Introduction

Due to their high hydrophobic and lipophilicity, polybrominated diphenyl ethers (PBDEs), brominated flame retardants used in a variety of industrial and consumer products (Cincinelli et al., 2012; LaGuardia et al., 2006), are strongly bound to soil and sediment particles, making the soil and sediment seriously contaminated (Chen et al., 2009; Luo et al., 2009), and posing a serious risk on human and environmental health (Hale et al., 2006; Hites, 2004; McDonald, 2002). As a consequence, the modified nanozero-valent iron has been intensively used for the degradation of PBDEs due to its large specific surface area and strong reducing potential (Fang et al., 2011; Qiu et al., 2011; Yu et al., 2012; Zhuang et al., 2011). However, several previous studies have demonstrated that nanoparticles were certified to have potential toxic effects on plants due to their novel physicochemical and structural (El-Temsah and Joner, 2012; Wang et al., 2016). For example, Wang et al.(2016) showed that higher concentrations of nZVI in soil induced rice chlorosis due to inhibited active iron transportation. Moreover, although the behaviors of PBDEs, such as uptake, bioaccumulation, translocation, and metabolism in the soil-plant system have been investigated by numerous studies (Huang et al., 2010, 2011; Vrkoslavová et al., 2010; Wang et al., 2011a), the effects of iron-based materials on the uptake of PBDEs by plants in the treated contaminated soil remain unclear. Furthermore, a series of process about translocation and accumulation of the residual contaminants and products have been taken place by the application of nZVI in soil (El-Temsah and Joner, 2013; Gardea-Torresdey et al., 2014). Our prior work successfully demonstrated that biochar-supported Ni/Fe nanoparticles (BC@Ni/Fe) performed significant removal of the decabromodiphenyl ether (BDE209) in soil with a removal efficiency of 87.7% (Wu et al., 2016a). However, the lack of understanding is particularly disconcerting in terms of the possible negative effects of the soil residual Ni/Fe nanoparticles and PBDEs on plants.

Biochar is a carbon rich by-product produced from the pyrolysis of organic matter under zero oxygen concentrations at relatively low temperatures (< 700 °C), which is widely used as a cost-effective soil amendment in environmental remediation due to its high sorption ability (Bolan et al., 2014; Mohan et al., 2014; Meyer et al., 2011). Many studies have shown that amendment with biochar in soil could enhance the removal of different heavy metals and organic contaminants, thus reducing their bioavailability, phytotoxicity, and uptake of contaminants on plants (Bolan et al., 2014; Mohamed, et al., 2017). For instance, Denyes et al. (2012) described that biochar additions significantly decreased the uptake of PCBs into plant tissue by 89% and the bioavailability of PCBs to earthworms by 88%. Mohamed et al. (2017) also showed that biochar not only reduced heavy metal (Pb, Ni, and Co) bioavailability and uptake but also improved soil fertility and increased plant growth rate by up to 145%. Nevertheless, there is limited information on the effect of biochar on the phytotoxicity, uptake, and translocation of PBDEs in the nZVI-based material-remedied soil.

Therefore, the objective of this study was to investigate the effects of the addition of biochar on the phytotoxicity, uptake, translocation, and accumulation of PBDEs by using Brassica chinensis as the model plants from the nZVI-based material-remedied soil. The influence of biochar on soil properties, such as pH and organic matter, was also studied.

Materials and methods

Materials and chemicals

A standard solution of decabromodiphenyl ether was purchased from Cambridge Isotope Laboratories (CIL, Andover, USA) and used to establish the standard curve. Decabromodiphenyl ether (BDE209 > 98%), ferrous sulfate (FeSO4·7H2O, > 99%), sodium borohydride (NaBH4 > 98%), nickel chloride (NiCl2·6H2O, > 99%), polyvinylpyrrolidone (PVP, K-30), and ethanol (EtOH, 99.7%) were purchased from Tianjin Damao Company. Acetonitrile and methanol (HPLC grade) were obtained from Tianjin Kermel Chemical Reagents Company. All chemicals were used as received without further purification.

