Chlorophenol compounds (CPCs) are frequently found in agricultural sites, water disinfected by chlorination, and pulp and paper mill effluents, which are carcinogens and widespread persistent organic pollutants in waters and soils (Vipul et al., 2021). As a priority pollutant, they are characterized as highly toxic, persistent, and easily mutagenic, and they can accumulate in organisms, such as algae, fishes, and mammals even humans (Nsh et al., 2021). 2-chlorophenol (2-CP), in particular, is one of the most representative chlorophenol compounds, which are highly stable and carcinogens because of their phenyl structure and the presence of chlorine and are listed as priority pollutants by China and the US Environmental Protection Agency (Li et al., 2020; Majumder and Gupta, 2007). The pollution of 2-CP in groundwater and wastewater should be concerned because of its toxicity and almost non-biodegradability. Removal of 2-CP from wastewater has been treated by biological and physicochemical methods (Loh and Wu, 2006; Miguel et al., 2018); these methods can effectively degrade 2-CP under certain conditions. However, the conventional methods are still problematic, such as when the content of 2-CP is relatively low, it cannot be completely degraded, or biodegradation requires strict environmental conditions. Therefore, it is urgently necessary to develop more widely applicable methods for 2-CP degradation.
In recent years, advanced oxidation processes (AOPs), including ozone (O3), persulfate (PS) activation, and hydrogen peroxide (H2O2) activation, have been developed as highly promising methods for degrading toxic and recalcitrant organic compounds (Liu et al., 2017; Xl et al., 2021; Zhou et al., 2020). Among them, persulfate-based (peroxodisulfate, PDS, and peroxymonosulfate, PMS) advanced oxidation processes have been widely applied to remove organic contaminants in wastewater and groundwater (Chen et al., 2021; Liu et al., 2020). Persulfate is found more stable in the subsurface as compared to H2O2 and O3, for it persisted in the subsurface and can be injected at high concentrations, transported in porous media, and would undergo density-driven and diffusive transport into low-permeability materials (Huang et al., 2002). Under mild conditions, PS can be activated via photolysis (UV and visible), alkali, heat, electron beam, and transition metals, forming the hydroxyl radical (∙OH) and sulfate radical anion (SO4∙−)(Matzek and Carter, 2016; Muhammad et al., 2019; Peng et al., 2015; Wang and Wang, 2018). SO4∙− has a high reduction potential (E0 = 2.5–3.1 V), and is nonselective compared with ·OH, and it can quickly mineralize most of the organic pollutants into inorganic compounds (Devi et al., 2016; Gao et al., 2012).
Transition metals have been widely applied to activate PS because of their effectiveness and cost-effectiveness(Wang et al., 2021). Among various transition metals, nanoscale zero-valent iron (nZVI) is considered a simple, economical, and environmentally friendly material, which can slow-release Fe2+ to activate PS (Kim et al., 2018). The removal efficiency of 2,4-dichlorophenol using the nZVI/PS system was better than those of the Fe2+/PS and nFe3O4/PS; the maximum degradation rate reached around 92.5% within 150 min (Li et al., 2015). However, nZVI catalysis has low PDS activation efficiency because of its aggregation, passivation, and poor electron transfer(Zhao et al., 2019b).
To improve stability, various porous media (e.g., coal fly ash (Chen et al., 2018), clay minerals (Li et al., 2016), carbon-based materials (Pirsaheb et al., 2018)) have been applied as supports of nZVI to boost their reactivity. Biochar (BC) is considered one of the best carriers for nZVI due to its low cost, porous structure, and high surface area (Hao et al., 2021; Kumar et al., 2021). However, this method is characterized by low degradation efficiency due to slow electron transfer rate and passivation induced by biochar (Dong et al., 2018; Vogel et al., 2019). Recently, some studies have reported a sulfide-modified method to improve the removal efficiency, which ascribes to the enhancements of hydrophobicity of nZVI, production of ∙OH, and salt resistance (Cai and Zhang, 2021; Wu et al., 2021; Xu et al., 2019). The presence of sulfur could regulate the morphology of S-mZVI with a dispersed and spherical shape, and it could improve the activation performance of PS (Zhang et al., 2021). The feasibility and mechanism of sulfide-modified nanoscale zero-valent iron supported on biochar for the removal of TCE in groundwater remediation is investigated; the results showed that the BC@S-nZVI, combining the high adsorption capacity of BC and the high reductive capacity of S-nZVI, had a much better performance than the single S-nZVI or BC (Chen et al., 2020). Therefore, sulfide-modified nZVI (S-nZVI) catalysts have been paid more and more attention, and it is of great significance to explore the new S-nZVI composite using BC and improve its efficiency in practical application.
In this study, soybean residue biochar supported S-nZVI (BC@S-nZVI) synthesized in a one-step method has been developed to activate PS for the degradation of 2-CP in an aqueous solution. The effects of initial concentration values of 2-CP and PS, BC@S-nZVI doses, pH values, reaction temperature, and some anion concentration values on degradation efficiency were also investigated. Furthermore, the degradation mechanism and the contribution of each chemical factor in the reaction system have also been explored using scavengers of radicals under optimum mass ratios of PS and BC@S-nZVI.