In order to investigate the function of submerged aquatic plants for recovery of water polluted by typical organic pollutants, and select potential plants for phytoremediation of polycyclic aromatic hydrocarbons contaminated water, removal efficiencies of four submerged macrophytes to phenanthrene were investigated, following 40-day exposure to phenanthrene solutions in an outdoor-simulated experiment. During the exposure period, phenanthrene concentration in water, sediments and the roots of submerged macrophytes were observed. Results showed that Elodeacanadensi exhibited the highest concentrations in roots, while Ceratophyllum demersum contained the lowest among these four submerged macrophytes. The disparity of phenanthrene in roots would come from plant properties including the shape and surface area of both shoots and roots. These plants enhanced the remediation of phenanthrene in solution through plant-promoted sedimentation and biodegradation. Potamogetoncrispu and Elodeacanadensi showed the higher performance to remove phenanthrene due to plant-promoted biodegradation.

Introduction

Polycyclic aromatic hydrocarbons (PAHs), by-products from the incomplete combustion or pyrolysis of organic materials, are widely distributed in the environment and cause serious problems (Grimmer, 1983). These lipophilic pollutants may be accumulated in vegetation and therefore pose a threat to plant health (Wild and Jones, 1994; Tao et al., 2004). There have been many reports concerning the presence of PAHs in the surface water and sediment of river and sea (Maskaoui et al., 2002; Wu et al., 2003; Zhang et al., 2004).

Submerged macrophytes (SMs) are those residing below the surface, which may have emergent bodies and may or may not be rooted to the substrate (Thomas et al., 1995). They play an important role in nutrient cycling, especially in shallow lakes (Chambers and Kalff, 1985; Qiu et al., 2001). As submerged plants are completely inundated and have the ability to take up pollutants directly from the water, they are suggested as useful species in reducing concentrations of nutrients (Wang et al., 2008) and metal ions such as Hg (King et al., 2002), Co, Cu, Ni and Zn (Lesage et al., 2007) and even Am (Bolsunovsky et al., 2005), in waste water and polluted lakes. There is, however, scarce research about uptake of organic pollutants, especially PAHs, by submerged aquatic plants.

In the present investigation, Ceratophyllum demersum (CD), Potamogeton maackianu (PM), Potamogeton crispu (PC), and Elodea canadensi (EC), were used to analyse their potential for accumulating phenanthrene, representing typical PAH compounds with three benzene rings (Yaws, 1999). This would provide the data necessary for investigating the function of these plants in recovery of water polluted by organic pollutants.

Materials and methods

Materials and plant cultivation

Mature SMs without detected phenanthrene were obtained from Dongshan town, Suzhou city, washed and then pre-cultured in aquaria. These glass aquaria were made to simulate shallow lake, with 100 centimeter in length, 50 centimeter in width and 80 centimeter in depth. They were arranged under a random block design in the open air with a natural sunlight and a temperature of 15–30°C. Sands without detected phenanthrene (< 0.005 mg l− 1) were purchased from market. Twenty-five kg (dry weight) of sands were washed with distilled water and put in the bottom of each aquarium with the thickness of about ten centimeter. Healthy and uniform plants (about 6000g in fresh weight) were chosen to cultivate in the aquaria with 280 l oxygenated water and keep plant density similar.

Experimental designs

SMs were exposed to concentrations of blank solution (without phenanthrene), 0.5 and 1.0 mg l− 1 phenanthrene for 40 d, 8 treatments in total (CT0.5 (control without SMs), CT1; EC0.5, EC1; PM0.5, PM1; CD1; PC1). A stock solution was prepared by dissolving 100 mg of phenanthrene in 100 ml methanol. Water samples (approximately 20 ml) and plant roots (approximately 5 g fresh weight) were collected from different aquaria at 0, 6, 9, 12, 19, 31 and 40 days after exposure, with three replicates. Water samples were stored at 4°C. The plant roots were washed with double distilled water and stored at −70°C. After the experiment, sediment samples (sands) were collected with a shovel from the bottom of the aquaria.

