The conventional technologies for removing arsenic from groundwater, include: oxidation, coagulation, adsorption, ion exchange and membrane technologies (Wang et al., 2007). The process characteristics and their main advantages and disadvantages are described below: Oxidation/filtration
Oxidation kinetics rate of As (III) species by dissolved oxygen is very slow in water, which can take weeks to complete. Chemical oxidants are used to increase the oxidation rate of soluble arsenite As (III) to arsenate As(V). Oxidation alone as a technology does not remove arsenic from the solution, thus it has always been added before the main treatment, such as adsorption, coagulation or ion exchange. For instance, oxidation is an important step for anoxic groundwater, since arsenite is the prevalent form of arsenic at neutral pH. There are many chemical oxidants such as, chlorine, ozone, potassium permanganate, manganese oxide and hydrogen peroxide, as well as bacteria that have been used to directly oxidize arsenite in water. Chlorine is considered a good oxidant although it produces unwanted disinfection by-products in the presence of organic matter and is responsible for bad taste and odor. Potassium permanganate produces no harmful compounds but may impart colour.
Coagulation- Flocculation
In the coagulation process, the chemicals used, positively charged coagulants, adsorb and co- precipitate arsenic ions in a particular pH solution, thereby making particles to aggregate and enlarge. Arsenic removal efficiency of different coagulants varies as a function of pH. Commonly used chemicals with this technique are ferric chloride (FeCl3) or aluminum sulphate
(Al2(SO4)3) (or ammonium sulphate).
Lime treatment is similar to coagulation, but instead of trivalent metal salts, the coagulant used is hydrated lime (Ca(OH)2) or solid form of Mg(OH)2. However, the method of lime treatment
cannot serve as a major arsenic removal technique due to its low removal efficiency. Other disadvantage of this process is the daily production of highly toxic sludge.
Flocculation involves the addition of anionic flocculants that causes bridging or charge neutralization between the formed larger particles, leading to the formation of flocs, which undergoes precipitation later (Matsui et al., 2017).
Several researchers reported that, between the two inorganic species, arsenate is more efficiently removed compared to arsenite and, therefore, a pre-oxidation would be beneficial. Moreover, it has also been reported that FeCl3 is a better coagulant than Al2(SO4)3 at pH higher
than 7.6. Below pH 7.6, Al2(SO4)3 and FeCl3 are equally effective in removing arsenic from
water. The pH adjustment and proper dosing are very critical to obtain a high process efficiency. In both processes, the formation of these solids allows the subsequent removal of arsenic through sedimentation and filtration processes. The major disadvantage of these techniques is the production of high amounts of an arsenic-concentrated sludge, which requires a careful management of this sludge to prevent a secondary environmental pollution. Additionally, the treatment of the sludge produced is costly. These limitations make this process less feasible, especially in field conditions (Pallier et al, 2010; Sun et al., 2013).
Adsorption
Adsorption is a process that uses solids as medium for the removal of substances from gaseous or liquid solutions. The substances are separated from one phase and accumulated at the surface of another. This process is driven mainly by Van der Waals forces and electrostatic forces between the adsorbate molecules and the adsorbent surface atoms. Therefore it is important to first characterize the adsorbent surface properties (e.g. surface area, polarity) before being used for adsorption.
Accordingly, to the review published by Giles et al., 2011, the main adsorbents of choice for As removal are Al2O3, Al(OH)3, carbon, FeO, Fe2O3, modified iron oxides and SiO2. The iron-
based adsorption is a widely used technique for the treatment of arsenic contaminated water due to the high affinity between inorganic species and iron. Iron can remove arsenic from water either by acting as sorbent, co-precipitant or contaminant-immobilizing agent or by behaving as a reductant (LeMire et al., 2010).
As per US Environmental Protection Agency (USEPA) classification, adsorption is amongst the best available technologies for As removal in potable water. Adsorption presents several advantages including relatively high arsenic removal efficiencies, easy operation and handling, cost-effectiveness, and no sludge production.
However, adsorption of arsenic strongly depends on the systems concentration and pH. At low pH, arsenate adsorption is favored, whereas for arsenite, maximum adsorption can be obtained between pH 4 and 9.
One significant disadvantage of the system is the presence of other ions in water such as phosphate and silicate, that compete for the adsorption sites. Furthermore, effectiveness of adsorption in arsenic removal can also be hindered by the type of adsorbent itself. Most conventional adsorbents have irregular pore structures and low specific surface areas, leading to low adsorption capacities. Lack of selectivity, weak interactions with metallic ions, and regeneration difficulties can also limit the ability of these sorbents in lowering arsenic concentrations to levels below MCL (Chatterjee S. et al., 2017).
