Study on the removal of hexavalent chromium using a new biosorbent

This art icle was downloaded by: [ Tariq Suhail Naj im ] On: 19 June 2013, At : 00: 52 Publisher: Taylor & Francis I nform a Lt d Regist ered in England and Wales Regist ered Num ber: 1072954 Regist ered office: Mort im er House, 37- 41 Mort im er St reet , London W1T 3JH, UK Desalination and Water Treatment Publicat ion det ails, including inst ruct ions for aut hors and subscript ion informat ion: ht t p:/ / www.t andfonline.com/ loi/ t dwt 20 Study on the removal of hexavalent chromium using a new biosorbent a a Tariq Suhail Naj im , Safana A. Farhan & Rasha M. Dadoosh a a Polymer Research Unit , College of Science, Must ansiriya Universit y , Baghdad , Iraq Phone: Tel. +964 1 7703608825 Published online: 18 Jun 2013. To cite this article: Tariq Suhail Naj im , Safana A. Farhan & Rasha M. Dadoosh (2013): St udy on t he removal of hexavalent chromium using a new biosorbent , Desalinat ion and Wat er Treat ment , DOI:10.1080/ 19443994.2013.808791 To link to this article: ht t p:/ / dx.doi.org/ 10.1080/ 19443994.2013.808791 PLEASE SCROLL DOWN FOR ARTI CLE Full t erm s and condit ions of use: ht t p: / / www.t andfonline.com / page/ t erm s- and- condit ions This art icle m ay be used for research, t eaching, and privat e st udy purposes. Any subst ant ial or syst em at ic reproduct ion, redist ribut ion, reselling, loan, sub- licensing, syst em at ic supply, or dist ribut ion in any form t o anyone is expressly forbidden. The publisher does not give any warrant y express or im plied or m ake any represent at ion t hat t he cont ent s will be com plet e or accurat e or up t o dat e. The accuracy of any inst ruct ions, form ulae, and drug doses should be independent ly verified wit h prim ary sources. The publisher shall not be liable for any loss, act ions, claim s, proceedings, dem and, or cost s or dam ages what soever or howsoever caused arising direct ly or indirect ly in connect ion wit h or arising out of t he use of t his m at erial. (2013) 1–8 Desalination and Water Treatment www.deswater.com doi: 10.1080/19443994.2013.808791 Study on the removal of hexavalent chromium using a new biosorbent Tariq Suhail Najim*, Safana A. Farhan, Rasha M. Dadoosh Polymer Research Unit, College of Science, Mustansiriya University, Baghdad, Iraq Tel. +964 1 7703608825; email: tariq_pru@yahoo.com Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 Received 7 June 2012; Accepted 7 May 2013 ABSTRACT Peppermint leaves (PML) have been explored as an effective and cheap adsorbent for removal of toxic Cr(VI) ions from aqueous solutions using batch system. Adsorption of Cr (VI) ions onto PML was found to be pH dependent and maximum removal of Cr(VI) ions was obtained at pH 2. It was also found that after 180 min of PML contact with chromium solution at the concentration of 0.3846 mmol/L, more than 95% of Cr(VI) ions can be removed. The equilibrium data were fitted with the Langmuir and Freundlich models. The adsorption kinetic data were best fitted with the pseudo-second-order. The activation energy Ea of the adsorption process was determined as 23 kJ mol 1, which may indicate a physisorption process. The Gibbs free energy, enthalpy, and entropy of the process were also determined, and their values revealed that the process is spontaneous and endothermic accompanied with randomness at the solid/solution interface. Keywords: Peppermint leaves; Adsorption; Thermodynamic; Activation energy; Cr(VI) ions 1. Introduction Chromium has long been used in electroplating, leather tanning, metal finishing, and chromate manufacturing industries. Effluents from these industries contain both trivalent chromium Cr(III) and hexavalent chromium Cr(VI), with concentrations ranging from tens to hundreds of mg/l [1]. Cr(VI) occurs as highly soluble and toxic chromate anions (HCrO4 or Cr2 O27 ), which are suspected to be carcinogens and mutagens [2]. In contrast Cr(III), having lower toxicity, is generally regarded as much less dangerous pollutant [3]. The chemistry of Cr(VI) is greatly dependent on pH of the solution. In acidic media Cr(VI) exists *Corresponding author. mostly in the form of chromate HCrO4 [4] ions. At pH between 4 and 6, Cr2 O27 and HCrO4–1 ions exist in equilibrium and under alkaline conditions pH > 8, it exists predominantly as chromate anion CrO24 [5]. Cr (VI) is most commonly encountered in the chromate CrO24 and dichromate Cr2 O27 anions. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate). The various treatment techniques available for the removal of Cr(VI) from aqueous effluents are chemical reduction [6], nanofiltration [7], bioaccumulation [8], ion exchange [9], and adsorption, which are the most widely used techniques for removing metal and dyes from industrial effluents. Adsorption is a well known equilibrium separation process, in which the adsorbent may be mineral, 1944-3994/1944-3986 Ó 2013 Balaban Desalination Publications. All rights reserved. 2 T.S. Najim et al. / Desalination and Water Treatment Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 organic, or biological origin. Activated carbon [10– 12], resins and polymers [13–16], agricultural wastes [17–19], and natural polymers were effectively used for the removal of toxic metal ions from aqueous solutions [20,21]. The presence of reactive chemical groups in polymer chains made these polymers an interesting and attractive adsorbents for water decontamination. Most leaves of plants and herbs are composed of biopolymers which include carbohydrates, fibers, protein, and tannin [22,23]. The peppermint plants are available in most part of Iraq as wild plants. In this work, the PML are employed for the removal of Cr(VI) ions from aqueous solutions under equilibrium conditions using batch technique. the remaining Cr(VI) in the filtrate was estimated by UV–visible spectrophotometer using calibration curve. The calibration curve obtained from standard solution of potassium dichromate absorbance vs. Cr (VI) concentration at pH P 12 using NaOH solution (kmax = 375 nm, e = 4,900 cm 1 M 1), [5,13]. Determination of Cr(VI) as chromate ion (yellow color) which is dominate at higher pH, is more sensitive than its determination as dichromate form (orange) in aqueous solution. The effect of initial pH from 1 to 6, adsorbent dosage from 1 to 14 g/L, contact time from 5 to 180 min, and Cr(VI) ions initial concentration of 20–120 mg/L on the adsorption process was performed. The equilibrium capacity qe was calculated according to Eq. (1). qe ¼ ðC0 2. Experimental 2.1. Materials Potassium dichromate K2Cr2O7 was supplied by Aldrich with purity 99.5%: hydrochloric acid and sodium hydroxide were of analytical grade reagents. Peppermint leaves (PML) were collected from Baghdad area, dried at room temperature away from sunlight, ground and then sieved to a particle size of 300–500 lm. Ce ÞV W ð1Þ The removal efficiency R% of Cr(VI) was determined using Eq. (2) R% ¼ C0 Ce C0  100 ð2Þ where C0 is the initial concentration, Ce is the concentration of unremoved Cr(VI) after certain contact time, V is the volume of sample (L) and W is the weight of PML powder (g). 2.2. Instrumentation FTIR spectrophotometer type (Jasco-4200) was used for determination of functional groups vibrations using KBr disc method. Temperature controlled shaking water bath type (Jeio Tech. BS-11) Korea, four-digit balance type Kern ABS Germany, UV–Visible spectrophotometer type Varian 100 Conc, and pH meter type Trans BP 300 were used throughout this work. 2.3. Adsorption study A stock solution of Cr(VI) ions was prepared by dissolving 1.4144 g of K2Cr2O7 in 1 L of deionized water to prepare 500 mg/L of Cr(VI), proper concentration of the adsorbate was prepared from the stock solution through dilution with deionized water. The pH of the adsorbate solution was adjusted during the dilution steps using 0.1 M hydrochloric acid and 0.1 M sodium hydroxide. The batch adsorption experiments were performed on a temperature controlled shaking water bath with a shaking rate of 140 rpm. At the end of a predetermined time interval, the Cr (VI) loaded adsorbent was removed by filtration and 3. Results and discussion 3.1. FTIR spectroscopy FT-IR spectra for both fresh and Cr(VI) loaded PML were obtained by KBr pellets method using FT-IR spectrophotometer type (Jasco 4200) to explore the functional groups present in the biomass and to look into possible Cr(VI) binding sites as shown in Fig. 1. The fresh biomass displays a number of absorption peaks, reflecting the complex nature of PML. The FTIR spectroscopic analysis shows broad peak at 3,500–3,200 cm 1, representing O–H and N– H stretching (the surface hydroxyl and