Which fraction of chromium is available to form the hexavalent species

In hides, when the tanning or retanning phases are completed, it is possible to find unbound chromium fractions which, if not removed, may be available to be oxidized in the hexavalent form. It has been shown that a large portion of the chromium used in leather processing, generally around 30-40%, does not take part in the process and is conveyed directly into the waste water (Wang et al., 2007). A part of it can become trapped inside the skins as free chrome available in the trivalent form. As for the operations of riviera, the tanning represents the conclusive phase. In the reccees, chromium is usually used in the initial phases (immediately after it breaks down). After use of chromium, the classic re-enamels continue with disacid, dyes and fatliquors. One would therefore expect to find a greater quantity of free chromium after tanning than after retanning. Without wanting to go too far into the treatment, relatively recent studies have shown that, of all the chromium contained within the skin, the one most susceptible to oxidation seems to be either free or blandly or incompletely linked to it. With regard to the alloyed chromium, the high level of coordination and the high stability of the chromium complexes make the metal nuclei very resistant to oxidizing attacks, therefore the oxidation of the polyoxometallate complexes is to be excluded. It is in the tanning that the best chromium fixation conditions occur because the times of penetration and polymerization of the tanning agent are much longer than the retanning times and the pH conditions are much more favorable. The tanning operation is carried out with the polymerization of the tanning salt directly inside the skin. The creation of side chains that bind the collagen fibers makes the skin stable over time. This process is characterized by low efficiency because, as already mentioned, only 65-70% of the chrome offered to the skin is bound by the support. The part of chromium bound to the skin is involved in the formation of transverse chains, but not all of them are tanning because not all of them lead to the formation of cross links:

Figure 3: examples of polymerization of the tanning salt inside the skin As simplified (a lot) in Figure 3, the tanning polymer that binds to the skin in very different ways:

• in the UNIPOINT form, in which the polymer has only one tropocollagen binding point. This situation is not conciliatory;
• in the MULTIPOINT form, in which the polymer has two binding points with different chains. This condition is tanning;
• as a branch of a polymeric chain linked in a unipoint manner, non-tanning situation;
• as a branch of a polymeric chain linked in a multipoint manner, a situation that may or may not be tanning.

The chromium bound to the skin through complete transversal ties does not seem to participate in the oxidation Cr (III) → Cr (VI). The whole part of chromium bound to the skin but which is not involved in cross-linking can be involved in the oxidation; this condition is clearer in Figure 4. The degree of fixation of the chromium inside the skin depends on the time and increases with the passing of it (think for example to the oxidation of the polymers that form cross links); however, some studies state that the final content of hexavalent chromium is independent of the presence or absence of chromium in the retanning phase. The opinion of Dr. Biagio Naviglio of the Experimental Station of Naples is that according to which it is the free chrome - that is the most easily extractable part of the Cr (III) - that can be available for the formation of Cr (VI). In his opinion, which is the most shareable, the most closely bound chromium to collagen is that used during tanning. This is true for a number of reasons. The first concerns the times in which chromium is made to penetrate and react. The absorption of chromium during the tanning is carried out in 1-2 hours while the polymerization (disacid) is completed in 6-8 hours. In retanning times are much narrower and, if penetration can be carried out at similar times, the disacide is completed in 60-90 minutes. The second concerns the pH of the end disacid. In the tanning the disacide reaches a pH of 3.8-4.0. At this pH value the polymerizing capacity of the tanning salt is maximum, the reactivity of the skin is sufficient. In retanning the disacide reaches pH 5.0 to 6.0. The purpose of the retachalic acidic de-icer is not to guarantee the polymerization of the re-used tanning salt but to bring the whole skin section to a pH higher than p.I.

In the reconciliation phase the working conditions for sale contribute to the formation of unipoint ties. It has been possible to obtain both the chromium used in the retanning process, more weakly bound to the collagen, to have an active role in the generation of Cr (VI). The fact that it is the free chromium fraction is a fact in hexavalent chromium is also verified by the fact that there are no decreases in the temperature of contraction of the skin in those leathers in which the hexavalent chromium is revealed. Further studies are needed. It is possible to formulate a further hypothesis regarding which fraction of chromium is available to oxidation, ie the bound chromium (polymerized or not) fraction, but which does not actively participate in the formation of transversal bonds, as analyzed in Figure 4:

Figure 4: chromium fraction bound to the skin but available for Cr (VI) formation Whatever may be the answer to this hypothesis it is good to take all the measures to minimize a situation like the one in Figure 4, recreating as best as possible the best conditions of polymerization (disacid).