The variation of pH and conductivity during the treatment is
shown in Fig. 12. In the first 10 min the solution pH values
increased for treatment in both air and oxygen, while the
conductivities dropped. After 10 min the solution pH values
dropped for treatment in air and oxygen, and the conductivities
increased. The pH values of solutions treated in oxygen
were lower than for those treated in air, but the difference in
the conductivities of solutions between air and oxygen
treatments was small. The variation of pH values differed
from that of Tichonovas et al. (2013) who also used tap water;
because of the variation in the first 10 min, the current results
differed from those in most of the studies (Benetoli et al., 2012;
Zhang et al., 2012; Dojčinović et al., 2011). However, He et al.
(2009) obtained similar results when treating Rhodamine B
using a photocatalytic method. The increase of pH values in
first the 10 min took place because the solution of R. aconiti
was acidic. When the parent Aconitum alkaloids and other
organic compounds that were acidic were decomposed, the
pH values rose. Then, some organic acids and even nitric acid
(products from the decomposition of nitro-organic compounds)
were produced after further degradation. Therefore,
pH values dropped after 10 min treatment.
The NO3 − concentration of solutions after 60 min treatment
in oxygen and air (discharge frequency 200 Hz) was 82.44 and
25.74 mg/L, respectively. The pH values of tap water before
and after the plasma treatment changed little in oxygen
plasma, while they decreased by about 0.5 pH units in air
plasma. So the generation of organic acids and nitric acid
(products from the decomposition of nitro-organic compounds)
was the reason that pH values changed in oxygen
plasma; the generation of organic acids and nitric acid
(products from nitrogen discharge and the decomposition of nitro-organic compounds) was the reason that pH values
changed in air plasma. The variation of conductivity followed
the changes in the concentrations of ions. The degradation
by-products during the treatment led to variation in the ion
concentrations.