Dichloride titanium oxide

Endpoint summary

Additional information

Target compound titanium oxychloride is an unstable chemical compound which occurs during the process of hydrolysis of parent compound titanium tetrachloride.

1. step: TiCl4 + H2O ↔ TiOCl2 + 2 HCl

2. step: TiOCl2 + H2O ↔ TiO2 + 2 HCl

Summary reaction: TiCl4 + 2 H2O ↔ TiO2 + 4 HCl

Addition of HCl to the reaction medium, or lack of H2O, lead to a partial hydrolysis only, which results in the formation of titanium oxide chlorides. The Raman spectrum of a yellowish TiCl4 solution in aqueous hydrochloric acid show the occurrence of [TiO2Cl4]4- and [TiOCl5]3- species, as TiOCl2 in aqueous hydrochloric acid reveal the same spectrum.

Target substance titanium oxychloride only can get stabilised in the system when hydrochloric acid is being added to the reaction medium, in this way shifting the chemical equilibrium towards the side of titanium oxychloride. This is the form in which the substance titanium oxychloride is being used technically. This form however is not of relevance for environmental fate discussion due to the chemical instability of the target substance. Therefore the discussion shall focus on the final hydrolysis products.

In aqueous medium at around neutral pH, above stated hydrolysis reaction proceeds immediately and completely to form the final hydrolysis products titanium dioxide and hydrochloric acid. Intermediate formation of hydroxides (titanium hydroxide, titanium dihydroxide oxide) is probable, however rapid transformation into the final product titanium dioxide is being assumed.

The eventually resulting titanium dioxide corresponds to the naturally occurring mineral rutile albeit in micro-disperse form. It is considered stable under environmental conditions and will sink to the sediment due to its density. The other hydrolysis product, hydrogen chloride, gets readily dissociated in contact with water into hydronium ions and chlorides.

Studies on direct phototransformation in water are not available but it is assumed the substance and its transformation products are not degraded by direct photolysis due to its spectral properties. It is concluded, therefore, that abiotic processes do not contribute significantly to the depletion of the transformation products in the aquatic environment.

Biodegradation is not relevant as titanium oxychloride and its transformation products are inorganic.

In conclusion the fate of both parent compound titanium oxychloride and target chemical titanium tetrachloride in the aquatic environment is rapid transformation to the inert inorganic compounds titanium oxide and hydrochloric acid.

Unlike parent substance titanium tetrachloride, the atmospheric fate of target substance titanium oxychloride is of no relevance. While titanium tetrachloride is smoking heavily when exposed to the air, there is no relevant evaporation of titanium oxychloride from an aqueous solution. The molecule only is stabilised in aqueous medium in its hydrated form in equilibrium with hydrochloric acid, thus no atmospheric emissions occur. To a certain extent the evaporation of hydrochloric acid still might be observed. Due to its physico-chemical properties hydrogen chloride distributed into the air will enter airborne water droplets and dissociate there into waterbound ions, which enter the surface waters via rain. In result it is assessed that the substance and/or its transformation products will neither stay in the atmosphere nor reach the troposphere in relevant amounts.

In soil, in the same way contact with pore water will initiate rapid hydrolysis into the above mentioned solid and ionised compounds.

A log Kd of 4.556 L/kg dw (solids-water in sediment), which is a median value derived from several literature sources, and an additional log Kd of 2.36 L/kg dw (solids-water in suspended matter) were reported. The Kd reflects the total titanium concentration ratio in equilibrium conditions for all titanium compounds under environmental conditions, where 12 °C are assumed. This indicates that the solid transformation product, titanium dioxide, ends up in sediments rather than in the water phase of natural water bodies. In conclusion the fate of titanium oxychloride in environmental soils and sediments is rapid transformation to inorganic inert compounds.

Bioaccumulation is not likely to occur for both parent compound titanium tetrachloride nor for target compound titanium oxychloride. Based on experimental BCF data after spiking water with titanium dioxide a metal-typical inverse relationship was established between water concentrations and the corresponding BCF (352 to 20 L/kg ww) and in consequence no bioconcentration is expected. Furthermore no accumulation of titanium species in wildlife biota is described in the literature. Accumulation in living organisms likewise is no issue for hydrogen chloride and/or its dissociation products due to its high water solubility and hydrophilicity (OECD SIDS 2002 Hydrogen chloride SIAR).

In conclusion, target compound titanium oxychloride, as parent compound titanium tetrachloride and their transformation products, are rapidly depleted from the air to soils and sediments of surface waters. The final hydrolysis products represent inert inorganic compounds, thus they are not subject to PBT assessment.