REMOVAL OF AQUEOUS PHOSPHATE AND PHENOL BY ADSORPTION ON CLAYEY PEAT, LATERITE AND RED EARTH

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1 REMOVAL OF AQUEOUS PHOSPHATE AND PHENOL BY ADSORPTION ON CLAYEY PEAT, LATERITE AND RED EARTH H.M.T.G.A. PITAWALA 1*, D.G.G.P. KARUNARATNE 2 and N. SILVA 3 1 Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka. 2 Department of Chemical and Process Engineering, Faculty of Engineering, University of Peradeniya, Peradeniya, Sri Lanka. 3 Industrial Technology Institute, 363, Bauddhaloka Mawatha, Colombo 7, Sri Lanka. * apitawala@pdn.ac.lk ABSTRACT Batch and column experiments were carried out to study the removal of phosphate and phenol in wastewater using three natural earth materials commonly found in Sri Lanka: peat, laterite and red earth. Solutions of 100 ml of 50 mg/l phosphate and 50 ml of 50 mg/l phenol were placed in Erlenmyer flask together with various masses of adsorbent for batch experiments. The results showed that a dosage of 3.5 g of clayey peat, 5 g of laterite or 6 g of red earth was required to remove 92.8%, 89.8% and 87.6% of phosphate, respectively. A dosage of 3 g of clayey peat was required to remove 95% of phenol. Adsorption isotherm data were interpreted through Langmuir and Freundlich equations. The results show that the Langmuir model best fits the equilibrium data for phosphate and phenol adsorbent systems. A comparative study shows that clayey peat is more effective than laterite or red earth in phosphate removal. Clayey peat can also be used as an efficient adsorbent material to remove phenol from wastewater. Keywords: isotherms, clayey peat, breakthrough curves INTRODUCTION Most industrial effluents contain phosphate and phenolic substances. Addition of these compounds into aquatic environment causes many problems. Accumulation of phosphates in water bodies results in eutrophication, which may occur when the total phosphate is greater than 0.05 mg/l in a stream, and 0.1 mg/l at a point where it enters a lake or reservoir (APHA, 2005). Phosphate concentration of domestic wastewater is in the range of 10 to 15 mg/l (Blackall et al., 2002). High applications of P-fertilizer, industrial and household wastes are the major sources of phosphate in natural water bodies. Aromatic compounds such as phenols occur in wastewater of a number of industries such as high temperature coal conversion, petroleum refining, resin and plastics. Such aromatic hydroxy compounds can be toxic when present in elevated levels. Higher concentrations of phenols present in water supplies are indicated by bad taste and odor. In the presence of chlorine in drinking water, phenols forming chlorophenol can have a pronounced and objectionable medicinal taste (Rengaraj et al., 2002). The United States Environmental Protection Agency recommends the maximum allowable limit of phenol as mg/l and the general effluent discharge standard for phenol is 0.05 mg/l (APHA, 2005). Therefore, the removal of such chemicals from industrial effluents prior to discharge is of vital importance. Adsorption is a well-established and a powerful technique for treating domestic and industrial effluents due to its high efficiency and relatively inexpensive operation (Huang et al., 2008). The 57

