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Environmental fate & pathways

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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); the hydrolysis reaction is the transformation of MG into its Carbinol derivative.


Furthermore MG degrades due to photolysis process with a half-life of 30 hours under natural sunlight conditions, with 8 hours of radiation/day (Perez et al, 2007). During this rapid photolytic degradation of MG, it has been shown that a large number of transformation products (Pérez et al 2007) 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 (Perez et al, 2007), 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.

Under standard test conditions no readily biodegradation was observed, but testing for inherent biodegradability showed 82% degradation presumably after 54 days in the Zahn-Wellens test.


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 decolourization 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 decolourization rate of MG by Cosmarium microalgae increased with an increased initial concentration of MG, an observation shared with the decolourization rate of MG by Kocuria rosea as shown by Parshetti et al, 2006. However, Ayed et al, 2009 reported a decrease in decolourization 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-1(mg of cells)-1, (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; two studies report by Bayer, 1998, provides a BCF estimated from logPow of >36 - <91 for MG hydrochloride and states that no bioaccumulation is expected. This result is consistent with the proposal of the Committee for Risk Assessment RAC of no considering Malachite Green (MG) as a non bioaccumulable substance based on a logKow < 3.

Not with standing after waterborne exposure of channel catfish to MG, MG and its metabolites can be traced in all fish tissues, with highest residues in adipose tissue and lowest residues in plasma. MG was rapidly and extensively metabolized to its reduced form, LG, which was slowly eliminated from fish tissues (Plakas et al. 1995). Bilandzic et al, 2010 studied residue levels of MG in carp and rainbow trout after treatment with MG in Croatian fish farms, using an in-house enzyme linked immunoassay (ELISA) validated to the criteria of Commission Decision 2002/657/EC with an LOD of 0.31 µg/kg. The highest concentration of MG residue was determined in rainbow trout at a concentration of 1.07 µg/kg. Schuetze et al, 2008, determined residue levels of MG and LG 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 LG were detected with total concentrations up to 0.765 µg/ kg fresh weight in the tissues of 25 out of 45 eels caught from different lakes, a river and a canal. In all cases, the occurrence of the residues could directly be linked to the presence of discharges by municipal STPs into the receiving surface waters. While no BAF/BCF values could be determined in these studies, it is established that the bioaccumulation of MG and its metabolite LG in fish is possible, also due to municipal releases i.e. by wash off from clothes or paper towels coloured with MG. Elimination from fish tissue appears to be slowest for LG. A study report by Bayer, 1998 provides a BCF value of <36-<91 for MG hydrochloride and states that no bioaccumulation is expected.


Exposure of the soil compartment is considered unlikely since environmental exposure only occurs through industrial waste water. Therefore, biodegradation in soil has not been considered to be relevant.

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 ionic 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).