Why is Calcite a Strong Phosphorus Sink in Freshwater? Investigating the Adsorption Mechanism Using Batch Experiments and Surface Complexation Modeling

Hilary Flower, Mark Rains, Yasemin Taşcı, Jia-Zhong Zhang, Kenneth Trout, David Lewis, Arundhati Das, Robert Dalton

Research output: Contribution to journalArticlepeer-review

Abstract

<p> <p id="x-x-abspara0010"> One of the primary drivers of Phosphorus (P) limitation in aquatic systems is P adsorption to sediments. Sediments adsorb more P in freshwater compared to other natural solutions, but the mechanism driving this difference is poorly understood. To provide insights into the mechanism, we conducted batch experiments of P adsorption to <a title="Learn more about calcite from ScienceDirect's AI-generated Topic Pages"> calcite </a> in freshwater and seawater, and used computer software to develop complexation models. Our simulations revealed three main reasons that, combining together, may explain the greater P adsorption to calcite in freshwater vs. seawater. First, aqueous speciation of P makes a difference. The ion pair CaPO <sub> 4 </sub> <sup> &minus; </sup> is much more abundant in freshwater; although seawater has more Ca <sup> 2+ </sup> ions, MgHPO <sub> 4 </sub> <sup> 0 </sup> and NaHPO <sub> 4 </sub> <sup> 0 </sup> are more thermodynamically favored. Second, the adsorbing species of P make a difference. The ion pair CaPO <sub> 4 </sub> <sup> &minus; </sup> (the preferred adsorbate in freshwater) is able to access adsorption sites that are not available to HPO <sub> 4 </sub> <sup> 2&minus; </sup> (the preferred adsorbate in seawater), thereby raising the maximum concentration of P that can adsorb to the calcite surface in freshwater. Third, water chemistry affects the competition among ions for surface sites. Other ions (including P) compete more effectively against CO <sub> 3 </sub> <sup> 2&minus; </sup> when immersed in freshwater vs. seawater, even when the concentration of HCO <sub> 3 </sub> <sup> &minus; </sup> /CO <sub> 3 </sub> <sup> 2&minus; </sup> is higher in freshwater vs. seawater. In addition, we found that under oligotrophic conditions, P adsorption is driven by the higher energy adsorption sites, and by the lower energy sites in eutrophic conditions. This study is the first to model P adsorption mechanisms to calcite in freshwater and seawater. </p></p>
Original languageAmerican English
JournalChemosphere
Volume286
DOIs
StatePublished - Jan 1 2022

Keywords

  • Calcium carbonate
  • Phosphates
  • Abiotic factors
  • Sediment dynamics
  • Nutrient

Disciplines

  • Earth Sciences

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