Ion-exchange
Ion-exchange (IEX) technology for arsenic removal is considered one of the best available technologies (BAT) (EPA, 200a). It is commonly based on the use of strong-base chloride or sulphate forms resins, and the exchange of Cl- or SO
42- anions for arsenic species and other
ions present in the water. The uncharged As(III) cannot be removed by an ion exchange method and a pre-oxidation step is needed if an arsenite solution has to be treated.
The IEX process has two major disadvantage: 1) sorption capacity, because a strong anion competitor such as sulphate (or others, e.g. nitrate) that are commonly present in groundwater thus decreasing the efficiency drastically (resins are highly selective for sulphate ions and there is the risk of arsenic release in the treated water) and; 2) large volumes of hazardous residuals are produced due primarily to frequent regeneration of the exhausted resin. Also an ion exchange method alone is not sufficient to remove arsenic below the maximum contaminant level (MCL) of <10 µg/L (Zhao et al., 2010; Zhao et al., 2012; Dominguez-Ramos et al., 2014; Guell et al., 2011; Jadhav et al., 2015).
Furthermore, it has been proposed the use of metal-loaded polymers (chelating or ion-exchange metal-loaded resins) due to the advantage of these materials to overcome interferences from other accompanying anions and the possibility for removing As(III, V). LeMire et al., 2010 studied the use of iron-impregnated ion exchange beads for As(V) removal and the influence of several factors (particle size, pH, As(V) concentration, competition, adsorbent, temperature and iron content) and concluded that it may be considered as a viable alternative to other iron based adsorbents in terms of durability and efficiency.
Membrane processes
There are two categories of pressure-driven membrane filtration: low pressure membrane such as microfiltratiom (MF) and ultrafiltration (UF), and high-pressure membrane processes such as reverse osmosis (RO) and nanofiltration (NF). According to Shih (2005) these membrane processes are effective to remove arsenic from water, especially high-pressure processes, NF and RO, in order to respect the maximum admissible concentrations. However, source water quality and effluent concentration to be reach are important design parameters (Criscuoli and Figoli, 2018).
As reported by Jekel and Amy, 2006, As(V) rejections observed in NF or RO ranged from 85% to 99% and As(III) rejections between 61 and 87%. Coagulation followed MF for arsenic removal was also shown to be more efficient than conventional filtration. Under optimal conditions, a 100 µg/L of arsenic level was reduced by 97% (Molgora et al., 2013; Pal et al., 2014). An integrated system combining NF + coagulation was proposed for arsenic removal from groundwater. A pre-oxidation step followed by a flat sheet cross-flow nanofiltration attained a 98% of As removal from an initial concentration of 180 µg/L. Arsenite can be rapidly oxidized to arsenate via a pre-oxidation step with, e.g. hypochlorite, permanganate and hydrogen peroxide. However, in this situation, a previous oxidation to convert As(III) to As(V) is not advisable due to the possible damage of the membranes with the chemical (oxidation agents) required to this step.
RO is probably the best practiced technology which can completely purify water and meet the strict water legislations (Holl, 2010; Katsoyiannis and Zouboulis, 2006). Both lab and pilot- scale experiments have shown more than 95% As(V) and 74% As(III) removal efficiencies achieved by RO.
Membrane processes have the main advantage of function without any chemical addition. However, there are some disadvantage such as high initial investment (apparatus, membrane and installation) and operational costs (including energy consumption) involved. Additionally, the presence of Fe and Mn in water prone to fouling due to precipitation of these ions as hydroxide and this type of fouling is irreversible in nature. For removing it, pretreatment of water, monitoring of the operating pressure, and a skilled operator are required. Moreover, in the case of a high arsenic containing water to be treated, the standard value MCL of arsenic is not achieved (Park et al., 2011; Dolores et al., 2017; Fang et al., 2013; Akin et al., 2011; Sen et al., 2010; Xu et al., 2015).
Amongst the conventional technologies presented, coagulation and ion exchange (IEX) are the most used ones due to lower costs, ease of handling and potential reuse of anion-exchangers. In each case, the removal efficiency is influenced by the chemical form of the arsenic present in water, usually as arsenate (As (V)) or arsenite (As (III)). The removal of arsenite is generally less effective by these techniques. However, arsenite can be rapidly oxidized to arsenate via a pre-oxidation step with, e.g. hypochlorite, permanganate and hydrogen peroxide.
The main disadvantages of chemical coagulation followed by settling and/or filtration of the treated water are primarily related to the need for the direct addition of the coagulant to the water, thus leading to residual levels of iron or aluminium, which is undesirable and can give rise to consumer complaints. The EU drinking water directive recommends a limit of 200 ppb for both Fe and Al in drinking water. Moreover, due to the fact that the coagulation process is sensitive to pH, appropriate reagents have to be often added to adjust the pH to the optimal value, additionally increasing the risk of secondary contamination of the treated water by these reagents.
One way to overcome these limitations is by transporting arsenate through anion exchange membranes via an ion exchange membrane (IEM) process - Donnan Dialysis, which is further explained in the following section.