Adsorption isotherms, degradation kinetics, and leaching behaviors of cyanogen and hydrogen cyanide in eight texturally different agricultural soils from China

Abstract

Cyanogen (C₂N₂) is a new and effective alternative soil fumigant to methyl bromide. The effects of soil properties on the fate of C₂N₂ and its degradation products, including hydrogen cyanide (HCN), are not fully understood. The objectives of this study were to determine the adsorption kinetics, adsorption isotherms, and degradation kinetics of C₂N₂ and HCN in texturally different soils and evaluate their leaching potentials using soil columns. Eight agricultural soils were collected throughout China: Luvisols (Hebei Province), Phaeozems (Heilongjiang Province), Gleysols (Sichuan Province), Anthrosols (Zhejiang Province), Ferralsols (Jiangxi Province), Lixisols (Hubei Province), Alisols (Shandong Province), and Plinthosols (Hainan Province). The adsorptions of C₂N₂ and HCN in C₂N₂-fumigated soils were positively correlated with organic matter and clay contents. For a C₂N₂ dose of 100 mg kg^-1, the adsorptions of C₂N₂ and HCN were highest in Phaeozems and lowest in Gleysols according to their adsorption coefficients (15.744 and 3.119, respectively). No significant difference in the half-life of C₂N₂ and HCN was observed between sterilized and unsterilized soils, indicating that abiotic degradation was predominant in the degradation of C₂N₂ and HCN. After leaching, the residual C₂N₂, HCN, NH₄⁺-N, and NO₃^--N concentrations in C₂N₂-fumigated Phaeozems were highest within 15 cm of the soil surface (30, 20, 19.68, and 10.41 mg kg^-1 soil, respectively). The results indicate that C₂N₂ and HCN have short lifetimes and low leaching potentials in agricultural soils, even under heavy rainfall conditions. The findings demonstrate that C₂N₂ and HCN resulting from fumigation will not accumulate in the soil and are not likely to contaminate groundwater.

Keywords: Adsorption; Agricultural soil; Cyanogen; Degradation; Hydrogen cyanide; Leaching.