Arsenic Mitigation Technologies from Ground Water: A Brief Review

Contamination of drinking water due to the presence of as has become a global environmental and socioeconomic threat. The appearance of high Arsenic (As) in drinking water causes a serious health issue around the world. Many countries in different parts of the world have reported high arsenic concentrations. Among all groundwater arsenic contamination affected countries, the position of Bangladesh is the worst. Therefore, it is very important to develop affordable and efficient techniques to remove As from drinking water to protect human health. The most used methods are oxidation, coagulation, adsorption, ion exchange, and membrane technologies. Oxidation is usually used as pretreatment for most of the methods. Coagulation is the most common arsenic mitigation technology in Bangladesh. This technique is effective from pH 6-8. Ion exchange resins can only remove arsenate. Activated alumina beds work best in slightly acidic waters and usually have much longer run times than ion exchange resins. A cost-effective method for mitigation of As from drinking water is the use of low-cost adsorbent. Membrane methods which are more costly than other arsenic mitigation techniques but very effective where very low arsenic levels are required. Providing a safe water source may not possible in some of the arsenic affected regions or sometimes this process becomes very expensive. Mitigation of As from drinking water may be more appropriate in these situations. This paper presents a review of the conventional methods used for mitigation of As from contaminated drinking water.

waters and As(III) in anoxic waters, But they could coexist in both types of waters (Shankar et al., 2014). On the other hand, in case of organic forms the dimethylarsinous acid and monomethylarsonous acid are more toxic than original compounds (Petrick et al., 2000). Four types of As are usually available in groundwater of Bangladesh, these arearsenite (H 2 AsO 3 ) and arsenate (H 2 AsO 4 -) (Shankar et al., 2014) and monomethylarsonic acid [CH 3 AsO(OH) 2 ] as well as dimethylarsinic acid [(CH 3 ) 2 As(OH)] (Abedin et al., 2002). At early age when knowledge on the toxicity of arsenicals was very poor, arsenic sulphide (realgar) was occasionally used, both as a medicine and as a poison in Asian civilizations (Abedin et al., 2002). Several As compounds were used as wood preservative, pesticides in agriculture and has also been taken as medicine to treat many diseases like malaria, asthma, leukemia, eczema, chorea, African trypanosomiasis, skin and breast cancers though these uses have become outdated in recent years (Shah, 2012). Arsenic concentration in fresh water represents a health problem, since this element is toxic and carcinogenic in small quantities (Asere et al., 2019).
Arsenic in water is invisible enemy because it does not possess a particular color or taste (Bhowmick et al., 2018). Ingestion of large doses of As could lead to gastrointestinal symptoms, disturbances of cardiovascular and nervous system functions and finally death (Wang et al., 2009). However, intestinal problems are caused because of consuming in determined amounts; long time consumption develops a range of serious diseases such as high blood pressure, skin discoloration, blood vessel diseases, cancer of skin, kidney and lung and reproductive disorders (Smith et al., 2000). The European Union (EU), The United States (US) and The World Health Organization (WHO) have established a value of 10 µg/L as the maximum contaminant level for total As in potable water (WHO, 2011). Due to presence of As in drinking water many countries have faced significant health problems such as arsenicosis which is responsible for various diseases in bladder, skin, kidney and lung. According to WHO 10 µg/l is set as the standard value for arsenic in water (WHO, 2011). But the concentration of arsenic has been found very high in many countries such as Bangladesh (Islam et al., 2017), India, Vietnam (Berg et al., 2007), China (Yang et al., 2012), Mexico (Armienta and Segovia, 2008), Pakistan (Muhammad et al., 2010), Japan (Ahn, 2011), Korea, New Zealand, Hungary, USA (Haque and Johannesson, 2006), Argentina, Chile, El Salvador, Peru and Nicaragua (Jain and Singh, 2012) WHO reported that at least million people of 50 countries are exposed to arsenic through arsenic-contaminated groundwater at levels above 10 µg/L (WHO, 2011) Bangladesh was detected as one of the worst arsenic-affected countries in the world (Muhammad et al., 2010) in terms of population exposure to arsenic-contaminated water when 400 measurements were carried out in 6 (Smith et al., 2000). In almost half of the measurements, arsenic concentrations were above the maximum permissible limit of 0.05 mg/L which is safe limit for drinking water in Bangladesh (WHO, 2011). Here about 50 million people being at risk of As exposure. Department of Public Health Engineering (DPHE), Bangladesh, detected four tube wells in Chamagram, Nawabganj that yielded arsenic-contaminated ground water, and in the next year, eight arsenicosis patients were found in the same village (BGS and DPHE 2001). Soon after the identification of arsenic in tube well water, Bangladesh government initiated a number of programs with support from the national and international non-government organizations (NGOs) to determine the extent of arsenic contamination problem (NAISU, 2002). Survey showed that 27% of the shallow groundwater aquifers have arsenic concentrations greater than 50 µg/L in Bangladesh .
The percentage seems remarkably low but it is a matter of thought that in Bangladesh, more than 90% of the rural population gets drinking water from tube wells (Islam et al., 2017). The south and east parts of Bangladesh is the high risk region; here more than 60% of the tube-wells contain arsenic over 1 mg/L. DPHE randomly checked about 23,000 tube-wells and they identified that the southeast Dhaka was the worst affected area. Almost 20% of the shallow tube-wells contain As exceeding 50 µg/L, which is more than the Bangladesh standard. 80% of the As-contaminated hand tube wells showed 50µg/L (Chakraborti et al., 2015). In Bangladesh the As concentration in the groundwater has been found in wide range (<0.5->4600 µg/L) (Chowdhury et al., 2018). Different technologies have been implemented for the remediation of arsenic levels in drinking water in many countries (Radloff et al., 2017).
The most commonly used conventional technologies include lime treatment, co-precipitation, coagulationflocculation, oxidation, adsorption, ion exchange resin, membrane techniques, cementation, biological process and emerging technologies (Ahmed et al., 2006; Banerji and Chaudhari, 2016; Gallegos-Garcia et al., Iervolino et al., 2016;Meher et al., 2015;Yavuz et al., 2010;and Yazdani et al., 2016). There are almost 14 technologies worldwide for the mitigation of arsenic with efficiency varying from 70 % to 99%. One of the most common technologies have been coagulation with metal salts, lime softening, and iron/manganese mitigation. Coagulation processes are sometimes unable to efficiently remove arsenic to permissible levels. Membrane processes have high mitigation efficiency but these methods are highly expensive. In case of Ion exchange process high levels of total dissolved solids, sulphate, fluoride, and nitrate affect the method's efficiency (Lin et al. 2017).
Among all methods adsorption can be considered as a low-cost, simple, and eco-friendly process for Arsenic mitigation. But treatments of natural adsorbents are required to enhance the mitigation efficiency which is very expensive. The aims of the present review is giving a scenario of possible sources of As contamination of groundwater, associated health risks, available technologies for mitigation of As pollution in groundwater, discussion about prospects and limitations of different treatment processes and delineate the areas of further improvement.

