Zinc tolerance and accumulation in Pteris vittata L. and its potential for phytoremediation of Zn- and As-contaminated soil
Introduction
Phytoremediation, the use of green plants to remove pollutants from soils, has been considered as a promising technique for the remediation of contaminated soils (Salt et al., 1995). Pteris vittata L., an arsenic (As) hyperaccumulator discovered independently by Chen’s and Ma’s groups, has a great capacity to phytoextract As from soils (Ma et al., 2001, Chen et al., 2002, Tu et al., 2002). The excellent potential of this plant to remediate As-contaminated soils was verified in the first field demonstration of As phytoremediation in Chenzhou City, Hunan Province of Southern China in 2000. For example, it was shown that, within 7 months, P. vittata L. could clean up to 7.8% of the As from soil where the As concentration was 64 mg kg−1 (Liao et al., 2004).
Zinc (Zn) is an essential element for normal growth and development of plants. This metal plays an important role in several metabolic processes in plants. Zn deficiency is one of the most widespread micronutrient deficiencies in many regions of the world (Cakmak, 2000, Fageria et al., 2002). On the other hand, excess Zn in soils caused by anthropogenic activities, such as mining, metal refining (Fialkowski et al., 2003), compost application (Ramos and López-Acevedo, 2004), and wastewater irrigating (Yediler et al., 1994), may retard the growth and development of plants (Alia et al., 1995, Kamal et al., 2004), and induce damage to the ecosystem (Nahmani and Lavelle, 2002, Lock et al., 2003). Because As usually occurs together with Zn in minerals, As contamination caused by mining and refining is often accompanied by Zn contamination (Black and Craw, 2001, Kim et al., 2003). Moreover, the use of chromated-copper–arsenate with Zn sulfate as a wood preservative (Hingston et al., 2001, Bhattacharya et al., 2002) may also cause co-contamination of soils with Zn and As. Therefore, whether P. vittata L. can be used to clean up As from soils co-contaminated with Zn is an important issue.
Fayiga et al. (2004) carried out pot experiments to study the effect of heavy metals on As concentration by P. vittata L., but the Zn concentration, 0.8 mg kg−1, in the soil used in their studies was much lower than in normal soil. In fact, we found in the present study that Zn contamination in soil, especially in mining and refining areas, was high (up to thousands of mg kg−1). Furthermore, whether P. vittata L. could be used in the phytoremediation of soil heavily contaminated with Zn has not been verified. Therefore, we conducted both a field investigation and pot experiments to understand the ability of P. vittata L. to endure Zn toxicity and to examine the impact of Zn on As accumulation. These studies should provide useful information for the application of P. vittata L. to remediate soils co-contaminated with As and Zn.
Section snippets
Field sites
A field investigation was carried out in Guangxi and Hunan Provinces of Southern China. Fronds and roots of well grown P. vittata L. and rhizosphere soils were sampled from 22 sites where P. vittata L. were found. Sites 1–7 were located in Guangxi Province with a tropical plateau monsoon-type climate, and the other 15 sites were located in Hunan Province with a subtropical monsoon-type climate. Except for the sites 1–5 and 7, the other sites were all located near metal mines or refineries. Of
Properties of soils and Zn concentrations in P. vittata L. at field sites
Soil pH varied greatly, from 4.6 at site 17–8.4 at site 19, among sites where P. vittata L. grew (Table 1). It indicated that P. vittata L. could survive at a wide range of soil pH in field naturally, and soil pH might not be a limiting factor for P. vittata L. to be used in field phytoremediation.
Concentrations Zn and As in the soils also varied over a wide range in soils investigated (Table 1). They were relatively low at sites 1–10 located in Guangxi Province, but high up to hundreds and
Discussion
This study showed that P. vittata L. grew well at many field sites where the total Zn concentration in soil was greatly higher than 400 mg kg−1 and the biomass of greenhouse-cultivated P. vittata L. was not reduced by 2000 mg kg−1 Zn treatment. Whereas, it is generally considered that a total Zn concentration in soil between 70 and 400 mg kg−1 is toxic to plants (Kabata-Pendias and Pendias, 1984). So it could be suggested that, except for high As tolerance which has been shown by Ma et al. (2001) and
Conclusion
The greenhouse experiments and field investigation in this study showed that P. vittata L. can tolerate high levels of Zn in the soil. Although excessive Zn in soil could reduce the accumulation of As by P. vittata L., As phytoremediation was effective even in soil with as much as 2000 mg kg−1 of Zn. In fact, the highest efficiency of As phytoextraction was 8.3 mg pot−1 at 1000 mg kg−1 of added Zn. In general, a relatively high capacity of P. vittata L. to accumulate Zn from soil was found in this
Acknowledgements
This research was supported by the National Foundation for Distinguished Youth of China (No. 40325003), the National Natural Science Foundation of China (No. 40232002), and the National Basic Science Research Program (No. 2002CCA03800).
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