2. MARCO TEÓRICO
2.5. Impuesto al valor agregado
Adsorption isotherms were performed on selected DRAW and DMAD chars in order to evaluate their adsorption capacities for BPA. The BPA adsorption data was modelled to Freundlich, Langmuir (using both linear equations 1-9 and 1-10) and Temkin equations in order to find the best fitted model equation and adsorption capacities of BPA by chars.
The adsorption parameters relating to these three model equations and normalized percent deviation (P) obtained from the BPA adsorptions by chars are reported in Table 4-10. The P values were given as better criteria to test the goodness of fit for adsorption data to isotherm model equation, the calculation method of these values is described in APPENDIX IV, section 12.5.2. There are many literature reports which use the P values and it has been mentioned that P value lower than 5 indicate an excellent fit to adsorption data [57, 58, 223- 225]. The plots of qe versus Ce of the BPA adsorption by DMAD and DRAW chars are shown
in Figure 4-25 and 4-26, respectively and the adsorption data fitted to model equations were also given for the chars produced between 800 and 1000°C.
Based on the adsorption data in Figure 4-25 and 4-26, it could be generally said that all of the BPA adsorption data for chars exhibit “type L isotherm” or “Langmuir” with the sub-group
2c according to Giles classification of liquid/solid isotherm shape [55]. The “type L isotherm”
indicates that the BPA molecules are adsorbed flat or occasionally on the vertical with particularly strong intermolecular attraction. The sub-group 2c indicate that the SBAs are microporous but probably the accessible internal area is relatively low and comparable with the external area [55], which is probably because micropore and accessible pores are still closed due to the result of carbon consolidation.
It can be seen from the results in Table 4-10 that the adsorption performance of the chars were quite low, especially when produced at a carbonization temperature below 700°C. As a result, the R2 and the P values of BPA adsorption by chars below 700°C are too low and too high,
Table 4-10 Parameter of Freundlich, Langmuir, Temkin adsorption isotherm equations of BPA adsorption by DMAD2 and DRAW2 and DRAW3
chars
ID*
Exp. Cond.*
Freundlich Langmuir, 1/qe vs 1/Ce (Eq.1-10) Langmuir, Ce/qe vs Ce (Eq.1-9) Temkin
k 1/n R2 P Qmax b RL R2 P Qmax b RL R2 P B** A R2 P PA4 5/500/0 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA PA5 5/600/0 0.02 1.26 0.752 36.5 -1.36 -0.012 -7.941 0.915 184.8 -4.73 -0.006 2.078 0.166 34.5 1.5 0.13 0.691 47.1 PA6 5/700/0 0.65 0.53 0.880 10.6 10.50 0.021 0.341 0.966 7.2 8.48 0.034 0.247 0.966 6.2 2.0 0.29 0.935 6.9 PA8 5/800/0 1.59 0.46 0.902 8.5 15.85 0.033 0.249 0.978 4.9 14.18 0.044 0.200 0.987 4.6 3.3 0.38 0.951 5.5 PA11 5/900/0 9.36 0.24 0.917 4.7 28.41 0.127 0.080 0.978 2.9 28.25 0.134 0.076 0.990 3.2 4.9 2.80 0.920 4.2 PA12 5/1000/0 14.96 0.22 0.980 2.2 40.00 0.173 0.060 0.985 2.5 41.84 0.129 0.079 0.997 3.2 6.6 4.30 0.989 1.5 PA14 5/600/120 0.13 0.87 0.869 46.3 -39.22 -0.002 1.215 0.948 41.4 16.18 0.007 0.627 0.372 47.9 2.2 0.14 0.937 45.7 PA16 5/700/120 1.29 0.44 0.947 6.1 11.22 0.041 0.214 0.983 3.7 11.26 0.040 0.216 0.983 3.7 2.6 0.36 0.963 4.3 PR4 5/500/0 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA PR5 5/600/0 0.30 0.61 0.775 11.9 6.90 0.019 0.368 0.630 14.1 11.90 0.008 0.591 0.424 12.3 2.3 0.09 0.778 14.0 PR7 5/700/0 0.17 0.89 0.886 14.2 -57.80 -0.002 1.193 0.951 20.0 27.17 0.005 0.691 0.292 12.5 3.1 0.13 0.924 11.6 PR10 5/800/0 1.71 0.48 0.932 7.1 18.69 0.032 0.254 0.984 4.2 17.67 0.037 0.230 0.962 4.4 4.1 0.33 0.966 4.7 PR14 5/900/0 8.15 0.29 0.968 3.7 31.25 0.104 0.096 0.961 4.3 33.78 0.074 0.129 0.987 4.5 6.4 1.19 0.961 3.1 PR15 5/1000/0 15.85 0.24 0.995 1.2 44.84 0.177 0.059 0.939 4.5 49.02 0.105 0.095 0.995 5.3 8.1 2.97 0.986 1.9 PR16 5/600/0 0.11 0.77 0.590 24.4 4.98 0.016 0.411 0.426 26.8 22.68 0.002 0.843 0.022 23.8 2.1 0.06 0.612 28.5 PR18 5/600/120 0.001 1.82 0.994 4.5 -1.82 -0.008 4.179 0.993 9.2 -2.05 -0.008 3.481 0.960 5.9 3.4 0.03 0.870 31.3 PR19 10/700/0 0.15 0.76 0.994 9.6 9.62 0.009 0.542 0.986 8.5 9.97 0.009 0.555 0.708 8.6 1.7 0.13 0.891 11.9 PR21 5/700/120 0.03 1.16 0.965 11.4 -7.85 -0.005 1.805 0.979 12.8 -11.81 -0.004 1.465 0.349 10.2 2.3 0.08 0.832 37.2 *experimental conditions (heating rate/carbonization temperature/dwell time)
* PR4-PR15 using DRAW2 sludge; PR16-21 using DRAW3 sludge; PA4-PA16 using DMAD2 sludge
Figure 4-25 BPA adsorption isotherms exhibited by DMAD2 chars; calculated from ---Freundlich, – – – Langmuir (1/qe vs 1/Ce), ···Langmuir
(Ce/qe vs Ce), —– Temkin model equations
0
5
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35
40
45
50
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90
q
e(mg
/g
)
C
e(mg/l)
PA5
PA6
PA8
PA11
PA12
PA14
PA16
Figure 4-26 BPA adsorption isotherms exhibited by DRAW2, 3 chars; calculated from ---Freundlich, – – – Langmuir (1/qe vs 1/Ce), ···Langmuir
(Ce/qe vs Ce), —– Temkin model equations
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0
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q
e(mg
/g
)
