Microwave–assisted hydrothermal conversion of crop straw into value–added products via magnesium acetate

Rational utilization of crop straw is conducive to sustainable development of society. This study focused on the mechanism for the effect of magnesium acetate on the microwave–assisted hydrothermal conversion (MHTC) products of crop straw for more efficient use of resources. The results showed that magnesium acetate improves the hydrothermal liquid yield and quality of hydrochar from crop straw under MHTC. With increasing amount of magnesium acetate, there were obvious increases in TOC concentration and organic acid concentration, while notable decreases in total sugar concentration of hydrothermal liquid. Magnesium acetate promoted the conversion of nitrogen and phosphorus elements to inorganic forms and the release of potassium, calcium, and magnesium ions, leading to increases in both conductivity and pH value of the hydrothermal liquid. Furthermore, magnesium acetate promoted the hydrolysis of crop straw, facilitated the formation of carbon microspheres, and greatly increased the surface area, mesopore volume, and average diameter of hydrochar. With increasing addition amount of magnesium acetate, there was an increase in oxygen–containing groups in the hydrochar. The equilibrium moisture content of the hydrochar remained at high levels (54.09–61.30 wt%), with the maximum values ranging from 60.71 wt% to 61.30 wt%. Finally, the concentrations of acetate and magnesium ions were found to be two parameters with the highest correlation and contribution with the organic matter concentration in hydrothermal liquid and also the greatest influence on the physicochemical properties of hydrochar by affecting the hydrolysis process of crop straw.