Abstract
The potential of granular-activated carbon (GAC) derived from agrowaste of Mangostene (Mangostana garcinia) fruit peel was investigated in batch and fixed bed system as a replacement of current expensive methods for treating wastewater contaminated by manganese, Mn(II) cations. Batch equilibrium data was analyzed by Langmuir, Freundlich, and Temkin isotherm models at different temperatures. The effect of inlet metal ion concentration (50 mg/L, 70 mg/L, and 100 mg/L), feed flow rate (1 mL/min and 3 mL/min), and activated carbon bed height (4.5 cm and 3 cm) on the breakthrough characteristics of the fixed bed sorption system were determined. The adsorption data were fitted with well-established column models, namely, Thomas, Yoon-Nelson, and Adams-Bohart. The results were best-fitted with Thomas and Yoon-Nelson models rather than Adams-Bohart model for all conditions. The column had been regenerated and reused consecutively for five cycles. The results demonstrated that the prepared activated carbon was suitable for removal of Mn(II) ions from wastewater using batch as well as fixed bed sorption system.
1. Introduction
Among various pollutants present in surface water, inorganic species of heavy metals and their metalloids are of major concern as they are difficult to remove owing to their smaller ionic size, complex state of existence, very low concentration in high volume, and competition with nontoxic inorganic species [1]. The presence of inorganic species especially divalent cations of manganese, Mn and its metalloids are commonly found in iron (Fe) bearing waste wastewater. The intake of manganese can cause neurological disorder in men when inhaled at concentration greater than >10 mg/day [1]. Even at lowest concentration, it produces objectionable stains on fabric [2–4]. Many industries, specially mining source discharge Mn(II) ions into natural freshwater bodies without sufficient prior treatment which is very difficult to remove as this is the last member of Irving William series which has least tendency to form stable surface complexes and thereby removed by sorption from wastewater.
Various technologies have been developed to address the deleterious effects of Mn(II) ions on the quality of fresh water, especially those emanating from mining sources. The most common approach to remove Mn(II) ions is to oxidize and subsequently precipitate it as MnO2. However, this process of abiotic and biological oxidation is relatively slow at pH below 8 and is significantly inhibited by presence of iron (Fe) [2]. Partial removal of Mn(II) ions under reducing condition was reported to produce secondary pollutant of rhodochrosite (MnCO3) [4]. Some previous studies reported to remove Mn(II) ions by using granular-activated carbon (41%), lignite (25.84%), and palm fruit bunch (50%) [5].
Adsorption onto commercial-activated carbon is an effective technique to remove heavy metals including manganese from waste effluents. Regardless of its extensive application in wastewater treatment, commercial-activated carbon remains an expensive material. Coal, lignite, peat, and wood are frequently used for production of commercial activated carbon. However, production of activated carbon from these nonrenewable starting materials makes it costly [6, 7]. Therefore, the use of renewable source of low cost agricultural waste biomass which needs little processing to produce activated adsorbent is considered as a better choice [8, 9]. Hence aqueous phase adsorption by utilizing different types of agroresidues has gained credibility in recent years because of its excellent performance, biodegradability, and simplicity of design for treating waste effluents [10–12].
This study examines the performance of granular activated carbon prepared from agroresidues of Mangostene (Mangostana garcinia) fruit peel for adsorption of Mn(II) ion bearing wastewater in batch as well as fixed bed sorption system. Column dynamics has been investigated by using Thomas, Yoon-Nelson, and Adams-Bohart models. Nevertheless, column regeneration and recycling has been carried out until five cycles by considering the industrial applicability of the prepared sorbent.
