Environ. Sci. Technol. 2010, 44, 8169–8174
Measurement of Dissolved Reactive Phosphorus Using the Diffusive Gradients in Thin Films Technique with a High-Capacity Binding Phase S H I M I N G D I N G , * ,† D I X U , † Q I N S U N , ‡ HONGBIN YIN,† AND CHAOSHENG ZHANG§ State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China, College of Environmental Science and Engineering, Hohai University, Nanjing, China, and School of Geography and Archaeology, National University of Ireland, Galway, Ireland
Received February 16, 2010. Revised manuscript received September 6, 2010. Accepted September 15, 2010.
Measurement of dissolved reactive phosphorus (DRP) by the diffusive gradients in thin films (DGT) technique was investigated using a new binding phase. Half-dried amorphous zirconium oxide (with 50 ( 5% of water content) was mixed with acrylamide solution for the preparation of the new binding phase. The resulting binding gel had a high binding capacity (223 µg P cm-2) for phosphate. The solution of NaOH (1 M) was used for elution of phosphate from the gel, and an elution efficiency of 0.95 was obtained. A test of DGT uptake with this gel showed its dependence on temperature, and there was no influence of pH (3 to 10) and ionic strength (10 nM to 0.1 M). Its capacity for DGT response exceeded 100 µg P cm-2, corresponding to a DRP concentration of more than 20 mg L-1 for a 24 h deployment with a standard DGT device at 25 °C, which was at least 50 times of the Fe-oxide gel commonly used in the present DGT technique. Measurements with this high-capacity DGT technique in a laboratory microcosm of homogeneously mixed sediments gave smooth and reproducible mass-depth profiles. This technique was well demonstrated by in situ measurements in algal- and macrophyte-dominated regions of Lake Taihu. The DGT-measured concentrations of DRP were on average 20% and 40% of the DRP concentrations in pore waters, respectively, indicating a partial resupply of the sediments to the pore waters with DRP.
Introduction Phosphorus (P) is the nutrient that most often limits the biological productivity in aquatic ecosystems. Elevated P concentration in the water column can result in water eutrophication and associated algae blooms, where P flux from sediments may be one of the controlling factors especially following a reduction in external P input (1). The substantial release of P from the sediments may occur from the rapid oxidization of organic matters when fresh materials * Corresponding author phone: 86-25-86882207; fax: 86-2586882207; e-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Hohai University. § National University of Ireland. 10.1021/es1020873
2010 American Chemical Society
Published on Web 10/01/2010
are recruited to surface sediments (2). The oxidation of organic materials is accompanied by the reduction of electron acceptors, resulting in a sharp redox boundary across the sediment-water interface (3). Therefore, concentration of dissolved P may be further controlled to a large extent by the redox-mediated distribution of ferric oxyhydroxides via adsorption-desorption processes. These processes may produce concentration gradients of dissolved P in the vicinity of the interface. Other localized concentration maxima of P within the sediments are also expected to be associated with organic decomposition in micro niches, as demonstrated for O2, pH, and pCO2 measured by microprobes (4). Successful observation of these chemical heterogeneities depends heavily upon the high resolution of the measurement. The technique of diffusive gradients in thin films (DGT) is a promising tool for in situ assessment of the kinetics of solute resupply from solid phase to solution in sediments at a high spatial resolution (5). DGT consists of a binding phase, which is separated from the sediment by a well-defined diffusion layer. The binding phase has affinity and capacity high enough to accumulate the solute, keeping concentration of the solute at effectively zero on its surface in contact with the diffusion layer. A diffusion-controlled flux of the solute is thereby maintained from the sediment pore water to the binding phase through the diffusion layer. This flux reflects the bulk pore water concentration and the supply from the sediment solid phases to pore waters (6). Numerous studies have used DGT for measurements of trace metal and sulfide remobilization fluxes and their concentration profiles at millimeter- and submillimeter scales (4). There have been relatively few applications of DGT measurements for dissolved P since Zhang et al. (7) developed this approach using ferrihydrite as a binding agent. Menzies et al. (8) applied this technique to soil systems, and recently Monbet et al. (9) and Pichette et al. (10) applied it to aquatic systems. The limited applications of this method may be related to the relatively low capacity of the ferrihydriteimpregnated binding gel (Fe-oxide gel) (8). The capacity of this phase for DGT response is ∼2 µg P cm-2 (7), which corresponds to a DGT-measured concentration (CDGT) of about 400 µg L-1 for a typical 24 h deployment with a standard DGT device at 25 °C. Saturation would reach for the DGT device under these deployment conditions, provided that the concentration of dissolved P in the surrounding matrix is maintained higher than 400 µg L-1. As concentrations of dissolved P in pore waters of productive lake sediments and fertilized soils are often higher than this value (11, 12), poor DGT measurements may be obtained under typical deployment conditions (8). Other potential drawbacks for the Feoxide gel include a transform of ferrihydrite to goethite during its preparation or storage making its binding capacity unstable (7, 13) and an inevitable heterogeneity of the particle size and distribution of ferrihydrite making the standard DGT device unsuitable for high-resolution analyses (9, 13). In this study, we propose to use amorphous zirconium oxide as an alternative binding agent for the preparation of the binding phase, which has been shown to have a high capacity for binding phosphate (14). The performance of DGT with this binding phase was tested under laboratory conditions and then validated by in situ measurements in Lake Taihu.
