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The objective of this study was the assessment of reduction of chemical oxygen demand (COD) of wastewater from textile industrial plant by using activated carbon prepared from locally available coal. Activated carbons were prepared by thermal activation at 700 0C and chemical activation with phosphoric acid (H3PO4) at 500 0C. The complete study was done in batch mode to investigate the effect of operating parameters. The result of COD concentration reduction with thermally activated carbon (TAC) and chemically activated carbon (CAC) were compared and optimum operating conditions were determined for maximum reduction. Adsorption isotherms were also studied besides the calculation of optimum treatment parameters for maximum reduction of COD concentration from effluent of the textile industrial plant. The maximum percentage reduction of COD concentration under optimum operating conditions using TAC and CAC was 86.9% and 95.9%, respectively. As the residual COD of wastewater with CAC was in NEQS (2000) permissible limit, therefore, it could be a lucrative technique for treatment of industrial wastewater.
Generally wastewater is about 99.9% normal pure water, with only 0.06% of the wastewater dissolved and suspended solid material. The cloudiness of wastewater is caused by suspended particles, which in untreated wastewater ranges from 100 to 350 mg/l (Al-Rekabi et al., 2007).
In the present study chemical oxygen demand (COD) parameter for analysis of pre and post treated wastewater was focused. In order to remove COD activated carbon was prepared from coal. The crystalline structure, surface edges, porosity, variable characteristics of surface chemistry, and high degree of surface reactivity in activated carbon (AC) regulate the adsorption efficiency (RodrÄ±gues-Reinoso, 1997; Bansal et al., 1988 ; Malik et al., 2004). Activated carbon is a commonly used adsorbent in sugar refining, chemical and pharmaceutical industrial water and wastewater treatment (Ng et al., 2003).
When the AC was in contact with an aqueous solution, an electric charge was generated. This charge resulted from either the dissociation of the surface functional groups of the carbon or the adsorption of ions from the solution, and dependent on the pH of the solution and on the surface characteristics of the adsorbent (Li et al., 2002).
Chemical activation (ChA) was a one-step method used for the preparation of AC; different chemical activating agents (H3PO4, KOH, H2SO4 etc.) might be used. Major advantages of ChA were the higher yield, lower temperature of activation (less energy costs), less activation time and, generally, higher development of porosity; among the disadvantages were the activating agents costs and the need to perform an additional washing stage to remove the chemical agent (Macia-Agullo et al., 2004).
Singh et al. (2008) prepared the low-cost activated carbon from Tamarind wood material by chemical activation with sulphuric acid for the adsorption of lead (II) from diluted aqueous solution. The isotherm equilibrium data were well fitted by the Langmuir and Freundlich models. Due to their particular porous characteristics, woody materials were very relevant and challenging raw materials to prepare AC, for the adsorption of solutes in the liquid phase (Wu and Tseng, 2006).
The objectives of the research work was to prepare activated carbon from indigenously available coal, investigating various optimum conditions for activated carbon efficiency, application of the prepared activated carbon in treating industrial wastewater and comparison of different application methods.
MATERIALS AND METHODS
The present research work was done jointly by Sustainable Development Study Centre (SDSC) of GC University, Lahore and Centre for Environmental Protection Studies (CEPS) of Pakistan Council of Scientific and Industrial Research (PCSIR) Ferozepur Road, Lahore.
Sample from Nishat Textile Industry (located at Rohi Nala Lahore) was collected by grab sampling technique (Kaul and Gautam, 2002) then these sample aliquots were mixed to form a composite sample and preserved in refrigerator. Sample after collection was analyzed for both physical and chemical parameters but only COD parameter was focused.
Activated Carbon preparation:
Indigenously available coal belonging to the wood of Acacia species was purchased from wood stall and used for the preparation of activated carbon. The coal obtained from wood stall was not only easily available but was least expensive. Activated carbon was synthesized from coal by thermal and chemical activation. For thermal activation, coal obtained from wood stall was first washed, dried at 110 0C then activated at 700 0C for 3 hours. For chemical activation, phosphoric acid was selected because it was very good in producing high surface area and pore structure (Attia et al., 2008). Coal was ground, washed, dried and treated with phosphoric acid at the rate of 35 ml/250 g of coal (Gueu et al., 2007). After treatment it was activated at 500 0C for three hours.
