The Adsorption and Precipitation of Phosphate Onto Calcite

Abstract

WITH TWO PLATES SummaryScanning electron micrographs show that the reaction products of solution phosphate and calcite are hemispherical, coral-like growths on calcite surfaces. Electron probe micro-analysis indicates that these are a calcium phosphate, and solution data plotted against solubility isotherms suggest that dicalcium phosphate (DCP) is formed rapidly and slowly changes to octa-calcium phosphate (OCP). X-ray diffraction shows that DCP and OCP are present with DCP predominating. The ratio of Ca:P in the treated calcite, after allowing for the CaC03 present, is that in DCP. and a surface coating of OCP on DCP is likely. The exchangeability of the reacted P falls from 100 per cent for small amounts (0-10 pg P per g calcite) to a constant 30 per cent when larger amounts are present (200-1000 p d g ) resulting from the porous structure of the coral-like growths. These cover only a small fraction of the calcite surface even when large amounts of P are present so allowing calcite to control solution pH. IntroductionTHE reaction of phosphate with calcite surfaces involves adsorption of small amounts of phosphate followed by precipitation of calcium phosphate at higher levels (Cole et al., 1953). The adsorption has been described by the Langmuir isotherm (Kuo and Lotse, 1972), with better fit when the isotherm is divided into two linear sections (Griffin and Jurinak, 1973) and has been described as a heterogeneous nucleation process leading to about 5 per cent of the calcite surface being covered with phosphate.The initial adsorption is thought to occur at certain sites where lateral interaction between phosphate ions produces surface clusters which then act as nuclei for subsequent crystal growth. These nuclei are presumably an amorphous calcium phosphate (Stumm and Leckie, 1970) which is slowly transformed into crystalline calcium phosphate, the form of which seems to depend on the adsorbing surface and on the solution conditions. For example Cole et al. (1 953) found evidence that dicalcium phosphate was formed, Clark and Peech (1955) concluded that octacalcium phosphate or dicalcium phosphate were formed depending on the conditions, Stumm and Leckie (1 970) showed that apatite formed in calcareous sediments, and Holford and Mattingly (1975) found octacalcium phosphate in soils on Oolitic Limestone. Time of reaction is also likely to be important, for Arvieu and Bouvier (1974) state that dicalcium phosphate is the first reaction product which by hydrolysis gives octacalcium phosphate, and

Forest Resource Islands in a Sub-tropical Marsh: Soil–Site Relationships in Everglades Hardwood Hammocks

“…Working with soils from the agricultural areas immediately east of the Everglades, Zhou and Li (2001) also found phosphorus-sorption to be strongly correlated with noncarbonate clays at low P-concentrations, though carbonates became important in binding P at high concentrations. Several studies have shown that amorphous and crystalline calcium phosphate compounds can form secondarily in association with calcite surfaces (Freeman and Rowell 1981;von Wandruszka 2006). However, if P-sorption in the hammocks was primarily through reaction with calcite, a positive association between P and IC likely would have resulted.…”

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

“…c Assuming a P:Fe ratio of 0.5 (Gunnars et al, 2002) and 10% of Fe(OH) 3 is available for fast sorption (Slomp et al, 1996); d Assuming a P:Ca ratio of 0.6 (Freeman and Rowell, 1981) and 5% of CaCO 3 is available for fast sorption (Cole et al, 1953). solids are denoted in mM (mmol dm − 3 pore volume ) and mmol dm − 3 (mmol dm − 3 total volume ), respectively. The reaction network includes aerobic dissolved organic carbon (DOC) degradation, denitrification, secondary reoxidation reactions, precipitation/dissolution of calcium carbonate and P minerals, acid-base equilibria and reversible P and NH 4 + sorption.…”

“…In the Fe oxide-rich aquifer, a maximum P:Fe molar ratio of 0.5 (Gunnars et al, 2002) is assumed, while for the calcareous aquifer the maximum P:Ca molar ratio is set to 0.6 (Table 1), the ratio for hydroxyapatite (Freeman and Rowell, 1981). Fast equilibrium sorption of P is described by a Langmuir isotherm: Table 2 Model parameters used to simulate N and P dynamics at Cambridge and Muskoka Symbol ( -not applicable; ⁎ additionally constrained through sensitivity analysis, which shows no significant further attenuation even at higher values of longitudinal dispersivities e.g.…”

“…During the whole time period, the groundwater in the plume remains supersaturated with the maximum SI indices for hydroxyapatite ranging from +6.5 after the first year of decommissioning to + 5.0 after 50 years. Although the relatively slow rate of P precipitation (Freeman and Rowell, 1981;Robertson et al, 1998) limits the aquifer's attenuation capacity, it can still be considered as an effective trapping mechanism for P on the long term.…”

Removal of orthophosphates in water by modified carbonate material of biological origin

“…However, a general consensus has affirmed that phosphate can be either adsorbed by calcium carbonate at low concentration or precipitated at high concentration [25][26][27] although it is not always easy to distinguish between these two mechanisms [29]. A previous work [30] suggested that the initial uptake of phosphate onto calcium carbonate occurs via chemisorption, which is then followed by a slow transformation of amorphous calcium phosphate to crystalline apatite [29].…”