Environ. Sci. Technol. 2002, 36, 4187-4194
Zero Discharge Tanning: A Shift from Chemical to Biocatalytic Leather Processing PALANISAMY THANIKAIVELAN, JONNALAGADDA RAGHAVA RAO,* BALACHANDRAN UNNI NAIR, AND THIRUMALACHARI RAMASAMI Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, India
Beam house processes (Beam house processes generally mean liming-reliming processes, which employ beam.) contribute more than 60% of the total pollution from leather processing. The use of lime and sodium sulfide is of environmental concern (1, 2). Recently, the authors have developed an enzyme-based dehairing assisted with a very low amount of sodium sulfide, which completely avoids the use of lime. However, the dehaired pelt requires opening up of fiber bundles for further processing, where lime is employed to achieve this through osmotic swelling. Huge amounts of lime sludge and total solids are the main drawbacks of lime. An alternative bioprocess, based on R-amylase for fiber opening, has been attempted after enzymatic unhairing. This totally eliminates the use of lime in leather processing. This method enables subsequent processes and operations in leather making feasible without a deliming process. A control experiment was run in parallel using conventional liming-reliming processes. It has been found that the extent of opening up of fiber bundles using R-amylase is comparable to that of the control. This has been substantiated through scanning electron microscopic, stratigraphic chrome distribution analysis, and softness measurements. Performance of the leathers is shown to be on a par with leathers produced by the conventional process through physical and hand evaluation. Importantly, softness of the leathers is numerically proven to be comparable with that of control. The process also demonstrates reduction in chemical oxygen demand load by 45% and total solids load by 20% compared to the conventional process. The total dry sludge from the beam house processes is brought down from 152 to 8 kg for processing 1 ton of raw hides.
Introduction A zero waste or discharge concept, in principle, should reduce the pollution completely. Approaching the discharge toward zero value is an intellectual as well as a global challenge. The various possible approaches are (a) the use of chemicals having low toxicity or less environmental impact, (b) recover water and valuable chemical inputs from each processspecific stream of wastes and reuse to as high an extent as possible, (c) near 100% utilization of chemicals, (d) process innovation, (e) product innovation, and (e) integration of * Corresponding author phone + 91 44 443 0273, 491 1386 extn 112, fax + 91 44 491 1589, e-mail
[email protected]. 10.1021/es025618i CCC: $22.00 Published on Web 08/30/2002
2002 American Chemical Society
ecobenign processes. Leather processing generally involves a combination of single-step and multistep processes that employ as well as expel various biological, organic, and inorganic materials (3). Hence, a strategy that involves the combined advantages of all possible approaches would be ideal. This implies that the study of suitable zero discharge approaches for every single-unit process is important. Leather processing involves a number of unit processes and operations, which are well addressed (4). Limingreliming processes are the inevitable steps in leather processing. The main objectives of liming are the removal of hair and flesh and splitting of fiber bundles by chemical and physical means (5). To achieve these, lime and sodium sulfide are employed along with a substantial quantity of water. Conventional liming-reliming process liquors contribute 50-70% of the total biochemical oxygen demand (BOD) and chemical oxygen demand (COD) load from a tannery wastewater and 15-20% in the case of total solids (TS) load (6). Apart from this, a great deal of solid waste containing lime sludge, fleshings, and hair are generated (5). The extensive use of sulfide bears unfavorable consequences for the environment and the efficacy of effluent treatment plants (7). Several lime and sulfide-free liming methods have evolved during the past century. They include dehairing methods based on proteolytic enzymes (8, 9), chlorine dioxide (10), alkaline hydrogen peroxide (11), nickel carbonate (12), and lactobacillus-based (13) enzymic applications. Attempts have also been made to replace lime with other alkalis (14, 15). Currently, none of these methods has found commercial application in the global leather sector. Enzyme-assisted lime-sulfide dehairing is being followed in some parts of the world. However, only partial replacement of sulfide has been feasible in such kind of applications. The conventional liming method removes all the interfibrous materials, especially proteoglycans, and produces a system of fibers and fibrils of collagen that are clean (16). This is achieved by the action of alkali as well as osmotic pressure built up in the skin matrix. Hence, in principle, it should be possible to produce pelt by removing the proteincarbohydrate conjugates through the action of substratespecific enzymes. It has been shown that R-amylase has specific activity on carbohydrate-containing proteins such as proteoglycans (17, 18). In this work, a biocatalytic beam house process has been attempted. In this process, enzymebased fiber opening has been demonstrated on a pelt that was dehaired using enzyme-based dehairing assisted by sodium sulfide at pH 8.0 without using lime. The reductions in COD and TS loads have been taken as benchmarks to analyze the impact on the environment. The extent of opening up of fiber bundles has been assessed through scanning electron microscope (SEM), stratigraphic chromium distribution analysis, and softness measurements. The strength properties of leathers have been analyzed through physical testing measurements, while the bulk properties have been evaluated manually.
