The isotherm adsorption of carbon microparticles prepared from pumpkin (Cucurbita maxima) seeds were studied and modelled. Experiments were done by evaluating carbon microparticles with various sizes (from 100 to 1000 um) for adsorbing curcumin (as a model of adsorbate) in an aqueous solution, and the results were derived and compared using the kinetics approach based on several standard isotherm adsorption models. Seven isotherm models were used to predict and determine the characteristic parameters: Langmuir, Freundlich, Temkin, Dubinin-Radushkevich, Flory-Huggins, Fowler–Guggenheim, and Hill-de Boer isotherm models. The results were then analyzed and accompanied by an adequate explanation related with the adsorption mechanisms and the determination techniques of its adsorption constants. The models showed that the interaction of adsorbates with carbon surface is done in multilayers with physical processes. Inorganic contents in the pumpkin seeds allow the formation of carbon with porosities, making more sites for the adsorption. The adsorbed molecules attract and associate with other free molecules. The adsorption is carried out on energetically different sites under an endothermic process. The Gibbs free energy confirmed that the adsorption is spontaneous. The results also confirmed that smaller adsorbent have direct impacts on the improving adsorption capacity (due to the existence of large surface area). Small-sized adsorbent (sizes < 500 um) has better additional adsorption (due to adsorbate-adsorbate interaction and possible existence of chemical interaction), resulting in the boosting adsorption capacity. This study is useful for further developments of carbon microparticles from organic waste material.