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padhi

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  1. Hello, I am using GWB X1t to simulate fluoride transport in a soil column. First, the column is contaminated with fluoride for a certain period of time and then the column was flushed with deionized water. In the simulation, first, I want to simulate the transport process during sorption, where fluoride sorption was described by surface complexation. For surface complexation, I used the general composite approach (I added a new basis species (X) with 1g/mol molecular weight, defined a new element and a mineral with the basis species). For the surface dataset, I defined the reactions in terms of a new surface species (>XOH). For the mineral,if I use the exact density (e.g., 3591 g/l, that I used for my column with 39% porosity) and a site density of 2.6e-5moles/g, the program ends with the message that the porosity is too small. Since the porosity at each grid block is calculated with each run, I decreased the mineral site density (e.g., 1g/l or 0.1 g/l) and accordingly increased the site density. Then the program doesn't converge. However, if I keep both site density and mineral density small, the program runs but the results are not satisfactory (fluoride appears in the outlet just after 1 pore volume because the sorption is small). I appreciate any suggestion on how to keep the desired site density? P.S.: My PHREEQC simulation gives good results with the same site density, mineral conc. and surface reactions. Padhi
  2. Hello Dr. Brian, Thank you for your help. Adding calcite as kinetic mineral may explain my system. Is it possible to slide pH, if I add calcite as kinetic mineral? The output shows only the results for initial pH. With best regards, Sakambari
  3. Hello Dr. Brian, Thank you for your reply and suggestions. I changed the surface dataset to include neutral species as the basis species. I attach the surface data. Although I am able to work with other surface reactions, I get error when I include the surface reactions >CaHCO3 = >CaOH + CO3-- + 2H+ - H2O, >CaCO3- = >CaOH + CO3-- + H+ - H2O and >CaF = >CaOH + H+ F- - H2O (reactions 3,4 and 8 in my surface dataset). In the script, now NO3- is balanced for charge instead of CO3-- (script is almost same as the previous one with small modification on charge balance). In my system, I use calcite equilibrated solution with a specific Ca2+ and CO3-- concentration; to which, I add calcite and NaF solution (this is to prevent calcite dissolution during the experiment). Hence, for pH 6 and pH 7, calcite might be in equilibrium. For other pH, Ca2+ in initial solution is lower than that at pH 6 and 7 (since I added NaOH to increase pH of the solution and by that, I removed Ca2+ as Ca(OH)2 precipitate). Would you suggest (1)how to model and include both calcite and Ca2+ in the system and (2) about the surface data set? With best regards, Sakambari Padhi calcite_fluoride_surface2.txt
  4. Hi, I am using GWB9.0 (React) to simulate surface complexation of fluoride on calcite. My experimental system contains a calcite saturated solution (Ca2+ = 0.005M and CO3 = 0.0075M (I considered total alkalinity)). I added 0.5M of calcite to the solution and maintained ionic strength to 0.1M (by adding KNO3). The F- conc. is 0.3937mM (in terms of NaF). For surface complexation modeling, I am using diffuse double layer model. Calcite surface properties and surface reactions considered are as mentioned in the surface data file. I have the following querries: 1.How to add calcite to the system (I need Ca2+, CO3 2- and H+ as basis species)? Now I swapped Ca2+ for calcite and balanced on CO3 2-. By this, the Ca2+ and CO3 2- conc. are different, as the system maintains equilibrium. 2.When I use the surface data; it shows the error: Warning: reaction mass balance for >CO3Ca+ = Reaction = >CO3Ca+ + H+ = >CO3H = Ca++ Exit:React stop:couple_rxns: bad rxn 3.If I exclude this reaction and try to run, the error that I receive is: Exit: React stop: set-basis: bad charge React is ending I attach the script and surface data for reference. Any help is kindly appreciated. Padhi calcite_fluoride_script.txt calcite_fluoride_surface.dat
  5. Dear Dr. Farrel, Thank you very much. I could get the correct results. Best regards, Sakambari Padhi
  6. Dear Dr. Farrel or others, I am a new user of GWB. I will use React program of GWB 9.0 to simulate surface complexation of Fluoride (F) on different minerals. Before using my experimental results and surface data, I want to reproduce the modeling results of Dixit and Hering (2003) (Dixit and Hering, 2003, Env.Sci.Technol.,37,4182-4189). They modeled As (V) and As(III) sorption to HFO, Goethite and Magnetite, using MINEQL+. Here I will show the simulation results that I got for As (V) sorption on to HFO (Fig. 1, Dixit and Hering, 2003) using both Visual MINTEQ and GWB 9.0 (react). I attach the surface file, the figures from Visual MINTEQ, GWB and the GWB input file. The results from Visual MINTEQ explains well the results of Dixit and Hering (2003), where as the results are not reproduced well using GWB (e.g., the maximum sorption is same to that of Dixit and Hering, 2003, where as the pH dependence is not correlated). I used the Thermo_minteq.dat thermodynamic database. To get better result, I changed the log K values to match to that of thermo.dat file of Visual Minteq at 25 degree C. However, the results did not improve. I would appreciate if you kindly help me to understand the problem. Best regards, Sakambari Padhi input_results.pdf modified_feoh_minteq.dat
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