Experimental Methods Materials. Wet salted cowhides, sourced from Kerala, India, were chosen as raw material. Since this work involves study of the extent of opening up of fiber bundles and the extent of diffusion and distribution of tanning agent, compact cowhides in the thickness range of 4-5 mm were chosen. All chemicals used for leather processing were of commercial grade. Biodart (a dehairing enzyme based on alkaline bacterial protease) was sourced from Southern Petrochemical IndusVOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4187
tries Corp. (SPIC) Ltd. (Chennai, India). Laboratory grade R-amylase was used as supplied (from S. D. Fine-Chem. Ltd., Mumbai, India; activity 2000 units/g). The chemicals used for the analysis of leather and spent liquors were of analytical grade. Preliminary Trials for Optimization of the Amount of Enzyme. Five soaked cow sides (halves of hides) were dehaired using a dehairing recipe, based on enzyme (Biodart) and a small amount of sodium sulfide (19). The dehaired weight was recorded. One cow side was used for each trial. Four different percentages of R-amylase was used, viz., 0.5, 1, 1.5, and 2% with 100% water (percentages based on weight/ dehaired weight). To optimize the time required for complete fiber opening, the length of treatment was varied from 3 to 6 h in a drum. A control trial was carried out using 10% lime (w/w) with a float of 350% (w/w) water. The length of treatment was 1 day with 1 min running/h for 6 h in a drum and left overnight in the bath. The remaining unit processes were similar to the manufacture of upper leathers. The crust leathers were evaluated manually for softness and other essential properties. Conventional and Experimental Schemes: Lime versus Enzyme. Ten cowhides were cut into halves, numbered, and soaked conventionally. The wet weight after soaking was noted for each hide and termed the soaked weight. Five left and five right sides were used for control (C) while the other sides were used for experiment (E) for matched pair comparison. Control liming was performed by using sodium sulfide (3.5%), lime (10%), and water (400%) (percentages based on weight/soaked weight) in a drum. The sides were handled (in drum) for 5 min every hour for 6 h and left overnight in the bath. Next day, the sides were dehaired using the conventional beam and blunt knife technique. Subsequently the sides were relimed for 1 day using lime (10%) and water (400%) in a drum (percentages based on weight/ soaked weight). The sides were handled (in drum) for 5 min every hour for 6 h and left overnight in the bath. Then the pelts (side/hide without hair/flesh) were fleshed (removal of flesh) and scudded (removal of remnants of epithelial tissue, short hair, dirt, etc., left in the hair follicles after dehairing), and the fleshed weight was recorded. Experimental sides (E) were dehaired using enzyme (Biodart) and a small amount of sodium sulfide in place of a conventional liming process (19). The sides were treated with an optimized amount of R-amylase (1.5%) and water (100%) for 3 h in a drum (percentages based on weight/dehaired weight) as given earlier. Then the pelts were fleshed and scudded. A commercial postfiber opening process sequence was adopted in order to convert the fleshed pelts into upper leathers. The pelts were delimed, bated, and pickled conventionally. Chrome tanning was done using a conventional tanning procedure with 8% (w/w) basic chromium sulfate salt. The leathers after chrome tanning were piled for 24 h. Then the leathers were sammed (removal of free water by pressing the wet chrome tanned leather between two felt rollers) and shaved to uniform thickness (1.1-1.2 mm). Rechroming was not done. The wet blue sides were converted into crust upper leathers using conventional post-tanning and mechanical operations. Scanning Electron Microscopic Analysis. Samples from experimental and control sides were cut from the official sampling position (20) after the fiber opening treatment, chrome tanning process, and crusting operations. Samples from crust sides were directly cut into specimens with uniform thickness without any pretreatment. Samples from opening up treatment and chrome tanning were first washed in water. Subsequently the samples were fixed by soaking them with buffered formalin for 18 h. Samples were then dehydrated gradually using acetone and methanol as per standard procedure (21). Samples were then cut into 4188
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 19, 2002
specimens with uniform thickness. All specimens were then coated with gold using a JEOL JFC-1100E ion-sputtering device. A JEOL JSM-5300 scanning electron microscope was used for the analysis. The micrographs for the grain surface and cross section were obtained by operating the SEM at an accelerating voltage of 20 kV with different lower and higher magnification levels. Stratigraphic Chrome Distribution Analysis. Samples from the official butt portion (20) of experimental and control wet blue hides were split into three uniform layers using a Camoga splitting machine and analyzed for layerwise chrome content. A known weight (∼1 g) of the sample was taken, and the amount of chromium was estimated as per standard procedures (22). Samples were initially analyzed for moisture content (23), and chrome content was expressed on a dry weight basis of leather. Objective Assessment of Softness through Compressibility Measurements. Softness of leathers can be numerically measured based on their compressibility (24). Circular leather pieces (2-cm2 area) from experimental and control crust leathers were obtained as per the IULTCS method (20) and conditioned at 80 ( 4 °F and 65 ( 2% RH over a period of 48 h. The initial load acting on the grain surface was 50 g. The thickness at this load was measured 60 s after the load was applied. Subsequent loads