1. It might be as a result of the NaOH having high affinity for the acid thereby increasing the solubility of the acid. Hence, increase in desorption.

2. At acid pH, ionization is high, hence the smaller -OH ions through ion exchange can displace the bulky dye anion.

Dear Idris-Hermann Tiotsop Kuete , since you did the adsorption experiment at an acidic pH, you will likely form the less soluble free acid of the anionic dye. I assume that desorption using NaOH is more effective, as the more soluble sodium salt of the anionic dye is regenerated. Please refer to the attached reference for similar observations.

Thank you very much for your answer Frank T. Edelmann

Dear Beniah thank you very much for your answer

Dear Idris-Hermann Tiotsop Kuete, it's certainly due to the inccrease of pH by NaOH which favorised the anionic exchanger regeneration with Na-salts formations normaly if more soluble in water, could also participate and could make the regeneration easier . Also, make sure how the anionic exchanger was made of.