Eutrophication and arsenic speciation in lake waters

Abstract

Arsenic (As) is widely distributed in aquatic environments in various forms. In natural waters, the dominant inorganoarsenicals (iAs) are incorporated into microorganisms such as phytoplankton, and are converted to methylarsenicals and/or more high order organoarsenicals. In addition, the organoarsenicals are mineralized to iAs and methylarsenicals by bacteria. The cycling of As species would depend on the bioactivity of organisms. Microorganisms, such as phytoplankton and organisms of higher trophic levels, produce methylarsenicals in natural waters with maximum concentrations in summer. The degradation and mineralization of organoarsenic compounds are thought to depend mostly on bacterial activities, which influence the As cycling in aquatic environment. Arsenic metabolism in aquatic organisms results in the occurrence of thermodynamically unstable arsenite and methylarsenic compounds in natural waters. The inorganic forms (As(V) and As(III)) and the methylated forms (methylarsonic acid (CH3AsO(OH)2); MMAA(V) and dimethylarsinic acid ((CH3)2AsO(OH)); DMAA(V)) are the main arsenic species present in natural waters. Although the predominant form of methylarsenicals is consistently DMAA(V) followed by MMAA(V), the existence of trivalent methylarsenic species in the environment has also been reported.Researchers reported the correlation between As(III)/methylarsenicals and chlorophyll-a concentrations and/or phytoplankton density, while others found that the seasonal changes of DMAA concentration is related to the temperature rather than the biological activity of phytoplankton. Eutrophication increases the concentration of nutrient salts and multiplies the primary producers, such as phytoplankton, in lake water. Lakes progress through the oligotrophic, mesotrophic and eutrophic process in the natural environment, and these transitions are very slow. Recently, the transition speed became faster because of discharged pollutants and nutrients from human activities, and the eutrophication affects the As circulation in lakes. Very recently, reports showed that the eutrophication influences As speciation in lake water too. In this chapter, the influence of eutrophication on arsenic speciation will be discussed. © 2010 Nova Science Publishers, Inc. All rights reserved