Toxic effects have been reported due to the accumulation of Cu in soils receiving long‐term applications of swine (Sus scrofa) manure, sewage, municipal composts, and mining waste. The objective of this study was to quantify sorption and desorption of Cu on various clay and clay‐organic matter fractions separated from a Zook soil (fine, smectitic, mesic Cumulic Vertic Endoaquolls) located in central Iowa. The clay was separated into four particle‐size fractions and portions of each fraction were treated with H2O2 and/or dithionite‐citrate‐bicarbonate (DCB) for removal of organic matter and free‐iron compounds, respectively. A batch‐equilibration technique was employed to evaluate sorption of Cu at pH 6.0 in the presence of 0.01 M CaCl2. The sorption isotherms show that Cu was preferentially sorbed on organic matter associated with the coarse clay fraction of the soil. After removal of organic matter, the fine clay (<0.02 µm) exhibited higher Cu retention than did the coarse (0.2–2.0 µm) and medium clay (0.02–0.2 µm). The DCB treatment created or exposed high‐affinity sites for Cu on the fine clay, but had little effect on Cu sorption for the coarse day. Copper appears to be specifically sorbed on the surfaces of silicate clays in excess of that which can be accounted for by variable charge sites. The results also suggest that Fe oxides may coat lateral surfaces of layer silicates, blocking access of Cu to potential sorption sites. Substantial sorption‐desorption hysteresis was observed for all samples, suggesting that Cu forms high‐energy bonds with both organic matter and layer‐silicate surfaces.

In vivo transformation of chlorpyrifos to chlorpyrifos oxon is believed to be a prerequisite for this insecticide to display acute toxicity to organisms. We discovered that active chlorine dispersed in water causes the rapid abiotic transformation of chlorpyrifos to chlorpyrifos oxon. The proposed mechanism for the transformation is an electrophilic attack by hypochlorous acid (HOCl) on the thion (P=S) double bond of chlorpyrifos followed by desulfuration of chlorpyrifos to its oxon analog. The pH of a Cl-aqueous solution controls the speciation of HOCl and hypochlorite (OCl-) in water and thereby influences the transformation process. Chlorpyrifos oxon is a potent anticholinesterase that is about 1,000 times more toxic than chlorpyrifos. Because chlorination is commonly used for treatment of domestic water supplies, the findings raise a new concern about the safety of domestic use of chlorpyrifos products.

Herbicide retention by residue mulch in vegetative filter strips could be an effective attenuation mechanism for removing herbicides from runoff. Adsorption studies were conducted to quantify the amount of atrazine and metolachlor that can be adsorbed and removed from runoff by thatch or fresh switchgrass residue. Adsorption of C 14 -atrazine and metolachlor on thatch or fresh switchgrass residue was conducted by using concentrations of 2.5, 7.5, 13.2, and 26.4 mmol L 21 and a 24-h equilibration period. Adsorption coefficients (K d ) decreased in the order, atrazine sorption on fresh switchgrass residue (81.1 L 21 kg 21 ), metolachlor sorption on fresh residue (32.9 L 21 kg 21 ), atrazine sorption on thatch residue (21.4 L 21 kg 21 ), and metolachlor sorption on thatch switchgrass residue (15.1 L 21 kg 21 ). On a volumetric basis (K dvol ), only atrazine showed a significantly greater K d-vol value on fresh residue than on the thatch residue. Absorption through cut ends of the residues (especially the fresh residue) may have produced adsorption capacities that would not be observed under field conditions. Fresh or thatch switchgrass residue in vegetative filter strips can help abate atrazine and metolachlor by intercepting and sorbing some of the herbicides.

An understanding of sorptive processes is key to describing the fate of chlorpyrifos [O,O-diethyl-O-(3, 5, 6-trichloro-2-pyridyl) phosphorothioate] in aquatic environments. The objectives of this study were to evaluate isotherms for adsorption and desorption of chlorpyrifos on colloidal materials and to advance understanding of interaction mechanisms between chlorpyrifos and colloidal materials. Six Ca-saturated reference smectites, one Ca-saturated humic acid (Ca-humate), and one suspended sediment sample, collected from the Upper Cedar River, Iowa, were studied. A batch equilibration technique was employed to quantify adsorption and desorption isotherms for chlorpyrifos over the 0 to 100 microg L(-1) concentration range in a 0.01 M CaCl(2) background. Large differences in sorption affinity and variation in desorption hysteresis were found among the smectites. Neither chlorpyrifos adsorption nor its desorption were correlated with cation exchange capacity, surface area, or surface charge density of the smectites. The evidence suggests that physical interaction between chlorpyrifos and smectites is the dominant mechanism for adsorption of chlorpyrifos in aqueous systems. Chlorpyrifos was very strongly sorbed on Ca-humate and was not desorbed from the Ca-humate back into the aqueous solution. Chlorpyrifos was moderately sorbed on river sediment, and a large adsorption-desorption hysteresis was also found. The study implies that the nature of both organic and inorganic materials in suspended sediment can influence the adsorption-desorption behavior of chlorpyrifos in aqueous systems.

A study was conducted to determine the half-life (t1/2), degradation rate, and metabolites of metolachlor in a water-sediment system and in soil with and without switchgrass. Metolachlor degradation in a laboratory was determined in sediment from Bojac sandy loam soil incubated at 24 degrees C. The study also was conducted in a greenhouse on tilted beds filled with Bojac soil and planted with switchgrass. In both experiments, samples were collected at days 0, 7, 14, 28, 42, 56, and 112 and analyzed for metolachlor and its major metabolites. The water-sediment oxidation-reduction potential took 28 d to reach -371 mV and the pH increased from 5.6 to 6.5 by the last sampling day (day 112). The average soil temperature of the tilted beds with or without switchgrass during the study was 21degrees C and the soil moisture content was 23% by volume. The t1/2 of metolachlor was 34 d in sediment and 8 d in the water phase. The t1/2 of metolachlor in soil from the switchgrass filter strip (6 d) was not different from that in soil without grass (9.6 d). The metolachlor metabolites ethanesulfonic acid (ESA) and oxanilic acid (OA) were detected in the water-sediment system and in soil from tilted beds. In both sediment and soil from tilted beds, the two metabolites peaked by day 56 of incubation and declined after that, indicating transformation to other products. In the water-sediment system, greater quantities of OA and ESA were detected in sediment than in the aqueous phase. The production of OA and ESA in the watersediment system occurred in the first 28 d, when the system was at an aerobic redox state. Metolachlor can degrade in sediment and the relatively high soil temperature and moisture level accelerated its breakdown in beds with and without switchgrass. Under warm and moist soil conditions, the presence of switchgrass has no effect on the degradation of metolachlor.