Deionized and Demineralized Water: Production Methods and Purity Measurement

Understanding Pure Water: Definitions and Production Methods

Achieving high-purity water is essential across numerous industries, from pharmaceuticals to semiconductor manufacturing. While terms like "distilled," "deionized," and "demineralized" are often used interchangeably, they refer to distinct purification processes and resulting water qualities.

Distilled Water

Distillation is one of the oldest and most recognized methods for water purification. It involves boiling water in a "still" to create vapor, which is then condensed back into liquid form in a cooling unit. This process effectively separates water from non-volatile dissolved contaminants, such as salts, which are left behind in the boiling vessel.

However, distillation has limitations:

• Volatile Contaminants: If contaminants are volatile (e.g., certain alcohols), they can also vaporize and recondense with the water, reducing purity.
• Carry-over: Bursting bubbles at the boiling surface can carry small amounts of non-volatile material into the vapor stream.
• Maximum Purity: Typical maximum resistivity for distilled water is around 1.0 MΩ·cm.
• pH: Due to rapid absorption of atmospheric carbon dioxide (CO₂), the pH of distilled water generally ranges from 4.5 to 5.0.
• Re-contamination: Distilled water is highly susceptible to re-contamination if not handled and stored properly.

Deionized Water

Deionization (DI) is a process that utilizes specially manufactured ion exchange resins to remove ionized salts from water. Theoretically, it can remove 100% of these salts. However, deionization typically does not remove non-ionic contaminants such as organics, viruses, or bacteria, except through incidental trapping within the resin bed or by specialized strong base anion resins designed to target gram-negative bacteria.

Another advanced method for producing deionized water is electrodeionization (EDI), which combines ion exchange resins with ion-selective membranes and a direct current to continuously remove ions without chemical regeneration.

Demineralized Water

Demineralization is a broader term encompassing any process used to remove minerals from water. While it can refer to various methods, it is most commonly associated with ion exchange processes.

Ultrapure Water (UPW)

Ultrapure water is highly treated water characterized by extremely high resistivity and minimal organic content. It is a critical component in sensitive industries such as semiconductor manufacturing, pharmaceuticals, and power generation.

Deionization Technologies

Deionization primarily relies on ion exchange, a process where electrically charged dissolved substances (ions) are bound to positively or negatively charged sites on a resin as water passes through a packed column. Different configurations of ion exchange systems are employed to achieve various qualities of deionized water.

Two-Bed Ion Exchange Systems

These systems typically consist of two separate vessels: one containing a cation-exchange resin in the hydrogen (H⁺) form, and the other containing an anion-exchange resin in the hydroxyl (OH⁻) form.

1. Cation Exchange: Water first flows through the cation column, where all cations (e.g., Na⁺, Ca²⁺, Mg²⁺) are exchanged for hydrogen ions (H⁺).
2. Anion Exchange: The decationized water then flows through the anion column, where all negatively charged ions (e.g., Cl⁻, SO₄²⁻, HCO₃⁻, SiO₃²⁻) are exchanged for hydroxide ions (OH⁻).
3. Water Formation: The released hydrogen and hydroxide ions combine to form pure water (H⁺ + OH⁻ → H₂O).

These systems effectively remove all ions, including silica. For waters with high CO₂ content, installing a CO₂ removal unit (degasifier) between the ion exchange vessels is often advisable. This reduces the CO₂ content to a few milligrams per liter (mg/L), thereby decreasing the load on the strong base anion exchanger and reducing regeneration reagent requirements.

Strong Acid Cation (SAC) + Strong Base Anion (SBA) Systems

This is the simplest two-bed arrangement, capable of producing deionized water suitable for a wide range of applications.

Strong Acid Cation (SAC) + Weak Base Anion (WBA) + Strong Base Anion (SBA) Systems

This variation provides the same high quality of deionized water but offers economic advantages when treating water with high concentrations of strong anions (e.g., chlorides and sulfates). An additional weak base anion exchanger is installed before the final strong base anion exchanger.

• CO₂ Removal: An optional CO₂ removal unit can be placed either after the cation exchanger or between the two anion exchangers.
• Regeneration Efficiency: Regeneration of the anion exchangers typically uses caustic soda (NaOH) solution, first passing through the strong base resin and then through the weak base resin. This method is more efficient, as the residual regeneration solution from the strong base anion exchanger is often sufficient to fully regenerate the weak base resin.
• Organic Protection: The weak base resin also helps protect the strong base resin from fouling by organic matter present in the raw water.

Mixed-Bed Deionization

In mixed-bed deionizers, cation-exchange and anion-exchange resins are intimately mixed and contained within a single pressure vessel. This configuration creates an "infinite" series of cation and anion exchangers, leading to exceptionally high water purity.

• Regeneration Process:

  1. Separation: During regeneration, the two resins are hydraulically separated; the lighter anion resin rises to the top, while the heavier cation resin settles at the bottom.
  2. Regeneration: Caustic soda is used to regenerate the anion resin, and a strong acid regenerates the cation resin.
  3. Rinsing: Excess regenerant is removed by rinsing each bed separately.
  4. Remixing: The resins are then remixed by agitation with compressed air.

