A review: The state-of-the-art of arsenic removal in wastewater

Clean water is one of the most important resources on Earth which is essential in our daily life (Boretti & Rosa 2019; Wang et al. 2021b). All around the world, pumping massive amounts of wastewater into rivers, oceans, and streams has become a routine practice now, over 44% of household wastewater is not safely treated (Water 2021). This has disastrous consequences for the ecology, fisheries, and animals, not to mention the fact that it is a water ‘waste’. Simply explained, wastewater is any water that has been contaminated because of a business or domestic procedure. This comprises sewage water and water produced as a by-product of large-scale businesses like mining (Li et al. 2019a; Meseldzija et al. 2019) and manufacturing (Poonam et al. 2018; Wang et al. 2018b).

Wastewater has toxic elements that can affect the environment and human health. If left untreated, chemical compounds and pathogens in wastewater can impair the health of the surrounding ecosystem. Also, it can potentially harm human health by contaminating crops and drinking water. Water-related diseases kill an estimated 3.4 million people worldwide each year as reported in Bangladesh (Hasan et al. 2019). In addition, some of the diseases that can be caused by water-related substances, such as cholera and schistosomiasis, remain common in developing nations (Magana-Arachchi & Wanigatunge 2020; Water 2021). Only a small fraction of the water in most areas is treated before it ends up in the environment (in some cases less than 5%) (Manikandan et al. 2022). As water shortages have become common in some regions of the world, wastewater, which is used in water, is important as a valuable resource (Water 2021). The need for a good water treatment process is so important for areas with water scarcity as it can be reused and returned to the water cycle for their daily life continuity instead of being wasted.

Therefore, wastewater treatment is critical for two reasons: to restore the water supply and to protect the environment and all living from pollutants (Manikandan et al. 2022). There are several methods for treating wastewater, and the better the process, the more likely it is to be reused before being thrown into the ocean. But to do that, we must identify the kind of hazardous compounds present in the wastewater. Metals in wastewater have long been a source of worry, and arsenic in nature has attracted a lot of attention in recent years (Badawi et al. 2021). Arsenic is a chemical element that is naturally occurring and widespread throughout the Earth. In sewage, the main sources of arsenic come from ore mining, fossil fuel combustion, non-ferrous metal smelting, arsenic-containing medicinal manufacture, volcanic eruption, arsenic-solidified wastes, and the use of pesticides (Sun et al.). Arsenic is commonly thought of as a dangerous heavy metal as it forms several poisonous compounds (Pratush et al. 2018; Sodhi et al. 2019). Heavy metals cause major health consequences, such as stunted growth and development, cancer, organ, and nervous system damage, and death in the worst-case scenario (Vardhan et al. 2019). Heavy metals can also cause permanent brain damage at higher doses (Engwa et al. 2019).

Hence, knowing the specific metal presence and how it further affects human health and other living things helps in finding the most suitable ways to remove the specific metal from the wastewater. Thus, this article aims to understand the characteristics of the arsenic metals that act as a pollutant in wastewater and review the available techniques for arsenic removal.

Arsenic (As) is found in nature primarily because of natural metalloid deposits in the earth's crust, which are typically found in old rock formations (Tabelin et al. 2018). Moreover, arsenic goes into groundwater by erosion or via synthetic sources including wood preservatives, petroleum production, semiconductor manufacturing, or the abuse of arsenic-containing animal feed additives and pesticides (e.g., Paris Green) (Ali et al. 2019; Bundschuh et al. 2021). As soluble arsenic has no taste or colour, its presence must be detected using a chemical water analysis. Arsenic can mix with other elements in the water to generate both inorganic and organic compounds. The inorganic forms are commonly found in two chemical valence states: arsenite (As III) and arsenate (As V).

In a study at arsenic endemic Gaighata block of West Bengal, the carcinogenic (ILCR) and non-carcinogenic risks (HQ) through dietary intakes (i.e., rice grains, vegetables, and drinking water) for adults were significantly higher than the suggested threshold level (Joardar et al. 2021). This metalloid can enter the blood–brain barrier, where it can cause mitochondrial membrane instability and calorie exhaustion, which makes it a strong neurotoxin that may cause symptoms like Parkinson's disease and Alzheimer's disease (Medda et al. 2020). Changes in diabetes-related gene expression pathways have been linked to arsenic pollution. A genome-wide DNA methylation investigation of 396 adults participating in the Health Effects of Arsenic Longitudinal Study (HEALS) has proven this (Demanelis et al. 2019; Navas-Acien et al. 2019). Inorganic arsenic is quickly absorbed from the gastrointestinal system and travels within the body's tissues and fluids, making chronic arsenic lethal in its most severe form. Some of the health implications of ingesting inorganic arsenic are cancerous effects such as skin, bladder, lung, kidney, nasal passages, liver, and prostate cancer (Briffa et al. 2020), while some other non-cancerous ones are cardiovascular, pulmonary, immunological, neurological, and endocrine disruption (Zheng 2020). An arsenic-related death rate of 1 in every 18 adult deaths is reported by about 20 million and 45 million people, especially in Bangladesh (Zheng 2020). Hence, WHO has provided a safe guideline for the maximum contaminant level of arsenic that is allowed in clean water which is only 0.010 mg/L (Uludag-Demirer et al. 2020).

It is necessary to develop realistic, precise, inexpensive, and user-friendly methods for determining the presence of inorganic arsenic in water, ideally with at least quick on-site monitoring capabilities to enhance population screening and identification. Even though many nations have been successful in replacing subpar home arsenic wells with alternative resources or treatments, ongoing effort is required to lower exposure among the dispersed rural population. Even high-income rural households are burdened by the high cost of arsenic treatment, thus, novel arsenic treatment from wastewater must be explored continuously.

