Mass transfer and diffusion regimes in hydrogen–based autotrophic denitrification biofilms: An integrative review

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.