Features
The aim of this special edition 
Yoko Fujikawa

Kyoto University

High arsenic concentrations which exit from environmental water are the by-product of layers of natural stratum and minerals in many cases. Bored wells used for groundwater withdrawal and human activities such as mining and refining bring high arsenic concentrations into water sources. In Bangladesh, groundwater with high-concentrations of arsenic cannot help being used for drinking water sources because surface water is polluted, even though this causes serious arsenic poisoning. In Japan there are closed or discarded wells because of groundwater containing arsenic. Thus, in order to make efficient use of groundwater resources, low cost arsenic removal technology with high removal capability is required. However, the required rate of arsenic removal changes with the situation of sites. The cost is also a factor affected by the situation of sites. Therefore, a variety of applicable technologies are expected.

The coagulation-sedimentation method together with iron or aluminum (Because arsenite has less effectiveness, chlorine oxidation is needed as a previous step.), the absorption method and the reverse osmosis method are the general technologies used to remove arsenic in water. Of the three methods the characteristics of the adsorption method provide high water quality when it is within their lifespan of absorbents. However, the operation and maintenance of an adsorption plant, because of the costs of purchases, regeneration and disposal of adsorbents, is likely to create higher costs than the coagulating sedimentation method. At any rate, the function of adsorbents and availability of its regeneration are vital for the application of the adsorption method.

The purpose of this paper titled “characteristics of arsenic and removal of it from environmental water” is to organize treating methods for the removal of arsenic from water sources in order to produce drinking water and purified water, especially using the adsorption method. In this series, the outline, principle and characteristics, example of construction, cost estimation, and limit of application of the arsenic removal technology are reported. This data is collected from universities which conduct research in academic-industrial cooperation and from some companies which install facilities and equipment employing the arsenic removal technology: serium filter media, hydrotalcite, manganese oxide and iron bacteria.

Knowledge on the chemical speciation and characteristics of arsenic in aquatic systems is necessary for efficient removal of arsenic from water. Occurrence of some arsenic species in environmental water is summarized in the following.
(1) Arsenate: Arsenate is the thermodynamically stable form of arsenic in oxygenated water. Arsenate mainly consists of HAsO42? in oxygenic seawater (,H2AsO4?in freshwater) as pKa values of H3AsO4 and H2AsO4? are 3.6 and 7.2. As arsenate is in ionic form at neutral paper (i.e., in most of the natural water), its removal from drinking-water supplies can be achieved by coagulation with alum, iron, lime or other salts , followed by precipitation or filtration at moderate ease and cost.
Dewatering of the arsenic-contaminated coagulation sludge (dewatering of iron coagulant sludge tends to be more difficult) and final disposal of the dewatered solid should be conducted according to the standards and regulations in each country. Generally the removal of arsenate is not difficult.
(2) Arsenite: Arsenite is a weak acid(pKa 9.2)and exists as non-ionic form, H3AsO3, in freshwater (at pH<9). Removal of arsenite by conventional technology such ascoagulation and precipitation,therefore, needs an oxidation step to convert arsenite to arsenate. At the higher pH of seawater, ca.13 % can be As(OH)4? as a result of the hydrolysis of H3AsO3 (Turner et al., 1981). In anoxic groundwater, 56 to 76 % of arsenic is in reduced form (arsenite).
(3) Organoarsenicals: Organoarsenicals most abundant in the aquatic systems are monomethylarsonic acid (CH3AsO(OH)(O?)) and dimethylarsinic acid ( (CH3)2AsO (O?)), formed by biomethylation of inorganic arsenic. In some fresh waters the methylarsenicals comprised up to 70 % of the total arsenic. More typically the ratio is 10 ? 20%, but in all cases the dominant organoarsenic compound was dimethylarsinic acid. So far organoarsenicals have never been detected in the groundwater.
The summary above shows that efficient removal of arsenite from water is one of the key issues in the advanced treatment of the groundwater. Removal of organoarsenicals and arsenate are of lower priority because the former are nonexistent in groundwater and the latter can be removed relatively readily by conventional procedures. The focus of this web content also is the development of the technology that is adopted to removal of arsenite through utilization of solid or biological oxidizing media, development of sorbents that can remove both arsenite and arsenate, and by placing oxidation / backwashing system.


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1 Arsenic removal technology - Cerium adsorbent