Sources of Arsenic
As released from both geogenic and anthropogenic sources (Alarcón-Herrera et al., 2013). The nature of the aquifers and the process responsible for the As in groundwater has been evaluated in several studies. As is commonly found in several minerals, among these oxides and hydroxides of metals (Mn, Al, and Fe), sulfides, arsenides, and arsenites are major minerals (Nriagu et al., 2007). Arsenic pyrite is the one of most important mineral source in the ore zones responsible for geogenic contamination of As. The presence of pyrite in the reduced alluvial aquifers sediments in Bangladesh causes release of As into the environment due to the desorption and dissolution of naturally occurring As bearing minerals (Das et al., 2018). Pyrite is found to be stable under reducing conditions, in presence of aerobic conditions it oxidizes and release As due to formation of iron oxides (Patel et al. 2019). Iron hydroxide acts as a sorbent to increase the amount of dissolved As in groundwater. A number of studies have reported that organic matter enriching fine-grained shale and clay are highly enriched with As (Bayatkashkoli et al., 2017;Reza and Jean, 2012). However, several studies have suggested that iron oxide coated sand and grains of mica of the sediment are the potential adsorbent of As (Freikowski et al., 2013). In the hydro geochemical environment, Arsenic is released under oxidizing as well as reducing conditions, the former is more common (Howladar, 2017;Nicolli et al. 2010;Su et al., 2016).
Under oxidizing conditions, the key mode of As is released by oxidation of As bearing sulfide minerals like arsenopyrite (FeAsS) (Kim et al., 2012;Yoshizuka et al., 2010) and in case of reducing aquifers, As is released by reductive hydrolysis of metal hydroxides (Berg et al., 2007). The principal cause of As release from aquifer sediments is the reductive dissolution of Fe oxides (Guo et al., 2011). The level of pH in ground water plays an important factor for As enrichment in groundwater (Wang et al., 2019). Positively charged minerals, namely, Fe or Al oxides can absorb As easily (Islam, 2004).
At high pH values, the colloids as well as clay minerals carry the positive charges. The high level of As in groundwater is accompanied by high pH values ranges from 7.65 to 8.3. High occurrence of As in the soil might be occurs due to irrigation runoff (Polizzotto et al., 2013) as well as flooding may also be responsible for high As in groundwater (Yu et al. 2015). Furthermore, As also increases due to mining activities burning of fossil fuels, use of arsenical fungicides, herbicides and insecticides in agriculture and wood preservatives (Bose and De, 2013). Emission of As takes place in the environment because of volatilization of As 4 O 6 by burning of coal, which condenses and ultimately transferred into water reservoirs.

