High Purity Water Production for Perchlorate Analysis

Perchlorate salts (ClO₄-) are stable and have excellent oxidizing properties (Figure 1). For this reason, they are widely used in explosives (including pyrotechnics), solid rocket fuel, matches, air bags and certain types of fertilizers. Perchlorate is chemically inert under most conditions.

As a result, it was not considered previously to be a hazardous substance and was commonly disposed of in wastewater.

Perchlorate is highly soluble in water and adheres poorly to both mineral surfaces and organic material. Therefore, it can remain in ground and surface water for extended periods of time.

Figure 1. Structure of the perchlorate anion

Perchlorate Contamination

Perchlorate present in water can be taken up by plants and concentrated in their tissues. For example, it has been found in lettuce and fruits in areas irrigated with perchlorate-containing water. It is not metabolized by animals, but has been found in cow and human milk.

Perchlorate now appears to be present in various foods and drinks in Europe, the Americas, Africa and Asia. Consequently, it is becoming a global concern.

Potential Health Effects of Perchlorate

Perchlorate inhibits the uptake of iodide by the thyroid gland. The health effects of perchlorate, which are dependent on iodine intake levels, are not understood completely. For those with diets poor in iodine, ingesting high levels of perchlorate may have negative effects on thyroid function.

Improper thyroid function may affect metabolic functions, growth, cardiovascular and central nervous systems. In pregnant women, thyroid malfunction may distress the fetus and result in behavioral changes, delayed development and decreased learning capabilities.

Need for Perchlorate-Free Water in Analytical Laboratories

As an increasing number of laboratories are monitoring perchlorate levels in water and food, there is a need for perchlorate-free, analytical-grade water. This is especially important in areas where the local tap water may contain perchlorate.

Indeed, water is the main component of ion chromatography mobile phases. It also is used in many steps of the analytical process, such as sample preparation, dilutions, standard preparation, blanks, glassware / plasticware rinsing, etc. Therefore, it is important for the water used in all these steps to be free of perchlorate.

Experimental Design

Perchlorate can be removed using anion exchange, reverse osmosis or bioremediation. EMD Millipore water purification systems use ion exchange and / or reverse osmosis combined with other purification technologies. In order to answer the needs of a variety of users, four different systems were evaluated on their ability to remove perchlorate from water:

• Direct-Q^® 3 UV system (with SmartPak^® DQ3 cartridge): combines reverse osmosis, UV photooxidation, activated carbon and ion exchange to produce both pure and ultrapure water from tap water
• Elix^® 5 system (with Progard^® S1 cartridge): combines activated carbon, reverse osmosis and electrodeionization to produce pure water from tap water
• Simplicity^® UV system (with SimpliPak^® 1 cartridge): combines UV photooxidation, activated carbon and ion exchange to produce ultrapure water from pure water
• Milli-Q^® Advantage A10 system (with Q-Gard^® T1 and Quantum^® TEX cartridges as well as a Millipak^® 40 end-filter): combines UV photooxidation, activated carbon and ion exchange to produce ultrapure water from pure water

Regulations and Norms for Perchlorate

Tap water used for the experiments was free of perchlorates. Therefore, large amounts of perchlorate (83-84 ppm) were added to the tap water before purifying it with the EMD Millipore water purification systems as shown in Figure 2.

For the systems fed by pure water, Elix^® system water was used and lower concentrations of perchlorate were used (84-117 ppb). Perchloric acid, 1 N (VWR International Group, West Chester, PA, USA) was used to spike the water. Potassium hydroxide was added for neutrality. Water was analyzed for perchlorate at the inlet and after each of the four systems' main purification steps.

Figure 2. Ion chromatograph of tap water (diluted 100-fold) spiked with 100 ppm of perchlorate

Figure 3. Schematic representation of the water purification

Analytical Method

An ion-chromatography method was developed to measure perchlorate in water. On-column preconcentration was used to provide greater sensitivity. The retention time of the perchlorate peak was 32.5 minutes.

The limit of detection (LOD) and limit of quantitation (LOQ) were calculated using the signal-to-noise ratio (S/N=3 and 10 respectively). The LOD of the method was 0.005 ppb and the LOQ was 0.016 ppb of perchlorate.

