Chemosphere 111 (2014) 587–595

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Dynamics and thermodynamics of toxic metals adsorption onto soil-extracted humic acid Medhat A. Shaker a,b,⇑, Hassan M. albishri c a

Chemistry Department, Faculty of Science – North Jeddah, King Abdulaziz University, Jeddah, Saudi Arabia Chemistry Department, Faculty of Science, Damanhour University, Damanhour, Egypt c Chemistry Department, Faculty of Science, King Abdulaziz University, Saudi Arabia b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Dynamics and thermodynamics of

Cr6+ and Cd2+ adsorption on humic acid are studied.  The adsorption data are well modeled using the second-order kinetic equations. 6+ 2+  The adsorption of Cr and Cd on HA was physical diffusion controlled reaction.  The experimental results well fit the Langmuir model.  The thermodynamics show endothermic, favorable and spontaneous sorption processes.

a r t i c l e

i n f o

Article history: Received 7 December 2013 Received in revised form 15 April 2014 Accepted 28 April 2014

Handling Editor: X. Cao Keywords: Adsorption Humic acid Heavy metals Dynamics Thermodynamics

a b s t r a c t Humic acids, HA represent a large portion of natural organic matter in soils, sediments and waters. They are environmentally important materials due to their extensive ubiquity and strong complexation ability, which can influence heavy metal removal and transportation in waters. The thermodynamics and kinetics of the adsorption of CdII and CrVI onto solid soil-derived HA have been investigated at optimum conditions of pH (5.5 ± 0.1), metal concentration (10–100 mmol L1) and different temperatures (293– 323 K). The suitability of adsorption models such as Freundlich and Langmuir to equilibrium data was investigated. The adsorption was well described by Langmuir isotherm model in multi-detectable steps. Adsorption sites, i (i = A, B, C) with different capacities, mi are characterized. The stoichiometric site capacity is independent of temperature and equilibrium constant, Ki. Adsorption sites A and B are selectively occupied by CrVI cations while sites A and C are selectively occupied by CdII cations. The thermodynamic parameters of adsorption systems are correlated for each adsorption step. The adsorption is endothermic, spontaneous and favorable. Different kinetic models are applied and the adsorption of these heavy metals onto HA follows pseudo-second-order kinetics and equilibrium is achieved within 24 h. The adsorption reaction is controlled by diffusion processes and the type of the adsorption is physical. Ó 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Current address: Chemistry Department, Faculty of Science – North Jeddah, King Abdulaziz University, Jeddah, Saudi Arabia. Permanent address: Chemistry Department, Faculty of Science, Damanhour University, Damanhour, Egypt. Tel.: +966 553471259, +20 35542422; fax: +966 26400376. E-mail address: [email protected] (M.A. Shaker). http://dx.doi.org/10.1016/j.chemosphere.2014.04.088 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Heavy metals are natural components of the Earth’s crust. They are relatively high density persistent and poisonous metals that cannot be biodegraded but can be accumulated by aquatic

