Adsorption properties and mechanisms of methylene blue and tetracycline by nano-silica biochar composites activated by KOH

Dyestuff wastewater and pharmaceutical wastewater have become typical representatives of water pollution. In this study, a novel nano-silica-biochar composite (NSBC) was synthesized based on corn straw as raw material, by a combination of ball milling, pyrolysis and KOH activation. The modified biochar with rough surface had higher specific surface area (117.67–132.82 m2/g), developed pore structure (0.12–0.15 cm3/g) and abundant surface functional groups (-OH, –COOH, Si–O and aromatic CC were dominated). These provided abundant active sites for the adsorption of pollutants. The adsorption capacities of NSBC for Methylene Blue (MB) and Tetracycline (TC) were both higher than that of other similar products, the maximum adsorption capacity of Langmuir were 247.22 and 86.95 mg/g, respectively. After five adsorption-desorption cycle experiments, the adsorption capacities of NSBC for both were still excellent, reaching 99.30 and 19.87 mg/g, respectively. Due to the different structure and molecular size of MB and TC, the adsorption capacities of NSBC were significantly different, especially the influence of solution pH value. The adsorption mechanisms were comprehensively discussed by FTIR and XPS of the samples before and after adsorption, and combining experimental results of BET and simultaneously, which were manifested as monolayer chemisorption, specifically surface complexation, hydrogen bonding, n-π/π-π conjugation, electrostatic interaction and pore filling.