March 2002 Vol

Environmental Engineering and Management Journal

March 2017, Vol. 16, No. 3, 569-580

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

MODELING EQUILIBRIUM DATA FOR Cd (II) ADSORPTION BY PEAT USING NON-LINEAR REGRESSION ANALYSIS Catalin Balan1∗, Brindusa Robu1, Petru Bulai2, Doina Bilba1 1

“Gheorghe Asachi” Technical University of Iasi, Department of Environmental Engineering and Management, 71A D.Mangeron Blvd, 700050 Iasi, Romania 2 Department of Mechanical and Technology, "Ştefan cel Mare" University, 13, Universităţii Street, Suceava, Romania

Abstract The Romanian peat was used as low-cost adsorbent to remove Cd(II) ions from aqueous solutions. The cadmium adsorption capacity of the peat was studied as a function of the solution pH (3-5.5) and temperature (293 – 333 K). Adsorption capacity was found to increase with the pH increasing and temperature decreasing and attained a maximum value of 27.9 mg/g at pH = 5±0.1 (acetate buffer) and 293 K. In addition, various thermodynamic parameters such as free energy change (ΔG0), enthalpy change (ΔH0), entropy change (ΔS0) along with isosteric heat of sorption (ΔHx) were calculated. The adsorption of Cd (II) ions by peat was found to be a spontaneous and exothermic process, which is governed by electrostatic interactions (physisorption). FT-IR analysis of the peat before and after Cd (II) sorption suggests electrostatic attraction between cadmium cations and negatively charged –COO- groups present on the peat surface. Key words: adsorption isotherm model, non-linear regression analysis, Romanian peat Received: February, 2016; Revised final: November, 2016; Accepted: Deecember, 2016

1. Introduction The release of large amounts of wastewater containing heavy metals into natural environment is one of the most important sources of pollution of inland surface water. Contamination of aqueous environment by heavy metals has become a major worldwide concern because the dissolved metal ions are non-biodegradable, persistent and toxic to many life forms. Thus, wastewater should be pre-treated and concentration of heavy metals must be lessened to permissible limits prior to its discharge to the surface water body. One of the most toxic pollutants is cadmium. The major sources of cadmium release into the environment are the wastes from industrial processes such as metal plating, metallurgical alloying, mining, manufacture of plastics, pigments, cadmium/nickel batteries. Toxicological studies have shown that ∗

Author to whom all correspondence should be addressed: e-mail: [email protected]

acute exposure to high levels of Cd (II) can produce harmful effects on human health such as nausea, vomiting, kidney damage, changes to the constitution of the bone, liver and blade, probable cancer (Bernard, 2008). According to the Romanian legislation, the maximum concentration permissible for Cd (II) discharge in surface waters is 0.2 mg/L (NTPA, 2005) and in drinking water is 0.005 mg/L (Law 458, 2002). Treatment processes for cadmium removal from wastewater include chemical precipitation, coagulation-flotation, membrane filtration, ion exchange, adsorption, solvent extraction and electrochemical methods, but selection of the appropriate treatment method takes into account the concentration of the pollutant and the cost of the process (Altaher et al., 2015; Rao et al., 2010). Among the conventional methods used for the cadmium removal from contaminated water of

Balan et al./Environmental Engineering and Management Journal 16 (2017), 3, 569-580

