The speciation of Fe(II) and Fe(III) in natural waters

Abstract

The interactions of Fe(II) and Fe(III) with the inorganic anions of natural waters have been examined using the specific interaction and ion pairing models. The specific interaction model as formulated by Pitzer is used to examine the interactions of the major components (Na + , Mg 2+ , Ca 2+ , K + , Sr 2+ , Cl - , SO 4 2- , HCO 3 - , Br - , CO 3 2- , B(OH) 4 - , B(OH) 3 and CO 2 ) of seawater and the ion pairing model is used to account for the strong interaction of Fe(II) and Fe(III) with major and minor ligands (Cl - , SO 4 2- , OH - , HCO 3 - , CO 3 2- and HS - ) in the watefs. The model can be used to estimate the activity and speciation of iron in natural waters as a function of composition (major sea salts) and ionic strength (0 to 3 M). The measured stability constants (K * FeX ) of Fe(II) and Fe(III) have been used to estimate the thermodynamic constants (K FeX ) and the activity coefficient of iron complexes (γ FeX ) with a number of inorganic ligands in NaClO 4 medium at various ionic strengths : ln(K * FeX /γ Fe γ X ) = ln K FeX - ln(γ FeX ) The activity coefficients for free ions (γ Fe , γ X ) needed for this extrapolation have been estimated from the Pitzer equations. The activity coefficients of the ion pairs have been used to determine Pitzer parameters (B FeX , B 0 FeX , C Φ FeX ) for the iron complexes. These results make it possible to estimate the stability constants for the formation of Fe(II) and Fe(III) complexes over a wide range of ionic strengths and in different media. The model has been used to determine the solubility of Fe(III) in seawater as a function of pH. The results are in good agreement with the measurements of Byme and Kester and Kuma et al. When the formation of Fe organic complexes is considered, the solubility of Fe(III) in seawater is increased by about 25%.