Why is calcite a strong phosphorus sink in freshwater? Investigating the adsorption mechanism using batch experiments and surface complexation modeling.

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.

Phosphorus sorption on marine carbonate sediment: phosphonate as model organic compounds.

Organophosphonate, characterized by the presence of a stable, covalent, carbon to phosphorus (C-P) bond, is a group of synthetic or biogenic organophosphorus compounds. The fate of these organic phosphorus compounds in the environment is not well studied. This study presents the first investigation on the sorption of phosphorus (P) in the presence of two model phosphonate compounds, 2-aminothylphosphonoic acid (2-AEP) and phosphonoformic acid (PFA), on marine carbonate sediments. In contrast to other organic P compounds, no significant inorganic phosphate exchange was observed in seawater. P was found to adsorb on the sediment only in the presence of PFA, not 2-AEP. This indicated that sorption of P from phosphonate on marine sediment was compound specific. Compared with inorganic phosphate sorption on the same sediments, P sorption from organic phosphorus is much less in the marine environment. Further study is needed to understand the potential role of the organophosphonate compounds in biogeochemical cycle of phosphorus in the environment.

Neutral persulfate digestion at sub-boiling temperature in an oven for total dissolved phosphorus determination in natural waters.

A simplified, easily performed persulfate digestion method has been developed to process a large number of water samples for routine determination of total dissolved phosphorus. A neutral potassium persulfate solution (5%, w/v, pH approximately 6.5) is added to the samples (at 10 mg potassium persulfate per mL of sample), which are then digested at 90 degrees C in an oven for 16 h. This method does not require pH adjustment after digestion because neither an acid nor a base is added to the samples prior to digestion. The full color of phosphoantimonylmolybdenum blue from the digested samples develops within 8 min. Compared with the autoclave method, digestion at sub-boiling temperatures in an oven is safer, and a large number of samples can be heated overnight requiring no constant monitoring. The apparent molar absorptivity (epsilon) of nine organic phosphorus compounds and two condensed inorganic phosphates ranged from 1.17 x 10(4) to 1.82 x 10(4)L mol(-1)cm(-1) in both distilled water and artificial seawater matrixes. The average recovery of these phosphorus compounds was 94+/-11% for the DIW matrix and 90+/-12% for the ASW matrix. No significant difference in molar absorptivity was observed between the undigested and digested phosphate, especially in the seawater matrix. It is, therefore, suggested that a phosphate solution be directly employed without digestion as the calibration standard for routine determination of total dissolved phosphorus. This method was used to study the spatial distribution of total dissolved phosphorus in the surface waters of Florida Bay.

Rate of phosphoantimonylmolybdenum blue complex formation in acidic persulfate digested sample matrix for total dissolved phosphorus determination: importance of post-digestion pH adjustment.

Acidic persulfate oxidation is one of the most common procedures used to digest dissolved organic phosphorus compounds in water samples for total dissolved phosphorus determination. It has been reported that the rates of phosphoantimonylmolybdenum blue complex formation were significantly reduced in the digested sample matrix. This study revealed that the intermediate products of persulfate oxidation, not the slight change in pH, cause the slowdown of color formation. This effect can be remedied by adjusting digested samples pH to a near neural to decompose the intermediate products. No disturbing effects of chlorine on the phosphoantimonylmolybdenum blue formation in seawater were observed. It is noted that the modification of mixed reagent recipe cannot provide near neutral pH for the decomposition of the intermediate products of persulfate oxidation. This study provides experimental evidence not only to support the recommendation made in APHA standard methods that the pH of the digested sample must be adjusted to within a narrow range of sample, but also to improve the understanding of role of residue from persulfate decomposition on the subsequent phosphoantimonylmolybdenum blue formation.

Relative importance of solid-phase phosphorus and iron on the sorption behavior of sediments.

Of all the metal oxide particles, amorphous iron oxides have the greatest adsorption capacity for phosphate. Coastal sediments are often coated with terrigenous amorphous iron oxides, and those containing high iron are thought to have a high adsorption capacity. However, this conventional wisdom is based largely upon studies of phosphate adsorption on laboratory-synthesized minerals themselves containing no phosphorus. Using natural sediments that contain variable phosphorus and iron, our results demonstrate thatthe exchangeable phosphate rather than the iron oxides of sediments governs the overall sorption behavior. The iron oxide content becomes important only in sediments that are poor in phosphorus. A total of 40 sampling sites across the Florida Bay provide detailed spatial distributions both of the sediment's zero equilibrium phosphate concentration (EPC0) and of the distribution coefficient (Kd) that are consistent with the distribution of the exchangeable phosphate content of the sediment. This study provides the first quantitative relationships between sorption characteristics (EPC0 and Kd) and the exchangeable phosphate content of natural sediments.