Ion exchange is a method in which the same signs are swapped reversibly between solid and liquid in a highly insoluble solution. Ion exchange chromatography is an ion exchange technique that is popular because of its high capacity and resolving power. The ion exchange method is used for water purification (especially converting hard water into soft water) and also for other purifications. It can detect inorganic salts. Cationic exchangers and ionic exchangers are two types of ion exchangers. Two techniques are used for ion exchange: a batch method and a column method ^[1][64].

For ages, the ion exchange method has been used to purify water from arsenic. The ion exchange method works for arsenic with few total dissolved solids (TDS) and little sulfate. For the arsenic ion exchange method, effectiveness is increased by different factors such as TDS and competing ions ^[2][58]. The study explains the tailored anion exchanger used to decrease the As concentration. Simulated ion exchange resins are effective for As(V) adsorption. Chloride ions are easily exchanged with As(III) or As (V). In water, the arsenic concentration, competing ions, and ion exchange resins are important elements for arsenic ^[3][37]. Graphene oxide and composites of graphite oxide iron-modified clinoptilolite are good composites used for arsenic removal using the ion exchange or adsorption method ^[4][65]. A study defines its goal to remove As(V) from a solution by using hybrid ion exchange/electrodialysis. Some other technologies can be used for arsenic removal, such as nanofiltration or reverse osmosis, but these have some drawbacks like regenerate ion exchange or membrane cleaning, so here, ion-exchange electrodialysis is more convenient, which overcomes the drawbacks of other technologies ^[5][66]. In the ion-exchange method, no pH acclimations are needed and do not rely on influent concentration. In the ion-exchange method, the removal of As(III) is low, and some precipitates block the process ^[6][67].

Arsenic contamination water is treated using coagulation, which is then followed by the neutralization and filtration processes. Various arsenic species are transformed into flocs for further filtration using various coagulants. The coagulation process is also preferable because of its simplicity and removal efficiency ^[7][52]. This process is applicable on small scales and large scales, but pH adjustments are obligatory ^[6][67]. A study briefly describes ferric coagulation. If the dosage of coagulant increases, there is more removal of arsenic ^[8][68]. Sludge is produced during the coagulation process; however, this process can remove very high amounts of arsenic concentrations ^[9][69].

According to studies, choosing a certain type of arsenic removal procedure is primarily influenced by the nature of the water. Numerous modifications to coagulation were being made, such as electrocoagulation as a substitute for aqueous arsenic removal ^[3][37]. Coagulation techniques, particularly electrocoagulation processes, are used to treat the various health problems brought on by consuming arsenic-contaminated water ^[10][70]. Although electrocoagulation has been used to cleanse water for human consumption, its use is not widespread because of the need for energy. Solids are easily removed; however, they cannot be disposed of directly because of the arsenic residue produced. The process of coagulation is economically viable ^[3][37]. Iron and aluminum are popular electrode materials for the electrocoagulation process. For the removal of arsenic from water, the researchers employed iron anodes rather than aluminum anodes. More than 27 researchers utilized iron or stainless-steel anodes to electrolytically generate iron hydroxides, whereas more than 15 employed aluminum anodes to electrolytically generate aluminum hydroxides. The major reasons for this electrode preference are its inexpensive cost, convenient availability, and improved efficiency ^[11][12][71,72]. The electrocoagulation method is capable of converting arsenite to arsenate, which is essential for the effective removal of aresenite. The efficacy of aluminum and iron for the removal of arsenite from an aqueous medium was investigated, and equal removal efficiencies were reported for both electrodes for arsenite concentrations ranging from 75 ppm to 500 ppm. Aluminum, on the other hand, has a lower effectiveness for arsenic removal than iron.

Phytoremediation is a cost-effective biological technique that can easily safeguard human health and the environment from the poisonous effects caused by arsenic ^[13][49]. It is a procedure in which plants accumulate pollutants from the soil.

To determine the phytoremediation capability of Pistia stratiotes L. to accumulate arsenite ^[14][60], an experiment was conducted with different factors in which, among four treatments, 10 µM As(III) showed a high accumulation of arsenite from the medium. A study conducted by Moreira, V., et al., to remove arsenic from the aqueous media under controlled conditions in which plant species Lemna Valdiviana showed arsenic reduction up to 82%, accumulating 1190 mg/kg of As from water ^[15][61]. Plant samples were collected from different areas of Enugu State, southeastern Nigeria. Arsenic in leaves, roots, and soil is monitored separately. The phytoaccumulation capacity of plants was measured by different factors (bioconcentration, translocation, and accumulation). Pteridium Aquilinum with 622.0 mg kg⁻¹ accumulation capacity is the most efficient plant species for the phytoremediation of arsenic from water, followed by Lasimorpha Senegalesis and Sacciolepis Cymbiandra ^[16][62]. Phytoremediation studies were led to assess the arsenic take-up capability of Salix atrocinerea in 30 days, in which plant roots showed higher intake of arsenic than leaves ^[17][63].

