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Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue
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Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue Deepak Yadav, Meghna Kapur, Pradeep Kumar, Monoj Kumar Mondal ∗ Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India
a b s t r a c t The aim of the present study was to investigate the possible use of fruit juice (Citrus limetta) residue and rice husk as adsorbents for phosphate removal from aqueous solutions. Batch experiments were performed to achieve maximal phosphate removal by varying process parameters, like pH, contact time, temperature, adsorbent dose and initial solute concentration. FTIR studies revealed that O H, N O and C N groups are responsible for phosphate binding process. The maximum removal of phosphate was achieved as 95.85% at 298 K, adsorbent dose 3 g/L and pH 6.0 with acid treated fruit juice residue. Adsorption process was fitted with pseudo-first order kinetics at 298, 308 and 318 K, respectively. Various isotherm models and mass transfer mechanisms were studied for the removal of phosphate ions from aqueous solutions. Among various adsorption isotherms, Freundlich isotherm showed a better correlation with experimental data. The adsorption energy calculated from Dubinin–Radushkevich isotherm for the most efficient adsorbent indicated physical nature of adsorption. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Adsorption; Activated fruit juice residue; Phosphate; Activated rice husk; Batch process
1.
Introduction
Phosphate enjoys the company of rest two macronutrients nitrogen and potassium and contributes in plant growth, human body upbringing and aquatic life survival. Phosphate is an inorganic element mined from hard phosphate rocks produced in America, China, Morocco, etc. and used as an irreplaceable plant growth nutrient because it allows the transfer of energy within plants cells, required in adequate level to stimulate growth and create changes to plant maturity. Phosphate is present as an ingredient in NPK (nitrogen, phosphorus, and potassium) fertilizer. The reserves of phosphate in soil are basically in the form of mineral phosphate and organic phosphate. Mineral phosphate, as the name suggests is the insoluble and strongly absorbed phosphate that exists in most soils. It is not available to plants as such through chemical transformation within the soil small amounts do become soluble and available. The organic phosphate is the return of organic crop residues,
compost and farm yard manures. It is dependent on the microbial activity in the soil. This living activity mineralizes the phosphates from the residues and makes them available to the plant (as one of the limiting macronutrients). The overabundance of phosphate in water (i.e. >0.1 mg/L) (USEPA, 1996) stimulate algal growth and other vegetation that consumes so much dissolved oxygen that an insufficient amount remains for aquatic life, cause eutrophication which is a danger for the denizen of water and the whole ecosystem in broader prospective. This nutrient loading affects water quality, aquatic life, cascading effect as well as increase the cost of water treatment. The permissible level of phosphorus in the water to prevent algae growth is 0.05 mg/L (Benyoucef and Amrani, 2011). Physical, chemical and biological treatment methods have been developed to remove aqueous phosphate prior to their discharge into natural water bodies and runoff (Bashan and Bashan, 2004; Morse et al., 1998; de Haas et al., 2000). Typically, phosphate is removed by adding aluminum, iron or calciumbased chemicals to separate it from the wastewater and
∗
Corresponding author. Tel.: +91 9452196638; fax: +91 5422367098. E-mail address: [email protected] (M.K. Mondal). Received 19 May 2014; Received in revised form 8 August 2014; Accepted 25 September 2014 http://dx.doi.org/10.1016/j.psep.2014.09.005 0957-5820/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Yadav, D., et al., Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.09.005
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allowing it to settle out (Zeng et al., 2004). Other processes force bacteria to consume and remove phosphate, which can be done by varying the amount of oxygen available to the bacteria. This is known as biological phosphorus removal (Sedlak, 1991). Other few techniques of phosphate removal are chemical precipitation (Oguz et al., 2003), advanced oxidation process (Crittenden et al., 2005), ion exchange (Kuzawa et al., 2006), enhanced biological phosphorus removal (Rittman and McCarty, 2001) and Wetland filtration (Kadlec and Knight, 1996). The conventional technology for phosphate removal in industrial wastewater treatment consists of treating the wastewater with iron oxide tailing (chemical phosphate removal) (Zeng et al., 2004). In this process the phosphorus reacts with the iron to form iron phosphates which precipitate. A significant drawback of this process is the formation of large amount of sludge contaminated with metal salts. This sludge can only be disposed of by landfill, incineration or dumping at sea. To overcome secondary pollution problem various phosphate removal technologies were developed including adsorption. Adsorption is a versatile treatment practice widely used in process industries for wastewater treatment with operational simplicity and adsorbents’ reuse potential separate it from the rest techniques. Activated carbon with porosity, internal surface area, relatively high mechanical strength and with very high removal efficiencies has proved to be the less expensive treatment option. Activated carbon mainly treats low concentration contaminants in wastewater streams to meet stringent treatment levels. For these reasons, activated carbon adsorption has been widely used for the treatment of wastewaters (Bansal and Goyal, 2005). The rice husk and fruit juice residue are available in plenty without any cost in the market. Present study was undertaken to evaluate the efficacy of both adsorbents with and without activation under different operating conditions for the removal of phosphate from aqueous solutions. The novelty of the work is that both the phosphate loaded adsorbents after adsorption can be used as the feed-stock for biogas production using anaerobic digestion as phosphate is used as an essential inorganic nutrient for microbes during biomethanation process. Thus it eliminates the disposal problem of phosphate loaded adsorbents and also can be efficiently used for resource generation in terms of biogas production. Rice husk has been used for the first time as adsorbent for phosphate removal whereas fruit juice residue was never used as an adsorbent. Although phosphorous removal from secondary effluent using fruit juice residue was available in literature (Harada et al., 2011). Various adsorption isotherms, kinetics, thermodynamic studies and mass transfer mechanisms have been discussed in regard to phosphate removal from aqueous solution.
2.
Materials and methods
2.1.
Adsorbents
juice residue were dried at 105 ◦ C for 24 h then grounded and used in the experiments. The other two adsorbents namely activated rice husk (ARH) and activated fruit juice residue (AFJR) were prepared by activation of the above-mentioned adsorbents. It required washing of rice husk and fruit juice residue with double distilled water then they were submerged in 0.1 N NaOH followed by 0.1 N H2 SO4 solution to remove the lignin based substances (Bhattacharya et al., 2008). Resulting masses were washed with double-distilled water and dried in sunlight for 12 h. There after oven-drying was carried out at 105 ◦ C till a constant weight to make the surface ready and active for adsorption (Pradhan et al., 1998). The dried material was washed with 98% H2 SO4 in the ratio of adsorbent to acid 1:1.5 by weight and cooled to room temperature with slow stirring. It was again washed with NaHCO3 and double distilled water to remove acid traces. Finally it was dried in an oven and carbonized in muffle furnace in absence of air at 650 ◦ C. Particles of the adsorbents were ground and sieved separately to different sizes and then stored in a vacuum desiccator for experimental purpose. The adsorbents having particle size of 250–350 m were used for adsorption studies.
2.2.
The chemicals required for the analytical determination of phosphate in the aqueous solutions were potassium dihydrogen phosphate, ammonium molybdate, stannous chloride, glycerol, H2 SO4 . Hydrochloric acid and sodium hydroxide pellets were used for pH adjustment. All the chemicals were of analytical grade.
2.3.
Adsorbate solution
The standard 50 mg/L phosphate stock solution was prepared by dissolving 219.5 mg of KH2 PO4 in 1000 mL of distilled water. The stock solution was further diluted to have the required initial concentration of phosphate solutions.
2.4.