Experimental Section Principle of DGT. Details of the DGT theory and technique have been reported previously (15). A typical DGT device consists of a binding phase overlain by a well-defined VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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diffusion gel and a membrane filter that is used as an extension of the diffusion layer in contact with the solution. When the device is deployed in the surrounding media, an analyte passes through the filter and the diffusion gel with a thickness of ∆g and is trapped by the binding phase, resulting in a steady-state linear concentration gradient through the diffusion layer. The accumulation of the analyte in the binding phase can then be related to its solution concentration (CDGT) according to the equation CDGT )
(M - M′)(∆g + δ) DA(t - t′)
(1)
where A is the exposed surface area of the DGT device, δ is the diffusive boundary layer (DBL) in solution, D is the diffusion coefficient of the analyte in the diffusive layer, t′ and t are the initial time for establishing the concentration gradient and the time of deployment, respectively, and M′ and M are the corresponding accumulation masses of the analyte during the two periods. As the establishment of the concentration gradient is rapid (5), the t′ and M′ in the equation can be neglected. For example, the linear concentration gradient is almost reached (r ) 0.9999) for phosphate after deployment of 10 min at 25 °C with a diffusion layer of 0.93 mm. Errors caused by neglecting the t′ and M′ can be negligible (100, corresponding to a DGT-measured concentration ∼2, corresponding to a DGT-measured concentration of of >20 mg L-1 for a 24 h ∼0.4 mg L-1 for a 24 h deployment at 25 °Ca,b deployment at 25 °Ca temperature 4 to 30 °C, pH 2 to 10, ionic strength 10 nM to pH 2 to 10, ionic strength 10 µM to 1 M b 0.1 Ma
From ref 7. c Determined in pure solutions.
FIGURE 2. Measured mass of P from replicate DGT deployments in a laboratory microcosm. The open and filled squares indicate back-to-back deployments of two DGT probes containing the Zr-oxide gel. The open and filled cycles show the same deployments with the Fe-oxide gel. The location of the sediment-water interface is represented by zero. Field Deployments. In situ deployments of the DGT probes containing the Zr-oxide gel were carried out in Meiliang Bay (algal-dominated) and Gonghu Bay (macrophyte-dominated) of Lake Taihu. The results showed that both the DRP profiles (expressed as DRP concentration calculated according to eq 2) had similar shapes to those in pore waters of the two sites, characterized by a constant
zone to a depth of about -40 mm and a progressive increase below the depth (Figure 3). The resulting average concentrations were 0.018 and 0.007 mg L-, respectively, which were 20% and 40% of the DRP concentrations (measured with the minipeepers) in pore waters (0.092 and 0.018 mg L-). Generally, two transport processes occur during the deployment of the DGT probe in sediments, including a removal of an analyte in the pore water to the probe and a further release of the analyte from the sediment to resupply the pore water (6). The difference observed between the DGTmeasured and the true DRP concentrations should thus be a consequence of the two processes. Their ratio, R, is a good indicator of sediment reactivity, which reflects the ability of the sediment to resupply local pore water concentration (9, 26). The R in the Meiliang and Gonghu bays were all lower than 1, with average values of 0.16 and 0.37, respectively (Figure S5). It demonstrates that sediments in the two sites only partially resupply the pore waters with DRP. A downward increase in the R value was observed below the depth of -40 mm for Meiliang Bay. This is likely related to the change in the redox state, since the color of the sediment core became black (anoxic) at the depth of about -40 mm. This feature is different from the observation in the Gippsland Lakes, where variation of R values seems to be independent of the redox state (9). No obvious trend was observed for R values in Gonghu Bay, which showed a considerable variation with the depth. It may reflect horizontal heterogeneity of the sediment affected by root growth. Such heterogeneity has been observed for solute distributions at a small-scale resolution (18).
