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Review ArticleEffects of Arsenic Toxicity on the Environment and

Its Remediation Techniques: A ReviewMuhammad Yousuf Jat Baloch a,b, Shakeel Ahmed Talpur a, Hafeez Ahmed Talpur c, Javed Iqbal a,

Sajid Hussain Mangi b, Shumaila Memon d

a School of Environmental Studies, China University of Geosciences Wuhan, Hubei, P. R. Chinab Institute of Environmental Engineering and Management, Mehran University of Engineering & Technology, Jamshoro, Pakistan

c Centre for Environmental Sciences, University of Sindh, Jamshoro, Pakistand School of Environmental Sciences, Northeast Normal University, Changchun, P. R. China

ABSTRACTContamination of arsenic (As) in water, especially groundwater, has been recognized as a major prob-lem across the world. The presence of arsenic in groundwater has become a global problem in the past decades. Health risks have also been reported for many years. Different areas of the world are affected by arsenic contamination of groundwater, the largest population at risk in Bangladesh, followed by West Bengal in India. Arsenic concentrations in drinking water cause severe health effects on human, more than 150 million people worldwide. The current drinking water standard regulation has become strict and requires a reduction in arsenic content. Therefore, the treatment of arsenic contaminants can be the only effective option to reduce health risks. This review paper briefly describes arsenic sources, arsenic chemistry, arsenic contamination in groundwater, its impact on human health and many conventional as well as advanced techniques that are used to remove arsenic from water.

Keywords: arsenic (As), effect of arsenic on environment, health hazard, arsenic chemistry, arsenic removal technologies

INTRODUCTION

Arsenic (As) occurs naturally at elevated levels of groundwater in many countries around the world, including mostly in South America and Southeast Asia [1,2]. The main arsenic regions are currently located in large delta and / or along major rivers emerging from the Himalayas [3], as in the Bengal Delta [4] and other parts of India [5], Nepal [6], Pakistan [7], Myanmar, Vietnam, Cambodia [8,9] and China [10]. The most dangerous inorganic pollutant in groundwater is arsenic and is recognized as an important environmental cause of cancer deaths worldwide [11–15]. Arsenic exerts its toxicity on multiple organ systems including cardiovascular, hematologic, hepatic, nervous, renal, and respiratory sys-tems. It has also been associated with many types of cancer (skin, lung, liver, and bladder) [16]. Nutrition deficiency in

woman also found due to arsenic exposure and has been reported in various studies [17–19]. However the organic arsenic is less toxic than inorganic arsenic. Arsenic can oc-cur in the environment with different chemical forms such as monomethyl arsenic acid [CH3AsO (OH)2], dimethylarsinic acid [(CH3)2AsOOH], trimethylarsine oxide [(CH3)3AsO], arsenobetaine (AsB) [(CH3)3As+CH2COOH], arsenocholine [AsC], arsenosugars [AsS], arsenolipids, etc [20]. Arsenate (As(V)) is a thermodynamically stable condition in aerobic water, while Arsenite (As(III)) is predominantly in a low oxidation environment. As(III) is usually more toxic than As(V) [21–23]. In literature, it is well reported that inorganic arsenic species are more toxic than the organic species mono-methylarsenate (MMA) and dimethylarsinate (DMA). Heavy metals can pose health risks to human and animal life if their concentration exceeds permissible limits. Arsenic is distrib-

Corresponding author: Muhammad Yousuf Jat Baloch, Email: engr.yousuf@yahoo.comReceived: September 30, 2019, Accepted: February 4, 2020, Published online: October 10, 2020

Open Access

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC BY) 4.0 License. http://

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Journal of Water and Environment Technology, Vol.18, No.5: 275–289, 2020doi: 10.2965/jwet.19-130

