Document ID: EPA-HQ-OPP-2007-1160-0005
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2008-02-20T05:00Z

Ammonium Thiosulfate (ATS)

(NH4)2S2O3

CAS Registry Number   HYPERLINK
"http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783
-18-8&Scope=CASSearch&btnSearch.x=1"  7783-18-8  (Solution)

CAS Registry Number 7664-41-7 (Anhydrous)

PC Code: 080103

		

Thiosulfate		Ammonium Counter Cation

  		

End-Use Product:	Oxalis/Spurge X

EPA Reg.No.		9499-1

Formulation:		Concentrate (55.5% ATS)

Uses:			Herbicide to control spurge and oxalis in dichondra lawns			

Prepared By:		Colleen Flaherty, M.S., Biologist

			James A. Hetrick, Ph.D., Senior Scientist

			Silvia Carlota Termes, Ph.D., Chemist

Reviewed by:		Stephanie Syslo, M.S., Environmental Scientist

			Ecological Risk Branch III

			Environmental Fate and Effects Division

Approved by: 	Karen Whitby, Ph.D.	

			Ecological Risk Branch III

			Environmental Fate and Effects Division

Date:			13 September 2007

Table of Contents

  TOC \o "1-5" \h \z \u    HYPERLINK \l "_Toc177444902"  I.	Executive
Summary	  PAGEREF _Toc177444902 \h  3  

  HYPERLINK \l "_Toc177444903"  A.	Nature of the Chemical Stressor	 
PAGEREF _Toc177444903 \h  3  

  HYPERLINK \l "_Toc177444904"  B.	Potential Risks to Non-target
Organisms	  PAGEREF _Toc177444904 \h  3  

  HYPERLINK \l "_Toc177444905"  C.	Environmental Fate and Exposure	 
PAGEREF _Toc177444905 \h  3  

  HYPERLINK \l "_Toc177444906"  D.	Ecological Effects	  PAGEREF
_Toc177444906 \h  4  

  HYPERLINK \l "_Toc177444907"  E.	Uncertainties and Data Gaps	  PAGEREF
_Toc177444907 \h  4  

  HYPERLINK \l "_Toc177444908"  II.	Problem Formulation	  PAGEREF
_Toc177444908 \h  5  

  HYPERLINK \l "_Toc177444909"  A.	Nature of Regulatory Action	  PAGEREF
_Toc177444909 \h  5  

  HYPERLINK \l "_Toc177444910"  B.	Stressor Source and Distribution	 
PAGEREF _Toc177444910 \h  5  

  HYPERLINK \l "_Toc177444911"  1.	Nature of the Chemical Stressor	 
PAGEREF _Toc177444911 \h  5  

  HYPERLINK \l "_Toc177444912"  2.	Identification of the Chemical
Stressor; Physical and Chemical Properties	  PAGEREF _Toc177444912 \h  5
 

  HYPERLINK \l "_Toc177444913"  3.	Overview of Pesticide Usage	  PAGEREF
_Toc177444913 \h  8  

  HYPERLINK \l "_Toc177444914"  C.	Receptors	  PAGEREF _Toc177444914 \h 
8  

  HYPERLINK \l "_Toc177444915"  1.	Aquatic and Terrestrial Effects	 
PAGEREF _Toc177444915 \h  8  

  HYPERLINK \l "_Toc177444916"  D.	Analysis Plan	  PAGEREF _Toc177444916
\h  9  

  HYPERLINK \l "_Toc177444917"  III.	Analysis	  PAGEREF _Toc177444917 \h
 10  

  HYPERLINK \l "_Toc177444918"  A.	Use Characterization	  PAGEREF
_Toc177444918 \h  10  

  HYPERLINK \l "_Toc177444919"  B.	Exposure Characterization	  PAGEREF
_Toc177444919 \h  10  

  HYPERLINK \l "_Toc177444920"  1.	Environmental Fate and Transport	 
PAGEREF _Toc177444920 \h  10  

  HYPERLINK \l "_Toc177444921"  2.	Aquatic Exposure Assessment	  PAGEREF
_Toc177444921 \h  13  

  HYPERLINK \l "_Toc177444922"  C.	Effects Characterization	  PAGEREF
_Toc177444922 \h  16  

  HYPERLINK \l "_Toc177444923"  IV.	Risk Characterization	  PAGEREF
_Toc177444923 \h  17  

  HYPERLINK \l "_Toc177444924"  A.	Aquatic Risk	  PAGEREF _Toc177444924
\h  17  

  HYPERLINK \l "_Toc177444925"  B.	Terrestrial Risk	  PAGEREF
_Toc177444925 \h  19  

  HYPERLINK \l "_Toc177444926"  C.	Federally Threatened and Endangered
(Listed) Species Concerns	  PAGEREF _Toc177444926 \h  19  

  HYPERLINK \l "_Toc177444927"  D.  	Discussion of Uncertainties,
Limitations, and Data Gaps	  PAGEREF _Toc177444927 \h  19  

  HYPERLINK \l "_Toc177444928"  1. Exposure For All Taxa	  PAGEREF
_Toc177444928 \h  19  

  HYPERLINK \l "_Toc177444929"  a. Maximum Use Scenario	  PAGEREF
_Toc177444929 \h  19  

  HYPERLINK \l "_Toc177444930"  b. Additive and/or Synergistic Effects	 
PAGEREF _Toc177444930 \h  19  

  HYPERLINK \l "_Toc177444931"  2. 	Exposure For Aquatic Species	 
PAGEREF _Toc177444931 \h  20  

  HYPERLINK \l "_Toc177444932"  3.	Data Gaps	  PAGEREF _Toc177444932 \h 
20  

  HYPERLINK \l "_Toc177444933"  V.	Cited Literature	  PAGEREF
_Toc177444933 \h  21  

 

Appendix A. Sulfur chemical species (other than thiosulfate) of
environmental significance

Appendix B. Aquatic Exposure Model Estimates (GENEEC Version 2.0)  

Executive Summary

Nature of the Chemical Stressor

Ammonium thiosulfate (CAS No.   HYPERLINK
"http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783
-18-8&Scope=CASSearch&btnSearch.x=1"  7783-18-8 ), an inorganic
compound, is the chemical stressor considered in this assessment.
Ammonium thiosulfate dissociates completely in water. The dissociation
products are the thiosulfate anion and its ammonium counter cation. This
is the first step in the behavior of thiosulfate in the environment.
Both thiosulfate and ammonium are considered in this risk assessment.

Thiosulfate is a metastable, moderately reducing oxyanion of sulfur.
Chemical reactions (redox reactions) of thiosulfate generate chemical
species of sulfur that differ in their oxidation state.  The ammonium
counter cation in ammonium thiosulfate is a source of “ammoniacal
nitrogen.” Its major chemical reaction in the environment is oxidation
to nitrate nitrogen, which is known as nitrification. Ammonium also
exists in equilibrium with ammonia gas.

