Download Modeling Uranium Sorption in Soil: Surface Complexation & Equilibrium Phases and more Study notes Geochemistry in PDF only on Docsity! Practical 4: Sorption, Precipitation and Complexation of UO2++ in Soil Background: Sherman et al. (2008) developed a surface complexation model for UO2++ sorption to FeOOH. The attached input file allows us to predict sorption as a function of pH, PCO2, [UO2]++ etc. The surface complexation model is based on the reactions: 2Fes_OH-1/2 +UO2++ = (Fes_OH)2UO2+ 2Fes_OH-1/2 +UO2++ + CO3-2 = (Fes_OH)2UO2CO3- Fes_OH-1/2 +UO2++ + CO3-2 = Fes_OCO2UO2+ Fes_OH-1/2 + H+ = Fes_OH2+1/2 We already know equilibrium constants for a variety of aqueous UO2++ complexes that may form and UO2++ minerals that may precipitate out. These are in the llnl.dat file. However, we have added some additional complexes in our input file. Use the attached input file and the llnl.dat to start a series of batch simulations that enable us to predict the solubility of UO2++ in a soil pore waters as a function of relevant chemical variables. Things in particular to explore: what is the effect of PCO2. What if phosphate and/or carbonate minerals are present? Explain what is going on during each simulation. Note that the soil has 0.09 g FeOOH (45 m2/g) per kg of soil pore water. What is the sorption capacity of this soil? Calculate a sorption isotherm for U at several pH values. What do these isotherms imply about a Kd model? After the batch simulations, set up an advective transport simulation with, say, 20 cells each with .09 g FeOOH (with 45 m2/g) per kg/soil pore water. Have cell 1 be contaminated with 0.1 g of schoepite and have the system be flused with rain water (SOLUTION 0). What is the effect of sorption by goethite on the downward transport of U? How many flushes would be needed to get rid of 99.9% the schoepite from cell 1? Docsity.com TITLE U sorption experiment SOLUTION_MASTER_SPECIES U UO2+2 0.0 U 238.0289 U(6) UO2+2 0.0 238.0290 SOLUTION_SPECIES UO2+2 = UO2+2 log_k 0 #primary master species for U #is also secondary master species for U(4) UO2+2 + H2O = UO2OH+ + H+ log_k -5.20 UO2+2 + 2H2O = UO2(OH)2 + 2H+ log_k -12.15 UO2+2 + 3H2O = UO2(OH)3- + 3H+ log_k -20.25 UO2+2 + 4H2O = UO2(OH)4-2 + 4H+ log_k -32.40 2UO2+2 + 2H2O = (UO2)2(OH)2+2 + 2H+ log_k -5.62 3UO2+2 + 5H2O = (UO2)3(OH)5+ + 5H+ log_k -15.55 UO2+2 + CO3-2 = UO2CO3 log_k 9.94 UO2+2 + 2CO3-2 = UO2(CO3)2-2 log_k 16.61 UO2+2 + 3CO3-2 = UO2(CO3)3-4 log_k 21.56 SURFACE_MASTER_SPECIES Fes_ Fes_OH-0.5 SURFACE_SPECIES Fes_OH-0.5 = Fes_OH-0.5 log_k 0.0 -cd_music 0 0 0 0 0 Fes_OH-0.5 + H+ = Fes_OH2+0.5 log_k 9.2 -cd_music 1 0 0 0 0 2Fes_OH-0.5 + UO2+2 = (Fes_OH)2UO2+ log_k 13.8 -cd_music 2 0 0 0 0 2Fes_OH-0.5 + UO2+2 + CO3-2 = (Fes_OH)2UO2CO3- log_k 21.05 -cd_music 2 -2 0 0 0 Docsity.com
