The study involved the preparation of activated carbon from tea waste and utilizing the same activated carbon to produce hydrogel composites. The main characteristics of the activated carbon were evaluated by iodine number and percentage yield while the composite hydrogels were evaluated for their swelling characteristics, thermal stability and their ability to adsorb colours.
Iodine Number
According to ASTM D 4607–94 Iodine number is defined as the amount of iodine absorbed by 1g of activated carbon within specific test conditions. The SI unit of iodine number is mg/g. The iodine number is calculated by an iodometric titration. In iodometric titration alkaline iodine (0.1N) is titrated against sodium thiosulfate (0.1N) and starch solution (1%) is used as an indicator.
The iodine number can be calculated by this formula [8].
Iodine number = C x Conversion factor (mg/g)
Here, B = Blank reading
A = absorbed reading (reading with AC)
The iodine number for activated carbon as a super adsorbent belongs to the range from 500 to 1200 mg/g [8]. Activated carbon produced from tea waste using 1.0M sulfuric acid as an activating agent shows the results within the above range. Activated carbon which was produced by using 0.5M sulfuric acid as an activating agent was not within the normal range. The result of the iodine number shows that the molarity affects the adsorption activity directly. In case of activated carbon produced by utilizing 1.0M concentration of sulfuric acid as activating agent, higher adsorption activity is exhibited when compared to activated carbon produced by utilizing 0.5M concentration of sulfuric acid as activating.
Table 1
Iodine Number of activated carbon
Tea waste
|
Molarity of sulfuric acid (H2SO4) M
|
Iodine Number
(mg/g)
|
Activated Carbon
|
0.5
|
320.00
|
Activated Carbon
|
1.0
|
710.97
|
Percentage Yield of activated carbon
The percentage of yield is an important parameter for the activated carbon. The amount that was produced from the dried precursor can be calculated by the formula
The specific yield of activated carbon for 1.0M activating agent obtained was 23.1% while that from 0.5M activating agent 24.12% of activated carbon was obtained. High molarity of activating agent affects the yield of the product. Similar results have been reported earlier [7].
Swelling and equilibrium water content. The hydrogel water content is of fundamental importance as it influences the overall hydrogel integrity and solubility, as well as the diffusion kinetics of substances. The swelling studies of the Hydrogel samples is used to ascertain that the basic network structure of the hydrogels has remained intact and addition of filler phase has not affected the basic water swelling nature of the Hydrogel. Equilibrium water content indicates the scale to which a Hydrogel can take up water in its fully swollen state.
In order to study the swelling of the Hydrogel composite samples, the samples were immersed in distilled water under room temperature and allowed to swell for 24 hours. The weight of the samples was measured after every hour. The percentage swelling was obtained by following equation [7]
Wt = weight of the swollen gel at time t
Wd = weight of the dry gel at time 0.
Similarly, equilibrium water content (EWC) was obtained by following equation
Ws = weight of the swollen gel at time t (equilibrium) Wd = weight of the dry gel at time 0.
Table 2
EWC of 1.0M batch samples
SAMPLE
|
Wd
|
Ws
|
EWC%
|
5%
|
0.20
|
3.58
|
94.413%
|
7%
|
0.22
|
4.27
|
94.817%
|
10%
|
0.18
|
3.54
|
94.915%
|
12%
|
0.20
|
3.88
|
94.845%
|
15%
|
0.19
|
3.90
|
94.128%
|
Fig- 1) Graph of dehydration in1.0M batch samples.
The dehydration rates of PAM/AC composite hydrogels show that there is no significant impact of the Activated carbon on swelling characteristics. It implies that the presence of AC does not affect the cross-linking behaviour of PAM. There is no hindrance caused by the AC on the network forming capacity of the PAM. The initial dehydration rate for PAM hydrogels was slightly higher compared to the unfilled hydrogel sample. For the composite hydrogels there may be retardation in the diffusion of water though the three-dimensional network due to the presence of AC formed in the porous hydrogel network. Also, a considerable amount of space inside the network is occupied by AC thereby displacing water from the network. It is appreciable to note that with changing weight of AC there is a slight variation in dehydration performance of the hydrogel composites. The highest water absorption is shown by the sample with 0.5g and 0.7g of AC. This may be ascribed to the fact that amount of AC inside the matrix of polyacrylamide will be least for this sample. The dehydration characteristics are least for sample with 0.5gand 0.7g of activated carbon.
