Modelling Biochemical Processes in Rocks: Analysis and exploratory simulations of competition of different processes imp
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0RGHOOLQJ%LRFKHPLFDO3URFHVVHVLQ5RFNV $QDO\VLVDQGH[SORUDWRU\VLPXODWLRQVRI FRPSHWLWLRQRIGLIIHUHQWSURFHVVHVLPSRUWDQWIRUIHUURXVPLQHUDOR[LGDWLRQDQGR[\JHQ GHSOHWLRQ Magnus Sidborn and Ivars Neretnieks Department of Chemical Engineering and Technology, Royal Institute of Technology, S-10044 Stockholm, Sweden $%675$&7 There are several oxygen consuming processes occurring in deep subsurface fractured rocks. The importance of each process depends on the prevailing conditions and what function it has in the overall oxygen scavenging process. In the present work, some of these processes are studied. The aim is to build a frame of maximum rates for each parallel process. Therefore, several ratelimiting factors are not included. Rate expressions were obtained from the literature and compared for different cases to address under which conditions each process is important. Some of the processes may be considered parallel and independent while others occur in series. The main reactions accounted for are abiotic and biotic pyrite and ferrous iron oxidation and the nonoxidative dissolution of matrix ferrous minerals. In addition, transport processes of oxygen and other substrates within the rock matrix are considered. The main conclusions are that initially, when the diffusion resistance in the matrix is nonexistent or small, reaction kinetics are important. However, in a longer time perspective, diffusion processes limit the oxygen scavenging process. ,1752'8&7,21 Figure 1 shows an undisturbed conducting fracture under reducing conditions suddenly exposed to oxygenated water infiltration. Oxidation of the reduced minerals in the infill material and fracture walls will then occur. In addition, dissolved oxygen is transported into the rock matrix by pore diffusion. The oxidation reaction may therefore occur both in the fracture and in the rock matrix. Microbes gaining energy for growth and maintenance from oxidation of reduced species such as pyrite thrive in this environment and increase the oxidation rate. However, the size of the microbes prevents them from growing inside the micropores of the rock matrix. Microbially mediated reactions therefore occur in the conducting fractures only. Even though the biotic reactions occur within the fractures, substrates may be transported to the microbes by matrix diffusion. )H ,, PLQHUDO
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)LJXUHDifferent oxygen consuming processes along a conducting fracture and adjacent rock matrix
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3\ULWHR[LGDWLRQ Pyrite may be oxidized by several mechanisms. Two important abiotic reactions in subsurface rock are the oxidation by oxygen and by ferric ions: FeS 2 + H 2 O + FeS 2 + 14 Fe 3+
7 O 2 → Fe 2+ + 2SO 24− + 2 H + 2 + 8H 2 O → 15Fe 2+ + 2SO 24− + 16 H +
Oxidation of pyrite, mediated by bacteria in direct contact with the mineral grains, can be described by an indirect mechanism where the pyrite grains are oxidized by ferric ions, which are
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