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Trace metals and polycyclic aromatic hydrocarbons ([PAH.sub.s]) levels in cattle skin (Kanda) processed with burning tyres.


Trace metals are natural components of the environment, but human activities notably industrial and mining processes have been responsible for the wider diffusion of these elements. They are metals in extremely small quantities, almost at the molecular level that reside in or are present in animal and plant cells and tissues. The concept of toxicity is usually associated with trace metals because they can be toxic if ingested in excess quantity. Examples of trace metals are Zinc (Zn), Chromium (Cr), Cadmium (Cd), Iron (Fe), Vanadium (V), Lead (Pb), arsenic (As), etc.

A number of toxic trace metals are introduced into the environment as a consequence of industrial activities. Important source of trace metals in the environment is combustion of crude, organic matter, rubber and rubber products, etc. Most of these trace metals occur in smalls amounts and are present in ash or fly ash resulting from combustion process. They are either carried about in the air or may be deposited in the soil and remain there ready indefinitely.

In most cases, potentially hazardous, trace metals found in urban garden, soils and on plants could be directly ingested, inhaled or are absorbed by plants. Scan (2003), reported that trace metals are accumulated in soils and plants. Animals fed with these plants, will tend to accumulate toxic metals themselves. According to Donald and Andre (2003), potentially harmful trace metals can accumulate in the soil and on plants and may pose a potentially health risk to people who breathe or especially swallowed contaminated soil or eat contaminated vegetables. Although plants always absorb trace metals to some extent, in nearly all instances, the quantities they take up are significant to cause alarm.

Goyer (1997), Lopez--Alonso (2002) opined that, the need to reduce trace metal contamination in animal feed in fact poses a significant problem for agricultural regions located in more or less industrialized areas in which animals are reared on locally produced feed. Loredo, et al (1991) reiterated that, any place subjected to human activity is likely to have trace metals at elevated levels in the environment, particularly in the soil. While this is generally no cause for alarm in some cases, these trace metals may accumulate to a level where it would be advisable to take measures that ensure they will not pose a health risk.

Automobile tyre is made up greatly of natural and synthetic rubber and to a little extent, some additives. To form the rubber into hardwearing vehicle tyres, an extensive range of chemicals including, zinc oxide, Xylene, benzene, chlorinated solvents e.g. 1, 1, 1--trichloroethene, china clay, carbon black, paraffin oils and waxes, sulphur, etc. are used. Some tyre manufacturers use wire and fabrics to reinforce the strength of the tyre. The presence of trace metals in tyres could arise from natural rubber (plant origin) and or from the wire and fabric in the tyre. Combustion of tyres could therefore easily introduce trace metals into the environment.

One of man's numerous activities which is the extraction and processing of natural crude oil (petroleum) produces hydrocarbons either in solid, liquid or gaseous form. Polycyclic aromatic hydrocarbons (PAHS) are among the numerous compounds of hydrocarbons obtained from petroleum (crude oil). They consist of fused aromatic rings and do not contain heteroatoms or carry substituent. There are over one hundred PAHS containing multiple benzene rings that are difficult to break down.

PAHS are one of the most wide spread organic pollutants. They are known to be carcinogenic (Capable of causing cancer). PAHS toxicity is very structurally dependent, with Isomers varying from being nontoxic to being extremely toxic. One PAH compound, benzo (a) pyrene is notable for being the first chemical carcinogen to be discovered and is one of the many carcinogens found in cigarette smoke (Culp et al, 1988). Other possible human carcinogens in the PAH family are benzo(a) arthracene, benzo (b) fluoranthene, benzo (k) fluoranthene, chlrysene, dibenz (a, h) arthracene and indeno (1, 2, 3--cd) pyrene (WHO, 1979; CAPCOA, 1993; 1ARC, 1998; EPA 2001).

