Environ. Sci. Technol. 2008, 42, 1731–1739
Sodium Metasilicate Based Fiber Opening for Greener Leather Processing SUBRAMANI SARAVANABHAVAN,† P A L A N I S A M Y T H A N I K A I V E L A N , †,‡ J O N N A L A G A D D A R A G H A V A R A O , * ,† BALACHANDRAN UNNI NAIR,† AND THIRUMALACHARI RAMASAMI† Chemical Laboratory, Centre for Leather Apparel and Accessories Development, Central Leather Research Institute, Adyar, Chennai 600 020, India
Received July 1, 2007. Revised manuscript received November 30, 2007. Accepted December 4, 2007.
Growing environmental regulations propound the need for a transformation in the current practice of leather making. The conventional dehairing and fiber opening process results in high negative impact on the environment because of its uncleanliness. This process accounts for most of the biochemical oxygen demand and chemical oxygen demand in tannery wastewater and generation of H2S gas. Hence, this study explores the use of a biological material and a nontoxic chemical for performing the above process more cleanly. In this study, the dehairing and fiber opening processes has been designed using enzyme and sodium metasilicate. The amount of sodium metasilicate required for fiber opening is standardized through the removal of proteoglycan, increase in weight, and bulk properties of leathers. It has been found that the extent of opening up of fiber bundles is comparable to that of conventionally processed leathers using a 2% sodium metasilicate solution. This has been substantiated through scanning electron microscopic analysis and softness measurements. The presence of silica in the crust leather enhances the bulk properties of the leather. This has been confirmed from the energy dispersive X-ray analysis. Performance of the leathers is shown to be on par with conventionally processed leathers through physical and hand evaluation. The process also exhibits significant reduction in chemical oxygen demand and total solid loads by 55 and 24%, respectively. Further, this newly developed process seems to be economically beneficial.
Introduction Green chemistry is a highly effective approach for pollution prevention. Principles of green chemistry are mainly applied to reduce or eliminate the use or generation of hazardous substance in the production process as well as product. Current practices of pretanning and tanning processes discharge enormous amount of pollutants, which accounts for nearly 90% of the total pollution from a tannery wastewater (1). This comprises biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), sulfides, sulfates, chlorides, and other minerals (2). Further, * Corresponding author phone: + 91 44 2441 1630; Fax: + 91 44 2491 1589. † Chemical Laboratory. ‡ Centre for Leather Apparel and Accessories Development. 10.1021/es071611v CCC: $40.75
Published on Web 01/23/2008
2008 American Chemical Society
toxic gases such as hydrogen sulfide, ammonia, and solid wastes such as lime sludge are generated during the pretanning process (3). Dehairing (Liming) and fiber opening (reliming) alone contribute to 60–70% of the total pollution load in leather processing (4). The main objectives of this process are the hair and flesh removal and splitting of fiber bundles by chemical and physical means (5). These processes employ lime and sodium sulfide, which generate severe pollution loads as well as unfavorable consequences to the environment (6, 7). Lime or sulfide free dehairing and fiber opening methods for hides and skins have been attempted in the past century (8–13). Presently, none of these methods has found commercial application in the global leather sector. Thanikaivelan et al. have developed a commercially viable lime free enzymatic dehairing process using low amounts of sodium sulfide (14). Enzyme only processes for dehairing the goatskins has been established (15). Recently, lime- and sulfide-free dehairing process for bovine hides using a commercial enzyme formulation along with sodium metasilicate by dip and pile method has been established (16). Fiber opening process removes all the interfibrous materials especially proteoglycans and produces a system of fibers and fibrils of collagen which are clean (17). This is achieved by the alkali action as well as osmotic pressure built up in the hide matrix. Attempts have also been made to replace the lime with other alkalis such as sodium hydroxide (18). Thanikaivelan et al. have successfully developed a lime free fiber opening process employing biocatalysts such as R–amylase (19). However, no successful attempt has been made to eliminate lime and sodium sulfide completely in leather processing. The application of soluble silicates in pretanning to eliminate the formation of chrome bearing leather waste has been reported (20). Saravanabhavan et al. have reported that the use of silicate in place of sodium sulfide in the enzymatic dehairing enhances the enzyme activity (16). A preliminary