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Copper sorption onto dried red alga Pterocladia capillacea and its activated carbon
Chemical Engineering Journal 168 (2011) 707–714
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Copper sorption onto dried red alga Pterocladia capillacea and its activated carbon Amany El-Sikaily, Ahmed El Nemr ∗ , Azza Khaled Department of Pollution, Environmental Division, National Institute of Oceanography and Fisheries, Kayet Bey, El Anfoushy, Alexandria, Egypt
a r t i c l e
i n f o
Article history: Received 1 November 2010 Received in revised form 16 January 2011 Accepted 17 January 2011 Keywords: Red algae Pterocladia capillacea Langmuir Freundlich Ion exchange model Error functions Copper sorption
a b s t r a c t Sorption of Cu(II) ions on dried red alga Pterocladia capillacea and its activated carbon have been studied. The sorption equilibrium was determined as a function of contact time at several Cu(II) ion concentrations and the effect of adsorbent concentration was also investigated. The pseudo second-order kinetic model provided the best correlation for the experimental data in compared to the pseudo-first order kinetic. Ion exchange was occurred in the initial reaction period. The experimental results were fitted to the Langmuir, Freundlich and Redlich–Petrson isotherms to obtain the characteristic parameters of each model. Both the Langmuir and Redlich–Peterson equations were significantly better for dried red alga biomass. On the other hand, Freundlich and Redlich–Peterson equation were significantly better than Langmuir for the activated carbon prepared from dried red alga. Error functions have been used to determine the alternative single component parameters by non-linear regression analysis. The error function method provided the best parameters for the isotherm equation in this system and is demonstrated for error comparison purposes. Red alga biomass and its activated carbon may be evaluated as an environmentally friendly and extra economic treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Marine environment and, in particular, coastal waters are subjected to increasing contamination by heavy metals [1]. Most of the metals in the fourth period of the periodic table are carcinogenic [2]. It can be assumed that the carcinogenicity is related to the electronic structure of transition metal. Since copper is an essential metal in a number of enzymes for all forms of life, problems arise when it is deficient or in excess. Excess copper accumulates in the liver and the most toxic form of copper is thought to be Cu(II) [2]. Toxicity of metals is highly pH dependent and it has been reported to be more toxic to fish at lower pH values [3]. In some respect the intake of essential elements is more critical than for toxic elements. However, epidemiological evidence such as a high incidence of cancer among copper smiths, suggests a primary carcinogenic role for copper [2]. Thus, the removal and recovery of heavy metals from waste water is important in the protection of the environment and human health. A number of technologies such as chemical precipitation, evaporation, electroplating and ion exchange processes have been used to remove copper(II) from waste water [4]. However, these technologies are most suitable in situations where the concentrations of the heavy metal ions are relatively high [4]. They are either ineffective or expensive when heavy metals are present in the waste water at low concentrations, or when very low concentrations of heavy metals in the
treated water are required [5]. Biosorption is an alternative technology which utilizes the ability of biological materials to accumulate heavy metals from aqueous solutions by either metabolically mediated or purely physico-chemical pathways of uptake [6]. Marine algae are biological resources which are available in large quantities in many parts of the world. The use of biomass of marine algae, Ascophyllum nodosum [7], Ecklonia radiate [8], Durvillaea potatorum [9], Ulva lactuca [10], algal bloom residue [11] and for heavy metal removal have been reported. Pterocladia capillacea is marine red alga in which the chlorophyll is masked by the red pigment phycoerthrin, they are always multicellular and usually of small to moderate size, frond is hollow and has a cartilaginous texture. In Mediterranean Sea, they habit on rocks on the shore and in shallow water [12]. Sulfuric acid was used to dehydrate some plants to produce activated carbon to use for heavy metal removal [13,15]. The objective of the present work was to test the ability of dried red alga (RA), P. capillacea, to remove copper(II) ions from aqueous solution, natural sea water, synthetic sea water and real waste water, furthermore, developing an activated carbon from dried red alga (CRA) and studying its sorption capacity for Cu(II) removal. Also, kinetic and isotherms of interaction were investigated. 2. Materials and methods 2.1. Biomass
∗ Corresponding author. Tel.: +20 35740944/107801845; fax: +20 35740944. E-mail address: [email protected] (A. El Nemr). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.01.064
Red alga P. capillacea was collected from Abu-Quir bay, Alexandria, Egypt and washed with sea water, tap water, and then distilled
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following equation: Nomenclature q C C0 qm n KL m r2
amount of metal ion adsorbed per specific amount of adsorbent (mg/g) residual metal ion concentration (mg/g) initial metal ion concentration (mg/g) amount of metal ions required to form a monolayer (mg/g) Freundlich equilibrium constant indicative of bond energies between metal ion and adsorbent Langmuir equilibrium constant related to the energy of sorption (L/mg) adsorbent mass correlation coefficients
water several times. The clean alga was sun dried in air for one week followed by air oven drying at 105 ◦ C for 24 h. The dried alga was milled and sieved to <0.063 mm [14]. 2.2. Activated carbon The dried red alga P. capillacea 0.5 kg was added in small portion to 500 mL of 98% H2 SO4 and the resulting mixture was kept for 24 h at room temperature followed by refluxing in fume hood for 5 h. After cooling, the reaction mixture was poured into ice water (2 L) and filtered. The filtrate was washed repeatedly with distilled water and soaked in 1% NaHCO3 solution to remove any remaining acid. The sample was then washed with distilled water until pH of the activated carbon reached 6, dried in an air oven at 160 ◦ C for 48 h to give activated carbon (168 g, 33.6% of alga weight). It was milled then sieved to an average particle size <0.063 mm and kept in a glass bottle until used [15]. The characteristics of the obtained activated carbon are as following: BET surface area (1014 m2 /g), ˚ pore volume 0.28 cm3 /g), density of particle pore radius (10.2 A), 3 (0.79 g/cm ), diameter of particle (0.06 mm), fixed carbon (82.0%) and ash (2.1%). 2.3. Preparation of synthetic solution Analytical grade reagents were used in all experiment set. A stock solution of copper(II) (1000 mg/L) was prepared in distilled water using copper sulfate. All working solutions (5–100 mg/L) were prepared by diluting the appropriate volume of the stock solution with distilled water. All the sorption experiments were carried out at room temperature (25 ± 2 ◦ C). The initial pH was adjusted with 1 M HCl or 1 M NaOH.
qt =
C0 − Ct V ms
(1)
where C0 and Ct are the Cu(II) concentrations (mg/L) initially and at a given time t, respectively. V is the volume of Cu(II) solution (L), and ms are the weight of adsorbent (g). The percentage of removed Cu(II) ions (% Removal) in solution was calculated using Eq. (2) % Removal =
C0 − Ct × 100 Co
(2)
2.5. Red alga characterization The functional groups present in the red alga and its activated carbon were characterized by a Fourier transform infrared (FT-IR), using KBr discs to prepare the alga samples. The X-ray diffraction spectrum was obtained by passing the sample through 44 copper target. Samples were exposed to X˚ with the 2 angle, scan range varying between ray ( = 1.5418 A) 4◦ and 9◦ and scan speed 2◦ /min. The applied voltage and current were 30 kV and 30 mA, respectively. The morphological characteristics of alga and its activated carbon were evaluated by using a JEOL JSM-6360 scanning electron microscope with an electron acceleration voltage of 20 kV. 3. Results and discussion 3.1. FT-IR analysis of red alga and its activated carbon The Fourier transform infrared spectroscopy (FT-IR) technique is an important tool to identify characteristic functional groups, which could capable of adsorbing metal ions. The FT-IR spectra for red alga (P. capillacea) and its activated carbon are shown in Fig. 1 and the FT-IR spectroscopic characteristic are shown in Table 1. As shown in Fig. 1, the spectra display number of absorption peaks, indicating the nature of the red alga. The bands observed at 3433 and 3436 cm−1 in both dried red alga and its activated carbon represent bonded –OH group on their surface [17]. Aliphatic C–H group is represented by the peak at 2925 cm−1 . The peaks located at 1637 and 1635 cm−1 are characteristics for carbonyl group stretching. These groups can be conjugated or non-conjugated to aromatic rings [18]. Deformations related to C–H and C–O bonds were observed in activated carbon of red alga at 1062 cm−1 probably due to carbonization method. Also, a peak observed at 1182 cm−1 in activated carbon of red alga indicate the presences of bisulfate (HSO4 − ) which present at 873, 445 and 381 cm−1 indicating the presences of H2 PO4− or PO4 2− and metal oxide. It has been
2.4. Batch sorption experiments Batch sorption experiments were performed at a constant temperature (25 ± 2 ◦ C) on a rotary shaker at 150 rpm using 250 mL capped conical flasks and agitated for the required contact time. The pH was adjusted to 5.0 by 1 M HCl and 1 M NaOH. Sorption of Cu(II) was studied using different weights of dried red alga and its activated carbon in 100 mL solution of 20, 30, 50, and 75 mg/L of initial Cu(II) concentration. The concentration of Cu(II) ions in the solution after and before sorption was determined spectrophotometrically using sodium diethyldithiocarbamate dissolved in 1.5 N NH3 solutions at 460 nm [16]. All experiments were duplicated, and only the mean values were reported. The maximum deviation observed was less than 5%. The amount of Cu(II) adsorbed (mg/g) at time t was computed using the
Fig. 1. The FT-IR spectra of red alga Pterocladia capillacea and its activated carbon biomass before adsorption. Note: (a) For carbon red alga Pterocladia capillacea biomass; (b) for red alga Pterocladia capillacea biomass.
