Biofilm formation remains one of the most persistent challenges in water treatment systems due to the complex architecture and protective properties of microbial communities embedded within extracellular polymeric substances (EPS). Traditional evaluation of oxidative treatment strategies has frequently emphasized oxidant concentration, oxidation-reduction potential (ORP), or individual oxidant strength. While these parameters provide useful operational information, they do not fully describe the diverse oxidative mechanisms required to prevent, destabilize, and remove established biofilm structures.

Biofilms represent dynamic biological and chemical systems composed of microbial cells, extracellular polymers, proteins, lipids, extracellular DNA (eDNA), mineral associations, and transparent exopolymer particles (TEP). Effective oxidative biofilm control may therefore involve the coordinated action of multiple reactive species capable of targeting different structural and functional components of the biofilm matrix.

This manuscript introduces the concept of Oxidative Diversity as a framework to describe the capacity of oxidative chemistry to establish and sustain a dynamic network of reactive oxygen species and related oxidizing species capable of supporting multiple complementary oxidative mechanisms simultaneously within a complex system.

Rather than evaluating oxidative technologies solely by the strength or concentration of a primary oxidant, this framework considers the broader oxidative functionality provided by the diversity, reactivity, selectivity, persistence, and interaction of oxidative species. Reactive oxygen species (ROS), reactive halogen species (RHS), and reactive nitrogen species (RNS) relevant to water treatment are reviewed according to their mechanistic contributions toward biofilm prevention, destabilization, and removal.

By integrating established principles of biofilm science and oxidative chemistry, the Oxidative Diversity framework provides a mechanistic approach for understanding how different oxidative systems interact with complex biofilm environments.

Introduction

Biofilm control has historically been approached primarily through the lens of microbial inactivation. In water treatment applications, oxidative agents are commonly evaluated according to their ability to reduce planktonic microorganisms, maintain disinfectant residuals, or achieve specific oxidation-reduction potential (ORP) targets.

While these parameters remain important, biofilms present a substantially different challenge than suspended microbial populations.

Microorganisms within biofilms exist as organized communities embedded within a hydrated extracellular matrix. This matrix creates physical and chemical barriers that influence oxidant transport, reaction pathways, and treatment effectiveness ^[1]. As a result, microbial survival within biofilms is not determined only by the susceptibility of individual cells, but also by the resilience of the surrounding structure.

The extracellular polymeric substance (EPS) matrix consists of polysaccharides, proteins, lipids, extracellular DNA, mineral associations, and other organic components that contribute to adhesion, cohesion, and protection ^[2]. In aquatic environments, transparent exopolymer particles (TEP) can further contribute to organic conditioning layers that promote microbial attachment and biofilm initiation ^{[3] [4]}.

The complexity of biofilms suggests that effective oxidative control requires more than microbial inactivation alone. Oxidative processes may contribute through several complementary mechanisms, including:

• prevention of initial attachment and matrix formation,
• destabilization of protective extracellular structures,
• disruption of microbial function,
• enhancement of biofilm detachment and removal.

These mechanisms are influenced not only by oxidant concentration or oxidation potential, but also by the specific chemical behavior of the reactive species involved.

Different oxidative species exhibit distinct chemical behaviors that influence their function within complex biofilm environments. Their effectiveness is determined not only by oxidation potential, but also by reaction kinetics, selectivity toward specific molecular targets, ability to diffuse through structured matrices, stability within the aqueous environment, and participation in secondary oxidative pathways that can extend or regenerate oxidative activity.

Therefore, understanding oxidative biofilm control requires a shift from evaluating individual oxidants based on isolated chemical properties toward evaluating the broader oxidative functionality generated by dynamic networks of reactive species.

Oxidative agents and reactive species in water treatment

Oxidative water treatment technologies are commonly classified according to the chemical agent applied to the system, such as chlorine, ozone, hydrogen peroxide, chlorine dioxide, bromine-based oxidants, or other advanced oxidation processes ^[5]. While this classification is useful from an operational perspective, the biological and chemical effects produced within a biofilm environment are ultimately determined by the reactive species generated and their interaction with organic and microbial structures.

Different oxidative agents may produce a single dominant oxidizing species or establish a broader network of reactive intermediates. These species can include reactive oxygen species (ROS), reactive chlorine species (RCS), reactive bromine species (RBS), and reactive nitrogen species (RNS), each contributing different chemical behaviors based on their electron-transfer properties, molecular structure, stability, and reaction pathways. ^{[5] [8]}

The major oxidative agents used in water treatment and the principal reactive species associated with their oxidative activity are summarized below.

The presence of a reactive species alone, however, does not fully describe its contribution to biofilm control. Oxidative species differ substantially in how they interact with complex organic matrices. Highly reactive species may provide rapid oxidation and fragmentation of resistant structures, while more selective or stable species may provide longer persistence, deeper penetration, or targeted modification of specific biomolecules.