Instruments used
Fourier transform infrared spectra of the samples was recorded by FTIR spectroscope (FTIR-8400, SHIMADZU) using KBr as reference. Digital pH meter (LI 120, Elico India) was used for measuring pH of the sample calibrated with standard buffer solutions (pH 4.0, 7.0 and 9.2). The Brunauer–Emmett–Teller (BET) surface area of the adsorbents were investigated by N2 adsorption–desorption method using Micromeritics ASAP 2020, V302G single port. Liquid nitrogen was used as the cold bath (77 K). The porous structures were studied through BJH (Barret–Joyner–Halenda) data. The densities of the adsorbents were determined using specific gravity bottles. Elico SL 159 UV-VIS Spectrophotometer was used for obtaining concentrations of the phosphate solutions before and after adsorption to check the accurate phosphate removal.
2.5. Rice husk (RH) and fruit juice residue (FJR) were used as lowcost natural biosorbents. Rice husk was collected from Banaras Hindu University farms and fruit juice residue (Citrus limetta) in solid form after extraction of juice collected from local fruit juice shop. The two adsorbents were thoroughly washed with double-distilled water to make the surface free of all the dust particles and muddy material adhered to it. Rice husk and fruit
Reagents
Batch adsorption techniques
Adsorption tests were performed batch wise in 100 mL Erlenmeyer flask immersed in a temperature controlled water bath. Fixed amount of adsorbent was placed in a 100 mL flask containing aqueous solution of KH2 PO4 used as phosphate source. The initial concentration was fixed at 10 mg/L for phosphate. After 3 h of vigorous stirring (180 rpm) at room temperature
Please cite this article in press as: Yadav, D., et al., Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.09.005
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Table 1 – Physico-chemical characteristics of activated rice husk and fruit juice residue.
Table 2 – IR bands of activated fruit juice residue for phosphate removal.
Parameters
Band position (cm−1 )
Moisture (%) Ash (%) Volatile matter (%) Fixed carbon (%) Bulk density (g/L) BET surface area (m2 /g) Total pore volume (cm2 /g) Mean pore radius (m)
Activated rice husk
Activated fruit juice residue
0.286 4.8 9.86 85.054 0.791 637.4 1.042 0.00188
0.519 5.047 26.06 68.374 0.871 927.3 1.241 0.00194
(298 ± 2 K), the suspension was filtered using Whatman filter no. 42 and the obtained filtrate solution was analyzed by UV-VIS spectrophotometer at 690 nm for phosphate content. All the experiments were done in duplicates. A blue color of the sample was developed by the presence of ammonium molybdate and stannous chloride in acidic medium as a complexing agent (APHA, 1976). Using the necessary adsorbents and 100 mL of test solution, batch adsorption studies were carried out at the desired pH value, contact time and adsorbent dosage, for all the adsorbents. Different initial concentration (10–30 mg/L) of phosphate solutions were prepared by proper dilution of stock (50 mg/L) phosphate solution. The pH of the solution was adjusted by adding 0.1 M HCl and 0.1 M NaOH solution. Required amount of adsorbent was then added and contents in the flask were shaken for the desired contact time. All experiments were executed at 298 ± 2 K except the study related to effect of temperature. Various parameters like initial phosphate concentration (10–30 mg/L), contact time (0–300 min), pH (2–12), temperature (298–318 K) and adsorbent dose (2–7 g/L) were investigated for phosphate removal from aqueous solutions onto the four different adsorbents. Batch experiments were conducted by mixing appropriate adsorbent dose at fixed shaking speed of incubator and temperature. Flasks were withdrawn at regular time intervals and analysis of samples was made by following standard procedure using UV–vis spectrophotometer. Removal efficiency of phosphate was calculated by the following equation: % Removal of phosphate =
co − cf co
× 100
(1)
Adsorption capacity q in mg/g was determined by the following equation: q=
(Co − Ce )V W
(2)
where q is the adsorption capacity in mg/g. The Co , Cf and Ce are the initial, final and equilibrium concentrations in mg/L of phosphate in the sample, respectively. W and V are mass of adsorbent in g and the volume of adsorbate solution in L.
3.
Results and discussion
3.1.
Characterization of the adsorbent
The physico-chemical characteristics of the two best adsorbents namely, activated rice husk and activated fruit juice residue were presented in Table 1. The proximate analysis of the adsorbents was carried out by using standard methods (ASTM D5142-90). The surface chemistry of the precursor and
Before adsorption
Assignment
After adsorption
3390.97
–
2858.6 1540.89
2924.18 –
– 1419.66
1599.04 1421.58
1116.82
–
1039.67
1043.52
873.78 553.59
–
O H stretch, H bonded, alcohol, phenols C H stretch, alkanes N O asymmetric stretch, nitro compounds N H bend, 1◦ amines C C stretch (in ring), aromatics C N stretch, aliphatic amines C N stretch, aliphatic amines C H “oop”, aromatics C Br stretch, alkyl halides
modified adsorbents were determined using Fourier transform infrared radiation (FTIR). FTIR studies were carried out for the adsorbents prior and after adsorption of phosphate using the KBr pellet. To study the nature of functional groups present on the surface (responsible for adsorption of phosphate), 1 mg of the adsorbent sample and 100 mg KBr were used in the spectra range of 400–4000 cm−1 . A number of peaks were found indicating the complex nature of a fruit juice residue. The functional groups participated in the process are listed in Table 2. It is evident that the O H stretch, H bonded (alcohol/phenols); N O asymmetric stretch (nitro compounds), C N stretch (aliphatic amines); C H “oop” (aromatics), C Br stretch (alkyl halides) play a pivotal role in the adsorption process. An ion exchange between phosphate and functional groups ( OH, C H, C C) present on the adsorbent surface is likely to be behind adsorption mechanism.
3.2.
Effect of solution pH on phosphate adsorption
Point of zero charge (pHPZC ) was determined by solid addition method (Singha and Das, 2011). 1.01 g KNO3 was dissolved in 1000 mL of distilled water to make 0.1 M KNO3 . 50 mL of 0.1 M KNO3 was kept in series of flasks and pH was adjusted using 0.1 N H2 SO4 and 0.1 N NaOH ranging from 2 to 12. One gram of adsorbent was added to each flask and capped immediately. The flasks were shaken using water bath shaker at 25 ◦ C for 24 h. The pH of supernatant was then measured using pH meter. Intersection point of curve at which pH = 0 gave point of zero charge. For activated fruit juice residue, activated rice husk, fruit juice residue and rice husk point of zero charge was found to be 7.2, 6.3, 8.6 and 9.1, respectively. Adsorption of anions is occurred at a pH lower than pHPZC , while that of cation is favored at a pH greater than pHPZC . This indicates that the phosphate adsorption will occur at a pH lower than respective pHPZC value (Kumar et al., 2010). The pH of the aqueous solution is an important monitoring parameter in the process of adsorption and thus its effect has been studied by varying the same in the range of 2–12 for the uptake of phosphate as shown in Fig. 1. Such study helps in optimizing the appropriate pH of the effluent/wastewater for achieving maximum
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AFJR ARH FJR RH
100
% Phosphate removal
80
60
40
20
Fig. 1 – Effect of initial pH on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; phosphate concentration: 10 mg/L; adsorbent dose: 3 g/L; adsorbent size: 230 m). efficiency in the removal of phosphate ions by the adsorbent. From Fig. 1, it is evident that the adsorption characteristics of the adsorbents are highly pH dependent. The optimum pH for phosphate removal was found to be 6 for all the adsorbents. Maximum removal efficiency was found to be 92.54, 89.65, 76.83 and 64.27% for activated fruit juice residue, activated rice husk, fruit juice residue and rice husk, respectively. All further studies on comparative adsorption of phosphate by the selected adsorbents were carried out by maintaining the solution pH at 6.