All chromatographic equipment was obtained from the Dionex Corporation (Sunnyvale, CA, USA).

• System: Dionex^® DX-500 system with a GP50 Gradient Pump, AS40 Autosampler, LC20 Module, CD20 Conductivity Detector
• Columns: IonPac^® TAC-2 (3x35 mm) preconcentration column, AG19 (2x50 mm) precolumn and AS19 (2x250 mm) column
• Eluent suppressor: ASRS^® Ultra II 2 mm, 100 mA suppressor current
• Eluent: EG 40 Eluent Generator generating KOH gradient 0.5-100 mM, flow rate: 0.25 ml/min. The eluent gradient was: t₀= 0.5 mM, t₂₀ = 35 mM, t₄₀ = 100 mM, t₄₅ = 100 mM, t_45.1 = 0.5 mM, t₅₅ = 0.5 mM.
• Standards: Potassium perchlorate standard, 1000 mg / l (SpexCertiPrep^® Inc. Metuchen, NJ, USA) was diluted as needed with water from an Elix^® 10 system / Milli-Q^® Element system water purification chain.
• Sample injection: All samples were collected in a polypropylene container rinsed following a defined protocol for trace analysis. Tap water samples were diluted (100x) before injection

Depending on the expected ion level in the water samples, one of two injection methods was used:

• Direct injection (10μl)
• On-column pre-concentration (10ml of sample) before injection

Results and Discussion

For each system, ion chromatography was used to measure perchlorate levels both in the feed water and in the purified water following each of the main purification steps as shown in Figure 3. For demonstration purposes, the quantity of perchlorate added to each system's feed water was much higher than the levels found in tap water, including contaminated areas, where it is usually less than 25ppb.

All four EMD Millipore systems tested removed high levels of perchlorate from water, yielding perchlorate-free water as shown in Figure 4 and Table 1.

The SmartPak cartridge combining activated carbon and reverse osmosis removed more than 97% of perchlorate present in the feed water (Figure 4 - A). This is comparable to the ionic rejection levels commonly observed for anions (from 95 % for Cl- to 99 % for SO₄^2-) and cations (from 90 % for Na⁺ to 99 % for Ca²⁺). When reverse osmosis was coupled with electrodeionization, perchlorate levels were below the detection limit (0.005 ppb).

All EMD Millipore systems combining ion exchange with other purification technologies removed perchlorate from the feed water (Figure 4 - B). Ultraviolet photooxidation had no effect on perchlorate and did not generate any degradation by products (chromatograms D' and E in Figure 4 - B).

In an additional test, the cartridge of the Simplicity^® system (containing a combination of activated carbon and ion exchange resins) was replaced with an experimental cartridge containing only ion exchange resins. In both cases, the resulting water was free of perchlorate (data not shown). This suggests that ion exchange resins can remove perchlorate efficiently without using activated carbon.

(A)

(B)

Figure 4. Chromatograms of water at the inlet and after each of the main purification steps represented in Figure 3. A: Pretreatment step B: Polishing step

Chromatograms are offset vertically for readability. Injection volume was 10μl except when samples were preconcentrated (10ml)

Table 1. Summary of the perchlorate levels present in water at the inlet and outlet of each of the four water purification systems

*N.D.: not detectable, below 0.005 ppb

Conclusion

Perchlorate is removed efficiently from water by deionization technologies such as ion exchange or electrodeionization. Reverse osmosis also can remove large amounts of perchlorate from water (over 97 %), but it should be used in combination with other purification technologies to generate perchloratefree water for analytical use.

Laboratories testing for perchlorate need a purification system that not only will provide appropriate water quality for their analytical procedures, but also will remove perchlorate efficiently. All EMD Millipore water purification systems delivering ultrapure water are suitable when combined with good quality pretreatment technologies.

About EMD Millipore - Lab Water Business Unit

Water is the most commonly used solvent in laboratories and constitutes often more than 99% of the mass of solutions used in experimentations. The quality of water used in the lab is therefore critical for the success of the tests performed.

This information has been sourced, reviewed and adapted from materials provided by EMD Millipore - Lab Water Business Unit.

For more information on this source, please visit EMD Millipore - Lab Water Business Unit.