Studies on an economically viable remediation of chromium rich waters and wastewaters by PTPS fly ash

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Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 222–228

Studies on an economically viable remediation of chromium rich waters and wastewaters by PTPS fly ash Y.C. Sharma a,∗ , Uma a , S.N. Upadhyay b , C.H. Weng c a

Environmental Engineering and Research Laboratories, Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India b Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221005, India c Department of Civil and Ecological Engineering, I-Shou University, Da-Hsu Township, Kaohsiung 84008, Taiwan Received 26 June 2007; received in revised form 8 October 2007; accepted 10 October 2007 Available online 13 October 2007

Abstract Application of fly ash, a waste material of thermal power plants for the removal of chromium from aqueous solutions and wastewaters has been investigated. The adsorption follows a first order rate kinetics and the value of Kad , rate constant for removal was found to be 3.0 × 10−2 s−1 at 298 K, 100 rpm, 2.5 pH and at other optimum conditions. Intraparticle diffusion was found to control the removal of Cr(VI) and the value of coefficient of intraparticle diffusion was found to be 2.25 × 10−11 cm2 s−1 at 298 K. The value of the coefficient of mass transfer, βl , 2.15 × 10−2 cm s−1 at 298 K, suggested the transfer of Cr(VI) onto the adsorbent surface to be rapid enough. The process of removal is highly dependent on pH of the solutions with maximum removal (89.12%) at pH 2.5. Values of Langmuir’s constants were also calculated. Various thermodynamic parameters namely free energy change (G◦ ), enthalpy change (H◦ ) and entropy change (S◦ ) were calculated and found to be −0.68, 5.26 and 1.77 K cal mol−1 at 298 K. Comparison of the adsorbent used with other non-conventional adsorbents shows that fly ash is a good adsorbent and can be recommended for treatment of metal rich wastewater in general and that of Cr(VI) in particular. © 2007 Elsevier B.V. All rights reserved. Keywords: Cr(VI); Removal; PTPS fly ash; pH; Mass transfer; Langmuir isotherm

1. Introduction Chromium is a very important metal with numerous applications. Its concentration levels on earth’s crust are of the order Abbreviations: qe , amount of Cr(VI) adsorbed (mg g−1 ) at equilibrium; q, amount of Cr(VI) adsorbed (mg g−1 ) at any time t; Kad , rate constant of adsorption (min−1 ); t, time (min); Kp , rate constant of pore diffusion (mg g−1 min−1/2 ); t1/2 , time for half adsorption (min); r0 , radius of adsorbent particles (cm); D, pore diffusion coefficient (cm2 s−1 ); C0 , initial concentration of chromium in solution (mg l−1 ); Ct , concentration of dye at time t (mg l−1 ); m, mass of adsorbrnt particles per unit volume of particle free slurry (g l−1 ); K, Langmuir’s constant (l g−1 ); βL , coefficient of mass transfer (cm s−1 ); Ss , outer surface of the adsorbent particles per unit volume of particle free slurry (cm−1 ); Ce , equilibrium concentration of chromium (mg l−1 ); Q0 , Langmuir’s constant related to capacity of adsorption (mg g−1 ); b, Langmuir’s constant related to energy of adsorption (l mg−1 ); G◦ , change in free energy (K cal mol−1 ); H◦ , change in enthalpy (K cal mol−1 ); S◦ , change in entropy (K cal mol−1 ); R, gas constant (cal mol−1 K−1 ); K, equilibrium constant at temperature T; K1 , equilibrium constant at temperature T1 ; K2 , equilibrium constant at temperature T2 . ∗ Corresponding author. Tel.: +91 542 2307025; fax: ++91 542 2316428. E-mail address: y sharma [email protected] (Y.C. Sharma). 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.10.015

