Methylene blue discoloration by heated persulfate in aqueous solution

Advanced oxidation process based on thermally activated sodium persulfate (SPS) is used in this work to degrade methylene blue (MB). The impact of temperature (30-70 degrees C), MB concentration (15.6-311.8 mu M), SPS load (1-100 mM) as well as phosphate buffer (PB) capacity were characterized in short term (1-2 h) and long term (1-2 weeks) experiments. Results showed complete MB discoloration under tested conditions. The observed degradation rates (k(obs)) from the pseudo-first order kinetics model changed upon concentration of MB and were found closely dependent on the [SPS](0)/[MB](0) ratio. An acceptable Arrhenius behaviour was noticed for solutions of [SPS](0)/[MB](0) >= 32 in which SPS is in excess. Upon addition of SPS to MB at room temperature, the solution turned purple due to the formation of a non-stable flocky precipitate. However, spectral analysis showed complete disappearance of MB and its derivative precipitate after heat as well. The HPLC/MS analysis indicated the formation of non-stable transformation products identified as sulfonic acid and hydroxylated MB derivatives via SO4- and HO center dot oxidation. Those disappeared totally by the end of the treatment making from thermally activated SPS an excellent choice for the treatment of industrial effluents of the dyes industry even at room temperature. (C) 2012 Elsevier B.V. All rights reserved.