Arsenite oxidation by H2O2 in aqueous solutions

The rates of the oxidation of As(III) with H2O2 were measured in NaCl solut ions as a function of pH (7.5-10.3), temperature (10-50C) and ionic strengt h (I = 0.01-4). The rate of the oxidation of As(III) with H2O2 can be descr ibed by the general expression: d[As(III)]/dt = k[As(III)] [H2O2] where k ( mol/L-1 min(-1)) can be determined from (sigma +/-0.12) log k = 5.29 + 1.41 pH - 0.57I + 1.40 l(0.5) - 4898/T. The effect of pH on the rates indicates that the reaction is due to AsO(OH)(2)(-) + H2O2 -->(k1) products AsO2(OH)(2-) + H2O2 -->(k2) products, AsO33- + H2O2 -->(k3) products where k = k(1) alpha(AsO(OH)2-) + k(2) alpha(AsO2(OH)2-) + k(3) alpha AsO33 - and alpha(i) are the molar fraction of species i. The values of k(1) = 42 +/- 20, k(2) = (8 +/- 1) x 10(4), and k(3) = (72 +/- 18) x 10(6) mol/L-1 m in(-1) were found at 25C and I = 0.01 mol/L. The undissociated As(OH), does not react with H2O2. The effect of ionic strength on the rate constants ha s been attributed to the effect of ionic strength on the speciation of As(I II). The rate expression has been shown to be valid for NaClO4 solutions, n orthern Adriatic sea waters, and Tiber River waters. The cations Fe2+ and C u2+ were found to exert a catalytic effect on the rates, Cu2+ plays a role at concentration levels (>0.1 mu mol/L) which are typical of polluted aquat ic systems, while Fe2+ is important at levels which may be found in lacustr ine environments (>5-10 mu mol/L). The reaction of As(III) with H2O2 may pl ay a role in marine and lacustrine surface waters limiting the accumulation of As(III) resulting from biologically mediated reduction processes of As( v). Copyright (C) 1999 Elsevier Science Ltd.