Physico-chemical investigations of interaction mechanisms between s-triazine herbicides and soil humic acids

Multiple binding mechanisms that may occur upon interactions between three soil humic acids of different origins and four s-triazine herbicides, differing in chemical structure and properties, have been investigated.Results of elemental, thermal, infrared and electron spin resonance analyses of the products of interactions demonstrate that different processes are involved and that they are controlled mainly by the content of the acidic functional groups and hydrogen binding capacity of humic acid and by the basicity of s-triazines.Experimental and theoretical evidence is given on the occurrence of electron donor—acceptor processes involving free rardical intermediates and leading to stable charge-transfer complexes between the adsorbant and the adsorbate. Parallels are suggested between the biological and chemical behaviour of s-triazines towards quinone-like structures in electron donor—acceptor systems.The feasible formation of covalent bonds, which leads to interaction products of enhanced molecular complexity, capable of stabilizing free radical intermediates, is discussed.Analysis of our data suggests that the higher the capacity of humic acids to form ionic and hydrogen bonds with s-triazines, the lower their effectiveness in forming electron-transfer complexes, as related to the lower ability to generate free radicals.Finally, our results show that the basicity of s-triazines, and hence their tendency to form ionic bonds, is not the main factor governing adsorption. Indeed the most basic prometone is not the most adsorbed; nevertheless, it appears to be the most efficient among s-triazines in giving rise to electron donor—acceptor processes with humic acids.