Silicate transport-controlled

Ionic form of transport. This is possible only in very acid solutions (pH < 3), the long existence of which is not likely at the Earth s surface due to interaction with rocks, due to dilution by surface waters, and due to the buffer effect of carbonate and silicate equilibria controlling the pH in the ocean. 

Equation (2.101) corresponds to transport-controlled kinetics (cf. Stumm 1990). White and Claassen conclude that after long times in natural water/rock systems parabolic rates tend to become linear. Helgeson et al. (1984) show that feldspar dissolution rates are linear if the feldspar is pretreated to remove ultrafine reactive particles. In other words initial parabolic rates are probably an artifact of sample preparation. It seems likely that, in general, the dissolution or weathering of most silicates in natural water/rock systems obeys zero-order kinetics. 

As was mentioned in the introduction to this chapter "diffusion-controlled dissolution" may occur because a thin layer either in the liquid film surrounding the mineral or on the surface of the solid phase (that is depleted in certain cations) limits transport as a consequence of this, the dissolution reaction becomes incongruent (i.e., the constituents released are characterized by stoichiometric relations different from those of the mineral. The objective of this section is to illustrate briefly, that even if the dissolution reaction of a mineral is initially incongruent, it is often a surface reaction which will eventually control the overall dissolution rate of this mineral. This has been shown by Chou and Wollast (1984). On the basis of these arguments we may conclude that in natural environments, the steady-state surface-controlled dissolution step is the main process controlling the weathering of most oxides and silicates. 

The processes described and their kinetics is of importance in the accumulation of trace metals by calcite in sediments and lakes (Delaney and Boyle, 1987) but also of relevance in the transport and retention of trace metals in calcareous aquifers. Fuller and Davis (1987) investigated the sorption by calcareous aquifer sand they found that after 24 hours the rate of Cd2+ sorption was constant and controlled by the rate of surface precipitation. Clean grains of primary minerals, e.g., quartz and alumino silicates, sorbed less Cd2+ than grains which had surface patches of secondary minerals, e.g., carbonates, iron and manganese oxides. Fig. 6.11 gives data (time sequence) on electron spin resonance spectra of Mn2+ on FeC03(s). 

Future studies should be pursued under controlled doping conditions and in atmospheres containing CO2 and O2. The known synergistic effect of CO2 on 02-solubility in silicate melts at very high gas pressures has, in fact, been interpreted in terms of Na2C03 formation in solution. Effects of this type could significantly enhance alkali vapor transport in practical combustion systems. 

Laboratory studies have suggested that there are three modes of transport for silicic acid (reviewed by Martin-Jezequel et al., 2000) first, silicic acid may be rapidly transported across the cell membrane, following surge uptake kinetics. This occurs primarily in Si-starved cells with cell quotas (Droop, 1968, 1973) near minimal values. Second, sdicic acid uptake can be controlled internally, presumably due to regulation ofsihcaprecipitation and deposition (e.g., Hildebrand et al., 1997). Third, silicic acid uptake may be controlled externally due to substrate hmitation. 

Hildebrand M. (2000) Silicic acid transport and its control during cell wall silicification in diatoms. In Biomineralization From Biology to Biotechnology and Medical Application (ed. E. Baeuerlein). Wiley-VCH, New York, pp. 171-188. 

Figure 13.6. (a) Linear dissolution kinetics observed for the dissolution of 6-AI2O3, representative of processes whose rates are controlled by a surface reaction and not by a transport step. (Data from Furrer and Stumm, (1986).) (b) Linear dissolution kinetics of frame silicates. Minerals used were pyroxenes and olivines their essential structural feature is the linkage of Si04 tetrahedra, laterally linked by bivalent cat-Fe -, Ca ). Plotted ate... 

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