For example, hydroxyl radicals (•OH) exhibit among the highest oxidation potentials and react rapidly with a broad range of organic molecules, resulting in strong but localized oxidative activity ^[5]. In contrast, species such as hydrogen peroxide (H₂O₂), monochloramine (NH₂Cl), and chlorine dioxide (ClO₂) exhibit greater persistence and diffusion capability, allowing oxidative activity to reach regions that may be less accessible to extremely short-lived species. ^[13]

Similarly, reactive halogen species such as hypochlorous acid (HOCl) and hypobromous acid (HOBr) provide effective oxidation of biological structures through reactions with proteins, enzymes, and other cellular components ^{[10] [11]} , while additional reactive species may contribute to extended oxidative pathways and redox cycling.

The relative characteristics of major oxidative species relevant to water treatment are summarized according to oxidation potential, reactivity, selectivity, and stability.

These differences demonstrate that oxidative effectiveness cannot be described by a single parameter such as oxidation potential, concentration, or residual persistence. Each characteristic represents only one dimension of oxidative behavior.

In complex systems such as biofilms, where physical barriers, chemical gradients, and diverse molecular targets coexist, effective oxidative control depends on the ability of reactive species to contribute complementary functions. This broader oxidative functionality forms the foundation for evaluating oxidative processes beyond individual oxidant strength.

Mechanistic requirements for oxidative biofilm control

The structural complexity of biofilms creates a challenge that extends beyond microbial inactivation. While destruction of embedded microorganisms is an important objective, long-term biofilm control requires addressing the extracellular structures and biochemical interactions that allow biofilms to attach, develop, resist treatment, and re-establish after exposure to oxidative stress.

The extracellular matrix acts as both a physical barrier and a reactive environment. Oxidative species entering this structure encounter a complex network of polysaccharides, proteins, lipids, extracellular DNA, mineral associations, and other organic compounds. These components create multiple oxidative targets with different chemical properties, requiring complementary mechanisms for effective disruption. ^{[1] [13]}

As a result, oxidative biofilm control can be understood through three major functional objectives:

Biofilm prevention focuses on limiting the early conditions required for attachment and development. This includes oxidation of organic precursors such as transparent exopolymer particles (TEP), reduction of conditioning layer formation, maintenance of oxidative pressure, and interference with microbial communication pathways involved in coordinated biofilm growth. ^[3]

Biofilm destabilization targets the structural integrity of established biofilms. Oxidative processes contribute to fragmentation of extracellular polymeric substances (EPS), disruption of organic and mineral interactions, oxidation of extracellular DNA, modification of proteins and enzymes, and damage to microbial cellular structures. These mechanisms weaken the protective matrix and reduce the resilience that allows biofilms to persist.

Biofilm removal involves processes that enable oxidative activity to penetrate beyond the outer biofilm surface, degrade internal matrix components, propagate oxidative reactions, and promote detachment of weakened structures. Removal therefore depends not only on microbial control but also on progressive loss of matrix stability and cohesion. ^{[12] [13]}

The contribution of individual oxidative species varies across these mechanisms. Highly reactive species may rapidly attack resistant organic structures, while more stable or selective species may support penetration, sustained oxidative activity, or targeted reactions within specific biofilm regions.

The following framework organizes oxidative biofilm control according to the primary mechanisms required for prevention, destabilization, and removal, and identifies the main oxidative species contributing to each process.

The concept of Oxidative Diversity shifts the evaluation of oxidative systems from the presence of a single dominant oxidant toward the collective functionality generated by multiple oxidative pathways. A diverse oxidative environment is not defined only by the number of reactive species alone, but by the ability of those species to provide complementary functions.

These functions may include rapid oxidation of resistant organic structures, selective modification of specific molecular targets, penetration into protected environments, propagation of secondary oxidative reactions, and maintenance of oxidative conditions over time. ^{[5] [13]}

Importantly, Oxidative Diversity does not replace established chemical parameters such as oxidation potential or reaction kinetics. Instead, it integrates these properties into a broader framework that considers how different oxidative behaviors contribute collectively within complex environments.

For example, a highly reactive species may provide rapid oxidation but limited transport distance due to immediate consumption ^[13]. Conversely, a more stable or selective species may contribute through persistence, diffusion, or targeted reactions rather than maximum oxidative strength. Both types of activity may provide important contributions depending on the structure and stage of biofilm development.

Therefore, oxidative biofilm control should not be interpreted solely as a competition between stronger and weaker oxidants, but as the interaction between oxidative species with different and potentially complementary functional roles.

The relationship between major oxidative species and their functional contributions to biofilm control is summarized below.