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Environmental fate & pathways

Adsorption / desorption

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Description of key information

For rhodium, the log Kd for suspended matter is 4.18 (Kd is 15100). The log Kd for soil is 0.58 (Kd 3.8).

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
4.18
at the temperature of:
23 °C

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
0.58
at the temperature of:
25 °C

Additional information

Two high quality studies have determined the partitioning of Rh between river water and suspended particulate matter (Cobelo-Garcia et al. 2008, Turner et al. 2006). Cobelo-Garcia et al. (2008) showed variation in partitioning for freshwaters, and Kd values were slightly affected by salinity, with a trend of increasing partitioning with increasing salinity. A mean log Kd of 4.18 (stdev 0.43) was observed for all salinities, a mean log Kd of 4.10 (stdev 0.51) was observed for freshwaters only, a mean log Kd of 4.27 (stdev 0.13) was observed for estuarine waters only, and a single log Kd of 4.57 was determined in seawater (salinity 33 ‰). The supporting study (Turner et al 2006) showed that experiments with Rh were at, or close to, steady state within 96 hours, and exhibited moderate changes in adsorption following chemical reduction or chemical oxidation of the sediment particles, indicating the role of metal(oxy)hydroxides to be more important than organic matter in Rh partitioning. Log Kd values of between 4.2 and 4.9 were derived, depending upon the treatment of the sediment material. The Kd values for Rh determined in the two studies were similar, and the mean log Kd value for all salinities of 4.18 (Kd 15100), taken from Cobelo-Garcia et al. (2008), will be used in the risk assessment.    

Sako et al. (2009) studied the partitioning of rhodium in two soils and one sediment. Although a standard guideline was not followed, the study is well described and meets generally accepted scientific methods. Kd values were reported at eight different metal concentrations, Freundlich adsorption model parameters were also reported as well as rate constants for adsorption and desorption. In order to account for the variability in Rh partitioning between different systems the overall mean (n=48, ± standard deviation) log Kd value will be used in the risk assessment, 0.58 ±0.38 for Rh.