John Sundar Paper

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Recovery and Utilization of Chromium-Tanned Proteinous Wastes of LeatherMaking: A ReviewV. John Sundar a , J. Raghavarao b , C. Muralidharan a & A. B. Mandalb

a Leather Process Technology Department, Central Leather ResearchInstitute, Council of Scientific & Industrial Research, Chennai, Indiab Chemical Laboratory, Central Leather Research Institute, Councilof Scientific & Industrial Research, Chennai, India

Available online: 04 Jul 2011

To cite this article: V. John Sundar, J. Raghavarao, C. Muralidharan & A. B. Mandal (2011): Recoveryand Utilization of Chromium-Tanned Proteinous Wastes of Leather Making: A Review, Critical Reviewsin Environmental Science and Technology, 41:22, 2048-2075

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Critical Reviews in Environmental Science and Technology, 41:2048–2075, 2011Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643389.2010.497434

Recovery and Utilization of Chromium-TannedProteinous Wastes of Leather Making: A Review

V. JOHN SUNDAR,1 J. RAGHAVARAO,2 C. MURALIDHARAN,1

and A. B. MANDAL2

1Leather Process Technology Department, Central Leather Research Institute,Council of Scientific & Industrial Research, Chennai, India

2Chemical Laboratory, Central Leather Research Institute, Council of Scientific & IndustrialResearch, Chennai, India

Hides and skins, by-products of the meat industry, are convertedinto a value-added product, leather, by tanners. Tanning essen-tially is the process of converting raw hides and skins into an impu-trescible substance. The tanning process has number of steps andgenerates significant quantities of by-products and wastes. Thesesolid and liquid wastes pose major a environmental problem if notmanaged effectively. Large-scale production systems are adopted forleather processing in clusters and, therefore, the industry receivesfocus of environmentalists and society. Consequently tremendouspressure is exerted by various pollution regulatory bodies. The hidesand skins are treated with chemicals, which cross-link the collagenfibers to form a stable, durable material. The chemicals used maybe derived from traditional vegetable products or inorganic metalsalts. During leather processing a number of size-reduction, lev-eling, and purification operations are carried out, which resultsin generation of untanned and tanned proteinous waste materi-als. The authors review various recovery processes and utilizationmethodologies of chrome-tanned proteinous solid wastes emanat-ing from leather processing operations.

KEY WORDS: buffing dust, chrome tanning, collagen, leatherprocessing, skins and hides

Address correspondence to V. John Sundar, Central Leather Research Institute, Adyar,Chennai 600020, India. E-mail: [email protected]

2048

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Recovery of Chromium-Tanned Wastes of Leather Making 2049

I. INTRODUCTION

The leather industry, which basically processes waste from the food-processing (meat-processing) industry, could easily be considered to be anenvironmentally friendly one if it were not for the large amounts of liquidas well as solid waste it generates. Leather products have been useful ma-terials since the dawn of human history. In the tanning industry, raw skinis transformed into leather by means of a series of chemical and mechani-cal operations. Leather processing technology has evolved naturally from atraditional practice to an industrial activity. The leather industry contributessubstantially to the pollution of the environment. Environmental challengesfrom leather processing arise from both the nature and the quantum of wastesdischarged (Gaidau et al., 2009; Popescu et al., 2008; Ramasami et al., 1998;Veeger, 1993). The leather-making process generates substantial quantitiesof solid, liquid, and gaseous wastes (Figure 1). Processing of one metricton of rawhide produces 200 kg of tanned leather, 200–250 kg of tannedwaste leather, 190–350 kg of nontanned waste, and 50,000 kg of wastewa-ter (Figure 2). Many cleaner processing approaches aimed at the reductionof liquid and solid wastes proved to be economically and environmentallybeneficial (Munz et al., 1997; Muralidharan et al., 2001; Sundar et al., 2008;Sundar et al., 2004; Sundar et al., 2002; Sundar et al., 2006; Taotao et al.,2009; Xuechuan et al., 2009).

There are numerous options available to contain liquid and gaseouswastes. They include in-process and end-of-pipe treatment solutions. At-tempts have also been made to combat the liquid discharge from the tan-ning processes to near zero levels. Solid wastes from the tanning industryare unavoidable at present. This is because leather processing is primarilyassociated with purification of a multicomponent, skin, to obtain a singleprotein, collagen. The intrinsic nature of the leather processing steps and thenature of chemicals employed are also responsible for the generation of cer-tain quantum of solid wastes. Most of these wastes are disposed of throughlandfill or incineration processes, although effective reutilization is greatlydesirable, as the methods of disposal involve economic and environmentallosses.

Chromium has been the dominant tanning agent for the better part ofthe century, and now accounts for approximately 85% of all leather pro-duced in the world. Other primary and combination tanning agents includevegetable tannin extracts; other metals such as aluminum, zirconium, andtitanium; aldehydes; organic syntans; and fish oil. An extensive worldwideresearch effort has been devoted to alternative tannages, but no satisfactoryreplacement for chromium has been or appears likely to be found (Jiao et al.,2009; Rutland, 1991; Sundar et al., 2001).