Journal of Water and Environment Technology, Vol. 18, No. 5, 2020 276

uted from these sources into air, water and soil and finds its way into the human system through direct inhalation or by contamination of food and consumer products. The World Health Organization (WHO) recommended the guideline value of 10 µg/L of arsenic in drinking water. Arsenic release into groundwater has become a health and environmental issue worldwide. In many countries, groundwater extracted from millions of wells is highly contaminated with arsenic up to 3500 µg /L, which is the only source of drinking water for people in many developing countries around the world [24,25]. Therefore, the removal of arsenic from groundwater is needed. The mobility of arsenic forms in water depends heavily on pH, Eh conditions and the existence of different chemical types [26,27]. The speciation of arsenic is sensi-tive to the pH conditions, oxidizing and reducing conditions [28]. However, under oxidizing conditions, H2AsO4

− is the dominant species at pH values lower than 6.9, while at higher pH conditions, HAsO4

2- becomes dominant. The pH of groundwater is generally in the range of 6.5 to

8.5 [29] with both oxidizing and reducing conditions, under which the predominant arsenic species are H3AsO3

0 for As(III) and HAsO4

2- for As(V). The X-ray Absorption Near Edge Structure (XANES) studies have demonstrated that arsenic speciation in the groundwater was usually associated with reducing conditions, and As(V) could be converted to As(III) in the sediments [30].

It has been well studied that at typical redox potentials of aerobic and oxygenated aquatic environment, As(V) species dominate As(III) species and at these redox potentials, a decrease in pH would cause an increase in the amount of As(III) over As(V). It is also predicted by the equilibrium thermodynamic calculation that As(V) should dominate over As(III) in oxidizing environment while As(III) can only be present in strongly reducing conditions. In addition, the coexistence of different arsenic species under the complex pH and redox conditions has raised new requirements for the proposal of remediation technologies. For the past three decades, several studies have shown that drinking arsenic-contaminated water should be one of the major concerns for the health of mankind. Thus, strategies to avoid arsenic con-tamination of the groundwater and/or to alleviate the impact of such contamination need to be developed in an attempt to reduce the health risks associated with the intake of arsenic-contaminated water. This review paper aims to provide a general description of the arsenic sources, arsenic chemistry, arsenic contamination, its impact on human health and many conventional as well as advanced techniques that are used to remove arsenic from water sources.

ARSENIC CHEMISTRY

Arsenic occurs in a free state, and it is mainly found in combination with oxygen as well as iron and sulphur [31,32]. In groundwater, arsenic combines with oxygen to form an inorganic arsenate pentavalent and arsenite trivalent. Unlike other heavy metals and component constituents of arsenic, arsenic can be mobilized at the pH values normally found in surface waters and groundwater (pH 6.5 to 8.5) and under oxidation and reduction conditions [33]. In natural water, their predominant forms are oxygen inorganic oxyanion of Arsenite trivalent As(III) or arsenate pentavalent As(V) [27]. Trivalent arsenic is about 60 times toxic than arsenic in oxidized five-state, and inorganic arsenic compounds are 100 times more toxic than arsenic compounds [32]. Arsenic can occur in the environment in many oxidation states (−3, 0, +3 and +5). Organic forms of arsenic are not quantitatively important and are found mostly in surface waters and also those areas severely affected by industrial pollution [27]. Increased risk of arsenic associated with drinking water was reported at concentrations below 50 µg/L [34]. The concentrations of As(III) to As(V) vary widely, depending on oxidation conditions in the geological environment [32].

Transformation and reactions of arsenicArsenic reaction with hydrogen

Arsenic dioxide can be reduced by hydrogen gas to form arsenic. Arsine (AsH3) is very toxic and more intense gas than air. When arsenic is heated, it breaks down into arsenic and hydrogen.

Arsenic trioxide reaction with hydrogen gas to form water and Arsine:

As2O3(s) + 6H2(g)→3H2O(l) + 2AsH3(g) (1)

Arsenic reaction with waterPure arsenic is not soluble in water; however many arsenic

compounds dissolve easily in water. Arsenic in the form of arsenic trisulfide, arsenic acid are examples of arsenic com-pounds that found their way into the water supply.

