Adsorption of methabenzthiazuron on six allophanic and nonallophanic soils: Effect of organic matter amendment

This article reports on methabenzthiazuron [1-(1,3-benzothiazol-2-yl)-1,3-d imethylurea] (MBT) adsorption process on six agricultural allophanic and no nallophanic soils. The effect of amendment with exogenous organic matter wa s also studied. Adsorption kinetic fits an hyperbolic model. MBT adsorption reached an apparent equilibrium within 2 h and followed a second-order rea ction. The maximum adsorbed amounts for natural soils ranged from 32 to 145 mug g(-1). Rate constants were considered relatively low (0.27-1.5 x 10(-4 ) [mug g(-1)](1-n) (s-1)); the slow process was attributed to a combined ef fect of diffussion and adsorption. MBT adsorption fits the Freundlich model with r values greater than or equal to0.998 at P less than or equal to 0.0 01 significance levels. K-f and Freundlich exponents (1/n) ranged from 5.3 to 82.1 cm(3) g(-1) and from 0.66 to 0.73, respectively. K-f values for soi ls with a low organic matter content were lower than that obtained from the only typical allophanic soil derived from volcanic ash under study. Lineal regression analysis between K-f and organic matter content of nonallophani c soils gave a correlation coefficient of 0.980 (P = 0.02). Dispersion of K -d values together with close values of K-OM indicate that organic matter ( OM) was the principal component responsible for MBT adsorption in unamended soils. Addition of peat decreased soil pH and increased adsorption capacit y for allophanic and nonallophanic soils. Kinetic experiments showed enhanc ements of X-max values and lower rate constants.