relatively low concentration, adsorption is found to be superior to other techniques in terms of initial cost, simplicity of design and ease of operation. Commercially available activated carbon is the most used adsorbent, although is expensive and not easily regenerated. In the past two decades, considerable attention has been focused on the removal of heavy metals from aqueous solutions using adsorbents derived from non-conventional and low-cost materials (agricultural or industrial wastes, clay minerals, natural zeolites, peat, biomass etc) (Ali, 2010; Altaher et al., 2015; Babel and Kurniawan, 2003; Gupta et al., 2009; Ponou et al., 2016). The major benefits of the adsorption using such materials as adsorbents are low operating costs, high efficiency in detoxifying dilute effluents, minimization of sludge, easy regeneration and possibility of metal recovery. Within this context, several studies have highlighted the use of the peat as a natural, inexpensive and available material for the removal of many heavy metal ions, because of its advantages in terms of both adsorption efficiency and economic viability (Balan et al., 2009, 2012; Bulgariu et al., 2011; Fine et al., 2005; Ho et al., 2002; Kalmykova et al., 2008; Kicsi et al., 2010; Lourie and Gjengedal, 2011; Priyanta et al., 2016; Qin et al., 2006) Peat is a complex soil resulted from biological and chemical degradation and partial carbonization of lignocellulosic biomass in aqueous media containing low oxygen amounts. The major constituents of the peat are of organic nature, namely lignin, cellulose and humic substances. Because of this, the peat structure contains many reactive sites represented by polar functional groups (like alcohols, aldehydes, ketones, carboxylic acids, phenolic hydroxides, ethers) that are mainly responsible for the retention of metal ions (Brown et al., 2000). Different studies suggest various mechanisms, including ion exchange, complexation, surface adsorption, chemisorption, to describe the interactions between metal ions and peat, although most probably the metal adsorption on peat is not the result of one mechanism but of several (Brown et al., 2000). The ability of the peat to retain metal ions is enhanced by their heterogeneous surface, large surface area and highly porous structure. One of the most important parameters to assess effectiveness of an adsorbent is its adsorption capacity, whose determination is based on equilibrium experiments and various isotherm models with two or three parameters (Langmuir, Freundlich, Temkin, Redlich–Peterson, KobleCorrigan etc). In addition, the different parameters of these isotherms models often provide information about the surface properties and affinity of the adsorbent and about adsorption mechanism (monolayer or multilayer adsorption). Due to the simplicity in estimation, the most common method to predict the model which best fits the adsorption isotherm is the linear regression analysis, where the constants of the isotherms are determined using the linearized equations of the models. However,

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transformation of nonlinear isotherm equations in linearized shape tends to change the distribution of the error, and thus distort the model parameters. In recent years, non-linear analysis has been proposed as a better method to obtain the isotherm parameters and determine the fit of the isotherm expressions (Foo and Hamed, 2010; Sharain-Liew et al., 2016; Subramanyam and Das, 2009). The non-linear regression method provides a more rigorous mathematically way for determining the isotherm parameters by using the original form of the isotherm equation. The method implies the minimization of error distribution between the experimental data and the predicted isotherm by an iterative procedure. Several error deviation functions have been used to adequately measure the goodness of fit of the models (Ncibi, 2008). In a previous study we described the effect of operating process parameters (contact phases time, solution pH, peat dose, initial Cd (II) concentration) on the cadmium removal from aqueous solutions using as adsorbent Romanian sphagnum moss peat (Balan et al., 2008). The objective of this work was to optimize adsorption process of Cd (II) on this peat by determining the adsorption capacity of the peat depending on the solution pH and temperature and to find, using non-linear regression analysis, the models that accurately describe the experimental results of adsorption isotherms. The information on the mechanism of Cd (II) adsorption on the peat is complemented by evaluation of the thermodynamic parameters of the process and FT-IR spectroscopy analysis. 2. Experimental 2.1. Materials Experiments were carried out using as adsorbent Sphagnum moss peat collected at 1 m depth from the peat bog Poiana Stampei, Romania. The material, previously air-dried, grounded and sieved to particles with size of 1-2 mm, has a water content of 10.5-11%, ash content 4.85%, total organic mater 84 - 85%, cation exchange capacity of 0.57 meq/g, pH in deionised water (1:5 w/v) = 4.0 ± 0.1 and pHZPC = 3.7 (Balan et al., 2008). Cadmium stock solution (1017 mg Cd (II)/L) was prepared using analytical reagent grade cadmium sulfate salt (3CdSO4 8H2O) by dissolving required quantity in deionised water; the working solutions were obtained by successive dilution of the stock solution. 2.2. Batch sorption studies Batch experiments were carried out with peat to investigate adsorption isotherms of cadmium at different pH values and temperatures. Optimal experimental conditions were selected according to previous results (Balan et al., 2008) and are following: peat dose = 4 g/L, phases contact time of 6

