Chemicals monitored in wastewater and drinking water analysis

¶ … Cadmium in Wastewater and Drinking Water

The importance of efficiently controlling and monitoring potential toxins in water systems is extremely important. The potential contaminant known as Cadmium (Cd) is a naturally occurring trace metal that is regularly found in various types of ores. Its most common commercial uses are in the metal plating and coating of transportation vessels, household-cooking utensils, machinery and nickel-cadmium batteries (Advanced Purification Engineering Corporation, 2010). As a result of its multitude of uses, there are an equally large number of ways in which Cd can find its way into water systems. The most common of these are leaching, pipeline corrosion, corrosion from transportation vessels, runoff from metal and ore refineries among others. Cd is also capable of resulting in various negative health effects to humans unfortunate enough to consume it. Failure to adequately monitor Cadmium levels can result in numerous unsolicited health outcomes such as: "nausea, vomiting, diarrhea, muscle cramps, salivation, sensory disturbances, liver injury, convulsions, shock and renal failure," and these are only the effects of short-term exposure (Advanced Purification Engineering Corporation, 2010, p. 1). Prolonged exposure to cadmium-infected water can result in life-threatening "kidney, liver, bone and blood damage" (Advanced Purification Engineering Corporation, 2010, p. 1). Additionally, The International Agency for Research on Cancer has recently included Cadmium on its list of carcinogenic substances that are potentially harmful to humans in the category 2A, (NWQMS 2004). As a result of this new classification, the WHO has set the protocol value for Cd in drinking water at 0.003 mg/L (WHO 2004). And the Australian Drinking Water Guideline (ADWG) is even stricter, mandating a value of 0.002 mg/L (NWQMS 2004).

The forthcoming report will proceed to specifically elucidate the most effective means of ensuring water systems' extended protection from the numerous aforementioned unwanted outcomes. This paper will give precise recommendations and guidelines for efficiently collecting and handling water samples through methods like grab sampling and sampling a depth (Green, 2004). By also highlighting some of the most valuable sample extraction techniques, this report will attempt to help in guaranteeing the accuracy of the eventual results. Furthermore, the techniques for intricately analyzing the collected samples will be examined. Also, this report will explain the essentiality of accurate measurement approaches and quality control mechanisms in the laboratory environment (NWQMS 2000). And in keeping with the assurance of accuracy and reliability in sample analysis, this report will conclude by distinctively highlighting the relevant analytical detection limits present in water-based Cadmium deposits. With the comprehensive nature of the forthcoming findings and recommendations, the intricacies and potentialities of Cd levels should be better understood and more easily monitored.

Sample Collection

The acquisition of reliable water samples is the foundation of the critical analysis and decision-making process. The ultimate findings and results of any subsequent testing procedures will only be as good and accurate as the samples from whence they came. Water samples potentially containing Cd are most commonly found in drinking water sources, though this report will also attempt to highlight some possible techniques for collection at wastewater sites. As a result, the primary drinking water sampling points that will be utilized are drinking water distribution lines and wells/bores. These locations comprise the majority of spaces where water is directly used for human consumption and are the most common sites suspected to be contaminated by Cd. Also, wastewater sample will conducted in locations directly outside of metal/ore refineries and battery plants. Strategically selecting sampling locations in these areas are highly essential. Such points will include the direct distribution systems such as pipelines outside of consumers' property, the water taps inside of homes, and runoff sources in the case of wastewater (NWQMS 2004). The goal of the two former samplings source locations will be to extensively investigate the contamination from the processes of leaching and corrosion occurring at the main pipelines and the plumbing structures in the household. In the case of samples taken from wells/bores, the objective will be to identify contamination sources in drinking water from land runoff and leaching of soil that may contain Cadmium. All sampling of drinking water sources must follow the guidelines in ADWG, which is quarterly in frequency and conducted at the pre-determined sampling points in the distribution network (NWQMS 2004). Though it is also important to note that more frequent samplings should be conducted immediately following any potentially contaminative or hazardous event.

Additionally, wastewater sampling will be conducted in locations directly outside of metal/ore refineries and battery plants. Though this is a much less common location for Cadmium deposits, ensuring that this potentially harmful element is not present in wastewater sources remains highly essential. The sampling process in these locations will primarily occur in order to examine direct pollution.

Grab Sampling

This is the most common and basic technique used in sample collection. Accordingly, it is frequently recommended for the accurate location of Cadmium contaminants. Grab samples targeting Cadmium are typically (and most effectively) retrieved from the drinking water treatment and distribution systems (NWQMS 2004). If accomplished properly, sample collection with the grab method can assure the sample quality is representative of the bulk quality. An example of this type of successful sampling completion would be the reliable acquisition of a sampling from a determined sampling location in the distribution pipeline or the outlet of a bore well pump. By strategically researching and selecting specific target sampling points, it is expected that the ultimate sample will be well-mixed, thus having many of the same and/or similar properties as the source water.

