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Phytoremediation of mercury by terrestrial plants
Stockholm University, Faculty of Science, Department of Botany.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mercury (Hg) pollution is a global environmental problem. Numerous Hg-contaminated sites exist in the world and new techniques for remediation are urgently needed. Phytoremediation, use of plants to remove pollutants from the environment or to render them harmless, is considered as an environment-friendly method to remediate contaminated soil in-situ and has been applied for some other heavy metals. Whether this approach is suitable for remediation of Hg-contaminated soil is, however, an open question. The aim of this thesis was to study the fate of Hg in terrestrial plants (particularly the high biomass producing willow, Salix spp.) and thus to clarify the potential use of plants to remediate Hg-contaminated soils.

Plants used for phytoremediation of Hg must tolerate Hg. A large variation (up to 30-fold difference) was detected among the six investigated clones of willow in their sensitivity to Hg as reflected in their empirical toxicity threshold (TT95b), the maximum unit toxicity (UTmax) and EC50 levels. This gives us a possibility to select Hg-tolerant willow clones to successfully grow in Hgcontaminated soils for phytoremediation.

Release of Hg into air by plants is a concern when using phytoremediation in practice. No evidence was found in this study that Hg was released to the air via shoots of willow, garden pea (Pisum sativum L. cv Faenomen), spring wheat (Triticum aestivum L. cv Dragon), sugar beet (Beta vulgaris L. cv Monohill), oil-seed rape (Brassica napus L. cv Paroll) and white clover (Trifolium repens L.). Thus, we conclude that the Hg burden to the atmosphere via phytoremediation is not increased.

Phytoremediation processes are based on the ability of plant roots to accumulate Hg and to translocate it to the shoots. Willow roots were shown to be able to efficiently accumulate Hg in hydroponics, however, no variation in the ability to accumulate was found among the eight willow clones using CVAAS to analyze Hg content in plants. The majority of the Hg accumulated remained in the roots and only 0.5-0.6% of the Hg accumulation was translocated to the shoots. Similar results were found for the five common cultivated plant species mentioned above. Moreover, the accumulation of Hg in willow was higher when being cultivated in methyl-Hg solution than in inorganic Hg solution, whereas the translocation of Hg to the shoots did not differ.

The low bioavailability of Hg in contaminated soil is a restricting factor for the phytoextraction of Hg. A selected tolerant willow clone was used to study whether iodide addition could increase the plant-accumulation of Hg from contaminated soil. Both pot tests and field trials were carried out. Potassium iodide (KI) addition was found to mobilize Hg in contaminated soil and thus increase the bioavailability of Hg in soils. Addition of KI (0.2–1 mM) increased the Hg concentrations up to about 5, 3 and 8 times in the leaves, branches and roots, respectively. However, too high concentrations of KI were toxic to plants. As the majority of the Hg accumulated in the roots, it might be unrealistic to use willow for phytoextraction of Hg in practice, even though iodide could enhance the phytoextraction efficiency.

In order to study the effect of willow on various soil fractions of Hg-contaminated soil, a 5-step sequential soil extraction method was used. Both the largest Hg-contaminated fractions, i.e. the Hg bound to residual organic matter (53%) and sulphides (43%), and the residual fraction (2.5%), were found to remain stable during cultivations of willow. The exchangeable Hg (0.1%) and the Hg bound to humic and fulvic acids (1.1%) decreased in the rhizospheric soil, whereas the plant accumulation of Hg increased with the cultivation time. The sum of the decrease of the two Hg fractions in soils was approximately equal to the amount of the Hg accumulated in plants. Consequently, plants may be suitable for phytostabilization of aged Hg-contaminated soil, in which root systems trap the bioavailable Hg and reduce the leakage of Hg from contaminated soils.

Place, publisher, year, edition, pages
Stockholm: Botaniska institutionen , 2004. , 41 p.
Keyword [en]
phytoremediation, phytoextraction, phytostabilization, bioavailability, willow, Hg
National Category
Botany
Identifiers
URN: urn:nbn:se:su:diva-307ISBN: 91-7265-975-0 (print)OAI: oai:DiVA.org:su-307DiVA: diva2:192263
Public defence
2004-12-21, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-11-30 Created: 2004-11-30Bibliographically approved
List of papers
1. Absence of Hg transpiration by shoot after Hg uptake by roots of six terrestrial plant species
Open this publication in new window or tab >>Absence of Hg transpiration by shoot after Hg uptake by roots of six terrestrial plant species
2005 (English)In: Environmental Pollution, ISSN 0269-7491, E-ISSN 1873-6424, Vol. 134, no 2, 201-208 p.Article in journal (Refereed) Published
Abstract [en]

In this paper we investigated if, and to what extent, six different plant species accumulate, translocate and emit mercury (Hg) into the air. The Hg uptake by roots, distribution of Hg to the shoot and release of Hg via shoots of garden pea, spring wheat, sugar beet, oil-seed rape, white clover and willow were investigated in a transpiration chamber. The airborne Hg was trapped in a Hopcalite trap or a gold trap. Traps and plant materials were analysed for content of Hg by CVAAS. The results show that all plant species were able to take up Hg to a large extent from a nutrient solution containing 200 μg L−1 Hg. However, the Hg translocation to the shoot was low (0.17–2.5%) and the Hg that reached the leaves was trapped and no release of the absorbed Hg to the air was detected.

