Silver

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Substance considered to fall within the scope of the read-across 'Silver metal: Justification of a read-across approach for environmental information requirements' (document attached in IUCLID section 13).

Reason / purpose for cross-reference:

read-across source

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

0.65 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: total NO3 production

Remarks on result:

other: Bordeaux soil, unleached, 95% CL = na-1.0

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

30 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: total NO3 production

Remarks on result:

other: Inman Valley soil, unleached, 95% CL = 14-44

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

26 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: total NO3 production

Remarks on result:

other: Charleston soil, unleached, 95% CL = 23-28

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

371 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: total NO3 production

Remarks on result:

other: Millicent soil, unleached, 95% CL = 322-609

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

61 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: total NO3 production

Remarks on result:

other: Bakalava soil, unleached, 95% CL = 52-72

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

85 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: total NO3 production

Remarks on result:

other: Port Kenny soil, unleached, 95% CL = 78-92

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

1.2 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: potential nitrification rate

Remarks on result:

other: Bordeaux soil, unleached, 95% CL = na-1.5

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

26 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: potential nitrification rate

Remarks on result:

other: Inman Valley soil, unleached, 95% CL = 23-29

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

14 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: potential nitrification rate

Remarks on result:

other: Charleston soil, unleached, 95% CL = 10-19

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

488 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: potential nitrification rate

Remarks on result:

other: Millicent soil, unleached, 95% CL = 369-613

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

36 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: potential nitrification rate

Remarks on result:

other: Bakalava soil, unleached, 95% CL = 32-41

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

38 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

other: potential nitrification rate

Remarks on result:

other: Port Kenny soil, unleached, 95% CL = 37-40

Details on results:

The control samples for all the soils showed sufficient production of NO3 over the 28 days of the test with the exception of the Kingaroy soil. The Kingaroy soil produced only approximately 1 to 2 mg NO3/kg over the 28 days of the test. Due to this low NO3 production in the controls and low and highly variable results in the remaining Ag rates, no further data analysis could be completed in this soil and no ECx values could be determined.

Results with reference substance (positive control):

The Ag concentration in the digested CRM was in good agreement with the certified Ag concentration and averaged 1.24 mg/kg with a relative standard deviation (RSD) of 7%.

Reported statistics and error estimates:

The data from the test endpoints were fitted to dose response models to determine the concentration that produced a 10% and 50% reduction relative to the controls (EC10 and EC50 respectively) using GraphPab Prism®. In cases where there was no significant increase (p > 0.05) in the measured response at low Ag concentrations, a standard dose log-logistic model was used to fit the data (Equation 1) and derive ECx values. For dose response curves that showed a significant increase (p ≤ 0.05) in the response at low Ag concentrations, a non-linear model that accounted for hormesis was fitted to the data (Equation 2) (Brain and Cousens, 1989).
y=c+ (d-c)/(1+(x/e)^b ) (1)
y=c+(d-c +fx)/(1+(x/e)^b ) (2)

The EC10 and EC50 values were then determined in each case through interpolation from the fitted curve at a 10% and 50% reduction from the fitted d values (i.e. fitted response in the control).

The EC10 and EC50 values were used to determine if significant relationships could be developed with soil properties (pH(CaCl2), OC, CEC and clay). This was conducted using stepwise multiple linear regression (MLR) analysis in GenStat® (15th Edition). Prior to analysis, the EC10 and EC50 values were transformed (square root or log10) to normalise the distribution. The transformed data were tested for normality using the Shapiro-Wilk test. The final relationship was deemed significant if the regression showed a p-value ≤ 0.05 and each of the properties was significant in the model (p ≤ 0.05).

Table1: Results from analyses of the selected study soils for pH, organic carbon, cation exchange capacity, particle size distribution and total silver concentration.

Soil	pH (CaCl2)	pH (water)	Organic carbon (%)	Cation exchange capacity (cmol^+/kg)	Particle size distribution (%)			Total Silver (mg/kg)
					clay	silt	Sand
Houthalen	3.6	4.9	1.5	5.3	1.4	1.7	93	< 0.04
Bordeaux	4.6	5.6	1.9	6.4	2.5	1.6	93	< 0.04
Inman Valley	5.0	6.0	5.3	25	42	22	26	< 0.04
Charleston	5.1	6.6	6.9	12	14	12	63	< 0.04
Kingaroy	5.5	6.1	0.9	13	60	17	19	< 0.04
Millicent	6.6	6.9	12	42	19	5.2	48	< 0.04
Balaklava	7.1	8.5	1.9	27	30	20	47	< 0.04
Port Kenny*	8.0	8.8	1.8	13	12	4.2	21	< 0.04

* note that particle size distribution for Port Kenny is equal to 37.2% due to the high concentration of CaCO₃that is present in this soil (60%)

 

Conclusions:

The EC10 values for 28-day NO3 production ranged from 0.65 to 371 mg Ag/kg and for PNR were between 1.2 and 488 mg Ag/kg in the unleached treatmen. The toxicity of Ag to the soil nitrification process was primarily controlled by soil pH and OC.

Executive summary:

The toxicity of silver to soil microorganisms was investigated using a soil nitrogen (N) transformation test (OECD guideline 216, 2000) for six study soils. Two test endpoints were calculated from the results: total NO3 production and potential nitrification rate (PNR). The 28-day EC10 values for NO3 production ranged from 0.65 (Bordeaux) to 371 mg Ag/kg (Millicent) and for PNR were between 1.2 (Bordeaux) and 488 mg Ag/kg (Millicent) in unleached soils. The toxicity of silver to the soil nitrification process was primarily controlled by soil pH and OC.

