Pre-existing apatite in estuarine sediments rapidly adsorbs phosphate from mining wastewater and forms mineral phases. This natural process limits pollutant mobility, reducing long-term environmental impact after large-scale discharge events.
Study: Rapid removal of mining waste-contributed phosphate from estuarine waters by pre-existing apatite in north Manatee County, Florida. Image Credit: Michalakis Ppalis/Shutterstock
A paper recently published in Communications Sustainability reported rapid mining waste-contributed phosphate removal from estuarine waters by pre-existing apatite in Florida.
Phosphate Pollution and Environmental Risk
Over the past 130 years, large-scale phosphate mining has occurred in central Florida. The phosphate fertilizer production generates calcium sulfate in large amounts as waste material (phosphogypsum), which is mostly stored in gypstacks. The Piney Point gypstack, located near Tampa Bay in northern Manatee County, Florida, is one of these sites where waste stacking ceased in the late 1990s.
Several lined ponds were built during the Piney Point gypstack closure to store water drained from gypstack portions and also the process water. As the retained water in the gypstack contained seawater, rainwater, and process water, it is referred to as stack water. A leak was reported in a lined compartment in March 2021 at the gypstack site.
Eventually, an emergency gravity siphon discharge was commenced from the site into Tampa Bay through Port Manatee to reduce pressure on retention walls. Almost 800 million liters of industrial phosphate wastewater, rainwater, and seawater were discharged over 10 days into Tampa Bay, a central Florida estuary. The long-term effects of discharging such a large phosphate quantity into an estuary remain unknown.
In aquatic systems, phosphate pollution can deplete oxygen and drive toxic algal blooms, adversely affecting native aquatic plant life. However, phosphate can precipitate as phosphate minerals when it gets mixed with ocean water. In ocean water, phosphate precipitation is a multi-stage process that eventually leads to apatite formation.
The Research Effort
In this work, researchers tracked the mobility of phosphate contributed by the Piney Point stack water release from July 2021 to May 2024 by sampling estuarine sediments along the shoreline of eastern Tampa Bay, both south and north of the discharge site.
The objective of the study was to investigate the impact of a rapid, large phosphate addition to the environment. In particular, researchers sought to determine the amount of phosphate remaining in the water after discharge and the movement of phosphate following the stack water release.
12 sampling sites were chosen for this work within Tampa Bay, with three sites away from the stack water plume point source acting as control sites. Background sediment total-phosphorus levels were established using control sites. In all sampling sites, water was sufficiently shallow for retrieving samples from the boat, and no vessel grounding required.
The sampling sites followed a general north-to-south transect along the coast, adjacent to where the stack water entered the Tampa Bay. At the first sampling point, researchers recorded the coordinates. In the ensuing samplings, they sampled within 15 m of the first sampling site to account for tide-level variations. Hence, sampling locations were 15 m radius zones centered on each coordinate set.
Researchers recorded water temperature, pH, and salinity at each site, and then collected sediment and water samples from the bay floor’s top 2 cm. A stainless-steel hand trowel attached to a 1.2 m polyvinyl chloride pole end was used to retrieve the samples. They performed sampling every month during their study period (May 2021 to May 2024).
However, the water sample phosphorus content was not analyzed after 2022. All samples were stored at 4°C in a lab refrigerator. Subsequently, researchers performed total-phosphorus extraction and analysis, mineralogical analysis of sediments using X-ray diffraction, and grain size distribution analysis.
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Key Findings of this Research
Results demonstrated that dissolved phosphorus from the Piney Point stack water release was rapidly removed from the water, primarily through interaction with sediment apatite. The fastest removal occurred within the first hour, indicating that pre-existing apatite in sediments near the release point played a key role in removing phosphate from the plume.
Although distinguishing between adsorption and precipitation was challenging, phosphorus adsorption onto reactive sediment grains likely occurred first, followed by crystallization into calcium phosphate minerals.
Sediment mineral fractions across all sites were dominated by calcite, quartz, and apatite, with higher apatite abundance observed at sites 2 and southward. Extractable phosphate concentrations varied with distance and plume flow direction, indicating immobility of phosphate in the environment.
Laboratory simulations using seawater and wastewater analogs, and native sediments and their mineral components, confirmed that natural apatite efficiently captured phosphate on its surface.
Overall, sediments, particularly natural apatite grains, within a 5 km radius of the release site rapidly attenuated phosphate and retained it for at least three years, significantly reducing the potential environmental impact of the discharge.
In conclusion, the findings of this study demonstrated the potential of using apatite to promote phosphate removal.
Journal Reference
Major, J. D., Feng, T., & Pasek, M. A. (2026). Rapid removal of mining waste-contributed phosphate from estuarine waters by pre-existing apatite in north Manatee County, Florida. Communications Sustainability, 1(1), 61. DOI: 10.1038/s44458-026-00060-8, https://www.nature.com/articles/s44458-026-00060-8
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