Arsenic Removal Technologies

In areas where the drinking water contains unsafe levels of arsenic, the immediate concern is finding a safe source of drinking water. There are two main options: Finding a new safe source or removing arsenic from the contaminated source. If an arsenic safe water source cannot be established, the short-term goal is to reduce arsenic levels.There are several methods available for removal of arsenic from water. The following important methods are discussed below:

• Oxidation
• Coagulation, precipitation and filtration
• Adsorption (sorptive filtration)
• Ion exchange
• Membrane techniques

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 (PIERCE & MOORE 1982). Air oxidation of arsenite can be catalysed by bacteria, strong acidic or alkali solutions, copper, powdered activated carbon and high temperature (EDWARDS 1994).

Oxidation with the oxygen contained naturally in the air during collection and subsequent storage in houses may cause a reduction in arsenic concentration in stored water, which is also known as passive sedimentation. For passive sedimentation, the water needs to be stored for a sufficiently long time allowing the exchange of oxygen from the air to the water. Arsenic reduction by plain sedimentation appears to be dependent on water quality, particularly the presence of precipitating iron in water. High alkalinity and presence of iron in the tube wells water increase arsenic removal by storage.

In-situ oxidation of iron and arsenic in the aquifer has been tested under the DANIDA (Ministry of Foreign Affairs of Denmark) Arsenic Mitigation Pilot project in Bangladesh. The process technology is to extract water from a tube well to let it oxygenate at the air with atmospheric oxygen. Oxygenated water is then allowed to run back into the iron and arsenic contaminated aquifer through the same tube well. This allows forming coating of iron hydroxide on sand grains around the strainer of the well. Water re-collected from the well will be significantly reduced in arsenic and iron.

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 (EDWARDS 1994).

• Precipitation:the formation of insoluble compounds.
• Co-precipitation:the incorporation of soluble arsenic species into a growing metal hydroxides phases (e.g. co-precipitation with Fe(III);
• Adsorption:the electrostatic binding of soluble arsenic to external surfaces of the insoluble metal hydroxide.

Coagulation technology has been used since 1970 in northern Chile for removing arsenic from drinking water. This experience suggests that coagulation is an effective technology for the removal of arsenic. It is currently possible to reduce arsenic from 400 μg/L to 10 μg/L at a rate of 500 L/sec, assuming pH, oxidising and coagulation agents are strictly controlled (SANCHA 2006).

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 (VU et al. 2009). 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. Arsenic removal by coagulation is mainly controlled by pH and coagulation dose. Coagulation with ferric chloride works best at pH below 8 and Alum has a narrower effective range from pH 6 to 8 (JOHNSTON and HEIJNEN 2001).

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 (BAMWSP et al. 2001).

Use of naturally occurring iron present in groundwater

The use of naturally occurring iron present in groundwater is a promising method of removing arsenic by adsorption, meaning that there is no need for adding chemicals. The iron precipitates formed by oxidation of dissolved iron has been found to remove arsenic by coagulation, adsorption, precipitation and filtration, but also by oxidation. The efficiency of the units depends on arsenic and iron contents of water. It can be heightened by increasing the contact time between arsenic species and iron flocs.

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 (AHMED 2001). Lime softening may be used as a pre-treatment to be followed by alum or iron coagulation.

Solar oxidation and precipitation of Fe(III)-oxides with adsorbed As(V)

SORAS is a simple method that uses irradiation of water with sunlight in PET- or other UV-A transparent bottle (see also SODIS) to reduce arsenic levels in drinking water. The SORAS method is based on two steps: the first step consists in photochemical oxidation (through the action of the solar UV light) of As (III) to As(V) followed by the second step consisting in precipitation or filtration of As(V) adsorbed on Fe(III)-oxides, which are either naturally present or added and kept in suspension by the addition of lime juice. It could be a water treatment method used at household level to treat small quantities of drinking water. Groundwater in Bangladesh naturally contains Fe (II) and Fe (III); therefore SORAS could reduce arsenic contents and would be available to everyone at virtually no cost (WEGELIN et al. 1999).

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

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. Regeneration of saturated alumina is carried out by exposing the medium to 4% caustic soda (NaOH) either in batch or by flow through the column resulting in highly arsenic-contaminated caustic wastewater. Arsenic removal by activated alumina is controlled by pH and the arsenic content of water. The efficiency drops as the point of zero charge is approached and at pH 8.2 where the surface is negatively charged, the removal capacities are only 2-5% of the capacity at optimal pH (CLIFFORD 1990). Some examples of activated alumina based sorptive media are: the “BUET Activated Alumina”, the “Alcan Enhanced Activated Alumina” and the “Apyron Arsenic Treatment Unit”.

