Towards a mechanistic understanding of carbon stabilization in manganese oxides - PubMed (original) (raw)

Graham Purvis 2, Elisa Lopez-Capel 3, Caroline Peacock 4, Neil Gray 2, Thomas Wagner 2, Christian März 2, Leon Bowen 5, Jesus Ojeda 6, Nina Finlay 1, Steve Robertson 1, Fred Worrall 7, Chris Greenwell 7

Affiliations

Towards a mechanistic understanding of carbon stabilization in manganese oxides

Karen Johnson et al. Nat Commun. 2015.

Abstract

Minerals stabilize organic carbon (OC) in sediments, thereby directly affecting global climate at multiple scales, but how they do it is far from understood. Here we show that manganese oxide (Mn oxide) in a water treatment works filter bed traps dissolved OC as coatings build up in layers around clean sand grains at 3%w/wC. Using spectroscopic and thermogravimetric methods, we identify two main OC fractions. One is thermally refractory (>550 °C) and the other is thermally more labile (<550 °C). We postulate that the thermal stability of the trapped OC is due to carboxylate groups within it bonding to Mn oxide surfaces coupled with physical entrapment within the layers. We identify a significant difference in the nature of the surface-bound OC and bulk OC . We speculate that polymerization reactions may be occurring at depth within the layers. We also propose that these processes must be considered in future studies of OC in natural systems.

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Figures

Figure 1

Figure 1. SEM, TEM and FIB images of birnessite coating.

(a) Scanning electron micrograph showing intact birnessite coating on sand grains. (b) Focused ion beam image of birnessite coating showing light and dark laminae at both μm and nm scale. (c) FIB image showing close-up of vugs within birnessite coating, apparently partially infilled with lighter (in colour) precipitates. (d) Transmission electron micrograph of birnessite coating showing both 7 and 10 Å poorly crystalline birnessite (akin to δMnO2). Images a–c are all in back-scattered mode showing atomic contrast.

Figure 2

Figure 2. Micro-FTIR images of birnessite coating.

(a) Micro-FTIR spectroscopy image and spectra of the surface of an intact birnessite coating (washed repeatedly in de-ionized water) showing presence of organic carbon (at three separate points representing low, medium and high OC concentrations). (b) Micro-FTIR spectroscopy image and spectra of a (different) surface of an intact birnessite coating (washed repeatedly with 1 M NaOH) showing presence of organic carbon (at 3 separate points representing low, medium and high OC concentrations) at lower concentrations than when washed in de-ionized water. Red-yellow colours show higher concentration of OC and green-blue colours show lower concentration of OC. In the spectra, the blue band shows the C-H stretch between 3,000 and 2,800 cm−1 and also labelled are the C=C (1,650 cm−1) and carbonyl (1,750 cm−1) peaks as well as the νsym COO- (1,400 cm−1) and νasym COO- (1,595 cm−1 in (a) and 1,565 cm−1 in (b)) peaks.

Figure 3

Figure 3. TGA-DSC data on birnessite coating.

(a) Carbon dioxide evolution with temperature and synthesized deconvolution components (see Methods section for more details) for bulk Mn-OC showing thermally labile Mn-OC (in pink), thermally refractory Mn-OC (in blue) and inorganic carbon (in purple; TIC is 0.5%w/wC). (b) Differential scanning calorimetry (black) curve showing labelled CO2 evolution at 270 and 600 °C (exotherms) and CO3 2− breakdown at 712 °C (endotherm); m/z 44 curve (green); m/z 18 curve (blue) and thermogravimetric weight loss (red).

Figure 4

Figure 4. XPS C1s data on thermally treated bulk birnessite coating.

Normalized and mean (_n_=9) XPS scans of the high-resolution C1s spectra of bulk birnessite coating both as received (MnOAR Bulk) and thermally treated by burning to 550 °C for 8 h (MnO550 Bulk), de-convoluted using synthetic component fitting (see Methods section). There are three main components evident in the bulk birnessite coating, and there are two main components evident in the bulk birnessite coating that has been burned to 550 °C. The most probable chemical assignments for these components are described in Table 2. The pie chart shows the proportion of hydrocarbon that is aromatic or aliphatic in character in each of the ‘as received' and ‘thermally treated' bulk coating. The inset shows N1s scan. Note ‘aromatic' in pie chart actually represents alkene/aromatic peak at ∼285 eV.

Figure 5

Figure 5. XPS C1s data comparing surface and bulk birnessite coating.

Normalized and mean (_n_=9) XPS scans of the high resolution C1s spectra of bulk birnessite coating (MnOAR Bulk) and the surface birnessite coating (MnOAR Surface, intact on the sand grain), both as received, de-convoluted using synthetic component fitting (see Methods section). There are three main components evident in the bulk birnessite coating, and there are four main components evident in the surface birnessite coating. The most probable chemical assignments for these components are described in Table 2. The pie chart shows the proportion of hydrocarbon that is aromatic or aliphatic in character in each of the ‘as received bulk' and ‘as received surface' coating. Note ‘aromatic' in pie chart represents alkene/aromatic peak at ∼285 eV and mean components are not Gaussian. Inset shows N1s scan.

Figure 6

Figure 6. Hypothesized reaction mechanism between DOC and birnessite.

Hypothesized reaction mechanism for conversion of DOC by birnessite coatings both at the surface ((a) points 1–3) and in the bulk material ((b) points 1–2). The dashed arrows represent phenol–birnessite interactions resulting in phenoxy radical formation. The original DOC molecule is oxidized (via the breaking of aromatic bonds6) at point 3 resulting in the release of LMW molecules into solution as the birnessite surface is subjected to backwashing in the WTW. The alkene/aromatic:aliphatic ratio (XPS signature) of the resulting smaller molecule DOCx attached to the birnessite surface is shown in pie-chart a. Once DOCx becomes trapped (shown in (b)) the reactions between phenols and the birnessite surface are likely to result in a build up of phenoxy radicals as these are less likely to be washed away at depth in the bulk material. The notably different XPS signature of the bulk material is shown in pie-chart (b).

Figure 7

Figure 7. Reaction scheme between birnessite and phenol groups.

Surface complex formation between birnessite and phenol groups in DOC. In the above reaction scheme the protonation state of the birnessite surface functional groups (as Mn(III/IV)OH2+2/3 or MnOH(III/IV)−1/3), the phenol ion (as ArOH or ArO−) and reaction intermediates are ignored. However, because the pKa of poorly crystalline hexagonal birnessite is ∼2 (ref. 26), while the pKa of most phenolic –OH groups is greater than 9 (ref. 52), then at pH∼9 in the WTW system, the majority of the birnessite surface groups will be present as negatively charged Mn(III/IV)OH−1/3, while phenol ions will be present as both ArOH or ArO−.

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