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Environmental fate & pathways

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Description of key information

Additional information

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 system

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 waters.

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 benthic organisms.

 

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 coefficient

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

Conclusions

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.