Enhancement of persulfate activation by Fe-biochar composites: Synergism of Fe and N-doped biochar

Highlights

• Graphitic and pyridine N atoms reinforced the anchor, dispersion and loading of nZVI.

• Synergistic effects of Fe and N species enhanced the PS activation.

• Fe-N @ BC activated PS to generate ${\text{O}}_2^{\text{•}-}$ and ¹O₂ for γ-HCH degradation.

• N-doped defects acted as reactive bridge to accelerate the electron transfer.

Abstract

Persulfate-based (PS-based) advanced oxidation process is a promising technology for degradation of organic pollutants. PS activation needs efficient and economical catalysts and heterogeneous Fe-carbon composites are competitive. Herein, novel N-doped biochar-loaded nanoscale zero-valent iron (nZVI) composites (Fe@N-BC) were synthesized and evaluated for PS activation. Detailed characterization data indicated that graphitic and pyridine N structures were introduced by N-doping, which enhanced the anchoring, dispersion and loading of amorphous nZVI on biochar. Remarkably, the optimized Fe@N-BC material, Fe@N₂-BC₉₀₀, presented excellent catalytic performance for PS activation for lindane removal. The N-doped defects in biochar acted as reactive bridges and accelerated the electron transfer between nZVI and PS, showing strong synergistic effects toward nZVI. ${\text{O}}_2^{\text{•}-}$ and ¹O₂ were the dominant active species in catalytic systems. Additionally, Fe@N₂-BC₉₀₀ catalyst showed effectiveness over a wide pH range for lindane removal. This work provides a new approach to the rational design and application of Fe@N-BC for persulfate activation in pollution control, which is certified by deep exploration of reaction mechanism.

Introduction

Advanced oxidation processes (AOPs) have been widely employed to rapidly and completely degrade organic pollutants in wastewater and groundwater via reactive oxygen species (ROSs) such as sulfate radicals (${\text{SO}}_4^{\text{•-}}$) and hydroxyl radicals (•OH) [1], [2]. In particular, AOPs based on persulfate (PS-based AOPs) have attracted much attention as an innovative oxidative treatment due to the relatively high stability, long-distance transmission and low cost of PS, and are expected to overcome the deficiencies of traditional AOPs [3], [4], [5]. PS needs external activation (e.g., UV, heat, microwave, transition metal ions and metal oxides) to break its critical O-O bond to produce ROSs with strong oxidation potential (mainly ${\text{SO}}_4^{\text{•-}}$, E₀ = 2.5–3.1 V) [5], [6]. Activation of PS by transition metals is simpler, more efficient and economical than energy-based activation methods (e.g., thermal, UV, electrochemical, ultrasonic catalysis) [7], [8]; therefore, has increasing application potential and attracts much attention.

Among all the tested transition metals, iron is the most preferred due to its abundant reserve, cost-effectiveness and environmental friendliness [9], [10]. Generally, iron-based catalysts for PS activation can be divided into two categories: homogeneous and heterogeneous catalysts. Heterogeneous iron-based catalysts show many advantages, such as good recyclability, wide pH response boundary, easy solid-liquid separation, and non-production of iron sludge [7], [11], [12], [13]. In recent years, nano- and micro-zero-valent iron (n(m)ZVI) has received particular attention for its ability to heterogeneously activate PS [14], [15] and has widely been studied for the degradation of various organic pollutants (e.g., pesticides [16], polycyclic aromatic hydrocarbons [17], antibiotics [18], [19]). However, the poor air stability, strong aggregation tendency, slow electron transfer cycle and low utilization of electrons of n(m)ZVI still constrain the efficiency of PS activation, and attempts need to be made to improve this approach [7], [15], [20].

Biochar, a new type of carbon-rich solid derived by pyrolyzing biomass under limited or no oxygen conditions, can behave as an effective carrier for transition metals including nZVI, which has been proved to be able to well distribute and protect nZVI to avoid reactivity defects [21], [22], [23]. It has been well documented that biochar-loaded nZVI composites exhibit higher catalytic activity than single ZVI since the biochar matrix can well distribute ZVI particles and can also participate in electron transfer and redox reactions due to its aromatic carbon [24]. For instance, Yan et al. [25] reported that nZVI reactivity to generate ${\text{SO}}_4^{\text{•-}}$ was significantly promoted with the support of rice hull-based biochar and the degradation of trichloroethylene was significantly enhanced. Moreover, a novel reaction path (nonradical pathway) of PS activation via electron transfer in Fe-biochar composites was proposed [26], [27], [28]. The preparation conditions (e.g., pyrolysis temperature, time) and precursors of biochar can affect its structural characteristics, leading to varied catalytic activity of the loaded nZVI and even change reaction pathways such as the radical and nonradical oxidation pathways [24], [27], which needs broad research to obtain an optimized performance.

Nitrogen is a fundamental component in biomass resources (e.g., straw, manure, and sludge waste), and the nitrogen atoms are trapped in the growing carbon network during pyrolyzing process, resulting in N-doped structures, such as pyridinic-N, pyrrolic-N and graphitic-N in biochar [29], [30]. These N-doped structures in biochars can lead to changes in the charge distribution, bond structure and Lewis basic sites in the carbon network [31], [32], thus may improving the loading, distribution and catalytic activity of nZVI and accelerating the reduction capacities of active sites and the electron transfer cycle between PS and catalysts [33], [34]. For instance, Cai et al. [35] reported that biochar prepared with N-rich waste-bean dregs exhibited superior catalytic performance in PS activation for bisphenol A removal. However, most natural biomass materials (straw, wood) have low nitrogen contents in the range of 0.1%− 5%, and the prepared biochars also have rare N-doped structures [30].

N-doping by extra N-dopants was claimed to be a promising approach to solve these problems [35]. For example, the synthesized Fe-N-C composite [26] and Fe/N codoped biochar [36] with additions of urea and dicyandiamide both showed good performance for PS activation. However, opposite results were also reported. Chen et al. [32] found that the N-doped active sites in biochar could be occupied or concealed by Fe species, which inhibited the synergistic catalytic effect in PS activation by Fe/N codoped biochar. Thus, due to the mistiness of diverse hybrid orbitals of N atoms in carbon network, the interactions among biochar, N-doped structures and Fe species have not been well clarified, and research on the effects of N-doping on the apparent structure formation or distribution of nZVI is also insufficient [37]. Therefore, further in-depth research is needed to accurately and fully explain the N-doping effects on biochar structure or nZVI distribution and the mechanism of PS activation to destruct organic pollutants.

Hence, in the present study, a series of novel Fe@N-BCs were synthesized by a combined route of pyrolysis and liquid phase reduction using different proportions of wood pulp, urea and ferric salt. The structure features of these composites were systematically characterized; particularly, the growth, anchoring and distribution of nZVI on the N-doped biochar and the effects of the N-doping were investigated. Owing to the characteristics of high toxicity, persistence, long-distance migration and hard-degradation, lindane (γ-hexachlorophene, γ-HCH) was selected as the target organic pollutant and its main properties are summarized in Table S1 in the Supporting Information (SI). Fe@N-BCs were used to activate PS for γ-HCH degradation under different conditions to optimize the degradation. To explore the underlying mechanism for γ-HCH degradation by Fe@N-BC-activated PS (Fe@N-BC/PS), chemical quenching experiments and electron paramagnetic resonance (EPR) detection for ROSs were both performed, and electrochemical analysis was also conducted to elucidate dynamic charge transfer between catalysts and PS intermediates.