The presence of elevated levels of arsenic in groundwater has become a major concern around the world, especially in South Asia. Up to date, there is no effective treatment for curing health impacts due to the intake of high levels of arsenic. A wide range of technologies has been developed for the removal of high concentrations of arsenic from drinking water. The most common arsenic removal technologies use oxidation, coagulation, precipitation adsorption, ion exchange and membrane techniques. Other potential approaches would include phytoremediation or the use of bacteria, which can play an important role in catalysing biological arsenic removal processes. All the arsenic treatment technologies ultimately concentrate arsenic in the sorption media, the residual sludge or in a liquid media. To avoid indiscriminate disposal and environmental pollution, these wastes need to be treated or disposed of properly.
Most arsenic removal technologies are most effective in removing the pentavalent form of arsenic (As(V), arsenate), since the trivalent form (As(III), arsenite) is predominantly non-charged below pH 9.2. Thus arsenate is much less mobile than arsenite, as it tends to co-precipitate out with metallic cations or to adsorb onto solid surfaces. Therefore, many treatment systems include an oxidation step to convert arsenite to arsenate. Arsenite can be oxidised by oxygen (O2), hypochlorite (HClO), permanganate (HMnO4) and hydrogen peroxide (H2O2). Atmospheric oxygen is the most readily available oxidising agent and many treatment processes prefer oxidation by air. However, air oxidation of arsenic is a very slow process and can take weeks for oxidation. Air oxidation of arsenite can be catalysed by bacteria, strong acidic or alkali solutions, copper, powdered activated carbon and high temperature.
Coagulation and filtration with metal salts and lime followed by filtration is the most heavily documented method of arsenic removal from water. In the process of coagulation, arsenic is removed from solution through three mechanisms.
1 - Precipitation: The formation of insoluble compounds
2 - Co-precipitation: The incorporation of soluble arsenic species into a growing metal hydroxides phases (e.g. co-precipitation with Fe(III)
3 - Adsorption: The electrostatic binding of soluble arsenic to external surfaces of the insoluble metal hydroxide
Coagulation-flocculation processes using alum, ferric chloride, or ferric sulphate are effective at removing arsenic. They are the most well known arsenic treatments and have been more extensively tested in both laboratory and field studies than other technologies. When added into water, they dissolve under efficient stirring for one to few minutes. During this flocculation process, all kinds of micro-particles and negatively charged ions are attached to the flocs by electrostatic attachment. Arsenic is also adsorbed onto coagulated flocs. It can be removed partially by sedimentation,while filtration may be required to ensure complete removal of all flocs.
The Bucket Treatment Unit (BTU) and the Stevens Institute Technology
The Bucket Treatment Unit (BTU), developed by the DANIDA project in Bangladesh is based on the above-described coagulation and filtration process. It consists of two buckets, each 20 Litres, placed one above the other. Chemicals are mixed manually with arsenic contaminated water in the upper bucket and stirred with for 30 to 60 seconds. The chemical used includes aluminium, sulphate and potassium permanganate supplied in powder form. The water from the top bucket is then allowed to flow into the lower green bucket via plastic pipe and a sand filter box installed in the lower bucket.
The Stevens Institute technology is similar to the BTU method. It uses also two buckets, one to mix chemicals (iron sulphate and calcium hypochlorite supplied in packets) and the other to separate flocs by the processes of sedimentation and filtration.
Coagulation with Lime
Water treatment by the addition of quick lime, CaO, or hydrated lime, Ca(OH)2 removes arsenic. Lime treatment is a process similar to coagulation with metal salt. The precipitated calcium hydroxide, Ca(OH)2 acts as a sorbing flocculent for arsenic. Excess of lime will not dissolve, but remains as a coagulant aid, which has to be removed along with precipitates through sedimentation and filtration process. It has been observed that the arsenic removal by lime is relatively low, usually between 40-70 %. The highest removal is achieved at pH 10.6 to 11.4. Lime softening may be used as a pre-treatment to be followed by alum or iron coagulation.
Several sorptive media like activated alumina, activated carbon, iron and manganese coated sand, kaolinite clay, hydrated ferric oxide, activated bauxite, titanium oxide, silicium oxide and many natural and synthetic media have been reported to remove arsenic from water. The efficiency of sorptive media depends on the use of oxidising agents as aids to provoke the sorption of arsenic on the media.
Activated alumina (Al2O3) has a good sorptive surface, in the range of 200-300 m2/g. The large surface area gives the material a very large area for adsorption of arsenic. When water passes through a packed column of activated alumina, the impurities including arsenic present in water are adsorbed on the surfaces of activated alumina grains. Eventually, the column becomes saturated, first at its upper zone and later downstream towards the bottom end, and finally the column gets totally saturated.
Iron Coated Sand and Brick Chips
Iron coated sand and iron coated brick chips are effective in removing both As(III) and As(V). The “Shapla arsenic filter” is an example of a household arsenic removal filter based on iron coated brick chips developed and promoted by the International Development Enterprises (IDE). The brick chips are treated with ferrous sulphate solution for iron coating. The water collected from contaminated tube wells passes through the filter media placed in earthen container having a drainage system underneath.
Ion exchange is similar to that of activated alumina; just the medium is a synthetic resin of better-defined ion exchange capacity. The synthetic resin is based on a cross-linked polymer skeleton, called the matrix. The charged functional groups are attached to the matrix through covalent bonding and fall into acidic, weakly acidic, strongly basic and weakly basic groups. The ion exchange process is less dependent on pH of water. Arsenite, being uncharged, is not removed by ion exchange process. Hence, pre-oxidation of As(III) to As(V) is required for removal of arsenite by ion exchange process but the excess of oxidant often needs to be removed before the ion exchange in order to avoid the damage of sensitive resins. As the resin becomes exhausted, it needs to be regenerated. Ion exchange resins can be easily regenerated by washing with a NaCl solution.
Synthetic membranes are used to eliminate many contaminants from water including pathogens, salts and various metal ions. Usually, two types of membrane filtration are used: low-pressure membranes such as microfiltration and ultrafiltration and high-pressure membranes such as nanofiltration and reverse osmosis. Arsenic removal by membrane filtration is independent of pH and presence of other solutes but adversely affected by presence of colloidal matters. Iron and manganese can also lead to scaling and membrane fouling. The membrane once fouled by impurities in water cannot be backwashed. The water having high concentrations of suspended solids requires pre-treatment for arsenic removal by membrane techniques to avoid clogging.