were added, and the change in thickness was recorded 1 min after the addition of each load. The logarithm of leather thickness (Y axis) was plotted against the logarithm of load (X axis). Physical Testing and Hand Evaluation of Leathers. Samples for various physical tests from experimental and control crust leathers were obtained as per the IULTCS method (20). Specimens were conditioned at 80 ( 4 °F and 65 ( 2% RH over a period of 48 h. Physical properties such as tensile strength, percent elongation at break, tear strength, and grain crack strength were examined by the standard procedures (25). Experimental and control crust leathers were assessed for softness, fullness, grain flatness, 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 experienced tanners, where higher points indicate the better property. Analysis of Spent Liquors from Dehairing and Opening Up Treatment Processes. Spent liquors from dehairing and fiber opening treatment were collected from control and experimental processes. Liquors were analyzed for COD and TS (dried at 103-105 °C for 1 h) by the standard procedures (23). The sulfide content was determined by an iodometric titration procedure (23). The amount of calcium was estimated using an oxalate-permanganate titration method and expressed as Ca(OH)2 (26). Effluent loads were calculated by multiplying concentration (mg/L) by volume of effluent (L) from the liming-reliming processes per ton of raw hides processed. Analysis of Composite Waste Liquor. Composite liquors from control and experimental leather processing were collected from all unit operations up to chrome tanning and analyzed for COD and TS (dried at 103-105 °C for 1 h) by the standard procedures (23). From this, effluent loads were calculated for 1 ton of raw hides processed.
Results and Discussion Preliminary trials were performed to select the amount of enzyme as well as the time required for optimum opening up of fiber bundles matching the requirements of the conventional leathers treated with lime. Softness is a property essentially associated with the opening up of fiber bundles. Hence, the leathers were assessed for softness and other essential properties by experienced tanners. It was found that the leathers treated with varying percentages of R-amylase for 6 h tend to have minor grain rupture and mild foul
TABLE 1. Typical Input-Output Audit for the Beam House Processes for Processing 1 Ton of Raw Hidesa control process liming/enzyme-based dehairing
reliming/enzyme-based opening up treatment
a
Weight of hides before soaking.
b
experimental
chemicals/raw material
input (kg)
output (kg)
soaked sides water lime Na2S enzyme (SPIC) dry sludge dehaired sides water lime enzyme (R-amylase) dry sludge
1000 4000 100 35
740 4180 11.0 17.6
740 4000 100
88 1020 3740 5.3
input (kg) 1000 60 5 10 720 720 10.8
output (kg) 720
neb 8 1080 410 neb
64
ne, not estimated.
smell at the completion of the process. This means that the degradation process has just started due to the action of microorganisms. However, it was noticed that there was neither visible grain damage nor bad smell in the leathers treated with varying percentages of R-amylase for 3 h. Further, the leathers treated with 1.5 and 2% R-amylase had comparable softness and other properties such as fullness and grain tightness with that of leathers treated with lime. On the other hand, leathers resulting from 0.5 and 1% R-amylase dose had insufficient softness and were firm in nature, indicating that the fiber opening was not adequate. Hence, it is evident from these results that a dose of 1.5% (w/w) R-amylase (with 100% water for 3 h running) is sufficient to produce leathers having a softness comparable with that of conventional leathers produced by lime treatment. Input-Output Audit of the Experimental and Control Processes. The input and output of the raw materials, chemicals, and water were monitored for the control and experimental beam house processes. Corresponding values for processing 1 ton of raw hides are given in Table 1. The reduction in the weight of the soaked hides after liming is mainly due to the removal of hair and is more or less similar for both control and experimental pelts. Only 10% of the lime employed was found in the spent lime liquor for the control liming, wherein the general notion is that during the pretanning processes the uptake of the chemicals is negligible. This is because the filtered spent liquor contains only the solubilized form of lime. The amount of sodium sulfide in the spent lime liquor is also estimated to be only 50% of the applied weight for the control process. This is because the pulping reaction, induced by sodium sulfide in the case of control liming, has produced sludge containing keratinsulfide reaction products. However, in the case of enzymebased dehairing, there was no spent liquor since the application method was by paint. Hence, there was no discharge of wastewater containing sodium sulfide and lime. The amount of dry sludge produced from the control liming process was 88 kg, of which majority of the fraction was from lime as evidenced from the following discussion. The remaining fraction, in principle, would be constituted of keratin degradation products, noncollagenous proteins, calcium, and sodium salts of protein degradation products and deaminated amino acids. No attempt was made to find out the individual components and origin of the sludge since they are too complex to analyze and this is beyond the scope of this work. A very low amount of dry sludge from the experimental process implies that there is no lime and only a negligible amount of hair degradation products is present. In other words, there is no pulping of hair. The visual observation of the hair from the experimental dehairing process also supports the above view since the hair seems to be straight and long without any damage.
