Uptake of Fe(II) and Mn(II) on chitin as a model organic phase

The sorption of Fe(II) and Mn(II) onto chitin has been measured in seawater as a function of pH (3-9), temperature (5-45°C), and salinity (0-35). The two metals showed markedly different pH effects. Uptake of Fe(II) was greatest at high pH, whereas Mn(II) showed negligible uptake above pH 6.3-7.3 and precipitated at high pH. The decrease in Mn(II) uptake above pH 6.3-7.3 could be related to the second acid dissociation site of chitin, which has p^*K_a2 = 6.4. Fe(II) follows a Langmuir isotherm at pH 8.2 at intermediate concentrations, as did Mn(II) below pH 6.3. Uptake of both metals increased with temperature from 5 to 45°C, yielding a calculated sorption enthalpy of 14.2 ± 0.6 kJ mol^-1 g^-1 chitin for Fe(II) and 11 ± 3 kJ mol^-1 g^-1 for Mn(II) at pH 4. Increasing salinity from zero to 35 increased the uptake of Fe(II) by 50%; at pH 4 the Mn(II) uptake also increased substantially. Calculations indicate that uptake onto chitin-like material cannot account for the partitioning between dissolved and particulate forms of either Mn(II) or Fe(II) in two typical anoxic systems (the Black Sea and Framvaren fjord).