11:00 〜 13:00
[HTT18-P09] 河川中のリン酸酸素安定同位体比分析のためのパッシブサンプリング方の開発
キーワード：リン、安定同位体比、方法論
Introduction
Phosphate oxygen isotope (δ18OPO4) is a promising tool for elucidating phosphorus (P) biogeochemistry in aquatic ecosystems (Davies et al., 2014). Previous studies using δ18OPO4 analysis have successfully identified the source of P inputs to rivers,(Ishida et al., 2019) and internal P loads from sediment in lakes(Paytan et al., 2017). However, despite these successful applications, δ18OPO4 has not been widely used, especially in freshwater systems, due to sampling and methodological difficulties in sample with low phosphate (PO4) concentration.
The δ18OPO4 values are analyzed by converting PO4 in the sample solution to pure Ag3PO4 (purification) using an isotope ratio mass spectrometer. Depending on the PO4 concentration in a water sample, several to several hundred liters of water are required to obtain enough PO4 (e.g., 10 μmol). Therefore, in such systems, sampling, filtration, and concentration of PO4 in the water sample are time-consuming and labor-intensive. Improvement of the sampling should be necessary for broader applications of δ18OPO4 analysis to aquatic ecosystems.
In the present study we developed a passive sampling method for δ18OPO4 analysis using zirconium (Zr)-loaded resin, which selectively adsorbs PO4 from solution without interference from chloride and nitrate ions (Okumura et al., 2001).
Material and method
Amberlite IRC-748 resin was used as an adsorbent for PO4. The conditioning and Zr loading for Zr-loaded Amberlite IRC-748 (ZrIRC) followed Yoshida et al., (1983). The passive sampling using ZrIRC resin (ZrIRC method) was applied to the two rivers with different soluble reactive phosphate (SRP) concentrations, Kurose River (KRS, 5.3 μmol L-1) and Kadowaki River (KDW, 0.2 μmol L-1) in Hiroshima Prefecture, Japan. The 20 ml of ZrIRC resin packed in a mesh bag was set in the rivers for 1 to 8 days. To desorb PO4 from the ZrIRC resin, 2 M NaOH was added to the resin and shaken for 2 days. The SRP concentration in the solution was measured by the molybdenum-blue method. The PO4 in sample solution was converted to Ag3PO4 by resin treatments and by adding AgNO3 solution. The subsequent Ag3PO4 samples were measured using a TC/EA-IRMS (thermal conversion elemental analyzer connected to an isotope ratio mass spectrometer, a Delta V advantage via ConFlo IV, Thermo Fisher Scientific) at the Research Institute for Humanity and Nature. The analytical precision (±SD) was better than 0.4‰.
To test the possibility of isotopic fractionation by ZrIRC method, the conventional sampling using a plastic bucket and plastic containers and concentration method (magnesium-induced co-precipitation method) were also conducted in KRS. The δ18OPO4 value in the samples was measured in the same manner as above.
Result and discussion
The amount of PO4 collected from KRS and KDW by ZrIRC method increased with the SRP concentrations in the rivers and the installation time of ZrIRC resin. From the KRS, 77.8 ± 22.6 μmol bag-1 of PO4 was collected in 1 day. In addition, the δ18OPO4 values obtained by the ZrIRC method (14.2 ± 0.2‰) were almost the same as those obtained by the conventional sampling (14.0 ± 0.0‰). From the KDW, 1.32 ± 0.06 μmol bag-1 in 4 days and 2.77 ± 1.42 bag-1 in 8 days were collected. The Ag3PO4 precipitation was obtained by integrating the three bags of ZrIRC resin (60 ml) into one sample. Regardless of the installed time, the δ18OPO4 values of KDW in 4 days (14.2‰) and in 8 days (14.1‰) were almost the same. These results demonstrated that by adjusting the amount of ZrIRC resin according to the SRP concentration in a river, enough PO4 for δ18OPO4 analysis can be collected within 4 days without significant isotope fractionation.
Reference
Davies et al., 2014. DOI: 10.1016/j.scitotenv.2014.07.057.
Ishida et al., 2019, DOI: 10.1021/acs.est.8b05837.
Okumura et al., 2001. DOI: 10.1007/s002160100739.
Paytan et al., 2017. DOI: 10.1016/j.scitotenv.2016.11.133.