Arsenic trisulfide reaction with water to form hydrogen sulphide and arsenious acid:

As2S3(s) + 6H2O(l)→3H2S(g) + 2H3AsO3(aq) (2)

Arsenic reaction with oxygenSolid arsenic is oxidized when exposed to oxygen. The

surface of the metalloid becomes black. When heated in

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oxygen gas, arsenic is associated with oxygen to form zinc tetraoxide or arsenic oxide.

Arsenic reaction with oxygen gas to form arsenious oxide:

4As(s) + 5O2(g)→As4O10(s) (3)

Arsenic reacts well with halogensArsenic pentafluoride is a very toxic colorless gas. When

liquid arsenic trichloride is cooled by a free chlorine mol-ecule, arsenic pentachloride can be formed. When solid arsenic reacts with bromine gas, it becomes a light yellow solid called arsenic tribromide.

The reaction of arsenious oxide with hydrochloric acid to form water and arsenic trichloride:

As4O6 + 12HCl(aq)→4AsCl3(l) + 6H2O(l) (4)

The reaction of arsenic trichloride with Cl2 to form arsenic pentachloride:

AsCl3(l) + Cl2(g)→AsCl5(s) (5)

The reaction of solid arsenic with Iodine gas:

2As(s) + I2(g)→2AsI3(s) (6)

SOURCES OF ARSENIC

Arsenic is an element found in the atmosphere, soil, rock, natural water and living organisms. It occurs as a key ele-ment in more than 200 metals, including primary arsenic, ar-senides, sulphides, oxides, arsenate and arsenites. The most common minerals are ore or modified products. However, these minerals are relatively very rare in the natural environ-ment. The largest concentrations of these minerals occur in mineral areas and are found in close association with transi-tion metals. It is generally accepted that arsenopyrite, along with other dominant substances in sulphide and its original mixtures, is formed only under high-temperature conditions in the Earth’s crust [27]. The primary source of As in the environment (aquifer, biosphere and atmosphere) is the re-lease of As enriched minerals. As sources include natural insecticides, herbicides, phosphate fertilizers, semiconduc-tor industries, mining, smelting, industrial processes, coal combustion, wood preservatives, etc [35,36]. Most element concentrations tend to occur in sulphide minerals, where pyrite is the most abundant. Arsenic may also exist in the crystalline structure of many other sulphide minerals as an alternative to sulphur. High concentrations are also present in many oxide minerals and aquatic oxides, either as part

of the metal structure or as sorbed species. The concentra-tions of arsenic in igneous rocks are generally low. Arsenic concentration in sedimentary rocks is usually between 5 and 10 mg/kg [37], slightly higher than the average ground abundance of 1.5 to 3 mg/kg [38].

OCCURRENCE OF ARSENIC IN GROUND-WATER

According to World Health Organization rules, the sug-gested measure of arsenic in drinking water is 10 µg/L. However, the levels as in non-contaminated surface water and groundwater usually vary from 1–10 µg/L. The WHO’s level of As in water is 10 µg/L. In a world in excess of 100 million individuals consume the water which is exceeds the WHO limit, out of these 100 more than 45 million people are mostly in developing countries from Asia [39]. Table 1 summarizes groundwater contamination in different parts of the world. Large areas of Bangladesh, West Bengal and other states of India and Vietnam based on contaminated groundwater to irrigate staple crops such as rice [40,41]. The consumption of food grown in polluted or irrigated soils, which poses a life-threatening problem to millions of people in large areas of south East Asia. As through food with some nutrients, especially rice and vegetables, being reported to have high inorganic concentrations, such as in elevated As in soils and irrigation water [42–45].