The weight of the pelts after fiber opening treatment was found to be increased compared to dehaired weight due to swelling for both control and experimental processes. It is higher for the biocatalytic process than the control process. This implies that the amount of water imbibed by the enzymatically opened hide is similar to or higher than that of the control pelt irrespective of the variation in the opening up method. Approximately 5% of the amount of lime employed during the reliming process is present in the spent relime liquor of the control process, due to the limited solubility of lime. The amount of dry sludge formed during the control reliming process is 64 kg. The sludge appeared to be pure white, which indicates that it contains mainly undissolved lime. Hence, it could be concluded that 64% of the applied lime is present as sludge and 5% as the solubilized form in the control spent relime liquor. The remaining 30% of the lime is present as suspended materials in the form of hydrated lime aggregates, which are removed during filtration before the calcium estimation. In the case of the experimental opening up process, the sludge formation and the presence of lime in the spent liquor have been completely avoided due to the replacement of lime with enzyme. Effect of Enzyme on Grain Structure. In this study, enzyme was used in both dehairing and fiber opening processes. It is of paramount importance to investigate the grain structure since it is expected that the use of enzyme could result in grain damage. The scanning electron micrographs of samples from lime- and enzyme-based fiber opening processes showing the grain surface at a magnification of ×100 are given in Figure 1a and b, respectively. It is clearly seen that the grain structure of the sample from enzyme-based fiber opening is clean without any grain damage. However, the sample from lime-based fiber opening seems to be devoid of hair pores. This is due to the presence of undissolved lime particles on the hair pores. During the subsequent operations (deliming), the undissolved lime particles are dislodged from the pores. This is evidenced from the scanning electron micrographs of chrome-tanned control (Figure 1c) and experimental samples (Figure 1d) showing the grain surface at a magnification of ×100. The arrangement of hair pores in the grain surface of both control and experimental samples looks the same without any grain damage. Pollution Load from Beam House Processes. The major constituents present in the beam house wastewater were analyzed for their concentration and effluent load. The results are presented in Table 2. The concentration of sulfide in the spent control lime liquor is in the range of 500 ppm, which is comparable to that already reported (6). Sulfide concentration in the reliming liquors could not be estimated under the present experimental conditions due to its meager presence. Sulfide load in the spent liquor from the control VOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4189
FIGURE 1. Scanning electron micrographs of control and experimental samples showing the grain surface: (a) after lime-based fiber opening (control) (×100); (b)after r-amylase-based fiber opening (experimental) (×100); (c) after chrome tanning (control) (×100); (d) after chrome tanning (experimental) (×100).
TABLE 2. Pollution Load Analysis on Process-Specific Streams spent liquor from liming/enzymatic dehairinga
spent liquor from reliming/enzymatic fiber openinga
parameters
C
(ppm) Ca(OH)2 (ppm) COD (ppm) TS (ppm) volume of effluent (L/ton of raw hidesa) effluent load (kg/ton of raw hidesc) S2Ca(OH)2 COD TS
562 ( 24 2624 ( 36 15060 ( 18 29755 ( 38 4180
1462 ( 26 2026 ( 14 15150 ( 20 5040 ( 34 17887 ( 24 3740 410
2.4 11.0 63 124
5.5 7.6 18.8
S2-
E
C
E
ndb
6.2 7.3
a C, control; E, experimental. b nd, not detectable under the experimental method and conditions. c Weight of hides before soaking.
liming process is 2.4 kg/ton of raw hides processed. However, it is eliminated by employing the enzyme-based dehairing process. The concentration of lime in the spent lime and relime liquors for the control process is in the range of 1.53.0 g/L despite their higher usage. This is due to the fact that lime tends to dissolve only up to 1.3 g/L in pure water. However, in the presence of some specific organic materials, it is known to show higher solubility (27). It is known that liming-reliming processes solubilize the keratinous and noncollagenous proteins present in the hide. This leads to the formation of calcium salts of protein hydrolysis products and of deaminated acids (28), which increases the solubility of lime. The pollution load due to lime for the experimental 4190
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 19, 2002
dehairing and opening up process is completely eliminated compared to the corresponding control processes. It is apparent that the COD and TS values for the control lime liquor is very high and absent for the experimental process since it does not generate effluent. The effluent loads for COD and TS from control lime liquor are 63 and 124 kg/ton of raw hides processed. The major contribution for COD is from the pulping of hair rather than the excretion of noncollagenous proteins as demonstrated earlier. This pulping is completely avoided in the case of the experimental dehairing process. Spent liquor analysis from the reliming/ opening up process shows that the COD and TS values for the experimental process are higher than the conventional process despite the elimination of lime with enzyme. This is because a part of the enzyme along with noncollagenous proteins is present in a comparatively small volume of spent liquor. This led to increase in the concentration of COD and TS. The COD and TS loads for the experimental opening up process are reduced by 18 and 62% compared to the conventional opening up method using lime. However, the total COD and TS loads from beam house processes (both liming/enzyme-based dehairing and reliming/enzyme-based fiber opening) for the experimental process are reduced by 91 and 95%, respectively, compared to the control process. Assessment of Opening Up of Fiber Bundles: Implicit Approach. Scanning electron micrographs of samples from lime- and enzyme-based fiber opening processes showing the cross section at a magnification of ×50 are given in Figure 2a and b, respectively. The sample from lime-based fiber opening displays a very fine opening up of fiber bundles compared to the sample treated with R-amylase, which seems to be devoid of fiber opening and carries a cemented appearance. Scanning electron micrographs of the same
FIGURE 2. Scanning electron micrographs of control and experimental samples showing the cross section: (a) after lime-based fiber opening (control) (×50); (b) after r-amylase-based fiber opening (experimental) (×50).