• Advantages:

  • Produces very high-purity water with consistent quality throughout the cycle.
  • Achieves an almost neutral pH.
  • Requires very low rinse water volumes.

• Disadvantages:

  • Lower exchange capacity compared to two-bed systems.
  • More complicated operating procedure due to the separation and remixing steps.

Reverse Osmosis (RO)

Beyond ion exchange, reverse osmosis (RO) is another highly effective process for producing deionized water. RO is considered the finest filtration technology available, capable of removing particles as small as individual ions from a solution.

• Mechanism: RO purifies water by applying pressure to force water molecules through a semi-permeable membrane, leaving behind salts and other impurities. This process improves the color, taste, and overall properties of the fluid.
• Rejection Capabilities: RO membranes can reject bacteria, viruses, pyrogens, salts, sugars, proteins, particles, dyes, and other constituents with a molecular weight greater than 150-250 Daltons. It achieves rejection rates of over 99.9% for viruses, bacteria, and pyrogens.
• System Configurations: Most water standards can be met with a single-pass RO system, while the highest purity requirements often necessitate a double-pass system.
• Operating Pressure: The driving force for RO purification is pressure, typically ranging from 3.4 to 69 bar (50 to 1000 psig).
• Efficiency: RO is significantly more energy-efficient than phase-change processes like distillation and requires fewer strong chemicals compared to ion exchange regeneration.
• Ion Separation: The separation of ions by RO is aided by their electrical charge. Dissolved ions, such as salts, are more likely to be rejected by the membrane than uncharged substances like many organics. Generally, the larger the charge and the larger the particle, the higher the rejection rate.

Reverse Osmosis

Measuring Water Purity

Water purity can be assessed using various analytical methods, each providing different insights into contaminant levels.

General Purity Indicators

• Weight of Dissolved Material: This involves determining the mass of dissolved solids (solutes) in a given volume of water.
• Colligative Properties: Impurities affect the physical properties of water, such as increasing its boiling point or lowering its freezing point.
• Refractive Index: The refractive index, which measures how transparent materials bend light waves, is also influenced by the presence of solutes.

Electrical Conductivity or Resistivity

One of the quickest and most common methods to estimate water purity is by measuring its electrical conductivity or resistance. Very pure water conducts electricity poorly, meaning it has high electrical resistance (or low conductivity). This method is highly sensitive to the presence of dissolved ions.

pH Value

Pure water, by definition, is slightly acidic. Freshly produced distilled water may initially have a pH value of approximately 7, but it rapidly absorbs carbon dioxide (CO₂) from the air until it reaches dynamic equilibrium with the atmosphere. Dissolved CO₂ reacts with water to form carbonic acid:

2 H₂O + CO₂ → H₂O + H₂CO₃ (carbonic acid) → H₃O⁺ (hydronium ion) + HCO₃⁻ (bicarbonate ion)

This reaction causes the pH of distilled water to settle around 5.8 within a few hours.

Challenges in Measuring Ultrapure Water pH

Measuring the pH of ultrapure water presents unique challenges:

• Rapid Contamination: High-purity water quickly absorbs atmospheric contaminants, particularly CO₂, which significantly affects its pH. Even a few parts per million (ppm) of CO₂ can cause the pH of ultrapure water to drop to 4.5, even though the water remains of essentially high quality.
• Low Conductivity: The extremely low conductivity of ultrapure water can interfere with the accuracy of standard pH meters, making reliable measurements difficult.

For ultrapure water, a more accurate estimation of pH is often obtained by measuring its electrical resistance. For example, if the resistance is 10.0 MΩ·cm, the pH typically lies between 6.6 and 7.6.

pH Values of Common Substances

Compared to many common beverages, deionized or distilled water appears to have a slightly acidic pH value. However, the human body effectively uses natural buffering systems to maintain pH balance. Consuming deionized water does not put the human body into an acidic state, as the blood will adjust its bicarbonate and carbon dioxide levels to neutralize any minor pH shifts.

AquaChain Engineering Tip

For ion exchange systems, effective pre-treatment (e.g., multi-media filtration, activated carbon filtration, or ultrafiltration) is crucial to protect resins from fouling by suspended solids, organics, and chlorine. Implementing robust pre-treatment extends resin life, reduces regeneration frequency, and maintains consistent high-quality water production.

Frequently Asked Questions

Q1: What is the primary difference between deionized and demineralized water? A1: Deionized water specifically refers to water treated by ion exchange to remove ionized salts. Demineralized water is a broader term for any process that removes minerals, often also referring to ion exchange but can include other methods like reverse osmosis.

Q2: Why is the pH of ultrapure water difficult to measure accurately? A2: Ultrapure water rapidly absorbs atmospheric carbon dioxide, forming carbonic acid and lowering its pH. Additionally, its very low conductivity can interfere with the accuracy of standard pH meters, making reliable measurements challenging.

Q3: Is drinking deionized water harmful to human health? A3: No, drinking deionized water is not harmful. The human body's natural buffering systems effectively neutralize any minor pH changes from consuming deionized water, preventing it from causing an acidic state in the body.