2 Pitawala, A. et al. / Removal of Aqueous Phosphate and Phenol by Adsorption most widely used water treatment method is activated carbon adsorption (Jadhav and Vanjara, 2004; Reynolds et al., 1982). High production and regeneration cost of activated carbon has led many workers to search for less expensive substitutes. These include fly ash (Kumar et al., 1987), sawdust carbon (Jadhav and Vanjara, 2004; Raji and Anirudhan, 1997), Eucalyptus stem bark (Sreenivasulu et al., 1996), wollastonite (Singh et al., 1984), tamarind nutshell carbon (Bhargava and Shelkar,1993), half-burnt or raw dolomite (Karaca et al., 2006; Roques et al., 1991) and red mud (Huang et al., 2008). However, there are many disadvantages in many of these techniques such as poor operational stability, high economic cost and scarcity of materials. Peat has been investigated by several researchers as a very effective adsorbent for the capture of dissolved metals and many other harmful substances from waste water (Brown et al., 2000; Couillard, 1994 and references therein; Nawar and Doma, 1989). Besides being plentiful and inexpensive, peat possesses several characteristics that make it an effective medium for removal of dissolved metal pollutants. Also raw clays, clays activated by acid-treatment or calcinations, organic-modified clays with small molecules or polymers were successfully used for adsorption and removal of organic dyes from aqueous solution in recent years (Liu and Zhang, 2007 and references therein) due to their high specific surface area, chemical and mechanical stabilities, and a variety of surface and structural properties. Therefore, peat with clay will be a more useful adsorbent medium, as it is made of naturally formed activated clay other than existing organic materials in normal peat. In addition clayey peats are widely available and has no commercial value compared to organic carbon rich peat. Red clay, bauxite and laterite, the other common earthy materials, are also used widely for the purification of waste water due to their high adsorbent capacity since they have free ions and clay to capture and fix strongly the pollutants (Akhurst et al., 2006; Tor et al., 2006; Wang et al., 2005). The present study was 58 undertaken to compare the performance and effectiveness of clayey peat, laterite and iron rich clayey soil (red earth) in the removal of phosphate and phenol by adsorption from wastewater. METHODS OF STUDY Adsorbent Materials Clayey peat in the northwestern coastal area of Sri Lanka is characterized by organic carbon (22-34 wt.%), Al (20,000-67,000 ppm), Fe (11,000-49,000 ppm), sulphur (1-4 wt. %) and nitrogen (~1 wt.%). Other materials include inorganic carbon and siliceous materials (Dissanayake et al., 1982). The clays present in the peat deposits are gibbsite, goethite and halloysite, with kaolin derived from laterites of surroundings (Dissanayake, 1987). The ph of peat deposits ranges from (Pitawala et al., 1994). Laterites occur in many parts of Sri Lanka, mainly in the Southwestern lowlands. They contain appreciable amounts of aluminum oxide (up to 50 wt. %) compared to red earth (Dissanayke, 1980). The other major constituents of laterites are iron oxides and secondary silicates (Dahanayake, 1982). Major clay minerals found in laterites are gibbsite, goethite and kaolin. The brick red coloured soil (locally called red earth) is a commonly found material overlying the Miocence limestone of the Northwestern coastal area of Sri Lanka. Similar types of red soil are ubiquitous in tropical countries. In Sri Lanka, it is a waste product of limestone quarrying. The material contains high amounts of iron oxide with low amount of aluminum oxide (Alwis and Pluth, 1976). The red earth contains typically high content of Fe 3+ and accessory Fe 2+ (Dahanayake and Jayawardana, 1979). As shown in Fig 1, selected materials were washed with distilled water to remove watersoluble impurities and surface adhered particles. Subsequently, the air-dried samples were ground using a hand-mortar and sieved with a mm mesh.

3 Adsorbates The adsorbate used in this study was synthetic wastewater prepared in the laboratory. The test solutions were prepared by diluting stock solutions of phosphates and phenols to the desired concentrations. Batch Studies Adsorption studies were conducted in a routine manner by batch technique. Various masses (25 o C) from 0.5 to 12 g by 0.5 g intervals) of adsorbents with 100 ml of 50 mg/l phosphate solution and various masses (0.5 to.7 g by 0.5g intervals) of clayey peat with 50ml of 50 mg/l phenol solutions were placed in a 250 ml Erlenmyer flask separately at natural ph and room temperature. The mixtures were agitated on a shaker at a 100 rpm constant shaking rate for 24 h to attain equilibrium. Test trials indicated that equilibrium was reached for phosphate in 16 h and for phenol in 6 to 7 hours. Once equilibrium was attained, the residual concentration of phosphate and phenol in the supernatant was analysed. Column Studies The diameter and the height of columns for phosphate and phenol treatments were 2.5 cm/53 cm and 3 cm/55 cm, respectively. A 10 mm plug of cotton wool was inserted at the bottom to retain the adsorbents. The space above the plug was packed with beds of 14 cm of mineral adsorbents for phosphate and 5 cm for phenol respectively. The column was then filled with 500 mg/l phosphate or 100 mg/l phenol solutions (at natural ph and room temperature), which was allowed to flow continuously through the bed at constant volumetric flow rate of 0.3, 0.5 and 0.3 ml/min for the beds of laterite, clayey peat, and red earth, respectively for phosphate and 1.5 ml/min for phenol. The concentration of phosphate and phenol in the effluent was monitored at 1hr time intervals by collecting 5 ml for analysis until the effluent concentration approached that of the influent. Adsorbent Washed with distilled water Air drying Grinding Sieving (0.005 mm mesh) Shaking with synthetic wastewater Column packing Supernatant for UV analysis Elution with synthetic wastewater Figure 1. Schematic representation of the experimental procedure Elute for UV analysis 59