FIGURE 3. Scanning electron micrographs of control and experimental samples showing the cross section: (a) after chrome tanning (control) (×50); (b) after chrome tanning (experimental) (×50).
FIGURE 4. Scanning electron micrographs of control and experimental crust leather samples showing the cross section: (a) at a magnification of ×500 (control); (b) at a magnification of ×500 (experimental). samples at relatively higher magnification (×500) confirm the above observation (See Figure 2c and d, Supporting Information.). However, the scanning electron micrographs of samples obtained by lime- and enzyme-based fiber opening, followed by the chrome tanning process (Figure 3a and b), clearly demonstrate that the extent of fiber opening for both lime- and enzyme-based treatment is similar or comparable. Higher magnification (×500) scanning electron micrographs of the same samples confirm the above observation (See Figure 3c and d, Supporting Information.). Hence, it is evident that the absence of fiber opening observed after enzyme-based treatment is a temporary phenomenon compared to the lime-based fiber opening. This is because lime-based fiber opening is an osmotic-driven splitting of
fiber bundles and the hydrostatic pressure built inside the matrix keeps the fibers apart from each other, while the R-amylase-based fiber opening is due to the disintegration of proteoglycans and its subsequent removal. The lack of hydrostatic pressure in the enzyme-based fiber opening resulted in a cemented appearance in the sample. Scanning electron micrographs of crust leather samples treated with lime and enzyme for fiber opening showing the cross section at a magnification of ×500 are given in Figure 4a and b, which confirm all the above observations. The fiber bundles of both samples show a very fine splitting and the individual fibers are clearly seen at higher magnification (×750) (See Figure 4c and d, Supporting Information). VOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4191
TABLE 3. Comparison of Stratigraphic Distribution of Chromium in Control (C) and Experimental (E) Wet Blue Leathers % Cr2O3 (dry weight basisa) sample
grain
middle
flesh
C E
3.87 ( 0.06 3.65 ( 0.07
3.81 ( 0.07 3.59 ( 0.06
3.65 ( 0.11 3.64 ( 0.05
a
Moisture free chrome tanned leather weight.
TABLE 4. Compression Measurement and Slope Angle Data for Control (C) and Experimental (E) Crust Leathers thickness after compression (mm) added loads (g)
total load (g)
C
E
86 100 200 100 100 300 300 400 400 200 200 slope angle (negative)
86 186 386 486 586 886 1186 1586 1986 2186 2386
1.480 1.458 1.438 1.429 1.418 1.392 1.371 1.356 1.340 1.332 1.328 1.98
1.426 1.398 1.369 1.360 1.347 1.332 1.319 1.300 1.287 1.282 1.276 1.94
Stratigraphic Chrome Distribution Analysis and Softness Measurements. It is important to look at the layerwise chromium distribution for assessing the extent of opening up of fiber bundles. The average values of stratigraphic chromium distribution are presented in the Table 3. In general, both the control and experimental leathers exhibit fairly uniform chromium distribution along the entire cross section, and the quantities are comparable. This means that the extent of opening up of fiber bundles using lime or enzyme is similar or comparable as evidenced from the uniform distribution of chromium along the entire cross section. It was shown earlier (29) that the amount of chromium in the middle layer is drastically reduced if the pelts have not undergone an opening up treatment. Softness is directly proportional to compressibility. The changes in thickness due to compression upon added loads are given in Table 4 for the control and experimental crust leathers. The logarithm of thickness plotted against logarithm of load for the control and experimental leathers showed a linear fit. The negative slope angles were calculated (24), and the values are given in Table 4. Higher negative angles imply more softness in the leather. It is apparent that the experimental leather (E) exhibits a more or less similar negative slope angle (compressibility index, CI) compared to the control leather (C). This indirectly shows that the opening up of fiber bundles of the leathers treated with R-amylase is comparable to that of osmotic swelling-based opening up. This is in agreement with the scanning electron microscopic analysis as well as stratigraphic chromium distribution analysis. Performance of the Leathers. The strength properties such as tensile, tear, and grain crack strength values were obtained by standard physical testing methods and are presented in Table 5 along with standard deviations. The experimental leathers (E) show strength values analogous to that of the control leathers (C), comparable to the Bureau of Indian Standards (BIS) norms (30). In fact, the grain crack strength values are slightly higher than the values of control leathers. The average of the rating values of five sides corresponding to each experiment has been calculated for 4192
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 19, 2002