trial on the use of soluble silicate (water glass) along with sodium sulfide and sodium hydroxide for dehairing and fiber opening process has been reported by Munz et al. (21). Sodium metasilicate plays different role in different industries such as detergents, oil, textile, paper, and automotive. The pH of 1% aqueous solution of sodium metasilicate is about 13.0. The optimum pH required to produce the osmotic swelling on the hide matrix is 12.0 (5). Hence, sodium metasilicate can be used to create osmotic pressure on the hide matrix thereby opening the fiber bundles. It is also possible that silica present in the aqueous solution can interact with the hide matrix and provide mild tanning effects during the fiber opening process. This may improve the chemical uptake and physical characteristics of the final leather. In the present work, a lime- and sulfide-free beamhouse process has been developed for cowhides to achieve a greener leather process as well as better quality of leather production through a biochemical approach. Sodium metasilicate was chosen to open the fiber bundles of a pelt that was dehaired using enzyme-based dehairing enhanced by sodium metasilicate (16). The extent of opening up of fiber bundles has been assessed through proteoglycan’s removal, scanning electron microscopy (SEM), stratigraphic chromium distribution analysis, and softness measurements. Spent dehairing and fiber opening liquor from both the control and experimental processes have been analyzed for pollution parameters. Composite liquors from both control and experimental processes have been analyzed for COD and TS. Energy VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dispersive X-ray analysis has been carried out to map the presence of elements in the hide matrix after fiber opening, chrome tanning, and crust process. Strength characteristics and bulk properties of the processed leathers have also been evaluated.
Experimental Methods Materials. Wet salted cowhides were chosen as raw material for this study. Compact cowhides in the thickness range of 4.5–5.0 mm were chosen since this work involves study of the extent of opening up of fiber bundles and the extent of diffusion and distribution of tanning agent. All the chemicals used for leather processing were of commercial grade. The enzyme used for dehairing, Biodart (protease derived from bacterial origin, active at pH 7.5–11.0 and temperature 25–40 °C), was purchased from Southern Petrochemical Industries Corporation (SPIC) Ltd., India. Commercial sodium metasilicate, 9-hydrate (Na2SiO3. 9H2O) was used in this study, and the SiO2 content was analyzed and found to be 16.5%. The chemicals used for the analysis of spent liquors were of analytical grade. Preliminary Trials for Standardization of Amount of Sodium Metasilicate for Fiber Opening. Twelve wet salted cow sides were soaked conventionally. Soaked sides were dehaired using the optimized dehairing process (16). The dehairing mixture, constituting 10% water, 1% dehairing enzyme (Biodart), and 1% sodium metasilicate (percentages based on soaked weight), was prepared and mixed well with the cow sides (16). Then the sides were piled overnight. Next day, the sides were manually dehaired. The dehaired weight of each side was noted. Two dehaired cow sides were used for each of the following trials. Concentration of sodium metasilicate was varied as 1, 2, 3, 4, and 5% with 200% water (percentages based on weight/ dehaired weight). The duration of the treatment was one day with 5 min running per hour for 6 h and left overnight in the bath. Next day, the pelts were fleshed. Control leather was made using a conventional liming and reliming process. Conventional liming process was performed on two soaked cow sides using 3% sodium sulfide, 4% lime, and 200% water (percentages based on weight/soaked weight) adopting pit method of liming. The sides were soaked in a pit containing the above chemicals for two days with handling twice a day, after which they were manually dehaired. Subsequently, the dehaired sides from control process were relimed for two days using 4% lime and 200% water in a pit (percentages based on soaked weight). Then, the relimed sides were fleshed and scudded. The prefleshed and fleshed weight of the sides from preliminary experimental trials as well as control were noted. The percentage increase in weight after swelling was calculated from the difference in weights between the soaked weight and prefleshed weight. The remaining unit processes from deliming were similar to the manufacture of upper leathers. The crust leathers from control and preliminary experimental trials were assessed for sensory properties by four experienced tanners. Standardized Experimental and Conventional Process Schemes. Ten wet salted cow sides were taken and soaked conventionally. The wet weight after soaking was noted for each side and termed as soaked weight. Five left sides were taken for control (C) and five right sides for experiment (E) for matched pair comparison. The control process was carried out as described above. The experimental process was also carried out as described above with optimized amount of sodium metasilicate (2%) for fiber opening. The pelts from control and experimental process were separately delimed, bated, and pickled conventionally. Chrome tanning was done using a conventional tanning procedure with 8% basic chromium sulfate salt. The leathers after chrome tanning were piled for 24 h. Then the leathers 1732
