Clean-up techniques

This part of the chapter lists, in detail, some techniques for the treatment of metal-containing effluents. The chemistry behind the techniques will be described with a view for use in effluent treatment. The advantages and disadvantages will be listed and some guide to the efficacy of the technique will be given. Wherever possible examples of successful use will be mentioned. Some of the techniques are tried and tested, whereas others are relatively new. 

For a given clean-up problem the eventual choice of technique is not only influenced by the chemistry but also by the form of waste. For example, metals contained in the hot flue gas from a power station cannot be treated in the same way as metals dissolved in a mine tailings lagoon. Most of the described techniques are applicable to aqueous effluent but the chemistry remains essentially the same whatever the situation. With an adequate grasp of the chemistry behind each technique, the reader will be able to select the one most appropriate for any form of metal-containing effluent. 

Precipitation is, by far, the most common method for dealing with metal-containing waste. The theory is simple. Some metal salts are very insoluble, precipitation generates these insoluble salts in the waste stream by the addition of an appropriate counter anion the precipitate is then filtered off (scheme 14.2). 

The anion Y is usually hydroxide (OH ), sulfide (S ) or carbonate (C03 ) and M is usually an alkali metal or alkaline earth. Broadly speaking, the effectiveness of a particular precipitant for a particular toxic metal M can be estimated by looking-up the solubility product of the precipitated metal salt M Y . The lower the solubility product the more effective the precipitant. Table 14.5 lists the solubility products of some common metal salts. 

All the solubility products are low, certainly low enough to meet very strict emission legislation. Some salts, especially sulfides are especially low indeed, sulfide salt precipitation can lead to very effective effluent treatment. In practice however these limits are seldom achieved. The numbers in Table 14.5 are obtained in ideal laboratory conditions, for practical use they can only be used as a guide. Many other factors affect the efficiency of precipitation of these the most important is the presence of an excess of other ions or complexing agents. For example low levels of citrate can strongly affect iron precipitation. 

The mass of sample taken for analysis is primarily dependent on four factors (1) the amount of material available, (2) the concentration of the analyte, (3) the heterogeneity of the sample, and (4) the method of analysis. Most conventional solvent extraction techniques currently start with more sample than is required, use more extraction solvent than is necessary, and ultimately only analyze 0.1% of the material prepared, e.g., 1 pi from 1 ml. Micro-extraction techniques [468] can be used in conjunction with on-line LC-GC or LC-MS to utilize the whole extract in the final determinations. This approach can significantly reduce the size of sample required and the volume of solvent used. Many workers have reported the use of solid phase microextraction (SPME) in different environmental matrices for various pollutants [288,342,345,469 - 477]. 

Normally an extraction technique is selected to give the highest recovery for a wide range of pollutants. Therefore, the extract will most likely contain a high proportion of co-extracted material. Many of the clean-up techniques have been tailored into a series of multi-residue schemes in order to maximize the use of each sample [189,402,453,454,478-481]. This is of particular value when the maximum amount of chemical information is required for each sample. 

The main requirement for any clean-up and group separation scheme is that it effectively removes not only the bulk of the co-extractants,such as lipids, sulfur, carotenoids, and other pigments, but also those compounds that may potentially interfere in the final determination. There are three main ways in which co-ex-tracted material may interfere in the final determination if not removed  

Gross contamination can overload the HPLC or GC columns with obvious and usually rapid deterioration of chromatographic performance. This can occur with so called rapid techniques where the detector is used as a filter, e.g., selected ion monitoring (SIM) MS, or where the clean-up method has been overloaded (e.g., excess of lipid). This problem can be overcome by using and monitoring more selective clean-up techniques. 

Interference occurs when compounds co-elute with the analytes and are not detected directly by a specific detector. The effect is to create negative peaks or an erratic response for the analyte. This problem can be identified by using a non-specific detector such as an ion trap MS detector, an MS in the electron impact ionization mode, or a flame ionization GC detector. 

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On-line LC-GC has frequently been used as a clean-up technique for the analysis of trace levels of contaminants (pesticides, plasticizers, dyestuffs and toxic organic chemicals) in water and food products. Several different approaches have been proposed for the analysis of contaminants by on-line LC-GC. Since pesticide residues occur at low concentration in water, soil or food, extraction and concentration is needed before GC analysis is carried out. 

Principles and Characteristics Solid-phase extraction (SPE) is a very popular sample preparation and clean-up technique. In SPE solutes are extracted from a liquid (or gaseous) phase into a solid phase. Substances that have been extracted by the solid particles can be removed by washing with an appropriate liquid eluent. Usually, the volume of solvent needed for complete elution of the analytes is much smaller (typically < 1 mL) than the original sample volume. A concentration of the analytes is thus achieved. 

There are a large number of extraction and clean-up techniques. Some of the more common ones are outlined in Table 4.5. The technique selected will depend on the nature of the sample and of the analyte. 

The solvent extraction of chlorinated pesticide residues from soil is often achieved by using mixtures of solvents such as hexane-isopropanol or hexane acetone, but can be unsatisfactory owing to the emulsification problems [2, 3] or, with hexane-isopropanol, poor recovery [2, 4], Acetone extraction of soil is efficient [4, 5] but problems can arise from large amounts of coextracted material unless an efficient clean-up technique [6] is used prior to analysis by gas chromatography. 

