Rethinking functional units in activated carbon life cycle assessment: toward product-based, waste intake-based, and performance-oriented evaluation

The selection of an appropriate functional unit (FU) is a critical yet often overlooked aspect in the life cycle assessment (LCA) of activated carbon (AC) systems. Existing studies commonly rely on product-based FUs, such as impacts per kg of AC produced, or waste intake-based FUs, such as impacts per kg of precursor or waste feedstock processed. While useful for evaluating production efficiency or waste management performance, these FUs do not fully capture the functional role of AC in pollutant removal. This perspective examines how FU selection can influence environmental interpretation, indicating that product-based and waste intake-based metrics may lead to incomplete or less representative sustainability conclusions when adsorption performance is not considered. Drawing on recent literature and case study evidence, it is suggested that performance-based FUs, such as impacts per unit of pollutant removed, provide a more application-relevant assessment, although they may introduce challenges related to comparability and data consistency. To address this, a structured FU framework is proposed that integrates product-based, waste intake-based, and performance-based metrics within a tiered assessment approach. The study further highlights the interaction between process-level parameters and functional performance in shaping environmental outcomes. This work provides a practical foundation for improving consistency, transparency, and decision relevance in AC LCA studies and offers guidance for future research and reporting.