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4.2. Effect of activation processes on Swiss Blue dye removal efficiency and
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Activation temperature played an important role on the yield of activated carbons prepared from the two precursors. It was found that weight loss increased with increase in activation temperature from 550oC to 9500C. The increase in temperature would release increasing volatiles as a result of intensifying dehydration and elimination reaction and also increase the C– KOH reaction rate, thereby resulting in decreasing yield (Lua & Yang 2004; Adinata et. al., 2007). Indeed, the increase in activation temperature quickens the gasification reaction of carbon and therefore, the attack of the amorphous components which obstruct the pores causes a decrease in the carbon yield (Bacaoui et. al., 2007). Activated carbon prepared from the two samples had the same pattern of decrease in yield as activation temperature increased, but the decrease was more with brewers‘ spent grain activated carbon with yield of 19.1% compared to hamburger seed shell activated carbon that had 25.6%. The high weight loss experienced with brewers‘ spent grain activated carbon can be attributed to the high volatile matter content of the sample which decreased as activation temperature was increased.
(a)
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0 200 400 600 800 1000
Swiss blue dye Removal Efficienct (%)
Activation Temperature (degree celsius)
Removal efficiency for spent grain activated carbon
Removal efficiency for hamburgar seed shell activated carbon
95 (b)
Fig: 4.2: Effect of conventional activation temperature on Swiss blue dye Removal Efficiency (a) and activated carbon yield (b) for both precursors
b) Effect of Conventional Activation Time on Swiss blue dye Removal Efficiency and Carbon Yield
Effect of activation time on the Swiss blue dye removal efficiency and carbon yield was studied at activation temperature of 850oC and 6M KOH solution at impregnation ratio of (1:1.5) carbon to KOH solution. It can be seen from the result (fig. 4.3) that increase in activation time increased the removal efficiency at 60 mins, beyond 60mins, there was reluctant increase in Swiss blue dye removal efficiency. As activation time was increased beyond 60mins, the surface area was not much affected due to low volatilization of organic matters from the biomass. The non-performance or insignificant activation time variable impact on uptake of basic blue dyes has been observed by a number of researchers. Gratuito et. al. (2008), Sentorum – Shalaby et. al.
(2006), and Auta and Hameed (2011) reported that activation time or prolonging time of activation does not necessary lead to yielding of high surface area and enlargement of pores during production of activated carbon. However, Gratuito et. al. (2008) indicated that it is not necessary to prolong activation time so much beyond the basic requirement, as doing so would cause pores enlargement, which may be undesirable depending on the requirement of a specific activated carbon application.
It can be seen that the two precursors used for the production of activated carbon showed the same behaviour of insignificancy at higher activation time. Hamburger
0 10 20 30 40 50 60
0 200 400 600 800 1000
Activated carbon Yield (%)
Activation temperature (degree celsius)
Yield for spent grain activated carbon
Yield for hamburgar seed shell activated carbon
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seed shell showed lower removal efficiency of 98.1% compared with activated carbon from spent grain that had 99.6%. Both had insignificant increase in surface area as the activated time was prolonged. This was evidenced from the similar removal efficiencies obtained. Effect of activation time on the carbon yield was equally observed for the activated carbons produced. It was observed that the yield of carbon was almost constant which confirmed the insignificancy of activation time on activated carbon production. Weight loss increases only if there are serious volatilities of the volatile matters. But since prolonging activation time did not appreciably increase volatilization, thus, carbon yield remained almost unchanged. It was equally interesting to know that carbon yield for hamburger activated carbon was higher with a value of 32.6% when compared with spent grain activated carbon that had 25.3%. This can be attributed to high volatile matter content of spent grain which went off during activation resulting to high weight loss.
