as pH decreased from 5.0 to 4.0, which should be associated with
acid catalysis under increased [H+] condition [30]. kapp experienced
two-orders of magnitude increase as pH increased from 5.0 to 9.0,
which was very similar to the results reported by Jiang et al. [15].
The removal rates of antibiotics, dyes and textiles by permanganate
in aqueous solution also increased with increasing pH from
5.0 to 9.0 [29,31]. However, the reaction of permanganate with
BPA slowed down at pH over 10.0.
A comparison of the pH-dependent second-order rate constants
of BPA oxidation by permanganate, chlorine and ozone was made
in Fig. 3. At near neutral pH, permanganate exhibited comparable
reactivity with chlorine but much lower reactivity than ozone.
The interesting phenomenon that kapp reached their maximum at
BPA’s first pKa (9.2) in all the three oxidative processes and then
decreased, similar to the behavior of chlorophenols oxidation by
permanganate [32]. But the mechanism about this phenomenon
was still unclear and further studies are necessary. The traditional
explanation that the increase of reaction rate with increasing pH
was caused by the stronger electron-donating strength of the hydroxyl
groups after their deprotonation, seemed questionable
based on our result. For example, the rate of BPA oxidation by permanganate
slowed down as pH increased from 10.0 to 11.0
although the fraction of BPA2 increased, which may be due to
the unfavorable repulsion between permanganate anions and
BPA2 anions.