Manganese (Mn) oxides have a high surface area and redox potential that facilitate sorption and/or oxidation of organic carbon (OC), but their role in regulating soil C storage is relatively unexplored. Small OC compounds with distinct structures were reacted with Mn(III/IV)-oxides to investigate the effects of OC/Mn molar ratios on Mn−OC interaction mechanisms. Dissolved and solid-phase OC and Mn were measured to quantify the OC sorption to and/or the redox reaction with Mn-oxides. Mineral transformation was evaluated using X-ray diffraction and X-ray absorption spectroscopy. Higher OC/ Mn ratios resulted in higher sorption and/or redox transformation; however, interaction mechanisms differed at low or high OC/Mn ratios for some OC. Citrate, pyruvate, ascorbate, and catechol induced Mn-oxide dissolution. The average oxidation state of Mn in the solid phase did not change during the reaction with citrate, suggesting ligand-promoted mineral dissolution, but decreased significantly during reactions with the other compounds, suggesting reductive dissolution mechanisms. Phthalate primarily sorbed on Mn-oxides with no detectable formation of redox products. Mn−OC interactions led primarily to C loss through OC oxidation into inorganic C, except phthalate, which was predominantly immobilized in the solid phase. Together, these results provided detailed fundamental insights into reactions happening at organo−mineral interfaces in soils.
Pest control effectiveness and residues of pesticides
are contradictory
concerns in agriculture and environmental conservation. On the premise
of not affecting the insecticidal effect, the pesticide residues in
the later stage should be degraded as fast as possible. In the present
study, composite nanoparticles in a double-layer structure, consisting
of imidacloprid (IMI) in the outer layer and plant hormone 24-epibrassinolide
(24-EBL) in the inner layer, were prepared by the W/O/W solvent evaporation
method using Eudragit RL/RS and polyhydroxyalkanoate as wall materials.
The release of IMI in the outer layer was faster and reached the maximum
within 24 h, while the release of 24-EBL in the inner layer was slower
and reached the maximum within 96 h. The contact angle of the composite
nanoparticles was half that of the 5% IMI emulsifiable concentrate
(EC), and the deposition of composite nanoparticles on rice was twice
that of 5% IMI EC, which increased the pesticide utilization efficiency.
Compared with the common pesticide, 5% IMI EC, the insecticidal effect
of the composite nanoparticles was stronger than that of planthoppers,
with a much lower final residue amount on rice after 21 days. The
composite nanoparticles prepared in this study to achieve sustained
release of pesticides and, meanwhile, accelerate the degradation of
pesticide residues have a strong application potential in agriculture
for controlling pests and promoting crop growth.
Methylmercury
(MeHg) is a potent neurotoxin and has great
adverse
health impacts on humans. Organisms and sunlight-mediated demethylation
are well-known detoxification pathways of MeHg, yet whether abiotic
environmental components contribute to MeHg degradation remains poorly
known. Here, we report that MeHg can be degraded by trivalent manganese
(Mn(III)), a naturally occurring and widespread oxidant. We found
that 28 ± 4% MeHg could be degraded by Mn(III) located on synthesized
Mn dioxide (MnO2–x
) surfaces during
the reaction of 0.91 μg·L–1 MeHg and
5 g·L–1 mineral at an initial pH of 6.0 for
12 h in 10 mM NaNO3 at 25 °C. The presence of low-molecular-weight
organic acids (e.g., oxalate and citrate) substantially enhances MeHg
degradation by MnO2–x
via the formation
of soluble Mn(III)-ligand complexes, leading to the cleavage of the
carbon–Hg bond. MeHg can also be degraded by reactions with
Mn(III)-pyrophosphate complexes, with apparent degradation rate constants
comparable to those by biotic and photolytic degradation. Thiol ligands
(cysteine and glutathione) show negligible effects on MeHg demethylation
by Mn(III). This research demonstrates potential roles of Mn(III)
in degrading MeHg in natural environments, which may be further explored
for remediating heavily polluted soils and engineered systems containing
MeHg.
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