amine groups). A change in peak position in the spectrum of the chromium-loaded PML indicates the binding of the metal with amino and hydroxyl groups. The band observed at 2,940 and 2,904 cm 1 are assigned to the stretching aliphatic C–H groups. The band present at about 1,732 cm 1 is assigned to C=O (band from carboxylic or ester groups). The peak around 1,650 cm 1 corresponds to C=O (amide band primarily a stretching band). The shifting of this peak to 1,639 cm 1 indicates the involvement of 3 Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 T.S. Najim et al. / Desalination and Water Treatment Fig. 1. FTIR spectrum of (A) PML (B) Chromium loaded PML. C=O of amides in the adsorption process. The peak around 1,396 cm 1 may indicate the stretching vibration of NO2. The peaks observed around 1,242 and 1,064 cm 1 could be assigned to SO3 stretching vibration, C–O stretching of polysaccharides, respectively. Several researchers [25,26] affirm that the hydroxyl, carboxyl, sulfonate, amine, amide, imidazole, and phosphate groups are the main functional groups responsible for the biosorption process. Some of these groups are present on the PML and may interact with the chromium ion during the adsorption process. 4 T.S. Najim et al. / Desalination and Water Treatment Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 3.2. Effect of pH Initial pH is one of the most important factors that affect the adsorption process. It affects both the surface charge of the adsorbent and the ionization degree of the adsorbate. To investigate the role of pH in Cr(VI) removal efficiency, the initial pH of Cr(VI) solution varied in the range of 1–5. The variation of pH vs. the percent removal of Cr(VI) which is shown in Fig. 2. It can be seen that the percent removal of Cr(VI) was 95% at pH 2 and 2 g/L of PML, at pH higher than 2 the percent removal of Cr(VI) decreased to 11%, at pH 5. As previously explained, the dominant form of Cr (VI) ions at pH 2 is HCrO4 . So, this anion can be attracted to the positive charge on the adsorbent which is generated in the acidic medium (due to protonation of the carbonyl, amine, amide, and hydroxyl groups). Based on this result, removal mechanism might be due to the attraction between the negatively charged Cr(VI) ions and the positively charged adsorbent groups. Amino, carboxyl, sulfonate, and hydroxyl groups of biomaterials are suspected to bind anionic Cr(VI) ions with the aid of protons in aqueous phase [5,27–30] as follows: B–NH2 ðsÞ þ HCrO4 þ Hþ ðaqÞ ! B–NH3þ . . . HCrO4 ðsÞ B–COOH ðsÞ þ HCrO4 þ Hþ ðaqÞ ! B–COOHþ 2 . . . HCrO4 ðsÞ B–SO3 H ðsÞ þ HCrO4 þ Hþ ðaqÞ ! B–SO3 Hþ 2 . . . HCrO4 ðsÞ B–OH ðsÞ þ HCrO4 þ Hþ ðaqÞ ! B–OHþ 2 . . . HCrO4 ðsÞ As the pH of the aqueous phase is lowered, the large number of protons can easily coordinate with these functional groups present on the biomaterial surface. Thus, low pH makes the biomaterial surface more positive, which enhance the binding of anionic Cr(VI) ion species with the positively charged groups on the adsorbent. The low pH also accelerates the redox reaction in aqueous and solid phase, since the protons participate in these reactions [31,32]. Thiol, phenolic, lignin, and tannin groups have been reported as electron-donor groups of biomaterials [33–36]. The possibility of Cr(VI) reduction to Cr(III) by PML powder is not excluded but not investigated in this study. 3.3. Effect of initial concentration of Cr(VI) The concentration of Cr(VI) in solution determines the toxicity of the solution. Therefore, the effect of initial concentration of Cr(VI) on the removal efficiency of Cr(VI) by PML powder was investigated. For this purpose 0.1 g of adsorbent was contacted for 180 min with 50 ml of Cr(VI) solutions with different initial concentrations (20–120 ppm). It was found that the removal efficiency is decreased and the adsorption capacity increased with increase of the initial concentration of Cr(VI), this trend was also found by other investigators [37,38]. The decrease in removal efficiency can be explained by the fact that all adsorbents had a limited number of active sites, which would have become saturated above a certain concentration. 