Viewpoint pubs.acs.org/est
Current Wet Persulfate Digestion Method Considerably Underestimates Total Phosphorus Content in Natural Waters Jia-Zhong Zhang* Ocean Chemistry Division, Atlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administration, Miami, FL 33149, United States 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.1 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 otal phosphorus (TP) in river and lake water is an technique for measuring carbon, hydrogen and nitrogen important environmental variable that can be used as an content of particulate samples. In PP determination, high indicator of nutrient status and water quality. TP was recently temperature combustion in the presence of oxidizing reagent selected as an environmental stressor and numeric criteria have Mg(NO3)2 was followed by acid extraction to recover DIP. been developed based on its background concentration in a This method has been used for analysis of PP in sediment and given environment. To minimize the eutrophication risk in TDP in water samples and achieved a close to 100% conversion aquatic ecosystems, U.S. Environmental Protection Agency efficiency. However, no commercial P elemental analyzer is water quality criteria require that TP is not exceed 10−128 available and manual analysis of PP by high temperature μg/L for river and streams. Following EPA method 365.1, TP combustion remains time-consuming and labor-intensive. in surface waters is routinely determined on unfiltered water It is worth to note that the first persulfate digestion method samples that are digested by autoclave in the presence of developed by Menzel and Corwin in 1965 was for measuring persulfate. Recent studies, however, indicated this method seawater TP. In that study, only a few DOP compounds were suffers low recovery of particulate phosphorus (PP) and we tested for recovery. A dried zooplankton tissue was suspended suggest reliable high temperature combustion method be used in distilled water and then the filtrate from the suspension was for PP determination. analyzed by persulfate digestion method. This extracted, filtered Raw water samples contain both suspended particles and sample contained only DOP and no PP sample was tested. dissolved constituents. In EPA TP method 365.1, unfiltered Ironically, this seawater method become obsolete in marine water sample is used for persulfate digestion but no attention community has since been widely adapted for TP measurement was given on how to keep sample homogeneous and in freshwater where particulate matter, dominated by clay representative to the field condition. Due to rapid settling of minerals, is much more abundant than oceanic waters. large particles in water sample containers, it is difficult to keep Recently, Suzumura pointed out the limitation of this method for determination of TP in natural waters.2 He sample homogeneous during subsampling and analysis. For compared wet persulfate digestion with high temperature example, it is practically impossible to avoid particle settling combustion on sediment, plankton and mineral samples. While when samples are seating in an autosampler during automated online digestion and analysis. TP consists of particulate phosphorus and total dissolved phosphorus (TP = PP + Received: October 25, 2012 TDP). Because most TP resides in particulate phase loss of Revised: November 8, 2012 particles during sampling and analysis can result in a Accepted: November 19, 2012 Published: December 5, 2012 significantly underestimated TP.
T
© 2012 American Chemical Society
13033
dx.doi.org/10.1021/es304373f | Environ. Sci. Technol. 2012, 46, 13033−13034
Environmental Science & Technology
Viewpoint
(5) Muller, B.; Berg, M.; Yao, Z. P.; Zhang, X. F.; Wang, D.; Pfluger, A. How polluted is the Yangtze River? Water quality downstream from the Three Gorges Dam. Sci. Total Environ. 2008, 402, 232−247.
marine waters containing predominantly biogenic particles showed >90% recoveries, low recoveries of 14% and 69% were observed for kaolinite and montmorillonite samples, respectively. The low recovery indicates that persulfate digestion is not capable to completely decompose the P bearing clay minerals. For example, apatite, an important P bearing mineral in the crust, is known to slowly dissolve only in a strong acid. This study provided strong evidence that persulfate digestion method could severely underestimate the TP content of river waters in which clay minerals dominate suspended particulate matter. The widespread use of wet persulfate digestion method has resulted in many underestimated TP measurements. This problem becomes apparent when high temperature combustion method was used for the same waters. For example, there is a 2fold difference in estimated P flux in Yangtze River during 1998 flood because different TP methods were used. Yan and Zhang3 obtained the TP of 10.6 μM at Datong based on PP measured in suspended particles with high temperature combustion method. Shen and Liu4 reported only 4 μM at same location with wet persulfate digestion on unfiltered water samples. In another study in Yangtze River, Muller et al.5 used persulfate digestion method to determine TP in unfiltered water samples and TDP in filtered samples. The PPs calculated from the difference between TP and TDP were all low, including some zero and even negative values. Their reported low PP is contrary to previous studies in that PP accounts for more than 90% of TP in Yangtze and other rivers around the globe. Because the PP dominates TP flux in the rivers accurate determination of PP is crucial in measuring P flux throughout different reaches of the rivers and eventually to the ocean. Monitoring watershed nutrient fluxes is currently taking place around the world including the U.S. Geological Survey ongoing effort in national streamwater quality monitoring networks. As discussed above, incomplete digestion of particles by current wet persulfate method has jeopardized the quality of TP data collected from many monitoring programs. To improve the data quality it is recommended the PP be separately determined on particulate samples with high temperature combustion. The TP should be obtained by the sum of PP and TDP, the latter can be readily measured by wet persulfate digestion method on filtered water samples.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 305 361 4512; fax: 305 361 4447; e-mail: jia-zhong.
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Huang, X.-L.; Zhang, J.-Z. Neutral persulfate digestion at subboiling temperature in an oven for total dissolved phosphorus determination in natural waters. Talanta 2009, 78, 1129−1135. (2) Suzumura, M. Persulfate chemical wet oxidation method for the determination of particulate phosphorus in comparison with a hightemperature dry combustion method. Limnol. Oceanogr. Methods 2008, 6, 619−629. (3) Yan, W.; Zhang, S. The composition and bioavailability of phosphorus transport through the Changjiang (Yangtze) River during the 1998 flood. Biogeochemistry 2003, 65, 179−194. (4) Shen, Z.; Liu, Q. Nutrients in the Changjiang River. Environ. Monit. Assess. 2009, 153, 27−44. 13034
dx.doi.org/10.1021/es304373f | Environ. Sci. Technol. 2012, 46, 13033−13034
