3.1 FTIR spectroscopic analysis of tanning agent
The tanning agent used is characterized by FTIR spectroscopic analysis. FTIR spectra indicate specific functional groups present in the chrome tanning agent. Hence, it is possible to determine the chemical structure of functional groups present in the chrome tanning agent. In this study, basic chromium sulfate was used as a tanning agent. FTIR spectrophotometer (IR Prestige-21, SHIMADZU) was used to study the functional groups present in the basic chromium sulfate. Fig. 1 shows the FTIR spectrum of pure basic chromium sulfate (CrOHSO4).
FTIR spectra reveal functional groups present in a compound. Figure 1 shows the FTIR spectra of basic chromium sulfate, Cr(OH)SO4. In the IR spectrum of basic chromium sulfate, the absorption peaks at 609 cm− 1 and 480 cm− 1 are attributed to the absorption due to vibration between Cr-O and SO42− [23]. The peak at 1132 cm− 1 are assigned for α-Cr2O3 [24]. The absorption peak at 609 cm− 1 also reveals the vibration of Cr-O. A broad peak centered at 3456 cm− 1 was observed due to the presence of -OH group [25].
3.2 Effect of ultrasound on % chromium uptake of leather
Chromium uptake was studied during tanning process for different tanning conditions. In this study tanning was carried out using 6%, 8%, 10% and 12% basic chromium sulfate for 120 minutes. Cr uptake of leather tanned with ultrasound and conventional methods are shown in Fig. 2. From the Fig. 2, it is evident that chromium uptake in case of tanning with ultrasound was more than those tanned without ultrasound. Ultrasound improves the penetration of chromium through the pores of collagen fibres and hence, enhances the percentage of chromium uptake by the leather sample. Thus 87%, 98%, 91% and 90% chromium uptakes were obtained in case of tanning with ultrasound for 120 minutes using 6%, 8%, 10% and 12% basic chromium sulfate, respectively while only 17%, 29%, 33% and 39% chromium uptakes were observed in case of conventional method under same tanning conditions.
The results showed that chromium uptakes of the leather samples tanned with ultrasound increased up to the concentration of basic chromium sulfate at 8% and then decreased. Because pickled pelts can’t absorb all the chromium present in tanning liquor if excessive amount of tanning agent is used [26]. Thus, the use of 8% basic chromium sulfate for 2 hours is the best condition for ultrasound assisted chrome tanning to get maximum percentage of chromium uptake.
3.3 Effect of time on chromium uptake
Cr uptake reached 98% when tanning was carried with ultrasound for 180 minutes while only 33% Cr uptake was observed in case of conventional method under same conditions (Fig. 3a). Further increase of tanning time proved to be detrimental for ultrasound process as was evidenced that 79% chromium uptake was observed with ultrasound for 360 minutes. However, it was increased to 63% with conventional process (Fig. 3b). This reduction of Cr uptake might be desorption of some non-chemically bounded chromium ions due to effect of excessive sound wave which generated in sonicator.
3.4 Determination of chromium content in the tanned leather
The chromium content in the tanned leather sample was determined by energy dispersive spectrophotometric (EDS) method. From the EDS spectrum of tanned leather sample, the elemental composition of the leather sample was derived. The tanned leather sample contained several elements e.g., C, H, N, O, Cr, S, P, Na, Ca, Cl etc but amount of H, Na, Ca, P etc. were ignored in EDS analysis. The EDS spectra of samples tanned in presence of ultrasound (a) and in absence of ultrasound (b) are shown in Fig. 4. The spectrum of other samples also showed similar trends.
Elemental analysis of chrome tanned leather with ultrasound and conventional method have been done and the data are tabulated in Table 2. Chromium uptake of ultrasound assisted tanned leather was higher than that of conventionally tanned leather. Uptake of other elements also showed similar trends.