The preparation and characterization of BC@Ni/Fe particles, Ni/Fe nanoparticles, and biochar were as demonstrated in our previous report (Wu et al., 2016a). The BET specific surface area of Ni/Fe, BC, and BC@Ni/Fe was 30.81, 352.59, and 82.18 m2/g, respectively. Also, the average particle size of Ni/Fe is 28.4 nm (Wu et al., 2016a, b).

Soil collection and preparation

The test soil (type of loam) was collected from the 0–20-cm depth zone at the Higher Education Mega Center, southern China (23° 03′ 12.59″ N, 11,323′ 25.81″ E), then air dried, homogenized, and stored at 4 °C after ground to pass through a 60 mesh. The soil sample characterizations are shown in Supplementary material, Table S1. Soil was artificially contaminated with a slight modification of the method previously reported by Wu et al. (2016a). The final concentration of BDE209 in the contaminated soil was 8.7 ± 0.9 mg/kg, and the recovery was 87.92 ± 7.5%.

The BC@Ni/Fe particles, Ni/Fe nanoparticles, and biochar (0.03 g/g) were added into 50 g of BDE209 contaminated soil, respectively. The mixture was homogenized and placed on a shaker (300 rpm, 25 °C). After reaction of 3 days, the soil samples were dried and stored for use. Control groups were also prepared.

Growth and harvest of plants

Soil culture experiments were conducted according to the methods of Wang et al. (2016) with a minor modification to test phytotoxicity. The soil was divided into the following five treatments: S 0 (BDE209-free soil), S 1 (BDE209-contaminated soil), S 2 (biochar-treated soil), S 3 (Ni/Fe nanoparticle-treated soil), and S 4 (BC@Ni/Fe-treated soil). The B. chinensis seeds were obtained from the Chinese Academy of Agricultural Sciences. The seeds were sterilized (0.5% w/v NaClO, 15 min), washed three times with tap water, and then thoroughly rinsed with deionized water.

The five kinds of soil were further used to conduct soil culture experiments in a set of 120-mm Petri dishes. All seed germination tests were conducted using the same plant species and seed densities. Each of Petri dishes were filled with 50 g of prepared soil and appropriate amount of distilled water. The dishes were placed in a biochemical incubator for balance at 25 °C for 48 h. Then, 20 seeds were placed inside each Petri dish and the seeded dishes were incubated in the biochemical incubator which was maintained at 25 °C during the day and 20 °C at night with a 16/8-h photoperiod and light intensity of 144 μmol m−2 s−1. During plant growth, DI water was added daily to maintain moisture content at 75% of its water holding capacity and seed germination was recorded. Triplicate pots of each treatment were carried out.

All treatments were harvested after 20 days and subsequently washed with tap water and DI water. The clean plant samples were then separated into roots and shoots and the length of shoots and roots was measured at once with a ruler (0.1-mm increments).The dry biomass was determined after the samples were kept in a drying oven at 70 °C for 48 h. For determining the concentration of Ni and Fe, the samples were dried at 105 °C for 48 h and then digested with 2 ml of nitric acid until less than 1 ml of the liquid. The concentration and constant volume of the solution were then measured via AAS.

The antioxidation ability analysis of B. chinensis seedlings

Plants treated for 20 days were harvested for enzyme assays. The antioxidation ability of the B. chinensis seedlings was evaluated by analyzing the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities. The ρ-nitroblue tetrazolium chloride (NBT) method was used to conduct the SOD activity analysis. The POD activity was detected via guaiacol assay. The CAT activity was determined following the literature (Wang et al., 2016).

Test of soil properties

To estimate the effect of different amendments on the soil characteristics after remediation, soil pH, available iron, organic matter, available phosphorus, and hydrolysable nitrogen were determined. Details of the analysis methods were described in Section S1.

