Pyrocatechol

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Remarks:

This study, using the batch equilibrium method, is very well described. Contrary to the precedent data assessing the adsorption potential to the soil organic phase, this study investigates the adsorption of catechol to mineral surfaces. Therefore, it gives interesting insights on the binding behaviour of catechol to mineral soils.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

Principles of method if other than guideline:

The OECD 106 method is adapted to investigate the adsorption potential in mineral soils.

GLP compliance:

not specified

Type of method:

batch equilibrium method

Media:

soil

Test temperature:

21 +/- 1.5°C

Analytical monitoring:

yes

Details on sampling:

No data

Details on matrix:

- Sorbents: Sorption experiment was conducted with the clay mineral Kaolinite [Al2Si2O5(OH)4]. Kaolinite was available as China Clay Supreme from the English Clays Lovering Pochin & Co Ltd (St. Austell/ Cornwall, UK) and was well-crystallized weathering product from granitic bedrocks.

- Pre-treatments: A pre-treatment was applied to raw kaolinite to obtain homoionic kaolinite, that was subsequently air-dried at 50 °C, and whose the cation-exchange capacity was determined to be 25 - 30 mequiv/kg.

Details on test conditions:

- Experimental design: Batch sorption experiments were conducted in 1.8-mL borosilicate glass vials with aluminium foil liners and spetum screw caps. Aqueous solutions of catechol, whose concentrations ranged from 0.5 to 300 µM, were spiked to dry sorbent (kaolinite) in a background electrolyte, and the suspension was then sonified for 30 sec. The suspension was shaken continuously until thermal and sorption equilibrium was attained. The equilibrium time was 30-60 min, and the temperature was maintained at 21 +/- 1.5°C. Phase-separation was achieved by temperature-controlled centrifugation at 120000 rpm for 1 min. The pH was around 8 (1.5 pH units below the pKa of catechol wich is 9.3). Blank samples, containing spike solutions of catechol but no sorbents, were processed in the same way as the suspension samples and were used as external standards in the HPLC analysis.

- Parameters followed: Sorbed concentrations were calculated from the difference between initial and equilibrium solution-phase concentrations. Individual sorption isotherm points were determined from three to five sample and blank replicates.

Duration:

60 min

Initial conc. measured:

other: No data

Duration:

0 min

Computational methods:

Standard deviations for the sorbed concentration as well as for the distribution coefficient Kd were calculated based on the error propagation method.

Type:

Kd

Value:

25

Temp.:

21 °C

Remarks on result:

other: Kd value was expressed in L/kg

Details on results (Batch equilibrium method):

When the mineral sorbent is Cs+-kaolinite, catechol presents a Kd value of 25 L/kg, under the following test conditions: Temperature = 21°C; I (= Ionic strength) = 0.1M CsCl; pH ~ 8 (1.5 pH units below the catechol's pKa which is 9.3). The sorption capacity of catechol was only ~250 µmol/kg.

Validity criteria fulfilled:

not specified

Conclusions:

Based on the Kd value of 25 L/kg, catechol is not considered as presenting a strong binding behaviour to mineral soil as kaolinite.

Reference 2

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

The study is well described and meets the general OECD Guideline 106 requirements, except that one soil type was used.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

GLP compliance:

not specified

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

no

Test temperature:

20°C

Analytical monitoring:

yes

Details on sampling:

- Concentrations: Samples were collected in each concentration treatment.
- Sampling interval and method: Samples were collected at the end of the 24-h equilibration period. They were centrifuged at 20 000 x g for 20 minutes in a Sorvall RC2-B refrigerated centrifuge set at 20°C. Portions of the aqueous phase was then filtered through small glass wool plugs held in 9-in. Pasteur pipets. Samples were collected in capped glass vials.
- Sample storage before analysis: Samples were analyzed immediately after sampling.

Details on matrix:

COLLECTION AND STORAGE
- Geographic location: Brockston clay loam soil from the Michigan State University Soils Research Farm, east Lansing, Michigan.
- Sampling depth (cm): 15
- Soil preparation: air-dried, and ground to pass through an 18-mesh (1 mm) screen.
No further data.

