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Thermodynamic and breakthrough column studies for the selective sorption of chromium from industrial effluent on activated eucalyptus bark
Bioresource Technology 97 (2006) 1986–1993
Thermodynamic and breakthrough column studies for the selective sorption of chromium from industrial effluent on activated eucalyptus bark Vikrant Sarin, Tony Sarvinder Singh, K.K. Pant
*
Department of Chemical Engineering, Indian Institute of Technology, New Delhi 110 016, India Received 6 August 2005; received in revised form 26 September 2005; accepted 1 October 2005 Available online 28 November 2005
Abstract Studies were carried out on adsorption of Cr(VI) on an adsorbent made from eucalyptus bark. Results revealed that sorption of chromium on activated eucalyptus bark (AEB) was endothermic in nature. Thermodynamic parameters such as the entropy change, enthalpy change and GibbÕs free energy change were found out to be 100.97 J mol1 K1, 33 kJ mol1 and 0.737 kJ mol1, respectively. Industrial chrome effluent of different chromium concentration at different pH was used as feedstock for the fixed bed adsorption studies. When effluent was fed to the column at low pH of 2, the breakthrough volume increased significantly compared to effluent at higher pH of 4.85. The surface properties of sorbent were characterized by the Scanning electron microscopy, X-ray diffraction technique and Infrared techniques. It was concluded that AEB sorbent column could be used effectively for removal of chromium from industrial effluents by reducing the pH of chrome effluent to two and at optimal column conditions. 2005 Elsevier Ltd. All rights reserved. Keywords: Fixed bed column; Adsorption; Activated eucalyptus bark; Breakthrough
1. Introduction Since the start of industrialization in the nineteenth century, the material cycle, amount of heavy metals, which impact upon water, soil, plants, animals, and people have increased considerably. Many industries, particularly in the trade of metal processing operations and refineries represent significant sources of heavy metal emissions. These include cement manufacturing units (thallium), battery manufacturers (lead), electroplating (Cu, Ni, Cr), tanning (Cr), pigments (Cr, lead), wood preservation (Cr), etc. Through refuse wastewater, heavy metals like chromium ions enter especially into fresh water or soil and thereby into plants, animals and humans. Considering process in biosphere, the geochemical cycle is connected via metabolic processes by microorganisms in the ocean floor, in sedi*
Corresponding author. Tel.: +91 11 2659 6172; fax: +91 11 2658 1120. E-mail address: [email protected] (K.K. Pant).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.10.001
ments or sludge, or in water. Microorganism plants and small animals serve as food source for fish. By means of plants and other animals, levels of heavy metals from water and soil find their way into our food, however when accumulated to high levels, more than the threshold limits of the organisms, it can generate serious problems and diseases. Chromium is highly reactive element. It exists in six oxidation states and the two most stable states are Cr(III) and Cr(VI). Chromium forms stable complexes as Cr2 O 7, HCrO , CrO , HCr O and fraction of these complexes. 2 7 4 4 Fraction of the above complexes varies with pH and at low pH Cr exists as negatively charged dichromate ions. Cr(VI) is the most toxic form thus it becomes imperative to remove Cr(VI) from industrial effluents before discharging into the water or on to the land. Chromium compounds are amphoteric in nature i.e. they exhibit both acidic and basic properties. These compounds also exhibit point of minimum solubility. The pH of minimum solubility for chromium is 7.5. Widespread use of chromium chemicals
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
in various industries leads to problem of contamination of natural waters due to improper disposal methods. Chromium effluent is generated by the following industrial uses of chromium like leather tanning, mining of chrome ore, production of steel and alloys, metal finishing industries, pigments manufacturing units, photographic industry, glass industry, wood preservation, chromium salts being used as corrosion inhibitor in the cooling water treatment in the industry, textile industry and many other industrial applications. Chromium is non-biodegradable and toxic beyond a given concentration. Chromium threshold concentration on inhibitory effect on heterotrophic organisms is 10 mg/ L for Cr(III) and 1 mg/L for Cr(VI). All living organisms require varying amount of Cr(III) in micro amounts for proper growth. The same metal becomes toxic when present in elevated concentration. As per studies carried out Cr(VI) is 500 times more toxic to Cr(III) (Kowalski, 1994). Cr(VI) is considered by International Agency for Research on Cancer (IARC) 1982 as a powerful carcinogenic agent that modifies the DNA transcription process causing important chromosomic aberration (US Department of Health and Human Services, 1991; CieslakGolonka, 1995). The current BIS (Bureau of Indian Standards) recommended guideline value for chromium (as total Cr) in drinking water as 0.05 mg/L (maximum). Chromium ions are removed from the effluent using precipitation technique. This technique is widely practiced by the industry. The common problem associated with this process is generation of large volume toxic sludge. Safe disposal of this toxic sludge is another problem. Further this method depends on the addition of chemicals to the effluent. Ion exchange (Tiravanti et al., 1997), adsorption (Dahbi et al., 1999) electrodialysis, membrane process, solvent extraction, freeze separation, bio-separation are some of the other methods for removal. The present study aimed at development of suitable method using low cost bio adsorbent for selective removal of chromium by adsorption from industrial effluent. 2. Methods 2.1. Materials All the chemicals used in the study were of analytical grade (Merck, Germany). Industrial effluent was obtained from automobile ancillary unit located at Sahibabad, Uttar Pradesh (India). Industrial chromium effluent was obtained from the outlet of the chrome plating section of the unit. Synthetic samples used in the study were prepared using double distilled/deionised water. Characteristics of different chromium effluent are given in Table 1. Measurements were made in triplicates for the analysis of metal concentration and data were recorded when the variations in two readings were less than 5%. All glass wares and sample bottles were soaked in hot water and rinsed with laboratory dish soap (Lavolene) for at least two hours and then rinsed
1987
Table 1 Characteristics of chromium effluent Parameters TDS (mg/L) pH Concentration Cr(VI) (mg/L) Concentration Cr(III) (mg/L) Hardness Ca (mg/L) Hardness Mg (mg/L) a
#
SS
2–8 200 – – –
IE-1a
IE-2
IE-3
IE-4
70 4.95 12.2 0 16.5 11.5
122 4.45 24.4 0 36 24
900 3.41 200 44.5 134 92
1934 4.2 488.5 14.6 240 184
IE—industrial effluent.
with de-ionized water. Hexavalent chromium [Cr(VI)] stock solution (1000 mg L1) was prepared by dissolving potassium dichromate (K2Cr2O7) in de-ionized water and stored in airtight polyethylene terphthalate (PET) bottles at a temperature less than 2 C. Further working solutions were freshly prepared from stock solution for each experimental run. Concentration of chromium Cr(VI) stock solution was measured periodically to monitor variations in concentration. Cr(III) concentration was determined by measuring the difference between total chromium concentration and Cr(VI) concentration. Total Cr concentration was determined by oxidizing Cr(III)–Cr(VI) using KMnO4 and then determining final Cr(VI) content in the sample (APHA, 1992). 2.2. Thermodynamic studies The adsorbent activated eucalyptus bark (AEB) used for the removal of Cr(VI) was prepared in the laboratory. The details of sorbent preparation, kinetics of adsorption and comparison of chromium removal efficiency with other adsorbents have been discussed elsewhere (Sarin and Pant, 2006). Adsorption, studies in the temperature range 293– 343 K were conducted to determine thermodynamics constants such as Gibbs free energy change (DG0), enthalpy change (DH0) and entropy change (DS0) for the system and to ascertain the sorption mechanism. For this study, adsorbent dosage selected was 5 g/L of chromium Cr(VI) effluent in a conical flask and allowed to equilibrate for 24 h at the different temperatures ranging from 293 to 343 K. Residual chromium concentration was determined in each sample after equilibrium was attained. 2.3. Fixed bed adsorption studies Fixed bed adsorption studies were conducted to evaluate dynamic behaviour for Cr(VI) removal on AEB. The experimental set-up consisted of up flow glass column (0.025 m ID, 0.4 m length) packed with different bed heights of sorbent. Influent feed flow rate was varied and maintained throughout the experiments by the use of variable flow peristaltic pump (Neolab India). At the exit of the column, flow rate was also measured so as to get steady state flow conditions in the column. Experiments were conducted with industrial effluent (pH 4.85) and at optimum pH 2, obtained from batch experiments.
