PROCEDIMIENTO DE APERTURA Y CIERRE DE CAJAS

Bioavailable trace metals play a most important role in the lake ecosystem. It has been shown that trace metal levels in various species are best related not to total metal concentrations in the adjacent sediments, but rather to more easily extracted fraction (Tessier and Campbell, 1987; Tessier et a l, 1984). Qian et a i (1996) also demonstrated that total metal content was a poor indicator of metal absorption by plants.

Forstner (1985) demonstrated that the state of knowledge on solid matter spéciation of trace metals is still somewhat unsatisfactory with respect to the assessment of the quantitative extent of bioavailable element concentrations. This is mainly due to the effect of competition between adsorption sites on the solid substrate and selective mechanisms of metal uptake by different organisms - a situation which cannot be improved by more sophisticated extraction data. For bioavailability studies,

concentrated nitric acid (HNO3) was the preferred extractant for determining ‘total’ metal concentrations in sediment because it was associated with minimal problems of precipitation (Forstner, 1985). Luoma and Bryan (1984) used multiple regression analyses of a large dataset involving measurements of Ag, Cd, Co, Cu, Pb, and Zn in

Scrobicularia plana, Nereis diversicolor and in various sediment extracts (together

with measurements of other sediment characteristics) and reached similar conclusions, that concentrations of metals extracted from the sediments with HCl or HNO3

generally explained the largest percentage of the total variation in metal concentrations in both deposit-feeding organisms.

Because metals distribute in different phases of sediments and soils, their activities differ. Usually, activity of a metal depends on the phases in which the metal exists, and

only part of a metal can influence an ecosystem. According to the critical load concept defined by UN/ECE Working Group on Nitrogen Oxides (mentioned in 1.7), only “significant harmful effects” are considered in critical loads. For a lake ecosystem, the

“significant harmful effects” considered are on the biota, and the fraction of metals which can have an effect on the biota in the lake ecosystem is the bioavailable fraction. In this study the bioavailable fractions of metals are measured. Sediments are digested with HNO3 and for consistency, the other materials are digested in the same way.

In order to select digestion conditions, standard samples, Buffalo River sediment

(Standard Reference Material 2704) and Stream Sediment Reference Material (GBW07305), were used.

In developing the digestion procedure, the volume of concentrated nitric acid, digestion temperature and time were the key stages. Different volumes (5ml, 8ml, and

10ml) of concentrated nitric acid were tested. In all digestion processes, blank and standard samples were treated in the same way. In general, when standard sediment samples (100°C on hot-plate for 1 h) were digested using different volumes of nitric

acid, similar results were obtained. For Lochnagar peat soil samples, if the sample weight was greater than 0.3 g, the 5 ml digestion still contained pieces of plant fragment whereas the 8 ml and 10 ml solution did not. Therefore 8 ml concentrated

nitric acid (Aristar) was selected for the digestion. In all digestion processes, blank samples were digested in the same way.

The results of different heating temperatures (80°C, 100°C, for Ih) and heating times

(heated at 100°C for 1 h, 1 h and leave to cool overnight, 3 hrs, 3 hrs and leave to cool

overnight) were as shown in Table 2.3 and Table 2.4.

Different heating times and temperatures show different abilities to take metals into

solution. In the digestions above, 100°C is better than 80°C in taking metals into

quickly. Heating a sample at 100°C for 1 h is sufficiant time, and is more practical in the laboratory when compared to longer times.

T able 2.3 Results of different heating temperature( 1 h)

Element Certified 80°C 100°C Al% 6 . 1 1 0.534 0.59 Ca% 2.60 1.87 1.93 Mg% 1 . 2 0 0.657 0 . 6 8 Fe% 4.11 1.99 2.25 Mn p.g/g 555 354 360 Co |Llg/g 14.0 12.4 15 Cr pg/g 135 63 78 Cu pg/g 9&6 80L8 81 Ni pg/g 44.1 34.2 37 Pb pg/g 161 142 158 Zn pg/g 438 260 318

T able 2.4 Results of different heating time(at 100°C)

Element Certified Ihr 1 hr+ on 3hrs 3hrs+on

Al% 6 . 1 1 0.59 0.583 0.597 &586 Ca% 2.60 1.93 1.94 1.93 1.94 Mg% 1 . 2 0 0 . 6 8 0.76 0.69 0 . 6 8 Fe% 4.11 2.25 2.23 2.3 2 . 2 Mnpg/g 555 360 378 358 390 Copg/g 14.0 15 16.9 14.7 14.5 Crpg/g 135 78 7T8 75 6 8 Cupg/g 9&6 81 84 80 81 Ni pg/g 44.1 37 42 37 45 Pbpg/g 161 158 167 168 179 Znpg/g 438 318 308 318 309 leave overnight.

The method for digestion in this study was therefore, to heat at 100°C for Ih with 8 ml

HNO3. The procedure is:

1. Weigh 0.2-0.3 sample into a Teflon beaker; 2. Add 8 ml concentrated nitric acid to the beaker;

3. Heat the beaker on the hot-plate for 1 hour;

4. Wait for the solution to cool (this is good for filtering the solution, especially for

peat samples), transfer the solution to a 2 0ml volumetric flask and add deionized

distilled water to make up to 20ml. Filter the solution into a plastic conical base test tube using a Whatman GF/A filter paper. Blank samples were filtered for monitoring blank value. The test tube was covered by a screw cap immediately.

For Hg analysis, the method was slightly changed. As the Hg measured by cold vapour AAS is the total value existing in the circular system (see Section 2.4.3.2). If the solution is filtered, some of the Hg will be lost (absorbed by the filter paper with some solution). Consequently, the digested solution was not filtered, but transferred into a plastic 30 ml Sterilin container directly. The residue was washed twice and the washings transferred into the same container. The container was covered with a plastic

flowed seal cap immediately. In this case, the Hg can be totally kept in solution.

Using centrifugation instead of filtering was tested. For the digested peat solution, even after centrifuging at 3000 rpm for 15 minutes, there were still many small fragments suspended in the solution.