conversion of leather wastes to useful products

Resources, Conservation and Recycling 49 (2007) 436–448

Conversion of leather wastes to useful products

Onur Yılmaz a, I. Cem Kantarli b, Mithat Yuksel c,Mehmet Saglam c, Jale Yanik b,∗

a Department of Leather Engineering, Faculty of Engineering, Ege University,35100 Izmir, Turkey

b Department of Chemistry, Faculty of Science, Ege University, 35100 Izmir, Turkeyc Department of Chemical Engineering, Faculty of Engineering, Ege University,

35100 Izmir, Turkey

Received 7 October 2005; received in revised form 18 April 2006; accepted 24 May 2006Available online 7 July 2006

Abstract

The main objective of the present study is to investigate the production of useful materials fromdifferent kinds of leather waste. Three different types of tannery wastes (chromium- and vegetable-tanned shavings, and buffing dust) were pyrolyzed in a fixed bed reactor at temperatures of 450 and600 ◦C under N2 atmosphere. Gas, oil, ammonium carbonate and carboneous residue were obtained bypyrolysis. The effect of temperature and type of leather waste on product distribution of pyrolysis wasinvestigated. Buffing dust gave the highest yield of oil (ca. 23%), while other wastes recorded yieldsof ca. 9%. Results of elemental analysis and column chromatography showed that pyrolysis oils couldbe used as fuel or chemical feedstock after re-treatment. The yields of carboneous residue (chars) werebetween 37.5% and 48.5% and their calorific value was between 4300 and 6000 kcal kg−1, suitablefor use as solid fuel. In addition, these chars were activated by CO2 to obtain the activated carbon.The activated carbon having highest surface area (799.5 m2 g−1) was obtained from chromium-tannedshavings. Activated carbons prepared from chromium-tanned leather were presented as an adsorbantfor the adsorption of dyes from aqueous solution.© 2006 Elsevier B.V. All rights reserved.

Keywords: Leather wastes; Biomass; Pyrolysis; Activated carbon

∗ Corresponding author. Tel.: +90 232 3884000x2386; fax: +90 232 3888264.E-mail address: [email protected] (J. Yanik).

0921-3449/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.resconrec.2006.05.006

mailto:[email protected]

dx.doi.org/10.1016/j.resconrec.2006.05.006

O. Yılmaz et al. / Resources, Conservation and Recycling 49 (2007) 436–448 437

1. Introduction

Leather industry is the one of the wide spread industries of Turkey. But, environmentalpollution is the main problem for leather industry. The leather making process generatessubstantial quantities of solid and liquid wastes (hides and skins, fats, shaving and trimmings,buffing dust, process effluents, sludge). The most common way to manage solid wastes isby disposing of them on land sites.

It is reported that the tanned wastes of 0.22 kg/kg of wet salted hides/skins is generatedper year in Turkey. Since the chromium metal is the most important tanning agent, thesolid wastes from chromium-tanned leather requires special attention because of the largeamount produced and because of the legislative restrictions. In literature, there are manystudies on the treatment of tannery wastes. Most of these studies concerns the extractionof chromium from wastes to re-use in the tanning process (Cassano et al., 1997; Imai andOkamura, 1991; Petruzelli et al., 1995; Sivaparvathi et al., 1986a, 1986b) and isolation ofprotein fractions (Mu et al., 2003; Cabeza et al., 1996, 1997, 1998).

On the other hand, pyrolysis may be one of the alternative route for treatment of solidwastes from tannery wastes. Pyrolysis have being widely applied to organic wastes, such asagricultural wastes, scrap tyres, sewage sludges and plastic wastes. The pyrolysis processinvolves heating the carboneous material in an inert atmosphere. The products of pyrolysisare gas, oil and carbonaceous residue. The gas can be used as fuel and the oil can eitherbe used as fuel or as raw material for chemicals. The carbonaceous residue can be burnt asfuel or safely disposed of—since the heavy metals are fixed in the carbonaceous matrix. Inaddition, this residue is also suitable for production of activated carbon.

