Environ. Sci. Technol. 2004, 38, 1195-1200
Removal of Chlorophenols Using Industrial Wastes AJAY K. JAIN,* VINOD K. GUPTA, SHUBHI JAIN, AND SUHAS Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247 667, India
Development of inexpensive adsorbents from industrial wastes for the treatment of wastewaters is an important area in environmental sciences. Blast furnace slag, dust and sludge from steel plants, and carbon slurry from fertilizer plants after their treatment have been utilized as inexpensive adsorbents for the removal of phenols, which are an important class of pollutants as they are highly toxic. The characterization of the four adsorbents prepared has shown that the carbonaceous adsorbent prepared from carbon slurry possesses high porosity and maximum surface area (380 m2/g) as compared to the other three adsorbents (4-28 m2/g). The adsorption of four phenols (phenol, 2-chlorophenol, 4-chlorophenol, and 2,4-dichlorophenol) on these adsorbents is parallel to their porosity and surface area order. The uptake of the phenols on carbonaceous adsorbent is substantial and found to be 17.2, 50.3, 57.4, and 132.5 mg/g for phenol, 2-chlorophenol, 4-chlorophenol, and 2,4-dichlorophenol, respectively. The detailed adsorption studies on carbonaceous adsorbent have indicated that the adsorption process follows the Langmuir isotherm, is first order, and is pore diffusion controlled. As adsorption of phenols on prepared carbonaceous adsorbent is significant, its performance has been evaluated with respect to standard activated charcoal. The results indicate that the phenols removal efficiency of carbonaceous adsorbent is about 45% to that of a standard activated charcoal sample. Thus, the carbonaceous adsorbent can be used for the removal of phenols as a low-cost alternative (∼0.1 U.S.$/kg) to activated charcoal.
Introduction Phenol is a basic structural unit for a variety of synthetic organic compounds; therefore, wastewater originating from many chemical plants and pesticide and dye manufacturing industries contain phenols. Besides this, wastewater originating from other industries such as paper and pulp, resin manufacturing, gas and coke manufacturing, tanning, textile, plastic, rubber, pharmaceutical, and petroleum also contain different types of phenols. In addition to phenols generated as a result of industrial activity, wastewaters also contain phenols formed as a result of decay of vegetation. In view of the wide prevalence of phenols in different wastewaters and their toxicity (1, 2) to human and animal life even at low concentration, it is essential to remove them before discharge of wastewater into water bodies. A number of methods such as oxidation with ozone/hydrogen peroxide (3, 4), biological methods (5), membrane filtration (6), ion exchange (7), * Corresponding author e-mail:
[email protected]; telephone: +91-1332-285809; fax: +91-1332-273560. 10.1021/es034412u CCC: $27.50 Published on Web 01/07/2004
2004 American Chemical Society
electrochemical oxidation (8), reverse osmosis (9), photocatalytic degradation (10), and adsorption (11-13) have been used for the removal of phenols. However, the methods based on chemical/biological oxidation, ion exchange, and solvent extraction have shown low efficiency for the removal of trace levels of phenols (4, 7). Despite the availability of the abovementioned processes for the removal of phenols, the adsorption process still remains the best as it can generally remove all types of phenols, and the effluent treatment is convenient because of simple design and easy operations. Furthermore, most methods already mentioned remain specific for phenol removal and may not remove other inorganic and organic pollutants. Thus, specific methods are not very useful in treating wastewater, which contains other pollutants also. With the adsorption process using a good and efficient adsorbent, it is possible to remove, in addition to phenols, a large number of other organic and inorganic pollutants. The adsorbent that is used in practice remains activated carbon (14-17). However because of high cost of activated carbon, its use in the field is sometimes restricted on economical considerations. As such, attempts have been made by different workers to develop alternative adsorbents, preferably of low cost, for the removal of phenols. Both the industrial wastes as well as some natural resource materials such as fly ash (18), corncob wastes (19), spent bleaching earth (20), apricot stone shells (21), rubber seed coat (22), waste tire rubber (23), etc. have been utilized for this purpose. A survey of literature shows that although a large number of alternative adsorbents have been studied to replace activated carbons, the results have not been very promising. Most of the adsorbents studied have poor capacity as compared to activated carbons and also are not versatile in adsorbing different types of pollutants. Furthermore, some of the adsorbents developed are not really low-cost materials to permit widespread use. Efforts are therefore still needed to have low-cost adsorbents that have a high-adsorption capacity for phenols. Thus, this study has been undertaken with the aim to study the adsorption behavior of four industrial wastes [blast furnace slag, dust, and sludge (from steel plants) and carbon slurry (from fertilizer plants)] for the removal of a number of chlorophenols.
