Current Wet Persulfate Digestion Method Considerably Underestimates Total Phosphorus Content in Natural Waters

Abstract

T phosphorus (TP) in river and lake water is an important environmental variable that can be used as an indicator of nutrient status and water quality. TP was recently selected as an environmental stressor and numeric criteria have been developed based on its background concentration in a given environment. To minimize the eutrophication risk in aquatic ecosystems, U.S. Environmental Protection Agency water quality criteria require that TP is not exceed 10−128 μg/L for river and streams. Following EPA method 365.1, TP in surface waters is routinely determined on unfiltered water samples that are digested by autoclave in the presence of persulfate. Recent studies, however, indicated this method suffers low recovery of particulate phosphorus (PP) and we suggest reliable high temperature combustion method be used for PP determination. Raw water samples contain both suspended particles and dissolved constituents. In EPA TP method 365.1, unfiltered water sample is used for persulfate digestion but no attention was given on how to keep sample homogeneous and representative to the field condition. Due to rapid settling of large particles in water sample containers, it is difficult to keep sample homogeneous during subsampling and analysis. For example, it is practically impossible to avoid particle settling when samples are seating in an autosampler during automated online digestion and analysis. TP consists of particulate phosphorus and total dissolved phosphorus (TP = PP + TDP). Because most TP resides in particulate phase loss of particles during sampling and analysis can result in a significantly underestimated TP. To obtain representative particulate samples, filtration through 0.45 μm pore size filters has been used to separate PP from TDP (it might require a large volume of water if water turbidity is low). Particles collected on the filters are used to determine the PP. The filtrates are used to determine dissolved inorganic phosphorus (DIP) and TDP. The difference between TDP and DIP is operationally defined as dissolved organic P (DOP). The sum of PP and TDP can provide an accurate estimate of TP in natural waters. DOP and PP are usually converted to DIP which can then be measured by the molybdenum blue method. Wet persulfate digestion has been widely used for digesting TDP samples with variable conversion efficiency for different DOP compounds, ranging from 66% for inositol phosphate to 100% for ADP. On the other hand, high temperature combustion is needed for PP determination. High temperature combustion at above 550 °C can decompose all organic matters and inorganic minerals to simple inorganic compounds. With commercial CHN elemental analyzer available, it has become a conventional technique for measuring carbon, hydrogen and nitrogen content of particulate samples. In PP determination, high temperature combustion in the presence of oxidizing reagent Mg(NO3)2 was followed by acid extraction to recover DIP. This method has been used for analysis of PP in sediment and TDP in water samples and achieved a close to 100% conversion efficiency. However, no commercial P elemental analyzer is available and manual analysis of PP by high temperature combustion remains time-consuming and labor-intensive. It is worth to note that the first persulfate digestion method developed by Menzel and Corwin in 1965 was for measuring seawater TP. In that study, only a few DOP compounds were tested for recovery. A dried zooplankton tissue was suspended in distilled water and then the filtrate from the suspension was analyzed by persulfate digestion method. This extracted, filtered sample contained only DOP and no PP sample was tested. Ironically, this seawater method become obsolete in marine community has since been widely adapted for TP measurement in freshwater where particulate matter, dominated by clay minerals, is much more abundant than oceanic waters. Recently, Suzumura pointed out the limitation of this method for determination of TP in natural waters. He compared wet persulfate digestion with high temperature combustion on sediment, plankton and mineral samples. While