title: Phosphate effects on cadmium(II) sorption to ferrihydrite.
abstract: OBJECTIVE
  Phosphate influences the sorption of metals to iron (hydr)oxides. An enhanced formation of inner-sphere complexes on the (hydr)oxide surface can be attributed to electrostatic interactions and/or to changes in metal coordination on the iron (hydr)oxide surface. Phosphate was expected to increase cadmium(II) sorption on ferrihydrite. It should be possible to identify changes in cadmium(II) coordination upon phosphate addition by Extended X-ray absorption fine structure (EXAFS) spectroscopy and implement the identified complexes in a surface complexation model (SCM).
  
  
  METHODS
  The effect of phosphate addition on cadmium(II) sorption to ferrihydrite was studied by a series of batch experiments covering the pH range from 4 to 8. EXAFS spectroscopy was performed on ferrihydrite from the batch experiments at the cadmium K edge. The identified surface complexes were incorporated in the Charge distribution multisite complexation (CD-MUSIC) model, and new surface complexation constants were optimized.
  
  
  RESULTS
  Without phosphate addition cadmium(II) formed inner-sphere bidentate complexes on the ferrihydrite surface. With phosphate there was an increased cadmium(II) sorption that could not be explained by electrostatic interactions alone. The enhancement was best explained by the formation of a ternary complex including cadmium(II), phosphate and ferrihydrite surface groups.
authors.affiliation: Department of Soil and Environment, Swedish University of Agricultural Sciences (SLU), Box 7014, SE-750 07 Uppsala, Sweden. Electronic address: charlotta.tiberg@slu.se.
authors.forename: Charlotta
authors.initials: C
authors.lastname: Tiberg
authors.affiliation: Department of Soil and Environment, Swedish University of Agricultural Sciences (SLU), Box 7014, SE-750 07 Uppsala, Sweden; Division of Land and Water Resources Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden. Electronic address: jon-petter.gustafsson@slu.se.
authors.forename: Jon Petter
authors.initials: JP
authors.lastname: Gustafsson
mesh_terms: D000327:Adsorption; D002104:Cadmium / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*
chemical_list: D005290:Ferric Compounds; D010710:Phosphates; D002104:Cadmium; C092844:ferric oxyhydroxide
keywords: Adsorption; Cadmium (Cd); Charge distribution multisite complexation (CD-MUSIC); Extended X-ray absorption spectroscopy (EXAFS); Iron hydroxide; Iron oxide; Phosphate; Surface complex