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Fig. 2. X-ray diffractograms of (a) Pterocladia capillacea and (b) carbon of Pterocladia capillacea.
well documented that several bio-molecules, proteins, polysaccharides and extra cellular polymers contain these functional groups, such as carboxyl, carbonyl, hydroxyl, amino, phosphoryl and sulfide groups. The different functional groups have a high affinity towards heavy metals so that they can form complex with metal ions [17]. Because of the complexity of most cell walls, several different mechanisms have been proposed to explain the uptake of metals by non-living biomass, including microprecipitation, ion exchange and complexation. Untreated biomass generally contains light metal ions such as K+ , Na+ Ca2+ and Mg2+ [20,21], the biosorption process of nickel, copper and cadmium can be mainly accounted for by ion exchange with calcium [19]. There was a significant release of Ca2+ , Mg2+ , K+ and H+ from the sorbent due to uptake of Cu(II) and Ni(II). This might indicate the displacement of these cations by the metals [22].
peared which indicate that the treatment arrange the crystal lattice of the samples. In the carbon of red alga the main peak is recorded at 2 = 25.8 and d-spacing 3.44 A˚ corresponding to the presence of silicate in quartz form [23]. 3.3. Scanning electron microscope (SEM) Scanning electron microscope of dried red alga and its activated carbon are shown in Fig. 3. The morphology of this material can facilitate the sorption of metals, due to the irregular surface of the
3.2. X-ray diffraction analysis The X-ray diffraction patterns for dried red alga and its activated carbon are shown in Fig. 2. The hump peak which appears in the front of sheet of X-ray diffraction of P. capillacea reflects the presence of high percent of organic compounds, whereas, the broad ˚ The middle zone of the peak at 2 = 77.45 and d-spacing (1.23119 A). X-ray sheet is occupied by significant peaks ranged from 2 = 11.95 to 24.178 with highest value recorded at 22.68 corresponding to d˚ This indicates the presence of spacing values 7.39, 3.67, and 3.91 A. silicate probably as orthoclase. This finding is in agreement with the FT-IR that revealed to the presence of silicate group. The carbonization of alga with sulfuric acid affects on the crystal form structure of the alga where, the hump which appeared in the red alga disappeared in its prepared carbon. In addition, the mean peak in the ˚ is also disapdried red alga at 2 (77.45 and d-spacing 1.23119 A) Table 1 The FT-IR spectral characteristics of dried red alga Pterocladia capillacea and its activated carbon. IR peak
Frequency (cm−1 ) Dried red alga
Carbon of red alga
1 2 3 4 5 6 7 8 9 10
3433.05 2922.00 2383.85 1637.45 – – 873.69 594.03 – –
3436.91 2925.81 2360.71 1635.5 1182.20 1062.7 873.69 584.39 445.53 381.88
Assignment
Bonded –OH group Aliphatic C–H group Dibasic phosphate (HPO4 2− ) C O stretching Bisulfate (HSO4 − ) C–H and C–O deformation Silicate H2 PO4 − or PO4 2− Metal compounds Metal oxide
Fig. 3. Scanning electron micrograph of (a) dried Pterocladia capillacea and (b) carbon Pterocladia capillacea.
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A. El-Sikaily et al. / Chemical Engineering Journal 168 (2011) 707–714 Table 2 Pseudo-first order kinetic model parameters for various adsorbent (red alga and its activated carbon).
100
% Removal
80
Initial copper concentration (mg/L)
60 40 RA 20
CRA
0 0
2
4
6
8
pH Fig. 4. Effect of pH on the uptake of Copper by dried red alga Pterocladia capillacea and its activated carbon (C0 = 50 mg/L, temp. 25 ± 2 ◦ C).
alga, thus makes possible the sorption of metal in different parts of this material. So, based on the morphology, as well as on the fact that high amounts of silicate which concentrated on the alga and its carbon, it can be concluded that this material presents an adequate morphological profile to adsorb metal ions. 3.4. Effect of contact time The rate of removal of Cu(II) is extremely rapid in the first 30 min, approximately 59% removal was attained for dried red alga and 70% for its activated carbon, and then the rate becomes almost constant up to the end of the experiment. Similar results have been reported in literature [1,2,24].
One of the most important parameters affecting the sorption process is suspension pH [25–27]. The metal uptake is pH dependence due to various functional groups on the cell walls. In this study, optimum pH was about 5.0 (Fig. 4). At the beginning of the sorption, both Cu(II) and protons are adsorbed, but when the Cu(II) concentration is increased, a partial desorption of protons occurs allowing the sorption of Cu(II) onto the sites left by protons at the surface of biomass. According to this, it may be suggested that there was a clear competition for the biomass sorption sites between Cu(II) and proton [28]. 3.6. Effect of sorbent biomass When the dried red alga concentration was increased from 2 to 15 g/L, the percentage removal increased while the amount of Cu(II) adsorbed decreased approximately from 11.5 to 2.0 mg/g and from 14.5 to 3.0 mg/g for RA and CRA, respectively (Fig. 5). This may be attributed to reduce of total surface area of sorbent, probably due to the aggregation during sorption and/or modification of the biomass surface [29]. 100
16
% Removal
80
12 10
60
8 40
6 4
20
2 5
10
15
20
0
Adsorption capacity, mg/g
14
0
k1 (min−1 )
RA
CRA
RA
CRA
RA
CRA
0.2
0.2
0.5
0.5
0.0326 0.0623 0.0218 0.3454 0.0689 0.0461 0.0272 0.0368
0.0405 0.0509 0.0401 0.0562 0.0207 0.0161 0.0161 0.0184
0.9818 0.8384 0.8854 0.7907 0.8939 0.9883 0.8376 0.9372
0.8188 0.9774 0.8339 0.9818 0.8891 0.9675 0.9491 0.7472
r2
3.7. Kinetic models Numerous kinetic models have been proposed to evaluate the mechanism by which pollutants may be adsorbed. The mechanism of sorption depends on the physical and/or chemical characteristics of the sorbent as well as on the mass transport process [30]. In order to investigate the mechanism of Cu(II) sorption, three models were used. 3.7.1. Pseudo-first-order kinetic model (Lagergren model) dqt = k1 (qe − qt ) dt
(3)
where qt (mg/g) is the amount of sorbed Cu(II) on the sorbent at time t and k1 (min−1 ) is the rate constant of first order sorption. The integrated form of equation is:
3.5. Effect of pH
0
20 30 50 70 20 30 50 70
Adsorbent weight (g)
%R(RA)
%R (CRA)
log(qe − qt ) = log qe −
k1 t 2.303
(4)
A straight line of log(qe − qt ) vs. t obtained with experimental data suggests the applicability of this kinetic model. qe and k1 can be determined from the intercept and slope of the plot, respectively [31]. Our data do not fall on straight lines indicating that this model is not appropriate. The Lagergren first-order rate constant (k1 ) and (qe )cal determined from the model are presented in Table 2 along with the corresponding correlation coefficients. Correlation coefficients were found to be between 0.791 and 0.988 for dried red alga and from 0.747 to 0.982 for activated carbon of red alga, but the calculated qe is not equal to experimental qe , suggesting the insufficiency of pseudo-first-order model to fit the kinetic data for the initial copper concentrations examined. 3.7.2. Pseudo-second-order The pseudo-second-order kinetic model (Ho equation) is expressed as: dqt = K2 (qe − qt )2 dt
(5)
where K2 (g mg−1 min−1 ) is the rate constant of second-order sorption and can be rearranged and linearized to obtain: t 1 1 = + t qt qe K2 q2e
(6)
qe (RA)
The plot t/qt vs. t of experimental data should give a straight line if second-order kinetics were applicable, and qe and K2 can be determined from the slope and intercept of the plot, respectively:
qe (CRA)
h = K2 q2e
Biomass concentration, g/L Fig. 5. Copper uptake by dried red alga Pterocladia capillacea and its activated carbon as a function of biomass concentration (C0 = 50 mg/L, pH 5, temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
(7)
where h is the initial sorption rate (mg g−1 min−1 ) [32]. By plotting t/qt against t for the studied different initial Cu(II) concentration and concentration of dried red alga and its activated carbon, a straight line is obtained in all cases, and the second-order rate constant (k2 )
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Table 3 Pseudo-second-order and ion exchange model parameters for various adsorbent (RA and CRA) at pH 5. Cu(II) conc.
20 30 50 70 20 30 50 70
Adsorbent weight (g)
qe(exp)
0.2
0.5
Pseudo-second order
Ion exchange model
qe (calculated)
k2 (g mg−1 min−1 )
h (mg/g min)
r2
S (min−1 )
r2
RA
CRA
RA
CRA
RA
CRA
RA
CRA
RA
CRA
RA
CRA
RA
CRA
7.43 10.83 15.99 20.95 7.81 11.54 18.92 25.97
9.55 13.95 21.35 28.07 9.64 14.46 23.78 32.81
7.58 10.44 16.46 20.82 7.806 11.63 19.12 26.32
9.35 13.67 21.69 27.90 10.10 15.06 25.06 35.09
0.051 0.042 0.023 0.016 0.175 0.105 0.030 0.023
0.029 0.025 0.024 0.014 0.032 0.028 0.012 0.009
2.94 5.22 6.18 7.53 10.66 14.25 11.12 16.56
2.75 5.22 11.36 11.31 3.38 6.36 7.59 11.74
0.999 0.999 0.999 0.999 1.000 1.000 0.999 0.999
0.997 0.999 0.999 0.999 0.998 0.999 0.997 0.998
0.033 0.062 0.021 0.028 0.042 0.040 0.027 0.036
0.041 0.051 0.040 0.056 0.020 0.017 0.016 0.017
0.982 0.838 0.885 0.922 0.879 0.979 0.838 0.937
0.982 0.977 0.983 0.962 0.989 0.968 0.995 0.975
and qe values were determined. The pseudo second-order kinetic model gives a good correlation (r2 > 0.999) for the sorption of Cu(II) onto red alga and its activated carbon (Table 3). 3.7.3. Ion exchange model In order to study the sorption mechanism, the rate of uptake of Cu(II) by dried red alga and its activated carbon were analyzed by an ion exchange phenomenon. Boyd et al. [33] developed a rate equation, which considered rates of ion exchange sorption in the exchange sorption of ions from aqueous solutions by organic zeolites. For the case of two monovalent ions the mass law applies to the exchange as: A+ + BR B+ + AR
(8)
If mA+ and mB+ denote the concentrations of ions A+ and B+ tion, nAR and nBR the moles of A+ and B+ in the sorbent,
in soluthe net
concentration during the sorption process. The pH variation could originate from the acidic groups of cellulose that are believed to be responsible for the cation exchange capacity or ion exchange reactions, such as hydrogen released when Cu2+ cation bind to the dried red alga and its activated carbon [26]. It is clear that ion exchange may have occurred in the beginning stage of sorption. Where, the structure of dried red alga is cellulose microfibrils network associated with material that includes amorphous polymers of sulfated galactans, mucilage and cellulose and their surface in contact with water is negatively charged. Metal compounds used in this study will dissolve to give the cationic metal and this will undergo attraction on approaching the anionic red alga structure. On this basis, it is expected that a metal cation will have a strong sorption affinity for dried red alga and its activated carbon. 3.8. Equilibrium studies
reaction can be written as follows: dnAR = K1 mA+ nBR − K2 mB+ nAR = −nAR (K1 mA+ + K2 mB+ ) dt + K1 mA+ E
(9)
where K1 and K2 are the forward and reverse specific rate constants, and E is a constant defined by E = nAR + nBR
(10)
When the concentrations of A+
and B+
in solution are kept constant,
integration of Eq. (9) becomes nAR =
K1 mA+ E (1 − e−St ) = q K1 mA+ + K2 mB+
(11)
where q is the sorption capacity at time t, S = K1 mA+ + K2 mB+ , Eq. (11) can be written as qe − q = qe e−St
(12)
where qe is the equilibrium capacity. Thus, log(1 − F) = −
S
2.303
t
Sorption isotherms describe how adsorbates interact with adsorbents and so are critical in optimizing the use of adsorbents. Thus, the correlation of equilibrium data with either theoretical or empirical equation is essential to the practical design and operation of sorption systems [34]. The copper ion is involved in a single-component system with one fixed pH 5 and several dose of adsorbent. The correlation coefficient (r2 ) values were used to predict the best-fit linear equation. Three isotherm equations have been tested in the present study, namely, Langmuir, Freundlich and Redlich–Peterson. The models and their linearized form are given in Table 4.
(13)
where F = (qt /qe ) is the fractional attainment of equilibrium and S (min−1 ) is a constant. Values of the constant, S, were calculated from the slopes of respective linear plots and are listed in Table 3, where r2 > 0.967. At the beginning of the sorption, both Cu2+ and H+ , from acidity of solution, are adsorbed onto dried red alga but when the Cu2+ concentration is increased, a partial desorption of H+ occurs allowing the sorption of Cu2+ onto the sites left by hydrogen at the surface of biomass. According to this, it may be suggested that there was a clear competition for the biomass sorption sites between the Cu2+ and H+ , and ion exchange was the main sorption mechanism. The increase in sorption depends on the surface properties and the chemical characteristics of the Cu2+ . There are two sources which may contribute to an increase in the hydrogen ion
3.8.1. Langmuir isotherm The Langmuir sorption isotherm has been successfully applied to many pollutants sorption processes and has been the most widely used sorption isotherm for the sorption of a solute from a liquid solution [35]. A basic assumption of the Langmuir theory is that sorption takes place at specific homogeneous sites within the sorbent. It is then assumed that once a metal ion occupies a site, no further sorption can take place at that site [35]. Application of Langmuir model indicates a straight line and represents the data reported in Table 5. 3.8.2. Freundlich isotherm In 1906, Freundlich studied the sorption of a material onto animal charcoal [36]. The Freundlich isotherm theory is the first known relationship explains the sorption process. It describes the ratio of the amount of solute adsorbed onto a given mass of sorbent to the concentration of the solute in the solution is not constant at different concentrations. Freundlich isotherm equation is applicable to the sorption on heterogeneous surfaces with interaction between adsorbed molecules and it can be employed to describe the heterogeneous systems. Where Kf is the Freundlich constant, nf is the heterogeneity factor represents the deviation from linearity
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Table 4 Isotherms and their linear forms. Isotherm
Formula
Freundllich Langmuir Redlich–Peterson
Linear form
Plot
qe =
Kf Ce1/nf
log(qe ) = log(Kf ) +
qe =
qm Ka Ce 1+Ka Ce
Ce qe
qe =
ACe g 1+BCe
ln
=
1 Ka qm
A Cqee
+
1 qm
1 nf
log(Ce )
log(qe ) vs. log(Ce ) Ce qe
Ce
vs. Ce
ln A Cqee − 1 vs. ln(Ce )
− 1 = g ln(Ce ) + ln(B)
Ce : equilibrium solute concentration (g/L) qe : amount of Cu(II) sorbed at equilibrium (mg/g); Kf : Freundlich isotherm constant (mg1−1/n g1/n L1/n ); nf : Freundlich exponent; Qm : maximum sorption capacity (mg/g); Ka : Langmuir constant (L/mg).
of sorption as follows: if the value of nf = 1, the sorption is linear; nf < 1, the sorption process is chemical; if nf > 1, the sorption is a favorable physical process (Table 5).
fit. RMSE can be defined as:
m 1 RMSE = (Qi − qi )2 m−2
3.9. Error analysis In the single component isotherm studies, the optimization procedure requires an error function to be defined in order to be able to evaluate the fit of the equation to the experimental data [34] by using SPSS computer program version 15. A part from the correlation coefficient (r2 ), the residual root mean square error (RMSE) and the Chi-square test were also used to measure the goodness-of
Table 5 Isotherm constants for copper sorption onto red alga and its activated carbon. Method
Model parameter
Langmuir
Linear method
Freundlich
Redlich–Peterson
Langmuir Non-linear method* Freundlich *
SPSS computer program (version 15).
Red alga
CRA
Qmax Ka r2 RMSE X2 Kf nf r2 RMSE X2 A B g r2 RMSE X2
49.50 0.042 0.991 0.392 0.022 2.838 1.435 0.987 0.594 0.056 3.980 0.555 0.504 0.998 0.410 0.316
60.60 1.000 0.991 24.40 33.30 12.69 1.495 0.993 1.340 0.184 16.92 0.304 0.962 0.998 0.520 0.043
Qmax Ka r2 Kf nf r2
40.56 0.437 0.994 2.110 1.789 0.990
56.12 0.963 0.989 12.16 1.589 0.980
where Qi is the observation from the batch experiment, qi is the estimate from the isotherm for corresponding Qi and m is the number of observations in the experimental isotherm. The smaller RMSE value indicates the better curve fitting [38]. The Chi-square test can be defined as X2 =
m (Qi − qi )2 i=1
(15)
qi
If data from model are similar to experimental data, X2 will be small number [40]. Among the three parameter models, high correlation coefficients and low RMSE and Chi-square values were observed for red alga (0.2 g and 0.5 g) with Langmuir and Redlich–Peterson models, Table 5. This indicates that in case of red alga R–P model the equation approaches the ideal Langmuir condition. When the g-values → 1. The main characteristic of the Langmuir equation is based on the assumption that all sites have equal sorption energies (homogeneous). If the metal sorption is based on ion exchange with a protonated amino group and a bond by sharing the lone pair on the nitrogen, then this is a reasonable description for a constant, equal energy of sorption and supports the extremely high correlation coefficients obtained from the Langmuir analysis [34]. On the other hand, the main model for carbon red alga (CRA) is Freundlich. R–P model also confirms the result where RMSE and Chi-square test have low values and high r2 , Table 5, and in this case g in R–P model tend to be zero and the equation becomes more Freundlich or heterogeneous sorption (Figs. 6 and 7). It is assumed that the stronger binding sites are occupied first and that the binding strength decreases with the increasing degree of site occupation. 3.10. Field study In this section we are reported the effects of salinity and of a real waste water on the ability of the red alga and its activated carbon on 25 20
qe, mg/g
3.8.3. Redlich–Peterson isotherm (R–P) The Redlich–Peterson isotherm [34] contains three parameters and incorporates the combination of Langmuir and Freundlich isotherms. The (R–P) isotherm has a linear dependence on concentration in the numerator and an exponential function in the denominator [37]. It has three isotherm constants A, B, and g which can be evaluated from linear plot by using a trial and error procedure [38]. The Redlich–Peterson isotherm constants can be predicted from the plot between ln[ACe /qe ) − 1] vs. ln(Ce ). However, this is not possible as the linearized form because it contains three unknown parameters A, B, and g. Therefore, a minimization procedure is adopted to maximize the coefficient of determination r2 between the theoretical data for qe predicted from linearized form of R–P isotherm equation and the experimental data [39]. SPSS computer program version 15 was used to calculate isotherm constants and their corresponding r2 values shown in Table 5.
(14)
i=1
15 exp. 10
Langmuir Freundlich
5
R-P
0 0
5
10
15
20
25
30
Ce, mg/L Fig. 6. Isotherms of copper sorbet on dried red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
A. El-Sikaily et al. / Chemical Engineering Journal 168 (2011) 707–714 Table 6 Comparison of copper(II) sorption by different seaweeds.
30 25
qe, mg/g
20 15
exp. Freundlich
10
R-P Langmuir
5 0 0
2
4
6
8
10
12
14
Sorbent
Cu (mg/g)
References
Saragassum filipendula Saragassum fluitans Saragassum vulgare Ulva reticulata Padina species Ascophyllum nodosum Durvillaea potatorum Ulva lactuca Pterocladia capillacea Carbon of Pterocladia capillacea
38.00 51.00 59.00 54.70 23.20 28.71 37.70 24.50 49.50 60.00
Volesky et al. [41] Davis et al. [42] Davis et al. [42] Vijayaraghavan et al. [43] Kaewsarn [4] Chong and Volesky [7] Matheickal and Yu [9] Abdelwahab [15] This study This study
16
Ce, mg/L Fig. 7. Isotherms of copper sorbet on activated carbon of dried red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
(NSW)R%
were suitable material for removal of Cu(II) from different types of aqueous solutions including real waste water. Comparison of biosorption of Cu(II) from aqueous solution by red alga in the present study with different seaweeds indicates that red alga is an effective adsorbent for removal of Cu(II) as reported in Table 6.
(SSW)R%
120
R% of Copper (pH=5)
713
(WW)R%
100 80 60 40 20 0 0
20
40
60
80
100
120
140
Time Fig. 8. Percentage removal of Cu(II) from synthetic sea water, natural sea water, waste water by dried red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
the sorption of Cu(II). A removal studies were achieved using synthetic sea water (SSW), natural sea water (NSW) and waste water (WW). Figs. 8 and 9 show that the removal percentage of Cu(II) from SSW, NSW, and WW were 89.75, 90.97 and 90.41%, respectively, for red alga and were 93.03, 94.30, and 95.83%, respectively, for activated carbon from red alga. These proved that the presence of salt had no effect on the sorption of Cu(II) on both adsorbents, which indicated that there was no interaction between salts and the surface of adsorbent. Also, the high concentration of Cu(II) makes them more applicable to adsorb by the adsorbents. These results indicate that the two sorbents dried red alga and its activated carbon
4. Conclusion The present work evaluated the removal of Cu(II) from aqueous solution using red alga (P. capillacea) and its activated carbon. The Cu(II) removal from aqueous solutions through red alga biomass may be considered as an environmentally friendly and economic treatment. The red alga biomass may be considered as an effective and inexpensive adsorbent for the removal of Cu(II) ions from aqueous solutions. The sorption process is a function of the adsorbent concentration, adsorbate concentration, and pH. The highest Cu(II) sorption onto red alga and carbon red alga was obtained at pH (5.0). The experimental results were analyzed by using Langmuir, Freundlich and Redlich–Peterson isotherm equations. The correlation coefficients (r2 ), RMSE, and Chi square test for fitting the Langmuir and Redlich–Peterson equations were significantly better than correlation coefficient, RMSE, and Chi-square test for Freundlich equation of red alga biomass. On the other hand, Freundlich and R–P equations and the RMSE, Chi-square test was significantly better than Langmuir for carbon red alga. The kinetics of sorption of Cu(II) onto red alga and carbon red alga followed the ion exchange rate model of Boyd, which considers rates of ion exchange sorption during the beginning of sorption. The order of the reaction for the sorption of Cu(II) onto red alga and carbon red alga followed a pseudo-second-order rate expression. Finally, the results showed that red algae and its carbon can be used for the removal of Cu(II) from different aqueous solutions. Acknowledgement
Carbon Red Algae
120
(NSW)R%
R% of Copper (pH=5)
(SSW)R% (WW)R%
100
The authors are gratefully acknowledges Dr. Abeer Abdelwahab of Mubarak city for Scientific Research & Technology Applications, for making IR, X-ray and scanning electron microscope analysis.
80
References 60 40
20
0 0
20
40
60
80
100
120
140
Time Fig. 9. Percentage removal of Cu(II) from synthetic sea water, natural sea water, waste water by activated carbon of red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
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Copper sorption onto dried red alga Pterocladia capillacea and its activated carbon Amany El-Sikaily, Ahmed El Nemr ∗ , Azza Khaled Department of Pollution, Environmental Division, National Institute of Oceanography and Fisheries, Kayet Bey, El Anfoushy, Alexandria, Egypt
a r t i c l e
i n f o
Article history: Received 1 November 2010 Received in revised form 16 January 2011 Accepted 17 January 2011 Keywords: Red algae Pterocladia capillacea Langmuir Freundlich Ion exchange model Error functions Copper sorption
a b s t r a c t Sorption of Cu(II) ions on dried red alga Pterocladia capillacea and its activated carbon have been studied. The sorption equilibrium was determined as a function of contact time at several Cu(II) ion concentrations and the effect of adsorbent concentration was also investigated. The pseudo second-order kinetic model provided the best correlation for the experimental data in compared to the pseudo-first order kinetic. Ion exchange was occurred in the initial reaction period. The experimental results were fitted to the Langmuir, Freundlich and Redlich–Petrson isotherms to obtain the characteristic parameters of each model. Both the Langmuir and Redlich–Peterson equations were significantly better for dried red alga biomass. On the other hand, Freundlich and Redlich–Peterson equation were significantly better than Langmuir for the activated carbon prepared from dried red alga. Error functions have been used to determine the alternative single component parameters by non-linear regression analysis. The error function method provided the best parameters for the isotherm equation in this system and is demonstrated for error comparison purposes. Red alga biomass and its activated carbon may be evaluated as an environmentally friendly and extra economic treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Marine environment and, in particular, coastal waters are subjected to increasing contamination by heavy metals [1]. Most of the metals in the fourth period of the periodic table are carcinogenic [2]. It can be assumed that the carcinogenicity is related to the electronic structure of transition metal. Since copper is an essential metal in a number of enzymes for all forms of life, problems arise when it is deficient or in excess. Excess copper accumulates in the liver and the most toxic form of copper is thought to be Cu(II) [2]. Toxicity of metals is highly pH dependent and it has been reported to be more toxic to fish at lower pH values [3]. In some respect the intake of essential elements is more critical than for toxic elements. However, epidemiological evidence such as a high incidence of cancer among copper smiths, suggests a primary carcinogenic role for copper [2]. Thus, the removal and recovery of heavy metals from waste water is important in the protection of the environment and human health. A number of technologies such as chemical precipitation, evaporation, electroplating and ion exchange processes have been used to remove copper(II) from waste water [4]. However, these technologies are most suitable in situations where the concentrations of the heavy metal ions are relatively high [4]. They are either ineffective or expensive when heavy metals are present in the waste water at low concentrations, or when very low concentrations of heavy metals in the
treated water are required [5]. Biosorption is an alternative technology which utilizes the ability of biological materials to accumulate heavy metals from aqueous solutions by either metabolically mediated or purely physico-chemical pathways of uptake [6]. Marine algae are biological resources which are available in large quantities in many parts of the world. The use of biomass of marine algae, Ascophyllum nodosum [7], Ecklonia radiate [8], Durvillaea potatorum [9], Ulva lactuca [10], algal bloom residue [11] and for heavy metal removal have been reported. Pterocladia capillacea is marine red alga in which the chlorophyll is masked by the red pigment phycoerthrin, they are always multicellular and usually of small to moderate size, frond is hollow and has a cartilaginous texture. In Mediterranean Sea, they habit on rocks on the shore and in shallow water [12]. Sulfuric acid was used to dehydrate some plants to produce activated carbon to use for heavy metal removal [13,15]. The objective of the present work was to test the ability of dried red alga (RA), P. capillacea, to remove copper(II) ions from aqueous solution, natural sea water, synthetic sea water and real waste water, furthermore, developing an activated carbon from dried red alga (CRA) and studying its sorption capacity for Cu(II) removal. Also, kinetic and isotherms of interaction were investigated. 2. Materials and methods 2.1. Biomass
∗ Corresponding author. Tel.: +20 35740944/107801845; fax: +20 35740944. E-mail address: [email protected] (A. El Nemr). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.01.064
Red alga P. capillacea was collected from Abu-Quir bay, Alexandria, Egypt and washed with sea water, tap water, and then distilled
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following equation: Nomenclature q C C0 qm n KL m r2
amount of metal ion adsorbed per specific amount of adsorbent (mg/g) residual metal ion concentration (mg/g) initial metal ion concentration (mg/g) amount of metal ions required to form a monolayer (mg/g) Freundlich equilibrium constant indicative of bond energies between metal ion and adsorbent Langmuir equilibrium constant related to the energy of sorption (L/mg) adsorbent mass correlation coefficients
water several times. The clean alga was sun dried in air for one week followed by air oven drying at 105 ◦ C for 24 h. The dried alga was milled and sieved to <0.063 mm [14]. 2.2. Activated carbon The dried red alga P. capillacea 0.5 kg was added in small portion to 500 mL of 98% H2 SO4 and the resulting mixture was kept for 24 h at room temperature followed by refluxing in fume hood for 5 h. After cooling, the reaction mixture was poured into ice water (2 L) and filtered. The filtrate was washed repeatedly with distilled water and soaked in 1% NaHCO3 solution to remove any remaining acid. The sample was then washed with distilled water until pH of the activated carbon reached 6, dried in an air oven at 160 ◦ C for 48 h to give activated carbon (168 g, 33.6% of alga weight). It was milled then sieved to an average particle size <0.063 mm and kept in a glass bottle until used [15]. The characteristics of the obtained activated carbon are as following: BET surface area (1014 m2 /g), ˚ pore volume 0.28 cm3 /g), density of particle pore radius (10.2 A), 3 (0.79 g/cm ), diameter of particle (0.06 mm), fixed carbon (82.0%) and ash (2.1%). 2.3. Preparation of synthetic solution Analytical grade reagents were used in all experiment set. A stock solution of copper(II) (1000 mg/L) was prepared in distilled water using copper sulfate. All working solutions (5–100 mg/L) were prepared by diluting the appropriate volume of the stock solution with distilled water. All the sorption experiments were carried out at room temperature (25 ± 2 ◦ C). The initial pH was adjusted with 1 M HCl or 1 M NaOH.
qt =
C0 − Ct V ms
(1)
where C0 and Ct are the Cu(II) concentrations (mg/L) initially and at a given time t, respectively. V is the volume of Cu(II) solution (L), and ms are the weight of adsorbent (g). The percentage of removed Cu(II) ions (% Removal) in solution was calculated using Eq. (2) % Removal =
C0 − Ct × 100 Co
(2)
2.5. Red alga characterization The functional groups present in the red alga and its activated carbon were characterized by a Fourier transform infrared (FT-IR), using KBr discs to prepare the alga samples. The X-ray diffraction spectrum was obtained by passing the sample through 44 copper target. Samples were exposed to X˚ with the 2 angle, scan range varying between ray ( = 1.5418 A) 4◦ and 9◦ and scan speed 2◦ /min. The applied voltage and current were 30 kV and 30 mA, respectively. The morphological characteristics of alga and its activated carbon were evaluated by using a JEOL JSM-6360 scanning electron microscope with an electron acceleration voltage of 20 kV. 3. Results and discussion 3.1. FT-IR analysis of red alga and its activated carbon The Fourier transform infrared spectroscopy (FT-IR) technique is an important tool to identify characteristic functional groups, which could capable of adsorbing metal ions. The FT-IR spectra for red alga (P. capillacea) and its activated carbon are shown in Fig. 1 and the FT-IR spectroscopic characteristic are shown in Table 1. As shown in Fig. 1, the spectra display number of absorption peaks, indicating the nature of the red alga. The bands observed at 3433 and 3436 cm−1 in both dried red alga and its activated carbon represent bonded –OH group on their surface [17]. Aliphatic C–H group is represented by the peak at 2925 cm−1 . The peaks located at 1637 and 1635 cm−1 are characteristics for carbonyl group stretching. These groups can be conjugated or non-conjugated to aromatic rings [18]. Deformations related to C–H and C–O bonds were observed in activated carbon of red alga at 1062 cm−1 probably due to carbonization method. Also, a peak observed at 1182 cm−1 in activated carbon of red alga indicate the presences of bisulfate (HSO4 − ) which present at 873, 445 and 381 cm−1 indicating the presences of H2 PO4− or PO4 2− and metal oxide. It has been
2.4. Batch sorption experiments Batch sorption experiments were performed at a constant temperature (25 ± 2 ◦ C) on a rotary shaker at 150 rpm using 250 mL capped conical flasks and agitated for the required contact time. The pH was adjusted to 5.0 by 1 M HCl and 1 M NaOH. Sorption of Cu(II) was studied using different weights of dried red alga and its activated carbon in 100 mL solution of 20, 30, 50, and 75 mg/L of initial Cu(II) concentration. The concentration of Cu(II) ions in the solution after and before sorption was determined spectrophotometrically using sodium diethyldithiocarbamate dissolved in 1.5 N NH3 solutions at 460 nm [16]. All experiments were duplicated, and only the mean values were reported. The maximum deviation observed was less than 5%. The amount of Cu(II) adsorbed (mg/g) at time t was computed using the
Fig. 1. The FT-IR spectra of red alga Pterocladia capillacea and its activated carbon biomass before adsorption. Note: (a) For carbon red alga Pterocladia capillacea biomass; (b) for red alga Pterocladia capillacea biomass.