Phenanthrene analysis for water, sediment and plant samples

The procedure used to extract PAHs from sediments was a modification of those of Gao and Zhu (2004). Prior to use, all methods were tested for the efficiency of recovery. For phenanthrene amended sediments, recovery averaged 76.6% (n = 5, RSD less than 2.9%; RSD, relative standard deviation) for phenanthrene. Recoveries of PAHs investigated by spiking plant samples were in average 69.1% (n = 5, RSD less than 8.1%) of phenanthrene for the entire procedure. After air drying and grinding in mortar, the sediment samples were sieved with an 18-mesh sieving screen (pore size 1000 × 1000 μ m) to remove large particles, leaves of plants, etc. All samples were analyzed in duplicate. The phenanthrene extracted from water, sediments and plants was analyzed by a Hewlett Packard (HP) 1100 HPLC equipped with a photodiode array detection (DAD) (Agilent, Germany), while the absorption wavelength was 254 nm, with an elute of methanol/water (80:20; v/v) and a flowing rate of 1.0 ml min− 1. The column was C18 (4.6 mm i.d × 250 mm length, Qrace Vydac, USA) and maintained at 30°C. Phenanthrene was not detected in the blank samples.

Results and discussion

Phenanthrene concentrations in water, roots of SMs and sediments

The concentrations of phenanthrene in solution (Table 1) decreased obviously with durations of exposure. It could be seen that the concentrations in the solution with cultivation of SMs were all lower than 0.02 mg l− 1 after 31 days exposure; furthermore, the concentrations in the solution of PM 0.5, PM 1 and CD 1 were lower than 0.02 mg l− 1 after exposure of less time, only 19 days.

Phenanthrene concentrations in SMs roots (shoots of CD, due to its lack of roots) with exposure time were illustrated in Table 2. As time passed, the concentrations of phenanthrene in roots of SMs all increased at first and then dropped after the maximum concentration was reached. Maximum concentrations of phenanthrene in various roots grown in solution with initial phenanthrene of 1 mg L− 1 were 7.38–44.87 μ g kg− 1. Higher abundance of the lower molecular weight PAHs including phenanthrene in plants has been also reported by other investigators (Wild et al., 1992; Voutsa and Samara, 1998). The domination of the lower molecular weight PAHs in plants may be attributed to their greater volatility and bioavailability (Kipopoulou et al., 1999).

Great variations of root maximum phenanthrene concentrations were observed among different plant species. EC exhibited the highest, while CD contained the lowest. However, the growth conditions of various plant species were identical. Thus the disparity of root uptake of phenanthrene would come from plant properties (Gao and Zhu, 2004). The shape and surface area of both shoots and roots probably influence the uptake (Wild and Jones, 1994). CD has no roots and less surface area of leaves. The surface area of PM leaves was also less than those of the other two plants. By contrast, the roots of EC and PC are stronger. Several studies suggest that the root uptake of lipophilic organic compounds can be in correlation with root compositions such as lipid contents (Simonich and Hites, 1995; Chiou et al., 2001). Results of our study implied that morphological properties would be useful in predicting of root accumulation of phenanthrene.

Sedimentation was found for all the treatments with or without plants. Concentrations of phenanthrene in sediments were 0.33(CT0.5), 0.74(CT1), 1.82 (EC1), 0.82 (PC1), 4.24 (CD1), 4.11 (PM1), 0.39 (EC0.5) and 3.43 (PM0.5) μ g.g-1, respectively. As can be seen, most of the concentrations in sediments from spiked aquariums were much greater than those beginning concentrations in water solutions. Phenanthrene concentrations in the sediments in planted aquariums were relatively larger.

