Sorption of chlorpyrifos to selected minerals and the effect of humic acid.
Sorption of chlorpyrifos (CPF) from 2.85 microM (1 mg/L) aqueous solutions in 0.01 M NaCl to montmorillonite, kaolinite, and gibbsite was investigated at 25 degrees C. Uptake of CPF by kaolinite and gibbsite was generally <10%, with pH having at most a small effect. Sorption to montmorillonite was significantly greater, with approximately 50% of the initial CPF being removed from solution below pH 5. Above pH 5 the sorption decreased to about 30%. About 70% of CPF was sorbed to kaolinite and gibbsite after 30 min, whereas on montmorillonite only 50% sorbed in an initial rapid uptake (approximately 30 min) followed by slower sorption, with a maximum achieved by 24 h. Although CPF desorbed completely from kaolinite in methanol, only about two-thirds was desorbed from montmorillonite. CPF has only a weak affinity for the surfaces of kaolinite and gibbsite. In the case of montmorillonite, sorption is significantly stronger and may involve a combination of sorption to external surfaces and diffusion into microporous regions. At pH >6 increased negative surface charge results in a lower affinity of CPF for the external surface. In the presence of 50 mg/L humic acid (HA) the amount of CPF sorbed on gibbsite and kaolinite was 3-4 times greater than that in the binary systems. The HA forms an organic coating on the mineral surface, providing a more hydrophobic environment, leading to enhanced CPF uptake. The HA coating on montmorillonite may reduce access of CPF to microporous regions, with CPF tending to accumulate within the HA coating.
Colloid and Environmental Chemistry Laboratory, La Trobe University, P.O. Box 199, Bendigo, Victoria 3552, Australia.
Tristan J
TJ
Van Emmerik
Michael J
MJ
Angove
Bruce B
BB
Johnson
John D
JD
Wells
D000327:Adsorption; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004390:Chlorpyrifos / Q000737:chemistry*; D006812:Humic Substances; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008903:Minerals / Q000737:chemistry*; D012996:Solutions; D014867:Water
D006812:Humic Substances; D008903:Minerals; D012996:Solutions; D014867:Water; D004390:Chlorpyrifos