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organisms. The inappropriate discharging of heavy metals into the ecosystem has resulted in potential threats to human health and environment. Toxic heavy metals may enter and accumulate in the human body through inhalation of dust, polluted drinking water and consumption of contaminated food crops (Ali et al., 2004; Angelova et al., 2004; Zhuang et al., 2009; Argun et al., 2009; Chen et al., 2012; Balistrieri and Mebane, 2014; Pietrzykowski et al., 2014). Lead, mercury, chromium, arsenic, cadmium, zinc, copper and nickel are the most common contaminants found in contaminated surface water and groundwater as well as industrial wastewater. Cadmium has been identified as a human carcinogen and teratogen substance severely impacting lungs, kidneys, liver and reproductive organs (Kerfoot and Jacobs, 1976; Nies, 2003; Banjerdkij et al., 2005; Kermani et al., 2010). Chromium is usually found as trivalent or hexavalent ion (Wang et al., 2011). The trivalent chromium is an essential trace element for human nutrition while the hexavalent chromium is toxic and carcinogenic even at low concentrations because of its high charge density (Cho et al., 2011). The World Health Organization (WHO), U.S. Environmental Protection Agency (USEPA) and many government environmental protection agencies have set the Maximum Contaminant Levels (MCLs) for the heavy metals in drinking water as well as trade effluent. As heavy metals are non-biodegradable, clean-up of contaminated water and soil is rather challenging. It is greatly urgent to develop cost-effective technologies that can effectively remove them from contaminated water and soil. Early research conducted in laboratory studies had demonstrated that biosorption was a promising, eco-friendly and cost-effective technology for the removal of heavy metals from aqueous solutions. Compared with such conventional methods (Pansini, 1996; Bailey et al., 1999; Porter et al., 2004; Gardea-Torresdey et al., 2004) as chemical precipitation, reverse osmosis, ion-exchange, filtration and evaporative recovery, biosorption technology possesses several advantages such as low operating cost, high efficiency in detoxifying heavy metals that have lower concentrations and less amount of spent biosorbent for final disposal (Sheng et al., 2007). Recently, the removal of hazardous heavy metals from water and soil environments and industrial waste streams has attracted considerable attention. A wide variety of adsorbents have been employed to sequester heavy metal ions from industrial effluents (Bhainsa and D’Souza, 2001; Wang, 2002; Huessien et al., 2004, 2007; Shaker and Huessien, 2005; Ghabbour et al., 2006a, 2006b; Shaker, 2007). Humic acids (HA), Fig. 1 are soluble in alkaline solutions but insoluble in acidic solutions. They are widely present in natural waters and soils and represent a key to separating heavy metals from their environments (Davies et al., 2003; Kalin et al., 2005; Wang and Chen, 2006; Ghabbour et al., 2006a, 2006b; Volesky, 2007; Nadeem et al., 2009). HA are organic polyelectrolytes having various functional groups such as carboxylic, phenolic, and hydroxyl groups, and some functional groups containing nitrogen, sulfur, and phosphorus which have shown significant effect on the sorption of toxic metal ions. As a continuum research, this work is aimed at investigating the adsorption thermodynamic and kinetics

of Cr(VI) and Cd(II) onto solid soil derived humic acids at optimum experimental conditions to focus on the environmental impact and significant role of humic acids in controlling removal, mobility and bioavailability of metal ions in the environment. 2. Materials and methods 2.1. Chemicals and reagents Analytical grade chemicals and reagents were purchased from Fisher Scientific Inc. The CdCl2 and K2Cr2O7 salts were dissolved in deionized distilled water to prepare different metal concentrations (0–100 mmol L1) for each metal. All glassware was washed with HCl acid (0.1 M) before and after each experiment to avoid the binding to metals. The pH was adjusted by addition of appropriate amounts of concentrated nitric acid or sodium hydroxide using an Orion pH meter model 420A fitted with an Orion combined glass electrode and calibrated according to conventional methods. 2.2. Batch adsorption procedure The adsorption experiments, for simplicity were performed as single component batch experiments in continuously stirred conical flasks containing a solution of Cr(VI) or Cd(II). In the present study, four parameters, viz., pH (1.0–7.0), initial metal concentration (0–100 mmol L1), temperature (293–313 K) and contact time (2–24 h) were varied. The adsorption experiments were conducted under optimum conditions of pH (5.5 ± 0.1). Various 50 mL of solutions with initial concentrations (0–100 mmol L1) of Cr(VI) or Cd(II) were prepared and then 50 mg of solid freeze-dried HA was added to each solution and shaken continuously at a shaking rate of 100 rpm, because above this value, the agitation has a little effect on the adsorption process (Mellah et al., 1992; Barkat et al., 2009). At the equilibrium point the suspensions were centrifuged to separate the solid metal-bound products and the supernatants were immediately measured using an inductively coupled plasma spectroscopy (ICP, Varian Model Liberty 150 AX Turbo). The differences between the initial and equilibrium Cr(VI) or Cd(II) concentrations determine the amount of metal ion adsorbed by HA using a standard mass balance equation (Das et al., 2012). At the equilibrium point the amount of metal remaining in solution became time invariant. The equilibrium data was analyzed using the two parameter isotherms; Langmuir and Freundlich models. The effect of the initial Cr(VI) or Cd(II) concentration on the adsorption rate was studied by shaking 50 mg aliquots of HA at 298 K and pH 5.5 using different initial metal concentrations (10, 15, 20 and 25 mmol L1). 2.3. Statistical design of experiments Factorial design is employed in this study to reduce the total number of experiments and achieve the best overall optimization

Fig. 1. (a) The building block of humic acid biopolymer and (b) a symbolic humic acid biopolymer.

