Volatility of arsenic

Arsenic occurs primarily in sulphide minerals associated with copper ores, and to a lesser extent with zinc, lead and gold ores. Arsenic is produced as a by-product of the smelting of these metals. Primary arsenic production has now ceased in the USA and Europe, and most arsenic is now imported from China and Mexico. The volatility of arsenic represents a significant concern, and there is at present no known natural mechanism by which arsenic is immobilized in the environment. Anthropogenic activities account for an input of some 19000 tonnes into the atmosphere, compared with 12000 tonnes from natural processes, such as volcanism and forest fires (Ayres and Ayres, 1996). 

Weathering, erosion, and sedimentation are also important in concentrating arsenic in continental crusts. Togashi et al. (2000) even argue that weathering, erosion, and sedimentation are more responsible for enriching arsenic in the upper crust of Japan than contributions from magmas and lavas. Additionally, as discussed in the next section, the moderate volatility of arsenic, its sufficient solubility in hot fluids, and its reluctance to enter and accumulate in the mantle should preferentially concentrate the element in the crust. 

The arsenic concentrations of metamorphic rocks generally decrease with increased metamorphism (Ryan et al., 1996), 265. As temperatures increase, more arsenic volatilizes out of the rocks. Specifically, metamorphic rocks that formed at <45 km in an ancient subduction zone in California lost about 80-85 % of their arsenic as metamorphic temperatures and pressures increased from about 275 °C and. 5 kilobars (kb) to approximately 750 °C and 12 kb (Bebout et al., 1999). As discussed in Section 3.6.2, the volatilization of arsenic during metamorphism may be important in transferring the element from subduction zones back into the crust (Figure 3.2). 

The volatilization of arsenic during the thermal destruction of CCA-treated wood may be reduced by utilizing low-temperature pyrolysis. Low-temperature pyrolysis uses temperatures of approximately 300-400 °C with a limited air supply (Helsen and Van den Bulck, 2004, 286, 290 Helsen and Van den Bulck, 2003). Pyrolysis includes slow and flash methods (Helsen and Van den Bulck, 2004). Flash pyrolysis, which produces an oil byproduct, is not effective with CCA-treated wood because only 5-18% of the arsenic... 

Although arsenic is less volatile during low-temperature pyrolysis than combustion, some arsenic still volatilizes during the process. The volatilization of arsenic during pyrolysis chiefly results from the reduction of As(V) to AS4O6 ( AS2O3 ) and other As(III) oxides (Helsen and Van den Bulck, 2003 Hata et al., 2003 Helsen et al., (2003)). To minimize arsenic volatilization, the characteristics of the wood must be known and pyrolysis operations must be carefully controlled at temperatures below 320 °C (Helsen and Van den Bulck, 2003). 

Several analytical methods for speciating arsenic have been reported. They include chromatographic techniques such as electrophoresis and ion-exchange (17), paper chromatography (18) and HPLC (19) selective volatilization of arsenic compounds to analogous arsines followed by GC-MES (20) boiling point separation/spectral emission (21) and atomic absorption (22). The above techniques have been applied to samples such as commercial pesticides (20),coal and fly ash (23),rocks, sediments, soils and minerals (24, 22),plant tissue (18), bovine liver (23),and water samples T25). 

Determination of the arsenic content of each sample was performed by the AgDDC method following wet ashing. Reductive volatilization of arsenic was carried out with sodium borohydride and subsequent analysis by GC-MS. 

Dobbs A J., Phil D, Grant C, (1978) The volatilization of arsenic on burning copper-chrome-arsenic (CCA) treated wood. Holzforschung 32,32-35. 

Figure 3 Volatilization of arsenic by Fusarium sp. under various oxic conditions with and without a soil column scrubber. (From Ref. 51.)...

RB Pearce, ME Callow, LE Macaskie. Fungal volatilization of arsenic and antimony and the sudden infant death syndrome. FEMS Microbiol Lett 158 261-265, 1998. 

JE Thomas, RD Rhue. Volatilization of arsenic in contaminated cattle dipping vat soil. Bull Environ Contam Toxicol 59 882-887, 1997. 

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