1. Need for arsenic removal
Environmental conservation against pollution of water sources and soil, in which natural and industrial pollutants bring about, is highly demanded recently. One of the typical pollutants is arsenic, which in majority of cases contaminates the soil. Arsenic eluted from rocks and minerals and the marine clay subsequently comes to the surface of the ground by surface water, hotspring water, and well water. Wastewater generated by human activities and especially industries also contributes to the concentration of arsenic through the production of leachate, wherein arsenic then becomes soluble in groundwater, from civil engineering structures. The arsenic concentration in the water discharged to the surface by these causes usually ranges from a few ppms to a few ppbs. The control criteria of the water quality are regulated to prevent health problems caused by arsenic. The effluent standard of Water Pollution Control Law for arsenic is 0.1mg/L. However, the level for standards, such as purification, running water, and soil elution is 0.01mg/L. In addition, specific underground penetration is 0.005mg/L. The severe management for water quality concentration is demanded.
2. Features of cerium adsorbent
It has been known that cerium hydroxide, which is one of the rare earth elements, has a high selective adsorption against negative ions, such as arsenic, fluorine, and boron. Cerium hydroxide has been put into practical use as an adsorbent called cerium adsorbent READ-As series by Nihonkaisui Co., Ltd.
Cerium hydroxide possesses the lowest solubility against acid among the rare earth elements. An adsorbent does not elute when harmful ions in water is removed. Since it is safe to use, cerium hydroxide is selected for the adsorption of arsenic. READ-As turns cerium hydroxide into a granulated polymer suitable for adsorbing, which is a 0.7mm spherical particle in average diameter. READ-As can operate for adsorption like the conventional ion-exchange resin.
Arsenic dissolved in water exists in two states, trivalent arsenite (AsO３3-) and pentavalent arsenite (AsO４3-). Arsenic in groundwater is in the trivalent state. When trivalent arsenite touches the air, it will be gradually oxidized into pentavalent arsenite. The conventional coagulation-sedimentation method and adsorbents like activated alumina work effectively only for the pentavalent state. Thus, when trivalent arsenite is oxidized with an oxidant in advance, a removal treatment is then processed. On the other hand, cerium adsorbent as shown in Fig. 1 shows a high adsorption for arsenic in both trivalent and pentavalent states. No pretreatment like oxidation is needed.
Fig. 1 Adsorption line of READ-As to trivalent and pentavalent arsenite
(pH=6.8?7.0 Ion-exchange water, agitation 90 hours)
3. Comparison of READ-As with other treatment systems
One of the methods for arsenic removal is the coagulation-sedimentation method using ferric chloride or Poly iron sulfate / PAC. This method exhibits a few problems as follows. The maintenance of the operation for treatment with arsenic concentration of less than 0.01mg/L is difficult. Greater amount of sludge containing arsenic is generated. Furthermore, a wide installation space is required. As a result, in case treatment of less than 0.01mg/L is needed especially for drinking water or the installation space is limited in a construction site, the treatment using adsorption method has an advantage.
The comparison of READ-As with iron, activated alumina absorbent, and coagulation-sedimentation method is shown in Table. Regarding Non-READ-As, pretreatment processes like the addition of an oxidant and pH control are necessary. On the other hand, pre- and post-treatments are no longer needed if the liquid is near neutral pH as concerning READ-As.
READ-As can be used repeatedly by regenerating it using an alkali. Basically, the recycling operation called “Commuting reproduction” is performed at the factory of our company. The expense of regeneration is to be paid by a user. In the case of the treatment for industrial wastewater, it is also possible to add a regenerative function to the site and to accomplish on-site performance.
Table Comparison of arsenic treatment systems
4. Specific case and treatment system by READ-As
The typical flow of READ-As is shown in Fig. 2. According to the standard specification, a two-line linking is employed. In case “breakthrough” occurred at one of the two raw water towers, all the adsorbents are extracted from the tower. The tower is then sent to our factory for reproduction. While the first tower is being reproduced, the treatment is performed only by using the second tower, the method of which is called the “Alternate recycle system”.
Fig. 2 The arsenic removal system by READ-As
Arsenic treatment device using READ-As runs in sites of industrial effluent treatment and arsenic removal performance from clean water, effluent from spring water, and groundwater discharged from tunnel, and in the repair of contaminated soil. The removal device of arsenic in tunnel spring water is shown in Fig. 3. The two towers filled with adsorbents of 1000L per tower is placed in series in two lines for spring water at 30?/hr with arsenic concentration of 0.63mg/L. Every year, one of the two towers in each line is taken back for recycling in order to maintain arsenic concentration of treated water at 0.01mg/L.
Fig. 3 A sample case of arsenic removal in tunnel spring water