There have been extensive investigations on arsenic removal methods in recent years to increase arsenic uptake from the environment. To date, various techniques have been implied such as adsorption using the advancement of metal oxide nanotechnology, oxidation, membranes techniques, coagulation, and electrochemical methods. Nonetheless, all these innovations have substantial limitations, particularly in terms of prices and efficiency, even though they are beneficial for removing arsenic from soil and water. A country's development stage, local efforts for reducing arsenic levels in water systems and trendy plants, and constraints on soil and water treatment technologies are just a few of the variables that affect the selection of appropriate arsenic treatments. Thus, in this section, we will thoroughly discuss novel studies in arsenic treatments.

Adsorption is a process that utilizes solids to remove compounds from liquid and gaseous solutions (Al-Ghouti & Da'ana 2020). Substances in adsorption will be separated from one phase and subsequently accumulated on another surface. Adsorption is governed by van der Waals forces and electrostatic interactions among adsorbate molecules and adsorbent surface atoms (Nordin et al. 2021a). Thus, before using an adsorbent for adsorption, it is necessary to first define its surface characteristics. Plus, adsorption is categorized among the widely applied techniques for removing arsenic from water and soil, and it is universally viewed as an affordable and excellent technique for water treatment.

Among the advantages of adsorption uncomplicated to run, it could supply widely sludge-free performance and also has the potential to restore (Wang et al. 2018a). Moreover, the process's cost is solely determined by the price of the absorbent. The downside of adsorption is when the adsorption bed becomes overly saturated and fatigued and eventually completely loses its ability to separate, sorbents must be replaced (Ghosal et al. 2018). For the adsorptive removal of arsenic from wastewater, several adsorbents have been widely used, including zeolites, carbon-based nanomaterials, agro-wastes, polymers, metal oxides, and composites (Alvarez-Cruz & Garrido-Hoyos 2019; Ashraf et al. 2019; Liu et al. 2020a; Poudel et al. 2020; Samah et al. 2020; Huo et al. 2021).

Metal oxide nanoparticles (MNOPs) represent an attractive candidate for the quick, efficient, and selective adsorptive removal of heavy metal and metalloid ions due to their abundance of surface-active sites which allows different pollutants to concentrate on the surface of MNOPs, non-toxicity, and economic feasibility. MNOPs have unique electrical, optical, and chemical capabilities because of their incredibly small size (1–100 nm). As an outcome, nano-sized adsorbents with a large surface area and a strong leaning to adsorb hazardous arsenic from water have evolved. MNOPs are often composed of metal content (0.1–46%), such as Fe, Al, Ce, Ti, Mg, and Si (Li et al. 2016).

For physical interaction, pore filling is the most common adsorption mechanism for metal oxides, especially those with amorphous structures (Chai et al. 2021). In the early stages of adsorption, arsenic ions almost fill the mesopores. Subsequently, when the arsenic ions go further and deeper into the micropores, they encounter more resistance, which reduces their driving power and adsorption rate. Although micropores have a much greater proportion of pore area, their short length prevents contaminants from diffusing within the adsorbent.

Despite that, the interparticle dipolar interaction facilitates the easy aggregation of pure nanoparticles into larger particles, which reduces their specific surface area (Wu et al. 2012). Numerous materials, including zeolite (Simsek et al. 2013), carbon nanotube (Tawabini et al. 2011), and polymer (Katsoyiannis & Zouboulis 2002) have been utilized as the template to disseminate the magnetic nanoparticles (MNPs) to prevent agglomeration. Saif et al. (2019) reported that polymers like chitosan and polyvinyl alcohol (PVA)–alginate were utilized to create nanocomposites of polymer–iron oxide nanoparticles (IONPs) and stabilize nanoparticles. Effective removal of arsenic was achieved by the adsorbents, even after five cycles of adsorption–desorption. This research also found that iron release in arsenic solution is facilitated by IONPs without polymer support. However, for polymeric-stabilized IONPs, the metal release was minimal, demonstrating their great stability in the solution.

Magnetic materials, notably IONPs, have been known to exhibit distinctive features for applications, including the treatment of water. The creation of high-capacity adsorbents may benefit from the usage of magnetic IONPs. Magnetic iron oxide-based nanocomposites offer an affordable method for adsorption and photocatalysis due to their saturation magnetization, reusability, and simplicity of recovery. For the adsorption of various types of arsenic, the minerals hematite (Fe₂O₃), magnetite (Fe₃O₄), limonite (FeO(OH)·nH₂O), maghemite (γ-Fe₂O₃), and siderite (FeCO₃) have been utilized (Nassar 2012). The most frequently investigated materials are magnetite and maghemite (Fe₃O₄ and γ-Fe₂O₃) because of the exceptional nanometric features they display, such as large specific surface area and superparamagnetism.

The oxide shell can absorb pollutants by electrostatic interactions and surface complexation, while the metallic core can transmit a capable electron from the core to the surface, acting as an electron source and decreasing character (Liu & Zhang 2014). The core, which primarily comprises zero-valent iron, supplies reducing power for interactions with pollutants from the environment. Because zero-valent iron was oxidized, iron oxides and hydroxides made up the shell. The shell serves as the foundation for the synthesis of chemical complexes (such as chemisorption). The hydroxyl groups of the arsenic species are crucial for interacting with adsorbent surface hydroxyl groups in the chemisorption process that removes arsenic from nano-sized iron oxide.

Water contaminated with hazardous arsenic can be effectively removed using superparamagnetic iron oxide nanoparticles (SPIONs) (Torasso et al. 2021). However, SPION's effectiveness is constrained by a propensity for agglomeration, and additional filtering is needed in practical applications to prevent potentially detrimental environmental consequences of nanoparticles released into the air (Sharma et al. 2022b). SPIONs encapsulated inside insoluble electrospun PVA nanofibres have an improved capacity for adsorbing arsenic (>52 mg/g) and exhibit a novel adsorption mechanism that involves a significant swelling of the super hydrophilic nanofibres before solution exchange (Torasso et al. 2021).

