Highlights
Abstract
Surface-dependent nonradical oxidation of N-doped carbon (NC)-based peroxymonosulfate (PMS) system features high stoichiometric efficiency and target degradation selectivity. However, these are severely limited by the ineffective mass transfer of organics and PMS to NC surface. Herein, we utilize membrane confinement to enhance the interaction between organics/PMS and NC catalyst, and propose the synergy mechanism of surface reaction and membrane confinement towards organics removal. The NC catalyst layer is firstly uniformly loaded throughout the ceramic membrane (NC@CM) via an innovative simultaneous polymerization-coating method. As expected, NC@CM/PMS system significantly expedites the surface reaction dominant oxidation in the filtration-through mode, and its kinetics constant for bisphenol A removal is 3700 times higher than that of NC powder/PMS system. Moreover, it has high selectivity in removal of targeted organics such as BPA (81–100 % removal rate) under various interferences (anions, NOM, water sources) due to the corporation of surface reaction and membrane sieving effect. Mechanism study reveals that the synergy of membrane confinement and surface reaction of NC@CM/PMS system removes large size disturbance and reinforces mass transfer process of reactive species towards target organics, and promotes the generation and transformation of reactive species on the NC surface. Our system can selectively and efficiently remove targeted micropollutants with strong anti-interference capability, which is of great significance in practical application.
Graphical Abstract
Introduction
Peroxymonosulfate (PMS) activation is perceived as a potential advanced oxidation process (AOP) for remediation of emerging toxic micropollutants (e.g., bisphenol A) in surface and ground water [1], [2], [3], [4]. Various PMS-activation catalysts, as core component in this process, have been explored for promoting electron transfer and reactive species production from PMS precursors [5], [6]. Compared to the homogeneous catalysts which employ dissolved transition metals as benchmark in PMS activation, heterogeneous metal catalysts have advantages of simple separation, low cost, and absence of sludge generation [7], [8], [9]. Nevertheless, some obstacles still limit their widespread applications: (i) the inevitable leaching of toxic metal ions leads to the secondary pollution risk; (ii) the inferior tunability of the coordination structure of metal atoms hinders the exploitation of catalytic activity [10].
Recently, N-doped carbon (NC), typical heteroatom doped carbonaceous materials, have become emerging heterogenous catalysts to overcome the mentioned drawbacks of traditional metal-based catalysts [11]. The introduction of N dopant can effectively regulate the spin density and charge distribution within the conjugated carbon network, inducing outperformed activity while ensuring safety of the treatment process [12]. Moreover, owing to the unique band structure of sp2 carbons tailored by electron-rich N dopants, the N-doped carbon catalyst is demonstrated to primarily mediate a surface reaction pathway in PMS activation, where organics are degraded via an electron-transfer pathway over the conducted carbon surface [13], [14]. This feature remarkably boosts specific selectivity of the target organics and stoichiometric efficiency of the reaction system, which makes NC a particularly promising catalyst in PMS activation for water treatment [3]. However, the surface reaction dominant AOPs is restricted by mass transfer among PMS, organics and NC in heterogeneous reactive system because the attachment of organics and PMS from bulk aqueous phase onto NC surface is extremely slow [15].
One strategy to enhance mass transfer process of surface dominant reaction is to confine the reactive substances in a spatial space, where the reaction occurs with concentrated reactive substances and short transfer paths [16]. Loading nanocatalysts into the membrane is a simple construction method of confinement reaction system, which has attracted great attentions [17], [18]. This catalytic membrane reactor highly accelerates the performance of heterogeneous catalysts in AOPs for selective removal of organic micropollutants [19]. However, the existing preparation methods for NC-based catalytic membrane reactor, such as blending or surface coating of bulk nanocatalysts inevitably cause the destruction of membrane structure, blockage of inner pores and uneven distribution of catalysts, leading to unsatisfactory performance for both permeation and catalytic activity [17], [19], [20], [21]. Furthermore, although the effect of spatial confinement on radical reaction has been well studied [16], [22], the promoting mechanism of spatial confinement has not been clearly studied on surface reaction dominant AOPs of NC-based catalytic membrane reactor, in which spatial condition has greatly influences on the surface reactive species [23].
Based on above analysis, we propose an innovative simultaneous polymerization-coating method to prepare N-doped carbon modified ceramic membrane (NC@CM) with uniform catalyst layer and adequate active sites inside of the membrane. Dopamine is used as a NC precursor, as it can fully infiltrate into membrane pores and gradually polymerize into a uniform coating layer of polydopamine (PDA), thus constructing a well-structured NC-based membrane reactor after being annealed [13]. The catalytic performance and mechanism of NC-based membrane reactor are explored for PMS activation to degrade BPA in a filtration-through mode. Meanwhile, the promoting mechanism of integrating membrane confinement with surface reaction dominant AOPs for micropollutant removal is systematically studied and proposed. Finally, the NC@CM/PMS system is evaluated to selectively and efficiently remove the micropollutants under various interferences, which highlights the critical roles of membrane confinement and surface reaction in practical application.
Section snippets
Materials
Dopamine hydrochloride (C8H11NO2·HCl, >98 %), Tris-buffer, 5,5-dimethyl-1-pyrroline (DMPO, >99.0 %), 2,2,6,6-tetramethyl-4-piperidinol (TEMP), furfuryl alcohol (FFA), and bisphenol A (BPA) were purchased from Sigma-Aldrich Co. Ltd. Peroxymonosulfate (2KHSO5·KHSO4·K2SO4, PMS), absolute methanol (MeOH, 99.5 %), t-butanol (TBA, 99 %), ammonia solution (NH4OH, 25 %), potash dichromate (K2Cr2O7), and polyethylene oxide (PEO, Mw = 50–500 kDa) were supplied by Aladdin Co. Ltd. The Suwannee River NOM
Characterizations
Seen from Fig. 1b and c, the color of the NC@CM changes from white to brownish black after NC is loaded. The NC layer is smooth and compact (pointed by the red arrow), and no obvious difference is observed in the structural characteristic between CM and NC@CM, indicating the even and thin NC coating throughout the surface of CM. Meanwhile, according to the EDS-mapping (Fig. 1d and e), the N and C elements are evenly scattered on the membrane cross-section of NC@CM, again suggesting the uniform
Conclusions
This study systematically investigates the availability of integrating surface dominant AOPs with membrane reactor for selectively efficient degradation of micropollutants. Firstly, an innovative simultaneous polymerization-coating method is employed for preparation of the N-doped carbon modified ceramic membrane (NC@CM). Characterization studies show that a uniform NC catalyst layer is loaded throughout the membrane. This feature guarantees adequate exposure of active sites in the limited
CRediT authorship contribution statement
Yufei Zhen: Conceptualization, Data curation, Formal analysis, Writing – original draft. Zhiqiang Sun: Writing – review & editing, Conceptualization. Hang Qie?Visualization, Investigation. Yixuan Zhang: Data curation, Software, Visualization. Caihong Liu: Investigation. Dongwei Lu: Software, Visualization. Wei Wang: Project administration. Yu Tian: Project administration. Jun Ma: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (Grant No. 52000050 and 51908162), Science Foundation of Heilongjiang Province (Grant No. LH2020E053), Postdoctoral Science Foundation of China (Grant No. 2020M670913), Heilongjiang Postdoctoral Fund (Grant No. LBH-Z20063) and State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (Grant No. 2021TS22).