Silica release during dissolution from

Figure 7.6. Silica release during the dissolution of untreated, sonified, and etched enstatite in buffer solution at pH = 6, T = 293 K, [From Schott et al. (1981), with permission.

Figure 6, Aqueous silica released to the aqueous solution during the dissolution of powders from three rock types. (A/ Eleana shale (UE-17e) (0), quartz mon-zonite (U15e-7) umtanum basalt (DC 3-3600).

There is no indication of a release of Zn from the sediments during the development of anoxia, unlike the release of phosphate and dissolved silicate. Zn bound to algal material may be dissolved upon mineralization of this material and Zn bound to manganese oxides upon dissolution of manganese oxides. It appears, however, that Zn is efficiently retained in the sediments, probably through bonding to other less soluble particles, such as iron oxides and silica parts of diatoms. In the presence of sulfide, Zn is probably retained in association with sulfide-containing particles. 

Addition of a secondary antifoam may represent another approach to mitigating the adverse effect of the deactivation of low -viscosity PDMS-hydrophobed silica antifoams by disproportionation during a wash cycle. Since wash cycles can finish at any tanperatures from 30°C to 95°C, a secondary antifoam would have to cut in at temperatures if it is to be universally applicable. However, release of a secondary antifoam need not be controlled only by temperature. It could, for example, be controlled by the delayed release associated with the dissolution of a coating. The time of release would have to be less than the shortest wash time at which the risk of overfoaming becomes significant. That will depend on the concentration of PDMS-hydrophobed silica antifoam present and the agitation conditions in a given machine. The concentration of course depends on the consumer dose. 

The tendency to hydrolyse increases with an increasing aluminium content in the zeolite [103]. An example of this is shown in Fig. 13, which shows the extensive level of hydronium exchange which incidentally occurred during a series of studies on Na/NH4 exchange equilibria in faujasitic zeolites [103]. This can also be seen in the increase of the selectivity coefficients for H30 /Na exchange with the increasing aluminium content of faujasite zeolites [73-75]. Zeolite hydrolysis also leads to several secondary phenomena. First, since the zeolite imparts an alkaline reaction to the water imbibed in the pores, carbon dioxide is picked up from the air. When the zeolite is then immersed in water, carbonate and bicarbonate ions are released into the solution. Secondly, hydroxyl ions released into the solution enhance the dissolution of silica and alvimina from the zeolite framework into the solution [73]. As a consequence, the following electroneutrality condition can be found to hold in pure water after it has been contacted with a zeolite such as NaX [73] ... 

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