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Please use this identifier to cite or link to this item: http://bsuir.bsum.edu.ng:8080/jspui/handle/11409/578

Title: SIMULTANEOUS REDOX CONVERSION OF CHROMIUM(VI) AND ARSENIC(III) IN AQUEOUS PHASE: A KINETIC AND MECHANISTIC APPROACH
Authors: IORHUNA, BONIFACE TERSOO
Keywords: SIMULTANEOUS, REDOX CONVERSION, CHROMIUM(VI), ARSENIC(III), AQUEOUS PHASE, KINETIC, MECHANISTIC APPROACH
Issue Date: Jun-2019
Publisher: NONE
Citation: NONE
Series/Report no.: NONE;NONE
Abstract: A kinetic and mechanistic approach was used to study the oxidation of As(III) and reduction of Cr(VI) simultaneously in aqueous phase, and in the presence of Fe(III) ions as well as dissolved organic matter (DOM) obtained from saw dust (SD), maize cob (MC), groundnut husk (GH) and rice husk (RH). Rate of reduction of Cr(VI) and oxidation of As(III) in aqueous phase by DOM was also carried out. Reactions were monitored by UV-visible spectrophotometry. The stoichiometry of the reaction was 1 : 3 (). Reaction products were identified as Cr(III) and As(III). DOM was confirmed by its UV absorption, having λmax of 250 nm, 260 nm, 256 nm and 260 nm for SD, MC, GH and RH respectively, and infrared spectral, having the major absorption bands as 2500 – 3500 cm−1 and 650 - 770 cm−1 (O-H stretching and out of plane bending groups), 2500 – 3500 cm−1, 1350 – 1470 cm−1 and 690 – 900 cm−1 (C–H stretching, deformation, and bending and ring puckering respectively), 1650–1800 cm−1 (C=O stretching of COOH), 1620 – 1680 cm−1 (alkene/aromatic C=C stretching), 970–1250 cm−1 (C–O stretching of alcohols/phenols) 880–995 cm−1 and 1395 – 1440 cm−1 (=C–H out of plane bending/C-O-H bending) respectively. Pseudo-first order plots for the oxidation of As(III) and reduction of Cr(VI) in aqueous phase were linear, indicating that the reactions were first order with respect to and . Hence, the overall reaction was second order. The rate equation for the reaction has been proposed as –d[]/dt = [][] where k2 is the second order rate constant. Rate of redox reaction of As(III) with Cr(VI) was not affected by ions within the concentrations range investigated and increased with increase in the volumes of DOM. However, the increment was highest with SD and fairly the same with the other agro wastes. The rate depended on pH with rate decreasing with increase in pH of the acidic range and increasing with increase in pH of the alkaline range, it increased with increase in ionic charge of the solution in the absence of DOM, and independent of the ionic strength when DOM was added. Reaction rate also increased with increase in temperature but having rate at 273 K seemingly lower than at 293 K for //DOM, almost the same with that at 305 – 308 K for /DOM and /DOM. The activation parameters; activation energy, Ea, enthalpy change, ∆H, and entropy change, ∆S were determined as; Ea of 83.31 kJmol-1 each for /GH, /RH and /SD couples were highest, while 44.38 kJmol-1 for //GH was lowest. ∆H were more negative (-80.85 kJmol-1) for /GH, /RH and /SD redox couples while // GH was lowest (-42.14 kJ mol-1). ∆S were most negative (-135.30 Jmol-1K-1) for //GH and less negative (-32.59 J mol-1 K-1) for the /GH, /RH and /SD redox couples. From the reactions /, a plausible mechanism involving an ester formation by the reactants leading to the eventual formation of As(V) and Cr(III) has been proposed.
URI: http://bsuir.bsum.edu.ng:8080/jspui/handle/11409/578
ISSN: NONE
Appears in Collections:Chemistry

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