Residence time mercury

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

The average residence times for mercury in the atmosphere, terrestrial soils, oceans, and oceanic sediments are approximately 1 yr, 1000 yr, 3200 yr, and 2.5 x 10 yr, respectively. (See Bergan et al. (1999) for more details on atmospheric residence times.)... 

Due to its chemical inertness, vaporizable nature (enthalpy of vaporization = 59.15 kJ/mol), and low water solubility (at 20°C, 2 x 10 6 g/g), elemental mercury vapor has over one year of residence time, long-range transport, and global distribution in the atmosphere [3-8]. 

Amount of mercury in some global reservoirs and residence time... 

Table 5.2 Amount of Mercury in Some Global Reservoirs and Residence Time...

The standard astm test method (D-1149-64) for rubber damage includes a test chamber (volume, 0.11-0.14 m ) through which ozonized air flows at a rate greater than 0.6 m/s. Because the residence time of the ozonized air in the test chamber is about 1 s, the ozone may be expected to reach the material in about 0.1 s. A somewhat similar test procedure (aatcc test method 109-1972 ansi L14, 174-1973) is used in testing colorfastness. The ozone generator is usually (but not necessarily) a mercury-vapor resonance lamp with emission lines at 184.9 and 253.7 nm. The 184.9-nm line is absorbed, and two ground-state oxygen atoms are produced ... 

Flow coulometry experiments were performed to study the reduction of U02 in nitric, perchloric, and sulfuric acid solutions [56]. The results of these studies show a single two-electron reduction wave attributed to the U02 /U + couple. The direct two-electron process is observed without evidence for the intermediate U02" " species because of the relatively long residence time of the uranium ion solution at the electrode surface in comparison to the residence time typically experienced at a dropping mercury working electrode. The implication here is that as the UO2 is produced at the electrode surface, it is immediately reduced to the ion. As the authors note a simplified equation for this process can be written, Eq. (7), but the process is more complicated. Once the U02" species is produced it experiences homogeneous reactions comprising Eqns (8) and (9) or (8) and (10) followed by chemical decomposition of UOOH+ or UO + to [49]. 

A flame AAS (FAAS) detector can monitor the GC effluent continuously to provide on-line analysis. However, as the gas flow rates for the flame are quite high, the residence time in the flame is short, and this can adversely affect the detection limits. Detection limits in the microgram range are usually achieved. Improved detection limits can be obtained if the additional techniques of hydride generation or cold vapour mercury detection are used as described in Section 4.6. 

Elemental mercury, a monatomic gas, is the dominant atmospheric form and has a long residence time in the troposphere (>1 yr Fitzgerald et al., 1981 Slemr et al., 1981 Lindqvist et al., 1991 Lamborg et al., 2000, 2002a). Such longevity allows emissions of mercury to the atmosphere from natural and... 

Watersheds are sources of mercury to the aquatic environment. However, and similar to biomass burning and evasion, the mercury released from watersheds is of mixed origin. Because the residence time of mercury within watersheds is fairly long (see Section 9.04.6), the potential for the buildup of legacy mercury exists. This feature is relevant when considering how rapidly a system might respond to decreased... 

There remains an intriguing inconsistency between experiments related to the mechanisms for mercury removal. Many lab, field, and model efforts indicate that the lifetime of mercury in the atmosphere must be 1 -2 yr, but there exist a number of plausible removal mechanisms (such as foliar mercury uptake followed by litterfall) that suggest the flux from the atmosphere is more consistent with lifetimes that are less than 1 yr. The likely resolution of this problem hes in the observation that majority of the Earth s surface is covered by areas that are not temperate or boreal forests, including the open ocean and tropical regions. The deposition to the ocean is consistent with an atmospheric residence time in excess of 1 yr, while the mercury cychng within tropical forests is understudied. 

Biological, chemical, and physical effects of airborne metals are a direct function of particle size, concentration, and composition. The major parameter governing the significance of natural and anthropogenic emissions of environmentally important metals is particle size. Metals associated with fine particulates are of concern particles larger than about 3-fjim aerodynamic equivalent diameter are minimally respirable, are ineffective in atmospheric interactions, and have a short air residence time. Seventeen environmentally important metals are identified arsenic, beryllium, cadmium, chromium, copper, iron, mercury, magnesium, manganese, nickel, lead, antimony, selenium, tin, vanadium, and zinc. This report reviews the major sources of these metals with emphasis on fine particulate emissions. 

These criticisms and possible action to eliminate or minimize them were discussed when the methodology was applied to study the complexation of Cd, Cu, Ph and Zn (32-34, 53, 104, 108). The results showed that Cu and Pb complexes were kinetically inert, with A ,/ values of between 10 and 10 s, which means that the lifetime of metal complexes, expressed by /ka, is some orders of magnitude higher than the residence time (1-100 ms) of complexes in the diffusion layer when Rotating Disk Electrodes (RDEs) are used. It can therefore be concluded that the reduction process is not appreciably affected by dissociation reaction inside the diffusion layer. Experiments showed instead that Cd complexes present a kinetic lability when Hanging Mercury Drop Electrode (HMDE) or RDE methods are used at low rotation speed (53). The results emphasized that dissociation from the electrode interface determines an underestimation of the conditional stability constant when low rotation speeds are used. To minimize the risk with respect to this problem the RDE method is normally used at the highest rotation speed. 

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