Modulation of selective cationic dye adsorption by alkali metal ions using quercetin cross-linked poly(vinyl alcohol) thin films

A series of polyvinyl alcohol films physically cross-linked with quercetin and loaded with different alkali hydroxides were synthesized through solution casting method. The incorporation of quercetin and alkali metals significantly modified the material properties resulting in the enhanced water up-taking, thermal stability and improved mechanical properties. The Li⁺-ion-embedded polymer composite (PQ-Li) exhibited the highest adsorption performance among the alkali metal ion-containing films. It achieved 96.3% removal of methylene blue from aqueous solution and showed excellent selectivity toward cationic dyes in mixed anionic–cationic dye systems. Furthermore, PQ-Li retained more than 90% of its initial adsorption efficiency after five consecutive adsorption–desorption cycles, demonstrating excellent reusability and long-term stability. The adsorption process was found to be influenced by the initial dye concentration, adsorbent dosage, pH and temperature. The adsorption kinetics follows a pseudo first order kinetics with the calculated adsorption capacity (Q e =4.747 mg g − 1 ) in close agreement with the experimental value (Q e =4.813 mg g − 1 ). The adsorption isotherm obtained was fitted well to the Langmuir model with high correlation coefficient of 0.9972. The thermodynamic studies revealed the endothermic nature of the adsorption. Overall, the PQ-Li sample combines with high adsorption efficiency, selectivity and recyclability, demonstrating its potential as an eco-friendly adsorbent for water remediation.