Activated carbons can be produced from carbonaceous materials by chemical (Girgisand Ishak, 1999; Jagtoyen and Derbyshire, 1998; Namasivayan and Kadirvelu, 1997; Philipand Girgis, 1996) and physical activation (Gonzalez et al., 1997; Lua et al., 2002; Miguelet al., 2003; Namasivayan and Kadirvelu, 1997). The physical activation method involvespyrolysis of the raw material and the subsequent activation at high temperature in a carbondioxide or steam atmosphere. The chemical activation method involves the pyrolysis of theraw material previously impregnated with a chemical agent such as zinc chloride, phosphoricacid, potassium hydroxide, etc. A large number of agricultural by products such as coconutshells, palm-kernel shells wood chips, sawdust, corn cobs, seeds, etc., have been successfullyconverted into activated carbons. The qualities and characteristics of activated carbonsdepend on the properties of the starting materials as well as the activation methods andprocesses.

In literature, there are few studies related to pyrolysis of tannery wastes. Caballero et al.(1998) carried out a kinetic analysis of the global thermal decomposition of leather. Thepyrolysis of chromium-tanned leather was modeled assuming that it was formed by twodifferent fractions which decompose by two independent reactions. They also studied onthe pyrolytic products evolved from the thermal degradation of chromium tannery wastesby two stages pyrolysis (Font et al., 1999). They concluded that the formation of pyrolyticproducts was influenced by the operation conditions (temperature, heating rate). They alsodetected significant levels of ammonia, hydrogen cyanide and sulfur dioxide.

On the other hand Martinez-Sanchez et al. (1989a, 1989b) and Martin-Martinez et al.(1989) prepared the activated carbons by carbonization in nitrogen at 900 ◦C followed by

438 O. Yılmaz et al. / Resources, Conservation and Recycling 49 (2007) 436–448

activation at 825 ◦C in carbon dioxide of chromium-tanned leather. The porous texture ofcarbons has been characterized by adsorption of N2, CO2 and iso-butane. To investigatethe effect of Cr2O3, they used also both pickled leather (without chromium) and extractedleather (after chromium extraction) as carbon precursors (Martinez-Sanchez et al., 1989b).The micropore volume (corresponding N2 at 196 ◦C) of activated carbon from chromium-tanned leather was 0.30 cm3 g−1 where as activated carbon from pickled leather had a verylow adsorption capacity. They concluded that tanning process favors the formation of thebasic structure needed to form a proper activated carbon. On the other hand, activated carbonfrom extracted leather exhibited a very high adsorption capacity. It was mentioned that theCr2O3 particles were partially blocking micropores in carbons and the adsorption capacity ofcarbons could be increased by elimination of chromium via previous extraction of the leather.

Taking the above considerations into account, the aim of this work was to investigatethe production of useful materials from different kinds of leather wastes by pyrolysis. Aprimary focus of the paper is on the production of activated carbon and investigation of itsaqueous adsorption characteristics.

2. Materials and methods

2.1. Materials

The tannery wastes – chromium- and vegetable-tanned shavings and buffing dust – weresupplied by Sepiciler Co., Izmir, Turkey. The chromium- and vegetable-tanned shavings(CTS and VTS) were shredded into the rectangular pieces (1 cm × 0.5 cm). Buffing dusts(BD) was used as received. Some properties of wastes are given in Table 1.