Extraction and analysis of PBDEs

The dried samples were pulverized and spiked with surrogate standards of 13C-PCB208 and 13C-PCB141 prior to extraction. Then, the samples were extracted with a mixture of hexane/acetone (1/1, v/v) by Soxhlet extraction for PBDE analysis. Finally, the extract was cleaned up, concentrated, and analyzed in a gas chromatograph (GC, Agilent 6890)/mass spectrometer (MS, Agilent 5975) with negative chemical ionization (NCI) in the selected ion monitoring (SIM) mode and equipped with a DB5-MS (20-m × 0.25-mm i.d., 0.1-μm film thickness) capillary column. The detailed analysis methods have demonstrated in previous report (Fang et al., 2011).

Statistical analysis

All statistical analyses were performed with the SPSS 19.0 for windows. One-way ANOVA tests were used to determine the significant differences among different treatments. Statistical significance was set at p < 0.05. Means and standard deviation were calculated for triplicates. The level of significance is indicated by a letter.

Results and discussion

Phytotoxicity test

Notable differences in B. chinensis morphology were observed in the five treatments. Compared with those in the control group (S 0), plants in the treatments S 1 and S 3 were withered and yellow and had fewer, shorter, and thinner roots (Fig. 1A). As shown in Table 1, the biomasses in the five treatments decreased in the following order: S 2 > S 0 > S 4 > S 1 > S 3. The dry biomass of plants cultivated in S 1 and S 3 treatments was decreased by 39.9 and 43.5 mg compared with that in S 0 (blank treatment), respectively. It has been clear that BDE209 and Ni/Fe nanoparticles were both found to be toxic to plants (Wu et al., 2016b). Moreover, the dry weight of plants in S 4 increased by 23 and 27.2 mg compared with those in S 1 and S 3, respectively. The above results suggested that the growth of B. chinensis in treatment S 4 was significantly improved.

Fig. 1
figure 1

A The physiognomy and B the length of roots and stems in each treatment. Within the same plant tissue, the different letters represent significant differences between the treatments and the same letters are not significantly different (p < 0.05)

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Table 1 Dry biomass of the seedlings grown in each treatment
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As shown in Fig. 1B, the growth of plants was significantly inhibited in S 1 treatment, accompanying with the length of roots and shoots were 63.8 and 65.4% shorter than that of S 0, respectively. Compared with those in S 1, the root lengths of B. chinensis in S 2 and S 4 increased by 8.88 and 1.35 cm, while the shoot lengths increased by 3.99 and 2.08 cm, respectively. However, the root lengths and shoot lengths in S 3 treatment decreased by 30.3 and 26.8%, respectively, compared to those in S 1.

As shown in Fig. 1 and Table 1, the high content of BDE209 in S 1 has obvious toxic effect on the growth of B. chinensis seedlings. The toxic effects on the plants cultivated in S 3 was the most significant because nanomaterials, heavy metal ions, and PBDE residual in the treated soil all had an inhibit effects on the plant growth. Just as reported in the literatures, the growth of plant would be significantly inhibited after being polluted by heavy metals and organic pollutants (Ahammed et al., 2012; Su et al., 2016). According to our results, the plants cultivated in S 2 and S 4 exhibited visually better growth than that in S 1 and S 3 because the presence of biochar could further remediate BDE209-contaminated soil and promote seedling growth and increase the biomass by mending soil structure, improving soil fertility and reducing the excessive leaching of mental ions (Jin et al., 2014). In particular, root elongations were significantly inhibited (except for S 2) and more sensitive to the toxicity than shoot elongations in our experiments. Therefore, root is a more sensitive indicator to the toxic effect of soil pollution.

