Modification of zero valent iron nanoparticles by sodium alginate and bentonite: Enhanced transport, effective hexavalent chromium removal and reduced bacterial toxicity
Graphical abstract
Introduction
Due to the strong reducing capacity and easy to fabricate, zero-valent iron (ZVI) particles especially the nanoscale ZVI (nZVI) have been recently demonstrated to be highly effective catalytic materials in environmental remediation and contamination treatment (Crane and Scott, 2012; Fu et al., 2014; Morshed et al., 2019; Wang et al., 2019). For example, nZVI has been found to have exceptional performances in heavy metals removal especially hexavalent chromium (Cr(VI)) remediation in groundwater (Park et al., 2019; Kanel et al., 2007; Lou et al., 2019; Li et al., 2019). However, the rapid agglomeration/sedimentation and the reduced mobility limit the practical application of nZVI in environmental remediation especially for groundwater remediation (Liu et al., 2007; Shi et al., 2011). To decrease the aggregation and improve their transport, different types of polymers or surfactants, such as sodium carboxymethyl cellulose (CMC) (He and Zhao, 2007), starch (Zhang et al., 2017), guar gum (Balachandramohan and Sivasankar, 2018), polyethylene glycol (PEG) (Wang et al., 2015), polyacrylic acid (PAA) (Lerner et al., 2012) and Tween 20 (Kanel et al., 2007), and so on, were employed to modify nZVI. Comparing with the bare nZVI, the stability and mobility of modified nZVI are greatly improved. In addition, zeolite (Huang et al., 2019), mesoporous carbon (MC) (Shi et al., 2019), resin (Chanthapon et al., 2018), active carbon (Kamarehie et al., 2018), diatomite (Sheng et al., 2017), graphene oxide (Mehrabi et al., 2019) and biomass (Wang et al., 2019) have also been recently shown to be effective supporters to promote the stability of nZVI.
Due to the presence of abundant free carboxyl groups on its molecular chain (Wang et al., 2017; Huang et al., 2016a; Fajardo et al., 2013), sodium alginate (SA) has been widely used to adsorb toxic pollutants from soil and groundwater (Ely et al., 2009; Zhao et al., 2017). SA has also been employed as dispersant to modify nanoparticles (Wu et al., 2017). Very recently, Li et al. (2019) used SA to modify nZVI and found that the mobility of modified nZVI was improved. However, the sedimentation, transport, as well as the bacterial toxicity of SA-nZVI have not been systematically examined and thus further studies are required. Because of the higher cation exchange capacity, inter-lamellar surface and expansion coefficient (Ayari et al., 2005; Zhang et al., 2011a; Li et al., 2015; Ezzatahmadi et al., 2017), bentonite (one type of typical clay particles commonly presents in natural environments), has been widely employed as adsorbent to remove pollutants from water (Wu et al., 2017; Eloussaief and Benzina, 2010). Previous studies have shown that bentonite can serve as mobile phase to facilitate the transport of colloids in porous media (Cai et al., 2014; Abdel-Fattah et al., 2013). Bentonite might be an ideal supporter for nZVI to decrease the aggregation and increase the transport of nZVI. To the best of our knowledge, bentonite has not been previously employed to modify and facilitate the transport of nZVI in porous media.
The objectives of this study were: 1) to fabricate and characterize both sodium alginate and bentonite modified nZVI; 2) to investigate the sedimentation and transport of these two types of modified nZVI in porous media, as well as the effects of flow velocity on their transport behaviors; 3) to determine the Cr(VI) removal efficiency of two types of modified nZVI as well as their toxicity towards bacteria. The results showed that two types of modified nZVI fabricated in present study have the potentials to be used to remediate Cr(VI) contaminated groundwater.
Section snippets
Fabrication and characterization of two types of modified nZVI
SA modified ZVI nanoparticles were prepared by a pre-aggregation stabilization method with slight modification (He et al., 2007). Specifically, 0.298 g FeSO4Ā·7H2O was dissolved in anaerobic ultrapure water with desired sodium alginate stock solution (1 g/L) in 500 mL conical flask (purged with nitrogen for 20 min). After stirring for 15 min in N2 environment to ensure formation of the Fe2+-SA complex (pre-stabilization stage), 8 mL fresh NaBH4 solution (BH4ā/Fe2+ molar ratio = 2) was added into
Characteristics of modified nZVI
The SEM images show that spherical zero-valent iron particles with diameters between 100 and 200 nm form chain-like aggregates due to the magnetic force (Fig.1a). Similar observation has also been reported previously (Jiemvarangkul et al., 2011; Liu et al., 2016). Due to the presence of ample functional groups such as carboxyl and hydroxyl groups in sodium alginate (SA) (Huang et al., 2016b), coating of SA on the surface of nZVI can greatly disperse and stabilize the zero-valent iron particles
Conclusion
In this study, sodium alginate (a natural polysaccharide) and bentonite (a natural clay) are employed to modify nZVI. Comparing with bare-nZVI, both SA- and Ben-modified nZVI exhibit higher stability (lower sedimentation) and higher mobility due to the smaller sizes and negative surface charges. Increasing amount of SA or bentonite would lead to the increased stability and mobility. Increasing flow velocity can increase the mobility of iron nanoparticles. The removal efficiency of Cr(VI) and
Contributions
Mengya Zhang and Meiping Tong conceived and designed the experiments.
Mengya Zhang, Meiping Tong, Wen Liu, and Peng Han analyzed the data.
Mengya Zhang, Kexin Yi, and Xiangwei Zhang performed the experiments.
Mengya Zhang and Meiping Tong wrote the manuscript. All authors discussed the results and commented on the manuscript.
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 work was supported by the National Nature Science Foundation of China under Grants No. 51779001 and Fund for Innovative Research Group of NSFC under Grant No. 51721006.
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