Exposure Pathways and Toxic Effects of Arsenic to Human Health
Several studies have been carried out to document the toxicity of arsenic and its impacts on human health in various arsenic-contaminated regions around the globe (Engel and Smith, 2004). The main routes of exposure are arsenic contaminated water and food cooked with that water. Water soluble inorganic arsenicals are rapidly absorbed from the gastrointestinal tract (Khairul et al., 2017). Arsenic usually enters the body in the As (III) form through a simple diffusion mechanism. A small amount of As (V) could cross cell membranes through an energy-dependent transport system then it is reduced to As (III) to binds to DNA or protein molecules (Jomova et al., 2011). Arsenic poisoning is undetectable in primary stages and depending on the amount of As consumed, and immune system of the individual, it takes above 8 years to impact health. Recent studies have reported that human intakes of arsenic in a range of 0.05 mgL -1 leads to arsenicosis. Arsenic is associated with cerebrovascular disease, cardiac disease, leucomelanosis and hyperkeratosis, diabetes mellitus, pulmonary disease as well as diseases of the capillaries, arteries and arterioles (Fontcuberta et al., 2011).
Chronic arsenic ingestion from drinking water causes several disorders of the digestive system, respiratory system, cardiovascular system, hematopoietic system, endocrine system, renal system, neurological system, and reproductive system (Santra et al., 2013). These diseases ultimately increase the risk for bladder, kidney, liver, lung and lymphatic cancer, and diseases of the blood-vessels of the legs and feet, and possibly high blood pressure. In Bangladesh, several studies reported that about 25 million people of 2000 village areas of Bangladesh are at risk of As contamination and 3695 out of 17,896 people tested are suffering from arsenicosis. A large number of people in rural Bangladesh are becoming affected by the arsenic contaminated groundwater, which they collect from the tube-wells. The rural people are also getting affected due to the consumption of arsenic-contaminated foods, which they produce by using ground water containing arsenic (Huq et al., 2006). Several skin diseases for example, melanosis, hyperkeratosis, keratosis and leucomelanosis, etc., are the most common effects of drinking arsenic contaminated water. The other effects are liver enlargement and cirrhosis, peripheral neuropathy, hypertension, chromosomal abnormality, cardiac failure, diabetes mellitus, goiter, skin cancers and gangrene (Yunus et al., 2011).
The prolonged drinking of arsenic-contaminated water has effect on children's cognitive and psychological development (Asadullah and Chaudhury, 2011). Higher fetal loss and infant deaths have also been found in the regions where groundwater is highly arsenic contaminated (Sohel et al., 2010). Arsenic is also known to cause cytotoxicity, epidemiological toxicity and genotoxicity (Gentry et al., 2010, Suzuki et al., 2007. Epidemiological studies on the effects of arsenic consumption from drinking water on public health indicated a carcinogenic effect. Expert's indicated that a concentration level of 50 µg/L could lead to cancer in 1 in 100 individuals. During chronic poisoning, As causes strong pigmentation of hand and foot known as keratosis, and problems in other body system such as respiratory, neurological, high blood pressure, endocrine, cardiovascular and metabolic disorders (Ferlay et al., 2015).

Treatment Technologies for Mitigation of As from Ground Water
Many conventional and advanced treatment methods have been proposed for mitigating As from ground water under both laboratory and field conditions. Common methods for mitigation of As from contaminated water are use of chlorine, ozone, etc., physical method such as UV treatment, other filtration techniques such as reverse osmosis, membrane filtration, flocculation, adsorption, ion-exchange, etc. The following sections will demonstrate and evaluate some efficient and practical techniques for the purpose of arsenic mitigation. The performance and the deficiency of the existing techniques are further illustrated specifically.