MNPs/chitosan-produced films (Kloster et al. 2020) and MNPs coated with ligands based on meglumine (Scurti et al. 2022) were used to adsorb arsenate ions, with adsorption capacity of 10.4 and 28.2 mg/g, respectively. MNPs integrated with chitosan showed increased adsorbent capacity were related to its significantly more uneven surface area, which resulted in an improved adsorption surface (Kloster et al. 2020). Both nanomaterials demonstrated a reusable up to five times (Scurti et al. 2022) and four cycles (Kloster et al. 2020). Other chitosan and PVA–alginate stabilized IONPs derived from Terminalia chebula had an adsorption capacity of 28.57 mg/g (Saif et al. 2019).

More environmentally friendly substances were used to create iron nanoparticles, including polymers, amino acids, bacteria, fungi, plant extracts, etc. Dissimilatory Fe(III)-reducing microbes have been linked to the oxidation of organic molecules and Fe(III) reduction, producing ultrafine-grained magnetite as a by product since the 1990s. Some of the environmentally friendly substances used to effectively adsorb arsenic and arsenate are Klebsiella oxytoca strain DSM 29614 (Casentini et al. 2019), rice-husk (Priyadarshni et al. 2020), bark (Dhoble et al. 2018), biochar (Ha et al. 2021), black tea leaves, oak tree leaves, green tea leaves, pomegranate leaves, and eucalyptus leaves (Kamath et al. 2020), Terminalia chebula (Saif et al. 2019), and activated carbon (Ha et al. 2021). Different plant extracts include various chemical components that can change how nanoparticles created with them behave.

Biochar made up of rice-husk innovation stabilized iron and copper oxide nanoparticles effectively removed arsenite and arsenate (Priyadarshni et al. 2020) through the adsorption mechanism of electrostatic and covalent interactions between the Fe/Cu–OH/OH²⁺ groups of the composites and the AsIII/V–OH/AsVO of the arsenic species. Fe/Cu nanoparticle's effectiveness in arsenic removal was increased in acidic conditions and achievable under basic circumstances (maximal sorption at a pH of 7.0) (Babaee et al. 2018). Bark-based magnetic iron oxide particles (BMIOPs) (i.e., saturation magnetization value of 38.62 emug⁻¹) may remove As(III) from drinking water efficiently for the magnetic separation of exhausted BMIOP following As(III) adsorption (Dhoble et al. 2018). On the other hand, activated carbon-based iron oxide (Fe₃O₄) nanoparticles were enhanced due to increased porosity and specific surface area of the adsorbent (Ha et al. 2021).

Five plant extracts from black tea leaves, oak tree leaves, green tea leaves, pomegranate leaves, and eucalyptus leaves were used to produce iron nanoparticles (Kamath et al. 2020) where eucalyptus leaf nanoparticles were determined to have the highest arsenic adsorption capacity of 39.84 mg/g, followed by oaktree leaf nanoparticles of adsorption capacity 32.05 mg/g. High gradient magnetic separation (HGMS) known as magnetic nanoparticle recovery devices (MagNERD), a promising green separation method that, when tuned, can permit the efficient removal of nanoabsorbents from water, can allay these worries (Powell et al. 2020). It was able to remove approximately 94% of the arsenic-bound Fe₃O₄ after mixing the nanopowder with simulated drinking water that included As. A summary of the adsorption properties of IONPs for arsenic removal is presented in Table 1.

Aluminium oxide is an amazing material to utilize in the fabrication of membranes because it is inexpensive, chemically stable, highly abrasive, and has a large surface area to volume ratio property (Ahmad & Guria 2022). Due to its acidic and basic characteristics, alumina exhibits a distinct ion exchange selectivity and offers both anion and cation exchange properties (Li et al. 2014). Additionally, the United Nations Environmental Program (UNEP) agency has recommended alumina-based adsorption as one of the finest methods currently available for removing arsenic from water (Sorg 2000).

Al₂O₃ NPs can remove arsenic from actual arsenic-contaminated groundwater as well as arsenite and arsenate from aqueous solutions (Ghosh et al. 2019). It is shown that sulphate and phosphate had a significant impact on the decline in removal efficiency (Prabhakar & Samadder 2018; Ghosh et al. 2019). The good regeneration performance of Al₂O₃ NPs suggested that the adsorbent was economically viable and operation costs would be minimal (Prabhakar & Samadder 2018; Ghosh et al. 2019). The incorporation of nano-aluminium oxide into cellulose acetate/polyphenylsulfone led to the formation of nano-Al₂O₃ crystals. As the quantities of nanoparticles increased, finger-like microvoids and micropores grew, thus increasing adsorption capability (Kumar et al. 2021).

Nonetheless, adding Fe₂O₃ and Al₂O₃ NPs to arsenic reduced the area under the frequency distribution curve of the sorption site energies, which in turn reduced the number of sorption sites that were open to arsenic (Turi et al.). This could be explained by the reduced hydrophobic interaction between synthetic materials and arsenic because of the hydrophobic surface of the Fe₂O₃ and Al₂O₃ NPs being blocked. Interestingly, Al₂O₃ NPs with activated carbon dopants not only demonstrated electrostatic attraction and ligand exchange adsorption mechanisms but also possessed antibacterial capabilities towards both Gram-negative (Escherichia coli) and Gram-positive (Bacillus subtilis) bacteria (Al-Gaashani et al. 2021). It is useful when used in water treatment applications; the improved material can reduce biofouling (Yu et al. 2019; Bi et al. 2023). A summary of the adsorption properties of Al₂O₃ NPs for arsenic removal is presented in Table 2.

Cerium oxide (CeO₂) is one of the rare-earth metal oxides that has been investigated the most in the areas of catalysis, soot particle reduction, oxidation of harmful environmental gases, catalytic reduction of nitric oxide, antibacterial action including the adsorption application of heavy metals. In determining the activity, stability, dispersion of the active component, and selectivity for toxin removal, CeO₂ properties such as crystalline size, bulk density, porosity, and surface area are crucial. Due to their distinct physical and chemical features towards arsenic affinity, cerium oxide-based composites stand out among the recently produced adsorbents (Zhang et al. 2016). The intriguing properties of using ceria as an adsorbent include its low solubility in acidic medium, low toxicity, a wide range of applications, and customization of charge from Ce³⁺ to Ce⁴⁺ during adsorption (Chen et al. 2018).

