Laboratory study on heterogeneous degradation of methyl chloroform (CH₃CCl₃) on aluminosilica clay minerals as its potential tropospheric sink

Abstract

Heterogeneous reactions of methyl chloroform (1,1,1-trichloroethane, CH3CCI3) with alminosilica clay particles were examined between 288 and 3 !3 K as a potential sink. Allophane and halloysite particles exhibited the activity to transform CH3CCI 3 to 1,1-dichloroethene (CH:=CCI•). The dependence of the reaction rate on the pretreatment, temperature, and relative humidity suggested that the reaction could proceed under environmental conditions. Illumination of light with wavelength longer than 300 nm did not affect CH3CCI 3 decay but caused heterogeneous degradation of CH:=CCI: on day particles. The lower limit of overall sticking coefficient for fresh allophane and halloysite particles was estimated to be 6 x 10 -9 and 1.1 x 10 -s, respectively. Heterogeneous degradation of CH3CCI 3 on mineral aerosols and/or soils is worth taking into consideration in deriving tropospheric OH concentrations from global budget concentrations of CH3CCI 3. IntroductionThe Other processes are not considered although ca3ccl 3 is known to be readily decomposed by the surface reactions under dry conditions. For example, ca3cel 3 was found to be converted to 1,1-dichloroethene We reexamined the interaction of CH3CC13 on the clay minerals kaolinitc, halloysite, montmorillonite, and allophane particles. Kaolinitc and halloysite are among the 1:1 type layer silicate minerals, while montmorillonite is a 2:1 layer silicate [Murray, 1988]. Allophane is an amorphous aluminosilica phase [Jackson, 1964] We here report that allophane and halloysite particles play an active role in the decomposition of CH3CC13. We also report on the activity as different reaction conditions such as the relative humidity. The overall uptake coefficients were obtained, and the accommodation coefficient for fresh clay particles was extrapolated. Finally, the implications of this reaction on clay particles as potential tropospheric sinks for CH3CCI3 are briefly evaluated using published modeling studies of mineral dust distributions [Tegen and Fung, 1994;Dentener et al., 1996]. The objective of the present study was to promote reanalyses of the AIF./GAGE/AGAGE data considering the impact of heterogeneous reactions on mineral aerosols and soils as tropospheric sinks. 6611

Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink

“…3.2.3. We also found previously that the decomposition rate of CH 3 CCl 3 on clay minerals (halloysite and allophane) decreased with increasing relative humidity (Kutsuna et al, 2000a). Figure 12 shows the k 1 value and the amount of CH 2 =CCl 2 produced in a 114-min reaction at different relative humidities.…”

“…For the montmorillonite samples, the k 1 value varied from 1 × 10 −5 to 41 × 10 −5 s −1 for 50 mg of sample. In a previous study (Kutsuna et al, 2000a), the montmorillonite sample examined showed a small value of k 1 (montmorillonite-p in Table 1), but in this study several of the montmorillonite samples exhibited much larger k 1 values. As for the illite samples examined, the k 1 value of one material (no.…”

“…3.1 and shown in Fig. 13, were performed in a 0.85-dm 3 closed-circulation Pyrex reactor, which is described elsewhere in more detail (Kutsuna et al, 2000a), so only a brief summary will be given here. The reactor consists of a removable cell in which clay samples are put, a magnetic-driven glass pump, and a White-type optical cell with an optical path length of 3 m by which the gas mixture is analyzed using an FTIR spectrometer (JEOL Winspec 50) at a 0.5 cm −1 resolution with 50 scans.…”

“…It is known that CH 3 CCl 3 can easily decompose heterogeneously on sands and metal surfaces under dry conditions (Rebbert and Ausloos, 1979). Kutsuna et al (2000a) reported that dehydrochlorination of CH 3 CCl 3 occurred easily on clay minerals (allophane and halloysite) at 313 K and indicated the possibility that the heterogeneous decomposition of CH 3 CCl 3 on clay minerals at the ground surface in arid regions would serve as a potential tropospheric sink. Clay minerals are among the main components of soils and dusts.…”

“…35*) are different lots for the same kind of clay mineral. Four kinds of clay minerals examined in the previous study (Kutsuna et al, 2000a) are also listed for comparison in Table 1, although they are not API standard clay materials. All of the samples were purchased from Iwamoto Mineral Co., Japan.…”