Modeling equilibrium data for Cd (II) adsorption by peat using non-linear regression analysis

h and an Cd (II) concentration range of 22.5 to 134.9 mg/L. The influence of the initial solution pH (pH = 3±0.1; 4±0.1; 5.5±0.1 adjusted with sulfuric acid and pH = 5±0.1 with acetate buffer, measured with a pHmeter Inolab pH/ion analyzer 735) was studied at a constant temperature of 293 K. The influence of temperature (293 K, 313 K and 333 K, thermostat Grant instrument of OLS 200 model) was monitored in solutions of pH = 5.5±0.1. The mixtures were mechanically shaken at 100 rpm. The solid was separated by filtration using Whatman filter paper (No.1) and the residual cadmium concentration in filtrate was analyzed by spectrophotometric method with xylenol orange (λ = 580 nm) using a VSU-2P spectrophotometer (Otomo, 1964). All experiments were conducted in duplicate and the average values were used for further calculation. The amount of cadmium adsorbed on peat qe (mg/g) was calculated using Eq. 1: qe =

(C0 − Ce ).V m

(1)

where C0 and Ce (mg/L) are the initial and residual (equilibrium) concentration of Cd (II) in solution,

respectively, V (L) is the solution volume and m (g) is the mass of the dry peat. Infrared spectroscopic analysis of the peat before and after adsorption of Cd (II) ions was performed using a FT-IR BioRad spectrometer FTS2000 on KBr pellets. Measurements were recorded in the wavelength range 4000 - 400 cm-1, using a resolution of 4 cm-1 and 32 scans / min. 2.3. Adsorption isotherms Equilibrium relationship between the adsorbate species and the adsorbent is described by adsorption isotherms which are obtained by plotting concentration of adsorbate retained in solid phase (qe, mg/g) against concentration of adsorbate that remaining in solution (Ce, mg/L), at a fixed temperature. There are several isotherm models for analyzing experimental equilibrium data; in this study the isotherms of two parameter (Langmuir, Freundlich, Temkin, Dubinin-Radushkevich) and three-parameter (Redlich-Peterson, Sips, Toth) were applied. Table 1 shows the equations and the parameters of these isotherms.

Table 1. Equations and parameters of the adsorption isotherm models used in this study Adsorption isotherm model Freundlich

Equation of the model

qe = K F Ce1/ n

Langmuir

Temkin

(2)

qe =

qm K L Ce 1 + K L Ce

(3)

qe =

RT ln( KT Ce ) bT

(4)

(

Dubinin= qe qDR exp − K DRε 2 Radushkevich (D-R)  1 

= ε RT ln 1 +   Ce  1 E= 2 K DR

Redlich-Peterson (R-P) Sips (Langmuir Freundlich)

Toth

Isotherm model parameters

–

)

(5) (6)

ARP Ce 1 + BRP Ceg

(8)

qe =

qm K S Ce1/ nS 1 + K S Ce1/ nS

(9)

(K

qmCe Th

+ Ce

)

t 1/ t

KF – Freundlich constant related to adsorption capacity, (mg/g)(L/g)n n – constant related to adsorption intensity

Freundlich, 1906

qm – maximum monolayer adsorption capacity, mg/g KL –Langmuir constant related to energy of adsorption, L/mg KT – Temkin equilibrium binding constant, L/mg bT - constant related to the heat of adsorption, J/mol

Langmuir, 1918

qDR – theoretical saturation capacity, mg/g KDR – D-R constant related to mean free energy of adsorption, mol2/kJ2 E – mean free energy of adsorption, kJ/mol

Dubinin and Radushkevich, 1947

ARP – R-P isotherm constant, L/g BRP – R-P isotherm parameter, L/mg g – exponent between (0 < g < 1) qm – maximum adsorption capacity, mg/g KS –Sips equilibrium constant, L/mg 1/ns-Sips model exponent (0< 1/nS Sips > Langmuir > Temkin > Dubinin-Radushkevich > Freundlich; it is noted that R2 values are higher and ARE, RMSE, χ2 values are lower for solutions of pH = 5±0.1 compared to the pH = 5.5 ± 0.1 (except Freundlich and DubininRadushkevich models). The experimental data concerning influence of temperature solution on the cadmium adsorption capacity of the peat were also analyzed using twoparameter (Freundlich, Langmuir, Temkin, DubininRadushkevich) and three-parameter (RedlichPeterson, Sips, Toth) isotherm models. Correlation between experimental and calculated adsorption equilibrium capacities qe using the Freundlich, Langmuir and Redlich-Peterson isotherms for sorption of Cd(II) by peat at different temperature values and pH=5.5±0.1 is shown in Fig. 2, while the

qm (mg/g) or (mmol/g)* 7.35 15.15 41.6 21.72 21.28 18 0.39* 0.59 ± 0.04* 42.19 85.47 0.729* 0.318* 19.8 18.52 32.57 74.9 0.285* 0.447* 27.92

