氯化银负载海藻酸钠吸附碘2011

Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 166– 171 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects j o u r n al h o m e p ag e : w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f a Adsorption of iodide ions on a calcium alginate–silver chloride composite adsorbent Huifang Zhanga,b,Xiaolei Gaoa,b,Tan Guoa,b,Quan Lia, Haining Liua, Xiushen Yea, Min Guoa,b,Zhijian Wua,∗ aKeyLaboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China bTheGraduate University of Chinese Academy of Sciences, Beijing 100049, China a r t i c l e i n f o Article history: Received 18 April 2011 Received in revised form 24 June 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Iodide Adsorption Silver chloride Calcium alginate Precipitation transformation 1. Introduction a b s t r a c t A composite spherical adsorbent for iodide ions was prepared with silver chloride as an adsorption active component, and calcium alginate as a carrier. The adsorbent had an adsorption active component content of 0.96mmol g−1,a water content of 83%, and a point of zero charge (PZC) of 7.2. The kinetics and thermodynamics of iodide adsorption onto the adsorbent in aqueous solutions were investigated comprehensively, by considering the effects of initial iodide concentration, temperature, solution pH, and coexisting NaCl. The results exhibited that the initial adsorption rate increases with the increase in initial iodide concentration and temperature. In general, the equilibrium adsorption amount was found to be insensitive to coexisting NaCl, and initial solution pH in a range of 1–6. The dominant adsorption mechanism has been proved to be the precipitation transformation reaction. The composite adsorbent used in this study is easy to prepare, avoiding silver chloride from leaching, and has been proved to be an effective iodide adsorbent with an adsorption capacity of 1.1mmol g−1. © 2011 Elsevier B.V. All rights reserved. exchanger, BiPbO2(NO3), may not be suitable for the applications in drinking water or groundwater [8]. The adsorption of iodide ions Superfluous iodide ions in drinking water have adverse impact on animals, because iodide ions take part in the metabolism pro- cess of thyroid gland and uptaking excess iodide ions may result in dysfunction of the thyroid gland mainly regulating growing and brain-developing processes of animals [1]. On the other hand, radioactive iodine isotopes,129I,127I,131I, etc. derived from nuclear reactions may pollute soil and interfuse into groundwater with- out proper treatment. Once released into environment, especially 129I,with a half-life of 1.57× 107years, should play a long-term deleterious role [2,3]. Depending on solution pH and redox con- ditions, these radioactive iodine nuclides are present in aqueous solutions primarily as iodide (I−) and iodate (IO3−) anions [4]. At low to neutral pH values and positive redox potentials, iodide ion is the dominant species in environment [4]. Various methods have been proposed to adsorb and remove iodide ions from aqueous solutions [5–7]. In prior research works, some inorganic anion exchangers were used to adsorb chloride or iodide anions from solutions [8–10]. However, such inorganic anion exchangers are usually not easy to prepare and not suitable for large-scale applications. The toxicity of some of the inorganic anion exchangers is also a problem. For example, inorganic anion ∗ Corresponding author. Tel.: +86 971 6307871; fax: +86 971 6307871. E-mail address: zjw6512@hotmail.com (Z. Wu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.07.014 onto cuprous-containing compounds was alsoinvestigated [11,12]. Cuprous oxide reacts with iodide ions with the participation of hydrogen ions to form cuprous iodide, but the adsorption is sensi- tive to solution pH, and the adsorption amount is rather low in neutral and slight basic solutions [12]. Common