Environ. Sci. Technol. 2003, 37, 2609-2617
Biointervention Makes Leather Processing Greener: An Integrated Cleansing and Tanning System PALANISAMY THANIKAIVELAN,† 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 and Centre for Leather Apparels & Accessories Development, Central Leather Research Institute, Adyar, Chennai 600 020, India
The do-undo methods adopted in conventional leather processing generate huge amounts of pollutants. In other words, conventional methods employed in leather processing subject the skin/hide to wide variations in pH. Pretanning and tanning processes alone contribute more than 90% of the total pollution from leather processing. Included in this is a great deal of solid wastes such as lime and chrome sludge. In the approach described here, the hair and flesh removal as well as fiber opening have been achieved using biocatalysts at pH 8.0 for cow hides. This was followed by a pickle-free chrome tanning, which does not require a basification step. Hence, this tanning technique involves primarily three steps, namely, dehairing, fiber opening, and tanning. It has been found that the extent of hair removal, opening up of fiber bundles, and penetration and distribution of chromium are comparable to that produced by traditional methods. This has been substantiated through scanning electron microscopic, stratigraphic chrome distribution analysis, and softness measurements. Performance of the leathers is shown to be on par with conventionally processed leathers 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 80%, total solids load by 85%, and chromium load by 80% as compared to the conventional process, thereby leading toward zero discharge. The input-output audit shows that the biocatalytic threestep tanning process employs a very low amount of chemicals, thereby reducing the discharge by 90% as compared to the conventional multistep processing. Furthermore, it is also demonstrated that the process is technoeconomically viable.
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 have been discussed elsewhere (1). Development of an eco-benign process requires commercial feasibility apart from achieving the required product * Corresponding author telephon: +91 44 2441 1630; fax: +91 44 2491 1589; e-mail:
[email protected]. † Centre for Leather Apparels & Accessories Development. ‡ Chemical Laboratory. 10.1021/es026474a CCC: $25.00 Published on Web 05/03/2003
2003 American Chemical Society
quality. The zero discharge approach would lead to several direct and indirect benefits such as reductions in the use of chemicals, water, energy, and effluent treatment cost (1). Conventional leather processing involves a number of processes and operations. Tanning alone involves 7-8 steps comprising soaking, liming, reliming, deliming, bating, pickling, chrome tanning, and basification, which are basically a combination of single- and multistep processes that employ as well as expel various biological, organic, and inorganic materials (2). These steps account for nearly 90% of the total pollution from a tannery (3). The tannery effluent has high biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), sulfides, chlorides, sulfates, chromium, etc. This is primarily due to the fact that the conventional leather processing employs “do-undo” process schemes such as swell-deswell (limingdeliming) and pickle-depickle (pickling-basification) (4). In other words, conventional methods employed in leather processing subject the skin/hide to wide variations in pH. Such pH changes demand the usage of acids and alkalis, which leads to the generation of salts (5). Furthermore, toxic gases such as ammonia and hydrogen sulfide are also emitted. Apart from this, a great deal of solid waste including lime sludge from the tannery and chrome sludge from the effluent treatment plant are being generated. This happens to be a major stumbling block for many of the tanners around the world because of the stringent environmental regulations. Attempts have been made to replace or minimize the use of toxic chemicals by revamping the individual processing steps in order to reduce the pollution load. Several lime-free and sulfide-free dehairing methods have evolved during the past century, namely, the use of enzymes (6, 7), chlorine dioxide (8), hydrogen peroxide (9), nickel carbonate (10), and Lactobacillus-based enzymes (11). Valeika et al. (12, 13) have attempted to replace lime for dehairing using sodium hydroxide and sodium sulfide. Most of these methods are operated at alkaline pH and hence need a dealkalization step. To reduce nitrification of soil, some ammonia-free deliming methods have been developed. These include the use of methods based on carbon dioxide (14) and esters of carboxylic acids (15). These methods, however, are not very successful. Spent pickle liquor has high dissolved solid content (3) since pickling involves the use of 8-10% sodium chloride. The use of nonswelling acids in pickling has been reported by Herfeld and Schubert (16) in order to reduce TDS. Several better chrome management methods based on high exhaust chrome tanning, pickle-less tanning, chrome recovery and reuse, and a closed pickle-tan loop system have been developed and reviewed by Rao et al. (17). These improvements are specific to a unit operation. Implementation of all the advanced technologies involves financial input as well as machinery requirements. This calls for the evolution of integrated process technologies by revamping the process sequence. Very few attempts have been made to revamp the whole leather processing technology. Thanikaivelan et al. (18) have attempted to make leather in a narrower pH range from 4 to 8.0. Although the dehairing process was feasible without employing lime, the attempt to open up the fiber bundles using urea or bating enzyme was not very successful. Furthermore, the uptake and distribution of the tanning agent was not adequate. The purpose of this work is to avoid the deficiencies of the earlier technologies and develop an integrated tanning system by the intervention of biotechnology. Hence, an VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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attempt has been made to produce leather within three steps, namely, an enzyme-based dehairing followed by enzymebased fiber opening and pickle- and basification-free chrome tanning in the pH range of 4.0-8.0. Reductions in the input and output of chemicals and bioproducts as well as COD, TS, and chromium loads have been taken as the benchmarks to analyze the impact on the environment. The quality of the leathers has been assessed through stratigraphic chromium distribution analysis, scanning electron microscope (SEM), softness measurements, physical testing, and tactile evaluation. The use of enzymes for dehairing is known to increase the area yield by 3-5%, which leads to direct monetary benefits (18, 19). Hence, a detailed techno-economic viability of the developed process has also been analyzed.
