Enhanced trichloroethylene dechlorination by carbon-modified zero-valent iron: Revisiting the role of carbon additives
Graphical abstract
Introduction
Zero-valent iron (ZVI) has been widely investigated over the past few decades for in situ remediation of groundwater and soil contaminated with chlorinated hydrocarbons (e.g., trichloroethylene (TCE)) (Gillham and O’ Hannesin, 1994; Matheson and Tratnyek, 1994; Wilkin et al., 2019). However, some intrinsic demerits of ZVI, including low reactivity and selectivity (i.e., reaction with target contaminants vs. H2O/H+/O2) toward target contaminants, still exist and limit its broad application (Fan et al., 2017b; Guan et al., 2015). To overcome these drawbacks, the combination of ZVI with other functional materials like noble/transition metals (e.g., Pd, Co, Ni, and Cu) (Cwiertny et al., 2006; Lien and Zhang, 2007; O’Carroll et al., 2013), inorganic clay minerals (e.g., silica (Zhan et al., 2008), zeolites (Kim et al., 2013), diatomite (Sheng et al., 2016), bentonite (Li et al., 2015)), and sulfur (Dong et al., 2018; Shao et al., 2018; Su et al., 2018) to form iron-based composites has been well documented (Guan et al., 2015). Historically, iron-based bimetals had gained great interests because of their ability to increase both the reduction rates and formation of more completely dechlorinated products through accelerating iron corrosion or serving as a hydrogenation catalyst. However, this positive effect of metal additives has now been confirmed to be short-lived, and on the other hand, it may cause a secondary contamination of heavy metals, so the interest in applying bimetals in practice has gradually waned (O’Carroll et al., 2013). With respect to the inorganic clay minerals, although their introduction could enhance the ZVI reactivity to some extent, they have not exhibited impressive performance or potentials guaranteeing their field application. Very recently, ZVI pretreatment with elemental sulfur or sulfur compounds has attracted increasing attentions since this process could significantly elevate both the reactivity and selectivity of ZVI (Fan et al., 2017b; Li et al., 2018). Although sulfidation was believed to be superior over many other ZVI performance enhancing methods (Fan et al., 2019), prior studies also demonstrated its enhancing effect was contaminant-specific and may impede the performance of ZVI toward some pollutants (e.g., carbon tetrachloride (CT), acetylene, nitrate, and so on) (Gu et al., 2017; Qiao et al., 2018). So this method has its own limitations and caution about its full scope of potential applications should be taken when it was applied. Exploring other suitable materials to effectively facilitate ZVI performance is thus still an urgent demand.
Carbonaceous materials (e.g., activated carbon (AC)), the commonly used materials in water purification, have many promising properties such as excellent molecular adsorption ability, electronic conductivity, chemical stability, and environmental friendliness (Fan et al., 2017a; Li et al., 2020a, b; Sun et al., 2019). These unique properties make them ideal candidates for ZVI modification and the benefits of integrating the redox reactivity of iron and the adsorptive ability of carbon are commonly expected by researchers. However, contradictory results about the influence of carbon additives on the decontamination process of ZVI are always reported in literature (Bleyl et al., 2012; Firdous and Devlin, 2018; Gao et al., 2015; Mackenzie et al., 2012, 2016; Wang et al., 2019; Zhan et al., 2011a). Some researchers contend that the introduction of carbonaceous materials could increase the reactivity of ZVI due to one or a combination of the following reasons: (i) facilitating the accumulation of contaminants (especially for hydrophobic contaminants at low concentrations) and thus their subsequent interaction with iron, (ii) accelerating ZVI corrosion by forming galvanic cells, and (iii) acting as carriers of ZVI particles and preventing them from aggregation (particularly for nanosized ZVI (nZVI)) (Mackenzie et al., 2012, 2016; Yang et al., 2017; Zhan et al., 2011a, b). For example, Mackenzie and co-workers have developed a Fe-C composite (named Carbo-Iron) by incorporating iron into colloidal activated carbon through wetness impregnation procedures and evaluated its decontamination performance in both laboratory and pilot scale (Mackenzie et al., 2012, 2016). They found that the Carbo-Iron materials not only hold advantages in facilitating the degradation rates of chlorinated hydrocarbons but also seemed to be able to extend the lifetime of nZVI particles (Mackenzie et al., 2012, 2016). Recently, these authors further reported that the introduction of sulfidation treatment could further improve the long-term performance of Carbo-Iron materials (Vogel et al., 2019a).
