Reducing MeBr Emissions
with a Sheet Containing Titanium Dioxide

Y. Kobara^a, Y. Ishii^a, S. Ishihara^a and K. Inao^b

^a Laboratory of Environmental Pesticide Assessment, National Institute of Agro-Environmental Sciences, 1–1, Kannondai 3–Chome, Tsukuba, Ibaraki 305–8604, Japan

^bAgricultural Chemicals Inspection Station 2–772, Suzuki-cho, Kodaira, Tokyo 187–0011, Japan

Restrictions on methyl bromide usage have led to an intensive search for improved technologies to reduce both dosage and emission from fumigated plots into the atmosphere, while maintaining its effectiveness for disease and weed control. Improved field management practices such as the use of gas-tight films, shallow injection in combination with irrigation, deep injection (ca. 60cm depth), and application of ammonium thiosulfate or a soil bacterium, etc. have been shown to limit methyl bromide emission in several countries. Machinery injection methods can reduce the amount of methyl bromide application and its emission during exposure period. However, such injection techniques are not entirely suitable in Japanese conditions, as fields are generally too small to employ those methods. In addition, agricultural fields and residential areas coexist, and farmers themselves usually apply methyl bromide without depending on special applicators. Soil surface applications such as cold or hot gas methods are currently in vogue.

In previous studies, we found that using a gas-tight film (Orgalloy film, elf atochem) in surface applications considerably reduced emission loss to 7.6% of the applied amount during 7 days of application (1.4% through the film and 6.2% from surrounding soil surface of the treated area). However, emissions were high soon after removing the film, amounting to 33% emission over the entire period. The total emission is thus largely similar to that after using conventional films such as polyethylene. The standard dose of methyl bromide in Japan varies from 15 to 30 g/m^-2, which is near threshold level, and it is difficult to dramatically reduce the dosage by using a gas-tight film alone.

The purpose this study is to develop and evaluate a new multi-layer sheet for use in surface application of methyl bromide. The sheet consists of three layers: (top to bottom)—an impermeable layer, a photocatalyst layer, and a support layer. We presumed that emission could be reduced significantly if methyl bromide degradation is enhanced by a photocatalyst, although this approach has not necessarily been well documented at this time.

Titanium dioxide (TiO₂) was chosen as a photocatalyst for various reasons. TiO₂ is a photo- semiconductor causing a redox reaction on the surface with ultraviolet (UV) radiation (<400 nm). It may be easier to understand if it is regarded as the generator of active oxygen, such as superoxide anion (0₂-) and hydroxide (OH) radicals. TiO₂ helps to degrade methyl bromide to carbon dioxide (CO₂), hydrogen bromide (HBr), and water. In addition to having a strong photocatalytic action, TiO₂ is considered to be environmentally safe.

The top impermeable (to methyl bromide) layer was selected by measuring the UV transmittance. Both EVOH (0.060 mm: low-density polyethylene/ethylene-vinyl alcohol co-polymer/low-density polyethylene) and fluorinated polymer films (0.050 mm) are excellent with respect to barrier properties and UV-transmittance but EVOH was chosen because of the ease of heat-sealing this material.

A non-woven high-density polyethylene fiber sheet (Tyvek, DuPont) was chosen as a support layer of TiO₂. Tyvek has near 100% reflectivity due to diffuse reflection by polyethylene ultra fine fiber, and good gas permeability.

The TiO₂ photocatalyst (ST–01, Ishihara Sangyo Kaisha Ltd.) was suspended in the solvent, spread ca. 3 g/m² on Tyvek sheet, and then heat-sealed with a barrier film. After placing the sheet in the center of a separable chamber (effective irradiation diameter: 10 cm, the upper and lower chamber volumes ca. 400 ml and ca. 280 ml, respectively), distilled water (1 ml) and methyl bromide (2.5ml) were introduced into the lower chamber. Irradiation was performed with a 500-W Xe arc lamp approximated to AM 1.5 G at room temperature. Two detectors were used for measuring gas concentrations: a Brüel & Kjœr 1301 FT-IR-photoacoustic spectrometer to measure methyl bromide, carbon dioxide, and water vapor, and a gas chromatograph for methyl bromide.

Methyl bromide concentrations at the beginning of the test were about 6000 ppm, which decreased to a few ppm within 48 hours after irradiation. Degradation products of methyl bromide were identified as CO₂ and HBr. As generated HBr was neutralized immediately by the soil in field conditions, most methyl bromide recovered in the field at the end of the experiment was near the soil surface and the sheet.

However, the ability of the multi-layer sheet to decompose methyl bromide decreased with repeated use (up to 5 times) because of detachment of TiO₂. It was possible to prevent the detachment of TiO₂ by mixing it with ca. 10% polytetra-fluoroethylene fine particles as a binder.

Although decomposition and removal rates of methyl bromide are slow and dependent on solar radiation, methyl bromide concentrations below the sheet declined rapidly while the sheet was covering the field (7 or 9 days). Just before the removal of the sheet, methyl bromide concentrations between the sheet and soil surface decreased to a few ppm with the multi-layer sheet, as opposed to 1,000 ppm with a gas-tight film.

Our experiments also showed that methyl bromide emission was reduced to less than 1% of the applied amount by using the sheet containing TiO₂, against about 57% and 33% with polyethylene (0.05 mm thickness in traditional method) and gas-tight film, respectively. Moreover, methyl bromide concentrations below the multi-layer sheet and gas-tight film were largely similar until the middle of the fumigation period. This indicates that under field conditions, the use of multi-layer sheet may not greatly reduce the efficacy of methyl bromide fumigation.

The multi-layer sheet can be used easily and repeatedly without any major modifications in current practice of soil surface application. Further, the problem in disposing of the sheet is minimal. We, therefore, believe that the technique is useful for substantially reducing methyl bromide emissions and that multi-layer sheet containing TiO₂ holds promise for commercial use. At the same time, we must study ways to improve methods of application of various chemical alternatives to methyl bromide.

The studies reported here form part of a research project sponsored by the Environmental Agency of Japan.

Last Updated: July 1, 1999