(a)
(b)
Fig. 4.3: Effect of Conventional activation time on Swiss blue dye Removal Efficiency (a) and activated carbon yield (b) for both precursors
95 96 97 98 99 100
0 50 100 150 200
Swiss blue dye Removal Efficiency (%)
Activation time (Minutes)
Removal efficiency for spent grain activated carbon
Removal efficiency for hamburgar seed shell activated carbon
0 5 10 15 20 25 30 35 40
0 50 100 150 200
Activated carbon Yield (%)
Activation time (Minutes)
Yield for spent grain activated carbon
Yield for hamburgar seed shell activated carbon
97
c) Effect of KOH Concentration on Swiss Blue Dye Removal Efficiency and Carbon Yield for spent grain and hamburger seed shell activated carbons Effect of concentration of the activating agent KOH was studied at activation temperature of 850oC, activation time of 1hour and 1:1.5 solid / KOH impregnation ratio. Impregnation of the chemical results in the dehydration of the carbon skeleton and widening of the interior canal of the botanic structure followed by the formation of a porous structure (Foo & Hameed, 2012f). It was observed from fig. (4.4) that increase in KOH concentration increased the Swiss blue dye removal efficiency to a point that further increase in KOH concentration led to decrease in Swiss blue dye removal efficiency. This phenomenon is probably due to the fact that upon the impregnation of carbon with KOH, K2CO3 was formed with a simultaneous evolution of CO2 and CO (Diaz-Teran et. al., 2003). As concentration of KOH increased, the catalytic oxidation also caused the widening of micropores to mesopores, therefore increasing the Swiss blue dye removal efficiency. At high KOH impregnation, the microporosity development is mostly due to the intercalation of potassium metal in the carbon structure (Sudaryanto et. al., 2006). Indeed, methylene blue molecules has a minimum molecular cross section of about 0.8nm, and it has been estimated that the minimum pore diameter it can enter is 1.3nm (Barton, 1987). This meant that when mesopores were developed, more swiss blue dye molecules could be absorbed by the activated carbon, therefore, enhancing the adsorption capacity of the activated carbons (Tan et. al., 2008).
However, at high KOH concentration of 7M, Swiss blue dye removal efficiency reduced, this was due to gasification reaction between the excess potassium and the carbon material resulting in the loss of some carbon and also translating to poor development of the pores and its surface area (Auto & Hameed 2011). Equally, El-Hendawy (2009) has observed that too high concentration of KOH might lead to the presence of K2CO3 and metallic potassium that was left in the carbon and cannot easily leached even after repeated washing. This may cause blocking of some pores leading to the observed drastic decrease in the accessible area for dye molecules that finally reduced the surface area of the activated carbon prepared.
Equally, effect of KOH concentration on the carbon yield was studied. Increase in KOH concentration decreased the yield and increased the carbon burn off. This was
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because when higher concentration was used, the weight loss was due to increasing release of volatile products as a result of intensifying dehydration and elimination reaction (Adinata et. al., 2007). KOH would promote the oxidation process, therefore with high concentration; the gasification of surface carbon atoms was the predominant reaction, leading to increase in the weight loss of carbon (Sudaryanto et. al., 2006).
Shevkoplyas and Saranchuk (2000) observed that impregnation of coal with KOH leads to the breaking of C-O-C and C-C bonds, facilitating coal decomposition during pyrolysis and as a result decreasing the carbon yield (Sahu et. al., 2010).
(a)
(b)
Fig. 4.4: Effect of KOH concentration on Swiss blue dye Removal Efficiency (a) and activated carbon yield (b) for both precursors
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0 2 4 6 8
Swiss blue dye Removal Efficiency (%)
KOH Concentration (M)
Removal efficiency for spent grain activated carbon
Removal efficiency for hamburgar seed shell activated carbon
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0 2 4 6 8
Activated carbon Yield (%)
KOH Concentration (M)
Yield for spent grain activated carbon
Yield for hamburgar seed shell activated carbon
99 4.2.2 Microwave Activation Process
In this study, microwave activation process was studied using a modified Sonik domestic microwave oven model SMW-90023 with a maximum power output of 900W, delivered at a frequency of 2450MHz. The operational parameters that were studied were power level (%), radiation time (minutes) and the KOH concentration (M).
a) Effect of Microwave power levels on the Swiss Blue Dye Removal Efficiency and Carbon Yield
Effect of microwave power levels were studied at 6M KOH and radiation time of 7minutes.The power level showed the percentage of the total power of the microwave used for a particular study. The effect of power levels was studied at defrost, low, medium high and high corresponding to 18%, 36%, 58%, 81% and 100% power output in watts. From the result (fig. 4.5), it can be seen that Swiss blue dye removal efficiency increased with increase in power level up to a point that further increase in power level resulted to decrease in the removal efficiency.
