Environmental compartment arsenic compounds

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 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). 

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. 

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