Adsorption of inorganic phosphates and organic polyphosphonate on calcite

Abstract

Adsorption of two inorganic phosphates (orthophosphate (P), tripolyphosphate (TPP)) and one organic polyphosphonate (EDTPH = ethylenediamine–N,N,N′,N′-tetrakis(methylenephosphonic acid)) on the surface of well-developed and crushed calcite crystals has been investigated using radioactive isotopes as tracers at 25.0 °C, where inorganic phosphates (P, TPP) and EDTPH were labeled with ³²P and ¹⁴C isotopes, respectively. The adsorbed amount of P and TPP systems on the calcite surface increases with an increase in their concentration in solution, whereas that of EDTPH was saturated at low concentration. The adsorbed amounts of P and TPP on the calcite surface also increase with an increase in Ca²⁺ activity in solution. This fact suggests that these phosphates are adsorbed in conjunction with the calcium ion. The adsorption of EDTPH, however, does not depend on the Ca²⁺ activity in solution. The adsorption of P and EDTPH were explained by Freundlich and Langmuir adsorption isotherms, respectively. On the other hand, TPP showed Freundlich adsorption isotherms on aged calcite but Langmuir adsorption isotherms on the crushed calcite. The adsorption of P, TPP and EDTPH on rough surfaces of calcite created by crushing the crystals, is much higher than on well-developed surfaces.