Treatability of hexabromocyclododecane using Pd/Fe nanoparticles in the soil-plant system: Effects of humic acids

Highlights

• Degradation of HBCD by Pd/nFe in presence of plant and humic acids was investigated.

• HBCD was debrominated by Pd/nFe in both aqueous and soil conditions.

• HAs showed considerable influence on the degradation and bioaccumulation of HBCD.

• The nano-bio remediation is a potential strategy to remove HBCD from the soil.

Abstract

Hexabromocyclododecane (HBCD) is a persistent organic pollutant that accumulates in soil and sediments, however, it has been difficult to degrade HBCD with developed remediation technologies so far. In this study, degradation of HBCD by bimetallic iron-based nanoparticles (NPs) under both aqueous and soil conditions considering the effects of humic acids (HAs) and tobacco plant was investigated. In the aqueous solution, 99% of the total HBCD (15 mM) was transformed by Pd/nFe (1 g L⁻¹) within 9 h of treatment and the HBCD debromination by Pd/nFe increased with the addition of HAs. In the soil system, 13%, 15%, 41% and 27% of the total HBCD were removed by treatments consisting of plant only, plant with HAs, plant with NPs and plant + NPs + HAs, respectively, compared to the HBCD removal in an unplanted soil. The 221–986 ng/g of HBCD were detected inside the plant after the treatments, and HAs showed considerable influence on the selective bioaccumulation of HBCD stereoisomers in the plant. Overall, this approach represents a meaningful attempt to develop an efficient and eco-friendly technology for HBCD removal, and it provides advantages for the sustainable remediation of recalcitrant emerging contaminants in soils.

Introduction

Thousands of xenobiotic pollutants from various human activities are being introduced into the environment. Over the last two decades, the generation of persistent organic pollutants (POPs) has emerged as a big environmental concern due to their strong potential for bioaccumulation, biomagnification, persistence, long-range transport and toxicity to the ecosystem (Santillo and Johnston, 2003). In particular, 1,2,5,6,9,10-hexabromocyclododecane (HBCD, C₁₂H₁₈Br₆), the most widely used cycloaliphatic brominated flame retardant (BFR), has been detected in various environmental matrices, especially in soil and sediment (Gao et al., 2011). Due to its physicochemical properties, HBCD was listed as a new POP under the Stockholm Convention treaty in 2014. As HBCD is an emerging contaminant, research on HBCD remediation is in its infancy (Davis et al., 2005). To date, several studies on the HBCD remediation have been reported; for example, it was proposed that HBCD could be photodegraded (Zhou et al., 2014). However, HBCD that is heterogeneously distributed in soils, sediments, and water is not readily exposed to light. Thermal degradation, electrochemical reduction, and mechanochemical technologies have been studied for HBCD degradation (Barontini et al., 2001; Wagoner et al., 2014; Zhang et al., 2014), but these technologies had low energy efficiencies and also could be costly for facilities.

Nanotechnology has been identified as a promising remediation strategy for organic pollutant degradation. In this category, nanoscale zerovalent iron (nZVI) and nZVI-based nanoparticles (NPs) are known to degrade various POPs with fast kinetics and high efficiency. To date, there have been several reports concerning the HBCD degradation using nanomaterials, particularly with bare nZVI (Tso and Shih, 2014), FeS (Li et al., 2016a) and sulfidated nZVI (S-nZVI) (Li et al., 2017). The usual strategy for increasing the reactivity of nZVI is the addition of transition metals such as Pd, Pt, Cu, and Ni on the nZVI surface for reducing their passivation (Stefaniuk et al., 2016). In particular, palladized nZVI (Pd/nFe) has been studied extensively with its hydrodehalogenation reactivity (Alonso et al., 2002). However, the application of bimetallic nZVI to promote the degradation of HBCD has not previously been investigated. In addition, most of the references were studied on the removal of POPs in aqueous solutions, and only a few reports dealt with soils and sediments. In most cases, the effectiveness achieved in soil remediation was not as good as that in water because of the high octanol-water partition coefficient (K_ow), the high tendency to adsorb onto the solid surfaces of environmental compartments and the formation of intermediates that may be more toxic than the parent contaminants. Nevertheless, it should be emphasized that the soil is a much more difficult and demanding matrix than water in the remediation field (Stefaniuk et al., 2016).

Employing appropriate technologies to remediate contaminated soils is crucial due to certain site-specifications such as public acceptability and environmental sustainability of technologies. These requirements increase the need to employ eco-friendly techniques, especially for the soil remediation (Abhilash et al., 2012). In case of HBCD, it has various stereoisomers with dissimilar physical properties, resulting in varying accumulation behaviors and interactions with plant and microbes in soils (Davis et al., 2005; Zhu et al., 2016). Our previous study determined that the microbes in an HBCD-contaminated rhizosphere were a promising candidate for the bioremediation of HBCD (Le et al., 2017). Therefore, applying the combined treatment of nanomaterials with plant could also be a meaningful research trial for the remediation of soils contaminated with POPs. Another aspect of the soil remediation is that natural environmental matrices commonly contain high amounts of natural organic matters (NOM), especially humic acids (HAs), which interact strongly with organic chemicals. It has been well demonstrated that the POPs sequestration capacity of the soil is strongly related to the proportion of NOM. On the other hand, the addition of dissolved organic matter (DOM) is currently considered to be a valid strategy to accelerate bioremediation in contaminated sites (Cai et al., 2017; Plaza et al., 2009). For example, HAs can accelerate the degradation of organic pollutants by increasing their solubility and enhance their diffusive mass transfer, promoting their bioavailability to microorganisms (Tejeda-Agredano et al., 2014). HAs can also act as an electron transfer mediator in the chemical reduction of organic pollutants (Li et al., 2016b), resulting in enhancement of the degradation rate of halogenated compounds. Therefore, HAs would be a good supplement for bioremediation of contaminated soils and it is important to understand the role of HAs before it is applied to the remediation field.

Here, we investigated the degradation of HBCD in a soil system using bimetallic nZVI (Pd/nFe) under the influences of plant and HAs. Since HBCD has not been tested by bimetallic nZVI yet, a pre-test was conducted in an aqueous solution to understand its HBCD degradation mechanism and pathway. Further, we expected to observe the HBCD remediation with nanomaterials in soils under the influences of HAs and plant. In short, our observations might be the beginning for the remediation strategy using integration of the nanotechnology and bioremediation, involving plant-assisted treatment can be provide promising advantages for the remediation of recalcitrant emerging contaminants in the soil environment.