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Critical Review of the Influence of Charge-Assisted Hydrogen Bonding on Fate, Transport, and Treatment of Representative Contaminants of Emerging Concern and Natural Organic Matter
Summary
Researchers reviewed how charge-assisted hydrogen bonding governs the adsorption of ionizable contaminants — including aged microplastics and nanoplastics, PFAS, pharmaceuticals, and pesticides — onto environmental surfaces and natural organic matter, showing that surface aging of plastic particles creates new CAHB-active sites that broaden their contaminant-binding pH range.
Understanding the fate and transport of contaminants of emerging concern (CEC) in natural and engineered systems is important to improve treatment. Adsorbents can retain and remove CEC; however, the widespread presence of natural organic matter (NOM) complicates such adsorption processes. The functional groups present in ionizable CEC, NOM, and adsorbent influence charge characteristics and adsorption affinity, with their effects varying according to pH conditions. Hydrophobic and electrostatic interactions are considered main driving forces. However, these two interactions cannot fully explain 1) the binding of ionizable CEC to adsorbents at specific environmental pHs and 2) the self-aggregation of NOM molecules to supramolecular aggregates. Charge-assisted hydrogen bond (CAHB) is a three-center-four-electron, low-barrier hydrogen bond with considerable covalent nature. Pignatello J.J., his contemporaries, and his colleagues demonstrated that the formation of CAHBs 1) facilitates adsorption characteristics at CAHB-favorable pHs, 2) increases pH by taking up protons from water, 3) shifts p K a upward, 4) contributes to homoconjugation of carboxylates, 5) affects surface charge of dissolved organic matter aggregates, and 6) holds NOM molecules together. This review article serves as a summary of CAHB papers with significant environmental implications, and as a critical evaluation of how CAHB influences the fate and transport of representative, ionizable CEC, such as (aged) microplastics and nanoplastics, per- and polyfluoroalkyl substances (PFAS), pharmaceuticals, pesticides, etc. This article also discusses how CAHB plays a role in the structure of NOM. Discussions herein will inform the community about the activity, reactivity, and treatability of ionizable CEC. • CAHB is a hydrogen bond with covalent characteristics. • CAHB-mediated adsorption follows the Δp K a rule (|Δp K a | < 3). • CAHB shifts apparent p K a upward, broadening favorable pH range to sorb ionizable CEC. • Environmental aging of microplastics and nanoplastics creates CAHB sites. • CAHB serves as a key cohesive force in supramolecular NOM architecture.