Two-step hydrothermal synthesis of manganese oxide impregnated fruit waste derived adsorbent for cationic dye removal / Fathimath Afrah Solih

Fathimath Afrah , Solih (2025) Two-step hydrothermal synthesis of manganese oxide impregnated fruit waste derived adsorbent for cationic dye removal / Fathimath Afrah Solih. PhD thesis, Universiti Malaya.

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Abstract

The global production of biomass waste is significant, with an estimated 180 billion tonnes generated yearly. In recent years, research has increasingly focused on resource recovery from non-toxic biomass waste due to its high carbon content, renewability, and abundant availability. Therefore, considering the notable characteristics of agricultural waste biomass, this study aims to develop a low-cost fruit-waste-based adsorbent to remove the cationic dye from aqueous solutions. Unlike conventional biomass-based adsorbents, this study introduces a novel hybrid synthesis combining avocado seeds and banana peels, chemically modified to optimize adsorption performance. These inexpensive, carbon-rich biomass materials have mesoporous surfaces and contain oxygenated functional groups, which are crucial for dye removal. Furthermore, hydrothermal carbonization is an innovative and energy-efficient thermochemical process that enhances the yield of adsorbent materials. To further enhance its performance, the adsorbent underwent phosphoric acid activation and manganese oxide impregnation. Chemical activation enhances the physicochemical characteristics of the biomass materials, while manganese oxide impregnation reduces agglomeration and facilitates electrostatic attractions for removing cationic dyes. The physicochemical analysis revealed the metal-impregnated adsorbent (HA/Mn) and acid-activated adsorbent (HA) from avocado seed and banana peel (RA) demonstrated a significant increment in surface area (666 m2/g) compared to activated HA (535 m2/g) and unmodified RA (1.19 m2/g). The functional group analysis indicates that HA/Mn is an efficient adsorbent as the modification introduces additional hydroxyl, carboxyl, carbonyl, phosphate, and manganese functional groups. Adsorption study also revealed that HA/Mn completely removed the colour, which is 37% to 33% higher than that of the raw avocado seed (63%) and raw banana peel (67%), respectively. HA/Mn also shows 29 % increase in adsorption capacity (199 mg/g) compared to the raw adsorbent banana peel (112 mg/g) and 18 % higher than raw avocado seed (181 mg/g). The validation study demonstrated that HA/Mn is the most efficient with a COD removal of 85 % with 18 % more colour removal and 23 % increased adsorption capacity compared to RA. Besides, kinetic and isotherm studies confirmed that HA/Mn fitted well with the pseudo-second-order kinetic model and the Redlich-Peterson isotherm model with a β value of 0.9412, suggesting a dominantly homogenous chemisorption process. This study also integrated machine learning in biomass-based dye removal, significantly improving large-scale predictability and efficiency. The Random Forest Regression Model demonstrated high accuracy in predicting dye removal efficiency, both at the laboratory and large-scale application levels, enhancing the scalability of this approach. The reusability study revealed that HA/Mn has a 48 % higher regeneration capacity, leading to an over 80 % reduction in adsorption costs, making it a highly economical alternative to commercial adsorbents. Additionally, HA/Mn has a higher removal colour efficiency of 6-18 % in industrial wastewater than in commercial AC with a surface area of >100 m2/g. In summary, these findings advance adsorption science by introducing a novel, scalable, and economically viable hybrid adsorbent, reinforcing the role of biomass-based solutions in sustainable dye wastewater treatment and circular economy initiatives.

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