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were sammed, split, and shaved to uniform thickness (1.1–1.2 mm). Rechroming was not done. A conventional post–tanning process was adopted to convert the tanned leathers into crust upper leathers. Analysis of Proteoglycans in Spent Liquors from Fiber Opening Processes. A known amount of sample from spent fiber opening process liquors from both preliminary experimental trials and control process was taken. The samples were neutralized to a pH 7.0 using 10% dilute sulfuric acid and made into a known volume. A known amount of neutralized sample was taken and analyzed for proteoglycan following standard procedure (22). Finally, the amount of proteoglycans removed from the cowsides was calculated by multiplying concentration (mg/L) by volume of spent liquor (L) collected from the fiber opening process and expressed as g/kg of raw cowhides. InputandOutputAnalysis.Acomprehensiveinput-output analysis for the raw materials, water, and chemicals was carried out for the conventional and experimental dehairing and fiber opening processes. The spent liquors from control dehairing and fiber opening processes were analyzed for S2and Ca2+ following standard procedures (23). Enzyme and sodium metasilicate were not estimated in the experimental dehairing process since they were present in the sludge either as such or in complexed form. Spent liquors from experimental fiber opening process were analyzed for Si4+ following standard procedure (23). The amount of dry sludge was estimated following the procedure described earlier (24). A known amount of sludge from experimental dehairing process was washed with water to remove the adhered materials with the hair. The weight of the washed hair was noted. The moisture content of the recovered hair was analyzed following standard procedure (23) and the amount of dry hair was calculated. This was not carried out for dry sludge from control dehairing process because the hair was in solublized form. The values were computed for processing of 1 t of raw materials. Analysis of Spent Dehairing and Fiber Opening Process Liquors. Spent liquors from dehairing and fiber opening processes were collected from control and experimental processes. The spent liquors were analyzed for S2-, Ca2+, Si4+, COD and TS following standard procedures (23). Effluent loads were calculated by multiplying concentration (mg/L) by volume of effluent (L) from the dehairing and fiber opening processes per metric ton of raw hides processed. Analysis of Spent Chrome Liquor. Spent chrome liquor was collected from both control and experimental processes. Liquors were analyzed for chromium following standard procedure (25). The percentage of exhaustion of chromium was calculated from the amount of spent liquor collected. Analysis of Composite Waste Liquor. Composite liquors from control and experimental leather processing were collected from all unit operations up to post-tanning except soaking and analyzed for COD and TS (dried at 103–105 °C for 1 h) following standard procedures (23). A direct correlation of the observed COD/TS values with the environment may not give proper consequences. Hence, emission loads were calculated by multiplying concentration (mg/L) with volume of effluent (L) per metric ton of raw hides processed. Stratigraphic Chrome Distribution Analysis. Samples from the official sampling position of control and experimental wet blue sides were split into three uniform layers using Camoga splitting machine and analyzed for layer wise chromium content (26). A known weight (∼1g) of the samples was taken and the amount of chromium was estimated following standard procedure (27). Samples were initially analyzed for moisture content (28) and chrome content was expressed on dry weight basis of leather. Physical Testing and Hand Evaluation of Leathers. Samples for various physical tests from experimental and
TABLE 1. Extent of Fiber Opening and Removal of Proteoglycan for Conventional Lime Based System and Sodium Metasilicate (SMS)a pelts treated with
% increase in weightb
proteoglycan removal (g/kg of soaked cow hides)
lime 4% SMS 1% SMS 2% SMS 3% SMS 4% SMS 5%
24.6 18.2 25.4 28.4 30.2 30.8
3.27 ( 0.02 2.40 ( 0.02 3.35 ( 0.02 3.42 ( 0.04 3.38 ( 0.02 3.44 ( 0.04
a ( Denotes standard error (n ) 3). b % Increase in weight ) prefleshed weight – soaked weight × 100 soaked weight.