each bulk property and is given in Figure 5 along with standard deviation values. The softness of leathers treated with R-amylase is equal to that of the control leathers. The ratings for fullness, grain flatness, and grain smoothness are slightly higher for the experimental leathers (E) compared to the control leathers (C). The rating for grain tightness or break is similar for both kinds of leathers. In general, the appearance of experimental leathers is slightly better than the control leathers. Environmental Benefits. The composite liquors were collected from control and experimental leather processing up to chrome tanning and analyzed for their impact on the environment. COD and TS are the two parameters that were chosen for analyzing the impact. A direct interpretation of the observed COD/TS values may not give their actual impact on the environment. Hence, the COD/TS values were converted into effluent loads by multiplying COD/TS values (mg/L) with volume of effluent (L) per ton of raw hides processed. Table 6 provides the amount of water employed for each operation for processing 1 kg of raw hides. It is apparent that the experimental leather processing enjoys a reduction in water consumption by 40% compared to the control leather processing. The COD and TS values and the calculated effluent loads are given in Table 7. It is expected that the COD and TS values for the experimental leather processing have to be higher than the control leather processing due to the reduced water usage in experimental leather processing. However, the COD value of the composite liquor of biocatalytic leather processing is lower than that of the control leather processing. This is due to the fact that hair from the control dehairing is pulped and dissolved in the spent liquor unlike the experimental dehairing process, where the application of depilant by painting (hair-saving process) avoids the discharge of the spent liquor and the use of enzyme avoids the pulping of hair. As expected, the TS value of composite liquor of experimental leather processing is higher than that of control leather processing despite the elimination of lime and the deliming process. This may be due to the reduction in the amount of water employed and the presence of unused enzyme (R-amylase) and noncollagenous proteins. The presence of unused enzyme increases not only the TS but also COD. It then seems relevant to think that the dose of enzyme can be reduced. However, the results from the preliminary trials show that 1.5% R-amylase is necessary to bring optimal opening up of fiber bundles. Further, this amount is critical due to the limited time frame since a reduced dose with longer duration resulted in grain damage. The reductions in the effluent loads of COD and TS for the experimental leather processing are 45 and 20% compared to the control leather processing. The reduction in the COD by 45% without employing lime, a low-solubility alkali, is remarkable in the context of a greener environment. The reduction in the TS load is not to the extent expected, since the experimental beam house process employs zero lime and the control process employs 20% lime (200 kg/ton of raw hides processed). The TS load represents mostly the dissolved solids and to some extent the suspended solids. This does not include solids in the sludges. A comparative table (Table 8) was made to account for the TS present in the effluent. The contribution of TS from the dehairing and opening up processes for the control and experimental leather processing is 143 and 7 kg/ton of raw hides processed, which would mean that a 95% reduction in the TS for the experimental beam house process is realistic. In other words, the percentage contribution of TS from the beam house processes of control and experimental leather processing in the composite liquor is 20 and 1%. This means that the TS contribution from the experimental process is almost zero. It is known that 70% of the TS load originates from the soaking
TABLE 5. Physical Testing Data of Control (C) and Experimental (E) Leathers expt
tensile strength (kg/cm2) (av valuea)
% elongation at break (av valuea)
tear strength (kg/cm) (av valuea)
C E
250 ( 4 244 ( 6
76 ( 3 55 ( 4
102 ( 6 96 ( 2
a
grain crack strength (av valueb) load (kg) distension (mm) 25 ( 1 45 ( 2
7.9 ( 0.1 10.0 ( 0.1
Average of mean of along and across backbone values for 10 halves. b Average of load and distention values for 10 halves.
FIGURE 5. Comparison of bulk properties for control (C) and experimental (E) leathers.
TABLE 6. Comparison of Water Requirement for Control (C) and Experimental (E) Leather Processing water requirement (L/kg of raw hidea) unit operations
C
E
soaking liming/enzyme-based dehairing reliming/enzyme-based fiber opening washing deliming and bating washing pickling chrome tanning total
9 4 4 2 1 2 1 1.5 24.5
9 0.06 1 2
a
1 1.5 14.56
Weight of hides before soaking.
TABLE 7. Composite Liquor Analysis for Control (C) and Experimental (E) Leather Processinga
process C E
COD (ppm)
TS (ppm)
5173 ( 18 28 205 ( 44 4802 ( 12 38 367 ( 38
effluent load vol of (kg/ton of raw effluent b (L/ton of hides processed) b raw hides ) COD TS 24 500 14 500
127 70
a Composite liquors were collected up to chrome tanning. of hides before soaking.