1988
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The empty bed contact time (EBCT) varied from 0.061 to 0.31 h during runs by varying the initial feed flow rate or bed height. Sampling of column effluent was done at specified time intervals in order to investigate the breakthrough point (BTP) and shape of breakthrough curve. Effects of inlet feed flow rate, initial sorbate concentration (10–25 mg/L) and adsorbent bed height (0.1–0.2 m) were investigated on the performance of breakthrough for the adsorption of Cr(VI). Initial chromium Cr(VI) concentration was also varied in the range of 12–24 mg/ L. For the analysis of mass transfer zone, breakthrough data were analyzed by Bohart and Adams model. 2.4. Characterization of activated eucalyptus bark X-ray diffraction (XRD) pattern, infra-red spectroscopy (IR) and scanning electron microscopy (SEM) studies were carried on AEB to determine its adsorption mechanism. For these IR spectra of fresh and used eucalyptus bark was carried out on IR spectroscope (PROTEGE 460, USA) in the range of wave numbers from 4000 to 500 cm1. SEM of the fresh and used sorbents was carried out to determine the qualitative analysis of surface morphology of the sorbent. The analysis was done on a model Cambridge Stereo Scan 360, UK. 3. Results and discussion 3.1. Thermodynamic studies In order to determine thermodynamic parameters, experiments were carried out at different temperatures in the range of 293–343 K for Cr(VI) adsorption. The thermodynamic parameters such as standard GibbÕs free energy change (DG0), enthalpy change (DH0) and entropy change (DS0) were estimated to evaluate the feasibility and nature of the adsorption process (Sarin and Pant, 2006). The GibbÕs free energy change, of the process is related to equilibrium constant by the equation DG0 ¼ RT ln K c
ð1Þ
where, T is temperature in K, R is ideal gas constant having value as 8.314, J mol1 K1 and Kc is thermodynamic equilibrium constant. The thermodynamic equilibrium constant, was determined as Kc ¼
Ca Ce
ð2Þ
where, Ca is mg of adsorbent adsorbed per liter and Ce is the equilibrium concentration of solution, mg/L. According to thermodynamics, the GibbÕs free energy change is also related to the enthalpy change (DH0) and entropy change (DS0) at constant temperature by the following equation
ln K c ¼
DS 0 DH 0 R RT
ð3Þ
The values of enthalpy change (DH0) and entropy change (DS0) were calculated from the slope and intercept of the plot ln Kc versus, 1/T with R2 value of 0.94. For the synthetic sample (C0 200 pm Cr(VI), pH 5.7) at temperatures 333 and 343 K the values of Kc were 1.305 and 1.913 respectively. The corresponding DG0 values were 0.737 and 1.850 kJ mol1 respectively. The values, enthalpy change (DH0) and entropy change (DS0) were found out to be as 33 kJ mol1 and 100.97 J mol1 K1 respectively. The calculated values of these parameters of various other synthetic and industrial samples are reported in Table 2. A positive value of DH0 as 33 kJ mol1 for chromium removal with AEB, indicated the endothermic nature of the process. A negative value of the free energy (DG0) (0.737 kJ mol1 at 333 K and 1.85 kJ mol1 at 343 K), indicated the spontaneous nature of the adsorption process. It was also noted that the change in free energy, increases with increase in temperature, which exhibits an increase in adsorption with rise in temperature. This could be possibly because of activation of more sites on the surface of AEB with increase in temperature. A positive value of DS0 as 100.97 J mol1 K1 showed increased randomness at solid solution interface during the adsorption of Cr(VI) on AEB. A value of Freundlich adsorption intensity (n) higher than 1, obtained from batch indicated that sorption of chromium on AEB is favourable and capacity is only slightly reduced at lower equilibrium concentration (Sarin and Pant, 2006). The endothermic nature of adsorption for chromium removal has also been reported on biogas residual slurry (Namasivayam and Yamuna, 1995). 3.2. Up flow fixed bed column adsorption studies on chromium removal Batch experimental data are often difficult to apply directly to fixed bed column adsorber because isotherms are unable to give accurate data for scale up since a flow column is not at equilibrium. Fixed bed column adsorption experiments were carried out to study the adsorption dynamics. The fixed bed column operation allows more efficient utilization of the adsorptive capacity than the batch process. The adsorbent at the inlet end is contacted continuously by the solution at the initial solute concentra-
Table 2 Thermodynamic parameters for the adsorption of Cr(VI) by AEB Effluent
Effluent concentration Cr(VI), mg/L
pH
Equilibrium constant, Kc
Gibbs free energy, kJ mol1
Synthetic effluent Synthetic effluent Synthetic effluent Industrial effluent Industrial effluent
200 [Cr(VI)] 200 [Cr(VI)] 200 [Cr(VI)] 200 [Cr(VI)] 44.5 [Cr(III)]
2 3 4.7 3.4 3.4
9.0 4.95 1.36 2.10 4.60
5.57 4.06 0.79 1.88 3.87
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
3.2.1. Effect of fixed bed column height on the performance of breakthrough The steepness of the breakthrough curves is a strong function of bed height. Two different industrial chromium effluents were passed through the fixed bed column. One having Cr(VI) concentration as 24 mg/L with pH 4.85 (without changing the pH) and the other effluent was treated at optimum pH 2. The breakthrough volume obtained for effluents treated at pH 4.85 were very less compared to the effluent having pH 2. Figs. 1 and 2 represent the performance of breakthrough curves at a bed height of 0.1, 0.15 and 0.20 respectively. As it is evident from these figures, an increase in column depth increased the throughput volume 25
Ct (mg/L)
20 15 10 Bed height - 0.1 m
5
Bed height - 0.15 m Bed height - 0.2 m
0 0
200
400
600
800 1000 Time (min)
1200
1400
Fig. 1. Effect of bed height, 0.10, 0.15 and 0.2 m on adsorption of Cr(VI) by AEB (flow rate: 3.6 · 104 m3/h, C0: 24.4 mg/L, pH: 4.85).
25
20
Ct (mg/L)
tion. The most important criterion in the design of fixed bed adsorption systems is the prediction of fixed bed column breakthrough or the shape of the adsorption wave front, which determines the operation life span of the bed. The design of up flow fixed bed column adsorber includes estimation of shape of breakthrough curves and the appearance of breakpoint. The dynamics of adsorption process was studied using bed depth service time (BDST) method based on a model proposed by Bohart and Adams (1920). The effect of fixed bed column parameters such as sorbent bed height, flow rate and initial chromium concentration on the performance of breakthrough curves were investigated. All the fixed bed column experiments were carried out at ambient conditions at pH 2 and 4.85 having Cr(VI) concentration as 24.4 mg/L. Flow rate of the effluent through the column was maintained at 3.6 · 103 m3/h. Breakthrough volumes obtained at bed heights of 0.1, 0.15 and 0.2 m for effluent having Cr(VI) concentration as 24.4 mg/L and pH 4.85 were found out to be 1.48 · 104 m3, 3.28 · 104 m3 and 5.76 · 104 m3 respectively, the corresponding values at a pH of 2 were 66.96 · 104 m3, 147.5 · 104 m3 and 242.0 · 104 m3 respectively. The total chromium adsorbed in the column for a given feed concentration is equal to the area under the breakthrough curve. The steepness of breakthrough curve was a strong function of fixed bed column parameters.
