The toxicity of arsenic in combination with its lack of taste, colour and odour made it an ideal homicidal agent from the time of the Roman Empire through the Middle Ages and the Renaissance.
The so called “king of poisons” continues to threaten public health, however chronically – with low dose ingested over long periods through drinking water. Arsenic in drinking water has been reported, for example, from Taiwan, Chile West Bengal (India), Mexico, China, UK, USA, Pakistan, Bangladesh, Canada, Vietnam as well as other regions in the world. Those who are most exposed live in South Asia, particularly in Bangladesh, India, Pakistan and Nepal. Irony is that the respective governments have so far failed to adequately address this matter. The scale of human exposure to arsenic is unprecedentedly huge, with latest estimates claiming more than 200 million people around the world are still exposed through drinking water.
How toxic is arsenic?
Arsenic is extremely toxic. It seems to affect nearly every major organ and system in the human body. Epidemiological studies of large populations exposed to wide ranges of arsenic concentrations in drinking water in regions such as Taiwan, Bangladesh, Chile, India, and Argentina have revealed that the chronic exposure to arsenic is associated with skin, lung, bladder, kidney, and liver cancer. Early-life exposure may be related to increased risks for several types of cancer and other diseases during adulthood. A multitude of other health effects are also linked to chronic arsenic exposure. Dermatological, developmental, neurological, respiratory, cardiovascular, immunological, and endocrine effects as a result of arsenic exposure have all been reported.
How does arsenic contaminate water sources?
Both anthropogenic and geogenic processes can release arsenic into drinking water sources. For example, the surface water bodies and groundwater aquifers around mining sites, industrial zones and agricultural areas where arsenic containing materials have been mined, manufactured, spread or disposed are typically at risk of anthropogenic arsenic contamination. Arsenic can also be of geogenic origin, mobilised into a drinking water source through complex natural processes involving various source minerals. Drinking water sources with high arsenic concentrations can exist in very close proximity to sources with low arsenic concentrations, with differences noted even in neighbouring individual wells. A significant amount of literature is available on this topic for developing readers’ further understanding on the key geogenic processes known to mobilise arsenic into drinking water sources, namely reductive dissolution, mineral dissolution, alkaline desorption and geothermal activity.
The arsenic limit at WHO evolved in time – with the availability of evidence on the carcinogenicity of arsenic at different exposure levels. The WHO International Standards for Drinking Water in 1953 included arsenic in the category of toxic substances and a maximum allowable concentration of 200 μg/L was proposed. In 1963, the 2nd Edition of the WHO International Standards for Drinking Water lowered the limit to 50 μg/L. In 1984, the 1st edition of the WHO Guidelines for Drinking Water Quality was published which maintained 50 μg/L as the guideline value for arsenic in drinking water. In 1993, the 2nd edition of the WHO Guidelines for Drinking Water recommended a lower value of 10 μg/L as a provisional guideline value for arsenic in drinking water. The same value and the designation – provisional – were also carried forward to the subsequent editions of the WHO Guidelines, including the one that is applicable today. In-line with WHO guidelines, the maximum allowable limit for arsenic in drinking water in United States and European Union is 10 μg/L. In several low income countries (e.g. Bangladesh, Pakistan etc.) the maximum allowable concentration in drinking water is still 50 μg/L. On the other hand, The Netherlands is considering options to reduce arsenic to <1 μg/L in drinking water supplies because of the uncertainty surrounding the actual risks associated with arsenic exposure at concentrations lower than the WHO provisional guideline of 10 μg/L.
“in water, arsenic can exist both in inorganic and organic forms, however in natural waters the former generally predominates”
Basic chemistry of arsenic in water
In water, arsenic can exist both in inorganic and organic forms, however in natural waters the former generally predominates. Redox potential (Eh) and pH strongly control arsenic speciation in water. Arsenic is stable in four oxidation states, +5, +3, 0, -3, but the latter two are rarely present in natural water, thus the relevant inorganic arsenic species are oxy-anions of trivalent arsenite [As(III)] and pentavalent arsenate [As(V)]. As(III) mainly occurs under reducing conditions such as in deep groundwater, whereas As(V) occurs mainly in well-oxidised systems at thermodynamic equilibrium. Oxidation of As(III) by dissolved oxygen (O2) alone is thermodynamically possible, but the reaction proceeds very slowly and since thermodynamic equilibrium is not achieved both As(III) and As(V) may be encountered together. In a particular oxidation state (i.e., +3 or +5), pH controls the level of protonation.
Detection, quantification and speciation of arsenic
Measurement of arsenic in drinking water is the first step in the assessment of the extent and severity of contamination. Water samples from groundwater wells, treatment trains and storage tanks can be analysed for arsenic either using a field kit or in the laboratory where several techniques including inductively coupled plasma–mass spectrometry (ICP–MS), inductively coupled plasma–atomic emission spectrometry (ICP–AES), atomic adsorption spectroscopy–hydride generation (AAS–HG), atomic adsorption spectroscopy–graphite furnace (AAS–GF) are available. Our experience from the field has indicated that the test kits may detect fairly accurately the presence of arsenic at high concentrations, however they can be very inaccurate in measuring lower concentrations.
“laboratories in western Europe increasingly use ICP–MS because it provides extremely low detection limit, with small sample volume”
Accurate measurement of arsenic in drinking water requires laboratory based analysis in well controlled conditions. Nowadays, laboratories in western Europe increasingly use ICP–MS because it provides extremely low detection limit, with small sample volume. Moreover, the analysis run takes only few minutes to complete. On the downside, it is important to note that the cost of installing and maintaining the proper conditions for an ICP-MS system may be over two to three times the cost of a highly suited ICP-OES system, making long-term budget an important factor in the purchasing consideration.
