Environmental compartment arsenic

Arsenic can exist in several oxidation states, as both inorganic and organometallic species, and in dissolved and gaseous phases (Table I). Dissolved arsenic species can adsorb to suspended solids and be carried down to the sediments in an aquatic system. Since gaseous arsenic compounds can form, arsenic can be removed from the sediments as dissolved gas or in gas bubbles (e.g. CH ). Thus, arsenic can cycle within aquatic ecosystems and this cyclic behavior has been reviewed by Ferguson and Gavis (1 ) and Woolson 2). In any given system, it is necessary to understand the behavior of a variety of different arsenic compounds as well as a variety of environmental compartments in order to totally characterize the cyclic behavior of this element. 

Arsenic moves between ditferent environmental compartments (rock-soil-water-air-biota) from the local to the global scale partly as a result of pH and redox changes. Being a minor component in the natural environment, arsenic responds to such changes rather than creating them. These changes are driven by the major (bio)geochemical cycles. 

Arsenic is mobile within all environmental compartments, and may circulate many times in various forms through the atmosphere, biosphere, water and soil before entering its sink in the sediments. Whether sediments are the ultimate As sink depends, however, on general and local conditions. For instance, pH and competing substances (ions) may remobilize part of the immobilized elements and compounds (see Part I, Chapter 8). 

Figure 14. Case study 1 contaminated situations. Geographic distribution of arsenic contents in different environmental compartments of the Malcantone area, Ticino /Switzerland soils, river sediments and aquatic mosses. Data from this study. State Laboratory, Canton Ticino, and Martini and Fillet (1996).The whole area is naturally contaminated with arsenic originating from numerous smaller and larger ore deposits dominated by Fe, As, Zn, Pb and Sb (indicated with black spots). The general background of As in the till material forming the aquifers varies between 50 and 150 ppm, but locally values up to 500 ppm occur.

Methylarsonate and dimethylarsinate are common arsenic metabolites found in most environmental compartments. They often occur together, with dimethylarsinate being the more abundant in most samples. Their joint occurrence might be... 

The major environmental compartments are considered here with respect to the types of arsenic compounds that have been found there. In most cases, the text merely summarizes the contents of the tables. Although the tables do not cover all available data, we have attempted to provide representative data wherever possible. In order to present the data in this manner, certain assumptions, approximations, and interpretations have been made. The quality of the analytical data from the many different studies varies considerably both in terms of quantification and, perhaps more importantly, the rigor applied when identifying the various arsenic compounds. We offer no judgment on these matters the reader may wish to refer to the original paper before making an assessment. 

Arsenic species that have been identified in the terrestrial environment are listed in Table 3. Apart from the inorganic species, which predominate in all environmental compartments, they are mainly methylarsenicals and are presumably formed via the same biological process outlined above. The formulations given for the methylarsenic(III) species are probably not correct because compounds of this type are unknown. It is likely that the species are actually thiols CH3As(SR)2 and (CH3)2AsSR (19). The reason for the uncertainty is that the analytical technique commonly used to determine arsenic species is hydride generation (HG) followed by some form of separation and detection, e.g, gas chromatography (GC) and atomic absorption (AA) spectrometry hence HG/GC/AA. 

In many previous chapters, the discussion of questions regarding various environmental aspects of environmental chemistry in air, water, and soil compartments touched upon the problems of heavy metals. However, we should pay more attention to these pollutants, which are of crucial environmental concern in the Asian region. In this chapter, the emphasis will be given to heavy metal emissions from coal (including lignite) burning power plants, and to the specific aspects of environmental behavior of the most dangerous contaminants, like arsenic, mercury and lead. The problems of heavy metal site remediation will be considered in Chapter 16. 

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