The fast decolourization of multi-purpose dye, Brilliant blue (BB-) oxidized by chlorine dioxide was investigated using the stopped flow technique under varied pH conditions by monitoring its oxidation kinetics. The products were identified and reaction mechanism is described, which is confirmed by kinetic simulations. Under [ClO2]0 > [OH-]0 > [BB-]0 conditions, the oxidation kinetics showed first-order dependence on BB- and chlorine dioxide. The overall second-order rate coefficient enhanced with increasing pH, and values were 30.2 ± 0.2 M-1 s-1,42.5 ± 0.8 M-1 s-1 and 117.9 ± 0.8 M-1 s-1 at pH 7.0,8.0 and 9.0, respectively. In the pH range 7.0 to 9.0, the catalytic constant for [OH-] catalyzed reaction was 9.0 X 10(6) M-2 s-1 with energy of activation of 50.06 kJ mol-1. Observed negative entropy of activation of -658.73 J K-1 mol-1 suggests the formation a compact transient activated complex.

Abstract Four triterpenoids, ergosta-4,6,8(14),22-tetraene-3-one 1, stigma-4-ene-3-one 2, 3β-hydroxy-21β-H-hop-22(29)-ene 3, sitosterol and a quinone, tectoquinone 4, were isolated from the leaf, stem bark and fruit extracts of Ficus natalensis subsp. natalensis, Moraceae, a medicinal fig found in Africa. The pure compounds 1–4 and crude extracts were tested for their antibacterial activity against five Gram-negative and seven Gram-positive strains and for their potential anti-biofilm activity. Antimicrobial susceptibility was observed with all pure compounds tested at 250 µg against the majority of Gram-negative and Gram-positive strains. The dichloromethane-soluble fruit extract was active against sensitive and resistant Staphylococcus aureus strains, Enterococcus faecalis and Staphylococcus xylosus. Compounds 2, 3 and 4 demonstrated broad-spectrum antibiotic effects against eight of the twelve bacterial strains tested. In the anti-biofilm assay, exposure to ethyl acetate, methanol and aqueous methanol leaf, stem bark and fruit extracts decreased adhesion with a biofilm reduction of ≥100% for all three tested organisms: Escherichia coli, Pseudomonas aeruginosa and S. aureus. The methanol leaf extract demonstrated the most potent anti-adhesion potential against E. coli (218% biofilm reduction). The greatest ability to decrease adhesion was observed with compounds 2, 3 and 5 against P. aeruginosa at the lowest concentration tested (100 µg ml−1).

The kinetics of the oxidation of a triphenylmethane dye, Brilliant Blue-R (BB- Na+), in aqueous solutionby hypochlorite as a function of pH was investigated. While the degradation of dye obeyed pseudo-first-order kinetics, the oxidation of the dye occurred through two competitive reactions facilitated by [OCl-] and [HOCl]. Both reactions exhibited first-order dependence on [OCl-] and [HOCl], respectively, but the hypochlorous acid initiated reaction was about ten times faster. The relative importance of the two paths rested on the pH-dependent concentrations of hypochlorite and hypochlorous acid. The overall second-order rate constants for the OCl- and HOCl initiated reactions are 1.2 ± 0.2 M-1 s-1 and 22.2 ±1.2 M-1 s-1, respectively. The reaction energy parameters were determined as Ea = 35.5 kJ mol-1, ∆H‡ = 33.1 kJ mol-1 and ∆S‡ = -191.9 J K-1 mol-1 for the OCl- driven oxidation; and Ea = 26.8 kJ mol-1, ∆H‡ = 29.3 kJ mol-1 and ∆S‡ = -204.6 J K-1 mol-1 for the HOCl facilitated reaction. The governing rate law and probable reaction mechanism were elucidated and validated by simulation. The three main oxidation products were 4-(4-ethoxyphenylamino)benzoic acid, 3-[(ethyl-hydroxyamino)methyl]benzene sulfonic acid and 6'-chloro-5'-hydroxy-bicyclohexylidene-2,5,2'-triene-4,4'-dione.