As suggested by the result, at low power of 18% and 36%, the pore structure was not adequately developed, and their removal efficiency were low indicating no continual reaction between the char and a activating agent. At higher power levels of 58% and 81%, the pore width was successively broadened and new miropores-mesopores were formed in the original pore walls, giving a sustaining increase in removal efficiency.
High microwave power improves the development of the pore structures of activated carbon which indicates that microwave power is important in the activation stage.
However, the removal efficiency decreased when the power level was increased to 100% due to decrease in the formation rate of new pores and beginning of pore destruction. According to Deng et. al. (2010), the decrease in removal efficiency with further increase in power levels might be due to the sintering effect at high power levels, followed by shrinkage of the char and realignment of the carbon structure which resulted in reduced pore areas as well as volume. Over gasification might occur causing destruction of pore structures, thus decreasing the removal efficiency.
Effect of microwave radiation power on the carbon yield was equally studied. From the resulted, it was ascertained that carbon yield decrease with increase in power levels. At high microwave power levels, absorbed microwave energy exceeded a
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certain level which led the plentiful energy to cause excessive destruction of pore structures, and a progressive decrease in carbon yield was observed. The weight loss of carbon increased proportional to the microwave power level, mainly due to the fierce reaction at higher thermal radiation which intensified devolatilization, dehydration and decomposition (Foo & Hameed, 2012e). It was observed that carbon prepared from hamburger seed shell had higher yield than those from spent grain. This can be attributed to the low volatile content of hamburger seed shell.
(a)
(b)
Fig. 4.5: Effect of Microwave power levels on Swiss blue dye removal Efficiency (a) and activated carbon yield (b) for both precursors.
0 20 40 60 80 100 120
0 20 40 60 80 100 120
Swiss blue dye Removal Efficiency (%)
Microwave power level (%)
Removal Efficiency for spent grain activated carbon
Removal Efficiency for hamburgar seed shell activated carbon
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0 20 40 60 80 100 120
Activated carbon Yield (%)
Microwave power level (%)
Yield for spent grain activated carbon
Yield for hamburgar seed shell activated carbon
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b) Effect of radiation time on Swiss Blue Dye Removal Efficiency and Carbon Yield
Microwave radiation time is another key parameter affecting the removal efficiency and carbon yield. Result (fig. 4.6) revealed that prolonging the radiation time exhibited an enhancement of removal efficiency. This phenomenon implied that prolonging time exposure promoted an acceleration of temperature, which in turn increased reaction rates thus developing porosity (Foo & Hameed, 2012f). A slight drop was observed at 8mins for activated carbon prepared from both precursors. However as the radiation time arrives at its optimum values (7mins) absorption and reflection of energy tends to balance and the activated carbon achieved their maximum removal efficiency. As activation proceeds, temperature increased dramatically and led to opening of microspores and mesopores which resulted in enlargement of the average diameter.
The drop in removal efficiency is probably due to a sintering effect, which largely destroyed the pore walls between adjacent pores. Moreover, high temperature might produce local hotspots, leading to external ablation and collapse of the carbon frame work, resulting in reduced accessibility of carbon actives sites. Higher pyrolytic temperatures could induce C-KOH, C-K2CO3, C-K, C-K2O, C-CO and C-CO2
reactions, facilitating breaking of the C-O-C and C-C bonds thus decreasing removal efficiency and carbon yield. That was why there were decreases in carbon yield as the radiation time increased. The decrease in carbon yield as radiation time was increased can equally be attributed to rapid evolution of volatile material to form stable compound. This is the reason why the carbon yield of spent grain activated carbon that has high volatile matter was lower than the activated carbon produced from hamburger seed shell.