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709
Fig. 2. X-ray diffractograms of (a) Pterocladia capillacea and (b) carbon of Pterocladia capillacea.
well documented that several bio-molecules, proteins, polysaccharides and extra cellular polymers contain these functional groups, such as carboxyl, carbonyl, hydroxyl, amino, phosphoryl and sulfide groups. The different functional groups have a high affinity towards heavy metals so that they can form complex with metal ions [17]. Because of the complexity of most cell walls, several different mechanisms have been proposed to explain the uptake of metals by non-living biomass, including microprecipitation, ion exchange and complexation. Untreated biomass generally contains light metal ions such as K+ , Na+ Ca2+ and Mg2+ [20,21], the biosorption process of nickel, copper and cadmium can be mainly accounted for by ion exchange with calcium [19]. There was a significant release of Ca2+ , Mg2+ , K+ and H+ from the sorbent due to uptake of Cu(II) and Ni(II). This might indicate the displacement of these cations by the metals [22].
peared which indicate that the treatment arrange the crystal lattice of the samples. In the carbon of red alga the main peak is recorded at 2 = 25.8 and d-spacing 3.44 A˚ corresponding to the presence of silicate in quartz form [23]. 3.3. Scanning electron microscope (SEM) Scanning electron microscope of dried red alga and its activated carbon are shown in Fig. 3. The morphology of this material can facilitate the sorption of metals, due to the irregular surface of the
3.2. X-ray diffraction analysis The X-ray diffraction patterns for dried red alga and its activated carbon are shown in Fig. 2. The hump peak which appears in the front of sheet of X-ray diffraction of P. capillacea reflects the presence of high percent of organic compounds, whereas, the broad ˚ The middle zone of the peak at 2 = 77.45 and d-spacing (1.23119 A). X-ray sheet is occupied by significant peaks ranged from 2 = 11.95 to 24.178 with highest value recorded at 22.68 corresponding to d˚ This indicates the presence of spacing values 7.39, 3.67, and 3.91 A. silicate probably as orthoclase. This finding is in agreement with the FT-IR that revealed to the presence of silicate group. The carbonization of alga with sulfuric acid affects on the crystal form structure of the alga where, the hump which appeared in the red alga disappeared in its prepared carbon. In addition, the mean peak in the ˚ is also disapdried red alga at 2 (77.45 and d-spacing 1.23119 A) Table 1 The FT-IR spectral characteristics of dried red alga Pterocladia capillacea and its activated carbon. IR peak
Frequency (cm−1 ) Dried red alga
Carbon of red alga
1 2 3 4 5 6 7 8 9 10
3433.05 2922.00 2383.85 1637.45 – – 873.69 594.03 – –
3436.91 2925.81 2360.71 1635.5 1182.20 1062.7 873.69 584.39 445.53 381.88
Assignment
Bonded –OH group Aliphatic C–H group Dibasic phosphate (HPO4 2− ) C O stretching Bisulfate (HSO4 − ) C–H and C–O deformation Silicate H2 PO4 − or PO4 2− Metal compounds Metal oxide
Fig. 3. Scanning electron micrograph of (a) dried Pterocladia capillacea and (b) carbon Pterocladia capillacea.
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A. El-Sikaily et al. / Chemical Engineering Journal 168 (2011) 707–714 Table 2 Pseudo-first order kinetic model parameters for various adsorbent (red alga and its activated carbon).
100
% Removal
80
Initial copper concentration (mg/L)
60 40 RA 20
CRA
0 0
2
4
6
8
pH Fig. 4. Effect of pH on the uptake of Copper by dried red alga Pterocladia capillacea and its activated carbon (C0 = 50 mg/L, temp. 25 ± 2 ◦ C).
alga, thus makes possible the sorption of metal in different parts of this material. So, based on the morphology, as well as on the fact that high amounts of silicate which concentrated on the alga and its carbon, it can be concluded that this material presents an adequate morphological profile to adsorb metal ions. 3.4. Effect of contact time The rate of removal of Cu(II) is extremely rapid in the first 30 min, approximately 59% removal was attained for dried red alga and 70% for its activated carbon, and then the rate becomes almost constant up to the end of the experiment. Similar results have been reported in literature [1,2,24].
One of the most important parameters affecting the sorption process is suspension pH [25–27]. The metal uptake is pH dependence due to various functional groups on the cell walls. In this study, optimum pH was about 5.0 (Fig. 4). At the beginning of the sorption, both Cu(II) and protons are adsorbed, but when the Cu(II) concentration is increased, a partial desorption of protons occurs allowing the sorption of Cu(II) onto the sites left by protons at the surface of biomass. According to this, it may be suggested that there was a clear competition for the biomass sorption sites between Cu(II) and proton [28]. 3.6. Effect of sorbent biomass When the dried red alga concentration was increased from 2 to 15 g/L, the percentage removal increased while the amount of Cu(II) adsorbed decreased approximately from 11.5 to 2.0 mg/g and from 14.5 to 3.0 mg/g for RA and CRA, respectively (Fig. 5). This may be attributed to reduce of total surface area of sorbent, probably due to the aggregation during sorption and/or modification of the biomass surface [29]. 100
16
% Removal
80
12 10
60
8 40
6 4
20
2 5
10
15
20
0
Adsorption capacity, mg/g
14
0
k1 (min−1 )
RA
CRA
RA
CRA
RA
CRA
0.2
0.2
0.5
0.5
0.0326 0.0623 0.0218 0.3454 0.0689 0.0461 0.0272 0.0368
0.0405 0.0509 0.0401 0.0562 0.0207 0.0161 0.0161 0.0184
0.9818 0.8384 0.8854 0.7907 0.8939 0.9883 0.8376 0.9372
0.8188 0.9774 0.8339 0.9818 0.8891 0.9675 0.9491 0.7472
r2
3.7. Kinetic models Numerous kinetic models have been proposed to evaluate the mechanism by which pollutants may be adsorbed. The mechanism of sorption depends on the physical and/or chemical characteristics of the sorbent as well as on the mass transport process [30]. In order to investigate the mechanism of Cu(II) sorption, three models were used. 3.7.1. Pseudo-first-order kinetic model (Lagergren model) dqt = k1 (qe − qt ) dt
(3)
where qt (mg/g) is the amount of sorbed Cu(II) on the sorbent at time t and k1 (min−1 ) is the rate constant of first order sorption. The integrated form of equation is:
3.5. Effect of pH
0
20 30 50 70 20 30 50 70
Adsorbent weight (g)
%R(RA)
%R (CRA)
log(qe − qt ) = log qe −
k1 t 2.303
(4)
A straight line of log(qe − qt ) vs. t obtained with experimental data suggests the applicability of this kinetic model. qe and k1 can be determined from the intercept and slope of the plot, respectively [31]. Our data do not fall on straight lines indicating that this model is not appropriate. The Lagergren first-order rate constant (k1 ) and (qe )cal determined from the model are presented in Table 2 along with the corresponding correlation coefficients. Correlation coefficients were found to be between 0.791 and 0.988 for dried red alga and from 0.747 to 0.982 for activated carbon of red alga, but the calculated qe is not equal to experimental qe , suggesting the insufficiency of pseudo-first-order model to fit the kinetic data for the initial copper concentrations examined. 3.7.2. Pseudo-second-order The pseudo-second-order kinetic model (Ho equation) is expressed as: dqt = K2 (qe − qt )2 dt
(5)
where K2 (g mg−1 min−1 ) is the rate constant of second-order sorption and can be rearranged and linearized to obtain: t 1 1 = + t qt qe K2 q2e
(6)
qe (RA)
The plot t/qt vs. t of experimental data should give a straight line if second-order kinetics were applicable, and qe and K2 can be determined from the slope and intercept of the plot, respectively:
qe (CRA)
h = K2 q2e
Biomass concentration, g/L Fig. 5. Copper uptake by dried red alga Pterocladia capillacea and its activated carbon as a function of biomass concentration (C0 = 50 mg/L, pH 5, temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
(7)
where h is the initial sorption rate (mg g−1 min−1 ) [32]. By plotting t/qt against t for the studied different initial Cu(II) concentration and concentration of dried red alga and its activated carbon, a straight line is obtained in all cases, and the second-order rate constant (k2 )
A. El-Sikaily et al. / Chemical Engineering Journal 168 (2011) 707–714
711
Table 3 Pseudo-second-order and ion exchange model parameters for various adsorbent (RA and CRA) at pH 5. Cu(II) conc.
