Monitoring Inorganic Anions and Cations During Desalination

Significance of the Topic

Monitoring inorganic anions and cations is critical in desalination to assess membrane performance, prevent scaling and fouling, and meet drinking water standards. As global freshwater resources become increasingly scarce, reliable ion monitoring supports sustainable production of potable and irrigation water from brackish or seawater.

Objectives and Study Overview

This study presents an ion chromatography method for simultaneous determination of common anions and cations in desalination process streams. Using a reagent-free system with electrolytic eluent generation and dual suppressed conductivity detection, the work evaluates linearity, detection limits, precision, and recovery across matrices from seawater to tap water.

Methodology and Instrumentation

The approach employs online generation of potassium hydroxide and methanesulfonic acid eluents, suppressed conductivity detection, and simultaneous injection into two parallel systems. Sample preparation involves 0.2µm filtration and, when needed, dilution. Calibration uses 1000 mg/L stock standards and daily prepared working solutions.

Used Instrumentation

• Thermo Scientific Dionex ICS-3000 RFIC-EG system with DP dual pump, EG eluent generator, DC detector module, and Chromeleon CDS
• IonPac AS18 and CS12A columns with AG18 and CG12A guard columns
• EGC II KOH and EGC II MSA cartridges and CR-ATC and CR-CTC trap columns for continuous suppressor regeneration
• ASRS 300 and CSRS 300 self-regenerating suppressors with suppressed conductivity detection
• Autosampler configured for simultaneous injections

Key Results and Discussion

All target anions (fluoride, chloride, nitrite, bromide, sulfate, nitrate, phosphate) and cations (lithium, sodium, ammonium, potassium, magnesium, calcium) were resolved within 20 minutes. Calibration curves showed excellent linearity (r2 > 0.9994). Detection limits were in the low microgram per liter range, with retention time precision < 0.1% and peak area precision < 1% in most cases. Recoveries in spiked seawater, artificial salt solutions, bay water, tap water, and mineral water ranged from 80% to 120%, confirming method robustness.

Benefits and Practical Applications

• Reagent-free eluent generation reduces chemical consumption and waste
• Simultaneous anion and cation analysis accelerates throughput
• Minimal sample preparation streamlines monitoring in municipal and industrial desalination facilities
• High precision and low detection limits support compliance with EPA and WHO guidelines

Future Trends and Potential Uses

Emerging developments include integration with online process control for real-time monitoring, extension to trace elements and emerging contaminants, coupling with mass spectrometry for detailed speciation studies, and application to advanced membrane processes such as nanofiltration and hybrid systems.

Conclusion

The reagent-free ion chromatography method with simultaneous suppressed conductivity detection offers a versatile, reliable, and cost-effective solution for comprehensive ion monitoring in desalination operations. Its performance across diverse water matrices supports quality assurance and regulatory compliance while reducing operational complexity.

References

1. IDA World Congress Information, International Desalination Association, Dubai, UAE, 2009
2. U.S. EPA Method 300.0, The Determination of Inorganic Anions in Water by Ion Chromatography, 1993
3. Potts D.E., Ahlert R.C., Wang S.S., Critical Review of Fouling of Reverse Osmosis Membranes, Desalination, 1981, 36, 235–264
4. Kester D.R., Duedall I.W., Connors D.N., Pytkowicz R.M., Preparation of Artificial Seawater, Limnology and Oceanography, 1967, 12, 176
5. U.S. EPA Drinking Water Health Advisory for Boron, 1993
6. WHO Guidelines for Drinking Water Quality, World Health Organization, Geneva, 2008
7. Lewis E.R., Schwartz S.E., Comment on Size Distribution of Sea-Salt Emissions as a Function of Relative Humidity, Atmospheric Environment, 2006, 40, 588
8. Thermo Scientific Dionex AS Autosampler Operator’s Manual, Document 065051-03, 2008