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of the adsorption system (Montgomery, 2001). For studying the Cr(VI) and Cd(II) biosorption on solid HA, the amount of adsorbate could be depend on acidity of the medium (pH), initial Cr(VI) or Cd(II) concentration, temperature and time of contact if the speed of agitation was kept constant. For data treatment, the Minitab Statistical Software release 14.1 was employed throughout in order to obtain the effects, coefficients, standard deviation of coefficients and other statistical parameters of the fitted models. 2.4. Preparation of HA adsorbent The soil sample was taken from freshly tilled farm in Abbis, Alexandria, Egypt. This soil was rich in organic matter such as peat and muck and classified as HISTOSOLS according to the USDA soil taxonomy. HA was extracted, purified and identified by the same our method as reported before (Ghabbour et al., 2006a, 2006b). The soil sample was air dried in a vacuum hood and then was Soxhlet pre-extracted with 2:1 (v/v) benzene-methanol solution to remove non-humic organic substances. Extraction was continued until the extracting solvent had negligible absorbance. The residue was removed and allowed to air dry in a vacuum hood overnight and then was washed many times with HCl acid (0.1 M) by stirring each time for 4 h, allowing to settle overnight and decanting the supernatant to remove most of the metals bound to the residue. The same washing protocol was performed 3 times using de-ionized water. The resulting sample was centrifuged and the residue was shaken with NaOH (0.1 M) for 24 h in polypropylene flasks. After centrifugation, HA was precipitated as an aqueous gel by reducing the supernatant pH to 1.0 with concentrated HCl. The base extraction/acid precipitation cycle was repeated until no more HA gel could be precipitated. The collected gel was washed five times by centrifugation with Khan’s solution (5 cm3 48% HF + 5 cm3 concentrated HCl/L) that removes the more tightly bound metals and minerals from the humic acid gels. The HA gel sample was then washed several times with de-ionized water until the supernatant was approximately of pH = 3. It was then dialyzed against water for four days. The resulting metal-free solid-derived HA was washed with deionized water several times and thereafter was further freeze-dried at 0 °C and reduced pressure and was used as a biosorbent. The acidities of carboxylic and phenolic groups in the extracted HA were determined quantitatively by using potentiometric titration method. 50 mL suspension of humic acid (550 mg L1) was titrated against 0.1 M NaOH from pH 3.0 to 11.00. Nitrogen gas was passed through the solution during titration in order to prevent CO2 interference. The value of the total acidity was taken as the sum of the acidities of both carboxylic and phenolic groups. The ash content was determined by combustion of 100.0 mg of extracted HA samples in air at 850 °C for 2 h. 2.5. The models for adsorption isotherms 2.5.1. Langmuir model At pH 5.5 and temperature (293–313 K), the relationship between the amount (A, mmol) of Cr(VI) or Cd(II) ions adsorbed on HA surface and the metal concentration (c, mmol L1) is called the adsorption isotherm. Langmuir (Langmuir, 1916) model was the first adsorption isotherm with a theoretical basis and assumed that the adsorbent surface is uniform with limited adsorption sites, only a monolayer is formed during adsorption and adsorbate molecules occupy separate surface sites, do not interact with each other and are equal in affinity. The Langmuir isotherm equation, Eq. (1) may be inferred as follows:

Ai ¼

K i mi c 1 þ Kic

ð1Þ

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where Ai (mmol metal g1 HA) is the amount of Cr(VI) or Cd(II) adsorbed onto 1 g of HA in binding step i, c (mmol L1) is the equilibrium concentration of Cr(VI) or Cd(II), Ki is the conditional adsorption equilibrium constant and mi (mmol metal g1 HA) is the stoichiometric saturated monolayer adsorption capacity of the adsorption site, i. When the adsorption data converge to horizontal plateaus, each plateau corresponds to the formation of a monolayer of adsorbate on the HA surface. Adsorption steps in an isotherm can be distinguished when a linear form of Langmuir isotherm is made, Eq. (2). The applicability of the Langmuir isotherm to the obtained data can also be examined from the linearity of the plots of 1/A vs. 1/c, where the values of Ki and mi for each adsorption step can be calculated from the intercept and slope of the line segments in these linear plots of Eq. (2).