Table 2
Elemental analysis of tanned leather
Sl. No | Tanning Conditions | Ultrasound | Conventional |
Element | Mass (%) | Element | Mass (%) |
1 | 8% BCS, 2h | Cr | 6.16 | Cr | 0.82 |
S | 1.47 | S | 1.43 |
Cl | 8.67 | Cl | 8.2 |
O | 16.49 | O | 21.18 |
N | 20.20 | N | 19.51 |
C | 47.01 | C | 48.86 |
2 | 8% BCS, 3h | Cr | 5.26 | Cr | 0.89 |
S | 1.52 | S | 0.85 |
Cl | 8.66 | Cl | 4.46 |
O | 21.33 | O | 25.03 |
N | 17.08 | N | 21.39 |
C | 46.14 | C | 47.38 |
3 | 8% BCS, 6h | Cr | 4.7 | Cr | 3.66 |
S | 1.82 | S | 2.29 |
Cl | 7.55 | Cl | 15.55 |
O | 21.41 | O | 14.85 |
N | 19.65 | N | 13.14 |
C | 44.86 | C | 49.43 |
3.5 Thermogravimetric analysis of the tanned leather
Thermal stability of leather tanned with and without ultrasound were compared to investigate the effect of ultrasound on leather tanning. Thermal degradation studies of the tanned leather samples were performed using thermogravimetric analyzer under N2 atmosphere.
The temperature needed for a fixed percentage of weight loss was increased for leathers tanned using ultrasound technique. The improvement of the thermal stability of ultrasound assisted tanned leather samples were due to higher penetration of basic chrome sulfate through the pores of leather leading to the formation of chromium-collagen complexes which acted as a barrier to the decomposition of leather sample [27].
3.6 Effect of ultrasound on hydrothermal stability of tanned leather
There are several thermal analysis methods but differential scanning calorimetry (DSC) comes into prominence in recent years for evaluation of thermal behaviors (e.g., denaturation and shrinkage temperatures) of the raw skin, pickled, tanned, and finished leathers.
Several important temperatures such as peak temperature, onset temperature, extrapolated onset temperature, and end set temperature can be determined from a typical DSC thermogram. In this study, the onset temperature was considered as the shrinkage temperature. The onset temperature is the temperature at which the slope of the thermogram first deviates from the baseline.
The DSC thermograms of the leather samples tanned with ultrasound and conventional method using 8% basic chromium sulfate for 2 hr is shown in Fig. 6 (a, b). The thermograms of the leather sample showed that the shrinkage temperature of sample tanned in presence of ultrasound was 111°C while that of conventionally tanned leather was 82°C. The shrinkage temperature of sample treated under ultrasound was 29°C higher than that of conventionally tanned leather. The main reason for the improvement of shrinkage temperature is that a very small amount of chromium fixation was happened within two hours in the conventional tanning method whereas most of the chromium present in tanning liquor was fixed with collagen fibre in ultrasound-assisted tanning process. Therefore, the fixation of chromium increased with ultrasound.
3.7 Boiling test of tanned leather for evaluating thermal stability
The influence of ultrasound on the hydrothermal stability of the tanned leather was also measured by evaluating the results of the boiling tests. In the present study, the percentage of area loss or area shrinkage was determined by the boiling test.
Table 3
Percentage of area loss of chrome tanned leather by boiling test
Sample No. | Experimental Technique | Initial area (cm2) | Final area (cm2) | Area loss (cm2) | Area loss or shrinkage (%) |
1 | With Ultrasound | 4 | 2.90 | 1.10 | 27.50 |
Conventional | 4 | 2.64 | 1.36 | 34.00 |
2 | With Ultrasound | 4 | 2.75 | 1.25 | 31.25 |
Conventional | 4 | 2.20 | 1.80 | 45.00 |
3 | With Ultrasound | 4 | 2.54 | 1.46 | 36.50 |
Conventional | 4 | 2.15 | 1.85 | 46.25 |
4 | With Ultrasound | 4 | 2.58 | 1.42 | 35.50 |
Conventional | 4 | 2.14 | 1.86 | 46.50 |
5 | With Ultrasound | 4 | 2.88 | 1.12 | 28.00 |
Conventional | 4 | 2.63 | 1.37 | 34.25 |
6 | With Ultrasound | 4 | 2.72 | 1.28 | 32.00 |
Conventional | 4 | 2.21 | 1.79 | 44.75 |
7 | With Ultrasound | 4 | 2.64 | 1.36 | 34.00 |
Conventional | 4 | 2.10 | 1.90 | 47.50 |
Table 3 shows that the reduction in the area of the samples tanned with ultrasound were less than that of the samples tanned by the conventional method. Shrinkage areas of leather samples tanned with ultrasound and conventional method are 31.25% and 45%, respectively. The conventionally tanned leather sample has a lower amount of fixed chromium and consequently, higher shrinkage occurred.
3.8 Field emission scanning electron microscopic (FESEM) analysis
The surface morphology of the wet-blue leathers produced by tanning under ultrasound and conventional techniques were observed by FESEM as shown in Fig. 7.
