Cadmium (Cd) in Drinking Water

by Dr. Yasir

Background

Cadmium (Cd) is a naturally occurring element found in different compartments of the environment including rocks, soil, water, air, and food. Its release into the environment takes place both naturally and through human activities. However, anthropogenic activities such as mining, smelting, and burning of fossil fuels contribute most to Cd contamination. Cd can contaminate the drinking water from both ground and surface water sources through multiple human activities including industrial emissions, mining operations, and agricultural runoffs. From these sources, Cd can either leach down to contaminate the groundwater reservoir or the surface water through runoff processes. Cd can also leach into drinking water through metal-made pipes such as brass and bronze and contaminate the drinking water. This leaching and runoff process further facilitates the uptake of metals by edible plants as well as metal accumulation in aquatic animals such as fish which can affect the human global food chain.

Although the levels of Cd reported in the drinking water are reported in relatively low concentrations, long-term exposure to the contaminant has been associated with multiple health concerns in epidemiological studies conducted in different regions. Therefore, it is important to ensure that the water supply is properly treated and monitored to minimize the risk of exposure. Some well-documented health impacts associated with Cd exposure include an elevated cancer risk, renal damage, damage to bone cells, and reproductive disorders in humans. Its chronic exposure to drinking water has been positively correlated with an increased risk of cardiovascular diseases and hypertension. Similarly, Cd exposure at higher concentrations may lead to health complications including anemia, nausea, vomiting, and abdominal pain. Children are considered as most vulnerable age group and exposure to Cd among children through drinking water may result in developmental delays and intellectual abnormalities. The International Agency for Research on Cancer (IARC) has classified Cd and Cd compounds as carcinogenic to humans (Group 1).

Importantly, Cd contamination in the US has mainly been attributed to industrial activities mainly smelting and refining processes, coal burning, and the use of phosphate fertilizers in agriculture. Furthermore, consumer products such as batteries and electronic items have also been reported as a cause of Cd contamination. The USEPA has identified Cd as a cause of public health concern and has regulated this contaminant to ensure a safe and Cd-free drinking water supply to its consumers. For this purpose, EPA has established an MCL of 5 parts per billion (ppb) for Cd in drinking water. Any concentration, above the prescribed MCL, should be considered toxic for human health therefore, regular drinking water monitoring for Cd has been recommended. Therefore, It is necessary to reduce your exposure to Cd by using an appropriate water filter or removal technique in your drinking water. Further, the EPA has also implemented regulations to reduce Cd emissions from industrial sources.

Moreover, the EPA has identified contaminated sites throughout the United States that may contain Cd and has taken steps to clean them up. It is important to note that the presence of Cd in drinking water does not necessarily mean that it is at levels that are harmful to human health. The EPA recommends that people who are concerned about the levels of Cd in their drinking water contact their local water utility or the state agency responsible for regulating drinking water. These organizations can provide information about the quality of drinking water in specific areas and help the consumer to understand any potential risks.

Scientific Aspects

It is well established that Cd exposure has been associated with multiple health impacts among the exposed populations. It has been suggested that the fate of Cd in water is dependent on various chemical and biological factors. For example, The pH and redox potential (ORP) of water can affect the solubility and toxicity of Cd. At a lower pH, Cd tends to be more soluble and therefore more readily available for uptake by plants and animals. This means that Cd levels may be higher in water with a lower pH. Similarly, water with a lower ORP (i.e. more reduced) may also have higher levels of Cd because the metal tends to be more stable and less reactive in these conditions (Kubier and Pichler 2019). It is important to note that Cd can exist concurrently with different other metals notable As, Pb, etc. which can be more toxic to the consumers drinking the contaminated water.

Once ingested by humans, numerous mechanisms have been suggested so far that demonstrate the manifestation of Cd toxicity inside the living system. These mechanisms mainly include altered gene expression (transcriptome), DNA methylation (epigenome), inhibition of damaged DNA repair, interference of apoptosis and autophagy, oxidative stress, and interaction with bioelements (Đukić-Ćosić et al. 2020; Rani et al. 2014). Furthermore, the epidemiological findings suggest increased cardiovascular disease mortality and high blood Cd levels among the subjects with high Cd urinary concentrations (Tinkov et al. 2018). Additionally, Co-exposure of Cd with As and Hg was found positively correlated with hypertension (da Cunha Martins Jr et al. 2018).

A recent review summarized the epidemiological findings of previously conducted studies on Cd exposure and suggested that Cd possesses a high potential of crossing placental barriers and can show its adverse effects at pre-natal stages as well as in the developmental stages of children (Chandravanshi et al. 2021). This also suggests the need for further research to explore mechanisms of Cd-induced toxicity and propose environmental doses of Cd exposure among exposed humans.

Detection Methods and removal Strategies

Several detection methods are available to monitor drinking water depending on the desired sensitivity, detection limits, and accuracy, as well as the availability of equipment and trained personnel. Some commonly used methods for Cadmium detection in drinking water include the Colorimetric method which uses a chemical reaction to produce a visible color change that can be measured spectrophotometrically.

The colorimetric method is relatively simple and easy to use but provides estimated information on Cd concentration in a water sample and thus possesses less sensitivity compared to AAS or ICP. 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), etc. This method uses a beam of light to measure the absorption of Cd in a sample. In case, ionic species of Cd are desired to be quantified, the preferred analytical method is ion selective electrodes (ISE) which can qualitatively and quantitatively measure the Cd ions but this method is also less sensitive compared to AAS and ICP.

