The role of oxygen-based functional groups in the photoluminescence of dehydrated carbon dots (DCs) and the adsorption mechanism of dye molecules onto the surface of DC is investigated. DC were prepared from orange peel for the first time via the chemical dehydration effect of sulfuric and phosphoric acid at 180 oC. We compared the emission spectra of DCs in different solvents in great detail. The solvatochromism of DC in different solvents is discussed. The role of oxygen-based functional groups in the light emission process is examined. Adsorption of methylene blue (MB) on the surface of DC was studied at different contact times, pH, concentrations, and temperatures. In this work, we used Langmuir and Freundlich adsorption models for the analysis. Sorption kinetic data were found to fit well with the pseudo-second-order model. Our results also showed increased MB adsorption capability with temperature. The results are essential for the application of CQDs, such as in wastewater treatment.