KL (L/mg) (L/mmol))* 0.7818 0.006 0.024 2.418* 2.94 0.0052 0.34* 5 ± 1* 0.0289 0.15 0.785* 1.771* 0.064 0.029 0.0513 0.082 4.18 3.07 0.2122

References Semerjian, 2010 Najam and Andrabi, 2014 Othman et al., 2011 Tofan et al., 2011 Kumar et al., 2010 Asuquo and Martin, 2016 Qayoom and Kazmi, 2012 Herrero et al., 2008 Horsfall and Spiff, 2005 Srivastava and Hasan, 2011 Gonzalez and Pokorsky, 2014 Gonzalez and Pokorsky, 2014 El-Sherif and Fathy, 2013 Yazuv et al., 2007 Yan et al., 2016 Anirudhan and Suchithra, 2010 Qin et al., 2010 Qin et al., 2010 This work

values of estimated isotherms parameters and error functions are listed in Table 5.

Fig. 2. Experimental data and calculated Freundlich, Langmuir and Redlich-Peterson isotherms of Cd(II) adsorption by peat at different temperatures and pH=5.5±0.1

As shown in Fig. 2, adsorption of cadmium on peat decreases when temperature increased from 293 K to 333 K indicating an exothermic process. Similar observations regarding the influence of the temperature on the Cd(II) adsorption on different materials have been communicated by many authors (Sari and Tuzen, 2008; Srivastava and Hasan, 2011). The data from Table 5 shows that Freundlich model produces the lowest R2 values and the highest ARE, RMSE and χ2 values and thus it is not best to describe Cd (II) adsorption on the peat. The Freundlich constant, KF, which is related to adsorption capacity, decreases with temperature, suggesting that the adsorption process is exothermic; the n values for all studied temperatures are greater than three indicating favorable adsorption.

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The values of error functions corresponding to Langmuir isotherm suggest that this model better describes distribution at equilibrium of cadmium ions between peat adsorbent and solution. The maximum adsorption capacity, qm, decreases from 15.48 mg Cd/g peat at 293 K to 12.63 mg Cd/g peat at 333 K and also, the values of Langmuir constant, KL, decrease with temperature increasing, suggesting lower binding energies and exothermic nature of the adsorption process. The Temkin isotherm model fits well to the experimental data (R2 = 0.994 - 0.997); if the values of bT constant (related to heat of the adsorption) increases with temperature increasing indicating exothermic nature of the adsorption process, the KT values decreases showing lower binding energies at higher temperatures. The values of the determination coefficient for the Dubinin-Radushkevich isotherm are the lowest compared with the other models (except Freundlich model). The qDR values are not consistent with maximum monolayer capacities, qm, and decreases with the temperature increasing. The magnitude of the mean free energy of adsorption (8.83 – 9.43 kJ/mol) corresponds to the electrostatic interactions between adsorbate and adsorbent (ion exchange).

By comparing the values of the error functions from Table 4 it was found that the three-parameter isotherm models (Sips, Toth and Redlich-Peterson) could be the best to describe the Cd(II) adsorption on peat at all studied temperatures. These models show high correlation coefficients (0.995 – 0.991) and low ARE, RMSE and Chi-square values. The results suggest that energy distribution of the peat adsorbent is not uniform and adsorption may consist of more than one monolayer of cadmium coverage. 3.2. Thermodynamic parameters Results of the study concerning the temperature effect on the Cd (II) ions adsorption by peat were completed by evaluating the thermodynamic parameters of the process. Calculation of the free energy change (ΔG0) requires the values of thermodynamic distribution coefficient (K0), defined by the ratio qe/Ce , where qe is the equilibrium concentration of Cd(II) on the peat (mg/L) and Ce is the equilibrium concentration of Cd(II) in solution (mg/L). The K0 values were obtained using the method proposed by Khan and Sing (Khan and Sing, 1987), by plotting ln(qe/Ce) versus qe and extrapolating to zero qe (Fig. 3a).

