This study examines the deposition/release mechanisms involved in colloid retention under unfavorable conditions through theoretical analysis and laboratory column experiments. A Maxwell approach was utilized to estimate the coupled effects of both primary- and secondary-minimum deposition. Theoretical analysis indicates that the secondary energy minimum plays a dominant role in colloid deposition even for nano-sized particles (e.g., 20 nm) and primary-minimum deposition rarely happens for large colloids (e.g. 1000 nm) under unfavorable conditions. Polystyrene latex particles (30 nm and 1156 nm) and clean sand were used to conduct three-step column experiments at different ionic strengths and a constant pH of 10. The results confirm that small colloids can also be deposited in the secondary minimum. Additional column experiments involving flow interruption further indicates that the deposited colloids can be spontaneously released to bulk solution when the secondary energy minimum is comparable to the average Brownian kinetic energy. Experimental collision efficiencies are in good agreement with Maxwell model predictions but largely different from the theoretical calculations by the interfacial force boundary layer approximation. Our study proposes a priori approach to estimate collision efficiencies accounting for both primary- and secondary-minimum deposition and suggests that the reversibility of colloid (e.g. viruses and bacteria) deposition must be considered in transport models for accurate predictions of their travel time in the subsurface environments.