RATE CONSTANTS FOR METAM-SODIUM CLEAVAGE AND PHOTODECOMPOSITION IN WATER

Metam-sodium is a soil fumigant with many important uses in agriculture. In soil the compound is rapidly and efficiently converted to methyl isothiocyanate (MITC), a volatile biologically active product. In this laboratory study the kinetics of conversion of metam-sodium to MITC, methylthiourea (MTU), and 1,3-dimethylthiourea (DMTU) in water were investigated. In the dark metam-sodium decomposed with a rate constant of 3.3 X 10(-4) min-1 (t1/2 = 35 h, 25-degrees-C, pH 7) and was converted to MITC with a yield of 15 mol %. In spite of weak absorption of near-UV light, metam-sodium photodegraded rapidly in the photoreactor with a near-UV quantum yield of 0.36 +/- 0.093 (n = 7). Photoproducts included MITC and DMTU, and the MITC yield was 19 mol %. These findings demonstrate that metam-sodium is considerably more stable in water than in soil. When exposed to midday, midsummer sunlight in shallow water, however, the compound is predicted by modeling to photodecompose with a half-life of less than 1 h.