According to the mandated guidelines, the minimum sample size for the determination of Cd concentration is 500 mL (NWQMS 2004). All samples collected should be stored in plastic or glass bottles that have been prewashed with 50% HNO3 (NWQMS 2004). This acid washing process is performed for the purpose of dissolving any and all metals that may have been previously attached to the walls of the bottle (such remnants cause analytical biases that can possibly interfere with eventual analyte measurements). The protocol below should be followed in the acid washing process (NWQMS 2000):

1. Wash the bottle and cap with non-ionic, metal-free detergent and tap water.

2. Rinse thoroughly with tap water.

3. Rinse with 50% HNO3, followed by 6.7% of HNO3.

4. Cap and keep until required, but a week as a maximum.

5. Before use, empty the bottle and rinse the inside with metal-free water such as distilled water.

Sampling at Depth

This type of sample extraction is typically utilized in lake and reservoir locations for the purpose of examining freshwater and drinking water sources. Though these source origins are far less common homes of Cadmium deposits, historical data has shown that Cadmium has been found in these locations (Burnsa, Rutherford, & Clayton, 1999). Being that shallow or surface sample cross-sections would not sufficiently represent the total contamination content in these locations, it is necessary to acquire samples from greater depths in order to achieve a more accurate representation of the chemical and elemental composition of these water sources. A very commonly used tool for this process is known as the LaMotte Water Sampling Bottle (Green, 2004). This unique device allows for the extraction of water samples at specific depths (Green, 2004). This instrument is basically comprised of a sample receptacle at the end of a long line with a two-pound weight at the bottom and a stopper above (Green, 2004). There is also a trip line, which opens the receptacle for filling when the desired depth is reached. Many of these devices are also equipped with the LaMotte Model 545 Armored Thermometer for more accurate sample data and temperature readings (Green, 2004). By setting this apparatus to a specific water depth, scientists can monitor different contaminant levels throughout the various degrees of collection sites. And being that different contaminants normally occur in greater or lesser quantities depending on water depth, this can be an extremely effectual tool.

Wastewater Sampling

Though wastewater locations are much less frequent locations of Cd contamination, historical research has shown that Cadmium has certainly been found in these locations (Singha, Rastogi, & Hasan, 2005). The most common wastewater sources for Cadmium are metal and ore refineries. Though since most wastewater sampling points are typically overrun with a cornucopia of elements and contaminants. The presence of such a wider range calls for a procedural protocol a bit different from that previously discussed. However, the actual sample collection process is quite similar to the grab sampling process. Although in the case of waste, it is even more important to make certain considerations when collecting samples. For instance, the elemental and molecular composition may change drastically or become biased if collected in a highly turbulent area (Singha, Rastogi, & Hasan, 2005). Additionally, due to the potentially high levels of numerous contaminants, it is recommended that samples be stored in a refrigerated location (after being rinsed and flushed like in the grab process) before the transportation to the laboratory takes place.

Sample Extraction

In order to accurately examine any water sample, the potential contaminants must first be concentrated. 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.

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

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). While these advantages are certainly attractive, this process also presents some significant shortcomings. Such disadvantages include: higher time requirements for measurement, subjection to several interferences from molecular absorption and potentially off-putting chemical and matrix effects (APHA 1992). The instrumental detection limits, which accompany this process, are listed below in Table 2.

Table 2. Instrument Detection Limits by Methodological Approach

Instrumental Detection Limit

Method

Reference

2 ?g/L

FAAS

APHA 1992

3.4 -- 4.0 ?g/L

ICP-AES

APHA 1992; U.S. EPA 2001

0.1 ?g/L

GFAAS

APHA 1992

While the table above does provide some important information, it also fails not consider some other factors. Though based on the above discussion, it can be seen that ICP-MS and GFAAS offer attractive solutions for the Cd analysis. Most notably, both alternatives offer detection limits that are consistently less than the guideline value of Cd in drinking water (which is 0.2 ?g/L). Additionally, the recent improvements in the pretreatment and measurement techniques of Cd for water samples using the GFAAS method have made this technique better able to suppress the potential interference problems and also improve its detection limits (Chuachuad & Tyson 2004; Harris 2007). Therefore, with the knowledge of these new procedural enhancements and its resultant presumable practical and scientific superiority, GFAAS becomes the alternative of choice. This fact is also supported by the lower operational and maintenance costs associated with GFAAS, as compared to ICP-MS (Tsogas et al. 2009). The analysis of Cd using GFAAS is also recommended in the Standard Method for the Examination of Water and Wastewater that the Australian Drinking Water Guidelines refers to for the determination of Cd levels in drinking water (APHA 1992; NWQMS 2004).