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e(mg/l)
PR5
PR7
PR10
PR14
PR15
PR16
PR18
PR19
PR21
mathematical models. Because of the low BPA adsorption performance exhibited by the chars, it is quite difficult to indicate which adsorption model equation is best fitted to the BPA adsorption data. Nevertheless, based on the BPA adsorption by chars at 700°C and over, it could be generally said that the adsorption capacities of BPA as determined by the k and Qmax
values from the Freundlich and Langmuir adsorption models respectively, are increasing with the carbonization temperature of the sludge. Both of the versions of the Langmuir linear equation provided a slightly better fit to most of adsorption data, as indicated by the P and R2
value, especially in the case of DMAD chars. Thus, this might indicate that the adsorption by most chars are of monolayer character and adsorption of each molecule has equivalent activation energy [226]. Based on the Langmuir linear equation 1-10, which gives the higher
P value for the BPA capture by high temperature chars, the maximum BPA adsorption
capacities (Qmax values) are 45 and 40_mg/g for DRAW and DMAD chars at 1000°C (PR 15
and PA12), respectively. Moreover, the magnitude of the Langmuir constants (b) is increasing with carbonization temperature from 0.032-0.177_mg/l and 0.021-0.173_mg/l for carbonized DRAW (PR10, 14, 15) and DMAD (PA6, 8, 11, 12), respectively. These values are largely determined by the heat of adsorption [226] and are indicating a high heat of adsorption (strong adsorption) with chars prepared at higher temperatures.
Nevertheless, it is also interesting to note that both of the 1000°C chars exhibited higher P values of Freundlich and Temkin models than the Langmuir model. This could be explained by the observation that, at this temperature, the micropores were widening and produced more mesopores and thus the BPA molecules can more easily access the SA. Maximum adsorption capacities determined by the Freundlich model (k values) were 16 and 15_(mg/g)(l/mg)1/n for
DRAW and DMAD chars, respectively.
Considering the influence of carbonization dwell time on the BPA adsorption capacities, it is quite interesting to note that the longer dwell time created a negative effect on the BPA adsorption in the case of DRAW sludge. Conversely, for DMAD sludge the adsorption capacities increased with carbonization dwell time. These results might be explained by the observation that the longer carbonization dwell time enhanced carbon consolidation. Thus some of the accessible pores may contract, creating a restriction so limiting the BPA molecules’ access to smaller pores inside the DRAW chars structure. In the case of DMAD chars, the high ash content may provide a dual benefit during carbonization. Firstly, the ash particles may help prevent pore contraction keeping pores open. Secondly, the inorganic content might also behave as a very mild activating reagent that catalyses burnt-off the
carbonaceous content in DMAD sludge, creating wider pores. Further investigation would be needed to confirm these hypotheses. Thirdly, it may be possible that basic-acid sites interact.
The BPA adsorption by chars, produced at 900°C and over (PR14-15 and PA11-12), had separation factor (RL) values very close to 0, indicating the adsorption isotherm were very
favourable (irreversible). All the intensity of adsorption (1/n) values by Freundlich are favourable, with values between 0 to 1 for the BPA adsorption by chars, except for PR18, PR21 and PA5, which were produced at low carbonization temperature ≤ 700°C and long carbonization dwell times.
The BPA Freundlich adsorption capacities observed for sludge char are comparable to those of carbonized sawdust (18_(mg/g)(l/mg)1/n) and better than commercial carbon obtained from
Wako pure chemical industries (9.2 (mg/g)(l/mg)1/n) and from Takeda chemical industry (9.9
(mg/g)(l/mg)1/n) [138, 139] even though the BET SA of these two commercial carbons are
much higher than the sludge chars (1350 and 1119_m2/g, respectively). The explanation of
this different behaviour is probably due to the case that the high BET SA of these two commercial carbons contain very narrow micropores which are not accessible to the BPA molecules.
Nevertheless, the BPA adsorption capacities obtained in this study are low compared to carbonized bamboo (25 (mg/g)(l/mg)1/n, Freundlich), carbonized almond shells (189_mg/g,
Langmuir) and other commercial carbons (see Table 1-9 in Chapter 1). This is simply due to the fact that sludge contains much lower carbon contents than the raw material used to make commercial carbons. Thus, there is less opportunity to form carbon pores. Moreover, some of the inorganic elements remaining in sludge chars that exhibited the hydrophilic property might expel the BPA molecule. A further acid washing step may help to increase the BPA adsorption capacity through removal of Ca, SO42- and PO43- ions, as reported elsewhere [118].
Nevertheless, HCl washing of sludge chars will unnecessarily increase the production cost and thus reduce the economic attractiveness of using the chars for BPA adsorption.