Techniques used to produce CeO₂ NPs include oxidizing cerium chloride in an H₂O₂ solution (Nakamoto & Kobayashi 2019) and one-pot, and surfactant-free aerogel technique (Mishra et al. 2018). On the other hand, 81.3% of As(III) could be effectively converted to As(V) by Ce and Cu in the Fe–Ce–Cu ternary oxide nanoparticle that was created using a coprecipitation–calcination process (Liu et al. 2023). This novel nanomaterial has enhanced surface hydroxyl groups that offer lots of places for arsenic to bind through surface complexation and redox.

Previous investigations showed a significant correlation between hydrophobicity and molecular weight (MW) in the rejection of pollutants, pinpointing size exclusion and adsorption as the main processes for their removal (Licona et al. 2018). The size and passage of neutral-hydrophilic and anionic contaminants showed a significant inverse association, according to Albergamo et al. (2019). A lower correlation was seen with the moderately hydrophobic pollutants. The poor removal effectiveness of RO membranes may be attributed to the hydrophobic moiety's attraction to the active layer, such as the hydrocarbon chains and aromatic rings. Concurrently, the negatively charged membrane surface and the pollutant experienced electrostatic repulsion, which aided in the pollutant's removal. Contrarily, positively charged impurities drastically decrease the removal efficiency due to their electrostatic interaction with the negatively charged membrane surface and subsequent diffusion.

NF, also known as a loose RO membrane, combines the hybrid functionality of both UF and RO membranes. The principal determinants of membrane performance in the removal of arsenic using NF are porosity, charge, concentration polarization at the membrane face, fouling of the membrane, concentration, and oxidation state of the arsenic and membrane module. As a result, predicting the removal mechanism only based on pore size is not possible. Most commercial NF membranes (e.g., NF-45, NF70 4040-B, HL-4040F1550, 4040-UHA-ESNA, ES-10, NTR-7250, NTR-729HF, BQ01, NTR-729HF, NF-90, NF-200, NE 90, and NF 300) feature a fixed surface charge caused by surface group dissociation, allowing ion separation by a combination of electrical effect, pore size, and ion interaction processes. It is significantly impacted by the water's temperature, pH, the presence of any other contaminants, the starting concentration of arsenic in the water, and the operation pressure, just like the other membrane processes (Siddique et al. 2020). Besides, arsenic rejection could be significantly reduced by extensive membrane fouling (Tanne et al. 2019).

The gold mining wastewater was treated using two stages of NF with NF90 membranes, resulting in 99.8%, however, both the increase in osmotic pressure and the presence of reversible fouling resistance have been attributed to the flux decline of the second-stage NF (Andrade et al. 2019). In gold mining wastewater treatment using NF90 membrane, pH of 8.5 was shown to be the most effective in terms of flux and rejection (Andrade et al. 2019), whereas pH of 5.0 produced the highest permeate flux with the least amount of fouling in another study (Andrade et al. 2017). Contrarily, complexation can improve As(III) retention with NF technology, as opposed to other techniques like oxidation or pH adjustment (Boussouga et al. 2021).

NF is an effective method for treating wastewater from sulphuric acid plants where 85% of the As(III) and 75% of the As(V) were removed. The most significant mechanism found was the retention of higher valence counterions to balance the retained co-ion loads; however, cake development was caused by membrane fouling (Reis et al. 2019). Membrane fouling becomes the biggest concern in the NF separation technique, and it acquires pre treatment before the NF process. Chemical coagulation with ferric chloride was used together with the NF separation technique and resulted in elimination efficiencies of 99 and 95%, respectively, for As(V) and As(III) (Zhao et al. 2020). NF270 showed that the size exclusion mechanism weakens (in the bulk solution) as the temperature rises as a result of the pore's thermal expansion (Dang et al. 2014) which lowers the As rejection. On the other hand, the rejection of As(V) with NF270 was also strongly impacted by the pH's effect on speciation where charge exclusion was the main mechanism (Boussouga et al. 2021). NF90 was evaluated to remove As(III) up to 87.5% (Jarma et al. 2021) but did not achieve the necessary limits for irrigation.

Arsenic removal from mining wastewater achieves a 97% rejection rate when using both NF90 and NF270 membranes (Jadhav et al. 2017). The NF270 membrane produced superior permeated flow values with a slight decrease in rejections, but it excelled at water recovery. Newly synthesized NF polyamide core–shell bio-functionalized matrix membrane for the purification of arsenic possesses increased hydrophilicity that improved rejections for As(lll) (99%) (Zeeshan et al. 2020). Polyamide is an aliphatic polymer that has recently shown good capability as an adsorbent (Basaleh et al. 2019; Li et al. 2019b). Higher fouling resistance membrane of NF membranes modified by functionalized graphene oxide and copper ferrodioxide was successfully removed 93.8 and 81.2% of As(III) and As(V), respectively (Gholami et al. 2023).

A purification of water method that uses a semipermeable membrane to reject ions, undesired molecules, and bigger particles from drinking water by forcing it through a semipermeable membrane under pressure to separate ions, unwanted molecules, and larger particles. The osmosis process occurs naturally without the need for energy, by which, the process of reversing osmosis requires the aid of energy. It works by forcing water through a special, selective membrane. Contaminants are rejected by a RO membrane based on their size and charge. Contaminants with a MW of more than 200 usually will be rejected by a RO system. Instead of MF and UF, where pollutants are captured within the filter medium, an RO system uses cross-filtration. Cross-filtration occurs when a solution passes through a filter with two outlets, one for filtered water and the other for contaminated water. It also removes pollutants by enabling water to flush them away while maintaining sufficient turbulence to keep the membrane surface clean.