The arsenite As(III) form of arsenic is highly versatile and needs conversion into a less mobile form, As(V), because most of the treatment procedures viably eliminate just As(V); that is why pre-oxidation of As(III) to As(V) is needed and, for this purpose, different oxidation processes like Fenton’s reagent, hypochlorite, and permanganate ^[18][13] and oxidizing agents like ozone, chlorine, bleaching powder, and hydrogen peroxide ^[18][13] are used.

Manganese-oxidizing aerobic granular sludge (Mn-AGS) is very effective in removing arsenic from organic wastewater, especially with the addition of Fe (II) attributed to the Fenton reactions ^[18][13]. pH dependence appears to be cons of Fenton reaction. Depending on the circumstances, As(III) removal from contaminated water can be effectively remedied using the CuFe₂O₄ reaction with peroxymonosulfate ^[19][84]. Commercial activated carbon has been showed promising results toward As(III) oxidation under different parameters including residence time, pH, dissolved oxygen, and initial As(III) concentration ^[20][85]. Photocatalytic oxidation is an efficient method because of its low cost ^[21][86]. Comparing the present study of photocatalytic oxidation with the previous studies to access its benefits, theirs study showed improved photocatalytic performance compared with different reports in view of comparative materials. Reference ^[22][87] studied the self-floating copper loading catalyst as an oxidizer of As(III) that can be used for the treatment of arsenic-contaminated water. Reference ^[23][88] reported that different lead extracts like eucalyptus (Eucalyptus globulus), mango (Mangifera indica), jamun (Syzygium cumini), and guava (Psidium guajava) also play an important role in arsenic oxidation. Zero-valent iron nanoparticles obtained from these leaf extracts are used as oxidants. Results showed 70% arsenic (III) oxidation at a period of 10 min and, among all the leaf extracts, guava leaves were able to oxidize arsenic at all pH values (3, 7, and 9).

Adsorption widely relies upon the porosity and movement of the adsorbent in disposing of or bringing down the grouping of a wide scope of toxins (natural, inorganic, and organic) from the arrangement ^[24][89]. Adsorption is an ex situ technique used to remove heavy metals, e.g., arsenic ^[25][90]. The different adsorbents used for the removal of As(III) and As (IV) include ferric hydroxide and activated alumina (SFAA) ^[26][91], chitosan–magnetic graphene oxide (CMGO) nanocomposite ^[27][92], magnetic gelatin-modified biochar ^[28][93], iron-modified activated carbons ^[29][94], iron-ore sludge ^[30][95], and magnetite nanoparticles ^[31][96].

By electrochemical adsorption with birnessite, Liu, L., et al. demonstrated a decrease in total arsenic (As T) and As(III) from 3808.7 to 73.7 µg/L and 682.8 to 21.4 µg/L, respectively ^[32][97]. With an adsorption capacity of 166.94 mg/g via a Langmuir isotherm model, Mn-doped MgAl-LDHs is the effective absorbent for removing As(V) from the aqueous medium ^[33][98]. Xu, F., et al. indicated that starch-stabilized ferromanganese binary oxide (starch-FMBO) generated with Fe/Mn at a 1:2 ratio has a greater adsorption influence on As (III), demonstrating substantial adsorption capacity ^[34][99]. Some binary oxides are widely employed for arsenic removal because they are affordable and environmentally friendly of their ecofriendly nature and cost-effectiveness ^[35][100]. Arsenic removal from water is greatly improved by microbial conversion of arsenite in combination with various adsorption approaches ^[36][101]. The combination of two techniques, adsorption (ADS) and dielectrophoresis (DEP) appear to be cost-effective and efficient; the adsorbent fly ash showed high adsorption capacity removing 91.4% As(V) from industrial wastewater ^[37][102]. Langmuir and Freundlich’s experiments were performed to evaluate As(III) and As(V) removal efficiencies by Fe-FeS2 prepared with mechanical ball milling at a pH ranging from 3–10 ^[38][39][103,104], and used Fe/olivine composite for arsenic removal that showed great adsorption capacity at a minimal expense. Another adsorbent, iron-coated S. Muticum, can be used as an alternative treatment for the expulsion of arsenic that also has an ecological benefit over other techniques ^[40][19].