2. Experimental
2.1. Preparation of Adsorbent
The fruit shells were first washed thoroughly to eliminate dust and inorganic matters on their surfaces. It was dried in an oven at temperature of 105°C for 24 h to remove all the moisture. The dried precursors were cut into small pieces and sieved to the size of 1.2 mm. 50 gm of dried fruit shell was placed on the metal mesh located at the bottom of the tubular reactor. Purified nitrogen gas was used to evacuate oxygen and create the inert atmosphere through the reactor. The flow rate of nitrogen gas and the heating, rate was maintained at 150 cm3/min and 10°C/min, respectively. The temperature was increased from room temperature to 400°C and held for 2 h to produce char. The char was mixed up with KOH at ratio 1 : 1 and activated under CO2 gas flow rate of 150 cm3/min for 750°C at heating rate of 10°C/min. The prepared activated carbon was washed with hot deionized water for several times until the pH becomes 6-7, dried and stored in air tight container for further application.
2.2. Surface Characterization of the Adsorbent
Surface area, pore volume and pore size distribution of the raw precursor and prepared adsorbent was determined by using Autosorb-1, Quantachrome Autosorb Automated gas sorption system supplied by Quantachrome. Prepared activated carbon was outgassed under vacuum at temperature 300°C for 4 hours to remove any moisture content from the solid surface before performing the nitrogen gas adsorption. Surface area and pore volume were calculated by Brunauer Emmett Teller (BET). Above-mentioned procedure was automatically performed by software (Micropore version 2.26) which was supplied with the instrument.
Iodine number is one of the most essential parameters widely used to characterize the prepared activated carbon. 0.1 gm of activated carbon is placed with 25 mL of iodine solution in a 100 mL conical flask and was shaken for 1 minute. After that the solution was filtered and 10 mL of filtrate was taken inside a 100 mL conical flask. The solution is titrated with 0.04 N sodium thio-sulphate solutions until it becomes clear. The iodine number of the activated carbon was determined by using (1) which represents the number of milligrams of iodine adsorbed by one gram of activated carbon [13]: where represents the volume of iodine solution (25 mL), is the volume of Na2SO4 solution used for titration of 10 mL iodine solution, is the volume of Na2SO4 solution used for titration of 10 mL of filtrate, represents the weight of activated carbon (0.1 gm), is the molar weight of Iodine (126.9044 g/mol), and is the concentration of iodine solution (0.045 N) [13].
2.3. Batch Adsorption Study
The batch experiment was carried out by adding 0.2 gm of activated carbon with 50 mL of 50, 60, 70, 80, 90, and 100 mg/L solution of Mn(II) ions and shaking at agitation speed of about 150 rpm until the equilibrium contact time in water bath shaker at temperature 30°C, 50°C, and 70°C. The remaining concentration of the cations was analyzed after set interval of time until equilibrium by using atomic absorption spectrophotometer (PerkinElmer Model 3100). The amount of adsorption of Mn(II) ions at equilibrium, (mg/g), was calculated by using the following (2) in batch sorption system: where (mg/g) is the amount of ion adsorbed at equilibrium. and (mg/L) are the liquid-phase concentrations of Mn(II) ions at initial and equilibrium conditions, respectively. (L) is the volume of the solution, and (g) is the mass of activated carbon used. The removal efficiency of the metal ion was calculated by dividing the residual metal ion concentration after equilibrium by initial metal ion concentration and the result is calculated on percentage basis.
2.4. Fixed Bed Adsorption Study
Figure 1 represents the schematic diagram of the fixed-bed adsorption system. Continuous flow adsorption studies were conducted in a column made of Pyrex glass tube having inner diameter of 4.5 cm and 25 cm height. A sieve made up of stainless steel was placed at the bottom of the column. Over the sieve, a layer of glass wool was placed to prevent loss of adsorbent. A peristaltic pump (Model Masterfiex, Cole-Parmer Instrument Co., USA) was used to pump the feed upward through the column at a desired flow rate. The solution was pumped upward to avoid channeling due to gravity.
Column regeneration was carried out by using 1 M HNO3 acid solution at flow rate 3 mL/min for 16 hours. After each cycle, the adsorbent was washed with hot distilled water and then packed inside the column. The regeneration efficiency (RE%) was calculated for bed height (4.5 cm), flow rate (1 mL/min), and initial concentration of 100 mg/L by using following (3): where is the adsorptive capacity of the regenerated column and is the sorption capacity (mg/g) of the adsorbent after each cycle.
