Determination of Total Nitrogen and Phosphorus in Wastewaters by Alkaline Persulfate Digestion Followed by IC

Significance of the Topic

Excess nitrogen and phosphorus in wastewater contribute to eutrophication, harmful algal blooms, oxygen depletion, and public health risks.
Accurate determination of total nitrogen (TN) and total phosphorus (TP) in wastewater effluents is essential for regulatory compliance, treatment optimization, and environmental protection.

Objectives and Study Overview

This study presents an alternative to traditional Kjeldahl-based methods for TN and TP by combining alkaline persulfate digestion with ion chromatography (IC) and suppressed conductivity detection.
The goal is to achieve simultaneous measurement of TN and TP in wastewater samples with improved safety, sensitivity, and simplicity.

Methodology and Instrumentation

The workflow consists of:

• Alkaline Persulfate Digestion: Mix 2 volumes of 1.5 M NaOH + K₂S₂O₈ reagent with 4 volumes of sample, heat at 120 °C for 60 min, cool, dilute 1:10–1:15, filter, and inject.
• Ion Chromatography: Thermo Scientific Dionex ICS-2100 RFIC System with Dionex IonPac AG19 guard (2×50 mm) and AS19 analytical column (2×250 mm). Gradient elution from 20 to 50 mM KOH at 0.30 mL/min, 5 µL full-loop injection, suppressed conductivity detection using AERS 500 suppressor at 38 mA.
• Reagents and Standards: Ultrapure water; NaOH; potassium persulfate; certified nitrite, nitrate, phosphate, and nitrogen‐containing organics for calibration (2.5–300 µg/L); continuous digest check solution containing glycine, glycerophosphate, and glucose.
• Software and Accessories: Dionex Chromeleon 7.2; CR-ATC trap column; EGC III KOH cartridge; autosampler; column heater; degasser.

Main Results and Discussion

The high-capacity AS19 column resolves nitrate and orthophosphate from co-eluting sulfate, chloride, and chlorate arising from digestion.
Chloride levels up to 1000 mg/L cause minimal retention shifts (<4 %) and recovery >95 % for nitrate and phosphate.
System LODs (3× S/N) are 0.76 µg/L (nitrite-N), 1.0 µg/L (nitrate-N), and 1.3 µg/L (phosphate-P); LOQs (10× S/N) are 2.5, 3.4, and 4.2 µg/L, respectively.
Sample LOQs post-digestion are ~58 µg/L (nitrate-N) and ~54 µg/L (phosphate-P) due to background from persulfate.
Recovery experiments with model nitrogen compounds yielded 93–100 % for glycine, urea, nicotinic acid, and ammonium chloride.
Phosphorus recoveries ranged from 85 to 99 %, with the highest from glycerophosphate and glucose-1-phosphate.
Precision over four days for a primary effluent sample: nitrate-N 36.4 ± 1.1 mg/L (RSD 3.1 %), phosphate-P 3.68 ± 0.14 mg/L (RSD 3.9 %).
Six wastewater samples (influent and effluent) showed low inorganic N and P before digestion and significant increases in TN (up to 40 mg/L) and TP (up to 4.5 mg/L) after digestion, indicating organically bound fractions.

Benefits and Practical Applications

This method replaces toxic Kjeldahl reagents, eliminates separate nitrite/nitrate assays, and provides simultaneous TN and TP results.
Electrolytic KOH generation simplifies eluent preparation and reduces background noise for low µg/L detection.
The approach supports routine monitoring in wastewater treatment, environmental compliance, and nutrient pollution studies.

Future Trends and Opportunities

Integration of on-line digestion modules with IC for real-time monitoring.
Advances in high-capacity and ultra-fast anion-exchange columns for shorter run times.
Coupling with mass spectrometry for speciation of nitrogen and phosphorus compounds.
Field-deployable IC systems for decentralized wastewater treatment monitoring.
Development of greener oxidants and miniaturized sample preparation techniques.

Conclusion

The combination of alkaline persulfate digestion and IC on the Dionex IonPac AS19 column provides a robust, sensitive, and safer method for simultaneous determination of TN and TP in wastewater.
This protocol offers improved detection limits, high recoveries, and reliable performance for environmental and compliance laboratories.

Reference

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