Inorganic arsenic compounds are the most toxic arsenic forms that occur naturally in the environment. The arsenate toxic effect results from the mechanism of oxidative phosphorylation uncoupling. The research into the contents of the arsenic chemical species in Lake Moira, which is one of the biggest lakes in Canada, indicated the complexity of the undergoing processes. The total arsenic concentration in the bottom sediments was determined after acid digestion. The result was many times higher than the background value. The arsenic extraction from the bottom sediments was performed with the mixture of the phosphoric acid (1 mol/L) and ascorbic acid (0.1 mol/L). The concentration of the arsenic species was determined in the HPLC-ICP-SF-MS system. It was observed that the As(III) concentration decreased with the increasing depth of the particular bottom sediment layers. The As(III)/As-complex ratio in the extracts also indicated the tendency to decrease with the increasing depth. The highest As(III)/As-complex ratio was obtained in the surface layer of the Lake Moira bottom sediments. The authors suggest that As(III) was released from the surface layer of the bottom sediments in the redox or decomposition process. Subsequently, it was moved into water through the bottom sediment/bottom water exchange. The research points to the complexity of the forming organic species of arsenic and the necessity to investigate fresh, not dried, and bottom sediments

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