Magnesium-induced Coprecipitation Research Articles

Pretreatment method for the analysis of phosphate oxygen isotope (δ18OP) of different phosphorus fractions in freshwater sediments

Accurate measurement of the oxygen isotopic composition of dissolved phosphate (δ18OP) of different phosphorus (P) fractions in lacustrine sediments is very difficult because of the influence of large amounts of impurities. In this study, we developed a five-step method for obtaining high purity Ag3PO4 sample for the analysis of δ18OP of different P fractions in freshwater sediments. Sedimentary P was divided into NaHCO3-P, NaOH-P and HCl-P by chemical sequential extraction. Pretreatment procedures for different sedimentary P fractions were improved in the following respects: 1) abandonment of the magnesium-induced coprecipitation method to avoid the introduction of impurity ions, such as Mg2+ and Cl−, and reduce the loss of P; 2) use of a small amount of non-phosphate activated carbon powder to efficiently remove organic matter in extracts of NaHCO3-P and NaOH-P, and reduce the loss of P; 3) adjustment of the HCl-P extract pH to 4 in order to form Fe(OH)3-PO43− coprecipitate, thereby removing most of metals and Cl−. This method reduces the pretreatment steps, simplifies the operation and increases the recovery of phosphate (90.98%–96.69%). The high purity Ag3PO4 sample can be obtained and the repeatability and accuracy of measured δ18OP is better than 0.3‰, demonstrating high reliability and accuracy. This new method was used to analyze the δ18OP of different P fractions in sediments of a eutrophic lake in southwestern China. The preliminary results indicated that the δ18OP in the sediments can be used to identify different P sources, and provide new insights into sedimentary P cycling. The method established in this study provides a powerful tool for investigating the sources and biogeochemical cycle of P in freshwater sediments.

Profile of David M. Karl

From the moment 17-year-old David Karl first glimpsed the ocean from the summit of Cadillac Mountain on Maine's Mount Desert Island, he was captivated by the blue waters surrounding the fog-veiled islands. Since then, he has contributed to some of the world's most pivotal discoveries in oceanography. Karl, who was elected to the National Academy of Sciences (NAS) in 2006 and is now a professor of oceanography at the School of Ocean and Earth Science and Technology at the University of Hawaii, has witnessed iconic moments like the discovery of hydrothermal vents at the Galapagos Rift. His work on 23 research expeditions to Antarctica helped to reveal the unanticipated thriving food web that exists in those frigid waters. He has developed or refined more than 50 analytical methods to probe ocean life and biogeochemistry. Many, like the MAGnesium Induced Co-precipitation (MAGIC) method for measuring phosphate, have been widely adopted. The work that has earned him the most recognition, however, is the Hawaii Ocean Time-series (HOT) program. David M. Karl. The C-MORE Hale laboratory. Since 1988, HOT program researchers have made monthly measurements of water column biogeochemistry and microbial dynamics at Station ALOHA, a site ∼100 km north of Oahu. Their datasets are freely available and have been used by hundreds of researchers worldwide. The most startling finding to emerge from HOT program data is that climate change is affecting the ocean ecosystem. Data from the HOT program provided the first “bulletproof” evidence that rising atmospheric carbon dioxide (CO2) from burning fossil fuels is making the ocean more acidic, a finding that was published in PNAS and won the 2009 Cozzarelli Prize (1). Karl's work has garnered many honors, including the G. Evelyn Hutchinson Medal from the American Society for Limnology and Oceanography in 1998, the A. G. Huntsman …

Assessment of the conventional molybdenum-blue and magnesium-induced coprecipitation procedures in phosphorus measurement in various aquatic environments

