[4-[α-[4-(dimethylamino)phenyl]benzylidene]cyclohexa-2,5-dien-1-ylidene]dimethylammonium acetate

Phototransformation in water

Description of key information

Malachite Green degrade due to photolysis; during the photolytic degradation process a large number of transformation products are generated.

Key value for chemical safety assessment

Half-life in water:

30 h

Additional information

Photolysis experiments (Perez et al, 2007) were performed by direct exposure of a solution of Malachite Green (MG) in water to natural sunlight. This resulted in a phototransformation half-life of 30 hours under natural sunlight conditions, with 8 hours of radiation/day. Due to the 8 hours radiation/day light cycle this photolytic half-life is an underestimation, and the result of a faster photolysis rate and a slower hydrolysis rate during non-radiation conditions, given that 45% hydrolysis was observed in the same study after 145 hours. Total degradation of MG in the closed batch system was observed after 210 hours at a temperature of 25°C. During this photolytic/hydrolytic degradation of MG, it has been shown that a large number of transformation products were generated. The kinetics of one of the, possible toxic, transformation products, 4-(dimethylamine)benzophenone (D20) indicated that photodegradation of D20 followed a similar photodegradation rate as MG (Pérez et al 2007).

The photodegradation of MG has been studied both under different pH values and varying amounts of TiO2 (Chen et al. 2006). Under the catalytic influence of TiO2, the photodegradation rate increased substantially, resulting in 99.9% degradation of the sample after 4 hours, indicating that the photodegradation of MG can be augmented substantially. Fischer et al, 2011 studied the influence of various wavelengths on the photodegradation of MG and MG carbinol, and concluded that MG carbinol is photodegraded more easily than MG. MG seemed to be rather resistant to photodegradation which is in contrast with the above mentioned study.