Plant contribution to plant-enhanced remediation of phenanthrene

Concentrations of phenanthrene in unspiked blank solutions were not detectable. This indicated that phenanthrene subsiding from air was negligible in this study. The dissipation ratio (%) of phenanthrene in vegetated or nonvegetated spiked solutions could be calculated as:

formula
where R (%) was the dissipation ratio; Ci was the solution initial concentration (mg L− 1); Ce was the residual concentration after 40 d (mg L − 1). The dissipation ratio of phenanthrene in unplanted aquaria with two initial concentrations was 19.80 and 15.10%, respectively. While the dissipation ratios in planted aquaria were relatively larger, and those ratios were more than 32%, even up to 82% (Table 3). This revealed that these plants enhanced the remediation of phenanthrene contamination. Dissipation of phenanthrene in planted solutions included sedimentation, abiotic dissipation (including surface sorption, photo-oxidation and volatization), biodegradation and plant uptake and accumulation. By contrast, the dissipation of phenanthrene in unplanted solutions was sedimentation, abiotic dissipation and biodegradation (Gao and Zhu, 2004). Variation of abiotic dissipation of PAHs between planted and unplanted solutions was negligible (Reilley et al., 1996). Thus the loss of phenanthrene in vegetated and non-vegetated solutions could be expressed as:
formula
where Runp and Rp were the dissipation ratios of phenanthrene in spiked unplanted and planted solutions. Rs, Rs* and Rb, Rb* were the loss by sedimentation and biodegradation in vegetated and non-vegetated solutions, respectively. Ra denoted the dissipation ratio of abiotic dissipation. Pa denoted the off-take ratio by plant accumulation (Gao and Zhu, 2004). Thus the dissipation ratio enhancement (Rd) of phenanthrene in planted versus unplanted solutions was:
formula
Rps and Rpb denoted the loss ratio of phenanthrene by the plant-promoted sedimentation and biodegradation. Obviously, Rd would overwhelmingly derive of plant direct uptake and accumulation, promoted sedimentation and biodegradation. Here, plant direct accumulation amount of phenanthrene in planted solutions was less, which accounted for from 0.84% to 10.01% in the presence of vegetation (Table 3). By contrast, plant-promoted biodegradation was dominant, and larger than 38% dissipation enhancement of phenanthrene in planted versus unplanted solutions came from plant-promoted biodegradation, considering solutions with initial phenanthrene concentration of 1 mg L− 1 (Table 3). Plant-promoted sedimentation of phenanthrene was also very important for treatments of PM and CD and the ratios were all larger than 30%.

Plant-promoted biodegradation of soil organic chemicals has been studied in the past. However, there has been scarce research about plant-promoted biodegradation of PAH in water by submerged aquatic plants. The factors that plants may contribute to the biodegradation of organic compounds are perhaps the stimulation of microbial population, the promotion of microbial activity, and the modification of microbial community in rhizosphere (Colleran, 1997; Joner and Leyval, 2003), which are the results of the promoted humification and adsorption of pollutants massive, root excretion of readily available carbon sources and increased substrates for co-metabolism of PAHs and the improvement of redox conditions, physical and chemical properties of the rhizosphere (Gregory, 2006; Rentz et al., 2005). As to plant-promoted sedimentation of phenanthrene, it was perhaps due to increased contents of organic matter in rhizosphere of submerged plants, in which there are large quantities of organic substrates and organic matter as root exudates mentioned above (Walker et al., 2003). Several authors have also observed that organochlorines and PAH in the sediments are mainly associated with the organic matter (Knezovich et al., 1987; Doong et al., 2002).

Additionally, the efficiency of plant-enhanced remediation for phenanthrene contaminants in water by various plant species was not correlated with plant accumulation of phenanthrene. For example, the accumulation ratio of phenanthrene (Pa) by PC was not larger than by other plants (Table 3), while PC showed the higher performance in the remediation of phenanthrene. Therefore, it was not reasonable to select plants with high remediation efficiency based on plant accumulation of target compounds in some sense.

Conclusions

Among these four submerged macrophytes, Elodea canadensi had the highest root concentrations of phenanthrene, while Ceratophyllum demersum had the lowest. The disparity of root uptake of phenanthrene would be due to intrinsic plant properties including the shape and surface area. These plants enhanced the remediation of phenanthrene through plant-promoted sedimentation and biodegradation. However, the efficiency of plant-enhanced remediation for phenanthrene was not correlated to phenanthrene accumulation in plant. Potamogeton crispu and Elodea canadensi showed the higher performance to remove phenanthrene due to plant-promoted biodegradation.

Acknowledgements

This work was funded by the national key basic research development program (“973” project) of China (No. 2002CB412307).

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