This method works effectively under various pH and pressure circumstances. The high removal efficiency, lack of chemical dependence, mechanical toughness, chemical stability, ability to withstand intense heat, reduced need for an expert operator, and relatively low energy usage of this technology are its further advantages. RO membranes used for arsenic removal include BW30, TFC-ULP, FT30, TFC-SR, PVD, XLE, AD, and BE membranes. RO was conducted as a post-treatment of mining wastewater by which a great rise, in the end, permeates quality before it is released into surface waters (Samaei et al. 2020). RO membrane was used to treat gold mining effluent and compared with NF90 membranes, nonetheless, the RO membrane showed lower efficacy compared with NF90 (Andrade et al. 2017). XLE BWRO membrane showed a high rejection rate of arsenic with 99.0% but still surpasses the necessary limits for irrigation (Jarma et al. 2021).

MF is a technique that uses a microporous membrane to remove micron-sized particles, bacteria, viruses, protozoa, contaminants, pollutants, and other micron-sized particles from a solvent. The MF process is similar to a low-pressure-driven membrane process, utilizing membrane pores ranging from 0.1 to 10 μm. Silica, ceramics, zirconia, alumina, PVC, polysulfone, PTFE, polypropylene, PVDF, polyamides, polycarbonate, cellulose acetate, cellulose esters, or composite materials are used in some MF membranes. Due to its poor removal capacity, the use of MF in heavy metal removal has not received adequate attention. It has, however, been applied by altering the feed solution's membrane or chemical pre-treatment. The MF process is offered in crossflow and dead-end versions, depending on the manner of application.

A flocculation-MF reactor used to remove arsenic from groundwater with a maximum removal rate of 91% was recorded in the absence of oxidation and increased up to 96% with oxidation (Wang et al. 2019). Whereas pre-treatment (adsorption and oxidation) of MF with iron-loaded activated carbon, the residual arsenic concentration was reduced to 0.105 mg/L. This suggests that heterogeneous Fenton combined with MF is effective for removing arsenic from the water medium (Pramod et al. 2020).

At both pilot and full-scale installations, pressure-driven processes of UF have been successfully used, either as a standalone process or coupled with other membrane techniques. As compared with NF or RO, UF membranes produce more flux and use less energy. However, since size exclusion is the primary mechanism, due to the greater pore size of the membranes, the single UF process might not offer the requisite arsenic rejection, and effective removal of dissolved species of arsenic is not anticipated unless paired with additional procedures. A combined UF and coprecipitation method using iron (oxyhydr)oxides results in arsenic concentrations that are below 1 g/L (Ahmad et al. 2020).

On the other hand, only 13–19% of the arsenic ions were removed from municipal wastewater utilizing hybrid adsorption/coagulation/UF using ferric chloride (PAC/FeCl₃/UF) (Marjanović et al. 2023). This interaction is not preferred in dynamic membrane filtering settings, most likely because of potential interactions between the natural coagulant and the membrane surface, which do not result in floc formation with arsenic. Due to the possibility that UF membrane holes could be larger than the heavy metal ions, additives could be bound to the metal ions to increase their size. Consequently, it is suggested to use polymer-enhanced ultrafiltration (PEUF) and micellar-enhanced ultrafiltration (MEUF) (Yaqub & Lee 2020; Yu et al. 2023). MEUF is created by combining UF and a surfactant. MEUF has a high flux and selectivity, resulting in low energy consumption, high removal efficiency, and a smaller footprint. MEUF is best suited for wastewater with low amounts of heavy metals (Huang et al. 2017; Rahmati et al. 2017). MEUF of arsenic-contaminated water was implemented using cetylpyridinium chloride as a surfactant and was able to remove 89.32% of arsenic from water. 1 mg/l NaOCl and 10 mg/l FeCl₃ allowed the coagulation-MF system to remove 89.32% of the arsenic (Yaqub & Lee 2021).

A promising technique for removing heavy metals (Pb, As, and Hg) from industrial wastewater is air gap membrane distillation (AGMD) (Shahu & Thombre 2019; Kebria et al. 2020). Because of the high quality of the permeate flux and the low likelihood of membrane wetting, AGMD was chosen to treat industrial wastewater. Three commercially available membrane pore sizes (0.2, 0.45, and 1 μm; TF200, TF450, and TF1000) were examined. According to the findings, AGMD successfully removed the heavy metals with a high removal efficiency, TF200 and TF450, which was above 96% for heavy metal ions in a variety of concentrations (Alkhudhiri et al. 2020). Another membrane method that has been reported to efficiently remove pollutants from wastewater is forward osmosis (FO), which is known as an alternative to RO. Nonetheless, the number of applications for the removal arsenic is limited, because NF and RO were found to be more effective (Kundu & Naskar 2021). Pham et al. (2020) reported that the rejection performance of FO at low concentrations of arsenic (50 mg/L) was 92%, indicating that FO is not as effective in treating arsenic in wastewater. A more effective arsenic removal was reported by Hubadillah et al. 2020 by using direct contact membrane distillation (DCMD) hydrophobic kaolin hollow fibre membrane. Throughout the continuous operation of 70 h, the membrane exhibits outstanding arsenic rejection of 100% and a constant flow of 23.3 kg/m² h. The results show that the performance of this membrane is superior to that of NF and RO in terms of permeate flux, arsenic rejection, as well as performance flexibility and consistency under different pH settings. The effectiveness of arsenic rejection using different membrane filtration technologies is shown in Table 4.

Arsenic may appear in several different forms in aqueous media, such as arsenite and arsenates. The oxidized forms of arsenic are protonated in acidic environments and deprotonated in basic environments (Sharma et al. 2007). The neutral forms of arsenate in aqueous media are species such as H₂AsO₄. Song et al. (2017) state that the predominant ions in the solution are negatively charged of and in pH ranges of 3.0–7.0 and 7.0–11.0, respectively.

