Trace-level metals and natural organic matter interactions: |oxidative/adsorptive removal pathways

Abstract

The destructive removal of humic acids in natural waters has gained increased attention, because these substances are known to be major precursors of carcinogenic disinfection byproducts which are produced during chlorination. Since the conventional treatment methods are expensive and complicated, heterogeneous photocatalytic degradation processes using Ti02 have been applied as an alternative treatment for the destruction of humic acid. This study was conducted to investigate the effects of chromium and manganese ions on the photocatalytic degradation of humic acid (10 mg L-1 ) in aqueous medium at neutral pH conditions (pH=6.7). Bench scale experiments were carried out using titanium dioxide (0.25 mg mL-1 ) as the photocatalyst and Black Light Fluorescent Lamp as the irradiation source. For comparison purposes, the photocatalytic degradation of humic acid was performed both in the presence and absence of metal ions. The degradation was followed by pseudo first order and Langmuir-Hinshelwood kinetics in terms ofColor436, Color4oo, UV280 and UV 254. The presence of chromium and manganese ions resulted in decreased removal rates on the photocatalytic degradation of humic acid compared to the baseline rate of humic acid alone. Pseudo first order reaction rate constants for Color436 and UV254 were found to be 3.86x10-2 min-1 and 2.73x10-2 min-1 in the absence of metal ions. Color436 removal rate constant decreased to 3.13x10-2 min-1 in the presence of 0.10 mg L-1 chromium ion and in the presence of 0.05 mg L-I manganese ion, a value of 3.37xlO-2 min-1 was attained. On the other hand, a similar trend was observed for UV254, the removal rate constant in the presence of chromium was calculated to be the lowest with a value of 2.06x10-2 min-1 whereas, in the presence of manganese it was found to be 2.31xlO-2 min-I. The decrease in the photocatalytic oxidation rate might be explained by the formation of a weak complex between humic acid and the metal ion which might have blocked the active sites on the Ti02 surface. Complimentary adsorption experiments were also performed to evaluate the effect of adsorption intensity on the photocatalytic degradation rates. The results indicated that no significant difference was observed with the adsorption capacity (KF) values, but compared to the other two, the humic acid + manganese system exhibited the highest values.