Geochemical buffers

Buffers are reactions that at least temporarily resist change to some aspect of a fluid s chemistry. A pH buffer, for example, holds pH to an approximately constant value, opposing processes that would otherwise drive the solution acid or alkaline. The bicarbonate-C02 buffer 

In this chapter we construct models of buffering reactions, both homogeneous and heterogeneous. We concentrate on buffering reactions that are well known to geochemists, taking the opportunity to explore reaction modeling 

1 Calculated effects on pH of reacting hydrochloric acid into a 0.2 molal NaCl solution and a 0.1 molal Na2CO3 solution, as functions of the amount of HC1 added. The two plateaus on the second curve represent the buffering reactions between CO3 and HCO3, and between HCOj and CO2(aq). 

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Reaction (5) is most likely to represent the major gas redox buffer because of the comparable abundances of the two sulfur-bearing gas species. The other effective geochemical buffer is the Fe0-Fc203 rock buffer, which can affect the redox equilibria above in magma, and as gas and wall-rock interact. 

Many geochemical processes occur in which a fluid remains in contact with a gaseous phase. The gas, which could be the Earth s atmosphere or a subsurface gas reservoir, acts to buffer the system s chemistry. By dissolving gas species from the buffer or exsolving gas into it, the fluid will, if reaction proceeds slowly enough, maintain equilibrium with the buffer. 

The reaction rate Rj in these equations is a catch-all for the many types of reactions by which a component can be added to or removed from solution in a geochemical model. It is the sum of the effects of equilibrium reactions, such as dissolution and precipitation of buffer minerals and the sorption and desorption of species on mineral surfaces, as well as the kinetics of mineral dissolution and precipitation reactions, redox reactions, and microbial activity. 

In this chapter we construct geochemical models to consider how the availability of oxygen and the buffering of host rocks affect the pH and composition of acid drainage. We then look at processes that can attenuate the dissolved metal content of drainage waters. 

Input map layers into Fuzzy Logic Operators include lithologies and hydrothermal alteration plus their ranges of influence (buffer zone), lineament density maps and multiplicative geochemical maps. Input map layers are explained in the following sections. 

The near field of the repository includes the engineered barrier system (EBS, i.e., canister and buffer) and the waste form. Also included in the near field is the interface between the buffer and the host rock, denoted as excavation disturbed zone (EDZ). In terms of waste/water interactions, the geochemical evolution of the near field is essential as it controls the composition of the fluids that will eventually contact the waste. 

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