control crust leathers were obtained following IUP method (26). Specimens were conditioned at 27 ( 2 °C and 65 ( 2% R.H. over a period of 48 h. Physical properties such as tensile strength, % elongation at break, tear strength, and grain crack strength were examined following standard procedures (29–31). Experimental and control crust leathers were assessed for softness, fullness, grain smoothness, grain tightness (break) and general appearance by hand and visual examination. The leathers were rated on a scale of 0–10 points for each functional property by four experienced tanners, where higher points indicate better property. Scanning Electron Microscopic Analysis. Samples from the control (C) and experimental (E) sides after fiber opening, chrome tanning and post tanning were cut from the official sampling position (26). The samples were dehydrated using acetone and methanol following standard procedure except crust leather samples (32). The crust leather samples from both experimental and control process with uniform thickness were directly taken for analysis without any pretreatment. Quanta 200 series scanning electron microscope, FEI Company was used for the analysis. The micrographs for the grain surface and cross section were obtained by operating the SEM at low vacuum and an accelerating voltage of 12 KV with different magnification levels. Energy Dispersive X-ray Analysis (EDAX). The samples prepared for the scanning electron microscopic analysis were subjected to EDAX analysis to find out the presence of various elements on the surface as well as cross-section. The X-ray spectrum produced by materials in a scanning electron microscope can be used to determine the composition of the materials. The high-energy electrons excite the electrons in the electron shells around the atoms in the material, causing them to jump to higher energy shells. When the electrons fall back to the lower energy shells, they emit electromagnetic radiation in the form of X-rays. The wavelengths, and hence energies, of the X-rays are characteristic of the electron shell energies, and the spectrum of X-rays can be used to identify different elements. INCA suite program, Oxford Instruments was used for the data analysis.
Results and Discussion Optimization of Sodium Metasilicate for Fiber Opening. Preliminary trials were carried out to select the amount of sodium metasilicate required for optimum opening up of fiber bundles and matching the functional properties of conventional leathers made using lime. Fiber opening trials were carried out using various concentration of sodium metasilicate namely, 1, 2, 3, 4, and 5%. Conventional fiber opening treatment was carried out using 4% lime for comparison. The percentage increase in weight after swelling for each trial is given in Table 1. It is evident that 1% sodium metasilicate dosage does not produce swelling comparable
to that of control. Higher dosages of sodium metasilicate (3–5%) produce higher swelling than the control. However, 2% sodium metasilicate dosage produces comparable swelling to that of the control. Fiber opening process removes the cementing substance, especially proteoglycans, and makes the fibers and fibrils separate and clean. The extent of fiber opening primarily depends on the removal of proteoglycans (33). Hence, the elimination of proteoglycan from hides/ skins can be used as a potential marker for quantifying the extent of fiber opening (33, 34). The proteoglycan removal after fiber opening for each trial is given in Table 1. A dose of 2% sodium metasilicate results in comparable removal of proteoglycan with that of the control (4% lime). No significant differences in the removal of proteoglycan were found among higher dosages of sodium metasilicate (3, 4, and 5%) compared to 2% sodium metasilicate dose. This may be due to the near-complete removal of proteoglycan at 2% sodium metasilicate dose. Softness is one of the bulk properties of leather, which is primarily related with the opening up of fiber bundles. Hence, the leathers were assessed for softness and also for other bulk properties such as fullness, grain smoothness, grain tightness, and general appearance by four experienced tanners. It was observed (data not given) that the leathers made using 1% sodium metasilicate offer resulted in insufficient softness and firm in nature indicating inadequate opening up of fiber bundles. It is also evident from Table 1 that the extent of increase in weight and proteoglycan removal is also low for 1% of sodium metasilicate treatment. However, the leathers made using 2, 3, 4, and 5% sodium metasilicate treatment provided comparable or even better softness with that of leathers processed conventionally with lime. Other bulk properties of leathers made using lime were also comparable with leathers made with a higher amount of sodium metasilicate (2, 3, 4, and 5%). Hence, it is evident