Abby Brewster would be the first to testify to the toxicity of Arsenic. While this fictitious character laced elderberry wine to kill off old bachelors, its use as a poison stretches back through history. Responsible for the demise of many kings and emperors, even Nero used it to murder his stepbrother, Britannicus, to become Emperor of Rome.
Arsenic kills by inhibiting enzyme production. Once an undetectable murder weapon of choice in the middle-ages, due to being colourless, odourless and inducing symptoms similar to Cholera, today arsenic poisoning is easy to prove. In the modern world it continues to kill people in their thousands through its natural presence in water courses. The World Health Organization (WHO) classify it as a group 1 human carcinogenic substance, recognising the condition arsenicosis that includes skin lesions, skin, lung, kidney and bladder cancer. Their guidelines set a limit of 10 μg/l for drinking water that is also adopted in the UK and regulated by the Drinking Water Inspectorate. The Environment Agency set their Health Criteria Values for inorganic arsenic around based on this limit.
While most of the fatalities through arsenicosis are found in third world countries, the threat of arsenic in water courses in the UK cannot be ignored. Arsenic is the 20th most abundant trace element in the earth’s crust and is present in more than 245 mineral ores that contain sulphide, copper, nickel, lead and cobalt predominantly. This presents challenges for companies sourcing water from wells, streams and boreholes in areas where mining activity is, or was, prevalent. Release of Arsenic into natural water sources is common from weathering of arsenic-containing rocks. Mining activities promote this and increase the potential for Arsenic to contaminate natural water sources. In 2016, the British Geological Survey (BGS) conducted a study of 512 properties in Cornwall served by private water supplies. Five percent of the wells and boreholes tested were above the WHO limit. This contamination is not industrial pollution. It is a result of Devon and Cornwall’s rich mining history. Cornwall was one of the most important mining areas in Europe until the early 20th century. It has a geology rich in high metal (tin and tungsten) in addition to rocks and sediments high in arsenic. During the 19th Century, Cornwall dominated the arsenic industry. By the 1870’s, a small number of mines in Cornwall produced over half the world’s arsenic.
Understanding Arsenic Chemistry
Arsenic is an element, so it cannot be destroyed. However understanding its chemistry is the secret to treating water contaminated with arsenic effectively. Arsenic is a metalloid that is very mobile in the environment. It can exist in four forms with varying levels of toxicity. The different forms of arsenic are known as valences. They are arsenite, As(III), arsenate, As(V), arsenic, As(0), and arsine As(III). The valence of arsenic plays an important role in its behaviour and toxicity in water. The inorganic forms of Arsenic are most commonly found in water and unfortunately, are the most toxic of the forms. The most toxic is arsine, followed by arsenate, often known as Pentavalent Arsenic. Elemental arsenic is the least toxic.
The environment can dictate what forms of arsenic are prevalent, based on how much air is available and how acidic the water is. Chemistry and composition of the water being treated is important in selecting effective removal methods. Most technologies are more efficient at removing arsenate than arsenite. Arsenate has a small electrical charge associated with it that lends itself to being removed by precipitation, adsorption or ion exchange technologies. Some treatment technologies use a two stage process to convert arsenite to arsenate before removing it.
Effective Arsenic Removal Technologies
The efficient removal of arsenic requires an understanding of the site from which you are drawing water and its environmental conditions. Testing the water for the various states of arsenic will determine which technologies to use to remove it reliably, efficiently and cost-effectively. “Culligan’s experience removing arsenic from water is extensive, employing a range of technologies in tailored packages depending on the volumes of water that need to be treated and a range of environmental parameters from pH to redox potentials and organic content in the water. In 2013, we supplied Italy’s first large scale water treatment system designed to remove arsenic and other contaminants from municipal water supplies. We took time to understand the concentrations of arsenic present in their various states to devise an effective system. Importantly, the system monitors the concentration of arsenic in the raw feed water, the treated water ready for consumption, and at various other stages in the process automatically,” explains John Kyle, Culligan UK’s Managing Director.
Several effective technologies can be employed to remove arsenic. Chemical oxidation of soluble arsenite to arsenate in a pre-removal stage is common where adsorption, coagulation or ion exchange processes are being employed. Membrane technologies can also be used from low pressure techniques including microfiltration and ultrafiltration through to high pressure techniques such as reverse osmosis and nanofiltration. Membrane systems operate best when arsenic is present as arsenate and there are low levels of suspended solids in the water. Again oxidation of any arsenite to arsenate is needed to remove it effectively using membranes.
Large scale systems, such as the one in Italy, use an absorption process to remove the arsenic. It relies on electrostatic forces between the media in the system and the charge that the arsenate molecule has. “We use an iron based media because there is a high affinity between inorganic arsenic species and iron. Iron not only acts as a sorbent but also has properties as a reductant, contaminant immobilizer and co-precipitant,” reveals John.
Arsenic removal by adsorption requires careful design of the system to maximise contact and contact time between the arsenic and the media in the treatment system. Surface area of the media, flow rates, retention times and an understanding of adsorption kinetics are all required to design an effective system.
Talk to Specialists
If you are considering abstracting water and need to remove arsenic, take time to discuss your requirements with experienced water treatment specialists who can advise on the best technologies for your site and provide the assurances you need to meet the regulatory requirements reliably and cost-effectively.