The widespread production and use of plastics with poor recycling practices have resulted in higher discharge of plastic waste into the aquatic systems. Nanoplastics (NPLs) resulting from the fragmentation of microplastics, are considered as the most hazardous fraction. Adsorption to mineral surfaces is considered as one of the most important processes controlling the fate and transport of nanomaterials both in the natural environment and conventional filtration systems. However, adsorption mechanisms, influencing physicochemical parameters, and adsorption capacities of different porous media are still poorly known. In this study, the adsorption process of polystyrene (PS) NPLs to quartz sand, used as a filter medium in the drinking water treatment plant in Geneva (Switzerland), is investigated. Contrasting conditions are considered: ultrapure water and Geneva Lake water. Results indicate that PS NPLs dispersed in ultrapure water are adsorbed to the sand surface mainly due to the electrostatic interactions and adsorption capacity of the sand grains varies from 0.05 ± 0.01 mg g^-1 to 0.10 ± 0.02 mg g^-1 for PS NPLs concentrations comprised between 10 and 50 mg L^-1. The adsorption process is successfully described by Langmuir isotherm, therefore indicating the formation of a monolayer on the sand grain surfaces. SEM micrography confirms PS NPLs monolayer formation and image analysis indicates a maximum surface coverage ranging from 9 to 12%. Adsorption mechanisms of PS NPLs are significantly modified under environmental conditions (i.e., pH, ionic composition, presence of DOM). The adsorption capacity of sand grains for the removal of PS NPLs increased and varies from 0.05 ± 0.01 mg g^-1 to 0.7 ± 0.2 mg g^-1 for PS NPLs concentrations ranging from 10 to 50 mg L^-1. Increased adsorption capacity is due to heteroaggregation and cation bridging processes. SEM micrography shows aggregates attached to smooth sand surfaces and deposited in the vicinity of surface irregularities.