Dry and wet deposition of mercury near a chlor-alkali plant

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Abstract

The dry and wet deposition near a Finnish chlor-alkali plant was estimated by using the moss-bag technique. The estimated net deposition was 130 ng g−1 per month or 480 μg m−2 per year. Two-thirds of the deposition was dry and one-third wet. The results emphasize the importance of the direct uptake of atmospheric mercury by vegetation.

Introduction

More than 90% of mercury in the atmosphere is in the gas phase, mainly as Hg0 and the rate of particle bound mercury is small [approx. 2% (Brosset, 1987, Iverfeldt, 1991a, Petersen et al., 1995)]. Mercury has a great evaporation rate even in a cool climate. Thus emissions to air may disperse over long distances. The oxidation to water soluble forms is slow and the half-life of elemental mercury is approx. 1 year (Brosset, 1987, Lindqvist et al., 1991). Mercury is emitted from a chlor-alkali plant as elemental mercury but little is known about the possible oxidation of chlorine compounds which may be present in this environment. Sorption and desorption are temperature dependent and differ for different mercury species. Gaseous mercury (Hg0) may be removed from the atmosphere by wet or dry deposition. Hg0 may be oxidized by chemical oxidants to water soluble forms, e.g. Hg(II). Irreversible sorption of mercury by canopy surfaces may occur for both wet and dry deposition, but sorbed mercury may also be released by leaching or evaporation (Lindberg et al., 1994). Dissolved mercury species are obviously rapidly attached to vegetation surfaces or particles while the process and velocity of uptake of gaseous mercury (mainly Hg0) by vegetation is poorly known.

The sorption to and re-emission from vegetation can be assumed to depend on:

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    Type and abundance of vegetation (different in summer and winter);

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    Light and weather (stomata open/closed); and

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    Air moisture and temperature.

The aim of this study was to measure dry and wet deposition of mercury near an emitter of Hg0 by using the moss-bag technique and evaluate the importance of uptake processes.

Section snippets

Material and methods

Moss bags were placed near a chlor-alkali plant (Finnish Chemicals) in Kuusankoski, SE Finland, for the period 26 May–24 July 1995 (59 days). The moss-bag technique is based on the exceptionally effective ion-exchange of metals in the tissue of Sphagnum moss where metals are absorbed directly from the air (Lodenius, 1989, Crist et al., 1996). The moss was washed with 0.5 M HCl, rinsed with deionized H2O and approx. 15 g (wet wt.) of moss tissue was placed in each nylon net (φ5–7 cm). This

Results

The accumulation was significantly higher in moss bags receiving both wet and dry deposition than in the covered moss bags (Table 1). The mean total (wet+dry) net accumulation of mercury was 130 (±20; S.D.) ng Hg per gram of moss per month. This can be estimated to correspond to a deposition of 480 (±75) μg m−2 per year [surface occupied by moss 32 mg m−2 (Lodenius and Tulisalo, 1984, Lodenius, 1995)]. Approximately two-thirds of the mercury deposition was dry and one-third wet (Fig. 2). The

Discussion

There is a continuous exchange of mercury between atmosphere and vegetation in both directions. Wet deposition is the most import form of removal of Hg from the atmosphere over water surfaces while both wet and dry processes are important over terrestrial ecosystems. Dry deposition includes absorption of gaseous and particle-bound mercury.

It is difficult to make reliable quantitative estimates of the deposition pathways of mercury. Different methods has been used: technical gauges, model

Acknowledgements

I am indebted to Mr Esa Tulisalo, B.Sc., for technical assistance.

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