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Analyses for the extractable components follow the methods discussed previously in Chapters 3 and 4, after taking into account the higher concentrations and more complex analytical matrix involved. The discussion questions given below are to encourage you to examine how these analytical procedures are used.
What modifications would you suggest would be needed for the BOD and COD methods described earlier in Section 3.3?
The BOD values shown above in Table 5.3 are extremely high and the samples would need to be diluted by up to a 1000 times. After dilution, the samples may have low nutrient levels, and thus additional nutrients would have to be considered. The microbial activity necessary for the test may also be inhibited by other components in the waste. For the COD analysis, note the high relative concentration of chloride ions (Table 5.3), particularly in the mature leachate. Mercury (II) sulfate may need to be included in the procedure in order to minimize any potential interference.
184.108.40.206 Trace Organics
In this case, analysis is most often carried out by GC. HPLC with UV detection can be used for the analysis of trace components in leachates which are liable to contain high concentrations of hydrocarbons (e.g. from dumped fuel). The hydrocarbons themselves exhibit no response. You could compare this to analysis by GC (with flame ionization detection) where the chromatogram would be swamped with hydrocarbon peaks unless there was a substantial clean-up stage. A second use of HPLC would be to investigate high-molecular-mass components
Analysis of Land, Solids and Waste
which are not sufficiently volatile for direct GC analysis. Ion chromatography may be used for non-extractable organic acids.
Which GC column type would you think best for determining trace organics in leachates?
Capillary columns are needed to separate the large number of expected components (see Section 4.2.3 above). Wide-bore, rather than narrow-bore, capillaries would be preferred as these are less likely to be affected by high-molecular-mass impurities. They may be necessary if sample introduction is by a purge-and-trap device.
220.127.116.11 Target Compounds
Preliminary investigations may be simplified if compounds can be identified which can act as markers for pollution. These are known as ‘target compounds’. A typical application would be to identify if groundwater was polluted. Often, volatile non-polar organics are used.
Why do you think volatile non-polar organics are used?
Volatile compounds tend to be relatively small molecules. These are more soluble than their higher-molecular-mass analogues (see Section 2.3 above) and so are more likely to have migrated away from the source.
The volatility of the compounds is not a problem in groundwater as there is little chance of vaporization. Highly polar compounds (e.g. acetone, ethanol and acetic acid) can be analysed by GC but they are more problematic than neutral compounds. Such compounds may produce tailing GC peaks or need prior derivitization before analysis.
Marker compounds can also be used to identify compound classes within complex mixtures. BTEX compounds (benzene-toluene-ethylbenzene-xylene(s)) may be used to indicate the presence of petroleum products.
18.104.22.168 Trace Metal Analysis
We have seen earlier in Section 4.3 that metal analysis in natural waters is now largely carried out by using atomic spectrometric techniques. This is also the case for leachate analysis. If spectrometric techniques were attempted, the additional metal ions present in the complex mixture could lead to interferences in the analyses. Leachate sample preparation may be different from what we
Introduction to Environmental Analysis
found with relatively pure natural waters samples. A decision first needs to be made as to whether analysis of dissolved or suspended metal is required. The metal content in the suspended solids may, in fact, be greater than in solution and so any dissolution stage would lead to an unrepresentative analytical concentration. It may be considered better to analyse the solid and dissolved components separately. The sample must be analysed as quickly as possible since the standard preservation technique for metals (acidification) could alter the relative proportions of the dissolved and undissolved phases. If atomic absorption spectrometry is being used, a background correction is necessary due to the complex and largely unknown matrix. Quantification should be by using the standard addition procedure.
5.6.5 Introduction to Gaseous Emissions
The major components and their change in relative concentrations with time are shown in Figure 5.8. Carbon dioxide is the main product when there is a plentiful
Figure 5.8 The main components of landfill gas emissions and their changes in relative concentrations with time.
Analysis of Land, Solids and Waste
supply of oxygen. After a short period of time, the oxygen becomes depleted and degradation continues under anaerobic conditions to produce methane. This continues for many years until all the biodegradable material has been consumed. Some landfill sites collect the methane for use as an energy source. Other components include volatile organics. Individual compounds can reach ppm (v/v) concentrations. These are greenhouse gases and contribute to the volatile organic compound (VOC) loading in the atmosphere. Unpleasant smells and local nuisance can also be produced from the volatile organics, particularly those containing sulfur, and also hydrogen sulfide. The total concentrations of sulfur-containing compounds can reach to ca. 1% under reducing conditions.