1 1 1 ¼ þ Ai mi K i mi c

ð2Þ

Sequential adsorption steps indicate that metal cations initially binding to a particular site have reached maximum capacity and that metal binding has moved to a second distinct site on HA surface. In order to confirm the number of steps, another linear form of Langmuir isotherm, Eq. (3) is made.

c c 1 ¼ þ Ai mi K i mi

ð3Þ

when plots of c/A vs. c are made, the changes of slope in the plot easily allow the number of steps for metal binding to be distinguished. 2.5.2. Freundlich model Freundlich (Freundlich, 1906) was the first to propose an empirical formula describing the adsorption isotherm. This model is often used for adsorbents with an irregular surface or single solute systems within a specific concentration range and it assumes that different adsorption sites exist on the solid phase surface with different adsorption energy. The Freundlich equation, Eq. (4) is expressed as follows:

A ¼ Kc1=n

ð4Þ 1

where A (mmol g ) is the amount of Cr(VI) or Cd(II) ions adsorbed per 1 g of HA adsorbent, c (mmol L1) is the equilibrium Cr(VI) or Cd(II) concentration, while K and n are constants. K is a function of energy of adsorption and temperature and is a measure of adsorptive capacity and n determines intensity of adsorption. The linear form of Freundlich isotherm is shown in Eq. (5).

log A ¼ 1=n log c þ log K

ð5Þ

The plots of log A vs. log c is made to test the Freundlich adsorption model and to determine the related adsorption constants, n and K. The derived n value measures the adsorbent–adsorbate bond strength and can help determine whether adsorption is favored. The value n > 1 describes favorable adsorption, whereas n = 1 characterizes linear adsorption and n < 1 describes situations unfavorable to adsorption. 2.6. Adsorption kinetics experiments For modeling the kinetic data of Cr(VI) or Cd(II) biosorption, pseudo-first order and pseudo-second order models have been used. The error values were estimated using APE% analysis (Das et al., 2010). Kinetics experiments were carried out by shaking certain amounts (50 mg) of HA adsorbent with 100 mL of 200, 250, 300 or 350 mg L1 solutions containing Cr(VI) separately at the desired temperature (293–313 K) and pH of 5.5. At pre-determined time intervals, portions of the mixture were drawn by a syringe, centrifuged and then the Cr(VI) concentration was determined as

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described before by ICP. The amount adsorbed, qt (mg g1) of Cr(VI) onto HA at time, t (min) was calculated by Eq. (6) as follows:

qt ¼

ðC o  C f ÞV W

ð6Þ

where Co and Cf are the concentration (mg L1) of Cr(VI) ion in the initial and final solutions, V (mL) is the volume of the solution, and w (g) is the weight of HA adsorbent. At equilibrium qt = q and the amount of metal ion adsorbed at equilibrium is calculated using Eq. (6). All the adsorption experiments were run in duplicate. The difference in results for the duplicates was typically less than 3%. The percent adsorption (%) was calculated from Eq. (7) as follows:

% adsorption ¼

Co  Cf  100 Co

ð7Þ

3. Results and discussion 3.1. Adsorption systems The amount of adsorbent and adsorbate, equilibration time, pH, initial concentration and temperature play significant roles in the adsorption of trace metal ions onto solid surfaces (Benes and Majar, 1980; Huessien et al., 2004; Shaker and Huessien, 2005; Ghabbour et al., 2006a, 2006b). The influence of the acidity of the solution, pH on the Cr(VI) and Cd(II) adsorption onto HA is investigated within pH range (1.0–7.0), Fig. 2a. It is found that the percentage of adsorption increases with increasing pH to a maximum value (pH 5.5 ± 0.1) and then declines rather rapidly with further pH increase. This optimum pH value (5.5 ± 0.1) is suitable to prevent hydrolysis, polymerization and precipitation of metal ions, to neutralize anionic HA functional groups and to minimize the double layer effects. The electrostatic interactions between cationic species and the negatively charged HA functional groups are responsible for metal binding (Nadeem et al., 2009). The mechanism of the metal uptake-pH dependence might be related to the nature of the surface binding sites and to the solution chemistry of metals in water. At low pH, surface binding sites were closely associated with the hydronium ions H3O+ and restricted the approach of metal cations as a result of the repulsive force. As the pH increased, more sites would be exposed and carried negative charges, with subsequent attraction of metallic ions with positive charge and biosorption onto the surface (Davies et al., 2003). The adsorption decreases at higher pH may be due to the occurrence of metal precipitation. The HA surface contains mainly carboxyl functional groups (Kalin et al., 2005; Wang and Chen, 2006) so that the binding sites are negatively charged under acidic pH conditions with a high affinity for metal ions in solution (Davies et al., 2003; Wang and Chen, 2006; Volesky, 2007). The effect of the contact time on the adsorption of Cr(VI) and Cd(II) onto HA surface was investigated and shown in Fig. 2b.

