Humic Solubility and Aggregation

Solubility of the organics was measured by preparing a sample of varying organic tjyie, concentration, salt composition, and pH. Solutions were prepared in 20 mL sample vials, rapidly shaken after addition of the various stock solutions, pH adjusted and then the UV scans to measure the aggregation of the organics were carried out after 1, 5 and 24 h. 

Aggregate sizes were measured using PCS (see section 4.9.5), strucmre using the humic fractal method (described. below), and 15 mL of the samples were then centrifuged to measure the solid content. Organic concentrations of 25-75 mgL as DOC, pH 3.5 to 10, calcium chloride concentrations of 0-25 mM were tested. 

Precipitation occurred at the pH extremes at which investigations were undertaken, i.e. at pH 3.5 and pH 10. The nature of the solids formed at these two pH extremes was very different. At low pH, only HA (not FA and NOM) precipitated. Calcium had no effect on this precipitation, which was observed even in the absence of calcium. This effect could be explained by the low solubility of HA, compared to FA at low pH. 

A variation of structure was observed for HA at low pH, indicating that a regation/precipitation is occurring over several hours after mixing. In the presence of large amounts of NaCl (75 mM), foam formation was observed. The aggregates formed were very large (see section 4.9.5). 

Both HA and FA were removed from solution in the presence of 25 mM calcium chloride at pH 10. At this pH, calcite precipitates and the interactions between organics and calcium are strongest. In the absence of calcium no precipitation occurred at pH 10. This may indicate the precipitation of a calcium-organic compound, co-precipation of organics with calcite or, simply adsorption of organics onto calcite surface sites. 

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