! r i _ ng I v i e i n m Acaitemr ol Science anJ Technoloay AJvances in Nalcial Sciences; Nanoicience and NanolecHnology
*"•• " • ' ^ ^ i l . N«,ol,cnnol, S (2015) 025014 (Bpp) aci:10.10B8/2043-lJ262;6/2/025014
Sorpd n of lead (II), cobalt (II) and copper (II) ions from aqueous solutions by 7-Mn02 nanostructure
Ngoc Chung Le^ and Dinh Van Phuc^
' Dalat University, 01 Phu Dong Thien Vuong street, Dalat City, Lam Dong Province, Vietnam
^Dong Nai University, 04 Le Quy Don Street, Tan Hiep District, Bien Hoa City, Dong Nai Province, Vielnam
E-mail: chung[n@dlu edu vn and [email protected] Received 8 December 2014
Accepted for pubhcation 23 January 2015 Published 2 March 2015
CrossMark Abstract
Manganese dioxide j'-Mn02 was synthesized via the reduction-oxidation reaction between KMn04 and C2H5OH at room temperature and characterized with x-ray powder diffraction (XRD), scanning election microscopy (SEM), transmission election microscopy (TEM) and Brunauer-Emmet-TeUer nitrogen adsorption (BET-N2 adsorption). The results showed that I'-Mn02 was about I 0 - 1 8 n m in size and the BET surface area was about 6 5 m ^ g " ' . The feasibility of j'-Mn02 used as a low cost adsorbent for the adsorption of Pb(II), Co(II) and Cu{II) from aqueous solutions was explored. During the adsorption process, batch technique was used, and the effects of contact time and pH on adsorption efficiency under room temperature were studied. The adsorption data showed that the Freundlich, Langmuir and Redlich-Peterson isotherms are a good model for the sorption of Co{II) and Cu(II), while tile Langmuir and Redlich-Peterson isotherms provide a reasonable fit to the experimental data for Pb(II). By using the Langmuir isotherm, the adsorption capacities for Pb(II), Co(II) and Cu(II) are found to be 200 m g g " ' , 90.91 m g g " ' and 83.33 m g g " ' , respectively. The effectiveness of y-Ma02 in the sorption of the three metal ions from aqueous system has tile order Pb(II)>Co(II)>Cu(n). Kinetic stiidies showed that a pseudo-second-order model was more suitable than the pseudo-Iirst-order model. Also, the intra-particle diffusion models were used to ascertain the mechanism of the sorption process. It is concluded that j'-Mn02 can be used as an effective adsorbent for removing Pb(II), Co(n) and Cu(II) from aqueous solutions.
Keywords: manganese dioxide j'-Mn02, nanostructure, adsorption, isotherm, kinetic
1. introduction considered as one of die most efficient and promising methods for the treatment of trace amount of heavy metal ions from The tremendous increase in the use of heavy metals over the large volumes of water because of its high enrichment efh- past few decades has inevitably resulted in an increased flux ciency, and the ease of phase separation [4-10].
of metallic substances in the aquatic environment [1-3]. Recentiy, tiie adsorption properties of nanostructured These pollutants enter the water environment through was- metal oxides have been applied for envu'onment pollution tewater from metal plating industries, battenes, phosphate removal. Because of their huge specific surface area and many fertilizer, mining pigments and stabilizers alloys [1-7]. unsaturated atoms on the surface, the adsorbability of nano- Various i r c - ".ent techtuques have been applied to remove materials to metal ions is very stiong. Nanostrucmred man- metal ions iHiii contaminated waters such as chemical pre- ganese oxides have attiacted increasing attenUon in view of cipitation. .i^cuption and ionic exchange, membrane technol- their applications in batteries, molecular sieves, catalysts, and ogy and soI%ent extraction [4-7]. Adsorption technology is adsortients [8-10].
acwa-P^saiS 025 , © 2015 Vietnam Academy of Science 8 Technology
Adv. NaL SCL. Nanosci. Nanotechnoi. 6 (2015) 025Q14 N C Le and D Van Phuc In tills study we report a simple method to syntiiesize
Mn02 nanostructiire which was used as a low cost adsorbent for tiie adsorption of Pb(n), Co(II) and Cu(n) fixim aqueous solutions. The sorption capacity of MnOi was evaluated using Freundlich, Langmuir and Redlich-Peterson isotherm models and the sorption dynamics were analyzed using pseudo-first- order and pseudo-second-order kinetic models. Intra-particle diffusion models were also used to ascertam the mechanism of the sorption process.
W7
1 1
\ w '"1 1. 1 .1
2. Experimental 2.1. Chemicals and instruments
Chemicals used included potassium permanganate (KMn04), etiiyl alcohol (CjHsOH), Pb(N03)2, Cu(N03)2-3H20 and Co (N03)2.6H20, HNO3 and NaOH. All reagents used in tiie experiment were of analytical grade and pure of Merck.
Pb(II), Cu(n), and Co(II) were used as adsorbate.
1000 mg L"' standard stock solution of each of the metal ions was prepared by dissolving Pb(N03)2, Cu(N03)2-3H20 and Co(N03)2.6H20, respectively, in distilled water. The con- centration of metal ions in the aqueous solutions was analyzed by using AA-7000 atomic absorption spectrometer (Shimadzu Corporation).
Instruments included x-ray diffractometer D5000 made in Germany by Siemens with x-ray radiation: CuKa with Jl=1.54056A, ultra high resolution scaiming electron microscopy S-4800, transmission electton microscope, phy- sical absorption system Micrometrics Gemmi VH, atomic absorption specttophotometer (Spectrometer Atomic Absorption AA-7000 made in Japan by Shimadzu).
The pH measiu^ments were done widi a pH-meter (Martini Instruments Mi-150 Romania). The pH-meter was standardized using HANNA instruments buffer solutions with pH values of 4.01 ± 0 . 0 1 , 7.01 ± 0 . 0 1 , and 10.01 ± 0 . 0 1 .
Temperature-conti-olled shaker (Model KIKA R 5) was used for equilibrium smdies
2 Ttieta (degree)
Figure 1. XRD images of as-prepared sample after different reaction limes: (a) 3 h, (b) 4 h, (c) 5 h, (d) 6h.
2.3. Adsorption study
The adsorption experiment was prepared by adding 0.1 g Mn02 to 50 mL heavy metal ion solution in a 100 mL conical flask. The effect of pH of the initial solution was analyzed over a pH range from 2 to 6 using HNO3 0.1 M or NaOH 0.1 M solutions (Merck). The adsorption studies were also conducted in batch experiments as a function of contact time (20, 40, 60, 80, 100, 120, 150, 180, 210 and 240min) and metal ions concentiation (from 100 mg L " ' to 500 mg L"') for maximum adsorption. An atomic absorption spectro- photometer was used to analyze the concentrations of the different metal ions in the filtrate before and after adsorbent process.
Adsorption capacity was calculated by using the mass balance equation for the adsorbent [8-20]
_ (Co - Ce)V
(1) where q is the adsorption capacity (mg g~') at equilibrium, Co and Ce are the initial concentration and the equilibrium con- centration (mg L~'), respectively. V is the volume (mL) of solution and m is the mass (g) of used adsorbent.
3. Resuits and discussions 2.2. Synthesis of Mn02 nanostructure
M n 0 2 nanostructure was synthesized via the reduction-oxi- dation between KMn04 and C2H5OH at room temperature for 4 h by adding gradually KMn04 saturated solution to the mixture of C2H5OH and H2O. The effect of reaction time as weU as the ratio between H2O and C2H5OH to the structure and size of crystal was studied. After the reaction was com- pleted, the solid precipitate was washed with distUIed water, and then dried at 80 "C for 12 h to get the product.
Charactenzation of the products: phase identification was carried out by x-ray diffraction. The surface morphology of the samples was monitored with SEM and TEM The specific surface area was evaluated by lutirogen adsorption-desorption isotiierm measurements at 77 K.
3.1. Synthesis of i-MnOg nanostructure and its adsorption to Pt^^, Cl/* and C</*
3.1.1. Characterization of manganese dioxide. The phase and purity of the products were firstly examined by XRD.
Figure 1 shows a typical XRD pattern of the as-synthesized samples. Curves (a) and (b) are the XRD pattems of flie two products obtained for 3 h and 4 h. Curves (c) and (d) are tiie XRD pattems of the two products obtained for 5 h and 6h, respectively. All reflection peaks can be readily indexed to hexagonal )'-Mn02 phase. However, the as-prepared sample achieved clearly crystal structure for 5 h.