porous materi- als, such as alumina and hydrotalcite, adsorb iodide ions mainly through surface physical adsorption or ion exchange [13,14]. The adsorption selectivity is poor and coexisting chloride ions behave remarkable competition adsorption against iodide ions [13]. The adsorption behaviors of iodide ions on some soils and minerals were investigated to exploit proper filling materials for restrain- ing transportation of the radioactive iodides into groundwater [15,2,16]. When soils and minerals were used to adsorb iodide ions, the adsorption primarily depends on surface complexation on iron ferric oxide or alumina in the soils and minerals [5]. Recently, silver or silver chloride impregnated activated carbons have been evaluated to remove iodide ions from aqueous solutions [4,17,18]. The advantage of taking silver or silver salts on porous solids as the adsorbents for iodide ions mainly relate to the strong chem- ical interaction between silver and iodide, so that this kind of adsorbents have a good adsorption selectivity [4,17,18]. However, the loading of silver or silver salts onto porous solids is not so easy and silver or silver salts on the adsorbents are quite easy to leach, and therefore, the adsorption capability is generally not satisfactory. H. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 166–171 167 In this work, a composite spherical adsorbent for iodide ions was prepared with silver chloride as an adsorption active com- ponent, and calcium alginate as a carrier. Compared with the reported iodide adsorbents, the adsorbent used in this study has several advantages: (1) easy to prepare; (2) avoiding silver chlo- ride from leaching during adsorption; (3) easy control of silver chloride loading in the composite adsorbent; (4) uniform distribu- tion of silver chloride in the carrier, calcium alginate. The kinetics and thermodynamics of iodide adsorption onto the adsorbent in aqueous solutions were investigated by considering the effects of initial iodide concentration, temperature, solution pH, and coexist- ing NaCl. The adsorption behavior and mechanisms were studied comprehensively. 2. Experimental 2.1. Reagents All reagents were of analytical grade except for sodium alginate (Chemical Grade), and used as received without further purifica- tion. Sodium chloride, sodium alginate (NaALG), potassium iodide, and calcium chloride were purchased from Tianjin Baishi Chemical Industry Co. Ltd., China. Silver nitrate was purchased from Shang- hai Boer Chemical Reagent Co. Ltd., China, and nitric acid from Rongyang Yellow River Chemical Industry Reagent, China. X-ray powder diffraction patterns of the adsorbent after adsorption were also collected for comparison. 2.4. Adsorption experiments In the adsorption experiments, 0.5 g of the wet adsorbent was added into 100mL solutions at the desired initial KI concentration, pH, temperature, and ionic strength. Before the wet adsorbent was weighted, water on the surfaces of the wet adsorbent spheres was removed using filter paper to reduce experimental error. All the adsorption experiments were carried out in a SHA-C shaking water bath (Changzhou Guohua Co. Ltd., China) with a shaking speed of 100 rpm. Solution pH was adjusted with dilute HCl and NaOH solutions, and ionic strength with NaCl solutions. Only in the exper- iments about the effect of ionic strength on adsorption kinetics and thermodynamics, NaCl was used to adjust the ionic strength. In the other conditional experiments, no NaCl was added. For the adsorp- tion thermodynamic experiments, the shaking time was fixed at 48 h. Iodide ion concentrations were recorded by a TU-1810 Ultra- violet Visible Spectrometer (Beijing Purkinje General Instrument Co. Ltd.) at the maximum absorption wavelength of iodide aque- ous solutions (226 nm). The adsorbed amount of iodide ions onto the adsorbent was calculated by a mass balance relationship: q =V(C0 − C) (2) 2.2. Preparation of the composite spherical adsorbent Silver chloride (AgCl) was prepared at room temperature by mixing a 1.0 mol L−1 AgNO3solution and a 5.0 mol L−1 NaCl solution through the precipitation reaction. 