The method of arsenic removal is divided roughly based on the following three principles: (1) the coagulation-sedimentation method by iron salt or aluminum salt, (2) the adsorption method by ion-exchange resin, chelating resin, activated alumina, or cerium agent, and (3) the separation method by reverse osmosis (RO). The processing of arsenic removal is composed of a single principle or a combination.
The practical utilization of the remediation processes for arsenic removal needs to consider the following factors: (1) accuracy of removal, (2) ions for removal (As(III), As(VI)), and (3) the amount of the generated sludge. In this paper, a chelating resin, which functions with selectivity of ions, and synthetic hydrotalcite (HT), which is one of the inorganic ion-exchangers, are described.
2. Chelating resin
The use of the chelating resin, which supports zirconium on the resin, in arsenic removal is through the adsorption method. Although its mechanism is not clear, the adsorption ability increases parallel to the level of coexisting salinity, thereby, more accurate removal of arsenic is produced. Chelating resin is spherical and easy to handle. However, there is a challenge that the adsorption ability to As(III) is inferior.
3. Inorganic ion-exchanger
The pertinent characteristic of HT, which is one of the clay minerals, is its function as a selective ion-exchanger. HT is a layered compound and it holds negative ions between the layers. The negative ions are exchanged with the negative ions in the medium, which are objects for adsorption. Generally, their chemical formulas are indicated as follows:
（In the formula, M2+, M3+, and Ａn- indicate bivalent metal, trivalent metal, and n-valent negative ions, respectively.）
HT, as well as the use of chelating resin and RO, pose difficulties in removing As(III). However, HT is an inorganic material, thus, it can oxidize As(III) into As(V) using an oxidizer. On the other hand, chelating resin and RO cannot function with an oxidizer because they are composed of high polymer materials. HT is in the form of a powder agent, hence, it is also feasible for use in the coagulation method. The only disadvantage is the difficulty in reproducing HT after the adsorption of arsenic ions. The isotherm of HT shown in Fig. 1 indicates that the amount of adsorption declines significantly as the residual arsenic concentration decreases. The pros and cons on the use of HT are described in Table 1.
Fig. 1 Arsenic adsorption isotherm by inorganic ion-exchanger
(temperature 25℃, pH6?7, no NaCl mixed)
Table 1 Pros and cons on arsenic removal in water
(Chelating resin method and inorganic ion-exchange method)
4. Practical method of arsenic removal in water
(1) Arsenic removal in certain sump water
The concentration of arsenic in sump water was 0.05mg/l, therefore, the removal technique applied was similar to the examination of natural water. Arsenic can be removed until less than 0.01m/l. However, the pH was initially high at 9.3 and subsequently, the adjustment of pH control after the removal of arsenic was necessary (Removal was just examined).
(2) Arsenic removal of certain industrial wastewater
Industrial wastewater with mean arsenic concentration (As(V)) of 1.8mg/l had been removed by the coagulation-sedimentation method using ferric chloride. However, arsenic concentration sometimes exceeded the effluent standard, which is 0.1 mg/l. Fig. 2 shows the examination of the processing method using HT and chelating resin together. In order to make the most of the existing equipment utilized for coagulation-sedimentation, the equipment for injecting ferric chloride was exchanged with the one for HT injection. Furthermore, the chelating resin tower was added. Consequently, the cost for the equipment was controlled. In this case, although the cost for adsorption method was higher, the total running cost was reduced because the amount of generated sludge decreased significantly (Table 2).
Fig. 2 Outline of processing flow on the effluent containing arsenic
(Highlighted areas show additional equipment.)
Table 2 Running cost of arsenic removal in water
In this paper, the removal for arsenic in water necessitates the determination of a flexible method catered for the different characteristics of the water to be treated. The presence of an existing wastewater treatment facility can be utilized for further remediation processes other than its original purpose or function. If and when initially considered, more and possibly different treatment systems can be employed.