Table of Contents
Background
Chromium (Cr) is a metallic element with odorless and tasteless properties which exists in various oxidation states predominantly in trivalent (+3) and hexavalent (+6) forms. Naturally, Cr is found in rocks, minerals, plants, soils, volcanic dust, and minerals. However, drinking water contamination through Cr mainly takes place through anthropogenic activities because of its broad application in industries and mining. The metal possesses high corrosion resistance properties and is widely used in electroplating, chromite mining, alloy forming, textiles, glass, photographic industries, leather tanning, etc. This widespread application of Cr in many industries makes the metal a potential candidate for drinking water contamination. Discharge from the industrial and mining sources results in the leaching and runoff of Cr to contaminate underground and surface water respectively.
The fate and toxicity of Cr in drinking water purely depend on its available oxidation state. For example, the trivalent Cr is considered non-toxic to humans and is an essential human dietary element found in many fruits, vegetables, meat, grains, yeast, etc. However, studies suggest that Cr+3 can also be toxic at chronic exposure levels. On the other hand, hexavalent Cr is considered a serious public health concern with established evidence of carcinogenic and other adverse health outcomes at both acute and chronic exposure conditions. This is because the hexavalent Cr possesses high water solubility and mobility as well as easy reduction potential compared to the trivalent form.
The mechanism through which the Cr+3 is converted to Cr+6 depends on many chemical factors that determine its fate in the aquatic environment. These factors notably include pH, redox potential, various organic and inorganic chelating agents along with the biological indicators in the form of various microbes that can convert Cr+3 to Cr+6 making it toxic for human consumption. The fraction of Cr+6 in drinking water depends on the type of water source mainly the groundwater, surface water, treated or untreated, geographical location, and redox potential of water. In the US, Cr has been reported in the drinking water of various regions and scientists have suggested both natural and anthropogenic sources are responsible for Cr contamination in drinking water.
USEPA regulates the levels of chromium in drinking water with an MCL of 0.1 mg/L for total chromium. This limit includes all the forms of Cr including the hexavalent and trivalent species. The reason for setting up MCL for total Cr but not individual species lies behind the mechanism of inter-conversion of Cr+3 and Cr+6 in water and the human body depending on the available environmental conditions. This makes it difficult to capture all Cr present in the drinking water. Therefore, EPA regulation considers the measurement of total Cr to be 100% Cr+6 (the most toxic specie). This means that once the levels of detected total Cr exceed the MCL, the consumers will be notified as per EPA regulations. Moreover, FDA has recommended that the Cr levels in bottled water should not exceed 0.1 mg/L. While WHO has set the permissible limits for total Cr to be <0.05 mg/L.
Literature also suggests that Cr may show toxic behavior in an antagonistic way to other metals. For example, there is growing evidence that demonstrates increased toxicity among humans when exposed to concurrent exposure of Cr and Arsenic (As). Human exposure to Cr has been associated with various health implications depending on acute and chronic exposure. Acute Cr intoxication mainly includes GI disorders, and injuries to the respiratory system, liver, and kidneys. Whereas, its chronic exposure has been positively correlated with dermatological impacts, neurological disorders, reproductive and developmental impacts, genotoxicity, and various cancers. This demands systematic monitoring of drinking water in the US to ensure Cr-free supply to the consumers and adoption of modern technologies that efficiently remove the Cr species from the drinking water.
Public health concerns related to chromium in drinking water are mainly centered around the hexavalent Cr which is an established human carcinogen (Zhitkovich 2011). Much of studies have been conducted to address the Cr+6 toxicity through the inhalation route in different occupational settings affecting the respiratory tract. Evidence also suggests that Cr+6 impacts on skin among individuals associated with leather processing. Recent epidemiological findings suggest Cr+6 is positively correlated with cancer when consumed through ingestion. Studies also support the evidence of concurrent exposure of Cr along with As through drinking water resulting in higher health risks associated with metal toxicity (Smith and Steinmaus 2009).
It is interesting to note that the human toxicity data related to Cr through the oral route is limited so far because of limited epidemiological studies conducted targeting human exposed populations via drinking water. Therefore, most of the international MCLs ranging between 50 ug/L (WHO) and 100 ug/L (USEPA) are based on experimental data on animals (Moffat et al. 2018). Several mechanisms have been proposed through which Cr+6 adversely affects the cellular systems once ingested by humans resulting in adverse health outcomes in the form of different diseases. These mechanisms notably include the induction of oxidative stress, DNA damage, mutagenicity, genetic polymorphisms, epigenetic changes mainly DNA methylation and miRNA alterations (Sun et al. 2015).
This toxic mechanism is further associated with various demographic factors such as ethnicity, age, gender, and levels of exposure among individuals. Among these, it is believed that the low-age group subjects are more prone to disease development because of biological development and growth. Once ingested, the Cr+6 gets reduced to Cr+3 by the gastric juices in the stomach. This absorption will result in 2-10% of ingested Cr into the bloodstream through the portal vein system to reach the liver where the reduction takes place (De Flora et al. 1997). Moreover, Cr+6 has also been reported in fetal tissues by crossing the placental barriers as well as in human breast milk with the potential to impact infants’ health at the early stages of life (Alvarez et al. 2021). Such mechanistic behavior of Cr toxicity among humans demands mechanistic epidemiological studies to be conducted targeting different age groups in the exposed US population from different states to further reveal the extent of Cr toxicity through drinking water.