The degree of rehydration for the PAM/AC composites hydrogels is summarized in Fig. 2. The water holding capacity of hydrogels is reduced by increasing the weight percentage of AC during synthesis of composite hydrogels. Since the voids or spaces in acrylamide network are occupied by AC molecules, there is less space available. Also, the network junction movement is retarded by the presence of rigid AC inside the hydrogel. It can be concluded that although the water uptake or swelling capacity of hydrogels decrease with the addition of AC, yet the decrease is small and the overall swelling properties are not hindered. The graph shows that increasing the weight percentage of AC during synthesis of composite hydrogels have some effect on the rehydration characteristics. Initial rehydration is high for samples. This is because lowest amount of AC is present in this sample. With the increase in amount of AC rehydration is slow.
Fig-2) Graph of rehydration in1.0M batch samples.
Thermogravimetric Analysis:
The thermal stability of the developed samples of polyacrylamide/activated carbon hydrogel composite were evaluated by using Thermo gravimetric analyzer Perkin Elmer TGA 8000, USA (Pyris 1), at a heating rate of 10°C/min under nitrogen atmosphere (flow rate, 10ml/min) from room temperature to 800°C. The TGA thermograms of the composite hydrogel samples was similar and TGA of sample with 10% AC is presented in Fig. 4.
It is evident in TGA graph of PAM/AC composite hydrogels with 1.0M activating agent that there is step wise degradation. The first step is weight loss of moisture present in the sample at 100ᴼC and thereafter the weight loss at 200-2900C is because the main thermal decomposition of the functional groups such as OH, NH2 etc. on the polymer backbone occurs at this temperature. Second weight loss is observed at 320–400ᴼC due to degradation of polyacrylamide polymer matrix and the major weight loss occurs here at this temperature. The third weight loss is observed at 550- 6500C which is the degradation of activated carbon. It seems that activated carbon does not affect decomposition mechanism and thermal stability of polyacrylamide hydrogel.
Fig-3) TGA of blank hydrogel without activated carbon.
Fig-4) TGA of sample with 1.0M activating agent and 5% activated carbon.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR studies were carried out on polyacrylamide/activated carbon composite hydrogel by ATR on horizontal cross section. In almost all the spectra the absorption peak appears at 3177cm− 1 which is ascribed to –NH stretching of polyacrylamide. The adsorption peak at 2922 cm− 1 is for the C = O stretching of PAM, and the absorption peaks at 1678 cm− 1 is for the C = O bending for PAM. It may be due to some increase of stretching vibration of AC at wavelength range of C-O vibration at 1041-1116cm− 1, C-H bending at 1319–1420 and C = O stretching at 1556–1636 cm− 1. The increase of these bands indicates that modification of activated carbon has developed surface oxygen functional groups on the activated carbon. Similar spectra were also reported for chemically synthesized activated carbon. The synthesized matrix was examined as well as along with samples with 5%, 7%, 10%, 12% and 15% of AC and the spectra are almost similar. A representative spectra of hydrogel composite with 10% AC is shown in Fig. 6.
Fig-5) FTIR spectra of Hydrogel sample with no activated carbon.
Fig-6) FTIR spectra of Hydrogel sample with 10% activated carbon.
Methylene blue absorption test
This test method involves measurement of adsorption of dye by calculating reduction of concentration of dye solution [9]. 1.0g of methylene blue was dissolved in 1L of deionized water to make a stock solution and subsequent concentrations of methylene blue (50–500 mg/L) were obtained by the dilution of the stock solution. In order to measure the decrease in concentration, hydrogel composites with varying percentage of activated carbon were put in 50 ml of diluted methylene blue solution for 24hrs at room temperature. The concentration of the methylene blue at the beginning and after 24 hours was calculated using spectrometry at the wavelength of 664 nm using a single beam spectrometer (Milton Roy Spectronic 20D Spectrophotometer).
Methylene blue (MB) was used as a dye. The initial concentration reading of the diluted MB measured with spectrophotometer was 3.70. All samples of PAM/AC composite hydrogels with different weight percent of AC were put in MB solution and the concentration variation was analyzed and evaluated. The results are presented in Table 3 and Fig. 5.
Table 3
Methylene blue adsorption of with 1.0M batch samples.
Batch
|
5%
|
7%
|
10%
|
12%
|
15%
|
concentration
|
2.75
|
2.55
|
1.85
|
2.13
|
2.65
|
Fig-7) Graph of Methylene blue absorption for 1.0M batch samples.
The sample with 10% AC treated with 1.0M of sulfuric acid shows the best dye absorption characteristics. It indicates that at 10 weight percent enough activated carbon is exposed to absorb dye while as beyond this concentration the AC active sites are reduced causing decline in the activity of AC.