|【Methods for disposal of arsenic 】|
Methods for disposal of arsenic
Water resources in developing countries in Asia are a very serious predicament. It hinders the growth of industrial projects and activities of Japanese companies as well as it creates inconvenience to the life of the people.
Groundwater is an example of a high-quality source of water as compared with surface water because the latter is easily contaminated. However, certain elements like arsenic, iron, manganese, ammonium ion, etc. seeping from the subsurface geologycould be extracted, in which excessive amount can be harmful or even fatal to the body.
By drinking arsenic-contained water, arsenic poisoning is possible. According to some reports, this is widespread across Asia, Africa, and South America. Statistically, patients with arsenic poisoning have reached an estimated number of 20 million, and they are mainly coming from the developing countries. As such, the United Nations and Japan decided to offer financial support amounting to 1,320,000 dollars in 2002 to the project implemented by the United Nations Industrial Development Organization (UNIDO) in the "improvement of human security through provision of safe drinking water supply corresponding to WHO standard about arsenic contamination".
Arsenic contamination of water resources in Thailand and Bangladesh is a serious problem. In addition to pollution of surface water, poisoning of groundwater by natural inorganic arsenic, in which water quality is relatively safe, has occurred. To date, various organizations as well as the WHO in Bangladesh have been working on to solve the problem. However, all their efforts are considered futile because the most suitable arsenic disposal water processing system is not yet established in developing countries.
Underwater arsenic is found in the form of arsenite ion (lll) or arsenite ion (V). In many cases, arsenite ion (lll) is the usual form. Since highly toxic arsenic, as well as iron and manganese, are found in groundwater, there is a need to dispose such elements before the water can be utilized. The arsenic removal methods, co-precipitation (condensation and precipitation) and absorption methods, are known methods for this purpose. These methods are introduced here including the technology currently being developed.
In the co-precipitation method, poly aluminum and ferric chloride are usually used. In this method, required infusion dose of the flocculating agent ranges between 20 and 40mg/L, 1000 times the pollutant arsenic level in trace amounts, which is between 10 and 30ug/L. This is a questionable treatment method because it causes mass generation of waste.
On the other hand, the absorption method is the most prevalent. It employs activated alumina and the other one is the use of an absorption filter of ferrioxide. Activated alumina requires ph between 4.5 and 5.0 for acidic region. When it exceeds absorption capacity, sodium hydroxide solution, which is strongly alkaline, is required in order to renew activated alumina. Changing the activated alumina at least biennially is common at the working place, therefore the process costs more and a large volume of waste is generated. However, ferrioxide absorbent is not renewable. Once a year, replacement for all the filters is generally required. This method likewise generates a large volume of waste similar to the use of activated alumina.
Recently, absorbent added bismuth to manganite has been developed but not yet manufactured. Although absorbent added bismuth to manganite is found to absorb arsenic efficiently, it is not used. It is because it is not a granular material making it impossible to dispose with water effectively.
Arsenic absorption using cerium hydroxylation is another method. It has a long-duration for filtering and is highly effective.
Many studies in the UK and France have described a method of biofiltering using bacteria. The procedure makes possible the removal of arsenic and iron, and oxidation of manganese at the same time. This study is currently applied in groundwater in Japan as well. This method is promising because low-cost agents can be utilized, and the volume of wasted filtering materials and production of sludge can be minimized.