An overview of As(V) removal from water by adsorption technology

Arsenic is a chemical element which is well known to human for its toxicity [1]. Chronic toxicity of arsenic via drinking arsenic-contaminated water will threat to human’s health [2]. Previous studies reported that long-term exposure to arsenic-contaminated water will cause harmful effects on people’s skins, lungs, digestive systems and nervous systems [1,3]. Arsenic is widely distributed in natural water, due to natural geological processes and human activities, such as mining, metallurgy and chemical industry [4,5].


Introduction
Arsenic is a chemical element which is well known to human for its toxicity [1]. Chronic toxicity of arsenic via drinking arsenic-contaminated water will threat to human's health [2]. Previous studies reported that long-term exposure to arsenic-contaminated water will cause harmful effects on people's skins, lungs, digestive systems and nervous systems [1,3]. Arsenic is widely distributed in natural water, due to natural geological processes and human activities, such as mining, metallurgy and chemical industry [4,5].
T he problem of water pollution caused by arsenic contamination is worldwide for a long time, and it is reported that drinking arsenic-contaminated water had seriously infl uence on more than 100 million people all over the world [6]. A rsenic removal from water has been an urgent problem to be solved, and reducing the limit of arsenic in drinking water is of great benefi t to human's health. Therefore, the contaminant acceptance threshold of arsenic in drinking water in the related standards were more stringent in the past decades [7]. According to the regulation of World Health Organization (WHO), the maximum permissible limit for arsenic concentration in drinking water was revised from 50 μg/L to 10 μg/L [8].
Arsenic usually exists in two valence states in natural water: As() and As(V). The toxicity of As(III) is much higher than that of As(V). As(III) mainly exists in the form of molecular, and As(V) mainly exists in the form of H 2 AsO 4 or HAsO 4 2under neutral conditions, in general, ionic arsenic is more easily removed by physical or chemical processes [9].
Therefore, As(III) can be converted into As(V) by chemical or biological methods to reduce its toxicity and the diffi culty of subsequent treatment [10][11][12].
The main methods for As(V) removal from drinking water include adsorption technology [13][14][15], coagulation technology [16,17] and membrane technology [18,19]. Arsen iccontaminated water treatment by adsorption technology has been widely adopted due to its advantages such as simple operation, economic benefi ts, and a variety of materials which Citation: Yao  are proven to be used as adsorbents. This study provided an overview of As(V) removal from water by adsorption technique, in order to give a reference for the application of adsorption technology to remove As(V) from water in practice.

The mechanism of arsenic removal by adsorption
Insoluble solid materials with high specifi c surface area or adsorption groups are chosen as adsorbents, and arsenic in drinking water can be fi xed to the adsorbent by physical or chemical process including electrostatic attraction, surface complexation, and ion exchange [20]. The characteristics and main infl uencing factors of adsorption pathways are summarized in Table 1. The adsorption capability of an adsorbent is an important indicator to evaluate its performance. The effects of As(V) adsorption are infl uenced by pH, temperature, initial concentration of As(V), contact time and the presence of other competing ions in water [14,21]. saturated adsorption capacity can reach 2.5 mg/g at pH 2-11.5 and 20℃ after 72 h, using 2 g/L GAC, which was of 950 m 2 /g specifi c surface area (BET), treating raw water with initial As(V) concentration of 0.5-10 mg/L. This study also indicated that

Metal based materials:
Metal based materials include metals, metal oxides and metal hydroxides [36]. Activated alumina is often used in drinking water treatment due to its large specifi c surface area and rich pore structure, which is often used for arsenic removal from water [37]. Previous studies reported that the adsorption capacity of As(V) using activated alumina as an adsorbent can be improved under acid conditions or by modifi cation methods [38][39][40].
Zero valent iron, iron oxides and their hydroxides also have good adsorption performances on As(V). Bang, et al. [41] showed that high dissolved oxygen (4.3-5.5 mg/L) and low pH

Traditional adsorption materials
Activated carbons: Activated carbon is well-known to be used as adsorptions, which is a kind of carbide made from coconut shell, charcoal, lignin, sawdust, rice husk and other carbonaceous materials by carbonization and activation processes. The strong adsorption capacity is attributed to rich pore structure, huge specifi c surface area and surface functional groups [13].
Both activated carbon and modifi ed forms can be used as adsorbents to remove As(V) from water. Natale, et al.
investigated the effect of As(V) removal using granular activated carbon (GAC) [29], and the results indicated that the Citation: Yao  The structure of adsorbent was also analyzed by Fourier transform infrared spectroscopy (FTIR) and extended X-ray absorption fi ne structure (EXAFS) to explore the adsorption mechanism of As(V).
In addition, the composite iron aluminum hydroxide, manganese oxide, rare earth oxide (such as lanthanum and cerium), zirconium oxide, titanium oxide and tin oxide can also be used for As(V) adsorption from water [13,42]. which needs to be treated properly. Altundoğan, et al. [44] reported that the adsorption capacity of As(V) can be effectively

