Nanoparticle surface charge properties represent key parameters to predict their fate, reactivity and complexation in natural, biological and industrial dispersions. In this context, we present here an original approach to better understand the surface charge electrostatic properties of spherical nanoparticles (NPs). The ion distribution around one nanoparticle is investigated using Monte Carlo simulations and by adjusting a wide range of parameters including NP properties (surface charge density and site distribution), salt concentration (ionic strength and cation concentration) and salt valency (mono-, di- and trivalent salt). A canonical Metropolis Monte Carlo method is used to reach equilibrium states and a primitive Coulomb model is applied to describe the electrostatic interactions between explicit discrete sites, counterions and salt particles. Our results show that the presence of explicit surface charges on the NP and in solution has a strong influence on the local ion distribution and on the effective surface charge of the nanoparticles. The increase of surface charge density reduces the NP effective charge by the formation of a condensation layer around the nanoparticle. However, a limit of condensation is achieved due to steric effects and electrostatic repulsions. The presence of di- and trivalent cations is also found to strongly modify the effective charge and improve condensation state as long as electrostatic repulsion between the cations close to the surface are not so strong. At high trivalent cation concentration, the NP effective charge is greatly reduced and the local environment around the nanoparticle becomes more structured with the formation of a multi layer structure composed by anions and cations.