The Role of Iron Minerals in Long-Term Phosphorus Sequestration along an Estuarine Salinity Gradient

EPA Grant Number: F5B30337
Title: The Role of Iron Minerals in Long-Term Phosphorus Sequestration along an Estuarine Salinity Gradient
Investigators: Hartzell, Jeanne
Institution: George Mason University
EPA Project Officer: Michaud, Jayne
Project Period: June 1, 2005 through May 31, 2007
Project Amount: $88,324
RFA: STAR Graduate Fellowships (2005) RFA Text |  Recipients Lists
Research Category: Academic Fellowships


The major objective of this study is to investigate the role of iron minerals in sequestering phosphorus in the sediments along a salinity gradient in Maryland’s Patuxent River, a nutrient over-enriched, sub-estuary of the Chesapeake Bay. The Patuxent River is ideal for this investigation because the salinity gradient is especially well-characterized, and this river is the subject of intensive and on-going investigations characterizing P delivered to the river and deposited in the surficial sediments. Insights gained about P-Fe interactions in the Patuxent River will be compared to other Chesapeake Bay sites to check for large-scale differences in P dynamics that may occur in different sedimentary environments.

Excessive phosphorus (P) in estuaries contributes to excess phytoplankton growth, oxygen depletion, and ecosystem destruction. Fortunately, P can be scavenged by iron-rich sediments, reducing the availability of this nutrient for algal growth. However, changes in surface water conditions, such as increases in salinity, can cause iron-bound P to be released to the water column. The goal of this research is to determine whether iron minerals can permanently sequester P and if changes in iron mineralogy can cause P to be released from sediments in saline environments. This research will also provide one of the most detailed assessments of the fate of particulate P in an estuary.


Approximately fifteen one-meter long sediment cores will be collected on the Patuxent River using a piston-core sampler that uses a slight vacuum that causes the sediment being cored to enter and move up the tube without disturbing the sediment layers. Salinity-induced changes in P-binding mechanisms are expected to occur at fairly low salinity levels; therefore most sediment cores will be collected from the region of freshwater to saltwater transition. Five cores from the tidal freshwater area and five cores from the oligohaline portion (i.e., seasonal salinities of approximately 0.5 to 5 ppt) will be collected for investigation. We will also collect an additional five sediment cores along a gradient of increasing salinity downriver into the mesohaline portion of the Patuxent to investigate any further changes in P pools with increasing salinity. The cores from the Patuxent will be compared to sediment cores collected from other Chesapeake Bay sub-estuaries. The Gunpowder River, a sub-estuary located primarily in a Piedmont watershed, is an example of one such site that will be used to compare to the primarily coastal plain Patuxent River.

In preparation for the phosphorus extraction, the one-meter sediment cores will be segmented under a nitrogen glove-bag into approximately ten 1- to 2-centimeter sections taken from 10-centimeter intervals. The sediments will be freeze-dried, and stored in closed Nalgene bottles. The chemical forms of P will be analyzed using a sequential sedimentary extraction procedure that can quantify the pools of P and identify those fractions that are important for biogeochemically reactive P (Ruttenberg 1992). While this extraction procedure has been found to be reliable in extracting P-binding Fe-oxides (Jensen and Thamdrup 1993, Schlesinger 1997), it does not distinguish between iron minerals. Therefore, X-ray powder diffraction will be used to determine the mineralogy of the sediments. A powder microdiffraction system that utilizes Mo radiation (for Fe-rich minerals) will be used. Selected samples will be also examined using Transmission Electron Microscopy (TEM) to investigate the chemical composition and texture of iron minerals. TEM work will be done with the assistance of Dr. Mark Krekeler (GMU) at the University of Illinois’s Research Resources Center using a 300 kV JEM 3010 microscope.

Linear regression analyses will be made to determine:

  1. if freshwater sediments are better at sequestering phosphorus than saltwater;
  2. if P stays associated with iron with depth;
  3. if any reduction in particulate P with salinity is correlated to changes in iron minerals;
  4. and if Coastal Plain sediments appear to be better at sequestering phosphorus than Piedmont sediments.

Expected Results:

We expect that the pool of iron-bound phosphorus will decrease with salinity, but will still remain the dominant form of P. However, as conditions change with burial, phosphorus sinks may change over time. Further research is required to determine whether P stays associated with iron with depth. Fe-oxides can persist in some deep marine sediments enabling Fe-bound P to become an important long-term sink for reactive P. However, it is also possible that the Fe-oxides will become reduced with depth, and with reduction release the associated P. Simultaneous release of P and fluoride from Fe oxides in marine environments may provide the necessary conditions for early diagenetic carbonate fluorapatite precipitation. Therefore, it is possible that P sinks may switch from Fe oxides to apatite minerals with depth – especially in saltwater. However, in freshwater, permanent P sinks may be associated with different minerals, such as vivianite or P may be adsorbed to other Fe-rich minerals.

We also expect that any changes in Fe-bound P that occur with salinity changes are directly correlated with changes in Fe mineralogy and crystallinity. The mineralogy and crystallinity of Fe-oxides strongly influences phosphate sorption. For instance, amorphous Fe-oxides which have a higher surface area than crystalline Fe-oxides have been found to play a dominant role in binding P. Additionally, reductions in the quantity of sedimentary amorphous iron minerals have been documented with increasing salinity. Therefore, a reduction in poorly-crystalline iron phases, such as ferrihydrite, may control the observed reduction in Fe-bound P across the salinity gradient of the Patuxent River. Variation in crystalline iron oxides could also influence P binding in the Patuxent River sediments. For instance, goethite has been shown to have a higher P adsorption capacity than hematite.

Given the apparent mineralogical control over P sorption, P sinks may vary between the Coastal Plain and Piedmont geologic provinces. Previous research in the Chesapeake Bay region has indicated that Coastal Plain sediments tend to be much richer in phosphates than sediments originating from the Piedmont. This indicates that mineralogical control over P differs between the two physiographic provinces. The watershed of the Patuxent River is primarily located in the Coastal Plain physiographic province. It may be that the minerals binding phosphates are different in a Piedmont watershed.

Supplemental Keywords:

phosphorus, phosphate, iron, estuaries, salinity gradient, Chesapeake Bay, Patuxent River, Coastal Plain, Piedmont, ferrihydrite, iron oxides, x-ray diffraction, TEM, sediments,, RFA, Scientific Discipline, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Aquatic Ecosystem, Aquatic Ecosystems, Environmental Monitoring, Ecology and Ecosystems, estuaries, ecosystem monitoring, transpher electron microscopy, oxygen depletion, phytoplankton, biogeochemcial cycling, iron, aquatic sediments, phosphorus, ecosystem stress

Progress and Final Reports:

  • 2005
  • Final