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EC number: 231-901-9
CAS number: 7778-39-4
Arsenic acid (AA) is an inorganic
compound of arsenic (As). There are no proprietary studies investigating
the partitioning and distribution properties of AA either in sediment or
in soil or with suspended matter. However, several studies have been
performed to investigate the adsorption of As compounds to soil, to
sediment and to suspended matter. The partitioning of As to sediment, to
suspended matter and to soil is driven by complex processes which
include environmental chemistry and biota; it is assumed to be not yet
fully understood. Partitioning and distribution of As will depend upon
the oxidation state of As and the interactions of As with the ligands
present in the system. All forms of As are adsorbed to most types of
soil and sediment to a greater or a lesser degree. The following
discussion aims to provide a concise overview of the partitioning
properties of As and is based on a few review documents:
-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.
-World Health Organization (WHO), 2001,
Arsenic and arsenic compounds, Environmental Health Criteria 224.
-Crommentuijn T, Polder M. and Van de
Plassche, E 1997. Maximum permissible concentrations and negligible
concentrations for metals, taking background concentrations into
account. RIVM, Report 601501001.
Partitioning in water/sediment
In water, arsenic is found mainly 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 arsenic. Precipitation of arsenic may occur
with calcium, sulphur, iron, aluminium and barium unless these reactions
are low. Adsorption is believed to be an important removal process of
arsenic from solution with 80% being removed on entering estuarine
In aerobic conditions, arsenic cycle
in sediment is dominated by inorganic forms. Both adsorption of arsenic
on iron-rich oxides on the surface of sediments and incorporation of
arsenic into sediments by co-precipitation with hydrous iron oxides are
factors controlling mobilisation of arsenic in sediment. Aluminium
compounds and organic matter have also been implicated in adsorption of
arsenic to sediments. The amount of arsenate adsorbed increases as the
pH of the system increases. The major arsenic 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 arsenic is non
bioavailable and it is expected that a fraction may be bioavailable to
Partitioning in soil system
Arsenic 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 arsenic 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
arsenic from soil is transported over only short distance. Therefore,
leaching does not appear to be a significant route of arsenic loss from
soil. In agricultural soils, arsenic 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. Arsenic 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 arsenic. The amount
of arsenic sorbed from solution increases as the free iron oxide,
magnesium oxide, aluminium oxide or clay content of the soil increases.
Identification of partition
The key values of arsenic partition
coefficients are coming from a RIVM report:
Partition coefficient ksoil/water -
Kpsoil = 190.5 L/kg
Partition coefficient ksediment/water
- Kpsed = 6,606.9 L/kg
Partition coefficient to suspended
solids - Ksusp-water = 10,000 L/kg
The identified partition coefficient
will be used as input data for the modelling/calculation works carried
out for the purpose of the chemical safety assessment of AA.
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