Diarsenic trioxide

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Not a guideline study, but well reported and conducted study

Qualifier:

equivalent or similar to guideline

Guideline:

other: APHA (1989): Standard methods for the examination of water and wastewater, 17th ed. American Public Health Association

Principles of method if other than guideline:

The study investigated the effect of arsenic (As(III)) and As(V)) on the respiration rate of activated heterotrophic sludge microorganisms.

GLP compliance:

not specified

Specific details on test material used for the study:

Additionally, analytical grade arsenic pentoxide (As2O5), purchased from Merck, was tested.

Analytical monitoring:

no

Vehicle:

no

Test organisms (species):

activated sludge, domestic

Details on inoculum:

A sequencing batch reactor (SBR) was used to simulate the activated sludge process and to provide biomass for the toxicity tests. The SBR was operated in a 24-h fill and draw cycle, each cycle consisted of four stages: FILL (10 min), REACT (22.5 h with aeration), SETTLE (1 h) and DECANT (20 min). Activated sludge from a municipal wastewater treatment plant was used to seed the reactor. Synthetic wastewater with a known COD concentration was used as a feed and phosphate buffer was added to maintain a constant pH. When biomass was acclimatized to the feed and steady state had been attained, activated sludge of known
concentration of suspended solids was mixed in a 300 mL BOD bottle with the appropriate amount of feed to yield a food-to-microorganisms (F/M) ratio of 0.1 mg COD/ mg MLVSS, and the test compound at various concentrations.

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

10 d

Test temperature:

20 ± 1 °C

pH:

7.0 ± 0.2

Nominal and measured concentrations:

Not reported

Details on test conditions:

TEST SYSTEM
- Test vessel: 300 ml BOD bottle
- Aeration: yes
- No. of vessels per concentration (replicates): 3
- No. of vessels per control (replicates): 3
- Sludge concentration (weight of dry solids per volume): not reported
- Nutrients provided for bacteria: Synthetic wastewater (ratio of 0.1 mg COD/mg MLVSS)
- Nitrification inhibitor used: Thiourea (at a final concentration of 10 mg/L)

OTHER TEST CONDITIONS
- Adjustment of pH: phosphate buffer was added to maintain a stable pH

EFFECT PARAMETERS MEASURED:
- Respirometric measurements of specific oxygen uptake rate (sOUR) using an oxygen probe

Reference substance (positive control):

no

Key result

Duration:

2.5 d

Dose descriptor:

EC50

Effect conc.:

6.1 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: lowest reported EC50 for As (III)

Duration:

5 d

Dose descriptor:

EC50

Effect conc.:

7.3 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: for As (III)

Duration:

10 d

Dose descriptor:

EC50

Effect conc.:

10.6 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: for As (III)

Duration:

2.5 d

Dose descriptor:

EC50

Effect conc.:

81 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: lowest reported EC50 for As(V)

Duration:

5 d

Dose descriptor:

EC50

Effect conc.:

97 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: for As(V)

Duration:

10 d

Dose descriptor:

EC50

Effect conc.:

198 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Remarks on result:

other: for As(V)

Validity criteria fulfilled:

not specified

Remarks:

Not assessable (control OUR not reported; no reference toxicant)

Conclusions:

In an activated sludge respiration inhibition test, the lowest EC50 for respiration inhibition was reported for 2.5-d old sludge at 6.1 mg As/L (nominal, As(III).

Description of key information

In an activated sludge respiration inhibition test, the lowest EC50 for respiration inhibition was reported for 2.5-d old sludge at 6.1 mg As/L (nominal, As(III).

Key value for chemical safety assessment

EC50 for microorganisms:

6.1 mg/L