Arsenic Mitigation by Oxidation
A common pretreatment step in most of the arsenic mitigation technologies is the oxidation of As (III) to As (V) because As (III) is the pre-dominant form of arsenic at neutral pH and adsorption of As (V) onto solid surfaces is easier than As (III) (Sharma et al., 2007). Thus, oxidation followed by adsorption is thought to be efficient for the mitigation of As (Leupin and Hug, 2005).
Arsenite could be oxidized by atmospheric oxygen, ozone or chemical oxidants. Chlorine, chlorine dioxide, hypochlorite, hydrogen peroxide, Fulton's reagent and potassium permanganate are usually applicable in this purpose, due to low cost and ease of availability (Lee et al., 2003).
54-57% of As (III) can be oxidized to As (V) in contaminated water with air and O 2 but complete oxidation (III) can be performed with ozone (Dodd et al., 2006). But this process is very expensive due to high energy input and mitigation of residual ozone and toxic. In general, oxidation with atmospheric air is a very slow process compared to chlorine and permanganate under wide range of conditions. However, chlorine dioxide is prohibited to use for surface water, American environmental agencies pay much attention to this factor. Chloroamine and hydrogen peroxide are slower than permanganate, chlorine and ozone. It is accepted that free Chlorine or hypochlorite is very effective for the oxidation of As (II), while the oxidation of As (III) is well achieved by ozone. However, chlorine dioxide is prohibited to use for surface water, American environmental agencies pay much attention to this factor. When manganese dioxide coated sand is combined with Fe containing compounds, this becomes efficient oxidizing agents because the treated products are easy to handle but in this process another treatment step is required for the mitigation of Mn from water. About the mitigation of arsenic, utilizing FeO 4 2for purifying water is recommended HFO (hydrous ferric oxide) appears to be the most important phase responsible for removing the arsenic from drinking water around the world. It is well documented that High arsenic came into contact with iron oxides in the shallow aquifer, which could possibly lead to arsenic mitigation.
Thus, oxidation is very effective process for the mitigation of arsenic from water. Interfering particles present in water such as Fe (II), Mn (II), sulfide (HSand S 2-), total organic carbon (TOC) and dissolved organic carbon affect this process. Due to presence of S 2and TOC, the oxidation rate of As (III) by ozone decreases significantly (Dodd et al., 2006). Thus, for the appropriate selection of oxidizing agents to accomplish high mitigation efficiency by oxidation it is very important to consider hydrophite chemistry and water composition.

Solar Oxidation Technique
In several studies photochemical and photocatalytic oxidation of As (III) has been investigated. UV irradiation increases the oxidation rate of As (III) with oxygen. This procedure can be catalyzed using sulfite, ferric iron or citrate. In case of solar light hydroxyl radicals generate by the photolysis of Fe (III) species. The oxidation rate is catalysed in presence of hydroxyl radicals. When As contaminated water in perchlorate and perchloric solution at pH 0.5-2.5was treated with Fe (III) followed by exposure to solar light the rate of mitigation of As increases. If lemon juice is applied after exposoure of solar light the mitigation rate of As from water becomes higher because the reaction of lemon juice (citrate) with strongly oxidizing radicals produces further radicals (Lara et al., 2006). Adsorption of As on TiO 2 after the oxidation of As (III) to As (V) by photocatalytic oxidation and TiO 2 was also investigated in several studies. This process can reduce arsenic levels to less than the standard limit given by WHO for drinking water (Miller et al., 2011). A TiO 2 impregnated chitosan bead (TICB) was synthesized and used for mitigation of As from aqueous solution. This study suggested that in the presence of UV light, a larger amount of As is adsorbed due to the increase of the surface area of the TICB and TiO 2 was able to photo-oxidize more As (III) to As (V) when comparing with the solution that was not exposed to UV light. In another study, nano-crystalline Al 2 O 3 and TiO 2 impregnated chitosan was prepared for As mitigation (Yamani et al. 2012). Several factors such as the initial As concentration, pH, the presence of natural organic matter (NOM) and anions also influenced the rate of As (V) adsorption on TiO 2 (Bang et al., 2005, Miller et al., 2011. When a very low amount of TiO 2 is present the TiO 2 /UV system has an inefficient As mitigation due to incomplete oxidation (Guan et al., 2012). In addition, an acidic pH was more effective for adsorption of As (V) on the TiO 2 surface (Dutta et al., 2004). Presence of silicate, fluoride and phosphate bicarbonate affect the photocatalytic oxidation of As (III), and adsorption of As on the TiO 2 based adsorbent (Guan et al., 2012). Moreover, the treatment required for mitigation of arsenic residues is very complex. Due to several limitations, only oxidation is not considered as a highly effective procedure for mitigation of As.

