3 Chromium salts

3.1 Chemistry of chromium

Chromium basic sulphate is the most widely used tanning substance today (unep 1994). The exhausted bath coming from the chromium tannage contains about 30% of the initial salt and is normally sent for cleaning up (cassano et al. 2001). Here chromium salts are entrained in the sludge creating serious problems for their disposal (gauglhofer 1986). Chromium is a micronutrient and cr salts such as chromium polynicotine, chromium chloride and chromium picolinate (crp) have been demonstrated to exhibit a significant number of health benefits in animals and humans (anderson 2000). Hazards due to environmental contamination, depend on its oxidation state (i.e. Six valence stage of chromium (cr⁶⁺) is more toxic than the cr³⁺ which precipitates at higher ph [27-28].

Trivalent chromium is unable to enter into cells but cr⁶⁺ enters through mem­brane anionic transporters. Intracellular cr⁶⁺ is metabolically reduced to cr³⁺. Cr⁶⁺ does not react with macromolecules such as dna, rna, proteins and lipids. However both cr³⁺ and the reductional intermediate cr⁵⁺ are capable of coordinated covalent interactions with macromolecules (shrivastava et al. 2002).

Chromium can exist in several chemical forms displaying oxidation numbers from 0 to vi. Trivalent and six valence forms of chromium are the most stable in the environment (shriver et al. (1994). Different chromium species also exhibit different energy levels for conversion to higher or lower oxidation states. This translitionary nature of chromium imparts certain environmental behaviour as shown earlier[29-30].

3.2 Trivalent chromium (cr³⁺)

Cr³⁺ presence, concentration and forms depend on different chemical and physical processes such as hydrolysis, complexation, redox reactions and adsorption (table 1). In the absence of complexing agents other than h₂o or oh^-, cr³+ exists as hexa-aquachromium (3+) and its hydrolysis products (rai et al. 1989). Cr (h₂o)₆³+ is a moderately strong acid (pk ~ 4).

Cr (h₂o)₆³⁺ + h₂o → cr (oh) (h₂o)₅² + + h₃o+ (1)

Cr (oh) (h₂o)₅²⁺ + h₂o → cr (oh)₂ (h₂o)₄ + + h₃o+ (2)

Cr (oh)₂(h₂o)₄+ + h₂o → cr (oh)₃.aq + h₃o⁺ (3)

The deprotonated forms are cro-h²+.aq, cr(oh)₂+.aq and cr(oh).aq dominating successively within ph 4-10. Trihydroxochromium is sparingly soluble within a ph range of 5.5-12 (minimum between ph 6.5 and 11.5 (rai et al. 1989; saleh et al. 1989). Croh²+ . Aq and cr(oh)₃ . Aq are expected to be the dominant forms of cr₃ in the environment (kotas and stasicka 2000). Cr(oh)₃ . Aq exhibits amphoteric behaviour and at higher ph is transformed into readily soluble tetrahydroxo com­plex, cr(oh)₄^-

Table 2. Summary of various cr oxidation numbers, type and environmental behavior

Valency	Environmental behaviour	Remarks
Cr	Unstable
Cr1+	Unstable
Cr2+	Readily oxidised to cr3 but stable only in the absence of any oxidant	Active under anaerobic condition
Cr3+	Most stable	Considerable energy required to convert to lower or higher states
Cr4*	Forms unstable intermediate reactions to	Exhibits this phase during oxidation
	Trivalent and oxidation states	And reduction
Cr5*	Unstable intermediate.	Observed during oxidation and reduction
Cr6+	In acidic conditions demonstrates very high positive redox potential and unstable in the presence of electron donors.	Strongly oxidizing

Cr³⁺ is a hard acid which exhibits a strong tendency to form hexacoordinate octahedral complexes with a variety of ligands such as water, ammonia, urea, eth- ylenediamine, and other organic ligands containing oxygen. The complexation of cr³+ by ligands other than oh^- increases its solubility when the ligands are in form of discrete molecules or ions. When, however, donor atoms are bound in a macro- molecular system, as humic acids than the cr³⁺ complex is more or less mobile. If the complexation from these ligands can be neglected, under redox and ph condi­tions normally found in natural systems, cr is removed from the solutions as cr(oh)₃, or in the presence of fe³+, in the form of (crfe_1-x)(oh)₃ [24-28].

3.3 Chromium in the environment

25 mole fraction of cr) (sass and rai 1987). Mediation by manganese oxides was found to be the effective oxidation pathway in environmental systems (nakayama et al. 1981a; schroeder and lee 1975; bartlett and james 1979). This was due to the fact that the redox potential of the cr⁶+/cr³+ couple is high enough, leaving only a few oxidants capable of oxidising cr³+ to cr⁶+ (kotas and stasicka 2000).