Persulfate oxidizing system for biomass pretreatment and process optimization

Highlights: • Biomass was degraded efficiently by persulfate activated systems. • Generated radical species were liable to modify and degrade biomass. • Base activated persulfate generated hydroxyl, sulfate, and superoxide anion radical. • The better radical generation was correlated to the higher sugar yield. • Oxidation pretreatment led to the lignin removal and the development of cracks and pores. - Abstract: The functions of radicals in the pretreatment of lignocellulose remain a fascinating topic about which there are more questions than answers. The effect of persulfate-based radical species on lignin degradation and carbohydrate yield was investigated. Wheat straw was pretreated with three persulfate activation systems which generated OH^• , SO₄^2−• , and O₂^{• −}. Radical species were monitored using nitrobenzene, anisole, and hexachloroethane, respectively. Pretreatment variables affecting sugar yield were optimized by using the response surface method based on central composite design. Enzymatic hydrolysis and compositional analysis were performed to quantify changes in biomass composition, lignin removal, and glucan yield. Fourier transmission infrared spectroscopic and scanning electron microscopy were used to characterize the effect of pretreatment on biomass structure. The maximum sugar yields were 66.8%, 49.4%, and 41.7% in base, heat, and hydrogen peroxide activated persulfate, separately, while untreated material showed only 3.7% glucan yield. The radical analysis confirmed the existence of radicals in these three systems. The results from compositional analysis and Fourier transmission infrared spectroscopic provided proof that parts of lignin and hemicellulose were degraded after the treatment. Scanning electron microscopy indicted that radical pretreatment converted the smooth surface of wheat straw to that of scattered, uneven morphology. Base-activated persulfate, capable of producing sulfate radical, hydroxyl radical, and superoxide anion radical was demonstrated as an efficient radical system to treat biomass.