3.3.
Effect of adsorbent dose
During the experimental investigations the initial phosphate concentration used was 10 mg/L and contact time maintained at 3 h at a pH 6. It is evident from Fig. 2 that percentage removal increased with increase in adsorbent dose and then comes to equilibrium. For activated rice husk and activated fruit juice residue maximum removal efficiency was achieved at an adsorbent dosage level of 3 g/L whereas for fruit juice residue and rice husk 5 g/L is the optimum dose after which there was no significant change on increasing the dosage. Therefore, the further experiments were followed at optimum adsorbent dosage of 3 g/L (for activated rice husk and activated
0 0
20
30
40
Fig. 3 – Effect of initial concentration of phosphate ion on its removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; pH: 6; adsorbent size: 230 m).
fruit juice residue) and 5 g/L (for fruit juice residue and rice husk) for adsorption of phosphate from aqueous solutions.
3.4.
Effect of initial concentration of phosphate
The effect of variation in initial phosphate ion concentration is shown in Fig. 3. For all the four adsorbents adsorption by all the adsorbents decreased with increase in initial concentration from 10 to 30 mg/L at constant pH 6. The best removal was shown by activated fruit juice residue and it varied from 91.18% for 30 mg/L to 95.65% for 10 mg/L initial phosphate concentration. At higher concentration more ions are left un-adsorbed in the solution due to active site saturation (Singha and Das, 2011).
3.5.
Effect of contact time
The experimental runs measuring the effect of contact time on the batch adsorption of 10 mg/L phosphate at 298 K and at initial pH 6 is shown in Fig. 4. During the experiments the contact time was varied from 1 to 5 h. It was observed that the increase in contact time enhanced significantly the percent removal of phosphate ions. The initial rapid adsorption gives away a very slow approach to equilibrium. The equilibrium was attained within the contact time of 3 h for all the adsorbents.
3.6.
Fig. 2 – Effect of adsorbent dose on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; pH: 6; phosphate concentration: 10 mg/L; adsorbent size: 230 m).
10
Initial phosphate ion concentration (mg/L)
Effect of temperature
Temperature has a significant effect on the adsorption process. The study was conducted in the temperature range of 298–318 K, and results presented in Fig. 5. Adsorption percentage of phosphate by the four adsorbents was found to be increasing with a decrease in temperature resulting exothermic nature of the process. With increase in temperature the mobility of the adsorbate increases there by resulting in decrease of adsorption due to desorption. Similar findings have been available in the literature (Kapur and Mondal, 2013).
Please cite this article in press as: Yadav, D., et al., Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.09.005
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Table 3 – Kinetic and thermodynamics parameters for phosphate adsorption onto activated fruit juice residue. Temperature (K) 298 Pseudo-first order kinetics 0.011515 k1 (min−1 ) 10.26 qe (mg/g) 0.994 R2 Pseudo-second order 0.000375 k2 (g/mg min) 16.39 qe (mg/g) 0.990 R2 Thermodynamics parameters −7.844 G◦ (kJ/mol) −7.174 H◦ (kJ/mol) −0.211 S◦ (kJ/mol/K)
Fig. 4 – Effect of contact time on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; pH: 6; phosphate concentration: 10 mg/L; adsorbent size: 230 m).
3.7.
The adsorption kinetics study describes the solute uptake rate and evidently these rates control the residence time of adsorbate uptake at the solid–solution interface including the diffusion process. The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass transfer process (Tchobanoglous et al., 2003). The results obtained from the experiments were used to study the kinetics of phosphate ion adsorption. The kinetics of phosphate adsorption on the activated fruit juice residue was analyzed using pseudo-first-order (Lagergren, 1898) and pseudo-second-order (Ho et al., 2000) models as expressed in Eqs. (3) and (4). The conformity between experimental data and the model predicted values was expressed by correlation coefficients (R2 ).
t 1 t = + qt qe K2 q2e
kad t 2.303
318
0.011515 9.20 0.987
0.016121 12.33 0.924
0.000633 13.51 0.981
0.000961 25 0.624
−5.561
−3.63
pseudo-first-order mechanism than second-order. Thus the adsorption of phosphate on activated fruit juice residue is supported by pseudo-first-order kinetics.
Adsorption kinetics study
log(qe − qt ) = log qe −
308
(3)
(4)
where qe and qt are values of q at equilibrium and at time t, respectively. The values of kinetic parameters are given in Table 3. The value of R2 shows better fit to
3.8.
Adsorption isotherm
Adsorption process can be well understood through isotherms resulting between adsorbate concentration in liquid (10–30 mg/L) and amount of adsorbate adsorbed by unit mass of adsorbent at a constant temperature (298 K). In the present work results obtained by the adsorption experiment were analyzed by Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich (D–R) isotherms. Langmuir adsorption isotherm model assumes that all the adsorption sites have equal adsorbate affinity and adsorption at one site is independent on the adsorption at an adjacent site. The Langmuir isotherm is represented by the following equation (Langmuir, 1918): 1 Ce Ce = + qe Qo b Qo
(5)
The Qo and b are the Langmuir constants representing the adsorption capacity and energy of adsorption, respectively. Values of the Langmuir constants can be obtained from the plots between Ce /qe and Ce . Langmuir isotherm can be interpreted to judge the adsorption condition using a dimensionless parameter, RL , as expressed by the following equation (Hall et al., 1966): RL =
1 (1 + bCo )
(6)
The condition based on RL values suggest that 0 < RL < 1 are more favorable, whereas RL > 1 are unfavorable, RL = 1 for linear, and RL = 0 for irreversible adsorption. The Freundlich adsorption isotherm model is represented by (Freundlich, 1906): log qe = log KF +
Fig. 5 – Effect of temperature on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (pH: 6; phosphate concentration: 10 mg/L; adsorbent size: 230 m).
1 log Ce n
(7)
Freundlich constants KF and n related to adsorption capacity and intensity, respectively, can be calculated from the linear plots of log qe vs. log Ce . The values of Langmuir and Freundlich constants are given in Table 4. The R2 values for both the models indicated a good fit between the experimental
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Table 4 – Isotherm constants of different models for phosphate adsorption onto activated fruit juice residue at 298 K. Langmuir constants Qo (mg/g) b (L/mg) R2 Freundlich constants KF n R2 Temkin constants KT (L/g) BT (J/mol) R2 D–R constants q (mg/g) B (mol2 /kJ2 ) R2
13.89 0.529 0.918 29.37 1.54 0.987 4.869 3.137 0.922 7.53 9 × 10−8 0.982
and isotherm data. The values of RL (0 < RL < 1) and n (between 1 and 10) was also indicative of favorable adsorption. Tempkin model is based on the assumption that the heat of adsorption of all the molecules in the layer decreases linearly with the coverage of molecules due to the adsorbate–adsorbate repulsions and the adsorption of adsorbate is uniformly distributed among the layers. The Tempkin model is given by equation: qe =
RT ln Ce bT
E=
1 √
2B
(13)
The magnitude of E determines the type of adsorption process. The value of E was found to be 2.357 kJ/mol at 298 K.
