HR: 14:40h
AN: B12D-05    [PDF]
TI: Mineral Surfaces and Their Implications for Microbial Attachment: Results from Monte Carlo Simulations and Direct Surface Observations
AU: * Luttge, A
EM: aluttge@rice.edu
AF: Rice University, Dept. of Earth Science P.O. Box 1892, Houston, TX 77251-1892 United States
AU: Arvidson, R S
EM: rsa4046@ruf.rice.edu
AF: Rice University, Dept. of Earth Science P.O. Box 1892, Houston, TX 77251-1892 United States
AB: Mineral surfaces in contact with aqueous solutions form the interface at which microbial attachment and development is likely to occur and reactions will progress. Therefore, it is key to understand how microbes attach to crystal surfaces, and how they eventually impact mineral reaction kinetics. A prerequisite for any attempt to define these processes is the understanding and quantification of surface area. For practical reasons, we use the total surface area, even if we agreed that it would be more correct to use the reactive surface area, which is understood to be a fraction of the total. Surprisingly, recent computer based Monte Carlo (MC) simulations of mineral dissolution reactions have indicated that this concept might be flawed. The following questions elucidate this conclusion: How large is the reactive surface area? AND: How reactive is it? The first answer will be given in units of surface area, the second answer, however, can not be given in this unit. This has the unfortunate consequence that the reactivity term is embedded within the rate constant itself. A comparison of reactive versus total surface area implies that one part of the surface is reactive while the other is not. This concept is arbitrary and otherwise incorrect, as shown by analytical techniques like AFM and VSI that provide spatially resolved information. Additional evidence comes from model calculations that indicate that every locus the crystal surface has a certain reactivity. Expressed in terms of a MC model, these reactivities are expressed as probabilities; - otherwise, parts of the surface would be inert. Focusing at the molecular scale, we understand that each surface molecule has a certain probability to leave the structure and that every adatom has a probability to become a new member of the structure. The probabilities for the processes to occur depend, beside other factors, on the energy configuration at each location. Therefore, it is the number, type, and distribution of surface sites that define the average reactivity of a crystalline surface at any given time. This insight has large implications for a number topics: (1) reactive surface area can not be expressed solely in units of area, (2) nor can it be compared directly with total surface area. (3) Even more important, the distribution of sites at a given surface have the inherent potential for dynamic change. Therefore, we can not expect a unique rate for a dissolution process, but a range of rates. This conclusion is in agreement with many very well constrained experimental results. An attempt to model microorganisms on mineral surfaces and their interactions should address this problem because all interactions occur at the molecular scale. For example, Shewanella MR-1 attaches to specific sites at carbonate surfaces, i.e., the outcrops of line defects. By blocking these sites MR-1 modifies the reactivity of the surface and therefore the dissolution or growth kinetics.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting