Owing to reactive functional groups on the surfaces of inorganic colloids and inside the macromolecular bodies of organic colloids, colloids are charged and stable in dilute aqueous media and sorb nutri- ents and contaminants. We particularly focus on relatively small colloids (called nano colloids hereafter) in this study, which may include various reactive colloids such as humic substances and very fine clay minerals or metal (hydr)oxides like ferrihydrite