The solubility of iron in seawater

The solubilities of iron(III) hydroxides in seawater were determined in Gulf Stream seawater as a function of pH (2 to 9), temperature (5 to 50 °C) and salinity (0 to 36). Our results at S=36 and 25 °C near a pH of 8 are in agreement with the measurements of Byrne and Kester [Mar. Chem. 4 (1976a) 255] and Kuma et al. [Limnol. Oceanogr. 41 (1996) 396] (0.2 to 0.3 nM). The solubilities at 5 °C are considerably higher than at 25 °C and decrease with a decrease in salinity. Near a pH of 8, the solubilities as a function of temperature (T/K) and ionic strength [I=19.922S/(1000−1.005S)] can be estimated from log[Fe(III)]=−10.53+322.5/T−2.524I0.5+2.921I (σ=0.2)The results at low pH (2 to 5) and S=36 have been used to determine the limiting solubilities of Fe(OH)3(s) which have been fitted to the equation (σ=0.08) logKFe(OH)3*=−9.17+3993/TThe values of KFe(OH)3* are in good agreement with those determined in 0.7 m NaCl. The solubilities as a function of pH, temperature and salinity have been used to determine the stability constants for the formation of Fe(OH)2+, Fe(OH)2+ and Fe(OH)3. The hydrolysis constants at I=0.74 (S=36) have been fitted to the equations logβ1*=1.86−1300/T (σ=0.1) logβ2*=0.18−1997/T (σ=0.2) logβ3*=−2.47−3185/T (σ=0.8)The results for log β1* and log β2* at 25 °C are in reasonable agreement with the values in 0.7 m NaCl. The value of log β3* in seawater is larger than the value in 0.7 m NaCl due to the formation of Fe(III) complexes with organic matter. The higher solubilities in seawater (0.3–0.5 nM) compared to the values in 0.7 m NaCl (0.011 nM) are due to the formation of Fe3+ complexes (FeL) with natural organic ligands (L). By diluting seawater (S=36) with 0.7 m NaCl, we have been able to show that the solubilities approach the values in NaCl at the same ionic strength and temperature. The differences in the solubility of Fe in seawater and 0.7 m NaCl near a pH have been used to calculate ligand concentrations [L′]=0.1–0.2 nM (not complexed with iron) using a literature value of the stability constant (K′FeL=1011.5). The results at different temperatures have been used to determine a value of ΔH=7±5 kcal mol−1 for the formation of FeL.