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

Highlights

• Novel nano iron-biochar composite (nZVI/BC) was tested to remediate contaminated groundwater.

• NZVI/BC followed by biostimulation transformed chlorinated ethenes (CEs).

• CEs were dechlorinated into less toxic nonhalogenated volatiles.

• 16S rRNA sequencing revealed development of an organohalide-respiring community.

• Living bacterial biofilm was detected on nZVI/BC using fluorescence microscopy.

Abstract

Abiotic and biotic remediation of chlorinated ethenes (CEs) in groundwater from a real contaminated site was studied using biochar-based composites containing nanoscale zero-valent iron (nZVI/BC) and natural resident microbes/specific CE degraders supported by a whey addition. The material represented by the biochar matrix decorated by isolated iron nanoparticles or their aggregates, along with the added whey, was capable of a stepwise dechlorination of CEs. The tested materials (nZVI/BC and BC) were able to decrease the original TCE concentration by 99% in 30 days. Nevertheless, regarding the transformation products, it was clear that biotic as well as abiotic transformation mechanisms were involved in the transformation process when nonchlorinated volatiles (i.e., methane, ethane, ethene, and acetylene) were detected after the application of nZVI/BC and nZVI/BC with whey. The whey addition caused a massive increase in bacterial biomass in the groundwater samples (monitored by 16S rRNA sequencing and qPCR) that corresponded with the transformation of trichloro- and dichloro-CEs, and this process was accompanied by the formation of less chlorinated products. Moreover, the biostimulation step also eliminated the adverse effect caused by nZVI/BC (decrease in microbial biomass after nZVI/BC addition). The nZVI/BC material or its aging products, and probably together with vinyl chloride-respiring bacteria, were able to continue the further reductive dechlorination of dichlorinated CEs into nonhalogenated volatiles. Overall, the results of the present study demonstrate the potential, feasibility, and environmental safety of this nanobioremediation approach.

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.