20 30 50 70 20 30 50 70
Adsorbent weight (g)
qe(exp)
0.2
0.5
Pseudo-second order
Ion exchange model
qe (calculated)
k2 (g mg−1 min−1 )
h (mg/g min)
r2
S (min−1 )
r2
RA
CRA
RA
CRA
RA
CRA
RA
CRA
RA
CRA
RA
CRA
RA
CRA
7.43 10.83 15.99 20.95 7.81 11.54 18.92 25.97
9.55 13.95 21.35 28.07 9.64 14.46 23.78 32.81
7.58 10.44 16.46 20.82 7.806 11.63 19.12 26.32
9.35 13.67 21.69 27.90 10.10 15.06 25.06 35.09
0.051 0.042 0.023 0.016 0.175 0.105 0.030 0.023
0.029 0.025 0.024 0.014 0.032 0.028 0.012 0.009
2.94 5.22 6.18 7.53 10.66 14.25 11.12 16.56
2.75 5.22 11.36 11.31 3.38 6.36 7.59 11.74
0.999 0.999 0.999 0.999 1.000 1.000 0.999 0.999
0.997 0.999 0.999 0.999 0.998 0.999 0.997 0.998
0.033 0.062 0.021 0.028 0.042 0.040 0.027 0.036
0.041 0.051 0.040 0.056 0.020 0.017 0.016 0.017
0.982 0.838 0.885 0.922 0.879 0.979 0.838 0.937
0.982 0.977 0.983 0.962 0.989 0.968 0.995 0.975
and qe values were determined. The pseudo second-order kinetic model gives a good correlation (r2 > 0.999) for the sorption of Cu(II) onto red alga and its activated carbon (Table 3). 3.7.3. Ion exchange model In order to study the sorption mechanism, the rate of uptake of Cu(II) by dried red alga and its activated carbon were analyzed by an ion exchange phenomenon. Boyd et al. [33] developed a rate equation, which considered rates of ion exchange sorption in the exchange sorption of ions from aqueous solutions by organic zeolites. For the case of two monovalent ions the mass law applies to the exchange as: A+ + BR B+ + AR
(8)
If mA+ and mB+ denote the concentrations of ions A+ and B+ tion, nAR and nBR the moles of A+ and B+ in the sorbent,
in soluthe net
concentration during the sorption process. The pH variation could originate from the acidic groups of cellulose that are believed to be responsible for the cation exchange capacity or ion exchange reactions, such as hydrogen released when Cu2+ cation bind to the dried red alga and its activated carbon [26]. It is clear that ion exchange may have occurred in the beginning stage of sorption. Where, the structure of dried red alga is cellulose microfibrils network associated with material that includes amorphous polymers of sulfated galactans, mucilage and cellulose and their surface in contact with water is negatively charged. Metal compounds used in this study will dissolve to give the cationic metal and this will undergo attraction on approaching the anionic red alga structure. On this basis, it is expected that a metal cation will have a strong sorption affinity for dried red alga and its activated carbon. 3.8. Equilibrium studies
reaction can be written as follows: dnAR = K1 mA+ nBR − K2 mB+ nAR = −nAR (K1 mA+ + K2 mB+ ) dt + K1 mA+ E
(9)
where K1 and K2 are the forward and reverse specific rate constants, and E is a constant defined by E = nAR + nBR
(10)
When the concentrations of A+
and B+
in solution are kept constant,
integration of Eq. (9) becomes nAR =
K1 mA+ E (1 − e−St ) = q K1 mA+ + K2 mB+
(11)
where q is the sorption capacity at time t, S = K1 mA+ + K2 mB+ , Eq. (11) can be written as qe − q = qe e−St
(12)
where qe is the equilibrium capacity. Thus, log(1 − F) = −
S
2.303
t
Sorption isotherms describe how adsorbates interact with adsorbents and so are critical in optimizing the use of adsorbents. Thus, the correlation of equilibrium data with either theoretical or empirical equation is essential to the practical design and operation of sorption systems [34]. The copper ion is involved in a single-component system with one fixed pH 5 and several dose of adsorbent. The correlation coefficient (r2 ) values were used to predict the best-fit linear equation. Three isotherm equations have been tested in the present study, namely, Langmuir, Freundlich and Redlich–Peterson. The models and their linearized form are given in Table 4.
(13)
where F = (qt /qe ) is the fractional attainment of equilibrium and S (min−1 ) is a constant. Values of the constant, S, were calculated from the slopes of respective linear plots and are listed in Table 3, where r2 > 0.967. At the beginning of the sorption, both Cu2+ and H+ , from acidity of solution, are adsorbed onto dried red alga but when the Cu2+ concentration is increased, a partial desorption of H+ occurs allowing the sorption of Cu2+ onto the sites left by hydrogen at the surface of biomass. According to this, it may be suggested that there was a clear competition for the biomass sorption sites between the Cu2+ and H+ , and ion exchange was the main sorption mechanism. The increase in sorption depends on the surface properties and the chemical characteristics of the Cu2+ . There are two sources which may contribute to an increase in the hydrogen ion
3.8.1. Langmuir isotherm The Langmuir sorption isotherm has been successfully applied to many pollutants sorption processes and has been the most widely used sorption isotherm for the sorption of a solute from a liquid solution [35]. A basic assumption of the Langmuir theory is that sorption takes place at specific homogeneous sites within the sorbent. It is then assumed that once a metal ion occupies a site, no further sorption can take place at that site [35]. Application of Langmuir model indicates a straight line and represents the data reported in Table 5. 3.8.2. Freundlich isotherm In 1906, Freundlich studied the sorption of a material onto animal charcoal [36]. The Freundlich isotherm theory is the first known relationship explains the sorption process. It describes the ratio of the amount of solute adsorbed onto a given mass of sorbent to the concentration of the solute in the solution is not constant at different concentrations. Freundlich isotherm equation is applicable to the sorption on heterogeneous surfaces with interaction between adsorbed molecules and it can be employed to describe the heterogeneous systems. Where Kf is the Freundlich constant, nf is the heterogeneity factor represents the deviation from linearity
712
A. El-Sikaily et al. / Chemical Engineering Journal 168 (2011) 707–714
Table 4 Isotherms and their linear forms. Isotherm
Formula
Freundllich Langmuir Redlich–Peterson
Linear form
Plot
qe =
Kf Ce1/nf
log(qe ) = log(Kf ) +
qe =
qm Ka Ce 1+Ka Ce
Ce qe
qe =
ACe g 1+BCe
ln
=
1 Ka qm
A Cqee
+
1 qm
1 nf
log(Ce )
log(qe ) vs. log(Ce ) Ce qe
Ce
vs. Ce
ln A Cqee − 1 vs. ln(Ce )
− 1 = g ln(Ce ) + ln(B)
Ce : equilibrium solute concentration (g/L) qe : amount of Cu(II) sorbed at equilibrium (mg/g); Kf : Freundlich isotherm constant (mg1−1/n g1/n L1/n ); nf : Freundlich exponent; Qm : maximum sorption capacity (mg/g); Ka : Langmuir constant (L/mg).
of sorption as follows: if the value of nf = 1, the sorption is linear; nf < 1, the sorption process is chemical; if nf > 1, the sorption is a favorable physical process (Table 5).
fit. RMSE can be defined as:
m 1 RMSE = (Qi − qi )2 m−2
3.9. Error analysis In the single component isotherm studies, the optimization procedure requires an error function to be defined in order to be able to evaluate the fit of the equation to the experimental data [34] by using SPSS computer program version 15. A part from the correlation coefficient (r2 ), the residual root mean square error (RMSE) and the Chi-square test were also used to measure the goodness-of
Table 5 Isotherm constants for copper sorption onto red alga and its activated carbon. Method
Model parameter
Langmuir
Linear method
Freundlich
Redlich–Peterson
Langmuir Non-linear method* Freundlich *
SPSS computer program (version 15).
