Methylation and demethylation of organic and inorganic compounds are pervasive biotransformation processes in various plants . Plant uptake, accumulation as well as volatilization of the contaminants can be significantly altered when methylation and demethylation processes occurred. Some researchers found that 6-MeO-BDE-47 was readily accumulated in maize shoots compared with 6-hydroxy-2,2′,4,4′-tetrabromodiphenyl ether Selenite and arsenic were inclined to form more volatile methylation products in wetlands which were liable to diffuse into the air phase . With regard to the two target compounds in this study, 2,4-DBA is more volatile than 2,4-DBP in seawater-air exchange system . Recent studies showed that over 80.0% of 2,4-DBA volatilize from surface seawater and reach a level of 97 pg L−1 in atmosphere . Although vegetation plays an important role in the exchange of volatile and semivolatile organic compounds  between contaminated soil or groundwater and atmosphere in terrestrial ecosystem,limited information is available to quantitatively evaluate the volatilization of 2,4-DBP and 2,4-DBA mediated by vegetation. Moreover, the roles of interconversion processes also need to be considered in the exchange of bromophenols and bromoanisoles between contaminated sites and atmosphere. Rice  is widely cultivated in Asia . A large amount of irrigation water is introduced into paddy fields during rice growth, and thus, many unintended compounds such as 2,4-DBP and 2,4-DBA could be uptaken into rice plants . In this context, the plant uptake, bioaccumulation, interconversion, as well as volatilization of 2,4-DBP and 2,4- DBA in rice plants were systematically investigated in hydroponic exposure experiments. Rice plants were significantly facilitated the volatilization of 2,4-DBP and 2,4-DBA from culture solution into air phase. Concurrently, methylation and demethylation processes help to volatilize 2,4-DBP and 2,4-DBA from rice plants. These results show that interconversion and phytovolatilization processes greatly affect the global cycles of 2,4-DBP and 2,4-DBA.

To systematically evaluate the plant uptake and bioaccumulation of 2,4-DBP and 2,4-DBA, as well as their interconversion,hydroponic gutter two target compounds were individually exposed to rice seedlings. In consideration of the transformation rate for phenol compounds, such as the methylation of hydroxyl polychlorinated biphenyls, were generally less than 1.0% in rice plants , a relatively high initial exposure concentration  was selected in exposure experiments. Five rice plants were planted in each reactor with the exposure solution which was prepared by adding 45 μL working solution of individual parent chemical  into deionized water and finally constant to 45 mL for each parent chemical. All exposure reactors were 55 mL brown glass bottles and wrapped with aluminum foil to avoid any photolysis of the target compounds. To minimize cross contaminations between reactors and ambient environment, three parallel reactors  were treated in an independent space  isolated by a pair of glass beakers which connected mouth to mouth, and the connection was sealed with commercial polytetra- fluoroethylene film as a treatment . Similar sealed exposure systems placed with three pieces of 1.50 g polyether-type polyurethane foam  inside the headspace were set as PUF treatments . PUF has been used as the sorbent traps to successfully capture bromophenols and bromoanisoles from atmosphere . The existence of PUF could affect the equilibriums of target compounds among different compartments in the sealed systems to some extent due to the strong adsorption capacity. Therefore, quantitative determination of PUF samples was introduced to evaluate the total amount of target compounds  volatilized from hydroponic solution  and from rice plants  into the headspace. Unplanted treatments with PUF for 2,4-DBP and 2,4-DBA were used to evaluate their direct volatilization from culture solution, which were treated with parent chemicals  and incubated with PUF but without rice plants in a sealed independent space. The differences between the volatilization of pollutants in unplanted treatments with PUF  and the planted treatments with PUF  were attributed to the phytovolatilization process in these sealed systems.Blank controls were used to evaluate any potential background contamination to plant and PUF samples.