691 556 b
Weight
and chrome tanning processes and 18-20% from the limingreliming processes (6, 31). Hence, now it is possible to reduce the contribution of TS from 20 to 1% by employing the enzymatic opening up method rather than the lime-induced osmotic swelling. It is evident from Table 1 that 100 kg of lime was employed for control reliming, of which 64 kg formed sludge, for processing 1 ton of raw hides. This is completely avoided by employing the enzyme-based opening up method. This is one of the pioneering achievements in the context of solid waste management and total solids reduction. Further, this approach calls for a paradigm shift in the leather sector from chemical-based leather processing to biocatalytic
TABLE 8. TS in the Effluent: A Comparative Figure (Values in kg/ton of Raw Hidesa Processed) process
control exptl
liming/enzyme-based dehairing 124.0 reliming/enzyme-based opening up treatment 18.8 7.3 total TS from beamhouse processes 142.8 7.3 composite liquor (soaking to chrome tanning) 691 556 a
Weight of hides before soaking.
processing. This is one of the untapped areas in leather processing, namely, beam house processes. Additionally, this biocatalytic leather processing sequence does not require a deliming process since the operating pH for the beam house processes is limited to 8.0 unlike the conventional lime-based processes. Economic Perspective of the Biocatalytic Process. Any development of an ecobenign process requires commercial feasibility apart from achieving the required quality product in order to capture the market. The developed biocatalytic process is not economically viable, if the tanner employs laboratory grade R-amylase. However, the use of a commercial R-amylase having similar or slightly higher activity would be economically viable. Preliminary studies using a commercial enzyme product (activity 3000 units/g; dose 1% (w/w)), sourced from SPIC Limited (price comparable to that of dehairing enzyme), showed promising results with respect to the quality of the leathers produced. It has been estimated that the cost of chemicals for chemical-based processing (liming-reliming-deliming) is U.S. $54/ton of raw hides processed, while it is only U.S. $55 for the biocatalytic processing. Hence, the use of commercially available enzymes makes the biocatalytic leather processing economically viable. It has been shown that the use of enzyme for dehairing results in 2% increase in area of the final leather (29). Since this biocatalytic process employs enzymes for both dehairing and fiber opening, the increase in area of the final leather would be higher compared to that achieved by the use of enzyme for dehairing alone. The increased area of the leather provides a saving of U.S. $70 (at 3% increase in area) per ton of raw hides processed. Further, the reduction in COD and TS loads through the biocatalytic process would provide VOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4193
additional benefit in effluent treatment costs (32). Apart from this, the disposal of lime-bearing sludge generated through conventional processing (a global estimate of 1.4 million tons per annum) causes both ecological and economic concerns. Plausible Mechanism of the Process. In the conventional fiber opening process, lime increases the pH from 7-8 to 12-13 and causes osmotic swelling due to ionic imbalances built up in the matrix (33). The hydrostatic pressure built up would enhance the splitting up of fiber bundles, separation of unwanted interfibrillary materials such as proteoglycans, globular proteins, reticulin, etc., and easy removal of flesh (16). However, in the case of R-amylase-based fiber opening, at least two different mechanisms can be proposed. Steven (17) suggested that the R-amylase breaks the O-linkage (34) between the protein and carbohydrate moiety, while Gilles et al. (35) reported that R-amylase catalyzes the hydrolysis of the R-(1,4) glycosidic linkages found in starch components, glycogen, and various oligosaccharides. In other words, it converts the polysaccharide units into their monomer. Hence, it is certain that the proteoglycan present in the skin/hide matrix can be disintegrated by R-amylase through either of the above-mentioned mechanisms. A detailed study on the characteristics of the liquor from the enzyme-based fiber opening process and the composition of protein and carbohydrate is being carried out in order to understand the mechanism of breakdown of the protein-carbohydrate moiety present in the skin matrix.
Acknowledgments P.T thanks the Council of Scientific and Industrial Research, India for providing a senior research fellowship to carry out the research work. Authors thank Dr. R. Rajaram for physical testing measurements. (Note: Standardized unhairing recipe (19) refers to the application of a paste comprising water 5-8%, Biodart 1% (sourced from SPIC Limited, Chennai, India), sodium sulfide 0.5%, and Noigen LS 0.25% (percentages based on weight/soaked weight) on the grain side of the cow side followed by piling for 18 h).
Supporting Information Available Higher magnification scanning electron micrographs of the samples treated with lime and R-amylase. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Ward, G. J.; Slabbert, N. P.; Shuttleworth, S. G. J. Am. Leather Chem. Assoc. 1976, 71, 551-559. (2) Ramasami, T.; Rao, J. R.; Babu, N. K. C.; Parthasarathi, K.; Rao, P. G.; Saravanan, P.; Gayathri, R.; Sreeram, K. J. J. Soc. Leather Technol. Chem. 1999, 83, 39-45. (3) Germann, H. P. Science and Technology for Leather into the Next Millennium; Tata McGraw-Hill Publishing Co. Ltd.: New Delhi, 1999; p 283. (4) Thorstensen, T. C. Practical Leather Technology, 4th ed.; Krieger Publishing Co.: Malabar, FL, 1993. (5) Ramasami, T.; Prasad, B. G. S. Proceedings of the LEXPO-XV, 1991; p 43.
