Enhanced removal of methylene blue dye via combined adsorption and photocatalytic degradation using CdS@activated pumice stone/carrageenan composite beads

In this work, CdS@activated pumice stone/carrageenan composite beads (CdS@PCar) were fabricated for methylene blue removal via adsorption and photocatalytic degradation approaches. Additionally, activated pumice stone (PS) and pumice stone/carrageenan composite beads (PCar) were prepared as reference materials for batch adsorption studies. The analyzed characteristics of CdS@PCar demonstrated an average pore size of 6.0 nm, a pH PZC of 5.7, an energy band gap of 2.31 eV, and a large surface area of 262.3 m 2 /g, compared with PS (172.3 m 2 /g) and PCar (113.1 m 2 /g). The efficiency of methylene blue batch adsorption was evaluated through various nonlinear isothermal and kinetic models. The results proved CdS@PCar's greatest adsorption capability ( Q m , 439.6 mg/g) (vs PS 380.2; PCar 355.8 mg/g) under various conditions: a 1.5 g/L dose, pH 8, 2 h shaking period, and 18 °C. After 100 min of irradiation, CdS@PCar achieved 94% photocatalytic degradation of methylene blue, compared to 75% for CdS under identical conditions. Kinetic and activation parameters were estimated using Arrhenius and Eyring-Polanyi; reaction kinetics were analyzed using Langmuir-Hinshelwood. CdS@PCar showed high photocatalytic stability, with only 4% activity loss after 10 cycles under irradiation (solution volume: 300 mL; MB concentration: 20 mg/L; temperature: 18 °C; irradiation time: 60 min per cycle; catalyst dosage: 0.3 g/L), while Cd 2+ remained below the LOD (0.01 mg/L) and mass recovery reached 95.8%. Overall, CdS@PCar demonstrated its efficacy as an adsorbent and photocatalyst for methylene blue removal. • A novel K + -crosslinked pumice–carrageenan supported CdS@PCar composite was developed. • CdS@PCar showed surface area of 262.3 m 2 /g, pH PZC = 5.7, and energy band gap of 2.31 eV. • CdS@PCar showed a maximum Langmuir adsorption capacity of 439.6 mg/g. • CdS@PCar showed excellent reusability, with only 1.7% adsorption loss after 7 cycles.