Comparative assessment of biochars from multiple sources based on persulfate activation capability: Role of iron component in enhancing thermal treatment effect on carbocatalytic performance
Graphical Abstract
Introduction
Water professionals have been increasingly focusing on heterogeneous carbocatalysis as an appealing technique for the activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) (collectively referred to as persulfate). This is owing to (i) high performance in oxidative treatment, (ii) minimal secondary contamination from toxic component release, (iii) chemical durability and biocompatibility, and (iv) facile modification (e.g., heteroatom doping and functionalization) to tune physicochemical and structural properties [1], [2], [3]. Engineered carbonaceous nanomaterials, such as carbon nanotubes, graphene, and their derivatives, have been demonstrated to effectively treat organic contaminants by persulfate activation based on three reaction routes: oxidation induced by highly reactive radicals, such as hydroxyl (•OH) and sulfate radicals (SO4•−) [4], [5], singlet oxygenation [6], and mediated electron transfer [7], [8]. Performance tests have shown that carbocatalysts are competitive to transition and noble metal-based activators [5], [9]. Their technical merits including insensitivity of the activation capacity to the type of persulfate precursor and high persulfate utilization efficiency have been suggested based on comparison with benchmark metal activators [1], [3]. However, the high cost of manufactured carbon nanomaterials is a major hurdle in their field-application as viable persulfate activators. Accordingly, to promote the practicability of carbocatalytic persulfate activation, utilizing naturally occurring and waste-derived carbon-rich substances as the precursors has been frequently considered as a feasible approach [10], [11].
Biochar (BC) is a high-carbon content byproduct (typically consisting of approximately 50–90 % carbon [12]) of the pyrolysis of biomass under oxygen-deficient conditions. This cost-effective carbocatalyst presents the potential for catalytic persulfate activation owing to its intrinsic characteristics. These include (i) electrical conductivity, (ii) abundance of oxygen-containing surface functional groups, (iii) large surface area (ranging from 8 to 800 m2/g [13], [14]), and (iv) high affinity toward organics [10], [11], [15]. Particularly, the moderate electrical conductivity of BCs originating from the redox activity of its graphitic carbon, quinone moieties, and persistent free radicals (PFRs) [16], enables radical and non-radical degradative pathways via single and double electron transfer reactions involving persulfate, respectively [10], [11], [15]. Functional groups on the BC surfaces activate persulfate by multiple mechanisms [17], [18], [19], [20]. Electron-rich carbonyl groups kinetically enhance non-radical oxidation by accelerating the self-decay of PMS (accompanied with singlet oxygen (1O2) production) [18], [20] and inducing the formation of surface persulfate complexes as electron-transfer mediators [19]. The positive correlation between the surface density of hydroxyl and carboxyl groups and the efficacy of radical oxidation suggests their roles as redox-active sites in BC-mediated conversion of persulfate to SO4•− [17], [21].
Thermal annealing of carbonaceous materials is often adopted to modulate sp2/sp3 carbon ratio [8], surface area [13], hydrophilicity [22], and type and surface density of functional groups [23]. Considering the aforementioned properties of BCs that exert a critical influence on the kinetics and mechanisms of persulfate activation, annealing is potential intelligent approach for upgrading a BC as a carbocatalyst for persulfate activation. This has been reflected in the recent demonstration of heat treatment increasing the efficacy of a BC for the catalytic oxidation of organics in the presence of persulfate by raising the degree of graphitization (i.e., sp3-to-sp2 carbon phase conversion) (accompanied with electrical conductivity improvement) [24]. Other examples supporting this are increase in the surface densities of oxygen functional groups [21] and change in the nature of nitrogen-containing moieties (originating from nitrogen impurity during pyrolysis) caused by thermal treatment [25]. Specifically, graphitic N initiated persulfate activation not reliant on •OH/SO4•− generation by inducing a positive charge on the adjacent carbon atoms [26] whereas pyrrolic N and pyridinic N possessing lone-pair electrons offered Lewis base sites, thus causing the conversion of persulfate into SO4•− [27] or promoting the local sorption capacity [28]. However, all studies on the beneficial effect of post-heat treatment are based on the performance enhancement of a single type of BC with a specific chemical composition, and little is still known about how thermal annealing can tune the carbocatalytic activity of BCs and modify the primary degradative pathway(s) induced by BC-activated persulfate. Furthermore, the impact of mineral constituents, such as iron and calcium, warrants in-depth investigation, considering that metal oxides formed by the exposure of metal impurities to heat serve as secondary reactive sites for persulfate activation [29], [30] and that selected metal components catalyze the carbon phase transformation [31]. In particular, metal impregnation, applied to promote the efficiency of BCs for redox reaction-based water treatment [32], [33], involves the secondary pyrolysis subsequent to exposure of BCs to metal solutions, and the resultant iron/carbon composites decomposed organics through persulfate activation by yielding diverse reactive intermediates, such as SO4•−, 1O2, and high-valent metals [33], [34], [35]. This implies the possible switching of the major oxidant(s) after heat treatment of metal-containing BCs.