from these results that an offer of 2% sodium metasilicate with 200% water (based on dehaired weight) is sufficient for opening up of fiber bundles to produce leathers with comparable softness and other bulk properties with that of conventional leathers processed by lime treatment. The optimized amount was used for further detailed experiments in comparison with the control. Lime Versus Silicate Based Fiber Opening: An Evaluation. SEM and EDAX Analysis. “Leather is made in the beamhouse” has been the conventional wisdom (35). Lime serves to open up the fiber bundles to the desired degree and also helps to remove the hair completely. In the present study, lime has been replaced by sodium metasilicate, which is also a mild alkali. Hence, it is necessary to analyze the opening up of fiber bundles and the effect of silicate on the grain quality. The scanning electron micrographs of control and experimental samples after fiber opening showing the grain surfaces at a magnification of ×500 are shown in Figure 1a and b. It is seen that hair pores of both the samples are visible and clear. A few undissolved lime particles are present in the control sample (Figure 1a), which will be removed during the deliming operation. It is known that the use of strong alkali like sodium hydroxide for fiber opening results in grain crackiness and pore damage (35). The scanning electron micrographs showing the grain surfaces of crust leathers at magnification (×50) are shown in Figure 1c and d. In general, the surface of both crust leather samples seems to be clean without any damage on the grain surface. Hence, it is evident that sodium metasilicate does not damage the grain surface unlike sodium hydroxide. This is primarily due to the mellowing nature of the sodium metasilicate like lime. Scanning electron micrographs of samples from lime and sodium metasilicate based fiber opening processes showing the cross section at a magnification (×50) are given in Figure 2a and b. Experimental sample shows separation of fiber bundles comparable to that of control sample. Scanning VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scanning electron micrographs of control and experimental samples after fiber opening treatment showing grain surface at a magnification of ×500: (a) lime based fiber opening, (b) silicate based fiber opening and scanning electron micrographs of crust leather samples showing grain surface at a magnification of ×50, (c) control leather, and d) experimental leather. electron micrographs of the same samples at relatively higher magnification ×250 are presented in Figure 2c and d. Both control and experimental samples clearly show void space between the fiber bundles. The void space is formed during the fiber opening operation due to the removal of interfibrillary materials induced by osmotic pressure. It is clearly seen that the void spaces for both samples seem to be equal, indicating the osmotic pressure developed on both the matrix is also similar. Scanning electron micrographs of control and experimental crust leather samples at a magnification ×200 is presented in Figure 2e and f. The fiber structure of the control crust leather sample seems to be compact compared to the experimental sample. Although separation between fiber bundles is comparable, separation within the fiber bundles seems to be poor for the control sample compared to the experimental sample. This may be because of the enhanced uptake of chemicals such as chromium, fatliquor, and syntan due to the presence of silicates in the matrix (21). Silica is known for the adsorption of metal ions and organic materials (36–39). This can be analyzed through EDAX analysis. The experimental and control samples were subjected to EDAX at various stages of leather processing to map the presences of elements. EDAX of cross section of control and experimental samples after fiber opening is shown in Figure 3a and b. The presence of calcium and silica are seen in the control and experimental samples, respectively. The experimental sample also shows the presence of iron, magnesium, aluminum, and potassium, which may be due to the impurities present in the commercial sodium metasilicate. Figure 3c and 3d show the EDAX of cross section of control and experimental samples after chrome tanning. Only chromium is seen in the chrome tanned control sample, 1734