1989
15
10
5
Bed height - 0.1 m Bed height - 0.15 m Bed height - 0.2 m
0 0
1000
2000
3000 Time (min)
4000
5000
6000
Fig. 2. Effect of bed height, 0.10, 0.15 and 0.2 m on adsorption of Cr(VI) by AEB (flow rate: 3.6 · 104 m3/h, C0: 24.4 mg/L, pH: 2).
treated due to higher contact time. At relatively lower contact time, the curve was steeper for both the effluents showing the faster exhaustion of the bed. For Cr(VI) removal the treated volume varied from 1.48 · 104 m3 to 5.76 · 104 m3 as the bed height was increased from 0.1 m to 0.2 m (Figs. 1 and 2). The curves followed characteristic S shape profile, which is associated with adsorbate of smaller molecular diameter and more simple structure. The increase in the metal uptake capacity at either pH of 4.85 or 2 with the increase of bed height in the fixed bed column was probably due to increased surface area of the adsorbent, which provide more binding sites for the adsorption. The breakthrough time was also increased with the increase in bed height. 3.2.2. Effect of flow rate on the performance of breakthrough curves Studies at different flow rates were conducted in the fixed bed of AEB. The flow rate was varied from 3.6 · 104 m3/h to 1.2 · 104 m3/h while the inlet Cr(VI) concentration was kept constant at 12.2 mg/L at pH 4.85. Effluent at optimal pH 2 was also run at a flow rate of 3.6 · 104 m3/h. The effect of flow rate on breakthrough performance, at a bed height of 0.10 m has been shown in Fig. 3. As can be seen from the breakthrough plots, higher efficiency of the process was achieved at lower flow rate. This can be explained by the fact at low empty bed contact time (EBCT), the diffusion process which controls the sorption becomes slow, and hence, the sorbent needs more time to bond the metals efficiently. These breakthrough curves demonstrate influence of sorption kinetics. The uptake of chromium decreased with increase in flow rate. It was also observed that sorbents get saturated easily at higher flow rates. Breakthrough volume obtained at pH 4.85 was significantly less than at pH 2. As volumetric flow rate increases, the velocity through the bed decreases, which results in the decrease in the length of adsorption zone. At higher flow rates breakthrough curves become steeper and breakpoint time and the adsorbed
1990
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
14
25
Cr(VI) conc 24.4 mg/L at pH 4.85 Cr(VI) conc 12.2 mg/L at pH 4.85 Cr(VI) conc 24.4 mg/L at pH 2 Cr(VI) conc 12.2 mg/L at pH 2
12
20
8
Ct (mg/L)
Ct (mg/L)
10
6
4 ml/min
4
15
10
6 ml/min 2
2 ml/min 5
0 0
500
1000
1500
2000 2500 Time (min)
3000
3500
4000
Fig. 3. Effect of flow rate on breakthrough curves for adsorption of Cr(VI) by AEB (C0: 12.2 mg/L, bed height: 0.10 m, pH: 4.85, flow rate m3/h · 104: (j) 6 (r) 4 (m) 2).
0 0
500
1000
1500 Time (min)
2000
2500
Fig. 4. Effect of initial Cr(VI) concentration on breakthrough performance (flow rate: 3.6 · 104 m3/h, pH: 4.85 and 2, bed height: 0.10 m).
ion concentration decreases. A higher uptake of chromium was observed at the beginning of fixed bed column operation but as solution continued to flow, the concentration of chromium in the effluent rapidly increased; the bed becomes saturated with chromium and the concentration of the solute in the effluent rise sharply. An increase in flow rate from 1.2 · 104 m3/h to 3.6 · 104 m3/h reduced the volume of effluent treated from 0.456 m3 to 0.162 m3 for Cr(VI) removal over AEB. There was a corresponding decrease in sorption capacity from 662 mg/kg to 617 mg/kg for effluent having Cr(VI) concentration as 12.2 mg/L and pH 4.85. Whereas sorption capacity for effluent having Cr(VI) concentration as 12.2 mg/L at pH 2 was 8748 mg/kg. As Cr(VI) ions moves from bulk solution to the surface of the film surrounding the sorbent particle, concentration gradient develops at the interface, which allows the solute particle to penetrate through the film, and reaches the surface of the particle where the positively active sites attract the negatively charged dichromate ions. At higher flow velocities, film surrounding the particle breaks thereby reducing the adhesion of sorbate to the sorbent particle. As can be seen from Fig. 3 the uptake of Cr(VI) decreased with the increase in flow rate. The breakthrough capacity of the sorbent decreased with increase in flow rate. As the velocity through the bed decreased, the depth of the adsorption zone decreased because there was more time for adsorption to occur. At low pH, the surface of AEB becomes positively charged because many more positive sites are created and amide functional groups become positively charged due to electromeric effect. This results in many more positive charges being created on the surface of the adsorbent, resulting in many folds increase in adsorption capacity of AEB at low pH. 3.2.3. Effect of initial chromium concentration on breakthrough curve Effect of initial chromium concentration on the performance of breakthrough curve is shown in Fig. 4. Break-
through volume obtained at bed height of 0.1 m, effluents having flow rate as 3.6 · 104 m3/h through the bed and having Cr(VI) concentration as 24.4 and 12.2 Cr(VI) both at pH 4.85 was 1.35 · 104 m3 and 3.6 · 104 m3 respectively. On optimising the pH to 2 of the above two effluents and keeping all the other experimental conditions same, breakthrough volume increased and was obtained as 66.96 · 104 m3 and 101.7 · 104 m3 respectively. A change in the inlet sorbate concentration affected the operating characteristics of the fixed bed column. At low initial concentration, breakthrough occurred late and the treated volume were higher since the lower concentration gradient caused a slower transport due to decreased diffusion coefficient or mass transfer coefficient (Aksu and Ferda, 2004). It was observed that at low pH the adsorption increased many folds. This could be due to increase in the number of binding sites. The sorbent gets saturated early at high initial concentration because binding sites become more quickly saturated in the system. These results indicated that increase in chromium concentration reduced the metal to sorbent ratio and the metal uptake as long as the sorbent was not saturated. Initial sorbate concentration affected the performance of fixed bed for adsorption of chromium (Fig. 4). Breakpoint time decreased with increasing inlet Cr(VI) concentration as the binding sites saturated faster. These results showed that performance of breakthrough curves for chromium removal by AEB was strongly affected by initial concentration and pH of solution and thus affected the treatment of effluent. 3.3. Modelling of breakthrough curves Many mathematical models have been developed to describe contaminant sorption over different adsorbents in many of the diverse applications for which it is used (Weber and Smith, 1987; Tien, 1994). The design of the adsorption process is based on the accurate generation of
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
breakthrough curves. The fluid velocity, concentration of solute in feed and the bed height affect the shape of the curve. Steep slopes of breakthrough curves were obtained for systems that exhibit high film transfer coefficients, high internal diffusion coefficients or flat adsorption isotherm that is smaller value of 1/n (Faust and Aly, 1987). The dynamic behaviour of the fixed bed column was predicted with the Adam–Bohart model in the present investigation. 3.3.1. Adam–Bohart model Bohart and Adams (1920) established the fundamental equations describing the relationship between Ct/C0 and t in a continuous system for the adsorption of chlorine on charcoal, known as bed depth service time (BDST) model. Although the original work by Adams–Bohart was done for the gas–charcoal adsorption system, its overall approach has been applied successfully in quantitative description of other systems (McKay, 1982; Singh and Pant, 2004). This model assumes that the adsorption rate is proportional to both the residual capacity of the sorbent and the concentration of the sorbing species. The Adams–Bohart model is used only for the description of the initial part of the breakthrough curve i.e. up to the breakpoint or 10–50% of the saturation points. The main objective of the fixed bed adsorption is to reduce concentration in the effluent so that it does not exceed permissible limit (breakthrough concentration Cb). This approach was focused on the estimation of characteristic parameters such as maximum adsorption capacity (N0) and kinetic constant (ka) from Adam–Bohart model using a quassi chemical kinetic rate expression. The mass balance obeys the following equation oq ¼ k a qC ot oC ka ¼ qC oz v
ð4Þ ð5Þ
The following equation can be obtained to relate the process conditions and operating parameters with service time. C0 ln 1 ¼ ln eka N 0 Z=v 1 k a C 0 t ð6Þ C A linear relationship between bed depth and service time given by Eq. (7) (Hutchins, 1973) N 0Z 1 C0 t¼ 1 ð7Þ ln C0v kaC0 C were C is the effluent concentration of adsorbate in the liquid phase (mg/L); C0 is initial concentration of sorbate in the liquid phase (mg/L); v is the linear flow velocity (m/h); N0 is adsorption capacity (mg solute/kg adsorbent); ka is rate constant in BDST model (m3/kg h); t is time (h) and Z is bed depth of fixed bed column (m). Eq. (7) enables the service time, t, of an adsorption bed to be determined for a specified bed depth, Z, of adsorbent. The service time and bed depth were correlated with the process parameter such as initial arsenic concentration, flow rate and adsorption capacity.
