Hydrothermal carbonization (HTC) is a novel thermal conversion process that can be used to convert municipal waste streams into sterilized, value-added hydrochar. HTC has been mostly applied and studied on a limited number of feedstocks, ranging from pure substances to slightly more complex biomass such as wood, with an emphasis on nanostructure generation. There has been little work exploring the carbonization of complex waste streams or of utilizing HTC as a sustainable waste management technique. The objectives of this study were to evaluate the environmental implications associated with the carbonization of representative municipal waste streams (including gas and liquid products), to evaluate the physical, chemical, and thermal properties of the produced hydrochar, and to determine carbonization energetics associated with each waste stream. Results from batch carbonization experiments indicate 49-75% of the initially present carbon is retained within the char, while 20-37% and 2-11% of the carbon is transferred to the liquid- and gas-phases, respectively. The composition of the produced hydrochar suggests both dehydration and decarboxylation occur during carbonization, resulting in structures with high aromaticities. Process energetics suggest feedstock carbonization is exothermic.

The compositions of humic acids (HAs) from various Histosols in North America and Europe, of similarly treated plant‐extracted materials (PEMs), of coal‐extracted humic acids, and of International Humic Substances Society (IHSS) Florida peat were quantified by solid‐state 13C nuclear magnetic resonance (NMR). In order to obtain quantitative intensities, the peak areas in direct‐polarization 13‐kHz magic‐angle spinning (DPMAS) 13C NMR spectra were corrected for incomplete relaxation by factors measured in cross‐polarization spin‐lattice relaxation time (CP/T1) experiments with total sideband suppression (TOSS). The elemental compositions (%C, %H, %O + N) of a peat sample, 8 HAs and 2 PEMs were estimated from the NMR results and compared with chemical analyses, as well as solution NMR for two of the HAs. The results are in good agreement, which shows that DPMAS corrected by CP/T1–TOSS permits quantitative characterization of HAs and PEMs. The compositions of the PEMs deviate significantly from those of the Histosol HAs. The compositions in terms of nine types of chemical groups were computed. The investigated HAs consist of more than 60% of aromatic and CO carbons (including both carbonyl and carboxyl groups). Previous cross‐polarization magic‐angle spinning (CPMAS) NMR experiments have significantly underestimated the ratio of sp2– to sp3–carbons; in particular, the true COO carbon fraction is a factor of two larger than estimated by CPMAS NMR. In spite of their wide range of geographical origins, the compositions of the Histosol HAs appear to be relatively uniform, suggesting that the search for a general model of Histosol HA structure is worthwhile. Eight models proposed in the literature do not reproduce the experimentally determined compositions, but a few models show promising partial agreement.

An improved approach for accurately determining the aromatic carbon fraction (fa) and nonprotonated aromatic carbon fraction (faN) in natural organic matter by solid-state 13C NMR is described. Quantitative peak areas are obtained from direct polarization 13C nuclear magnetic resonance (NMR) under high-speed magic angle spinning (MAS). The problem of overlap between aromatic and alkyl carbon resonances around 90-120 ppm in 13C NMR spectra is solved by a 13C chemical shift anisotropy (CSA) filter technique. After correction for residual spinning sidebands, an accurate value of the aromaticity fa is obtained. To obtain a quantitative faN fraction, dipolar dephasing was adapted for high-speed MAS 13C NMR; the separation of the signals of nonprotonated alkyl and aromatic carbons was achieved by CSA filtering plus dipolar dephasing. The method is demonstrated on a peat humic acid, yielding fa = 45 +/- 2% and faN = (0.64 +/- 0.07) x 45%.

Dissolved black carbon (BC) released from biochar can be one of the more photoactive components in the dissolved organic matter (DOM) pool. Dissolved BC was mainly composed of aliphatics and aromatics substituted by aromatic C-O and carboxyl/ester/quinone moieties as determined by solid-state nuclear magnetic resonance. It underwent 56% loss of absorbance at 254 nm, almost complete loss of fluorescence, and 30% mineralization during a 169 h simulated sunlight exposure. Photoreactions preferentially targeted aromatic and methyl moieties, generating CH2/CH/C and carboxyl/ester/quinone functional groups. During irradiation, dissolved BC generated reactive oxygen species (ROS) including singlet oxygen and superoxide. The apparent quantum yield of singlet oxygen was 4.07 ± 0.19%, 2-3 fold higher than many well-studied DOM. Carbonyl-containing structures other than aromatic ketones were involved in the singlet oxygen sensitization. The generation of superoxide apparently depended on electron transfer reactions mediated by silica minerals in dissolved BC, in which phenolic structures served as electron donors. Self-generated ROS played an important role in the phototransformation. Photobleaching of dissolved BC decreased its ability to further generate ROS due to lower light absorption. These findings have significant implications on the environmental fate of dissolved BC and that of priority pollutants.

The structure of synthetic nanodiamond has been characterized by (13)C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range (1)H-(13)C dipolar couplings and (13)C relaxation times. The surface layer of these approximately 4.8-nm diameter carbon particles consists mostly of sp(3)-hybridized C that is protonated or bonded to OH groups, while sp(2)-hybridized carbon makes up less than 1% of the material. The surface protons surprisingly resonate at 3.8 ppm, but their direct bonding to carbon is proved by fast dipolar dephasing under homonuclear decoupling. Long-range (1)H-(13)C distance measurements, based on (13)C{(1)H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains approximately 60% of all carbons, predominantly resonate more than +8 ppm from the 37-ppm peak of bulk diamond (i.e., within the 45-80 ppm range). Nitrogen detected in (15)N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location of unpaired electrons (approximately 40 unpaired electrons per particle) was studied in detail, based on their strongly distance-dependent effects on T(1,C) relaxation. The slower relaxation of the surface carbons, selected by spectral editing, showed that the unpaired electrons are not dangling bonds at the surface. This was confirmed by detailed simulations, which indicated that the unpaired electrons are mostly located in the disordered shell, at distances between 0.4 and 1 nm from the surface. On the basis of these results, a nonaromatic core-shell structural model of nanodiamond particles has been proposed.