Materials and Methods Phenol, 2-chlorophenol, 4-chlorophenol, and 2,4-dichlorophenol were procured from Spectrochem (India), and standard activated charcoal was from E. Merck (India). Blast furnace (BF) slag, dust, and sludge were collected from Malvika Steels Ltd. (Jagdishpur, India), and carbon slurry was from National Fertilizer Limited (NFL) (Panipat, India). Double-distilled water was used throughout for the preparation of solutions. All reagents used in the present study were of analytical grade. Preparation of Adsorbents. The wastes used in this study are of two types: (i) waste with mainly inorganic constituents and (ii) wastes with mainly organic constituents. The three wastes (blast furnace slag, dust, and sludge) obtained from steel plants are basically inorganic in nature whereas the fourth waste (carbon slurry) is organic in nature because of its high carbon content. These wastes have been processed in the following manner so that they can be used as adsorbents. Carbonaceous Adsorbent. Carbon slurry waste was procured from the NFL plant (Panipat), which operates with fuel oil/low sulfur heavy stock (LSHS) as feedstock. It was treated with hydrogen peroxide (24, 25) to oxidize the adhering organic material and then washed with water and VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dried. The dried material was then activated at different temperatures in a muffle furnace for 1 h in air atmosphere. The yield of the activated product (now called carbonaceous adsorbent) was found to be ∼90%. The cost of prepared adsorbent comes out to be ∼0.1 U.S.$/kg. It was sieved to get different mesh sizes and stored in a desiccator. The optimum temperature of activation was found to be 500 °C. Blast Furnace (BF) Slag, Dust, and Sludge Adsorbent. Blast furnace slag, dust, and sludge were treated as described earlier (24) to impart adsorption characteristics. The products were then sieved and stored in a desiccator. Instrumentation. Phenols were determined spectrophotometrically on a Shimadzu 160A UV-Vis spectrophotometer. The pH of solutions was measured with a Elico LI 127 pH meter, and a LEO 435 VP was used for scanning electron microscopy. Carbon content was measured by Elementar CHNS analyzer model Vario EL III. IR spectra of the samples were recorded on a Perkins-Elmer FTIR spectrophotometer model 1600. X-ray measurements were done on a Phillips X-ray diffractometer employing nickel-filtered Cu KR radiations. Particle Sizing of Adsorbents. All the powdered adsorbents were passed through different British Standard Sieves (BSS), and fractions corresponding to 100-150, 150-200, and 200-250 mesh were collected. Adsorption on particles of different sizes was studied in order to understand the effect of particle size. Adsorption Studies. Adsorption was determined by batch method, which is simple and easy to perform. Furthermore, it permits convenient evaluation of parameters that influence the adsorption process. In batch method, a fixed amount of the adsorbent (0.01 g) was added to 10 mL of phenols solution of varying concentrations taken in stoppered glass tubes, which were placed in the thermostat shaking assembly. The solutions were stirred continuously at constant temperatures for 8 h to achieve equilibration. The concentration of the phenols in the solution after equilibrium adsorption was determined spectrophotometrically at λmax 210, 274, 280, and 284 nm for phenol, 2-chlorophenol, 4-chlorophenol, and 2,4dichlorophenol, respectively. The experiments were repeated a number of times, and the average values are reported. Standard deviations were found to be within (2.0%. Furthermore, the error bars for the figures were smaller than the symbols used to plot the graphs and hence are not shown. Kinetic studies of adsorption were also carried at two concentrations of the adsorbates wherein the extent of adsorption was investigated as a function of time. The pH of all solutions in contact with adsorbents was found to be in the range of 6.5-7.5.
