Effect of stabilizer on silver nanoparticles that immobilized on activated carbon derived from durian skin for methylene blue removal

Abstract

Wastewater from textile industry resulted in organic pollutants, mainly dyes which could impair the quality of water and give negative effects to human and aquatic life, especially after a prolonged exposure to the pollutants. Thus, sustainable, and efficient technology to remove them is needed. Catalytic removal of dyes is one of the solutions that have been extensively studied. Silver nanoparticles (AgNPs) are among metal nanocatalysts that have been studied for the removal of dyes in wastewater. The most common dyes being tested is methylene blue (MB). However, using the catalysts alone are not convenient as the catalysts cannot be recycled. Therefore, one of the common solutions is to use support materials such as activated carbon. In this study, we use activated carbon synthesized from durian skins (abbreviated as AC-DS) to immobilize AgNPs. Nevertheless, AgNPs can easily leach from the surface of AC-DS if not properly immobilized. Therefore, research was conducted to investigate the role of stabilizers in the formation of AgNPs and how these stabilizers affect the efficiency of immobilized AgNPs on AC-DS in removing methylene blue. In this study, two types of stabilizers were employed: polyvinylpyrrolidone (PVP) and citrate (Cit). The results were compared to those of the AgNPs synthesized directly on AC-DS without any stabilizers. The effects of different activating agents, temperature and with and without acid washing were also determined. Then, the AgNPs were immobilized on AC-DS by precipitate-deposition method. The efficiency of AC-DS was evaluated based on percentage of MB removal. The faster change from blue MB solution to colorless solution indicate fast and more of MB being absorbed by the AC-DS. UV-Vis’s absorption was used to indicate these results quantitatively. Meanwhile, the performance of AgNPs immobilized on AC-DS was evaluated by catalytic removal of methylene blue using oxidation reaction in which hydrogen peroxide (H2O2) was used as oxidizing agent. Results obtained from this study show that the shape of AgNPs/Citrate were mainly round-like shape while for AgNPs/PVP, about half of the nanoparticles were rod-like or oval-like shape. 1:3 molar ratio of silver precursor-to-stabilizer was the best condition for the synthesis of AgNPs based on the highest AgNPs formation and longer stability. SEM analysis revealed that AC-DS activated by KOH shows more pores. Second, MB adsorption analyses resulted in the best MB removal by AC-DS synthesized using KOH, at 400oC and with acid-washing The AgNP/Citrate@AC-DS resulted in the highest percentage of MB removal (77.44±13%)) as compared to AgNPs/PVP@AC-DS (56.19±18%). In addition, AC-DS immobilised with both AgNPs had better recyclability than the AC-DS alone (only 42.33 ±33.34% of MB removal) showing about 50% improvement to the percentage of MB removal after being used once when the AC-DS was immobilised with AgNPs/Citrate, as compared to the AC-DS alone. These results indicate that the AgNPs that catalyzed the degradation of adsorbed MB by oxidation reaction had facilitated the regeneration of the adsorption sites on the AC-DS surface, thus improving its recyclability. The AgNPs (AgNPs /Citrate) immobilised with AC-DS yielded the highest MB removal percentage (77.4 ± 13%) compared with AgNP/PVP (AgNPs /PVP) immobilised (56.9 ± 18%). The AgNPs also improved the recyclability of AC-DS compared to AC-DS on its own (47.7 ± 4.7%). The AgNPs / Citrate immobilised AC-DS showed the best recyclability (47.3 ± 4.7%) compared with AC-DS (47.9 ± 4.7) on its own (45.3 ± 3.6%). The percentage of MB removed after use once when AC-DS is AgNP immobilised (47.33 ± 3.34%) showed approximately 50% improvement compared to the percentage after use alone. These results suggest that AgNPs, which catalyze the degradation of adorbed MB through oxidation reaction, have enabled the regeneration of adhesion sites on the surface of AC-DS, resulting in improved recyclability.