The adsorption efficiency increases rapidly where over 80% of the Cr(VI) and Cd(II) cations are adsorbed during the first 8 h and then becomes constant. The adsorption equilibrium was established after 12 h. The effect of the initial Cr(VI) and Cd(II) ion concentrations on their percentage of adsorption onto HA surface was investigated and shown in Fig. 2c which shows that the percentage of metal adsorption increased with increasing its initial concentration. The adsorption is fast at 8 h and approach equilibrium after 12 h.

3.2. Adsorption isotherms 3.2.1. Freundlich adsorption model The relationship between the amount (A, mmol) of Cr(VI) and Cd(II) cations per 1 g of HA and their solution concentrations, c (M) is represented as isotherm plots, Fig. 3a and b in the temperature range (293–313 K). When the adsorption results are modeled by the Freundlich isotherm, the adsorption parameters presented a poor linear fit for the adsorption of Cr(VI) and Cd(II) by solid soil derived HA. A representative plot of log A vs. log c for the adsorption of Cr(VI) cations on solid HA surface is illustrated in Fig. 3c which suggests that unfavorable adsorption situation (n < 1). Moreover, the low value of R2 values reveal that the Freundlich isotherm model is not suitable for describing the adsorption behavior in this study.

3.2.2. Langmuir adsorption model Linear plots of Langmuir models Eqs. (2) and (3) and Fig. 3d and e (representative figures) indicated multi-binding steps, i where i = A, B and C. In these plots, linear segments of positive intercepts with excellent R2 values are observed which are indicative of the number of steps involved in the binding process (Bhainsa and D’Souza, 2001; Wang, 2002; Shaker and Huessien, 2005; Ghabbour et al., 2006a, 2006b). The examination of the adsorption data revealed three different binding sites labeled A, B and C whose capacities (vA, vB and vC) are (0.22 ± 0.01), (0.31 ± 0.01) and (0.40 ± 0.01) mmol g1, respectively. If the calculated site capacity (vi) of site i for the investigated systems approaches one of those values, this means that the given metal cations bind selectively to this site. Therefore, according to the values of site capacities for the investigated Cr(VI)-HA and Cd(II)-HA systems, it is concluded that Cr(VI) cations bind to sites A and B with vA = 0.22 and vB = 0.31 mmol g1, respectively while Cd(II) cations bind to sites A and C with vA = 0.22 and vC = 0.40 mmol g1, respectively. The trends of the site capacities to increase on going from step A to B to C and of the equilibrium constants to decrease from step A to B to C are seen for all systems, Table 1. The essential characteristics of the Langmuir isotherms may be expressed in terms of dimensionless separation factor, RL as shown in Eq. (8) which is defined in terms of the equilibrium constant, and initial

Fig. 2. (a) Effect of pH; (b) effect of contact time and (c) effect of initial concentration of metal on the adsorption efficiency onto 50 mg of HA at 25 °C.

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Fig. 3. (a) The isotherms for Cr(VI) adsorption onto HA. (b) the isotherms for Cd(II) adsorption onto HA. (c) a linearized Freundlich equation for Cr(VI) adsorption onto HA at 293 K. (d) a linearized Langmuir equation for Cd(II) adsorption onto HA at 293 K. (e) a plot of c/A vs. 1/c for Cd(II) adsorption onto HA at 293 K.

concentration, Co (Hayward and Trapnell, 1964; Thomson and Webb, 1968; Webber and Chakravarti, 1974).