Results and Discussion Characterization of the Prepared Adsorbents. The BF slag, dust, and sludge were chemically analyzed, and the results indicate that silica and calcium oxides are the main constituents of BF slag (32.7 and 31.7%, respectively) whereas coke, silica, and R2O3 (mainly iron oxide, R ) Fe or Al) are the prominent components of BF dust and sludge (24.6%, 15.8%, 44.9% and 40.5%, 12.7%, 35.4%, respectively). Thus, BF slag has mainly inorganic constituents, and so it may be said that it is inorganic in nature. On the other hand, although BF dust and sludge have inorganic constituents in substantial proportions, they also have partial organic character (carbon content). As compared to BF slag, dust, and sludge, the analytical data of carbonaceous adsorbent indicates carbon content of 89.8% and ash content of 0.9%. Thus, a carbonaceous adsorbent consisting mainly of carbon may be treated as organic in nature. The emphasis here is that adsorbents possessing organic content (mainly carbon) generally show greater porosity as compared to adsorbents of inorganic nature, except gels, which show poor porosity. Thus, the 1196
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FIGURE 1. Effect of contact time on the uptake of phenols on carbonaceous adsorbent (temperature: 25 °C; particle size: 200250 mesh). carbonaceous adsorbent is expected to show greater porosity as compared to the other three adsorbents. The scanning electron microscopy revealed that the carbonaceous adsorbent is significantly porous whereas other adsorbents exhibit poor porosity. The porosity imparts a higher surface area to the adsorbents. To confirm the expected relationship between surface area and porosity, the surface area was determined by nitrogen gas adsorption and was found to be 380, 28, 13, and 4 m2/g for carbonaceous adsorbent, BF sludge, BF dust, and BF slag, respectively. The decrease in surface area of these adsorbents is parallel to their decreasing organic (carbon) content/porosity. The samples of carbonaceous adsorbent, slag, dust, and sludge were stirred with deionized water for 2 h and left for 24 h to see any interaction. It was seen that in the case of BF slag, dust, and sludge an enhancement of pH was observed, indicating alkaline hydrolysis of inorganic constituents. In the case of carbonaceous adsorbent, the pH of water was lowered, which indicates that carbonaceous adsorbent as per Steenberg classification (26) comes under “L” type carbon. X-ray spectra of carbonaceous adsorbent does not show any peak, thereby indicating its amorphous nature. The X-ray diffraction peaks in the spectra of BF sludge and dust are due to iron oxides while in case of BF slag indicates the presence of silicates of calcium and aluminum and quartz. The IR spectra of the sample of carbonaceous adsorbent taken indicates the presence of two prominent bands lying at 1605 and 3340 cm-1. The first peak may be assigned to the presence of a carbonyl group, and the latter one may be assigned to the presence of the OH group. Effect of Contact Time and Concentration. To determine equilibration time for the maximum uptake of phenols, their adsorption at fixed concentration on carbonaceous adsorbent was studied as a function of contact time, and the results are shown in Figure 1. It is seen that the rate of uptake of all the phenols is rapid in the beginning and that 50% of ultimate adsorption is completed within 2 h in all cases. Figure 1 also shows that the time required for equilibrium adsorption is 8 h. Thus for all equilibrium adsorption studies, the equilibration period was kept at 10 h. The effect of concentration on equilibration time was also investigated at different concentrations as shown in Figure 2 for 2,4-dichlorophenol. Similar plots were obtained for the other phenols. The plots
FIGURE 2. Effect of contact time on the uptake of 2,4-dichlorophenol on carbonaceous adsorbent at different initial concentrations (temperature: 25 °C; particle size: 200-250 mesh). FIGURE 4. Adsorption isotherms of 2,4-dichlorophenol on different adsorbents at 25 °C (b, standard activated charcoal; O, carbonaceous adsorbent; 0, BF sludge; ×, BF dust). Adsorption on BF slag negligible.
