Laboratory weathering

Since weather varies from day to day, year to year, and place to place, no precise correlation exists between artificial laboratory weathering and natural outdoor weathering. However, standard laboratory test conditions produce results with acceptable reproducibility and in general agreement with data obtained from long-time outdoor exposures. 

Nickel (8) calculated the thickness of the proposed "residual layer" on albite from the mass of dissolved alkalis and alkaline earths released during laboratory weathering and the measured surface area, and determined that the thickness ranges from 0.8 to 8.0 nm in the pH range of natural surface waters. Although he interpreted his results differently, they anticipate later findings on the pH dependence of residual layer compositions (see below). 

White and Brantley (in press) suggest several factors to explain these observations. Whereas the chemical affinity of the laboratory weathering solutions are similar and relatively under saturated (AG = —13.3 kcal mol and —15.6 kcal mol ), groundwaters associated with natural weathering of the Panola granite were only slightly undersaturated (—2.6 kcal mol ) with... 

Inskeep W. P., Nater E. A., Bloom P. R., Vandervoort D. S., and Erich M. S. (1991) Characterization of laboratory weathered labradorite strrfaces using X-ray photoelectron spectroscopy and transmission electron microscopy. Geochim. Cosmochim. Acta 55, 787—800. 

This laboratory weathering procedure has four methods. 

TABLE 7.10 Laboratory weathering (dissolution) rates of calcite, corundum, and some silicate minerals measured at pH = 4... 

Suu.ivAN, P. J., A. A. Sober, and J. Rybarczyk, 1986. Evaluating mineral dissolution in laboratory weathering experiments using leaehate data. Soil Sci. Soc. Am. J. 50 251-54. 

Henin, S., and G. Pedro. 1965. The laboratory weathering of rocks, p. 29-39. In E.G. Hall-sworth and D.V. Crawford (ed.) Experimental pedology. Butterworths, London. 

In the accelerated laboratory weathering of mica, Na tetraphenylboron is an extremely effective reagent because it reacts with released K+ to form the insoluble K tetraphenylboron precipitate, thereby maintaining a very low concentration of ions in solution. 

Several simple experimental systems that simulate some aspect of the groundwater environment have been used to study the breakdown of individual minerals. These kinetics studies have encompassed quartz (Brantley et al., 1986), feldspars (Holdren and Berner, 1979 Holdren and Speyer, 1985), pyroxenes and amphiboles (Berner and Schott, 1982 Schott and Berner, 1985), carbonates (Berner, 1978), and glasses (White, 1983). The relative stability observed in laboratory weathering is consistent with field-based observations however, experimental rates appear to be faster than those in natural systems. 

A tanperature measuring device consisting of a metal panel, having a black coating that absorbs all wavelengths uniformly, with a thermally sensitive element firmly attached to the center of the exposed surface. The black panel thermometer is used to control a laboratory weathering device and to provide an estimate of the maximum tanperature of samples exposed to a radiant energy source. 

Numerous workers have studied simulated weathering. Brown et al. (26) and Frankenfeld (27) conducted two-year studies under Coast Guard contract on the weathering of oil and devised some effective rapid simulation methods that could be the basis for future standardized methods. Ahmadjian et al. (28) studied simulated weathering by infrared spectroscopy. Flanigan et al. (29) investigated the effects of several methods of laboratory weathering on the results of various analytical methods. They also attempted to separate the relative importance of variables such as dissolution, evaporation, photooxidation, etc. as to their effects. Dissolution and evaporation were the variables that most affected GC and IR photooxidation most affected fluorescence. 

The standard tests for assessing the durability of polymers in outdoor exposure (and other degradative environments) have been listed and discussed by Brown [5]. He concluded that the standard tests are of little value in predicting service lifetime and discussed some of the reasons for this. He also noted the lack of correlation between the results of natural weathering and laboratory weathering, a topic also discussed by other authors [2, 4, 5]. 

Artificial laboratory weathering conditions are usually chosen to accelerate degradation. Elevated temperature is almost always used. This accelerates all of the degradation reactions and diffusion rates but in doing so it may distort the balance between the various reactions. Many reactions involve the products of other reactions and depend on diffusion. These factors will also affect the degradation depth and, consequently, the fracture mechanics. 

In order to determine the valnes for K, k2, and k in the SLP model, the absorbance data obtained as a function of exposure time for all eight laboratory weathering... 

Correlating a temperature that is available from climate data with the WPT parameter that was used to fit the laboratory degradation data by means of the SLP model also poses a number of challenges. Typically, climate data include only ambient (air) temperatures. These must somehow be converted to the WPTs used to control the laboratory weathering devices. Even then, one must consider whether such a conversion would translate to equivalent film specimen temperatures for both the outdoor and laboratory exposures. If the resulting film specimen tanperatures are different, the correlation between outdoor air temperature and WPT in a laboratory weathering chamber may not be sufficient to provide a useful SLP estimate. 

The SLP model determined via laboratory weathering experiments was employed to predict the degradation of peak absorbance for the fluorescent vinyl film weathered in the Arizona45S exposnre from Jnly 23 to October 21, 2008. The resnlting prediction, calculated at 1-min intervals, is plotted in Fig. 2.7. The zigzag appearance in the curve comes from the fact that the SLP model predicts no degradation at... 

Full exploitation of the SLP model demands the ability to make lifetime estimates for specimens in other orientations and geographic locations. Obviously, measured climate data will usually not be available to make such lifetime estimates. Instead, calculated climate data using models based on averages over many years must be used. One such model, METEONORM, provides hourly data for total solar irradiance and air temperature in numerous locations around the world [16]. Since TUVR and WPT data are not provided, conversions for total solar irradiance and air temperature to the laboratory weathering device parameters of irradiance at 340 nm and WPT, respectively, must be derived. 

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