Adsorption Characteristics of Polyvinyl Alcohols in Solution on Expanded Graphite

Expanded graphite (EG) adsorbent was prepared with 50 mesh graphite as raw materials, potassium permanganate as oxidant, and vitriol as intercalation compound. Three kinds of polyvinyl alcohol (PVA) with different degree of polymerization (DP) in aqueous solution were used as adsorbates. We have studied the influence of initial PVA concentration, temperature and ionic strength on adsorption capacity. Langmuir constants and Gibbs free energy change (⊿G°) were calculated according to experimental data respectively. Thermodynamic analysis indicates the equilibrium adsorbance of PVA on EG increase with the rise of SO4 concentration. Adsorption isotherms of PVA with different degree of polymerization are all types and we deduce PVA molecules lie flat on EG surface. Adsorption processes are all spontaneous. Kinetic studies show that the kinetic data can be described by pseudo second-order kinetic model. Second-order rate constants and the initial adsorption rate rise with the increasing of temperature and half-adsorption time decreases with the increasing of temperature. The adsorption activation energy of each PVA is less than 20 kJ·mol, physical adsorption is the major mode of the overall adsorption process.


Introduction
Polyvinyl alcohol (PVA) is a sort of widely used polymer in industry and the molecular structure of [-CH 2 -CH(OH)] n makes it possess of well water solubility.PVA has many unique characteristics such as strong adhesion, film flexibility, smoothness, oil resistance, solvent resistance, protective colloid, obstruction of gas and abrasion resistance, so it has extensive use in food, medicine, textile, papermaking, agriculture, polymer chemical industry 1 .However, the wasted textile solution may become one of the major wastewater sources of PVA, especially in sizing production process 2 .The principal treatment methods of PVA wastewater are biodegradation [3][4][5] , chemical coagulation 6 and ultrafiltration technique 7,8 .Lee et al 3 isolated PVA-degrading bacteria (SB68+SB69) from various sources such as activated sludge from the water-course of textile and dyeing factories, cultivating soil and leaf mold.They showed high activity for PVA degradation so that 75% of PVA with an initial concentration of 0.01% was mineralized during 46 days of the modified Sturm test.A dyeing factory in beijing 6 made continuous condensation recycling of PVA slurry desizing water of polyester-cotton fabric with Na 2 SO 4 (10~12 g/L) and Borax (1~1.5 g/L) under temperature 40~50 o C, both of PVA condensate recovery and wastewater COD removal rate achieved 80%.With hollow fiber ultra-filtration membrane equipment 8 , Dong studied the influence of liquid running time, pressure on both sides of membrane, liquid temperature and mainstream flow rate et al on filter flux and rejection rate, gained optimum conditions of PVA desizing wastewater.
In wastewater treatment, it is well known that adsorption process has been considered available method for eliminating organic pollutants.Adsorption of PVA on adsorbents such as silica, polystyrene particles, kaolin, fuller's earth, montmorillonite and powered activated carbon has been studied [9][10][11][12][13][14] .Adsorption of PVA on silica illustrated silanols and siloxane bonds were the main adsorption sites.Very low affinity was found between PVA and kaolin minerals.At fixed pH (4.8) and ionic strength of the medium, adsorption isotherm for PVA on fuller's earth surfaces resembles with LIII type.The maximum adsorbance of PVA (pH=6) on montmorillonite Ca-Mt was 151.2 mg/g, which is considerably lower than that on Na-Mt (496.2 mg g -1 ).Adsorption isotherm of PVA on powered active carbon was similar with H-type and the maximum removal of 92% was obtained at a pH of 6.3 and contact time of 30 min for an adsorbent dose of 5 g/L.
Expanded graphite (EG) is a kind of porous adsorbent; it can be easily prepared with chemical method or electrical chemical method through oxidation and intercalation reaction.The pore in EG ranges from several nm to hundreds µm 15 , it has been used as absorbent for its high adsorption capacity for organic materials, such as heavy oil and biomedical molecules [16][17][18][19][20][21] .Pang et al have studied the adsorption capacities of EG for oil, dyes, aromatic sulfonates, polyethylene glycol (PEG) [22][23][24][25] , results indicated EG show definite adsorption capability for these organic substances, especially for oils.Both adsorbates molecular weight and molecular structure affected adsorption type, saturation adsorbance.
The aim of this work is to study the adsorption thermodynamics and adsorption kinetics characteristics of PVA on EG in water solution.Investigate the effect of ionic strength, adsorbate concentration, temperature on adsorption capacity and do further evaluation of applicability of common isotherm model (i.e., Langmuir and Freundlich) and pseudosecond-order rate model.In the research, PVA with different PDs of 500, 700 and 1750 in aqueous solution are investigated.