Batch Adsorption Experiment:
To reduce the COD of wastewater by activated carbon, batch adsorption study was carried out. In this method 100 ml sample of wastewater was treated with different concentration (1 to 10 g) of AC at different pH (2 to 12) and contact time (1 to 5 h). Pre and post treated COD of sample was measured and percentage reduction in COD was measured as follows:
% Reduction in COD = (X - Y) x 100
'X' is the initial COD of wastewater
'Y' is the COD of filtrate
Results and Discussion:
In the present work, activated carbon was prepared by physical (TAC) and chemical (CAC) activation methods. Suhas et al. (2007) also used physical and chemical methods for the preparation of activated carbon. By physical activation, Bagno et al. (1978) designed and constructed a pilot plant to produce char and AC from a hydropyrolysis kraft chemical recovery.
Activated carbon prepared from coal was used in this study which was observed to be efficient in removal of COD from industrial wastewater i.e. 87% and 96% with TAC and CAC at treatment time of 4 hrs respectively. This result matched with Devi and Dahiya (2008) who observed the percentage reduction of COD with mixed adsorbent carbon was 93.84% after treatment time of 150 minutes whereas maximum reduction with commercial activated carbon was 94% after a treatment time of 180 minutes.
The yield of thermally activated carbon was found to be 69% while that of phosphoric acid activated carbon give good yield of 82% the rest was lost during washing and activation process (Table 3.2). This is in accordance with the yield calculated by Alcaniz-Monge et al. (2008) from physical and chemical activation of coal 68 and 89%, respectively. One of the important advantages of the chemical process over the physical process was that the yield tends to be greater as carbon burned off was negligible in chemical process.
Batch adsorption study was carried out at room temperature 25+3 0C. 100 ml of wastewater sample was taken in a 250 ml conical flask in which maximum adsorption was obtained. Similar work was done by Kadirvelu et al. (2003) which was also carried out in batch mode. They also investigated the influence of the agitation time on the rate and extent of uptake on the adsorbent. Their experiment was carried out at room temperature 31+2 0C. 250 mg adsorbent was taken in a 100 ml conical flask and agitated at 160 rpm. Attia et al. (2008) used both batch adsorption and column adsorption study for the adsorption of dye using activated carbon impregnated with 70% phosphoric acid and carbonized at 500 0C exhibited the best properties which prevailed upon raising treated dye concentration to150 and 200 mg/l.
From the Figure 3.1 and 3.2, it is evident that COD removal was increased from neutral to acidic pH. It was assessed that maximum values were obtained at 2pH and 4pH with TAC and CAC, respectively because at acidic pH the positively charged species start dominating and the surface tends to acquire positive charge, while the adsorbate species were still negatively charged. As the adsorbent surface was positively charged, the increasing electrostatic attraction between negatively charged adsorbate species and positively charged adsorbent particles would lead to increased adsorption of reactive dye (Shukla et al., 2002). Higher adsorption rate of fluoride in the acidic range can be explained by Karthikeyan and Illago (2007) that the surface charges of the adsorbent play the role.
However, results are against the result obtained by Yi et al. (2008) who observed that with the initial pH increasing from 3 to 7, the COD removal efficiency of dye after 60 min of electrolysis increased accordingly. Reddy and Kotaiah (2006) also observed that equilibrium sorption capacity of the activated carbons decreased with increasing pH values of reactive dye solution, while increase in adsorption towards basic pH (8 to12) with TAC and CAC may be due to increase in OH- ions as a result of coagulation of pollutants, and thus, are adsorbed. Malik et al. (2004) observed the similar findings, that in alkaline medium, the extent of dye color removal increase as pH increased from 8 to 9.
Adsorption increased with increased in contact time because there was much time for pollutants to adsorb on the surface of activated carbon. At 4 hrs there was no chance for them to be left behind than adsorption. Wang et al. (2008) showed similar observation that the degree of coloration and total organic carbon of the dye solution decreased significantly with an increase in the contact time until equilibrium was attained.
It was observed that adsorption capacity increased with increased amount of activated carbon due to increased surface area, as more sites were becomes available for adsorption. In case of thermally activated carbon maximum removal of COD was observed at 8 g and after that it became constant while in case of chemically activated carbon removal became maximum at 7 g (Table 3.3).
It was also observed that adsorption capacity of CAC was more than that of TAC due to the formation of more adsorption sites by chemically activated carbon (Figure 3.3), but with both the carbons adsorption density decreased. That was also observed by Reddy and Kotaiah (2006) that by increasing the dose of sludge derived activated carbon and commercial activated carbon, the percentage of dye removal from aqueous solution increased but adsorption density decreased. The decrease in adsorption density with increase in the adsorbent dosage was mainly because of unsaturation of adsorption sites through the adsorption process (Yu et al., 2003; Shukla et al., 2002).
Namasivayam (2004) was of the view that increase in the adsorption capacity of the material with dosage indicated the availability of a large number of active sites on its surface. Singh et al. (2008) showed that initially the percentage removal increases very sharply with the increase in adsorbent dosage (1-5 g/l) but beyond a certain value, the percentage removal reached almost a constant value.
Agitation speed directly effected the removal in COD of industrial wastewater. The results showed that adsorption increased with both TAC and CAC by increasing agitation speed and removal in COD was maximum at 600 and 500 rpm (Table 3.3), respectively. Agitation speed influence was reported by Devi and Dahiya (2008). They observed maximum removal in COD at 600 rpm with mixed adsorbent carbon and commercially available activated carbon.
In the present study detailed cost analysis for the preparation of carbons was not carried out but as the raw materials obtained from indigenous coal was only 0.31 $/kg and available freely and abundantly. Yield was good and methods of activation were cheap, hence the cost for preparation of activated carbons was expected to be low, and these methods can be adopted.
Adsorption isotherm studies:
Data analysed by Langmuir isotherm indicated the homogenous nature of coal carbon surface; the results demonstrated the formation of monolayer coverage of waste substances at the outer surface of coal carbon (Fig. 3.4 and 3.6). Pehlivan (2005) observed the amount of fluoride adsorbed was smooth and continuous indicating the formation of monolayer coverage of the outer interface of the adsorbent. Values of qmon and KL , the Langmuir constants (McKay et al.,1982) were calculated from intercept and slope of the linear plots, respectively and are presented in Table 3.4. These values showed that data completely fit in the Langmuir isotherm.
The equilibrium adsorption capacities evaluated from the Langmuir equation showed that evaluated values are reasonable. qmon values of present study are 58.5mg/g and 62.9mg/g with TAC and CAC respectively. Reddy and Kotaiah (2006) determined Langmuir constants qmon and b from the slope and intercept of the plot and found to be 33.5 mg/g and 0.070 l/mg, for CAC and 25 mg/g and 0.44 l/mg, for SAC, respectively. Mohan et al. (2002) reported the qmon value for adsorption of dye from aqueous solution on commercial activated carbon was 7.69 mg/g. A qmon value of 6.72 mg/g was observed by Namasivayam and Kavitha (2002) for adsorption of Congo Red on activated carbon prepared from coir pith. Juang et al. (2001) obtained a qmon value of 273 mg/g for adsorption of of dye on activated carbon prepared from bagasse pith by steam activation at 750 0C.
The RL values are more than 0 and less than 1, which indicated that data was according to Langmuir equation. The RL values indicated the shape of the isotherm to be irreversible (RL = 0), favorable (0< RL<1), linear (RL = 1) or unfavorable (RL > 1) (Karthikeyan, 2005). By processing the above equation RL values for investigated COD removal system are shown in Table 3.4. From the value of RL it was confirmed that prepared coal carbon was favorable for removal of COD from wastewater.
Freunlich isotherm is the earliest known relationship describing the sorption equation. This farely satisfactory empirical isotherm can be used for non-ideal sorption that involves heterogenous surface energy system (Lee et al., 1995). The Freunlich isotherm corresponding to the experimental measurements for TAC and CAC were plotted on log scale as shown in Fig. 3.5 and 3.7, respectively. Values of regression coefficient (R2) had been calculated from the linear fit and are based on the fit, the respective values of the slope 1/n and intersect on y-axis taken as k was also calculated. Values 1/n, k and regression coefficient R2 for TAC and CAC are shown in Table 3.5, corresponding to COD concentration reduction. As value of R2 was closer to 1 it showed the fittest of the result of this study in Freunlich isotherm. Amuda and Ibrahim (2006) reported that the constants 1/n and k were of definite importance in determining the adsorption capacity of organic pollutants from wastewater and reduction of COD concentration by adsorbents. The slope 1/n was dependent on the order of the change in reduction of COD concentration with the adsorbent dose, while k was dependent on extent of removal of COD by the adsorbents.
Present study showed that both activated carbons (TAC and CAC) are effective for reduction of COD concentration from effluent of textile industry. Adsorption of COD was found to be dependent on pH, treatment time and adsorbent dose. The studied adsorption data fitted well to Langmuir and Freunlich adsorption isotherms. This adsorbent prepared from coal could be a good alternative to expensive activated carbon and hence wastewater treatment process can become very economical. A certain amount of work has already been done on the production of ACs from coal, as well as on the adsorption of inorganic and organic pollutants on coal and coal-derived activated carbons (ACs). Although the amount of published work is still comparatively small, the results so far obtained are promising and there is clearly a need for more detailed systematic studies.