3.4. Effect of contact time and determination of adsorption kinetic Fig. 2. Effect of pH on Cr(VI) adsorption onto PML. In order to evaluate the optimal time required for nearly complete removal of Cr(VI) from aqueous solution, 0.1 g of PML powder was exposed to 50 ml of chromate solution with concentration of 20 ppm. The Cr(VI) concentration was measured after different contact times by the measurement of the UV–vis absorption peaks spectrophotometrically. The reduction of the absorption peak intensity during contact time indicates the reduction of Cr(VI) concentration in solution in contact to the adsorbent. The Cr(VI) removal efficiency after different contact times were calculated using Eq. (2); results showed that after 130 min of contact at 35˚C, over 96% of chromate ions in the solution has been removed by PML powder. The data obtained from the effect of time on the T.S. Najim et al. / Desalination and Water Treatment 5 Table 1 The kinetic parameters of the adsorption of Cr(VI) onto PML powder. Temp (oC) 25 35 Qe exp (mg /g) Pseudo-first-order kinetics 9.53 9.67 1 Qe cal (mg/g) k1 (min ) 6.06 5.15 0.026 0.033 Pseudo-second-order kinetics Dqe (%) 36.5 46.7 Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 adsorption of Cr(VI) onto PML powder were then regressed against the pseudo-first-order Eq. (3) [39], and second-order Eq. (4) [40], kinetic models. logðqe qt Þ ¼ log qe t=qt ¼   1 1 t þ k2 q2e qe   k1 t 2:303 ð3Þ ð4Þ where k1 and k2 are the rate constants of the pseudofirst and second-order kinetics, respectively. From the slope and intercept of plot of log (qe qt) vs. time, k1 and qe were determined, the results are shown in Table 1. The second-order rate constant k2 and qe were determined from intercept and slope of Fig. 3, and presented in Table 1. However, the pseudo-secondorder kinetic model provided a near-perfect match between the calculated and experimental qe values. Furthermore, the correlation coefficient of the pseudosecond-order plot Fig. 5 is 1.00. As a result, the sorption system appears to follow pseudo-second-order reaction kinetic. On the other hand, the second-order-rate constant increases with increasing temperature from 25 to 35oC, which indicates that the adsorption is enhanced with increasing temperature, as shown in Table 1. Fig. 3. Pseudo-second-order plot at 25 and 35oC. R 2 0.997 0.981 Qe cal (mg/g) k2 (g mg 10.06 10.02 0.46 0.80 1 1 Dqe (%) R2 5.5 3.6 0.997 0.999 min ) 3.5. Adsorption isotherms The relation between Cr(VI) initial concentration and its extent of removal from aqueous solutions was studied at various Cr(VI) concentrations at fixed PML dose and temperature. Adsorption data for a wide range of adsorbate concentrations are the most commonly described by adsorption isotherms, such as Langmuir and Freundlich, which relate adsorption capacity, qe (adsorbate uptake per unit weight of the adsorbent) to equilibrium adsorbate concentration in the bulk liquid phase Ce. The Langmuir isotherm [41] is valid for monolayer adsorption onto a surface containing a finite number of identical sites. The model assumes uniform energies of adsorption onto the suface. The langmuir isotherm is represented by the following equation: Ce ¼ qe  1  Qmax  b þ  1 Qmax  Ce ð5Þ where Ce is the equilibrium concentration of adsorbate (mg/l), qe is the amount of metal adsorbed at equilibrium (mg/g), and Qmax (mg/g) is the maximum quantity of metal per unit weight of adsorbent, whereas b (L/mg) is a constants related to the affinity Fig. 4. Langmuir adsorption isotherm of Cr(VI) onto PML. 6 T.S. Najim et al. / Desalination and Water Treatment with increasing temperature (Table 2), it decreases from 0.225 at 25oC to 0.102 at 55oC, which indicates that the reaction is more favorable at higher temperature. Furthermore, the Freundlich adsorption isotherm [44], can be applied to the adsorption data using the linear form of the Freundlich equation: Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 1 log qe ¼ log KF þ log Ce n Fig. 5. Freundlich adsorption isotherm of Cr(VI) onto PML powder. of binding sites with the metal ions [42]. Plotting of Ce/qe vs. Ce, at 25, 35, 45 and 55oC, gives straight lines, Fig. 4, with correlation coefficients (R2) of 0.966, 0.982, 0.989, and 0.997, respectively as given in Table 2. The values of b is increased with increasing temperature, which implies increasing affinity between the adsorbent and metal ions with increasing temperature. The essential characteristics of Langmuir can be expressed in terms of a dimensionless