Kinetics and Thermodynamics of the Adsorption of Copper( Ⅱ) onto a New Fe-Si Adsorbent 1

34 卷 1 期结构化学 (JIEGOU HUAXUE) Vol. 34, No. 1 2015. 1 Chinese J. Struct. Chem. 49 55 Kinetics and Thermodynamics of the Adsorption of Copper( Ⅱ) onto a New Fe-Si Adsorbent 1 FU Cheng MA Cong-Cong WU Qiong YU Yan 2 (College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China) ABSTRACT A new kind of Fe-Si adsorbent was synthesized by iron oxide and diatomite after calcining and hydrothermal process. The influences of the initial Cu 2+ concentration, ph and adsorption time on the Cu 2+ removal efficiency were discussed. Three adsorption empirical kinetics equations and two thermodynamics equations were used to simulate the adsorption process. The microstructures of newly developed copper removal materials and properties of copper removal are characterized in details by SEM and EDS. Adsorption mechanism of the adsorbent was discussed. The suitable ph value for Cu 2+ removal is 5.0 to 6.0 and the adsorption capacity increases with increasing the initial Cu 2+ concentration. The adsorption kinetics of the adsorbent could be better described by pseudo second order kinetic model, whereas the adsorption isotherms highly conform to the Freundlich equation. The main crystalline phase of the adsorbent is Fe(SiO 3 ) which can build porous structures conducive to the Cu 2+ adsorption. Keywords: diatomite, copper( Ⅱ ) absorption, kinetics, thermodynamics DOI: 10.14102/j.cnki.0254-5861.2011-0407 1 INTRODUCTION The copper ion, as well as its compounds, is a toxic metal ion which universally exists in the polluted water. They enter organism mainly through the respiratory tract and alimentary canal, causing great harm to human survival and biological diversity. Cu 2+ is difficult to adsorb by microorganism in water, so it is easy to concentrate in the body and further transform to heavy metal organic compounds with toxicity, which may lead to peroxidation of human organism and thus disease [1-4]. Adsorption, as a traditional water treatment technique, has such advantages as high capacity, ability to remove various metal ions and anions, simple process, convenient operation, high treatment efficiency, etc., and is still one of the main ways to treat copper containing wastewater now [5-8]. The conventional adsorbents are in the form of powders, having a certain effect on the improvement of adsorption efficiency, but it is easy to have sludge and cause secondary pollution. The direction for the development of modern adsorbent is to prepare the shaped materials which are easy to separate from water and recycle [9-12]. In this study, iron oxide and diatomite were used as raw materials to develop a recyclable wastewater copper removal adsorbent in a calcining-hydrothermal way for the first time. In this paper, the copper removal efficiency of samples will be mainly discussed with different formulas under different environmental conditions so as to determine the best Received 9 June 2014; accepted 19 August 2014 1 Sponsored by the National Natural Science Foundation of China (No. 51102047 and No. 51472050) and Fujian Science Foundation for Distinguished Young Scholars (2012J06011) 2 Corresponding author. Yu Yan, female, born in 1972, PhD, professor, E-mail: yuyan@fzu.edu.cn Fu Cheng, male, born in 1992, postgraduate