of 100 ppm [1]. It has a wide variety of applications and is extensively used in leather manufacturing, explosives, ceramics, paint, pigments, photography, wood preservation, etc. [2]. It is an alloying element of steel and many other alloys as well. Disposal of untreated wastes and effluents from these applications into water resources is one of the major causes of water pollution by chromium [3]. Chromium exists in four valency states namely zero (metallic chromium), bi-, tri- and hexavalent chromium but in nature it occurs in trivalent form as chromite. Tri- and hexavalent forms of chromium are biologically active with later being important from environment view point but its hexavalent form is reported to be 100 times more toxic than its trivalent form [4]. There are no evidences regarding toxicity of Cr(III) but there are several reports regarding toxic and harmful nature of Cr(VI) [5–7]. Exposure to Cr(VI) results in skin allergies like dermatitis, gastro-intestinal ulcers and bronchogenic cancer. It has also been reported to be acutely toxic and a mutagen. A number of treatment technologies viz. reduction, ion-exchange, electro dialysis, electrochemical precipitation, evaporation, solvent extraction, reverse osmosis and chemical

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precipitation are used for removal of chromium from wastewater. The above methodologies are costly and labour intensive in nature and at the same time require land and site for waste disposal. Adsorption on activated carbon, both in batches as well as in columns has been a popular choice especially for treatment of metal rich industrial effluents [8–11], but its large-scale application is limited by its high cost for developing nations like India. Many scientific workers have reported development of low cost activated carbons from cheaper and easily viable materials and waste materials [12–17]. These activated carbons have been used for removal of metallic species from aqueous solutions and wastewaters. Millions of tons of fly ash, a by-product of thermal power plants are produced everyday with India producing 100 million tons of it every year. The biggest problem posed by fly ash is its safe disposal. In India alone 4000 acres of land is utilized for disposal of fly ash. In addition to many applications of fly ash, scientific workers have used fly ash as an adsorbent for removal of a variety of metallic pollutants from aqueous solutions [18–22]. Main emphasis of the present work is mainly two fold: first being the application of waste material viz. fly ash and second the removal of the Cr(VI) from aqueous solutions and waste waters. Effect of various important parameters namely concentration, contact time, temperature, pH, ionic strength, adsorbent particle size, etc. on process of removal of Cr(VI) has been studied. Kinetic and equilibrium modeling of the process of removal has also been carried out. 2. Materials and methods 2.1. Materials All the chemicals used in the experiments were procured from B.D.H., Mumbai, India and were of A.R. grade. The adsorbent, fly ash was obtained from a nearby thermal power plant, Patratu Thermal Power Station (PTPS), Jharkhand, India. Fly ash is a cheap, abundant and well known waste material of thermal power plants. Fly ash is basically a mixture of a number of metallic oxides with silica (53.80% by weight), alumina (28.24% by weight) as its major constituents. Other constituents of fly ash are iron oxide, magnesia and titanium dioxide. Composition of fly ash varies and depends on the quality of coal used for combustion. Indian coal is mainly lignite and contains ash up to 30–40% [23]. Combustion of this coal produces more amount of fly ash. In order to remove earthen impurities, fly ash was washed several times with normal water and then with de-ionized water prior to its use in the experiments. 2.2. Methods 2.2.1. Physiochemical characterization of fly ash The selected adsorbent was sieved mechanically through sieves to maintain a uniform particle size. The average particle size of the fly ash was determined by particle size analyzer, Model HIAC-320 (Royco Instrument Division, USA). Surface area was measured by Lazer zee-meter, Model QS 7 (Quantachrome Corporation, USA) and porosity by mercury

