N-butyltin trichloride

Endpoint summary

Description of key information

Environmental stability:

The substance is not readily biodegradable. The key study for biodegradability (Hansveit, 2003), reported 0% degradation of the registered substance after 35 days.

The substance has recently been determined not to exhibit rapid hydrolysis. The key study Kleinmaier and Heitmann (2018), indicated that irreversible hydrolysis occurs on a timeframe of one to six days upon dissolution of MBTCl in neutral water, mildly or strongly acidic aqueous solutions of hydrochloric acid in water. The substance is therefore considered as persistent.

Bioaccumulation:

The bioaccumulation of MBTC was studied in carp by Tsuda in 1988. After an uptake period of 14 days BCFs of 2, 126, 50 and 126 were obtained in muscle, liver, kidney and gall bladder respectively. These BCFs are significantly below 2000 and indicate that the risk of bioaccumulation and subsequent biomagnification in the food chain is low.

The worst case BCF value does not meet the criteria for classification as a bioaccumulative or very bioaccumulative substance as set out by Annex XII of EC Regulation 1907/2008.

Adsorption/desorption:

The study by Berg et al (2001) investigated the reversibility of sorption of n-butyltin trichloride in different sediment samples from different sites. Sorption-desorption batch experiments were conducted using contaminated freshwater harbour sediments and two certified organotin containing marine sediments. To distinguish between the organotins originally present in these sediments and the freshly sorbed organotins, sorption experiments were carried out using perdeuterated standard compounds. Findings from the study indicate that sorption of organotin compounds to sediments is a fast and reversible process involving primarily particulate organic matter constituents as sorbents. The study found that, in general, organotin compounds will readily desorb and, therefore, be more readily bioavailable. The log Kd values that were determined for n-butyltin trichloride ranged from 3.15 to 4.92 L/kg. Experimental results normalised for carbon content in the sediment indicated that log Doc values were in the range of 5.12 to 4.65 which are in the same range as the in situ measured values of 4.61 to 4.92 L/Kg.

The experimentally derived Log Doc value of 4.65 was selected for chemical safety assessment as it is considered the most reliable (due to being obtained in controlled experimental conditions) and results in a worst-case scenario with regards to potential exposure to the substance in the modelling conducted in the CSR.

Until very recently, the substance was considered to exhibit properties, such as rapid hydrolysis, such that waivers were submitted to cover this endpoint as certain studies were considered technically not feasible.  As part of the ongoing CoRAP evaluation additional investigations have been, and are currently being, conducted and studies are still ongoing.  Current evidence suggests that the substance may in fact behave differently than originally thought and as such the results presented in Berg et al. (2001) are considered to accurately reflect the behaviour of the substance with regards to its adsorption/desorption.