Quantitative Evaluation of Noncovalent Interactions between 3,4-Dimethyl‑1<i>H</i>‑pyrazole and Dissolved Humic Substances by NMR Spectroscopy

Nitrification inhibitors (NI) represent a valid chemical strategy to retard nitrogen oxidation in soil and limit nitrate leaching or nitrogen oxide emission. We hypothesized that humic substances can complex NI, thus affecting their activity, mobility, and persistence in soil. Therefore, we focused on 3,4-dimethylpyrazole phosphate (DMPP) by placing it in contact with increasing concentrations of model fulvic (FA) and humic (HA) acids. The complex formation was assessed through advanced and composite NMR techniques (chemical shift drift, line-broadening effect, relaxation times, saturation transfer difference (STD), and diffusion ordered spectroscopy (DOSY)). Our results showed that both humic substances interacted with DMPP, with HA exhibiting a significantly greater affinity than FA. STD emphasized the pivotal role of the aromatic signal, for HA-DMPP association, and both alkyl methyl groups, for FA-DMPP association. The fractions of complexed DMPP were determined on the basis of self-diffusion coefficients, which were then exploited to calculate both the humo-complex affinity constants and the free Gibbs energy (Kd and ΔG for HA were 0.5169 M and −1636 kJ mol–1, respectively). We concluded that DMPP-based NI efficiency may be altered by soil organic matter, characterized by a pronounced hydrophobic nature. This is relevant to improve nitrogen management and lower its environmental impact

Novel Nanomaterials Made of Humic Substances from Green Composts and Chitosan Exerting Antibacterial Activity

Green compost represents a sustainable source of humic substances (HS), which are supramolecular associations of heterogeneous natural compounds bearing a variety of functional groups. Novel humic-chitosan nanoconjugates (NC) were synthesized by using HS from three green composts [coffee husks (HS-COF), artichoke residues (HS-CYN), and fennel residues (HS-FEN)]. Solid-state NMR spectra showed that HS-CYN and HS-FEN contained the largest relative amount of polysaccharidic and phenolic carbons, whereas HS-COF had the greatest relative content of alkyl and carboxyl groups. While size and z-potential of NC depended upon the HS origin, NC-HS-FEN revealed the greatest thermal stability, followed by NC-HS-CYN and NC-HS-COF. Scanning electron microscopy and transmission electron microscopy analyses suggested the formation of spherical nanoparticles only for NC-HS-FEN and NC-HS-CYN. This was explained by more stable conformations of HS-FEN and HS-CYN that enabled regular spherical nanoparticles, as assessed by high-pressure size-exclusion chromatography measurements. NC bioactivity against bacterial human pathogens was strain-specific, and the inhibition of bacterial development was attributed to the positive z-potentials that facilitated NC adhesion on bacterial cell walls, smaller nanosizes favoring cell penetration, and subsequent release in the cell of NC toxic components that altered microbial biochemical functions. This study indicates that not only novel NCs can be obtained by humic matter derived from recycled biomass, such as green compost, but they can also be employed as effective and sustainable antimicrobial agents

Waste to Wealth Approach: Improved Antimicrobial Properties in Bioactive Hydrogels through Humic Substance–Gelatin Chemical Conjugation

Exploring opportunities for biowaste valorization, herein, humic substances (HS) were combined with gelatin, a hydrophilic biocompatible and bioavailable polymer, to obtain 3D hydrogels. Hybrid gels (Gel HS) were prepared at different HS contents, exploiting physical or chemical cross-linking, through 1-ethyl-(3-3-dimethylaminopropyl)carbodiimide (EDC) chemistry, between HS and gelatin. Physicochemical features were assessed through rheological measurements, X-ray diffraction, attenuated total reflectance (ATR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscopy (SEM). ATR and NMR spectroscopies suggested the formation of an amide bond between HS and Gel via EDC chemistry. In addition, antioxidant and antimicrobial features toward both Gram(−) and Gram(+) strains were evaluated. HS confers great antioxidant and widespread antibiotic performance to the whole gel. Furthermore, the chemical cross-linking affects the viscoelastic behavior, crystalline structures, water uptake, and functional performance and produces a marked improvement of biocide action