The burning (combustion) of vehicle tyre to remove the fur on cattle skin is a common site in most of the abattoirs especially in the South--Eastern and Southern parts of Nigeria. This operation is characterized with the emission of thick, black, sooty smoke into the environment. The resultant skin (aka "Kanda" or "Kpomo") is washed and sold as meat to the public. Trace metals emanating from the combustion process could be deposited on the processed skin. Moreover, because PAHS are fat soluble, they can be absorbed by the fatty tissue beneath the skin.

The aim of this study is therefore to determine the trace metals and PAH levels of cattle skin processed with vehicle tyre. This paper also proffers suggestions on alternative ways to process cattle skin.

Experimental Procedure

Three categories of samples namely; unprocessed, tyre-processed and hot water-processed cattle skins were used in the study. To rule out errors that might arise from dietary origin, the three samples were obtained from the same animal. This is equivalent to a set of samples. To establish or confirm any trend in the results, skins of five different animals were used to provide five set of samples.

Trace Metals: (ASTM, 2000)

Atomic Absorption and Emission spectroscopy (AAES) was used to determine trace metals (Zn, Fe, Cd, V, Cr).

The samples were digested with concentrated trioxonitrate (V) acid (HN[O.sub.3]), tetraoxosulphate (VI) acid ([H.sub.2]S[O.sub.4]) and Oxochlorate (VIII) acid (HCl[O.sub.4]). Standard solutions of known concentration of the metals of interest were prepared and aspirated in the AAES in order of increasing concentration. Calibration curve was generated. The absorbances of the digested samples were also measured. The concentrations of the absorbing species in the samples (metals) were then determined from the calibration curve.

Polycyclic Aromatic Hydrocarbons (PAHS) (ASTM, 2000)

Known weights of samples were dried with anhydrous sodium sulphate. Thereafter, extraction solvent (dichloromethane) was added to the samples respectively in extraction bottles, and shaken in a water bath for 5 hours. The extracts were allowed to settle for 1-2 hours, filtered through a funnel fitted with cotton wool and sodium sulphate, into clean amber coloured extraction bottles washed with dichloromethane. The resultant residue was washed with the extraction solvent and filtered again to ensure complete recovery of the extract (PAH). Finally, sample extract was fractionated (separated) through elution in a 10mm ID Chromatographic column using a mixture of 40% dichlorometane and 60% pentane as the eluting solvent.

Standard PAH solution of known concentration was prepared. Using Agilent 6890N gas chromatogram (GC), several dilutions of the PAH standard solution were injected into the chromatographic chamber to obtain retention times and calibration peaks for the PAH standards. Using dichloromethane as blank, sample extracts were also run through the GC and PAH concentrations in the sample identified and determined through the calibration peaks of PAH standards.

Results and Discussion

The results of trace metals and [PAH.sub.S] are given in tables I, II and III respectively.



From the results obtained, using those for animal 2 as example, the tyre-processed cattle skin recorded the highest level of PAH (23.9 mg/kg) while the hot water-processed skin had the least value (1.09 mg/kg).

To establish PAH toxicity, benzo (a) pyrene (Bap) is usually used as reference because of its high toxicity (Culp et al, 1988). For this reason this study was taken a step further to assay Bap in the cattle skins. The same trend of result was observed (Table III).

For better clarification, one set of results, that is, PAHS of unprocessed, tyre-processed and hot water-processed skins, as well as Bap values of Animal 2, are depicted as Histograms (Figs 1 and 2).

It is obvious that burning of tyre deposited PAHs on the cattle skin. Values obtained from the unprocessed skin might be of dietary origin of the cattle. The reduction of PAH concentration in the hot water-processed skin might be as a result of dissolution of the fatty tissues underneath the skin. PAHs are soluble in fat; because of their hydrophobic nature, washing the cattle skin with cold water may not remove them. It is possible that during processing with hot water, PAH might have been released into the hot water medium. This might explain the reduction of PAHs in the hot water-processed skin.

PAH is synthesized from hydrocarbons (pyrene, the major component of tyre is a hydrocarbon), under oxygen--deficient conditions. Hydrocarbons with very low molecular masses including methane, may act as precursors for the polycyclic aromatic compounds. PAHs are formed by pyrolysis. This happens at temperatures exceeding approximately 500[degrees]C at which carbon-hydrogen and carbon-carbon bonds are broken to form free radicals. These undergo dehydrogenation and combine chemically to form aromatic ring structure, which are resistant to thermal degradations.

Pyrolysis could be of significance during tyre burning operation in the abattoir. Deposition of PAH on the cattle skin is likely to have occurred from the burning tyre. No critical values are set as acceptable guidelines for PAHs. The compounds are very toxic and so are threats to life. Ingesting low quantities may not be immediately dangerous. However, over time, their levels may exceed the total body burden through bioaccumulation.

Experimental work indicated that the actual carcinogen is a more polar substance, which can combine with DNA (a high molecular weight compound which contains genetic information) in cells, (Robert, 1997). In the case of 1, 2-benzo pyrene, the polar compound produced by enzymatic oxidation is a diol epoxide derivative of 1,2-benzo pyrene.

Some variations were also observed for trace metals. The level of iron increased from 13.0 mg/kg in the unprocessed skin to 37.3mg/kg after being processed with burning tyre. Processing with hot water reduced iron level to 1.44mg/kg. The same trend was observed for zinc (Table 1).

Cadmium and Vanadium were below detectable limit of this study (<0.01 mg/kg).

Zinc and iron are natural constituents of the body; hence their presence in the unprocessed skin is not strange. The elevated difference between the unprocessed and tyre-processed skin might have come through deposition from the combusted tyre.

Zinc is a constituent of tyre. It is added to tyre as zinc oxide (ZnO), filler during tyre manufacturing. Iron too is present in tyre as steel wire to increase the tensile strength of tyres. Also trace metals in tyre could have dietary origin (via the natural rubber). During tyre combustion, the possibility of depositing some of the trace metals on the cattle skin may not be ruled out.

Some trace metals are essential in the body. For instance, zinc aids in normal growth and tissue respiration, and also plays an important role in the development of normal coat of hair. Iron, the blood-building element is essential in the formation of haemoglobin, in oxygen transfer, and in all cellular respiration. However, whether a metal is essential or not, exposure above certain levels may cause adverse health effects. Individuals who consume the tyre-processed cattle skin frequently may stand the risk of exceeding the total body burden for these metals through bioaccumulation.

Conclusion and Recommendations

In spite of its indigestibility within the digestive system of man, cattle skin (aka kanda or kpomo) remains a delicacy on most tables in Nigeria. Many people eat it for textural satisfaction. Besides, it could be good meat for the elderly due to the poor bioavailability of its protein content. For the poor or low income earners, it is relatively cheap. For these reasons, cattle skin may not be completely removed from the Nigerian market.

Those who burn used tyres to process their cattle skin say it is a cheaper method. From this study, it has been established that the use of tyre to process cattle skin, deposits carcinogens like polycyclic aromatic hydrocarbons and some trace metals on the processed skin.

In this study, hot water was used as an alternative procedure for removing the fur on cattle skin. It proved to be better because the processed skins contained lesser values of trace metals and PAHs respectively. Another alternative method that could be used for processing cattle skin is the utilization of sharp objects such as razor blades or knives to shave off the fur from the cattle skin. This method is cheaper than the hot water method and it could be conceived as a more environmentally friendly procedure.


[1] American Society for Testing Materials ASTM (2000). Published by United States Environmental Protection Agency, Washington DC 20402

[2] California Air Pollution Control Officers Association, (APCOA) (1993). London, Imperial College Press.

