Abstract Summary background Montmorillonite (Mt) as a low-cost and high-efficient adsorbent for cationic dyes has a promising application in dye wastewater treatment. However, proper disposal of the used Mt is critical to its application. Objective In this work, the used Mt after the adsorption of crystal violet (CV) was calcined under the protection of N 2 , with the purpose of transforming the used Mt into carbon monolayer–Mt nanocomposites. Methods Mt, the used Mt after CV adsorption, and the calcined Mt have been characterized by XRD, FTIR, Raman spectra, thermal analysis and elemental analysis. Results X-ray diffraction results showed that calcination led to the decrease of basal spacing of the used Mt from 2.06 nm to 1.34 nm. As such, the interlayer spacing of the calcined Mt is 0.38 nm, close to the thickness of a single graphene layer (0.34 nm), suggesting the formation of carbon monolayer within the interlayer space of Mt. FTIR characterization results showed that the infrared absorption bands of CV, which were clearly displayed on the infrared spectra of the used Mt, disappeared after the calcination. On the other hand, Raman spectra clearly showed the presence of D-band and G-band on the calcined Mt. These spectroscopy characterization results further proved the formation of carbon monolayer within the interlayer spaces of Mt. Elemental analysis results showed that about 40% carbon and 22% nitrogen from CV were transformed into the carbon monolayer at 600 °C calcination, and the transformation ratios decreased to 37% and 12% respectively at 800 °C calcination. Therefore, the obtained carbon monolayer was an N-doped graphene-like material. Conclusions As such, this work provided an easy way to transform the used Mt after the adsorption of cationic dyes into carbon monolayer–Mt nanocomposites.