In-Situ Oxidation
In the in situ oxidation process of arsenic and iron, the tube well water is allowed to oxygenate arsenite to arsenate by the oxygen which is present in the air and the ferrous iron in the aquifer is oxidized to ferric iron (Nicomel et al., 2016). This leads to a reduction in arsenic content in tube well water. The oxygenated water which containing As, and iron is flow back into the same tube well. When water is extracted again from the tube well, the concentration of arsenic will be low due to underground-precipitation and adsorption on ferric iron. The probable reactions of hydrous iron oxide with arsenate are shown below. This process is suitable where the source of water is tube well or deep wells and others. To avoid contamination of the subsurface by introducing microbes from the surface care must also be taken. Some pore spaces may also become clogged with precipitates if dissolved iron levels are high in water. The potentiality of this process for the mitigation of arsenic is little; the results also indicate that arsenic concentrations can be minimized in the groundwater zone before water extraction (Sharma et al., 2007).

Arsenic Mitigation by Biological Oxidation
Several bacteria known as arsenate respiring bacteria (ARD) may couple anaerobic oxidation of organic substrates to the reduction of arsenates such as Geospirillum arsenophilus, Geospirillum barnesi, Desulfutomaculum auripigmentum, Bacillus arsenicselenatis, and Crysiogenes arsenatis (Oremland et al., 2009). Due to formation of undesirable byproducts, application of chemicals in drinking water treatment is often discouraged. Several species of bacteria have been applied to carry out biological oxidation of As. Bacterial activity acts as catalyst to remove the arsenic from water. Some microorganisms such as Gallionella ferruginea and Leptothrix ochracea accelerate bioticoxidation of iron (Katsoyiannis et al., 2004;Ekhlas et al., 2014). Iron-oxides and micro-organisms were settled in a filter medium, which has a adjuvant environment for the adsorption of arsenic. These microorganisms oxidize As (III) to As (V), which adsorbed on Fe (III). This leads to up-to 95% mitigation of arsenic. Bacterial oxidation of As (III) followed by mitigation of As (V) by sorption onto biogenic manganese-oxides was also studied (Sharma Mitigation of As (III) and As(V) from groundwater by biological oxidation of dissolved Fe and Mn in a pipe reactor (PR), followed by microfiltration (MF) was also studied. The latest PR-MF process is very efficiently removes Fe, Mn, and As without application of toxic chemicals for oxidation purpose or pH adjustment and there is no need of regeneration or backwashing and follows the principles of green chemistry.

Arsenic Mitigation by Coagulation-Flocculation
Coagulation followed by flocculation is another widely used treatment for the mitigation of arsenic from ground water ( . It is commonly used for larger capacity facilities and it requires the production of a floc used to mitigate As from groundwater. Among other various chemical coagulants, this process usually requires Fe and Al based coagulants i.e. ferric chloride, ferric sulfate, aluminium sulfate. These chemicals need to be added and dissolved in water under efficient stirring for 1-10 minutes. In this process, Cationic coagulants have to decrease the negative charge of colloids and aggregation of particles forms larger particles (Choong et al., 2007). More than 90% of As (V) and 77% of As (III) can be removed by this technique. Oxidation of As (III) to As (V) with the addition of hypochlorite or potassium permanganate is required for effective mitigation. Aluminium chloride and polyaluminium chloride are able to reduce the concentration of As below the MRL . When kaolinite and FeCl 3 are used as a coagulant/flocculent, mitigation efficiency is over 90%and 77% for As (V) and As (III), respectively (Pallier et al., 2010). Fe based coagulants have been found to be most efficient in water treatment than the Al based coagulants (Katsoyiannis et al., 2004).
For efficient mitigation of arsenic from water, the arsenic needs to be adsorbed on amorphous metal hydroxides formed from coagulant. The rate of As mitigation is dependent on the quality and pH of the water before coagulation. The presences of organic matter in groundwater also affect the mitigation efficiency of this technique. The optimum mitigation was observed at pH below 8.5. There is critical limitation in the process of coagulation/flocculation; it produces a large amount of sludge along with a big concentration of arsenic. The management of this sludge is required to prevent the impact of secondary pollution of the environment and the treatment procedure of sludge is expensive (Mondal et al., 2013). Moreover, in many cases it becomes difficult to lower the arsenic concentration to the acceptable level by this technique (Shakoor et al., 2017).