3. Results and Discussion
3.1. Surface Characterization of the Prepared Adsorbent
Surface area, pore volume, and pore size distribution of the prepared activated adsorbent is listed in Table 1. The raw fruit shell had BET surface area of 1.034 m2/g, micro pore volume 0.0.0051 cc/g and pore diameter 4.087 Å. It was observed that, after the activation process, BET surface area and total pore volume increased significantly. This might be due to the reaction of both chemical and physical activating agents of KOH and CO2 with the cellulosic precursor at high temperature during the activation process. Thus, it would increase the surface area by developing new pores inside the carbon matrix of the semicarbonized char [14]. Based on the International Union of Pure and Applied Chemistry (IUPAC 1972) classification, the pores can be categorized into three main types depending on pore diameters, such as micropores (pore size < 2 Å), mesopores (pore size 2–50 Å), and macro pores (pore size > 50 Å) [15]. Here, the activated carbon prepared had the average pore diameter of 28.9 Å which is in the range of mesoporous type of activated adsorbent [14].
3.2. Batch Adsorption Study
Batch equilibrium data obtained at 30°C–70°C were analyzed by using the linear form of Langmuir isotherm [16] equation which is expressed by (4): where (mg/g) is the maximum amount of the Mn(II) ions per unit weight of the activated carbon to form a complete monolayer on the surface whereas (L/mg) is Langmuir constant related to the affinity of the binding sites.
The essential characteristics of the Langmuir equation can be expressed in terms of a separation factor, which is given below:
The linear form of Freundlich [17] isotherm is
Here, (mg/g) represents the affinity factor or multilayer adsorption capacity and is the intensity of adsorption, respectively.
According to Temkin isotherm [18], the linear form can be expressed by (7):
Here, (J/mol), which is Temkin constant related to heat of sorption, whereas (L/g) represents the equilibrium binding constant corresponding to the maximum binding energy. (8.314 J/mol K) is universal gas constant and (°K) is absolute temperature. The model parameters at different temperature are listed in Table 2.
The results from Table 1 suggested the applicability of Langmuir model which reflected homogeneous texture of the prepared where adsorption of each cations of Mn(II) had equal activation energy. The values obtained were less than 1 demonstrating that the adsorption of Mn(II) ions onto the prepared activated carbon is favorable. The positive value of and the Freundlich exponent, ranging between 0 and 1, showed surface heterogeneity and favorable adsorption of Mn(II) ions onto the surface of prepared activated carbon [14]. The experimental data were further analyzed by Temkin isotherm which showed a higher regression coefficient, values, showing the linear dependence of heat of adsorption at low to medium coverage [14].
3.3. Fixed Bed Adsorption Study
3.3.1. Effect of Adsorbate Inlet Concentration
The effect of adsorbate Mn(II) ions concentration on the column performance was studied by varying the inlet concentration of 50, 70, and 100 mg/L for while the same adsorbent bed height of 4.5 cm and feed flow rate of 1 mL/min were used. The breakthrough curve is illustrated by Figure 2.
As can be observed from the plots (Figure 2), the activated carbon beds were exhausted faster at higher adsorbate inlet concentration that is, for 100 mg/L. That is earlier breakthrough point was reached at higher concentration. The breakpoint time was found to decrease with increasing adsorbate inlet concentration as the binding sites became more quickly saturated in the column. A decrease in inlet concentration gave an extended breakthrough curve, indicating that a higher volume of solution could be treated. This is due to the fact that lower concentration gradient caused a slower transport due to a decrease in diffusion coefficient or mass transfer coefficient [19, 20].
