Removal properties of arsenic compounds with Water Science and Technology: Water Supply synthetic hydrotalcite compounds Y. Kiso*, Y. J. Jung*, T. Yamada**, M. Nagai*** and K. S. Min**** *Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi, Japan (E-mail: [email protected]) **Department of Architecture and Civil Engineering, Toyohashi University of Technology, Japan ***Department of Human Environment, University of Human Environment, Okazaki, Japan ****Department of Environmental Engineering, Kyungpook National University, Daegu, Korea Abstract The contamination of underground water with inorganic arsenic compounds has caused serious problems, particularly in developing countries. For water containing low-level arsenic compounds, an adsorption process may be more effective than other processes such as RO membrane and precipitation. In this study, the removal performance for arsenic compounds was examined with synthetic hydrotalcite (HTAL) Vol 5 No 5 pp 75–81 compounds as an adsorbent process from the following viewpoints: the adsorption capacity, adsorption isotherm, the effects of pH and co-existing anions. The HTAL-Cl, which contains Cl2 ions as an intercalate, showed very high adsorption capacity in the neutral pH region. The maximum adsorption capacity was 105 mg-As(V) g21. The adsorption isotherm was approximated by the following modified Langmuir equation: pffiffiffiffi 263 Ce qe ¼ pffiffiffiffi 1 þ 2:39 Ce Q The equation suggests that one mol of As(V) occupies two adsorption sites of HTAL-Cl, and the IWA Publishing 2005 experimental result indicated that 2.64 mol of Cl2 ions in the HTAL-Cl were substituted with one mol of As(V). The interfering effects of co-existing anions were relatively low, and the magnitude of the effects was 2 22 22 2 observed in the order of HCO3 . HPO4 . SO4 . Cl . Keywords Arsenic compounds; adsorbent; hydrotalcite; adsorption isotherm; co-existing anions Introduction Many people in the world are dependent on groundwater as their drinking water sources, and it has been reported that contamination with trace elements has caused serious health problems in developing countries (Hodi, 1995; Karim, 2000; Roberts et al., 2004). One of typical environmental incidences is caused by the inorganic arsenic compounds, and new regulations for arsenic levels in drinking water have been established for each country: 0.01 mgL21 for EC, 0.025 mgL21 for Canada, 0.01 mgL21 for the US EPA, and 0.01 mgL21 for Japan (Smedley and Kinniburgh, 2002). The arsenic compounds in a natural system are primarily found in the forms of arsenate and arsenite depending on the prevailing redox conditions. Their environmental behavior and mobilization are also influenced by some factors such as pH and the pre- sence of co-existing ions, which should be considered in the selection of the removal technologies for arsenic compounds. For water containing low-level arsenic compounds, sorption processes may be more effective than other methods, such as RO membrane and precipitation, from the view- points of treatment efficiencies and water losses under normal conditions (EPA, 2000). In addition, the sorption process appears to be the most attractive technique owing to the following advantages: sludge-free operation, easy operation, and reuse of the adsorbent after regeneration (Nriagu, 1994). 75 Several adsorbents, such as aluminium oxide, cerium oxide, and manganese dioxide, have been developed and examined for the removal of arsenic compounds. However, the adsorbents except cerium oxide have relatively low adsorption capacity, and cerium oxide is very expensive. In addition, the currently available adsorbents require acidic con- ditions for the effective removal of arsenic. Some alternative adsorbents have been inves- tigated to remove arsenite and/or arsenate: ferrihydrite (Pierce and Moore, 1982), iron Y. Kiso oxide-coated sand (Joshi and Chaudhuri, 1996), hematite/feldspar (Singh et al., 1996), and biopolymers (Zouboulis and Katsoyiannis, 2002). The similar subjects mentioned et al. above also remain for these adsorbents. Hydrotalcite compounds (HTALs), which have a layered structure consisting of mag- nesium and aluminium hydroxides and which include the interlayer anions, are known as anionic clay (Orthman et al., 2003). They have been developed as adsorbents for toxic anions as well as for the removal of Cl2 and NO32 from water (Lazaridis et al., 2002; Toraishi et al., 2002). Considering that the chemical properties of arsenate are similar to those of phosphate, the appropriate adsorbents for phosphate removal may be also useful for arsenic removal in spite of competing for the same adsorption sites. In our previous investigations (Kindaichi et al., 2002; Kuzawa et al., 2005), we reported that HTAL-Cl, which contains Cl2 ion as an intercalate, was an effective adsor- bent