Ammonium thiosulfate is registered to kill oxalis and spurge in
dichondra lawns. The only end-use product is “Oxalis/Spurge X,
Concentrate Foliar Spray” (USEPA Reg. No. 9499-1; Approval date of
last label 9/30/2003). This product is a concentrate foliar spray
containing 55.0% ammonium thiosulfate. Ammonium thiosulfate is also used
as a fertilizer.

Potential Risks to Non-target Organisms

This ecological risk assessment considers the limited use of ammonium
thiosulfate as an herbicide on dichondra lawns in residential/homeowner
settings in California. This assessment is qualitative in nature; that
is, the risk quotient approach is not utilized.  The nature of ammonium
thiosulfate and its predicted behavior in the environment make it
difficult to provide meaningful quantitative exposure estimates for
aquatic and terrestrial systems.  Since ammonium thiosulfate is being
used as an herbicide, there is a possibility that non-target plants may
be at risk; however, given that it is only used in residential/homeowner
settings, the potential for adverse effects to non-target plants seems
highly unlikely.  Based on this qualitative assessment for the
pesticidal use of ammonium thiosulfate, risk to non-target aquatic and
terrestrial organisms (including Federally-listed species) is presumed
to be negligible.  

Environmental Fate and Exposure

The thiosulfate anion is only stable in neutral or alkaline media. It is
unstable in acid media. Thiosulfate acid cannot be generated by
acidification in aqueous solutions of its salts because it decomposes to
sulfuric acid and a mixture of elemental sulfur, hydrogen sulfide,
polysulfide, and sulfur dioxide. In aqueous media, thiosulfate
irreversibly disproportionates to sulfide and sulfate. 

The chemistry of sulfur in soils (and in water) is complex because of
its many oxidation states of sulfur, which not only involves oxysulfur
species, but also species containing only sulfur (e.g., elemental
sulfur, sulfide). The predominance of a given species is dependent on pH
and redox potential of the media, as well as by the type of bacteria
present in soils.  The terminal, thermodynamically-favored reaction
product of thiosulfate is sulfate, except in highly reduced soils.  As
an anion, sulfate is expected to be mobile in soils. Sulfate is
ubiquitous and is present in most natural waters, where it is considered
a permanent solute. Sources of sulfate in natural water are dissolution
of naturally occurring sulfate salts, runoff of oxidized sulfide ores,
acid rain deposition, industrial waste, and fertilizers. No aquatic
exposure modeling was performed for sulfate, and risk was discussed
qualitatively.

For the ammonium counter cation, its major chemical reaction in the
environment is oxidation to nitrate. Ammonium is expected to be in
equilibrium with ammonia gas in the soil air voids. The volatilization
of ammonia is dependent on temperature and pH of the media and increase
with increasing temperature and pH.  As a cation, ammonium can adsorb to
soil colloid or organic matter through electrostatic cation exchange.
For ammonia, a screening-level aquatic exposure assessment using the
GENEEC model (Version 2.0) was conducted to estimate surface water
concentrations of ammonium from use of ammonium thiosulfate as a
pesticide.

Ecological Effects

There are no acceptable registrant-submitted ecotoxicity data available
for fish, aquatic invertebrates, aquatic plants, birds, terrestrial
invertebrates, or terrestrial plants for consideration in this risk
assessment for ammonium thiosulfate.  The aquatic assessment relies on
the USEPA ambient water quality criteria for ammonia (1999 Update).  The
terrestrial assessment is qualitative in nature, but it does consider
mammalian toxicity data provided by the Health Effects Division. 
Ammonium thiosulfate is practically non-toxic to mammals on an acute
oral basis.

Uncertainties and Data Gaps

No acceptable environmental fate or ecotoxicity guideline studies are
available for ammonium thiosulfate.  However, given the nature of the
chemical and its predicted behavior in the environment, this was not an
impediment to the risk assessment. At this time, the EFED is not
requesting any environmental fate or ecotoxicity studies to be submitted
for ammonium thiosulfate.

II.	Problem Formulation

The purpose of this problem formulation is to provide the foundation for
the ecological risk assessment being conducted for ammonium thiosulfate
(ATS).  As such, it articulates the purpose and objectives of the risk
assessment, evaluates the nature of the problem, and provides a plan for
analyzing the data and characterizing the risk (EPA, 1998).  

A.	Nature of Regulatory Action

This ecological risk assessment is in support of the Reregistration
Eligibility Decision (RED) for ammonium thiosulfate use as an herbicide.
There is one end-use product considered in this assessment,
Oxalis/Spurge (55.0% active ingredient ammonium thiosulfate), which is
used to kill oxalis and spurge in dichondra lawns in California.

B.	Stressor Source and Distribution

	

1.	Nature of the Chemical Stressor

Ammonium thiosulfate (CAS No.   HYPERLINK
"http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783
-18-8&Scope=CASSearch&btnSearch.x=1"  7783-18-8 ) is the chemical
stressor considered in this assessment. Ammonium thiosulfate is an
inorganic compound. Both ammonium and thiosulfate are considered in the
assessment.

Ammonium thiosulfate is not listed as a High Production Volume (HPV)
chemical or in the Toxic Release Inventory (TRI), but it is listed in
the Toxic Substance Control Act (TSCA) (8b) Inventory of Chemical
Substances (Substance of Commerce).

2.	Identification of the Chemical Stressor; Physical and Chemical
Properties

The first step in the environmental fate of ammonium thiosulfate will be
dissociation into ammonium and thiosulfate.  Thiosulfate is one of the
oxyanions of sulfur. The chemical reactions of thiosulfate in the
environment are mostly controlled by the redox potential of the media,
as well as pH. The mode of action of ammonium thiosulfate as an
herbicide to control oxalis and spurge is presumably via fertilizer burn
(salt injury).  

Identification and information on ammonium thiosulfate/thiosulfate anion
is summarized in Table II-1. Figure II.1 presents the ammonium counter
cation and its equilibrium with ammonia. Sulfur compounds have an
extensive and often ambiguous nomenclature, and the chemical names for
sulfur chemical species relevant to thiosulfate are summarized in
Appendix A.

