The term “compounds of emerging concern” (CEC) is used to identify chemical contaminants that have no regulatory standard, have been found in environmental samples, and have potentially adverse effects on aquatic life. Of particular concern are compounds that may have always been present, but at concentrations previously below limits of detection, or compounds that pass through water treatment processes. One may ask why so many of these CECs remain CECs even though we knew about them years ago. Contrast current CEC with the original priority pollutants, or, in particular, the routinely monitored conventional pollutants, trace metals, radionuclides, organic solvents, and pesticides. At one time, each of the mentioned classes of compounds were CEC. A 1978 response to a consent decree between the newly formed United States Environmental Protection Agency (EPA) and several environmental groups named 65 classes of compounds and gave EPA 15 months to develop test methods and establish effluent guidelines.1
As with pharmaceutical and personal-care products (PPCP), one of the new CEC we will discuss below, the 65 “compounds” could include thousands of potential single analytes. EPA was challenged to weed through the list and decide on targeted analytes. Criteria used by EPA included whether calibration standards were available, frequency of occurrence, and if there were methods capable to test for them. Many of the compounds became groups, such as “total” metals, and “total” cyanide. EPA eventually settled on 129 compounds, known today as priority pollutants. Similarly, modern CEC are often classes of compounds. For compliance testing, laboratories need a specific list of what to look for, how to look for it, and at what concentrations. Laboratories need methods; preferably, they need standardized methods so that if several labs analyze the same samples for the same analytes, results would be reasonably the same. Many CEC are still waiting for the standardized list and the standard methods.
Analyzing for unknowns in water
Unknowns present a problem because you don’t know they are there and no one is really looking for them. With the advent of inductively couple plasma/mass spectrometry (ICP/MS), unknown metals are easier to find and detect because ICP/MS can scan virtually the entire periodic table. Organic contaminants are different, however, because you need to know what you are looking for before you look. This is because the technique used and any preliminary approaches taken, such as sampling, preservation, and extraction, limit the compounds you will be able to “see.” In other words, the instrument technique you use can “search” for different classes of compounds based on chemical and physical characteristics. Other compounds that do not meet the criteria could still go undetected.
The problem remains that compounds you did not look for and do not know are there could still enter and pass through a treatment facility and end up in the environment or in drinking water. The good thing is that as technology improves, we become more capable of finding and measuring unknown pollutants in our water supplies.
One such group of compounds, mentioned previously, is PPCP. This diverse group of organic compounds includes antibiotics, hormones, human and veterinary drugs, over-the-counter drugs and painkillers, illegal drugs, laundry and cleaning products, cosmetics and sunscreen, dietary supplements, and antimicrobial agents. PPCP are introduced into the environment by intentional disposal (flushing), bathing or swimming, leaching from landfills, runoff from animal feeding lots, and discharge from storm events or septic systems. The most commonly found are steroids and nonprescription drugs; however, antibiotics, prescription drugs, detergents, pesticides, and hormones may also be present. Not much is known about the effect of trace PPCP on humans in their drinking water. However, there is concern that trace amounts of PPCP in wastewater could result in serious harm to aquatic life.2
It is difficult to establish a “toxicity” of antibiotics in drinking water because we take them in such high concentrations. In addition, the agricultural industry may use large amounts of antibiotics in keeping their animals healthy. Antibiotics are not regulated for NPDES and may not be monitored in wastewater treatment or in the final effluent. Wastewater treatment plants serve as a means to make polluted water suitable for release into the environment. The technology at most plants takes advantage of the high nutritional value of the organic content of sewer water to feed microorganisms leading to biosolids and a fairly clean effluent. Not only does the traditional wastewater treatment plant rely on vast amounts of bacteria, but other microbes are entering the plant, in the wastewater, every day. When low concentrations of antibiotics are also present, resistant strains of bacteria can evolve and enter the wastewater effluent.3,4
The survival of PPCP through a wastewater treatment process is highly dependent on the technology used. As mentioned above, the presence of antibiotics in an influent can produce disease-resistant strains of bacteria that can pass through into receiving streams. In addition, depending on the process, other PPCP may pass through. Once in the environment, trace antibiotics and PPCP can further interact with bacteria and aquatic life. Antibiotics are persistent pollutants because the supply is continuously replenished. And, since there is no NPDES pretreatment or effluent limit guidelines, they may not even be monitored.
Perchlorate is a highly oxidized polyatomic anion. It is highly water-soluble and very difficult to remove from water. Perchlorate can occur naturally in very arid regions, form in the atmosphere, and is a manufactured product used in rocket propellants, explosives, and fireworks. Perchlorate has been found in some public water supplies, usually near sites where rocket fuel has been manufactured or used.5
Perchlorate was included in the First Unregulated Contaminants Monitoring Rule (UCMR1). In UCMR1, 160 systems out of 3870 tested had perchlorate concentrations above 4 parts per billion (ppb). Detections were identified in 26 states and two territories.6 Of those with detectable levels, the mean concentration was 9.85 ppb and the mode was 6.40 ppb. As expected, concentrations were much higher in groundwater closest to military operations.