ARSENIC TOXICITY AND ITS EFFECTS ON HUMAN HEALTH

Various studies have been conducted to assess As toxicity and its effects on human health in different contaminated areas [46,47]. The arsenic in vegetables, followed by inges-tion, may contribute fundamentally [48]. Recent studies have shown that chronic arsenic exposure is linked to lifestyle-related diseases such as metabolic syndrome, diabetes, and respiratory complications followed by ingestion of contami-nated vegetables. Because the groundwater is used for irriga-tion to cultivate a variety of crops and vegetables, so irriga-tion of fertile groundwater is the main pathway for the entry of the human food chain [49,50]. Recently potential risks to human health, considerable attention, particularly for rice [51]. The accumulation of As is also important in wetland species [52]. Thus co-deposition of As with iron hydroxides absorbed on the plant surface [53]. In West Bengal (India) and Bangladesh, most crop fields are contaminated due to high concentration of arsenic in soil [54,55]. Excessive and

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long-term human intake (for example, with the concentration of 50 µg/L for 5–10 years) of toxic inorganic arsenic impacts on wellbeing including skin problems, skin cancer and in-ternal cancers (bladder and kidneys lung disease), vascular disease in the legs and feet, and possibly diabetes, high blood pressure and reproductive disorder [12]. However, the results of these studies are inconsistent. The different impacts of harmfulness on human health are melanin, leukocyte fluid, cornea, hyper-circulation, non-contaminated edema, gangrene and skin cancer. Amongst them, the melanocytes and keratosis are the most common to infect the people [56]. Patients with white melanin and hyper circulation were found in many cases. The chronic exposure to the organism also causes many disorders on various biological systems for example, the stomach related framework, respiratory system, cardiovascular system, hematopoietic system, endo-crine system, renal system, sensory system and regenerative system which prompts cancer disease [57]. The nutrition deficiency is an important promoter of arsenic, especially for younger women [58]. Recently studies about the high blood pressure and cardiovascular disease (CVD) mortality have been linked to chronic arsenic intake [59–65]. A series of arsenic concentrations has been reported over the past two decades and now it is one of the world’s most significant environmental risks [4,66–68]. It is suggested that ingestion of a large amount of arsenic can increase the chances of cancer development, especially the chances of skin disease,

lung disease, liver malignancy, cardiovascular impact, neu-rological impacts and lymphoma infertility and miscarriage in women heart disorders, and brain damage in men and women. To overcome this problem in this study we discuss about the advanced techniques to remediation arsenic from water.

ARSENIC REMOVAL TECHNOLOGIES

Water contaminated with arsenic is a major threat to hu-manity. It is known to be exceptionally dangerous whenever taken in enormous dosages of arsenic. High levels of arsenic are found in groundwater in certain areas of India, China, Argentina, Mexico, United States, Hungary, Taiwan, Viet-nam, Japan, New Zealand, Bangladesh, Chile, and Germany due to the natural arsenic found in the deposits of aquifers [69]. Treatment of arsenic from water has become a signifi-cant environmental issue; a few techniques for treatment of arsenic. Sensitive operating conditions, low efficiency, and secondary sludge production that require disposal of addi-tional charges are constraints associated with the application of these methods [70,71]. The techniques for arsenic removal is oxidation-precipitation, coagulation-coagulation filtration and adsorption, ion exchange and membrane filtration such as Microfiltration (MF), Ultra Filtration (UF), Nanofiltra-tion (NF) and Reverse Osmosis (RO) [72,73]. Adsorption procedures are successful strategies that have been utilized

Table 1 Concentration of arsenic in groundwater of the arsenic affected countries.Countries Arsenic Sources Concentration in μg/l ReferencesBangladesh Well Waters 10–41,000 [117]Calcuta, India Arsenic rich sediments 50−23,080 [57]Fukuoka, Japan Natural origin 0.001–0.293 [118]Hanoi, Vietnam Arsenic rich sediments 1–3,050 [40]Hungary Deep groundwater 1–174 [119]Inner Mongolia Bores 1–2,400 [120]Lagunera region, Mexico Well waters 8–624 [121]Nepal Drinking water 8–2,660 [6]Northeast Ohio Natural origin 1–100 [122]Peru Drinking water 500 [119]Romania Drinking water bores 1–176 [123]Ronpibool, Thailand Water contaminated due to tin mining waste 1–5,000 [124]Shanxi, PR China Well Water 0.03–1.41 [125]South-west Finland Well water, natural origin 17–980 [126]West Bangal, India Arsenic rich Sediments 3 −3,700 [57]The western USA Drinking Water 1 −48,000 [127]Xinjiang, PR China Well Water 0.05–850 [125]