Adsorption
Adsorption is a simple process in which water is flown through a packed bed of solid adsorption media filled in a column (Shakoor et al., 2017). Solutes adsorb on the adsorbent surface and its concentration become reduces in the solvent (

Activated Carbon
Activated carbon is graphite with amorphous structure with a wide range of pore sizes. Activated carbon is used either in powdered or granular form for arsenic mitigation. Ancient Hindus in India used Charcoal for filtration of drinking water and carbonized wood was a purifying agent and medical adsorbent in Egypt by 1500 b.c. Bone-char was replaced by activated carbon in sugar refining in 1901. Activated carbon was first used in the US for treatment of water in 1930. Arsenic adsorption onto pure activated carbon is very poor and regeneration is also difficult, so it is not directly applied for water treatment (Daus et al., 2004).
Increase of arsenic absorption capacity by activated carbon could be achieved when it is treated with various metal compounds. Impregnating iron compound onto activated carbon and treatment of activated carbon with Zr are two way in this purpose. The latter was not suitable for drinking water because of toxic nature. Active carbons are prepared from bones, coconut shells, bagasse, carbon cobs, cereals blood, coal, coffee beans, bark, fish, fertilizer waste slurry, wood, coal, lignite, coconut shell peat etc (Deng et al., 2016). Adsorption capacity depends on activated carbon properties, adsorbate chemical properties, temperature, pH, ionic strength, etc. Although availability of activated carbons, this process remains expensive and vast quantities of activated carbon is required (Ahmad et al., 2018).

Activated Alumina
Activated alumina (AA) successfully removed arsenic from ground water (Xie et al., 2013). Activated alumina is a granulated form of aluminum oxide (Al 2 O 3 ) having good sorptive surface. This is an effective medium for arsenic mitigation from water with high dissolved solid content (Golami et al., 2009). The mechanisms of arsenic mitigation are similar to ion exchange resin, and are often collectively referred to as 'adsorption'. Its efficiency is greater than 95% under acidic conditions when Alumina surface is protonated. This process is controlled primarily by pH and mitigation capacity is effective in the narrow pH range from 5.5 to 6.0 where the alumina surfaces are protonated. Above pH 8.5, Arsenic mitigation capacity is reduced to only 2-5% of capacity at optimal pH. Therefore, pH adjustment is required for efficient arsenic mitigation from neutral and basic waters. For arsenic mitigation, fine (28-48 mesh) particles of activated alumina are commonly used. Arsenic adsorbed on the alumina surface during the flow of water through surface. After that column becomes saturated first at upstream zone later at downstream zone and finally it is completely saturated. The advantages of activated alumina are that it requires no chemical addition, it can treat thousands of bed volumes before breakthrough, and filters could be operated for months before the media need to be regenerated.
Regeneration of saturated alumina is done by exposing the medium to 4% caustic soda, NaOH, resulting caustic waste water with high arsenic content. After washing out of the residual caustic soda, 2% solution of sulfuric acid is used to neutralize the medium. During regeneration, the efficiency of the regenerated medium is decreased by 30-40%. If aluminium based sludge is produced during rinsed with sulfuric acid, this sludge will contain a high amount of arsenic because of its arsenic adsorption characteristics. This sludge and the remaining liquid fraction of the solution will require disposal because both residuals contain arsenic. When the AA has reached the end of its useful life, the media itself will also become a solid residual that must be disposed (Tripathy and Raichur 2008).

Natural Wastes and Agriculture Wastes
The adsorption technique using agricultural waste has been investigated as a cost-effective method for the mitigation of arsenic in wastewater. Lignin and cellulose in agricultural material contained aldehydes, ketones, carboxylic, alcohols, ether and phenolic groups, which may bind heavy metal ions through complex formation (Ahluwalia et al., 2005). The technique has advantages over others due to simplicity, low cost, high efficiency, flexibility and recovery (Gueye et al., 2016). Different agro-wastes are used as adsorbents for heavy metal mitigation such as orange peel, sawdust (Memon et al., 2008), banana peels (Israt et al., 2008), potato peel, rice straw, seaweed (Basha et al., 2008), wood and bark, tea waste (Malkoc et al., 2007), maize corn cob, jatropha oil cake, sugarcane bagasse (Garg et al., 2007), tamarind hull (Verma et al., 2006), rice husk, saltbush (Sawalha et al., 2005), marine algal biomass, olive pomace, activated sludge, sugar beet pulp, wool, olive cake, sawdust, pine needles, almond shells, cactus leaves and charcoal, seafood processing waste sludge and pine bark (Charlet et al., 2007). The fresh agro wastes were washed with distilled water to remove all dirt, cut into smaller sizes and air dried to remove the free water before oven dry. Then dried in oven at 100°C for 24 hours and homogenized in a blender to utilize as an adsorbent.