Antioxidative-enzyme activity in B. chinensis shoots

Effective antioxidant-enzyme systems can eliminate ROS build-up in plants and protect living organisms against oxidative stress (Lin et al. 2009). This study evaluated oxidative stress by quantitatively determining the activity of the enzyme system in terms of the SOD, POD, and CAT activities. Compared with those in the control treatment, the activity of SOD, POD, and CAT in shoots of plants grown in treatment S 2 increased by 29.1, 32.5, and 15.6%, respectively. This suggested that BDE209 induced the increasing SOD, POD, and CAT activity to remove excess radicals. According to the experiment results (Fig. 2), the SOD, POD, and CAT activity in S 3 increased by 20.4, 54.5, and 23.4% compared with those in S 2 treatment. The significant increase in the enzyme activity may be attributed to the excess O2− caused by co-pollution (BDE209 and Ni/Fe nanoparticles). However, compared with those in S 3, the SOD, POD, and CAT activities in S 4 treatment were found to decreased by 33.8, 47.2, and 24.1%, respectively, indicating that the oxidative stress to plants was reduced in treatment S 4. These results all inferred that the phytotoxicity of the BDE209-contaminated soil was decreased as a result of the effects of the BC@Ni/Fe particles.

Fig. 2
figure 2

A SOD activities, B POD activities, and C CAT activities in shoots of B. chinensis. The different letters represent significant differences between the treatments (p < 0.05)

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Uptake and translocation of PBDEs in plants

Uptake and accumulation of BDE209 and total PBDEs in plants

The contents of BDE209 and total PBDEs in roots and shoot of each treatment are shown in Fig. 3. It is clear to see that the uptake of BDE209 and PBDEs in plant tissues was greater in roots than those in shoots. For instance, the concentration of BDE209 in shoots of treatments S 1, S 2, S 3, and S 4 decreased by 157, 168, 232, and 138 ng/g, respectively, compared with the roots of the corresponding treatment, indicating that the plant roots were more likely to take up and accumulate BDE209. The main function of roots is to absorb water and other molecules from the soil. Thus, BDE209 can be adsorbed to the outer surface of the roots, which contribute to the higher concentrations of BDE209 detected in roots. It is worth noting that the concentration of BDE209 in the shoots of plants cultivated in biochar-containing treatments (S 2 and S 4) was only 12 and 25 ng/g, which was 6.7 and 15.3% of the corresponding plant roots. The results can be speculated that biochar may have certain inhibitory effect on the upward translocation of BDE209 in plants, but the detailed mechanism of action remains to be studied.

Fig. 3
figure 3

Accumulation concentration of A BDE209 and B total PBDEs in plant roots and shoots

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It is also clear that the contents of BDE209 and PBDEs in both above and below ground parts were lower for plants grown in the soil with the presence of biochar (S 2 and S 4 treatments). It can be concluded that the addition of biochar reduced the bioavailability of PBDEs to plants. For example, the concentration of total PBDEs in the plant tissues of S 1 and S 3 decreased from 755.8 and 999.9 ng/g to only 197.8 and 397.5 ng/g for the treatments S 2 and S 4, which was amended with biochar. The results indicated that biochar can effectively prevent the uptake and accumulation of PBDEs in plants, thereby slowing down the toxic effects on plants. Adding biochar into the contaminated soil resulted in a sharp decrease in the accumulation of PBDEs, which is explained primarily by the strong physical adsorption and binding of PBDEs to the adsorbents, as demonstrated in our previous work and other studies (Vasilyeva et al., 2010; Wang et al., 2013; Wu et al., 2016a;). From Fig. 3, we can also notice that the accumulation of BDE209 and total PBDEs in treatment S 3 was highest. It may be due to the BDE209 removal efficiency in S 3 which was lower than that in S 4, resulting in higher residual content of BDE209 in the soil of S 3 and thus causing an increased concentration in the plants. The literatures also report that the amount of PBDEs uptaken by plants is positively correlated with the concentration of PBDEs in the soil (Vrkoslavová et al., 2010; Inui et al., 2008). As a result, the plants in S 3 showed a maximal phytotoxicity.

In summary, the above results indicated that using BC@Ni/Fe to remediate BDE209-contaminated soil could reduce the uptake and accumulation of PBDEs in plants, thereby reducing the phytotoxicity of the remediated soil and facilitating soil reproduction.

