CVEN 6414 Aquatic Surfaces and Particles

Problem Set 1: Proton Exchange

Using Visual MINTEQ, complete the following assignment.

1. Calculate the distribution of surface species for goethite (a-FeOOH) using the two pK_a double-layer model from pH 0 to 14 for three different ionic strengths: 10^-5, 10^-3, and 10^-1.
   Use sodium chloride as the salt to adjust ionic strength.  For goethite, the concentration is 1 g L^-1, the surface area is 50 m² g^-1, and the site density is 3.84 sites nm^-2.  The pK_a^int values are 7.5 and 10.2.
   
   (a) what is the surface hydroxyl site concentration in units of molar? 
   (b) what are the effective pK_a values for the three ionic strengths?
   (c) explain the difference in the distribution of the ºFe-OH₂⁺,  ºFe-OH, and  ºFe-O^- as a function of ionic strength.
    

Last updated on May 15, 2007 by Joe Ryan