Experimental Methods Materials. Wet-salted cowhides were chosen as raw material. Since this work involves the study of the extent of opening up of fiber bundles and the extent of diffusion and distribution of the tanning agent, compact cowhides in the thickness range of 3-4 mm were chosen. All chemicals used for leather processing were of commercial grade. A dehairing enzyme, Biodart (protease derived from bacterial origin, active at pH 7.5-11.0 and temperature 25-40 °C) and R-amylase (activity 3000 units/g) were sourced from Southern Petrochemical Industries Corporation (SPIC) Limited, Chennai, India. The chemicals used for the analysis of leather and spent liquors were of analytical grade. Conventional Tanning Process. Ten wet-salted cowhides were cut into halves (sides), numbered, and soaked conventionally. The wet weight after soaking was noted for each side and termed as soaked weight. Five left sides and five right sides were used for control (C), while the other sides were used for experiment (E). Control liming was performed by using 3% sodium sulfide, 10% lime, and 300% water (percentages based on weight/soaked weight) in a drum. The duration of treatment was 1 day with 5 min running per 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 10% lime and 300% water in a drum (percentages based on weight/soaked weight). The duration of treatment was 1 d with 5 min running per hour for 6 h and left overnight in the bath. Then the pelts (side/hide without hair) 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 noted. The pelts were washed with 200% water (percentage based on weight/ fleshed weight). Subsequently, the pelts were delimed by treating with 100% water and 1.2% ammonium chloride (w/ w) for 90 min. Completion of deliming was ascertained by checking the cross-section of the delimed pelt for colorless to phenolphthalein indicator. Bating was carried out in the same bath for 30 min by the addition of 0.5% commercial alkali bate (percentage based on weight/fleshed weight). The bated pelts were washed with 200% water. Pickling was carried out by treating the pelts initially with 100% water and 10% sodium chloride for 10 min followed by the addition of 1.2% sulfuric acid (percentages based on weight/fleshed weight) in three installments at the interval of 15 min and running for 3 h. The pH of the cross-section was found to be 2.8. Chrome tanning was initiated by the addition of 8% basic chromium sulfate (BCS) (w/w) after draining out 50% of the pickle liquor. Drum was run for 2 h after which complete penetration of chromium was ascertained and 50% water was added. After 30 min running, basification was carried out by the addition of a mixture of 0.5% sodium formate, 1% sodium bicarbonate, and 20% water (percentages based on weight/fleshed weight) in three installments at the interval of 10 min. Finally, the drum was run for 3 h, and the pH was 2610
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found to be 3.9. The chrome-tanned sides were washed with 200% water. Three-Step Tanning Process. Experimental sides were dehaired by the application of a paste comprising 6% water, 1% Biodart, and 0.5% sodium sulfide (percentages based on weight/soaked weight) on the grain side of the cow sides followed by piling for 18 h (5). The next day, the sides were dehaired using conventional beam and blunt knife technique. The dehaired weight was noted. The sides were treated with 1% R-amylase and 100% water for 3 h in a drum (percentages based on weight/dehaired weight) (1). Then the pelts were fleshed and scudded. The fleshed weight of the sides was noted. The pelts were washed with 100% water (percentage based on weight/fleshed weight). The pH of the cross-section was found to be 7.5. A pickle-basification-free chrome tanning was followed (20) by treating the pelts with 100% water and 0.3% sulfuric acid (percentages based on weight/ fleshed weight). Sulfuric acid was added in two installments at the interval of 15 min, and the drum continued to run for 45 min. The pH of the cross-section was found to be 5.2. This was followed by the addition of 8% BCS (percentage based on weight/fleshed weight). The drum was run for 5 h after which complete penetration of chromium was ascertained by the appearance of uniform blue color throughout the cross-section of the leather without any white streak. The pH of the cross-section was found to be 3.8. The chrometanned sides were washed with 100% water (w/w). The leathers after chrome tanning were piled for 24 h. The shrinkage temperature of the chrome-tanned leathers was measured using a Theis shrinkage tester in a glycerolwater (3:1 ratio) medium (21). Then the leathers were sammed (removal of free water by pressing the wet chrome-tanned leather between two felt rollers), split, and shaved to uniform thickness (1.1-1.2 mm). Rechroming was not done. A commercial post-tanning process sequence was adopted in order to convert the tanned leathers into crust upper leathers. Input-Output Audit. A comprehensive input-output audit for the raw materials, water, chemicals, and other reaction products was carried out for the conventional and three-step tanning processes excluding soaking and posttanning processes. Essential parameters such as the amount of lime, sodium sulfide, and dry sludge were estimated as described earlier (22), and the mass balance was calculated for the volume of effluent collected for that particular process. The estimation of acid and base was not carried out since they remain in the effluent in the form of reaction products. Similarly, enzymes are also not analyzed as they are neither absorbed nor fixed with the hide matrix. The amount of chromium in the wastewater from chrome tanning was analyzed by employing a a Perkin-Elmer Lambda 35 UVvisible spectrophotometer after digesting the sample using an acid mixture as per the standard procedure (23), and the mass balance was calculated based on the volume of chrome liquor collected. Scanning Electron Microscopic Analysis. Samples from experimental and control sides were cut from the official sampling position [details of the sampling position are given in Supporting Information] (24) after chrome tanning process and crusting operations. Samples from crust sides were directly cut into specimens with uniform thickness without any pretreatment. Samples from chrome tanning were first washed in water. Subsequently, the samples were dehydrated gradually using acetone and methanol as per standard procedure (25). Samples were then cut into specimens with uniform thickness. All specimens were then coated with gold using Edwards E306 sputter coater. A Leica Cambridge Stereoscan 440 scanning electron microscope was used for the analysis. The micrographs for the grain surface and crosssection were obtained by operating the SEM at an accelerating
TABLE 1. Typical Input-Output Audit for Conventional and Three-Step Tanning for Processing 1 t of Raw Hidesa control process liming/enzyme-based dehairing
reliming/enzyme-based opening up treatment
washing deliming
washing pickling
chrome tanning
washing a
Weight of hides before soaking.
experimental
chemicals/raw material
input (kg)
output (kg)
input (kg)
output (kg)
soaked/green sides water lime Na2S enzyme (SPIC) dry sludge dehaired sides water lime enzyme (R-amylase) dry sludge fleshed sides water fleshed sides water NH4Cl alkali bate (enzyme based) delimed sides water delimed sides water sodium chloride H2SO4 pickled/enzyme opened up sides water H2SO4 basic chromium sulfate Na2SO4 from BCSd water sodium formate sodium bicarbonate chrome tanned sides water
1000 3000 100 30
880 2530 11.0 16.6
1000 60
900
880 3000 100
85 930 2670 5.3
930 1860 930 930 11.16 4.65 ne 1860 ne 930 93 11.16 ne
64 930 1860 ne 930 rpc ne ne 1860 ne 465 ne ne ne
b
74.4 465 4.65 9.3 ne 1860
ne, not estimated. c rp, reaction products.
voltage of 20 kV with different lower and higher magnification levels. Stratigraphic Chromium Distribution Analysis. Samples from the official butt portion (24) of experimental and control wet blue sides were split into three uniform layers using a Camoga splitting machine and analyzed layer-wise for chrome content. A known weight (∼1 g) of the sample was taken, and the amount of chromium was estimated as per standard procedure using a Perkin-Elmer Lambda 35 UVvisible spectrophotometer (26). Samples were initially analyzed for moisture content (27), 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 (28). Circular leather pieces (2 cm2 area) from experimental and control crust leathers were obtained as per IULTCS (International Union of Leather Technologists and Chemists Societies) method (24) and conditioned at 80 ( 4 °F and 65 ( 2% RH over a period of 48 h. The samples were spread uniformly over the solid base of the C & R (compressibility and resilience) tester. 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 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 (24). Physical properties such as tensile strength, percent elongation at break, tear strength, and grain crack strength were examined as per the standard procedures (29-
d
12 22.3 1130 rp rp ne 1860
5 10 900 900
neb 14 800 230
9
ne
800 800
800 400
800 640 2.4 64
ne 400 ne 2.2 19.2
ne 800
ne 730
Assuming BCS contains 30% sodium sulfate.
31). 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 better property. Analysis of Composite Waste Liquor. Composite liquors from control and experimental leather processing were collected from all unit operations except soaking up to chrome tanning as well as post-tanning and analyzed for COD and TS (dried at 103-105 °C for 1 h) as per the standard procedures (27). From this, emission loads were calculated by multiplying concentration (mg/L) with volume of effluent (L) per metric ton of raw hides processed.
Results and Discussion Input-Output Audit of the Conventional and Three-Step Tanning Processes. The input and output of the raw materials, chemicals, and water have been monitored for the control and experimental tanning processes. Corresponding values for processing 1 t 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, epidermis, soluble proteins, and water and is more or less similar for both control and experimental pelts. It is known that the uptake of chemicals by the hide matrix in the pretanning processes is negligible. However, the conventional dehairing process discharges only 11% lime and 55% sodium sulfide of the dose into the effluent. This is evidenced from the fact that the control process leads to the formation of 85 kg of dry sludge as against 14 kg from enzyme-based dehairing process as seen in Table 1. VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Cumulative Input-Output of Chemicals and Biochemicals for Conventional and Three-Step Tanning (for Processing 1 t of Raw Hides) control process liming/enzyme-based dehairing
reliming/enzyme-based opening up treatment deliming pickling chrome tanning
chemicals/raw material lime lime in suspended forma Na2S enzyme (SPIC) dry sludge lime enzyme (R-amylase) dry sludge NH4Cl alkali bate (enzyme based) sodium chloride H2SO4 H2SO4 basic chromium sulfate Na2SO4 from BCSb sodium formate sodium bicarbonate
input (kg) 100 30
11.0 56 16.6
100
85 5.3
11.16 4.65 93 11.16 74.4 4.65 9.3
chromium as BCS in leather total a
Ref 22.
b
9
438.32
input (kg)
output (kg)
5 10 14 9
9
2.4 64
2.4 2.2 19.2
64 11.16 4.65 93 11.16 12 22.3 4.65 9.3 40 446.12
43 90.4
89.8
Assuming BCS contains 30% sodium sulfate.
The reliming process produces about 64 kg of dry sludge per metric ton of rawhides. Audit for remaining portion of lime is discussed elsewhere (22). The biocatalyzed fiber opening process eliminates the above problems as seen in Table 1. Deliming and bating steps lead to the discharge of nearly 15 kg of solids in various forms. However, biocatalyzed pretanning processes do not require deliming and bating processes, as both dehairing and fiber opening processes are carried out at pH 8.0 and the purpose of bating has already been met during biocatalytic fiber opening process. Conventional pickling process employs nearly 93 kg of sodium chloride and 11 kg of sulfuric acid (to reduce the pH to 2.8) for processing 1 t of raw hides. This leads to the discharge of nearly 104 kg of solids into the effluent at one stage or the other. This emission is reduced to a maximum extent, if not completely, in the three-step tanning technique. Furthermore, the conventional chrome tanning requires nearly 14 kg of solids (weak bases) for the basification (complexing chromium with the protein carboxyl groups) step. The chrome tanning-basification processes lead to the discharge of nearly 48.3 kg of solids comprising about 12 kg of BCS and 22.3 kg of sodium sulfate (assuming that 30% of BCS contains sodium sulfate) for processing 1 t of raw hides. This emission is reduced to nearly 24 kg by employing a pickle-basificationfree chrome tanning method as seen in Table 1. The cumulative input-output of chemicals and biochemicals for conventional and biocatalyzed three-step tanning processes is given in Table 2. It is evident that the conventional processing sequence employs about 433 kg of chemicals and 4.7 kg of biochemicals for converting 1 t of raw hide into tanned matrix. While the biocatalytic threestep tanning technique employed in this study requires only 71.4 kg of chemicals and 19 kg of biochemicals, of which, 64 kg of the chemicals is the tanning agent. In other words, the biocatalyzed three-step tanning technique requires only 7.4 kg of chemicals in addition to the tanning agent against the conventional requirement of 359 kg. This means that the developed three-step tanning technique reduces the chemical consumption by 98%. With regard to the discharge of materials into the effluent, the conventional methods discharge about 201 kg of chemicals and biochemicals. However, the biocatalyzed three-step tanning method discharges only 33 kg of materials, of which the biochemicals constitute about 9 kg. In other words, the developed three2612
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output (kg)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
TABLE 3. Mass Balance of Chemicals and Bioproducts for Conventional and Biocatalyzed Three-Step Tanning Methods
parameter input output in effluent (excluding suspended lime) in effluent (suspended lime) in sludge in leather
control experimental (kg/t of raw (kg/t of raw hidesa) hidesa) 438
90
201 56b 149 40c
33 14 43c
a Weight of hides before soaking. b Includes the suspended lime (56 kg from liming and reliming (22)). c Amount of chromium present as BCS.
step tanning method reduces the discharge of chemicals by 88%. The other form of discharge of chemicals into the environment is through sludge. The conventional processing sequence leads to the formation of 149 kg of dry sludge against 14 kg from the biocatalyzed three-step tanning technique, effecting a 91% reduction as seen in Table 2. It is also important to look at the total input-output audit of chemicals and bioproducts employed in the conventional and three-step tanning techniques. The results are presented in Table 3. It is seen that the amount of materials present in the effluent collected from conventional processing is more than what was shown in Table 1. This is because of the presence of lime in suspended form. The suspended solids escape from the estimation of chemicals as well as sludge as already shown (22). It is also seen that the amount of chemicals (tanning agent) present in the leather from the three-step process is slightly more than that of conventionally processed leather. The data demonstrate that the mass balance is more or less accurate for both methods, and hence, the above arguments are legitimate. It is seen that an excess of materials is present in conventional process. This excess of materials in conventional process may be due to the presence of noncollagenous materials in the sludge unlike biocatalyzed dehairing, which leads to an additional amount of 8 kg. This is in accordance with the earlier report by Money and Adminis (32). In totality, the biocatalyzed threestep tanning technique has an environmental benefit of reducing the discharge of chemicals (both in effluent and
FIGURE 1. Scanning electron micrographs of control and experimental samples showing the grain surface after chrome tanning: (a) control (×250), (b) three-step process (×250).
FIGURE 2. Scanning electron micrographs of control and experimental samples showing the cross-section after chrome tanning: (a) control (×80), (b) three-step process (×80). sludge) by nearly 90% as compared to conventional processing sequence. This is one of the pioneering achievements in the context of solid waste management and total solids reduction. Scanning Electron Microscopic Analysis. The biocatalyzed three-step tanning process employs enzyme for both dehairing and fiber opening processes. It is important to investigate the grain structure since it is expected that the use of enzyme could result in grain damage. Furthermore, the pickle-basification-free chrome tanning process is carried out at pH 5.0 against the conventional pH of 2.8-3.8. Although it has been established (20) that the application of BCS at pH 5.0 without any masking (stabilization of coordinated complexes such as chromium with organic ligands against hydrolysis and precipitation) eliminates the above problems, it is of paramount importance to ascertain this in the present study also. The scanning electron micrographs of chrome-tanned samples from conventional and threestep tanning processes showing the grain surface at a magnification of ×250 are given in Figure 1a,b, respectively. It is clearly seen that the grain structure of the sample from the biocatalyzed three-step tanning process is clean without any severe grain damage. The hair pores are clearly visible without any surface deposition of chromium. This is comparable to that of conventionally processed leather sample. Higher magnification micrographs reinforce the above observations (see Figure 1c,d in Supporting Information).
Control and experimental crust leather samples (see Figure 1e,f in Supporting Information) exhibit similar arrangement of hair pores at the grain surface without any grain damage and surface deposition of chromium. Scanning electron micrographs of chrome-tanned samples from conventional and three-step tanning processes showing the cross-section at a magnification of ×80 are given in Figure 2a,b, respectively. Both the samples display identical fiber structure irrespective of the mode of fiber opening. Scanning electron micrographs of the same samples at relatively higher magnification (×500) confirm the above observation (see Figure 2c,d in Supporting Information). This demonstrates that the extent of fiber opening brought about by the selected enzyme is comparable to that achieved using lime. Scanning electron micrographs of crust leather samples from conventional and three-step tanning processes showing the cross-section at a magnification of ×160 are given in Figure 3a,b, respectively, which confirm all the above observations. The fiber bundles of both samples show a very fine splitting. Some portions of both samples seem to be dark. This is due to the charging of fatty materials (oil/fat components are added during post-tanning processes for fiber lubrication) with the incoming electrons. Higher magnification (×750) scanning electron micrographs (for Figure 3c,d, see Supporting Information) confirm the fine splitting of fiber bundles in both control and experimental crust leather samples. VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Scanning electron micrographs of control and experimental crust leather samples showing the cross-section: (a) control (×160), (b) three-step process (×160).
TABLE 4. Shrinkage Temperature and Stratigraphic Distribution of Chromium in Control (C) and Experimental (E) Wet Blue Leathers sample
3.55 ( 0.04 4.45 ( 0.10
C E a
% Cr2O3 (dry weight basisa) grain middle flesh 3.51 ( 0.06 3.73 ( 0.12
3.30 ( 0.10 4.48 ( 0.20
shrinkage temp (°C) >120 >120
Moisture-free chrome tanned leather weight.
Stratigraphic Chrome Distribution Analysis and Softness Measurements. This study employs a bioproduct for fiber opening. Hence, it is important to look at the layer-wise chromium distribution as well as hydrothermal stability for assessing the efficacy of the pickle-free chrome tanning. The average values of stratigraphic chromium distribution are presented in Table 4 along with shrinkage temperature data. Shrinkage temperature of the leathers from conventional and three-step tanning processes is more than 120 °C. In general, both control and experimental leathers exhibit fairly uniform chromium distribution along the entire cross-section. However, the amount of chromium in all the layers, especially grain and flesh, is higher in the case of leathers from the biocatalyzed three-step tanning process. This is due to the fact that pickle-basification-free chrome tanning provides higher uptake of chromium especially in the grain and flesh layers (20). Hence, it is noteworthy to mention that it is possible to reduce the offer of the tanning agent (basic chromium sulfate) such that the leather contains chromium equivalent to that derived using conventional process and dosage, leading to material saving as well as pollution reduction. It has been shown earlier (18) that the amount of chromium in the middle layer is drastically reduced if the pelts have not undergone the opening up treatment. The fact that the middle layer of the leather from the biocatalyzed three-step tanning process contains slightly more chromium than the conventionally processed leather demonstrates that the extent of opening up of fiber bundles using lime or enzyme is similar or comparable. It was earlier suggested (33) that the liming process involves deamination of protein amino groups, thereby leading to improved fixation of chromium. Although the biocatalyzed three-step tanning method does not involve lime or alkali treatment, the resultant leathers contain more chromium than the leathers produced by lime treatment. Furthermore , previous study demonstrates that the distribution and quantity of chromium does not vary when conventional chrome tanning is followed for the hide 2614
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TABLE 5. Compression Measurement and Slope Angle Data for Control (C) and Experimental (E) Crust Leathers added loads (g)
total load (g)
change in thickness due to compression (mm) C E
50 100 200 100 100 300 300 400 400 200 200
50 150 350 450 550 850 1150 1550 1950 2150 2350
1.381 1.348 1.265 1.252 1.236 1.219 1.198 1.185 1.17 1.163 1.156
1.561 1.509 1.416 1.399 1.391 1.357 1.334 1.314 1.301 1.287 1.283
2.75
3.02
slope angle (negative)
matrix for which fiber opening has been achieved through biocatalytic process (1). Hence, it appears that chromium fixation does not depend on the modification of collagen side-chain amide groups. This is in agreement with the results obtained by Covington et al. (34). Changes in thickness due to compression upon added loads are given in Table 5 for the crust leathers from conventional and biocatalyzed three-step tanning procedures. The logarithm of thickness plotted against the logarithm of load for the control and experimental leathers show a linear fit. The negative slope angles were calculated from the line equation (28), and the values are given in Table 5. Higher negative angles imply more softness in the leather. It is apparent that the experimental leather (E) exhibits a slightly higher negative slope angle (compressibility index, CI) as compared to the control leather (C). This is due to the softening of grain as well as fiber structure upon the twostage action of the enzyme. 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 6 along with standard deviations. The experimental leathers (E) show analogous strength values with that of the control leathers (C), comparable to the BIS (Bureau of Indian Standards) norms (35). The average of the rating values of each bulk property for the crust leathers corresponding to conventional and biocatalyzed three-step
TABLE 6. Physical Testing Data of Control (C) and Experimental (E) Leathers average valuea
expt
tensile strength (kg/cm2)
% elongation at break
tear strength (kg/cm)
C E
234 ( 5 224 ( 6
64 ( 4 58 ( 3
108 ( 5 110 ( 2
b
grain crack strength (average valueb) load (kg)
distension (mm)
35 ( 1 34 ( 2
9.3 ( 0.3 9.6 ( 0.4
a Average of mean of along and across backbone values for 10 halves. Average of load and distention values for 10 halves.