2.2. Pyrolysis procedure

Pyrolysis experiments were carried out under nitrogen atmosphere at the temperaturesof 450 and 600 ◦C. The pyrolysis experiments were performed in a fixed bed design and

Table 1The properties of tanned shavings and buffing dust

Chromium-tanned Vegetable-tanned Buffing dust

Proximate analysis (wt.%)Moisture 7.1 5.9 5.2Volatile matter 67.0 59.0 67.7Ash 9.6 3.9 6.7

Ultimate analysis (dry, wt.%)C 44.3 52.4 42.8H 3.1 0.9 6.1N 14.2 6.6 11.0S 1.8 1.1 2.1Oa 36.6 39.0 38.0

a From difference.


stainless steel reactor (L, 210 mm; Ø60 mm) under atmospheric pressure using a semi-batchoperation. The reactor was purged before experiments by nitrogen gas flow of 25 ml/minfor 10 min to remove air inside. In a typical pyrolysis experiment, a quantity of 50–60 g ofleather wastes was loaded and then the reactor temperature was increased by a heating rateof 5 ◦C/min up to pyrolysis temperature and hold for 2 h at the desired temperature. Thenitrogen gas swept the volatile products from the reactor into the condensation unit wherethe crystals of ammonium carbonates were accumulated on the wall of connection glassline and traps and the liquid products were condensed in the traps. The non-condensablevolatiles (gases) were vented to the atmosphere.

The liquid product contained the aqueous and organic phase. The aqueous phase in liquidproduct was separated from the organic phase (oil) by centrifugation. In each experiment;char, oil, (NH4)2CO3 and aqueous fraction yields were determined by weighting the amountof each obtained and calculating the corresponding percentage. Gas yield was calculatedby the difference.

2.3. Demineralization and activation of char

The char obtained from the pyrolysis was demineralized to decrease its inorganic content.Thus, char was treated with HCl solution (10 wt.%) at 100 ◦C for 2 h and then it waswashed with distillated water until no chlorine ions could be detected and dried at 100 ◦Cfor 24 h.

Activated carbons were prepared from chars by physical activation method. Activationprocess was carried out in the pyrolysis reactor by carbon dioxide. In activation process, non-demineralized and demineralized char was heated up to 900 ◦C under a flowing (25 ml/min)nitrogen atmosphere. When 900 ◦C was reached, the inert atmosphere was rapidly substi-tuted by flowing carbon dioxide (350 ml/min). The tested activation times were 4, 6, 8 and10 h. At the end of desired the activation time, reactor was cooled to room temperature undernitrogen atmosphere. The resulting carbons (activated carbon) from activation process wereweighted to calculate the burn-off value.

2.4. Analysis

Elemental analysis (C, H, and N) of wastes used and oils was determined with an elemen-tal analyzer (Carlo Erba 1106). Sulfur amount in oil was determined by using UltravioletFluorescence according to ASTM D5453. The asphaltenes of the oil were precipitated withn-hexane and soluble in n-hexane portions were fractioned by column chromatography intoaliphatic, aromatic and polar fractions by using hexane, toluene and methanol, respectively(Yanik et al., 1995).

The BET surface area of the activated carbons was calculated from the adsorptionisotherms by using Brunauer–Emmett–Teller equation. The BET (Brunauer–Emmett–Teller) surface area measurements were obtained from nitrogen adsorption isotherms at77 K using a Micrometrics FlowSorb II-2300 surface area analyzer. The scanning electronmicroscopy (SEM) analyses were performed on some of activated carbons using JEOL-FEGwith EDS detector. The ash content and calorie values of activated carbons were determinedby using the following standard methods: ASTM D3172, ASTM D 5865.


2.5. Aqueous adsorption characteristics

The methylene blue and phenol adsorption isotherms were carried out using a batchequilibration technique in a 250 ml conical flask at room temperature. Each flask was filledwith 100 ml of methylene blue (MB) or phenol at a known concentration (ranging between100 and 400 mg l−1 for MB and 25–200 mg l−1 for phenol) and 0.1 g of activated carbon.The flask was then shaken for a determined equilibrium time (24 and 4 h for MB andphenol, respectively) and filtered through Whatman No.1 filter-papers. The filtrate wasanalyzed for adsorbate concentration using the UV spectrophotometer at λmax (665 nm).Selected activated carbon was investigated for their aqueous adsorption characteristics usingphenol and MB. Residual concentrations were determined using Perkin-Elmer UV–visspectrophotometer at 665 nm (MB) and 269 nm (phenol).

