Discovering the potential of an nZVI-biochar composite as a material for the nanobioremediation of chlorinated solvents in groundwater: Degradation efficiency and effect on resident microorganisms
Graphical abstract
Introduction
Originally, biochar-based carbon materials (BC) were utilized as a soil amendment to increase the fertility and stability of disturbed soils (Lehmann et al., 2011). Currently, interest in BC is increasing dramatically, and the material has become intensively studied as an inexpensive sorbent for many environmental applications (Cha et al., 2016). Its sorption capability allows, in addition to other aspects, the removal and immobilization of many pollutants on the surface of its porous carbon structure (Sophia A. and Lima, 2018). BC was proven to have positive effects on resident microbes, and its surface can serve as a support for biofilm formation or as a source of carbon, which is represented by adsorbed organic compounds (Han et al., 2017; Zhu et al., 2017). Currently, composite materials based on BC or modified BCs are being designed and developed to improve the sorption efficiency or specificity to target pollutants. Depending on the type of particles in the composite, the resulting material can have a combination of sorptive and reactive properties (Tan et al., 2016). One of the promising composite materials designed for remediation applications is a composite of nanoscale zerovalent iron (nZVI) BC, where the nZVI particles are incorporated into the BC matrix (Wang et al., 2019). Currently, materials based on nZVI are commonly used in the remediation industry for the degradation of many pollutants, including chlorinated solvents, e.g., chlorinated ethenes (CEs), as well as for the removal of various hazardous elements. Despite the high efficiency of nZVI in the removal of inorganic and organic pollutants, there are certain concerns regarding toxic nZVI properties toward microorganisms and possible environmental risks at relevant application concentrations (Hjorth et al., 2017; Semerád et al., 2018; Semerád and Cajthaml, 2016). In contrast, BC, which has been recently reported as an environmentally friendly material, can serve as a carrier matrix, reducing the negative properties of nZVI (e.g., unspecific reactivity, toxicity, etc.), and improving the nZVI degradation efficiency (Tan et al., 2016). In some studies, the positive effects of the BC matrix on ZVI particles are also discussed. These effects include the prevention of particles against aggregation and corrosion, the formation of an oxide film on ZVI as well as the increase in contact area with contaminants or the buffering function of acid-washed BC maintaining the reactivity of ZVI (Devi and Saroha, 2014; Han et al., 2015; Ying et al., 2015). Moreover, nZVI together with BC can act as an electron-transfer mediator enhancing the reductive transformation of adsorbed contaminants, where the surface functional groups are involved in the catalytic enhancement of the electron transfer chain (Oh et al., 2017; Yan et al., 2015), The BC matrix can also contribute to better stability and mobility of nZVI in groundwater compared to bare nZVI (Su et al., 2016). BC composites containing nZVI particles were prepared and studied for the removal of selected contaminants mostly under laboratory conditions and using model water solutions. The treatment of real contaminated water was documented only in a few studies (Wei et al., 2018). The mechanism is based on a combination of adsorption and degradation, and the reactivity is often linked with the pH of the solution (Fan et al., 2019).
The biotransformation of chlorinated solvents is well described, and the ability of microbes to reduce the number of chlorine substituents of CEs (and eventually to produce nonchlorinated transformation products) has been proven by many authors and is summarized, e.g., by Dolinová et al. (2017). Despite the advantageous combination of biotic and abiotic remediation, and the current success of this process in removing many CEs, the reaction mechanisms are not yet well described. Therefore, the main goal of this study was to enhance knowledge regarding the possible involved mechanisms and to evaluate the potential of a novel nZVI/BC composite material (prepared by a method following “green chemistry principles”) for nanobioremediation (nanoremediation with a subsequent biostimulation step) under simulated in situ conditions in contaminated groundwater. The combination of the positive aspects of BC with regard to microbes and the degradation efficiency of nZVI toward various pollutants has the potential for promising results that is reflected in this study. Taken together with the decrease in contamination caused by nZVI, BC could improve the environment for the resident microbial communities and for the implementation of the biostimulation step. The results of this study, employing iron (bio) cycling during nanobioremediation by nZVI, could be consequently used in situ to improve the nanobioremediation efficiency of groundwater restoration.
Section snippets
Preparation of materials
Softwood sawdust (pine and spruce from local sources; diameter was below 2 mm) was mixed with an iron precursor (hematite; Bayferrox 110 powder; iron oxide red pigment, α-Fe2O3; and LANXESS inorganic pigments; predominant particle size was 90 nm) in a water suspension (1 g hematite + 5 g sawdust + 20 mL water) using a using a laboratory homogenizer (IKA Ultra-Turrax 25 T; IKA®-Werke GmbH & Co. KG, Staufen, Germany) for 5 min. Then, the modified sawdust was dried in a laboratory drier
Material preparation and analysis
The sawdust pretreated by a water suspension of iron oxide pigment was pyrolyzed, resulting in the reduction of iron oxides to nZVI particles on the surface or within the porous BC matrix in this one-step procedure. Fig. 1 shows a comparison of SEM images between the unmodified BC and the nZVI/BC composite. In the case of the composite, the biochar matrix was decorated by isolated iron nanoparticles or their aggregates. EDS measurements confirmed the presence of iron in the composite material
Conclusion
The results of this study showed promising properties of an organic carbon-based nZVI composite for combination with a biostimulation step to remove CEs from groundwater. Although the content of bare iron dramatically decreased within the first 7 days of the experiment, the presence of nZVI in the composite material was found to be crucial for the success of the CE transformation experiment. The respective BC-treated and only biostimulated samples showed a certain decrease in the content of
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
The work was supported by the Center for Geosphere Dynamics funded by Charles University (UNCE/SCI/006). The authors acknowledge the assistance provided by the Research Infrastructures NanoEnviCz (Project No. LM2018124), which is supported by the Ministry of Education, Youth and Sports of the Czech Republic and the Investment Funds in the frames of Operational Program Research, Development and Education—project Hybrid Materials for Hierarchical Structures “HyHi” (Project No.
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