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porosimeter. Chemical characterization of the adsorbent was carried out by Indian Standard Methods [24]. 2.3. Adsorption experiments 2.3.1. Kinetic adsorption experiments Batch adsorption experiments were carried out with 1.0 g of fly ash sample with 50 ml aqueous solution of Cr(VI) of desired concentrations at known temperature, pH and ionic strength in polythene bottles on a shaking thermostat with a constant speed of agitation at 100 rpm. Ionic strength of solutions was maintained a 1.0 × 10−2 M NaClO4 . The progress of adsorption was assessed by determining residual concentration of Cr(VI) by UV–vis spectrophotometer (Spectronic 20, Bausch and Lomb; USA) at 540 nm [25]. After equilibrium time, the adsorbent was separated by centrifugation from reaction mixture centrifuging at 10,000 rpm for 10 min. 2.3.2. Equilibrium adsorption experiments The batch method was used during the entire experiments. The experiments were carried out as follows. A series of 300 ml polythene bottles containing 50 ml of Cr(VI) solution of different concentrations was prepared. Ionic strength was maintained by 1.0 × 10−2 M NaClO4 and pH by 1.0 M, 1.0 × 10−1 M and 1.0 × 10−2 M HCl/NaOH, respectively. Then 1.0 g of the oven dried adsorbent of known particle diameter was added to the solutions. These bottles were shaken on a reciprocal shaker at 100 excursions per minute up to equilibrium time. pH of the reaction mixture was recorded each time. Amount of residual chromium was determined by a Bausch and Lomb (B&L) spectrophotometer at 540 nm. 3. Results and discussion 3.1. Characterization of the adsorbent Fly ash, the adsorbent chosen for the proposed work contains silica (53.80%) and alumina (28.24%) as major constituents. Oxides of Ca, Mg, K, Fe and Mn are also present. Its surface area was found to be 61.82 m2 g−1 . Other parameters are given in Table 1. Table 1 Physiochemical analysis PTPS fly ash Components

% by weight

Silica Alumina Calcium oxide Iron oxide Magnesia Magnesium oxide Potassium oxide Others Loss on ignition Density (g cm−3 ) Porosity Surface area (m2 g−1 )

53.80 28.24 2.42 2.21 1.47 0.72 0.38 10.75 3.02 1.57 0.29 61.82

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Fig. 1. Effect of contact time and concentration on removal of Cr(VI) by adsorption on fly ash.

Fig. 2. Lagergren’s plot for the dynamics of removal of Cr(VI) by adsorption on fly ash.

3.2. Effect of contact time and initial concentration on removal

Table 2 Values of rate constant for adsorption kad , rate constant for pore diffusion and kp and rate constant for mass transfer, βl

Contact time and concentration have pronounced effect on removal of pollutant species from aqueous solutions and industrial effluents [26–29]. A rapid transfer of pollutant species from bulk to the surface of adsorbent cuts short the equilibrium period. Time taken to attain equilibrium is important to know the efficacy and feasibility of the adsorbent for its use in removal of the pollutants. The development of charge on the surface of adsorbent is also governed by the contact time. In the present studies the removal of Cr(VI) decreased from 89.12 to 51.3% by increasing the concentration of Cr(VI) from 2.5 × 10−5 to 10.0 × 10−5 M (Fig. 1) at 2.5 pH, 150 ␮m adsorbent particle size, 100 rpm, agitation speed, 1.0 × 10−2 M NaClO4 ionic strength and 298 K temperature. The removal is rapid in initial stages, decreases gradually and finally attains equilibrium in 2.4 ks. An increase in initial concentration of Cr(VI) resulted in increase in the amount adsorbed from 0.12 to 0.27 mg g−1 by increasing the concentration of Cr(VI) in solutions from 2.5 × 10−5 to 10.0 × 10−5 M. This can be attributed to an increase in driving force of the concentration of Cr(VI) in aqueous solutions.

Temperature (K)

Kad (ml−1 )

Kp (mn−1/2 )

βl (×10−11 cm2 s−1 )

298 303 308

3.0 × 10−2 2.09 × 10−2 1.44 × 10−2

1.24 1.08 0.73

2.25 1.50 0.32

mined by slopes of Fig. 2 and are given in Table 2. Values of Kad are exhibiting a decreasing trend with temperature and this proves exothermic nature of the process of Cr(VI) removal. Intraparticle diffusion may also play role in the removal of Cr(VI) and this possibility was investigated by plotting graph between ‘amount adsorbed vs t1/2 (Fig. 3). These graphs show a double nature: straight line in initial stages and curved in

3.3. Kinetic studies In order to decipher the kinetics of removal of Cr(VI) by adsorption on PTPS fly ash, the data was tested with many models and finally the Lagergren’s model [30,31] was found to satisfy the experimental data: log(qe − q) = log qe − (Kad /2.303)t

(1)

where qe and q (both mg g−1 ) are the amounts of Cr(VI) adsorbed at equilibrium and at the time ‘t’ respectively and Kad (min−1 ) is rate constant of adsorption. The straight line graphs of ‘log (qe − q) vs t’ (Fig. 2) are single and smooth indicating suitability of proposed model. The values of Kad were deter-

Fig. 3. Intraparticle diffusion plot for the removal of Cr(VI) by adsorption on fly ash.