1991
BDST plots for Cr(VI) adsorption onto AEB were made. The slope of the BDST curves represents the time required for the adsorption zone to travel a unit length through the adsorbent under the selected experimental conditions at a given concentration. This is used to predict the performance of the bed, if there is a change in the initial solute concentration, C0, to a new value of solute concentration. The critical bed depth Z0 is obtained for t = 0 and for a fixed outlet concentration Ct = Cb, where Cb is the concentration at the breakthrough defined as a limit concentration or a fixed percent of initial concentration. v C0 Z0 ¼ ln 1 ð8Þ kaN 0 Cb The critical bed depth (Z0) represents the theoretical depth of adsorbent, which is necessary to prevent the sorbate concentration to exceed the limit concentration Cb. The critical bed depth is the theoretical depth of the sorbent sufficient to prevent the adsorbate concentration from exceeding Cb (breakthrough concentration) at t = 0. After applying Eq. (7) to the experimental data at the breakthrough point, a linear relationship was found while plotting service time (t) against bed height, Z for Ct 0.1 mg/L (breakthrough point). As could be seen from the Table 3, the values of kinetic constants were influenced by flow rate and increased with increase in flow rate indicating that overall system kinetics was dominated by external mass transfer in the initial part of the adsorption in the fixed bed column. Calculated values of adsorption capacity (N0) showed a marginal decrease with increase in flow rate. These results correlate well with the observed sorption capacity and showed poor adsorber performance at high flow rate. Time required to pass a unit length of primary adsorption zone (td) also decreased with increase in flow rate. 3.4. Study of adsorption mechanism of chromium on AEB Maximum Cr(VI) adsorption over AEB is 99.9% at initial solution pH 2.0 (Sarin and Pant, 2006). To determine the behaviour of AEB towards adsorption of Cr(VI), X-ray diffraction (XRD), scanning electron microscopy (SEM) and infra red spectroscopy (IR) on fresh AEB and exhausted AEB (with chromium) were carried out. The results obtained explain the mechanism for uptake of Cr(VI) exhibited by AEB at low pH. From SEM of adsorbent (AEB) it was observed that adsorbent was fibrous in nature whereas SEM of adsorbent (AEB) saturated with chromium Cr(VI) clearly indicated the adsorption of Cr(VI) on the surface of the adsorbent did not affect the crystallinity of the surface. XRD of both adsorbent (AEB) before and after saturation with Cr(VI) was also carried out to identify the phase present on sorbent. The structure of AEB remained the same before and after saturation with Cr(VI). There was no change in the structure, indicating that the structure
1992
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
Table 3 BDST parameters for sorption of Cr(VI) over AEB Adsorbate
Bed depth, m · 102
Flow rate, m3/h · 104
Linear velocity, m/h
td, H/m
pH
Feed concentration, mg/L
N0, mg/kg
ka, m3/kg H · 102
Z0 critical bed depth, m
Cr(VI)
10
3.6
0.71
15
3.6
0.71
Cr(VI)
20
3.6
0.71
Cr(VI)
10
3.6
0.71
Cr(VI)
10
2.4
0.473
Cr(VI)
10
1.2
0.237
2 4.85 2 4.85 2 4.85 2 4.85 2 4.85 2 4.85
24.4
Cr(VI)
485 10 485 10 485 10
8407 173.24 8407 173.24 8407 173.24 8407 173.24 8401 173.12 8418 173.48
0.423 0.2215 0.423 0.2215 0.423 0.2215 0.65 0.2244 0.65 0.2244 0.65 0.2244
0.063 0.0583 0.063 0.0583 0.063 0.0583 0.0482 0.0441 0.0321 0.0075 0.016 0.0037
24.4 24.4 12.2 12.2 12.2
of AEB remains intact after Cr(VI) adsorption (Fig. 5). IR of the adsorbent (AEB) before and after saturation with Cr(VI) was carried out to identify the functional group effective for adsorption. IR wavelength used was in the range of 2.5–15 lm. Functional groups involved in the removal of chromium were identified from analysis of IR spectrum of AEB before and after saturation with Cr(VI) shown in the Fig. 6. The IR bands observed at wave numbers 2360.15, 889.36 and 667.31 cm1 with fresh sorbent were found missing with AEB saturated with Cr(VI) ions. These results indicated that these bonds were involved with sorption of Cr ions. The bond involved in adsorption at the wave numbers 2360.15 cm1 is triple or cumulative bond þ of ammonium salts (R2 –NHþ 2 or R3 –NH2 ) attached to polyphenols or straight organic chains of bark. Where as the bonds found missing at wave number 889.36 and 667.31 cm1 are of C–H bonds of unsaturated groups particularly aromatic rings and cis alkenes respectively. At low pH (pH 6 2) all the chromium complexes exist as negatively charged dichromate ions (HCr2 O 7 ). Further the concentration of hydronium ion is large at low pH. This result in electromeric effect on the ammonium salts attached to aromatic rings or straight chains of bark. Thus due to electromeric effect the sites becomes positively charged and the
Fig. 6. IR of fresh AEB and used, saturated with chromium, AEB.
Fig. 5. Plots of XRD of fresh AEB (a) and used (saturated with Cr) AEB (b).
number of sites increases, thereby attracting negatively charged dichromate ions. This phenomenon results in high and selective removal of Cr(VI) at low pH. Removal of poisonous hexavalent form of chromium from solutions was possible using selected adsorbents. The cost of preparation of AEB is low since the raw material required for its preparation is cheaply and abundantly available. AEB saturated with chromium, after drying, can be used as fuel. The ash obtained after burning of
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
this AEB shall contain chromium metal. This ash would be of very low volume compared to conventional chromium sludge, thus can be disposed off easily. 4. Conclusions Activated eucalyptus bark was found to be an effective biosorbent for removal of Cr(VI) from industrial effluent. The adsorption of Cr(VI) by AEB is endothermic spontaneous in nature. The adsorption capacity increases with increase in temperature range of 293–343 K. Gibbs free energy change has been observed as 5.572 kJ mol1 and negative value indicates the feasibility of adsorption for system. Up flow fixed bed column adsorption studies have shown that chromium removal was a strong function of initial flow rate, bed height and initial sorbate concentration. Mutual effects of the adsorption capacities and adsorption rate satisfactorily explain the dependence of shape of the breakthrough curves on experimental parameters. Higher bed volume of wastewater could be treated at low chromium concentration and lower flow rate. Relatively higher sorption capacities were observed in fixed bed column operation compared to batch operation. The kinetic constants were influenced by flow rates and increased with increase in flow rate. It was concluded that at low pH concentration of hydronium ion is large which results in electromeric effect on the ammonium salts attached to the aromatic rings or straight chain of bark. References Aksu, Z., Ferda, G.F., 2004. Biosorption of phenol by immobilized activated sludge in a continuous packed bed: prediction of breakthrough curves. Process Biochem. 39 (5), 599–613.
1993
APHA, 1992. Standard Methods for the Examination of Water and Wastewater, 18th ed. APHA, Washington, DC. Bohart, G.S., Adams, E.Q., 1920. Behavior of Charcoal towards Chlorine. J. Chem. Soc. 42, 523–529. Cieslak-Golonka, M., 1995. Toxic and mutagenic effects of chromium(VI). Polyhedron 15, 3667–3689. Dahbi, S., Azzi, M., de la Guardia, M., 1999. Removal of hexavalent chromium from wastewaters by bone charcoal. Fresenius J. Anal. Chem. 363, 404–407. Faust, S.D., Aly, O.M., 1987. Adsorption Processes for Water Treatment. Butterworth Publishers, USA. Hutchins, R.A., 1973. New method simplexes design of activated carbon system. Chem. Eng. J. 80, 133–138. Kowalski, Z., 1994. Treatment of chromic tannery wastes. J. Hazard. Mater. 37, 137–144. McKay, G., 1982. Adsorption of dyestuffs from aqueous solution with activated carbon I: equilibrium and batch contact time studies. J. Chem. Technol. Biotechnol. 32, 759–772. Namasivayam, C., Yamuna, R.T., 1995. Adsorption of chromium(VI) by a low cost adsorbent: biogas residual slurry. Chemosphere 30, 561– 578. Sarin, V., Pant, K.K., 2006. Removal of chromium from industrial waste by using eucalyptus bark. Bio Resour. Technol. 97, 15–20. Singh, T.S., Pant, K.K., 2004. Equilibrium, kinetic and thermodynamics studies for adsorption of As(III) on activated alumina. Sep. Pur. Technol. 36, 139–147. Tien, C., 1994. Adsorption Calculations and Modeling. ButterworthHeinemann, USA. Tiravanti, G., Petrluzzelli, D., Passino, R., 1997. Pretreatment of tannery wastewaters by an ion exchange process for Cr(III) removal and recovery. Water Sci. Technol. 36, 197–207. US Department of Health and Human Services, 1991. Toxicological Profile for Chromium. Public Health Services Agency for Toxic substances and Diseases Registry, Washington, DC. Weber, W.J., Smith, E.H., 1987. Simulation and design models for adsorption processes. Environ. Sci. Technol. 21, 1040–1050.
