Adsorption and biodegradation of the azo dye methyl orange using Ralstonia pickettii immobilized in polyvinyl alcohol (PVA)–alginate–hectorite beads (BHec-RP)

Abstract

Biological methods are widely used to treat dye waste, particularly methyl orange (MO) dye. The importance of MO degradation stems from its classification as a toxic dye. Within the scope of this research, successful bio-decolorization of MO was achieved through the use of Ralstonia pickettii bacteria immobilized in a PVA–alginate–hectorite matrix (BHec-RP). The optimum conditions for the degradation were observed at a composition of PVA (10%), hectorite (1%), static incubation, 40 °C, and pH 7. Subsequently, the adsorption kinetics of BHec-RP (dead cells) as well as the degradation kinetics of BHec-RP (live cells) and MO using free R. pickettii cells were evaluated. The decolorization of MO using BHec-RP (dead cells) is an adsorption process following pseudo-first-order kinetics (0.6918 mg g⁻¹ beads) and occurs in a monolayer or physical process. Meanwhile, the adoption of BHec-RP (live cells) and free R. pickettii cells shows a degradation process under pseudo-first-order kinetics, with the highest rates at an initial MO concentration of 50 mg L⁻¹ being 0.025 mg L⁻¹ h⁻¹ and 0.015 mg L⁻¹ h⁻¹, respectively. These results show that the immobilization system is superior compared to free R. pickettii cells. Furthermore, the degradation process shows the inclusion of several enzymes, such as azoreductase, NADH-DCIP reductase, and laccase, presumed to be included in the fragmentation of molecules. This results in five fragments based on LC-QTOF/MS analysis, with m/z values of 267.12; 189.09; 179.07; 169.09; and 165.05.