Mercury translocation to shoots was low.

Keyword
Mercury, Accumulation, Translocation, Hg release
National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-23475 (URN)10.1016/j.envpol.2004.08.007 (DOI)
Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2010-10-28Bibliographically approved
2. Clonal differences in mercury tolerance, accumulation and distribution in Willow
Open this publication in new window or tab >>Clonal differences in mercury tolerance, accumulation and distribution in Willow
2004 In: Journal of Environmental Quality, ISSN 0047-2425, Vol. 33, 1779-1785 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23476 (URN)
Note
Part of urn:nbn:se:su:diva-307Available from: 2004-11-30 Created: 2004-11-30Bibliographically approved
3. Changes in Hg fractionation in soil induced by willow
Open this publication in new window or tab >>Changes in Hg fractionation in soil induced by willow
2005 (English)In: Plant and Soil, ISSN 0032-079X, E-ISSN 1573-5036, Vol. 275, no 1-2, 67-75 p.Article in journal (Refereed) Published
Abstract [en]

This study investigated the effect of willow (Salix viminalis ×  S. schwerinii) on soil characteristics, including changes in Hg fractionation in the soil solid phase, and Hg accumulation and distribution in pot-grown plants cultivated for 32 and 76 days in aged Hg-contaminated soil (30 mg Hg kg−1DW). Changes in soil pH and organic carbon content as well as in Hg fractionation were monitored in both rhizospheric soil and in soil without plants. Mercury fractionation was performed by a 5-step sequential soil extraction procedure. Organic carbon content increased while pH decreased in the rhizospheric soil. Both chemically defined exchangeable Hg (0.1) and Hg bound to humic and fulvic acids (1.1) decreased in the rhizospheric soil, whereas plant accumulation of Hg increased with cultivation time. The sum of the decrease of these two soil Hg fractions after 76 days of cultivation was approximately equal to the amount of Hg accumulated in plants. Furthermore, the major Hg fractions (Hg bound to residual organic matter (53), sulphides (43), and the residual fraction (2.5)) remained stable. Neither whole plant accumulation of Hg from the soil, approximately 0.2 of total Hg in soil after 76 days cultivation, nor the fraction of total plant Hg in the shoots, which accounted for about 3 of the total plant Hg pool regardless of the cultivation time, were high. The overall results suggest that plants might be suitable for phytostabilization of aged Hg-contaminated soil, where root systems trap bioavailable Hg and help to control both leaching of Hg and re-entrainment of Hg-containing particulates from a contaminated site.

Keyword
bioavailability - mercury - phytoremediation - rhizosphere - Salix - soil extraction
Identifiers
urn:nbn:se:su:diva-23477 (URN)10.1007/s11104-004-6108-x (DOI)
Note
Part of urn:nbn:se:su:diva-307Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2010-08-05Bibliographically approved
4. Use of iodide to enhance the phytoextraction of mercury-contaminated Soil
Open this publication in new window or tab >>Use of iodide to enhance the phytoextraction of mercury-contaminated Soil
2006 (English)In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 368, no 1, 30-39 p.Article in journal (Refereed) Published
Abstract [en]

Low bioavailability of mercury (Hg) in soil is a restricting factor in phytoextraction of Hg-contaminated soil. Iodide is known to mobilize Hg in soil and, therefore, the aim of this study was to investigate the possibility to use iodide to increase phytoextraction of Hg. The sensitivity of willow to iodide was investigated in both hydroponics with 0–10 mM KI and in soil with 0.2 and 1 mM KI addition. The capacity to accumulate HgI2 was compared with that of Hg(NO3)2 in hydroponics. Soil extractions with 0–2 mM KI were used to study mobilization of Hg in aged Hg-spiked soil. Additions of KI (0.2–1 mM) were used in pot tests with aged Hg-spiked soil as well as in field trials in an industrial Hg-contaminated soil to study whether iodide addition increased the accumulation of Hg from contaminated soil and the translocation of Hg to the shoots. The total Hg contents in plants, soils and extracts were analyzed by CVAAS. The results showed that too high KI concentration was toxic to plants. Moreover, KI was toxic to plants at lower concentrations in hydroponics than in soil additions. KI (85 μM) in hydroponics gave 50% growth decrease in terms of dry weight of shoot biomass, whereas 0.2 mM KI in soil addition could be tolerated by the plant without growth disturbance. Willow accumulated Hg from HgI2 solution slower than that from Hg(NO3)2 solution. KI mobilized Hg in contaminated soil and thereby increased the bioavailability of Hg in soil. Up to 1 mM KI addition increased the Hg concentrations to about 5, 3 and 8 times, respectively, in the leaves, branches and roots. However, the Hg translocation to the shoots did not show an increase with KI addition. Only less than 1% of the total Hg accumulated in the plant was distributed in the shoots. We may conclude that iodide addition enhances the phytoextraction of Hg, however, the translocation of Hg to the shoots is still too low and therefore it will not be realistic to use this method for phytoextraction of Hg-contaminated soil in practice.

Place, publisher, year, edition, pages
Elsevier B.V., 2006
Keyword
Bioavailability; Iodide; Mercury; Phytoextraction; Soil; Toxicity; Willow
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-23478 (URN)10.1016/j.scitotenv.2005.09.034 (DOI)
Note
Part of urn:nbn:se:su:diva-307Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2010-11-03Bibliographically approved

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