Oxidative and Photoxidative Polymerization of Humic Suprastructures by Heterogeneous Biomimetic Catalysis

Abstract

The meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of manganese(III) chloride [Mn-(TDCPPS)Cl] biomimetic catalyst immobilized on spacer-functionalized kaolinite clay mineral was employed in the oxidative coupling reaction of a dissolved humic acid (HA) suprastructure with either chemical (H2O2) or UV-light oxidation. The changes in molecular size of humic matter subjected to catalyzed oxidative reaction were followed by high-performance size exclusion chromatography (HPSEC) with UV-vis and refractive index (RI) detectors in series, and by thermogravimetric (TGA) analysis. Both the enhanced molecular size shown by differences between HPSEC chromatograms of humic reaction mixtures at either pH 6 or 3.5 and the increase of thermogravimetric stability suggest that the heterogeneous biomimetic catalysis promoted the stabilization of humic conformations by new intermolecular covalent bonds during oxidative coupling. The similarity between chemical and light-induced oxidation results suggests potential multiple applications of the kaolinite-supported heterogeneous catalyst in controlling the reactivity of natural organic matter within biogeochemical cycles and environmental reactions.

Influence of biodiversity and iron availability on soil peroxide: Implications for soil carbon stabilization and storage

“…More work is needed to confirm the occurrence of the free radical catalysis reaction as explained here and the stability of SOC (Nuzzo & Piccolo, 2013) or soil respiration (Huang et al, 2017;Piccolo et al, 2011). Because manure application rate is a major factor influencing soil C storage and microbial diversity (Liang et al, 2016), it may have an interactive effect within experimental places.…”

“…Possible mechanisms to stabilize SOC in soils via free radical reactions include those depicted in Figure 4. First, free radicals may benefit SOC stabilization and storage via rapid formation of intermolecular covalent bonds among soil components (Nuzzo & Piccolo, 2013;Piccolo et al, 2011), which increase the recalcitrance of SOC. Second, mobilized iron can be easily transformed into the newly formed reactive Fe (hydro)oxides (especially poorly crystalline Fe oxides; Kleber, Mikutta, Torn, & Jahn, 2005;Yu et al, 2017), which promote the formation of organo-mineral associations that are chemically stable (Baldock & Skjemstad, 2000;Huang et al, 2018;Kaiser & Guggenberger, 2007;Kleber et al, 2015;Koegel-Knabner et al, 2008;Lehmann & Kleber, 2015;Mikutta, Kleber, Torn, & Jahn, 2006;Wen, Liu, Deng, He, & Yu, 2019).…”

Effective carbon sequestration in Italian agricultural soils by in situ polymerization of soil organic matter under biomimetic photocatalysis

“…The evidence of SOC fixation in the Fe‐amended soil was also supported by the significant shift of carbon content in WSA towards larger size‐aggregates after the second and third experimental year, whereby the relative SOC enhancement in larger particles (0.50–0.25 mm) corresponded to a concomitant decrease of OC content in the <0.25 mm microaggregates (Figure ; Table S3). This indicates that the occurred photo‐oxidative coupling among humic phenolic molecules and consequent enhancement of SOM hydrophobicity induced the association of microaggregates and the formation of larger soil particle‐sizes (Kaiser, Kleber, & Berhe, ; Nuzzo & Piccolo, ; Spaccini, Piccolo, Conte, Haberauer, & Gerzabek, ).…”

“…As a more convenient alternative to fragile enzymes, synthetic biomimetic metal‐porphyrin catalysts that mimic the activity of oxidative enzymes, were found to produce larger and chemically stable humic molecules during oxidative (H 2 O 2 ) treatments of soluble humus (Piccolo, Conte, & Tagliatesta, ; Smejkalova & Piccolo, ; Smejkalova, Piccolo, & Spiteller, ). Being more suitable for soil applications than H 2 O 2 , dioxygen molecules under solar irradiation and in the presence of both homogeneous and heterogeneous metal‐porphyrin catalysts provide the highly oxidizing singlet oxygen that induces the photo‐oxidative formation, via a free radical mechanism, of intermolecular C–C and C–O–C bonds among humic phenolic molecules (Nuzzo & Piccolo, , ; Smejkalova & Piccolo, ).…”

“…Being more suitable for soil applications than H 2 O 2 , dioxygen molecules under solar irradiation and in the presence of both homogeneous and heterogeneous metal-porphyrin catalysts provide the highly oxidizing singlet oxygen that induces the photo-oxidative formation, via a free radical mechanism, of intermolecular C-C and C-O-C bonds among humic phenolic molecules (Nuzzo & Piccolo, 2013a, 2013bSmejkalova & Piccolo, 2005).…”