Characterization of Acid-Mechanical Milling Pretreated Rice Straw for Subcritical Water Hydrolysis

Ball milling of rice straw impregnated with sulfuric acid (RS-S), hydrochloric acid (RS-H), acetic acid (RS-A), or nitric acid (RS-N) were carried out in this study. Physicochemical analysis and subcritical water hydrolysis were performed to evaluate the effect of acid species on ball milling treatment of rice straw. Acetic acid and solo ball milling treatment showed little effect on solubility, thermal stability and crystalline structure of rice straw, while hydrochloric acid, acetic acid and nitric acid significantly improved the solubility and decreased the crystallinity index and thermal stability of rice straw. Sulfuric acid was found to be the most efficient acid to destroy the rice straw structure during ball milling followed by nitric acid and hydrochloric acid, attributed to its long retention on rice straw surface after drying. The effective cleavage of holocellulose-lignin chemical linkages in RS-S during pretreatment made the hydrolysis products of RS-S easier to be hydrolyzed to biochar at high reaction temperatures, resulting in the increased solid residue yield. The breakage of crystallites and holocellulose-lignin chemical linkages greatly improved the reactivity of RS-S, resulting in the lower temperature and activation energy required to initiate the hydrolysis reaction compared with those of rice straw and RS-A. H2SO4 was the most efficient acid to destroy the RS structure during ball milling. Acid-associated mechanical treatment can cleave the glycosidic bonds in RS. The solubility of RS was greatly improved after acid impregnation and ball milling. The destruction of hydrogen and glycosidic bonds made RS easier to be hydrolyzed. Biomass is a promising platform for both bioenergy and feed stock chemicals. However, the rigid structure of biomass made it difficult to be hydrolyzed and converted to biofuels and value-added chemicals in subcritical water. Acid/alkaline pretreatment, a broadly employed biomass pretreatment method could efficiently cleave the chemical bonds in biomass. However, it suffers equipment corrosion and environmental pollution because of the high concentration of acid/alkaline used. In this study, diluted acids (H2SO4, HCl, CH3COOH and HNO3) were employed to impregnate rice straw, followed by ball milling, in order to improve the accessibility of rice straw during hydrolysis in subcritical water. H2SO4 was found to be the most efficient acid to destroy the rice straw structure during ball milling, which resulted in the lower activation energy needed to initiate the hydrolysis reaction in subcritical water, and higher contents of value-added chemicals in the hydrolysate.