Reference 2

Endpoint:

toxicity to soil microorganisms

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Substance considered to fall within the scope of the read-across 'Silver metal: Justification of a read-across approach for environmental information requirements' (document attached in IUCLID section 13).

Reason / purpose for cross-reference:

read-across source

Duration:

28 d

Dose descriptor:

NOEC

Effect conc.:

0.13 mg/kg soil dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

element

Basis for effect:

nitrate formation rate

Duration:

28 d

Dose descriptor:

EC10

Effect conc.:

0.3 mg/kg soil dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

element

Basis for effect:

nitrate formation rate

Remarks on result:

other: 95% confidence 0.16-0.41 mg/kg dw

Details on results:

Nitrate concentrations increased with test material dose, as silver nitrate was used. To determine the difference in nitrate-N concentration between day 28 and 0, average soil nitrate-N concentration measured on day 0 was subtracted from the individual soil nitrate-N concentration measured on day 28.

Results with reference substance (positive control):

Not applicable

Reported statistics and error estimates:

Hypothesis testing of the NOEC was determined using Toxstat version 3.5.

Validity criteria fulfilled:

yes

Conclusions:

The 28 day NOEC was 0.13 mg Ag/kg dw and the EC10 was 0.30 mg Ag/kg dw.

Executive summary:

The chronic toxicity of silver nitrate to indiginous soil microorganisms was tested in an OECD 216 test. The test was conducted as a static exposure with a single soil type. Six test concentrations and a control were included, and the results are expressed based on the mean measured total silver concentrations at the start and end of the test. Alfalfa was added to half the replicates as an organic substrate. The rate of nitrogen transformation was studied for 28 days. The 28 day NOEC was 0.13 mg Ag/kg dw and the EC10 was 0.30 mg Ag/kg dw.

Description of key information

Read across from ionic silver

Plus supporting data from a soil nitrogen (N) transformation test comparing the effects of the smallest nanosilver form registered under REACH previously (‘Nano 8.1’) and ionic silver (silver nitrate) in 3 natural soils, demonstrating nanosilver is equally toxic than ionic silver in 1 soil and less toxic than ionic silver in 2 soils (based on EC10 and EC50 values)

Plus supporting published data from 1 study included in the REACH dossier as Endpoint Study Record, reporting a NOEC for nanosilver and silver ions in loam soil of >3 mg/L

Additional information

Summary of available data for uncoated and coated nanosilver

Two reliable studies investigating the effects of nanosilver on soil microorganisms are available (Calder et al. 2012, and Smolders and Willaert, 2017)).

Calder et al. (2012) evaluated the toxicity of 10 nm spherical silver nanoparticles (1 and 3 mg/L) to Pseudomonas chlororaphis O6 (a beneficial soil bacterium) in sand and soil matrices (loam soil). In sand, both concentrations of nanosilver resulted in a loss of bacterial viability whereas in loam soil, no cell death was observed. The addition of clays (30% v/v kaolinite or bentonite) to sand did not protect the bacterium when challenged with Ag NPs. However, viability of the bacterium was maintained when sand was mixed with soil pore water or, to a lesser extent, humic acid. Despite being considered a reliable study the experimental design employed should be considered to represent rather artificial conditions that may not reflect the behaviour of nanosilver materials in conventional soils or the response of natural microbial assemblages. Specifically, this relates to the fact that the effects of nanosilver in soil matrices were tested in aqueous suspension and that only a single microbial species was present. The study reports a NOEC for nanosilver and silver ions in loam soil of >3 mg/L (note that conventionally the results of terrestrial ecotoxicity tests are expressed as units of soil mass e. g. mg/kg (dry weight).

 

The effects of nanosilver and silver nitrate to soil microorganisms were compared by Smolders and Willaert (2017), using a soil nitrogen (N) transformation test (OECD guideline 216, 2000). The toxicity of nanosilver and silver nitrate were evaluated in three natural soils (Rots, Poelkapelle and Lufa 2.2), based on two test endpoints: potential nitrification rate (PNR) and substrate induced nitrification (SIN).Soil samples from three different arable soils (Rots, Poelkapelle and Lufa 2.2) were collected from the plough layer (0-20 cm). The soils were selected to have a pH between 4.3 and 7.3, %OC between 1.1 and 6.8% and CEC between 9.7 and 33.9 cmol_c/kg.The nanosilver material used was a powder in aqueous suspension with the following particle size distribution: D25 = 7 nm, D50 = 8 nm, D75 = 9 nm (see section 4.5 of IUCLID), and contained 37% silver. Based on 0-14 day PNR, the EC10 values for nanosilver (as measured total silver) ranged from 3.8 to 29mg Ag/kg. Based on 28 day SIN,the EC10 values for nanosilver (measured total silver) ranged from 35 to 132mg Ag/kg. Corresponding 0-14 day PNR EC10 values for silver nitrate (measured total silver) ranged from 3.8 to 9.1 mg Ag/kg, and 28 day SIN EC10 values ranged from 30 to 45 mg Ag/kg. These results inicate that silver toxicity to soil microorganisms is similar for both ionic silver and nanosilver.