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The micrographs show the chromium penetration across the cross-sectional view of the wet-blue leathers tanned with 8% basic chromium sulfate for 2 hours using both ultrasound and conventional techniques. In the micrographs, the darker region represents the concentration of chromium. It was observed that ultrasound assisted tanned leather showed higher uniform distribution of chromium throughout the samples than that of conventionally tanned leather. Therefore, FESEM further confirmed that the tanning agent completely penetrates the full cross-section of leather in the process of ultrasound-assisted chrome tanning.
The fibril bundles of the tanned leather were also analyzed from FESEM micrographs which showed that the fibrils and fibril bundles of collagen in the flesh, corium, and grain layers of the chrome tanned leather and observed that the collagen fibrils are intact for the leather tanned with both ultrasound and conventional techniques. These results revealed that collagen fibre structures were not damaged because of the usage of ultrasound in the tanning for 2 hours. But it was observed that in the case of the tanning process for 6 hours, the distance between fibrils is more with ultrasound than that of the conventional process. It means exposure to ultrasound for longer period is harmful for the structure of leather. Thus, in ultrasound-assisted chrome tanning process, 2–3 hours tanning time is optimum to complete the tanning operation.
3.9 Diffusion of tanning agent into leather: Stereo microscopic observations
Ultrasound enhances the penetration of the tanning agent and the degree of penetration was examined with a stereomicroscope. The cross-sectional images of the tanned leather obtained from the stereo microscope are shown in Fig. 8. The depth of penetration of the tanning agent depends on the type of sample and the tanning agent used.
It was observed that the tanning agent distribution in the leather samples treated with ultrasound was about 30–70% higher than that of the conventional method. The principal reason behind this is the effect of acoustic cavitation which enhances the diffusion rate of tanning agents through the pores of collagen fibres [21]. Almost 98% penetration of tanning agent was observed for tanned leather after 2 hours of tanning with ultrasound whereas only 29% penetration was observed for that sample after 2 hours of tanning without ultrasound. Besides this, uniform distribution of tanning agents throughout the whole cross-section of the pelt, even the inner portion of the pelt is possible in the case of ultrasound-assisted chrome tanning, which is quite impossible in the conventional tanning process. Hence, ultrasound can improve the penetration or diffusion of the tanning agent and impart the better-quality leather.
3.10 Determination of leachable chromium content
All the absorbed chromium present in tanned leather is not chemically bonded to the collagen chains. The chemical bonding within the carboxyl groups of collagen fibres and chromium ion is supposed to happen. In this study, chromium physically attached to the fibres was extracted by the TCLP. It helps in determining the quantity of free chromium that can be leached out from the tanned leather to the environment when it is disposed off. The leached chromium present in TCLP liquor was measured by UV-Vis spectrophotometric analysis (Table 4).
Table 4
Leachable chromium of tanned leather
Sample No. | Sample specification | Amount of leachable chromium (mg/gm) | Improvement (%) |
With ultrasound | Without ultrasound |
1 | 8% BCS, 2h | 0.027 | 0.047 | 42.55 |
2 | 10% BCS, 2h | 0.128 | 0.165 | 22.42 |
3 | 8% BCS, 3h | 0.113 | 0.140 | 19.29 |
4 | 8% BCS, 6h | 0.241 | 0.247 | 2.43 |
These results indicated that the amounts of leachable chromium with ultrasound-assisted tanned leathers are lower than that of conventionally tanned leather. It is assumed that chemically bonded chromium in the tanned leather is not easily leached out [28]. Thus, higher hydrothermal stability for the tanned leather might be obtained for ultrasound assisted tanning.
3.11 Estimation of environmental benefits
Ultrasound may be applied to reduce the effluent load to the environment. Ultrasound can improve the diffusion, penetration and fixation of chromium even at a lower concentration of tanning chemicals. As a consequence, Cr content in the spent liquor is reduced (Table 5) resulting the lower pollution load to the environment.
Table 5
Amount of chromium present in the spent tanning liquor
Sample No. | Sample specification | Amount of chromium in the spent liquor (%) | Difference (%) |
Ultrasound | Conventional |
1 | 8% BCS, 2h | 2 | 71 | 69 |
2 | 8% BCS, 3h | 2 | 67 | 65 |
3 | 8% BCS, 6h | 21 | 37 | 16 |
There is a significant reduction in the amount of unused chromium in the spent tanning liquor due to the use of ultrasound was observed. The reductions were 69%, 65%, and 16% for samples 1, 2, and 3 that were tanned with 8% BCS for 120 minutes, 180 minutes, and 360 minutes, respectively.