3.9.
qe = BT ln AT + BT ln Ce
(9)
where BT = (RT)/bT , T is the absolute temperature in K and R is the universal gas constant (8.314 J/mol K). The constant bT is related to the heat of adsorption, AT is the equilibrium binding constant (L/min) corresponding to the maximum binding energy. The values of isotherm constants are shown in Table 4. The D–R isotherm investigates the nature of the adsorption process. The D–R isotherm does not assume a homogeneous surface or constant sorption potential. It is not based on the assumption of homogeneous surface or constant adsorption potential, but it is applied to estimate the mean free energy of adsorption (E). If the value of E is between 1 and 16 kJ/mol, then physical adsorption prevails, and if the value is more than 16 kJ/mol, then chemisorption prevails (Dubinin and Radushkevich, 1947). The D–R equation can be expressed as follows: qe = BT exp(−Bε2 )
(10)
The linear form of above equation can be expressed as ln qe = ln qm − Bε2
9 × 10−8 mol2 /kJ2 and 7.53 mg/g, respectively at 298 K (Table 4). The mean free energy of sorption per molecule of the adsorbate (kJ/mol) E is given by
(8)
And the linear form of this equation along with binding energy term can be written as:
ε = RT ln 1 +
Fig. 6 – Weber and Morris model for phosphate removal by Activated fruit juice residue (AFJR) at different concentrations (temperature: 298 K; pH: 6; adsorbent dose: 3 g/L; adsorbent size: 230 m, time: 3 h).
1 Ce
(11)
(12)
where R is the gas constant (8.314 J/mol K), T is the absolute temperature and ε is D–R isotherm constant. The values of qm and B can be obtained by the intercept and slope of the graph of ln qe vs. ε2 . The values of B and qm were found to be
Adsorption mechanism
Prediction of rate-limiting step is the best way to understand the solid-liquid adsorption mechanism, where intra-particle diffusion or external mass transfer or the two are responsible for solute mass transfer. Mass transfer mechanisms involve adsorption onto porous adsorbent, film and particle diffusion. During the batch adsorption process the transfer of adsorbate particle into the adsorbent pores is the rate controlling step. Intraparticle diffusion model given by Weber and Morris can be represented as: qt = Kid t1/2
(14)
where Kid is the intraparticle diffusion rate constant(mg/g min1/2 ). According to this model, a plot of solute adsorbed against the square root of the contact time should yield a straight line passing through origin (Weber and Morris, 1963). When the plot does not pass through origin then it gives an indication of some degree of film diffusion control and intraparticle diffusion is not only the rate limiting step. The values of Kid were found as 0.986, 0.979 and 0.951 mg/g min1/2 at initial phosphate concentrations of 10, 20 and 30 mg/L, respectively. It was indicated from Fig. 6 that the intraparticle diffusion was not alone rate controlling step; film diffusion was also expected to affect the adsorption of phosphate by activated fruit juice residue.
3.10.
Thermodynamic study
Phosphate adsorption decreased with increasing temperature indicating the exothermic nature of phosphate adsorption by the activated fruit juice residue and activated rice husk. The adsorption process of phosphate on the above said adsorbents were investigated by calculating thermodynamic parameters
Please cite this article in press as: Yadav, D., et al., Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.09.005
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(G◦ , H◦ and S◦ ). The Gibbs free energy (G◦ ) of adsorption was estimated as follows (Chabani et al., 2006): ◦
G = −RT ln Kd
(15)
where R, T, and Kd are gas constant, temperature, and the distribution coefficient, respectively. The Kd values were calculated by the following expression (Islam and Patel, 2009): CAe Kd = Ce
(16)
where CAe is the amount of phosphate adsorbed per unit mass of the adsorbents and Ce is the concentration of phosphate per unit volume of solution. The enthalpy (H◦ ) and entropy (S◦ ) of adsorption were calculated using the slope and intercept of Van’t Hoff equation, respectively as follows:
ln Kd = −
H◦ S◦ + RT R
(17)
The thermodynamic parameters of phosphate adsorption onto two adsorbents are reported in Table 3. The negative values of G◦ indicated the spontaneous nature of adsorption. Higher negative G◦ values showed that adsorption on it was more energetically favorable than that of other. The negative values of H◦ confirmed the exothermic nature of adsorption. These findings for phosphate ion are in agreement with the findings of Mandal and Mayadevi (Mandal and Mayadevi, 2009) for fluoride, and (Das et al., 2006) for phosphate, but in disagreement with the reports of Islam and Patel (Islam and Patel, 2009) for nitrate. The negative values of S◦ indicate the less degree of freedom of adsorbing phosphate at the solid–solution interface during adsorption. Although positive values of S◦ have been widely reported for anion adsorption on clays or Mg/Al based layered double hydroxide (Mandal and Mayadevi, 2009; Das et al., 2006). Some studies have shown negative values for it (Mandal and Mayadevi, 2009; Islam and Patel, 2009).
4.
Conclusions
The adsorbent of present work proves its efficacy to be an attractive alternative for treatment of phosphate from aqueous solution. Beside this phosphate loaded adsorbent can be used for biogas generation in an eco-friendly manner. Comparison of adsorption capacity of adsorbents showed that activated fruit juice residue can be considered as the efficient adsorbent for treating phosphate rich wastewaters. The percentage removal of phosphate for activated fruit juice residue was found to be maximum at pH 6 and the optimum adsorbent dose was 3 g/L. Adsorption kinetics obeyed a pseudo-first order kinetic model. An increase in temperature decreased the removal of phosphate by activated fruit juice residue. Freundlich isotherm was found to represent the adsorption equilibrium data well. The thermodynamic parameters indicate that the adsorption of phosphate onto activated fruit juice residue is feasible and spontaneous. The experimental data predict that the pore diffusion is not the sole rate-controlling step. Film-diffusion was also found as the rate-controlling step for phosphate adsorption from bulk of the liquid to the surface of the adsorbent.
7
Acknowledgement Authors are grateful to Indian Institute of Technology (Banaras Hindu University), India for extending all necessary facilities to undertake the work.
