FLOW-THROUGH AND BATCH METHODS FOR DETERMINING CA-MG AND MG-CA SELECTIVITY

Authors: Desutter, Thomas; PIERZYNSKI, GARY - KANSAS STATE UNIVERSITY; BAKER, LUKE - KANSAS STATE UNIVERSITY

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2005
Publication Date: N/A
Citation: N/A

Interpretive Summary: One of the key functions of soil is to supply nutrients to plants, thus, the study of how nutrients are absorbed or released is important in determining soil fertility. For one group of these soil nutrients, called cations, measurement of adsorption and release is difficult. This study was done to compare two different methods for determining cation adsorption by soil. Determining whether one cation is selectively adsorbed onto soil over another cation is most commonly achieved using flow-through or batch methodologies. Flow-through and batch methods were statistically compared by determining whether calcium or magnesium was preferentially adsorbed onto soil. No statistical difference was observed between the two methods but the results of the flow-through method were less variable. For both methods, calcium was preferred on the soil over magnesium. This research is important for scientists investigating the adsorption behavior of soils, as it will affect whether they use a flow-through or batch method to achieve their objectives.

Technical Abstract: Conducting binary-exchange experiments is a common way to identify cationic preferences of exchanger phases and the two most common approaches are the flow-through and batch methods. Although batch reactions tend to be more laborious than flow-through reactions, the overall time requirement from onset to conclusion of the experiment may be less for batch experiments. However, there is more increased risk of errors in batch compared to flow-through experiments because of additional handling. The objectives of this research were to provide the materials and methods for both flow-through and batch equilibration techniques and to compare calcium-magnesium and magnesium-calcium selectivity when using these two methods. Binary-exchange reactions between calcium-magnesium and magnesium-calcium were performed on a montmorillonitic soil using both methods. The methods were evaluated by comparing the Gibb's free energy values ('G) derived from the exchange reactions. The 'G values for the calcium-magnesium reaction were 630 and 441 J moles per liter as determined by the flow-through and batch methods indicating an exchanger preference for calcium. Exchanger preference for calcium was also evident in the magnesium-calcium reaction with 'G values of '869 and-775 mol 1 for the flow-through and batch methods. The flow-through and batch methods worked very well for determining cation selectivity and results indicate no significant differences existed between the two methods. Although, the coefficient of variation (CV) of the 'G values for both calcium-magnesium and magnesium-calcium equilibrations, achieved using the flow-through method, were 17.3 and 18.1%, respectively, and were less than the CV values determined using the batch method. Choosing an equilibrating technique for determining cation selectivity will be a function of the time, laboratory resources, and goals of the experiment.