Abstract:
Algal infestation in water bodies causes the release of soluble organic compounds that impact
negatively on the taste and odour of the water. With increasing pollution in water bodies and
increasing nutrient loading from agricultural activities, most water reservoirs in South Africa
and around the world have become affected by this problem. In this study, an advanced
oxidation process (AOP), namely, photocatalysis was evaluated for its potential to degrade
aromatic compounds; and taste and odour causing bi-cyclic compounds originating from
algae. Semiconductor photocatalysis is an environmentally friendly technology requiring no
chemical inputs which is capable of completely mineralising organic pollutants to CO2 and
H2O thereby eliminating production of unwanted by-products. Although processes involved
in the photo-degradation have been reported for a wide range of pollutants, the degradative
pathway in this process has not been fully established. In this study, compounds including
phenol, 2-chlorophenol, 4-chlorophenol and nitrophenol were successfully eliminated from
simulated wastewater. Degradation of geosmin at an environmentally significant initial
concentration of 220 ng/L to levels below the lowest detectable concentration was achieved
with an optimum catalyst concentration of 60 mg/L at a rate of 14.78 ng/L/min. Higher
catalysts loading above 60 mg/L resulted in a decrease in degradation rates. An increase in
initial geosmin concentration resulted in a decrease in rates. Ionic species commonly found in surface waters (HCO3
-, and SO4
2-) significantly reduced the
efficiency of geosmin degradation. Degradation of geosmin produced acyclic intermediates
from ring fission tentatively identified as 3,5-dimethylhex-1-ene, 2,4-dimethylpentan-3-one,
2-methylethylpropanoate and 2-heptanal.
The results obtained indicate that the degradation of organic pollutants in aqueous solution is
as a result of synergic action from hydroxyl radicals, positive holes and direct photolysis by
UV radiation, though the predominant pathway of degradation is via hydroxyl radicals in
solution. Major aromatic intermediates of phenol degradation include catechol, resorcinol
and hydroquinone produced in the order catechol > resorcinol > hydroquinone. All three are
produced within 2 minutes of photocatalytic reaction of phenol and remain in solution until
all phenol is degraded in aerated systems. Production of resorcinol in non-aerated systems is
transient, further supporting the hydroxyl radical dominant reaction pathway.