Nonetheless, other researchers claim that, although carbon additives may mediate the reduction pathways of contaminants, they cannot enhance the ZVI reactivity in general and even dampen it because the reactive sites provided by carbon inclusions are not as reactive as that of iron (Dries et al., 2004; Gao et al., 2015; Jafarpour et al., 2005; Oh et al., 2002, 2004). For instance, Gao et al. (2015) found that, compared to the unamended iron, the Fe-C material composite slightly decreased the TCE dechlorination rate and they attributed this phenomenon to the carbon additives’ role as a barrier for electron transfer between TCE and ZVI. In addition, previous studies about the influence of carbon impurity on the performance of cast iron also reflected a negative role of carbon inclusions (Burris et al., 1998; Velimirovic et al., 2013a, b). Specifically, compared to the high purity electrolytic iron, the less pure cast iron always contains 0.1–4.0 wt.% carbon and this carbon impurity is found to be able to retard the rates of contaminants removal by a factor of 4–100 (Sun et al., 2016).
Taken together, some key questions have arisen as to why carbon materials can exhibit opposite influences on ZVI performance, which kind of carbon materials are suitable for preparing iron-carbon composites and which properties of carbon determine the performance of amended ZVI. The porosity and adsorption ability have been proposed to be crucial for obtaining the expected synergistic effect of carbon and iron in Carbo-Iron systems (Mackenzie et al., 2012). However, this explanation is ambiguous and cannot fully address the abovementioned issues. In our opinion, although it is not emphasized in most previous studies, the electronic properties of carbon should play important roles in carbon-modified ZVI system due to (i) electron transfer from iron to target contaminants is fundamental for the occurrence of reduction reactions and (ii) the major reactive sites will shift from iron surface to carbon surface upon the introduction of carbon materials (Gao et al., 2015; Tang et al., 2011). So it can be inferred that if the electron conducting property of carbon is poor, the accumulation of contaminants on carbon surface may not favor their subsequent reaction with ZVI. Instead, an inhibiting effect of carbon on ZVI performance would be more likely to be observed. One means of validating this hypothesis would be to compare the reactivity of a suite of iron-carbon composites with different characteristics. However, previous studies considering multiple iron-carbon composites have been limited, and most of them failed to estimate the influence of additive loading on ZVI performance under identical conditions. As the additive loading is related with the configuration of iron-carbon composites, its omission complicates attempts to develop a reactivity trend that could help identify the potential roles of carbon additives.
Therefore, in the current study, four carbon materials, including two activated carbons with high surface area and two carbon fibers (CF) with high electronic conductivity, are selected and their influences on ZVI performance are systematically examined by fabricating carbon-modified ZVI with a ball-milling method. Note that, as a commonly used mechanochemical synthesis approach (Kubota et al., 2019; Xiao et al., 2020), the ball-milling process has many advantages, including free of potentially harmful solvents and external heating, short reaction times, and simple operational handling. TCE, a typical chlorinated solvent in contaminated groundwater, was chosen as a probe and the dechlorination rate, products, and H2 generation were employed as indicators of ZVI performance. In sum, the specific objectives of this work are to (i) quantitatively compare the reactivity and selectivity of ZVI toward TCE reduction with and without carbon modification, (ii) gain deep insights into the possible roles played by carbon additives. This study may also shed lights on the development of strategies to improve ZVI performance through carbon amendment.
Section snippets
Materials
The high purity (>99% Fe°) granular iron particles with a d50 of 46.2 μm were purchased from Alfa Aesar Chemicals Co. (China). Two powder activated carbons were obtained from Macklin Chemical Co. (China) (termed AC1) and Aladdin Chemicals Co. (China) (termed AC2), respectively. Two carbon fibers were purchased from Cangzhou Zhongli New Materials Co. (China) (termed CF1) and Shenzhen Zhongsenlinhang Technology Co. (China) (termed CF2), respectively. Additional details on these materials and the
Influence of different carbon additives on the reactivity of ZVI
Fig. 1 depicts the kinetic of TCE disappearance and its daughter products formation in different ZVI systems. Note that the iron-carbon particles shown in Fig. 1 were prepared with different carbon materials at a constant loading of 1.0 wt.%. As a comparison, the adsorption of TCE by each carbon material and the degradation of TCE by unamended ZVIbm were also examined and shown in Fig. 1a. Although TCE could be adsorbed by all the four tested carbon materials, negligible degradation products of
Conclusions
Surface modification with carbon represents an attractive approach to improve the ZVI performance but general conclusions about the role of carbon additives have not been reached previously, which limits its use in practice. The results of this study confirm that carbon modification is capable to enhance both the reactivity and selectivity of ZVI, while this enhancing effect is highly dependent on the carbon loadings considering the carbon contents could not only affect the adsorption of target
CRediT authorship contribution statement
Xiaohong Guan: Conceptualization, Methodology. Xueying Du: Validation, Formal analysis, Investigation. Meichuan Liu: Validation, Resources. Hejie Qin: Formal analysis. Junlian Qiao: Methodology, Visualization. Yuankui Sun: Writing - review & editing, Project administration, Funding acquisition.
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.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (Grants 21876129 and51608431).
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