To address the knowledge gap existing in the variation of the effect of thermal annealing with the type of BC, in this study, we examined 12 standard BCs of five different origins (provided by UK Biochar Research Center (UKBRC)) and their annealed counterparts for oxidative organic degradation by persulfate activation. The properties required for a BC to be the best performing activator were explored and the roles of inorganic impurities in improving the catalytic activity of a BC via annealing were investigated. For these purposes, pristine and heat-treated BCs were characterized in terms of their morphological features, carbon and metal phases, and surface functionality. Moreover, their persulfate activation efficiencies were correlated to their electron transfer-mediating capacity and binding affinity toward persulfate, determined by the magnitude of the negative shift of the open circuit potential (OCP) and isothermal titration calorimetry, respectively. Major degradative pathways were elucidated based on (i) the retarding effects of radical scavengers, (ii) multi-activity assessment using diverse model substrates, (iii) electron paramagnetic resonance (EPR) spectral features, and (iv) product distribution. Finally, the catalytic performance of the annealed BC was assessed based on the variation in the treatment efficiency during multiple use in PDS activation followed by thermal regeneration.
Section snippets
Chemical reagents
The following reagent-grade chemicals were used in this study: potassium monopersulfate (Oxone®), sodium peroxydisulfate, acetaminophen (ACT), benzoic acid (BA), bisphenol A (BPA), carbamazepine (CBZ), 4-chlorophenol (4-CP), furfuryl alcohol (FFA), 4-hydroxybenzoic acid (4-HBA), nitrobenzene (NB), phenol (PH), 2,4,6-trichlorophenol (TCP), 2,2,6,6-tetramethyl-4-piperidone (TEMP), 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO), methanol (MeOH), tert-butanol (t-BuOH), sodium bromide,
Persulfate activation capability of UKBRC biochars
UKBRC BCs, prepared by pyrolyzing the biomasses of six different origins at 550 and 700 ℃, were tested for oxidative elimination of 4-CP as a model substrate (nonsusceptible to direct persulfate oxidation (data not shown)) in the presence of PMS and PDS (Fig. 1 and S1). Considering the extent of 4-CP removal by adsorption in dark (performed without persulfate addition), all tested BCs except for SS700 exhibited marginal carbocatalytic activities, regardless of the persulfate precursor type and
Conclusion
In this study, we explored the role of iron, which is frequently detected as a mineral component in biomasses, in fabricating and modifying BC-based carbocatalysts for persulfate activation via thermal treatment based on the comparative assessments using UKBRC BCs of different origins prepared at 550 and 700 ℃. SS BCs outperformed the other UKBRC BCs in PDS activation and the gradual improvement of catalytic activity with increasing annealing temperature was unique to the iron-containing sewage
CRediT authorship contribution statement
Sae-In Suh: Investigation, Validation, Writing - Original Draft; Heesoo Woo: Investigation, Validation, Methodology; So-Yeon Song: Investigation, Methodology; Dongjoo Park: Methodology; Yong-Yoon Ahn: Methodology; Eun-Ju Kim: Methodology; Hongshin Lee: Conceptualization, Investigation; Dong-Wan Kim: Investigation, Validation; Changha Lee: Writing - Revised Manuscript; Yong Sik Ok: Conceptualization, Validation, Writing - Original Draft; Jaesang Lee: Conceptualization, Formal Analysis, Writing -
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.
Acknowledgments
This study was supported by a National Research Foundation of Korea grant funded by the Korean government (grant no. 2021R1A2C2003763) and the Korea Ministry of Environment (grant no. RE202201829) and Korea Environment Industry and Technology Institute (KEITI) through the Developing Innovative Drinking Water and Wastewater Technologies Project (2022002710001).
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These authors contributed equally to this work.