Electrocoagulation (EC) is regarded as a competitive alternative to chemical coagulation for the treatment of highly contaminated wastewater due to its high removal rate and lack of chemical addition. A less volumetric final sludge with greater stability is produced when EC is used to remove arsenic from wastewater (Hansen et al. 2023). However, this electrochemical technique has some significant drawbacks, including the need to change the sacrificial electrodes regularly and the production of passivated films (Sandoval et al. 2021; Zhang et al. 2021). Current density, electrode type, solution pH, electrolysis time, supporting electrolyte, and inter-electrode spacing are the main factors that affect the efficiency of the EC process. The utilization of various electrode connection types, ideal reactor designs, simulation models, the creation of sophisticated electrode materials, and all these factors taken together may significantly contribute to improving the treatment efficiency of the EC process.

The EC process has one of the highest arsenic removal efficiency among separation processes, at over 90% (Syam Babu & Nidheesh 2021). The effect of electrolyte concentration was studied in EC using aluminium-air fuel cells which is useful for treating wastewater in rural areas where electricity is challenging (Kim et al. 2017). It was effective when operated in low concentrations of Na₂SO_4, which produced an unstable formation of Al³⁺ ions with and ions complexes, leading to an increase in the arsenic removal efficiency (Maitlo et al. 2018). A combination of statistical and mathematical techniques known as the response surface methodology (RSM) is used for experimental problem optimization and modelling including the EC process. In a study conducted by Gilhotra et al. (2018), the RSM analysis reported that pH 5.2, current density of 20 A/m², and inter-electrode distance of 15 mm resulted in removal efficiency of 99.6 and 86% for 10 and 100 ppm of arsenic from arsenic high-strength wastewater, respectively (Gilhotra et al. 2018).

A two-dimensional bentonite/g-C₃N₄ composite (B/g-C₃N₄) with a layer–layer cross-structure and substantial surface area for photocatalytic oxidation was created to remove As(III) in wastewater (Wang et al. 2021a). The composite demonstrated a strong photocatalytic oxidation efficiency over a wide pH range of 3–8.5 and the results show that the As(III) oxidation rate of the B/g-C₃N₄ composite was three times that of pure g-C₃N₄. The photocatalytic oxidation of As(III) in simulated and real wastewater was investigated using carbon-modified titanium oxide nanoparticles supported on activated carbon (C–TiO₂/AC) using natural sunlight (Alfarawati et al. 2020). It took 150 min to remove all arsenic (2.66 ppm) from wastewater (Alfarawati et al. 2020); the process is quicker when using a new Fe₂O₃–Mn₂O₃ nanocomposite photocatalytic oxidation technique in As(III) removal from contaminated water, reduction of arsenic to ≥99% As(III) at 30 min (Eslami et al. 2018). The quicker process is due to the application of bimetal nano-oxide compounds promote unique oxidation and adsorption of arsenic from aqueous environments because using two or more metals in a single combination has an additive effect on each metal.

TiO₂ with an anatase crystal structure offers high photosensitivity, chemical stability, low toxicity, and low cost, making it a popular photocatalyst material. However, it possesses limitations such as a large band gap and limited transit efficiency of photogenerated carriers in TiO₂ (Wang et al. 2020c). Loading metal and non-metal materials with TiO₂ is an efficient strategy for improving the photocatalytic activity of titanium dioxide because it promotes charge separation and interface electron transport. TiO₂@Fe₃O₄ composites were created for efficient photocatalytic oxidation of As(III). The effective formation of Ti–O–Fe interface bonds between TiO₂ and Fe₃O₄ considerably increased the transfer efficiency of photogenerated carriers and accelerated the photocatalytic oxidation of As(III) in just 4 min (Xiao et al. 2022). Fe₃O₄ loading can also improve TiO₂ photocatalytic properties by allowing for the efficient separation of h⁺ and e produced by the reduction of Fe³⁺ into Fe²⁺ or F (Bi et al. 2019; Wang & Zhang 2020).

The nanomaterials WO₃/TiO₂ were produced for As(III) photo-oxidation (Navarrete-Magaña et al. 2021), enhanced photo-oxidation of As(III) correlates with effective charge transfer at the interface of TiO₂ and WO₃ where efficiency reached 99% and greater than bare TiO₂. Besides, challenges produced in employing xenon and fluorescent lamps to activate photocatalyst nanoparticles are generated at high temperatures, producing UV light, high costs, and operating problems. This can be overcome by using LED lamps as demonstrated by the BiVO₄/TiO₂/LED system, total arsenic removal from aqueous solution was reported by using this innovation (Rahimi & Ebrahimi 2019).

Furthermore, because adsorption of organic compounds on the surface of the photocatalyst is the first step in photocatalytic oxidation of heavy metals, combining adsorption with photocatalysis may result in more efficient materials. Many studies have implemented this strategy. A magnetic-Fe₂O₃ core–shell heterojunction nanocomposite was created by hydrothermal crystallization of TiO₂ on the surface of a magnetic core–shell loaded with polyaniline (γ-Fe₂O₃@PANI@TiO₂) to facilitate photocatalysis adsorption (Wang et al. 2020b). The efficient photocatalyst is dominated by the synergy of numerous active substances (i.e., superoxide free radicals and photogenerated holes). The photocatalyst composite ZnFe₂O₄/Ag/AgCl was used to remediate As(III) wastewater by activating peroxymonosulfate (PMS) (Lei et al. 2021). Particularly, the dual active sites (i.e., Fe atoms were active sites for As(III) adsorption while Ag nanoparticles were sites for PMS activation) on the photocatalyst could effectively reduce the distance between the oxidizing product and the arsenite molecule, facilitating As(III) fast oxidation. Lastly, sulphite was used to boost the photocatalytic oxidation ability, As(III)-containing wastewater was removed in 30 min (Lei et al. 2022; Table 6).

