The adsorption and desorption of phosphate on calcite and aragonite were investigated as a function of temperature (5–45 °C)and salinity (0–40) in seawater pre-equilibrated with CaCO
3. An increase in temperature increased the equilibrium adsorption; whereas an increase in salinity decreased the adsorption. Adsorption measurements made in NaCl were lower than the results in seawater. The higher values in seawater were due to the presence of Mg
2+ and Ca
2+ ions. The increase was 5 times greater for Ca
2+ than Mg
2+. The effects ofCa
2+ and Mg
2+ are diminished with the addition of SO
4
2- apparently due to the formation of MgSO
4 and CaSO
4 complexes in solution and/or SO
4
2- adsorption on the surface of CaCO
3. The adsorbed Ca
2+ and Mg
2+ on CaCO
3 (at carbonate sites) may act as bridges to PO
4
3- ions. The bridging effect of Ca
2+is greater than Mg
2+ apparently due to the stronger interactions of Ca
2+ with PO
4
3-.The apparent effect of salinity on the adsorption of PO
4 was largely due to changes in the concentration of HCO
3
- in the solutions. An increase in the concentration of HCO
3
- caused the adsorption of phosphate to decrease, especially at low salinities. The adsorption at the same level of HCO
3
- (2 mM) was nearly independent of salinity. All of the adsorption measurements were modeled empirically using a Langmuir-type adsorption isotherm[ [PO
4]
ad = K
mC
m[PO
4]
T/(1 +K
m [PO
4]
T) , ]where [PO
4]
ad and [PO
4]
T are the adsorbed and total dissolved phosphate concentrations, respectively. The values of C
m (the maximum monolayer adsorption capacity, (mol/g) and K
m (the adsorption equilibrium constant, g/(mol) over the entire temperature (t, °C) and salinity (S) range were fitted to[ C
m = 17.067 + 0.1707t - 0.4693S + 0.0082S
2 ( = 0.7) ][ ln K
m = - 2.412 + 0.0165t - 0.0004St - 0.0008S
2 ( = 0.1) ]These empirical equations reproduce all of our measurements of[PO
4]
ad up to 14 mol/g and within ±0.7 mol/g.The kinetic data showed that the phosphate uptake on carbonate minerals appears to be a multi-step process. Both the adsorption and desorption were quite fast in the first stage (less than 30 min) followed by a much slower process (lasting more than 1 week). Our results indicate that within 24 hours aragonite has a higher sorption capacity than calcite. The differences between calcite and aragonite become smaller with time. Consequently, the mineral composition of the sediments may affect the short-term phosphate adsorption and desorption on calcium carbonate. Up to 80 % of the adsorbed phosphate is released from calcium carbonate over one day. The amount of PO
4 left on the CaCO
3 is close to the equilibrium adsorption. The release of PO
4 from calcite is faster than from aragonite. Measurements with Florida Bay sediments produced results between those for calcite and aragonite. Our results indicate that the calcium carbonate can be both a sink and source of phosphate in natural waters.
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