Granular ferric hydroxide

Granular ferric hydroxide is also used for the adsorptive removal of arsenate, arsenite and phosphate from water. Granular ferric hydroxide reactors are fixed bed adsorbers operate like a conventional filter with a downward flow of water. The water containing high dissolved iron and suspended matters should be aerated and filtered through a gravel/sand bed as a pre-treatment to avoid clogging of the adsorption bed.

Hydrous cerium oxide

Hydrous cerium oxide is also a good adsorbent; laboratory test and field-testing of the materials at several sites showed that the absorbent is highly efficient in removing arsenic from groundwater.

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.

Household-level arsenic filters

Some filters like the SONO 3 KALSHI, the KanchanTM or the SAFI arsenic filter use zerovalent iron fillings (solid iron), sand, brick chips and wood coke for removing arsenic, and other trace metals from ground water (see also arsenic filters). Arsenic is removed by adsorption on the mixture of partly oxidised zerovalent iron filling and sand.

The KanchanTM filter was developed by Massachusetts Institute of Technology (MIT), the Environment and Public Health Organisation (ENPHO) and the Rural Water Supply and Sanitation Program (RWSSSP) of Nepal. It combines slow sand filtration and adsorption on iron hydroxide and is efficient in removing arsenic, pathogens, iron, turbidity, odour and some other contaminants in drinking water. The filter is made up of a concrete or plastic box that is filled with layers of sand and gravel much like a biosand filter. On the top of the filter, as the first stage, a layer of 5 kg of iron nails is installed. These nails when exposed to air and water rust very quickly producing ferric hydroxides particles, which is an excellent absorbent of arsenic. When arsenic-containing water is poured into the filter, surface complexation reaction occurs, and arsenic is rapidly adsorbed onto the surface of the ferric hydroxide particles. The arsenic loaded iron particles are then flushed into the sand layer below. Because of the very small pore space in the fine sand layer, the arsenic loaded iron particles will be trapped in the top few centimetres of the fine sand layer. As a result, arsenic is effectively removed from the water.

The SAFI filter is an adapted ceramic candle filter that works by the principles of adsorption and filtration on chemically treated active porous composites materials of the candle. The filter is made of composite porous materials like kaolinite and iron oxide on which hydrated ferric oxide is deposited by sequential chemical and heat treatment. The oxy-hydroxides of iron, aluminium and manganese are involved in the removal of arsenic, iron and bacteria.

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 (CLIFFORD 1990). 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.

Other potential approaches would include the use of phytoremediation or rhizofiltration. Phytoremediation use green plants to remove pollutants from the environment and render them non available. Rhizofiltration is defined as the use of plant roots to adsorb, absorb, concentrate and precipitate metal from solution. Several plants species are known to accumulate metals from environment including water hyacinth (Eichhornia crassipes), which grows in waterways in many parts of the world and has been used as a model system for studying the uptake of metal ions by aquatic plants. Biomaterial produced from dried water hyacinth roots can provide a simple, effective and yet cheap method for removing arsenic from contaminated water. It has been found that more than 93% of arsenite and 95% of arsenate were removed within 60 minutes of exposure to a powder produced from dried roots. (RMALLI et al. 2005). Some researchers have shown that certain types of bacteria can play an important role in catalysing biological arsenic removal processes.

All the arsenic treatment technologies ultimately concentrate arsenic in sorption media, sludge or liquid media and indiscriminate disposal of these may lead to environmental pollution. Hence, environmentally safe disposal of sludge, saturated media and liquid wastes rich in arsenic is of high concern. Experiments were conducted to assess transformation of arsenic from aqueous solutions in the presence of cow dung. Some studies suggested that bio-chemical (e.g., bio-methylation) process in the presence of fresh cow-dung may led to significant reduction of arsenic from arsenic rich treatment wastes (ALI et al. 2001). Another option would be to blend the arsenic contaminated material into stable waste or engineering materials such as glass, bricks, concrete or cement blocks. However, there is also possibility of air pollution or water pollution downstream of kilns burning bricks containing arsenic contaminated sludge due to volatilisation of arsenic during burning at high temperature.