RL ¼

1 1 þ KC o

ð8Þ

The RL value indicates the adsorption nature to be unfavorable if RL > 1, linear if RL = 1, favourable if 0 < RL < 1 and irreversible if RL = 0. All calculated RL values for the investigated sorption systems were found to be from 0.91 to 0.13 for the initial concentrations range 1.0  101–1.0  101 M of metal cations, indicating that the equilibrium sorption of Cr(VI) and Cd(II) cations by HA was favorable. Also, the R2 values (1.0) proving that the sorption data fitted well to Langmuir isotherm model and suggest the homogeneous monolayer sorption. This suggested homogeneous monolayer sorption and those adsorption systems were better described by

the Langmuir model. The binding reaction is stoichiometric. Eq. (9) corresponds to inner-sphere ligand exchange with the release of aquo ligands.  MðH2 OÞnþ m þ nRCOOðsÞ $ ½MðH2 OÞmn ðRCOOÞn ðsÞ þ n H2 O

ð9Þ

Since HA has many different organic functional groups but carboxylic groups are predominant, we may conclude that in Eq. (8), at least two carboxylic groups of HA are involved in the adsorption process by ligand exchange and charge neutralization. 3.3. Thermodynamic studies The experiments were carried out at 293, 298, 303, 308 and 313 K. The thermodynamic parameters obtained for the sorption processes were calculated using Eq. (10).

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Table 1 Adsorption isotherm parameters and thermodynamic data at optimum pH values and different temperatures.. T (K)

Cd(II) Calculations from Eq. (2)

Calculations from Eq. (2)

Calculations from Eq. (3)

Site A

Site A

Site A

mA 293 298 303 308 313 Average mi DH (kJ mol1) DG (kJ mol1) DS (J K1) The site capacity,

ln K i ¼

Cr(VI)

Site C KA

0.23 67.9 0.21 78.2 0.22 149.7 0.21 179.2 0.22 216.2 0.22 48.1 ± 0.1 12.2 ± 0.1 198.8 ± 0.1

mC

KC

0.41 10.6 0.42 12.0 0.38 21.7 0.39 28.4 0.41 34.9 0.40 49.5 ± 0.1 7.5 ± 0.1 188.2 ± 0.1

Site C

Calculations from Eq. (3)

Site B

mA

KA

mC

KC

mA

0.21 0.21 0.22 0.23 0.23 0.22

68.0 79.5 151.0 183.1 217.5

0.43 0.40 0.39 0.39 0.41 0.40

10.1 11.4 23.3 28.4 33.7

0.22 67.4 0.22 79.8 0.22 111.3 0.22 137.8 0.21 182.1 0.22 38.6 ± 0.1 11.8 ± 0.1 166.5 ± 0.1

KA

mB

Site A KB

0.29 23.5 0.29 28.3 0.32 31.4 0.32 39.3 0.31 51.9 0.31 29.1 ± 0.1 8.9 ± 0.1 125.3 ± 0.1

Site B

mA

KA

mB

KB

0.19 0.22 0.23 0.24 0.20 0.22

65.8 82.0 109.8 138.3 184.7.8

0.31 0.31 0.31 0.30 0.32 0.31

23.1 26.9 32.4 41.4 52.8

vi, in mmol g1, the equilibrium constant, Ki, in L mmol1.

DSoads DHoads  R RT

ð10Þ

where Ki is the conditional equilibrium constant of adsorption step i, DSoads is its standard entropy change (J mol1 K1), DHoads is its standard enthalpy change (kJ mol1), T is the absolute temperature (K), and R is the gas constant (8.314 J mol1 K1). The values of DHoads and DSoads were calculated from the slopes and intercepts of linear regression of ln K i vs. 1/T (Fig. 4a). All of the points lying on a straight line in a ln Ki vs. 1/T plot, Eq. (10) can be taken to mean that a metal cation binds to the same average site i in solid HA over the experimental temperature range. The standard Gibbs free energy change for the adsorption step, i, DGoads (kJ mol1) can be calculated from Eq. (11).

