WaterSubject

Extensive arsenic contamination of surface and groundwater has been reported in many parts of the world. The possible long term effects and the risks associated with the ingestion of arsenic contaminated water have compelled the regulatory agencies to promulgate a lower standard for arsenic in drinking water. The World Health Organization (WHO) standard for arsenic is 10 ?g/l. The maximum acceptable concentration for arsenic is 25 ?g/l in Canada; recently the United States Environmental Protection Agency (USEPA) adopted a new arsenic standard for drinking water at 10 ?g/l. The existing arsenic standard for drinking water in Australia is 7 ?g/l, whereas European Union has stipulated an arsenic standard of 10 ?g/l.

Arsenic, a common toxic element, is present in both inorganic and organic forms in water. Arsenic contamination of surface water is mainly due to weathering of rocks, geochemical reactions, contact of arsenic bearing sediments with aquifer, industrial waste discharges, volcanic emissions, fertilizers, and mining and smelting operations. Though arsenic contamination of groundwater in Taiwan was reported about three decades ago, the general awareness and attention of the public has been focussed recently because of similar reports of contamination of surface and sub surface waters by As from many other regions in the world, such as Bangladesh, Mexico, China, Chile, USA, Canada and India.

Arsenic occurs in several different species depending upon the pH and oxidation potential of the water. The four arsenic species commonly reported are arsenite [As(III)], arsenate [As(V)], monomethylarsonate (MMA) and dimethylarsinate (DMA). Despite the fact that inorganic species are predominant in natural waters, the presence of MMA and DMA has also been reported.

In the studies conducted, one of the water samples collected from the wells in the Lagunera Region, Northern Mexico had a high DMA concentration of 20 ?g/l. The results of studies showed that most of the water samples from the wells in northeast and northwest Taiwan (endemic area for blackfoot disease) had a high DMA concentration of 7 ?g/l. reported that the methylated species (dominant species was DMA) comprised on average of 24% of the total dissolved As in surface waters of the various lakes examined in California, USA, with the exception of Mono and Pyramid lakes. In recent studies , the concentration of organic arsenic species was 10% of the total arsenic concentration in the groundwater samples from a superfund site in the northeastern United States, where the total As concentration was 30 mg/l. showed that chronic exposure to DMA enhanced tumor development in the kidney, liver and urinary bladder of F 344 rats. The toxicity of different arsenic species varies in the order: arsenite>arsenate.monomethylarsonate (MMA)>dimethylarsinate (DMA). Inorganic arsenic species are about 10 to 60 times more toxic than organic arsenic compounds (MMA and DMA). Methylation of inorganic arsenic in the body is a detoxification process, which reduces the affinity of the compounds for tissues.

Recent research pointed out, however, that organic arsenic species are more toxic than initially thought. Therefore more research is needed for an estimation of the global arsenic toxicity based on speciation. It is anticipated that the USEPA may approach the wide range in arsenic species toxicity as it is done with mercury, whereby the more toxic species are the most closely regulated. It is unlikely that organoarsenic species will be as tightly regulated unless they are in a high concentration. Only a few studies have been conducted to remove organic arsenic from drinking water. reported that the adsorption behaviour of both monomethylarsonate (MMA) and dimethylarsinate (DMA) onto activated alumina were similar to that of arsenate, in their studies with ferrous and ferrate ions reported that better removal of MMA than DMA was obtained. In the present study, column studies were conducted using manganese greensand (MGS) to examine the removal of organic arsenic (DMA) spiked in tap water. Further, studies with iron oxide-coated sand (IOCS) prepared by two different methods (IOCS-1 and IOCS-2) and ion exchange (Amberlite IR-120 resin activated with ferric ions) resin were also conducted to remove organic arsenic (DMA) spiked in tap water, and the results were compared with the data from the MGS studies.

1. Batch study results with IOCS-2 showed that the arsenic (DMA) adsorption can be best described by the Freundlich isotherm based on the correlation co-efficient.

2. The constants estimated from the Freundlich isotherm indicated that the arsenic (DMA) adsorption capacity of IOCS-2 was low.

3. The arsenic (DMA) removal capacities of the media used in the column studies decreased in the following order:

4. High bed volumes and high bed capacity were achieved with ion exchange resin in the column studies; these results suggested that ion exchange resin can be effectively used to remove organic arsenic (DMA) in drinking water. Poor performance of DMA removal was observed with MGS and IOCS-1.