ARTICLE pubs.acs.org/IECR
Analytical Pyrolysis As a Method to Characterize Tannery Wastes Antonio Marcilla,†,* Angela Nuria García,† Milagros Leon,† Pascual Martínez,‡ and Elena Ba~non‡ † ‡
Department of Chemical Engineering, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain Footwear Technological Institute (INESCOP), Polígono Industrial Campo Alto, P.O. Box 253 E-03600 Elda, Spain
bS Supporting Information ABSTRACT: The main objective of this paper is to study the thermal decomposition of waste products from leathers tanned with different tanning agents, from two different points of view: (i) thermogravimetric analysis and (ii) flash pyrolysis using a Pyroprobe device connected to a gas chromatograph with a mass spectrometer (Py/GC-MS). Both techniques allow us to characterize the samples by evaluating their potential differences regarding the decomposition process as well as by identifying the significant volatile compounds obtained depending on the tanning processes. The results have been treated using a multivariate statistical analysis method.
1. INTRODUCTION The leather and tannery industry produces a huge amount of waste. Leather waste is mainly composed of collagen protein and the chemical compounds introduced during the tanning process. There are at least six types of collagen; however, type I is the most common in leather.1 Tanning is the stabilization stage of the leather, and as a consequence of this process the thermal stability of the leather is increased. The degree of hydrothermal stabilization under normal usage conditions is determined by the type of tanning agent used. The leather industry uses the parameter of shrinkage temperature (Ts) as an indicator and a control measure for the execution of the tanning process. The Ts is the temperature at which denaturation of collagen occurs when the sample is immersed in water. This temperature is macroscopically detected by a sudden shrinkage of the leather. The Ts of an untanned hide sample is about 60 °C; for vegetable tanned leather, 75 85 °C; and for chromium sulfate tanned leather, 100 120 °C.2 Generally, tanning agents can be classified as organic or inorganic products. Examples of tanning with organic agents use products such as vegetable extracts and syntans, diverse aldehydes, and quinones, as well as sulphochlorinated paraffin and multiple resins. Of these, tanning with vegetable extracts is the most important from an economic viewpoint. In tanning with inorganic products, inorganic salts are used. For an inorganic salt to be capable of acting as a tanning agent, it is necessary that its water-based solution hydrolyses and the basic salts formed remain dissolved in the solution so that they can penetrate into the hide and react with it to increase the shrinkage temperature. Apart from chromium salts, whose tanning action is well-known, aluminum, zirconium, and titanium salts are also used in tanning on an industrial level. It is also known that other salts such as those from copper, tungsten, vanadium, zinc, mercury, chlorine, cobalt, cadmium, tin, lead, and silver have some tanning effect on leather, but these are not used in industry. According to the nature of the tanning agent, its fixation to the protein takes place through one of the following binding or interaction mechanisms:3 a. covalent bonds, present in tanning with aldehydes, oxazolidine, and methacrylic polymers, which react with the free amino groups of the lateral collagen strands r 2011 American Chemical Society
b. coordinate covalent bonds, which are present in what is known as inorganic or mineral tanning using chromium, aluminum, zirconium, etc. salts, together with bis[tetrakis(hydroxymethyl)phosphonium] sulfate (TPHS), and which fix preferably to free carboxyl groups in the protein c. hydrogen bridge bonds and dipolar bonds, which are present in vegetable extract tanning; these bonds are less stable and, therefore, easy to reverse Currently, and mainly due to the fact that the waste is in most cases spread in different manufacturing industries, the waste from tanned leather is disposed of together with other urban waste. In addition, used footwear is also part of municipal solid wastes. One of the environmental problems that must be borne in mind is the presence of chromium in these leather waste products, where it has been used as a tanning agent. The pyrolysis process could represent a very promising alternative to resolving treatment problems of these type of wastes. In this process, the waste is exposed to high temperatures in an inert atmosphere, being converted into products of interest for their energy content and chemical utilization properties. Pyrolysis of tannery wastewater tanneries has been studied with the aim of recovering the chromium and obtaining bio-oil as a byproduct, with a low to medium heating value.4 Two experimental techniques have been widely used to characterize the behavior of wastes under decomposition processes. On one hand, thermogravimetry (TG) allows one to establish the range of temperatures at which the thermal decomposition of wastes takes place. TG is a widely used technique in the study of the kinetics of thermal decomposition reactions of many materials.5 8 In spite of the simplicity of the technique, there are few studies in which this technique has been used to evaluate the thermal behavior of leather. One of these is presented by Plavan,9 who used thermogravimetric analysis in an oxidizing atmosphere. Most of these studies use thermogravimetry combined with differential scanning calorimetry (DCS) to evaluate Received: March 22, 2011 Accepted: June 21, 2011 Revised: June 14, 2011 Published: June 21, 2011 8994
