Dual functionalized coconut shell bioadsorbent with chlorella microalgae and magnesium oxide for enhanced CO2 capture / Nuradila Zahirah Mohd Azmi

Nuradila Zahirah , Mohd Azmi (2025) Dual functionalized coconut shell bioadsorbent with chlorella microalgae and magnesium oxide for enhanced CO2 capture / Nuradila Zahirah Mohd Azmi. PhD thesis, Universiti Malaya.

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Abstract

The increasing carbon dioxide emissions resulting from industrial and human activities require the implementation of effective carbon capture systems. Adsorption using solid adsorbents, such as activated carbon has emerged as the most preferred among other technologies due to its simplicity and wide range of applications for CO2 adsorption and utilization. Nevertheless, conventional solid adsorbents have limitations such as the need for complex regeneration processes, poor efficiency, dependence on non-renewable resources, and high energy consumption for production. Hence, this study aims to develop a green bio-adsorbent from coconut shell (CS) which is a type of agricultural waste, to capture CO2, aligning with the principles of the circular economy. CS was chosen as a precursor for the synthesis of a bio-adsorbent due to its abundant access, with a worldwide production of 63.7 million metric tonnes annually. However, raw CS exhibits low adsorption efficiency due to lower carbon-to-surface ratio and surface area availability. To overcome this limitation, potassium hydroxide was used to activate the carbonised CS, hence promoting the development of a porous structure in an energyefficient and alkaline environment. Further enhancement involved dual-functionalisation using Chlorella microalgae and magnesium oxide (MgO) to introduce nitrogen and metal oxide functional groups, enhancing CO2 capture further due to basicity. The characterisation revealed the successful development of the ternary composite bioadsorbent, functionalised with microalgae and MgO (HCS-N-Mg), with specific surface area of 1045 m2/g, which is as high as the conventional activated carbon, and containing nitrogen and MgO functional groups. XRD analysis of HCS-N-Mg reveals crystalline peaks at 36.8°, 42.9°, and 62.4°, confirming the successful impregnation of MgO. The CO2 adsorption experiments were conducted using High-Pressure Volumetric Analysis, examining the effects of temperature and pressure using a design of experiments. The dual functionalised CS with microalgae and MgO, HCS-N-Mg, exhibits 50% higher CO2 adsorption capacity (2.63 mmol/g) than pristine bio-adsorbent, HCS (1.75 mmol/g). Statistical analyses suggest that the adsorption capacity of HCS-N-Mg is influenced by pressure, temperature, and their interaction. Adsorption isotherms show that the nonlinear Sips model best describes the CO2 adsorption onto HCS-N-Mg, indicating multilayer adsorption with surface heterogeneity of n=2.3, providing various adsorption sites for CO2 binding. Additionally, thermodynamic analysis confirms the chemisorption and stability of HCS-N-Mg at 25-75°C, with an enthalpy of -20.45 kJ/mol, compared to -13.23 kJ/mol for HCS, with stable adsorption at 25-50°C, indicating higher stability at elevated temperatures. Density functional theory (DFT) was applied to determine the CO2 adsorption mechanism onto bio-adsorbent surfaces. DFT result demonstrated that the dual-functionalisation enhances the binding interactions between the carbon surface and CO2. Compared to conventional adsorbent, HCS-N-Mg outperforms by 40%, highlighting the potential of microalgae and MgO in advancing carbon capture technology and addressing environmental challenges.

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