Cation exchange properties of humic acids and their importance in the geochemical enrichment of UO 2++ and other cations

Laboratory studies led us to conclude in 1951 that the insoluble humic acid content of peat is the geochemical agent responsible for the accumulation of uranium in bioliths. The accumulation of uranium occurs after the death of plants, the lignin of which is, in the course of humification, transformed into insoluble humic acids which are able to concentrate uranium from very dilute solution in natural waters. Adsorption isotherms can be adequately described by means of the Langmuir adsorption equation and can be characterized by two numerical constants: one represents the sorption capacity of peat, and the other is called "the geochemical enrichment factor". The sorption capacity is usually about 1 m-equiv./g of peat. Under-saturation is the rule in nature and the concentration of uranium in bioliths and natural waters is very low. The enrichment factor has a very high value (10,000 : 1) for uranium, and this is very important in its geochemistry. This is considered to be the governing factor of the association of uranium with organic substances in nature. Laboratory experiments have indicated that the sorption is a cation exchange process that is identical to the cation exchange property of humic acids which had long ago been observed for calcium, potassium and other cations in soil chemistry. The cation exchange property of humic acids has its origin in their carboxylic groups. They are comparable to synthetic cation exchange resins which are based on a carboxylated polyaromatic skeleton. Our investigations demonstrated that the cation exchange capacity for UO ₂⁺⁺ has a maximum value in the peat state of 2-3 m-equiv./g. The cation exchange capacity of lignites never exceeds about 0.7 m-equiv. The role of humic acids in the geochemical enrichment of other cations is more than a possibility, although it has not yet been sufficiently explored. A number of cations were studied and found to be sorbed and exchanged by humic acids. Similar observations have been made by other investigators. Inasmuch as the overwhelming majority of nuclear fission products can be sorbed, humic acid presents a very useful substance for waste disposal problems of the growing atomic energy industry. The enrichment constant is high for many other cations, and at least for some of them it is higher than for uranium; examples are Th ⁺⁴, rare earths, Zr ⁺⁴, etc. The enrichment constant, however, is not the sole factor for geochemical enrichment. The cation must be mobile in natural waters and soluble in the pH range of 4-7, which is favorable for sorption. This presents a bottleneck for study of geochemical enrichment, for to investigate the general role of humic acids in geochemistry the mobility of cations must first be studied.