Accelerated Reduction Of Azo Dyes By Redox-active Humin

Azo dyes have been widely used in dyeing,food,rubber and many other industries.Because the release of azo dye into the environment during its production and application could cause great risks,the treatment of azo dye pollution has attracted wide attentions.As redox-active natural macromolecules,humic substances existed widely in water,soil and sediments.It has been found that soluble humic acid and fulvic acid could act as electron shuttle to improve the biological and abiotic reduction of azo dyes.However,whether insoluble humin,which is an essential component of humic substances,could also stimulate the(a)biotic reduction of azo dyes,and how would humin and other coexisting redox-active components in soil and sediment(e.g.,soluble humic substance and iron-containing mineral)together impact azo dye reduction,remained unknown.Redox-active humin was isolated from a typical paddy soil and then used to investigate detailedly its mediating activities in microbe-and sulfide-mediated azo dye reduction.Humin and humic acid samples were obtained from paddy soil via HF/NaOH extraction.The results of cyclic voltammetry,electron paramagnetic resonance and Fourier transform infrared spectroscopy indicated that,similar to humic acid,humin also possessed quinone and other redox-active groups.Reduced humin could stimulate azo dye reduction by Shewanella oneidensis MR-1.When 100-1000 mg/L humin was added,the reduction extent of Acid Red27(AR27)increased with the increase of humin concentration and was 5.9%-28.0% higher in15 h in comparison to those without humin.For systems containing different biomass(0.04-0.12 g/L)or AR27(100-1000 mg/L)concentrations,improved azo dye reduction was observed in the presence of reduced humin.Additionally,humin demonstrated stable mediating effects in four rounds of continuous reduction of 200 mg/L AR27.Scanning electronic microscopy analysis found that MR-1 cells attached on the surface of humin particles,indicating that humin might help electron transfer from MR-1 cells to azo dye via direct contact.It was also found that humin could stimulate the abiotic reduction of AR27 and other four azo dyes by sulfide.When 100-1000 mg/L humin was added,the AR27 reduction efficiency was increased from 56.3% to 79.3%-92.5%.The stimulation effects could be observed with 100-300 mg/L AR27 and increased with the increase of sulfide concentration(1.2-3.0 mM).Reduced humin showed better stimulation performance than the original one.Moreover,when humin coexisted with 10-30 mg/L humic acid or 1-4 g/L iron-containing smectite,the reduction performance was higher in comparison to those obtained in systemscontaining any single component.In reduction systems containing coexisting humin and SWy-2,humin could stimulate the production of Fe(II)in SWy-2,resulting in indirect stimulation of AR27 reduction and synergistic stimulation effects.These results demonstrated the improvement of azo dye reduction by redox-active humin and were conducive to treatment and remediation of contaminated soil.