Ion Exchangers
Ion exchangers are also used for mitigation of As from water (Oehmen et al., 2006). Tetrahedron ion exchange resin filter tested under rapid assessment program in Bangladesh (BAMWSP et al., 2001) showed promising results in arsenic mitigation. In ion exchange, through dissolved ion the reversible displacement occurs of an ion that will adsorb on the surface of solid materials. For the treatment of As strong base anion exchange resins are commonly used where the oxyanionic species of arsenate are effectively exchanged with the anionic charged functional group of the resin (Chang et al., 2010). Thus produces effluents with low concentration of As (V) (Choong et al., 2007). Sulfate, phosphate and nitrate are mostly used for the mitigation of As via ion exchange. In case of arsenite oxidation of As (III) to As(V) is an important pretreatment step for ion exchange processes because arsenite usually exists as a neutral molecule. In this process at first the resin bed are usually flowed through HCl to implant labile Clon the surface of resin, which is later exchanged with As. Thus, the effluent contains a large amount of Cl-and additional step is needed to enhance the quality. The arsenic exchange and regeneration equations with common salt solution are as follows: There are several disadvantages of this process such as. During the presence of sulfate in the treated water exchange of sulfate is more preferable than As for commercial resins. Additionally, impedence of the resin takes place due to iron precipitation and solid particles in aqueous solution. So the resin becomes exhausted, it needs to be regenerated which increases the cost of operation. Due to the generation of large quantities of toxic sludge and is considered as a more expensive treatment compared with other techniques (Ahmad et al., 2017). Moreover, during regeneration of resins, As rich brine solutions are produced which may again lead to environmental release of As (Cundy et al., 2008).

Membrane Filtration
Membrane filtration technique is also used for the mitigation of arsenic from groundwater. Membranes are synthetic materials having billions of pores serving as selective barriers which allow some dissolved compounds to pass through but retains contaminants. Pressure difference between the feed and the permeate sides is the driving force which is needed to transport the water through the membrane. There are two types of pressure-driven membrane filtrations: (i) lowpressure membrane processes, such as ultrafiltration (UF) and microfiltration (MF); and (ii) high-pressure membrane processes, such as nanofiltration (NF) and reverse osmosis (RO) (Shih, 2005).
These Separation processes mostly depend on the the membrane's pore size and increasing driving pressure increases the selectivity of the processes. Separation is accomplished via mechanical percolation for MF and UF membranes while, capillary flow or diffusion is responsible for the separation by NF and RO membranes (Shih, 2005). The size of the soluble As are small enough to pass through the MF membrane. Thus the efficiency of the process is highly dependent on the size distribution of As bearing particles in water (Shih, 2005) for improving the mitigation efficiency, such as coagulation and flocculation (Singh et al., 2015). The mitigation of As using flocculation followed by MF was better than flocculation sedimentation when FeCl 3 and Fe 2 (SO 4 ) 3 are used as flocculants. But adsorption of As on the Fe (III) complex was interfered by the pH and other ions present in solution. UF is also a low pressure technique which is alone not effective for mitigation of As due to large membrane pores (Velizarov et al., 2004). The membrane's pores are not sufficient small to remove the dissolved As in water. As a result, surfactant-based separation processes such as micellar-enhanced ultrafiltration (MEUF) can be utilized to increase the mitigation of As (Iqbal et al., 2007). Micelles are formed after the adsorption of As onto the surface of the cationic surfactant, which are removed by UF. The study conducted by (Brandhuber and Amy 2001) found a moderate rejection of 65%and 53%for As (V) and As (III), respectively. Among different cationic surfacetants hexadecylpyridinium chloride (CPC) showed the highest mitigation efficiency, i.e., 96%. However, the mitigation of As was depended on the pH of the solution, the initial As concentration and the presence interfering particles (Mondal et al., 2013).