Tanning with chromium was invented in 1858 (Sarkar, 2005). Tanningis the main process that protects leather against environmental effects such

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2050 V. J. Sundar et al.

Process Waste

Raw hides

Leather

Pretanning

Tanning

Wet finishing

- Soaking - Fleshing - Unhairing + liming - Bating - Pickling

Trimmings, Fleshings

BOD, COD, SS, Salts, Organic N

H2S, NH3

- Chrome tanning - Sammying - Sorting - Splitting - Shaving

Shavings, Trimmings

BOD, COD, SS, Salts, Chrome

- Neutralisation - Retanning - Dyeing - Fatliquoring - Sammying - Setting - Drying

Trimmings

BOD, COD, Chrome, Dyes, Fat

Finishing - Conditioning - Staking - Buffing - Trimming - Finishing

Solid residue

Liquid residues

Solvents, Formaldehyde

Solid wastes; ---- Liquid wastes; ……. Gaseous emissions

Unprocessed Trimmings

FIGURE 1. Description of the tanning process and outputs.

as microbial degradation, heat, sweat, and moisture. Basic chromium sulfateis the most widely used tanning material for converting putrescible collagenfibers into a nonputrescible leather matrix (Gauglhofer, 1986; Groenestijnet al., 2002). One of the reasons for chromium being indispensable is thatno other single tanning material can give the desired characteristics to theleather. Hides that have been tanned with chromium salts have improvedmechanical resistance, extraordinary dyeing suitability, and better hydrother-mal resistance in comparison with hides treated with a vegetable tanningagent or other tanning agents. Chromium is bound to collagen by morethan one mechanism. However, all of the bound chromium contributes toan equilibrium that determines the thermal stability of the resulting leather.

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FIGURE 2. Environmental impact of leather processing (Chakraborty, 2003).

Chromium salts also have a higher rate of penetration into the interfibrillarspaces of the skins. This results in substantial reduction in process time andbetter quality of the finished product. Sulfates and chlorides are the moststudied chromium salts. Basic chromium chloride solutions contain a signif-icant proportion of polynuclear complexes. This results in poor diffusion ofchromium salts through the pelt, leading to superficial tanning. Chromiumsulfates contain sulfate radicals, which can readily penetrate the complexion and mask chromium. With basic chromium sulfates, products can beobtained easily and rapidly with a high molecular weight, giving colloidalsolutions of a high tanning ability. This is the reason why chromium sulfateis used in preference to chloride in the tanning process (Chagne et al., 1996).

Most tannery and other leather product wastes contain significantamounts of chromium, which is present exclusively as Cr (III) salts. Pro-duction of chrome-tanned leather is done exclusively with Cr (III), usuallyin the form of basic trivalent chromic sulfate [Cr (OH)(H2O)5]SO4. This pro-cess depends on the unique ability of Cr (III) to form stable, kinetically inertcoordination complexes, which can bind and cross-link hide protein fibers(collagen). Figure 3 shows the molecular structure of chrome-tanned leatherand shavings. Hexavalent chromium compounds are not tanning substances,as they do not have the ability to form such coordination complexes. In addi-tion, typical tanning formulas contain masking agents (organic acid radicalssuch as formate or acetate), which help control the tanning reaction and

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FIGURE 3. Molecular structure of chrome-tanned leather and shavings (Gammoun et al.,2007).

further ensure the chromium is exclusively trivalent. Given the high organiccontent of these waste materials, it is highly unlikely that soluble Cr (VI)compounds could even persist as such in environment (Brown and Taylor,2003; Kolomaznik et al., 2008).

The products resulting from the tanning are grain leather, usable splits (aproduct obtained on horizontal sectioning of leather), and a certain amountof unusable splits (i.e., chrome-containing solid waste). At the end of chrometanning about 75% of chrome offer (Cr2O3) remains in the collagen structure.Other chemicals and auxiliaries such as tensides, calcium, acids, and bases(in the form of soluble reaction salts) remain in wet blue leather in smallamounts (Buljan et al., 1999).

II. TECHNOLOGICAL SOLUTION FOR THE LEATHER WASTE

A. Chrome Shavings, Splits, and Trimmings

Because tanned hides are too thick for most purposes, they are split using amachine similar to a horizontal band saw. After splitting, the thickness of thehide must be uniform throughout. This is achieved with a shaving machineused in the process line. The helical-shaped cutting blades level the overallthickness to meet exact specifications and open the fiber structure to improvethe response to subsequent chemical processing. Such chromium-containingshaving and trimming wastes of wet blue leather amount to more than 10%based on the dry weight of hide and skin. Chrome shavings constitute 75%of the solid wastes containing chromium in the leather-making process. Ithas been estimated that about 0.8 million tons of chrome shavings could begenerated per year globally. This waste is partly used in the manufactureof leather board, but most are disposed of as landfill. However, such direct

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TABLE 1. Characteristics of chrome shavings (Swarnalatha et al., 2009)

Parameter Values

Moisture content 11.15 ± 2%Ash content 45.4 ± 3.8%Bulk density 0.16 ± 0.02 g cm−3

Fat 4.69 ± 0.26%Dermal 74.9 ± 3.6%COD 840 ± 58 mg g−1

TOC 315 ± 41 mg g−1

Chromium (III) 10.68 ± 1.98 mg g−1

Chromium (VI) BDL (< 1 µg g−1)Carbon 37.230 ± 3%Hydrogen 6.193 ± 0.7%Nitrogen 6.443 ± 0.72%Sulfur 1.422 ± 0.24%Oxygen 24.285 ± 3.1%

discharge is not accepted in many countries because of the chromium con-tent (Lipsett, 1982; Mukherjee et al., 2005). When the chrome shaving wascontacted with water, the chromium in it released into water in the concen-tration above the maximum allowable concentration limit given as 5 mg/Lby the U.S. Environmental Protection Agency. It was determined that theamount of chromium dissolved increased with the increasing contact timeand decreased liquid/solid ratio. A thermal stabilization process was appliedto the chrome shaving due to its high organic compounds content. Effectivestabilization of chromium in the chrome shavings was achieved by heating ofchrome shavings at 350 ◦C under the CO2 atmosphere. This was found to beenvironmentally stable and can be directly discharged (Erdem and Ozverdi,2008; Sempere et al., 1997). The characteristics of chrome shavings are listedin Table 1.

The chromium recovery from this waste is necessary for environmentalprotection and economic reasons. The costs connected with the gradualintroduction of clean technologies into the leather industry could to a certainextent be compensated for by the processing of leather waste (not onlyfrom tanneries but also from other leather processing industries) into rawmaterials, which could be used in other industries (Langmaier et al., 1999).

B. Treatment of Wastes

The most problem-causing type of solid waste is chrome-tanned leatherwaste, consisting mainly of shavings and trimmings, especially dyed or fin-ished and buffing dust. It is most often taken to industrial waste dumps.From such waste, acid rains release chromic compounds, which infiltrategroundwater. Their transition includes oxidation processes, which turn Cr+3

into Cr+6 compounds, which are carcinogenic in nature. This type of waste

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is therefore potentially risky and very expensive for storing in identifiedplaces. Studies have been carried out to investigate safe disposal, recov-ery of chromium and protein, and their reuse in various fields of industry.The processing of tanned waste can hardly in itself represent a financiallylucrative industry, but in connection with the growing pressure on usingclean—waste-free—technologies, their processing into secondary raw mate-rials is one of the preconditions for the survival of the leather industry.

1. INCINERATION

Shavings discharged as wastes from tanneries may cause environmental pol-lution by chromium regardless of whether they are incinerated or dumpedinto the ground. Chrome shavings have a significant amount of proteins(78.6–75.2%) and chromium oxide (4.4–4.3%). Attempts were made to de-compose organic components in shavings by wet air oxidation and separateonly chromium from the oxidized liquor. The separation of Cr (III) fromcollagen can be achieved by a combination of the protein (collagen) sta-bilization (protective cross-linking) and a subsequent labilization of the Cr(III) species bound to the protein. During the separation collagen retains thetertiary triple helical and higher (fibril) structures. The process of chromiumremoval takes place in a restricted aqueous environment, and it can be char-acterized as semihomogeneous or semiheterogeneous. Investigations wereperformed on the incinerating conditions of chrome leather scraps to re-duce the toxic gas compounds and on the effect of removing them usingthe scrubber. The amount of the various components of gas was within ac-ceptable levels and the recovered ash could be used as a material in theprocess of preparing bichromate (Imai and Okamura, 1991). The respectiveashes contain meaningful amounts of chromium and some in hexavalentform, and are used as a source of chromium for various applications, in-cluding pigment for ceramic glazes. Chromium recovered in the form ofsoluble chromate by oxidation has been reused in tanning by reduction withNa2SO3. This method was effective in chrome recovery, is environmentallyfriendly, and has economical benefits (Carneiro et al., 2003; Erdem, 2006;Ferreira et al., 1999; Okamura and Shirai, 1976; Poulopoulou et al., 1998;Tahiri et al., 2007). Swarnalatha et al. (2009) demonstrated that the organicfractions of chrome shavings could be incinerated without oxidizing trivalentchromium into hexavalent chromium through starved air combustion and forbetter recovery of energy.

2. ALKALINE HYDROLYSIS

Tanned solid wastes have a highly organized structure in the form of fibers,which are held closely to each other. A technological alternative for thedetanning of chrome wastes is through alkaline hydrolysis. The alkaline di-gestion of chromium wastes to recover protein products uses three alkaline

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agents; calcium hydroxide, sodium hydroxide, and sodium carbonate werefound to be effective. Alkaline agent concentration and reaction time weremain parameters and the comparisons of various alkali and enzymatic hy-drolysis showed that calcium oxide had advantage over MgO, NaOH, andenzymes plus MgO pretreatment. Analyses of amino acid compositions in-dicate the residue protein in chrome cake was difficult to hydrolyze com-pletely under alkaline conditions due to hydrophobic interaction, covalentbridges, or cross-linking induced by chromium/carboxyl groups complex-ation. A three-step hydrolysis process was helpful in extracting valuableproducts at different stages. The collagen hydrolysate obtained and chromecake had potential applications in the tanning industry (Basak and Vibhavari,2003; Cantera et al., 1997; Martinelango and Shelly, 2004; Mu et al., 2003;Saha et al., 2003; Tahiri et al., 2006; Zhao et al., 2009). Detanning also canbe done using strong oxidizing agents such as hydrogen peroxide or chlo-rine under slightly alkaline conditions to produce protein hydrolysates. Butthe less expensive way of detanning is treatment with acid. This technologyis developed for production-scale hydrolysis for commercial applications,as the endproduct had many potential uses (Amir et al., 2008; Heidemann,1991; Kosmac et al., 1995; Stockman, 1996).

Cot et al. (1999a, 199b) found that the presence of peroxochromatesgenerated in situ during the oxidation, in alkaline conditions, can producea partial hydrolysis to the fibers of collagen, accelerating the process ofisolation of gelatin. But alkaline digestion with sodium hydroxide allowed therecovery of proteins in the aqueous phase and the metallic salts in the solidphase. After lyophilization of the liquid, solid proteins with a high nitrogenpercentage and very low chrome content were recovered. The feasibility ofusing organic chelates as decontamination agents to isolate chromium fromthe organic matrix of leather waste was found to be efficient in alkaline mediaand potassium tartrate was found to be the important extracting agent. Theoxidative dechroming using hydrogen peroxide augmented with ultrasound,the chrome removal rate can be over 99%, which would greatly favor widerutilization of the collagen products obtained (Malek et al., 2009; Sun et al.,2003; Tahiri et al., 2004).

3. ENZYMATIC DEGRADATION METHODS

Enzymic processing of chrome shavings and trimmings is a viable treatmentand provides a 50–60% yield of hydrolysate, which shows low ash contentand a low content of chromic compounds. Commercially available prote-olytic enzymes used at moderate temperatures and for short periods of timeto give a chromium product and a protein product that has potential useas a fertilizer is a simple treatment that provides a practical and economicalsolution. Proteolytic enzymes, active at moderate temperatures, are effectivein solubilizing the protein and, as the reaction takes place at an alkaline pH,

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the chromium remains insoluble. The chemical composition of the isolatedproducts is dependent on the type of treatment and on the composition ofthe original chromium-containing leather waste product. The process can bemade cost-effective by reduction in enzyme concentration and isolation ofa gelable protein. In two-step process the chrome shavings are treated withalkali to isolate gelatin and in a second step, the enzyme is used to recoverthe remaining protein so that the chrome cake can be treated and recycled.Protein products with varying properties can be obtained using alkaline pro-tease with reproducibility (Taylor et al., 1998; Taylor et al., 1992; Taylor et al.,1994; Taylor et al., 1991; Taylor et al., 1990). The effects of various alkalinesalts on polypeptide size and the stability of triple helical conformations wereevaluated to reduce the amount of enzyme used in the recovery process andincrease the value of the protein product recovered. Pepsin behaved as amild enzyme with a controllable effect on leather wastes, trypsin gave a bet-ter yield, and isolated gelatin was of high quality and cost-effective. Gelatinproducts can be deionized and the recovered chrome cake was purified ina chemical process. This was used in the chrome tanning without decreasein the quality of the leather (Brown et al., 1996; Brown et al., 1994; Cabezaet al., 1999a; Cabeza et al., 1998a; Cabeza et al., 1999b; Cabeza et al., 1998b;Cabeza et al., 1997; Cabeza et al., 1999d; Taylor et al., 2000). To furtherthe potential utilization of chrome cake, aerobic biological degradation inan aqueous environment was developed (Dvorackova et al., 2007; Hrnciriket al., 2005; Kupec et al., 2002).

Sivaparvathy et al. (1986) explored the possibility of biodegradation ofchrome shavings using microbial enzymes (P. aeruginosa) to minimize thetime required for the hydrolysis. Using bovine pancreatic homogenate, asource of alkaline protease, for hydrolyzing the pretreated chrome shavingswas also found to be feasible. When enzyme Paecilomyces lilacinus wasused, the efficient removal of chrome and protein hydrolysate from shavingswas observed (Chakraborty, 2004; Sastry et al., 1999).

The combination of enzymatic and acid hydrolysis resulted in tailor-made, predefined molecular weight products fitting the intended use. Lowmolecular weight products are used as the plant biostimulator (Kasparkovaet al., 2009). The use of organic bases (Amines) increased the quality ofprotein hydrolysates and chrome sludge. Protein hydrolysate recovered andchanneled as an organic nitrogenous fertilizer to increase the yield of thecrop. Recovered chromic salts can be used to tan hides, as chromic pigmentsfor glass making, and in the manufacture of heat-resistant bricks (Kolomazniket al., 1999).

4. PYROLYSIS

Pyrolysis may be one of the alternative routes for the treatment of solid wastesfrom tannery wastes. Three different types of tannery wastes (chromium- and

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vegetable-tanned shavings, and buffing dust) can be pyrolyzed, resulting ingas, oil, ammonium carbonate, and carboneous residues. The gas can beused as fuel and the oil can either be used as fuel or as raw material forchemicals. The carbonaceous residue can be burned as fuel or safely dis-posed of. In addition, this residue is also suitable for production of activatedcarbon (Yilmaz et al., 2007). The elimination of the organic matter resultedin chromium-rich material, which could be suitably used as a ceramic pig-ment (Abreu and Toffoli, 2009; Lollar, 1981). The addition of coal ash fromfluid bed combustion technologies, at a suitable temperature and pH, re-sults in effective removal of Cr (III) compounds present in the wastes. Themethod is very simple, cheap, and effective, and could be used for differentcompositions (Bulewicz et al., 1997).

III. USES OF RECOVERED MATERIAL

A. Animal Feed/Chicken Feed/Fertilizer

The possibility of transforming recovered materials from chrome wastesinto useful proteins seemed to present an interesting challenge, being ofeconomic value as well. High-value-added, industrially reusable bioprod-uct whose application fields could include veterinary medicine, medicine,and pharmacology were isolated (Cot et al., 1986). Good-quality food-gradegelatin can be produced by hydrolysis if the hide has been tanned withconventional chrome tanning salts. Hydrolyzed protein, because of its highnitrogen content, has potential applications as animal feed additive, whichprovides food supplement amino acids (Bataille et al., 1983; Brown et al.,1996; Hauck, 1974; Smith and Donovan, 1982). Hydrolysates resemble gela-tine or glues, which are used in industry, mainly due to their easy gel-soltransition. This sol-gel ability is significant when used as a secondary rawmaterial (Langmaier et al., 1999; Langmaier et al., 2001). It is possible to pro-duce an almost chromium-free leather meal (less than 0.1 ppm Cr), which ishigh in protein with a high digestibility. Its amino acid pattern is of a betterquality than feather meal and is equal to meat meal and soybean meal withregard to cost competitiveness. Recovered protein also used as food supple-ment for Tilspia oreochromis niloticus (Boushy et al., 1991; Chakraborty andSarkar, 1999; Katsifas et al., 2004; Langmaier et al., 2002; Montoneri et al.,1994; Nogami et al., 2000; Reis and Beleza, 1991).

B. In Leather Processing

A new method was evolved for utilizing leather waste in preparing newertypes of surfactants from protein hydrolysates. These low-cost surfactantsare used in soaking and fatliquoring of leather processing (Narasimhanet al., 1980). Collagen hydrolysates have proven to be useful contributors

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to increase the chromium oxide content of the leathers at the end of thechrome-tanning process. Collagen hydrolysate in posttanning showed attrac-tive properties by its cosmetic, lubricating, and restoring effects by develop-ing a synergetic action with acrylic-retanning agents, by forming polyelec-trolyte complexes by enhancing the leather grain properties and providingsoftness and improved tensile strength. Collagen hydrolysates are also usedas dye exhaust aids (Afsar et al., 2009; Aslan et al., 2006; Cantera et al.,1997; Cantera et al., 1999; Cantera et al., 2002; Cantera 2003; Munoz et al.,2002). The gelatin has potential to be chemically modified to produce value-added leather finishing agents. Glutaraldehyde-modified low-quality gelatinand collagen hydrolysate was used in filling of veiny areas in chrome tannedcalfskin leather. Modified gelatin products are also used in leather process-ing, more specifically in preparation of coatings and fillers for leather (Caoet al., 2005; Chen et al., 2001; Crispim and Mota, 2003; Mu et al., 2003; Tayloret al., 2005). The dechromed collagenic residues can be transformed into araw material for the leather and paper industries as a substitute for caseinand as a pretanning/retanning agent (Cot et al., 2003). The high protein con-tent of chrome shavings has been utilized for reduction of chromium (VI)in the preparation of chrome tanning agent and developed products exhibitmore masking due to the formation of intermediate organic oligopeptides(Rao et al., 2002, 2004).

C. Adhesives/Cosmetics/Films

High-purity and demineralized protein hydrolysates have potential uses incosmetics (e.g., moisturizing creams, lotions, hair sprays) and biomedicalproducts (e.g., burn dressings, implant coverings; Cakl et al., 1998).

Historically, hydrolysis products from collagen, such as technical gelatinand animal glue, were used as eco-friendly adhesives, paints, encapsulatingagents, flocculating agents, and fireproofing agents. The gelatin was used inthe application of microencapsulation to microencapsulate drugs, essentialoils, perfumes, and other materials. Depending on the application of thegelatin, different modifiers can be used to get the desired functional proper-ties. Films were prepared from commercial gelatins by enzymatic treatmentand glycerol as a plasticizer to be used in the preparation of coatings, ediblefilms and sausage casings, and also in the packaging material. The additionof polyvinyl alcohol further improved the tensile strength and mechanicalproperties of films and were biodegradable (Cabeza et al., 1998a, b, c, andd; Cabeza et al., 1999a; Cabeza et al., 1999b; Cabeza et al., 1999c; Tayloret al., 1998; Taylor et al., 1997a; Taylor et al., 1995; Taylor et al., 1997b;Taylor et al., 2001; Taylor et al., 2002; Taylor et al., 2003). Modified proteinhydrolysates have potential to be used as good biodegradable films (e.g., foragriculture applications; Krncirik et al., 2009).

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D. Biological Uses

Shanthi and Shelly (2003) established the usefulness of the collagen hy-drolysate from chrome shavings as a support matrix for immobilization oforganophosphorus hydrolyse enzyme. The collagen hydrolysate representsa good mixture and a good source of amino acids as a carbon and nitrogensource (Aslan et al., 2007). The manufacture of chromium-enriched brew-ers yeast from chromium-containing leather waste is feasible because of itschromium (III) content, which is an essential trace element and cofactorfor insulin has important biological functions in metabolism (Liu and Yang,2006).

E. As Carbon

As collagen fiber is an abundant natural biomass, the preparation of porouscarbon fiber from collagen fiber as precursor is facile, cost-efficient, andsustainable. The collagen fiber, with hierarchical super molecular structures,can be used to prepare well-defined porous carbon fiber, which has largesurface area, high mesopore ratio, and controllable pore size, which hasgreat potential in selective adsorption and as chemical sensors and catalyst.In another method, the wet-blue leather waste after controlled pyrolysis wastransformed into chromium-containing activated carbons with microporousand mesoporous structure (Deng et al., 2008; Oliveira et al., 2008a).

F. In Waste Treatment

Tannery solid wastes are formed mainly by proteins and have a highly or-ganized structure in the form of fibers (� = 100 nm). The waste has thepotential to be used as a low-cost adsorbent for the removal of surfac-tants from wastewater (Na et al., 2006; Oliveira et al., 2007). The use ofchromium shavings also brings potential adsorbents for vegetable tanninsfrom mixed effluents. The tannin-bearing chrome shavings were reused inBCS preparation as reductant. The removal of dyes was also studied and thedye-adsorbed solid wastes were used for the preparation of pigments. Thechrome wastes have a possibility to remove the dyes from textile effluents(Saravanabhavan et al., 2004, 2007; Sreeram et al., 2004; Tahiri et al., 2002).Another ability of wet-blue shavings to remove motor oils, oily wastes, andhydrocarbons from contaminated water was studied and found that they arecapable of absorbing many times their weight in oil or hydrocarbons. Lowdensity, high buoyancy of fibers, porosity, and nontoxicity are the main ad-vantages of tanned solid wastes, and these wastes can be easily treated byincineration. The detanned hide powder also was utilized for the manufac-ture of an ecological agent suitable for removing environmentally hazardous

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organic substances. (Gammoun et al., 2007a;Gammoun et al., 2007b; Przepi-orkowska et al., 2003). The chrome wastes showed good potential for theremoval of chromate and arsenate from aqueous media. Fe (III)- and Al (III)-loaded adsorbents can be prepared by using skin-split waste of tannery assupporting matrix and can be transformed into low-cost and environmentalprotection material (Huang et al., 2009; Oliveira et al., 2008b).

G. In the Pigment Industry

The recovered chromium from solid wastes showed the possibility of pro-ducing commercial pigments for paints (Berry et al., 2001, 2002; Tahiri et al.,2001a; Tahiri et al., 2001b).

The possibility of thermoresistant pigments for the ceramic industrywas explored and the improvement of environment quality and decrease inpigments cost was noticed (Lazau et al., 2007).

H. In Energy

The tannery wastes disposed contain more than half of the energy valueof coal, and if recovered and converted into useful energy could satisfy allof a tannery’s own heat energy requirements for leather processing. So, anattempt was made to combine drying and gasification to eliminate tannerysolid wastes while providing combustible gas as a renewable energy sourcethat the tannery can directly reuse (Bowden, 2003).

