Chemistry: analytical and immunological testing – Metal or metal containing – Zn – cd – hg – sc – y – or actinides – or lanthanides
Reexamination Certificate
1998-09-02
2002-11-05
Soderquist, Arlen (Department: 1743)
Chemistry: analytical and immunological testing
Metal or metal containing
Zn, cd, hg, sc, y, or actinides, or lanthanides
C422S050000, C422S062000, C422S078000, C422S080000, C422S081000, C422S082080, C422S082090, C422S088000, C436S073000, C436S155000, C436S158000, C436S160000, C436S177000, C436S181000, C436S182000
Reexamination Certificate
active
06475802
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to detecting and collecting mercury for air or other gas, and particularly is concerned with an apparatus for and a method of mercury speciation, which can differentiate between different components of gaseous mercury. More particularly the inventor is concerned with differentiating between the different components of mercury in ambient air and stack gas, namely gaseous elemental mercury vapour, gaseous ionic water soluble compounds of mercury, and particulate bound mercury.
BACKGROUND OF THE INVENTION
This invention is concerned with the detection of mercury and speciation of mercury, both in ambient air and in stack gases. There are three basic or largest components of gaseous or airborne mercury, namely: gaseous elemental mercury vapour, gaseous ionic water soluble compounds of mercury, and particulate bound mercury. Of the three basic components of gaseous or airborne mercury, the largest is gaseous elemental mercury Hg
0
, i.e. non-ionized mercury vapour. Gaseous ionic water soluble compounds of mercury are generally known by the collective designations: reactive gaseous mercury (RGM), ionic mercury, or Hg (II), or Hg
2+
; this class of compounds consists almost exclusively of mercury chloride, HgCl
2
, since this compound is produced by combustion processes that have free chlorine present (for example, coal-fired power plants, waste incinerators, etc.). Particulate bound mercury comprises particles having mercury bound to them. In ambient air, the large majority of mercury in particulate form is contained in small particles <2.5 &mgr;m (microns) in diameter.
In background ambient air, elemental mercury generally comprises 90-98% of the total mercury; in stack gases, the proportions tend to be more equal. However, even for ambient air, the small reactive gaseous portion is extremely important, since it deposits locally and, being water soluble is a much larger local concern. Elemental mercury, which generally has a lifetime in the atmosphere 6-12 months, will often be deposited well away from its source. The RGM is present at very low concentration, e.g. 10-50 pg/m
3
so it must be preconcentrated before being analyzed.
There have been proposals in the art for measuring mercury and providing some degree of mercury speciation, i.e. measuring separately two or more components of mercury. Thus, one proposal is found in the paper entitled “Atmospheric Mercury Speciation: Concentrations and Behaviour of Reactive Gases, Mercury in Ambient Air” by S. E. Lindberg et al;
Environmental Science and Technology
1998, Vol. 32, No. 1, pp. 49-57. There, the proposal is to use a high-flow refluxing mist chamber. This operates by drawing the sampled air through water disbursed as a fine aerosol. Water-soluble gases are adsorbed by the nebulised mist, and the droplets containing the scrubbed reactive gases of mercury coalesce on the surface of a hydrophobic membrane and then drain back into the chamber. The small solution volume required enables sampling times of the order of one hour to be used. Simultaneously, total gaseous mercury can be collected on old-coated sand adsorbers, but it should be noted that this technique collects all gaseous mercury, including the RGM. Particulate mercury is trapped by an external filter, which also traps some of the RGM. The mist chamber also has a particulate filter. The use of these filters tend to bias some of the RGM results low. The solutions from the mist chamber were analyzed by a wet chemistry technique, requiring reduction of the water soluble mercury ions to Hg
0
by stannous chloride. The mercury is purged from the mixture into a carrier gas using standard techniques and preconcentrated onto a gold cartridge. The total gaseous mercury was detected on the gold adsorber in known manner using cold vapour atomic fluorescence. What is noteworthy about this technique is that all the mercury species are not determined together, but require separate detection techniques, and moreover due to the wet chemistry techniques required to analyze the collected RGM, the technique is neither continuous nor suitable for automation.
A further proposal is found in the paper entitled “Sampling and Determination of Gas Phase Divalent Mercury in the Air using a KCI Coated Denuder” by Z. Xiao et al. in
Fresenius Journal of Analytic Chemistry
(1997) 358: 386-391. The technique proposed there used KCl denuders, formed by coating appropriately dimensioned glass tubes with KCl from a saturated KCl solution. The experiments reported show that gaseous elemental mercury (Hg
0
), simply passes through the denuders without being adsorbed, and apart from some apparent system errors, 100% “passage” of elemental mercury vapour was reported. For field sampling, two lines were set up, one for collecting gaseous Hg
2+
species using a KCl denuder and the other for conventional sampling of total gaseous mercury. It is noted that the effective recovery and analysis of the collected mercury from a denuder is an important step. Here, the mercury is recovered by HCl extraction followed by CVAFS (Cooled Vapour Atomic Fluorescent Spectroscopy). Again, such a technique is complex, and is not suited for continuous monitoring and cannot readily be automated.
A further series of developments and experiments have been reported by K. Larjava and others as follows:
“Development and Laboratory Investigations of a Denuder Sampling System for the Determination of Heavy Metal Species in Flue Gases at Elevated Temperatures” by K. Larjava et al,
International Journal of Environmental
&
Analytical Chemistry,
38 (1990), pp. 31-45;
Application of the Diffusion Screen Technique to the Determination of Gaseous Mercury and Mercury (II) Chloride in Flue Gases by K. Larjava et al., accepted for publication in the
International Journal of Environmental
&
Analytical Chemistry;
and
On the measurement of Volatile Metal Species at Elevated Temperatures by K. Larjava—Dissertation for the degree of Doctor of Technology to be presented at Helsinki University of Technology on May 21, 1993.
Here again, there is a disclosure of the use of annular denuder tubes coated with potassium chloride for collecting mercury species. These papers focus on the basic research and do not teach any complete, functioning instrument, capable of speciating mercury vapour.
Accordingly, known techniques suffer from a number of disadvantages. Commonly, they require a denuder collector or the like to be extracted from the equipment and analyzed using wet chemical techniques. Such a method is not suited to continuous monitoring, nor to an automated system.
A further problem with detection of RGM, which has not been fully identified in the art, is that common ionic mercury compounds have a strong affinity to adsorb onto a wide variety of surfaces. In this sense, ionic mercury is “sticky”, and extremely difficult to handle. Bearing in mind that the problem is to detect very low levels of ionic mercury, the small amount present readily adsorbs onto surfaces of tubing, valves and other fittings distorting any reading made.
Another problem with the determination of RGM is that it is very difficult to separate the RGM from the particulate mercury. Previously, this has been extremely difficult, as one known technique is to provide a particulate filter upstream of a denuder, to filter out the particulate mercury. However, due to the characteristics of reactive gaseous mercury, some fraction of it would collect on the particulate filters, leading to a false low reading for the measured amount of reactive gaseous mercury.
SUMMARY OF THE INVENTION
The present inventors have realized that, when measuring mercury vapour, including reactive gaseous mercury, it is necessary to take extra steps, to ensure that the reactive gaseous mercury does not collect onto surfaces before reaching a detection device intended to detect its presence.
Additionally, the present invention is directed to a technique which will allow continuous and automated measurement of mercury vapour, including speciating
Schaedlich Frank H.
Schneeberger Daniel R.
Bereskin & Parr
Soderquist Arlen
Tekran Inc.
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