FIGURE 4. Comparison of bulk properties of crust leathers derived from conventional (C) and three-step (E) tanning.
TABLE 7. Comparison of Water Requirement and Discharge for Control (C) and Experimental (E) Leather Processing of 1 kg of Raw Hidea control unit operations soaking liming/enzyme-based dehairing reliming/enzyme-based opening up treatment washing deliming and bating washing pickling chrome tanning washing washing neutralization washing I washing II post tanning washing total a
input (L)
experimental
output (L)
input (L)
output (L)
9 3
8.33 2.53
9 0.06
8.33
3
2.67
0.9
0.23
1.86 0.93 1.86 0.93 0.56 1.86 0.350 0.350 0.466 0.466 0.233 0.466
1.86 0.93 1.86 0.465 1.13 1.86 0.332 0.399 0.466 0.466 0.266 0.466
0.8
0.4
0.64 0.8 0.330 0.330 0.440 0.440 0.220 0.330
0.4 0.73 0.200 0.356 0.440 0.440 0.270 0.330
25.331
24.03
14.29
12.126
Weight of hides before soaking.
tanning process has been calculated and given in Figure 4. Softness of leathers from the biocatalyzed three-step tanning process is similar or better than the control leathers. The ratings for fullness, grain flatness, grain smoothness, and grain tightness of the experimental leathers (E) are comparable to the control leathers (C). In general, the appearance of experimental leathers is analogous to the control leathers. Environmental Benefits. COD and TS have been chosen for analyzing the impact of the composite liquors on the environment. Table 7 provides the amount of water employed and the discharge for each process for processing 1 kg of raw hides. It is observed that the reduction in total water consumption is 44% and that discharge is reduced to 49% by employing an integrated three-step tanning method. It is apparent that the biocatalyzed three-step tanning method enjoys a reduction in water consumption by 77% and in
discharge by 87% as compared to the conventional leather processing, assuming that the green hides (unsalted) are processed till chrome tanning. It has been estimated that, by 2025 AD, 1.8 billion people will live in countries or regions with absolute water scarcity (36). In this context, the achievement of reduction of water consumption to 3.2 L for preserving 1 kg of green hides/skins permanently is remarkable. The COD and TS values and the calculated emission loads are given in Table 8. It is expected that the COD and TS values for the biocatalytic three-step tanning based leather processing have to be higher than the conventional leather processing due to the reduced water usage. Consequently, the COD and TS values of the composite liquor of biocatalytic three-step tanning process are higher than that of the conventional leather processing. This is not only due to the reduction in the amount of water employed but also the presence of enzyme (R-amylase) and noncollagenous proteins in a very low volume of water. Hence, it is relevant to discuss the consequent environmental problems in terms of emission load. The TS load up to chrome tanning for the conventional and three-step tanning process is 196 and 29 kg for processing 1 t of green hides. However, the input-output audit shows that the effluent contains 201 and 33 kg of solids (excluding suspended lime) as seen in Table 3. The lower TS values in the effluent load analysis could be due to the removal of the fraction of neutral salts along with the water present in the hide matrix. The reductions in the emission loads of COD and TS till chrome tanning for the biocatalyzed three-step tanning process are 80 and 85%, respectively, as compared to the conventional leather processing. In the case of composite liquors collected up to post-tanning, the reductions are 71 and 80%. These reductions, without compromising the quality of the leather, are remarkable in the context of greener environment. The discharge norms for the COD and TDS set by the pollution control boards in most of the countries are 250 and 2100 ppm, respectively (37). This means that the allowed emission loads for COD and TS are 6.2512.5 and 52.5-105 kg for processing 1 t of raw hides, assuming that the wastewater discharge is 25-50 L/kg of raw hides in the conventional leather processing. It is seen from Table 8 that the COD and TS loads from the biocatalytic three-step tanning process are 9 and 44 kg/t of green hides processed. The values are comparable to the lower end of the emission loads calculated from the discharge norms. This shows that the biocatalyzed three-step tanning process leads toward zero discharge of pollutants and obliquely meets the emission norms prescribed by the pollution control board authorities. Another important pollutant from the tannery wastewater is chromium(III). The chromium uptake is found to be 77 and 95% of the dose for the conventional and biocatalyzed three-step tanning processes. The chromium concentration of the spent chrome liquor from conventional and biocatalytic three-step tanning process is 2477 and 1490 ppm, respectively. Thus, it seems that the concentration of chromium in the composite liquor is 100 and 124 ppm for the conventional and biocatalyzed three-step tanning processes. Although the