3. Results and discussion

3.1. Pyrolysis yields

The product distributions from pyrolysis of leather wastes at different temperatures arepresented in Table 2. It can be seen that the effect of temperature on product yield varieddepending on the waste type. In the case of the pyrolysis of chromium-tanned shaving (CTS)and buffing dust (BD), high temperature led to an increase in gas yields but a decrease inchar yields. However, in the case of vegetable-tanned shaving (VTS), the gas yield did notsignificantly changed when the temperature increased from 450 to 600 ◦C.

Table 2Product yields from the pyrolysis of leather wastes (wt.%)

Waste type

Chromium-tanned Vegetable-tanned Buffing dust

Temperature = 450 ◦CGasa 17.8 22.4 16.8

LiquidAqueous 21.1 21.5 14.7Oil 10.9 7.7 22.2

Char 44.5 48.5 40.1(NH4)2CO3 5.7 Nil 6.2

Temperature = 600 ◦CGasa 23.6 23.6 20.0

LiquidAqueous 19.5 23.5 15.1Oil 9.4 8.9 23.5

Char 38.1 43.8 37.5(NH4)2CO3 9.3 0.3 3.9

a Calculated from mass balance.


It must be noted that the formation of (NH4)2CO3 was mainly observed in the pyrolysisof CTS and BD. It may be assumed that (NH4)2CO3 is formed via reaction between NH3and CO2 in the cold zone (connection lines and the wall of traps). As known NH3 isformed from the degradation of polypeptides and the carbon dioxide is formed from thedecarboxylation of carboxylates. The fact that the formation of (NH4)2CO3 from chromium-tanned wastes rather than from vegetable-tanned wastes may be explained as follows. Sincethe decarboxylation can be more easily occurred in presence of chromium, the evolution ofNH3 and CO2 might take place at the same time. Therefore, they can be reacted with eachother. But, in degradation of vegetable-tanned wastes, decarboxylation might be delayed. Forthis reason, the formed NH3 at first was condensed in aqueous phase, without any reaction.

In pyrolysis of CTS at 600 ◦C, the amount of (NH4)2CO3 was higher than that of at450 ◦C. This shows that high temperature improves the decarboxylation. The increment intemperature decreased the formation of (NH4)2CO3 in the case of BD. This difference canbe mainly due to the particle size. It is known that particle size is one of the parametersthat influence the formation of pyrolytic products, for example mass transfer restrictions tothe volatile evolution and escape of the evolved volatiles from the inside of particle. Dueto the high reactivity of small particles, evolved NH3 can easily further react with otherdecomposition products at high temperatures.

The particle size also affected the oil yields obtained from pyrolysis. For two temperatures(450 and 600 ◦C), the maximum oil yields were obtained from pyrolysis of BD.

3.2. Oil properties

Pyrolysis liquids can be separated into water soluble and organic phases, of which latter,named as oil, consists of mainly a brown tar containing the high molecular weight com-pounds, while the water-soluble fraction contains the lower molecular weight substances.

Elemental composition of oils obtained from pyrolysis of leather wastes is shown inTable 3. The oils from pyrolysis of chromium-tanned shaving and buffing dust were denotedas CTSO and BDO, respectively.

The biomass-derived oils is consistent of very complex mixture. To characterize thepyrolysis oil, the oil was fractionated as asphaltenes, aliphatics, aromatics and polars. Thecomposition of oils obtained from BD is shown in Table 4.

Table 3Elemental composition of oils obtained from leather wastes pyrolysis (wt.%)

Pyrolytic oil type

CTSO BDO

450 ◦C 600 ◦C 450 ◦C 600 ◦C

C 63.1 59.3 65.9 68.4H 1.0 6.8 3.7 6.3N 14.2 15.0 8.5 7.2S 0.3 0.4 0.5 0.5Oa 21.4 18.5 21.4 17.6

a From difference.