[3] Culp S. J. Gryar D. W.; Sheldon W. G.; Gold Stein L. S. and Beland F. A (1988). A Compassion of the Tumurs induced by Coaltar and Benzo (a) pyrene in a 2-year Bioassay, Carcinogenesis: 19, pp. 117-124

[4] Donald H. R. and Andrew C. C. (2003): Effects of Trace Elements on Plants. University of California: Cooperative Extension, pp. 57-66.

[5] EPA (2001) US Environmental Protection Agency, Reviews of the Environmental Effects of Pollutants. New York International press.

[6] Goyer R. A. (1997). Toxic and Essential Metals Interactions. Annu: Rev. Nutr. pp. 37-50

[7] International Agency for Research on Cancer, IARC (1998). London, London press.

[8] Lopez--Alonso, M. (2002). Shore of Cattle as Biomonitors of Soil Arsenic, Copper and Zinc Concentrations in Garlician (NW Spain). Arch Environmental Contamination Toxicol. pp. 43, 103-8.

[9] Robert J. B. (1997) Essentials of Organic Chemistry. The McGraw Hill Companies. Dubique, IA 52001 USA pp 163-179

[10] Scientific Committee on Animal Nutrition SCAN (2003). Opinion on the undesirable Substances in Feeds pp. 99-105

[11] World health Organisation, WHO (1983). Health Hazards of Human Environment--Geneva. 88-96.

(1) Dibofori-Orji Amalo Ndu and (2) Braide S.A.

(1) Senior Lecturer, Department of Chemistry Rivers State University of Education Rumuolumeni, Port Harcourt


(2) Institute of Pollution Studies, Rivers State University of Science & Technology, Nkpolu, Port Harcourt
Table I: Trace Metals in Cattle Skin.

 Observed Effects on Trace
 Metals Level

 Fe Zn Cd
Group Treatment (mg/kg) (mg/kg) (mg/kg)

1. Unprocessed Skin 13.0 6.73 <0.01
2. Processed with burning Tyre 37.3 7.04 <0.01
3. Processed with Hot water 1.44 0.55 <0.01

 Observed Effects on Trace Metals Level

 Cr V
Group (mg/kg) (mg/kg)

1. <0.01 <0.01
2. 0.85 <0.01
3. <0.01 <0.01

Table II: Polycyclic Aromatic Hydrocarbons in Cattle Skin (mg/kg).

Animal 1:

 Processed with
 Unprocessed Skin Processed with Tyre Hot Water

 1.76 6.70 0.09
 1.76 6.28 0.08
 1.75 5.37 0.04
 X = 1.756 = 1.76 X = 6.116 = 6.12 X = 0.07

Animal 2:

 Processed with
 Unprocessed Skin Processed with Tyre Hot Water

 5.10 18.7 1.09
 5.10 19.3 1.09
 5.10 33.8 1.09
 X = 5.10 X = 23.93 = 23.9 X = 1.09

Animal 3:

 Processed with
 Unprocessed Skin Processed with Tyre Hot Water

 1.97 5.49 1.12
 1.95 5.28 0.09
 1.97 3.78 0.05
 X = 1.963 = 1.96 4.85 X = 0.42

Animal 4:

 Processed with
 Unprocessed Skin Processed with Tyre Hot Water

 3.96 9.48 1.07
 3.96 9.28 1.07
 3.96 7.07 1.07

Animal 5:

 Processed with
 Unprocessed Skin Processed with Tyre Hot Water

 0.90 3.70 0.01
 0.90 8.28 0.01
 0.91 5.78 0.01
 X = 0.90 X = 5.92 X = 0.01

Table III: Summary of Total Mean Benzo (a) Pyrene (Bap) in Cattle
Skin for Animal 2.

Group Treatment Total (mg/kg)

1. Unprocessed skin 0.59
2. Tyre-processed skin 1.89
3. Hot water-processed skin 0.04
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Author:Dibofori-Orji Amalo, Ndu; Braide, S.A.
Publication:International Journal of Applied Environmental Sciences
Date:May 1, 2011
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