ESTIMATION OF THE DIFFUSION-COEFFICIENTS OF ADSORBED AND NON-ADSORBEDSOLUTES IN SOIL

A method is proposed which follows Darrah's experimental procedure and takes advantage of a mathematical solution provided by Carslaw & Jaeg er to estimate the diffusion coefficients of adsorbed and non-adsorbed solutes in soil. The method requires only the values of the concentra tion of the solute at the input face of a uniform column of soil, C(s) , and of the total amount, Q(t), that has entered the soil after a spe cified time during which the surface of the block is in contact with a thin porous pad containing a known initial amount of solute, Q0, at c oncentration C0, expressed in the same units as C(s). In the C(s)/C0 v s. Q(t)/Q0 space there is a unique relationship between the effective diffusion coefficient, D(e), of the solute in the soil and the contact conductance for this solute, h, between the pad and the soil surface. The proposed procedure is firstly to determine D(e) and h for a non-a dsorbed solute in the experimental soil using the experimental values of C(s)/C0 and Q(t)/Q0 for that solute. This value of D(e) gives the d iffusion impedance factor for the solute in the soil, f, which is assu med also to apply to adsorbed solutes. A first estimate of the effecti ve diffusion coefficient of an adsorbed solute, 1D(ea), is then made u sing f and the diffusion coefficient of the free solute in water, D(L) , obtained from the literature (i.e. 1D(ea) = D(L)f). Only if the solu te is weakly adsorbed will the values of C(s)/C0 and Q(t)/Q0 lie in C( s)/C0 vs. Q(t)/Q0 space as defined by 1D(ea) and the contact conductan ce, h. Instead a second space relating C(s)/C0 and Q(t)/Q0 is now cons tructed from nominated values of h and D(e), where D(e) is defined in terms of 1D(ea), the adsorption coefficient, F, and the volumetric moi sture content of the soil, theta. The position of the experimental val ues of C(s)/C0 and Q(t)/Q0 within this new space defines h and the act ual D(e) and F of the solute as it diffuses and is adsorbed in the soi l. The advantages and limitations of the method are discussed. In part icular, the method assumes that the adsorption process is linear and r eversible.