FU C. et al.: Kinetics and Thermodynamics of the 50 Adsorption of Copper( Ⅱ) onto a New Fe-Si Adsorbent No. 1 technical conditions of copper removal. The adsorp- tion processes are discussed by adsorption thermo- dynamics and kinetics, and the microstructures of newly developed copper removal materials and properties of the copper removal are characterized in details by SEM and EDS. 2 EXPERIMENTAL METHODS AND PROCESS 2. 1 Raw materials and preparation of the samples For iron oxide and diatomite (see its main compositions in Table 1), Fe 2 O 3 /SiO 2 (molar ratio) = 1:1; based on calculation, the optimum weight ratio between iron oxide and diatomite was 2.67. After homogenous mixing, Φ20 30 (mm) cylindrical specimens were formed. After sintering at 950 and hydrothermal treatment at 130 for 13 h, Fe Si adsorbent material was produced. Table 1. Chemical Composition of Diatomite (wt%) Composition SiO 2 Al 2O 3 Fe 2O 3 K 2O Na 2O I.L. Total Content 91.94 2.06 1.65 0.90 0.30 3.15 100.00 2. 2 Copper removal effectiveness evaluation Cu(NO 3 ) 2 2H 2 O of 3.799±0.001 g was weighed and distilled water was added into a 1 L volumetric flask to prepare copper stock solution with the concentration of 1000 mg/l, and then such solution was diluted into copper containing solution with the concentration of 5 mg/l. The evaluation conditions of copper adsorption capacity are as follows: the ambient temperature for experiment was 22, and the dosage proportion of sample to wastewater was 1 g to 25 ml. Cu 2+ removal effect of the sample was tested after an interval; the influence of different initial copper concentrations (10, 20, 30, 40, 50 and 100 mg/l), different adsorption time (3, 6, 9, 12, 24, 36, 48, 72, 84 and 96 h) and different initial ph values of wastewater (2, 3, 4, 5, 6, 7, 8, 9,10, 11 and 12) on the efficiency of copper removal was discussed, respectively, and the accumulated Cu 2+ adsorption capacity was calculated. Then the adsorption capacity of copper-bearing wastewater with high concentrations (10, 20, 30, 40, 50 and 100 mg/l) was calculated, and adsorption thermodynamics and kinetics were used to simulate the adsorption process. The concentrations of copper ions were determined by TAS-986 Atomic Absorption Spectrophotometer. The adsorption capacity was represented by Qe(mg/g), and the equation is as follows: V Q = e ( C0 Ce ) 3 m 10 (1) where V volume of the solution (ml); C 0 concentration of Cu 2+ in the wastewater before treatment (mg/l); C e concentration of Cu 2+ after treatment (mg/l); m mass of adsorbent (g). in the wastewater 2. 3 Kinetic and thermodynamic analyses Langmuir equation and Freundlich equation were used to simulate the adsorption thermodynamic process. First-order kinetic model, Pseudo second order kinetic model and Elovich model were used to simulate the adsorption kinetic process. 2. 4 Characterization of the microstructures of the samples ShimadzuXD-5A X-ray diffractometer (Japan) (CuKα target; current of 40 ma, voltage of 40 kv, diffraction angle range: 5~65º, scanning speed: 4º /min) and X-ray powder diffractometer were utilized for determining the crystalline phase of specimens; Philips XL30ESEM environment scanning electron microscope was utilized to observe the microstruc- ture of specimens before and after Cu 2+ removal. 3 RESULTS AND DISCUSSION 3. 1 Influence of ph value on the Cu 2+ adsorption

2015 Vol. 34 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 51 For results of Cu 2+ removal experiment, which was conducted under the conditions with the ambient temperature of 22, the initial Cu 2+ con- centration of 5 mg/l and the adsorption time of 96 h, of the adsorption material at different ph values, refer to Fig. 1. According to Fig. 1, the Cu 2+ adsorp- tion capacity of the Fe-Si adsorbent increased with increasing the ph value at first, and increased most rapidly when ph = 3.0~4.0, reached a saturation value at ph = 5.0, then kept stable (the value of that was 0.1108~0.1095 mg/g) at ph = 5.0~6.0. It indicates that the ph value had fairly greater effect on the Cu 2+ removal capacity, which was mainly because changing the ph value of the solution could affect the Cu 2+ existence form in aqueous solution. The relationship between ph values and the form of an aqueous solution of Cu 2 + is as shown in Table 2. Table 2. ph Value vs. the Form of an Aqueous Solution of Cu 2+ Ions ph