THE EFFECTS OF SOLUTION CONDITIONS ON PARTIAL SPECIFIC VOLUMES OF HUMIC SUBSTANCES

The volumes of humic substances isolated from surface water and peat s oil and characterised in terms of partial specific volume (<(upsilon)o ver bar>) measurements by digital densimetry, have been obtained over a range of solution conditions, Specifically, the effects of ionic str ength, pH, temperature and several metal ion concentrations for both a quatic fulvic, humic acids and two peat humic acids have been investig ated. The volumes have been found to contract at high salt concentrati ons and low pH for the aquatic materials due to a decrease in electros tatic charge repulsions. The volumes of pear humics are only slightly affected by pH changes. The volumes of all humics go through maxima in the temperature range 20-30 degrees C possibly as a consequence of ch anges in hydrophobic interactions. Addition of cadmium ions leads to m olecular contractions due to ion binding and charge neutralisation whe reas both copper and lanthanum ions which more easily flocculate humic s, result in expansion of the molecules and give rise to increases in partial specific volume. This effect is attributed to the entrapment o f water in the Aocs. The results show that the volumes of humic substa nces in the aquatic environment and in peat soils are sensitive to med ia conditions. The results have implications in the characterisation o f these materials by techniques such as low angle X-ray scattering and analytical ultracentrifugation where a knowledge of partial specific volumes and their dependence on solution conditions are required. (C) 1998 Elsevier Science B.V.