engleski

Interactions of phosphate and phosphonate with calcite surface

Crytsal growth mechanisms of calcite have been studied in detail in the past. The ability of calcite to adsorb P as phosphate from raw, sea and artificial water has been investigated intensively to recover P as a valuable nutrient. Moreover, a detailed understanding of the interaction between phosphonates and calcite is of great interest for large–scale application as scale inhibitor, dispersant or chelating agent. To date, understanding atomic–level processes that govern the kinetics of calcite growth with respect to phosphate and phosphonate anions is still incomplete. As part of the ReCaWa joint project (Reactivity of Calcite/Water Interfaces), we investigate the sorption and coprecipitation processes of phosphate and phosphonate on calcite surfaces from (super)saturated calcite solutions. Solutions were prepared from p. a. grade chemicals (CaCl2 , NaHCO3 ) in double distilled water (18.2 MM·cm), with pH adjusted to 8.2 and I=0.1 mol/L. Phosphate (H3 PO4 , Na3 PO4·6H2 O) or phosphonate (etidronic acid, HEDP) were added in concentrations from 3–1000 µmol/L and 10– 25 µmol/L, respectively. For batch experiments differing amounts precipitated calcium carbonate or limestone powder with varying specific surface area (SSA) and crystal morphology were added to the solutions. Additional and upcoming coprecipitation experiments under strongly defined conditions are performed using a mixed flow reactor (MFR) to allow coprecipitation of homogeneous phosphate or phosphonate containing calcite onto the calcite seed powder. Experimental results show that phosphate uptake is a function of SSA and formation of HEDP precipitates dependent on the calcite morphology, respectively. Forthcoming analytical work includes the identification of different fixation mechanisms and phosphorus– containing phases using XRD and XAFS spectroscopy at grazing incidence.

Calcite; crystal growth; dissolution; Phosphate; Phosphonate

Coautor list was extended after the abstracts were printed.