FIGURE 3. Effect of particle size on the adsorption of 2,4dichlorophenol on carbonaceous adsorbent (temperature: 25 °C). show that time of equilibrium as well as time required to achieve a definite fraction of equilibrium adsorption is independent of initial concentration. Similar observations were also made by Zogorski et al. (14) while studying the kinetics of adsorption of phenols on granular activated carbon. The results obtained indicate that the adsorption process is first order, which is confirmed by Lagergren’s plots discussed later under Dynamic Modeling. Effect of Particle Size on Adsorption. The adsorption of the four phenols was investigated at three particle sizes of 100-150, 150-200, and 200-250 BSS mesh, respectively, and the results for 2,4-dichlorophenol adsorption on carbonaceous adsorbent are shown in Figure 3. It was found that the adsorption capacity increases to some extent with a decrease in particle size of the adsorbent. This could not be due to substantial increase in surface area (27). It is possible that phenol molecules are not able to penetrate to some of the interior pores of the particles, especially when their size is large. The access to all pores is facilitated as particle size becomes smaller. Similar results were also obtained by McKay et al. (28). As particles of 200-250 mesh show maximum adsorption capacity, all studies were carried out with this fraction only. Adsorption Isotherms. To assess the efficiency of prepared adsorbents for the removal of phenols, the equilibrium adsorption of the four phenols was studied as a function of concentration on the four prepared adsorbents, and the
adsorption isotherms obtained for 2,4-dichlorophenol are shown in Figure 4. Similar adsorption isotherms were also obtained for the other three phenols. The adsorption capacity (maximum adsorption) has been evaluated and compiled in Table 1. It is seen that the adsorption of phenols is in the following order: carbonaceous adsorbent > BF sludge > BF dust > BF slag. The adsorption on BF sludge and dust is small, whereas on slag it is negligible. The order of phenols adsorption is parallel to surface area and porosity of adsorbents. The carbonaceous adsorbent, which has maximum surface area and porosity, adsorbs phenols to the maximum extent whereas the other three adsorbents having low surface area show little or negligible adsorption. Thus it is concluded from these studies that only the carbonaceous adsorbent that possesses appreciable surface area and porosity is the potential material for the removal of phenols. As studies have shown that the extent of adsorption depends on surface area, it is reasonable to conclude that the adsorption, to some extent, is a surface phenomenon where van der Waals forces operate. It is further seen from Table 1 that the adsorption capacity for various phenols follows this order: 2,4-dichlorophenol > 4-chlorophenol > 2-chlorophenol > phenol. These results indicate that as the number of chloro group increases, phenol adsorption also increases and that the cause appears to be the solubility factor. It is seen from Table 1 that the solubility of these phenols in aqueous media is of the order phenol > 2-chlorophenol > 4-chlorophenol > 2,4-dichlorophenol. A comparison of solubility and adsorption capacity (Table 1) clearly indicates that there exists an inverse relationship between the extent of adsorption and solubility. Chlorosubstituted phenols, which have smaller solubility (lesser affinity for water), would obviously have a higher tendency to get adsorbed at the solid-liquid interface. Thus, 2,4dichlorophenol (having the least solubility) is adsorbed to the maximum extent. To assess the adsorption efficiency of the prepared adsorbents, their performance was further analyzed by comparing the results with standard activated charcoal (surface area: 710 m2/g). A comparison of adsorption capacity (Table 1) with standard activated charcoal shows that carbonaceous adsorbent, BF sludge, and BF dust are respectively 40-45%, 15-20%, and 10-15% as efficient as standard activated charcoal in removing phenols. As the VOL. 38, NO. 4, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Solubility (in water) at 25 °C and Adsorbability of Phenols (mg/g) on Various Adsorbents adsorbability (mg/g) 25 °C
45 °C
phenols
solubility (g/100 g of water)
standard activated charcoal
carbonaceous adsorbent
BF sludge
BF dust
BF slag
carbonaceous adsorbent
phenol 2-chlorophenol 4-chlorophenol 2,4-dichlorophenol
9.3 2.8 2.7 1.5
42.4 121.2 136.5 281.2
17.2 50.3 57.4 132.5
7.5 20.4 22.8 38.6
5.3 14.0 16.2 29.1
negligible negligible negligible negligible
20.7 61.3 69.1 145.1
FIGURE 6. Langmuir adsorption isotherms of 2,4-dichlorophenol on carbonaceous adsorbent at different temperatures.