value Form of an aqueous solution of Cu 2 + ions ph < 4.0 Cu 2+ 4.0 ph < 5.0 Cu 2+ and CuOH + 5.0 ph < 6.0 CuOH + and Cu(OH) 2 6.0 ph Cu(OH) 2 Table 2 shows that, when ph = 5, a small amount of blue Cu(OH) 2 floc formed in the solution. It could cover the surface of Fe-Si adsorbent, and then affected the adsorption and led to adsorption saturation further. When ph 6.0, a large amount of blue Cu(OH) 2 sediment formed. At that time, the Cu 2+ removal mechanism was no longer a simple adsorption, as Cu 2+ reacted with OH - to form Cu(OH) 2 sediment. Hence, the appropriate ph value is 5.0 to 6.0. 3. 2 Influence of the initial concentration of Cu 2+ -bearing wastewater on the Cu 2+ adsorption When the ambient temperature was 22 and the ph value was 6.0, for results of the adsorption of Cu 2+ bearing wastewater with different initial concentration under different adsorption time, refer to Fig. 2. Q e /(mg g -1 ) 0.12 0.10 0.08 0.06 0.04 0.02 0.00 1 2 3 4 5 6 7 8 9 10 11 12 ph Q e / (mg g -1 ) 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 12 24 36 48 60 72 84 96 108 Time/h 10mg/L Fig. 1. Influence of ph on the Cu 2+ adsorbing capacity Fig. 2. Influence of initial concentration and time on the Cu 2+ removal efficiency According to Fig. 2, the adsorption capacity of Fe-Si adsorbent increased with increasing the initial Cu 2+ concentration of wastewater under different adsorption time. The adsorption capacity changed slowly at the concentration of 10~50 mg/l, but increased rapidly when the concentration increased from 50 to 100 mg/l, which was mainly because the initial concentration of Cu 2+ -bearing wastewater could influence the ionic strength of solution, and then affect the diffusion of Cu 2+ in the solution as

FU C. et al.: Kinetics and Thermodynamics of the 52 Adsorption of Copper( Ⅱ) onto a New Fe-Si Adsorbent No. 1 well as on the surface of adsorbent further. Electric potential formed on the surface between the adsor- bent and solution, and would increase with an increase of ionic strength. At that time, the concentration of Cu 2+ diffusing onto the adsorbent surface increased, so that the adsorption capacity kept increasing before the adsorption saturation. Adsorption time also had direct influence on the adsorption capacity. The adsorption capacity increa- sed gradually with the extension of adsorption time and then tended towards stability. After adsorbing for 48 h, the adsorption capacity basically reached equilibrium in all of the wastewater with different initial concentration of Cu 2+. It indicates that the Fe-Si adsorbent has high-efficiency adsorption effect on copper containing wastewater with different concentration, the adsorption equilibrium can be reached in a very short time, and such adsorbent is able to treat copper-containing wastewater with a wider scope of concentration. For wastewater with the Cu 2+ concentration of 100 mg/l, the maximum adsorption capacity of Cu 2+ could reach 0.4625 mg/g. 3. 3 Adsorption kinetic model According to the data in Fig. 2, Lagergren pseudo first order kinetic equation, Ho pseudo second order kinetic equation and Elovich equation were used for linear fitting with the adsorption curve, so curves of log(q e q t ) to t, t/q t to t and q t to lnt were plotted respectively. The fitting graphs and parameters are shown as Fig. 3 and Table 3, respectively. Table 3. Parameters of Kinetic Equation for Cu 2+ Adsorption C 0 (mg/l) Pseudo second order Elovich model First-order kinetic model kinetic model K 1 R 2 Qe(exp) Qe(cal) K 2 R 2 Qe(cal) R 2 a b 10 0.0012 0.97 0.21 0.15 0.0069 0.99 0.23 0.96 0.04 0.10 20 0.0010 0.99 0.26 0.17 0.0057 0.99 0.28 0.96 0.04 0.11 30 0.0010 0.99 0.30 0.19 0.0052 0.99 0.32 0.97 0.05 0.11 40 0.0011 0.99 0.34 0.21 0.047 0.99 0.36 0.96 0.05 0.11 50 0.0010 0.97 0.38 0.24 0.041 0.99 0.40 0.96 0.06 0.13 100 0.0012 0.99 0.46 0.28 