ABSTRACT
The presence of high level of arsenic in groundwater is a major concern around the world, especially in South Asia. Only in Bangladesh, an estimated 60 million people are exposed to unsafe levels of naturally occurring arsenic in their drinking water, radically raising the risk for cancer and other chronic diseases [1,2]. In areas where drinking water contains unsafe levels of arsenic, the pressing need is nding a safe source of drinking water. Two main options are adopted to solve the problem: nding a new safe source or removing arsenic from the contaminated source. Since most of the contaminated water is near the surface, many people now in Bangladesh have installed deep wells to tap the groundwater that is relatively free of arsenic-a practice that is essentially compromising on access to clean drinking water across the country, according to a report in the journal Science in 2010 [3]. Not only that, people in Bangladesh have started pumping clean water from the deep aquifers for irrigation. This can be a major cause of concern, as when irrigation wells pump high enough volumes, it can simultaneously pull down arsenic-contaminated water from the surface and jeopardize the quality of the groundwater below, which is used for drinking [3]. So, if deep wells are not a rational solution to the dreadful menace, a short-term goal can be to reduce arsenic levels through ltration techniques from the contaminated water. A wide range of technologies have been developed for the removal of high concentrations of arsenic from drinking water [4]. To our understanding, arsenic water ltration is therefore vital to many communities inside and outside Bangladesh where arsenic has been detected at dangerous levels in public drinking water supplies. In this context, this chapter shall discuss the various methods currently in use to lter arsenic from contaminated water. In this regard, it is worthwhile to mention that the most common valence states of arsenic in raw water sources are As(V) or arsenate and As(III) or As(III). In the pH range of 4-10, the As(III) species are neutral in charge, while As(V) compounds are negatively charged [5,6]. Therefore, the removal efciency of As(III) is less compared to that of As(V) by any of the conventional technologies discussed later; hence, for effective and complete removal of arsenic from water, an oxidation of As(III) to As(V) is a necessary prerequisite. As(III) can generally be oxidized by the use of any one of the following chemicals: oxygen, hypochlorite, permanganate, and hydrogen peroxide. Additionally, to complicate the issue, all the arsenic treatment technologies discussed later ultimately concentrate arsenic in the adsorbing media, the residual sludge, or in a liquid media, and, therefore, disposal or treatment of these wastes is as equally important as arsenic water ltration itself. Therefore, the requirements for a suitable technique for removal of arsenic from water must include the following: high efciency, safe technology to ensure the maintenance of the maximum contaminant level, simple operation, and minimum residual mass. As of now, there are numerous arsenic removal technologies developed by several sectors starting from universities to government organizations, groups, and private sectors. Some of the widely used technologies classied into categories based on the dominant removal process (although at times, a given technology may use multiple treatment processes) are discussed next.
