Determination of Total Phosphorus in Wastewater Using Caro's Reagent and Ion Chromatography

Importance of the Topic

Phosphorus plays critical roles in aquatic ecosystems but excess loads from agricultural runoff and wastewater discharge cause eutrophication and oxygen depletion. Monitoring total phosphorus in wastewater is essential for environmental protection and regulatory compliance.

Objectives and Study Overview

This study presents and compares two ion chromatography methods for determining total phosphorus in wastewater following oxidation with potassium peroxymonosulfate (Caro reagent). One approach uses a manual carbonate/bicarbonate eluent and the other employs automated hydroxide eluent generation. Performance parameters such as detection limits, linearity and recovery are evaluated.

Used Instrumentation

• Dionex ICS-3000 system with dual pump, eluent generator module, suppressed conductivity detector and autosampler
• IonPac AS22 column and AG22 guard for carbonate/bicarbonate eluent method
• IonPac AS18 column and AG18 guard for hydroxide eluent generated by EGC II KOH cartridge
• CRD 300 chemical suppressor and ASRS 300 suppressor for conductivity detection

Methodology and Instrumentation

Wastewater samples are filtered and oxidized by boiling with Caro reagent for 30 minutes. For the first method samples are diluted with carbonate/bicarbonate eluent; for the second method samples are neutralized with KOH prior to analysis with a potassium hydroxide gradient eluent generated inline. Calibration standards are prepared accordingly and method blanks are used for background correction in the automated eluent method.

Main Results and Discussion

Both methods demonstrate excellent linearity over the 20 to 320 µg/L phosphorus range (r2 > 0.99). The manual eluent method detection limit is 5.6 µg/L limited by reagent background, while the automated hydroxide method achieves 2.4 µg/L. A real wastewater sample yielded 202 µg/L using the manual method and 205 µg/L after blank correction with the automated method. Spike recoveries at 50 µg/L were 100.1% and 100.5% respectively.

Benefits and Practical Applications

• Complete conversion of organic and inorganic phosphorus forms to orthophosphate for reliable quantification
• Choice between manual eluent preparation and reagent free eluent generation
• High accuracy, low detection limits and reproducible retention times

Future Trends and Opportunities

Further integration of inline oxidation and eluent generation, development of compact systems for field monitoring and real time control, and advances in suppressor technology and reagent free approaches are expected to improve throughput and reduce maintenance.

Conclusion

Both ion chromatography methods are suitable for accurate total phosphorus measurement in wastewater following oxidation with Caro reagent. The automated hydroxide eluent method offers superior detection limits and workflow efficiency by eliminating manual eluent preparation. Analysts can select the approach that best fits laboratory capabilities and throughput demands.

References

• Masson P; Morel C; Martin E; Oberson A; Friesen D Commun Soil Sci Plant Anal 2001 32 2241–2253