Arsenic contamination also originates from organic sources. The main method that the general population is exposed to organic arsenic is by eating seafood, which includes shellfish, finfish, and seaweed (Taylor et al. 2017). Organic arsenic comes in the forms of dimethyl arsenic acid, monomethyl arsenic acid, dimethyl arseneous acid, and monomethyl arsenic acid. Approximately 10–60 times as deadly as organic arsenic compounds are inorganic arsenic species (Thirunavukkarasu et al. 2002). However, considering the possible health dangers connected to certain types of arsenic, organic arsenic removal from wastewater is an essential part of water treatment. In recent years, the removal of organic arsenic has been concentrated using adsorption techniques. Table 7 summarizes the technologies used for the treatment of organic arsenic.

The cost for each technology must be taken into consideration to develop cost-effective techniques in removing arsenic from wastewater. Different technologies possess different costs that need to be understood and analysed carefully. This section discussed cost analysis based on the scenario of current technology applications in wastewater treatment. The adsorption technique depends on the cost of adsorbents where they are varied and influenced by several variables, including availability, source, treatment conditions, recycling, stability, and economic status of the country. Besides, the cost of the adsorbent per gram of the adsorbate removed, annual capital expenditures (CAPEX) and operating expenditures (OPEX), and the cost of applying the adsorbent in an adsorption operation included in the calculation of the cost of the adsorbent (Ighalo et al. 2022). The price of the various adsorbents is also influenced by several variables, including stability, availability, synthesis process, source, and recycling. In general, activated carbon (PAC, $1.8–2.1/kg) is more expensive than biochar ($0.35–1.2/kg) (Thompson et al. 2016). In comparison to ACs, which range in price from $1,100 to $1,700 per ton, biochar-based materials are an affordable substitute which are often less expensive, ranging from $350 to $1,200 per ton (Rehman et al. 2023). The low average cost of natural clays can be used as option such as bentonite, montmorillonite at $0.04/kg is a clear benefit over AC. Natural adsorbents like clay, ash, and peat are around 12 times less expensive than chitosan and could range from $10 to $1,000 (Gkika et al. 2019) and organic/inorganic composite, CNT-based hybrid adsorbents, was roughly four times greater than the cost of natural adsorbents (Shahadat & Isamil 2018). The cost of a kilogram of pure solid graphene oxide and melamine sponge sheets was about $72.87/kg and $2.91/1 m × 1 m × 2 cm, respectively (Xu et al. 2020).

Membranes techniques that employ various types should be considered carefully, NF, for instance, is not currently utilized commercially in water treatment because of the high expense of NF operation where installation costs can be relatively high and due to the membrane's durability is jeopardized, thus RO and UF are favoured (Mohammad et al. 2007). Another challenge of membrane technology is polymeric membranes used are prone to fouling which raises operating costs by necessitating additional cleaning procedures. As reported the total investment and unit running costs as roughly $2,000,000 and $1 per m³ processed, respectively, based on the results of pilot-scale research and a cost analysis for full-scale applications of membrane application (Cinperi et al. 2019). For FO recoveries of 80 and 90%, respectively, the cost increased to 1.01 and 1.27 €/m³ (Vinardell et al. 2020) while a 7 years cost analysis stated that the overall cost per m³ of water for the UF system ascended by 50% (Yu et al. 2020). Novel low-cost ceramic MF was made from natural materials such as sand (Tomina et al. 2017; Addich et al. 2022), natural magnesite (Manni et al. 2020), and Moroccan clay (Iaich et al. 2021) can be used to replace high-cost polymeric materials. Based on feasibility analysis, the investment cost of a pilot-scale plant containing ceramic membrane was approximately $1,100,000, and the operating cost varied between $910,000 and $2,120,000 depending on the temperature of the water to be used in the system (Ağtaş et al. 2020). Approximately $1,340,000 could be saved from both electricity and water reuse by using ceramic membranes.

The EC process is widely used because it is easy to install and maintain, has minimal operating costs, produces little sludge, and can be combined with other treatment methods including ozone, microwave, and ultrasonic (Table 5; Hashim et al. 2020; Das et al. 2021b). In addition to operating costs (i.e., cost of chemicals, sludge disposal, electrode/coagulant consumption) (Gasmi et al. 2022) and the small amount of sludge generated for disposal, the cost of operation in EC is correlated with energy consumption and electrode dissolution. The cost of electrodes per weight is $10.557/kg for iron and $23.15/kg for aluminium, respectively. Consequently, for the Al–Al and Fe–Fe combinations, the total operating cost was $4.15/m³ and $4.01/m³, respectively (Ebba et al. 2021). The findings demonstrated that, under optimal conditions, the first reactor (i.e., using cylindrical titanium cathode inside a Plexiglas cylinder) configuration removed 99.3% of the arsenic at an operating cost of $1.55/m³, while the second setup (i.e., two plate anodes and two plate cathodes both made of iron and connected in a monopolar parallel mode inside a Plexiglas cylinder) removed 96.9% of the arsenic at an operating cost of $0.1/m³ (Kobya et al. 2015). A detailed cost analysis of EC has been discussed by Moussa et al. (2017). In comparison, the cost for EC and chemical coagulation varied from $0.314 to $4.59 and from $0.113 to $0.467/m³, respectively. Chemical coagulation was almost twice as expensive as EC (Khor et al. 2020). Additionally, the costs associated with EC treatment were lower for the Fe electrode than for the Al electrode in terms of $/m³ of treated wastewater or $/kg of COD eliminated (Kobya et al. 2020). The treatment cost of 1 m³ of wastewater was estimated at €13.8/m³ for EC_iron (Gaied et al. 2019).

