Deionized water is often associated with purity, leading to questions about whether it is also free from living microorganisms. This confusion stems from the understanding that deionization removes impurities. Understanding the difference between chemical purity and biological sterility is important for various applications, from laboratory work to medical procedures.
What is Deionized Water?
Deionized (DI) water is water that has undergone a process to remove mineral ions. This purification involves passing water through ion-exchange resins, which attract and bind to dissolved salts and minerals. Cations are exchanged for hydrogen ions, and anions are exchanged for hydroxide ions. These then combine to form water molecules, reducing ionic impurities.
The deionization process produces water with extremely low conductivity, indicating an absence of dissolved inorganic solids. This method targets charged particles. Its purpose is to achieve chemical purity, making the water suitable for sensitive applications where mineral contamination could interfere.
What Does “Sterile” Mean?
In a scientific context, “sterile” signifies the complete absence of all living microorganisms, including bacteria, viruses, fungi, and their reproductive forms like spores. This means an item or substance is entirely free from any viable microbial life.
This definition is distinct from terms like “clean” or “purified,” which indicate a reduction in contaminants but not their total elimination. For instance, purified water may have reduced levels of chemicals or particulates, but it could still harbor microorganisms. Understanding this definition is fundamental to appreciating why deionized water is not automatically considered sterile.
Is Deionized Water Inherently Sterile?
No, deionized water is not inherently sterile. The deionization process, which relies on ion-exchange resins, removes dissolved mineral ions, not living microorganisms. Microbes like bacteria, viruses, and fungi are typically uncharged or too large to be captured by the resins. Therefore, while chemically pure, water from a deionization system can still contain viable microbial populations.
Even if deionization reduced some microbial load, the water can easily become re-contaminated during or after production. Microorganisms in the air, storage tanks, or plumbing lines can readily colonize deionized water. Biofilms, communities of microbes encased in a protective matrix, can also form on water system surfaces, continuously shedding microorganisms. Unlike tap water, which often contains disinfectants like chlorine, deionized water lacks these protective agents, making it more susceptible to microbial proliferation.
How to Achieve Water Sterility
Achieving true water sterility requires additional processing steps beyond deionization, specifically targeting the elimination of all microbial life. One common method is autoclaving, which involves heating water under high pressure. This process exposes water to steam at 121 degrees Celsius (250 degrees Fahrenheit) and 15 pounds per square inch (psi) for 15 to 30 minutes, destroying cells and spores.
Another effective method is filtration, using specialized membranes with extremely small pore sizes, such as 0.22 micrometers. These filters physically block and remove bacteria and other larger microorganisms. Ultraviolet (UV) irradiation is also employed, where UV-C light at approximately 254 nanometers damages the DNA and RNA of microorganisms, preventing their reproduction. These post-deionization treatments are important when water is needed for sensitive applications like cell culture, pharmaceutical manufacturing, or medical procedures.