Phosphorus (P) is an essential nutrient utilized by all organisms for energy transport and growth. Both the conventional molybdenum-blue method and the magnesium-induced coprecipitation (MAGIC) procedures were applied for the measurement of dissolved inorganic phosphorus (DIP) and total dissolved phosphorus (TDP) in more than 840 water samples collected between 2003 and 2005, including seawater (the Huanghai Sea, the East China Sea, and the northern South China Sea), water from rivers and estuaries (the Changjiang, the Huanghe, and the major rivers emptying into the Jiaozhou Bay), groundwater (in the drainage basin surrounding the Jiaozhou Bay), rainwater, and aquaculture water samples. The MAGIC method allows the investigation of phosphorus distributions and cycling for systems in which DIP is below the detection limits of conventional methods. Comparison between the two methods demonstrated that the concentrations obtained with both methods were significantly correlated. The differences of DIP and TDP concentrations measured with the two methods were higher when the concentrations of DIP and TDP were lower, implying the lower reproducibility at low concentrations. The concentration differences increase with the increase in the proportion of DOP in TDP, which indicates that the discrepancy of DIP concentrations measured with the two methods increases when the DOP concentration is high. The discrepancies indicated that the composition of P compounds differs depending on sample sources and water type; it would be useful to infer the presence of different phosphorus compound pools from differences between the two methods. This study indicates the potential presence of a pool of alkaline-labile compounds for samples from rainwater and rivers and estuaries surrounding the Jiaozhou Bay; the potential presence of acid-labile compounds that were adsorbed by Mg(OH)2 for samples from the Changjiang Estuary, Huanghai Sea, East China Sea, and groundwater; the potential presence of a substantial pool of acid-labile compounds that were not adsorbed by Mg(OH)2 for samples from the Huanghe Estuary, aquaculture water, the East China Sea algal bloom water, and the northern South China Sea.

Determination of arsenic(V) in freshwaters by flow injection with luminol chemiluminescence detection

A simple and rapid flow injection (FI) method is reported for the determination of arsenic(V) based on luminol chemiluminescence (CL) detection. The molybdoarsenic heteropoly acid formed by arsenic and ammonium molybdate in the presence of ammonium vanadate in acidic conditions generated chemiluminescence emission via the oxidation of luminol. The limit of detection was 0.15 μg L−1, with a sample throughput 120 h−1. A linear calibration graph was obtained over the range 0.15 to 7.5 μg L−1 (r 2 = 0.9989; n = 9) with relative standard deviation (n = 4) in the range 0.8 to 2.5%. Interfering cations were removed by passing the sample through an in-line iminodiacetate chelating column and phosphate (at 0.6 mg L−1) was removed off-line by magnesium-induced coprecipitation (MAGIC) method. The method was applied to freshwater samples and the results obtained were in reasonable agreement with the results obtained using HGAAS as the reference method.

Intercomparison and coupling of magnesium-induced co-precipitation and long-path liquid-waveguide capillary cell techniques for trace analysis of phosphate in seawater

Currently, two common techniques for nanomolar-level phosphate measurements in seawater are magnesium-induced co-precipitation (MAGIC) and long-path liquid-waveguide capillary cell (LWCC) spectrophotometry. These techniques have been applied in the open ocean, and our understanding of phosphate distributions in oligotrophic subtropical gyres is based on those data. However, intercomparison of these methods has not previously been performed at nanomolar levels. Here, we report experimental results directly comparing the MAGIC and LWCC techniques. We also evaluated the impact of various commonly employed filters on phosphate determinations, as well as interferences from dissolved organic phosphorus (DOP) and arsenate. Our results find agreement between these methods at phosphate concentrations <100 nM. We found that filter selection is important for accurate determinations of phosphate, and that DOP hydrolysis affects both techniques similarly. Finally, we demonstrate the advantage of combining MAGIC preconcentration and LWCC spectrophotometry for analysis of very low nanomolar concentrations.

Comparison of Offshore and River-Influenced Coastal Waters in the Eastern Mediterranean

Depending on the environmental condition and antropogenic nutrient supply, organic matter production and chemical composition show spatial and temporal variabilities in marine environment. The Eastern Mediterranean is known to be one of the oligotrophic seas, where the surface inorganic phosphate and nitrate concentrations vary in the range of 10 to 20 nM and 0.10 to 0.30 µM, respectively. Primary production is, therefore, mainly controlled by phosphate. The exceptionally low phosphate concentrations have limited reliable estimates of the N/P (nitrate/phosphate) ratio by traditional measurement methods. In this study, we have used Magnesium induced coprecipitation (MAGIC) technique to measure inorganic phosphate with a sensitivity of 1 nM. Changes in nutrient and hydrographic conditions are monitored at three near-shore stations on Erdemli (Mersin) coastal zone and four off-shore stations in December 2003. Nutrient concentrations in the surface water displayed a strong offshore gradient, decreasing from 0.018 to 0.009 µM for phosphate; 0.40 to 0.06 µM for nitrate; 1.76 to 1.69 µM for reactive silicate towards open sea. On the other hand, deep water concentrations exhibited an opposite trend; phosphate rises from 0.024 to 0.214 µM, nitrate from 0.35 to 5.52 µM, reactive silicate from 1.77 to 13.78 µM in the region.