Table II.1   Chemical Identification and General Information on
Ammonium Thiosulfate (ATS) and the Thiosulfate anion

Type of Information	Information

Common Name(s) 	Ammonium Thiosulfate; ATS

Chemical Name- Synonyms

2− anion is referred as thiosulfites or hyposulfites) 

Thiosulfuric acid, diammonium salt

Diammonium Thiosulfate

Ammonium thiosulfate solution (60% or less)

As per US EPA Substance Registry System

CAS Registry Number

EPA PC Code No.	  HYPERLINK
"http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783
-18-8&Scope=CASSearch&btnSearch.x=1"  7783-18-8  (Solution)

7664-41-7 (Anhydrous)

080103

Empirical Formula	(NH4)2S2O3

  

Note the S-S bond (disulfide linkage)

  

Note: The purple symbol stands for Na+ or K+ cations. See below for
ammonium

Crystal Structure:

S. T. TENG, H. FUESS AND J. W. BATS. 1979. Acta Cryst. B35, 1682-1684

The structure consists of

NH 4 and S2O3 tetrahedra interconnected by hydrogen bonds. All but two H
atoms were located by different methods. One of the NH4 groups is
possibly disordered at room temperature.

Oxidation state of sulfur in thiosulfate 

Smiles Notation	The two different S atoms in thiosulfate have charge
densities corresponding to V and -I oxidation states

[NH4+].[S-]S(=O)(=O)[O-].[NH4+]

  

↔ [NH4+];  Log Ko=11.04, where Ko is the equilibrium constant
expressed in terms of activity.

The ammonium ion has a tetrahedral structure. The formal oxidation state
of nitrogen in ammonia and ammonium cation is –III. Ammonia-based
fertilizers serve as a source of nitrogen.

Physical and Chemical Properties

The physical and chemical properties of ammonium thiosulfate are shown
in Table II.2.

Table II.2 Physical and Chemical Properties of Ammonium Thiosulfate	

Property	Value

Melting Point

Boiling Point	Decomposes at 160 C

Vapor Pressure

Henry’s Law Constant	Reported as 0 mmHg at 25C

Not applicable

Solubility in Water

(Thiosulfate is not stable under acidic pHs)	103.3 g/100 ml water @ 100
deg. C.

Complete dissociation

Solubility in Non-aqueous Solvents	Easily soluble in cold water, hot
water.

Slightly soluble in acetone.

Insoluble in diethyl ether.

Insoluble in alcohol

Unstable in acid pH

Behavior of Ammonium Thiosulfate, Thiosulfate and Ammonium in the
Environment

Ammonium thiosulfate dissociates completely in water. The dissociation
products are the thiosulfate anion and the ammonium cation. This is the
first step in the behavior of thiosulfate in the environment.

Thiosulfate is a metastable, moderately reducing oxyanion of sulfur.
Chemical reactions (redox reactions) of thiosulfate generate chemical
species of sulfur that differ in their oxidation state.

The main types of chemical reactions of thiosulfate relevant in the
environment are:

Disproportionation (chemical and microbially mediated)

Chemical and microbially mediated redox reactions

Complexation with metal ions 

Bimolecular nucleophilic substitution reactions (SN2 mechanisms);
dihalogenation

The ammonium counter cation in ammonium thiosulfate is a source of
“ammoniacal nitrogen”. Its major chemical reaction in the
environment is oxidation to nitrate nitrogen, that is N(-III)→ N(V),
which is known as nitrification.	

3.	Overview of Pesticide Usage

Ammonium thiosulfate (ATS) is registered to kill oxalis and spurge in
dichondra lawns. The only end-use product is “Oxalis/Spurge X,
Concentrate Foliar Spray” (USEPA Reg. No. 9499-1; Approval date of
last label 9/30/2003). This product is a concentrate foliar spray
containing 55.0% ATS.

The major use of ammonium thiosulfate is not as a pesticide, but rather
as a fertilizer (soil nutrient and amendment). In 2005, over 700 million
pounds (350,580 short tons) of ammonium thiosulfate was used as a
fertilizer (USDA/NASS).  Compared to its use as a pesticide, less than
6500 pounds of ammonium thiosulfate is used annually.

In minerals processing, ATS is increasingly used as a leaching agent for
gold and silver extraction from auriferous ores. It is increasingly used
as an alternative for conventional cyanide leaching.

	C.	Receptors

		1.	Aquatic and Terrestrial Effects

The receptor is the biological entity that is exposed to the stressor
(EPA, 1998).  In general, the receptors considered in ecological risk
assessments for pesticides are aquatic organisms, such as fish
(surrogate for aquatic-phase amphibians), invertebrates, and plants
(non-vascular and vascular), and terrestrial organisms, such as birds
(surrogate for reptiles and terrestrial-phase amphibians), mammals,
invertebrates, and plants.

D.	Analysis Plan

Available Data

No environmental fate guideline studies (Subdivision N) are available
for ammonium thiosulfate; thus, the environmental fate and exposure
assessment will be based on open literature data.  EPISuite Version 3.2,
a tool commonly used in the ecological risk assessment process when
environmental fate data are unavailable, cannot be used to estimate the
environmental fate and transport of ammonium thiosulfate because it is
not an appropriate model for inorganic chemicals.  

There are no acceptable registrant-submitted ecotoxicity data available
for fish, aquatic invertebrates, aquatic plants, birds, terrestrial
invertebrates, or terrestrial plants for consideration in this risk
assessment for ammonium thiosulfate.  The aquatic assessment will rely
on the U.S.E.P.A. ambient water quality criteria for ammonia (1999
Update).  The terrestrial assessment will rely on the mammalian toxicity
data provided by the Health Effects Division.

Aquatic Risk

For sulfate, the most likely sulfur species in water, the Agency has
established a Secondary Maximum Contaminant Level (SMCL) of 250 mg/L. It
is highly unlikely that the aquatic exposures of sulfate would approach
that level as a result of its limited use as an herbicide to control
spurge and oxalis in dichondra lawns.  Therefore, no aquatic exposure
modeling will be performed for sulfate.  

For ammonium, aquatic exposure estimates will be generated using the
Tier I GENEEC model (GENeric Estimated Exposure Concentrations, Version
2.0). Assumptions will be made to select the environmental fate input
parameters. Further, since the label does not specify an application
rate in terms of pounds of ATS per acre, assumptions must be made to
estimate the application rates in terms of pounds per acre. Given that
the product is applied in residential/homeowner settings, assumptions
will be made to estimate the percent of treated area, which will be used
to correct the model-generated aquatic exposure estimates. It is
recognized that these assumptions will result in highly conservative
aquatic exposure estimates.  

Given the multiple assumptions and the complexity of the sulfur and
nitrogen systems (multiple chemical species in different oxidation
states), the aquatic exposure assessment carries a very high degree of
uncertainty.  Despite this uncertainty, the model estimates will be
compared to the U.S.E.P.A. ambient water quality criteria for ammonia to
determine if there is a potential risk to aquatic animals as a result of
the pesticidal use of ammonium thiosulfate. 

