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Abstract：
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 calcite 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 CaPO4- is much more abundant in freshwater; although seawater has more Ca2+ ions, MgHPO40 and NaHPO40 are more thermodynamically favored. Second, the adsorbing species of P make a difference. The ion pair CaPO4- (the preferred adsorbate in freshwater) is able to access adsorption sites that are not available to HPO42- (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 CO32- when immersed in freshwater vs. seawater, even when the concentration of HCO3-/CO32- 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.
摘要：
水生系统中磷（P）限制的主要驱动因素之一是P对沉积物的吸附。与其他天然溶液相比，沉积物在淡水中吸附更多的P，但是驱动这种差异的机制知之甚少。为了提供对机制的见解，我们在淡水和海水中进行了方解石吸附磷的批量实验，并使用计算机软件来开发复杂模型。我们的模拟揭示了三个主要原因，结合在一起，可以解释淡水中方解石对磷的吸附海水。首先,P的水形态有所不同。离子对CaPO4-在淡水中更丰富；尽管海水中含有更多的Ca2离子，MgHPO40和NaHPO40在热力学上更有利。第二,P的吸附种类有所不同。离子对CaPO4-（淡水中优选的吸附质）能够进入HPO42-（海水中优选的吸附质）不可用的吸附位点，从而提高了淡水中可以吸附到方解石表面的P的最大浓度。第三，水化学影响离子之间对表面位点的竞争。其他离子（包括P）在浸入淡水时与CO32-的竞争更有效海水,即使淡水中HCO3-/CO32-的浓度较高，海水。此外,我们发现在贫营养条件下，P吸附是由较高能量的吸附位点驱动的，以及富营养化条件下能量较低的地方。本研究首次模拟了淡水和海水中方解石对磷的吸附机制。