References APHA, 1976. Standard Methods for the Examination of Water and Waste Water, 14th edition. APHA, AWWA and WEF, Washington, DC. Bansal, R.C., Goyal, M., 2005. Activated Carbon Adsorption. CRC Press, USA. Benyoucef, S., Amrani, M., 2011. Adsorption of phosphate ions onto low cost Aleppo pine adsorbent. Desalination 275, 231–236. Bhattacharya, A.K., Naiya, T.K., Mondal, S.N., Das, S.K., 2008. Adsorption, kinetics and equilibrium studies on removal of Cr(VI) from aqueous solutions using different low-cost adsorbents. Chem. Eng. J. 137, 529–541. Chabani, M., Amrane, A., Bensmaili, A., 2006. Kinetic modelling of the adsorption of nitrates by ion exchange resin. Chem. Eng. J. 125, 111–117. Crittenden, J.C., Trussell, R.R., Hand, D.W., Howe, K.J., Tchobanoglous, G., 2005. WaterTreatment – Principles and Design. John Wiley & Sons. Das, J., Patra, B.S., Baliarsingh, N., Parida, K.M., 2006. Adsorption of phosphate from aqueous solution in layered double hydroxides. Appl. Clay Sci. 32 (3–4), 252–260. de-Bashan, L.E., Bashan, Y., 2004. Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997–2003). Water Res. 38, 4222–4246. de Haas, D.W., Wentzel, M.C., Ekama, G.A., 2000. The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal Part 1: Literature review. Water, SA 26 (4), 439–452. Dubinin, M.M., Radushkevich, L.V., 1947. Equation of the characteristic curve of activated charcoal. Proc. Acad. Sci. Phys. Chem. Sect. USSR. 55, 331–333. Freundlich, H.M.F., 1906. Uber die adsorption in losungen. Zeitschrift fur Physikalische Chemie (Leipzig) 57A, 385–470. Hall, K.R., Eagleton, L.C., Acrivos, A., Vermeulen, T., 1966. Poreand solid-diffusion kinetics in fixed-bed adsorption under constant-pattern conditions. Ind. Eng. Chem. Fund. 5, 212–223. Harada, H., Kondo, M., Biswas, B.K., Ohura, S., Inoue, K., Ishikawa, S., Kawakita, H., Ohto, K., 2011. Phosphorus recovery using zirconium-loaded saponified orange juice residue. J. Environ. Conserv. Eng. 40 (4), 240–246. Ho, Y.S., Mckay, G., Wase, D.A.J., Foster, C.F., 2000. Study of the sorption of divalent metalions onto peat. Ads. Sci. Technol. 18 (7), 639–650. Islam, M., Patel, R., 2009. Nitrate sorption by thermally activated Mg/Al chloride hydrotalcite-like compound. J. Hazard. Mater. 169, 524–531. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. CRC Press/Lewis Publishers, Boca Raton FL, USA. Kapur, M., Mondal, M.K., 2013. Mass transfer and related phenomena for Cr (VI) adsorption from aqueous solutions into Mangifera indica sawdust. Chem. Eng. J. 218, 138–146. Kumar, P., Sudha, S., Chand, S., Srivastava, V.C., 2010. Phosphate removal from aqueous solution using coir-pith activated carbon. Sep. Sci. Technol. 45, 1–8. Kuzawa, K., Jung, Y.J., Kiso, Y., Yamada, T., Nagai, M., Lee, T.G., 2006. Phosphate removal and recovery with a synthetic hydrotalcite as an adsorbent. Chemosphere 62 (1), 45–52. Lagergren, S., 1898. Zurtheorie der sogenannten adsorption gelosterstoffe. KungligaSevenska Vetenskapasakademiens, Handlingar 24, 1–39. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1367.
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Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Adsorptive removal of phosphate from aqueous solution using rice husk and fruit juice residue Deepak Yadav, Meghna Kapur, Pradeep Kumar, Monoj Kumar Mondal ∗ Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India
a b s t r a c t The aim of the present study was to investigate the possible use of fruit juice (Citrus limetta) residue and rice husk as adsorbents for phosphate removal from aqueous solutions. Batch experiments were performed to achieve maximal phosphate removal by varying process parameters, like pH, contact time, temperature, adsorbent dose and initial solute concentration. FTIR studies revealed that O H, N O and C N groups are responsible for phosphate binding process. The maximum removal of phosphate was achieved as 95.85% at 298 K, adsorbent dose 3 g/L and pH 6.0 with acid treated fruit juice residue. Adsorption process was fitted with pseudo-first order kinetics at 298, 308 and 318 K, respectively. Various isotherm models and mass transfer mechanisms were studied for the removal of phosphate ions from aqueous solutions. Among various adsorption isotherms, Freundlich isotherm showed a better correlation with experimental data. The adsorption energy calculated from Dubinin–Radushkevich isotherm for the most efficient adsorbent indicated physical nature of adsorption. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Adsorption; Activated fruit juice residue; Phosphate; Activated rice husk; Batch process
1.
Introduction
Phosphate enjoys the company of rest two macronutrients nitrogen and potassium and contributes in plant growth, human body upbringing and aquatic life survival. Phosphate is an inorganic element mined from hard phosphate rocks produced in America, China, Morocco, etc. and used as an irreplaceable plant growth nutrient because it allows the transfer of energy within plants cells, required in adequate level to stimulate growth and create changes to plant maturity. Phosphate is present as an ingredient in NPK (nitrogen, phosphorus, and potassium) fertilizer. The reserves of phosphate in soil are basically in the form of mineral phosphate and organic phosphate. Mineral phosphate, as the name suggests is the insoluble and strongly absorbed phosphate that exists in most soils. It is not available to plants as such through chemical transformation within the soil small amounts do become soluble and available. The organic phosphate is the return of organic crop residues,
compost and farm yard manures. It is dependent on the microbial activity in the soil. This living activity mineralizes the phosphates from the residues and makes them available to the plant (as one of the limiting macronutrients). The overabundance of phosphate in water (i.e. >0.1 mg/L) (USEPA, 1996) stimulate algal growth and other vegetation that consumes so much dissolved oxygen that an insufficient amount remains for aquatic life, cause eutrophication which is a danger for the denizen of water and the whole ecosystem in broader prospective. This nutrient loading affects water quality, aquatic life, cascading effect as well as increase the cost of water treatment. The permissible level of phosphorus in the water to prevent algae growth is 0.05 mg/L (Benyoucef and Amrani, 2011). Physical, chemical and biological treatment methods have been developed to remove aqueous phosphate prior to their discharge into natural water bodies and runoff (Bashan and Bashan, 2004; Morse et al., 1998; de Haas et al., 2000). Typically, phosphate is removed by adding aluminum, iron or calciumbased chemicals to separate it from the wastewater and
∗
Corresponding author. Tel.: +91 9452196638; fax: +91 5422367098. E-mail address: [email protected] (M.K. Mondal). Received 19 May 2014; Received in revised form 8 August 2014; Accepted 25 September 2014 http://dx.doi.org/10.1016/j.psep.2014.09.005 0957-5820/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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allowing it to settle out (Zeng et al., 2004). Other processes force bacteria to consume and remove phosphate, which can be done by varying the amount of oxygen available to the bacteria. This is known as biological phosphorus removal (Sedlak, 1991). Other few techniques of phosphate removal are chemical precipitation (Oguz et al., 2003), advanced oxidation process (Crittenden et al., 2005), ion exchange (Kuzawa et al., 2006), enhanced biological phosphorus removal (Rittman and McCarty, 2001) and Wetland filtration (Kadlec and Knight, 1996). The conventional technology for phosphate removal in industrial wastewater treatment consists of treating the wastewater with iron oxide tailing (chemical phosphate removal) (Zeng et al., 2004). In this process the phosphorus reacts with the iron to form iron phosphates which precipitate. A significant drawback of this process is the formation of large amount of sludge contaminated with metal salts. This sludge can only be disposed of by landfill, incineration or dumping at sea. To overcome secondary pollution problem various phosphate removal technologies were developed including adsorption. Adsorption is a versatile treatment practice widely used in process industries for wastewater treatment with operational simplicity and adsorbents’ reuse potential separate it from the rest techniques. Activated carbon with porosity, internal surface area, relatively high mechanical strength and with very high removal efficiencies has proved to be the less expensive treatment option. Activated carbon mainly treats low concentration contaminants in wastewater streams to meet stringent treatment levels. For these reasons, activated carbon adsorption has been widely used for the treatment of wastewaters (Bansal and Goyal, 2005). The rice husk and fruit juice residue are available in plenty without any cost in the market. Present study was undertaken to evaluate the efficacy of both adsorbents with and without activation under different operating conditions for the removal of phosphate from aqueous solutions. The novelty of the work is that both the phosphate loaded adsorbents after adsorption can be used as the feed-stock for biogas production using anaerobic digestion as phosphate is used as an essential inorganic nutrient for microbes during biomethanation process. Thus it eliminates the disposal problem of phosphate loaded adsorbents and also can be efficiently used for resource generation in terms of biogas production. Rice husk has been used for the first time as adsorbent for phosphate removal whereas fruit juice residue was never used as an adsorbent. Although phosphorous removal from secondary effluent using fruit juice residue was available in literature (Harada et al., 2011). Various adsorption isotherms, kinetics, thermodynamic studies and mass transfer mechanisms have been discussed in regard to phosphate removal from aqueous solution.