Thermodynamic and breakthrough column studies for the selective sorption of chromium from industrial effluent on activated eucalyptus bark Vikrant Sarin, Tony Sarvinder Singh, K.K. Pant
*
Department of Chemical Engineering, Indian Institute of Technology, New Delhi 110 016, India Received 6 August 2005; received in revised form 26 September 2005; accepted 1 October 2005 Available online 28 November 2005
Abstract Studies were carried out on adsorption of Cr(VI) on an adsorbent made from eucalyptus bark. Results revealed that sorption of chromium on activated eucalyptus bark (AEB) was endothermic in nature. Thermodynamic parameters such as the entropy change, enthalpy change and GibbÕs free energy change were found out to be 100.97 J mol1 K1, 33 kJ mol1 and 0.737 kJ mol1, respectively. Industrial chrome effluent of different chromium concentration at different pH was used as feedstock for the fixed bed adsorption studies. When effluent was fed to the column at low pH of 2, the breakthrough volume increased significantly compared to effluent at higher pH of 4.85. The surface properties of sorbent were characterized by the Scanning electron microscopy, X-ray diffraction technique and Infrared techniques. It was concluded that AEB sorbent column could be used effectively for removal of chromium from industrial effluents by reducing the pH of chrome effluent to two and at optimal column conditions. 2005 Elsevier Ltd. All rights reserved. Keywords: Fixed bed column; Adsorption; Activated eucalyptus bark; Breakthrough
1. Introduction Since the start of industrialization in the nineteenth century, the material cycle, amount of heavy metals, which impact upon water, soil, plants, animals, and people have increased considerably. Many industries, particularly in the trade of metal processing operations and refineries represent significant sources of heavy metal emissions. These include cement manufacturing units (thallium), battery manufacturers (lead), electroplating (Cu, Ni, Cr), tanning (Cr), pigments (Cr, lead), wood preservation (Cr), etc. Through refuse wastewater, heavy metals like chromium ions enter especially into fresh water or soil and thereby into plants, animals and humans. Considering process in biosphere, the geochemical cycle is connected via metabolic processes by microorganisms in the ocean floor, in sedi*
Corresponding author. Tel.: +91 11 2659 6172; fax: +91 11 2658 1120. E-mail address: [email protected] (K.K. Pant).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.10.001
ments or sludge, or in water. Microorganism plants and small animals serve as food source for fish. By means of plants and other animals, levels of heavy metals from water and soil find their way into our food, however when accumulated to high levels, more than the threshold limits of the organisms, it can generate serious problems and diseases. Chromium is highly reactive element. It exists in six oxidation states and the two most stable states are Cr(III) and Cr(VI). Chromium forms stable complexes as Cr2 O 7, HCrO , CrO , HCr O and fraction of these complexes. 2 7 4 4 Fraction of the above complexes varies with pH and at low pH Cr exists as negatively charged dichromate ions. Cr(VI) is the most toxic form thus it becomes imperative to remove Cr(VI) from industrial effluents before discharging into the water or on to the land. Chromium compounds are amphoteric in nature i.e. they exhibit both acidic and basic properties. These compounds also exhibit point of minimum solubility. The pH of minimum solubility for chromium is 7.5. Widespread use of chromium chemicals
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
in various industries leads to problem of contamination of natural waters due to improper disposal methods. Chromium effluent is generated by the following industrial uses of chromium like leather tanning, mining of chrome ore, production of steel and alloys, metal finishing industries, pigments manufacturing units, photographic industry, glass industry, wood preservation, chromium salts being used as corrosion inhibitor in the cooling water treatment in the industry, textile industry and many other industrial applications. Chromium is non-biodegradable and toxic beyond a given concentration. Chromium threshold concentration on inhibitory effect on heterotrophic organisms is 10 mg/ L for Cr(III) and 1 mg/L for Cr(VI). All living organisms require varying amount of Cr(III) in micro amounts for proper growth. The same metal becomes toxic when present in elevated concentration. As per studies carried out Cr(VI) is 500 times more toxic to Cr(III) (Kowalski, 1994). Cr(VI) is considered by International Agency for Research on Cancer (IARC) 1982 as a powerful carcinogenic agent that modifies the DNA transcription process causing important chromosomic aberration (US Department of Health and Human Services, 1991; CieslakGolonka, 1995). The current BIS (Bureau of Indian Standards) recommended guideline value for chromium (as total Cr) in drinking water as 0.05 mg/L (maximum). Chromium ions are removed from the effluent using precipitation technique. This technique is widely practiced by the industry. The common problem associated with this process is generation of large volume toxic sludge. Safe disposal of this toxic sludge is another problem. Further this method depends on the addition of chemicals to the effluent. Ion exchange (Tiravanti et al., 1997), adsorption (Dahbi et al., 1999) electrodialysis, membrane process, solvent extraction, freeze separation, bio-separation are some of the other methods for removal. The present study aimed at development of suitable method using low cost bio adsorbent for selective removal of chromium by adsorption from industrial effluent. 2. Methods 2.1. Materials All the chemicals used in the study were of analytical grade (Merck, Germany). Industrial effluent was obtained from automobile ancillary unit located at Sahibabad, Uttar Pradesh (India). Industrial chromium effluent was obtained from the outlet of the chrome plating section of the unit. Synthetic samples used in the study were prepared using double distilled/deionised water. Characteristics of different chromium effluent are given in Table 1. Measurements were made in triplicates for the analysis of metal concentration and data were recorded when the variations in two readings were less than 5%. All glass wares and sample bottles were soaked in hot water and rinsed with laboratory dish soap (Lavolene) for at least two hours and then rinsed
1987
Table 1 Characteristics of chromium effluent Parameters TDS (mg/L) pH Concentration Cr(VI) (mg/L) Concentration Cr(III) (mg/L) Hardness Ca (mg/L) Hardness Mg (mg/L) a
#
SS
2–8 200 – – –
IE-1a
IE-2
IE-3
IE-4
70 4.95 12.2 0 16.5 11.5
122 4.45 24.4 0 36 24
900 3.41 200 44.5 134 92
1934 4.2 488.5 14.6 240 184
IE—industrial effluent.
with de-ionized water. Hexavalent chromium [Cr(VI)] stock solution (1000 mg L1) was prepared by dissolving potassium dichromate (K2Cr2O7) in de-ionized water and stored in airtight polyethylene terphthalate (PET) bottles at a temperature less than 2 C. Further working solutions were freshly prepared from stock solution for each experimental run. Concentration of chromium Cr(VI) stock solution was measured periodically to monitor variations in concentration. Cr(III) concentration was determined by measuring the difference between total chromium concentration and Cr(VI) concentration. Total Cr concentration was determined by oxidizing Cr(III)–Cr(VI) using KMnO4 and then determining final Cr(VI) content in the sample (APHA, 1992). 2.2. Thermodynamic studies The adsorbent activated eucalyptus bark (AEB) used for the removal of Cr(VI) was prepared in the laboratory. The details of sorbent preparation, kinetics of adsorption and comparison of chromium removal efficiency with other adsorbents have been discussed elsewhere (Sarin and Pant, 2006). Adsorption, studies in the temperature range 293– 343 K were conducted to determine thermodynamics constants such as Gibbs free energy change (DG0), enthalpy change (DH0) and entropy change (DS0) for the system and to ascertain the sorption mechanism. For this study, adsorbent dosage selected was 5 g/L of chromium Cr(VI) effluent in a conical flask and allowed to equilibrate for 24 h at the different temperatures ranging from 293 to 343 K. Residual chromium concentration was determined in each sample after equilibrium was attained. 2.3. Fixed bed adsorption studies Fixed bed adsorption studies were conducted to evaluate dynamic behaviour for Cr(VI) removal on AEB. The experimental set-up consisted of up flow glass column (0.025 m ID, 0.4 m length) packed with different bed heights of sorbent. Influent feed flow rate was varied and maintained throughout the experiments by the use of variable flow peristaltic pump (Neolab India). At the exit of the column, flow rate was also measured so as to get steady state flow conditions in the column. Experiments were conducted with industrial effluent (pH 4.85) and at optimum pH 2, obtained from batch experiments.
