The solubility of iron hydroxide in sodium chloride solutions

The solubility of iron(III) hydroxide as a function of pH was investigated in NaCl solutions at different temperatures (5–50°C) and ionic strengths (0–5 M). Our results at 25°C and 0.7 M in the acidic range are similar to the solubility in seawater. The results between 7.5 to 9 are constant (close to 10−11 M) and are lower than those found in seawater (>10−10) in this pH range. The solubility subsequently increases as the pH increases from 9 to 12. The solubility between 6 and 7.5 has a change of slope that cannot be accounted for by changes in the speciation of Fe(III). This effect has been attributed to a solid-state transformation of Fe(OH)3 to FeOOH. The effect of ionic strength from 0.1 to 5 M at a pH near 8 was quite small. The solubility at 5°C is considerably higher than at 25°C at neutral pH range. The effects of temperature and ionic strength on the solubility at low and high pH have been attributed to the effects on the solubility product and the formation of FeOH2+ and Fe(OH)4−. The results have been used to determine the solubility products of Fe(OH)3, K∗Fe(OH)3 and hydrolysis constants, β∗1, β∗2, β∗3, and β∗4 as a function of temperature (T, K) and ionic strength (I): log K∗Fe(OH)3 = −13.486 − 0.1856 I0.5 + 0.3073 I + 5254/T (σ = 0.08) log β∗1 = 2.517 − 0.8885 I0.5 + 0.2139 I − 1320/T (σ = 0.03) log β∗2 = 0.4511 − 0.3305 I0.5 − 1996/T (σ = 0.1) log β∗3 = −0.2965 − 0.7881 I0.5 − 4086/T (σ = 0.6) log β∗4 = 4.4466 − 0.8505 I0.5 − 7980/T. (σ = 0.2) Both strong ethylenediaminetetraacetic acid and weak (HA) organic ligands greatly affect iron solubility. The additions of ethylenediaminetetraacetic acid and humic material were shown to increase the solubility near pH 8. The higher solubility of Fe(III) in seawater compared to 0.7 M NaCl may be caused by natural organic ligands.