dx.doi.org/10.1021/ie200582k | Ind. Eng. Chem. Res. 2011, 50, 8994–9002
Industrial & Engineering Chemistry Research Table 1. Tanning Products Used, Percentage of Product Added, and Nomenclature Used for Each Sample Type
ARTICLE
Table 2. Proximate and Ultimate Analysis of the Samples Studied
nomenclature
quantitya
Ml
8
basic chromium sulfate
M2 M3
3 30
glutaraldehyde tannins (mimosa-quebracho- chestnut extract)
moisture 12.8 12.0
8.8 10.2 13.3
M4
8
titanium ammonium sulfate
ash
1.3
M5
5
oxazolidine
volatiles
M6
5
THPS (bis[tetrakis
chemical compound
proximate analysis (tanning wastes %) M1
M2
7.6
1.7
a
8
aluminum oxide
M8
8
modified methylacrylic polymer
2. MATERIALS AND METHODS 2.1. Materials. The experiments were carried out on a bovine leather tanned with different tanning agents. Table 1 shows the products used and the percentage of the product in relation to the weight of the leather used. As a reference, an experiment was carried out on collagen powder from the Achilles tendon of a bovine species supplied by SIGMA-Aldrich without any tanning treatment. All samples were analyzed by (a) scanning microscopy with energy disperse X-ray microanalysis, by using a sequential X-ray spectrometer (PHILIPS MAGIX PRO) to measure the elements with an atomic weight higher than 9 (F), and (b) a Carlo Erba Elemental Analyzer to carry out the ultimate analysis of the samples. Table 2 shows these results together with the percentage of moisture, ash, and volatiles estimated for each sample. Some aspects are worthy of being remarked upon, such as the differences in the ash content and in the nitrogen percentage among the samples. Thus, collagen, M2, M3, and M5 samples
M5
6.3
M6
M7
M8
12.8 12.5 12.7
0.64
3.8
2.7
collagen 7.5
2.8
1.5
75.4 78.5 79.3
78.9
ultimate analysis (tanning wastes %) M1
Weight product/weight skin.
the deterioration of parchments or historical leathers; however, studies on leather for footwear are more limited.10 12 On the other hand, the analytical device Pyroprobe has been used by many researchers as a way to approach the study of primary pyrolytic reactions of different biomasses. Due to its design, the residence time of volatile compounds in the high temperature zone is very short, and therefore, the cracking of tar is minimal. Just as examples, we mention the work of pyrolysis of cellulose,13 wood,14 almond shell,15,4 urban solid waste,16 lignin,17 etc. In this paper, we apply thermogravimetric analysis to the study of waste from leather tanned in different ways, with the aim of gaining knowledge of the effect of the tanning process on the decomposition process of the corresponding leather. Furthermore, a Pyroprobe device connected to a GC-MS has been also used. Thus, flash pyrolysis of different tanned leather samples has been performed in order to analyze the volatiles evolved from the pyrolysis of the different samples studied. For the pyrolytic data treatment, multivariate statistical analysis techniques were used. These techniques have been applied previously in numerous characterization studies, for example, the analysis of the pyrolysis results of different types of agricultural waste;18 samples of environmental studies for the analysis of water, soil, or air;19 23 studies related to food technology,24 26 pharmaceutical products, or biochemical compounds;27,28 the evaluation of vegetable substances;29 31 etc., with excellent results.
M4
79.5 79.6 69.8 79.0 78.3
(hydroxymethyl) phosphonium]sulfate) M7
M3
M3
M4
M5
M6
M7
M8 collagen
N 11.1
12.1
8.6
10.2
12.5
12.9
11.0
12.2
16.1
C 42.2
49.6
48.3
49.7
49.9
45.7
50.9
47.9
43.6
6.8
7.1
6.2
7.3
7.4
6.7
7.7
7.0
Oa 35.4
26.5
35.4
28.3
28.5
28.7
27.8
26.5
H
6.72 29.7
Cr
2.07
0.014
S
1.3
3.6
0.55
0.55
Ca 0.61 Na 0.14
0.36 0.23
0.83 0.029
0.610 0.088 1.04 0.55 0.55
0.083 0.59 0.069 0.53
0.99
Cl
0.19
0.23
0.011
0.049
0.42
0.095 0.30
2.7
P
0.061 0610
0.054 0.78
0.22
0.004 0.025 0.009
0.0610 0.16
1.3
2.8
1.7
0.13
4.6
0.15
Si
0.058
0.064 0.014
0.036 0.032 0.17
Al
0.015
0.056 0.004
0.046 0.051 0.039 0.49
Fe
0.015
0.015
0.028 0.032 0.077
0.015
K
0.009
0.022 0.004
0.006 0.011 0.043 0.000 0.027
0.000
Mg 0.017 Ti 0.016
0.007 0.064 0.003
0.013 0.013 0.01 3.504 0.032
0.011
Sr
a
M2
0.17
0.023
0.015
0.005
0.021 0.056 0.021
0.002
Nb
0.011
Ce
0.012
Calculated from diference to 100 present).