Table 5. Equilibrium parameters and error functions for the adsorption of Cd(II) by peat at different temperatures and pH=5.5±0.1 Two-parameter isotherm

T=293K

Freundlich KF 4.7846 n 3.6898 R2 0.9857 ARE 5.7737 RMSE 0.7194 χ2 0.3713 Langmuir qm 15.480 KL 0.1724 R2 0.9940 ARE 2.9349 RMSE 0.4644 χ2 0.0964 Temkin KT 2.8957 bT 841.23 R2 0.9940 ARE 3.2863 RMSE 0.4639 χ2 0.1171 Dubinin- Radushkevich qDR 35.148 KDR 0.00641 E 8.8331 R2 0.9912 ARE 4.6336 RMSE 0.5621 χ2 0.2226

576

T=313K

T=333K

4.3481 3.7942 0.9871 5.2869 0.6118 0.2243

3.8565 3.8221 0.9849 5.4101 0.5925 0.2242

13.965 0.1583 0.9943 3.3783 0.4035 0.1115

12.632 0.1385 0.9943 3.1924 0.3626 0.0877

2.5353 1023.2 0.9978 1.6773 0.2516 0.0278

2.2046 1196.2 0.9954 2.0837 0.3256 0.0488

30.7274 0.00592 9.1885 0.9928 3.8659 0.4541 0.1214

27.3149 0.00562 9.4345 0.9906 3.9836 0.4656 0.1286

Threeparameter isotherm Redlich- Peterson ARP BRP g R2 ARE RMSE χ2 Sips qm KS 1/nS R2 ARE RMSE χ2 Toth qm KTh t R2 ARE RMSE χ2

T=293K

T=313K

T=333K

4.4599 0.5201 0.8571 0.9966 2.3391 0.3503 0.0525

3.3770 0.3909 0.8896 0.9988 1.0901 0.1827 0.0136

2.4491 0.2929 0.9072 0.9971 1.8791 0.2595 0.0313

19.476 0.2035 0.6537 0.9952 2.8243 0.4206 0.0778

16.247 0.2057 0.7024 0.9991 0.8884 0.1596 0.0106

14.312 0.1841 0.7382 0.9977 1.4933 0.2299 0.0237

21.379 1.5696 0.4930 0.9956 2.6851 0.3996 0.0697

17.091 1.9342 0.5749 0.9991 0.8832 0.1626 0.0108

14.859 2.4268 0.6268 0.9976 1.5796 0.2368 0.0252


Fig. 3. Plots of ln qe/Ce versus qe (a) and of thermodynamic distribution coefficient K0 versus 1/T for the adsorption of Cd(II) ions by peat Table 6. Thermodynamic parameters for the adsorption of Cd (II) ions by peat T, K 293 313 333