The morphologies and structure information were further obtamed firom SEM and TEM images. Figures 2(a)-(c) show an SEM image of tiie as-prepared / - M n 0 2 which was
Nanosd. Nanotechnoi 6 (2015) 025014 N C Le and D Van Phuc
Figure 2. SEM images of r-Mn02 at tiie different ratios between HjO C2H5OH: (a) sample Ml, (b) sample M2, (c) sample M3; and (d) TEM image of sample M3
synthesized at the different ratios between H2O and C2H5OH:
(a) H20:C2H50H = 2:I (sample M l ) , (b) H20:C2H50H= 1:1 (sample M2), (c) H20:C2H50H= 1:2 (sample M3). As a result, y-Mn02 nanospheres with nanostructure were formed in tiie alcohol (KMn04:C2H50H= 1:2). It is clear fliat tiie flocculation occurred in the water solution (figure 2(a)).
Figure 2(c) also shows that the products of )'-Mn02 consisted of a large amount of uniform nanospheres, with size of about 10 nm. Figure 2(d) shows the TEM image of the as-prepared )'-Mn02 nanospheres (sample M3) and the TEM image further demonstrates that the obtained product has uniform sphere morphology. The TEM image also provides the size of )'-Mn02 nanospheres from 10 to 18 nm. The BET surface area of tile aS'Synthesized product (sample M3) was determined to be about 6 5 m ^ g ~ ^
3.1.2. Effect of pH on adsorption of heavy metals. The pH is one of the imperative factors governing the adsorption of tiie metal ions. The effect of pH was studied fjom a range of 2 to 6 under the precise conditions (at optimum contact time of 120 min, 240 rpm shaking speed, with 0.1 g of the adsorbents used, and at a room temperature of 24 °C). From figure 3, with y-Ma02 used as adsorbent, it was observed that with increase in the pH (2-6) of the aqueous solution, the adsorption percentage of metal ions (lead, cobalt and copper) all incre"sed up to flie pH 4, as shown above. At pH 4, the maxirr^in adsorption was obtained for all flie three metal ions v^ itli 98.9% adsorption of Pb(n), 54.1% of Co(n) and 41.?''^ adsorption of Cu(n).
00 - 80 - 60 40 20 n •
• Pb
• Co A Cu
•
—•—
•
•
A
«
•
A
*
•
A
«
•
A
0 2 4 6 8 pH
Figure 3, Effect of pH on the adsorption of heavy metals by j'-MnOa nanostructure. (Measurement parameters; time is 120 min, agitation speed is 240 rpm, mass is 0.1 g and temperature is 24 °C).
The mcrease in adsorption percentage of the metal ions may be explamed by the fact that at higher pH tiie adsorbent surface is deprotonated and negatively charged, hence attraction between the positively metal cations occurred [10].
3.1.3. Effect of contact time on adsorption of heavy metals.
The relationship between contact time and the adsorption percentage of heavy metals from aqueous solution with y- M n 0 2 adsorbent is shown in figure 4. The effect of contact time was studied at a room temperature of 24 °C, at intervals of 20 min. From the obtained result it is evident that the adsorption of metal ions increased as contact time increased.
Adv Nat Sci. Nanosci Nanotechnoi 6(2015)025014 N C Le and D Van Phuc
80 - 60 40 20 -
II -
t
m
t
• t
• t
m
• Pb
• Co A C U
0 50 100 150 200 250 300 Time (min)
Figure 4. Effect of contact time on adsorption of heavy metals by y- Mn02 nanostmcmre. (Measurement parameters: pH is 4, agitation speed is 240 rpm, mass is 0.1 g and temperature is 24 "C).
The adsorption percentage of metal ions approached equilibrium within 80 min for Pb(n), 120 min for Co(II) and 180 min for Cu(n); witii Pb(II) recording 92.47% adsorption, Co(U) 81.51 % and Cu(n) 89.24% adsorption. This experiment shows that different metal ions attained equilibnum at different times.
3.2. Equilibnum isotherm studies
Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among liquid and solid phases, based on a set of assumptions that are related to the heterogeneity/homogeneity of the solid surface, the type of coverage, and the possibiUty of interaction between the adsorbate species In this study, equilibrium data were ana- lyzed using the Freundlich, Langmuir and Redlich-Peterson isotherms expression.