These problems are overcome by applying a tailored LC separation prior to the final determination and having a built-in feedback to monitor the success of the separation or to give a warning of any failure. The following are the most commonly used clean-up techniques in organic analysis of environmental pollutants. 

Modem extraction and clean-up techniques, such as pressurised liquid extraction and microwave-assisted extraction, have almost not applied to the analysis of PFCs yet. Llorca et al. [49] reported the development and application of a PEE method for PFCs determination in fish. This technique provided rapid and accurately clean extracts for sensitive analysis. 

Walters SM. 1990. Clean-up techniques for pesticides in fatty foods. Anal Chim Acta 236 77-82. 

V-Nitrosodiethanolamine has been found in many complex matrices such as cutting and grinding fluids and cosmetics. Analysis for V-nitrosodiethanolamine is complicated by the matrix and a clean-up technique with derivatization is typically required before quantitation of the analyte to achieve adequate sensitivity and selectivity. Ammonium sulfamate may be added to the sample to prevent the artifactual formation ofV-nitrosamines. Derivatives of V-nitrosodiethanolamine have been prepared by acylation, trifluoroacylation, trimethylsilylation and methylation. The derivatives have been analysed by gas chromatography using flame ionization and mass spectro-metric detectors (Occupational Safety and Health Administration, 1990). 

CG Rimkus, M Rummler, I Nausch. Gel permeation chromatography high-performance liquid chromatography combination as an automated clean-up technique for the multiresidue analysis of fats. J Chromatogr A 737 9-14, 1996. 

This paper shows a nice example for solving an important analytical problem using MISPE. Mycotoxins and particularly zearalenone (ZON) and /nmv-a-zearalenol (a-ZOL) present an everyday problem in food analysis. Existing sample clean-up techniques have different drawbacks. Liquid-liquid extraction is characterized by... 

Thin-layer chromatography is well established as a clean-up technique for environmental samples and it has been used successfully for many years to separate pesticide residues from interfering co-extractives. 

Extraction and clean-up techniques for the analysis of PBDE residues in biological samples are similar to those developed for PBB. Table 6 shows several methods to determine PBDEs in various media. Most methods are based on extraction with organic solvents, purification of the extracts by gel permeation or... 

Mplc is a separation technique and not a clean-up technique, and since the columns are re-used, any base-line material or inorganic sohd should be removed from the sample before you start. This can be done either by simple filtration or by filtration through a small pad of silica in a short column. While the mplc column is equilibrating, make up a... 

A4.3.6.3.3 Provide any other issues relating to spills and releases. For example, including advice on inappropriate containment or clean up techniques. 

Tn recent years much concern has risen, especially in o Bcial regulatory circles, about the problem of misidentification or uncertain identification in pesticide residue analysis. Since the introduction of the electron-capture detector (EC) in 1960 (i) and its rapid exploitation for the determination of organochlorine residues by gas-liquid chromatography (GLC) (2) the combined EC-GLC system has become, from 1963, the most commonly used end-method for quantitative pesticide residue analysis. It was quickly discovered that even after the application of the more common clean-up techniques (3, 5, 4) EC-GLC interferences occurred not only from peak-overlap of the various pesticides themselves (6, 7) but also from extraneous contamination—e,g, the laboratory or... 

SO that none or only part of them were being eluted. A simple treatment with hydrochloric acid tied up the metal ions and reproducible results were obtained then from the diflEerent wood charcoals. Both column elution and batchwise clean-up techniques are used. 

SPE is a sample preparation or sample clean-up technique used to separate compounds of interest from interfering matrix components. The actual process of SPE has been explained in as little as four steps— conditioning, sample introduction, washing, and elution. In reality it can be significantly more complicated. In order to develop a useful method, the entire analytical scheme should be considered. A decision must be made to use a sample cleanup step, based on the requirements of downstream... 

While microbial bioremediation is usually the fastest and most widely applied clean-up technique, phytoremediation can prolong or enhance degradation over longer timescales on sites where microbial techniques have been used first. It is also useful at remote sites missed during the main remediation campaign, and can be aesthetically pleasing. 

Saari-Nordhaus, R. and Anderson, Jr., J. M. 1995. Membrane-based solid-phase extraction as a sample clean-up technique for anion analysis by capillary electrophoresis, J. Chromatog. A, 706 563-569. 

Extractions were performed with different solvents after melting the fat at 40 C, e.g. petroleum ether, hexane or iso-octane. Various clean-up techniques were used to remove the fat and other compounds that could interfere with GC (either by masking the GC... 

As with all techniques there may be limitations to the use of clean-up techniques. Sample loading may exceed the capacity of the clean-up column. Some of the interfering compounds may have similar structures and their behaviour with regard to polarity and sorbent phases may be similar, thus evading clean-up. Some analytes of interest may be lost due to poor technique or choice of solvent or sorbent. An understanding of the chemistry of the analytes of interest and interferences may overcome some of the limitations. 

More recently, researches have been utilizing PLE in combination with automated clean-up techniques. Suchan and co-workers (56) extracted indicator-... 

In this section, we will introduce in general the extraction and clean-up techniques used for EDCs in aqueous, solid, and biological samples. Only those commonly used sample preparation methods will be discussed in the following. 

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