1988
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The empty bed contact time (EBCT) varied from 0.061 to 0.31 h during runs by varying the initial feed flow rate or bed height. Sampling of column effluent was done at specified time intervals in order to investigate the breakthrough point (BTP) and shape of breakthrough curve. Effects of inlet feed flow rate, initial sorbate concentration (10–25 mg/L) and adsorbent bed height (0.1–0.2 m) were investigated on the performance of breakthrough for the adsorption of Cr(VI). Initial chromium Cr(VI) concentration was also varied in the range of 12–24 mg/ L. For the analysis of mass transfer zone, breakthrough data were analyzed by Bohart and Adams model. 2.4. Characterization of activated eucalyptus bark X-ray diffraction (XRD) pattern, infra-red spectroscopy (IR) and scanning electron microscopy (SEM) studies were carried on AEB to determine its adsorption mechanism. For these IR spectra of fresh and used eucalyptus bark was carried out on IR spectroscope (PROTEGE 460, USA) in the range of wave numbers from 4000 to 500 cm1. SEM of the fresh and used sorbents was carried out to determine the qualitative analysis of surface morphology of the sorbent. The analysis was done on a model Cambridge Stereo Scan 360, UK. 3. Results and discussion 3.1. Thermodynamic studies In order to determine thermodynamic parameters, experiments were carried out at different temperatures in the range of 293–343 K for Cr(VI) adsorption. The thermodynamic parameters such as standard GibbÕs free energy change (DG0), enthalpy change (DH0) and entropy change (DS0) were estimated to evaluate the feasibility and nature of the adsorption process (Sarin and Pant, 2006). The GibbÕs free energy change, of the process is related to equilibrium constant by the equation DG0 ¼ RT ln K c
ð1Þ
where, T is temperature in K, R is ideal gas constant having value as 8.314, J mol1 K1 and Kc is thermodynamic equilibrium constant. The thermodynamic equilibrium constant, was determined as Kc ¼
Ca Ce
ð2Þ
where, Ca is mg of adsorbent adsorbed per liter and Ce is the equilibrium concentration of solution, mg/L. According to thermodynamics, the GibbÕs free energy change is also related to the enthalpy change (DH0) and entropy change (DS0) at constant temperature by the following equation
ln K c ¼
DS 0 DH 0 R RT
ð3Þ
The values of enthalpy change (DH0) and entropy change (DS0) were calculated from the slope and intercept of the plot ln Kc versus, 1/T with R2 value of 0.94. For the synthetic sample (C0 200 pm Cr(VI), pH 5.7) at temperatures 333 and 343 K the values of Kc were 1.305 and 1.913 respectively. The corresponding DG0 values were 0.737 and 1.850 kJ mol1 respectively. The values, enthalpy change (DH0) and entropy change (DS0) were found out to be as 33 kJ mol1 and 100.97 J mol1 K1 respectively. The calculated values of these parameters of various other synthetic and industrial samples are reported in Table 2. A positive value of DH0 as 33 kJ mol1 for chromium removal with AEB, indicated the endothermic nature of the process. A negative value of the free energy (DG0) (0.737 kJ mol1 at 333 K and 1.85 kJ mol1 at 343 K), indicated the spontaneous nature of the adsorption process. It was also noted that the change in free energy, increases with increase in temperature, which exhibits an increase in adsorption with rise in temperature. This could be possibly because of activation of more sites on the surface of AEB with increase in temperature. A positive value of DS0 as 100.97 J mol1 K1 showed increased randomness at solid solution interface during the adsorption of Cr(VI) on AEB. A value of Freundlich adsorption intensity (n) higher than 1, obtained from batch indicated that sorption of chromium on AEB is favourable and capacity is only slightly reduced at lower equilibrium concentration (Sarin and Pant, 2006). The endothermic nature of adsorption for chromium removal has also been reported on biogas residual slurry (Namasivayam and Yamuna, 1995). 3.2. Up flow fixed bed column adsorption studies on chromium removal Batch experimental data are often difficult to apply directly to fixed bed column adsorber because isotherms are unable to give accurate data for scale up since a flow column is not at equilibrium. Fixed bed column adsorption experiments were carried out to study the adsorption dynamics. The fixed bed column operation allows more efficient utilization of the adsorptive capacity than the batch process. The adsorbent at the inlet end is contacted continuously by the solution at the initial solute concentra-
Table 2 Thermodynamic parameters for the adsorption of Cr(VI) by AEB Effluent
Effluent concentration Cr(VI), mg/L
pH
Equilibrium constant, Kc
Gibbs free energy, kJ mol1
Synthetic effluent Synthetic effluent Synthetic effluent Industrial effluent Industrial effluent
200 [Cr(VI)] 200 [Cr(VI)] 200 [Cr(VI)] 200 [Cr(VI)] 44.5 [Cr(III)]
2 3 4.7 3.4 3.4
9.0 4.95 1.36 2.10 4.60
5.57 4.06 0.79 1.88 3.87
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3.2.1. Effect of fixed bed column height on the performance of breakthrough The steepness of the breakthrough curves is a strong function of bed height. Two different industrial chromium effluents were passed through the fixed bed column. One having Cr(VI) concentration as 24 mg/L with pH 4.85 (without changing the pH) and the other effluent was treated at optimum pH 2. The breakthrough volume obtained for effluents treated at pH 4.85 were very less compared to the effluent having pH 2. Figs. 1 and 2 represent the performance of breakthrough curves at a bed height of 0.1, 0.15 and 0.20 respectively. As it is evident from these figures, an increase in column depth increased the throughput volume 25
Ct (mg/L)
20 15 10 Bed height - 0.1 m
5
Bed height - 0.15 m Bed height - 0.2 m
0 0
200
400
600
800 1000 Time (min)
1200
1400
Fig. 1. Effect of bed height, 0.10, 0.15 and 0.2 m on adsorption of Cr(VI) by AEB (flow rate: 3.6 · 104 m3/h, C0: 24.4 mg/L, pH: 4.85).
25
20
Ct (mg/L)
tion. The most important criterion in the design of fixed bed adsorption systems is the prediction of fixed bed column breakthrough or the shape of the adsorption wave front, which determines the operation life span of the bed. The design of up flow fixed bed column adsorber includes estimation of shape of breakthrough curves and the appearance of breakpoint. The dynamics of adsorption process was studied using bed depth service time (BDST) method based on a model proposed by Bohart and Adams (1920). The effect of fixed bed column parameters such as sorbent bed height, flow rate and initial chromium concentration on the performance of breakthrough curves were investigated. All the fixed bed column experiments were carried out at ambient conditions at pH 2 and 4.85 having Cr(VI) concentration as 24.4 mg/L. Flow rate of the effluent through the column was maintained at 3.6 · 103 m3/h. Breakthrough volumes obtained at bed heights of 0.1, 0.15 and 0.2 m for effluent having Cr(VI) concentration as 24.4 mg/L and pH 4.85 were found out to be 1.48 · 104 m3, 3.28 · 104 m3 and 5.76 · 104 m3 respectively, the corresponding values at a pH of 2 were 66.96 · 104 m3, 147.5 · 104 m3 and 242.0 · 104 m3 respectively. The total chromium adsorbed in the column for a given feed concentration is equal to the area under the breakthrough curve. The steepness of breakthrough curve was a strong function of fixed bed column parameters.
1989
15
10
5
Bed height - 0.1 m Bed height - 0.15 m Bed height - 0.2 m
0 0
1000
2000
3000 Time (min)
4000
5000
6000
Fig. 2. Effect of bed height, 0.10, 0.15 and 0.2 m on adsorption of Cr(VI) by AEB (flow rate: 3.6 · 104 m3/h, C0: 24.4 mg/L, pH: 2).