concentration of chromium is slightly more in the composite liquor from the biocatalyzed three-step tanning process as compared to conventional process, the emission load is reduced by 80% in the novel three-step process. Corresponding emission loads are 2.8 and 0.6 kg as chromium. This quantity is comparable to the amount of BCS in the effluent as estimated in Table 1. In most of the countries, the tanners are required to manage the chromium discharge at 2 ppm level (37). Hence, the segregation of the chrome liquor from the main stream is essential. Various options for the management of chromium include reduction in the BCS dose, direct recycling of segregated liquor to the next batch instead of freshwater, or recover the chromium VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 8. Composite Liquor Analysis for Control (C) and Experimental (E) Leather Processinga
type of liquor
process
COD (ppm)
TS (ppm)
vol of effluent (L/t of raw hidesb)
composite up to chrome tanning
C E C E
2172 ( 16 3412 ( 14 1986 ( 16 2361 ( 18
14766 ( 32 16605 ( 38 13868 ( 22 11478 ( 34
13305 1760 15700 3796
composite up to post tanning a
emission load (kg/t of raw hidesb processed) COD TS 29 6 31 9
196 29 218 44
Composite liquors were collected from all the processing steps excluding soaking. b Weight of hides before soaking.
TABLE 9. Cost Estimates of the Conventional (C) and Biocatalytic Three-Step Tanning (E) Processes (US$/t of raw hides) unit operations
control
lime sodium sulfide Biodart (SPIC) R-amylase (SPIC) ammonium chloride alkali bate sodium chloride sulfuric acid BCS sodium formate sodium bicarbonate
24.48 15.92
total
1.94 6.64 3.80 1.37 45.55 1.90 2.66 104.27
experimental 2.65 24.49 36.73
0.30 39.18
103.35
using a semi-continuous chrome recovery system and reuse (38). Techno-Economic Feasibility Study. The techno-economic analysis includes reductions in cost arising from the use of chemicals/bioproducts, power, area yield in leather, water, effluent treatment, and sludge disposal cost. It was evident from the input-output audit that the biocatalyzed three-step tanning process requires very low amount of chemicals as compared to the conventional process. It is known that the cost of bioproducts is generally higher than the common chemicals. Hence, it is important to compare the economic viability of the conventional and biocatalytic three-step tanning process. Costs of chemicals and bioproducts used in the conventional and biocatalytic threestep tanning processes are given in Table 9. It is surprising to see that the biocatalytic three-step tanning process provides a savings of 1 US$/t of raw hides processed. It is estimated that the total time required for the conventional and biocatalytic three-step tanning processes is 60 and 27 h, respectively, for tanning 1 t of green hides. This means that the interest on the investment for purchasing the raw hides and converting into leather is reduced significantly. The total running time of tanning drums is nearly 13 and 9.3 h, and the resultant power consumption is 390 and 280 kWh for the conventional and biocatalytic three-step tanning processes. This provides an additional saving of 10 US$/t of raw hides while processing through biocatalytic three-step tanning process. It has been shown that the use of enzyme for dehairing results in a 2% increase in area of the final leather (18). It has also been shown that the increase in area of the final leather would be 3-5% if the enzymes are used in leather processing (19). The biocatalytic three-step tanning process employs enzymes for both dehairing and fiber opening. The increased area of the leather provides a saving of 70 US$ (at 3% increase in area)/t of raw hides processed. Furthermore, the reduction in COD and TS loads would provide additional benefits in effluent treatment costs (19). Apart from this, the disposal of lime-bearing sludge generated through a conventional 2616
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chemical-based process (a global estimate of 1.4 million t/yr) causes both ecological and economic concerns. Considering the process followed in developed countries, where about 5% lime for liming and 8% sodium chloride for pickling are employed, the application of the present threestep tanning process leads to the reduction in chemicals usage by 96%, TS load by 84%, and formation of dry sludge by 62%. Consequently, the overall savings as compared with the tailored conventional process is 62 US$/t of raw hides against savings of 81 US$ in developing countries. Hence, the intervention of biotechnology in combination with pickle-basification-free chrome tanning leads to a viable eco-alternative to the conventional multistep leather processing adopted in both developed as well as developing countries because of its near zero discharge potential without compromising the qualities of the processed leather.
Acknowledgments P.T. thanks the Council of Scientific and Industrial Research, India, for providing a senior research fellowship to carry out the research work. The authors thank Dr. R. Rajaram for physical testing measurements.
