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Adsorption / desorption

Adsorption study was conducted for evaluating the adsorption capacity of test chemical (Einat Magal et. al., 2008). Adsorption study was performed according to batch experiments in sediments at a temperature of 25 deg.C.Two types of sediments were used for the sorption experiments:First, a group of pure minerals include clays (bentonite,kaolinite), quartz, calcite and dolomite. Two different grain sizes of quartz, calcite and dolomite were used.Second, a group representing natural sediments. Samples of sediments were collected from three boreholes drilled along the Dead Sea shore. Test chemical conc. used for the study were 0, 50 ,100, 150, 200, 250 mg/l, respectively.A sediment sample and dyed solution were placed in a 50 ml tube. The tubes were continuously rotated 360 deg at 8 rpm by Labquake tube rotator (Thermolyne, USA) in the dark at room temperature (17–25 deg C) for 12–14 h in order to achieve equilibrium concentration. In the first set of experiments, 5 g of sediment was placed together with 5 ml of solution, except for the clays (bentonite and kaolinite) for which, due to difficulty of separating liquids from the sediment, a higher volume of solution was needed (20 ml for bentonite and 10 ml for kaolinite).The solution was removed from the sediments by centrifuging for 20 min at a rate of 3000 rpm and was analyzed by fluorescence spectrophotometry (Cary Eclipse Fluorescence Spectrophotometer, Varian[1], Palo Alto, CA).The concentration of the dyes in the samples was determined separately for each experiment by using calibration curves prepared with solutions of similar salinity and composition to those of the experiment solutions. Blanks were prepared by repeating experimental procedures with solutions without dyes. The fluorescence intensities (FI) of the blanks were analyzed and calculated to determine the ‘‘apparent dye concentration’’ from the FI measurements. It was found that except for Naph, FI of the blank solutions was close to zero. The FI’s in the emission–excitation wavelength similar to that of Naph in the blank solutions were found to be comparable in values to 20–30 ppb. The amount of dye sorption on sediments equals the amount of dye loss after mixing with sediment during the batch experiment. Other dye losses such as dye precipitation are thought to be negligible since in the experiment the dye concentrations were a few orders of magnitude lower than its solubility (a few grams per liter). Dye sorption was calculated as percentage of initial concentration. Percent adsorption of test chemical was 29–53% (0.01 DSW), 38–59% (0.5 DSW) and 64–88% (DSW), except for sample B3 (41%, 83% and 98%, 0.01 DSW, 0.5 DSW and DSW, respectively). On the basis of this, test chemical have low to moderate sorption on sediments and therefore have moderate to slow migration potential to groundwater.

Additional information

Adsorption / desorption

Experimental result of the test chemical and various supporting studies for its structurally similar read across substance were reviewed for the adsorption end point which are summarized as below:

 

In an experimental key study from peer reviewed journal (Einat Magal et. al., 2008), adsorption experiment was conducted for evaluating the adsorption capacity of test chemical. Adsorption study was performed according to batch experiments in sediments at a temperature of 25 deg.C. Two types of sediments were used for the sorption experiments: First, a group of pure minerals include clays (bentonite,kaolinite), quartz, calcite and dolomite. Two different grain sizes of quartz, calcite and dolomite were used. Second, a group representing natural sediments. Samples of sediments were collected from three boreholes drilled along the Dead Sea shore. Test chemical conc. used for the study were 0, 50 ,100, 150, 200, 250 mg/l, respectively. A sediment sample and dyed solution were placed in a 50 ml tube. The tubes were continuously rotated 360 deg at 8 rpm by Labquake tube rotator (Thermolyne, USA) in the dark at room temperature (17–25 deg C) for 12–14 h in order to achieve equilibrium concentration. In the first set of experiments, 5 g of sediment was placed together with 5 ml of solution, except for the clays (bentonite and kaolinite) for which, due to difficulty of separating liquids from the sediment, a higher volume of solution was needed (20 ml for bentonite and 10 ml for kaolinite).The solution was removed from the sediments by centrifuging for 20 min at a rate of 3000 rpm and was analyzed by fluorescence spectrophotometry (Cary Eclipse Fluorescence Spectrophotometer, Varian[1], Palo Alto, CA).The concentration of the dyes in the samples was determined separately for each experiment by using calibration curves prepared with solutions of similar salinity and composition to those of the experiment solutions. Blanks were prepared by repeating experimental procedures with solutions without dyes. The fluorescence intensities (FI) of the blanks were analyzed and calculated to determine the ‘‘apparent dye concentration’’ from the FI measurements. It was found that except for Naph, FI of the blank solutions was close to zero. The FI’s in the emission–excitation wavelength similar to that of Naph in the blank solutions were found to be comparable in values to 20–30 ppb. The amount of dye sorption on sediments equals the amount of dye loss after mixing with sediment during the batch experiment. Other dye losses such as dye precipitation are thought to be negligible since in the experiment the dye concentrations were a few orders of magnitude lower than its solubility (a few grams per liter). Dye sorption was calculated as percentage of initial concentration. Percent adsorption of test chemical was 29–53% (0.01 DSW), 38–59% (0.5 DSW) and 64–88% (DSW), except for sample B3 (41%, 83% and 98%, 0.01 DSW, 0.5 DSW and DSW, respectively). On the basis of this, test chemical have low to moderate sorption on sediments and therefore have moderate to slow migration potential to groundwater.