DGoads ¼ DHoads  T DSoads

ð11Þ

Linear plots in Fig. 4a indicate that all metal cations bind to one average site in solid HA. The enthalpy, DHoads , free energy, DGoads and entropy changes, DSoads for the adsorption of Cr(VI) and Cd(II) cations onto solid HA for each characterized site are recorded in Table 1, which were found to vary with the identity of the metal cation. The values of DHoads , DGoads and DSoads were found to be 38.6 ± 0.1 (kJ mol1), 11.8 ± 0.1 (kJ K1 mol1) and 166.5 ± 1 (J K1), respectively for step (A) and 29.1 ± 0.1 (kJ mol1), -8.9 ± 0.1 (kJ mol1) and 125.3 ± 1 (J K1) for step (B) in the adsorption of Cr(VI) on HA surface while those values were found to be 48.1 ± 0.1 (kJ mol1), 12.2 ± 0.1 (kJ mol1) and 198.8 ± 1 (J K1) for step (A) and 49.5 ± 0.1 (kJ mol1), 7.5 ± 0.1 (kJ mol1) and 188.2 ± 1 (J K1) for step (C) in the adsorption of Cd(II) on HA surface. It is clear that the positive values of the enthalpy changes for the adsorption processes confirms the endothermic nature of these adsorption processes. This can be noticed from the increase of the

conditional equilibrium constants (KA, KB, KC) in Table 1 with increasing temperature. The Cr(VI) and Cd(II) ions have to displace more than one water molecule for their adsorption and this results in the endothermicity of the adsorption processes. The positive values of DSoads suggest increased randomness at the solid/solution interface with some structural changes in the Cr(VI) and Cd(II) cations and HA adsorbent and thus an affinity of HA biosorbent towards metal ions (Aksu, 2001). DGoads values were negative indicating the feasibility and spontaneity of the adsorption processes. Given that the metal ions are binding to the same sites, then it must be some property of the cations that is responsible for the order of reactivity, the importance of DS and the correlation between DH and DS. Surely the most likely explanation is that the difference between the cations is due to the different amounts of water that are released from the metal ion hydration shells when the metal ions are bound to the surface. The increase in disorder caused by the release of these waters would explain the observed behavior, since the degree of order imposed on the aqueous solution by each cation is a function of its charge density. This even explains the correlation of DH and DS, since for metal cations in aqueous solution, the partial molar entropies are directly correlated with the enthalpies of hydration, and so we would expect the reaction to become more endothermic as DS increased. The same behavior at all five temperatures supports use of the Langmuir model to describe adsorption of the investigated metal cations to solid HA. The straight line in Fig. 4b has slope T = 293.4 K and intercept DG = 6.8 kJ mol1. This linearity confirms the appropriateness of the Langmuir model and strengthens the conclusion of correlated thermodynamic parameters for metal binding by solid HA. The calculated DG = 6.8 kJ mol1 means favorable but mildly exergonic metal binding and indicates that solid HA acts as free energy buffer on binding to metals.

Fig. 4. (a) Effect of temperature on the equilibrium constants for the two sites and (b) correlation of enthalpy and entropy changes for all investigated systems.

M.A. Shaker, H.M. albishri / Chemosphere 111 (2014) 587–595

3.4. Adsorption dynamics It is generally believed that HA is a good complexing agent for many metal ions and its binding to metal ions can improve the adsorption of heavy metal ions. In order to investigate the reaction order, rate constant, adsorption mechanism and potential rate controlling step, different kinetic models should be applied to experimental kinetic data obtained from the investigation of

593

adsorption kinetic of Cr(VI) onto HA in the temperature range (293–323 K). At first, it is assumed that HA particles are spherical and only surface adsorption is occurring and that the effect of external film diffusion on adsorption rate is not significant since the experimental data are obtained in a well-agitated batch system and the measured concentrations are equal to HA surface concentrations (Aksu, 2001). The experimental kinetic data can be modeled by several models such as the pseudo first-order Lagergren

Fig. 5. (a) The plot of pseudo-first-order equation for different initial concentrations of Cr(VI). (b) the plot of pseudo-second-order equation for different initial concentrations of Cr(VI). (c) effect of the temperature on the kinetics of Cr(VI) adsorption onto solid HA (Co = 200 mg L and pH = 5.5) and (d) effect of temperature on k2ads for the Cr(VI) adsorption on HA (Co = 200 mg L and pH = 5.5).