Reverse Osmosis
Reverse Osmosis is high pressure technique and able to remove dissolved As from water to an appreciable level (Figoli et al., 2010). Both As (V) and As (III) can be effectively removed (up to 99%) from water RO membranes (BAMWSP et al., 2001). Here size exclusion regulates the separation and not the charge interaction. Charged membranes generally have a higher rejection for charged solutes than for noncharged solutes (Seidel et al., 2001). The size of pore of the membrane does not have a significant effect on As rejection, but rather charge exclusion is predominant over the size exclusion mechanism. RO membranes are non-porous so transport of the solvent occurs through the free volume between the segments of the polymer of which the membrane is composed. These membranes are used for water desalination and low molecular mass compounds can be highly rejected (Velizarov et al., 2004). It was shown that the mitigation efficiency for As (V) exceeded 85% while that of As (III) was far too low (Uddin et al., 2007). In case of RO membranes initial concentration of As had no effect on the removal rate but the removal of As was affected by the pH of the solution and operating pressure. The effect was much higher for As (III) than As (V). Yoon et al. (2009) concluded that mitigation of As (III) below pH 10 was low because of the existence of uncharged As (III) species in solution. When As (III) was the dominant As species, the mitigation efficiency of As from ground water became less than 50% (Walker et al., 2008). Therefore, RO is not effective for As contaminated water where As (III) is dominanted. Oxidation is required as a pretreatment step to mitigate total As at the desired concentration. As RO is very expensive, it is not favorable in developing countries (Uddin et al., 2007).

Nano-Filtration
NF is suitable for the mitigation from water of dissolved compounds with a molecular weight above 300 g/mol (Seidel et al., 2001). In neutral and alkaline solution NF membranes are generally possesses negatively charged. Here separation of As is occurred due to electrostatic repulsion between the anionic As species and the charge of membrane (Velizarov et al., 2004). NF is more sensitive than RO with respect to pH and the solution's ionic strength. The rejection of As (V) was better than As (III) and As (III) could not be reduced to MCL (Uddin et al., 2007). Thus oxidation of As (III) to As (V) is required pretreatment step. Saitua et al. (2005) found that As rejection was independent of transmembrane pressure, cross-flow velocity as well as temperature. But a recent study demonestrated that the efficiency of As (V) mitigation increased with increase of pH, decrease of operating temperature and the initial As concentration (Figoli et al., 2010). Although NF gives desired results in mitigation of As (V) from solution, the problem of this technique is the mitigation of the uncharged As (III) species in the near neutral pH range. Thus, without oxidative pretreatment, NF is not enough to remove total As (Uddin et al., 2007).

Comparison between Different Techniques
Many factors can affect arsenic mitigation efficiency including arsenic concentration, speciation, pH and cooccurring solutes. Therefore, any method should be tested using the actual water to be treated, before implementation of arsenic mitigation systems at the field scale.

CONCLUSION:
The accumulation of arsenic in drinking water from different sources plays an important role in water pollution and this is a major worldwide problem. Arsenic has created health problems around the world; arsenic has been documented in the Americas, Africa, Asia, Europe and Pacific countries. Bangladesh, India, Nepal and North American countries are having severe condition of arsenic contamination. Many Governments and WHO had made several efforts to improve the condition of contaminated groundwater and its severe effect on human health. But as Bangladesh is highly mass infected, this problem is not in control here and it is going bad day by day. Among different types of sources anthropogenic sources are getting more effective day by day in modern ways of development throughout the world. Arsenic enters in the human body by direct consumption of drinking water contaminated by arsenic and the indirect intake through foods and crops cultivated using arseniccontaminated water. For arranging safe drinking water, it is required to mitigate of arsenic from the water source. The conventional arsenic mitigation technologies include oxidation (biological and chemical), adsorption, ion exchange resin, membrane processes, co-precipitation and bacterial treatment. Most of these technologies for mitigation of arsenic involve the direct mitigation of As(V) or converting As(III) to As(V) followed by mitigation of As(V). Many water purifier manufacturing companies are using reverse osmosis due to its high efficiency. But in case of mass infected people, it seems that it is unable to fulfill the requirement. However, more research is required to evaluate the practicability and feasibility of these treatment processes as well as to lower the expenses of treatment. The government should monitor and document industrial and agricultural activities as they brought the arsenic pollution issue to the bodies of water in the first place. There should analysis of the discharge from industrial plants, which aim to supply safe drinking water to people in rural areas. The government should take steps and put restrictions about the handling of industrial waste. Investment should be employed on the great engineering system for water transportation and documentation of water quality.

ACKNOWLEDGEMENTS:
The authors gratefully acknowledge the financial supports of the Bangladesh Council of Scientific and Industrial Research (BCSIR) by providing "D.
Abdullah Al Muti-Shorfuddin Memorial Fellowship". The authors' are thankful to INARS, BCSIR for providing technical support and facilities.

CONFLICT OF INTEREST:
The Authors declares that there is no potential conflict of interest.