MNM1D: a code for modeling colloid transport under variable ionic strength in saturated porous media

Beside advection-dispersion phenomena, colloid transport in saturated porous media is controlled by particle-particle and particle-collector interactions, that result in deposition (attachment) and release (detachment) kinetics, strongly affected by hydrochemical parameters. The present study develops and depicts a novel colloid transport model, where mobilization and immobilization onto the solid matrix are taken into consideration, and the effect of transients in ionic strength is systematically addressed and quantified by explicitly including a salt transport equation in the set of the partial differential model equations. Semi-empirical correlations between attachment/detachment coefficients and ionic strength are proposed, and used to tie the salt and colloid transport equations. The model is implemented in a finite-differences numerical code, called MNM1D (Micro- and Nanoparticle transport Model in porous media), that takes into consideration two distinct interaction sites with both linear and blocking (i.e. limited to a maximum colloid concentration onto the solid phase) attachment kinetics. The code can be downloaded from www.polito.it/groundwater/software. MNM1D code was validated comparing the results with well-established analytical and numerical solutions for colloid transport, applied in constant ionic strength conditions. The model was then used for interpreting a number of laboratory tests in 1D geometry, exploring a wide range of ionic strengths (1 to 300 mM) both in constant and transient conditions.