Institute of Chemistry, A. Gotauto 9, 01108 Vilnius, Lithuania
Two representatives of dyes widely used in textile industry azodye, direct light resistant blue, and acid ntrachinone blue were decontaminated using metallic iron. When the excess of iron powder is used, the destruction of dyes obeys the second order reaction equation with the rate constants of 0.0024 lmg-1min-1 and 0.0002 lmg-1min-1 for direct light resistance blue and acid antrachinone blue respectively in neutral solutions and 0.0035 lmg-1min-1and 0.0014 lmg-1min-1 respectively in acidic solutions. UV/Vis and FT-IR investigations showed that a part of the dye degradation products remains in the solutions and a part is adsorbed on the precipitate formed during the treatment. The increased values of COD and BOD with the increase in time of destruction indicate that both dyes are decomposed to easily degradable products.
Key words: direct light resistance blue, acid antrachinone blue, metallic iron, Discoloration
Colorants are widely used in different branches of industry, i.e. textile, painting, leather, printing, photography etc. In addition to natural organic and inorganic dyes, synthetic organic dyes comprise the main part (~80%) of all the colorants used in practice. Synthetic dyes used in textile industry are not utilized for fabrics completely. The remaining part gets into the wastewater, which requires careful decontamination. These dyes are toxic to aquatic organisms and resistant to natural biological degradation. Due to their high chemical stability and xenobiotic nature, they cannot be destroyed during classical wastewater treatment procedures.
In order to achieve the decomposition of these persistent organic compounds, lots of solutions have been proposed. Acid dyes form insoluble precipitate with Ba(II) salts in the presence of alkaline substances. However, the precipitation of acid dyes containing the amino groups is complicated and not complete. Besides, such treatment has no practical value due to the use of poisonous soluble Ba(II) salts and the difficulties in the disposal of the formed sludge. Promising results in dye treatment were obtained using various sorbents [1, 2].
For the treatment of the dyes containing effluents oxidative destruction is mainly used in practice. Ozone, hydrogen peroxides, Fentons reagent are used as oxidizing agents . In the resent years the reductive destruction of azodyes using metallic iron has been investigated very intensively [4, 8]. The authors of the work  established a strong relationship between the composition of the iron oxidized surface layer and kinetics and reaction pathways of azodyes decomposition. The proposed mechanism for azodyes decomposition by zerovalent iron involves the formation of free amino groups
RN=NR' + 2Fe0 + 4H+ ---- RNH2 + 2Fe2+ + H2NR'.
This reaction is catalyzed by iron surface. Actually, the dissolution of iron is more complicated. The hydrogen evolved during the iron dissolution can act as a reducing agent as well:
Fe0 + 2H2O --- Fe2+ + H2+ 2OH
The iron dissolving reaction depending on pH and the presence of an oxidizing agent, for instance oxygen, proceeds with the formation of Fe2+ or Fe3+. The latter forms amorphous precipitate.
2Fe2+ + 1/2 O2 + 3H2O --- 2FeOOH + 4H+,
2Fe2+ + 1/2 O2 + 2H2O --- 2Fe2O3 + 4H+.
It is well known [10, 11] that this precipitate act as a good sorbent for organic and inorganic substances. Fe3+ ions form insoluble precipitate with some organic compounds as well . Synthetic organic dyes having many unsaturated linkages and different functional groups may act as powerful complexing agents for iron ions.