0.043 0.99 0.49 0.95 0.07 0.12 Qe(exp): The experimental values of adsorption quantity (mg/g) Qe(cal): The theoretical value of adsorption quantity (mg/g) The experimental values q e (exp) of the equili- brium absorption capacity of adsorbent should equal to the calculated values q e (cal) of that in ideal conditions. We can see from Table 3 that the q e (cal) calculated by Ho pseudo second order kinetic equa- tion was the closest to the experimental value q e (exp), while there was great difference between the q e (cal) calculated by pseudo first order kinetic equation and q e (exp). Thus, the pseudo first order kinetic equation is more suitable for the Cu 2+ adsorption process of the adsorbent, that is to say, chemical adsorption existed in the Cu 2+ adsorption process of the adsorbent. According to Fig. 3, pseudo second order kinetic equation at different initial concentration can be represented as below: t/q t = 4.38t + 1368.56 10.00 mg/l t/q t = 3.63t + 1122.73 20.00 mg/l t/q t = 3.12t + 930.80 30.00 mg/l t/q t = 2.81t + 813.83 40.00 mg/l t/q t = 2.48t + 759.24 50.00 mg/l t/q t = 2.06t + 484.07 100.00 mg/l 3. 4 Adsorption thermodynamic model According to the data in Fig. 3, Langmuir adsorption isotherm and Freundlich adsorption isotherm were used for isothermal fitting, so curves of 1/Q e to 1/C e and lgq e to lgc e were plotted respectively. The fitting graphs and thermodynamic parameters are shown as Fig. 4 and Table 4, respectively.

2015 Vol. 34 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 53 log(q e -Q t ) -0.6 10mg/L -0.8-1.0-1.2-1.4-1.6 t/q t 30000 25000 20000 15000 10000 10mg/L -1.8-2.0 5000 0 500 1000 1500 2000 2500 T/min 0 1000 2000 3000 4000 5000 6000 T/min (a) (b) 0.50 0.45 0.40 0.35 0.30 10mg/L Q t 0.25 0.20 0.15 0.10 0.05 0.00 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 lnt (c) Fig. 3. Linear fitting curves of three empirical kinetic equations (a) First-order kinetic model; (b) Pseudo second order kinetic model; (c) Elovich model 5.0-0.30 1/Q e 4.5 4.0 3.5 3.0 lgq e -0.35-0.40-0.45-0.50-0.55 2.5-0.60-0.65 2.0-0.70 0.00 0.02 0.04 0.06 0.08 0.10 1.0 1.2 1.4 1.6 1.8 2.0 1/C e lgc e (a) Fig. 4. Linear fitting of Cu 2+ adsorption isotherms (a) Langmuir isotherm, (b) Freundich isotherm (b) Table 4. Thermodynamic Parameters for Cu 2+ Adsorption C 0 (mg/l) Langmuir isotherms Freundlich isotherms Q m b R 2 n k R 2 0~100 0.45 0.08 0.90 2.89 0.09 0.99

FU C. et al.: Kinetics and Thermodynamics of the 54 Adsorption of Copper( Ⅱ) onto a New Fe-Si Adsorbent No. 1 After the linear fitting of adsorption results by Langmuir isotherm and Freundich isotherm, the correlation coefficients were 0.90 and 0.99, respectively. It indicates that, the Cu 2+ adsorption isotherms of the adsorbent could be better described by Freundlich model, and the adsorption process on the surface of the adsorbent was heterogeneous physical adsorption. Freundlich constant n > 1, suggesting that the adsorption process was spontaneous, and Cu 2+ adsorption of the adsorbent was not monolayer absorption but multilayer absorption, including physical and chemical adsorption. 3. 5 Microstructure analysis of the adsorbent Fig. 5 shows that, principal crystalline phases of the Fe-Si adsorbent were Fe(SiO 3 ) and SiO 2, mixed with a small amount of Fe 3 O 4. The phases of samples before and after adsorption were almost the same, so the whole adsorption process was mainly physical adsorption and the adsorption had not changed the phase composition on the surface of adsorbent. a b SiO2 Fe(SiO 3 ) Fe 3 O 4 0 10 20 30 40 50 60 70 2θ Fig. 5. XRD pattern of adsorbent before and after adsorption SEM images before and after adsorption of the samples are as shown in Fig. 6. Fig. 6. SEM images before and after adsorption (a) before adsorption 2000, (b) before adsorption 5000 (c) after adsorption 2000, (d) after adsorption 5000