Nutrient gradients in the western North Atlantic Ocean: Relationship to microbial community structure and comparison to patterns in the Pacific Ocean

Studies of nitrogen and phosphorus dynamics in the oligotrophic surface waters of the western North Atlantic Ocean have been constrained because ambient concentrations are typically at or below the detection limits of standard colorometric methods, except during periods of deep vertical mixing. Here we report the application of high-sensitivity analytical methods—determinations of nitrate plus nitrite (N+N) by chemiluminescence and soluble reactive phosphorus (SRP) by the magnesium induced co-precipitation (MAGIC) protocol—to surface waters along a transect from the Sargasso Sea at 26°N through the Gulf Stream at 37°N, including sampling at the JGOFS Bermuda Atlantic Time-series Study (BATS) station. The results were compared with data from the BATS program, and the HOT station in the Pacific Ocean, permitting cross-ecosystem comparisons. Microbial populations were analyzed along the transect, and an attempt was made to interpret their distributions in the context of the measured nutrient concentrations. Surface concentrations of N+N and SRP during the March 1998 transect separated into 3 distinct regions, with the boundaries corresponding roughly to the locations of the BATS station (∼31°N) and the Gulf Stream (∼37°N). Although N+N and SRP co-varied, the [N+N] : [SRP] molar ratios increased systematically from ∼1 to 10 in the southern segment, remained relatively constant at ∼40–50 between 31°N and 37°N, then decreased again systematically to ratios <10 north of the Gulf Stream. Dissolved organic N (DON) and P (DOP) dominated (⩾90%) the total dissolved N (TDN) and P (TDP) pools except in the northern portion of the transect. The [DON] : [DOP] molar ratios were relatively invariant (∼30–60) across the entire transect. Heterotrophic prokaryotes (operationally defined as “bacteria”), Prochlorococcus, Synechococcus, ultra- and nanophytoplankton, cryptophytes, and coccolithophores were enumerated by flow cytometry. The abundance of bacteria was well correlated with the concentration of SRP, and that of the ultra- and nanophytoplankton was well correlated with the concentration of N+N. The only group whose concentration was correlated with temperature was Prochlorococcus, and its abundance was unrelated to the concentrations of nutrients measured at the surface. We combined our transect results with time-series measurements from the BATS site and data from select depth profiles, and contrasted these North Atlantic data sets with time-series of N and P nutrient measurements from a station in the North Pacific subtropical gyre near Hawaii [Hawaii Ocean Time-series (HOT) site]. Two prominent differences are readily observed from this comparison. The [N+N] : [SRP] molar ratios are much less than 16 : 1 during stratified periods in surface waters at the BATS site, as is the case at the HOT site year round. However, following deep winter mixing, this ratio is much higher than 16 : 1 at BATS. Also, SRP concentrations in the upper 100 m at BATS fall in the range 1–10 nM during stratified periods, which is at least one order of magnitude lower than at the HOT site. That two ecosystems with comparable rates of primary and export production would differ so dramatically in their nutrient dynamics is intriguing, and highlights the need for detailed cross ecosystem comparisons.

Trace analysis of phosphorus in water by sorption preconcentration and luminol chemiluminescence

A new, highly sensitive chemiluminescence method for the determination of sub-ppb quantities of phosphorus in water is described. The method is based on sorption preconcentration of phosphorus as a yellow vanadomolybdophosphoric heteropoly acid (HPA) in the presence or absence of a cationic surfactant on a paper filter, followed by direct chemiluminescence detection of the phosphorus concentrate via reaction with an alkaline luminol solution. The molar ratio of cationic surfactant to HPA in the ion associate sorbed on the filter is 4:1. The detection limits for phosphorus are 0.02 microgram of P L-1 in the presence of surfactant and 0.1 microgram of P L-1 in the absence of surfactant for a sample volume of 150 mL. The calibration plot is linear from 0.06 to 1.7 micrograms of P L-1 in the presence of a surfactant, and the time required for analysis is 25 min. In the absence of surfactant, the selectivities against Si4+ and As5+ are 5 and 40 times greater than those for the standard colorimetric method based on the formation of the blue molybdophosphoric HPA. Applications of the method to the analyses of river water, seawater, and the turbine vapor condensate from a coal-fired power plant are described. It is demonstrated that the sensitivity advantage of the chemiluminescence technique can be combined with the magnesium-induced coprecipitation (MAGIC) method for a more selective measurement of soluble reactive phosphorus.