Detection Methods and Removal Strategies
Several detection methods are available to monitor drinking water. These methods possess varying sensitivity, accuracy, and detection limits as well as the costs to measure Cr in drinking water. In case, an estimated quantification is desired, usually, the gravimetric and colorimetric methods are preferred. However, for accurate quantification, the available analytical platforms include spectrophotometric methods mainly atomic absorption spectrophotometer (AAS) coupled with various detection accessories such as a flame (FAAS), graphite furnace (GFAAS), and X-ray fluorescence (XRF). In case, ionic species such as Cr+3 and Cr+6 are desired to be quantified, the preferred analytical method is ion chromatography (IC) which can qualitatively and quantitatively measure the Cr species.
For more sensitive and accurate analysis of water with low detection limits, the most preferred instrument available so far is inductively coupled plasma (ICP) with various detectors (OES, and MS) that can accurately measure Cr at micro and nanogram levels. Similarly, high-performance liquid chromatography (HPLC) when combined with AAS or ICP can perform the organic speciation of compounds chelated with Cr. Selection and preference of methods to quantify Cr by various laboratoriesmainly depend on the available resources, affordability, and desired results. For example, AAS-based quantification is usually cheaper and can efficiently measure the Cr levels in drinking water at ppm level i.e. samples having total Cr higher than the MCL can be quantified using this approach. ICP-based methods are preferred where high accuracy is desired to quantify Cr at µg/L levels. However, the ICP-based methods are relatively costly compared to AAS.
A major challenge related to the removal of Cr arises once it is detected in drinking water. Several methods and approaches have been proposed so far for the efficient removal of Cr to ensure a safe water supply to consumers. Among the widely practiced Cr removal methods include precipitation which involves the addition of coagulating agents such as sulfates of aluminum or ferric or ferric chloride to treat drinking water. The added coagulant tends to bind with Cr ions to form a flocculant that can easily be removed through settling or filtration processes. This process is among the most commonly used Cr removal approaches (Sharma et al. 2008).
More recent approaches for hexavalent Cr include the electrochemical methods involving ultrafiltration membranes under high salinity conditions (Duan et al. 2017). It has also been suggested that both electrocoagulation and chemical coagulation methods involving Fe as a coagulating agent are effective in removing total Cr from drinking water (Martín-Domínguez et al. 2018). The ion exchange method involving passing the water through a resin bed or beads has also been reported to remove Cr as this method involves an exchange of ions with Cr ions (Rengaraj et al. 2001).
Further, the reverse osmosis method involving forced movement of water from a membrane is also among the most widely used and cost-effective Cr removal approaches from drinking water (Sharma et al. 2008). Some other methods have also been proposed for Cr removal which mainly involve adsorption through activated carbon (Yue et al. 2009), activated alumina (Mor et al. 2007), and chemical precipitation. Preference for adopted methods for removal mainly depends on the available resources and economic costs.
Public Perspective
Following frequently asked questions (FAQs) try to address some general public concerns in the US, especially the NYC region.
At low levels, usually, it is not harmful. The hexavalent form of Cr is more toxic than its trivalent form.
Currently, the regulated MCL for Cr in drinking water set by USEPA is 100 µg/L. This means any concentration of total Cr above MCL can be toxic to human health.
Some reports suggest Cr causes occasional irregular heartbeats, sleep disturbances, headaches, mood changes, and allergic reactions. Cr may increase the risk of kidney or liver damage.
WHO has set an MCL of 0.05 mg/L for total Cr in the drinking water.
Quality filters such as Berkely water filters have the potential to efficiently remove Cr from the drinking water.
Epidemiological findings based on urinary Cr levels among occupational groups suggest kidney damage be associated with Cr. However, its impacts on the general population consuming Cr-contaminated water are poorly understood so far.
This mainly includes lung cancer, intestinal tract problems, liver and kidneys due to chronic exposure to Cr.
The hexavalent form of Cr (Cr+6) has been proven an established human carcinogen in various epidemiological and experimental studies.
Widely used methods for Cr removal from drinking water include electrochemical methods, chemical coagulation, reverse osmosis, and chemical precipitation.
Drinking bottled water might seem like a good way to avoid Cr exposure, but there is no guarantee that bottled water contains less of this contaminant. Furthermore, there is no legal limit for Cr+6 in bottled water, so consumers cannot assume it is free of it.
Trivalent Cr (Cr+3) is an essential trace mineral with many health benefits. However, its exposure for longer periods should be avoided.
Conclusion
Cr contamination in drinking water is a serious public health concern. This is mainly due to the existence of Cr in different ionic forms Cr+3 and Cr+6. These ionic species are interconvertible in aquatic and biological systems. Trivalent Cr possesses beneficial properties for humans and is considered an essential nutrient. Whereas, its hexavalent form is the most toxic one that can trigger various health issues through different proposed cellular and molecular mechanisms.
Detectable concentrations of Cr in US drinking water have been reported from different states which demands the need for further epidemiological research to demonstrate Cr toxicity among different age groups chronically exposed to Cr via drinking water. Further, the government authorities must ensure the supply of Cr-free drinking water by adopting state-of-the-art removal strategies and routinely monitoring the Cr levels to ensure its concentration should be under the prescribed MCLs (100 µg/L).
References
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- Yasir A. Rehman, Ph.D.
Dr. Rehman was born in Rawalpindi, Pakistan. He completed his MSc from PMAS – Arid Agriculture University Rawalpindi in 2011 where his thesis comprised a health risk assessment of subjects living in the vicinity of wastewater channels in urban settings and its relationship with the incidence of Malaria.
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OTHER RESEARCH ON WATER CONTAMINANTS BY DR. YASIR