3.2.1. Freundlich isotherm. The Freundlich (1906) equation [11-20] is an empirical equation based on adsorption on a heterogeneous surface. The equation is commonly represented as
(2) l o g ^ ^ = l o g K ' p -I- ( 1 / r t ) l o g C e ,
w h e r e Ce ( m g L ~ ) i s t h e e q u i l i b r i u m c o n c e n t r a t i o n a n d q^
( m g g " ' ) is t h e a m o u n t o f a d s o r b e d m e t a l i o n p e r u n i t m a s s o f t h e a d s o r b e n t . T h e c o n s t a n t n i s t h e F r e u n d U c h e q u a t i o n e x p o n e n t tiiat r e p r e s e n t s t h e p a r a m e t e r c h a r a c t e r i z i n g q u a s i - G a u s s i a n e n e r g e t i c h e t e r o g e n e i t y o f t h e a d s o r p t i o n s u r f a c e . Kp is t h e F r e u n d l i c h c o n s t a n t w h i c h i n d i c a t e s t h e r e l a t i v e a d s o r p t i o n c a p a c i t y o f t h e a d s o r b e n t . T h e F r e u n d l i c h m o d e l w a s c h o s e n t o e s t i m a t e t h e a d s o r p t i o n i n t e n s i t y o f t h e s o r b a t e o n t h e s o r b e n t s u r f a c e T h e e x p e r i m e n t a l d a t a from t h e b a t c h s o r p t i o n s t u d y o f t h e t h r e e m e t a l i o n s o n y - M n 0 2 n a n o s t r u c t u r e s w e r e p l o t t e d l o g a r i t h m i c a l l y ( f i g u r e 5 ) u s i n g t h e l i n e a r F r e u n d l i c h i s o t h e r m e q u a t i o n .
T h e l i n e a r F r e u n d l i c h i s o t h e r m c o n s t a n t s f o r P b ( I I ) , C o ( I I ) a n d C u ( I I ) o n y - M n O i n a n o s t r u c t u r e a r e p r e s e n t e d i n t a b l e 1. T h e F r e u n d l i c h i s o t h e r m p a r a m e t e r I / n m e a s u r e s tiie a d s o r p t i o n m t e n s i t y o f m e t a l i o n s o n t h e j ' - M n 0 2
o-
1 lo g
1.5
Z.O
1.5 1.0
0.5
R' = 0.993
R' = 0.846
R^ = 0.974
• P b
• Co A Cti
-0.5 0.0
ioflC.
Figure 5. Freundlich equilibrium isotherm model for the sorption of tiie three metal ions (Pb(II), Co(n}, Cu(II) onto y-MnOz-
Table 1. Freundhch isotherm parameters.
Samples 1/n ^ F (L g ) /?
Pb(II) 0.067 137.40 0.846 Co(n) 0.164 40.55 0.974 Cu(n) 0.064 59.98 0.993
nanostructure. The low lin values of Pb(II) (0.067), Cu(II) (0.064) and Co(n) (0.164) less than I show the favorable sorption and confirm the heterogeneity of the adsorbent. Also, they indicate tiiat the bond between heavy metal ions and y- Mn02 are strong. The adsorption capacity Kp of the adsorbent was calculated from the isothermal linear regression equation.
The Kp value of Pb(n) ( 1 3 7 . 4 0 L g " ' ) is greater than tiiat of Cu(II) (59.98 L g - ' ) and Co(II) (40.55 L g " ' ) , suggesting and confirming that Pb(iri has greater adsorption tendency towards the y-Mn02 nanostructure than the other two metals.
3.2.2. Langmuir isotherm. The Langmuu- model [11-20]
assumes that uptake of metal ions occurs on a homogenous surface by monolayer adsorption without any interaction between adsorbed ions. The linearized form of the Langmuir equation is given by the relation
9m
2 (3)
where q^ is the sorption capacity and Ki^ is Langmun constant.
The Langmuir isotherm model was chosen for the estimation of maximum adsorption capacity corresponding to complete monolayer coverage on the )'-Mn02 surface. The plots of specific sorption (CJqe) against tiie equihbrium concentration (C^) for Pb(n), Co(II) and Cu(n) are shov™ in figure 6 and the Unear isotherm parameters, q^,, A^L and the coefficient of determinations are presented in table 2.
