Prominent effectiveness of leather industry is amplified by high input and expenditure but on other side it causes huge waste of resource, incredible environmental pollution and biological chain destruction . Streams of gaseous, liquid and solid waste are resulted by environmental blow of tanneries. Global leather industry generates 4 million tones of solid waste per year . People use products of the leather-processing industry on a daily basis. These include especially shoes, leather and textile goods; we normally encounter leather products even in both public and private transport. The primary raw material for final products is hide from animals from slaughter houses and hide from game—i.e. waste from the meat industry, which is processed in tanneries and turned into leather. Therefore, the tanning industry can be considered one of the first industries to use and recycle secondary raw materials.
Although the tanning industry is environmentally important as a principal user of meat industry waste, the industry is perceived as a consumer of resources and a producer of pollutants. Processing one metric ton of raw hide generates 200 kg of final leather product (containing 3 kg of chromium), 250 kg of non-tanned solid waste, 200 kg of tanned waste (containing 3 kg of chromium), and 50,000 kg of wastewater (containing 5 kg of chromium) . Thus, only 20% of the raw material is converted into leather, and more than 60% of the chromium is in the solid and liquid waste. During the production of leather goods, especially shoes, manipulation waste is produced, whichmakes about 15–20% of the entry material—leather. The last kinds ofwaste are used leather products which have lost their utility value.
1.1. The possibility of oxidation of CrIII to CrVI
The basic question is the possible oxidation reaction from chromium III to chromium VI. In basic solutions, the oxidation of CrIII to CrVI by oxidants such as peroxides and hypohalide occurs with ease . Such strong oxidation conditions are realized in the process of the sterilization of drinking water. This is the first threat to human health and life. Rain (especially acid rain) can leach chromium III from waste dumps, and soluble salts can then reach sources of drinking water. During the sterilization process by ozone or hypochloride, chromium III is converted into chromium VI and reacts with magnesium and calcium ions occurring in drinking water to produce carcinogenic magnesium and calcium chromate or dichromate salts. Another problem concerns the possibility of oxidation of CrIII into CrVI in gentle conditions by air in the wide range of pH. Principally, oxidation can be realized after the following equations:
According to the European Commission (EC) the quantities of solid waste produced by tanneries depend on the type of leather processed, the source of hides and skins, and the techniques applied .On an average, at the end of the process, about 20% of the weight of the raw hides is (grain side) leather . On the other hand, in Rio Grande do Sul, approximately 40% of the initial raw material is transformed into solid and liquid wastes . In the tanning industry, raw skin is transformed into leather by means of a series of chemical and mechanical operations [4,5]. Chromium salts (in particular, chromium sulfate) are the most widely used tanning substances today. Hides that have been tanned with chromium salts have a good mechanical resistance, an extraordinary dyeing suitability and a better hydrothermal resistance in comparison with hides treated with plant substances. Chromium salts also have a high rate of penetration into the inter fibrillar spaces of the skin, what represents a saving in terms of production time and a better control of the process . In Brazil, approximately 90% of the leather industry uses chromium in hide processing, resulting in hazardous
The conventional tannery methods lead to...