Table 4Composition of oils obtained from pyrolysis of buffing dust at 450 and 600 ◦C (wt.%)

450 ◦C 600 ◦C

Asphaltene 54.8 59.7Aliphatics 11.9 11.5Aromatics 7.0 4.5Polars 26.3 22.2

As seen from Table 4, oil contained large amount asphaltenes and the pyrolysis temper-ature had no considerable effect on the composition of oil. The amount of aromatic andpolar fractions slightly decreased as temperature increased. This may be due to the repoly-merization of aromatics and polars to asphaltenic compounds. The unchanged amount ofaliphatic compounds shows that these compounds are resistant in the pyrolysis conditions.

The high asphaltene and polar contents of oil affect the quality of pyrolysis oil and itsuse. For this, the oil obtained from leather waste can be used as fuel or chemical feedstockafter re-treatment, such as, steam cracking, hydrogenation, Fisher–Tropsch synthesis, etc.

3.3. Char properties

Pyrolysis of leather wastes gave the char yields of about 40–48%. The char consisted ofcarboneous material generated during the leather thermal decomposition, polymerizationand polycondensation reactions occurring in the pyrolysis reactor and almost all of theinorganic compounds originally present in leather wastes.

In principle, three methods can be used for the disposal of char obtained from pyrolysisof any carboneous material, such as waste sludges, agricultural wastes, etc. (Inguanzo etal., 2002). The char can be used as fuel, alone or mixed with other fuels. It can be used as acheap adsorbent. As third option, it could be safely disposed of by landfill, since the heavymetals present in char are relatively resistant to natural lixiviation (Caballero et al., 1997).

The resulting carbonaceous residue of wastes (both CTS and VTS) was in solid block,which was easily disintegrable into small particles. However, the char from BD was a finedispersed material. The main fractions were 1–5 mm and their share was about 49 wt.% ofthe total char mass.

The calorific value and ash content of chars are given in Table 5. Because of the low ashcontent, the char derived from VTS had the highest calorific value, which is comparableto those of lignite. It could be used alone as solid fuel; the others could be incinerated bymixing with coal.

3.4. Demineralization and activation of chars

One of the aims of this study was to produce activated carbon from solid residue of leatherwastes pyrolysis. In contrast to studies done by Martinez-Sanchez et al. (1989a, 1989b) andMartin-Martinez et al. (1989), carbonization step of activated carbon production was carriedout at low temperatures and the chars were demineralized by acid treatment before activationstep in this study.


Table 5Some properties of washed and unwashed chars with acid

Pyrolyis temperature

450 ◦C 600 ◦C

Before washing After washing Before washing After washing

Ash content (wt.%)Char from CT 19.5 8.5 22.6 16.2Char from VT 8.0 2.3 9.7 4.2Char from BD 14.0 4.7 15.5 13.5

Calorific value (kcal kg−1)Char from CT – 4578 – 4314Char from VT – 5919 – 6009Char from BD – 4338 – 4884

By demineralization, the ash content of chars obtained by pyrolysis at 450 ◦C was con-siderably decreased, however acid washing had no considerable effect on demineralizationof chars obtained at 600 ◦C, especially obtained from CT and BD. This shows that inor-ganic materials in leather wastes were converted to acid-insoluble form at high pyrolysistemperature.

As expected, the maximum decrease in ash content by acid washing was obtained inchars derived from VT (Table 5); the ash content of chars obtained at 450 and 600 ◦C wasdecreased at the ratio of 71.2% and 56.7%, respectively.

To obtain the activated carbon, acid washed and non-washed chars were activatedwith CO2. Fig. 1 shows the influence of activation time on the degree of burn-off inCO2 achieved for both washed and non-washed chars. The type of char used in each runis seen in Table 6. The chars from pyrolysis at 450 ◦C of buffing dust, chromium- andvegetable-tanned shaving were denoted as CBD-450, CCT-450 and CVT-450, respectively,in Table 6. The carbon burn-off exhibited an increase with increasing activation time for

Fig. 1. Carbon burn-off in CO2 for chars obtained from pyrolysis of leather wastes.