2015 Vol. 34 结构化学 (JIEGOU HUAXUE)Chinese J. Struct. Chem. 55 We can see from (a) and (b) in Fig. 6 a large num- ber of irregular crystals formed in the Fe Si adsor- bent after hydrothermal process. These irregular crystals constructed space porous structures which could provide attachment site for Cu 2+ randomly, so that the strong adsorption ability of samples was guaranteed. (c) and (d) in Fig. 6 show that the surface topography of samples before and after adsorption was almost the same, except for a large amount of lump, which could be caused by the interaction between Cu 2+ and the sample surface after adsorption. 4 CONCLUSION (1) This paper discussed Cu 2+ removal efficiency of copper-bearing wastewater by a new adsorbent prepared by using iron oxide and diatomite as the raw materials. The results show that the adsorption capacity increased with the initial concentration increasing and the suitable ph for Cu 2+ removal was 5.0 to 6.0. When the ambient temperature was 22, the initial Cu 2+ concentration was 5 mg/l, adsorption time was 96 h and the ph value was 6.0, the maximum Cu 2+ adsorption capacity could reach 0.1108 mg/g. It indicates that the Fe-Si adsorbent can be perfect for copper removal. (2) In our research, Cu 2+ adsorption process of the adsorbent was better described by Freundlich model. It suggests that, the adsorption process on the surface of the adsorbent was heterogeneous physical adsorption, multilayer adsorption was dominant in the process, and both physical and chemical adsorp- tion coexisted at the same time. Freundlich constant n > 1, indicating that the adsorption process was spontaneous. (3) Study of kinetics shows that, the adsorption processes at different initial concentration of copper- bearing wastewater highly conform to pseudo second order kinetic equation, and the correlation coefficient was 0.99. It indicates that chemical absorption existed in the Cu 2+ adsorption process of the adsorbent. REFERENCES (1) Xie, D.; Meng, X.; Tang, J. S. Study on the water and soil pollution in karst area, harm from coal mine waste water and evaluation. Chinese Agricultural Science Bulletin 2013, 29, 296 302. (2) Qin, F.; Wen, B.; Shan, X. Q. Mechanisms of competitive adsorption of Pb, Cu, and Cd on peat. Environmental Pollution 2006, 144, 669 680. (3) Lu, L. L.; Tao, X. J.; Hu, J. W. Research progress on determination and treatment of Cu 2+ in water environmental. Environmental Science and Technology 2010, 23, 125 127. (4) Xu, X. Q.; Zhu, Y.; Yang, T. Harm and remediation of water pollution of heavy metals. Pollution Control Technolog 2007, 20, 67 69. (5) Liu, P.; Zeng, G. M.; Huang, J. H. Research progress of biosorption in treatment of waste water containing heavy metals. Industrial Water&Wastewater 2004, 35, 1 5. (6) Zhou, Z. H.; He, S. F.; Han, C. Y. Progress of heavy metals liquid waste processing technique. Technology of Water Treatment 2010, 36, 17 21. (7) Iakovleva, E.; Sillanpää, M. The use of low-cost adsorbents for wastewater purification in mining industries. Environmental Science and Pollution Research 2013, 20, 7878 7899. (8) Ahluwalia, S. S.; Goyal, D. Microbial and plant derived biomass for removal of heavy metals from wastewater. Bioresource Technology2007, 98, 2243 2257. (9) Ali, I.; Gupta, V. K. Advances in water treatment by adsorption technology. Nature Protocols 2007, 1, 2661 2667. (10) Brown, P. A.; Gill, S. A.; Allen, S. J. Metal removal from wastewater using peat. Water Research 2000, 34, 3907 3916. (11) Li, P. F.; Lei, F. H.; Yan, R. P. Study on the adsorption behavior of the rosin based functional polymer for Cu(Ⅱ). Ion Exchange and Adsorption 2010, 26(006), 533 541. (12) Liu, C.; Bai, R.; San, L. Q. Selective removal of copper and lead ions by diethylenetriamine-functionalized adsorbent: behaviors and mechanisms. Water Research 2008, 42, 1511 1522.