The data in table 2 with high values of correlation coefficient iR^ = 0.998-0.999) mdicate good agreement between the parameters and confirm the monolayer
Mvj^ Nanosci Nanotechnoi 6(2015)025014 N C Le and D \fen Phuc
C.(mgn)
Figure 6. Langmuir equibbrium isotherm model for the sorption of the three metal ions (Pb(II), Co(II), Cu(II) onto j'-Mn02.
adsorption of Pb(II), Co(n) and Cu(n) ions on to y-MnOi nanostructure surface. Furthermore, the sorption capacity q^, which is a measure of the maximum sorption capacity corresponding to complete monolayer coverage, showed that the / - M n 0 2 nanostructure has a mass capacity 200 mg g~' for Pb^*, 90.91 m g g " ' for Co^* and 83.33 m g g " ' for Cu^*
An important characteristic of the Langmuir isotherm is expressed in a dimensionless constant equilibrium parameter, Sp, also known as the separation factor, given by
1
1 + KLCO (4)
The data in table 2 shows that the dimensionless parameter Sp remained between 0.008 and 0.059 ( 0 < S F < 1 ) at lowest concentration and between 0.0016 and 0.025 ( 0 < 5 F < 1 ) at highest concentiation, i.e. the separation parameters Sp for the three metals are less than unity indicating that }'-Mn02 nanostructure is an appropriate adsorbent for tiie three metal ions. The smaUer Sp value indicates a highly favorable adsorption [20, 21]. However, Sp value of Co ( n ) > C u (ID>Pb (II) mdicates that in a mixed meta! ion system, Pb(II) will be adsorbed faster tiian Co(II) and Cu(II).
3.2.3. Redlich-Peterson isotherm. Redlich-Peterson isotherm [11-20] is a hybrid isotiierm featuring botii Langmuir and Freundlich isotherms, which incorporate three parameters into an empirical equation. Then, the Redlich and Peterson equation, designated tiie 'tiiree parameter equation', may be used to represent adsorption equilibria over a wide concentration range. The linearized form of tiie Redlich-Peterson equation is given by tiie relation
nl KKP —
l 1'
n Ce -I- In ORP. (5) where KRP ( L g""'), " R P ( L mg^') and /3 are tiie Redlich- Peterson isotherm constants. The value of fi is the exponent which lies between 0 and 1. In the limit when the fi values tend to zero fiie Redhch-Peterson isotherm approaches Freundli^-h isoihcrm model at high concentration and is in
accordance with the low concentration limit of flie ideal Langmuir condition when the fi values are all close to 1.
The Redlich-Peterson isotiierm constants could be predicted from the plot between hi I Ksp-^ - 1 1 versus InC^.
However, tills is not possible as die linearized form of Redlich-Peterson isotherm equation contains three unknown parameters ORP, .KRP and fi. Therefore, a minimization procedure is adopted to maximize the coefficient of determination if, between tiie tiieoretical data for q^ predicted from the Unearized form of Redlich-Peterson isotherm equation and the experimental data. The Redlich-Peterson isotiierm plots for die tiiree metal ions (Pb(II), Co(II) and Cu (II)) are presented in figure 7 and tiie isotherm parameters are given in table 3.
The data in table 3 indicated that, tiie higher R^ values for Redlich-Peterson show the agreement of experimental equihbrium data wifli the Redlich-Peterson isotherm equation. This was expected, because degree of heterogeneity ( ^ is included and this equation can be used successfully at high solute concentrations. Langmuir is a special case of Redhch-Peterson isotherm when constant fi is unity.
3 2.4. Coefficients of determination. It has been suggested that linearization plots may not be a significant basis to reject or accept a model. To further analyze tine suitability of the three models (Freundhch, Langmuir and Redhch-Peterson), their fitness to the experimental data was assessed. The fitness of the data was established using a single statistical parameter iR-) which is called the coefficient of determination [18, 19].
The coefficient of determination values for the three models as shown in table 4 shows that the three models are appbcable in describing the data of Co(n) and Cu(ID (as aU I^>0.90), whereas tiie adsorption process of Pb(n) is described by the Langmuir and Redlich-Peterson isotherm models.
