Causes and detection methods of hexavalent chromium in leather1

Causes and detection methods of hexavalent chromium in leather: The raw leather itself does not contain chromium, but it is an objective fact that hexavalent chromium is detected after being processed into finished leather. The causes of hexavalent chromium in leather are generally considered to be of two types. One is that tanning materials contain hexavalent chromium; the other is that part of trivalent chromium in chrome-tanned leather is oxidized to hexavalent chromium during the process of tanning or storage. Since tanning is a complex and systematic project involving many kinds of materials and multiple processes, the formation of hexavalent chromium in leather products is the result of many factors, which are summarized as follows. Influence of chrome-containing materials: In the tanning process, the use of hexavalent chromium-containing chemicals is the most direct cause of excessive hexavalent chromium content in finished leather. Chromium-containing materials include chrome powder, chrome-containing tanning agents, Chromium salt dyes, pigment pastes, fixatives, etc. Chromium powder is the largest and most direct source of chromium in leather. Experiments have shown that once the chromium powder exceeding the standard of hexavalent chromium is used, even if some remedial measures are taken during the pre-tanning or tanning process, such as adding a certain amount of soda, At the same time, the pH value is controlled as low as possible, and the reduction effect cannot be achieved. It can even be said that it has no effect on the hexavalent chromium measured in mg [s]. In addition to chrome powder, some chrome-containing retanning agents, chrome-containing dyes and pigments also contain high levels of hexavalent chromium, which will inevitably lead to excessive hexavalent chromium content in leather products after use. Influence of bath pH value: The pH value of bath solution not only plays a vital role in the tanning process, but also has a certain influence on the cause of hexavalent chromium. According to the thermodynamic principle, the oxidizing property of hexavalent chromium decreases sharply with the increase of the pH value of the medium. Under acidic conditions, trivalent chromium is not easily oxidized to hexavalent chromium; under alkaline conditions, trivalent chromium is easily oxidized to hexavalent chromium. Theoretically speaking, when the pH value of the solution is greater than 1, the oxygen in the air can oxidize trivalent chromium, but in fact, the pH value required for the oxidation of trivalent chromium by oxygen in the air is greater than 5t. Therefore, in the tanning process In the processes of neutralization, retanning and fatliquoring, when the pH value is greater than 5, it is possible to oxidize trivalent chromium in leather to form hexavalent chromium. Effect of fatliquor (or grease): Although the fatliquor itself does not contain chromium, it has a structure that allows hexavalent chromium to be produced. Studies have found that a fatliquoring agent or esterified fatty acid containing one or more unsaturated bond components can lead to the production of hexavalent chromium in leather products, while a fatliquoring agent that does not contain unsaturated bonds, whether natural or Synthetic, will not lead to the production of hexavalent chromium. In addition, some studies have shown that the iodine value of the fatliquor is different, and the amount of hexavalent chromium produced in the leather is also different, and the general trend shows that the higher the iodine value of the fatliquor used, the conversion of trivalent chromium to hexavalent chromium. more likely. This is because the unsaturated bond structure in the molecule of fatliquoring or grease containing unsaturated bonds is easily oxidized by oxygen in the air to generate peroxides and peroxide free radicals. The free radicals have strong oxidizing properties, and can easily oxidize the free trivalent chromium in the leather to hexavalent chromium. Effects of heat and light: Both heat and light can increase the hexavalent chromium content in leather. Under the conditions of heat and light, the covalent bonds in substances such as collagen, synthetic tanning agents, fatliquors or dyes will be broken, resulting in free radicals, which are in contact with oxygen in the air to generate extremely strong oxidizing effects. The oxygen-containing free radicals can oxidize trivalent chromium to hexavalent chromium. In addition, there is a kinetic equilibrium in the conversion between trivalent chromium and hexavalent chromium, and heat is a necessary condition for kinetic conversion, and the heat provided by heating and light has become a favorable condition for the conversion of trivalent chromium to hexavalent chromium . Influence of air humidity: During the storage process of finished leather, the content of hexavalent chromium will change greatly. Studies have shown that air humidity during storage is an important factor affecting the hexavalent chromium content in leather. With the increase of air humidity, the content of hexavalent chromium in leather will decrease; otherwise, it will increase. Storing leather under high humidity conditions is beneficial to inhibit the formation of hexavalent chromium in leather. Because the organic matter (often reducing) content in leather is relatively high and generally acidic, even if hexavalent chromium is generated, it is easily reduced in the case of high relative humidity (equivalent to being in solution). After understanding the formation causes of hexavalent chromium, factors that may affect the results can be avoided as much as possible in the detection of hexavalent chromium, which has an important guiding role in the detection of hexavalent chromium in leather products.