here, magnetic N-doped sludge biochar (MS-biochar) was prepared and applied for PDS activation, and similar to “top-down”, a research thought from the whole to specific part was also presented. Using TC as target contaminants, MS-800 exhibited better removal efficiency than many other typical graphitic carbon materials (GP, GO, MWCNT) and biochars (WBIO and LBIO), and ranked only second to SWCNT, indicating its superior catalytic property. EDAX manifested the uniform dispersion of elements in
Notes
The authors declare no competing financial interest.
Acknowledgment
The study was financially supported by Projects 51579096, 51521006 and 51409024 supported by National Natural Science Foundation of China, the Key Research and Development Program of Hunan Province of China (2017SK2241), the National Innovative Talent Promotion Program of China (2017RA2088), the National Program for Support of Top–Notch Young Professionals of China (2012), and the Training Program for Excellent Young Innovators of Changsha (kq1802022). The authors would like to thank the
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