3.3.2. Effect of Activated Carbon Bed Height
Figure 3 shows the breakthrough curve obtained for adsorption of Mn(II) on MFSAC for two different bed height of 3 and 4.5 cm (3.56 and 4.86 g of MFSAC) at constant adsorbate feed flow rate of 1 mL/min and adsorbate inlet concentration of 100 mg/L.
As can be seen from the plots (Figure 3), both the break through time, , and exhaustion time, , were found to increase with increasing bed height. The plots represent that the shape and gradient of the breakthrough curves were slightly different with the variation of bed depth which is expected also. A higher uptake was observed at higher bed height due to the increase in the amount of the activated carbon which provided more fixations of the cations with active binding sites for the adsorption process to proceed. The increase in bed height will increase the mass transfer zone. The mass transfer zone in a column moves from the entrance of the bed and proceed towards the exit. Hence for same influent concentration and fixed bed system, an increase in bed height would create a longer distance for the mass transfer zone to reach the exit subsequently resulting an extended breakthrough time. For higher bed depth, the increase of adsorbent mass would provide a larger service area leading to an increase in the volume of the treated solution [21].
3.3.3. Effect of Feed Flow Rate
The effect of feed flow rate on the adsorption of Mn(II) on MFSAC was investigate by varying the feed flow rate (1 and 3 mL/min) with constant adsorbent bed height of 4.5 cm and inlet adsorbate concentration of 100 mg/L, as shown by the breakthrough curve in Figure 4. The curve showed that at higher flow rate, the front of the adsorption zone quickly reached the top of the column that is the column was saturated early. Lower flow rate has resulted in longer contact time as well as shallow adsorption zone. At higher flow rate more steeper curve with relatively early breakthrough and exhaustion time resulted in less adsorption uptake.
3.4. Column Dynamics Study
The sorption performance of the cations through the column was analyzed by Thomas, Yoon-Nelson, and Adams-Bohart models starting at concentration ratio, that is 10% breakthrough until , that is, 90% breakthrough for manganese by considering the safe water quality standards and operating limit of mass transfer zone of a column [21–23].
3.5. Application of the Thomas Model
Thomas model is based on the assumption that the process follows Langmuir kinetics of adsorption-desorption with no axial dispersion. It describes that the rate driving force obeys the 2nd order reversible reaction kinetics [24]. The linearized form of the model is given as: where (mL/mg min) is the Thomas rate constant (mg/g) is the equilibrium adsorbate uptake and is the amount of adsorbent in the column.
The experimental data were fitted with Thomas model to determine the rate constant () and maximum capacity of sorption (). The , and , values were calculated from slope and intercepts of linear plots of ln against using values from the column experiments (Figures not shown). From the regression coefficient () and other parameters, it can be concluded that the experimental data fitted well with Thomas model. The model parameters are listed in Table 3.
As the concentration increased, the value of decreased whereas the value of showed a reverse trend, that is, increased with increase in concentration [19, 25]. The bed capacity () increased and the coefficient () increased with increase in bed height. Similarly, values decreased and values increased with increase in the flow rate. Similar trend has also been observed for sorption of Cr(VI) by activated weed fixed bed column [26]. The well-fitting of the experimental data with the Thomas model indicate that the external and internal diffusion will not be the limiting step [19, 25].
3.6. Application of the Yoon-Nelson Model
A simple theoretical model developed by Yoon-Nelson was applied to investigate the breakthrough behavior of Mn(II) ions on MFS-based activated carbon. This model was derived based on the assumption that the rate of decrease in the probability of adsorption for each adsorbate molecule is proportional to the probability of adsorbate adsorption and the probability of adsorbate breakthrough on the adsorbent [27]. The linearized model for a single component system is expressed as: where (min−t) is the rate constant and is the time required for 50% adsorbate breakthrough.
The values of and were estimated from slope and intercepts of the linear graph between versus at different flow rates, bed heights, and initial cation concentration (figures are not shown). Values of was found to decrease with decrease in bed height whereas, the corresponding values of increased with increasing bed height. With increase in initial cation concentration, the and values decreased. With increase in flow rate, increased but decreased. Similar trend was flowed for sorption of azo dye and Cd(II) for column mode sorption [19, 21]. The values of and along with other statistical parameter are listed in Table 4.
