UV induced degradation of herbicide methyl viologen: Kinetics and mechanism and effect of ionic media on degradation rates

Abstract

A treatment technology which involves the utilization of UV light for the degradation of the herbicide, methyl viologen, (MV²⁺), in aqueous solutions is developed. Kinetics and mechanism of photodegradation process is also investigated. The photolysis of methyl viologen was found to be a first order process, however, the observed first order rate constant decreased from 0.0908 to 0.0141 min^‐1 when starting methyl viologen concentration was changed from 3.0 mg/L to 18.0 mg/L. It is proposed that ion‐pair formation between the species ^∗MV²⁺, counter ion X^‐, and MV²⁺ exits, and is responsible for the observed decrease in rate at higher methyl viologen concentrations. The effect of the ionic strength upon the rate of degradation was also investigation, and the are constant was found to decrease further at higher ionic strength. The photodecomposition of methyl viologen appears to be a homolytic process leading to the formation of methyl pyridinium radical which then undergo photolysis at a much faster rate producing environmentally safe by‐products, present studies suggests that while developing new. treatment technologies for the destruction of charged species, the effect of medium upon degradation rates may be important factors and should not be ignored.

Key words:

• UV photolysis
• methyl viologen
• kinetics and mechanism