4 Pitawala, A. et al. / Removal of Aqueous Phosphate and Phenol by Adsorption Analytical Measurements Analysis of phosphate: A 5 ml sample containing 0.05 to 1 mg phosphorus was placed in a 100 ml volumetric flask. A volume of 10 ml vanadate molybdate reagent was added to the sample. For the blank, 5 ml of distilled water was used instead of the sample. After 10 minutes, the concentration of phosphate was measured using a Helios α Thermospectronic UV-visible spectrophotometer, at wavelength 470 nm. Analysis of Phenol: A 20 ml distillate or a portion containing not more than 0.5 mg phenol was placed in a 100 ml volumetric flask and add 2.5 ml 0.5N NH 4 OH solution and immediately adjusted the ph 7.9 ±0.1 with phosphate buffer. One mililitre each of 4-aminoantipyrine solution and potassium ferricyanide solution were added and mixed well. After 15 minutes, the sample was transfered to cells and absorbance of sample and standards against the zero absorbance of the reagent blank was measured at 500 nm using a Varian DMS 80 UV-Vis spectrophotometer (Andrew et al., 2005). All the experiments were performed in duplicate and mean values were used for calculations. Maximum deviation was found to be 5%. Changes of the crystalline phase of the materials used for the treatments were studied using X-ray Diffraction (XRD) analysis with a Siemens D5000 X-ray powder diffractometer. RESULTS AND DISCUSSION Adsorption is usually described through isotherms such as Langmuir and Freundlich. The linear form of the Langmuir isotherm model can be represented by the following relation; evaluated from the intercept and slope of the linear plot of experimental data of 1/qe versus 1/Ce (Aksu and Yener, 2001; Banat et al., 2000; Benefield, 1982; Khalid et al., 2000; Rengaraj et al., 2002; Reynolds and Richards, 1982). The linear form of the Freundlich isotherm model is given by the following relationship....(2) Where k and 1/n is the Freundlich constants related to adsorption capacity and adsorption intensity respectively of the sorbent, ranging between 0 and 1, becoming more heterogeneous as its value gets closer to zero. The values of k and 1/n can be obtained from the intercept and slope, respectively, of the linear plot of experimental data of ln qe versus ln Ce. Adsorption Isotherms The adsorption isotherms obtained from the phosphate and phenol trials (Figures 2 and 3) closely corresponded to the type I isotherm curve described by Brunauer et al., 1938 and Rastogi, The results showed that, in the case of 50 mg/l phosphate in 100 ml of solution, a maximum dosage of 3.5 g of clayey peat, 5 g of laterite, or 6 g of red earth were required for 92.8%, 89.8 % and 87.6 % phosphate removal, respectively. Furthermore, it was noticed that the equilibrium uptake and adsorption yield of clayey peat is higher than that of the other two materials.... (1) Where q e is the amount of solute adsorbed per unit weight of adsorbent (mg/g), Ce is the equilibrium concentration of adsorbate in the solution after adsorption (mg/l), Q 0 (mg/g) and b (l/mg) are the Langmuir constants related to the maximum adsorption capacity and the energy of adsorption respectively. These constants can be 60 Figure 2. Adsorption isotherm of phosphate