TABLE 2. Langmuir Constants for Adsorption of Phenols on Carbonaceous Adsorbent at Different Temperatures FIGURE 5. Adsorption isotherms of phenols on carbonaceous adsorbent at 45 °C (O, 2,4-dichlorophenol; 4, 4-chlorophenol; 0, 2-chlorophenol; ×, phenol).
qm phenols
temp (°C) (mg/g)
phenol
carbonaceous adsorbent appears to be a promising material with good adsorption efficiency, all further studies were done with it only. Effect of Temperature. To understand the effect of temperature on the adsorption of phenols, adsorption experiments were conducted at 45 °C. The results are shown in Figure 5. The maximum adsorption estimated from these isotherms is incorporated in Table 1. A comparison of adsorption capacity at 25 and 45 °C (Table 1) shows that adsorption increases with an increase in temperature, indicating that the process is apparently endothermic. Similar results were obtained by other workers also (14, 29). The effect of temperature can be explained on the basis of hydrogen bonding. In aqueous solutions of phenols, there exists extensive hydrogen bonding between the phenol molecules and water resulting in appreciable solubility. These hydrogen bonds get broken at higher temperatures, and this would cause phenols to be less soluble and therefore exhibit higher tendency to go to the adsorbent surface and get adsorbed rather than remaining in the solution. This would result in more adsorption at higher temperature. The adsorption data were further analyzed in light of the Freundlich and Langmuir equations and found to conform best to the folowing Langmuir equation:
1 1 1 ) + qe qm qmbCe
9
4-chlorophenol 2,4-dichlorophenol
18.3 22.0 51.8 64.9 58.1 70.4 137.0 150.2
b (L/mol)
19.4 × 10-2 23.4 × 10-2 40.3 × 10-2 50.5 × 10-2 45.2 × 10-2 54.8 × 10-2 84.0 × 10-2 92.1 × 10-2
10.8 × 103 11.1 × 103 11.9 × 103 13.9 × 103 12.2 × 103 14.4 × 103 15.3 × 103 18.9 × 103
(qm) of the adsorbent for the phenols is comparable to the maximum adsorption obtained from adsorption isotherms. As b values reflect the equilibrium constant for the adsorption process, they also reflect the affinity of the adsorbent for phenols. Thus, b values indicate that the adsorbent has maximum affinity for 2,4-dichlorophenol and minimum affinity for phenol. The free energy change (∆G°), enthalpy change (∆H°), and entropy change (∆S°) for adsorption process were calculated using following equations (30):
∆G° ) -RT ln(b)
(
∆H° 1 1 ln (b2/b1) ) R T2 T1 ∆G° ) ∆H° - T∆S°
(1)
where qe is the amount adsorbed at equilibrium concentration, qm is the Langmuir constant related to maximum monolayer capacity, b is the Langmuir constant related to energy of adsorption, and Ce is the equilibrium concentration. The plot between 1/qe and 1/Ce for the adsorption of 2,4dichlorophenol is presented in Figure 6. Similar plots were obtained for other three phenols. The values qm and b have been evaluated from the intercept and slope of these plots and given in Table 2. It is seen that the monolayer capacity 1198
2-chlorophenol
25 45 25 45 25 45 25 45
(mmol/g)
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(2)
)
(3) (4)
in order to know the nature of adsorption, and the values are summarized in Table 3. The ∆H° values are positive. As the adsorption process is pore diffusion controlled, an increase in molecular diffusion occurs at higher temperatures leading to endothermic enthalpy of adsorption. Furthermore, negative ∆G° values indicate spontaneous process. Positive ∆S° values indicate the affinity of the adsorbent for phenols. Dynamic Modeling. The kinetics of adsorption is important from the point of view that it controls the process efficiency. Various kinetics models have been used by different workers where the adsorption has been treated as
TABLE 3. Thermodynamic Parameters for Adsorption of Phenols on Carbonaceous Adsorbent at Different Temperatures phenols phenol 2-chlorophenol 4-chlorophenol 2,4-dichlorophenol
temp (°C)
∆ G° (kJ/mol)
∆ S° (J/mol‚K)
∆ H° (kJ/mol)
25 45 25 45 25 45 25 45
-23.0 -24.6 -23.3 -25.2 -23.3 -25.3 -23.9 -26.0
80.9 80.8 98.7 98.4 100.0 100.0 108.1 107.9
1.1 6.1 6.5 8.3
FIGURE 8. Bangham’s plot for phenols on carbonaceous adsorbent. where C′0 is the initial concentration of adsorbate in solution (mmol/L), V is the volume of solution (mL), m′ is the weight of adsorbent used per liter of solution (g/L), q′ (mmol/g) is the amount of adsorbate retained at time t, and R (