2.
Materials and methods
2.1.
Adsorbents
juice residue were dried at 105 ◦ C for 24 h then grounded and used in the experiments. The other two adsorbents namely activated rice husk (ARH) and activated fruit juice residue (AFJR) were prepared by activation of the above-mentioned adsorbents. It required washing of rice husk and fruit juice residue with double distilled water then they were submerged in 0.1 N NaOH followed by 0.1 N H2 SO4 solution to remove the lignin based substances (Bhattacharya et al., 2008). Resulting masses were washed with double-distilled water and dried in sunlight for 12 h. There after oven-drying was carried out at 105 ◦ C till a constant weight to make the surface ready and active for adsorption (Pradhan et al., 1998). The dried material was washed with 98% H2 SO4 in the ratio of adsorbent to acid 1:1.5 by weight and cooled to room temperature with slow stirring. It was again washed with NaHCO3 and double distilled water to remove acid traces. Finally it was dried in an oven and carbonized in muffle furnace in absence of air at 650 ◦ C. Particles of the adsorbents were ground and sieved separately to different sizes and then stored in a vacuum desiccator for experimental purpose. The adsorbents having particle size of 250–350 m were used for adsorption studies.
2.2.
The chemicals required for the analytical determination of phosphate in the aqueous solutions were potassium dihydrogen phosphate, ammonium molybdate, stannous chloride, glycerol, H2 SO4 . Hydrochloric acid and sodium hydroxide pellets were used for pH adjustment. All the chemicals were of analytical grade.
2.3.
Adsorbate solution
The standard 50 mg/L phosphate stock solution was prepared by dissolving 219.5 mg of KH2 PO4 in 1000 mL of distilled water. The stock solution was further diluted to have the required initial concentration of phosphate solutions.
2.4.
Instruments used
Fourier transform infrared spectra of the samples was recorded by FTIR spectroscope (FTIR-8400, SHIMADZU) using KBr as reference. Digital pH meter (LI 120, Elico India) was used for measuring pH of the sample calibrated with standard buffer solutions (pH 4.0, 7.0 and 9.2). The Brunauer–Emmett–Teller (BET) surface area of the adsorbents were investigated by N2 adsorption–desorption method using Micromeritics ASAP 2020, V302G single port. Liquid nitrogen was used as the cold bath (77 K). The porous structures were studied through BJH (Barret–Joyner–Halenda) data. The densities of the adsorbents were determined using specific gravity bottles. Elico SL 159 UV-VIS Spectrophotometer was used for obtaining concentrations of the phosphate solutions before and after adsorption to check the accurate phosphate removal.
2.5. Rice husk (RH) and fruit juice residue (FJR) were used as lowcost natural biosorbents. Rice husk was collected from Banaras Hindu University farms and fruit juice residue (Citrus limetta) in solid form after extraction of juice collected from local fruit juice shop. The two adsorbents were thoroughly washed with double-distilled water to make the surface free of all the dust particles and muddy material adhered to it. Rice husk and fruit
Reagents
Batch adsorption techniques
Adsorption tests were performed batch wise in 100 mL Erlenmeyer flask immersed in a temperature controlled water bath. Fixed amount of adsorbent was placed in a 100 mL flask containing aqueous solution of KH2 PO4 used as phosphate source. The initial concentration was fixed at 10 mg/L for phosphate. After 3 h of vigorous stirring (180 rpm) at room temperature
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Table 1 – Physico-chemical characteristics of activated rice husk and fruit juice residue.
Table 2 – IR bands of activated fruit juice residue for phosphate removal.
Parameters
Band position (cm−1 )
Moisture (%) Ash (%) Volatile matter (%) Fixed carbon (%) Bulk density (g/L) BET surface area (m2 /g) Total pore volume (cm2 /g) Mean pore radius (m)
Activated rice husk
Activated fruit juice residue
0.286 4.8 9.86 85.054 0.791 637.4 1.042 0.00188
0.519 5.047 26.06 68.374 0.871 927.3 1.241 0.00194
(298 ± 2 K), the suspension was filtered using Whatman filter no. 42 and the obtained filtrate solution was analyzed by UV-VIS spectrophotometer at 690 nm for phosphate content. All the experiments were done in duplicates. A blue color of the sample was developed by the presence of ammonium molybdate and stannous chloride in acidic medium as a complexing agent (APHA, 1976). Using the necessary adsorbents and 100 mL of test solution, batch adsorption studies were carried out at the desired pH value, contact time and adsorbent dosage, for all the adsorbents. Different initial concentration (10–30 mg/L) of phosphate solutions were prepared by proper dilution of stock (50 mg/L) phosphate solution. The pH of the solution was adjusted by adding 0.1 M HCl and 0.1 M NaOH solution. Required amount of adsorbent was then added and contents in the flask were shaken for the desired contact time. All experiments were executed at 298 ± 2 K except the study related to effect of temperature. Various parameters like initial phosphate concentration (10–30 mg/L), contact time (0–300 min), pH (2–12), temperature (298–318 K) and adsorbent dose (2–7 g/L) were investigated for phosphate removal from aqueous solutions onto the four different adsorbents. Batch experiments were conducted by mixing appropriate adsorbent dose at fixed shaking speed of incubator and temperature. Flasks were withdrawn at regular time intervals and analysis of samples was made by following standard procedure using UV–vis spectrophotometer. Removal efficiency of phosphate was calculated by the following equation: % Removal of phosphate =
co − cf co
× 100
(1)
Adsorption capacity q in mg/g was determined by the following equation: q=
(Co − Ce )V W
(2)
where q is the adsorption capacity in mg/g. The Co , Cf and Ce are the initial, final and equilibrium concentrations in mg/L of phosphate in the sample, respectively. W and V are mass of adsorbent in g and the volume of adsorbate solution in L.
3.
Results and discussion
3.1.
Characterization of the adsorbent
The physico-chemical characteristics of the two best adsorbents namely, activated rice husk and activated fruit juice residue were presented in Table 1. The proximate analysis of the adsorbents was carried out by using standard methods (ASTM D5142-90). The surface chemistry of the precursor and
Before adsorption
Assignment
After adsorption
3390.97
–
2858.6 1540.89
2924.18 –
– 1419.66
1599.04 1421.58
1116.82
–
1039.67
1043.52
873.78 553.59
–
O H stretch, H bonded, alcohol, phenols C H stretch, alkanes N O asymmetric stretch, nitro compounds N H bend, 1◦ amines C C stretch (in ring), aromatics C N stretch, aliphatic amines C N stretch, aliphatic amines C H “oop”, aromatics C Br stretch, alkyl halides
modified adsorbents were determined using Fourier transform infrared radiation (FTIR). FTIR studies were carried out for the adsorbents prior and after adsorption of phosphate using the KBr pellet. To study the nature of functional groups present on the surface (responsible for adsorption of phosphate), 1 mg of the adsorbent sample and 100 mg KBr were used in the spectra range of 400–4000 cm−1 . A number of peaks were found indicating the complex nature of a fruit juice residue. The functional groups participated in the process are listed in Table 2. It is evident that the O H stretch, H bonded (alcohol/phenols); N O asymmetric stretch (nitro compounds), C N stretch (aliphatic amines); C H “oop” (aromatics), C Br stretch (alkyl halides) play a pivotal role in the adsorption process. An ion exchange between phosphate and functional groups ( OH, C H, C C) present on the adsorbent surface is likely to be behind adsorption mechanism.
3.2.