5 clay minerals with lower hydrated iron oxides (Dahanayake, 1982; Dissanayake, 1980). As the aluminum compounds and clay minerals have a higher capacity to remove aqueous phosphates, it might be that laterite can remove higher amounts of phosphate compared to red earth (Wang et al., 2005). Slight changes of representative peaks for kaolin of the XRD diagram after the treatment may be a result of replacement of ions in the clay mineral lattice. Figure 3. Adsorption isotherm of phenol The charged peat surface, due to the polar function groups and negatively charged groups, may have stimulated adsorption of free pollutants. Then the chelation and the formation of other complexes by chemical bond formation during adsorption are considered to be the major interactions between polar groups and peat (Dissanayake and Weerasooriya, 1981). Couillard (1992) reported that an addition of 0.4% iron resulted in 99% of the phosphorus being removed. The water soluble free iron content in clayey peat is also high (Dissanayake, 1987) and therefore it removes a higher amount of phosphate, as iron phosphates by precipitation (Zenga et al., 2004). The formation of iron phosphate (vivianite) in treated clayey peat is confirmed by XRD analysis [see Figure 4(i)]. In addition, the clay minerals in peat may also the other agents to adsorb phosphate as they have large specific surface area (Velde, 1992). Structural changes of kaolin in clayey peat after the treatment was indicated from the XRD analysis [see Figure 4(i)]. Therefore, it can be assumed that phosphate ions may have incorporated into the clay structure. The amount of phenol removed from the solutions increased with the mass of adsorbent (Figure 3). However, there is no marked increase between 3 to 7 g weights of clayey peat. Therefore, a maximum dosage of 3 g of clayey peat is required for 95% phenol removal at equilibrium. Results of the present study show that clayey peat has high capacity to adsorb phenol compared to pure peat (Viraraghavan et al., 1998). It indicates that other than organic complexes, clay minerals in peat also have great potential for the retention of phenol as described by Liu and Zang (2007). To facilitate estimation of the adsorption capacities the two well-known equilibrium adsorption models, Freundlich and Langmuir, were employed. The Freundlich equation, which is an indication of surface heterogeneity of the sorbent, is an empirical one and the Langmuir equation assumes that the maximum adsorption occurs when the surface is covered by a monolayer of adsorbate. Red Earth has higher concentrations of ferric oxides with lower amounts of aluminum oxide and clay minerals (Alwis and Pluth, 1976, Dahanayake and Jayawardana, 1979). However, the present study revealed that red earth did not contain clay minerals [see Fig 4(ii-a)]. In contrast, laterite is characterized by higher contents of hydrated oxides of aluminium and Figure 4. (i) XRD patterns of untreated and treated clayey peat Q-quartz, K-kaolin V- Vivianite 61

6 Pitawala, A. et al. / Removal of Aqueous Phosphate and Phenol by Adsorption RL >1 (unfavourable) 0<RL<1 (favourable) RL = 0 (irreversible) RL = 1 (linear) Figure 4. (ii) XRD patterns of untreated and treated red earth (a and b respectively) and laterite (c and d respectively) Q-quartz, K-kaolin Adsorption model constants, the values of which express the surface properties and affinity of the adsorbent, can be used for different adsorbents. The parameters in the models were estimated by nonlinear regression and fitting transformed data to the linearised forms of the models. Freundlich and Langmuir adsorption isotherms of phosphate on clayey peat, laterite, and red earth and phenol on clayey peat are shown in Figures 5 and 6 respectively. It is seen from the linearity of plots in the Figures that adsorption of phosphate on clayey peat, laterite and red earth and phenol on clayey peat follow both Freundlich and Langmuir isotherm models with regression constants greater than The essential characteristics of the Langmuir isotherm have been described by the term separation factor or equilibrium constant RL = 1/1+ bc 0 (where C 0 is the initial concentration of adsorbate and b is its Langmuir constant) (Batzias and Sidiras, 2004; Malik, 2003 and 2004). This indicates the nature of adsorption as, 62 The values of RL in the present investigation have been found to be below 1.0 for phosphate and phenol, showing that the adsorption of phosphate and phenol is very favourable. Regression values (R 2 ) and RL presented in Tables 3 and 4 indicate that the adsorption data fit well with the Langmuir isotherm, when the adsorption is on favourable material. The values of Freundlich and Langmuir constants are listed in Table 1. The adsorption capacity K for Freundlich isotherm plot is higher for both the clayey peat and laterite phosphate system than the red earth phosphate system. The value of Q o (i.e. maximum uptake) is significantly higher for clayey peat phosphate system than laterite and red earth. The magnitude of maximum adsorption capacity indicated that the amount of phosphate per unit weight of sorbent to form a complete monolayer on the surface appears to be significantly higher for the phosphate peat system in comparison to red earth and laterite. Similar observations have been reported in phosphate adsorption studies using carbon prepared from stem bark of Eucalyptus teriticornis (Sreenivasulu et al., 1996) and sorption of phenol on saw dust carbon (Jadhav and Vanjara, 2004; Raji, and Anirudhan, 1997), fly ash (Kumar et al., 1987) and palm seed coat activated carbon (Rengaraj et al., 2002).

