Effects of C/Mn Ratios on the Sorption and Oxidative Degradation of Small Organic Molecules on Mn-Oxides

Abstract

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.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

“…For instance, introducing organic acids can modify the relative rates of olation and oxolation reactions, causing the transformation of ferrihydrite into goethite, magnetite, and lepidocrocite, facilitated by the formed Fe(II) [ 67 ]. Similarly, partial reduction of Birnessite by organic carbon can increase the proportion of Mn(II, III) on the edge sites of Birnessite and further lead to the formation of MnOOH and Mn 3 O 4 [ 18 , 20 , 66 , 70 , 73 , 74 ].…”

“…For organic carbon, the co-precipitation with iron minerals will alter its biogeochemical cycling, offering higher stability against leaching, runoff, and biodegradation [ 194 , 202 , 205 , 207 , 208 ]. Mn minerals can also be co-precipitated with organic carbon during the oxidative precipitation of Mn(II) [ 18 , 20 ], although fewer studies have been reported. Metals such as Ca and Al can be precipitated with alkalinity and involve organic carbon to form co-precipitates, while only several studies reported the formation of Ca-organic carbon [ 209 , 210 ] or Al-organic carbon coprecipitates [ 48 , 211 ], as they assumably often remain in the soluble form in the environment.…”

“…Polymerization reactions of organic carbon, aided by minerals, can stabilize due to the increased size/complexity and the higher energy threshold against degradation [ 18 , 25 , 212 ]. Mn(IV)-oxides, as a strong and efficient oxidant, can accelerate oxidative polymerization of polyphenols with the combination of semi-quinine radicals, which is referred to as the “browning” process that has been acknowledged as the potential pathway for the abiotic formation of humic substances [ 20 , 25 , 63 , [212] , [213] , [214] ]. In addition, Fe minerals in either soluble ions or solid forms can catalyze the Maillard or oxidative reaction for geo-polymerizing small organic molecules into larger ones [ 26 , 215 ].…”

“…Polymerization could stabilize the organic carbon by increasing its size and complexity, thereby raising the energy threshold in microbial respiration [ 212 , 256 , 257 ]. For example, Birnessite could enhance the binding and polymerization of small organic carbon ( e.g., phenolic monomers) with the formation of large molecules [ 18 , 20 , 214 , 258 ]. Iron (hydro)oxides have also been reported to promote the transformation of small molecules to bigger organic molecules, generally accompanied by sorption and co-precipitation, leading to further enhancement of carbon stability [ 215 ].…”

“…The interactions between specific organic carbon and minerals are generally coupled with multiple routes, leading to a complicated impact on the carbon sequestration potential. It has been confirmed that interaction with minerals could significantly change the speciation and properties of the SOC via redox reaction, polymerization, and catalytic reaction, directly affecting its stability and bioavailability [ [17] , [18] , [19] , [20] , [21] ]. In addition, sorption, complexation, and co-precipitation between organic carbon and minerals would form the mineral-organic carbon association, protecting the organic carbon against degradation [ [22] , [23] , [24] , [25] ].…”