Many studies demonstrated that adsorption is a beneficial technique for the treatment of contaminated water ^[41][105]. During adsorption, As(III) oxidizes to As(V) to improve removal efficiency as As(III) is not easy to remove ^[42][106]. Ferric salts and iron oxides are most convenient when they are used to purify water from As contamination ^[41][42][105,106]. Iron oxide is cost effective and has a higher charismatic character. Initially, adsorption performance is low but activated carbon enhances its effectiveness. After the result, activated carbon can be removed with the help of a magnetic process ^[43][107]. Activated carbon is a venerable adsorbent. There are several types of adsorptions; some can regenerate after use and become impregnated ^[44][108]. It is expected in the next 10 years that some advanced adsorbents will be made that will be very effective to clean the environment ^[44][108].

Adsorption is a worthy technique that can be used at home. It is an effective technique and increases the quality of things after removing contamination ^[25][90]. This process can be processed with the water having multiple pollutants and there is less waste production at the end of processing ^[43][107]. This technique gets attention because of its low cost, ease, simplicity of operation, and high efficiency standard ^[45][109].

Many adsorbents are neither practically nor financially viable for usage in underdeveloped nations. As a result, locally accessible natural adsorptive materials may provide viable and affordable alternatives for removing As pollution in low-income nations. These organic substances include hydroxylapatite and struvite, zeolites, clays, rocks, soils, and sorbents [0]. Only the right pH is suitable for this technique. Sometimes the performance is affected by the minerals or nutrients in the water and soil ^[25][90]. Due to its weight, unstructured nature, and flocculant properties, the iron-based adsorbent is relatively limited in removing arsenic ^[43][107]. Sometimes there is no revival of adsorbents, which become secondary pollutants ^[44][108].

Arsenic is hazardous to health—not only to human health, but it is also dangerous for marine life when it is present in water ^[46][112]. Now it extends around the globe, so bioremediation is a technique used to remove arsenic ^[47][50]. Bioremediation is a helpful technique to remove arsenic from water, soil, and mine tailings ^[48][49][113,114]. Bioremediation is of two types, in situ and ex situ, which are further classified, too ^[50][115]. Though we had some old technologies to remove arsenic, they consume a lot of money and are complicated. The use of microorganisms is a little tricky but removes the As contamination more in a less expensive way ^[51][116]. Some bioremediation techniques used for As removal are Prangos ferulacea (Pf) and Teucrium polium ^[52][117], microbial fuel cells ^[47][50], field-pilot bioreactor ^[49][114], biogenic pyrite ^[48][113], etc. In some bioremediation processes, arsenic mobility increases so it can be removed easily ^[48][113]. As removal capability also depends upon its bonding with other metals, which makes its removal easy or tough in the bioremediation process ^[49][114]. Studies show that different elements are present with arsenic, so the removal of all, including arsenic microorganisms, is used. Sometimes for As, the tolerance level is unpredictable, and bacterial activity is done on it, which then clarifies its tolerance concentration. Arsenic-resistant bacteria are used for arsenic removal from soil because the soil contaminates food. A study had been done for the removal of As in which for 193.7 nm As the line is segregated for processing so, after all, the processing result shows that greater than 90% As(III) and As(V) had been removed ^[53][118].

There are some drawbacks to the bioremediation process. Bioremediation needs sometimes aerobic and sometimes anaerobic conditions to run a process; otherwise, fluctuations may occur. The nutrient requirement is very important in this technique. Sometimes the process is slow and faces difficulties when it works for inorganic things. The most severe drawback is that sometimes it converts into a more highly toxic material after processing than the original ^[54][123]. Suitable soil, climate, and other suitable conditions are required for good performance ^[25][90].

As the name suggests, it is a membrane structure that serves as a barrier, forcing contamination to stop as molecules flow through it. A porous membrane serves as the material for the barrier. Nano-filtration, micro-filtration, ultra-filtration, and electrodialysis are the four basic forms of membrane technology ^[55][124]. Electrodialysis is a technique used to remove salts and chemicals from water. It works when an electric current is applied to it and ions pass through the selective membrane ^[56][125]. Nanofiltration needs pretreatment, removes arsenic and bacteria and viruses, and makes water drinkable. It can remove 93.8% of arsenic at pH 8. With microfiltration, with the help of electrocoagulation and micro-coagulation, As is removed at a minimum cost ^[18][13]. In ultrafiltration, pressure is applied to the semipermeable membrane to split the contamination. Its pore size is large ^[57][24]. In the separation technique, pressure is applied to a harmful substance to produce a contaminant-free product ^[58][126].