To expand upon the aforementioned improvements to this process, it is important to delve a bit deeper into the specificities of the necessary instruments and processes, which provide the foundation for the superior efficiency of this method. The most commonly used GFAAS instrumentation features a small graphite tube that is capable of manipulating the sample's temperature by using an electric heating element. The atmosphere inside the heating chamber is saturated with a premixed gas composed of 95% argon -- 5% hydrogen. This environment is explicitly created to make an inert condition during the process, while simultaneously reducing the condition as a result of the presence of hydrogen, which promotes the production of more ground state atoms (APHA 1992; U.S. EPA 2001). The ultimate structure of this temperature-based procedure can be categorized into three distinct stages or steps which include the drying phase, the charring phase and the atomization phase. As can probably be inferred by its name, the first stage aims to dry the sample. Heightened temperatures existent in the second stage are intended to vaporize the interfering matrix without volatilizing the Cd. Finally, during the last stage, the temperature is rapidly elevated to the point of atomization of the Cd. This multi-tiered process subsequently creates an atomic cloud that absorbs the element's specific wavelength released from a hollow cathode lamp that incorporates the Cd in the electrode. The amount of light intensity that is subsequently absorbed is then immediately measured and the concentration is calculated based on the Beer-Lambert law. The procedural steps taken immediately following the measurement are (in order): the cooling down phase and the purging of the heating chamber.

Despite the great practicality and efficiency of this process, GFAAS also presents some potential disadvantages that should not be overlooked. The predominant weakness of GFAAS arises mainly from the potential for matrix interference (U.S. EPA 2001; Tsogas et al. 2009). However, pretreatment techniques with SLE and cation exchange resins have been found to be capable of adequately and appropriately removing these interferences (Tsogas et al. 2009). In addition, matrix modifiers have also been used to reduce the volatility of Cd, increase the volatility of the matrix, and/or increase the ultimate atomization efficiency (Harris 2007; Nollet 2000). All of these techniques can help to improve the volatilization of the interfering matrix at the charring stage and enhance the atomization of Cd at the atomizing stage. Examples of these modifiers include: NH4H2PO4 & Mg (NO3)2, (NH4)2HPO4 & Mg (NO3)2, and (NH4)2SO4 (APHA 1992).

Instrumental modifications can also improve the performance of GFAAS. Improvements in the limit of detection can be achieved through the implementation of chemical vapor generation mechanisms and in-atomizer trapping (Chuachuad & Tyson 2004). In this technique, a larger sample volume is often required in order to give rise to a higher instrumental sensitivity (thus a lower limit of detection), while at the same time maintaining simplicity within operation. Making this type of instrumental modification typically mandates the affixing of a coating that is usually applied to the walls of the graphite tube. During the vaporization process, this coating material traps cadmium hydride, which results from the hydride generation system. In the ensuing stage of charring, the matrix interferences are purged from the process in the form of vapor. Eventually, the atomization takes place and consequently releases the Cd in the atomic cloud to be measured by the spectrometer. Recent scientific research in this area suggests that Zr-Ir is the compound best used for the coating material of the graphite tube (Chuachuad & Tyson 2004). This literature also makes specific recommendations regarding the optimal operating temperatures for trapping, charring and atomizing at 75oC, 350oC and 1,200oC respectively. Ultimately, improvements in the tolerance level towards interferences and increases in the detection limits of one to two orders of magnitude are achievable by the application of this suggested method (Chuachuad & Tyson 2004; Lampugnani et al. 2003).

Approaches for the Accurate Quantitation of Cadmium

The useable approaches for obtaining an accurate quantitation of the Cadmium involve integration of practices aimed at achieving optimization in sample preparation and instrument operational procedure. The applicable techniques in the preparation step include the preconcentration of the analyte and the aforementioned tactics for the suppression of matrix-based interferences. All of these approaches primarily aim to increase the concentration of Cd to the extent that it falls within the optimal range analyzable by the instrument, and also to remove the matrix elements that can potentially cause disruptions in the measurement phase.

When examining the instrument operational procedure, the assurance of accurate measurements can be garnered through the strategic utilization of several scientific approaches. These methodologies include: performing sample dilution, conducting proper calibration procedures, running standards between sample analyses, and conducting replicate measurements.

When the measurement results are found to occur in a concentration above the maximum quantity mandated by the given standard, the sample must be diluted and reanalyzed. If the final concentration measurement does fall within the linear portion of the calibration curve an addition to the standard can be applied when the concentration of the analyte is below the optimum range.

Correlating the Cd concentration in the sample and the instrument response are both essential steps necessary to successfully perform the calibration procedure. Calibration is a relatively simple process conducted by using standard solutions prepared by diluting a metal stock solution of analytical grade. The dilution should use a matrix similar to the one present in the sample. The preparation of standard solutions is to be carried out on daily basis. Such standard solutions for Cadmium are composed of a blank and at least three calibration standards within the optimum concentration range of 0.5-10 ?g/L using GFAAS (APHA 1992).

The measurement(s) and analyses of the standard solution between operational processes must be regularly performed in order to ensure that instrumental drift does not occur. If the result of this procedure fluctuates between 5-10% of the expected value, the analysis should be terminated (APHA 1992). In such an instance, the analyst should attempt correct the problem, which can normally be done by simply recalibrating the instrument (APHA 1992).