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because calcium was removed during the deliming and pickling processes. However, the chrome tanned experimental sample demonstrates the presence of silica along with chromium. Further, EDAX of experimental crust leather sample at grain surface and cross section is shown in Figure 3e and f. Silica is present on the grain surface as well as cross section of the experimental crust leather. This shows that the presence of silica can enhance the uptake of tanning and post tanning chemicals, thereby improving the physical characteristics of the leather. These results are in agreement with the scanning electron microscopic analysis. Input and Output Analysis. The input-output audit in leather manufacture is to evaluate the impact of the developed process on environment as against the existing process. The input and output of raw materials, chemicals, and water were analyzed for the control and experimental dehairing and fiber opening processes. The observed values have been calculated for processing 1 t of raw cowhides and are given in Table 2. The reduction in the weight of control and experimental soaked hides and the amount of lime and sodium sulfide present in the spent liquor after control and experimental dehairing is comparable to that of an earlier study and hence related discussion can be found elsewhere (16). Dry sludge produced from control dehairing process is about 29 kg, which is essentially a mixture of lime, sulfide, some meager quantities of unstructured hair, and noncollagenous proteins. The experimental dehairing process does not yield any spent liquor due to the low use of water. It generates dry sludge containing the enzyme, silicate, some meager amount of noncollagenous proteins, and hair. The hair is in undamaged form, which is about 25 kg. This can be further used for several applications such as manufacture of hair brushes,
FIGURE 2. Scanning electron micrographs of control and experimental samples after fiber opening treatment showing cross section at lower and higher magnifications: (a) lime based fiber opening (×50), (b) silicate based fiber opening (×50), (c) lime based fiber opening (×250), (d) silicate based fiber opening (×250); scanning electron micrographs of crust leather samples showing cross section at a magnification of ×200, (e) control leather, and (f) experimental leather. carpet and keratin based syntan (40, 41). The amount of dry sludge (excluding dry hair) produced from experimental process is about 21 kg. After fiber opening, the weight of the dehaired pelts from both control and experimental processes is increased and is almost similar. The percentage weight gain of control and experimental pelts are about 23 and 24%, respectively, and this is due to the osmotic swelling associated with lime and sodium metasilicate treatments, respectively. The spent liquor from control fiber opening process contains about 4.2 kg lime, which is about 10% of the input. The experimental spent fiber opening liquor contains about 5 kg of sodium metasilicate, which is in dissolved form. The total amount
of dry sludge formed from the control and experimental processes is about 57 and 21 kg, respectively. Higher amount of dry sludge is formed in the case of control process. This is mainly due to the poor solubility of lime. The remaining 10–15% of the lime input is present in the spent liquor as suspension. This is in accordance with the earlier observation (24). There is no dry sludge formed in the experimental fiber opening process due to the higher solubility of sodium metasilicate. Pollution Load from Dehairing and Fiber Opening Processes. The sectional liquid stream pollutants concentration and their load on the environment, if discharged directly into the water bodies, are given in Table 3. The amount of VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Energy dispersive X-ray analysis of hide samples after fiber opening treatment at cross section: (a) control and (b) experimental; chrome tanned leather samples at cross section: (c) control and (d) experimental; experimental crust leather samples (e) grain surface and (f) cross section. 1736
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TABLE 2. Input and Output Audit of Control and Experimental Dehairing and Fiber Opening Processes for Processing One Metric Ton of Raw Cowhidesa control process dehairing
fiber opening
a
experimental
chemicals/raw material
input (kg)
output (kg)
soaked sides water lime sodium sulfide biodart (SPIC) sodium metasilicate dry hair dry sludge dehaired pelts water lime sodium metasilicate dry sludge
1000 2000 40 30
920 1680 5.27 15.9
input (kg)
output (kg)
1000 100
840 0b
10 10 29.4 1230 1780 4.2
920 2000 40
NE NE 25.4 20.6 1240 1340
840 1680 16.8
5.1
27.2
Weight of hides before soaking. NE, not estimated.
b
Spent liquor was not yielded.
TABLE 3. Analysis of Spent Dehairing and Fiber Opening Liquors from Control (C) and Experimental (E) Processesa spent dehairing liquor parameters S2- (ppm) Ca2+ (ppm) Si4+ (ppm) COD (ppm) TS (ppm) volume of effluent (L/t of raw hidesb) emission load (kg/t of raw hidesb) S2Ca2+ Si4+ COD TS
C
spent fiber opening liquor E
C
1268 ( 12 1870 ( 18
Nd 1420 ( 14
26432 ( 16 40128 ( 32 1680
11258 ( 24 16752 ( 48 1780
0c
2.13 3.14
2.52
44.4 67.4
20.0 29.8
a Nd, not detectable; C, control; E, experimental. ( denotes standard error (n ) 3).
b
Weight of hides before soaking.