(N% + C% + H% + metals
present very low ash contents, while M1 and M4 show higher values. With respect to the nitrogen percentage, the vegetabletanned sample shows the lowest content (about 9%), inorganic tanners (M1, M4, and M7) show percentages around 10 11%, and for the organic tanners it remains around 12 13%. The collagen sample shows the highest value (around 16%). As was expected, the highest content of metals in the samples is related to the tanning agent. Thus, the highest percentage of chrome corresponds to M1, titanium to M4, phosphorus to M6, and aluminum to M7. Of note is the high percentage of chlorine in collagen, compared with that in the tanned samples. 2.2. Thermobalance. The thermogravimetric analysis was carried out in a TGA/SDTA 851 Mettler-Toledo thermobalance in a nitrogen atmosphere with a flow rate of 50 mL min 1. The temperature of the sample was measured with an R-type thermocouple. The temperature values used were those recorded by the probe of the device, located under the crucible, and not the programmed values. The temperature range selected for the study was 25 to 800 °C with a nominal heating rate of 10 °C min 1. The amount of sample analyzed was around 8 mg, taken as two cylindrical portions of 2 mm in diameter. The tanned leathers were approximately 2-mm-thick. The tests were carried out in duplicate on different days to check the reproducibility of the test, which was found to be acceptable, as the difference in measurements was less than 1 °C. 8995
dx.doi.org/10.1021/ie200582k |Ind. Eng. Chem. Res. 2011, 50, 8994–9002
Industrial & Engineering Chemistry Research
ARTICLE
Table 3. Temperature of Maximum Decomposition Rate tannage (nomenclature)
Figure 1. Experimental DTG curves from leather with different tanning methods.
At the start of the study, the equipment was calibrated using indium and aluminum standards; later, periodic checks were made to ensure that the equipment remained in compliance with the calibration specifications. 2.3. Pyroprobe Connected Online with a Gas Chromatograph (Py-GC/MS). For the flash pyrolysis of the samples, a pyroprobe 2000 device was used. The amount of sample pyrolyzed in each experiment was approximately 600 μg, introduced into a quartz capillary tube. The capillary was automatically introduced into the center of a platinum resistor, which was heated up in an inert atmosphere. The parameters used in this process were nominal heating rate, 20 °C/ms; pyrolysis time, 20 s; and process temperature, 500 °C. Each experiment was repeated twice; the results show the reproducibility of the experiments. The products generated in pyrolysis were rapidly removed from the reaction zone (quartz capillary surrounded by the resistor) using a flow of helium, through a transfer line at a temperature of 280 °C until introduced into the gas chromatograph provided with a mass spectrometry detector for analysis (HP-5973 MSD). The volatile compounds generated were analyzed using an HP-5MS (30-m-long and 0.250 mm internal diameter) capillary column. The conditions used for the analysis were Tinitial = 37 °C, Tfinal = 320 °C, heating rate = 12 °C/min, and ttotal = 33.5 min. The qualitative analysis of the chromatogram obtained was carried out using the mass spectra commercial libraries WYLEY275 and NIST02.
3. RESULTS AND DISCUSSION 3.1. Thermogravimetric Study. Figure 1 shows the DTG curves obtained for each one of the eight samples of leather tanned with different chemical agents and for the powdered collagen. As can be seen, in all cases, two thermal decompositions take place, the first around 80 100 °C due to the loss of humidity and other volatile compounds and the second between 300 and 550 °C due to the decomposition of the leather. Although all of the DTG curves are very similar, it can be seen that the second decomposition slightly varies from one type of tanning process to another, as the curves show different slopes. The final residue values obtained were similar for collagen and most types of tanning (around 21 25%); however, vegetable
Tmax.decompositiona (°C)
collagen (COL)
323.1
chromium (M1)
330.7
glutaraldehyde (M2)
320.5
vegetable (M3)
317.5
titanium (M4)
326.7
oxazolidine (M5)
327.7
phosphonium (M6)
321.4
aluminum (M7) resin (M8)
326.2 323.2
tanning produces around 30% residue. Table 1 shows that the quantity of vegetable tanning agent used in these experiments represents 30% of the initial sample weight, much higher than that used in the rest of the samples (3 8%). Thermogravimetric analysis of the vegetable tanning agents (TG data available in the Supporting Information) indicated that the mimosa, quebracho, and chestnut extracts lose weight in the temperature range used in this study, leaving approximately 38 44% of the residue. Therefore, in vegetable tanned products, around 13% of the final residue comes from the tanning agent. In the rest of the samples, the residue due to the tanning agent is expected to be lower (
