Synthesis and characterization of carbon quantum dots derived from oil palm mesocarp fibre embedded in xerogel for CO₂ removal

Malaysia’s palm oil industry generates substantial quantities of lignocellulosic biomass residues, including oil palm mesocarp fibre (MF), which remain underutilised for highvalue environmental applications. Concurrently, CO2 emissions have increased by 10.74% over the past decade, underscoring the need for sustainable and low-cost carbon capture technologies, underscoring the need for sustainable carbon mitigation strategies. This study addresses these dual challenges by valorising MF, a lignocellulosic by-product, as a carbon precursor for CO2 adsorbent development. Despite extensive research on carbon-based adsorbents, the instability of carbon quantum dots (CQDs) in aqueous systems and their limited application in fixed-bed CO2 capture remain key research gaps. In this study, MF was valorised into nitrogen/sulfur/oxygen doped carbon quantum dots embedded within a xerogel matrix (X-N/S/O-CQDs-MF) via hydrothermal synthesis using thiourea and KOH as doping and activating agents. The research gap addressed lies in the limited understanding of how ternary-heteroatom doping, CQDs embedment, and process parameters collectively govern CO2 adsorption mechanisms in biomass-derived CQD–xerogel systems. Among the synthesised formulations, X-N/S/O-CQDs-MF 133, prepared at an MF: thiourea: KOH ratio of 1:3:3 and a dilution ratio of 1:50, exhibited the highest CO2 adsorption capacity of 583.56 mg/g, supported by a high quantum yield (36.86%), effective heteroatom incorporation, and a well-developed mesoporous structure (20–50 Å). Breakthrough experiments revealed best adsorption capacity performance at 30 °C and a total gas flow rate of 100 mL/min, with enhanced CO2 uptake under dilute conditions (678.05 mg/g at 2 wt.% CO2). Isotherm analysis showed that CO2 adsorption followed the Redlich–Peterson model (R2= 0.9991), indicating heterogeneous surface interactions involving both monolayer and multilayer adsorption. Kinetic data fitted well with the pseudo-second-order model (R2 = 0.9935), indicating surface-controlled interactions consistent with chemisorption tendencies. However, the relatively low enthalpy change (ΔH = –8.75 kJ/mol) suggests that physisorption also contributes significantly. This supports a hybrid adsorption mechanism combining surface functional-group (i.e., (O–H, N–H, C–S) interactions (chemisorption) with weaker physical adsorption processes. Overall, this study demonstrates that xerogelimmobilised, heteroatom-doped CQDs derived from oil palm mesocarp fibre constitute a stable, tuneable, and effective CO2 adsorbent, operating through a temperaturesensitive hybrid adsorption mechanism suitable for low-concentration gas streams.