3.7. Application of the Adams-Bohart Model
This model was established based on the surface reaction theory and it assumed that equilibrium is not instantaneous. Therefore the rate of adsorption was proportional to both the residual capacity of the activated carbon and the concentration of the sorbing species [28]. The mathematical equation of the model can be written as: where and are the inlet and outlet adsorbate concentrations, respectively, (cm) is the bed height, (cm/min) is the superficial velocity. (mg/L) is the situation concentration and (L/mg min) is the mass transfer coefficient. Adams-Bohart model was applied to experimental data for the description of the initial part of the breakthrough curve. This approach focused on the estimation of characteristics parameters such as maximum adsorption capacity () and the mass transfer coefficient (). Linear plots of against time, at different flow rates, bed heights and initial cation concentrations (Figures are not shown) were plotted. The mass transfer coefficient () and saturation concentration () values were calculated from the slope and intercept of the linear curves respectively and listed in Table 5.
Although, Adams-Bohart models gives a simple and comprehensive approach for evaluating column dynamics, its validity is limited to the range of condition used. Thus the poor correlation coefficient reflects less applicability of this model [28]. The mass transfer coefficient and experimental uptake capacity along with and and other statistical parameters are shown in Table 5. From the Table, it is observed that, mass transfer coefficient increased with increase in bed height and flow rate but decreased with initial concentration. This showed that the overall system kinetics was dominated by external mass transfer [19, 28]. However, the sorption capacity increased for increasing initial concentration, flow rate, and bed height [24, 26, 29].
3.8. Regeneration of the Activated Carbon
It is essential to reuse the cation loaded sorbent for metal removal in industrial applications for economical feasibility of the process. Reusability of any sorbent can be determined by its adsorption performance in consecutive sorption/desorption cycles. MFSAC were tested for four cycles after the initial application, using 1 M HNO3 as an eluting agent at flow rate of 3 mL/min for 16 hours.
Based on, Yoon-Nelson model, amount of adsorbate being adsorbed in a fixed bed column is half of total adsorbate entering within period [21]. Thus, the sorption capacity of a column, or (mg/g) is calculated from following equation and tabulated in Table 6 for each cycle: Here, is the initial concentration, is flow rate and is mass of the activated carbon in fixed bed. However, the breakthrough time, and complete exhaustion time, and regeneration efficiency, according to (2) for different condition were determined and listed in Table 6.
From the tables, it can be seen that the breakthrough time is less at higher flow rate, lower bed height, and at higher inlet concentration. Experimental equilibrium uptake, (mg/g) for initial concentration of 50 mg/L, 70 mg/L, and 100 mg/L solution obtained was 9.978 mg/g, 13.110 mg/g and 17.260 mg/g for batch sorption system which was higher than fixed bed system for the same concentration used. This might be due to the less effective surface area in packed bed system than the stirred batch vessels [20, 30].
4. Conclusion
This investigation showed that the granular activated carbon prepared from Mangostene fruit peel (MFSAC) was promising for removing Mn(II) ions from wastewater batch and fixed bed sorption column. The column performs better with lower feed flow rate and concentration with higher bed height. Experimental data followed Langmuir isotherm better than Freundlich at all the temperature range being studied. Column data were best-fitted with Thomas and Yoon-Nelson models. The adsorbed Mn(II) ions were desorbed quantitatively by 1 M HNO3 and the adsorbent can be used repeatedly without significant loosing of sorption capacity reflecting its feasibility for commercial application.
Acknowledgments
The authors are grateful for the financial support of this project by Research Grant (UMRG 056-09SUS) of University Malaya, Kumoh National Institute, Republic of Korea (KIT), and Malaysian Agricultural Research and Development Institute (MARDI), Malaysia, for their continuous encouragement.