All the PUFs were pre-washed with dichloromethane/hexane  in Dionex ASE 350 before use. All treatments were placed in an exposure chamber with the same conditions as the cultivation chamber for 5 days.Distributions of parent and daughter compounds in the exposed plants were characterized in plant treatments, which are incubated without PUF. Their hydroponic solutions and rice seedlings were sampled at time intervals of 6 h, 12 h, 24 h, 48 h, 72 h and 120 h. Treatments with PUF was sampled for hydroponic solutions, plant samples and PUF samples at the end of exposure . The PUF samples were used to determine the target compounds and corresponding methylated or demethylated metabolites entering into the head space. Root samples were washed with DI water before harvest, and then those rinse water was combined with the exposure solution for further analysis. And all solution samples were directly stored at −20 °C before further pretreatment. Plant samples were vacuum freeze-dried in a lyophilizer at −50 °C for 2 days  and stored at −20 °C. PUF samples were all sampled from planted and unplanted treatments and blank control. To avoid the release of analytes from PUF samples which might occur in storage process, PUF samples were immediately extracted after sampling. The extraction of plant tissues, PUF samples and culture solution, as well as cleanup procedure for rice tissues were described in supporting information .For the planted blank controls in the absence of PUF, none of 2,4- DBP and 2,4-DBA were detected in any of the samples, suggesting that no background contamination influenced the determination of hydroponic solutions and plant samples. Variations of parent 2,4-DBP and 2,4-DBA in culture solution and plant tissues during exposure period are summarized in Fig. 2. For the 2,4-DBP exposure system in the absence of PUF, a rapid disappearance of parent compound within the first 24 h was followed with a slow decrease observed in the culture solution . Only 0.40% of the initial amount of 2,4-DBP was left in the hydroponic solution after exposure for 120 h. The photo-degradation of parent chemicals was avoided by wrapping aluminum foil outside the brown glass reactors, so major losses of parent compounds from solution resulted from plant absorption and adsorption as well as direct volatilization. Concurrently, a significant amount of 2,4-DBP was detected in rice root.

The highest root concentration  was occurred in the first 6 h, and then decreased rapidly to around 304 mg kg−1 after 12 h of exposure . 2,4-DBP was also found in rice leaf sheath and leaf. However, the concentrations of 2,4-DBP in leaf sheath and leaf was 6.99–54.4 and 95.9–358 times lower than those in the rice root during the period of exposure. It was reported that plants can take up contaminants through the developing root system . Once uptaken into the root, those pollutants were translocated from the rice root to upper plant parts Some volatile pollutants were concurrently phytovolatilized into atmosphere. Detection of parent compound in leaf sheath and leaf confirmed that plant uptake and translocation of 2,4-DBP in rice plants. Levels of 2,4-DBP in the leaf sheath and leaf showed similar trends-initially increasing and then decreasing . But the maximum concentration of 2,4-DBP  in leaf sheath was later than in leaf . As well known, distributions and bioaccumulations of pollutants in different tissues are the results of comprehensive processes within the rice plant. Translocation of 2,4-DBP from rice root into leaf sheath was higher than their loss in leaf sheath  before 24 h, and then those elimination processes were predominant, making the highest concentration occurred at 24 h. While, the amount of 2,4-DBP translocated from rice root and leaf sheath into leaf was lower than those elimination processes just after 6 h. Therefore, the highest concentrations in different rice tissues were observed in different time. Furthermore, the sum of 2,4-DBP decreased in rice plants over time indicating that the losses of the parent compound probably resulted from plant phytovolatilization and metabolism . For the exposure group of 2,4-DBA in the absence of PUF, similarly, high concentrations of parent compound were detected in rice roots compared with other plant tissues. This phenomenon was consistent with the bioaccumulation of organophosphate esters in various plants . Concentrations of 2,4-DBA associated with the root gradually increased during 0–72 h, then slightly decreased at 120 h  and showed comparable levels to that of 2,4-DBP during 12–120 h . Meanwhile, the level of 2,4-DBA in leaf sheath  and leaf  continuously increased during the whole exposure period , which differed from the trends of 2,4- DBP. The concentrations of 2,4-DBA in rice leaf sheath and leaf increased up to 75.2 mg kg−1 and 9.24 mg kg−1 after 120 h exposure, respectively, which were 3.11 and 5.96 times higher than those of parent 2,4-DBP at 120 h.