Terrestrial Risk

Given the expected behavior of ammonium thiosulfate in the environment,
it is inappropriate to use the Tier 1 T-REX model (Version 1.3.1) to
estimate terrestrial dietary exposures. The potential risk of ammonium
thiosulfate (when used as a pesticide) to terrestrial organisms will be
discussed qualitatively.

III.	Analysis

A.	Use Characterization

Ammonium thiosulfate (CAS No.   HYPERLINK
"http://www.sigmaaldrich.com/catalog/search/SearchResultsPage?Query=7783
-18-8&Scope=CASSearch&btnSearch.x=1"  7783-18-8 ) is the chemical
stressor considered in this assessment. The end-use product,
Oxalis/Spurge X (55.0% active ingredient ammonium thiosulfate), is used
to kill oxalis and spurge in dichondra lawns in California.

Dichondra (Dichondra repens; kidney weed dichondra) is a low growing,
creeping perennial that spreads by underground runners. The leaves are
kidney-shaped and dark green in color. The overall appearance is that of
a flat cover. It has a neat rich velvety appearance throughout the year
and needs no mowing. 

According to the product label, the dichondra lawn is to be watered the
day before applying the ATS product, but the foliage must be dry at the
time of application. The range of temperature at which Oxalis/Spurge X
is to be applied is between 72 and 80° F (22 to 26.7° C). Above 80°
F., it could cause severe, temporary damage to the dichondra foliage.
The product can be used on young dichondra.

The label also indicates that the product can be used in bladegrass
lawns. Bladegrass can tolerate temperatures up to 95° F (35° C), but
it is not to be used until a minimum of three mowings. The product
should not be applied immediately after mowing. Weeds should be killed
before they go to seed. 

The product is claimed to work best in sunlight.

The label recommends mixing 4 oz/gallon of water per 100 feet of area to
be treated. It is recommended the addition of a “good
spreader-sticker” and use of pressure type tank sprayer because
hose-end garden sprayers and water cans are ineffective. Leaves must be
thoroughly saturated. According to the label, application can be
repeated every 10 days or two weeks, but the number of applications per
year (maximum total application rate) is not specified in the label.

B.	Exposure Characterization

1.	Environmental Fate and Transport

The first step in the environmental fate of ammonium thiosulfate,
(NH4)2S2O3, will be dissociation [i.e., (NH4)2S2O3 ↔ 2 NH4+ + S2O3 2-
].  Because (NH4)2S2O3 is a salt, dissociation will be controlled by the
water solubility of (NH4)2S2O3 (i.e., 103.3 g/100 mls of H2O @ 100oC or
1,033,000 mg/L; 1,033 g/L). Concentrations of (NH4)2S2O3 below the water
solubility will cause complete dissociation to 2NH4+ and S2O32-.  As a
first approximation, (NH4)2S2O3 concentrations in soil solution based on
the maximum application rate and different depths of soil incorporation
are not expected to exceed the water solubility of (NH4)2S2O3 (Table
II.2).  Therefore, (NH4)2S2O3 is not expected to exist as the (NH4)2S2O3
salt (i.e., as a discrete, non-dissociated entity) when applied because
it will completely dissociate to 2 NH4+ and S2O32– ions.

The thiosulfate anion is only stable in neutral or alkaline media. It is
unstable in acid media. Thiosulfate acid cannot be generated by
acidification in aqueous solutions of its salts because it decomposes to
sulfuric acid and a mixture of elemental sulfur, hydrogen sulfide,
polysulfide, and sulfur dioxide. In aqueous media, thiosulfate
irreversibly disproportionates to sulfide and sulfate. The rate of
disproportionation is pH-dependent, with the rate of disproportionation
being directly proportional to the hydrogen ion concentration (i.e, with
decreasing pH). Sulfite is a transient intermediate in this reaction
Another reaction intermediate of thiosulfate is tetrathionate (S4O62-),
which undergoes further reactions to sulfate, the thermodynamically
stable sulfur species.

Chemical and microbial reactions are involved in the transformation of
thiosulfate (and of other sulfur species) in the environment. Both
sulfide and thiosulfate are the most abundant reduced species of sulfur
in the environment and both are converted to sulfate in the oxidative
half of the sulfur cycle. 

Redox and disproportionation reactions of thiosulfate (chemical;
bacterial) are involved in the sulfur cycle in the environment. For
example, disproportionation has been identified as an important pathway
in bacterial energy metabolism in anoxic marine sediments. In the
environment, sulfur (elemental sulfur, S°) is cycled between 

S(VI)-Sulfate and S(-II)-sulfide soluble and insoluble sulfur species,
such as thiosulfate. Because thiosulfate, sulfite, and sulfate are
chemical species relevant to the natural sulfur cycling, it would be
difficult to distinguish between these reactions from those arising from
the addition of thiosulfate as an herbicide. In any case, added
thiosulfate would be incorporated into the natural sulfur cycle, where
the thermodynamically-stable end product is sulfate. 

Among the types of enzymes involved in the oxidation-reduction of
thiosulfate and other sulfur species are rhodanese (thiosulfate
cleaving) and sulfite oxidase (plus electron transfer). Several
Thiobacillus species are known to be involved in the formation or
transformation of thiosulfate.  The enzymatic reaction products are
dependent on the conditions of the media (e.g., presence of absence of
oxygen). An important Thiobacillus species involved in the oxidation of
thiosulfate is Thiobacillus thiooxidans. 

Besides enzymatic reactions, thiosulfate can coordinate with several
metals (i.e., thiosulfate is a ligand; thiosulfate complexes). These
metal complexes serve as catalysts to the oxidation of thiosulfate
(e.g., Co(III)-thiosulfate complexes). In leaching of gold from
auriferous ores using ammonium thiosulfate, Cu(II)-amine complexes can
catalyze the undesirable conversion of thiosulfate (oxidation) to
polythionates instead of the desirable of thiosulfate with gold and
prevent the desirable complexation of thiosulfate with gold. The
photoreduction of thiosulfate in semiconductor dispersions (TiO2) reduce
the efficiency of thiosulfate conversion to sulfide and sulfite/sulfate.
However, the extent of complexation and surface reactions of thiosulfate
in the environment is not well documented. 

In summary, the chemistry of sulfur in soils (and in water) is complex
because of its many oxidation states, which not only involve oxysulfur
species, but also species containing only sulfur. The predominance of a
given species is dependent on pH and redox potential of the media, as
well as by the type of bacteria present in soils (see the “Ammonium
Thiosulfate Behavior in Soils” section). The terminal,
thermodynamically favored reaction product of thiosulfate is sulfate,
except in highly reduced soils. Sulfate would form insoluble compounds
by precipitation with cations such as Ba2+ and Ca2+ if the
concentrations of sulfate and cation exceed the solubility product.
Other sulfur species formed from thiosulfate, such as sulfide and
polysulfide(s), may be transient and unstable products under most
environmental conditions. 

