Date of Graduation
The objective of this study was to investigate the reduction of inorganic As(V) with Suwannee River fulvic acid (FA) in aqueous solutions and in real samples. Samples of inorganic As(V) were incubated with FA under various solution conditions and aliquots from the incubated samples were taken at various time intervals and analyzed for As(III) using square wave cathodic stripping voltammetry at a hanging mercury drop electrode. The study demonstrated the following important aspects of As speciation: (1) FA can significantly reduce As(V) to As(III), (2) Reduction of As(V) to As(III) is a function of pH, (3) Reduction of As(V) to As(III) is a function of time, (4) Reduction of As(V) to As(III) can occur under both dark and light conditions, (5) Fe(III) increases the reduction reaction, and (6) Oxidation of As(III) to As(V) is promoted at pH 2 more than at pH 6, (7) The reduction of As(V) to As(III) does not follow first order kinetics, (8) Reduction of As(V) can occur in a real sample. The reduction of As(V) by FA is hypothesized to occur via complexation, which is greatly enhanced by the presence of iron through intermetallic bridging. Fe(III) plays two roles: (i) facilitates binding of arsenate by FA which results in the reduction of As(V) to the intermediate As(IV), and (ii) Fe(II) produced by the reduction of Fe(III) by FA reduces As(IV) to As(III). The reduction reaction is postulated to follow two one-electron step mechanism, with Fe(II) reducing As(IV) and not As(V). In solutions where no Fe(III) was added, some binding of the negative arsenate by the negative FA occurs through intermetallic bridging by cationic metals inherent in the FA. Competition from H+ ions for the binding sites on FA at lower pH results in the diminished reduction at low pH. The kinetics of the reduction of As(V) by FA was also evaluated. The reaction does not follow first order kinetics, therefore the reaction order could be two or higher. The study also demonstrated that As(III) can be quantitatively analyzed in the presence of FA as As(III)-FA complex using SWCSV on a hanging drop electrode.
Tongesayi, Tsanangurayi, "Effects of various environmental parameters on the reduction of arsenic(V) to arsenic(III) by dissolved organic matter." (2006). Graduate Theses, Dissertations, and Problem Reports. 9908.