Biotite is one of the most important rock-forming minerals and vulnerable to weathering. Certain elements such as K and F can be greatly affected by the weathering of biotite. The effects of different cation species in solution on biotite weathering have not been throughly studied. 137Cs is a highly soluble nuclear element and can be fixed on the mica and clay minerals
The purpose of this study is to investigate the effects of cation species, pH, and reaction time on the weathering of biotite and on the sorption characteristics of Cs on weathered biotite.
Biotite was cut into small particles by scissors and a blender, and finally ground by mortar agate (55 and 100 ㎛). 0.1 g biotite was reacted in 50 ml solution of 1 M Na, K, Rb, Cs, Ca, and Mg chloride at pH 2 and 4 and at 40℃. Different reaction time, 20, 40, 60, 80, 100, and 120 days were used. For Cs sorption experiment, weathered biotite samples (0.5 g) were reacted with 50 ml 10-5 M CsCl solutions at 25℃ for 36 hours. The concentrations of K and Cs were analyzed by ICP-MS and that of Si was determined by colorimetry. XRD and SEM were used to investigate the mineral changes during weathering process.
The Si concentrations are greatly affected by pH and reaction time and low pH and longer reaction time increase the dissolution of biotite sheet. Dissolution rate is the greatest in Na solution based on Si contents. However, pH and reaction time do not influence that much on the K concentrations, indicating that cation species are more important in the release of K by ion exchange process.
There are no significant changes in XRD patterns after reaction with the exception of samples reacted in Na and Mg solutions. The samples reacted in Na solution show additional peaks at 12 and 14 A, indicating formation of expandable layers. The samples reacted in Mg solution only show small 14 A peak after long reaction time. The increases of peak width of (001) peak are also obserbed. The most significant changes are in the samples reacted in Na, Mg, and Ca solutions. The increasing peak width in the Na-sample is probably related with the formation of expandable layers. The increasing peak width of Mg, and Ca-samples are attributed to the ion exchange reaction of K with alkaline-earth cations with different charge and size.
Na- and Mg-samples show most significant changes after reaction, which is correlated with the XRD results.
Reaction time and different pH''s do not affect the amount of sorbed Cs. These mineralogical changes are also well correlated with the Cs sorption on biotites. Na-samples have the highest sorption capacities of Cs, and Mg-samples have the second higest sorption capcities.
In conclusion, different cations in solution affect on the weathering of biotite in two different ways, the dissolution of biotite sheet and cation exchange. Longer reaction time and low pH increase the dissolution of biotite sheet, but do not influence on the ion exchange much. Na is the cation affecting dissolution of biotite most greatly. Na increases the K release by forming expandable layers while other alkali cations increase the K release by ion exchanges. Cs exchange is greatly affected by the mineralogical changes caused by Na, Mg, Ca, rather than increased dissolution of biotite sheet.