 → CH3S2O3 + Br-

Although the chemical reactions of thiosulfate are independent of the
nature of the counter cation, the use of ATS as a sulfur and nitrogen
fertilizer/soil amendment appears to present several advantages. Both
thiosulfate and its intermediate tetrathionate have been found to
inhibit nitrification (oxidation reaction) of ammonia and hydrolysis of
urea, thus increasing nitrogen efficiency. 

For the ammonium counter cation, its major chemical reaction in the
environment is oxidation to nitrate. Ammonium is expected to be in
equilibrium with ammonia gas in the soil air voids. The volatilization
of ammonia is dependent on temperature and pH of the media and increases
with increasing temperature and pH.  As a cation, ammonium can adsorb to
soil colloid or organic matter through electrostatic cation exchange.

2.	Aquatic Exposure Assessment

For sulfate, the most likely sulfur species in water, the Agency has
established a Secondary Maximum Contaminant Level (non-enforceable),
SMCL of 250 mg/L for drinking water.  Ambient surface water
concentrations of sulfate range from 0.1 to 12,000 mg/L (USGS NWIS,
March 21, 2005). The maximum sulfate concentration was detected at sites
in Montana, Wyoming, and Arizona.  These sites were generally associated
with mining activities.  It is very unlikely the concentration of
sulfate from ammonium thiosulfate would approach the maximum
concentration sulfate in surface water or the SMCL as a result of its
limited use as an herbicide to control spurge and oxalis in dichondra
lawns.  No aquatic exposure modeling will be performed for sulfate, and
risk will be discussed qualitatively.

For ammonia, a screening-level aquatic exposure assessment using the
GENEEC model (Version 2.0) was conducted to estimate surface water
concentrations of ammonium from use of ammonium thiosulfate as a
pesticide. Ammonium concentrations in surface water were used to
estimate ammonia concentrations using chemical equilibria. Additionally,
the Henry’s constant was used to estimate the partitioning of ammonia
between water and air.  It is important to note that the GENEEC model is
not designed to address environmental behavior of inorganic compounds.
Therefore, the Tier I modeling required conservative assumptions for
input parameters (Table III.1).  This exposure assessment is
conservative because it assumes 100% of ammonium from ammonium
thiosulfate is available for runoff.  Competing environmental fate
processes for ammonium fixation (sorption on sediment and soil) and
transformation (nitrification, denitrification, etc.) are not considered
in the assessment.   

The Tier 1 aquatic exposure modeling scenario assumes a 10-hectare field
is 100% treated with ammonium thiosulfate at 74.31 lbs/A.  Because the
registered pesticidal use of ammonium thiosulfate is in
residential/homeowner settings, it is highly conservative to assume 100%
of the area is treated.  According to the U.S. Census data, a typical
house (residence) area is 1000 sq-foot (0.023 acre) located within a
0.25 acre plot.  It is then assumed that the area in the plot not
occupied by the house is a dichondra lawn and that all of the lawn is
treated with ATS at the estimated application rate. With these
assumptions, the percent treated area is 91%.  This percent treated area
correction is used as a refinement in the exposure assessment.     

Table III.1. Environmental fate, physical/chemical, and use information
input parameters selected GENEEC model to estimate ammonia
concentrations in surface water resulting from ammonium thiosulfate
application to dichondra lawns.

Input parameter	Value	Source	Comments

Hydrolysis half-life (days)	0	Assumed	pH-dependence of redox reactions
are not taken into account

Photolysis in Water half-life (days)	0	Assumed	Photooxidation reactions
not taken into account

Aerobic Soil Metabolism half-life (days)	0	Assumed	Microbial
nitrification not considered

Anaerobic Aquatic Metabolism half-life (days)	0	Assumed	Microbial
nitrification not considered

Solubility in Water, mg/L	1030	Open literature	This solubility is based
on the solubility of ATS, which is mostly reported as “very
soluble”; “totally soluble”. The value used to run GENEEC is the
reported solubility of ATS at 100°C. The solubility would be lower at
25°C.

Koc	0	Assumed	Ammonium is known to sorb onto clay surfaces. For
screening level purposes, a Koc of zero was assumed

Method of Application	Ground, Broadcast	Assumed from the available label
Spot applications are probably more typical for this product

Frequency of Application	4 times per year	Assumed	The label does not
specify an upper limit of number of applications per year

Interval between applications (days)	10	Shorter interval specified in
the label	N/A

Application Rate, lb Ammonium/A	18.09	Estimated	The application rate for
ammonium was based on an estimate of the application rate of ATS in
terms of lb ATS/A

The GENEEC estimated environmental concentrations (EECs) for the
ammonium cation are summarized in Table III.2. It includes uncorrected
concentrations corrected for percent treated area. The concentrations
appear to remain stable as a result of the environmental fate
assumptions used for model input parameters. The GENEEC model output is
provided in Appendix B.

Table III.2.  Aquatic EECs (in mg/L) for Ammonium Cation in surface
water (Source: Ammonium Thiosulfate; Uncorrected and corrected for
percent of treated area, PTA, 0.91)

↔ [NH4+];

Ko = [NH4+]/ [NH3(g)] [H+], 

where Ko is the equilibrium constant expressed in terms of activity

Assuming the activity coefficient (γ) to be 1 (i.e., ionic strength not
considered), the equilibrium constant is then expressed in terms of
molar concentration. That is, Ko becomes K and Log Ko=11.04 becomes Log
K = 11.04 (Lindsay, 1979):

Log[NH3(g)]= -11.04 + log[NH4+]+ pH

This equation is used to estimate the relative concentrations of
ammonium and ammonia gas in the receiving water body as controlled by
the pH of the water. This equation indicates that theoretically, the
concentration of dissolved ammonia gas increases with increasing pH. 

Table III.3. GENEEC Environmental Exposure Concentration (EECs) of
Ammonia in Surface Water (Corrected for percent of treated area, 0.91)

GENEEC EECS	Peak Ammonium EEC (mg/L)	Peak Ammonia EEC1

(mg N/L)

pH 5	pH 7	pH 9

PCT Corrected	3.90	2.77E-6	2.79E-4	0.027

1Estimated using following equation: log(NH3(g))=-11.04 + log (NH4+)+ pH

These are overall, upper bound concentrations resulting solely from the
use of ammonium thiosulafate on dichondra lawns. These concentrations do
not differentiate between solvated and ammonia gas. Ammonia has a
Henry’s Law Constant of ammonia at 25° C is 3.5 x 10-6 atm-m3/mole
(1.41 x 10-4, unitless). Volatilization of ammonia is dependent on
temperature and pH of the media and increase with increasing temperature
and pH. In addition, these exposure estimates do not take into account
assimilation (uptake by organisms), binding to humic acids and clays in
soils, nitrification in soil/water body, atmospheric oxidation reactions
of volatilized ammonia, or the contribution from ammonia fertilizers. 

