Removal of colour from solutions of azo dyes using bacterial cells (Shewanella strain J18 143)

Water pollution control is presently one of the major areas of scientific activity. While coloured organic compounds generally impart only a minor fraction of the organic load to wastewater, their colour renders them aesthetically unacceptable. Effluent discharges from the textile industries to neighbouring water bodies and wastewater treatment systems have been given of much concern especially in recent years. Colour removal, in particular, has recently become of major scientific interest, as indicated by the large number of related research reports. The bacterium, Shewanella strain J 18 143, has been reported to be capable of directly inducing the reduction of azo bonds in reactive azo dyes and, indirectly being involved in the reduction of azo/ketohydrazone chromophores in pigment molecules. Two approaches have been used to extend the applications of Shewanella strain J 18 143 in the treatment of textile wastewaters. The first approach concerned the use of this bacterium, in an immobilised form, in removing the colour from reactive azo dye solutions. The second approach was the use of this bacterium, in a free form, in reducing colour from selected metal-complex azo dye solutions. A grafted cellulose copolymer was chosen as the immobilisation substrate. The cotton fabric was firstly pre-treated and then underwent copolymerisation with the use of acrylic acid as the monomer and potassium persulphate as an initiator. Soxhlet extraction as used for after-treatment of the modified cellulose. The grafting yield of the graft copolymer was controlled at 5.5%. The bacterial cells were immobilised by three methods. The first method, "growing-in", was carried out by growing the bacterial starter culture with the presence of graft cellulosic copolymer. The cells were therefore bound to the substrate. The immobilisation method of adsorption was carried out by allowing the pre-grown, resting cells to physically adsorb onto the copolymeric substrate. The third method, chemical coupling, was carried out by coupling the bacterial cells onto the substrate using a coupling agent, carbodi-imide (CMC). Protein assay of immobilised cells was studied. The method derived from the use of the BCA kit was applied for a qualitative confidence of the presence of the immobilised cells. Decoloration of Remazol Black B solution was carried out using the immobilised cells. In all of the three methods the immobilised cells were able to decolorise the dye. The immobilisation methods of "growing-in" and chemical coupling were found more effective in decoloration. Complete decoloration of the dye solutions was observed. The potential use of the graft copolymer substrate was confirmed. Four Irgalan series metal-complex azo dyes, Irgalan Grey GLN, Irgalan Black RBLN, Irgalan Blue 3GL and Irgalan Yellow 3RL KWL, were decolorised using planktonic cells of Shewanella strain J 18 143. The bacterial cells completely decolorised the solution of Irgalan Grey GLN, apart from that some coloured by-products were observed in the system. Irgalan Grey GLN is a mixture of Irgalan Black RBLN and Irgalan Blue 3GL. Colour reductions of these two metal-complex azo dyes were achieved using planktonic bacterial cells. The results obtained were similar to that of the Irgalan Grey GLN. The colour of the solutions was reduced. Some coloured materials were produced at the bottom and at the top of the aqueous treating system. Colour reduction of Irgalan Yellow 3RL KWL was carried out using the planktonic biological cells Shewanella strain J18 143. Not too much colour reduction can be found from the visual results and from the UV-visible spectra. However, some changes were made by the biological cells. Some precipitation was observed at the bottom the evaluating system which contained Irgalan Yellow 3RL KWL and the cells. This observation was further proved by the particle sizing analysis. The particle cells analysis has shown the standard dye solution has 35% of particles with a size no more than 1.1 nm. The biological treated the aqueous system contained particles, up to 20%, with a size range of 500 nm to 7000nm. The effect of the dye concentration, the incubation temperature and the pH on the colour reduction of the selected metal-complex azo dyes was studied. The evaluations were carried out by evaluating the colour reduction rate against the varied factors. For the colour reduction of Irgalan Grey GLN. the colour reduction rate was increased as the dye concentration increased, the optimum temperature of for the colour reduction was around 50°C, and the reduction rate has shown to be higher at pH range 6 to 9. Same results have shown to the colour reduction of the Irgalan Black RBLN and Irgalan Blue 3GL, except that the optimum operating temperature for the Irgalan Blue 3GL was at 40°C. Although the colour reduction of the Irgalan Yellow 3RL KWL was not significant, the effects of these factors were investigated. The maximum colour reduction was achieved at a dye concentration of 0.11 g dm-3, at a temperature of 40°C and pH range from 6 to 8.