For more sensitive and accurate analysis with low detection limits, the most preferred instrument available so far is inductively coupled plasma (ICP) which uses a plasma torch for the vaporization of the sample and quantification using a mass spectrometer. Known detectors for quantifying metals include OES and MS which can accurately measure Cd 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 Cd metal. Immunoassay-based methods are also used for Cd detection which use antibodies to detect Cd in a sample but this method also has less sensitivity.

Selection and preference of methods to quantify Cd by various laboratories mainly depend on the available resources, affordability, and desired results. For example, AAS-based quantification is usually cheaper and can efficiently measure the Cd levels in drinking water at ppm level. Since the EPA-prescribed MCL for Cd is 5 µg/L, therefore for such low detection, ICP should be preferred over other methods to ensure the accurate detection of Cd in drinking water under the EPA regulatory limits. For opting for AAS-based detection, care must be taken during analytical procedures related to proper instrument calibration and ensuring the precision and accuracy of the instrument.

Several methods have been proposed for the removal of Cd from drinking water. These methods mainly include reverse osmosis in which water is passed through a semipermeable membrane with pressure which results in the removal of Cd from water. Moreover, nanofiltration membranes have also been reported to efficiently remove Cad from drinking water (Kheriji et al. 2015). More recently, the application of biochar to treat water for Cd removal has also been claimed to be a cost-effective method and it has been reported that peanut shells biochar can remove up to 99% of Cd from drinking water (Puglla et al. 2020). Various biochar-based methods have been proposed so far with a removal efficiency of more than 90% for Cd in drinking water. Another effective method for Cd removal comprises of ion exchange method that utilizes a bed of resin beads charged with ions.

Water is subjected to pass through the resin bed and Cd ions are attracted to the beads and are removed from the water (Simonescu et al. 2020). Similarly, the precipitation approach by using chemicals that precipitate Cd molecules from water is also a cheap and effective Cd removal strategy from drinking water (Pal et al. 2021). Some other methods for Cd removal from drinking water comprise activated carbon filtration (Jusoh et al. 2007) and electrolysis in which the electric current break down the Cd ions to facilitate the effective removal of Cd from drinking water (Escobar et al. 2006). Despite a range of Cd-removal methods proposed so far, the choice of preference is based on available resources and the affordability of the approach.

Public Perspective

Following frequently asked questions (FAQs) try to address some general public concerns in the US, especially the NYC region.

How much Cadmium is toxic to humans?

According to ATSDR, fatal poisoning from Cd is rare. However, high doses above the MCL for a long period can cause gastrointestinal irritation, vomiting, abdominal pain, and diarrhea.

Can your body get rid of Cadmium once ingested?

Absorbed Cd gets eliminated from the body primarily through urine. The excretion rate is relatively low, probably because Cd remains tightly bound to metallothionein (MTN) which is almost completely reabsorbed in the renal tubules. Because excretion is slow, Cd accumulation in the body can be significant.

What organs are affected by Cadmium toxicity?

Chronic exposure to Cd may lead to different health abnormalities mainly cancer and organ system toxicity such as skeletal, urinary, reproductive, cardiovascular, central and peripheral nervous, and respiratory systems.

What Cancers are caused by Cadmium?

Cd is primarily associated with human lung, prostate, and kidney cancers, and recently pancreatic cancer. It has also been associated with cancers of the breast and urinary bladder.

Why Cadmium is so toxic?

Ingestion of any significant amount of Cd causes immediate poisoning and damage to the liver and the kidneys. Compounds containing Cd are also carcinogenic

What are the top sources of Cadmium contamination?

The most important sources of Cd are smelters. Other sources of Cd include industries, burning fossil fuels such as coal or oil, and incineration of municipal waste such as plastics and Ni-Cd batteries.

Where is Cadmium stored in the body?

Most of the Cd in the human body is stored in the kidney and liver. It can stay there for many years. The body can metabolize Cd to a harmless form. However, too much Cd can make it difficult for the liver and kidneys to process it, and that leads to dangerous health effects.

How much Cadmium intake is safe per day for humans?

The EPA reference dose for daily exposure to the general population that is likely to be without appreciable risk of deleterious effects during a lifetime is 5 x 10^-4 mg/kg/day for water.

What household items contain Cadmium?

Everyday household products such as second-hand plastic toys, drinking glasses, alcoholic beverage bottles, ceramics, and artists’ paints, may contain high levels of Cd.

What is the WHO standard for Cadmium in drinking water?

WHO has set an MCL of 0.003 mg/L for Cd in drinking water compared to the EPA standard of 0.005 mg/L.

Conclusion

Cd contamination in drinking water has been associated with various health issues therefore, USEPA has regulated this contaminant to ensure a safe drinking water supply to US residents. Once entered into the human body, its main target organs include the kidneys and liver where it alters the biological functions through various cellular and molecular mechanisms. Owing to its toxic potential among humans, the best possible step to avoid Cd exposure is its safe removal at the source. Therefore, the government authorities must ensure a Cd-free drinking water supply to the consumers to minimize Cd exposure through the ingestion route. This can be done by adopting state-of-the-art Cd-removal strategies and routinely monitoring the Cd levels to ensure its concentration should be under the prescribed MCLs (5 µg/L).

References

Chandravanshi L, Shiv K, Kumar S. 2021. Developmental toxicity of cadmium in infants and children: A review. Environmental Analysis, Health and Toxicology. 36(1).
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Đukić-Ćosić D, Baralić K, Javorac D, Djordjevic AB, Bulat Z. 2020. An overview of molecular mechanisms in cadmium toxicity. Current Opinion in Toxicology. 19:56-62.
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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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