3 3. Kineses studies
The kinetics of adsorption describes the rate of Pb(II), CofH) and Cu(n) ions uptake on y-Mn02 and this rate controls the equibbrium time The kinetics of adsorbate uptake is required for selectmg optimum operating conditions for the full-scale batch process.
The kinetic parameter, which is helpful for the prediction of adsorption rate, gives important information for designing and modehng the processes. The kinetics of the adsorption data was analyzed using different kinetic models such as pseudo-first-order and pseudo-second-order models. The intra-particle diffusion models are also used to ascertain the mechanism of the sorption process.
3.3.1. Pseudo-firstorder model. The pseudo-first-order rate model equation given by Lagergren in 1898 is [20-29]
^ = kiiq, - q), (6) d(
where q^ is the amount of solute adsorbed at equilibnum per unit weight of adsorbent (mg g"'), q is the amount of solute
Adv. NaL SCL. Nanosci. Nanotechnoi 6 (2015} 025014 N C Le and D Van Phuc Table 2. Langmuir adsorption isotherm constants for ions on j'-MnOi
Sample KL 5F (at lowest Co= 100mgL"') Sp (at highest Co = 500 mgL ') Pb (U) 1.25 200
Co (II) 0.16 90.91 Cu(ID 1.09 83.33
0.0079 0 0588 0.0091
0.0016 0.0123 0.0018
Figure 7. Redlich-Peterson equilibnum isotherm model for the sorption of the three metal ions (Pb(II), Co(n), Cu(ll)) onto j-MnOz-
Table 3. Redlich-Peterson isotherm parameters.
Metal ion Pb Co Cu
^RP 307.346
24.134 3693.189
«RP fi 1.562 0.998 0.378 0.932 60.167 0.941
le
0.999 0 999 iOOO
4. Adsorption isotheim coefficients of determmatio
Adsorption isotherm Freundlich Langmuir Redlich-Peterson
Heavy metals pb(n) co(ii) 0.846 0.974 0.999 0.998 0.999 0.999
Cu(II) 0.992 0.999 1.000
iqe-qt) versus time t was not Imear over the entne time range, the correlation coefficients in this model for all metal ions were low. Also, the theoretical q^, values estimated from the first-order kinetic model were not in accordance with the experimental values. This suggests that this adsorption system is not a first-order reaction, indicating that more tiian one mechanism was involved in adsorption.
3.3.2. Pseudo-second-order model Adsorption kinetics was explained by the pseudo-second-order model given by Ho and McKay as follows [20-29]
adsorbed at any time (mg g" ) and Ai is the adsorption constant. Integrating equation (6) with conditions g(0) = 0 and qit) = q„ the kinetic rate expression becomes
\0giq^-q,) = \<:igq^- k\t
2.303' (7)
The first-order rate constant k^ (min" ) can be obtained from the slope of the plot of \ogiqe-q,) against time (, as shown in figure 8(a) and table 5. The adsorption first-order rate constants were found to be 4.606 10"^, 0.0115 and 4.606.10"^ min"' for metal ions Pb(II), Co(II) and Cu(II), respectively.
If the adsorption process can be described by pseudo-first order equation, there should be good linear relationship between Xogiq^-q,) and t. In the present study, the plot of log
d^ .
kiiq^ - qf- (8)
With the boundary conditions ^(0) = 0, qit)-q, tiie solution of equation (8) is
q kjq; q^
Additionally, the initial adsorption rate h (mg mg"^ min~^) can be determined as
h - k2q^. (10)
where k^ (g mg"' min" ) is the second-order rate constant determined from tiie plot of tlq against t, as shown in figure 8(b) and table 5.
The plots of tlq versus / are hnear for different metal ions.
The adsorption second-order rate constants were found to be 1.88.10-\ 1.25.10"^ and 1.45.10"^ g m g " ' m m ' ' for metal ions Pb(n), Co(II) and Cu(II), respectively. The imtial sorption rate, h, is the highest for Pb(II) and the lowest for CuCH), which reflects the affinity of the sorbent for tiiese absorbate species.