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Table 2 Kinetics data, values of R2 and effect of temperature on the rate constants for HA at pH = 5.5. co (mg L1)

(mg g1) qexp e Pseudo-1st order

Pseudo-2nd order

Pseudo-1st order

Pseudo-2nd order

200 250 300 350 T (K)

0.25 0.22 0.21 0.20 k2ads  103 (g mg1 min1) 4.4 6.3 8.1 13.6

0.25 0.22 0.21 0.20

0.21 0.19 0.15 0.16

0.21 0.19 0.18 0.17

293 303 313 323

1 qcal e (mg g )

ð12Þ

ð13Þ

On the other hand the corresponding pseudo-second-order equations based on adsorption equilibrium capacity may be expressed by Eqs. (14) and (15) as follows:

dqt ¼ k2ads ðqe  qt Þ2 dt

ð14Þ

1 1 ¼ þ k2ads t ðqe  qt Þ qe

ð15Þ

where k2ads (g mg1 h1) is the rate constant of pseudo-secondorder adsorption. The kinetic parameters obtained from the kinetic models which include the values of qe, k1ads and k2ads obtained from the slopes and the intercepts of the straight lines shown in Fig. 5a and b are collected in Table 2. As evidenced by the values of the regression coefficients obtained, the two kinetic models can well simulate the experimental data. The values of the rate constant k1ads were found to increase from 1.34  102 to 2.61  103 g mg1min1, while those for k2ads were found to increase from 2.2  103 to 4.4  103 g mg1min1, with a decrease in the initial chromium concentration from 350 to 200 mg L1. 3.4.1. Statistical tool to predict optimum adsorption kinetic Adsorption kinetic data are the basic requirements for the design of adsorption systems. It is important to establish the most appropriate correlation for the experimental kinetic data. Applicability of some statistical tools to predict optimum adsorption kinetic of Cr(VI) onto HA after linear regression analysis showed that the normalized standard deviation Dq calculated using Eq. (16) could be suitable and meaningful tools to predict bestfitting equation models to the experimental data (Wu et al., 2001):

#1=2  exp 2 ðqt  qcal t Þ Dq ¼ 100  sum =ðn  1Þ qcal t

0.0261 0.0215 0.0192 0.0134

0.0044 0.0034 0.0030 0.0022

R2 Pseudo-1st order

Pseudo-2nd order

0.98 0.98 0.99 0.99

0.98 0.98 0.99 0.95

28.5 ± 2

where k1ads (h1) is the rate constant of pseudo-first-order adsorption and qe, qt (mg g1) denotes the amount of adsorption at equilibrium and at time t, respectively. The Eq. (13) is the integrated form of Eq. (12) by applying the initial conditions qt = 0 at t = 0 as follows:

ln ðqe  qt Þ ¼ ln qe  k1ads t

k2ads (g mg1 min1)

1 E– a (kJ mol )

equation and pseudo second-order rate equation to examine the controlling mechanism of the adsorption process (Glasston et al., 1941; Ruthven, 1984; Ho and McKay, 1999; Ho and Chiang, 2001). Pseudo-first order equation, Eq. (12) represents a simple kinetic analysis of adsorption as follows:

dqt ¼ k1ads ðqe  qt Þ dt

k1ads (min1)

"

ð16Þ

where n is the number of data points (n = 7). Based on the values of Dq (43 kJ mol1 while if boundary layer diffusion of the aqueous species is the rate controlling step, Ea is generally 628 kJ mol1. It is obvious form Table 2 that the values of Ea is 28.5 ± 2 kJ mol1 which indicated that the adsorption reaction is controlled by diffusion processes and the type of the adsorption of Cr(VI) onto HA is physical (Cooney, 1998; Wu et al., 2001; Ho and Chiang, 2001). 4. Conclusion The Egyptian soil used in this work to extract HA was rich in organic matter such as peat and muck and classified as HISTOSOLS according to the USDA soil taxonomy. At the test pH value, Cr(VI) cations bind to adsorption sites labeled A and B while Cd(II) cations bind to adsorption sites labeled A and C located on the solid HA surface. The binding isotherms fit the Langmuir model and the calculated positive DH and negative DG values of the adsorption indicate spontaneous endothermic nature of sorption. Adsorption DH and DG are linearly correlated for the Cr(VI) and Cd(II) cations.

M.A. Shaker, H.M. albishri / Chemosphere 111 (2014) 587–595

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