treated due to higher contact time. At relatively lower contact time, the curve was steeper for both the effluents showing the faster exhaustion of the bed. For Cr(VI) removal the treated volume varied from 1.48 · 104 m3 to 5.76 · 104 m3 as the bed height was increased from 0.1 m to 0.2 m (Figs. 1 and 2). The curves followed characteristic S shape profile, which is associated with adsorbate of smaller molecular diameter and more simple structure. The increase in the metal uptake capacity at either pH of 4.85 or 2 with the increase of bed height in the fixed bed column was probably due to increased surface area of the adsorbent, which provide more binding sites for the adsorption. The breakthrough time was also increased with the increase in bed height. 3.2.2. Effect of flow rate on the performance of breakthrough curves Studies at different flow rates were conducted in the fixed bed of AEB. The flow rate was varied from 3.6 · 104 m3/h to 1.2 · 104 m3/h while the inlet Cr(VI) concentration was kept constant at 12.2 mg/L at pH 4.85. Effluent at optimal pH 2 was also run at a flow rate of 3.6 · 104 m3/h. The effect of flow rate on breakthrough performance, at a bed height of 0.10 m has been shown in Fig. 3. As can be seen from the breakthrough plots, higher efficiency of the process was achieved at lower flow rate. This can be explained by the fact at low empty bed contact time (EBCT), the diffusion process which controls the sorption becomes slow, and hence, the sorbent needs more time to bond the metals efficiently. These breakthrough curves demonstrate influence of sorption kinetics. The uptake of chromium decreased with increase in flow rate. It was also observed that sorbents get saturated easily at higher flow rates. Breakthrough volume obtained at pH 4.85 was significantly less than at pH 2. As volumetric flow rate increases, the velocity through the bed decreases, which results in the decrease in the length of adsorption zone. At higher flow rates breakthrough curves become steeper and breakpoint time and the adsorbed
1990
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
14
25
Cr(VI) conc 24.4 mg/L at pH 4.85 Cr(VI) conc 12.2 mg/L at pH 4.85 Cr(VI) conc 24.4 mg/L at pH 2 Cr(VI) conc 12.2 mg/L at pH 2
12
20
8
Ct (mg/L)
Ct (mg/L)
10
6
4 ml/min
4
15
10
6 ml/min 2
2 ml/min 5
0 0
500
1000
1500
2000 2500 Time (min)
3000
3500
4000
Fig. 3. Effect of flow rate on breakthrough curves for adsorption of Cr(VI) by AEB (C0: 12.2 mg/L, bed height: 0.10 m, pH: 4.85, flow rate m3/h · 104: (j) 6 (r) 4 (m) 2).
0 0
500
1000
1500 Time (min)
2000
2500
Fig. 4. Effect of initial Cr(VI) concentration on breakthrough performance (flow rate: 3.6 · 104 m3/h, pH: 4.85 and 2, bed height: 0.10 m).
ion concentration decreases. A higher uptake of chromium was observed at the beginning of fixed bed column operation but as solution continued to flow, the concentration of chromium in the effluent rapidly increased; the bed becomes saturated with chromium and the concentration of the solute in the effluent rise sharply. An increase in flow rate from 1.2 · 104 m3/h to 3.6 · 104 m3/h reduced the volume of effluent treated from 0.456 m3 to 0.162 m3 for Cr(VI) removal over AEB. There was a corresponding decrease in sorption capacity from 662 mg/kg to 617 mg/kg for effluent having Cr(VI) concentration as 12.2 mg/L and pH 4.85. Whereas sorption capacity for effluent having Cr(VI) concentration as 12.2 mg/L at pH 2 was 8748 mg/kg. As Cr(VI) ions moves from bulk solution to the surface of the film surrounding the sorbent particle, concentration gradient develops at the interface, which allows the solute particle to penetrate through the film, and reaches the surface of the particle where the positively active sites attract the negatively charged dichromate ions. At higher flow velocities, film surrounding the particle breaks thereby reducing the adhesion of sorbate to the sorbent particle. As can be seen from Fig. 3 the uptake of Cr(VI) decreased with the increase in flow rate. The breakthrough capacity of the sorbent decreased with increase in flow rate. As the velocity through the bed decreased, the depth of the adsorption zone decreased because there was more time for adsorption to occur. At low pH, the surface of AEB becomes positively charged because many more positive sites are created and amide functional groups become positively charged due to electromeric effect. This results in many more positive charges being created on the surface of the adsorbent, resulting in many folds increase in adsorption capacity of AEB at low pH. 3.2.3. Effect of initial chromium concentration on breakthrough curve Effect of initial chromium concentration on the performance of breakthrough curve is shown in Fig. 4. Break-
through volume obtained at bed height of 0.1 m, effluents having flow rate as 3.6 · 104 m3/h through the bed and having Cr(VI) concentration as 24.4 and 12.2 Cr(VI) both at pH 4.85 was 1.35 · 104 m3 and 3.6 · 104 m3 respectively. On optimising the pH to 2 of the above two effluents and keeping all the other experimental conditions same, breakthrough volume increased and was obtained as 66.96 · 104 m3 and 101.7 · 104 m3 respectively. A change in the inlet sorbate concentration affected the operating characteristics of the fixed bed column. At low initial concentration, breakthrough occurred late and the treated volume were higher since the lower concentration gradient caused a slower transport due to decreased diffusion coefficient or mass transfer coefficient (Aksu and Ferda, 2004). It was observed that at low pH the adsorption increased many folds. This could be due to increase in the number of binding sites. The sorbent gets saturated early at high initial concentration because binding sites become more quickly saturated in the system. These results indicated that increase in chromium concentration reduced the metal to sorbent ratio and the metal uptake as long as the sorbent was not saturated. Initial sorbate concentration affected the performance of fixed bed for adsorption of chromium (Fig. 4). Breakpoint time decreased with increasing inlet Cr(VI) concentration as the binding sites saturated faster. These results showed that performance of breakthrough curves for chromium removal by AEB was strongly affected by initial concentration and pH of solution and thus affected the treatment of effluent. 3.3. Modelling of breakthrough curves Many mathematical models have been developed to describe contaminant sorption over different adsorbents in many of the diverse applications for which it is used (Weber and Smith, 1987; Tien, 1994). The design of the adsorption process is based on the accurate generation of
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
breakthrough curves. The fluid velocity, concentration of solute in feed and the bed height affect the shape of the curve. Steep slopes of breakthrough curves were obtained for systems that exhibit high film transfer coefficients, high internal diffusion coefficients or flat adsorption isotherm that is smaller value of 1/n (Faust and Aly, 1987). The dynamic behaviour of the fixed bed column was predicted with the Adam–Bohart model in the present investigation. 3.3.1. Adam–Bohart model Bohart and Adams (1920) established the fundamental equations describing the relationship between Ct/C0 and t in a continuous system for the adsorption of chlorine on charcoal, known as bed depth service time (BDST) model. Although the original work by Adams–Bohart was done for the gas–charcoal adsorption system, its overall approach has been applied successfully in quantitative description of other systems (McKay, 1982; Singh and Pant, 2004). This model assumes that the adsorption rate is proportional to both the residual capacity of the sorbent and the concentration of the sorbing species. The Adams–Bohart model is used only for the description of the initial part of the breakthrough curve i.e. up to the breakpoint or 10–50% of the saturation points. The main objective of the fixed bed adsorption is to reduce concentration in the effluent so that it does not exceed permissible limit (breakthrough concentration Cb). This approach was focused on the estimation of characteristic parameters such as maximum adsorption capacity (N0) and kinetic constant (ka) from Adam–Bohart model using a quassi chemical kinetic rate expression. The mass balance obeys the following equation oq ¼ k a qC ot oC ka ¼ qC oz v
ð4Þ ð5Þ
The following equation can be obtained to relate the process conditions and operating parameters with service time. C0 ln 1 ¼ ln eka N 0 Z=v 1 k a C 0 t ð6Þ C A linear relationship between bed depth and service time given by Eq. (7) (Hutchins, 1973) N 0Z 1 C0 t¼ 1 ð7Þ ln C0v kaC0 C were C is the effluent concentration of adsorbate in the liquid phase (mg/L); C0 is initial concentration of sorbate in the liquid phase (mg/L); v is the linear flow velocity (m/h); N0 is adsorption capacity (mg solute/kg adsorbent); ka is rate constant in BDST model (m3/kg h); t is time (h) and Z is bed depth of fixed bed column (m). Eq. (7) enables the service time, t, of an adsorption bed to be determined for a specified bed depth, Z, of adsorbent. The service time and bed depth were correlated with the process parameter such as initial arsenic concentration, flow rate and adsorption capacity.