The results showed that the theoretical q^ values calculated from pseudo-second order kinetic model were very close to the experimental values of equilibrium sorption with correlation coefficients of determination higher than 0.999. The pseudo-second-order adsorption mechanism was predominant for adsorption of metal ions Pb(n), Co(n) and Cu(n) by y-MnOi. The applicability of this model showed that the sorption process is complex and involves more than one mechanism.
3.3.3. Intra-particle diffusion model Because tiie pseudo- first-order and pseudo-second-order kinetic models cannot identify the diffusion mechaiusms, the most commonly used technique for identifying the diffiision mechanism involved in the adsorption process is to use flie intra-particle diffusion model.
Nanosci. NanotBchnol 6(2015)025014 N C Le and D Van Phuc
100 150 200 250 300 Time (min)
3U - 2-5 • 2 0 1 i.
1 0 Ob
nn
(b)
J ^ ^ .!^^^
^f^^
x^ y
^ ^ ^ .Pb
-"'^ ICo
100 150 2(
Time (min)
Figure 8. (a) Pseudo-first-order kinetic plots for the adsorption of metal ions, (b) pseudo-second-order kinetic plots for the adsorpuon of metal ions.
Table 5. Pseudo-first-order and pseudo-second-order for the adsorption of metal ions on y-yinO^
PbCE) 126.10
4.606.10-' 0.886 14.62
1.88.10"^
38.46 1
Co(n) 89.76
0.0115 0.898 19.23
1.25.10-' 12.5
0.999
Cu (II) 78.58
4.606.10"^
0.870 19.91
1.45 10"^
10.10 0 999 Pseudo-first-order kinetic
Pseudo-second-order kinetic Ki (g mg ' h (mg mg min )
The intra-particle diffiision model used here refers to the theory proposed by Weber and Morris, based on the following equation for the rate constant [26-30]
- k^t"^ + C, (11)
where q, is the quantity of metal ions adsorbed at time t (mg g~'), ka the initial rate of intra-particular diffiision (mg L"' mm ~"^), and C is the y-intercept which is proportional to the boundary layer thickness.
According to this model, when the metal ion solution is mixed with the adsorbent, transport of the metal ions from the solution through the interface between the solution and the adsorbent occurs into the pores of the particles. If adsorption of a solute is controlled by the intra-particle diffusion process, a plot of q, versus r"^ gives a straight line [26-28]. Figure 9 illustrates the diffusion of Pb(II), Co(n) and Chi(II) ions within )'-Mn02 as a function of time and shows that intira-particle diffiision occurred in two stages, first linear portion (stage I) and second curved path foUowed by a plateau (stage II). In stage I, the metal ions were up taken by j'-Mn02. This is attnbuted to the utilization of the most readily available adsorbing sites on the adsorbent surfaces. In stage H, very slow diffiision of adsorbate from surface site into the inner-pores is observed. Thus, the initial portion of adsorption of Pb(II), Co (ID and Cu(II) '""^ by /-Mn02 adsorbents may be governed by the initial inti,. ^-uticle transport of Pb(U), Co(n) and Cu(n)
• Pb
• Co A C U
500
Figure 9, Plot of the intra-paiticle diffusions kinetics for the three heavy metal ions Pb(II), Co(n) and Cu(n).
Table 6. The values of the mtra-paiticular diffusion coefficients.
Metal ion Ka, (mg L"' min""^) ^d2(nig L"' mm""^) PbOD
cu(n) Co(II)
o.ii 0 147 0.159
0.011 0.004 0.006
Adv Nat SCI Nanosci. Nanotechnoi 6(2015)025014
ions controlled by surface diffiision process and the later part is controlled by pore diffusion. The intra-particle diffiision constants for all two stages (iji, k^) are given in table 6.
4. Conclusion
Manganese dioxide }'-Mn02 was synthesized via the reduc- tion-oxidation reaction between KMn04 and C2H5OH at room temperature. The results showed that y-MnOa was about 10-18 nm in size and the BET surface area was about 65m^g"V The feasibihty of y-MnOi used as a low cost adsorbent for tiie adsorption of Pb(II), Co(II) and Cu(II) from aqueous solutions was explored. The results of adsorption performance showed that the pH of aqueous solution, adsorp- tion tune has a great influence on the adsorption performance.
