Just read the Derivation of the Freundlich equation by Sposito (1980). Sposito cites that van Bemmelen was the first to suggest the equation for solution-to-solid sorption of ions in trace amounts.
Its a mater of shortness and a better credit goes always to the researcher who popularized it. You can find also articles with the use of van Bemmelen–Freundlich name. Here are some examples: