BINDING MECHANISMS OF METALS IN MOSS

Gyda May and Eiliv Steinnes (Department of Chemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway). gydam@stud.ntnu.no

 

ABSTRACT

Moss monitors atmospheric pollution by absorbing metals from wet and dry deposition. By way of sequential extraction, it is possible to separate the metals into fractions, each belonging to a different part of the moss. Each element is then determined in each fraction to see the distribution in the moss. The moss Hylocomium spendens was taken from two locations near Trondheim, Norway, and the cellular locations of different metals and their variation with age were determined. The divalent elements Ca, Mg, Zn, and Mn were found mostly in exchangeable form whereas K and Pb were mainly found in intracellular locations. In the particulate fraction, the sample segments tended to increase in the amount of K, Mg, Ca, Zn, and Mn with age, while the intracellular fraction showed the opposite trend for K, Mg, and Zn. A slight increase in the amount of Zn was visible with increasing age of the samples in the exchangeable fraction.

 

INTRODUCTION

Mosses are frequently used as monitors of atmospheric deposition of metals, because they have no root system and thus pick up chemical substances mainly from wet and dry deposition. Metal ions in solution may be fixed electrostatically to the negatively charged moss surface and eventually replaced by exchange with other cations (Gjengedal and Steinnes 1990). Some cations may possibly form complexes with ligands on the moss surface. Certain metals, in particular those which are biologically essential, may also be stored intracellularly after penetration of the cell wall. Moreover metals may ble supplied to the moss as part of aerosols physically trapped on the moss surface. The areosols may remain on the moss as such or be gradually eroded and thereby releasing metals in ionic form.

 

When using mosses as biomonitors the total concentration of  a metal is usually taken as a relative measure of the atmospheric deposition of the metal, regardless of the distribution of the metal between different binding forms. Few attempts have been reported in the literature to elucidate the speciation of metals in moss, and no such work has been attempted in Norway so far in spite of a long history of biomonitoring with mosses. In UK however Brown and coworkers have published several papers  where attempts are being made to distinguish between different binding forms by sequential extractions (e.g. Brown 1990, 1991, Brown and Brumelis 1996). In the present work  this sequential elution approach was tested on moss samples from the Trondheim area moderately or little exposed to air pollutants. Elements selected for study were the macronutrients Ca, K and Mg, the micronutrient Mn, the pollutant element Pb, and Zn which may occur in the moss partly as a micronutrient, partly as a pollutant.

 

EXPERIMENTAL

Samples of the feather moss Hylocomium splendens, which grows on the forest floor and has been frequently been used in northern Europe as a biomonitor of atmospheric metal deposition, were selected for this study. Samples were collected in October 1999 at two locations near Trondheim: Trolla, which is a pristine site and Heimdal, where some influence from a highway and a domestic waste incinerator was to be expected.

 

The mosses were subjected to a sequential extraction procedure consisting of 5 steps, as indicated in Table 1. The residual fraction from the last extraction step was defined as particulate.

 

Table 1 Extraction times and respective fraction of the moss involved

 

order

elution

shaking time

location

1

deionized water (20ml)

40 minutes

intercellular

2

deionized water (20ml)

30 minutes

intercellular

3

20mM NiCl2 (20ml)

40 minutes

exchangeable

4

20mM NiCl2 (20ml)

30 minutes

exchangeable

5

1M HNO3  (32ml)

60 minutes

intracellular

 

 

 

 

 

The extractions were carried out with field moist moss samples of about 3g. Dry weight of the moss was determined in a separate aliquot. Prior to extraction the moss was separated into 3 annual segments, representing the  period 1997-1999, in order to investigate possible changes in distribution as a function of time. Concentrations of the metals in the eluates were determined by flame atomic absorption spectrometry against standard solutions made up with the chemicals used as extractants. The concentration of Pb in the extracts from the first four steps were below the analytical detection limit.

Total concentrations in the moss were determined after decomposition with concentrated nitric acid. The sum of extracted amounts in the various fractions was in most cases in good agreement with the total concentration determined separately.

 

RESULTS AND DISCUSSION

Results from the sequential extractions of the Trolla and Heimdal mosses are shown in Figs.1-2, where the percent distribution of the various elements between the different fractions is shown for moss segments from the three years. A brief discussion of the results follows:

K: The major fraction (70-80%) was present intracellularly, whereas the remaining amount, somewhat surprisingly, appeared to be in particular form. The intracellular fraction showed a small decrease with time, while the particulate fraction tended to increase.

Mg: The exchangeable fraction contained the largest amount of Mg, except for the older Heimdal segments, where the particulate fraction was higher. The particulate fraction tended to increase with time, whereas the intracellular fraction showed a small decrease with time.

Ca: The highest amount of Ca was always found in the exchangeable fraction. The next largest amount found within the cells was, on average, about half of that found in exchangeable form. The particle fraction always tended to increase with age.

Zn:  Zn had the highest amount in the exchangeable fraction, but the intracellular concentration was only slightly lower. An increase in Zn occurred with increasing age of the segments in the exchangeable and particulate fractions, whereas a similar decrease was evident in the intracellular fraction.

Mn: The largest amount of Mn was in the exchangeable fraction (with one exception). Next largest was the intracellular fraction, on average around half of the exchangeable fraction.  Mn increased with age in the particulate fraction.

Pb:  According to the results, lead was mostly found within the cell. The particulate fraction had half to one fourth of that amount. The amount of Pb in the intercellular and  exchangeable fractions consisted of less than 2% of the total amount in most cases.

It should be noted however that Pb may form strong complexes with functional groups on the outer cell surface. If so, the complex-bound Pb would have a tendency to occur in the 1M HNO3 fraction along with the intracellular amount.

General: The divalent elements Ca, Mg, Zn, and Mn all occurred predominantly in exchangeable form whereas K was mainly found within the cell. These findings confirm results from other investigations (Brown and Brumelis 1996).   

 


 

 

 

 

 


REFERENCES

E. Gjengedal and E. Steinnes (1990), Environ. Monitoring and Assessment 14: 77-87

 

D. H. Brown and G. Brumelis (1996), The Science of the Total Environ. 187: 153-161

 

D. H. Brown (1991), Symbiosis 11: 207-223