Concentrations of 2,4-DBA in the culture solution displayed a decreasing trend , but the decrease was slower and less than that for 2,4-DBP. Finally, 33.6% of initial amount of 2,4-DBA remained in the culture solution, which was considerably higher than that of 2,4-DBP . Overall, compared with 2,4- DBP, 2,4-DBA showed less loss in hydroponic solution and more remaining in the rice plants, especially in the above ground tissues of rice. This might be attributed to their different properties and biotransformation abilities. In comparison, 2,4-DBA was less water-soluble and more volatile, hydroponic nft channel making it was probably less taken up by rice roots and more volatilized from the solution. Those comprehensive processes  finally resulted in less loss of 2,4-DBA in the hydroponic solution. Once 2,4-DBA and 2,4-DBP were uptaken into rice plants, numerous enzymes within the rice plants would make them metabolized. 2,4-DBP contains active phenolic hydroxyl group which was reported being readily catalyzed by glycosyltransferase and other Phase II enzymes to form saccharide and amino acid conjugates . Thus, it was inferred that less 2,4-DBA was biotransformed within rice plant due to the lack of active hydroxyl group but more 2,4- DBP was metabolized by rice plants in the exposure period. Therefore, more 2,4-DBA was remained in the rice plant tissues .Same as plant samples in the planted blank control without PUF, no 2,4-DBP and 2,4-DBA was detected in plant samples for the planted blank control with PUF. However, concentrations of PUF samples in blank control  were 0.01 mg kg−1 for both 2,4-DBP and 2,4- DBA due to the strong adsorption capacity of PUF for bromophenols and bromoanisoles. The amounts of 2,4-DBP and 2,4-DBA in blank control were only accounted for 0.73–1.16% and 0.12–0.13% of PUF samples in unplanted treatments  and exposure groups . Therefore, the average amount of 2,4-DBP and 2,4-DBA in PUF sample of blank control was considered as the background contamination and deducted from the results of concentrations from the PUF samples in unplanted treatments and exposure groups. Direct volatilization of parent 2,4-DBP and 2,4-DBA from culture solution to head space was determined by PUF samples in unplanted treatments with PUF.

As shown in Fig. 3, the measured fluxes of 2,4- DBA , 4.78 Pa m3 mol−1 through direct volatilization are 8006 ng, 5.66 times higher than that of 2,4-DBP , 0.037 Pa m3 mol−1, consistent with the finding that volatile compounds with low octanol-air partitioning coefficients are more likely to be volatilized from water . The direct volatilization accounted for 0.80% and 4.42% of the initial amounts of 2,4-DBP and 2,4-DBA after 120 h exposure , respectively. Those results indicated that the properties of compounds, such as Henry’s law constant and octanol-air partitioning coefficients, related the volatilization of 2,4-DBA and 2,4-DBP in exposure system. The total volatilization of 2,4-DBA  was 3.43 times higher than that of 2,4-DBP  by comparing the results of PUF samples from their separate planted exposure groups with PUF. Then, it was calculated that 0.32% and 0.58% of initial amounts of 2,4-DBA and 2,4- DBP  entered into the air phase through phytovolatilization though only three reactors with fifteen rice plants were incubated in the sealed exposure systems for just 5 days. 2,4-DBA and 2,4-DBP was phytovolatilized at rates of 7830 and 4422 mg kg−1 fresh biomass  day−1 which were higher than that of mercury with estimated values between 14.4 and 85.0 mg Hg kg−1 fresh biomass day−1 from tobacco plants . Overall, 6.78% and 41.7% of total volatilization of 2,4-DBA and 2,4-DBP was attributed to rice plants, respectively, distinctly larger than the contribution of phytovolatilization to the total volatilization of arsenic Obviously, phytovolatilization facilitated the volatilization process between culture solution and atmosphere within the hydroponic exposure systems.