Cadmium in Wastewater and Drinking Essay

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Furthermore, any possible interferents must be localized and discarded. Both of these objectives comprise the essence of sample extraction. And as can be inferred, the process is ultimately utilized to achieve a more accurate measurement. The reliable and accurate analysis of Cd from a given water sample most often requires the use of an instrument such as a flame atomic absorption spectroscopy (FAAS) or an inductively coupled plasma atomic emission spectroscopy (ICP-AES), both of which require pre-concentration steps. Such prerequisites must occur because these instruments have a detection limit that is not low enough to sufficiently detect the concentration levels mandated by the given guideline (Ferreira 2007; NWQMS 2004). On the other hand, the are other instruments such as graphite furnace atomic absorption spectroscopy (GFAAS) or inductively coupled plasma mass spectrometry (ICP-MS), which have detection limits low enough to meet the given regulatory guidelines. Though unfortunately the measurements garnered from the use of these tools are highly prone to matrix interferences (APHA 2004; Senkal et al. 2007). Hence, knowing the realities implied from the data presented above, the sample extraction step is extremely beneficial in the reduction and subsequent removal of many possible sources of interference prior to measurement.

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With the essentiality of the extraction step in mind, there are several techniques available for Cadmium extraction from a sample. These methodologies most notably include: electrochemical deposition, coprecipiation and precipitation, cloud point extraction (CPE), solid-liquid extraction (SLE) and liquid-liquid extraction (LLE) (Ferreira et al. 2007). This report will proceed to illustrate the benefits of the utilizing the LLE and SLE approaches as these are the most common techniques used in water sample analysis (Ferreira et al. 2007, Nollet 2000).

Solid-Liquid Extraction

Essay on Cadmium in Wastewater and Drinking Assignment

The process of solid liquid extraction (SLE) uses a solid sorbent attached to a support material deemed adequate depending on the targeted subject matter. This procedural composition exists in order to capture the analyte from a water sample that passes through the selected items. A preselected organic solvent is used to wash out the target analyte. The SLE technology is rapid and relies heavily upon chromatographic retention. It also has the attractive potential for easy technological conversion and automation (Nollet 2000).

Beign that the sorbent represents such a vital aspect of SLE, sorbent selection is crucial in assuring accurate measurements and contaminant representation. Accordingly, there are several appropriate sorbents that can be used to extract Cd using this method. These commonly include groupings of both natural and synthetic materials (Ferreira et al. 2007). Examples of useable natural materials are: purified humic acid, vermicompost, and the yeast S. cerevisiae (Pereira & Arruda 2004; Bag et al. 1999). However, these natural absorbents have a limited application because they have been shown to be prone to contamination from other foreign ions and thus present relatively low levels of selectivity (Pereira & Arruda 2004; Tarley et al. 2004).

Numerous synthetic materials, such as polyurethane foam, zeolites and divinylbenzene polymers can also be utilized to preconcentrate Cd ions (Ferreira et al. 2007). From these materials, higher selectivity can be achieved through the creation of chelating resins. This process occurs through the chemical and physical binding of chelating agents (Ferreira et al. 2007). The range of synthetic sorbents usable in this process is massive and includes substances like Amberlite, Silica gel and Chromosorb-106 (Xie et al. 2005; Minamisawa et al. 2006; Tuzen et al. 2005). It is also important to remember the list of solvents normally utilized during the desorption phase. Such substances most often include kerosene, toluene and chloroform (Ferreira et al. 2007).

The SLE extraction method has been widely used over the years for the purposes of quantifying Cd levels in water samples (Ferreira et al. 2007). Some of the most striking advantages associated with this process include lesser amounts of toxic chemical usage and more expressive preconcentration factors. Though, specific considerations must be made and several factors must be accounted for before the most appropriate materials can be selected for use in this procedure. Some of these factors requiring deliberation include the sample throughput, the Cd concentration in the sample, and the desired preconcentration factor.