equilibrium parameter, RL, which describes the type of isotherm [11,43], and 1 is defined by: RL ¼ 1þbc where b (L/mg) is the o Langmuir constant and Co is the initial concentration of Cr(VI) solution. The RL value indicates the type of the isotherm as follows: where KF and n are the Freundlich constants, related to the capacity of adsorbent and favorability of the adsorption, respectively. Plotting of log qe against log Ce at different temperatures, (Fig. 5), straight lines were obtained. The Freundlich constants and correlation coefficients are presented in Table 2. As shown by the results, the values of n range from 2.13 at 25oC to 1.59 at 55oC (i.e. n > 1) showing that the adsorption of Cr(VI) onto PML powder is favorable and physical in nature [38]. 3.6. Effect of temperature and determination of thermodynamic parameters The temperature dependence of the adsorption was calculated by the linearized Arrehenius equation [45] at two temperatures 25 and 35oC: ln K ¼ ln A ln RL RL > 1 RL = 1 0 < RL < 1 RL = 0 Type of isotherm Unfavorable Linear Favorable Irreversible The values of RL for different Cr(VI) initial concentrations at 55oC are listed in the following table: Cr(VI) concentration (mg/l) 20 40 60 80 RL value 0.102 0.054 0.036 0.027 It is clear that all the values of RL range between 0 and 1, indicating favorable adsorption of Cr(VI) onto PML powder, furthermore, the values of RL decreases ð6Þ Ea RT  k1 Ea 1 ¼ k2 R T2 1 T1 ð7Þ  ð8Þ where Ea is the activation energy of the adsorption (kJ mol 1), k1 and k2 are the pseudo-second-order rate constants at 25 and 35oC, respectively, R is the gas constant (8.314 J mol 1K 1), and T is the solution temperature (K). The activation energy value gives information on whether the adsorption is mainly physical or chemical. Physisorption process normally had activation energy of 5–50 kJ mol 1, while chemisorption had higher activation energy 40–800 kJ mol 1 [46]. From Eq. (8) the activation energy was calculated and found to be 23 kJ mol 1, and given in Table 3. It was concluded from these results that the adsorption process involved physisorption. The thermodynamic parameters, like Gibbs free energy, enthalpy, and entropy of adsorption were calculated from the values of Langmuir constant (b) at different temperatures: DG ¼ RT ln b ð9Þ T.S. Najim et al. / Desalination and Water Treatment 7 Table 2 The Langmuir and Freundlich constants and correlation coefficients of isotherm models at different temperatures. Temperature (K) Langmuir isotherm 298 308 318 328 Freundlich isotherm 2 Qmax (mg/g) b R 39.3 48.7 49.2 50.8 0.17 0.21 0.31 0.44 0.966 0.982 0.989 0.997 RL 1/n Kf R2 0.225 0.191 0.139 0.102 0.469 0.582 0.568 0.627 0.97 1.76 2.11 3.05 0.985 0.932 0.988 0.934 Note: Cr(VI) concentration, 20–80 mg/L, adsorbent concentration, 2 g L 1,agitation speed, 140 rpm, contact time,180 min at pH 2. Table 3 Thermodynamic parameters of Cr(VI) adsorption by PML powder. Downloaded by [Tariq Suhail Najim] at 00:52 19 June 2013 T (K) ln b (L/mg) 298 308 318 328 ln b ¼ 1.76 1.55 1.17 0.82 DS R DG (kJ mol 1) 4.36 3.97 3.10 2.24 DH RT ð10Þ The plot of ln b vs. 1/T is given in Fig. 6, DS and DH were calculated from the intercept and slope of the plot and tabulated with other thermodynamics parameter in Table 3. The negative values of DG suggest that the adsorption process is spontaneous. The positive value of DH indicates the endothermic process, while the positive value of DS shows the increased randomness at the sorbent/solution interface during the adsorption of chromate ions onto PML powder. DS (J mol 1K 1) DH (kJ mol 1) Ea (kJ mol 1) – – 72.80 – – – 26.22 – – – 23 – 4. Conclusions The PML as solid phase extractor have the following advantages:(i) Stable, inexpensive, environment friendly, and rich in functional groups that have the ability to bind metal ions. (ii) It has the pronounced capability for the uptake of Cr(VI) ions in aqueous solution at strongly acidic medium pH 1–2 with no need to chemical modification. (iii) It was applicable for the removal of Cr(VI) ions with percentage recovery >95% using batch technique. (iv) Its sorption data were fitted well with Langmuir and Freundlich models with correlation factor r2 = 0.997 and 0.934 at 55oC, respectively, and obeying pseudo-second-order model r2 = 0.999. 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