Arsenic acid

Endpoint summary

Additional information

There are no existing studies investigating the environmental fate of arsenic acid (AA), however there is an extensive array of published information on the environmental fate of arsenic (As). The published data on As has been previously compiled and reviewed in several published reviews. The technical dossier of AA compiles some of these reviews and recommendations. The relevant reviews/publications are listed here under:

-ATSDR, 2007, Toxicological profile for Arsenic, In: Agency for toxic substances and disease registry, Division of toxicology and environmental medicine/applied toxicology branch, 297, Atlanta, USA.

-Lepper P, Sorokin N, Maycock D, Crane M, Atkinson C, Hope S-J, Comber, S, 2007, Proposed EQS for Water Framework Directive Annex VIII substances: arsenic (total dissolved), Published by UK Environment Agency, Science report: SC040038/SR3, 101 p.

-United States Environmental Protection Agency (US EPA), 2004, Volume II - Geochemistry and Available Kd Values for Selected Inorganic Contaminants, In: Understanding Variation in Partition Coefficient, Kd, Values, Office of Air and Radiation, EPA 402-R-04-002C, July 2004, 188p.

-United States Environmental Protection Agency (US EPA), 2004, Technical Summary of Information Available on the Bioaccumulation of Arsenic in Aquatic Organisms, Office of Science and Technology, Office of Water, EPA-822-R-03-032.

-WHO, 2001, Arsenic and arsenic compounds, Environmental Health Criteria 224, United Nations Environment Programme, International Labour Organisation, World Health Organization, International Programme on Chemical Safety.

 

The environmental fate of arsenic species and bioavailability are influenced by environmental factors such as redox conditions, pH, and presence of ligands and biotransformations reactions. Irrespective of the species that As is introduced into the environment, the actual species present vary according to various reversible equilibria which are driven according to the local conditions. In addition, arsenic is a naturally occurring chemical which concentration varies greatly according to region.

 

Concentrations of As in the environment

The FOREGS Database provides the median concentrations of As in the environment: 6.02 mg/kg (subsoil), 7.03 (topsoil), 0.63 µg/L (water, filtered < 0.45 µm), 6 mg/kg (stream sediment, floodplain sediment). The FOREGS Data provide a present overview of As in the environment. Since they do not provide a historical variation, these concentrations cannot be considered as true natural background concentration. Furthermore, those concentrations include several fractions as bioavailable and non bioavailable ones.

 

Stability in contact with water

As(+5) is a thermodynamically stable form of As. In water and under aerobic conditions, arsenic acid exists as a mixture of arsenate ions based on its pKa:

H3AsO4 <-> H+ H2AsO4-           pKa1=2.22

H2AsO4- <-> H+ + HAsO42-      pKa2=6.98

HAsO42- <-> H+ + AsO43-           pKa3=11.53

Considering the influence of pH alone, the protonated form H3AsO4 is the dominant form of arsenate at very low pH. Under most environmental conditions (pH 5 - 9), H2AsO4- and HAsO42- are the dominant forms of arsenate ions.

 

Stability in contact with light

Photolysis has no influence on the behaviour and transformation of arsenate in waters.

 

Biotransformation

Modes of biotransformation involve redox transformation between arsenite and arsenate, reduction and methylation of As, biosynthesis of organoarsenic compounds. The biomethylation and bioreduction are considered the most important pathways. Such biotransformation of As species occurs in the soil, in sediments, in plants and animals, and in zones of biological activity in the oceans.

Under aerobic conditions, the mixed microbial cultures of lake sediments were able to reduce arsenate (the predominant from of arsenic in water) to arsenite and a variety of methylated arsenicals, and also to oxidise arsenite to arsenate. Under anaerobic conditions, however, only reduction occurs.

 

Partitioning and distribution

Partitioning and distribution of arsenic will depend upon the oxidation state of arsenic and the interactions with the ligands present in the system. All forms of arsenic are adsorbed to most types of soil and sediment to a greater or a lesser degree. The following discussion aims to give an overview of the main current understandings.

 

Partitioning in water/sediment system

In water, As is mainly found in the particulate phase and it is suggested that sorption/desorption and co-precipitation processes are responsible of the regulation of the dissolved concentrations of As. Precipitation of As may occur with calcium, sulphur, iron, aluminium and barium unless these reactions are low. Adsorption is believed to be an important removal process of As from solution with 80% being removed on entering estuarine waters.

In aerobic conditions, As cycle in sediment is dominated by inorganic forms. Both adsorption of As on iron-rich oxides on the surface of sediments and incorporation of As into sediments by co-precipitation with hydrous iron oxides are factors controlling mobilisation of As in sediment. Aluminium compounds and organic matter have also been implicated in adsorption of As to sediments. The amount of arsenate adsorbed increases as the pH of the system increases. The major As species leached is arsenate which is found to be related to total iron and free iron oxides in the sediments. Finally, not all the adsorbed As is non bioavailable and it is expected that a fraction may be bioavailable to benthic organisms.

 

Partitioning in soil system

As compounds tend to adsorb to soils to a greater or a lesser degree being stronger adsorbed in clay soil than in sandy soil. The main factors that affect As adsorption to soil include: Fe and Al soil content, OC content, soil clay content, soil pH, phosphate concentrations and concentrations of other cations. Among them, the most influential factor is the iron content of the soil.

Under oxidizing conditions, leached As from soil is transported over only short distance. Therefore, leaching does not appear to be a significant route of As loss from soil. In agricultural soils, As is largely immobile and it tends to concentrate and remain in upper soil layers.

Under reducing conditions, arsenite compounds are dominant forms in soil but As(-3) and As(0) can also be present. As would be present as H2AsO4 - in well-drained and acids soil or as HAsO42 - in well-drained and alkaline soils. In the porewater of aerobic soils, arsenate is the dominant form of As. The amount of As sorbed from solution increases as the free iron oxide, magnesium oxide, aluminium oxide or clay content of the soil increases.

 

Transport

As is emitted to atmosphere as dust and fine particles which can be transported by winds and air prior to deposition to terrestrial compartment.

 

Bioaccumulation

As bioaccumulation depends on various factors such as the compartment considered (e.g. freshwater, seawater), the exposure concentration and the route of exposure. The range of BCF values for aquatic organisms suggest that As is accumulated to a greater or a lesser degree with fish BCF being generally below 2000. As does not appear to biomagnify between trophic levels.