Biochar modification significantly promotes the activity of Co3O4 towards heterogeneous activation of peroxymonosulfate
Graphical abstract
Introduction
Sulfate radical-based advanced oxidation processes (SR-AOPs) have received considerable concern in the environmental remediation applications during the recent decades. According to the comparable standard redox potentials, sulfate radicals (SO4−, 2.5–3.1 V [1]) possess strong oxidation capacity like hydroxyl radicals (OH, 1.8–2.7 V [2]) [3], [4]. However, due to the fact that the SO4− preferably attacks the organic compounds through the electron transfer reactions, while the OH more likely do that through the hydrogen abstraction or addition reactions [5], SO4− exhibits better selectivity towards organic pollutants than OH. Accordingly, SO4− is deemed as a competitive candidate for the abatement of recalcitrant organic pollutants from eco-systems (including water and soil).
Cobalt oxides coupled with peroxymonosulfate (PMS) has been demonstrated to be a highly effective process for the generation of SO4− [6], [7], [8]. The reactivity of the solid catalyst usually dominates the efficiency of the whole heterogeneous system. For this reason, the rational design and synthesis of the highly active cobalt oxides deserve continuous efforts. Anipsitakis and colleagues investigated the heterogeneous activation of PMS with two commercially available cobalt oxides (Co2O3 and Co3O4) for the first time and found that only Co3O4 activated PMS heterogeneously [6]. After that, various Co3O4 nanostructures were constructed and investigated for the activation of PMS [7], [9], [10], [11], [12], [13], [14], [15]. For example, a Co3O4 nano-sphere with a small size (20 nm) and high specific surface area (SSA, 18 m2/g) possessed satisfied performance in decomposing PMS [9]. Noting that, a large SSA is definitely necessary for the heterogeneous activation process because the available reactive sites are directly proportional to the SSA [16]. Inspired by this, a mesoporous bouquet-like Co3O4 nano-product with an extremely high SSA (37.0 m2/g) was synthesized by our group and investigated for the activation of PMS [8]. However, a severe drawback, that was aggregation occurred, significantly inhibiting the exposure of the active sites. As a matter of fact, due to the high surface energy of the nano-level particles, they very much tend to get together during the preparation. To inhibit the aggregation, two-dimensional (2D) materials such as graphene are usually served as matrix of the nanoparticles [17], [18]. For example, Co3O4-reduced graphene oxide (rGO) was investigated for the catalytic activation of PMS [15]. Very recently, MXene, a novel 2D layered material was applied as a supporting material for Co3O4 nanoparticles [19]. However, though these advanced 2D materials possess extremely large surface area, their industrial-scale production is still a big challenge.
Biochar is an emerging and low-cost carbonaceous material produced from low-cost/waste biomass residues. Its industrial-scale production is already possible. Its promising potential in agricultural and environmental applications makes it receiving considerable interests [20], [21]. Previous studies have demonstrated the significant role of biochar amendment in improving the soil fertility as well as reducing the emission of greenhouse gas (GHG) [22], [23]. Moreover, the biochar is recognized as a competitive candidate for the removal of organic and inorganic contaminants from water and soil [24], [25], [26], [27], [28], [29], though the efficacy is highly dependent on its physical and chemical properties. Recently, Fang et al. [30] revealed that biochar could effectively activate H2O2 to generate OH. The significant role of persistent free radicals (PFRs) in the biochar was elucidated. Similarly, the authors also investigated the persulfate (PS) activation by biochar and detected OH and SO4− in the biochar/PS system through the electron paramagnetic resonance (EPR) technique [31]. These findings provide a new insight into the biochar’s properties and environmental implications.
Despite of the inspiring findings, the role of biochar for water treatment is sometimes overestimated due to its limited functionalities, inherited from the feedstock after pyrolysis. The raw biochar has limited ability to remove contaminants from aqueous solutions [26], particularly for heavily polluted water. Alternatively, due to its abundant functional groups and well-defined porous structure, biochar exhibits intriguing properties for the rational design of functional materials. Typically, biochar can be utilized to disperse and stabilize nanoparticles to enhance their reactivity for catalytic reactions [32], [33]. More importantly, biochar-based adsorbents/catalysts can exert beneficial win-win effects for both carbon sequestration and water pollution control when using for the removal of aqueous contaminants [34]. For instance, biochar was reported as supporter for nanoscale zero valent iron (nZVI) to reduce the aggregation [33], [35]. The resulted composite exhibited excellent activity in decomposing PS to produce SO4− [35].
In this study, the Co3O4 nanoparticles were synthesized through a facile process with the assistance of biochar for the first time. Unexpectedly, the resultant product, BC-Co3O4, showed an extraordinary reactivity towards decomposition of PMS. The degradation of ofloxacin (OFX), a ubiquitous antibiotic by the developed SR-AOP was investigated. The reactive species were identified by radical scavenging experiments and electron paramagnetic resonance (EPR) technique. The underlying mechanism was tentatively elucidated based on the surface elemental analysis.
Section snippets
Materials and reagents
Rice straw derived biochar (pyrolysis temperature: 450 °C) was supplied by a local manufacturer. The powdered biochar was passed through a sieve (60 mesh) to ensure the particle size was regularly smaller than 250 µm prior to the use.
A triple salt of the sulfate commercially known as Oxone (KHSO5·KHSO4·K2SO4, ≥47% KHSO5 basis) obtained from Aladdin, China was used as the PMS source. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), ammonia solution (28%), and dimethyl formamide (DMF) were obtained
Characterizations
From the electron microscopic images, it was observed that the Co3O4 nanoparticles aggregated together and formed micro-scale rod-like structures, whereas, the regular structure seemed to be cracked in the BC-Co3O4 product (Fig. S1). Through the TEM images, it could be found that the size of the Co3O4 nanoparticle was around 10–20 nm. Although no obvious difference was observed from the SEM images, an apparent porous structure was clearly recognized in the TEM image of BC-Co3O4 sample. It was
Conclusions
In summary, an extraordinary degradation of OFX was observed in biochar-assisted-synthesized Co3O4/Oxone system. The derived BC-Co3O4 exhibited high specific surface area and small pore size. Accordingly, more oxygen species were adsorbed on the BC-Co3O4, which was beneficial for the formation of Co-OH groups. Through the radical scavenging study and EPR analysis, it was found that both SO4− and OH contributed to the degradation process. The degradation process was favored at neutral and weak
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No. 51608274), National Science and Technology Major Project (No. 2017ZX07204001-06), the Research Foundation of Jiangsu Environmental Protection Department (No. 2017002), and the Fundamental Research Funds for the Central Universities (KYZ 201619, KJQN 201749). Dr. Rongjun Bian from Nanjing Agricultural University is appreciated for his kind help in providing biochar samples and discussion.
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