Distribution and translocation of PBDEs in plants

In this study, the uptake of PBDEs by B. chinensis seedlings in the four treatments was analyzed by using GC-MS after planting for 20 days. As shown in Fig. 4, a total of 13 PBDE homologs were detected in this study. The congener profiles of PBDEs differed slightly among four soil groups. For treatments S 1 and S 2, the congeners in the plant tissues comprised of deca- and nona-BDE206 and 207. Previous studies have shown that the removal of BDE209 by biochar was only through adsorption and complexation, which would not lead to further de-bromination (Wu et al., 2016a). As a result, we hypothesized that the nona-congeners in the plant tissues of S 1 and S 2 were derived from the metabolism of the B. chinensis, agreeing with reports that BDE209 could be metabolized inside plants in the process of uptake and inclined to lose one bromine atom to form nona-BDE (Wang et al., 2011a, b; Huang et al., 2010; Chow et al., 2015). In addition to BDE209, BDE207, and BDE206, the lower bromine products of octa- through penta-BDE congeners in plants were detected in samples S 3 and S 4, including BDE205, 183, 181, 154, 153, 138, 100, 99, 49, and 47. As demonstrated in our previous work (Wu et al., 2016b), low brominated diphenyl ether in plant tissues can be derived from the plant metabolism, high brominated diphenyl was de-brominated by Ni/Fe nanoparticles in plants and uptake directly from soil. It is worthwhile to notice that the distribution of PBDEs was significantly different in S 3 and S 4. For example, BDE209, 207, 206, 205, 183, 138, 153, 154, and 99 were detected in both treatment groups. However, hepta-bromide BDE-181 was only detected in the plants of S 4, while BDE-100 (penta-BDE) and lower brominated products, BDE47 and BDE49, were detected only in the plants of S 3 treatment. The result showed that BC@Ni/Fe composites can effectively adsorb and immobilize the low brominated products produced from degradation in soil, so that the plants are less likely to uptake the higher toxic by-products and reduce the toxic effect. In addition, hepta-BDE was not detected in the plant roots of S 3 treatment, but a small amount of hepta-BDE (BDE-183, 2%) was detected in the aboveground part of the plant. Also, the content of BDE205 in the aboveground part of S 3 treatment was slightly higher than that in the plant roots. These results indicated the translocation and further metabolism of PBDEs were happened in the root-aboveground system. Although some literatures reported that accumulation of PBDEs in shoots may result from a combination of uptake through the root-to-shoot pathway and foliar uptake from the air (Huang et al., 2010, 2011; Wang et al., 2011a). However, in this study, the constant presence of water in the soil surface prevents the PBDEs from evaporating off the soils, so the foliar uptake had no appreciable contribution for the plant accumulation of PBDEs. Therefore, the PBDEs in the aboveground parts of the plants in this study are considered to be derived from the root-to-shoot pathway and the metabolism inside plants.

Fig. 4
figure 4

A Composition profiles of PBDEs in the roots and shoots of plant. B Relationship between the translocation factor (TF) and the concentration of PBDEs in plants

