Hydrothermal Wood Decomposition and Influence of the Degradation Products on the Uranium Adsorption on Metamorphic Rocks and Sediments

The influence of mine-wood leachates on the uranium(VI) adsorption (1) on typical rocks and minerals of the ore mountains mining sites and (2) on sediments of the Königstein mine (Saxony) and its near field was studied under aerobic condition.

Wood decomposition was carried out by boiling of shavings with water under reflux. The leachate were analysed regarding DOC, phenolic and saccharic compounds, molecular weight distribution and content of carboxylic and phenolic hydroxyl groups.

The adsorption experiments were carried out by a 4-6 weeks equilibration of the geomaterial with the aqueous phase in the presence of U(VI) and DOC. The uranium concentration ranged from 3.2∙10^-6 to 3.2∙10^-5 mol/L (0.76-7.62 mg/L) and the DOC content was 8.3-166 mg/L. The distribution ratio (R_s [mL/g]) was determined from liquid scintillation counting of the added ²³⁴U tracer (10-40 Bq/sample) after distribution and a careful phase separation.

Uranium adsorption behaviour at the ore mountains rocks and minerals: The adsorption from a DOC-free synthetic mine water takes mainly place at diabase (R_s=4.8 mL/g) and calcite (R_s=4.5); about 80% of the uranium is bound on this minerals. The adsorption under the influence of the wood degradation products and pine wood lignin (PWL) is increased to 90-95 % (calcite: R_s=13.0/108 mL/g; diabase: R_s= 9.9/59). Granite and basalt adsorb from DOC-free mine water 50-60% of the uranium (R_s=0.97/1.6). In the presence of the wood leachate and lignin the adsorption increases to about 80% (granite: R_s=6.5/15.5; basalt: R_s=5.9/18.5). In the case of gneiss the effect of the wood leachate and lignin is not so significant (R_s: 1.1-1.5). The uranium adsorption on phyllite takes place to a much lower level. From all solutions practically no uranium is bound (R_s≤ 0.1), i.e., uranium adsorption is smaller than 2%.

Uranium adsorption behavior at the Königstein sediments: It is evident that uranium is prefferently bound to sandstone (91-97%); especially with a high distribution ratio to the limonithe-rich turonian sandstone (142 mL/g). Even under the influence of the organic compounds the sorption ranges from 68 to 93% (R_s: 10-70). A sorption degrease is caused by the wood leachate and lignin (2-4%). The level of adsorption on lime marl is much lower (22-68%, R_s: 1.4-11). The wood leachate and the lignin enhance the adsorption by 10-20%. The studied claystone, in general, was not able to bind uranium. Only in the presence of the wood leachate about one third of the uranium is adsorbed (R_s= 2.9). The reason may be that this sediment acidifies the aqueous phase (from pH 7.3 to 3.5) and some of the DOC with part of the uranium is precipitated on the sediment.

As a general tendency it can be seen that the strongly polymer wood breakdown substances strengthen the uranium(VI) adsorption at the geomaterials from the typical mine waters.