I. In Ceramic/Asphalt Mixtures

The addition of chrome shavings, which is used in the production of Portlandcement, produced better distribution and pore volume results. Incineratedchrome-tanned shavings were characterized and used to develop a refrac-tory product with the addition of ashes. The technological properties of theobtained product were also evaluated and found to be beneficial (Basegioet al., 2006; Basegio et al., 2009; Trezza and Scian, 2007). Krummenauerand Andrade (2009) investigated the usability of leather sawdust in asphaltmicrosurface layer mixtures and found it to be technically and economicallyfeasible. Biocomposite layers of silica obtained from coatings of silica solsmixed with proteins in water/dioxane revealed that such coatings are highlybiocompatible, with excellent mechanical properties (Brasack et al., 2000).

J. In Chromate Manufacturing

The possibility of substituting chromic ore (the typically used natural rawmaterial) with chromic waste in the sodium chromate production process

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has been proven (Kowalski and Walawska, 2001; Vieira and Marcilio, 2004;Walawska and Kowalski, 2000, 2001).

K. In Plastics/Rubber/Resin

Successful commercial applications for the hydrolysate include its use inthe manufacture of biodegradable polymers for agriculture sowing tapes.Potential applications include its use in the production of antiskid agentsin PVC and rubber compounds, agents for increasing adhesion to textilebacking in conveyor belt manufacture, heat stabilizers in PVC paste, andadditives to concrete and plaster (Kolomaznik et al., 1999; Pearson, 1982).

There has been an increasing interest in the development of biodegrad-able polymers owing to the growing problem of waste disposal of plastics.Protein hydrolysate is easily and uniformly blended with metallocene-basedlinear low-density polyethylene (mLLDPE) and the obtained polymer filmretains mechanical strength properties and biodegradability behavior. Thehydrolysate is also used to modify biodegradable plastics. The film basedon polyvinyl alcohol (PVA) is modified with protein hydrolysate to be usedin agricultural applications (Kresalkova et al., 2002; Saha et al., 2003; Shinet al., 2007).

The utilization of collagen as a filler for isoprene and butadiene-acrylonitritle rubber mixes was found to have good resistance to aging andalso provides increased recaptivities to microbiological decomposition. Theleather powder mixed with zinc oxide is the best form of its addition torubber mixes. The mechanical properties of collagen-added carboxylatedbutadiene-acrylonitrile rubber indicates an increase in tensile strength andelongation at break (Przepiorkowska et al., 2006; Przepiorkowska et al.,2007; Przepiorkowska et al., 2004; Przepiorkowska et al., 2005). The chromewastes used as filler in a polymer matrix and PVC–leather fiber compositeswere prepared due to their leather-like appearance. The leather-like sheetsare flexible and exhibit sufficient water absorption to be suitable for severalapplications in the footwear and clothing industry (Santana et al., 1998). Fur-ther, the short leather fibers were modified by emulsion polymerization ofmethyl methacrylate (MMA) in order to increase the compatibility of leatherfibers with several commodity polymers used in shoe industries. The treat-ment significantly improved the thermal stability of fibers and reduced wateradsorption capacity, as a coating of PMMA is produced over the leather sur-face (Babanas et al., 2001; Santana et al., 2002). By reacting hydrolysateswith glutaraldehyde, inexpensive UF adhesives was developed with reducedformaldehyde emissions (Langmaier et al., 2003; Langmaier et al., 2004a;Langmaier et al., 2004b).

The application of collagen hydrolysates in the cross-linking of epoxideresins resulted in solvent-free biodegradable epoxide films. Cross-linkingof hydrolysate with epichlorhydrin is an advantage for applications such

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as degradable packing materials. Collagen hydrolysate was plasticized withglycerol and low-molecular-weight polyethylene glycols, and the resultanthydrophilic plasticizers efficiency and their action mechanisms are useful forthe production of soft capsules on collagen hydrolysate (Langmaier et al.,2006; Langmaier et al., 2008; Langmaier et al., 2007).

L. In Leather Boards

Chrome-shaving wastes were used to make leather boards for industrial pur-poses. Chrome tanned grain shavings, trimmings, and splits, in combinationwith cotton and kraft pulp, were used to manufacture good-quality leatherboards, which are strong, flexible, and have a high elongation at break(Crispim and Mota, 2003; Gish, 1999; Okamura and Shirai, 1972).

IV. BUFFING DUST

During the leather manufacturing process, the inner-collagen layer of the fin-ished leather is buffed to get a smooth and fine feel. This process generates afine powder of collagen fibrils in large quantities in leather industries, whichis called buffing dust. Buffing dust is a proteinous solid waste impregnatedwith chromium, synthetic fat, oil, tanning agents, and dye chemicals, whichis generated during manufacture of leather. About 2–6 kg of buffing dust isliberated as a solid waste per ton of skin or hide processed. Buffing dust is amicrofine solid particulate and carries about 2.7% chromium on dry weightbasis. It is carcinogenic in nature and it causes clinical problems such as res-piratory tract ailments, as well as allergic dermatitis, ulcers, perforated nasalseptum, kidney malfunctions, and lung cancer in humans. The characteristicsof buffing dust are listed in Table 2.

