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Gas analysis is both of the ambient air and sub-surface atmospheres for major and trace components. Details of gas analysis procedures will be discussed later in Chapter 6.
Why do you think care in quality assurance and control is particularly important when analysing waste samples?
5.7 Specific Considerations for the Analysis of Sediments and Sewage Sludge
5.7.1 Sampling and Storage
The first problem with sediment analysis is to obtain the sample from the river or sea beds. Core samplers are available for shallow areas. A simple core sampler used for this purpose is shown in Figure 5.9(a). Using this device, a cylindrical tube is first driven into the sediment. On withdrawal, the valve system closes which allows the sample to be withdrawn from the sediment. Just before breaking the surface of the water, the tube is then sealed to preserve the sediment structure. In this way, sections corresponding to different depths in the sediment can be analysed, which can provide a historical record of the deposition of pollutants. Grab samplers (Figure 5.9(b)) may be used for greater depths, or where the sediment is loose so that there is no vertical structure. Dredging can be used for coarse sediments. Samples are often stored deep-frozen.
5.7.2 Pretreatment DQ 5.17
What is the main difference between sediment samples and other samples we have looked at, which may modify the pretreatment?
166 Introduction to Environmental Analysis
Figure 5.9 Schematics of sediment samplers: (a) simple core sampler (b) grab sampler. Answer
Quite simply, it is the high water content.
On return to the laboratory the sample is thawed and screened to remove large contaminants such as stones and twigs, if necessary by using pressure. Most of the previous methods we have looked at then dry the sample. It would be
Analysis of Land, Solids and Waste
quite impracticable to remove the water from sediment by air-drying at room temperature and if the analysis is to determine organic material, the analysis proceeds by using wet samples. Samples for metal ion analysis may be oven dried at 110° C before further treatment. Pretreatment may also include the separation of the sample into size fractions by wet sieving.
A map showing the distribution of trace metals around a marine site for the dumping of solid waste showed a wider geographic distribution of metals in the lower size fractions. What do you think might be happening here?
The smaller particles are drifting more in the local currents.
Can you remember the other reasons why an analysis of size fractions may be useful? Look again at Section 5.1 above if you are unable to do this.
5.7.3 Extraction Techniques for Organic Contaminants
Organic analysis is once again based on solvent extraction of the homogenized slurry (produced by using a high-speed blender), often using a Soxhlet apparatus. The extracting solvents we have previously discussed are often extremely hydrophobic. If they were used with a wet sediment sample, we would end up with water-solvent emulsions. In order to overcome this, more polar extractants such as acetonitrile or acetone are used. Concentrations of organic pollutants are often in the ^g kg-1 range.
5.7.4 Dissolution Techniques for Trace Metals
Acid dissolution is used for the analysis of adsorbed metal ions but care has to be taken not to dissolve the bulk sediment itself. A suitable acid mixture would be concentrated nitric acid and hydrogen peroxide, with metal concentrations in the mg kg-1 region being expected.
Extreme solubilization techniques have to be used for analysis of the less-soluble portion of the sediment. A typical method uses hydrogen fluoride under pressure in a ‘Teflon’-lined ‘Parr’ bomb. We will be looking later at other methods for insoluble solid analysis in Chapter 7.
Why do you consider that in some cases analysis of adsorbed metal ions is most relevant for a study of environment problems, whereas in other cases analysis of the whole of the sediment is more appropriate?
Introduction to Environmental Analysis
Environmental analysis is concerned with ions and compounds which are available to living species. Only loosely adsorbed metal ions will always be readily available. The availability of the ions in the bulk of the sediment will depend, among other criteria, on the particle and the chemical composition.
5.7.5 Analysis of Sewage Sludge
Sewage sludge is included in this section due to the high water content of such samples, which require similar pretreatment for trace-level organics to those discussed above for sediment samples. The material, however, has a high organic content and so digestion is necessary before any metal analysis is carried out. Typically, this would involve heating with concentrated nitric acid in a Kjeldahl apparatus (see Figure 5.3) and extraction of the metal ions after dilution with water.
What are the relative merits of investigating metal or insoluble organic compound pollution in a river or sea by: