Dissolution chemical weathering

Another major process at the Earth s surface not involving rapid exchange is the chemical weathering of rocks and dissolution of exposed minerals. In some instances the key weathering reactant is H30 in rainwater (often associated with the atmospheric sulfur cycle), while in other cases H30" comes from high concentrations of CO2, e.g., in vegetated soils. 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

In this chapter, we build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

Blum, A.E. Stillings, L.L. 1995. Felsdpar dissolution kinetics. In White, A.F. Brantley, S.L. (ed.), Reviews in mineralogy, 31 Chemical weathering rates of silicate minerals, Mineralogical Society of America, USA, 291-352. 

Various devices can be used to determine the kinetics and rates of chemical weathering. In addition to the batch pH-stats, flow through columns, fluidized bed reactors and recirculating columns have been used (Schnoor, 1990). Fig. 5.15a illustrates the fluidized bed reactor pioneered by Chou and Wollast (1984) and further developed by Mast and Drever (1987). The principle is to achieve a steady state solute concentration in the reactor (unlike the batch pH-stat, where solute concentrations gradually build up). Recycle is necessary to achieve the flow rate to suspend the bed and to allow solute concentrations to build to a steady state. With the fluidized bed apparatus, Chou and Wollast (1984) could control the AI(III) concentration (which can inhibit the dissolution rate) to a low level at steady state by withdrawing sample at a high rate. 

What is the chemical weathering rate in this catchment area Can it be subdivided to dissolution rates of individual minerals ... 

Grandstaff, D. E. (1986), The Dissolution Rate of Forsteritic Olivine from Hawaiian Beach Sand", in S. M. Colman and D. P. Dethier, Eds., Rates of Chemical Weathering of Rocks and Minerals, Academic Press, pp. 41 -59. 

The rate of chemical weathering is related to the rate of C02 consumption. Consider first the dissolution of carbonates... 

The terrestrial weathering of organic matter derived from shales and soils results in the oxidation of carbon, which generates CO2. Dissolution of this CO2 in water produces carbonic acid. This weak acid serves to enhance chemical weathering reactions... 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

The rate of chemical weathering of minerals in the subsurface depends on a number of factors, including mineralogy, temperature, flow rate, surface area, presence of ligands and CO, and H+ concentrations in the subsurface water (Stumm et al. 1985). Figure 2.3 shows the rate-limiting steps in mineral dissolution consisting of... 

Table 1 Important weathering reactions in order of ease of chemical weathering and solubility, which goes along with the reaction rate of the mineral dissolution, except for bacterial mediated pyrite oxidation [9, 10]...

Many technical-chemical processes take maximum benefit of similarities with ongoing processes in Nature, with increased purity or reaction speed as the most important differences. The production of carbonates is a typical example of this, and the process of C02 mineralization for carbon capture and storage (CCS) (see Section 14.4) is in fact the accelerated version of what is known as the natural weathering of minerals. This is a combination of the interacting processes of mechanical and chemical weathering, and relevant to the current discussions are the chemical weathering processes of dissolution and hydrolysis that involve C02 [6, 7]. A dissolution equilibrium reaction that proceeds in Nature with dissolved C02 in water and calcite gives a bicarbonate solution ... 

The fluidized bed reactor has been used by several researchers to study the kinetics of chemical weathering (Holdren and Speyer, 1985, 1987 Chou and Wollast, 1985). One of the advantages in using the fluidized bed reactors for studies of this type is that there are no strong concentration gradients in the aqueous and solid phases. Additionally, the concentration of the dissolved species can be maintained at levels well below saturation with respect to possible precipitates. This means, for example, that one could study mineral dissolution exclusively without secondary precipita-... 

Holdren, G. R., Jr., and Speyer, P. M. (1986). Stoichiometry of alkali feldspar dissolution at room temperature and various pH values. In Rates of Chemical Weathering of Rocks... 

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