3.	Terrestrial Exposure Assessment

Based on basic physical-chemical principles, ammonium thiosulfate will
dissociate to form ammonium (NH4+) and thiosulfate (S2O32-) ions when
used as an herbicide application in a residential setting to control
spurge and oxalis in dichondra lawns.  Dissociation in water is the
first step involved in the environmental fate of ammonium thiosulfate. 

Given the environmental fate of ammonium thiosulfate in terrestrial
systems (see Section III.B.1), it is not appropriate to model
terrestrial dietary residues using the T-REX model.  The potential
terrestrial risks associated with using ammonium thiosulfate as a
pesticide will be discussed qualitatively.

	C.	Effects Characterization

For ammonium thiosulfate, there are no acceptable registrant-submitted
ecotoxicity data available for any receptors other than mammals.  That
is, there are no acceptable data for the following taxa: fish (surrogate
for aquatic-phase amphibians), aquatic invertebrates, aquatic plants,
birds (surrogate for reptiles and terrestrial-phase amphibians),
terrestrial invertebrates, and terrestrial plants.  A search of the
publicly-accessible ECOTOX website (  HYPERLINK
"http://www.epa.gov/ecotox"  http://www.epa.gov/ecotox ) revealed one
aquatic toxicity study that is unacceptable for use in this risk
assessment, and there were no terrestrial toxicity data for ammonium
thiosulfate.

As described in the Exposure Characterization section above, the aquatic
risk assessment for ammonium thiosulfate will focus on ammonia.  Ammonia
is known to exhibit aquatic toxicity and has been studied extensively in
the open literature.  The available evidence indicates that the toxicity
of ammonia can depend on ionic composition, pH, and temperature. The
Agency (Office of Water) has developed ambient water quality criteria
for ammonia, which will be used in this risk assessment for ammonium
thiosulfate use as a pesticide.

The terrestrial risk assessment will be qualitative for ammonium
thiosulfate due to its environmental fate properties and predicted
behavior in the environment.  For the sake of being complete, Table
III.4 summarizes the available toxicity data for mammals (provided by
the Health Effects Division). Ammonium thiosulfate is practically
non-toxic to mammals on an acute oral basis, and the acute dermal and
inhalation toxicity studies revealed no mortalities. 

Table III.4.  Toxicity of ammonium thiosulfate to mammals. 

Test Type	Test Organism	Results	Toxicity Category	MRID

Acute oral 	Rat 

(Sprague-Dawley)	5-day LD50 (both sexes) = 3824 mg/kg

5-day LD50 (males) = 4054 mg/kg

5-day LD50 (females) = 3500 mg/kg	Practically non-toxic	41647405

Acute dermal	Rabbit	3-day LD50 > 2000 mg/kg bw

(Male and female rabbits were treated dermally with 2000 mg/kg bw
ammonium thiosulfate. All rabbits survived the treatment. No abnormal
clinical signs or gross findings were noted.  Erythema was noted in 3
males and in all 5 females; however, there were no skin reactions after
day 3.)	N/A	41647406

Acute inhalation	Rat 

(Sprague-Dawley)	Male and female Sprague-Dawley rats were exposed to
ammonium thiosulfate aerosol (1.79 mg/L) for four hours. Signs included
polypnea and dyspnea, languid behavior, hunched appearance, prostration
and tremors, rhinorrhea, chromodacryorrhea, and salivation.  There were
no effects on body weights and all animals survived the treatment.  	N/A
41647407

IV.	Risk Characterization

A.	Aquatic Risk 

As described in the Exposure Characterization (Section III.B), ammonium
thiosulfate will quickly dissociate in the environment into ammonium and
thiosulfate, both of which are considered in this aquatic risk
assessment.  Thiosulfate would be incorporated into the natural sulfur
cycle, where the thermodynamically-stable end product is sulfate. Since
the Agency has established an SMCL of 250 mg/L for sulfate in drinking
water, and it is highly unlikely that sulfate levels would approach this
level (as a result of this ammonium thiosulfate use), no aquatic
exposure modeling was performed for sulfate. The risk of sulfate to
aquatic organisms via the pesticidal use of ammonium thiosulfate is
presumed to be negligible.

For ammonium, aquatic exposures were estimated using the Tier 1 GENEEC
model and adjusted for the percent of the area treated.  Ammonium EECs
were then adjusted using equilibrium equations to estimate ammonia
concentrations at three pH levels (5, 7, and 9). Based on the Agency’s
national criterion for ammonia in fresh water, aquatic life should be
protected if both of the following conditions are satisfied for the
temperature, T, and pH of the waterbody:

The one-hour average concentration of total ammonia nitrogen (in mg N/L)
does not exceed, more than once every three years on the average, the
CMC (acute criterion) calculated using the following equations: 

Where salmonid fish are present:

CMC = (0.275 / (1 + 10 7.204 – pH)) + (39.0 / (1 + 10 pH - 7.204))

Or, where salmonid fish are not present:

CMC = (0.411 / (1 + 10 7.204 – pH)) + (58.4 / (1 + 10 pH - 7.204))

The thirty-day average concentration of total ammonia nitrogen (in mg
N/L) does not exceed, more than once every three years on the average,
the CCC (chronic criterion) calculated using the following equations.

When fish early life stages are present:

CCC  = [(0.0577 / (1 + 10 7.688 – pH)) + (2.487 / (1 + 10 pH
–7.688))]

* MIN(2.85, 1.45*100.028 * (25-T) )

		

When fish early life stages are absent:

CCC  = [(0.0577 / (1 + 10 7.688 – pH)) + (2.487 / (1 + 10 pH
–7.688))]

* 1.45*100.028 * (25-MAX (T,7)) 

Using these equations, these criteria were calculated assuming a
temperature of 25oC at pH 5, 7, and 9.  As shown in Table IV.1, the
highly conservative GENEEC model estimated aquatic EECs for ammonia are
all below the national water criteria.  It is important to keep in mind
that the GENEEC modeled EECs are upper bound concentrations and do not
take into account uptake by organisms, binding to humic acids and clays
in soils, nitrification in soil/water body, or atmospheric oxidation
reactions of volatilized ammonia.  This analysis suggests that the
pesticidal use of ammonium thiosulfate poses negligible risk to aquatic
organisms.

Table IV.1. Comparison of GENEEC modeled aquatic EECs versus national
water criteria for ammonia.  