Low concentrations of perchlorate may only be removed from water by advanced treatment, such as ion exchange, reverse osmosis, and biological reduction. There is little data to support the effectiveness of these treatments to meet proposed public health goals (PHG). Until January 7, 2011, California had a public health goal (PHG) of 6 ppb, and an MCL of 6 ppb. However, on January 7 of that year, the California Office of Environmental Health Hazard Assessment (OEHHA) revised the proposed PHG to 1 ppb. While it is possible to analyze perchlorate at concentrations below the proposed PHG, particularly using Method 331.0, care is required to ensure accurate results.
In 2011, EPA announced a decision to regulate perchlorate under the Safe Drinking Water Act (SDWA); however, EPA has yet to establish a maximum contaminant level goal (MCLG). In the absence of a national MCLG, states are establishing their own maximum contaminant levels (MCL) and notification levels ranging from 1 to 6 ppb.
Microplastics are defined as plastic particles smaller than 5 millimeters (mm).7 The lower size is not defined; however, it is common practice to use about 0.3 mm, although some bottled water manufacturers are filtering to as low as 0.1 µm. Microplastics exist as primary microplastics and secondary microplastics. Primary microplastics consist of manufactured raw materials such as plastic pellets, scrubbers, and microbeads that enter the environment from runoff. Secondary microplastics occur when larger plastic products such as bottles, bags, old carpet, etc., undergo mechanical, photooxidation, or biological degradation that progressively break the larger pieces into smaller pieces until they are less than 5 mm.
The impact of microplastics on human and aquatic health is not known. However, microplastics have a tendency to accumulate toxins at a factor many times the concentration in the water.8 This means microplastics could provide a mechanism to transport concentrated toxins to organisms.
There are currently no known standardized regulatory methods for sampling, sample preparation, or analysis of microplastics in water, nor are there any regulations. Without standardized methods and practices, we cannot accurately assess and compare the amounts present in the environment. Sampling and sample preparation are likely the biggest issue. Sampling protocols will need to vary depending on what is sampled. Sampling drinking water from a tap or bottled water is entirely different than sampling ambient water, which is extremely different from sampling wastewater influents. In addition, since microplastics are not dissolved, or necessarily have chemical interactions with water, they are not distributed homogeneously; some may float and some may sink. In addition, particle shape will impact their behavior.
The common practice for sampling microplastics in ambient water is to deploy a manta net from the sides of a vessel far enough from the wake to avoid turbulence using a 0.3-mm mesh size or less. More than just plastic will be captured.9 Sample preparation is needed to remove the unwanted material while at the same time leave the plastic intact.
If you are familiar with asbestos analysis, you know that its fibrous nature is what makes it hazardous. We do not know if particle shape of microplastics matters, but we do know that the shapes vary, ranging from spheres, flakes, rods, and rounded cubes. They also have a tendency to break easily, possibly even during the sampling process. Unlike asbestos, where every variety has a known and distinct chemical composition, plastics contain many different polymers. Optical methods for microplastics can count particles, estimate their grain size and shape, and with micro-IR or Raman can estimate their composition based on spectra. The particle count yields a mass estimate. Other techniques, such as GC/MS coupled with pyrolysis, can measure the mass of different polymers, but do not measure particle size or shape. As yet, no one has determined whether microplastics should be determined by particle count, as with asbestos, or by mass, as with most chemical methods.
In the end, no one single method will yield all the information we may need on microplastics. We may need a combination of GC/MS for chemical mass and optical microscopes with IR to estimate the number of particles and their size and shape.
Per- and polyfluorinated alkyl substances (PFAS)
PFAS are a large group of synthetic chemicals used widely as surfactants in industrial products and applications, and in aqueous film forming foam (AFFF) firefighting products. PFAS have very unique chemical and physical properties that result in bioaccumulation and a propensity to migrate from areas of soil contamination into ground and surface water. Carbon is one of the few elements that can form long chains of carbon-to-carbon bonds. A hydrocarbon chain, such as hexane, consists of six carbon atoms bonded to each other in single bonds with all other available bonding to hydrogen atoms (saturated). Branched alkanes may have a few carbon atoms bonded, in single bonds, to two carbon atoms.
A surfactant is a linear or branched alkane terminated with a highly polar, water-soluble, functional group. When this terminating functional group or “head” is negatively charged, the surfactant is considered an anionic surfactant. A fatty acid is essentially a surfactant with a carboxylic acid head.
PFAS are linear and branched carbon-based compounds, similar to fatty acids but with normal carbon-hydrogen (C-H) bonds substituted with carbon-fluorine (C-F) bonds. The C-F bond is the strongest there is, making PFAS fire-resistant and very persistent in the environment. PFAS compounds include per- and polyfluorinated substances. Perfluorinated means that every carbon is surrounded by fluorine atoms, and polyfluorinated means that there is either a carbon without saturation, or there is a break in the C-C chain by replacement of one carbon with another element, such as oxygen.
