Methanaminium, N-[4-[[4-(dimethylamino)phenyl]phenylmethylene]-2,5-cyclohexadien-1-ylidene]-N-methyl-, ethanedioate

Endpoint summary

Description of key information

Not readily biodegradable

Hydrolysis, dark conditions, ambient temperature t1/2= 145h

Phototransformation, t1/2= 30h

Additional information

The primary source of environmental pollution of Malachite Green (MG) is represented by industrial waste water and secondary by release during washing of dyed textile, thus the fate and pathways of MG in surface water is of primary concern.

In the aquatic environment MG degrades due to hydrolysis with a measured approximate half-life of 145 hours at 25 °C (45 % decrease in 145 hours, Perez et al, 2007).

Based on the structure of MG, the triarylmethyl cation is expected to be stabilized by the conjugation that delocalizes the positive charge and a good leaving group. Therefore, the hydrolysis mechanism should be S_N1 mechanism and the reaction rate is unrelated to the concentration of the nucleophile H₂O. The pH varied from 8.98 to 7.87 in the reaction process. Results from LC–MS conformed to the fact that MG, MG leucocarbinol and Leucomalachite Green can transform naturally to each other in water matrix (Yong et al, 2015).

 

MG degrades due to photolysis process with a half-life of 30 hours under natural sunlight conditions, with 8 hours of radiation/day. During this rapid photolytic degradation of MG, it has been shown that a large number of transformation products were generated, and the reaction mixture after degradation of MG still showed toxicity to bacteria which was attributed to some of the reaction products of MG. No in depth identification of the transformation products was performed in this study, although a possible toxic transformation product, 4-(dimethylamine)benzophenone, was mentioned both in this study. The kinetics of 4-(dimethylamine)benzophenone (D20) indicated that photodegradation of D20 followed a similar photodegradation rate as MG (Pérez et al 2007).

The primary transformation product Leucomalachite Green (LG), has been tested separately for its effects on Vibrio fischeri and did not exhibit 50 % effect at the maximum concentration tested (EC50 > 39.9 mg/l) i.e. roughly 300 times less toxic than MG (Hernando et al, 2007). The transformation of LG by Cunninghamella elegans exhibited a similar pathway as for MG, i.e. identical patterns of metabolites (mono-, di-, tri-, and tetra-desmethyl leucomalachite green) were observed as studied by Cha et al, 2001. This indicates that the toxicity of LG and its transformation products is covered by the toxicity of the parent compound MG. For further discussion on the toxicity of MG and its transformation products see section 6.

 

Most studies conducted on the biodegradation of MG have focused on the decolourisation of the test solution, which has been obtained using for instance bacteria, fungi or algae. The study by Daneshvar et al, 2006 shows that Cosmarium microalgae species have the capability to decolorize MG. Daneshvar et al further studied the effect of initial concentration, pH and temperature on the decolorization efficiency of Cosmarium species and determined that MG was decolorized 80 - 90 % under environmental pH conditions (i.e. pH 5-8). Furthermore they determined that, with a standard European temperature of 12 °C, Cosmarium species decolorized MG for 60 % and that increased decolourization (max > 95 %) occurred with increasing temperature (max 45 °C). The decolorization rate of MG by Cosmarium microalgae increased with an increased initial concentration of MG, an observation shared with the decolorization rate of MG by Kocuria rosea as shown by Parshetti et al, 2006. However, Ayed et al, 2009 reported a decrease in decolorization by the soil bacillus S. paucimobilis with increasing initial MG concentration and Jadhav et al, 2006 determined that the yeast Saccharomyces cerevisiae MTCC 463 decolourized MG by biosorption and biodegradation and about 85% decolorization in distilled water (< 7 h), and 95.5 % in 5 % glucose medium (< 4 h) was observed, under aerobic conditions at room temperature. The fungus Cunninghamella elegans(ATCC 36112) is able to degrade MG with a first order rate constant of 0.029 mmol/h (mg of cells), (Cha et al, 2001). In this study the biodegradation pathway has been elucidated for MG and its primary transformation product LG. Both MG and LG follow the same reduction pathway to form N-demethylated and N-oxidized metabolites including primary and secondary arylamines. Isolates of another fungal species Fusarium solari (Martius) Saccardo from dye containing effluents have been shown to degrade MG for 96 % after 2 days of shaking (Hazrat, 2010).

 

No reliable BCF was found. After waterborne exposure, MG and its metabolites can be traced in all studied fish tissues.

MG is rapidly and extensively metabolized to its reduced form, Leucomalachite Green (LG): concentrations of LG declined more slowly than those of MG in catfish muscle and plasma.Tissue levels of MG and LG increased dramatically with pH of the exposure water, suggesting that the MG carbinol form is more readily absorbed than the chromatic one.

Residue levels of MG and LG were detected in wild eels caught in catchment areas after municipal sewage treatment plants (STP) in Berlin, Germany. LG was the dominating residue with LG:MG ratios varying between 5:1 and 7:1. MG and its metabolite. The occurrence of the residues could directly be linked to the presence of discharges by municipal STPs into the receiving surface waters. 

Despite no BAF/BCF values could be determined on the basis of the available studies, it is established that the bioaccumulation of MG and its metabolite LG in fish is possible.

 

Exposure of the soil compartment is considered unlikely since environmental exposure only occurs through industrial waste water.

Volatilization from moist soil and water surfaces is not expected to be an important fate process based upon an estimated Henry's Law constant of 9.79 x 10 -9 atm- m^3/mole (Estimated by Epi Suite, 4.1).

However, for a correct assessment of the behaviour of the substance, it is necessary to take into account the transformations which MG undergoes in aqueous solution with a normal pH of the environmental matrices. The MG-carbinol formed by oxidation has a very limited solubility. The solubility loss of the positive charge present in the form of MG from the carbinol (Ginzburg 1953), means that the affinity for the organic material increases significantly favouring thus the deposition of the substance in sediments and in suspended particulate matter in water and aquatic organisms.

 

In case of the possible release of MG in wastewater after production or use as a colouring agent, several methods have been developed recently most of which focus on the sorption of MG on natural materials such as carbonised rice husks and oxihumolite (Rahman et al, 2005 and Janos et al, 2005). In addition methodology for the enhanced biodegradation of MG following the photolysis pathway by the addition of nanoparticles has been published recently (Chen et al. 2006), or degradation by specific bacterial strains have been proposed (Hazrat, 2010).

REFERENCE

Ginzburg O.F. 1953. On the dissociation of the triarycarbinol.Translated from J. Gen. Chem. USSR, 23, (1953)9 9, 1504-1509.