7 (a) (b) (c) (d) Figure 5. (a) and (b) Linearized Freundlich and Langmuir adsorption isotherms for phosphate with clayey peat, respectively; (c) and (d) linearized Freundlich and Langmuir adsorption isotherms for phosphate with laterite, respectively (a) (b) (c) (d) Figure 6. (a) and (b) Linearized Freundlich and Langmuir adsorption isotherms for phosphate with red earth respectively; (c) and (d) Linearised Freundlich and Langmuir adsorption isotherms for phenol with clayey peat, respectively 63

8 Pitawala, A. et al. / Removal of Aqueous Phosphate and Phenol by Adsorption Table 1. Adsorption parameters obtained from Langmuir and Freundlich isotherms of adsorption of phosphate and phenol Adsorbent Adsorbate Langmuir model Freundlich model b Q o R 2 RL K 1/n R 2 Clayey peat Laterite Phosphate Red earth Clayey peat Phenol Table 2. Breakthrough data for phosphate and phenol Adsorbent Adsorbate t E (h) V T (cm 3 ) V Z (cm 3 ) h Z (cm) Clayey peat Laterite Phosphate Red earth Clayey peat Phenol Table 3. Saturation capacities for phosphate and phenol Adsorbent Mass (g) Volumetric flowrate (ml/h) t u / t t H UNB (cm) Flow of adsorbate (mg/h) Total adsorbate (mg) C T (mg/g) Adsorbate Phosphate Clayey peat Laterite Red earth Adsorbate Phenol Clayey peat Table 4. Adsorption capacity of phosphate and phenol in batch and column experiments Adsorbent Adsorbent Batch capacity (mg/g) Column capacity (mg/g) Clayey peat Laterite Phosphate Red earth Clayey peat Phenol

9 Column Studies For a fixed bed unit operating at a steady liquor flow rate and for which a symmetrical breakthrough S curve have been obtained experimentally, the height of the mass transfer zone, h Z and the saturation capacity C T are given by the relations as given below (Geankoplis, 1993). h Z = h T [V Z / (V T 0.5 V Z )]...(3) C T = amount of adsorbate / amount of adsorbent H UNB = (1-t u / t t ) H T... (4) H B = t u / t t H T,... (5) H T = H UNB + H B....(6) Where h T = bed height, V T = volume of effluent collected upon exhaustion of the bed, V E = volume of effluent collected up to breakthrough, V Z = V T - V E, t t = time equivalent to the total capacity, t u = time equivalent to the usable capacity, H UNB = length of unused bed, H T = total length, H B = length of bed needed to achieve the required usable capacity at the break point, C T = saturation capacity The data obtained for the breakthrough experiments for phosphate and phenol on mineral adsorbents are shown in Table 2 and the corresponding breakthrough curves are represented in Figures 7 and 8. Initially, most of the phosphate and phenol ions were adsorbed; hence the solute concentration in the effluent was low. As adsorption continues the effluent concentration rises. Figure 7 reveals that up to 34 h, 27 h, 25 h, there is no trace of phosphate in the effluent when clayey peat, laterite and red earth are used as adsorbents respectively. The adsorption capacity of the clayey peat, laterite and red earth were found to be exhausted after 63 h, 57 h and 56 h of column operation. Thus, the Figure 7. Column studies of phosphate breakthrough time (t E ) for phosphate by using clayey peat, laterite and red earth was found to be 34 h, 27 h, and 25 h respectively. Thus for phosphate, it was found that the highest breakthrough time is in clayey peat. Figure 8 reveals that up to 8 h, there is no trace of phenol in the effluent when clayey peat was used as an adsorbent and thus the breakthrough time for phenol was found to be 8 h. The adsorption capacity of the clayey peat was found to be exhausted after 34 h of column operation. Figure 8. Column studies of phenol 65