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final stages. The initial stages indicate boundary layer diffusion [30] and the straight line portions indicate dominance of intra particle diffusion [31]. The values of rate constant of intra particle diffusion, Kp were calculated from slopes of Fig. 3 and are given in Table 2. Value of Kp was found to be 1.24 × 10−2 mg g−1 min−1/2 at 298 K. The following equation [32]: D = 0.03r02 /t1/2

(2)

was used to determine the value of coefficient of intra particle diffusion D. Here, D (cm−2 s−1 ) is the coefficient of intra particle diffusion, r0 (cm) is radius of adsorbent particles and t1/2 is time for half adsorption of Cr(VI) by fly ash. The values of D were found to be of order of 10−11 and this indicates that intra particle diffusion plays a significant role in the process of removal [32]. The value of D at 298 K was, however, found to be 2.25 × 10−11 cm2 s−1 . Fig. 4. Plot for transfer of mass for removal of Cr(VI) by adsorption on fly ash.

3.4. Mass transfer studies To understand suitability of adsorbent for any adsorption process, it is necessary to know the transfer of adsorbate to the surface of adsorbent and this possibility was studied by using the following well known model [32]:     Ct 1 mk ln − = ln − {(1 + mk)/mk}βl Ss t C0 1 + mk 1 + mk (3) where Ct and C0 (both mg l−1 ) are the concentrations of adsorbate, Cr(VI), at time ‘t’ and initial concentration, m (g l−1 ) is the mass of the adsorbent, k(l g−1 ) is Langmuir’s constant, βl (cm s−1 ), coefficient of mass transfer and Ss (cm−1 ) is the outer surface of the adsorbent per unit volume of particle free slurry. Value of ‘m’ was calculated as follows [33,34]: m=

W V

(4)

and Ss was determined by following relation: Ss =

6m dp ρp (1 − εp )

(5)

where W(g) is weight of the adsorbent, V(l), volume of the adsorbent sample, dp (cm) is diameter of adsorbent particle, ρp (g cm−3 ) density of adsorbent, and ‘ε’, a unit less parameter is the porosity of the adsorbent. The value of the coefficient of mass transfer, βl , is measure of knowing rapidness of the transfer of adsorbate onto adsorbent surface. Its values were calculated at different temperatures by plotting ‘ln(Ct /C0 − 1/(1 + mk) vs t’ (Fig. 4). It is clear from perusal of this figure that the graphs are linear at all the values of temperature. This validates applicability of proposed mass transfer model for removal of Cr(VI) by adsorption on fly ash. The deviation of plots near saturation can be attributed to the difference in extent of mass transfer at the initial and final stages of the process of removal. Values of the coefficient of mass transfer, βl , were calculated from slopes and intercepts of the

graphs of Fig. 4 and are given in Table 3. The values of ‘βl ’ was however, found to be βl , 2.15 × 10−2 cm s−1 at 298 K. The values βl indicate a rapid rate of mass transfer for the process undertaken [30]. The product of ‘βl Ss ’ has the dimensions of rate constant of adsorption and its values have also been found in the same order. This gives additional certification to our findings. 3.5. Effect of pH on removal of Cr(VI) by adsorption on fly ash The pH of solution is reported to affect metallic species by adsorption [35,36] significantly as it has pronounced impact on surface charge of the adsorbent phase. As a result of its prominent effect on removal of pollutants from aqueous solutions, it has been termed as ‘master variable’. pH also controls the degree of ionization and speciation of the adsorbate. In the present studies the removal of Cr(VI) by PTPS fly ash was studied at pH values of 2.5, 6.0 and 8.0, respectively and the data has been plotted in Fig. 5. It is clear from this figure that removal was maximum (89.12%) at pH 2.5 and it decreased with increase in the pH value. At pH 6.0 maximum removal was 61.2% and it drastically decreased (25.4%) at pH 8.0 (Fig. 5). Fig. 6 also shows maximum removal of Cr(VI) at different values of pH. The variation in removal of Cr(VI) by fly ash can be understood as follows. The pHzpc of fly ash is 2.8 and this indicates that the surface is positively charged at 2.5 and will be quite favorable for removal HCrO4 − species. Significant adsorption at neutral and negatively charged surface cannot be explained on the basis Table 3 Values of coefficient of mass transfer and rate parameter at different temperatures Temperature (K)