GENEEC EECS	Peak Ammonium Concentration 

(mg/L)	Peak Ammonia Concentration1

(mg N/L)

pH 5	pH 7	pH 9

PTA Corrected	3.90	2.77E-6	2.79E-4	0.027

National Water Acute Criterion (Salmonid)2 	38.759	24.103	0.885

National Water Acute Criterion (Non-Salmonid)2	58.039	36.093	1.324

National Water Chronic Criterion2

(Fish early life stage-present)@ 25oC	3.599	3.000	0.233

National Water Chronic Criterion2

(Fish early life stage-absent)@ 25oC	3.599	3.000	0.233

1 Ammonia concentration estimated using following equation:
log(NH3(g))=-11.04 + log (NH4+)+ pH

	B.	Terrestrial Risk

Terrestrial exposures of ammonium and thiosulfate were not
quantitatively estimated in this assessment. As described in the
Exposure Characterization (Section III.B), the environmental fate of
thiosulfate will be dependent on soil redox potential.  In suboxic and
oxic environments (where oxygen, nitrogen and iron act as electron
acceptors), thiosulfate will be oxidized to form sulfate.  In anoxic
environment, thiosulfate will be reduced to form sulfide. The
environmental fate of ammonium is predominantly dependent on
nitrification and sorption on soil colloids and organic matter. Ammonium
may also form ammonia (NH3(g)) in very alkaline and sodic soil
environments (pH 9-10). 

Sulfur and nitrogen are essential nutrients for plants and animals.
Available mammalian data indicate that ammonium thiosulfate is
practically non-toxic on an acute oral basis.  There are no toxicity
data available for birds, terrestrial invertebrates, or terrestrial
plants.  Since ammonium thiosulfate is being used as an herbicide, there
is a possibility that non-target plants may be at risk.  However, given
that it is only used in residential/homeowner settings in California on
dichondra lawns, the potential for adverse effects to non-target plants
(including Federally-listed species) seems highly unlikely.  

Given the nature of the chemical and its expected behavior in the
environment, and because sulfur and nitrogen are essential nutrients for
plants and animals, risk to terrestrial organisms is not expected as a
result of ammonium thiosulfate use as an herbicide on dichondra lawns. 

C.	Federally Threatened and Endangered (Listed) Species Concerns  tc "

4.	Federally Threatened and Endangered (Listed) Species Concerns " \l 3 

This risk assessment suggests that Federally-listed threatened and
endangered aquatic and terrestrial species are not at risk as a result
of the limited use of ammonium thiosulfate as an herbicide on dichondra
lawns.

D.  	Discussion of Uncertainties, Limitations, and Data Gaps

	1. Exposure For All Taxa  tc "Assumptions, Limitations, Uncertainties,
Strengths and Data Gaps  Related to Exposure For All Taxa " \l 3 

			a. Maximum Use Scenario

The screening-level risk assessment focuses on qualitatively
characterizing potential ecological risks resulting from a maximum use
scenario, which is determined from labeled statements of maximum
ammonium thiosulfate application rate and number of applications with
the shortest time interval between applications.  The frequency at which
actual uses approach this maximum use scenario may be dependent on
herbicide resistance, timing of applications, cultural practices, and
market forces.  

				b. Additive and/or Synergistic Effects  tc "Additive and/or
Synergistic Effects " \l 4 

It was assumed that aquatic and terrestrial organisms were exposed only
to ammonium thiosulfate use as an herbicide.  Ecological risks
associated with exposure to a mixture of ammonium thiosulfate and other
pesticides, adjuvants, heavy metals, industrial chemicals,
pharmaceuticals, etc. were not considered in this risk assessment. 

2. 	Exposure For Aquatic Species 

The Tier I (screening) simulation model GENEEC was used to estimate the
exposure concentrations (EECs) of thiosulfate and the ammonium cation.
This model is not suitable for inorganic chemicals, particularly those
that undergo extensive, pH-dependent redox reactions. Assumptions had to
be made for the selection of environmental fate parameters (hydrolysis,
photolysis, biotransformation). It was assumed that thiosulfate and
ammonium were stable (i.e, half-lives = 0). Therefore, these EECs
represent an upper bound concentration of thiosulfate and ammonium
cation that do not consider any redox reaction products in soil and
water.

The available product label does not provide clear information on
application rates and frequency of application. Given that the product
is only used in residential settings, an application rate had to be
estimated from U.S. Census 2000 data for a typical residential plot and
assumed treated area, which then was used to estimate the percent
treated area (PTA). Since GENEEC estimates assume that all of the area
gets treated, the PTA served to correct the aquatic EECs.

		3.	Data Gap tc "a.	Data Gaps " \l 4 s 

There were no acceptable registrant-submitted environmental fate or
ecotoxicity data available for ammonium thiosulfate. However, given the
nature of the chemical and its predicted behavior in the environment,
this was not an impediment to the risk assessment.   At this time, the
EFED is not requesting any environmental fate or ecotoxicity studies to
be submitted for ammonium thiosulfate.

V.	Cited Literature

Bamford, V.A., Bruno, S., Rasmussen, T., Appia-Ayme, C., Cheesman, M.R.,
Berks, B.C., and Hemmings, A.M. 2002. Structural basis for the oxidation
of thiosulfate by a sulfur cycle enzyme. The EMBO (European Molecular
Biology Organization) Journal, Vol.21(21), pp.5599-5610.

Boparal, H.K., Shea, P.J., Comfort, S.D., and Snow, D.D. 2006.
Eviron.Sci.Technol., Dechlorinating  chloroacetanilide herbicides by
dithionite-treated aquifer sediment and surface soil.  Vol 10(9),
pp.3043-3049.

Borgello, E., Desilvestro, J., Grätzel, and Pelizzetti. 1998.
Phoreduction of Thiosulfate in Semiconductor Dispersions. Helvetica
Chimica Acta, Vol.66(6), pp 1827-1834.

Brown, T., Fischman, A., Spiccia, L., and McPhail, D.C. 2003.
Hydrometallurgy 2003-Fifth International Conference in Honor of
Professor Ian Ritchie-Volume 1: Leaching and Solution Purification,
Edited by 

Cotton, F.A. and Wilkinson, G. Advanced Inorganic Chemistry, Fifth
Edition, John Wiley and Sons, New York, 1988.

Gan, J., Yates, S.R., Ole-Becker, J. and Wang, D. 1998. Surface
Amendment of Fertilizer Ammonium Thiosulfate to Reduce Methyl Bromide
Emission from Soil. Environ. Sci. Technol. Vol 32, pp 2438-2441.

Gan, J., Becker, J.O.., Ernst, F.F., Hutchinson, C., Knuteson, J.A., and
Yates, S.R. 2000. Surface application of ammonium thiosulfate fertilizer
to reduce volatilization of 1,3-dichloropropene from soil Pest
Mnanagement Science, Vol. 56(3) pp.264-270.