Red alga
CRA
Qmax Ka r2 RMSE X2 Kf nf r2 RMSE X2 A B g r2 RMSE X2
49.50 0.042 0.991 0.392 0.022 2.838 1.435 0.987 0.594 0.056 3.980 0.555 0.504 0.998 0.410 0.316
60.60 1.000 0.991 24.40 33.30 12.69 1.495 0.993 1.340 0.184 16.92 0.304 0.962 0.998 0.520 0.043
Qmax Ka r2 Kf nf r2
40.56 0.437 0.994 2.110 1.789 0.990
56.12 0.963 0.989 12.16 1.589 0.980
where Qi is the observation from the batch experiment, qi is the estimate from the isotherm for corresponding Qi and m is the number of observations in the experimental isotherm. The smaller RMSE value indicates the better curve fitting [38]. The Chi-square test can be defined as X2 =
m (Qi − qi )2 i=1
(15)
qi
If data from model are similar to experimental data, X2 will be small number [40]. Among the three parameter models, high correlation coefficients and low RMSE and Chi-square values were observed for red alga (0.2 g and 0.5 g) with Langmuir and Redlich–Peterson models, Table 5. This indicates that in case of red alga R–P model the equation approaches the ideal Langmuir condition. When the g-values → 1. The main characteristic of the Langmuir equation is based on the assumption that all sites have equal sorption energies (homogeneous). If the metal sorption is based on ion exchange with a protonated amino group and a bond by sharing the lone pair on the nitrogen, then this is a reasonable description for a constant, equal energy of sorption and supports the extremely high correlation coefficients obtained from the Langmuir analysis [34]. On the other hand, the main model for carbon red alga (CRA) is Freundlich. R–P model also confirms the result where RMSE and Chi-square test have low values and high r2 , Table 5, and in this case g in R–P model tend to be zero and the equation becomes more Freundlich or heterogeneous sorption (Figs. 6 and 7). It is assumed that the stronger binding sites are occupied first and that the binding strength decreases with the increasing degree of site occupation. 3.10. Field study In this section we are reported the effects of salinity and of a real waste water on the ability of the red alga and its activated carbon on 25 20
qe, mg/g
3.8.3. Redlich–Peterson isotherm (R–P) The Redlich–Peterson isotherm [34] contains three parameters and incorporates the combination of Langmuir and Freundlich isotherms. The (R–P) isotherm has a linear dependence on concentration in the numerator and an exponential function in the denominator [37]. It has three isotherm constants A, B, and g which can be evaluated from linear plot by using a trial and error procedure [38]. The Redlich–Peterson isotherm constants can be predicted from the plot between ln[ACe /qe ) − 1] vs. ln(Ce ). However, this is not possible as the linearized form because it contains three unknown parameters A, B, and g. Therefore, a minimization procedure is adopted to maximize the coefficient of determination r2 between the theoretical data for qe predicted from linearized form of R–P isotherm equation and the experimental data [39]. SPSS computer program version 15 was used to calculate isotherm constants and their corresponding r2 values shown in Table 5.
(14)
i=1
15 exp. 10
Langmuir Freundlich
5
R-P
0 0
5
10
15
20
25
30
Ce, mg/L Fig. 6. Isotherms of copper sorbet on dried red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
A. El-Sikaily et al. / Chemical Engineering Journal 168 (2011) 707–714 Table 6 Comparison of copper(II) sorption by different seaweeds.
30 25
qe, mg/g
20 15
exp. Freundlich
10
R-P Langmuir
5 0 0
2
4
6
8
10
12
14
Sorbent
Cu (mg/g)
References
Saragassum filipendula Saragassum fluitans Saragassum vulgare Ulva reticulata Padina species Ascophyllum nodosum Durvillaea potatorum Ulva lactuca Pterocladia capillacea Carbon of Pterocladia capillacea
38.00 51.00 59.00 54.70 23.20 28.71 37.70 24.50 49.50 60.00
Volesky et al. [41] Davis et al. [42] Davis et al. [42] Vijayaraghavan et al. [43] Kaewsarn [4] Chong and Volesky [7] Matheickal and Yu [9] Abdelwahab [15] This study This study
16
Ce, mg/L Fig. 7. Isotherms of copper sorbet on activated carbon of dried red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
(NSW)R%
were suitable material for removal of Cu(II) from different types of aqueous solutions including real waste water. Comparison of biosorption of Cu(II) from aqueous solution by red alga in the present study with different seaweeds indicates that red alga is an effective adsorbent for removal of Cu(II) as reported in Table 6.
(SSW)R%
120
R% of Copper (pH=5)
713
(WW)R%
100 80 60 40 20 0 0
20
40
60
80
100
120
140
Time Fig. 8. Percentage removal of Cu(II) from synthetic sea water, natural sea water, waste water by dried red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
the sorption of Cu(II). A removal studies were achieved using synthetic sea water (SSW), natural sea water (NSW) and waste water (WW). Figs. 8 and 9 show that the removal percentage of Cu(II) from SSW, NSW, and WW were 89.75, 90.97 and 90.41%, respectively, for red alga and were 93.03, 94.30, and 95.83%, respectively, for activated carbon from red alga. These proved that the presence of salt had no effect on the sorption of Cu(II) on both adsorbents, which indicated that there was no interaction between salts and the surface of adsorbent. Also, the high concentration of Cu(II) makes them more applicable to adsorb by the adsorbents. These results indicate that the two sorbents dried red alga and its activated carbon
4. Conclusion The present work evaluated the removal of Cu(II) from aqueous solution using red alga (P. capillacea) and its activated carbon. The Cu(II) removal from aqueous solutions through red alga biomass may be considered as an environmentally friendly and economic treatment. The red alga biomass may be considered as an effective and inexpensive adsorbent for the removal of Cu(II) ions from aqueous solutions. The sorption process is a function of the adsorbent concentration, adsorbate concentration, and pH. The highest Cu(II) sorption onto red alga and carbon red alga was obtained at pH (5.0). The experimental results were analyzed by using Langmuir, Freundlich and Redlich–Peterson isotherm equations. The correlation coefficients (r2 ), RMSE, and Chi square test for fitting the Langmuir and Redlich–Peterson equations were significantly better than correlation coefficient, RMSE, and Chi-square test for Freundlich equation of red alga biomass. On the other hand, Freundlich and R–P equations and the RMSE, Chi-square test was significantly better than Langmuir for carbon red alga. The kinetics of sorption of Cu(II) onto red alga and carbon red alga followed the ion exchange rate model of Boyd, which considers rates of ion exchange sorption during the beginning of sorption. The order of the reaction for the sorption of Cu(II) onto red alga and carbon red alga followed a pseudo-second-order rate expression. Finally, the results showed that red algae and its carbon can be used for the removal of Cu(II) from different aqueous solutions. Acknowledgement
Carbon Red Algae
120
(NSW)R%
R% of Copper (pH=5)
(SSW)R% (WW)R%
100
The authors are gratefully acknowledges Dr. Abeer Abdelwahab of Mubarak city for Scientific Research & Technology Applications, for making IR, X-ray and scanning electron microscope analysis.
80
References 60 40
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0 0
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120
140
Time Fig. 9. Percentage removal of Cu(II) from synthetic sea water, natural sea water, waste water by activated carbon of red alga Pterocladia capillacea (temp. 25 ± 2 ◦ C, agitation rate 150 rpm, contact time 120 min, pH 5.0).
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