Arsenic accumulation by aquatic macrophyte coontail (Ceratophyllum demersum L.) exposed to arsenite, and the effect of iron on the uptake of arsenite and arsenate

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Abstract

In this study, the aquatic macrophyte Ceratophyllum demersum L. (coontail or hornwort) was tested for its efficiency of arsenic (As) uptake under laboratory conditions. Our results revealed that the solution pH had a significant effect on As accumulation by C. demersum (p < 0.001). The accumulation was highest at pH 5 and decreased as pH values increased. Plants that were exposed to various concentrations of arsenite (As(III)) for 24 and 48 h, exhibited tolerance and toxic responses, respectively. As accumulation by C. demersum depended on the concentrations of As(III) and the duration of exposure (p < 0.001). At 40 μM after 24 h, plants accumulated 227.5 μg As g−1 dw and showed no visible symptoms of toxicity. However, after 48 h, As level reached 302.4 μg g−1 dw and biomass production decreased significantly. Toxic effects were evident by plant necrosis and negative biomass production, leading to a decrease in the amount of accumulated As. Also, the addition of iron (Fe) into the nutrient solutions (0.18 mM) had contrasting effects on the uptake of 2 As species – the uptake of As(III) was enhanced by the presence of Fe, but the uptake of arsenate (As(V)) was considerably inhibited.

Highlights

► Arsenic accumulation by Ceratophyllum demersum was pH-dependent. ► C. demersum accumulated high concentrations of arsenic after exposed to As(III)-containing solutions. ► The addition of iron enhanced arsenic adsorption on plant surfaces of C. demersum exposed to both As(III) and As(V), but had contrasting effects on the uptake of two arsenic species.

Introduction

Arsenic (As) is a naturally occurring element in the earth's crust. It ranks 20th in natural abundance, 14th in sea water, and 12th in the human body (Mandal and Suzuki, 2002). Natural As elevation of drinking water supplies has been reported from more than 70 countries, posing a serious health hazard to an estimated 150 million people world-wide (Brammer and Ravenscroft, 2009). As-contaminated groundwater used for irrigation may pose an equally serious health hazard to people eating food from irrigated crops (Williams et al., 2006), and As accumulating in irrigated soils poses a serious threat to sustainable agriculture in affected areas (Heikens, 2006).

Phytoremediation, a plant-based green technology, is a promising technology for environmental pollution caused by unavoidable limitations of traditional technologies (Rahman et al., 2008a). The use of some submerged aquatic macrophytes and floating plants in the process of phytoremediation is more commonly known as phytofiltration. Recently, As accumulation by aquatic plants, such as Eichhornia crassipes, Lemna minor (Alvarado et al., 2008), Salvinia natans (Rahman et al., 2008a), Spirodela polyrhiza (Rahman et al., 2008b, Rahman et al., 2008c), Hydrilla verticillata (Srivastava et al., 2007), and Wolffia globosa (Zhang et al., 2009) have been reported in the literature. The promising results of phytoremediation technology for As removal from contaminated water has gained the attention of researchers.

Thus far, 3 mechanisms have been proposed for the uptake of As species in aquatic macrophytes: (i) active uptake through phosphate uptake transporters, (ii) passive uptake through aquaglyceroporins, and (iii) physicochemical adsorption on plant surfaces (Rahman and Hasegawa, 2011). Physicochemical adsorption, an alternative mechanism for As accumulation by aquatic plants, was hypothesized by Robinson et al. (2006). In this mechanism, oxides/hydroxides of iron (Fe plaque) suspended on the aquatic plant surfaces adsorb and accumulate As. In most As-affected areas of South and South East Asia, groundwater is rich in Fe (Brammer and Ravenscroft, 2009). Water soluble Fe2+ is oxidized when water is exposed to the air and is then precipitated as Fe hydroxides that adsorb As. If aquatic macrophytes are used for phytoremediation of As, the elevated Fe concentrations in water could be presumed to be a factor affecting the accumulation characteristics.

Coontail (Ceratophyllum demersum L.), a completely submerged aquatic macrophyte is reportedly a scavenger of As(V) in contaminated water (Mishra et al., 2008). However, available knowledge of the accumulation characteristics of plants exposed to As(III) is insufficient. Therefore, the present study was focused on investigating As uptake efficiency when C. demersum is grown in As(III)-containing nutrient solutions. Additionally, the effects of Fe on As adsorption and accumulation properties of C. demersum exposed to As(III) and As(V) were also elucidated. These results may be useful when this aquatic plant is used as a phytoremediator for As-contaminated water.

Section snippets

Plant material

C. demersum plants were purchased from a local shop and were grown for 2 weeks in a large aquarium. Before treatments, plants (approximately 5 cm tip portion) were acclimatized in 10% Hoagland solution (Hoagland and Arnon, 1950) for one week in a growth chamber with the conditions set to a light intensity of 70 W m−2, a 16 h photoperiod at 25 ± 2 °C, and a relative humidity of 70%. After acclimatization, the plants were used for subsequent studies. All experiments were set up in triplicate and each

As accumulation of C. demersum in response to different values for solution pH

As accumulation of C. demersum varied significantly (p < 0.001) when the experiment was carried out at different pH values (Fig. 1). Within a pH range of 5–10, maximum As uptake by the plant occurred at pH 5, and decreased with an increase in solution pH. As uptake decreased abruptly as pH was increased from 5 to 6, and was not appreciably affected by changes in pH from 8 to 10. No visual negative effects on plant growth such as necrosis/chlorosis or the weakening of plants was observed.

The

Conclusion

Our results demonstrated that As accumulation by the aquatic macrophyte C. demersum was decreased with an increase in solution pH. The plant tolerated higher concentrations of As than those normally present in contaminated areas. In addition, an Fe coating on the plant surfaces and other dissolved elements in the nutrient solution were found to pose significant effects on plant As uptake. This information is important when defining strategies for the most efficient As removal in a real

Acknowledgements

This work was partially supported by A-STEP, Exploratory Research of the Japan Science and Technology Agency (JST) and Mitsui & Co. Ltd., Environment Fund. The authors are thankful to Dr. Yasumasa Ogawa for ICP-MS analysis.

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