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TFs of PBDEs were also calculated and plotted in Fig. 4B for the further analysis of acropetal translocation in different treatments. TFs are calculated as the ratio of the concentration of PBDEs in the aboveground part of the plant to that in the root, expressing the ability of pants to transfer PBDEs from roots to shoots. The results further indicated that PBDEs can transfer from the roots to the aboveground parts. The TFs of total PBDEs and BDE209 for the S 1, S 2, S 3, and S 4 treatments followed the orders as S 1 > S 3 > S 4 > S 2, suggesting notable differences in the translocation capability of plants in the four treatments. The TFs of S 1 and S 3 treatments in this study showed the different trends as the concentrations, that is, the TFs of deca-BDE and nano-BDE in S 1 increased by 45.2 and 56.7% compared to those in S 3, respectively. This is due to the higher accumulation of BDE209 in the plant roots in S 1 than that in S 3. In addition, compared with those in S 3, the total PBDEs, deca-, nano-, and octa-BDE in S 4 were decreased by 51.8, 57.7, 70.3, and 99.4%, respectively, indicating that the remediation of BDE209 by BC@Ni/Fe can have an inhibition effects on translocation of PBDEs in the plants. In addition, the order of TFs in S 4 treatment were as follows: deca- > nano- > octa- > hexa- > hepta-BDE. The TFs of the higher brominated diphenyl ethers were slightly higher than those of the corresponding lower brominated diphenyl ethers. The results indicated that higher brominated diphenyl ethers are more likely to translocate upward. However, the results of the different studies were different. For example, as reported by Wang et al. (2011a), the TF values were in the order: BDE28 > BDE15 > BDE47 after maize being exposed in PBDEs for 150 h. This may be due to the notable differences in the translocation capability of PBDEs in different plants (Wang et al., 2011a; Deng et al., 2016). Furthermore, as Huang et al. (2010) reported, accumulation of BDE209 in roots might be affected not only by root uptake from the soil but also by root-to-shoot translocation. The more PBDEs transferred upward, the fewer PBDEs accumulated at the roots. And the acropetal translocation of PBDEs in plants was more complicated than their root uptake of PBDEs. Thus, the combined contribution of both root-to-shoot translocation and metabolism of PBDEs inside plants would make much uncertainty of TFs and make more detailed explanation of TFs of PBDEs in root-aboveground system difficult.

Uptake and accumulation of Fe and Ni in plants

Numerous studies have reported that excessive accumulation of Fe and Ni in the organism will cause some toxic effects (Auffan et al., 2008). Therefore, it is necessary to investigate the uptake and transport of Fe and Ni among five treatments and evaluate the detrimental effects of Fe and Ni on plants in the application of BC@Ni/Fe. As shown in Fig. 5, the application of the materials had significant effects on the concentration of Fe and Ni in plant tissues. Compared to those in S 0, the contents of Fe in roots and shoots of plants in S 3 were 2.4 times (3895 mg/kg) and 3.7 times (1221 mg/kg) higher than those of S 0, respectively, which might be attributed to the highest activity of bimetallic in S 3. In addition, the contents of Fe in plant roots and aboveground parts in S 4 were reduced by 35.9 and 53.6% compared with those of S 3. This indicated that biochar could effectively inhibit the excessive absorption of Fe, thereby reducing the toxic effects on plants. Meanwhile, the accumulation of Ni in plants showed the same trend as the result of Fe. The total amount of Ni uptake by plants in plants in the S 4 treatment was 7.4 mg/kg, decreasing by 39.8% compared with that of S 3 (12.3 mg/kg). This once again showed that biochar could inhibit over-accumulation of metal elements by plant to reduce its toxic effects. The results can be explained primarily by the capability of biochar to immobilize metals such as Ni and Fe in the soil by sorption and complexation due to their large surface area, the rich pore structure and the rich surface functional groups (carboxyl, carbonyl, and hydroxyl) (Song et al., 2014; Zhou et al., 2014). In addition, the accumulation of Fe and Ni in the roots and shoots of the three treatment groups was found to be similar, that is, the content of Ni and Fe in root of plants was higher than that in aboveground parts.

Fig. 5
figure 5

Uptake and accumulation of A Fe and B Ni of Chinese cabbage. Within the same plant tissue, the different letters represent significant differences between the treatments and the same letters are not significantly different (p < 0.05)

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Effects of the materials on soil properties

As shown in Table 2, the pH of the pure soil was 5.83 which is moderate acidic, with no significant difference in pH for soil + BDE209 (S 1) compared to the pure soil (p > 0.05). However, the pH of S 3 treatment was 6.39, which was due to the fact that the Ni/Fe nanoparticles that participate in the reaction may consume the H ions of the system. Compared to those in S 0, the soil pH of S 2 and S 4 increased by 0.83 and 1.11, respectively, demonstrating that the addition of biochar can improve soil pH. Soil pH is the main factor that controls the potential release of the immobilized heavy metals or metalloids and surface precipitation. The increased soil pH after biochar addition will result in the precipitants of metal oxy/hydroxides that are formed due to increased hydrolysis of heavy metals (Jiang et al., 2012), which will reduce heavy metal bioavailability.