Table 6Surface areas of activated chars

Char type Activation time (h) BET surface area (m2 g−1)

C-1, –CBD-450 (washed with acid) 10 617.2C-1 (CBD-450, washed with acid) 8 551.6C-2 (CCT-450, washed with acid) 10 799.5C-3 (CCT-450, unwashed with acid) 6 82.9C-4 (CCT-600, unwashed with acid) 6 61.8C-5 (CVT-600, unwashed with acid) 8 295.0

both demineralized and non-demineralized of chars. As can be seen from Fig. 1, in thecase of demineralized CCT-450, a burn-off value of 70% was obtained for the activationtime of 10 h. However, an activation time of 6 h was enough to reach the similar burn-offvalue in the case of non-demineralized char (C-1 and C-2). It is clear that acid treatmentdecreased the reactivity of char, since char reactivity is related to the ash content of thecarbonaceaus materials (Ucar et al., 2005). Several researchers have also mentioned thatsome inorganic compounds showed catalytic effect on gasification (Cazorla-Amoros et al.,1996; Cunliffe and Williams, 1999; Samaras et al., 1996).

A key property of activated carbon is their surface area. Commercial activated carbonstypically have a surface area in the range from 400 to 1500 m2 g−1. The BET surface areasof activated carbons derived from leather wastes are presented in Table 6.

The ash content, most of which were chromium compounds, and activation tempera-ture were main parameters for the enhancement of activated carbons. There is extremedifference between BET surface areas of activated carbons obtained from mineralized andnon-demineralized chars. The surface area of activated carbon from C-2 is approximately10 times higher that that of activated carbon from C-3. This different behavior is not sur-prising considering the large ash content of the carbons, especially for high CO2 burn-off.In previous report (Martin-Martinez et al., 1989), it was concluded that the Cr2O3 particlesare partially blocking micropores in carbons from chromium-tanned leather; if a portion ofthese particles is eliminated by previous extraction of the leather, the adsorption capacityis considerably increased. It should be noted that the surface area of activated carbon fromC-1 (10 h) was lower than that of activated carbon from C-2, although former activatedcarbon has lower ash content than later activated carbon. The reason may be due to the charsample. It may be suggested that the inorganics up to limited amounts improve the surfacearea. It has been reported that although Cr2O3 is blocking the micropores is also acting asa frame for the structure of the activated carbon (Martinez-Sanchez et al., 1989c).

Activation time had also an effect on the surface area. In activation of demineralizedCBD-450, the increase of activation time from 8 to 10 h, the BET surface area increased atthe ratio of approximately 10%.

As conclusion, to obtain the activated carbon, leather wastes should be pyrolyzed at450 ◦C and then demineralized with HCl before activation at 900 ◦C. The activated carbonhaving highest surface area was obtained from chromium-tanned shavings.

The EDS results (Table 7) are based on elemental analysis of selected particles on theactivated carbon surface. It merits mentioning that there was a noticeable scattering ofinorganics particles, mainly chromium compounds, present on activated carbons.


Table 7Summary of EDS analysis of activated carbons

Activated carbon Element (wt.%)

C Mg S Cl Cr Fe Cu Zn

A-BD 59.08 2.47 2.08 3.58 27.46 2.39 1.63 1.30A-CTS 34.98 1.88 2.16 – 42.16 16.97 0.71 1.14

3.5. Aqueous adsorption characteristics

Aqueous adsorption tests were conducted on selected activated carbons with the aim ofassessing potential applications in the water treatment industry. Two adsorbate compoundswere used in this study cover a range of molecular sizes, which makes them useful forthe investigation of adsorption in pores of different dimensions. Phenol is preferentiallyadsorbed in small and medium sized micropores while methylene blue is mainly adsorbedin medium and large micropores (Miguel et al., 2003).