Coefficient of mass transfer, βl (×10−2 cm s−1 )

Rate parameter (×10−3 s−1 )

298 303 308

2.15 1.73 0.56

2.61 2.10 0.66

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Fig. 5. Effect of pH on the removal of Cr(VI) by adsorption on fly ash.

Fig. 7. Langmuir’s plot for equilibrium modeling for removal of Cr(VI) by adsorption on fly ash.

Such type of model has also been reported by other scientific workers as well [38] and thus support our findings. 3.6. Adsorption isotherms Equilibrium studies were undertaken to understand the behaviour of the adsorbent at equilibrium conditions. For these studies the well-known Langmuir model [39] was used: Ce 1 Ce = 0 + 0 qe Q b Q

Fig. 6. Maximum removal of Cr(VI) by adsorption on fly ash at different values of pH.

of electrostatic attraction only. Specific chemical interactions and surface complexation have been suggested to describe the adsorption of Cr(VI). The adsorption beyond pH 4.5 will necessarily include CrO4 2− ions and the following complexation scheme has been suggested [37]: (SOH)2 + 2H+ + CrO4 2− ↔ CrO4 2− (SOH2 + )

(6)

(7)

where Ce (mg l−1 ) is the concentration of adsorbate at equilibrium in solution, qe (mg g−1 ), amount of Cr(VI) adsorbed at equilibrium, Q0 (mg g−1 ) and b (l mg−1 ) are Langmuir’s parameter related to capacity and energy of adsorption. For equilibrium studies, the experiments were conducted with all the four concentration viz 2.5 × 10−5 , 5.0 × 10−5 , 7.5 × 10−5 and 10.0 × 10−5 M at three temperatures namely 298, 303 and 308 K, respectively. The equilibrium data thus obtained was plotted as ‘Ce /qe vs Ce ’ (Fig. 7). The linear plots of this figure confirm that the above method is valid for the studies undertaken. The values of the Langmuir’s constants were calculated from slopes and intercepts of these plots and are given in Table 4. The values of Q0 and b, however were found to be 0.266 mg g−1 and 0.584 l g−1 at 298 K. The decrease in values of both the parameters with increasing temperature fur-

Table 4 Values of Langmuir constants at different temperature Temperature (K)

298 303 308

Q0 (mg g−1 )

b (l mg−1 )

k (l g−1 )

Regression values

Graphical values

Regression values

Graphical values

0.266 0.225 0.192

0.258 0.221 0.192

0.584 0.424 0.217

0.569 0.422 0.216

0.1.55 0.095 0.042

Y.C. Sharma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 222–228 Table 5 Values of thermodynamic parameters at different temperatures Temperature (K)

−G◦ (K cal mol−1 )

H◦ (K cal mol−1 )

S◦ (K cal mol−1 )

298 303 308

0.68 1.74 3.28

5.26 6.13 7.18

1.77 1.47

ther confirms exothermic adsorption involved in the present studies. 3.7. Thermodynamic studies for the removal of Cr(VI) by adsorption on fly ash There are many aspects of reactions which can be efficiently understood by knowledge of thermodynamic parameters. Further, thermodynamic explanations to any reaction provide additional support to the findings. For the present studies, the thermodynamic parameters namely free energy change (G◦ ), enthalpy change (H◦ ) and entropy change (S◦ ) during the reaction were determined using well known equations [40]: G◦ = −RT ln k   T2 T1 H ◦ = R T2 − T 1 S ◦ =