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in the water and wastewater plants to decontaminate pollut-ants for ease of handling, minimum sludge production and regeneration capacity. The cost of the procedure depends just on the expense of the adsorbents. Different arsenic removal technologies are shown in Fig. 1. Here we define currently available technologies, specifically, coagulation flocculation, Membrane filtration, Ion Exchange, Adsorption, Oxidation, Phytoremediation, and Electrocoagulation (EC) arsenic remediation.

Coagulation flocculationThe coagulation reaction between arsenic and coagulants

can transform soluble arsenic into floc which is easier to be separated from water by sedimentation or filtration. Floc or large aggregates are formed due to the reduction of negative charge of arsenic colloids by the positively charged coagulants. The innovation expulsion procedure relies upon the convergence of arsenic, dose of coagulant, acidity alkalinity and arsenic valency. Ferric salts are normal use as a coagulant, iron coagulants are available as ferric sul-phate, ferric chloride and ferrous sulphate. Ferric salts are very corrosive acidic liquids. Ferrous sulphate on its own

is used as a coagulant in processes utilising high pH values such as lime softening. Aside from the expulsion of arsenic, this treatment can adequately expel many suspended and broke down segments from water [74]. The ferric sulphate coagulation method for arsenic removal is effective [75]. Removal of arsenic by applying a conventional coagulation and flocculation process. Coagulation has found to be ef-ficient and effective for arsenic removal and achieved up to 99% removal. Sarkar et al. & Karcher et al.’s studies reported the use of ferric chloride and lime poly ferric sulphate as coagulants [72,76], the ferric chloride and ferric sulphate are efficiently used as a floc for the removal of arsenic [77]. Wickramasinghe et al. describe that the utilization of ferric based coagulants treatment of groundwater in those areas, which has polluted with arsenic [78]. The particles colloid is converted into bigger particles. Rapid mixing is required to scatter coagulation all through the fluid. Flocculation is the activity of polymers to shape connects between bigger square particles or flocs and interface atoms in huge agglom-erates. The anionic flocculants will react against the positive suspension, adsorb on the particles and destabilize either by bridging or neutralizing the charge.

Fig. 1 Various techniques used for the removal of arsenic from groundwater.

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Membrane filtrationThis technique has been widely used to remove arsenic

in developed countries [79]. Few membranes are permeable and soluble compounds. The specific permeability of various compounds, these membranes can act as a molecular filter to remove dissolved arsenic, along with many other soluble compounds and molecules. High pressure is needed to cause water to pass through the membrane from concentrated to the diluted solution. Therefore, membrane is treated as a pressure driven procedure. Membrane technology processes are sepa-rated into four classifications: MF, UF, NF and RO. MF can be utilized to evacuate microscopic organisms just as suspended solids with pore sizes to 0.1 micron. UF will expel colloids, infections and a few proteins extending in size from 0.0003 to 0.1 microns. NF relies upon physical dismissal dependent on charge, and atomic size and its pore sizes go from 0.001 to 0.003 microns. The RO pore size is about 0.0005 microns and can be utilized to desalinate water. High pressure is ex-pected to water through the membrane from concentrated to the diluted solution. It is desirable to get the required degree of separation (rejection) at the maximum flow specific flux [71]. Advances in membrane technology have occurred in the name of electro-ultrafiltration [80], which has been found to have good potential for the arsenic treatment of water [81]. An overview of arsenic removal in the process of pressure-driven membrane and explored parameters that may affect arsenic removal efficiency by membrane techniques such as water source parameters, membrane material, membrane types and membrane process.