PROPERTIES
- Soil taxonomic classification: A typic Argiaquoll, fine, loamy, mixed, mesic.
- pH: 5.7
- Organic matter (%): 5.10
- CEC (meq/100 g): 22.22
No further data

Details on test conditions:

In order to obtain adsorption isotherms, the following experimental procedure was applied. Batch equilibrations were set up inside an anaerobic glove box (90% N2, 10% H2) using a 1:10 soil:solution ratio and 5 initial concentrations of catechol: 5, 10, 20, 30, and 50 ppm. Triplicate samples were measured for each initial concentrations. Aqueous solutions (100 mL) were prepared in 0.0025 M CaCl2 that had been deoxygenated by purging for 10 min with purified N2. Oxygen was excluded in an attempt to eliminate aerobic biodegradation and to prohibit oxidative coupling. For each isotherm, 2.5-g soil samples were weighed into 50-mL Oak Ridge-type polypropylene centrifuge tubes and transferred into an anaerobic glove box. 25-mL aliquots of the catechol solution were added to triplicate soil samples, and a 25-mL aliquot was saved for use as an analytical standard. The centrifuge tubes were subsequently capped, removed from the glove box and shaken for 24 h at 20 +/- 1°C. Preliminary experiment demonstrated that adsorption equilibrium was reached within 24 h and that no detectable amount was adsorbed on the polypropylene centrifuge tubes.

Duration:

24 h

Initial conc. measured:

other: no data

pH:

5.7

Duration:

0 h

Type:

Koc

Value:

118.44

Type:

log Koc

Value:

2.07

Adsorption and desorption constants:

Freundlich isotherm constants for adsorption of catechol:
- K = 3.18
- 1/n = 0.36
- R2 = 0.998

Details on results (Batch equilibrium method):

The equilibrium adsorption was obtained by the Freundlich adsorption equation: x/m = K C exp(1/n) 
where: x/m: amount adsorbed (µmoles/g soil),
C: equilibrium concentrations (µmoles/mL),
K and n: constants, which are for catechol: K = 3.18; 1/n = 0.36

The relatively strong adsorption of catechol was likely due to the formation of H-bonds involving both -OH groups.

The Koc measured in the experiment (118.44) was compared to the Koc predicted by the linear relationship developped by Chiou et al. (1979), Means et al. (1980) and Karichkoff (1981), which correlates Koc values with water solubilities for hydrophobic organics. This equation is the following:    
Predicted log Koc = -0.686 log S + 4.273          where: S = Solubility (mg/L)                       
Predicted log Koc = -0.686 log 451000 + 4.273
Predicted log Koc = 0.394
Predicted Koc = 2.479                              

Adsorption of catechol was much greater than predicted for hydrophobic sorption; this was likely due to the formation of two H-bonds involving both -OH groups.

Validity criteria fulfilled:

not specified

Conclusions:

Based on a Koc measured value of 118.44 and the associated Log Koc of 2.07, catechol is not considered as presenting a strong binding behaviour to particles of a clay loam soil.

Executive summary:

The adsorption potential of catechol was studied according to the batch equilibrium method (OECD guideline 106), except that only one soil type was used. It was a clay loam soil containing 5.10% of organic matter and presenting a pH of 5.7 and a cation exchange capacity of 22.22 meq/100 g. A 1:10 soil:solution ratio was used together with five catechol concentrations (5, 10, 20, 30 and 50 ppm). Based on preliminary experiment, an equilibration period of 24 hours was applied. after 24 hours, the samples were collected and analyzed using HPLC. Measurements concluded to a Koc of 118.44 and a Log Koc of 2.07; implying that catechol is not considered as presenting a strong binding behaviour to particles of a clay loam soil.

Description of key information

Catechol is not considered as presenting a strong binding behaviour to organic or mineral soils.

Key value for chemical safety assessment

Koc at 20 °C:

118.44

Additional information

Two studies are available (Boyd, 1982; Haderlein & Schwarzenbach, 1993). The first of them, scored as reliability 2 according to Klimisch, was flagged as a key study. It was performed accordingly to OECD guideline 106, except that one soil type was used. A Koc value of 118.44 and an associated Log Koc of 2.07 were reported; implying that catechol is not considered as presenting a strong binding behaviour to soil organic particles. The second study, also scored as Klimisch 2, investigated the adsorption potential of catechol to mineral surfaces. A Kd value of 25 L/kg was reported; supporting a low potential for adsorption in mineral soils also. This study was selected as a supporting study.