A HYDROGEOCHEMICAL TRANSPORT MODEL FOR AN OXIDATION EXPERIMENT WITH PYRITE CALCITE/EXCHANGERS/ORGANIC MATTER CONTAINING SAND/

We report the hydrogeochemical modeling of a complicated suite of reac tions that take place during the oxidation of pyrite in a marine sedim ent. The sediment was equilibrated in a column with MgCl2 solution and subsequently oxidized with H2O2. The oxidation of pyrite triggers dis solution of calcite, cation and proton exchange, and CO2 sorption. The composition of the column effluent was modeled with PHREEQC, a hydrog eochemical transport model. The model was extended with a formal 1D tr ansport module which includes dispersion and diffusion.The algorithm s olves the advection-reaction-dispersion equation with explicit finite differences in a split-operator scheme. Also, kinetic reactions for py rite oxidation, calcite dissolution and precipitation, and organic C o xidation were included. Kinetic relations fdr pyrite oxidation and cal cite dissolution were taken from the literature, and a coefficient equ ivalent to the ratio A/V (surface over volume), was adjusted to fit th e experimental data. The comparison of model and experiment shows that ion exchange and sorption are dominant chemical processes in regulati ng and buffering water quality changes upon the oxidation of pyrite. C ation exchange was assigned to the colloidal fraction (<2 mu m) and de protonated organic matter, proton buffering to organic matter, and CO2 sorption to amorphous Fe-oxyhydroxide. These processes have been negl ected in earlier modeling studies of pyrite oxidation in natural sedim ents. (C) 1998 Elsevier Science Ltd. All rights reserved.