FIGURE 3. DGT-measured (open) and the true (filled, measured by minipeepers) concentrations of DRP in pore waters of two sites in Lake Taihu. The location of the sediment-water interface is represented by zero. Values are means ( SD of three replicates. VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Further simulations using the DIFS (DGT-induced fluxes in sediments) model (6) (the input parameters are listed in Table S1), with an example of the top surface sediment (4 mm), showed that the resupply rate of DRP from the sediments was different for the two sites. The Meiliang Bay is partially sustained and nonsteady, characterized by visible decreases in pore water concentration (Figure S6A) and accumulation mass of P with the deployment time (Figure S6D). The Gonghu Bay also displayed partially sustained feature but exhibited a steady state, characterized by constant pore water concentration (Figure S7A) and accumulation mass over the deployment time (Figure S7D). The simulation results can reflect how the sediment responds to a sink, such as macrophyte roots or the overlaying water depletion of DRP. Further studies are necessary to determine the relationship between sediment reactivity and its geochemical properties (such as the redox state and P speciation) and biological conditions (such as macrophyte growth) and elucidate how these factors affect resupply of the sediments with DRP to the pore waters. Application of the Zr-oxide binding phase will significantly promote these studies by enabling the measurement of DRP in various aqueous environments.
Acknowledgments This study was sponsored by the Project of Knowledge Innovation for the third period, CAS (KZCX2-YW-JS304), the National Scientific Foundation of China (40730528), the National High Technology Research Development Plan (863) (2007AA06Z411), and the Nanjing Institute of Geography and Limnology, CAS (NIGLAS2010KXJ01). We thank the two anonymous reviewers for their constructive suggestions in improving this manuscript.
Supporting Information Available Locations of sampling sites in Lake Taihu; uptake of P by the Zr-oxide gel as a function of time; recovery of P loaded on the Zr-oxide gel when treated with different concentrations of NaOH; relationship between accumulation mass of P in the Fe-oxide DGT devices and concentration of P in wellstirred solution; variation of R with depth in sediments of Lake Taihu; and DIFS model output for surface sediment layer of Lake Taihu. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Sondergaard, M.; Jeppesen, E.; Lauridsen, T. L.; Skov, C.; Van Nes, E. H.; Roijackers, R.; Lammens, E.; Portielje, R. Lake restoration: successes, failures and long-term effects. J. Appl. Ecol. 2007, 44, 1095–1105. (2) Dondajewska, R. Internal phosphorus loading from bottom sediments of a shallow preliminary reservoir. Oceanol. Hydrobiol. Stud. 2008, 37, 89–97. (3) Konovalov, S. K.; Luther, G. W.; Yucel, M. Porewater redox species and processes in the Black Sea sediments. Chem. Geol. 2007, 245, 254–274. (4) Stockdale, A.; Davison, W.; Zhang, H. Micro-scale biogeochemical heterogeneity in sediments: A review of available technology and observed evidence. Earth-Sci. Rev. 2009, 92, 81–97. (5) Zhang, H.; Davison, W.; Miller, S.; Tych, W. In-situ high resolution measurements of fluxes of Ni, Cu, Fe, and Mn and concentrations of Zn and Cd in porewaters by DGT. Geochim. Cosmochim. Acta 1995, 59, 4181–4192. (6) Harper, M. P.; Davison, W.; Tych, W. DIFS - a modelling and simulation tool for DGT induced trace metal remobilisation in sediments and soils. Environ. Modell. Softw. 2000, 15, 55–66.