“installing and maintaining the proper conditions for an ICP-MS system may be over two to three times the cost of a highly suited ICP-OES system”
The development of strategies for arsenic remediation strongly depends upon the speciation analysis. The ICP–MS method coupled with high-performance liquid chromatography (HPLC) is considered as the most efficient laboratory arsenic speciation analysis method available today. This combination of two processes provides a good separation of arsenic compounds together with an excellent detector sensitivity.
“the ICP–MS method coupled with high-performance liquid chromatography (HPLC) is considered as the most efficient laboratory arsenic speciation analysis method available today”
Arsenic remediation of drinking water
Whatever the origin, suitable remediation measures should be taken once arsenic is detected in drinking water sources, to ensure the provision of safe drinking water. There is no one ‘silver bullet’ when it comes to removal of arsenic from groundwater. Arsenic remediation is challenging and different approaches may be appropriate under different circumstances. Removing arsenic from a particular source water requires expertise, a good understanding of the complex aqueous chemistry of arsenic and above all expensive treatment methods. The handling and disposal of treatment residuals may introduce additional complications. Source substitution and treatment (Figure 1) are two approaches for arsenic remediation of drinking water.
By far the simplest way to reduce arsenic exposure via drinking water is to use an alternative drinking water source which is free of contamination, such as appropriately treated surface water or rain water. However, this is often challenging, expensive and in some cases simply not possible. The tradeoff between microbial versus chemical contamination is longstanding. In some cases, it is relatively straightforward for a user to switch to a low(er) arsenic groundwater well if testing has been done and a low arsenic source is available. There is also the possibility of blending low-arsenic water with high-arsenic water to achieve an acceptable arsenic concentration. Groundwater abstracted from the red/reddish brown sediments in the Bengal Basin Aquifers have indicated promise to deliver low arsenic water. These aquifers, however, have concerns for compliance with the drinking water safety plan due to elevated manganese concentrations. For Bangladesh, the most promising and speedy solution in the author’s view point is to enable the poor rural communities to target arsenic-safe aquifers by themselves. In this regard local drillers have a vital role to play. They are the main driving force in tube well installation process and they should be trained and equipped to target safe aquifers. Researchers at KTH Groundwater Arsenic research Group (KTH-GARG) in Sweden have developed a tool that links the colour and textural attributes and the geochemical characteristics of the targeted aquifer sediments to the groundwater pH, redox and a series of water quality parameters. This tool will certainly improve the situation in Bangladesh, provided its use is promoted and practiced. The abstraction of water from deep aquifers in Bangladesh must not be blindly adopted. Groundwater overuse can push arsenic deeper, rendering high future costs in terms of health and water-purification.
Techniques to remove arsenic from water can be broadly grouped into precipitation, adsorption and ion exchange, membranes, oxidation and bioremediation based processes. These methods have been described and critically evaluated in our recent publication1. Removal of As(V) is generally more efficient compared to As(III) in various treatment processes, mainly due to the fact that As(V) in the pH range of 6.5-8.5 (pH range of most drinking waters) occurs as a singly or doubly charged specie. Oxidation of As(III) to As(V) by dissolved oxygen is a very slow process and the slow oxidation kinetics of As(III) renders the conventional aeration techniques (e.g. cascade aeration) ineffective in oxidizing As(III). Rapid oxidation of As(III), however, can be achieved by chemical oxidants such as chlorine or potassium permanganate. The choice of oxidant for a certain water type is critical and dictated by factors such as availability, water composition (particularly with regard to (i) scavenger substances and (ii) the potential formation of dangerous disinfection by products) and the desired/existing treatment configuration.
Coprecipitation of arsenic with iron or aluminum hydroxides is one of the most common approaches to remove arsenic from water. Coprecipitation may be defined as the adsorption of an ion during the simultaneous precipitation of the adsorbent in solution. Several studies have reported “incidental” removal of arsenic during iron removal from groundwater and they attributed the removal to coprecipitation with hydrous ferric oxide that precipitated due to the oxidation of the indigenous ferrous to ferric ions and their subsequent hydrolysis. In order to achieve higher arsenic removal, the iron content may be increased by dosing a ferrous or ferric based coagulant. Dosing of iron, in any of the two forms, results in an increased production sludge, that may also contain considerable concentration of arsenic. Low-pressure membranes (e.g. UF) which are typically characterised by high flux and low fouling potential solely could not be a viable technique for direct arsenic removal due to the large pore size of membrane, however they may replace granular media filtration to serve as a more effective barrier to hydrous ferric oxide flocs in a coprecipitation based arsenic removal intervention. Adsorption of arsenic to preformed metallic hydroxides is also widely practiced. Adsorptive removal of arsenic is an active research area and new materials are increasingly being developed. A large number of commercial adsorbents are available for the removal of As(III) and As (V). Most are oxides and hydroxides of metals such as iron, aluminum, zirconium and titanium. Some of the key advantages of using adsorption media include treatment of water without generating sludge, selective removal of arsenic and (relatively) easy operation of the process compared to previously described coprecipitative removal of arsenic.
Selection of the most appropriate method for arsenic removal needs careful pre-evaluation of water quality characteristics, target finished water arsenic concentration, ease of implementation on an existing system and residual management options. Piloting the potential mitigation techniques is an essential procedure to optimise treatment variables and avoid implementing a technology that may not work for unforeseen reasons. Applying the advanced arsenic treatment options to rural settings is generally hindered by the de-centralised nature of the populations, however the basic principles of water treatment can be shared. Many of the conventional technologies can be reduced in scale and conveniently applied at community and household scale. Socio-economic reality of the target geographical region should never be overlooked. It is undoubtedly the most important aspect influencing the choice of arsenic remediation strategy.