INFLUENCE OF DISSOLVED HUMIC-ACID AND CA-MONTMORILLINITE CLAY ON PESTICIDE EXTRACTION EFFICIENCY FROM WATER USING SOLID-PHASE EXTRACTION DISKS

Intermittent rain can influence the sediment load in surface runoff from agricultural fields,thereby causing variability in amounts of sediment and dissolved organic matter (DOM) in the water that could adversely affect extraction efficiency and ultimately the method sensitivity of pesticide analyses in water monitoring studies, Therefore, a study was conducted to determine the effect of purified sediment components, Ca-montmorillinite clay and commercial humic acid, on extraction efficiency of 12 pesticides from water using solid-phase extraction (SPE) disks. Batches of water at pH 6.0 and 8.0 were prepared at an ionic strength of 3 x 10(-3) M. Individual water samples (250 mL) at each pH containing 20 mu g L(-1) each pesticide were amended with all possible combinations of (a) commercial humic acid at either 0, 5, 10, or 25 mg L(-1) dissolved organic carbon (DOC) and (b) Camontmorillinite amounts of either 0, 0.01, 0.1, or 1 g. Samples were prefiltered to remove clay and then extracted using solid-phase extraction (SPE) disks. Pesticides eluted from disks were analyzed by gas chromatography/mass spectroscopy (GC/MS). Pesticides within chemical families reacted similarly to treatments of pH, Ca-montmorillinite, and humic acid. The effects of Ca-montmorillinite and humic acid were generally pH-dependent and acted independently in affecting extraction efficiency. Lower recovery of most pesticides was observed at pH 8 when Ca-montmorillinite was greater than or equal to 0.1 g and was attributed to greater dispersion of clay, increased surface area, and subsequent adsorption. Concentrations of DOC in humic acid had less effect on extraction efficiency when water was at pH 8 compared to water at pH 6, which was probably due to greater nonpolar interactions of pesticides to the charge-neutralized humic acid polymer.