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(7) Zhang, H.; Davison, W.; Gadi, R.; Kobayashi, T. In situ measurement of dissolved phosphorus in natural waters using DGT. Anal. Chim. Acta 1998, 370, 29–38. (8) Menzies, N. W.; Kusumo, B.; Moody, P. W. Assessment of P availability in heavily fertilized soils using the diffusive gradient in thin films (DGT) technique. Plant Soil 2003, 269, 1–9. (9) Monbet, P.; McKelvie, I. D.; Worsfold, P. J. Combined gel probes for the in situ determination of dissolved reactive phosphorus in porewaters and characterization of sediment reactivity. Environ. Sci. Technol. 2008, 42, 5112–5117. (10) Pichette, C.; Zhang, H.; Sauve, S. Using diffusive gradients in thin-films for in situ monitoring of dissolved phosphate emissions from freshwater aquaculture. Aquaculture 2009, 286, 198–202. (11) Maassen, S.; Uhlmann, D.; Roske, I. Sediment and pore water composition as a basis for the trophic evaluation of standing waters. Hydrobiologia 2005, 543, 55–70. (12) McDowell, R. W.; Condron, L. M. Chemical nature and potential mobility of phosphorus in fertilized grassland soils. Nutr. Cycling Agroecosyst. 2000, 57, 225–233. (13) Stockdale, A.; Davison, W.; Zhang, H. High-resolution twodimensional quantitative analysis of phosphorus, vanadium and arsenic, and qualitative analysis of sulfide, in a freshwater sediment. Environ. Chem. 2008, 5, 143–149. (14) Chitrakar, R.; Tezuka, S.; Sonoda, A.; Sakane, K.; Ooi, K.; Hirotsu, T. Selective adsorption of phosphate from seawater and wastewater by amorphous zirconium hydroxide. J. Colloid Interface Sci. 2006, 297, 426–433. (15) Zhang, H.; Davison, W. Performance characteristics of diffusion gradients in thin films for the in situ measurement of trace metals in aqueous solution. Anal. Chem. 1995, 67, 3391–3400. (16) Warnken, K. W.; Zhang, H.; Davison, W. Accuracy of the diffusive gradients in thin-films technique: Diffusive boundary layer and effective sampling area considerations. Anal. Chem. 2006, 78, 3780–3787. (17) Murphy, J.; Riley, J. P. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 1962, 27, 31–36. (18) Ding, S. M.; Sun, Q.; Xu, D. High-resolution determination of soluble reactive phosphate profiles in sediment porewaters of lakes using DET gel probes. Int. J. Environ. Anal. Chem. 2010, 90, 1130–1138. (19) Duan, H. T.; Ma, R. H.; Xu, X. F.; Kong, F. X.; Zhang, S. X.; Kong, W. J.; Hao, J. Y.; Shang, L. L. Two-Decade Reconstruction of Algal Blooms in China’s Lake Taihu. Environ. Sci. Technol. 2009, 43, 3522–3528. (20) Ma, R. H.; Duan, H. T.; Gu, X. H.; Zhang, S. X. Detecting aquatic vegetation changes in Taihu Lake, China using multi-temporal satellite imagery. Sensors 2008, 8, 3988–4005. (21) Trolle, D.; Zhu, G. W.; Hamilton, D.; Luo, L. C.; McBride, C.; Zhang, L. The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large, shallow lake. Hydrobiologia 2009, 627, 31–44. (22) Aminot, A.; Andrieux, F. Concept and determination of exchangeable phosphate in aquatic sediments. Water Res. 1996, 30, 2805–2811. (23) Li, W. J.; Zhao, H. J.; Teasdale, P. R.; John, R.; Wang, F. Y. Metal speciation measurement by diffusive gradients in thin films technique with different binding phases. Anal. Chim. Acta 2005, 533, 193–202. (24) Li, W.; Zhao, H.; Teasdale, P. R.; John, R.; Zhang, S. Application of a cellulose phosphate ion exchange membrane as a binding phase in the diffusive gradients in thin films technique for measurement of trace metals. Anal. Chim. Acta 2002, 464, 331– 339. (25) Li, Y. H.; Gregory, S. Diffusion of ions in seawater and in deepsea sediments. Geochim. Cosmochim. Acta 1974, 38, 703–714. (26) Harper, M. P.; Davison, W.; Zhang, H.; Tych, W. Kinetics of metal exchange between solids and solutions in sediments and soils interpreted from DGT measured fluxes. Geochim. Cosmochim. Acta 1998, 62, 2757–2770.
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