CVEN 6414 Aquatic Surfaces and Particles

Problem Set 3: Anion Adsorption

Using Visual MINTEQ, complete the following assignment.

1. Calculate the extent of adsorption of sulfate on hydrous ferric oxide (HFO) as a function of pH using the two-pK_a double-layer model from pH 2 to 12.  For HFO, the concentration is 90 mg L^-1, the surface area is 600 m² g^-1, and the site density is 3 sites nm^-2.  The pK_a^int values are 7.29 and 8.93.
   
   (a) Create log C versus pH diagrams showing the distribution of the five surface species for two total concentrations of sulfate (10^-4 M and 10^-6 M, added as sodium sulfate) in the presence of 0.001 M sodium nitrate.  Discuss the effect of pH and sulfate concentration on the species distribution.
   
   (b) Create log C versus pH diagrams showing the distribution of the five surface species for two different sodium nitrate concentrations (0.001 and 0.1 M) with a total sulfate concentration of 10^-6 M.  Discuss the effect of ionic strength on the sulfate adsorption.
   
   (c) Create a log C versus pH diagram showing the distribution of the four surface species for the adsorption of chromate (total concentration of 10^-6 M) to HFO at a sodium nitrate concentration of 0.001 M.  The chromate adsorption reaction is
        >FeOH  +  CrO₄^2-  +  H⁺  =  >FeCrO₄^-  +  H₂O      K₁^s = 10^10.85
   Discuss the difference between chromate and sulfate adsorption.
   
    

Last updated on May 16, 2007 by Joe Ryan