Effect of solution pH on phosphate adsorption
Point of zero charge (pHPZC ) was determined by solid addition method (Singha and Das, 2011). 1.01 g KNO3 was dissolved in 1000 mL of distilled water to make 0.1 M KNO3 . 50 mL of 0.1 M KNO3 was kept in series of flasks and pH was adjusted using 0.1 N H2 SO4 and 0.1 N NaOH ranging from 2 to 12. One gram of adsorbent was added to each flask and capped immediately. The flasks were shaken using water bath shaker at 25 ◦ C for 24 h. The pH of supernatant was then measured using pH meter. Intersection point of curve at which pH = 0 gave point of zero charge. For activated fruit juice residue, activated rice husk, fruit juice residue and rice husk point of zero charge was found to be 7.2, 6.3, 8.6 and 9.1, respectively. Adsorption of anions is occurred at a pH lower than pHPZC , while that of cation is favored at a pH greater than pHPZC . This indicates that the phosphate adsorption will occur at a pH lower than respective pHPZC value (Kumar et al., 2010). The pH of the aqueous solution is an important monitoring parameter in the process of adsorption and thus its effect has been studied by varying the same in the range of 2–12 for the uptake of phosphate as shown in Fig. 1. Such study helps in optimizing the appropriate pH of the effluent/wastewater for achieving maximum
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AFJR ARH FJR RH
100
% Phosphate removal
80
60
40
20
Fig. 1 – Effect of initial pH on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; phosphate concentration: 10 mg/L; adsorbent dose: 3 g/L; adsorbent size: 230 m). efficiency in the removal of phosphate ions by the adsorbent. From Fig. 1, it is evident that the adsorption characteristics of the adsorbents are highly pH dependent. The optimum pH for phosphate removal was found to be 6 for all the adsorbents. Maximum removal efficiency was found to be 92.54, 89.65, 76.83 and 64.27% for activated fruit juice residue, activated rice husk, fruit juice residue and rice husk, respectively. All further studies on comparative adsorption of phosphate by the selected adsorbents were carried out by maintaining the solution pH at 6.
3.3.
Effect of adsorbent dose
During the experimental investigations the initial phosphate concentration used was 10 mg/L and contact time maintained at 3 h at a pH 6. It is evident from Fig. 2 that percentage removal increased with increase in adsorbent dose and then comes to equilibrium. For activated rice husk and activated fruit juice residue maximum removal efficiency was achieved at an adsorbent dosage level of 3 g/L whereas for fruit juice residue and rice husk 5 g/L is the optimum dose after which there was no significant change on increasing the dosage. Therefore, the further experiments were followed at optimum adsorbent dosage of 3 g/L (for activated rice husk and activated
0 0
20
30
40
Fig. 3 – Effect of initial concentration of phosphate ion on its removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; pH: 6; adsorbent size: 230 m).
fruit juice residue) and 5 g/L (for fruit juice residue and rice husk) for adsorption of phosphate from aqueous solutions.
3.4.
Effect of initial concentration of phosphate
The effect of variation in initial phosphate ion concentration is shown in Fig. 3. For all the four adsorbents adsorption by all the adsorbents decreased with increase in initial concentration from 10 to 30 mg/L at constant pH 6. The best removal was shown by activated fruit juice residue and it varied from 91.18% for 30 mg/L to 95.65% for 10 mg/L initial phosphate concentration. At higher concentration more ions are left un-adsorbed in the solution due to active site saturation (Singha and Das, 2011).
3.5.
Effect of contact time
The experimental runs measuring the effect of contact time on the batch adsorption of 10 mg/L phosphate at 298 K and at initial pH 6 is shown in Fig. 4. During the experiments the contact time was varied from 1 to 5 h. It was observed that the increase in contact time enhanced significantly the percent removal of phosphate ions. The initial rapid adsorption gives away a very slow approach to equilibrium. The equilibrium was attained within the contact time of 3 h for all the adsorbents.
3.6.
Fig. 2 – Effect of adsorbent dose on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; pH: 6; phosphate concentration: 10 mg/L; adsorbent size: 230 m).
10
Initial phosphate ion concentration (mg/L)
Effect of temperature
Temperature has a significant effect on the adsorption process. The study was conducted in the temperature range of 298–318 K, and results presented in Fig. 5. Adsorption percentage of phosphate by the four adsorbents was found to be increasing with a decrease in temperature resulting exothermic nature of the process. With increase in temperature the mobility of the adsorbate increases there by resulting in decrease of adsorption due to desorption. Similar findings have been available in the literature (Kapur and Mondal, 2013).
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Table 3 – Kinetic and thermodynamics parameters for phosphate adsorption onto activated fruit juice residue. Temperature (K) 298 Pseudo-first order kinetics 0.011515 k1 (min−1 ) 10.26 qe (mg/g) 0.994 R2 Pseudo-second order 0.000375 k2 (g/mg min) 16.39 qe (mg/g) 0.990 R2 Thermodynamics parameters −7.844 G◦ (kJ/mol) −7.174 H◦ (kJ/mol) −0.211 S◦ (kJ/mol/K)
Fig. 4 – Effect of contact time on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (temperature: 298 K; pH: 6; phosphate concentration: 10 mg/L; adsorbent size: 230 m).
3.7.
The adsorption kinetics study describes the solute uptake rate and evidently these rates control the residence time of adsorbate uptake at the solid–solution interface including the diffusion process. The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass transfer process (Tchobanoglous et al., 2003). The results obtained from the experiments were used to study the kinetics of phosphate ion adsorption. The kinetics of phosphate adsorption on the activated fruit juice residue was analyzed using pseudo-first-order (Lagergren, 1898) and pseudo-second-order (Ho et al., 2000) models as expressed in Eqs. (3) and (4). The conformity between experimental data and the model predicted values was expressed by correlation coefficients (R2 ).
t 1 t = + qt qe K2 q2e
kad t 2.303
318
0.011515 9.20 0.987
0.016121 12.33 0.924
0.000633 13.51 0.981
0.000961 25 0.624
−5.561
−3.63
pseudo-first-order mechanism than second-order. Thus the adsorption of phosphate on activated fruit juice residue is supported by pseudo-first-order kinetics.
Adsorption kinetics study
log(qe − qt ) = log qe −
308
(3)
(4)
where qe and qt are values of q at equilibrium and at time t, respectively. The values of kinetic parameters are given in Table 3. The value of R2 shows better fit to
3.8.
Adsorption isotherm
Adsorption process can be well understood through isotherms resulting between adsorbate concentration in liquid (10–30 mg/L) and amount of adsorbate adsorbed by unit mass of adsorbent at a constant temperature (298 K). In the present work results obtained by the adsorption experiment were analyzed by Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich (D–R) isotherms. Langmuir adsorption isotherm model assumes that all the adsorption sites have equal adsorbate affinity and adsorption at one site is independent on the adsorption at an adjacent site. The Langmuir isotherm is represented by the following equation (Langmuir, 1918): 1 Ce Ce = + qe Qo b Qo
(5)
The Qo and b are the Langmuir constants representing the adsorption capacity and energy of adsorption, respectively. Values of the Langmuir constants can be obtained from the plots between Ce /qe and Ce . Langmuir isotherm can be interpreted to judge the adsorption condition using a dimensionless parameter, RL , as expressed by the following equation (Hall et al., 1966): RL =
1 (1 + bCo )
(6)
The condition based on RL values suggest that 0 < RL < 1 are more favorable, whereas RL > 1 are unfavorable, RL = 1 for linear, and RL = 0 for irreversible adsorption. The Freundlich adsorption isotherm model is represented by (Freundlich, 1906): log qe = log KF +
Fig. 5 – Effect of temperature on phosphate removal by activated fruit juice residue (AFJR), activated rice husk (ARH), fruit juice residue (FJR) and rice husk (RH) (pH: 6; phosphate concentration: 10 mg/L; adsorbent size: 230 m).
