Application of laser-induced graphene in water treatment processes : from desalination to heavy-metal removal

Abstract

Growing pressures on water resources require the development of advanced materials that can be tailored for different water-treatment processes. Laser-induced graphene (LIG) is a porous carbon material produced through direct laser irradiation on polymeric precursors, offering a rapid and adaptable alternative to conventional graphene synthesis and functionalization methods. This dissertation investigated LIG as a material for environmental applications, focusing on how precursor chemistry and fabrication conditions influence its physicochemical properties and functionality. LIG was fabricated from polyimide (PI) and polyvinylidene fluoride (PVDF) and deposited on porous ceramic supports. Structural, chemical, morphological, and wetting characterization demonstrated that the polymer precursor remained an important factor for the characteristics of the material after laser conversion. PVDF derived LIG retained fluorinated carbon functionalities and exhibited a higher specific surface area, larger pores, and greater hydrophobicity than PI-derived LIG. The water contact angles of PVDF-LIG and PI-LIG were approximately 140 degrees and 129 degrees, respectively, while their specific surface areas were 242 and 130 m²/g. These results demonstrate that precursor selection can be used as a material-design strategy to tailor LIG surface chemistry and structure without requiring extensive post-synthesis functionalization. The feasibility of transferring LIG fabrication from planar surfaces to a tubular geometry was also evaluated by producing a PVDF-LIG coating on a ceramic tube and the resulting membrane was tested for water desalination in a direct contact membrane distillation process. It was observed that membrane performance was strongly dependent on coating thickness. Decreasing the LIG thickness increased total water transport and vapor fraction by reducing mass-transfer resistance, whereas thicker coatings increased tortuosity and restricted vapor transport. Although the tubular membranes initially achieved high salt rejection, rejection decreased during operation because of liquid penetration and pore wetting. These results identified pore structure, liquid-entry pressure, coating durability, and hydrodynamic conditions as critical parameters that require further optimization. The potential of PI-LIG and PVDF-LIG for Pb2+ removal was evaluated through kinetic, equilibrium, pH-dependent, and ionic-strength experiments. Adsorption was rapid and was better represented by pseudo-second-order kinetics and Langmuir equilibrium models. PVDF-LIG exhibited substantially higher maximum adsorption capacities than PI-LIG, reaching 2.68 and 1.60 mg/g at ionic strengths of 1 and 10 mM NaNO3, respectively. Surface complexation modeling indicated that Pb2+ uptake was not controlled exclusively by electrostatic attraction but was better described by complex formation with multiple effective reactive-site populations. Overall, this work demonstrates that LIG should be considered a tunable family of materials rather than a carbon material with fixed properties. Its precursor-dependent chemistry and adaptable architecture provide significant potential for desalination, adsorption, and environmental remediation. However, improvements in mechanical robustness, pore control, fabrication reproducibility, and long-term stability are required for practical implementation.