In order to optimize the design of an adsorption system to remove dye or phenol fromeffluents, it is important to find the best isotherm equations. The isothermal equilibriumdata were processed employing Langmuir and Freundlich isotherm equations.

The Freundlich model is considered to be suitable for highly heterogeneous surfaces andindicates that significant adsorption takes place at low concentrations, but the increase in theamount adsorbed with concentration becomes less significant at higher concentration (Tengand Hsieh, 1998). On the other hand, Langmuir model is used for homogenous surfaces anddemonstrate monolayer coverage of the adsorbate at the outer surface of the adsorbent.

Table 8 shows the Langmuir (Ce/Qe = 1/KLSM + Ce/SM, where SM is the saturated adsorp-tion and KL is the constant related to the adsorption energy) and Freundlich (Qe = KFC

1/ne ,

where Qe is the amount adsorbate, Ce the equilibrium concentration of the adsorbate, KFand n are the constants) parameters obtained by fitting the MB and phenol adsorptionson activated carbons. In the case of MB, Langmuir model was found to fit the data well(R2 > 0.95). The Langmuir capacity (SM) of A-BD was more than that of A-CTS. For phenoladsorption, both equations were found to fit the data well (R2 > 0.95).

Results in Table 8 shows that activated carbon obtained from BD have higher adsorptioncapacity for phenol and methylene blue than activated carbon from CTS. Tseng et al.

Table 8Parameters of the Langmuir and Freundlich adsorption models of MB and phenol

Activated carbon Langmuir model Freundlich model

SM (mg g−1) KL (l mg−1) R2 1/n KF R2

Methylene blueA-BD 200 7.14 0.99 0.1913 116.98 0.55A-CTS 166.7 30.00 0.99 0.1715 88.94 0.59

PhenolA-BD 84.745 0.0282 0.99 0.4783 7.0048 0.98A-CTS 56.179 0.04052 0.97 0.4369 6.014 0.96


(2003) obtained SM values of 556 and 240.6 mg g−1 for the adsorption of MB and phenol,respectively, on the activated carbon prepared from pinewood. A SM value of 46.95 mg g−1

for adsorption of phenol onto activated carbon from sewage sludge (Chen et al., 2002) anda SM value of 225.64 mg g−1 for adsorption of MB onto activated carbon from jute fiber(Senthilkumaar et al., 2005).

As conclusion, owing to their poorly developed micropore structure, activated carbonsobtained from leather wastes showed low adsorption capacity for phenol. In contrast, acti-vated carbons possessed relatively higher adsorption capacity for methylene blue due totheir highly developed mesopore structure. It can be concluded that one important aspect ofactivated carbons prepared from chromium-tanned leather is the high volume of macropores,which is important for the adsorption from solutes, such as dyes, in aqueous solution.

4. Conclusion

In this study, three types of leather wastes were pyrolyzed at the temperatures of 450 and600 ◦C under nitrogen atmosphere. Pyrolysis of leather wastes yielded the charred residueand ammonium carbonate besides gas and oil products. Pyrolysis yields varied with the typeof leather waste. Pyrolysis temperature significantly affected the acid solubility of inorganicconstituents in chars.

The char from vegetable-tanned leather wastes was suitable to use a solid fuel becauseof its high calorific value and low ash content. On the other hand, the activated carbonshaving the high volume of macropores were produced from the chars obtained at 450 ◦Cchar by physical activation. The experiments related to aqueous adsorption characteristics ofactivated carbons showed that the prepared activated carbons could be used as an adsorbentfor the effective removal of dyes from aqueous solutions.

Consequently, the results of this work showed that waste leather is a useful recyclingresource and the conversion of wastes into activated carbon and fuels may be recognized asan attractive approach. This work can be considered complimentary to other studies whichhave concentrated on protein isolation and chromium recovery. However, in our case, as inthe case of other works further works are necessary before large-scale application of thesestudies is realisable.

Acknowledgement

We would like to thank to Ege University for financial support under the contract 04-CSUM-001.

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conversion of leather wastes to useful products

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