Jeffery, R., Masau, Y, Oh, J.K., and Suzuki, I. 2001. Mechanism of
oxidation of inorganic sulfur compounds by thiosulfate-grown
Thiobacillus thiooxidans. Can. J. Microbiol., Vol 47, pp 348-358.

Johnson, M.D. and Balahura, R.J. 1988. Kinetics and Mechanism of the
Oxidation of Coordinated Thiosulfate by Peroxymonosulfate. Inorg.Chem.,
Vol.27, pp 3104-3107.

Lens, P.N.L and Kuenen, J.G. 2001. The biological surface cycle: novel
opportunities for environmental biotechnology. Water Science and
Technology, Vol. 44(8), pp.57-66.

Lindsay, W.L. Chemical Equilibria in Soils, John Wiley and Sons, New
York, 1979.

Pryor, W.A. 1960. The Kinetics of Disproportionation of Sodium
Thiosulfate to Sodium Sulfide and Sulfate. J.Am.Chem.Soc. Vol. 82,
pp.4794-4798.

Schippers, A.L. and Wolfang, S. 1999. Bacterial Leaching of Metal
Sulfides Proceeds by Two Indirect Mechanisms via Thiosulfate or via
Polysulfides and Sulfur.  Applied and Environmental Microbiology, Vol 65
(1), pp. 319- 321.

Suzuki, I. 1999. Oxidation of inorganic compounds: Chemical and
enzymatic reactions. Can. J. Microbiol., Vol 45, pp 97-105

Termes, S.C. and Pope, M.T.  1979. Stabilization of Uranium(V) in
Heteropolyanions. Transition Metal Chemistry, Vol.3 (1), pp 103-108

Vairavamurthy, A., Manovitz, B., Jeon, J., and Luther, G.W. (III), 1993.
Oxidation State of Sulfur in Thiosulfate and Implications for Anaerobic
Energy Metabolism. Geochimica et Cosmochimica Acta, Vol. 57(7), pp
1619-1623.

Wan, R-Y. and Le Vier, K.M. 2003. Solution chemistry factors for gold
thiosulfate heap leaching. Int. J. Miner. Process, Vol. 72, pp. 311-322.

Wang, G.J., Yates, S.R., Koskinen, W.C, and Jury, W.A. 2002. 
Dechlorination of chloroacetanilide herbicides by thiosulfate salts.
Proc. Natl. Acad. Sci (PNAS), USA Vol.99(8), pp. 5189-5194.

Young, C.A., A.M. Alfantazi, C.G.Anderson, D.B. Dreisinger, B.Harris,
and A.James, TMS (The Minerals, Metals & Materials Society), pp.
213-219.

Appendix A. Sulfur chemical species (other than thiosulfate) of
environmental significance

Name of Sulfur Species	Oxidation State of Sulfur	Chemical Name of the
Acid Form	Compounds 

Sulfide

S2-	-II	Hydrogen sulfide, H2S

Hydrosulfide, HS-	Metal Sulfides , mostly minerals such as:

Galena (PbS)

Sphalerite (ZnS)

Polysulfide

Sn2-	-II	Acid form is not stable	In aqueous solution only

S32- and S42- are stable. 

Sulfur (elemental sulfur)	0	No acid form

The thermodynamically stable form is cyclooctasulfur (cyclo-S8);
orthorhombic 	Not applicable

Sulfur Dioxide, SO2	IV	Not applicable	Not applicable

It can be generated from burning of S-fuels It is of environmental
concern because it forms sulfuric acid

Hydrosulfite, sulfite, HSO3-

Sulfite, SO3-	IV	Acid form not stable	As salts

Sulfate, SO42-	VI	Sulfuric acid, H2SO4	Sulfate salts (sodium potassium)

Naturally occurring sulfate minerals (e.g., barite)

Sulfur oxyanions containing 3 or more sulfurs:

Polythionates (Sn)O62-, of which tetrathionate is an example, S4O6 2-

Appendix B.  Aquatic Exposure Model Estimates (GENEEC Version 2.0)  

RUN No.   1 FOR Ammonium Cation  ON   Dichondra     * INPUT VALUES *

  --------------------------------------------------------------------

   RATE (#/AC)   No.APPS &   SOIL  SOLUBIL   APPL TYPE NO-SPRAY INCORP

    ONE(MULT)    INTERVAL    Koc   (PPM )    (%DRIFT)   (FT)     (IN)

  --------------------------------------------------------------------

18.090( 72.360)   4  10        .0 1030.0   GRHIFI(  6.6)     .0    .0

  FIELD AND STANDARD POND HALFLIFE VALUES (DAYS)

  --------------------------------------------------------------------

  METABOLIC  DAYS UNTIL  HYDROLYSIS   PHOTOLYSIS   METABOLIC  COMBINED

   (FIELD)   RAIN/RUNOFF   (POND)     (POND-EFF)    (POND)     (POND)

  --------------------------------------------------------------------

      .00        2          N/A       .00-     .00      .00       .00

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  GENERIC EECs (IN MILLIGRAMS/LITER (PPM))     Version 2.0 Aug 1, 2001

  --------------------------------------------------------------------

      PEAK      MAX 4 DAY     MAX 21 DAY    MAX 60 DAY    MAX 90 DAY

      GEEC      AVG GEEC       AVG GEEC      AVG GEEC      AVG GEEC

  --------------------------------------------------------------------

       4.29           4.29                       4.29                  
4.29                    4.29

 http://www.epa.gov/waterscience/criteria/ammonia/99update.pdf

 Disproportionation reactions apply to reactions in which an element in
one of the reaction products is in a lower oxidation state and the other
is of a higher oxidation than the oxidation state of the parent species.
For example, 2U(V)→ U(IV) + U(VI) in water.

 Another important Thiobacillus species is Thiobacillus ferroxidans. It
not only oxidizes sulfide to sulfate, but above pH 3 it also oxidizes
Fe(II) to Fe(III), mostly from pyrite and/or marcasite. As the pH
increase, Fe(III) generates a series of hydrolytic reactions 
hydroxyoxides, with the concomitant formation of hydronium ions (H3O+).
These reactions are responsible for acid mine drainage, which has
negative impact on the environment.  Thiobacillus thiooxidans oxidizes
sulfur only.

 S2- may polymerize to polysulfides (Sn2-). In aqueous media, the
polysulfide with the highest number of sulfurs is S42-. Polysulfide can
also oxidize to oxyanions of sulfur (oxic condition) or breakdown
polysulfides with a lower number of sulfurs, including S2-.

 Ecotox. Ref. No. 14566. Fathead minnow acute toxicity study. Substance
purity and mortality data were not reported.

 For more information visit:
http://www.epa.gov/waterscience/criteria/ammonia/99update.pdf

ia/ammonia/99update.pdf

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