1 log Ce n
(7)
Freundlich constants KF and n related to adsorption capacity and intensity, respectively, can be calculated from the linear plots of log qe vs. log Ce . The values of Langmuir and Freundlich constants are given in Table 4. The R2 values for both the models indicated a good fit between the experimental
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Table 4 – Isotherm constants of different models for phosphate adsorption onto activated fruit juice residue at 298 K. Langmuir constants Qo (mg/g) b (L/mg) R2 Freundlich constants KF n R2 Temkin constants KT (L/g) BT (J/mol) R2 D–R constants q (mg/g) B (mol2 /kJ2 ) R2
13.89 0.529 0.918 29.37 1.54 0.987 4.869 3.137 0.922 7.53 9 × 10−8 0.982
and isotherm data. The values of RL (0 < RL < 1) and n (between 1 and 10) was also indicative of favorable adsorption. Tempkin model is based on the assumption that the heat of adsorption of all the molecules in the layer decreases linearly with the coverage of molecules due to the adsorbate–adsorbate repulsions and the adsorption of adsorbate is uniformly distributed among the layers. The Tempkin model is given by equation: qe =
RT ln Ce bT
E=
1 √
2B
(13)
The magnitude of E determines the type of adsorption process. The value of E was found to be 2.357 kJ/mol at 298 K.
3.9.
qe = BT ln AT + BT ln Ce
(9)
where BT = (RT)/bT , T is the absolute temperature in K and R is the universal gas constant (8.314 J/mol K). The constant bT is related to the heat of adsorption, AT is the equilibrium binding constant (L/min) corresponding to the maximum binding energy. The values of isotherm constants are shown in Table 4. The D–R isotherm investigates the nature of the adsorption process. The D–R isotherm does not assume a homogeneous surface or constant sorption potential. It is not based on the assumption of homogeneous surface or constant adsorption potential, but it is applied to estimate the mean free energy of adsorption (E). If the value of E is between 1 and 16 kJ/mol, then physical adsorption prevails, and if the value is more than 16 kJ/mol, then chemisorption prevails (Dubinin and Radushkevich, 1947). The D–R equation can be expressed as follows: qe = BT exp(−Bε2 )
(10)
The linear form of above equation can be expressed as ln qe = ln qm − Bε2
9 × 10−8 mol2 /kJ2 and 7.53 mg/g, respectively at 298 K (Table 4). The mean free energy of sorption per molecule of the adsorbate (kJ/mol) E is given by
(8)
And the linear form of this equation along with binding energy term can be written as:
ε = RT ln 1 +
Fig. 6 – Weber and Morris model for phosphate removal by Activated fruit juice residue (AFJR) at different concentrations (temperature: 298 K; pH: 6; adsorbent dose: 3 g/L; adsorbent size: 230 m, time: 3 h).
1 Ce
(11)
(12)
where R is the gas constant (8.314 J/mol K), T is the absolute temperature and ε is D–R isotherm constant. The values of qm and B can be obtained by the intercept and slope of the graph of ln qe vs. ε2 . The values of B and qm were found to be
Adsorption mechanism
Prediction of rate-limiting step is the best way to understand the solid-liquid adsorption mechanism, where intra-particle diffusion or external mass transfer or the two are responsible for solute mass transfer. Mass transfer mechanisms involve adsorption onto porous adsorbent, film and particle diffusion. During the batch adsorption process the transfer of adsorbate particle into the adsorbent pores is the rate controlling step. Intraparticle diffusion model given by Weber and Morris can be represented as: qt = Kid t1/2
(14)
where Kid is the intraparticle diffusion rate constant(mg/g min1/2 ). According to this model, a plot of solute adsorbed against the square root of the contact time should yield a straight line passing through origin (Weber and Morris, 1963). When the plot does not pass through origin then it gives an indication of some degree of film diffusion control and intraparticle diffusion is not only the rate limiting step. The values of Kid were found as 0.986, 0.979 and 0.951 mg/g min1/2 at initial phosphate concentrations of 10, 20 and 30 mg/L, respectively. It was indicated from Fig. 6 that the intraparticle diffusion was not alone rate controlling step; film diffusion was also expected to affect the adsorption of phosphate by activated fruit juice residue.
3.10.
Thermodynamic study
Phosphate adsorption decreased with increasing temperature indicating the exothermic nature of phosphate adsorption by the activated fruit juice residue and activated rice husk. The adsorption process of phosphate on the above said adsorbents were investigated by calculating thermodynamic parameters
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(G◦ , H◦ and S◦ ). The Gibbs free energy (G◦ ) of adsorption was estimated as follows (Chabani et al., 2006): ◦
G = −RT ln Kd
(15)
where R, T, and Kd are gas constant, temperature, and the distribution coefficient, respectively. The Kd values were calculated by the following expression (Islam and Patel, 2009): CAe Kd = Ce
(16)
where CAe is the amount of phosphate adsorbed per unit mass of the adsorbents and Ce is the concentration of phosphate per unit volume of solution. The enthalpy (H◦ ) and entropy (S◦ ) of adsorption were calculated using the slope and intercept of Van’t Hoff equation, respectively as follows:
ln Kd = −
H◦ S◦ + RT R
(17)
The thermodynamic parameters of phosphate adsorption onto two adsorbents are reported in Table 3. The negative values of G◦ indicated the spontaneous nature of adsorption. Higher negative G◦ values showed that adsorption on it was more energetically favorable than that of other. The negative values of H◦ confirmed the exothermic nature of adsorption. These findings for phosphate ion are in agreement with the findings of Mandal and Mayadevi (Mandal and Mayadevi, 2009) for fluoride, and (Das et al., 2006) for phosphate, but in disagreement with the reports of Islam and Patel (Islam and Patel, 2009) for nitrate. The negative values of S◦ indicate the less degree of freedom of adsorbing phosphate at the solid–solution interface during adsorption. Although positive values of S◦ have been widely reported for anion adsorption on clays or Mg/Al based layered double hydroxide (Mandal and Mayadevi, 2009; Das et al., 2006). Some studies have shown negative values for it (Mandal and Mayadevi, 2009; Islam and Patel, 2009).
4.
Conclusions
The adsorbent of present work proves its efficacy to be an attractive alternative for treatment of phosphate from aqueous solution. Beside this phosphate loaded adsorbent can be used for biogas generation in an eco-friendly manner. Comparison of adsorption capacity of adsorbents showed that activated fruit juice residue can be considered as the efficient adsorbent for treating phosphate rich wastewaters. The percentage removal of phosphate for activated fruit juice residue was found to be maximum at pH 6 and the optimum adsorbent dose was 3 g/L. Adsorption kinetics obeyed a pseudo-first order kinetic model. An increase in temperature decreased the removal of phosphate by activated fruit juice residue. Freundlich isotherm was found to represent the adsorption equilibrium data well. The thermodynamic parameters indicate that the adsorption of phosphate onto activated fruit juice residue is feasible and spontaneous. The experimental data predict that the pore diffusion is not the sole rate-controlling step. Film-diffusion was also found as the rate-controlling step for phosphate adsorption from bulk of the liquid to the surface of the adsorbent.
7
Acknowledgement Authors are grateful to Indian Institute of Technology (Banaras Hindu University), India for extending all necessary facilities to undertake the work.
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