Novel Humo-Pectic Hydrogels for Controlled Release of Agroproducts

Abstract

Sustainable and precision agriculture requires that agroproducts be employed for plants’ benefits with maximum efficiency and reduced environmental hazard. Since hydrogels may become efficient vehicles for agroproduct spreading, we prepared novel humo-pectic hydrogels by first blending low methyl-esterified pectin with either humic substances (HS) extracted from different composted biomasses or humic-like substances (HULIS) from depolymerized lignocellulose waste derived by biorefinery activities and, then, by cross-linking with calcium chloride through ionotropic gelation. Hydrogels were characterized by solid-state 13C NMR spectroscopy, scanning electron microscopy (SEM), magnetic resonance imaging (MRI) and rheological analyses. Their ability to work as controlled-release systems was tested by kinetically monitoring the release of a previously incorporated phloroglucinol. Our results indicated that the hydrogels made with HULIS (HP-HULIS) showed a better controlled-release property than those containing HS (HP-HS). This highly depended on the larger content of oligosaccharides, lesser porosity, and greater cross-linking density of HP-HULIS as compared to HP-HS. Our study suggests that the development of a technology based on such novel humo-pectic hydrogels as carriers of bioactive compounds may represent a promising strategy for the controlled and environmentally safe delivery of agroproducts to crops.

Bio-Based Hydrogels Composed of Humic Matter and Pectins of Different Degree of Methyl-Esterification

“…The signals in the 110-145 ppm interval derived from unsubstituted phenyl carbons in either lignin monomers, lignans, flavonoids or unsaturated lipid structures, while those in the phenolic region (145-160 ppm) were indicative of O-and C-substituted aromatic structures, pertaining to lignin and lignan components. Finally, the intense signal in the carbonyl region (160-190 ppm) at about 172 ppm referred to carboxyl and amide groups in aliphatic acids and peptides, respectively [24].…”

“…Molecules 2020, 25, 2936 3 of 13 signal in the carbonyl region (160-190 ppm) at about 172 ppm referred to carboxyl and amide groups in aliphatic acids and peptides, respectively [24]. Spectral integration revealed a composition dominated by carbohydrates (60-110 ppm) for all hydrogels, but H-HMP, H-HMP-HS and H-HMP-HULIS showed a smaller content of carboxyl groups (190-160 ppm) and a larger presence of methoxyl substituents (60-45 ppm) than H-LMP, H-LMP-HS and H-LMP-HULIS (Table 1).…”

“…Furthermore, MRI experiments for the different hydrogels allowed us to measure the apparent diffusion coefficients (Diff), which enabled an evaluation of the extent of free diffusion of water molecules within the hydrogel matrix. Diff values decrease when water molecules encounter physical barriers to their diffusion, whereas they increase if water can move more freely through a less compact hydrogel (longer free diffusion distance) [24]. H-HMP-HS showed the greatest Diff value (Figure 4), thus indicating a fast diffusion rate of water molecules through this sample, probably because of the large amount of micro-size pores, whereas H-LMP and H-HMP revealed the smallest Diff values, which proved a much more compact structure in these hydrogels and a limited water diffusion capacity.…”

“…Up to strains of about 3%, the storage G' modulus was independent of the applied strain for all the investigated systems, thus allowing the identification of the linear viscoelastic region. Since all the rheological experiments should be conducted in the linear viscoelastic region to avoid any disruption of the sample structure [24], a strain of 2% was chosen for the subsequent frequency sweep tests.…”

“…Finally, for H-LMP-HS and H-HMP-HS, the n values considerably smaller than 0.5 suggested that phloroglucinol was released by a pseudo-Fickian diffusion. Deviations from the ideal Fickian diffusion may be explained by the erosion of the hydrogel structure due to a dissolution of HS molecules in the aqueous solution [24]. The data obtained by measuring phloroglucinol release from hydrogels were used in the Korsmeyer-Peppas mathematical model [38,39] and the relative kinetic parameters are shown in Table 2.…”