On the other hand, large-scale wastewater treatment plants for photocatalysis are currently not feasible due to their high capital costs. This entails extensive research into models before plant building (Pandey et al. 2021). The disadvantage of solar energy is that it can only treat wastewater during the day, whereas wastewater treatment plants work poorly at night. The need for energy storage systems raises the total cost of the wastewater treatment plant. The high preparation cost of photocatalysts, combined with the limited availability of materials, has raised concerns in the scientific community (Das et al. 2021a). Specifically, catalyst separation and recovery remain a key challenge, as their compatibility with photocatalytic reactors which can be costly (Constantino et al. 2022). To overcome these constraints, a tunable reaction system consisting of immobilized photocatalytic materials and appropriate photocatalytic reactors is necessary to expand the use of photocatalytic technology in water treatment (Zhang et al. 2023). In this context, the development of new and unique low-cost alternatives with Fenton-like catalytic ability, such as coal fly ash, holds considerable promise for pilot-scale wastewater treatment (Wang & Tang 2021a, 2021b). The inclined plate collector (IPC) is one example of a basic and low-cost photocatalytic reactor (Zhang et al. 2022). Along with the ease of separation after use, the as-prepared TiO₂/FeZ derivatized photocatalyst provided various benefits, including the use of a low-cost substrate and the capacity to maintain acceptable long-term catalytic activity (Srikanth et al. 2017). When developing a photocatalytic reactor, it is important to carefully manage the reaction time to minimize high costs from extended cycle times (Ahmed & Haider 2018). Improving the photocatalyst's recyclability can help reduce costs. Several TiO₂ and g-C₃N₄-based photocatalysts have been tested for recyclability (Balakrishnan & Chinthala 2022; Wang et al. 2023). Overall, a thorough comprehensive cost analysis must be done for each of the technologies.

In recent years, the emergence of various technologies for arsenic wastewater treatments offers vast advantages; however, large-scale application is still in the early stage. The pollutant adsorption activity of bimetal oxide adsorbents is greater than that of single-metal oxide adsorbents. Lanthanum and cerium are both members of the lanthanide series where the combination of lanthanum oxide and cerium oxide can create more efficient and cost-effective adsorbents (Nhiem et al. 2021; Wei et al. 2023). It appears that the current studies of membrane methods focus on the commercially available membrane due to low cost; however, this will limit the capacity to explore other newly synthesized membranes. NF is foreseen as having great potential in arsenic removal from wastewater where comparison investigation reveals various insights into the structure–property–performance link and its practical application, but it possesses limitations such as high membrane fouling that reduces the removal capacity and reduces water flux with time. Future research should be directed towards fabricating NF membranes that have high antifouling mechanisms through methods such as plasma treatment, photo-grafting polymerization, nanoparticles, electron beam irradiation, layer-by-layer, and interfacial polymerization. Polymer-based NF membranes using thin-film nanocomposites (TFNs) and thin-film composites (TFCs) have been extensively researched while incorporating different nanomaterials such as TiO₂ NPs (Ingole et al. 2017; Borpatra Gohain et al. 2022; Bhattacharyya et al. 2023).

Precautionary actions must be taken to help prevent membrane fouling not only through chemical treatment and assessment but developing advanced technology to enhance fouling mitigation and autopsy. Significant membrane fouling mitigation of integrated electrooxidation (EO) with membrane bio-reactor (MBR) with 28-day increased operational time before approaching the trans membrane pressure (TMP) limit of 30 kPa (Gharibian & Hazrati 2022). Besides, novel non-wetting solid-infused surfaces (SIS) also demonstrated superior fouling mitigation compared with lubricant-infused surfaces (LIS) and conventional smooth surfaces (Hatte & Pitchumani 2022). Besides, hybrid technologies of EC methods with other arsenic removal methods have opened a window of opportunities to the industry players, the advantages and disadvantages of each hybrid technology have been presented clearly in this review. By overseeing these details, technical challenges related to EC alone can be overcome as presented by the hybrid EC-membrane technique where 100% removal of the heavy metal from steel industry effluent was demonstrated (Changmai et al. 2022).

Each technology for arsenic removal from wastewater has its own set of advantages and disadvantages as tabulated in Table 8. The choice of the appropriate technology depends on factors such as the concentration of arsenic, the specific requirements of the water treatment application, cost considerations, and environmental impacts.

This article comprehensively reviewed the key strategies of technologies for arsenic removal from wastewater including adsorption by MNOPs, membrane methods, EC, and photocatalysis. With the rapid development of various industries, these technologies must be conducted on a large scale and most of the technologies are still in the premature stage. The key findings of this review are summarized below:

• The key advantages and limitations of each technology have been discussed which help future research to choose the best yet simpler method for arsenic treatment in wastewater.

• Bimetal oxides NP adsorbent is more efficient compared with single-metal oxides NP adsorbent and antibacterial properties of metal oxide NPs help to reduce membrane fouling.

• The primary issue of membrane technology, however, must be addressed more specifically using antifouling strategies to sustain the use of the membrane while cutting the cost of operation. For instance, chemical treatment and fouling mitigation must be advanced.

• Hybrid technology of EC with other treatment technology provides a more efficient system, 100% removal of the heavy metal from steel industry effluent using EC-membrane technology.

• Photocatalysis has been set as a promising technology in combination with adsorption due to the adsorption mechanism of organic compounds on the surface of the photocatalyst is the first step in photocatalytic oxidation.

• Further research is recommended to overcome the challenges of each arsenic removal treatment and to choose treatment with most cost-effective and productive. Future research on life cycle evaluation and economic analysis must be rigorous.

The authors wish to thank Universiti Teknologi Malaysia for their financial support toward the project titled ‘The influence of chemical and fibre traits of Malaysian bamboos on their pulp and paper characteristics’, under grant number PY/2022/02318-Q.J130000.3851.21H99. The research has been carried out under the program Research Excellence Consortium (JPT (BPKI) 1000/016/018/25 (57)) provided by the Ministry of Higher Education Malaysia (MOHE). The authors also acknowledge the financial support funded by Kurita Water and Environment Foundation through Kurita Overseas Research Grant 2023 (23Pmy220 and PY 2023/02465). The authors would like to express gratitude for the financial support from the Universiti Pertahanan Nasional Malaysia (UPNM).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.