1/T, 1/K 0.003413 0.003195 0.003003

ln K0 1.986 ± 0.234 1.761 ± 0.154 1.574 ± 0.084

ΔG0, kJ/mol - 4.813 ± 0.59 - 4.562 ± 0.402 - 4.358 ± 0.232

The changes in enthalpy (ΔH0) and entropy (ΔS ) (at zero surface coverage) were calculated from the slope and intercept of Van’t Hoff plots of ln K0 versus 1/T, respectively, (Fig. 3b). The values for the thermodynamic distribution coefficient (K0) and thermodynamic parameters of the adsorption of Cd(II) by peat are given in Table 6. The Gibbs free energy (ΔG0) is negative at all studied temperature, showing that adsorption of Cd (II) by peat is feasible and follows a spontaneous trend. It is known that the values ΔG0 between -20 and 0 kJ/mol are compatible with the electrostatic interaction between adsorption sites and metal ions (physisorption) while values more negative than – 40 kJ/mol (-80 to - 400 kJ/mol) involve charge sharing and transfer from adsorbent surface to the metal ions to form a coordinative bond (chemisorption) (Horsfal and Spiff, 2005). The relatively low ΔG0 values obtained in this study (< -10 kJ/mol) indicate that physisorption is the dominating mechanism of the adsorption of Cd(II) by peat. The negative value of enthalpy change (ΔH0) confirms that adsorption is accompanied by the heat evolution and so, the adsorption process is exothermic in nature. Similar results have been reported for cadmium adsorption onto oxidized granular activated carbon (Huang et al., 2007), Caladium bicolor biomass (Horsfal and Spiff, 2005), coconut copra meal (Ho and Ofomaja, 2006; Srivastava and Hasan, 2011). Generally, the heat evolved during physical adsorption is in the range of 2.1 – 20.9 kJ/mol, but the enthalpy changes for ion exchange reactions are usually smaller than 8.4 kJ/mol (Saha and Chowdhury, 2011). The calculated value of ΔH0 suggest that the adsorption of Cd(II) ions by peat is reached via physisorption including electrostatic interactions. The entropy change (ΔS0) is negative, indicating a decreased randomness at the solid-liquid interface during the 0

ΔH0, kJ/mol

ΔS0, J/mol K

- 8.356 ± 0.324

- 12.030 ± 1.038

adsorption process of Cd (II) ions by peat (the system becomes more structured); however the value of ΔS0 is very small suggesting rather that no significant change occurs in the peat structure during adsorption. Also, it was found that ΔH0 > T ΔS0, indicating that the influence of enthalpy is more remarkable than entropy in this study (the process is enthalpy driven). 3.3. Isosteric heat of adsorption The differential isosteric heat of adsorption (ΔHx), defined as the heat absorbed or released during adsorption reaction at constant surface coverage and different temperatures, was calculated from the slope of plot lnCe vs 1/T, according to the Clausius-Clapeyron equation; the equilibrium concentration of adsorbate, Ce (mg/L), at constant qe is obtained from the adsorption isotherm data at different temperatures. The isosteres corresponding to different equilibrium surface loading (qe = 6, 8, 9, 10, 11 mg Cd(II)/g peat) are shown in Fig. 4a and the appropriate values of the isosteric heat of adsorption are given in Table 7. As can be seen, for all constant amount of Cd(II) adsorbed by peat, the ln Ce varies linear with temperature and the calculated values of ΔHx are negative confirming the exothermic nature of adsorption. The dependence of the isosteric heat of adsorption on surface loading is used as an indication of the degree of heterogeneity of the adsorbent surface (Saha and Chowdhury, 2011). The variation of ΔHx with surface loading (Fig. 4b) shows that the adsorbent surface is energetically heterogeneous and both adsorbate - adsorbate and adsorbate - adsorbent interactions are present. The observed increase in the isosteric heat of adsorption with increasing loading may be due to the increase in lateral interactions

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at ~1732 cm-1 is assigned to C=O stretching of carbonyl and carboxyl groups (carboxylic acids and aromatic esters), while the peak at ~1631.7 cm-1 is indicative of aromatic C=C and asymmetric COOgroup vibrations (lignin and other aromatics or aliphatic carboxylates). In the region below 1500 cm1 (fingerprint region), absorption peaks at 1450-1350 cm-1 can be assigned to OH deformations and C-O stretch of phenols or CH deformation in phenolic and aliphatic structures. Absorption in the region 12001000 cm-1 shows stretching of C-O bonds in carbohydrates; typically in this area the band around 1040 cm-1 is attributed to polysaccharides. After Cd(II) ions loading, the most of characteristic peaks are unchanged, however are evident some changes in position, shape and intensity of the peaks assigned to carboxyl and hydroxyl groups of the peat. Thus, the peak at 1732 cm-1 assigned to C=O in un-dissociated carboxyl group practically disappear, while absorption peaks at 1631.7 cm-1 (asymmetric COO-) is clearly stronger and shifted to 1612.4 cm-1 suggesting that the Cd (II) ions binding may occur via carboxylic groups of the sorbent.

