title: Hydrolysis of carbaryl by carbonate impurities in reference clay SWy-2.
abstract: The influence of clay preparation methods on the sorption and hydrolysis of carbaryl (1-naphthyl, N-methyl carbamate) by K+-saturated reference smectite SWy-2 was studied. Four methods were utilized: (1) The reference (or specimen) clay used as received was K+-saturated (hereafter referred to as whole clay). (2) High-speed centrifugation (3295g) of whole clay resulted in a pellet with three discrete bands. The upper, light-colored, low-density band was obtained by manual separation (light fraction). The high-density, dark-colored material comprising the lower band (heavy fraction) was also obtained manually. (3) SWy-2 was subjected to overnight gravity sedimentation to obtain the <2 microm particles (clay-sed.) and then K+-saturated. (4) SWy-2 was subjected to low-speed centrifugation (58-60g) to separate the <2 microm particle size (clay-cent.) and then K+-saturated. Each preparation of mineral fractions manifested significantly different abilities to hydrolyze carbaryl to 1-naphthol, decreasing in the order whole clay > heavy fraction >> clay-sed. > light clay > clay-cent. The extent of 1-naphthol disappearance from solution, accompanied by a progressive darkening of the clay, followed the order whole clay > heavy fraction >>> light clay > clay-sed. > clay-cent. Using ring labeled [14C]carbaryl, approximately 61 and 15% of the total 14C activity added to the whole clay and light fraction, respectively, remained unextractable. X-ray diffraction of the heavy fraction revealed several peaks corresponding to minor impurities, including calcite and dolomite. Aqueous slurries of whole clay, light fraction, clay-sed., and heavy fraction were alkaline, whereas the pH of slurried clay-cent. was neutral. It was concluded that dissolution of inorganic carbonate impurities in SWy-2 caused alkaline conditions in the slurries leading to the hydrolysis of carbaryl. Dissolution of carbonates with sodium acetate buffer eliminated hydrolytic activity associated with SWy-2. None of the four preparation methods reliably removed inorganic carbonates. The use of commercial or reference smectites in surface chemistry studies should be accompanied by a treatment with acetate buffer to remove carbonate impurities.
authors.affiliation: Department of Crop and Soil Sciences, and Environmental Science and Policy Program, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: L Jacqueline
authors.initials: LJ
authors.lastname: Arroyo
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Teppen
authors.forename: Cliff T
authors.initials: CT
authors.lastname: Johnston
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Boyd
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D012721:Carbaryl / Q000737:chemistry*; D002254:Carbonates / Q000737:chemistry*; D000077215:Clay; D006868:Hydrolysis; D007306:Insecticides / Q000737:chemistry*; D012987:Soil / Q000032:analysis
chemical_list: D000538:Aluminum Silicates; D002254:Carbonates; D007306:Insecticides; D012987:Soil; D012721:Carbaryl; D000077215:Clay