(H ◦ − G◦ ) T

(8) (9) (10)

where R (cal mol−1 K−1 ) is universal gas constant, K1 and K2 are equilibrium constants at temperature T1 and T2 (K) respectively. ‘k’ (l g−1 ) is Langmuir’s constant, which is the product of ‘Q0 and b’. Values of the above three parameters for the present system were calculated and are given in Table 5. Negative values of G◦ reveal that the process of removal is spontaneous in nature with high preference to Cr(VI) in this case. The values of G◦ (Table 5) are found to be less than −3.82 K cal mol. This indicates that the adsorption of Cr(VI) onto fly ash is accompanied by electrostatic attractions and is physical in nature. The values of G◦ >7.82 K cal mol−1 , however, suggest that the process of adsorption would have been activated and chemical in nature [41]. 4. Comparison of removal of chromium by fly ash by other adsorbents Chromium is not most toxic metallic species but it is the most important metal ion from environment view point. Because of this reason significant amount of work has been done and is being carried out worldwide. Rice husk powder was reported to remove 100% chromium from aqueous solutions [42]. Blast furnace flue dust also displayed appreciable removal of chromium but loses its removal capacity soon which does not happen with fly ash [43]. Acid treated saw dust was reported to remove 80% Cr [44] but it took quite some time to reach equilibrium. Bagasse and coconut jute carbon [45], tree bark [46], waste tea leaves [47], rice husk [48], waste slurry from a

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fertilizer plant [49], fly ash [50] and soil [51] are the other non-conventional adsorbents reported for the removal of Cr(VI) mainly from aqueous solutions. In most cases the authors have claimed that the process could be applied for industrial effluents. In most cases the physical or chemical treatment of the adsorbent is required and this incurs financial liability on the user. In the present studies no treatment—physical or chemical, except sieving the fly ash to get a ‘sized adsorbent’, has been provided. Further, the removal observed in the process is significant. The process can be recommended for an economic treatment of Cr(VI) containing industrial effluents in particular and for treatment of metal rich effluents in general. 5. Conclusions Fly ashes are by-products and are understood to be nuisance to the environment. Because of their regular generation especially from coal burning, scientists are trying their other applications. Removal of metals by adsorption on fly ash is one such important application. On the basis of the studies carried out, the following conclusions can be drawn: (i) Fly ash has been successfully used for removal of Cr(VI) from aqueous solutions. (ii) Time of equilibrium was found to be 8.4 ks and higher removal was obtained at lower concentrations. (iii) The removal is governed by first order rate kinetics. (iv) Mass transfer studies approve suitability of application of fly ash for removal of Cr(VI) and the process of uptake is governed both by pore and intraparticle diffusion. (v) Process of Cr(VI) removal is exothermic in nature with maximum removal (89.12%) at 298 K. (vi) Maximum removal is obtained at pH 2.5 and removal decreases with increasing pH values. (vii) Thermodynamic parameters have been calculated and indicate that the process of removal is spontaneous. The data obtained can be successfully used for designing treatment plants for the treatment of Cr(VI) rich water and wastewaters. References [1] U. Forstner, G.T.W. Wittman, Metal Pollution in Aquatic Environment, Springer and Verleg, NY, 1979. [2] K. Kannan, Fundamentals of Environment Pollution, S Chand and Co. Ltd, N. Delhi, 1995. [3] H. Cho, D. Oh, K. Kin, J. Hazard. Mater. 127 (2005) 187. [4] J.O. Nriagu, Environ. Pollut. 50 (1988) 139. [5] M. Kobya, Adsorption Sci. Technol. 22 (2004) 51. [6] Y.C. Sharma, V. Uma, M. Srivastava, Mahto, Chem. Eng. J. 127 (2007) 151. [7] Y.C. Sharma, C.H. Weng, J. Hazard. Mater. 142 (2007) 449. [8] G. McKay, J. Chem. Technol. Biotechnol. 32 (1982) 59. [9] G. Hernandez, R. Rodriguez, J. Non-Cryst. Solids 246 (1999) 209. [10] A. Seco, P. Marzal, C. Gabaldon, J. Ferrer, J. Chem. Technol. Biotechnol. 68 (1999) 23.

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