Sorption of cyromazine on humic acid: Effects of pH, ionic strength and foreign ions

Cyromazine (CY) is a triazine pesticide used as an insect growth inhibitor for fly control in cattle manure, field crops, vegetables, and fruits. Sorption of CY onto humic acid (HA) may affect its environmental fate. In this study, HA was used to investigate the sorption of CY at different solution chemistry conditions (pH, ionic strength) and in the presence of foreign ions and norfloxacin. All sorption isotherms fitted well with the Freundlich and Langmuir models. The sorption reached a maximum at initial pH 4.0 over the initial pH range of 3.0-7.0, implying that the primary sorption mechanism was cation exchange interaction between CY+ species and the negatively charged functional groups of HA. Increasing Ca2+ concentration resulted in a considerable reduction in the Kd values of CY, hinting that Ca2+ had probably competed with CY+ for the cation exchange sites on the surfaces of HA. The sorption of CY on HA in different ionic media followed the order of NH4Cl ≈ KCl > K2SO4 > ZnCl2 ≈ CaCl2 at pH 5.0. Spectroscopic evidence demonstrated that the amino groups and triazine ring of CY was responsible for sorption onto HA, while the carboxyl group and the O-alkyl structure of HA participated in adsorbing CY.