Liquid-Liquid Extraction

The LLE technique is primarily founded upon the relative solubility of the elements in two distinctive phases. The targeted analyte is concentrated or isolated in the same phase in which the analytical signal would be acquired (Ferreira et al. 2007). During this process, the desired analyte is quantitatively removed from the aqueous matrix, ideally leaving any and all interference materials behind. Though, the efficiency of the extraction depends on the affinity of the analytes to the extracting solvent, the ratio between the phases, and the number of extractions (Ferreira et al. 2007). Despite the great efficiency of this process in removing interferents, LLE is somewhat expensive, slow and presents high consumption rates of toxic organic compounds that can be damaging to the environment and to the greater public health (Ahmed 2001). Also, its ultimate analyte recovery rates seem to fail in comparison to those associated with the SLE process. Therefore, accounting for all of these drawbacks, this process seems relatively disadvantageous when compared to SLE. Table 1 exemplifies this fact through a study done in which the two procedural approached were compared side-by-side.

Table 1. SLE vs. LLE

(Seiler, Kohler, & Arlt, 2002)

Techniques for Sample Analysis

There are several methodologies that have been effectually utilized to analyze Cd levels in a given water sample. The most common of these techniques are flame atomic absorption spectrometry (FAAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), ICP-mass spectrometry (ICP-MS), the Dithizone method and graphite furnace AAS (GFAAS) (APHA 1992; Nollet 2000).

The FAAS method is one that involves the examination of the potential absorption intensity of a certain wavelength by the atomized element Cd presented within the flame. Instrumentation requirements are relatively minimal and simplistic, although the process in its entirety is often accompanied by substantial limitations. Such constraints include the presence of a relatively low detection limits and significant propensities for the exposure to a wide range of chemical interferences (APHA 1992; Ferreira et al. 2007). The latter drawback is most often considered to be a result of the inevitable existence of some atoms that are still present in a chemically bound form in the flame (APHA 1992). The presence of such atoms can cause a reduction in the absorption of the main analyte atoms. Table 2 below provides a quantitative example of the typical instrument detection limit presentable through this process.

In the ICP-AES method, the critical atomization process occurs in the plasma of an argon gas having a temperature of over 6,000oC (Harris 2007). At this temperature, the electrons in the atoms become excited and when they return to the eventual ground state they emit light within a certain wavelength. The intensity of this light is measured by the spectrometer and is then correlated to the concentration of the element in the sample. The most regular instrumental detection limit are elucidates in Table 2 below.

The underlying methodological cornerstones of the ICP-MS approach are very similar to those found in the ICP-AES technique. In fact, this approach utilizes precisely the same atomization technique as ICP-AES. However, instead of using a normal spectrometer for the detection of ionized atoms, the ICP-MS process uses the mass spectrometer to determine the amount of ionized atoms generated in the plasma. Because this instrument classifies the ionized atoms by their mass-to-charge ratio and then magnifies the signal by means of electron multiplier, this system has an enhanced sensitivity and does not present problems with spectroscopic interferences. Nevertheless, due to the greater levels of sophistication associated with this instrument, it typically necessitates higher operational and maintenance costs (Tsogas et al. 2009). ICP-MS is able to detect Cd in water samples within three orders below the detection limit of ICP-AES (Harris 2007).

The Dithizone method is based on the reaction of Cd and dithizone, which forms a pink-red color that can be easily extracted using chloroform. The extract is then measured photometrically to determine the color intensity, which is then correlated to the concentration of Cd by comparison to the given calibration curve. Though because this method has historically exhibited relatively low levels of precision, it not recommended unless other methods such as FAAS and ICP are not available (APHA 1992).

The method of electrothermal atomic absorption spectroscopy (ETAAS) or GFAAS operates on the same principle as FAAS. However, these processes additionally integrate an electrically heated atomizer or graphite furnace in place of the standard burner head. The temperature profile can be modulated during the analysis to allow for the drying, charring and atomization stages to take place in turn. The concentration of the targeted element is measured by using the proportion of the light intensity being absorbed by the atomized element. There are several advantages associated with the use of this method. Such benefits normally consist of its excellent detection limits and its requirement of much smaller sample amounts (Harris 2007).… [END OF PREVIEW] . . . READ MORE

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