New adsorption materials
Nanomaterials: Nanomaterials have smaller particle size, higher specifi c surface area and stronger adsorption capacity, compared with the traditional adsorption materials [14,47]. , with a BET of 152 m 2 /g, to treat raw water with initial As(V) concentration of 100 μg/L. The contact time was extended from 10 min to 60 min, correspondingly, the removal ratio was increased from 50% to 99%. The results indicated that As(V) can be removed effectively from water, and its concentration was as low as 1 μg/L in effl uent. The adsorption capacity was 5000 μg/L calculated by Langmuir and Freundlich adsorption isotherms. Kanematsu, et al. [53] fi lled the adsorption fi xed bed with nano goethite particles (BET 158.1 m 2 /g), for treating As(V)-contaminated water with initial As(V) concentration of 120 μg/L at pH=7. The empty bed contact time (EBCT) was controlled at 0.328 min, to ensure the discharge to meet the standards of As(V) in drinking water. In addition, silicate was not conducive to be used for As(V) removal because of strong adsorption competition with the existing As(V) ions.

Mesoporous materials:
Mesoporous materials is a kind of adsorption material with pore sizes ranging from 2 nm to 50 nm, with large specifi c surface area, highly ordered pore structure and high adsorption capacity [28]. Patra, et al. [54] showed that mesoporous γ-Al 2 O 3 spherical nanoparticles with BET of 497 m 2 /g had a high affi nity for As(V) in water. The dosage of adsorbent was 0.1 g/100 mL, and the contact time was prolonged from 1 h to 6 h, then, the removal rate can be increased from 60% to 80% when treating the water with

Biological adsorption
Biological adsorbents include plants, microorganisms and biomaterials, which can adsorb, transform and degrade As(V) in water through a series of physical, chemical and biological processes. Biological adsorbents have promising application prospects due to the advantages of high effi ciency, cost effective and less secondary pollution.
Many species of a l gae in huge quantities are widely distributed in natural water, which have been reported to be associated with adsorption and degradation of pollutants in water. As(V) can be adsorbed on the surface of algae and accumulated in their cells, and then reduced to As(III), which can be further degraded and detoxifi ed by methylation process, to achieve arsenic removal from water [56].
Microbial cell walls are composed of polysaccharides, lipids and proteins, which are rich in binding sites, therefore, they can be used as adsorbents for water treatment [13]. As(V) in water is adsorbed on the surface of microorganism and degraded by a series of metabolic activities. It has been proven that several fungal strains of Aspergillus and Trichoderma, and other fungi were isolated from soil [57]. Aspergillus candidus belonging to a facultative marine fungus also showed good adsorption performance on As(V) removal from water [58].
In addition, previous studies have shown that biomaterials can also adsorb As(V) in water [59]. For example, chitosan converted from crustacean shell by deacetylation process is used as an adsorbent to treat As(V)-contaminated water, which has good adsorption capacity for As(V), due to its high affi nity for ions caused by its high molecular chain structure, rich hydroxyl and amino groups [13].
Furthermore, the adsorbents can also be modifi ed by chemical methods to improve the adsorption capacities of As(V). A novel Fe(III)-loaded ligand exchange cotton cellulose adsorbent was prepared by loading Fe(III) onto cotton cellulose (BET 2.23 m 2 /g, water content 87%) by Zhao,et. al. [60], and this adsorbent was fi lled into a glass fi lter column (φ 9.5 × 300 mm). The raw water with initial As(V) concentration of The results showed that the raw water with initial As(V) concentration of 101 mg/L can be treated until there was no As(V) in effl uent (downward fi ltration, fi ltration rate 2.5 mL/min, pH=4), thus, the adsorption capacity was as high as 96.46 mg/g. It has also indicated that acidic condition was more conducive to the removal of As(V), and 0.1 M NaOH solution can be used as regeneration agent.

Problems and suggestions
As(V) removal from water by adsorption will not produce chemical sludge and concentrated water needed subsequent treatment, compared with treating As(V) using coagulation or membrane technologies. However, the adsorption capacity of an adsorbent is limited, and the effect of arsenic removal is easily affected by the competition of coexisting ions in water, therefore, it is not benefi cial to select adsorption technology to treat As(V)-contaminated water with complex ionic composition, and the costs will be highly increased due to the needed extra pretreatment process. Furthermore, the se paration of adsorbents from water has been considered as a big challenge for drinking water treatment processes.
In general, the adsorption technology is suitable for treating