Experimental
With the mass ratio of 1: 0.12: 5.0 of graphite (C): KMnO 4 : H 2 SO 4 (96%), the definite amount of C, KMnO 4 and H 2 SO 4 (diluted to a mass concentration of 50% with de-ionized before mixed with graphite) were mixed under room temperature.The reaction lasted 30 min under stirring, then products were washed with de-ionized water until pH reached to 6.0 ~ 7.0.After filtration and dryness under 80 °C, the solid scalelike grains were heated under 900 °C for about 10 sec and then EG could be gained.The pore distribution of the prepared EG is detected by autopore II 9220 mercury porosimeter (Micromeritics Inc.USA) under the condition of 0.58~1301PSIA.Results given in Table 1 show pores in EG are mainly micron pore.2) and its solution is initially calibrated for concentration in terms of absorbance units.Static adsorption of PVA 0.20 g of EG is mixed in different conical glass flasks with 100.0 mL solution at the desired PVA concentration and ionic strength.Ionic strength is adjusted with NaCl or Na 2 SO 4 solution.Mass of EG to volume of solution is standardized as M/V = 0.200 / 0.l = 2.0 g/L.PVA concentration changes are recorded with spectrophotometer at λ max and equilibrium adsorbance is determined according to equation (1).q e = V (C 0 -C e ) / M (1) q e: Equilibrium adsorbance of PVA on EG, mg/g; C 0: Initial concentration of PVA in solution, mg/L; C e : Equilibrium concentration of PVA in solution, mg/L; V: Volume of solution, L; M: Mass of EG, g

Adsorption kinetics of PVA
Adsorption kinetics experiments are carried out using a HZS-D shaking water bath (Donglian Haerbin, China).A series of desired PVA concentration and fixed 100.0 mL are placed in vessels, where they are brought into contact with EG at 25 °C, 35 °C and 45 °C, respectively.Amount of PVA captured by EG at different time is determined according to equation (2):

Adsorption isotherm and thermodynamic parameters
Static adsorption capacities of EG for PVA (500, 700, 1750) are measured.Figure 1 (a) and (b) illustrates a typical I type isotherm.Equilibrium adsorbances are less than 20 mg/g among the detected PVA solution and it decrease with the increase of PVA' DP.The results are just as the adsorption of linear PEG on EG 25 .The planar structure and large molecules of PVA might form certain kinds of conformation on the surface of EG, which might reduce the adsorbed sites and make the further adsorption difficult.To providing a description of PVA concentration and possible conformation in the interfacial region, Langmuir and Freundlich isotherm equations ( 3) and ( 4) are used to treat the isotherm data.The molecule area (a) of PVA with different DP is calculated according to q 0 and total pore area.As showed in Table 3, Langmuir isotherm gives a better fit than Freundlich isotherm and the higher DP PVA possessed, the smaller A is gained, the lower equilibrium concentration of PVA corresponding to half saturation adsorbance.This might be caused by the lower water solubility of higher DP of PVA and then cause stronger appetency with EG.At the same time, there is an almost linear relationship between PVA DP and PVA molecule area (Figure 2).We can deduce PVA molecule lies flat on the EG surface 25 .
Langmuir equation 28 : 1/q e = 1/q 0 + A•/ (q 0 ×Ce) (3) Freundlich equation: lnq e = lnK F + (1/n) lnCe (4) q 0 : Maximum adsorption amount of PVA in forming complete monolayer coverage on EG pore surface, mg/g; A: Equilibrium concentration of PVA corresponding to half saturation adsorbance, mg/mL; K F : Freundlich equation constant; 1/n: adsorption intensity for Freundlich equation.Base on temperature and Langmuir constant, adsorption free energy change (⊿G°) of the adsorption process is calculated according to equation ( 5) 29 , negative ⊿G° (Table 4) indicates that adsorption of these reference compounds on EG are all spontaneous and the higher DP the PVA possessed, the easier adsorption is carried.

Influence of ionic strength on adsorption capacity
At room temperature, the influence of ionic strength on adsorption capacity is investigated using a 100 mg/L PVA (500, 700,) solutions which contain NaCl or Na 2 SO 4 with ionic strength ranging from 0 to 0.6 mol/L.The results showed in Figure 3 indicate that the presence of SO 4 2-can improve the adsorption capacity of EG for PVA and it may be caused by the electrostatic interaction between PVA and adsorbent, hydrophobic attraction of PVA increases due to the "salting-out" effect.

