The Characteristics And Mechanism Of Arsenic Accumulation By Submerged Plants

In recent years, arsenic (As) contaminated water and its remediation have become of increasing concern to government and industry. Phytoremediation is considered as the most promising approach for the remediation of soil or water pollution. Submerged plants, at the bottom of the food chain, are exposed to both overlying water and sediment, so active and passive circulation of As by plants would be expected to play important roles in the biogeochemistry of As in aquatic systems. Some species of submerged macrophytes have considerable potential to accumulate As and to act as phytofiltrators of As-contaminated water. To investigate suitable plant species for the phytoremediation of arsenic contaminated water and ensure the safety of people and animals, five common submerged plant species (Vallisneria natans (Lour.) Hara., Potamageton crispus L., Myriophyllum spicatum L., Ceratophyllum demersum L. and Hydrilla verticillata (L.f.) Royle were investigated to determine their potential to remove As from contaminated water. The characteristics and mechanism of As accumulation by these submerged plants were studied. The main results were as follows:(1) After exposure to5mg/L As(V) for7d, the arsenic concentrations in the five plants each exceeded1000mg/kg (dw), bioconcentration factor (BCF, concentration in plant/concentration in water)>220. This suggested that each of five plants have the potential to remove As from water. Their phytoextraction abilities were:V. natans>H. verticillata>P. crispus>M. spicatum>C. demersum. After exposure to5mg/L As(V) for7d, the arsenic concentrations in V. natans exceeded1800mg/kg (dw), BCF>360. There was a significant difference between V. natans and the other plants (p<0.05). The arsenic concentrations in C. demersum, a rootless plant, was the lowest, indication that root may play an important role on the As uptake by plants. Therefore, V. natans had the highest potential of all tested plants to efficiently remove As from water and was further studied. (2) With different arsenic levels (CK,0.1,0.2,0.5,1.0,2.0mg/L), the V. natans grew very well with time. During the whole experimental period(28d), there were no significant differences in plant growth compared to controls (CK), even at the maximum As(V) concentration (2.0mg/L)(p>0.05). This suggests that V. natans is highly tolerant to As and stable to environmental changes.At different arsenic concentrations, the As concentrations in all plants increased with time. After21d with0.2mg/L As treatment, the bioconcentration factor (BCF) of V. natans reached1300. The As concentration in the environment and exposure time are major factors controlling the As concentration in V. natans. The As accumulation in the plant was significantly correlated with both the exposure timeand the As concentration in the environment.(3) The results of diferent pH (5,7and9) showed that the optimum pH for submerged V. natans growth is close to7.0. Higher pH (9), lower pH (5) and As treatment caused considerable loss of chlorophyll. The loss increased with the increasing As concentration. However, the above results indicated that the biomass of V. natans did not significantly change with As(V) concentration ranging from0to2mg/L. It is suggested that other factors such as temperature, nutrients, and time may also influence the plant biomass.(4) The accumulation of As in the plant increased with increasing pH (5-9). This may have been due to the concentration of AsO43-increased with the increase of pH, arsenic/phosphate transporters having a higher affinity for the more highly electronegative AsO43-than for HAsO42-and H2ASO4-. With increasing pH, the arsenic in all fractions (Fraction I:protoplast fraction, Fraction II:organelle containing fraction, Fraction III:soluble fraction) increased. The increasing amount in the soluble fraction was the major proportion of increasing As accumulated by plants with increasing pH.(5) After uptake by V. natans, As(V) is almost completely reduced to As(III) in the plant cells. After7d of V. natans being supplied with0.5or2mg/L As(V), more than69%of As in the plant was As(Ⅲ). It seems likely that reduction of As(V) to As(III) is an essential process for As detoxification by V. natans during As accumulation. However, increasing the pH (from5to9) in the nutrient solution significantly decreased the percentage of As(III) in the plant (from95%to82%)(p<0.05), and increasing arsenic in the plants also can inhibit the capacity for As(V) reduction.(6) Superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and thiols can protect V. natans from As stress. At different As treatments (0.5or2mg/L), the concentration of thiols in V. natans increased, suggesting that increased exposure to As can stimulate the synthesis of thiols. Therefore, thiols may play an important role in helping to decrease As stress. However, increasing As concentration in the nutrient solution (from0.5to2mg/L), did not increase the synthesis of thiols further. This may be because the ability for synthesis was already maximal.(7) In our experiment, most As (>64%) was measured in the soluble fraction (Fraction Ⅲ). So the vacuole is presumed to be the predominant sink of As for V. natans. Only18%of accumulated As was bound to the cell wall fraction(Fraction I). Although the cell walls of submerged plants as the first barrier protecting cell from heavy metal toxicity, the function is smaller compared to the terrestrial plants. The results of observation under SEM+EDX (Scanning electron microscopy and Energy dispersive spectrometer) are the same. Therefore, the major of accumulated As was stored in the vacuole and the capacity of As(V) reduction were the important mechanism of As detoxification in submerged plants.(8) The results of As(V) and phosphate interactions showed that As(V) is taken up by by Ⅴ.natans via the P transport systems. At low P levels (<0.1mg/L), plant As concentrations increased. This may be caused by P deficiency at these levels resulting in increased As(V) uptake. Increasing P concentrations in nutrient solution significantly decreased the As concentrations in the plant. This may be because P transporters have a higher affinity for P than As(V), and if the external P status is high, P will be taken up more efficiently than As(V) and subsequently decrease the amount of As(V) sorbed. Therefore, better phytoremediation of As-contaminated water may be achieved by controlling the phosphate levels in the water, where this is possible.(9) Increasing P concentrations in nutrient solution significantly increased the As concentrations in the plant. If sulfur levels above30mg/L, the As concentration in the plant did not increase further. Higher aqueous sulfur concentration in solution can enhance the enzymatic activity (SOD, CAT, APX) and stimulate the synthesis of thiols which can reduce As stress.Therefore, increasing the use of sulfur fertilizer can increase As uptake by V. natans and decrease the As stress.