various pollutants from conventional and experimental dehairing processes is comparable to that of earlier study and hence related discussion can be found elsewhere (16). Sulfide concentration in the spent fiber opening liquors could not be estimated under the present analytical conditions due to its low concentration. The total calcium present in the liquid waste discharged from control process is about 5.7 kg. The silica present in the spent liquor from experimental fiber opening process is about 0.51 kg, which is a negligible amount. It is seen that the COD and TS values for the experimental process are slightly higher than the conventional process. This is mainly due to the presence of noncollagenous matter in small volume of spent liquor from experimental fiber opening process. However, it is seen that the COD and TS loads for the experimental fiber opening process are reduced by 13 and 17% respectively, compared to the conventional fiber opening using lime. When the emission loads from dehairing and fiber opening processes are combined, the reduction in the COD and TS loads for the experimental beamhouse process is about 73 and 75%, respectively, compared to the control process. It is observed that the reduction in TS load is higher than that of earlier study (16), since lime is totally eliminated in the developed experimental process. It should be noted that these reductions are without including other streams such as from soaking, deliming, pickling, etc. Stratigraphic Chrome Distribution Analysis. Fiber opening is known to remove noncollagenous proteins and loosen the hide matrix. Loosening makes easier for the
c
E
380 ( 12 12860 ( 16 18456 ( 24 1340
0.51 17.32 24.73 Spent liquor was not yielded;
tanning agent, dyestuff, fatliquors, and other substance to diffuse into the hide. Hence, while examining the extent of opening up of fiber bundles it is imperative to look at the chromium distribution layer-wise. The average values of layer-wise chromium distribution are presented in the Table 4 along with the standard error. It is seen that the control and experimental leathers exhibit uniform chromium distribution along the entire cross section. This implies that the extent of opening up of fiber bundles using lime or sodium metasilicate is similar. The quantity of chromium present in all the layers of the experimental leather is slightly higher as compared to the control. In addition, the uptake of chromium is also higher for the experimental process. This could be due to the slight improvement in the degree of fiber opening by sodium metasilicate and the presence of silicate throughout the cross section of the hide matrix after fiber opening, which enhances the uptake of chromium (21). The presence of silica is evidenced from EDAX analysis (Figure 3b). Strength and Bulk Properties. Control and experimental crust leathers exhibit comparable strength and bulk properties (for detailed information, see the Supporting Information). Environmental Impact. The COD and TS values and the calculated emission loads are given in Table 5. The COD and TS values for experimental leather processing are lower than the control leather processing. The reduction in COD and TS VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Comparison of Uptake in Spent Chrome Liquors and Layer Wise Distribution of Chromium in Wet-blue Leathers from Control (C) and Experimental (E) Processesa % Cr2O3 (dry weight basisb)
a
sample
grain
middle
flesh
% uptake of chromium
C E
3.92 ( 0.04 4.14 ( 0.02
3.82 ( 0.02 3.96 ( 0.06
3.88 ( 0.02 4.06 ( 0.04
69 ( 2 82 ( 2
( Denotes standard error (n ) 3).
b
Moisture free chrome tanned leather weight.
TABLE 5. Composite Liquor Analysis for Control (C) and Experimental (E) Processesa emission load (kg/t of raw hidesb processed) b
process
COD (ppm)
TS (ppm)
volume of effluent (L/t of raw hides )
COD
TS
C E
6018 ( 32 3396 ( 24
18836 ( 48 17718 ( 52
10300 8240
62 28
194 146
a Composite liquors were collected excluding soaking. ) 3).
b
TABLE 6. Total Cost Consumption for Control (C) and Experimental (E) Dehairing and Fiber Opening Processes for Processing One Metric Ton of Raw Cowhidesa (U.S. $/t of raw cowhides) cost chemicals lime sodium sulfide biodart (SPIC) sodium metasilicate labor energy total cost of chemicals and labor
control
experimental
10.7 17.3 2.2 30.2
Acknowledgments S.S. thanks the CSIR, New Delhi for providing a senior research fellowship. We thank Dr. R. Rajaram for physical testing measurements.
Details of physical and bulk properties of control and experimental leathers as well as glossary of leather terms. This information is available free of charge via the Internet at http://pubs.acs.org.
30.7
Note: all the costs are based on commercial value in India at the time of publication. The Indian rupee value was converted into U.S.$ for the convenience of global readers (1 U.S. $ ) Rs. 45.00).
loads in experimental composite liquors are 55 and 25%, respectively, compared to the control process. Control dehairing and fiber opening processes are the major contributors for the COD in the composite liquor. This has been modified using enzyme and sodium metasilicate, which results in significant reduction in COD. The reduction in TS is mainly due to the avoidance of lime in leather processing. Hence, the developed process demonstrates significant reductions in COD and TS loads. Economic Perspectives. The total cost consumption for processing one metric ton of cowhides through conventional and experimental dehairing and fiber opening process is given in Table 6. Total cost of chemicals, labor, and energy for experimental process is comparable to control process. However, it is reported that the enzymatic dehairing would increase the area of the final leather by about 8% (16). This would lead to a saving of about U.S. $270 per metric ton of cowhides processed. Hence, experimental process provides additional revenue of about U.S. $266 for processing one metric ton of raw hides. Further, the experimental process exhibits significant reduction in COD and TS loads in the final composite effluent (excluding soaking), which would lead to savings in the effluent treatment costs. Moreover, this process 9
completely eliminates the lime bearing sludge, which is one of the major concerns for environment.
Supporting Information Available 24.0 4.1 1.2 1.4
a
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Weight of hides before soaking; ( denotes standard error (n
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