Influence of PVA concentration and temperature on adsorption kinetics
The influence of PVA (500, 700) concentration and temperature on adsorption equilibrium time is detected and showed in Figure 4 & 5. Results suggest that adsorbance is the function of PVA concentration, temperature and adsorption time.Equilibrium adsorbances and adsorption rate increase with the increasing of temperature, which might be caused by the change of solution viscosity under different temperature.

Adsorption kinetic models
Both pseudo first-and second-order adsorption models are used to describe the adsorption kinetics data 30,31 .In both models, all the steps of adsorption such as external diffusion, internal diffusion and adsorption are lumped together and it is assumed that the difference between the average solid phase concentration and the equilibrium concentration is the driving force for adsorption and the complete adsorption rate is proportional to either the driving force (as in the pseudo first-order equation) or the square of the driving force (as in the pseudo second-order equation).
First-order model : ln(q e −q)=lnq e −kt ( 6) Second-order model : t/q=1/(k q e 2 )+t/q e (7) K: Adsorption rate constant (min -1 for first-order adsorption, g•mg -1 •min -1 for secondorder adsorption); t: Adsorption time (min) Since q reaches q e at equilibrium, q values smaller than 0.9q e are used for analysis.Plots of ln(q e −q) versus t and t/q versus t are used to test the first-and second-order models and the fitting results are given in Table 5 and Table 6.Both line curve fit and q e,cal , second-order model gives more well results than first-order model.Second-order model is more suitable to describe the adsorption kinetics data.Similar results were observed in the adsorption of PEG on EG 25 .Based on the second-order model, initial adsorption rate and half-adsorption time are estimated according to equations ( 8) and (9).As showed in Table 7 and Table 8, u is found to increase with the increase of temperature and t 1/2 decrease with the increase of temperature.

Internal diffusion analysis
Among the external diffusion, internal diffusion and actual adsorption of the adsorption process, the long adsorption equilibrium time of PVA on EG (20~60 min corresponding to adsorption temperature of 25~45 °C) suggests that the internal diffusion may dominate the overall adsorption kinetics.To provide definite information on the rate-limiting step, an internal diffusion model based on fick's second law is used to test if the internal diffusion step is the rate-limiting step 33 : q =k id t 1/2 (11) k id : Internal diffusion constant, (mg/g•min 1/2 ) According to the internal diffusion model, the plot of q versus t 1/2 should give a straight line with a slope k id and an intercept of zero if the adsorption is limited by the internal diffusion process.The relationships between q of PVA (500, 700) and t 1/2 at different temperature are showed in Figure 6 and Figure 7.In the range of the tested temperature, a linear relationship between q versus t 1/2 with a zero intercept is found when the adsorption temperature is high.Increase of temperature can decrease the viscosity of PVA solution and reduce the external diffusion and then internal diffusion is control step.(c) t 1/2 /min 1/2 t 1/2 /min 1/2 t 1/2 /min 1/2 q/mg/g q/mg/g q/mg/g q/mg/g q/mg/g

Conclusion
Adsorption of EG for PVA with different DPs have been investigated.The results are summarized as follows: • Adsorption isotherms of EG for PVA can be well described with Langmuir equation, and the present of SO 4 2-can improve the adsorbance.
• Adsorption of EG for PVA is spontaneous, adsorption isotherms of PVA (500, 700, 1750) are all type I and PVA molecule lies flat on EG surface.• Adsorption kinetics of EG for PVA can be described by pseudo-second-order model equation.Equilibrium time and half-adsorption time t1/2 decreases with the increase of temperature.The adsorption belongs to physical adsorption and internal diffusion is tested to be the rate-limiting step of the complete adsorption process.

Figure 2 .
Figure 2. Relationship between PVA, DP and molecule area

Table 1 .
Structural parameter of EG 27stewaters are prepared by dissolving PVA in distilled deionized water at various concentrations.In quantitative analysis27, H 3 BO 3 -I 2 in solution is used as colored reagent of PVA and absorbance of the colored complex (color reaction lasted 15 min) is detected with T6 new century UV spectrophotometry (Puxi Tongyong Instrument Limited Company of Beijing).Absorbance values are recorded at the wavelength for maximum absorbance (λ max ) (Table

Table 2 .
DP Accumulative adsorbance of adsorbate on EG at the moment of t, mg/g; C t : concentration of PVA in solution at the moment of t, mg/L;

Table 3 .
Langmuir and Freundlich isotherm constants at 25 °C

Table 4 .
Free energy change of PVA adsorption on EG at 25 o C

Table 5 .
Comparison of the adsorption kinetic models of PVA (500) on EG

Table 6 .
Comparison of the adsorption kinetic models of PVA (700) on EG

Table 7 .
Kinetic parameters for the second-order adsorption model of PVA (500)

Table 8 .
Kinetic parameters for the second-order adsorption model of PVA (700)