Competitive sorption of metsulfuron-methyl and tetracycline on corn straw biochars

The application of biochars into soil may influence the fate of contaminants. Sorption and competitive sorption of metsulfuron-methyl (ME) and tetracycline (TC) on corn straw biochars prepared at 100 to 600°C (referred to as CS100-CS600) were examined. The Freundlich model described sorption isotherms well. The lower sorption capacity () of ME compared with TC on the biochars was attributed to the electrostatic repulsion between the anionic ME and the negatively charged biochars. No electrostatic repulsion between the zwitterionic TC and the biochars occurred. Tetracycline exhibited little competition with ME for CS100-CS300 in bi-solute systems, suggesting that partitioning of ME into noncarbonized rubbery organic matter might be the dominant sorption mechanism. The nonlinear isotherms of ME on CS400-CS600 and those of TC on CS100-CS600 suggested adsorption as the dominant mechanism. A significant increase in the Freundlich values in the bi-solute systems might be attributed to TC/ME occupying and blocking ME/TC-specific adsorption sites. The percentage decreases in of TC on CS400-CS600 with low ME concentrations were lower than those on CS200-CS300. This might be attributed to the lower steric restriction of ME for TC on CS400-CS600 because of their larger specific surface areas. The larger percentage decrease in of ME with TC from CS400 to CS600 may be explained by a decrease in the hydrophobicity and the increased sorption of TC with numerous polar groups. When using biochars as engineered sorbents to prevent the transport of ME from soil, the effect of coexisting TC needs to be taken into account.