between the adsorbed Cd(II) ions. According to the Table 7, the values of the isosteric heat of adsorption ranges between -10 and -20 kJ/mol indicating that adsorption of Cd(II) by peat is a physical process involving electrostatic interactions. 3.4. FT-IR analysis To investigate the nature of interactions between the functional groups of peat and Cd(II) ions a FT-IR study was carried out and the spectra of the peat before and after adsorption of Cd(II) are shown in Fig. 5. The FT-IR spectra of the peat show the presence of a large number of the functional groups, reflecting the complex nature of the adsorbent material. The most important peaks could be assigned as follows (Krumins et al., 2012; Biester et al., 2014). The broad absorption band at 3700-3000 cm-1 (centered at 3414 cm-1) is assigned to stretching vibrations of –OH groups in alcohols, phenols and carbohydrates, while absorption in the range 29002800 cm-1 reflect aliphatic C-H vibrations of -CH3 and -CH2 groups. Absorption in the range from 2000 to 1500 cm-1 is characteristic for functional groups with double bonds, usually C=O and C=C. The peak

Fig. 4. Plots of ln Ce against 1/T for adsorption of Cd(II) ions by peat at constant surface coverage (a) and variation of isosteric heat of adsorption with surface loading (b) Table 7. Isosteric heat of the adsorption of Cd(II) ions by peat qe, mg/g 6 8 9 10 11

Regression equation y = - 1400 x + 6.065 y = - 1697 x + 7.598 y = - 1938 x + 8.679 y = - 2312 x + 10.22 y = - 2443 x + 10.98

R2 0.991 0.990 0.989 0.986 0.998

ΔHx, kJ/mol -11.642 ± 1.102 -14.109 ± 1.395 -16.112 ± 1.701 -19.222 ± 2.283 -20.311 ± 0.835

Fig. 5. FT-IR spectra of the peat before and after adsorption of Cd(II) ions (pH=5.5, T=293K)

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Also, the peak at around 3414.7 cm-1, attributed to the -OH vibrations in carboxylic and alcoholic groups, is slightly shifted to the higher wave numbers (3419.7 cm-1) indicating involvement of these groups in metal ions binding on the peat surface. 4. Conclusions The present study shows that the Romanian peat may be an effective adsorbent for removal of cadmium ions from aqueous solutions. Adsorption of Cd(II) on peat increased with an increase in pH (3 5.5) and decreased with temperature increasing (293333 K). Generally, equilibrium data fitted better in three-parameter isotherm models than two-parameter models. Langmuir isotherm effectively described the experimental data; the maximum adsorption capacity is comparable with those of other nonconventional and low cost adsorbents. The mean free energy of sorption (E) estimated from Dubinin-Radushkevich model shows that the ion exchange is the predominant mechanism of the adsorption. The thermodynamic parameters indicated that the adsorption process is spontaneous and feasible, exothermic and accompanied by negative entropy, suggesting a physical adsorption process involving electrostatic interactions. The isosteric heat of adsorption increased with an increase in surface loading indicating that surface of the peat is energetically heterogeneous and there may be some lateral interactions between adsorbed ions. The FT-IR spectra of the peat before and after cadmium loading shows the changes in position, shape and intensity of the peaks assigned to carboxyl and hydroxyl groups of the peat, suggesting the Cd (II) ions binding via carboxylic groups of the sorbent. References Ali I., (2010), The quest for active carbon adsorbent substitutes: inexpensive adsorbents for toxic metal ions removal from wastewater, Separation and Purification Reviews, 39, 95-171. Altaher H., Alghamdi A., Omar W., (2015), Innovative biosorbent for removal of cadmium ions from wastewater, Environmental Engineering and Management Journal, 14, 793-800. Anirudan T.S., Suchithra P.S., (2010), Equilibrium, kinetic and thermodynamic modeling for the adsorption of heavy metals onto chemically modified hydrotalcite, Indian Journal of Chemical Technology, 17, 247-259. Asuquo E.D., Martin A.D., (2016), Sorption of cadmium (II) ion from aqueous solutions onto sweed potato (Ipomoea batatas L) peel adsorbent. Characterization, kinetic and isotherm studies, Journal of Environmental Chemical Engineering, 4, 4207-4228. Babel S., Kurniawan T.A., (2003), Low-cost adsorbents for heavy metals uptake from contaminated water: A review, Journal of Hazardous Materials, 97, 219-243. Balan C., Bilba D., Macoveanu M., (2008), Removal of cadmium (II) from aqueous solutions by sphagnum

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