| Abstract: | The adsorption of paraquat dichloride (1,1′-dimethyl-4,4′-bipyridylium dichloride) on a soluble sodium humate fraction of a Fenland soil was studied by gel filtration (on Sephadex G10 and G100) and by ultrafiltration (through an Amicon Diaflo UM-2 ultrafilter). Both methods depend upon the separation, on a molecular weight basis, of the unadsorbed molecules of herbicide from the adsorption complex (consisting of polymeric organic materials and the adsorbed paraquat). Separations were obtained on columns of Sephadex G10 (Method I) and in the ultrafiltration experiments (Method II), and isotherms were prepared from data for adsorption in water (by Method II) and in sodium chloride (by Methods I and II) solutions. Results from the two methods were comparable over the concentration range examined. The increased adsorption of paraquat by Na+-compared with Ca2+-humate is explained on the basis of the selectivity sequence of humate for exchangeable cations. Attempts to prepare isotherms from gel filtration data, for the adsorption of paraquat on two soluble model humic polymers (polyacrylic acid and a polymer prepared by the oxidative coupling of benzoquinone and ammonium chloride) were unsuccessful because binding to the gel matrix did not permit quantitative recoveries of the adsorption complexes. Paraquat was adsorbed to the same extent on each of four fractions of Na+-humate separated on Sephadex G100. |