The PFAS compounds that we can analyze are water- and oil-repelling C-F chains, or tail, with an anionic water-soluble head. The number of carbon atoms in the tail and the functional group define its chemistry and naming convention. The water-soluble functional groups for perfluorinated PFAS include, among others:
- Carboxylic acids (R-COOH) Surfactants (A)
- Sulfonic acids (RSO3H) Surfactants (S)
- Sulfonamides (R-SO2NH2) Raw material or intermediate
The water-soluble functional groups for polyfluorinated PFAS include, among others:
- Fluorotelemer alcohols (R-CH2CH2OH) Raw material
- Fluorotelemer sulfonic acid (R-CH2CH2SO3H) Surfactant
- Fluorotelemer carboxylic acid (R-CH2COOH) Intermediate
The number of carbons using the same naming conventions as used for alkanes contributes to how we name each compound:
4 carbons—Buta (B)
5 carbons—Penta (Pe)
6 carbons—Hexa (Hx)
7 carbons—Hepta (Hp)
8 carbons—Octa (O)
Considering the above, a saturated chain with eight carbons and the carboxylic acid functional group becomes perfluorooctanoic acid (PFOA) and a saturated chain with eight carbons and a sulfonic acid functional group becomes perfluorooctane sulfonate (PFOS). PFOA and PFOS each have eight carbons and are presumably the most persistent, bioaccumulative, and biologically active. As a result, these compounds have been voluntarily banned globally,10 and are the compounds most subject to state and federal health advisories. PFOA, for example, very closely resembles octanoic acid, or MCT,11 and can be assimilated into biological cells and tissue.
Generally, PFOS was formulated as an ingredient of firefighting foam, and PFOS was used in the manufacture of fluoropolymers for commercial products such as stain-resistant carpet, moisture-repellent clothing, and no-stick cookware. Unlike priority pollutants, such as PCBs, organochlorine pesticides, and dioxin, PFAS are highly water-soluble. The anions do not bind well with soil, do not degrade, and readily leach into groundwater.
PFAS compounds are being detected in wastewater treatment effluents, urban runoff, and landfill leachates. Standard wastewater treatment processes do not remove them. Although PFOS may have a tendency to attach to biosolids, it still finds its way to the effluent. PFOA is not retained by the sludge. In addition, widespread PFOA and PFOS contamination has been linked to industrial sites and military fire training areas. Newer compounds, such as the polyfluorinated compounds, with breaks in the carbon chain by elements such as oxygen, may be more susceptible to environmental degradation; however, many of them may break into saturated PFAS chains, such as PFOA, or into smaller chains that are not routinely monitored and with greater water solubility.
This has been a short introduction to compounds of emerging concern. As can be seen, these compounds are a moving target and may change, depending on what research reveals. In the early 1960s, DDT was the new CEC. Alarm bells were raised and actions taken. Now, DDT is still routinely monitored but is rarely detected. Many compounds that would be detected today are not even monitored. For some of them there may not even be a method or an instrument capable of detecting them. However, as needs arise, new instruments will be developed with lower detection limits and discoveries will be made. Eventually, governments will establish limits and new regulations, and current compounds of emerging concern will become obsolete like DDT. The challenge will always be to find better ways of detecting and quantitating new contaminants, understanding what they are and their environmental fate, and ultimately determining their effect on living organisms and the environment.
- Keith, L.H. and Telliard, W.A. Priority pollutants I—a perspective view. ES&T Special Report Apr 1979, 13(4), 416–23
- Water Sci. Technol. 2008, 58(8), 1541–6; doi: 10.2166/wst.2008.742.
- Water Sci. Technol. 2018 May, 77(9–10), 2320–6; doi: 10.2166/wst.2018.153.
- https://aem.asm.org/content/76/11/3444, accessed April 4, 2019.
- https://www.epa.gov/dwstandardsregulations/perchlorate-drinking-water, accessed April 3, 2019.
- https://www.epa.gov/sites/production/files/2015-09/documents/12004-exhibita.pdf, accessed April 3, 2019.
- http://www.waterrf.org/resources/NewsletterStories/Microplastics.html, accessed April 3, 2019.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5918521/, accessed April 3, 2019.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5226407/, accessed April 3, 2019.
- https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/fact-sheet-20102015-pfoa-stewardship-program, accessed April 3, 2019.
- https://www.healthline.com/nutrition/mct-oil-101, accessed April 3, 2019.
William Lipps is CSO, Eurofins Eaton Analytical, LLC, 750 Royal Oaks Dr., Ste. 100, Monrovia, CA 91016, U.S.A.; tel.: 626-386-1127; mobile: 626-260-2307; e-mail: [email protected]; www.EurofinsUS.com/Eaton