Understanding the contaminant & its evolving treatment technologies
We all want our families to be drinking and using clear, clean water. There are a number of products on the market that claim to do just that — provide clean drinking water. What, in reality, is “clean,” and does “clean” mean healthy? Take two glasses of water. One full of silt and dirt, the other clear and colorless. Which one would you drink? The likelihood is that you would probably drink the one that is not muddy.
Water that looks, smells and tastes good, may contain contaminants that evade the human senses. Heavy metals like lead (Pb), copper (Cu), mercury (Hg) and arsenic (As) may be prevalent in the water, which is why the U.S. EPA and other governmental organizations regulate levels of contaminants.
Lead is one of the most concerning metals often found in drinking water. In Flint, Michigan; Newark, New Jersey; and several other areas across the country, high levels of lead in water resulted in harm to humans.
Lead has no natural role in the human body, unlike some other contaminants which may be toxic at higher concentrations, such as selenium, for example (1). When lead gets into the body, it can substitute for similar divalent cations, in particular calcium (Ca2+). Calcium is such a key element in the human body, that the substitution of Pb2+ can have a serious health effect. There is no one, single biological mechanism to describe the toxicity of Pb. It can interfere with many biological functions. Young children are the most affected by Pb toxicity because Pb (similar to Ca) can travel to the brain and disrupt development and kill neurons. This typically happens because lead interrupts the equilibrium of Ca2+ in the cell, causing certain organelles (our mighty mitochondria) to release Ca2+ and induce apoptosis, the programmed death of a cell (2).
Lead gets into the body mainly through ingestion. Governments regulate lead in water to keep the public safe. In the U.S., there are two regulatory bodies that deal with drinking water: the EPA and the U.S. Food and Drug Administration (FDA). The FDA is responsible for bottled water, while the EPA is responsible for water from municipalities and the environment. The maximum contaminant goal is 0 parts per billion (ppb) lead, but it is not always feasible because of the historic use of lead as water service pipes. For this reason, the EPA action level is set at 15 ppb for drinking water, while for bottled water, the lead limit is 5 ppb. However, the EPA is in the process of changing its Lead and Copper Rule, so these numbers may change. Note that in Canada, the guideline for maximum acceptable concentration of Pb in drinking water is 5 ppb.
What Causes Lead Contamination?
Lead poisoning happens all over the world, especially in third-world countries, so why does lead contamination continue to be a problem for the U.S.? The health crises surrounding lead in the U.S. stems from aging infrastructure. Millions of Americans get water from their municipalities. The water is clear. It tastes and smells good. Why is it contaminated with lead (3)?
Water gets pumped from a municipality to homes through water mains. Service lines are the pipes that branch off of the main and take the water directly into the home. These lines can be made of a variety of materials, including galvanized steel, wrought iron, polyethylene, lead and more. Even in circumstances where the service lines are not made of lead, lead can get in the water through pipes that have been connected with lead solder.
Water passing through lines made of or contaminated with lead can corrode. Like biological cells, an equilibrium between the water and pipes is formed, and small amounts of lead can leach into the water stream. Municipalities can use a variety of techniques to prevent the corrosion, but lead can get into homes. Municipalities and water companies recognize that the lead service lines need to be replaced, but there are two problems, including high costs and the challenges associated with locating lead service lines.
How Can Consumers Be Prepared to Mitigate Lead Contamination?
While governments try to understand and plan for the replacement of these lead service lines, what can everyday citizens do?
There are two choices for residents effected by high lead concentrations in their water: bottled water or in-home water treatment. The companies who bottle water use the same lead treatment technologies that can be used in your own home.
To understand the treatment of lead in water, it is necessary to understand the type of lead in the water. To do that, chemists often rely on Pourbaix diagrams (often called Eh – pH diagrams), which provide theoretical insights as to the lead speciation — the type of lead that needs to be removed (4). These are not hard and fast rules but are guidelines. Marcel Pourbaix was a Belgian chemist who studied corrosion of metals (5). The diagrams he first plotted used the Nernst equation to visualize the relationship between molecular speciation in a system, typically water. Overtime, these diagrams began to be used in environmental chemistry to describe the chemical behavior of species in the hydrosphere (6).