 

In a supporting study from study report (2016), adsorption coefficient Koc in soil and in sewage sludge of test chemical was determined by the Reverse Phase High Performance Liquid Chromatographic method according to OECD Guideline No. 121 for testing of Chemicals. The solutions of the test substance and reference substances were prepared in appropriate solvents. A test item solution was prepared by accurately weighing 50mg of test item and diluted with mobile phase up to 100ml. Thus, the test solution concentration was 500mg/l. The pH of test substance was 7.18. Each of the reference substance and test substance were analysed by HPLC at 210 nm. After equilibration of the HPLC system, Urea was injected first, the reference substances were injected in duplicate, followed by the test chemical solution in duplicate. Reference substances were injected again after test sample, no change in retention time of reference substances was observed. Retention time tR were measured, averaged and the decimal logarithms of the capacity factors k were calculated. The graph was plotted between log Koc versus log k(Annex - 2).The linear regression parameter of the relationship log Koc vs log k were also calculated from the data obtained with calibration samples and therewith, log Koc of the test substance was determined from its measured capacity factor. The reference substances were chosen according to estimated Koc range of the test substance and generalized calibration graph was prepared. The reference substances were 4-chloroaniline, 4-methylaniline, N methyl aniline, 2-Nitrophenol, Nitrobenzene, 4-Nitrobenzamide, N,N-dimethylbenzamide, N-Methylbenzamide, Benzamide, Phenanthrene having Koc value ranging from 1.239 to 4.09. The Log Koc value of test chemical was determined to be 3.024 ± 0.020 at 25°C. This log Koc value indicates that the test chemical has a moderate sorption to soil and sediment and therefore have slow migration potential to ground water.

 

For the test chemical, the adsorption coefficient Koc in soil and in sewage sludge of test chemical was determined by the Reverse Phase High Performance Liquid Chromatographic method according to OECD Guideline No. 121 for testing of Chemicals (Experimental study report, 2016). The solutions of the test substance and reference substances were prepared in appropriate solvents. A test item solution was prepared by accurately weighing 50mg of test item and diluted with mobile phase up to 100ml. Thus, the test solution concentration was 500mg/l. The pH of test substance was 8.10. Each of the reference substance and test substance were analysed by HPLC at 210 nm. After equilibration of the HPLC system, Urea was injected first, the reference substances were injected in duplicate, followed by the test chemical solution in duplicate. Reference substances were injected again after test sample, no change in retention time of reference substances was observed. Retention time tR were measured, averaged and the decimal logarithms of the capacity factors k were calculated. The graph was plotted between log Koc versus log k(Annex - 2).The linear regression parameter of the relationship log Koc vs log k were also calculated from the data obtained with calibration samples and therewith, log Koc of the test substance was determined from its measured capacity factor. The reference substances were chosen according to structural similarity with the test substance and calibration graph was prepared. The reference substances were Phenol, Toluene, Xylene, Ethylbenzene, Naphthalene, Phenenthrene having Koc value ranging from 1.32 to 4.09. The Log Koc value of test chemical was determined to be 3.313 ± 0.007 at 25°C. This log Koc value indicates that the test chemical has a moderate sorption to soil and sediment and therefore have slow migration potential to ground water.

 

On the basis of above overall results for test chemical, it can be concluded that thetest chemicalhas a low to moderate strong sorption to soil and sediment and therefore have moderate to slow migration potential to ground water.