Response Characteristics of Colloid Migration to pH and Ionic Strength and Its Numerical Simulation
Colloids significantly influence contaminant transport in groundwater, yet their behavior under varying hydrochemical conditions remains inadequately characterized. This study employed quartz sand as a surrogate porous medium to investigate colloid transport through column experiments under controlled pH and ionic strength (IS). Breakthrough curves (BTCs) were obtained for a conservative tracer and for colloids under nine combinations of pH (5, 7, 9) and IS (1, 5, 10 mmol·L−1). Hydrus-1D, incorporating a two-site kinetic sorption model, was used to simulate colloid transport and derive key parameters: attachment rate (k1a), detachment rate (k1d), straining rate (k2a), and maximum retained concentration on site 2 (Smax2). Results demonstrated that increasing pH and decreasing IS enhanced colloid mobility. Specifically, at IS = 1 mmol·L−1, normalized peak concentrations (C/C0) were 0.33, 0.40, and 0.72 for pH 5, 7, and 9, respectively. At pH = 7, C/C0 decreased from 0.40 to 0.18 and 0.12 as IS increased from 1 to 5 and 10 mmol·L−1. The fitted transport parameters accurately captured these trends, with R² ≥ 0.95 across all conditions. Mechanistically, higher pH increases negative surface charge and electrostatic repulsion, while higher IS compresses the double layer and reduces repulsion, thereby inhibiting transport. These findings provide quantitative insights for predicting colloid-facilitated contaminant migration in subsurface environments.