The expenmental results were analyzed using three adsorption isotherm models, the Freundhch, Langmuir and Redlich-Peterson, isotiierm models. By usmg tiie Langmuir isotherm, the adsorpnon capacities for Pb(II), Co(II) and Cu(n) are found as 200mgg""', 90.91 m g g " ' and 8 3 . 3 3 m g g " \ respecbvely. The effectiveness of /-Mn02 nanostmcture m the sorption of the three metals from aqueous system has Pb(II) > Co (II)>Cu(II). The separation parameters, Sp, for the three metals are less flian unity indicating that y-MnOz nanostructure is an appropriate adsorbent for the three metal ions. However, Sp values of Pb(II) < Cu(II) < Co(II), indicate that m a mixed metal ion system, Pb(II) will be adsorbed faster than Co(II) and Cu(II).
Kmetic studies showed that a pseudo-second-order model was more suitable than tiie pseudo-first-order model. The intra-par- ticle diffusion models were also used to ascertain the mechanism of the sorption process.
[1 ] Gaikwad S S and Kamble N A 2014 Octa J. Environ. Res. 2 67 [2] Karakaya N and Karakaya M C 2014 Turkish J. Earth Sci.
23 113
N C Le and D Van Phuc
[3] Dhanalakshmi B 2013 Asian J. Sci TechnoL 4 021 [4] Jiuhui Q U 2008 J Environ. ScL 20 1
[5] Gaikwad R W and Gupta D V 2008 AppL EcoL Environ Res.
6 81
[6] Dubey S P, Gopal K and Bersillon J L 2009 J. Environ Biology 30 327
[7] Syafalni AbduUah R and Ushaa Nair P S 2013 World AppL Sci.
J. 27 614
[8] Li J, Xi B, Zhu Y, Li Q, Yan Y and Qian Y 2011 J. Alloys Compd. 509 9542
[9] Andjelkovic I, Manojlovic D, Skrivanj S, Pavlovic B M, Amaizah N R and Roglic G 2013 Int. J. Environ Resour.
7 395
[10] Huo Q and Xiao H 2014 J. Chem. Pharm. Res. 6 270 [11] Sulaymon A H, Mohammed T J and Ai-Najar J 2012 Canadian
J. Chem. Eng. TechnoL 3 86
[12] Foo K Y and Hameed B H 2010 Chem. Eng. J. 156 2 [13] Rafatullah M, Sulaiman O, Hashim R and Ahmad A 2009
J. Hazard. Mater. 170 969
[14] Rout K, Mohapatra M, Mohapatm B K and Anand S 2009 Int.
J. Eng. Sci. Technol 1 106
[15] Dawodu F A, Akpomie G K and Ogbu I C 2012 Int J.
Multidiscip. ScL Eng. 3 9
[16] Subramanyam B and Das A 2009 Int J. Environ. ScL TechnoL 6 633
[17] Dawodu P A, Akpomie G K and Abuh M A 2012 Int. J. Sci.
Eng. Res 3 1
[18] Shahmohammadi-Kalalagh S, Babazadehl H, Nazemi A H and Manshoun M 2011 Caspian J Environ. ScL 9 243 [19] HorsfaU M J and Spiff A I 2005 Acta Chim. Slovenica 52 174 [20] Kumar K V 2006 J. Hazard. Mater. B 137 1538 [21] Ho Y-S 2006 /. Hazard. Mater. B 136 681
[22] Ho Y-S and Oforaaja A E 2006 J. Hazard. Mater B 137 1796 [23] Ho Y-S and McKay G 1999 Process Biochem. 34 451 [24] Hossain M A and Hossain M L 2014 Int. J. Adv. Res. 2 360 [25] Chowdhury S and Saha P 2010 IIOAB J. (Institute of
Integrative Omics & Applied Biotechnology Joumal) 1 3 [26] Kumar P S and Gayathri R 2009 J. Eng. Scl Technol 4 381 [27] Argun M E, Dursun S, Ozdenur C and Karatas M 2007
J Hazard Mater. 141 77
[28] Salaryana P, Noroozifaib M, Atashic H, Sajadid S M, Mokaria F and Salaryana H 2013 Inter. J. Chem. Biochem.
Sci. 3 110
[29] Yasemin B and Zeki T 2007 J. Environ. Scl 19 160 [30] Igwe J C, Abia A A and Sonde C U 2011 ABSU J. Environ
Sci. TechnoL 1 25