Supporting Information Available Details of the official sampling portion for chemical/physical testing and scanning electron micrographs of the chrome tanned and crust leather samples derived from conventional and biocatalyzed three-step tanning. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Thanikaivelan, P.; Rao, J. R.; Nair, B. U.; Ramasami, T. Environ. Sci. Technol. 2002, 36, 4187-4194. (2) Germann, H. P. Science and Technology for Leather into the Next Millennium; Tata McGraw-Hill Publishing Company Ltd.: New Delhi, 1999; p 283. (3) Aloy, M.; Folachier, A.; Vulliermet, B. Tannery and Pollution; Centre Technique Du Cuir: Lyon, France, 1976. (4) Bienkiewicz, K. Physical Chemistry of Leather Making; Krieger Publishing: Malabar, FL, 1983. (5) Thanikaivelan, P.; Rao, J. R.; Nair, B. U. J. Soc. Leather Technol. Chem. 2000, 84, 276-284. (6) Bose, S. M. Leather Sci. 1955, 2, 140-144. (7) Dhar, S. C. Leather Sci. 1974, 21, 39-47. (8) Rosenbusch, K. Das Leder 1965, 16, 237-248. (9) Morera, J. M.; Bartoli, E.; Borras, M. D.; Marsal, A. Proceedings of the XXIV IULTCS Congress, London, 1997; p 436. (10) Sehgal, P. K.; Ramamurthy, G.; Muralidharan, C.; Gupta, K. B. J. Soc. Leather Technol. Chem. 1996, 80, 91-92. (11) Schlosser, L.; Keller, W.; Hein, A.; Heidemann, E. J. Soc. Leather Technol. Chem. 1986, 70, 163-168. (12) Valeika, V.; Bale`iu ˆniene¨, J.; Beleoˇka, K.; Skrodenis, A.; Valeikiene¨, V. J. Soc. Leather Technol. Chem. 1997, 81, 65-69. (13) Valeika, V.; Bale`iu ˆniene¨, J.; Beleoˇka, K.; Skrodenis, A.; Valeikiene¨, V. J. Soc. Leather Technol. Chem. 1998, 82, 95-98. (14) Munz, K. H.; Toifl, G. Das Leder 1992, 43, 41-46. (15) Streicher, R. Leder Hautemarkt 1987, 39, 7-12. (16) Herfeld, H.; Schubert, B. Das Leder 1975, 26, 117-128. (17) Rao, J. R.; Chandrasekaran, B.; Nair, B. U.; Ramasami, T. J. Sci. Ind. Res. 2002, 61, 912-926. (18) Thanikaivelan, P.; Rao, J. R.; Nair, B. U. J. Soc. Leather Technol. Chem. 2001, 85, 106-115.
(19) Shrewsbury, C. World Leather 2002, February, 40-42. (20) Legesse, W.; Thanikaivelan, P.; Rao, J. R.; Nair, B. U. J. Am. Leather Chem. Assoc. 2002, 97, 475-486. (21) McLaughlin, G. D.; Theis, E. R.; The Chemistry of Leather Manufacture; Reinhold Publishing Corp.: New York, 1945. (22) Thanikaivelan, P.; Rao, J. R.; Nair, B. U.; Ramasami, T. J. Cleaner Prod. 2003, 11, 79-90. (23) Vogel, A. I. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; Longman Inc.: Essex, 1989. (24) IUP 2, Sampling. J. Soc. Leather Technol. Chem. 2000, 84, 303309. (25) Echlin, P. In Scanning Electron Microscopy, Vol. 4; Heywood, V. H., Ed.; Academic Press: London, 1971; p 307. (26) IUC 8, Determination of chromic oxide content. J. Soc. Leather Technol. Chem. 1998, 82, 200-208. (27) Clesceri, L. S., Greenberg, A. E., Trussell, R. R., Eds. In Standard Methods for the Examination of Water and Wastewater, 17th ed.; American Public Health Association: Washington, DC, 1989. (28) Lokanadam, B.; Subramaniam, V.; Nayar, R. C. J. Soc. Leather Technol. Chem. 1989, 73, 115-119. (29) IUP 6, Measurement of tensile strength and percentage elongation. J. Soc. Leather Technol. Chem. 2000, 84, 317-321. (30) IUP 8, Measurement of tear loadsDouble edge tear. J. Soc. Leather Technol. Chem. 2000, 84, 327-329.
(31) SLP 9 (IUP 9), Measurement of distension and strength of grain by the ball burst test. Official Methods of Analysis; The Society of Leather Technologists and Chemists: Northampton, 1996. (32) Money, C. A.; Adminis, U. J. Soc. Leather Technol. Chem. 1974, 58, 35-40. (33) Gustavson, K. H. The Chemistry of Tanning Processes; Academic Press: New York, 1956. (34) Covington, A. D.; Menderes, O.; Brown, E. M.; Collins, M. J.; O’Duwole, A. Proceedings of the XXVI IULTCS Congress, Cape Town, South Africa, 2001. (35) Specification for Chrome Retan Upper Leather; IS 2961; Bureau of Indian Standards: New Delhi, India, 1964. (36) International Water Management Institute. Projected Water Scarcity in 2025; http://www.cgiar.org/iwmi/home/wsmap.htm (accessed June 2002). (37) Buljan, J. World Leather 1996, November, 65-68. (38) Sreeram, K. J.; Rao, J. R.; Sundaram, R.; Nair, B. U.; Ramasami, T. Green Chem. 2000, 2, 37-41.
Received for review December 28, 2002. Revised manuscript received March 28, 2003. Accepted April 2, 2003. ES026474A
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