Silvain Rafini Posted March 3, 2020 Share Posted March 3, 2020 Hi Brian/all I'm trying to predict the concentration of Zn at equilibrium with sphalerite at neutral pH and Eh = -50 mV (groundwater), taking into account only the components that are present into the dissolution reaction : ZnS + 2 O2 <--> Zn++ + SO4-- My objectives, after that, are to complexify the system and investigate the influence of some major ligands that are present in natural groundwater (PO4, SO4, Cl), on Zn solubility in the presence of sphalerite. In order to predict natural Zn concentrations in various types of groundwater. Below are the input parameters in REACT, water is defined only by components Zn-SO4-pH-Eh and Na for charge balance, reacting with 10 g of sphalerite : The command panel is : > # React script, saved Fri Feb 28 2020 by Utilisateur > data = thermo.tdat verify > conductivity = conductivity-USGS.dat > time start = 0 day, end = 1 yr > temperature = 25 C > H2O = 1 free kg > pH = 7 > Zn++ = .1 ug/l > SO4-- = 1 mg/l > swap e- for O2(aq) > Eh = -50 mV > balance on Na+ > react 10 g of Sphalerite This model predicts as low value as Zn = 1.01 E-9 mol/L, as shown below : Trying to corroborates this result with reaction balancing using RXN : Sphalerite + 2 O2(aq) = Zn++ + SO4-- Log K's: 0 ? 140.0535 150 ? 81.8395 25 ? 126.8808 200 ? 69.6068 60 ? 111.2955 250 ? 59.0203 100 ? 96.7075 300 ? 49.4066 Polynomial fit: log K = 140 - .5635 T + .001593 T^2 - 3.229e-6 T^3 + 2.744e-9 T^4 Log K at 25 C = 126.8808 Assumptions implicit in equilibrium equation: temperature = 25 C activity of SO4-- = 10^-3 Equilibrium equation: 129.9 = log a[Zn++] - 2 log a[O2(aq)] This seems to me a unrealistic result... This would give Zn concentrations as high as log aZn++ = 315.46, if we make the conversion from Eh = -50 mV (pE = -0.84) to log aO2 = -92.78 (see below RXN window with H2O dissociation reaction) At this point, I guess I need help... It seems to me that I should obtain the same results with these two approaches, but I obtain unrealistic values. Moreover, if I specify e- activity ae-, or Pe = -0.84 in the above RXN model, the reaction changes (see below) and the resulting Zn value also, quite drastically... Sphalerite + 4 H2O = Zn++ + SO4-- + 8 H+ + 8 e- Log K's: 0 ? -47.3593 150 ? -40.0723 25 ? -45.1228 200 ? -39.5912 60 ? -42.9247 250 ? -39.7529 100 ? -41.2689 300 ? -40.7818 Polynomial fit: log K = -47.35 + .1002 T - .0005183 T^2 + 1.452e-6 T^3 - 1.981e-9 T^4 Log K at 25 C = -45.1228 Assumptions implicit in equilibrium equation: temperature = 25 C activity of SO4-- = 10^-3 activity of H+ = 10^-7 activity of e- = 10^.8451 activity of H2O = 10^0 Equilibrium equation: 7.116 = log a[Zn++] Could you please help me by explaining what is wrong in these approaches, and what would be the more appropriate one. Thank you very much in advance. Silvain Quote Link to comment Share on other sites More sharing options...
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.