The study illustrated that by modifying an adsorptive membrane, seven tons of water can be purified to drinking level by using a 1 m² adsorptive membrane and giving a high level of water security ^[59][127]. A study has been done that shows reverse osmosis can clean water up to 70–90% from As(III) and As(V) ^[60][128]. Another study encapsulates that under reverse osmosis and nanofiltration using their different membranes, up to 91% arsenic was removed, which are very efficient results ^[61][129]. The study has been undertaken to determine membrane technology’s cost- and energy-efficiency benefits. The results showed that for 20,000 residents, water costs USD 1041 per day, producing drinking water costs USD 0.52 per cubic meter, and electricity costs just 35 percent of the total cost ^[58][126]. Nanofiltration is a well-founded technique proved by an experiment in which contaminated groundwater was treated, decreasing its contamination level from 435 µg/L to 10 µg/L ^[62][130].

This technology not only helps to remove arsenic but also removes the total dissolved solids, salt, turbidity, and other unsuitable material. This method eliminates significant waste because it has a high filtration potential ^[25][63][90,131]. There is no sludge production in the membrane technique as in adsorption and chemical precipitation ^[64][132]. In addition, less energy is used in this method ^[60][128]. The main advantage distinguishing membrane technology from other methods is that there is no use of chemicals here ^[55][60][124,128]. Being easy to scale added a plus point to the membrane technique ^[6][67].

Membrane filtration required a very good price to process, so it burdens the economy ^[65][133]. A heavy amount is required for its maintenance, not only for processing. Substantial waste production is less but water contamination production is high. Linear scaling is not very feasible ^[25][90]. Temperature maintenance is also required ^[17][63], as is high expenditure and utilization of electricity ^[6][67]. A high amount of water could not filter here ^[55][124]. The defilement of the membrane can disrupt the whole process ^[6][67]. It is not preferable to change As(III) oxidation to As(V) because there is a chance of membrane damage ^[55][124].

The increasing population has elevated the requirement for water resources ^[66][134]. On the other hand, electrodialysis is a technique used to remove water contamination ^[67][135]. It is an electrochemical process ^[68][136]. Electrodialysis is a type of membrane technology in which anion and cation are migrated through the membrane by electric force. Initially, this technique was not used for As removal but for salt removal ^[68][69][136,137]. With the help of an electric field, ions are separated, shifted, and concentrated ^[70][138]. Electrodialysis is a very useful technology to enhance water quality and alter salt concentration ^[68][136]. The study was performed to remove arsenic with the help of electrodialysis, deep eutectic solvent enhancement, by which 82% As was removed successfully ^[71][139]. A study shows that As(V) removal is faster than As(III) under the mass transfer coefficient. It is increased in the case of As(V) because it is negatively charged, but not for As(III) as it has had no charge and the pH is not affected ^[72][140]. Another experiment has been done to show the efficiency of this technology. The result was electrodialysis removal efficiency of 80% at pH 10 ^[71][139].

Electrodialysis has a disadvantage in that it requires a labor force. Defilement can affect performance. Prior treatments and restoring minerals are required in this technique ^[67][135]. Another technical problem is that ED has less storage capacity. There would be chances of energy losses and blocking ^[73][144].

In the present era when there is an economic race everywhere, developing countries are unable to reach water purification technologies that are expensive. Simply, they cannot afford them, so in this case, ultrafiltration is a viable technique to remove contamination ^[74][145]. It is a low-pressure membrane technique ^[75][146]. Hydrogen peroxide is helpful to oxidize As(III) into As(V) at the membrane ^[76][147]. Micellar-enhanced ultrafiltration (MEUF) is a modification to ultrafiltration where a little decrease in the surface tension of the heavy metal is made to improve removal efficiency ^[77][148].

A study demonstrated the reinforced conventional treatment is 96.9% successful by ultrafiltration ^[78][149]. An experiment has shown that there is efficient arsenic removal with the help of ultrafiltration in collaboration with cetylpyridinium chloride, by which 91% and 84% of contaminated water were obtained ^[79][150]. The study elaborated that a negatively charged membrane efficiently removes particles at neutral pH. It has been studied that there is more arsenic removal caused by the increase in UF membrane by increasing negative charge and increasing pH ^[57][24].

Ultrafiltration membrane technology helps to increase the validity and solidity of the process. In this process, unwanted contaminants are removed and color and turbidity are improved, bringing the water to drinking level. Even then, if its membrane life-span is short, it is feasible because of its suitable price ^[78][149]. This technique helps to decrease the fouling potential ^[80][153]. The flux rate is high and requires less energy, and is economically feasible ^[18][13].

Some enhancements are required in ultrafiltration because, alone, it will not give an effective result. Modifications are required to increase its rejection rate. The modification can be on the membrane or the whole system. The large pore size of ultrafiltration affects the purity and helps small particles pass out ^[81][154]. Some essential ions like Ca and Mg are not filtered here ^[18][13].