4194
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 19, 2002
(6) Aloy, M.; Folachier, A.; Vulliermet, B. Tannery and Pollution; Centre Technique Du Cuir: Lyon, France, 1976. (7) Bailey, D. G.; Tunick, M. H.; Kuzma, M. A. Proceedings of the 14th Industrial Waste Mid Atlantic Conference; 1982. (8) Bose, S. M. Leather Sci. 1955, 2, 140-144. (9) Dhar, S. C. Leather Sci. 1974, 21, 39-47. (10) Rosenbusch, K. Das Leder 1965, 16, 237-248. (11) Morera, J. M.; Bartoli, E.; Borras, M. D.; Marsal, A. Proceedings of the XXIV IULTCS Congress; London, 1997; p 436. (12) Sehgal, P. K.; Ramamurthy, G.; Muralidharan, C.; Gupta, K. B. J. Soc. Leather Technol. Chem. 1996, 80, 91-92. (13) Schlosser, L.; Keller, W.; Hein, A.; Heidemann, E. J. Soc. Leather Technol. Chem. 1986, 70, 163-168. (14) Valeika, V.; Bale`iu ˆniene¨, J.; Bele∂Ä ka, K.; Skrodenis, A.; Valeikiene¨, V. J. Soc. Leather Technol. Chem. 1997, 81, 65-69. (15) Valeika, V.; Bale`iu ˆniene¨, J.; Bele∂Ä ka, K.; Skrodenis, A.; Valeikiene¨, V. J. Soc. Leather Tehnol. Chem. 1998, 82, 95-98. (16) Campbell, D. W.; Donovan, R. G. J. Am. Leather Chem. Assoc. 1973, 68, 96-102. (17) Steven, F. S. Biochim. Biophys. Acta 1965, 97, 465-471. (18) Burton, D.; Reed, R.; Flint, F. O. J. Soc. Leather Technol. Chem. 1953, 37, 82-87. (19) Thanikaivelan, P.; Rao, J. R.; Nair, B. U. J. Soc. Leather Technol. Chem. 2000, 84, 276-284. (20) Official methods of analysis; Society of Leather Technology and Chemistry: Herts, U.K., 1965. (21) Echlin, P. In: Scanning Electron Microscopy; Heywood, V. H., Ed.; Academic Press: London, 1971; Vol. 4, p 307. (22) In The Chemistry and Technology of Leather; O’Flaherty, F., Roddy, W. T., Lollar, R. M., Eds.; Krieger Publishing: Malabar, FL, 1977; Vol. IV. (23) In Standard Methods for the Examination of Water and Wastewater, 17th ed.; Clesceri, L. S., Greenberg, A. E., Trussell, R. R., Eds.; American Public Health Association: Washington, DC, 1989. (24) Lokanadam, B.; Subramaniam, V.; Nayar, R. C. J. Soc. Leather Technol. Chem. 1989, 73, 115-119. (25) Bangaruswamy, S. Technological Controls in Leather Manufacture; CLRI Publications: Madras, India, 1980. (26) Vogel, A. I. A Textbook of Quantitative Inorganic Analysis, 4th ed.; Longman Inc.: New York, 1978; p 354. (27) In Kirk Othmer Encyclopaedia of Chemical Technology, 3rd ed.; Mark, H. F., Othmer, D. F., Overberger, C. G., Seaborg, G. T., Grayson, M., Eckroth, D., Eds.; Wiley-Interscience Publication: New York, 1978; Vol. 14, p 350. (28) Thompson, F. C. J. Am. Leather Chem. Assoc. 1923, 18, 662-678. (29) Thanikaivelan, P.; Rao, J. R.; Nair, B. U. J. Soc. Leather Technol. Chem. 2001, 85, 106-115. (30) Specification for Chrome Retan Upper Leather; IS 2961; Bureau of Indian Standards: New Delhi, India, 1964. (31) Thanikaivelan, P.; Rao, J. R.; Nair, B. U.; Ramasami, T. J. Cleaner Prodn. 2003, 11, 79-90. (32) Shrewsbury, C. World Leather 2002, (Feb), 40-42. (33) Bienkewicz, K. Physical Chemistry of Leather Making; Krieger Publishing: Malabar, FL, 1983. (34) Butler, W. T.; Cunningham, L. W. J. Biol. Chem. 1966, 241, 38823888. (35) Gilles, C. B.; Rousseau, P.; Rouge, P.; Payan, F. Structure 1996, 4, 1441-1452.
Received for review March 4, 2002. Revised manuscript received July 11, 2002. Accepted July 19, 2002. ES025618I
