1770 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Detection of Hydroxyl Radicals
The change in colour of the methylene blue (MB) dyed test
strips that had been exposed to ultrasound leaching from
a dark blue to an almost white colour (Figure 6 )suggest
the presence of hydroxyl radicals in a sample. According
to literature methylene blue (MB) dye is decolorized by
hydroxyl radical due to the cleavage of the chromophoric
group in the molecule, (Liu, Y et al., 2017). The chromo-
phoric group in MB consists of a thiazin-2-yl ring and two
quaternary ammonium groups, which provide the dye with
its characteristic blue colour.
The IR spectrum of phenols shown below by Figure 7(a)
and Figure 7(b), the O-H stretch appears at roughly
3230 cm–1. The breath is caused by interference of hydro-
gen bonding interactions. As confirmed by these results,
indeed hydroxyl radicals are formed during sonication.
The ·OH has very strong oxidizing property. The redox
potential of ·OH is 2.80 V, which is much higher than Fe
ion and O2 (EFe3+/Fe2+ =0.73V, EO2 =1.23V). This means
that under ultrasonic cavitation, the pyrite surface will be
hydroxylated by hydroxyl radicals primarily. In the case
where oxygen is present during sonication, O2 reacts with
·H to form ·OH, indicating that the presence of dissolved
O2 in water sonolysis gives additional amount of ·OH
hence increasing the rate of pyritic oxidation. After that,
more ·OH or H2O will interact with hydroxyl for further
anodic reaction. The SO42–, S2O32– and H+ released with
electron transfer. Meanwhile, the iron will dissolve into
water in the form of Fe2+. The Fe2+ will further be oxi-
dized to Fe3+ by hydroxyl radicals, (Rooze et al., 2013). The
results of the study also showed the generation of hydroxyl
radicals which were postulated to help with the oxidation
of pyrite and subsequently causing the breakdown of the
sulphide matrix, realizing locked gold and increasing the
gold yield in solution.
The fate of radicals produced during ultra-sonication:
H2O → ·OH +·H (4)
2·H +2O2 → 2·OH +O2 (5)
FeS2 +·OH → SO4 2– +H+ +Fe2+ (6)
FeS2 +·OH → S2O3 2– +H+ +Fe2+ (7)
Figure 5. (a) SEM images of pyrite crystal before ultrasonic pre-treatment (b) SEM images of pyrite crystal after ultrasonic
pre-treatment
(a) (b)
Figure 6. Methylene blue dye test results showing
discoloration of the methylene blue dye when ultrasound
pre-treated water is applied and no discoloration when
distilled water is applied
Detection of Hydroxyl Radicals
The change in colour of the methylene blue (MB) dyed test
strips that had been exposed to ultrasound leaching from
a dark blue to an almost white colour (Figure 6 )suggest
the presence of hydroxyl radicals in a sample. According
to literature methylene blue (MB) dye is decolorized by
hydroxyl radical due to the cleavage of the chromophoric
group in the molecule, (Liu, Y et al., 2017). The chromo-
phoric group in MB consists of a thiazin-2-yl ring and two
quaternary ammonium groups, which provide the dye with
its characteristic blue colour.
The IR spectrum of phenols shown below by Figure 7(a)
and Figure 7(b), the O-H stretch appears at roughly
3230 cm–1. The breath is caused by interference of hydro-
gen bonding interactions. As confirmed by these results,
indeed hydroxyl radicals are formed during sonication.
The ·OH has very strong oxidizing property. The redox
potential of ·OH is 2.80 V, which is much higher than Fe
ion and O2 (EFe3+/Fe2+ =0.73V, EO2 =1.23V). This means
that under ultrasonic cavitation, the pyrite surface will be
hydroxylated by hydroxyl radicals primarily. In the case
where oxygen is present during sonication, O2 reacts with
·H to form ·OH, indicating that the presence of dissolved
O2 in water sonolysis gives additional amount of ·OH
hence increasing the rate of pyritic oxidation. After that,
more ·OH or H2O will interact with hydroxyl for further
anodic reaction. The SO42–, S2O32– and H+ released with
electron transfer. Meanwhile, the iron will dissolve into
water in the form of Fe2+. The Fe2+ will further be oxi-
dized to Fe3+ by hydroxyl radicals, (Rooze et al., 2013). The
results of the study also showed the generation of hydroxyl
radicals which were postulated to help with the oxidation
of pyrite and subsequently causing the breakdown of the
sulphide matrix, realizing locked gold and increasing the
gold yield in solution.
The fate of radicals produced during ultra-sonication:
H2O → ·OH +·H (4)
2·H +2O2 → 2·OH +O2 (5)
FeS2 +·OH → SO4 2– +H+ +Fe2+ (6)
FeS2 +·OH → S2O3 2– +H+ +Fe2+ (7)
Figure 5. (a) SEM images of pyrite crystal before ultrasonic pre-treatment (b) SEM images of pyrite crystal after ultrasonic
pre-treatment
(a) (b)
Figure 6. Methylene blue dye test results showing
discoloration of the methylene blue dye when ultrasound
pre-treated water is applied and no discoloration when
distilled water is applied