The present practice of disposing of buffing dust consists of (a) incin-eration and (b) land codisposal. Incineration causes serious air pollutionproblems because of the release of toxic Sox and Nox gases, and it has beenobserved that at 800 ◦C about 40% of Cr (III) is converted into Cr (VI). Theland codisposal method poses a threat to groundwater resources becausethe dust carries organic compounds that can be leached by solvents andwater at pH 5–9. The leachates from buffing dust interact with soil particles

TABLE 2. Characteristics of buffing dust (Sekaran et al., 1998)

Parameter Value

Carbon 461 mg/gTKN 93 mg/gChromium 28 mg/gAsh 88.49 mg/g

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to a level affecting physicochemical characteristics of soil. So there are manystudies undertaken for possible use of buffing dusts.

An investigation was done by subjecting chrome buffing dust to starvedincineration and the bottom ash was effectively solidified and stabilized us-ing Portland cement and fine aggregate. The study indicated that the organicfractions of chrome buffing dusts can be destructed without oxidizing triva-lent chromium into hexavalent chromium through starved air incineration.The solidified block can be used as low-cost hollow cement blocks for itscompressive strength and metal fixation capacity (Swarnalatha et al., 2008).

Conversion of buffing dust into activated carbon is a rational idea ofcombating solid waste pollution and is able to remove more dye, comparedwith commercially activated carbon. Activated carbons with high adsorptioncapacity have good market potential for use as the tertiary treatment ofwastewater because they can decrease the cost of operation and upgradethe quality of the treated effluents. Buffing dusts have proven to be a muchbetter adsorbent than chrome shavings for cationic dyes. Buffing dusts ofcrust leather wastes are capable of absorbing oil many times their weight(Gammoun et al., 2007b; Sekaran et al., 1998; Tahiri et al., 2003).

The thermoplastic polymer, acrylonitrile-butadiene-styrene (ABS), wasused as the matrix and leather buffing powder material as the filler to pre-pare a particulate-reinforced composite and the process indicated reducedcost, resource utilization, and environmental benefits (Ramaraj, 2006). Buff-ing dust also was used as an active filler of XNBR and NBR vulcanizates andfound higher cross-linking density, better strength properties, and higherhardness, as well as lower elasticity and shock absorption (Chronska and

TABLE 3. Critical appraisal of waste management methodologies

Method ofType of waste treatment Benefits/Limitations

Chrome shavings Incineration • Generation of toxic gases• High ash content• Need for starved air combustion to

prevent Cr (VI) formationChrome shavings Alkaline hydrolysis • Possibility of complete hydrolysis

with chemicals such as sodiumhydroxide

• The resultant chrome cake also canbe reused in tanning sector.

Chrome shavings andtrimmings

Enzymaticdegradation

• Products with low ash content• Shorter duration• Variety of products can be obtained

through controlled hydrolysis formultiple end uses

Chromium/vegetabletanned shavings andbuffing dust

Pyrolysis • High energy requirement• Not cost competitive

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TABLE 4. Application of various recovered products

Waste Product Application

Chrome waste Proteins Veterinary, medicine, pharmacologyGelatin Animal/chicken mealProtein hydrolysates Cosmetics/adhesives/films/biodegradable

polymers/rubber, leather processingCollagen fiber/

hydrolysatesCarbon/nitrogen source

Protein fiber Removal of oil wastesChromium Pigments/chromate production

Chromium waste EnergyIncinerated chrome

shavingsCement/ceramic/asphalt mixtures

Chromium splits/shavings/trimmings

Leather boards

Buffing dust Powder Polymer filler, absorbent of dyes/oilAsh Portland cement, activated carbonSheet Wallets, key cases

Przepiorkowska, 2008). A leather-like sheet has been developed using acombination of natural and synthetic polymers for its use in manufacture ofleather-like products such as wallets and key cases (Vedaraman et al., 2002).

V. CONCLUSION

The leather industry has outgrown the principal meat industry in termsof global turnover. Most solid waste generated from tannery industries ispresently unutilized and wasted. Leather wastes can be recovered and pro-cessed at industrial scales to generate value-added products, which canemerge as an important commercial activity on its own while eliminatingenvironmental concerns of the leather process industry. A critical appraisalof various waste management methodologies and application of recoveredproducts is provided in Tables 3 and 4.

The growing environmental concerns pose a new challenge to leatherchemists, calling for renewed research and development to facilitate suste-nance of the traditional industry. Generation of industrial bioproducts anddevelopment of biomedical applications for some of them would ensurehigher profitability for leather making or lead to the development of neweconomic models exclusively for waste processing.

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