Yoshida et al., 1983. DOI: 10.1080/01496398309438126.
Phosphate oxygen isotope (δ18OPO4) is a promising tool for elucidating phosphorus (P) biogeochemistry in aquatic ecosystems (Davies et al., 2014). Previous studies using δ18OPO4 analysis have successfully identified the source of P inputs to rivers,(Ishida et al., 2019) and internal P loads from sediment in lakes(Paytan et al., 2017). However, despite these successful applications, δ18OPO4 has not been widely used, especially in freshwater systems, due to sampling and methodological difficulties in sample with low phosphate (PO4) concentration.
The δ18OPO4 values are analyzed by converting PO4 in the sample solution to pure Ag3PO4 (purification) using an isotope ratio mass spectrometer. Depending on the PO4 concentration in a water sample, several to several hundred liters of water are required to obtain enough PO4 (e.g., 10 μmol). Therefore, in such systems, sampling, filtration, and concentration of PO4 in the water sample are time-consuming and labor-intensive. Improvement of the sampling should be necessary for broader applications of δ18OPO4 analysis to aquatic ecosystems.
In the present study we developed a passive sampling method for δ18OPO4 analysis using zirconium (Zr)-loaded resin, which selectively adsorbs PO4 from solution without interference from chloride and nitrate ions (Okumura et al., 2001).
Material and method
Amberlite IRC-748 resin was used as an adsorbent for PO4. The conditioning and Zr loading for Zr-loaded Amberlite IRC-748 (ZrIRC) followed Yoshida et al., (1983). The passive sampling using ZrIRC resin (ZrIRC method) was applied to the two rivers with different soluble reactive phosphate (SRP) concentrations, Kurose River (KRS, 5.3 μmol L-1) and Kadowaki River (KDW, 0.2 μmol L-1) in Hiroshima Prefecture, Japan. The 20 ml of ZrIRC resin packed in a mesh bag was set in the rivers for 1 to 8 days. To desorb PO4 from the ZrIRC resin, 2 M NaOH was added to the resin and shaken for 2 days. The SRP concentration in the solution was measured by the molybdenum-blue method. The PO4 in sample solution was converted to Ag3PO4 by resin treatments and by adding AgNO3 solution. The subsequent Ag3PO4 samples were measured using a TC/EA-IRMS (thermal conversion elemental analyzer connected to an isotope ratio mass spectrometer, a Delta V advantage via ConFlo IV, Thermo Fisher Scientific) at the Research Institute for Humanity and Nature. The analytical precision (±SD) was better than 0.4‰.
To test the possibility of isotopic fractionation by ZrIRC method, the conventional sampling using a plastic bucket and plastic containers and concentration method (magnesium-induced co-precipitation method) were also conducted in KRS. The δ18OPO4 value in the samples was measured in the same manner as above.
Result and discussion
The amount of PO4 collected from KRS and KDW by ZrIRC method increased with the SRP concentrations in the rivers and the installation time of ZrIRC resin. From the KRS, 77.8 ± 22.6 μmol bag-1 of PO4 was collected in 1 day. In addition, the δ18OPO4 values obtained by the ZrIRC method (14.2 ± 0.2‰) were almost the same as those obtained by the conventional sampling (14.0 ± 0.0‰). From the KDW, 1.32 ± 0.06 μmol bag-1 in 4 days and 2.77 ± 1.42 bag-1 in 8 days were collected. The Ag3PO4 precipitation was obtained by integrating the three bags of ZrIRC resin (60 ml) into one sample. Regardless of the installed time, the δ18OPO4 values of KDW in 4 days (14.2‰) and in 8 days (14.1‰) were almost the same. These results demonstrated that by adjusting the amount of ZrIRC resin according to the SRP concentration in a river, enough PO4 for δ18OPO4 analysis can be collected within 4 days without significant isotope fractionation.
Reference
Davies et al., 2014. DOI: 10.1016/j.scitotenv.2014.07.057.
Ishida et al., 2019, DOI: 10.1021/acs.est.8b05837.
Okumura et al., 2001. DOI: 10.1007/s002160100739.
Paytan et al., 2017. DOI: 10.1016/j.scitotenv.2016.11.133.
Yoshida et al., 1983. DOI: 10.1080/01496398309438126.