No abstract

Two types of swine-manure chars, hydrothermally produced hydrochar and slow-pyrolysis pyrochar, and their raw swine-manure solid were characterized using advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. In comparison to raw swine-manure solid, both hydrochars and pyrochar displayed significantly different structural features, with lower alkyl carbons, NCH, OCH3, O-alkyl, and COO/N−CO groups but higher aromatic/olefinic and aromatic C−O groups. The chemical structures of four hydrochars varied with different processing conditions. In comparison to the hydrochar with only water wash (HTC-swine W), washing hydrochar with acetone (HTC-swine A) removed the soluble intermediates deposited on the hydrochar, as shown by the decrease of O-alkyl (primarily carbohydrates), corresponding increase of aromatic/olefinic carbons and complete removal of OCH3 groups. With citric acid prewash and acetone wash (HTC-AW-swine A), aromatic C−O and aromatics/olefinics were increased and alkyls were decreased, with O-alkyls totally removed in comparison to just acetone wash (HTC-swine A). Citric acid catalysis and acetone wash (HTC-AC-swine A) increased aromatic C−O and non-protonated aromatics/olefinics, decreased alkyls further, and reduced protonated aromatics/olefinics compared to citric acid prewash and acetone wash (HTC-AW-swine A). The ratios of non-protonated to protonated aromatic/olefinic carbons for HTC-swine W, HTC-swine A, and HTC-AW-swine A hydrochars were quite similar but enhanced for HTC-AC-swine A hydrochar. Obviously, citric acid catalysis and acetone wash (HTC-AC-swine A) provided deeper carbonization than other hydrothermal processes. Hydrothermal carbonization (HTC) processes were associated with the hydrolysis and subsequent decomposition of major biopolymer components in swine manure. The increase of aromaticity during HTC was likely due to condensation polymerization of the intermediates from the degradation of carbohydrates. Pyrochar produced from slow pyrolysis was structurally different from HTC hydrochars. The dominant component of pyrochar was aromatics, whereas that of hydrochars was alkyl moieties. The aromatic cluster size of pyrochar was larger than those of hydrochars. Slow pyrolysis at 620 °C provided deeper carbonization than HTC processes.

Temperature is one of the controlling factors determining the chemical structure of char. We employed advanced solid-state 13 C NMR techniques to characterize maple wood and its chars produced under N 2 at temperatures from 300 to 700 °C. Our results indicated that 300 °C char was primarily composed of residues of biopolymers such as lignin and cellulose. Carbohydrates are completely lost for char prepared at 350 °C. At 400 °C, the char lost most of the ligno-cellulosic features and consisted predominantly of aromatic structures. By 500 °C, sp 3 -hybridized carbon had all but disappeared. Protonated aromatic carbons increased up to 400 °C chars but then decreased. Aromatic C−O groups decreased, whereas nonprotonated aromatic carbons, especially bridgehead carbons, increased as temperature increased. The minimum aromatic cluster sizes estimated from spectral analysis increased from 8 carbons in 300 °C char, to 20, 18, 40, 64, and 76 carbons, respectively, in 350 °C, 400 °C, 500 °C, 600 °C, and 700 °C chars. 1 H− 13 C long-range dipolar dephasing displayed the same increasing trend of aromatic cluster sizes of wood chars with increasing temperature. We show for the first time quantitative changes of different aromatic C forms and aromatic cluster size as a function of heat treatment temperature.

Chars from wildfires and soil amendments (biochars) are strong adsorbents that can impact the fate of organic compounds in soil, yet the effects of solute and adsorbent properties on sorption are poorly understood. We studied sorption of benzene, naphthalene, and 1,4-dinitrobenzene from water to a series of wood chars made anaerobically at different heat treatment temperatures (HTT) from 300 to 700 °C, and to graphite as a nonporous, unfunctionalized reference adsorbent. Peak suppression in the NMR spectrum by sorption of the paramagnetic relaxation probe TEMPO indicated that only a small fraction of char C atoms lie near sorption sites. Sorption intensity for all solutes maximized with the 500 °C char, but failed to trend regularly with N2 or CO2 surface area, micropore volume, mesopore volume, H/C ratio, O/C ratio, aromatic fused ring size, or HTT. A model relating sorption intensity to a weighted sum of microporosity and mesoporosity was more successful. Sorption isotherm linearity declined progressively with carbonization of the char. Application of a thermodynamic model incorporating solvent-water and char-graphite partition coefficients permitted for the first time quantification of steric (size exclusion in pores) and π-π electron donor-acceptor (EDA) free energy contributions, relative to benzene. Steric hindrance for naphthalene increases exponentially from 9 to 16 kJ/mol (∼ 1.6-2.9 log units of sorption coefficient) with the fraction of porosity in small micropores. π-π EDA interactions of dinitrobenzene contribute -17 to -19 kJ/mol (3-3.4 log units of sorption coefficient) to sorption on graphite, but less on chars. π-π EDA interaction of naphthalene on graphite is small (-2 to 2 kJ/mol). The results show that sorption is a complex function of char properties and solute molecular structure, and not very predictable on the basis of readily determined char properties.

Effects of biomass types (bark mulch versus sugar beet pulp) and carbonization processing conditions (temperature, residence time, and phase of reaction medium) on the chemical characteristics of hydrochars were examined by elemental analysis, solid-state ¹³C NMR, and chemical and biochemical oxygen demand measurements. Bark hydrochars were more aromatic than sugar beet hydrochars produced under the same processing conditions. The presence of lignin in bark led to a much lower biochemical oxygen demand (BOD) of bark than sugar beet and increasing trends of BOD after carbonization. Compared with those prepared at 200 °C, 250 °C hydrochars were more aromatic and depleted of carbohydrates. Longer residence time (20 versus 3 h) at 250 °C resulted in the enrichment of nonprotonated aromatic carbons. Both bark and sugar beet pulp underwent deeper carbonization during water hydrothermal carbonization than during steam hydrothermal carbonization (200 °C, 3 h) in terms of more abundant aromatic C but less carbohydrate C in water hydrochars.