Ion exchangeIon exchange is a chemical or physical procedure where

electrolytic particles are exchanged on the surface of the solid phase of ions with similar charge in the solution. It is a reversible exchange where there is no lasting change in the strong structure. The strong material is generally manu-factured anion exchange resin, which is utilized to remove certain contaminants. Ion exchange is commonly used in the treatment of drinking water for softening (for example expulsion of calcium, magnesium.) as well as the removal of nitrates, selenite arsenic and other contaminates in water [82]. Ion exchange, generally stored chloride in “exchange sites.” Arsenic-containing water is passed through the vessel and the arsenic and “exchange” of chloride ions. Water the out in the vessel is lower in arsenic but higher in chloride than the water entering the vessel. In the end, the resin be-comes weary; this means that all or most exchange sites that filled with chloride ions become filled with arsenic or other

ions. The chloride particles available for treatment of arsenic and different anions that is in treated water.

AdsorptionAdsorption technology is accepted worldwide because of

its application in environmental engineering and sciences [83]. Adsorption is a mass transfer process through which a substance from the liquid phase is transferred to the surface of the solid material and becomes associated with physical and chemical reactions. Adsorption phenomena operate in the most natural physical, biological and chemical systems. Physical adsorption occurs primarily due to the van der Waals forces and the electrostatic forces between the adsor-bate molecules and the atoms that form the adsorbing sur-face. Chemical absorption occurs by binding the adsorbed molecules. Bio-adsorption depends on the different bio sor-bent material such as fruits waste (peels of orange, banana, sapodilla) [83] and peels of different vegetables. Other bio organic waste is also used for the remediation of arsenic [84]. Adsorption operations employing solid such as activated car-bon, metal hydrides and synthetic resins are widely used in industrial applications for water and wastewater purification. Activated carbon is also commonly used as a substance in the treatment of arsenic [85,86]. The capacity of adsorption depends on the properties of the activated carbon, and its chemical properties. Selective adsorption that uses bio-logical agents, mineral oxides, activated carbon, or polymer resins has resulted in increased agitation [87]. Carbon was also used in history to filter the drinking water by ancient in India. The carbonized wood was an extensive and refined medical adsorbent and purifying agent in Egypt by 1500 BC [88]. Modern industrial production of activated carbon has been established. In 1901 the bone-char was replaced with sugar refining [89]. Many other adsorbents such as activated carbons are available, but few are selective. Activated carbon was first commercially produced from wood in Europe in the early 19th century and was widely used in the sugar industry and was first reported for water treatment in the United States in 1930. It can be prepared from coconut husk, bone, cane sugar cane, bark, blood, charcoal, coffee beans, cereals, carbon cubes, fertilizer waste grout, fish, etc. For dilute concentration, the adsorption is an appropriate tech-nique for removing arsenic. Thus, attempts have been made to find new simple and effective techniques and its removal efficiency depends on activated carbon properties, adsorbate chemical properties, temperature, pH, time etc. Many others factors also influence the removal of arsenic by adsorp-tion, such as redox conditions in the soil, arsenic species

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in the aquifer, and the presence of phosphates, citrates, and oxalates play an important role [90–92], however the natural presence organic acid that is oxalate may play an important role to remove the arsenic in presence of iron based minerals. The addition for the recycling of Fe (II)/Fe (III) at adsorbent surface plays an important role in order to minimize the concentration of arsenic (As). However with the increase of oxalate dosage the iron pyrite become dissolution which reduce the removal efficiency of arsenic removal. Therefore, oxalate in low concentration with iron pyrite is a promising method to remediate arsenic. Thus, the presence of oxalate and iron based minerals present in groundwater recovered as a ferrous oxalate phase, which is useful resources, and con-tributes to the sustainable development and also improves the overall hydrogeochemistry.

