The effects of solution conditions on partial specific volumes of humic substances

The volumes of humic substances isolated from surface water and peat soil and characterised in terms of partial specific volume (ῡ2) measurements by digital densimetry, have been obtained over a range of solution conditions. Specifically, the effects of ionic strength, pH, temperature and several metal ion concentrations for both aquatic fulvic, humic acids and two peat humic acids have been investigated. The volumes have been found to contract at high salt concentrations and low pH for the aquatic materials due to a decrease in electrostatic charge repulsions. The volumes of peat humics are only slightly affected by pH changes. The volumes of all humics go through maxima in the temperature range 20–30°C possibly as a consequence of changes in hydrophobic interactions. Addition of cadmium ions leads to molecular contractions due to ion binding and charge neutralisation whereas both copper and lanthanum ions which more easily flocculate humics, result in expansion of the molecules and give rise to increases in partial specific volume. This effect is attributed to the entrapment of water in the flocs. The results show that the volumes of humic substances in the aquatic environment and in peat soils are sensitive to media conditions. The results have implications in the characterisation of 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.