10 Pitawala, A. et al. / Removal of Aqueous Phosphate and Phenol by Adsorption Available results showed that the column capacity was found to be higher than the batch capacity in the phosphate and phenol for different adsorbents (as shown in Tables 7 and 8). Theoretically, adsorption capacity obtained from column study should be less than the amount predicted by the Langmuir model. This might be due to the fact that the Langmuir model predicts the adsorption capacity based on a monolayer adsorption, whereas what is actually taking place can be multi-layer adsorption. Similar results have been published in literature and the explanation given is that it may be due to the fact that the continuously large concentration gradient at the interface zones occurred as it passes through the column, while the concentration gradient decreases with time in batch experiments (Ahmaruzzaman and Sharma, 2005). Further, this indicates the importance of carrying out both column and batch tests, in adsorption studies. The results show that raw clayey peat can successfully be used for the adsorption and removal of phosphate and phenol from aqueous solutions. The clay minerals are in nano-meter scale so they have large specific surface area. Also the mineral structure can be modified by the available organic compounds in peat. This is very much in favour of adsorption. However, the structural changes of clay minerals in peat cannot easily be identified using XRD analysis since organic matter covers the clay particles. CONCLUSIONS The present study reveals that clayey peat has good adsorption capacity to remove phosphate and phenol from wastewater. Presence of organic complexes, clay minerals and iron as well as acidic conditions facilitate the adsorption phosphate and phenol into clayey peat. Laterite and red earth also have considerable adsorption capacities for the removal of phosphate from wastewater. The column capacity was found to be higher than the batch capacity for phosphate and phenol with different adsorbents. The Langmuir and Freundlich models 66 also fit well the isotherm. The equilibrium data for all the phosphate and phenol sorbent systems fitted the Langmuir model best. The estimated adsorption capacities estimated through batch experiments are significantly lower than that obtained from column tests, indicating possible multilayer adsorption. The order of adsorption capacities of the materials studied for their ability to remove phosphates was found to be clayey peat > laterite > red earth. All investigated materials have sufficient potential to remove phosphate and phenols from aqueous solutions compared to the materials previously used in literature. Thus, the data obtained may be helpful for designing and establishing a continuous treatment plant for water and wastewater enriched with phosphate and phenol. REFERENCES Ahmaruzzaman, M. and Sharma, D.K. (2005). Adsorption of phenols from waste water, J. Colloid Interface Sci., Vol. 287, p Akhurst D.J., Jones, G.B., Clark, M. and McConchie D. (2006). Phosphate removal from aqueous solutions using neutralized bauxite refinery residues (Bauxsol (TM)), Environ. Chem., Vol. 3, p Aksu, Z. and Yener, J. (2001). A comparative adsorption/ biosorption study of Monochlorinated phenols onto various sorbent, Waste Management, Vol. 21, p Alwis K.A. and Pluth D.J. (1976). The red latosols of Sri Lanka. Soil Sci. Soc. Am. J., Vol. 40, p Andrew, D.E., Lenore, S.C., Eugene, W.R. and Arnold, E.G. (2005). Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C. APHA (2005). Standard Methods for the Examination of Water and Wastewater. 21 st edition. American Public Health Association, Washington, D.C. Banat, F.A., Al-Bashir, B., Al-Asheh, S. and Hayajneh, O. (2000). Adsorption of phenol by bentonite. Environ. Pollut., Vol. 107, p Batzias, F.A. and Sidiras, D.K. (2004). Dye adsorption by Calcium Chloride treated beech sawdust in batch and fixed bed systems. J. Hazard. Mater., Vol. 114, p Benefield, L.D. (1982). Process chemistry for water and wastewater treatment. Prentice Hall Inc. New Jersey.

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