PhytoremediationPhytoremediation is the use of plants to extract heavy

metals from water and soil. Under different environmental conditions, arsenic and arsenic absorption in plants will vary. An environmentally friendly, low cost method for removing arsenic from treated water and plant-soil systems, where live plants are used to remove arsenic from the envi-ronment or reduce its toxicity. Many factors, such as redox conditions in the soil, arsenic species in the soil. The trans-location factor is an important feature of plant classification due to its repair ability. The phytoremediation process is the transfer of arsenic from the species soil to the root surface plant factors. The transfer of arsenic from soil to plant roots depends on the oxygen content of the rhizosphere plants. A type of arsenic depends heavily on the redox state of the soil [90–92]. For example, arsenic is more readily available under aerobic conditions and more arsenic is found under anaerobic conditions [93]. Previous studies have proposed some physiological characteristics (improved root absorption rate, root-tip compensation rate and tolerance to mineral pol-lution) [94]. The physiological role of high accumulations in the rhizosphere of the root zone has been described in mod-ern literature [95–97]. In addition, certain microorganisms may promote the transformation of trace elements through various mechanisms, including methylation, demethyl-ation, complex formation, and oxidation [98]. However, the mechanism by which monomethyl arsenic acid and dimethyl arsenic acid are produced in plant roots is unclear. Plants use three independent systems to absorb arsenic: 1. passive absorption through the Apo plastic; 2. intercellular transport directly from the environment to the vascular system; and 3. active absorption through the symplast. There are many

root factors that control the uptake of trace elements in the soil. These elements acidify the soil through root exudates, activity, selectivity of the translocators, root membrane activity, strategy of avoidance, plant root excretion, plant phytosiderophores, and hydrogen ions [99–103]. In addition to the fundamental factors, several elements (e.g., Fe, S, P, and Si) also play an important role in the mechanism of plant absorption of inorganic arsenic [93].

Different Phytoremediation TechniquesThe term phytoremediation includes a series of plant-based

treatments such as phytostabilisation, phytoimmobilisation, rhizofiltration, phytoextraction and phytovolatilization. However phytoextraction was given priority from all tech-niques because of economical and scientific value [104]. Since phytostabilisation is a long-term process, local plant species will be given priority to reduce ecological conflicts with local ecosystems [105].

Phytoimmobilisation is an area where data is scarce [106] in soil science, which focuses on reducing the availability of pollutants in water and plants by altering soil factors by forming a precipitate and insoluble compounds and by sorption roots. Rhizosphere techniques is used to filter contaminated water using aquatic plants and large algae, including groundwater, rainwater and other wastewater [107]. During plant synthesis, plants spray or spread flying arsenic from their roots, leaves or stems. The process used in phytoextraction technology is the biological concentration of the aerial parts of plant species. Arsenic-tolerant plants known for a long time for possible phytostabilisation [108]. During phytovolatilization, plants transpiration or diffuse volatile arsenic from roots, leaves or stems. Arsenic and volatile arsenic compounds are involved in this process [98]. Under this mechanism, plants absorb organic and inorganic pollutants from the soil or water in the transpiration stream and volatilize them into the atmosphere in a modified or unmodified form at relatively low concentrations.

However, the literature on arsenic phytovolatilised scarce. It has been observed that the presence of sulphates and salts in the soil hinders the evaporation process [109].

ElectrocoagulationElectricity has been used for water treatment in the UK

since 1887 [110]. Electrochemical methods for wastewater treatment are cheap and environmentally friendly. EC was patented by A.E. Dietrich in 1906 for treating water on ships. It is an alternative to CF (coagulation / flocculation). The pro-cess involves an electrochemical phenomenon that uses metal