5.8 g of AgCl, 1 g of NaALG, and 50 mL of distilled water were mixed under vigorous stirring to get an even suspension. This suspension was added dropwise to a 4% CaCl2solution to obtain the composite gel spheres. After 48 h aging, the gel spheres were separated from the solution, washed several times with distilled water, and stored in distilled water away from light. The average diameter of the gel spheres was 2.9 mm. These wet composite gel spheres were used for the adsorption experi- ments. 2.3. Characterization of the composite adsorbent The water content of the adsorbent was determined by the weight change before and after drying at 30 ◦ C [19]. Before the wet adsorbent was weighted, water on the surfaces of the wet adsor- bent spheres was removed using filter paper. Point of zero charge (PZC) of the adsorbent was determined by the method described in Ref. [20]. The adsorption active component content in the adsorbent was determined by gravimetric method. 1.0171 g of the wet adsorbent was added into 15 mL of 68.0% HNO3solution, followed by heating to dissolve the gel adsorbent matrix. After dissolution, the mixture was cooled to room temperature. Pure adsorption active compo- nent, AgCl precipitate, was separated, washed, dried, and weighed. The AgCl content in the adsorbent was calculated according to the following equation: =m2 W where q is the adsorbed amount of iodide onto the adsorbent (mmol g−1),V the volume of the solution (L), C0and C the iodide concentrations in the solutions before and after adsorption (mmol L−1),and W the wet weight of the adsorbent used (g). When Ce is used instead of C in Eq. (2), qe is obtained. 3. Results and discussion 3.1. Preparation and characterization of the composite adsorbent The adsorption active component content in the adsorbent directly affects the adsorption capacity of the adsorbent. In the preparation of the composite adsorbent, if overmuch AgCl was used, it was difficult to get the composite gel spheres, which was used as the adsorbent. If insufficient AgCl was used, although it was easy to get the composite gel spheres, however, the adsorption capacity of the obtained adsorbent was low. In this study, under the condition that good composite gel spheres were obtained, a AgCl loading of 0.96 mmol g−1 was achieved, which was much higher than that of the reported adsorbents [4,17]. In iodide adsorbents of silver-impregnated activated carbon and sil- ver chloride-impregnated activated carbon, the maximum loading of the adsorption active components, Ag [4] and AgCl [17], was all 0.097 mmolg−1. The water content of the composite adsorbent was detected to be 83%, which was distinctly less than that of pure calcium alginate gel spheres (96%) prepared in the same way, due to the effective entrapment of AgCl in the composite gel spheres. For the determination of the PZC of the adsorbent, the relation- M · m1×1000 (1) ship between pH change and initial pH of the solutions is shown in Fig. 1. pH change is the difference between the final and initial where is the AgCl content in the wet adsorbent (mmol g−1),m1 the weight of the wet adsorbent (1.0171 g), m2the weight of AgCl, M the molecular weight of AgCl (143.32g mol−1). The composite adsorbent before adsorption was dried at 30 ◦ C, grinded to powder, and used to obtain the XRD patterns, which were collected on an X’ Pert PRO (PANalytical) diffractometer with Cu K radiation ( = 0.15419 nm) over a 2 range from 5 to 80◦ . The pH values of the solutions after acid–base equilibrium is attained between the adsorbents and the solutions. If the initial pH is lower than the PZC of the adsorbent, the pH change is positive; if the ini- tial pH is higher than the PZC, the pH change is negative; only if the initial pH is equal to the PZC, the pH change is zero. Therefore, when the pH change is zero, the corresponding initial pH is the PZC, which is estimated to be 7.2. 