Table 2 The influence of the materials on soil properties
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The content of available Fe in S 3 was obviously higher than the control group S 1 (about three times), which was mainly due to excessive iron ions released during the remediation process by Ni/Fe nanoparticles. Compared to that of S 3, the available iron of S 4 was decreased by 61.2%, indicating that BC@Ni/Fe could effectively reduce the release of iron ions and resolve the problem of the excessive release of iron caused by Ni/Fe nanoparticles.

In this paper, soil organic matter content in S 3 had a slight decrease compared to that in S 0 (5.5 mg/kg), which may be due to the reaction of Ni/Fe nanoparticles with macromolecular compounds such as amino acids, proteins, and organic acids. Noteworthily, the soil organic matter increased from 65.49 to 140.88 and 88.56 mg/kg after the biochar and BC@Ni/Fe remediation, respectively. The results proved that the presence of biochar can significantly increase the content of soil organic matter, mainly because biochar is rich in biomass. The application of biochar is beneficial to the accumulation and formation of soil organic matter and has important significance in improving the soil fertility and stabilizing the soil organic carbon pool (Kimetu and Lehmann, 2010).

Moreover, the changes of available phosphorus and available nitrogen in different treatments were also compared. The available phosphorus of S 3 was lower than that of S 1 and S 0, indicating that Ni/Fe nanoparticles could destroy soil available phosphorus and reduce soil fertility, which was not conducive to plant growth. The contents of available phosphorus in soil were increased from 9.81 to 17.33 and 11.9 mg/kg after the remediation of biochar and BC@Ni/Fe, respectively. The results showed that the addition of biochar could significantly increase the content of available phosphorus in soil, probably because the biochar contains large amounts of phosphorus and its availability is high (Glaser et al., 2005). Furthermore, the determination results of available nitrogen in the soil showed a similar trend to that of available phosphorus. Compared with those of S 1, the available nitrogen of S 3 decreased by 3.05 mg/100 g, while those of S 2 and S 4 treatments increased by 58.12 mg/100 g and 18.7 mg/100 g, respectively.

In summary, the remediation of BC@Ni/Fe has beneficial effects on the soil physical and chemical properties, effectively improves the soil fertility, and is conducive to the growth of plants.

Conclusion

This study suggested that the remediation of BDE209-contaminated soil by BC@Ni/Fe has the potential to reduce the uptake and translocation of PBDEs from soil into B. chinensis, especially for lower brominated PBDEs. Soil culture experiments showed that the morphological growth of plants cultivated in S 1 and S 3 were significantly inhibited, suggesting that BDE209 and Ni/Fe nanoparticles were both found to be toxic to the plants. Moreover, the phytotoxicity in S 4 treatment was significantly decreased compared with that in S 1 and S 3 treatments, accompanied plant biomass, root, and shoot length were significantly increased, while the contents of BDE209 and the total PBDEs were significantly decreased. In addition, the content of Ni and Fe accumulated in plants was the least in S 4. Furthermore, the concentrations of BDE209, Ni, and Fe accumulated in plant tissues were greater in the roots than those in the shoots, which indicated that roots were more sensitive to the toxicity than shoots. Results also showed that higher brominated PBDEs were more readily to translocate upward than the lower brominated PBDEs. Meanwhile, BC@Ni/Fe can effectively improve the soil physical and chemical properties and increase soil fertility, which is conducive to the plant growth. For example, the enhanced soil pH and soil organic matter may contribute to a decrease in the available concentration of heavy metals by reducing metal bioavailability. Further studies should be conducted to determine the long-term effects of biochar on phytotoxicity and translocation of polybrominated diphenyl ethers at the field scale.