Other
Scientific paper
Feb 1973
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1973gecoa..37..269b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 37, Issue 2, pp.269-281
Other
6
Scientific paper
Humic acid extracted from a podzolic soil developed under Eucalyptus delegatensis and Pteridium aquilinum in northwestern Tasmania exhibits very strong solvent activity towards a number of minerals and metals. Aqueous solutions (0.1 per cent w/v) of this acid acting for 24 hours on mineral grains ranging in size from 297 to 590 , extracted varying amounts of metal. Chalcopyrite yielded 140 g Cu whereas chalcocite released 15,000 g Cu. Some correlation is found between relative bond strengths of sulphides and their degradation by humic acids. For example, galena is far less stable than sphalerite. Haematite and pyrolusite are quite vulnerable to humic acid attack and this may be a factor in the lack of development of extensive gossans over mineralization in western Tasmania during the current erosional cycle. Metals are particularly strongly attacked with a maximum release of 291,000 g Pb in 24 hours. Contrary to earlier findings, Ag and Au were found to release 400 g and 20 g respectively of metal in a period of 6 weeks. Humic acid extracted from soils below other vegetation types in northwestern and western Tasmania are all active in mineral degradation. The variable effect of the acids is possibly the result of overall differences in complexing sites active under the conditions of experimentation and selective complexation. Several examples of minor soil organic compounds show no greater activity than humic acids and on the basis of their very low content in the soils studied, they are considered inferior to the latter as agents of weathering. Many metal humates display low solubility in water, but they are readily mobilized in the presence of humic acids. Humic acids developed under varying vegetation in a cool temperate climate are potentially very powerful solvents in the weathering cycle. Their ability to mobilize precipitated metal humates suggests that classical concepts of relative metal mobility may need modification in environments where appreciable concentrations of these substances are found.
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