Investigations of interactions of arsenic with humic substances

Interest in arsenic in groundwater has greatly increased in the past decade because of the increased awareness of human health effects linked to prolonged drinking of untreated ground water supplies high in arsenic content. A number of studies on arsenic bioavailability in soils have linked the presence of organic matter to high arsenic content. The question of whether arsenic interacts with organic matter chemically becomes significant. The work reported in this thesis investigates the chemistry of arsenic interaction with humic acid which comprises 50 % of soil organic matter. The thesis begins with an overview of arsenic environmental chemistry including its signifitcance, distribution, metabolism and toxicity. A literature review outlines the aqueous chemistry of arsenic specifically speciation, adsorption and mobility. The first part of the study focuses on the investigation of arsenic humic acid reactions over a range of conditions, for example changes in pH and ionic strength. Hydrolysed species of inorganic arsenic M and (111), and an organic form of arsenic (CH3)2AsOOH, were employed. Results show that the extent of reaction generally increased with pH and decreased with ionic strength. The results were interpreted firstly by assuming simple association and then by postulating ligand exchange. The derived equilibrium constants showed weak arsenic interaction with humic acid. The second part of the study examined the identification of arsenic bearing phases in a reference soil sample and contaminated soil samples. The results were processed using Chemometric Identification of Substrates and Elements Distribution (CISED). The research concluded that arsenic was mainly associated with an iron oxide phase in soils.