1991
BDST plots for Cr(VI) adsorption onto AEB were made. The slope of the BDST curves represents the time required for the adsorption zone to travel a unit length through the adsorbent under the selected experimental conditions at a given concentration. This is used to predict the performance of the bed, if there is a change in the initial solute concentration, C0, to a new value of solute concentration. The critical bed depth Z0 is obtained for t = 0 and for a fixed outlet concentration Ct = Cb, where Cb is the concentration at the breakthrough defined as a limit concentration or a fixed percent of initial concentration. v C0 Z0 ¼ ln 1 ð8Þ kaN 0 Cb The critical bed depth (Z0) represents the theoretical depth of adsorbent, which is necessary to prevent the sorbate concentration to exceed the limit concentration Cb. The critical bed depth is the theoretical depth of the sorbent sufficient to prevent the adsorbate concentration from exceeding Cb (breakthrough concentration) at t = 0. After applying Eq. (7) to the experimental data at the breakthrough point, a linear relationship was found while plotting service time (t) against bed height, Z for Ct 0.1 mg/L (breakthrough point). As could be seen from the Table 3, the values of kinetic constants were influenced by flow rate and increased with increase in flow rate indicating that overall system kinetics was dominated by external mass transfer in the initial part of the adsorption in the fixed bed column. Calculated values of adsorption capacity (N0) showed a marginal decrease with increase in flow rate. These results correlate well with the observed sorption capacity and showed poor adsorber performance at high flow rate. Time required to pass a unit length of primary adsorption zone (td) also decreased with increase in flow rate. 3.4. Study of adsorption mechanism of chromium on AEB Maximum Cr(VI) adsorption over AEB is 99.9% at initial solution pH 2.0 (Sarin and Pant, 2006). To determine the behaviour of AEB towards adsorption of Cr(VI), X-ray diffraction (XRD), scanning electron microscopy (SEM) and infra red spectroscopy (IR) on fresh AEB and exhausted AEB (with chromium) were carried out. The results obtained explain the mechanism for uptake of Cr(VI) exhibited by AEB at low pH. From SEM of adsorbent (AEB) it was observed that adsorbent was fibrous in nature whereas SEM of adsorbent (AEB) saturated with chromium Cr(VI) clearly indicated the adsorption of Cr(VI) on the surface of the adsorbent did not affect the crystallinity of the surface. XRD of both adsorbent (AEB) before and after saturation with Cr(VI) was also carried out to identify the phase present on sorbent. The structure of AEB remained the same before and after saturation with Cr(VI). There was no change in the structure, indicating that the structure
1992
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
Table 3 BDST parameters for sorption of Cr(VI) over AEB Adsorbate
Bed depth, m · 102
Flow rate, m3/h · 104
Linear velocity, m/h
td, H/m
pH
Feed concentration, mg/L
N0, mg/kg
ka, m3/kg H · 102
Z0 critical bed depth, m
Cr(VI)
10
3.6
0.71
15
3.6
0.71
Cr(VI)
20
3.6
0.71
Cr(VI)
10
3.6
0.71
Cr(VI)
10
2.4
0.473
Cr(VI)
10
1.2
0.237
2 4.85 2 4.85 2 4.85 2 4.85 2 4.85 2 4.85
24.4
Cr(VI)
485 10 485 10 485 10
8407 173.24 8407 173.24 8407 173.24 8407 173.24 8401 173.12 8418 173.48
0.423 0.2215 0.423 0.2215 0.423 0.2215 0.65 0.2244 0.65 0.2244 0.65 0.2244
0.063 0.0583 0.063 0.0583 0.063 0.0583 0.0482 0.0441 0.0321 0.0075 0.016 0.0037
24.4 24.4 12.2 12.2 12.2
of AEB remains intact after Cr(VI) adsorption (Fig. 5). IR of the adsorbent (AEB) before and after saturation with Cr(VI) was carried out to identify the functional group effective for adsorption. IR wavelength used was in the range of 2.5–15 lm. Functional groups involved in the removal of chromium were identified from analysis of IR spectrum of AEB before and after saturation with Cr(VI) shown in the Fig. 6. The IR bands observed at wave numbers 2360.15, 889.36 and 667.31 cm1 with fresh sorbent were found missing with AEB saturated with Cr(VI) ions. These results indicated that these bonds were involved with sorption of Cr ions. The bond involved in adsorption at the wave numbers 2360.15 cm1 is triple or cumulative bond þ of ammonium salts (R2 –NHþ 2 or R3 –NH2 ) attached to polyphenols or straight organic chains of bark. Where as the bonds found missing at wave number 889.36 and 667.31 cm1 are of C–H bonds of unsaturated groups particularly aromatic rings and cis alkenes respectively. At low pH (pH 6 2) all the chromium complexes exist as negatively charged dichromate ions (HCr2 O 7 ). Further the concentration of hydronium ion is large at low pH. This result in electromeric effect on the ammonium salts attached to aromatic rings or straight chains of bark. Thus due to electromeric effect the sites becomes positively charged and the
Fig. 6. IR of fresh AEB and used, saturated with chromium, AEB.
Fig. 5. Plots of XRD of fresh AEB (a) and used (saturated with Cr) AEB (b).
number of sites increases, thereby attracting negatively charged dichromate ions. This phenomenon results in high and selective removal of Cr(VI) at low pH. Removal of poisonous hexavalent form of chromium from solutions was possible using selected adsorbents. The cost of preparation of AEB is low since the raw material required for its preparation is cheaply and abundantly available. AEB saturated with chromium, after drying, can be used as fuel. The ash obtained after burning of
V. Sarin et al. / Bioresource Technology 97 (2006) 1986–1993
this AEB shall contain chromium metal. This ash would be of very low volume compared to conventional chromium sludge, thus can be disposed off easily. 4. Conclusions Activated eucalyptus bark was found to be an effective biosorbent for removal of Cr(VI) from industrial effluent. The adsorption of Cr(VI) by AEB is endothermic spontaneous in nature. The adsorption capacity increases with increase in temperature range of 293–343 K. Gibbs free energy change has been observed as 5.572 kJ mol1 and negative value indicates the feasibility of adsorption for system. Up flow fixed bed column adsorption studies have shown that chromium removal was a strong function of initial flow rate, bed height and initial sorbate concentration. Mutual effects of the adsorption capacities and adsorption rate satisfactorily explain the dependence of shape of the breakthrough curves on experimental parameters. Higher bed volume of wastewater could be treated at low chromium concentration and lower flow rate. Relatively higher sorption capacities were observed in fixed bed column operation compared to batch operation. The kinetic constants were influenced by flow rates and increased with increase in flow rate. It was concluded that at low pH concentration of hydronium ion is large which results in electromeric effect on the ammonium salts attached to the aromatic rings or straight chain of bark. References Aksu, Z., Ferda, G.F., 2004. Biosorption of phenol by immobilized activated sludge in a continuous packed bed: prediction of breakthrough curves. Process Biochem. 39 (5), 599–613.
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APHA, 1992. Standard Methods for the Examination of Water and Wastewater, 18th ed. APHA, Washington, DC. Bohart, G.S., Adams, E.Q., 1920. Behavior of Charcoal towards Chlorine. J. Chem. Soc. 42, 523–529. Cieslak-Golonka, M., 1995. Toxic and mutagenic effects of chromium(VI). Polyhedron 15, 3667–3689. Dahbi, S., Azzi, M., de la Guardia, M., 1999. Removal of hexavalent chromium from wastewaters by bone charcoal. Fresenius J. Anal. Chem. 363, 404–407. Faust, S.D., Aly, O.M., 1987. Adsorption Processes for Water Treatment. Butterworth Publishers, USA. Hutchins, R.A., 1973. New method simplexes design of activated carbon system. Chem. Eng. J. 80, 133–138. Kowalski, Z., 1994. Treatment of chromic tannery wastes. J. Hazard. Mater. 37, 137–144. McKay, G., 1982. Adsorption of dyestuffs from aqueous solution with activated carbon I: equilibrium and batch contact time studies. J. Chem. Technol. Biotechnol. 32, 759–772. Namasivayam, C., Yamuna, R.T., 1995. Adsorption of chromium(VI) by a low cost adsorbent: biogas residual slurry. Chemosphere 30, 561– 578. Sarin, V., Pant, K.K., 2006. Removal of chromium from industrial waste by using eucalyptus bark. Bio Resour. Technol. 97, 15–20. Singh, T.S., Pant, K.K., 2004. Equilibrium, kinetic and thermodynamics studies for adsorption of As(III) on activated alumina. Sep. Pur. Technol. 36, 139–147. Tien, C., 1994. Adsorption Calculations and Modeling. ButterworthHeinemann, USA. Tiravanti, G., Petrluzzelli, D., Passino, R., 1997. Pretreatment of tannery wastewaters by an ion exchange process for Cr(III) removal and recovery. Water Sci. Technol. 36, 197–207. US Department of Health and Human Services, 1991. Toxicological Profile for Chromium. Public Health Services Agency for Toxic substances and Diseases Registry, Washington, DC. Weber, W.J., Smith, E.H., 1987. Simulation and design models for adsorption processes. Environ. Sci. Technol. 21, 1040–1050.