A typical, simple, Pourbaix diagram for a lead-water system looks like Figure 1 (7).
This figure shows that depending on the potential (ORP) the Pb phase is cationic from 0 to 8.5 and then neutral from 8.5 to 10.2, and anionic from 10.2 to 14. This is a simple system. When variables change (other ions, temperature, pressure, etc.) the diagram can change as well. The Pourbaix diagram of Figure 2 (page 26) shows lead bound with carbonate and zero charge (note the difference between zero charge and solid). It is estimated that Pb can be in many forms and there are many different treatment techniques. The Water Quality Association (WQA) says that common forms of lead in water are Pb(OH)2 , PbCO3 and Pb2O (8). Other forms of lead are also likely depending on the circumstances.
Technologies to Remove Lead in Drinking Water
The treatment of water contaminated with lead can be accomplished by a few techniques. All of the techniques have advantages and disadvantages. Two of the most popular techniques are water filtration by a reverse osmosis (RO) membrane and filtration by sorption (carbon blocks, ion-exchange media).
An in-home RO system works by rejecting ions based on the size and sometimes charge. The rejected ions are concentrated in a waste stream that drains to the sewer. These technologies use membranes that may need to be cleaned or replaced. While RO is efficient for providing clean water, the biggest downside is the waste. In fact, many who are water-poor are trying to move away from RO because of the wasted water.
Sorption technologies remove lead from waste streams by selective interaction with the lead. Some common carbon blocks found in refrigerator filters have added specialty sorbents to address heavy metals. MetSorb, for example, is a line of sorbents that remove metals, including lead, copper, mercury and arsenic. MetSorb products can be incorporated into blocks or used directly in a cartridge to remove metals and provide healthy drinking water.
When water travels through a filter (pitcher cartridge or carbon block), the ions will interact with the sorbent. The sorbent can either transform the lead so that it gets stuck in a porous matrix, or the sorbent can chemically pull the lead out of the water and onto its surface or pore structures. In both cases, the result is that the water coming out of the filter is lead-free or highly reduced. While there is no water wasted as compared to an RO, waste is generated. The filter itself must be discarded over some time.
The difficulty of water treatment is that the water chemistry is different everywhere and constantly changing. However, the water treatment industry is also constantly changing to bring better, healthier and more cost-effective technologies to market. Many technologies already exist on the market to filter water, but it can be difficult to determine which ones will and will not work for your home.
As more service lines degrade and are not replaced, there will likely be more news stories about how lead is contaminating drinking water. While not everyone will face these issues, those who do have a variety of treatment technologies at their disposal to ensure that their families do not have to worry about elevated levels of lead. As new technologies are invented and improved, the goal of lead-free water can be achieved.
- http://staging.wqa.org/Portals/0/Tec hnical/Technical%20Fact%20Sheets/2015_Selenium.pdf2)
- https://academic.oup.com/brain/arti cle/126/1/5/299373
- Pourbaix, M., Atlas of electrochemical equilibria in aqueous solutions. 2d English ed. 1974, Houston, Tex.: National Association of Corrosion Engineers.
- DOI: 10.1016/B978-044452787-5.00098-6
- https://www.researchgate.net/publication/215622195_Mode_of_Occurrence_Treatment_and_ Monitoring_Significance_of_Tetravalent_Lead/figures?lo=1
Additional Resources Used:
- https://www.wqa.org/Portals/0/Resources/LeadinDrinking Water%20FAQ.081319%20.pdf
- Mode of Occurrence, Treatment and Monitoring Significance of Tetravalent Lead. Conference Paper, Jan 2005. Proc. AWWA Water Quality Technology Conference
- https://www.fda.gov/consumers/consumer‐ updates/bottled‐water‐everywhere‐keeping‐it‐safe