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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Titanium tetrachloride hydrolyzes fast revealing eventually rutile, which is considered immobile in soils. Measured Kd values are obtained from environmental measurement of the total titanium concentration ratio, regardless its compounds. The risk assessment is based on this field observed values. Kp values are influenced by speciation and the speciation behaviour must therefore be accounted for in Kp. 

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
4.61
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
2.36
at the temperature of:
12 °C

Additional information

Due to rapid hydrolysis after contact with pore water the assessment of suspended matter, sediment, and soil mobility is required for the final products, titanium dioxide (CAS 13463-67-7) and hydrogen chloride (CAS 7647-01-0), which is almost dissociated in into to hydronium ions (CAS 12586-59-3) chlorides (CAS 16887-00-6). Thus no experimental data on adsorption / desorption can be experimentally obtained for the parent substance and no experimental results are available for the adsorption/desorption of titanium dioxide.

In order to assess the impact of the solid transformation products, data from measurements of the sum of titanium were used. The available information is derived from monitoring of water and corresponding sediment or suspended matter. No soil data exist in the recent literature (until 2009). A log Kd sed of 4.61 L/kg dw (solids-water in sediment, Kp = 40’738, Roychoudhury & Starke 2006, organic matter mean 3.9% with a range of 0.9-15.5%), a world major rivers average log Kd of 5.75 L/kg dw (Gaillardet et al 2003) and a log Kd susp of 2.36 L/kg dw (solids-water in suspended matter, Kp = 230, Veselý et al 2001) were reported. The Kd reflects the total titanium concentration ratio in equilibrium for all titanium compounds under environmental conditions, where 12 °C are assumed. It is likely that the data reflect the environmental speciation behaviour as a significant part of the total environmental titanium may be derived from titanium dioxide from human production and/or ore materials.

In the risk assessment and based on field observed values, the partition coefficients log Kp susp 2.36, log Kp sed 4.61 are used.

It is considered inappropriate to use the Kow and Koc concept for inorganic compounds. Nonetheless and in order to give quantification the following calculations are provided for instance. 2% organic matter and 1.16% organic carbon contents in soil are considered as these weight percentages are common default values as used e.g. in the U.S. EPA GENEEC Version 2.0. This means that 58% weight of the organic matter represents organic carbon or that the organic matter mass represents 1.724 times the one of organic carbon (% organic carbon = % organic matter / 1.724). For the calculations the information of a recent review (Pribyl 2010) is applied. The author states that the factor 1.724 may be too low for most soils and recommends replacement by 2 with a range of 1.4 to 2.5 instead, which suggests that about 50% (weight) of the organic matter represents carbon. The adsorption coefficient per organic carbon (Koc) is by definition calculated as a function of the organic carbon and organic matter contents of the soil using: Koc = Kp·100/% organic carbon. In result the log Koc assuming 2% organic matter and the mean factor of 2 were 6.60 for solids-water in sediment. Thus the Koc [L/kg dw] ranges in the order of 2’818’382. Lower values would result in case of higher percentages of organic matter. Assuming 15.5% organic matter and using the lower range for the factor, i.e. 1.4, would result in a Koc of 371’535 (log Koc 5.57).

Concerning data interpretation of the measured Kd (and the resulting Koc) in sediments, all available data uniformly are pointing in one direction and clearly indicate that the analogue materials and the target chemical has to be considered as “immobile” (Kd or Koc > 5000) or “non-mobile” (Kd or Koc > 4000) in soils or sediments according to the scales of McCall et al (1981) or Hollis (1991) respectively. Nonetheless the sorption to suspended matter is less strong.

Due to their high water solubility and hydrophilicity hydronium ions and chlorides (Bohn 1979) belong to the “very high” or “very mobile” mobility class according to the scales of McCall et al (1981) or Hollis (1991) respectively.

 

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