for the removal of phosphate in a wide range of pH. In addition, the phosphate adsorption capacity of HTAL was effectively recovered by desorption with alkaline solution and regeneration with MgCl2 solution. In the current study, the removal perform- ance of HTAL-Cl for arsenic compounds was examined from the following viewpoints: the adsorption capacity, adsorption isotherm, the effects of pH and co-existing anions. Experimental Materials Three kinds of synthetic HTALs listed in Table 1 were used in this study: HTAL-Cl and 2 22 HTAL-CO3 containing Cl and CO3 as intercalates, respectively, and HT-500 prepared by baking HTAL-CO3 at 5008 C. Both stock solutions of As(III) and As(V) were prepared z with NaAsO2 and Na2HAsO47H2O, respectively, and they were diluted to an appropriate concentration for the experiments. Adsorption procedure The adsorption experiments were performed by batch type procedures. When the adsorp- tion properties of three type of HTALs were compared, HTALs (1.0 g of As(III) or 0.3 g of As(V)) were brought come into contact with an aliquot of the 100 mL solution (100 mg-As(III) L21 or 450 mg-As(V) L21)at258 C for 24 hours. Since HTAL-Cl indicated the effective adsorption properties for As(V), the adsorption isotherm was measured with HTAL-Cl under the following conditions: 0.3–1.0 g of HTAL-Cl and 100 mL of As(V) solution (450 mg-As L21). The effects of pH on the adsorption capacity were examined under the following conditions: 0.3 g of HTAL-Cl and 100 mL of As(V) solution (450 mg-As L21) and pH adjustment with dil-HCl and dil- NaOH. Table 1 Chemical formula and denotation of HTALs used in this work Chemical formula Denotation z Mg0.683Al0.317(OH)1.995(CO3)0.028Cl0.2260.54H2O HTAL-Cl z Mg6Al2(OH)16CO34H2O HTAL-CO3 Mg0.7Al0.3O1.15 HT-500 76 The adsorbents in the mixed solutions were removed by filtration with a H-PTFE membrane filter (0.1 mm), and the arsenic concentrations of the filtrates were analyzed by the membrane extraction method for molybdenum blue, which was developed in our lab- oratory (Kiso et al., 2005) and which can detect up to the level of 1 mgL21 of As(V). The analysis of As(III) was conducted after oxidation to As(V) with potassium peroxodisul- fate. During the experiments, no pH adjustment was conducted with the exception of the case of the experiments for the pH effects. Y. Kiso Effects of co-existing anion et al. The effects of co-existing anions on the adsorption capacity were examined by the addition of NaCl, Na2SO4,Na2HPO4, or NaHCO3 under the following conditions: 450 mgL21 of As(V), 0–2000 mgL21 of the concentrations of the other salts, and 22 2 22 24 hours of the reaction time at 258 C. Anions (HPO4 ,Cl SO4 ) were analyzed by ion-chromatography under the following conditions: ion-chromatograph (DX-120, DIO- NEX, USA); suppressor column: ASRS-ULTRA (4 mm, 50 mA); separation column: AS14A (4 mm i.d. £ 250 mm long); mobile phase: 8.0 mM-Na2CO3-1.0 mM-NaHCO3. The concentration of carbonate ions was determined by a TOC analyzer (TOC 5000, Shi- madzu, Japan). Results and discussion Adsorption capacity The adsorption amounts of As(III) and As(V) on the three kinds of synthetic HTAL are summarized in Figure 1, where the adsorption amounts are plotted against the equilibrium pH. As(III) was not adsorbed effectively on each adsorbent, because As(III) exists as a nonionic species under neutral pH conditions. On the other hand, As(V) was entrapped effectively by HTAL-Cl and HT-500. The adsorption amount on HTAL-CO3 was very 22 low, because the property that HTAL has the highest selectivity for CO3 . In the case of HT-500, since the equilibrium pH shifted to alkaline conditions, pH adjustment is required before or after the adsorption process. In addition, since HT-500 can easily adsorb carbonate from the atmosphere, HT-500 should be stored in a container isolated from air. However, in the case of HTAL-Cl, only a slight increase of pH was observed. Therefore, HTAL-Cl was used in further experiments. The relation between the adsorption amount of As(V) and equilibrium pH is shown in Figure 2, where the HTAL-Cl was used as an adsorbent. The maximum adsorption was observed under neutral pH conditions, and the adsorption amounts decreased in acidic and alkaline regions. It is pointed out that the HTAL-Cl was dissolved under the con- dition of pH , 2. In the case of phosphate adsorption on the HTAL-Cl, the phosphate removal rate was not affected in the region of pH ¼ 5.5–8.8 (Kindaichi et al., 2002), Figure 1 Adsorption amounts for As(III) and As(V) on HTALs 77 Y. Kiso et al. Figure 2 Effects of pH on As(V) adsorption and the results shown in Figure 2 indicate the characteristics of HTAL-Cl adsorbent for As(V) removal. In the neutral pH region, a stable adsorption amount was observed, its deviation was less than 3% in the region of pH ¼ 6.3–7.8.