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plates / electrodes instead of chemicals while using electricity to remove heavy metals, suspended solids, emulsified solids, and other pollutants from water. EC is a simple and effective technology in which no condensate is added to reduce the amount of sludge [110,111]. In EC, electrolytic oxidation of sacrificial iron anodes produces Fe(III) oxyhydroxides / pre-cipitation in As contaminated water. Generally, As(V) forms bi-dentate binuclear bridging complexes with an elevated Fe to As weight ratio, which allows the reduction of arsenic to low levels [112]. Electro-condensation of arsenic has been achieved through iron and aluminum electrodes. However, some authors have also reported the use of titanium, copper and zinc electrodes [113,114]. The complete process of EC can be divided into three parts: 1. the electrolytic oxidation of the sacrificial anode, thus forming a coagulant; 2. the destabilizing effect of pollutants, the suspension of particles and the destruction of the emulsion; and 3. the instability of aggregation. The acting particles form a flock.

The EC includes a simple electrochemical cell made of an anode and a cathode. When a suitable potential is applied from an external source, the anode material is oxidized.

M(S) →Mn+(aq) + ne− (6)

Considering arsenic removal by using Fe electrodes in basic medium, there are two possible ways as described by [114,115]. Maldonado-Reyes et al. described As(V) removal as per the following equations:

Fe(S) →Fe2+(aq) + 2e− (7)

Iron dissolves from the anode to form ferrous cations (Equation 7). The ferrous cations are easily oxidized to the form of trivalent iron and hydrolyzed to polymerized iron oxyhydroxide, namely goethite (FeO(OH)) (Equation 8). Goethite has been identified by X-ray diffusion.

Fe2+(aq) + 3OH− →FeO(OH) + H2O+e− (8)

As(V) is mainly in the arsenate H3AsO4 state with 3 pKas: 2.19, 6.84, and 11.5. At pH 7, arsenic acid is in the basic form of HAsO4

2− (60%) and the acid form is H2AsO4− (40%).

Both forms produce insoluble complexes with fresh goethite FeO (OH):

H2AsO4− + 2FeO(OH) → (FeO)2 HAsO4 + H2O + OH−

(9)

HAsO42− + 3FeO(OH) → (FeO)3 AsO4 + H2O + 2OH−

(10)

In general, As(V) forms a bidentate and binuclear bridged complex, and the weight ratio of iron to arsenic is increased, thereby reducing arsenic to a minimum level [114]. In EC, iron ions are generated from iron electrodes and form vari-ous hydrate species, such as Fe (H2O)6

3+, Fe (H2O)5(OH)2+,

Fe (H2O)4(OH)2+, Fe2(H2O)8(OH)2

4+ and Fe2(H2O)6(OH)44+

and goethite (FeO (OH)); which depends on the pH of the aqueous medium [115]. All of these trivalent iron compounds have a strong affinity for arsenic. Among them, the maximum cohesion of As(III) and As(V) is water-containing hydrated ferric oxide (Fe2O3.xH2O) and goethite [(FeO (OH)] [36,37]. Arsenate anions form naturally occurring Arsenate miner-als, namely FeAsO4.2H2O (scorodite) and Fe3 (AsO4)2.8H2O (symplesite) as the dominant solid phases [116].

CONCLUSION

Many technologies can be effective at removing arsenic from water with high availability of arsenic. However, the technologies adopted have some disadvantages, and their production can be a potential source for secondary pollution. Therefore, in order to clean up the environment, new tech-niques are needed with modern technology to take the risk seriously. Adsorption is an important way to removing arse-nic. Most studies focus on the type of adsorbent. Membrane technology, especially nanofiltration, is a promising method for eliminating arsenic, and can also be widely understood to meet the regulation of arsenic concentration in drinking water.

Phytoremediation and electrocoagulation are more suit-able for arsenic removal. Other alternative methods have also been studied for their feasibility in modifying existing methods. Future requirements for arsenic removal technol-ogy should be taken into consideration to reduce treatment costs, facilitate the operational complexity of the technology and reduce arsenic treatment residuals.

ACKNOWLEDGMENTS

We are thankful to the China University of Geosciences (Wuhan), who gave us a platform and all possible availabili-ties for this review paper.

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