'Hidden' arsenic in estuarine systems

The nature, distribution and cycling of dissolved hydride-reducible and 'hidden' arsenic in the surface and interstitial waters of estuarine systems has been studied by cryogenic trap hydride-generation atomic absorption spectroscopy (HG-AAS) methods. With 'hidden' arsenic compounds no volatile hydride is formed upon treatment with sodium borohydride and pretreatment methods are necessary. Batch ultra-violet irradiation pretreatment processes for the detection of 'hidden' arsenic were further improved by the addition of persulphate resulting in the effective breakdown of non-hydride reducible arsenic compounds to 'inorganic' arsenic forms in 4-8 hours. Photolysis with persulphate using a microwave driven lamp reduced the time required to 2.5 minutes for freshwater samples, but concurrent heating effects made it unsuitable for estuarine water samples. A novel inline photooxidation HG-AAS technique was also developed which converted both hydride-reducible organoarsenic and non-hydride reducible arsenic species to 'inorganic' arsenic with ca. 92±4% efficiency.A seasonal study of the surface waters of Southampton Water and the estuary of the River Itchen revealed that during the summer months dissolved hydride-reducible 'inorganic' arsenic concentrations decreased as the hydride-reducible methylated arsenic concentrations increased. Dissolved 'hidden' arsenic concentrations were consistent with the release of complex organoarsenicals from planktonic organisms by active excretion, during senescence and grazing. These complex organoarsenic compounds are then broken down to simpler forms by chemical and biological processes. Deposition of these organoarsenic compounds into the sediments is accompanied by further breakdown and release into the overlying water column. Only once the decomposition has reduced the chemical complexity of the organoarsenicals can they be detected as hydride-reducible arsenic species.