Elsevier

Chemical Engineering Journal

Volume 364, 15 May 2019, Pages 146-159
Chemical Engineering Journal

Magnetic nitrogen-doped sludge-derived biochar catalysts for persulfate activation: Internal electron transfer mechanism

https://doi.org/10.1016/j.cej.2019.01.163Get rights and content

Highlights

  • MS-biochar exhibited better catalytic ability than GP, GO and MWCNT.

  • ASS was the most important contributor for MS-400/600 while CM dominated MS-800.

  • ASS was effective for SO4radical dot formation and CM was primarily committed to radical dotOH.

  • Iron compounds, doped-nitrogen and graphitic carbon were proved as catalytic sites.

  • Two internal electron transfer paths among MS-biochar’s CM were proposed.

Abstract

Persulfate-based advanced oxidation process is a powerful countermeasure for water remediation, where effective and low-cost catalysts are still needed. Herein, a one-pot synthetic method for magnetic nitrogen-doped sludge biochar (MS-biochar) was presented, which exhibited better catalytic property with PDS for tetracycline degradation than typical graphitic carbon (graphite powder, graphene oxide and multiwalled carbon nanotubes) and two other types biochars. EDAX manifested the uniform dispersion of elements in MS-biochar. Similar to “top-down”, a research thought from whole to part of MS-biochar for contribution measurement was presented, where acid-soluble substance (ASS) was the most important contributor for MS-400/600 while carbon matrix (CM) was dominant for MS-800. Quenching and EPR demonstrated a free-radicals pathway in MS-biochar/PDS, where ASS mainly assumed to be effective for SO4radical dot and CM was primarily committed to radical dotOH generation. EDAX, XPS and Raman studies proved three kinds of catalytic sites, namely the iron compounds, doped nitrogen and graphitic carbon. And their activating mechanism were discussed where one internal electron migration path (from sp3 to nanocrystalline sp2 carbon) has been first proposed. Reusability, metal leaching detection and pharmaceutical wastewater application indicated the potential of MS-biochar. This work not only presents a potential resource-based disposal of sewage sludge, a novel research thought from whole to part for materials performance measurement, but also provides guidance for carbon materials’ design for persulfate activation, especially for sp2 and sp3 co-hybridized carbons.

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 (radical dotOH) 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 radical dotOH-dominated AOPs such as ozonation and Fenton reaction, persulfate-based AOP with introduced SO4radical dot (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 single bondNH2, single bondOH and -NHCOR) and aromatic compounds [5], but also presents a higher redox potential (2.5–3.1 V) than that of radical dotOH (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

References (55)

  • Y. Pang et al.

    Preparation and application of stability enhanced magnetic nanoparticles for rapid removal of Cr(VI)

    Chem. Eng. J.

    (2011)
  • Y. Pang et al.

    PEI-grafted magnetic porous powder for highly effective adsorption of heavy metal ions

    Desalination

    (2011)
  • G. Fang et al.

    Photogeneration of reactive oxygen species from biochar suspension for diethyl phthalate degradation

    Appl. Catal., B

    (2017)
  • L. Tang et al.

    Sustainable efficient adsorbent: alkali-acid modified magnetic biochar derived from sewage sludge for aqueous organic contaminant removal

    Chem. Eng. J.

    (2018)
  • F. Liu et al.

    Mechanism of the selective catalytic reduction of NOx with NH3 over environmental-friendly iron titanate catalyst

    Catal. Today

    (2011)
  • K. Keyvanloo et al.

    A novel CeO2 supported on carbon nanotubes coated with SiO2 catalyst for catalytic cracking of naphtha

    Appl. Catal., A

    (2012)
  • L. Kemmou et al.

    Degradation of antibiotic sulfamethoxazole by biochar-activated persulfate: factors affecting the activation and degradation processes

    Catal. Today

    (2018)
  • Y. Bao et al.

    Urea-assisted one-step synthesis of cobalt ferrite impregnated ceramic membrane for sulfamethoxazole degradation via peroxymonosulfate activation

    Chem. Eng. J.

    (2018)
  • M.C. Biesinger et al.

    Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni

    Appl. Surf. Sci.

    (2011)
  • A.M. Dehkhoda et al.

    Effect of activated biochar porous structure on the capacitive deionization of NaCl and ZnCl2 solutions

    Microporous Mesoporous Mater.

    (2016)
  • X. Duan et al.

    Nanodiamonds in sp2/sp3 configuration for radical to nonradical oxidation: core-shell layer dependence

    Appl. Catal. B

    (2018)
  • X. Duan et al.

    Surface controlled generation of reactive radicals from persulfate by carbocatalysis on nanodiamonds

    Appl. Catal. B

    (2016)
  • Y. Deng et al.

    Facile fabrication of mediator-free Z-scheme photocatalyst of phosphorous-doped ultrathin graphitic carbon nitride nanosheets and bismuth vanadate composites with enhanced tetracycline degradation under visible light

    J. Colloid Interface Sci.

    (2018)
  • R. Matta et al.

    Removal of carbamazepine from urban wastewater by sulfate radical oxidation

    Environ. Chem. Lett.

    (2010)
  • R.J. Watts et al.

    Treatment of contaminated soils and groundwater using ISCO

    Pract. Period. Hazard. Toxic. Waste Manage.

    (2006)
  • G.V. Buxton et al.

    Critical review of rate constants for reactions of hydrated electrons chemical kinetic data base for combustion chemistry. Part 3: propane

    J. Phys. Chem. Ref. Data

    (1988)
  • P. Shao et al.

    Identification and regulation of active sites on nanodiamonds: establishing a highly efficient catalytic system for oxidation of organic contaminants

    Adv. Funct. Mater.

    (2018)
  • Cited by (0)

    View full text