Abstract

Hydrogen-based autotrophic denitrification is a promising approach for nitrate removal in groundwater treatment systems. In biofilm reactors, process performance is often strongly constrained by mass-transfer limitations, particularly when hydrogen delivery, nitrate penetration, or biofilm structure restrict substrate availability to active microbial zones. However, these transport constraints operate together with biological, chemical, and matrix-related factors that can influence nitrogen selectivity and long-term stability. This review critically examines how diffusion regimes, biofilm structural properties, and reactor configuration govern substrate penetration, active-layer formation, and hydrogen utilization efficiency in hydrogen-based autotrophic denitrification. By integrating reported operational data with one-dimensional diffusion-reaction modeling and sensitivity analysis, the review distinguishes the dominant transport resistances in co- and counter-diffusional systems and identifies the parameters that most strongly affect predicted substrate profiles. The analysis consolidates current understanding of geometry-dependent transport constraints and highlights unresolved challenges related to effective diffusivity, biofilm structure control, parameter uncertainty, real groundwater matrices, and long-term operational stability. These insights provide a process-oriented perspective to support improved design, monitoring, and operation of hydrogen-based biofilm reactors for groundwater treatment.