(a)
0 20 40 60 80 100 120
0 2 4 6 8 10
Swiss blue dye Removal Efficiency (%)
Radiation Time (Minutes)
Removal Efficiency for spent grain activated carbon
Removal Efficiency for hamburgar seed shell activated carbon
102 (b)
Fig. 4.6: Effect of Radiation time on Swiss blue dye Removal Efficiency (a) and Activated Carbon yield for both precursors.
c) Effect of KOH concentrationon on Swiss Blue Dye Removal Efficiency and Carbon Yield for Microwave Activation Process
Effect of KOH concentration on the Swiss Blue dye removal efficiency and carbon yield was studied at the microwave power output of 81% and irradiation time of 7mins at 1:1.5 carbon to KOH ratio. Fig. (4.7) shows that augmenting KOH concentration from 2M to 6M showed an increase in Swiss blue dye removal efficiency and carbon yield. Beyond 6M, further increase in KOH concentration illustrated a gradual decrease in Swiss blue dye removal efficiency and carbon yield. KOH activation involves the redox reduction and carbon oxidation to generate porosity. It can be deduced that the pore enlargement related to KOH activation is associated to the redox reduction and oxidative modification responsible for the development of micro and mesoporosity.
During which the reaction of CO, CO2 and H2 constituents, and additional reactions between the active Intermediates with the carbon surfaces are possible (eqns. 4.1 to 4:3) (Pinero et. al., 2005).
6𝐾𝑂𝐻 + 2𝐶 → 2𝐾 + 3𝐻2+ 2𝐾2𝐶𝑂3 (4.1) 𝐾2𝐶𝑂3+ 𝐶 → 𝐾2𝑂 + 2𝐶𝑂2 (4.2)
2𝐾 + 𝐶𝑂2 → 𝐾2𝑂 + 𝐶𝑂 (4.3) Concurrently, the alkaline and carbonate metal formed during the activation stage are
Intercalated to the carbon matrix responsible for both stabilization and widening of pores between the carbon atomic layers. Therefore, by increasing the KOH concentration, the activation process was accelerated and correspondingly, the removal
0 20 40 60 80 100
0 2 4 6 8 10
Activated carbon Yield (%)
Radiation Time (Minutes)
Yield for spent grain activated carbon
Yield for hamburgar seed shell activated carbon
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efficiency was further enhanced (Foo & Hameed, 2011). Beyond the optimum value, the excessive KOH would promote vigorous gasification reaction, which destroys the carbon frame work leading to a drastic decrease of accessible areas besides, excessive KOH molecules might decompose following the reaction (Cao et. al., 2006)
2𝐾𝑂𝐻 → 𝐾2𝑂 + 𝐻2𝑂 (4.4)
𝐻2𝑂 + 𝐶 → 𝐶𝑂 + 𝐻2 (4.5) Therefore, the catalytic oxidation entailed widening of mesopores structures and
carbon burn off. Effect of KOH concentration on carbon yield was equally studied. It was observed that carbon yield increased with initial increase in KOH concentration and decreased with further increase in KOH concentration beyond the optimal value.
The increase in yield at low KOH concentration was as a result of intercalation of potassium metals on the carbon matrix which resulted to increase in weight.
Additionally, a further increase in KOH Concentration beyond the optimal value would intensity a vigorous activation reaction, which leads to carbon burn off and transition of microspores- mesopores into macrospores lowering the carbon yield.
(a)
0 20 40 60 80 100 120
0 2 4 6 8
Swiss blue dye Removal Efficiency (%)
KOH concentration (M)
Removal efficiency for spent grain activated carbon
Removal efficiency for hamburgar seed shell activated carbon
104 (b)
Fig. 4.7: Effect of KOH Concentration on Swiss blue dye Removal Efficiency (a) and Activated carbon yield (b) for microwave activation.
4.3. Optimization of Activation Conditions for Maximum Removal Efficiency of