168 3 2 1 0 -1 -2 -3 H. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 166–171 1.2 0.8 PZC = 7.2 0.4 0.0 pH 3 4 5 6 4 6 pH 8 10 0 50 100 150 200 t (h) Fig. 1. PZC of the calcium alginate–AgCl composite adsorbent. 3.2. Adsorption kinetics 3.2.1. Effect of initial iodide concentration and temperature on adsorption kinetics The effect of initial iodide concentration and temperature on the adsorption kinetics is shown in Fig. 2. The adsorbed amount increases obviously in the first 10 h, after which the adsorbed amount changes slowly. The relatively fast adsorption rate in the beginning may be due to the fact that initially all sites on the sur- faces of the adsorbent are vacant and the iodide concentration 1.2 0.8 0.4 Fig. 3. Effect of initial solution pH on the adsorption kinetics. No NaCl was added to adjust ionic strength. Initial iodide concentration was 5.5 mmolL−1.Adsorption temperature was 25◦C. gradient is high. As the adsorption continues, the number of vacant sites and the iodide concentration gradient decrease, resulting in the decrease in the adsorption rate [21]. 3.2.2. Effect of initial solution pH and coexisting NaCl on adsorption kinetics The effect of initial solution pH and coexisting NaCl on the adsorption kinetics is depicted in Figs. 3 and 4. In the initial solution pH range tested, the initial solution pH does not affect the adsorp- tion kinetics in general. Fig.4 demonstrates that the coexisting NaCl does not affect the adsorption kinetics obviously. 3.2.3. Adsorption kinetic analysis In order to further investigate the adsorption kinetics, both pseudo first- and second-order adsorption models were applied to deal with the adsorption data in Fig. 2 [22–24]. First-order model : q = qe −qe (3) 25oC 0.0020 40 60 80 100 t (h) ekt Second-order model : q =kqe2t 1 + kqet (4) 1.6 1.2 0.8 0.4 0.0 1.2 40oC 0 20 40 60 80 100 t (h) In Expressions (3) and (4), q and qe are adsorption amount at time t and at equilibrium. k denotes the corresponding adsorption rate constant. The fitting results are given in Table 1. In Table 1, the experimental equilibrium adsorption amount, qe,exp, was deter- mined accordingto the experimentalresults in Fig. 2. The calculated equilibrium adsorption amount, qe,cal,was determined from the fitting results using Expressions (3) and (4). Based on the data in Table 1, in general, the first-order model is more suitable to describe the adsorption, especially at higher initial iodide con- centration and higher temperature. For the judgment of kinetic 1.2 0.000 0.8 0.4 0.0 55oC 0 20 40 60 80 100 0.8 0.4 0.0 0.004 0.008 0.010 0.014 0.018 0.020 0.030 0.040 0.050 0 10 20 30 40 50 t ( h) t (h) Fig. 2. Effect of initialI− concentration and temperature onthe adsorption kinetics. Initial I− concentration (mmol L−1)was ( ) 5.5, ( ) 7.5 and ( ) 10.0. No HCl, NaOH or NaCl was added to adjust pH or ionic strength. Fig. 4. Effect of coexisting NaCl on the adsorption kinetics. No HCl or NaOH was added to adjust the solution pH. Initial iodide concentration was 5.5mmol L−1. Adsorption temperature was 25◦C.The concentrations of NaCl were (mmolL−1)0 ( ), 0.004 ( ), 0.008 ( ), 0.010 ( ), 0.014 ( ), 0.018 ( ), 0.020 ( ), 0.030 ( ), 0.040 ( ), and 0.050 ( ). Δ pH q ( mmol g -1 ) q ( mmol g -1 ) q ( mmol g -1 ) q ( mmol g -1 ) q ( mmol L -1 ) Table 1 Kinetic fitting results of the adsorption. H. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 166–171 169 C0 (mmolg−1) T (◦C) qe,exp (mmolg−1) First-order qe,cal (mmolg−1)k (h−1) r2 Second-order qe,cal (mmol g−1) k (g mmol−1h−1)r2 5.5 7.5 10.0 Table 2 25 40 55 25 40 55 25 40 55 1.140 1.129 1.095 1.052 1.070 1.046 1.048 1.198 1.094 1.105 ± 0.026 1.106 ± 0.020 1.048 ± 0.024 1.040 ± 0.008 1.057 ± 0.010 1.044 ± 0.006 1.055 ± 0.009 1.200 ± 0.007 1.094 ± 0.005 0.129 ± 0.010 0.232 ± 0.017 0.276 ± 0.029 0.235 ± 0.007 0.365 ± 0.017 0.461 ± 0.015 0.337 ± 0.014 0.483 ± 0.016 0.635 ± 0.017 0.991 0.993 0.988 0.999 0.998 0.998 0.998 0.999 0.999 1.210 ± 0.027 1.202 ± 0.016 1.130 ± 0.011 1.144 ± 0.022 1.153 ± 0.020 1.133 ± 0.023 1.151 ± 0.034 1.300 ± 0.027 1.179 ± 0.024 0.167 ± 0.010 0.316 ± 0.013 0.437 ± 0.017 0.300 ± 0.004 0.485 ± 0.031 0.620 ± 0.049 0.443 ± 0.046 0.565 ± 0.046 0.796 ± 0.067 0.993 0.997 0.998 0.994 0.994 0.990 0.983 0.989 0.988 Parameters for the first-order adsorption model. C0(mmolg−1)T (◦C) 5.5 25 40 55 7.5 25 40 55 10.0 25 40 55 u (mmol g−1h−1) 0.143 0.257 0.289 0.244 0.386 0.481 0.356 0.580 0.695 t1/2(h) 5.37 2.99 2.51 2.95 1.90 1.50 2.06 1.44 1.09 Ea (kJ mol−1) 20.8 18.4 17.2 r −0.958 −0.987 −0.998 model for the adsorption process, the other key factor is whether the fitting (calculated) equilibrium adsorption amount (qe,cal)is equal to the experimental equilibrium adsorption amount (qe,exp). If there is a great difference between qe,calandqe,exp,the kinetic model is not likely to be suitable for the description of the adsorp- tion process, even the fitting has a high correlation coefficient. As shown in Table 1, though correlation coefficients of the second- order model in case of C0=5.5 mmolg−1arebetter than those of 3.3. Adsorption thermodynamics 3.3.1. Effect of iodide concentration and temperature on the adsorption thermodynamics – adsorption isotherms The adsorption isotherms are shown in Fig. 5. They were fitted with Langmuir and Freundlich isotherms [23]: Langmuir isotherm :Ce1Ce the first-order model, qe,calofthe first-order model is approximate equal to qe,exp not only for C0 = 5.5 mmol g−1,but also for C0 = 7.5 qe=aQm+ Qm (8) and 10.0 mmol g−1.Consequently, it is appropriate that the adsorp- tion kinetics can be well described by the pseudo first-order kinetic Freundlich isotherm : ln qe = ln KF +1nlnCe (9) model. Based on the first-order model, the initial adsorption rate and half-adsorption time are calculated (Table 2) according to the fol- lowing two expressions, respectively: u = kqe (5) where Ce and qe are the equilibrium concentration in solution and equilibrium adsorption amount. a is the adsorption equilibrium constant and Qmthe saturated adsorption amount. In Freundlich equation, KFand n are empirical constants [27]. The fitting results indicate that the adsorption can be well described with Langmuir equation (Table 3). Adsorption thermodynamic parameters, H, t1/2=ln 2 (6) G, and S can be determined through the following equations k Half-adsorption time, t1/2, is defined as the time required for the adsorption to take up half as much iodide as its equilibrium value. This time is often used as a measure of the adsorption rate. Based [28]: ln a = −H RT+const (10) on the results listed in Table 2, the initial adsorption rate increases with increasing initial iodide concentration and temperature. On the contrary, the half-adsorption time decreases with the increase in initial iodide concentration and temperature. The first-order rate constants listed in Table 1 are used to esti- mate the adsorption activation energy using Arrhenius equation: ln k = ln A −Ea (7) G = −RT ln a 1.2 0.8 25oC (11) RT The slope of plot of ln k versus 1/T (T, absolute temperature) is used to evaluate Ea, which was found to be 20.8–17.2 kJ mol−1 dependingon the initialiodide concentration(Table 2).We suppose the potential barrier of the whole adsorption process include diffu- sion resistance and adsorption interaction potential. The increase in iodide concentrationresults in weakening of diffusionresistance, 0.4 0.0 40oC 55oC 70oC 0 1 2 3 4 5 6 Ce (mmol L-1) as a result, the activation energy of the adsorption diminishes cor- respondingly [25,26]. Fig.5. Adsorptionisotherm