Partial characterization of the dissolved organic phosphorus pool in the oligotrophic North Pacific Ocean

The combined application of two independent methods for phosphorus (P i determination in seawater, continuous‐flow UV photodecomposition and magnesium‐induced coprecipitation (MAGIC), was used to provide a partial characterization of the dissolved P pools at Sta. ALOHA (22°45′N, 158°W) in tl e oligotrophic subtropical North Pacific Ocean. Comprehensive laboratory analyses of the UV light‐induced photodecomposition of a variety of specific organic P compounds dissolved in seawater confirmed that submicromolar concentrations of monophosphate compounds could be analytically separated from nucleotide di‐ and triphosphates based upon a previously described temperature‐controlled low‐pressure, mercury vapor UV irradiation treatment. When combined with a recently described high‐precision P‐detection system, the total dissolved phosphorus (TDP) in seawater could be reproducibly subdivided into three chemically distinct pools: soluble reactive phosphorus (SRP; presumably dominated by orthophosphate), UVlabile phosphorus (PUV‐L; containing primarily monophosphate esters), and UV‐stable phosphorus (PUV‐S; containing primarily nucleotide di‐ and triphosphates, nucleic acids, and other compounds that are resistant to the UV treatment). Field application of these procedures to samples collected at Sta. ALOHA during the period September 1991 to March 1992 (HOT‐30 to HOT‐35) revealed the presence of all three operationally defined pools. In the upper portion of the water column (0–100 m) the TDP pool (29.26 ± 2.32 mmol P m−2) contained, on average, 23% SRP, 26% PUV‐S, and 51% With increasing depth, the concentration of P decreased while that of the PUV‐S pool increased; the PUVL,: PUV‐S ratios decreased from values of 2–5 in the upper water column to ≤ 1.0 at 200 m.

Temporal variability in dissolved phosphorus concentrations in the subtropical North Pacific Ocean

Dissolved phosphorus pool measurements were made on approximately monthly intervals during the period October 1988-November 1994 at an oligotrophic, deep-ocean station near Hawaii (Station ALOHA; 22 °45′N, 158 °W). Data include depth profiles (0–200 m) of soluble reactive phosphorus (SRP), measured by two independent techniques (a high-sensitivity, magnesium-induced coprecipitation (MAGIC) method, and standard autoanalysis procedure), and total dissolved phosphorus (TDP) based on UV photo-oxidation treatment followed by automated SRP analysis. Considerable variability in SRP was observed both with depth and with time over the 6-yr period. Upper water column (0–100 m) SRP concentrations varied in time from > 150 to 60 nM, respectively. Throughout the entire 6-yr observation period, the upper water column (0–100 m) inventory of SRP displayed a systematic decrease from ∼ 10 to ∼ 5 mmol m−2 P, a result that is consistent with the hypothesized role of N2 fixation and P control of new and export production at Station ALOHA. Three independent, but not mutually exclusive, models are presented to explain these time-varying SRP concentrations: (1) convective mixing; (2) atmospheric deposition; and (3) upward P flux. The upward P flux model including both passive (upward particle flux) and active (biological transport) processes, is the most consistent model for the available data set.

MAGIC: A sensitive and precise method for measuring dissolved phosphorus in aquatic environments

A simple and straightforward method has been developed for the precise determination of nanomolar concentrations of soluble reactive phosphorus (SRP) and total dissolved phosphorus (TDP) in marine and freshwater environments. The method relies on the quantitative removal of P from solution by in vitro formation of brucite [Mg(OH)2], initiated by the addition of NaOH. The precipitate is collected by centrifugation and either dissolved in HCl for SRP determinations or hydrolyzed (8 M HCl, 80°C, 1 h), dried, and ashed (450°C, 3 h) for TDP determinations. Both procedures are completed by reaction with the standard Murphy‐Riley molybdenum blue reagents. This magnesium‐induced coprecipitation (MAGIC) procedure can be used to effect a 100‐fold concentration, thereby providing a method which can reliably detect 1 nmol SRP liter‒1 of seawater. MAGIC can also be used to determine P in Mg2+‐amended freshwater samples. The method is highly reproducible; typical estimates of precision for triplicate SRP determinations in the 10–100 nM range are from 1 to 3%. Field trials of the MAGIC procedure in the North Pacific Ocean reveal mixed‐layer SRP concentrations from 30 to 100 nM, with substantial temporal variability. The method is versatile and has many other potential applications.