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This study investigated the sorption potential of hydrochars, produced from hydrothermally carbonizing livestock wastes, toward organic pollutants (OPs) with a wide range of hydrophobicity, and compared their sorption capacity with that of pyrochars obtained from conventional dry pyrolysis from the same feedstock. Results of SEM, Raman, and C NMR demonstrated that organic carbon (OC) of hydrochars mainly consisted of amorphous alkyl and aryl C. Hydrochars exhibited consistently higher log K of both nonpolar and polar OPs than pyrochars. This, combined with the significantly less energy required for the hydrothermal process, suggests that hydrothermal conversion of surplus livestock waste into value-added sorbents could be an alternative manure management strategy. Moreover, the hydrochars log K values were practically unchanged after the removal of amorphous aromatics, implying that amorphous aromatic C played a comparable role in the high sorption capacity of hydrochars compared to amorphous alkyl C. It was thus concluded that the dominant amorphous C associated with both alkyl and aryl moieties within hydrochars explained their high sorption capacity for OPs. This research not only indicates that animal-manure-derived hydrochars are promising sorbents for environmental applications but casts new light on mechanisms underlying the high sorption capacity of hydrochars for both nonpolar and polar OPs.

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The benefits and disadvantages of hydrochar incorporation into soil have been heavily researched. However, the effect of hydrochar application on the soil microbial communities and the molecular structure of native soil organic carbon (SOC) has not been thoroughly elucidated. This study conducted an incubation experiment at 25 °C for 135 days using a soil column with 0.5 and 1.5% hydrochar-amended paddy soil to explore the interconnections between changes in soil properties and microbial communities and shifts in native SOC structure using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) and NMR after hydrochar application. Hydrochar addition decreased the labile SOC fraction by 15.6–33.6% and increased the stable SOC fraction by 10.3–27.0%. These effects were significantly stronger for 1.5% hydrochar-treated soil. Additionally, hydrochar addition induced the native SOC with 1.0–3.0% more carbon and 6.0–13.0% higher molecular weight. The SOC in hydrochar-amended soil contained more aromatic compounds but fewer carbohydrates and lower polarity. This was resulted by a statistically significant reduction in Sphingobacterium, which was active in polycyclic aromatic hydrocarbon degradation, and an increase in Flavobacterium, Anaerolinea, Penicillium, and Acremonium, which were the efficient decomposers of labile SOC. These findings will help elucidate the potential influence of hydrochar on the carbon biogeochemical cycle in the soil.

Natural sorbents including one humic acid (HA), humins (HMs), nonhydrolyzable carbons (NHCs), and engineered sorbents (biochars) were subject to bleaching to selectively remove a fraction of aromatic C. The structural properties and sorption isotherm data of phenanthrene (Phen) by original and bleached sorbents were obtained. Significant correlations between Phen Koc values by all sorbents and their organic carbon (OC)-normalized CO2 cumulative surface area (CO2-SA/OC) suggested that nanopore-filling mechanism could dominate Phen sorption. After bleaching, natural sorbents still contained large amounts of aromatic C, which are resistant to bleaching, suggesting that they are derived from condensed or nonbiodegradable organic matter (OM). After eliminating the effect of aromatic C remaining in the bleached samples, a general trend of increasing CO2-SA/OC of natural sorbents with increasing aliphaticity was observed, suggesting that nanopores of natural sorbents are partially derived from their aliphatic moieties. Conversely, positive relationships between CO2-SA/OC or Phen logKoc of engineered sorbents and their aromaticity indicated the aromatic structures of engineered sorbents primarily contribute to their nanopores and dominate their sorption of HOCs. Therefore, this study clearly demonstrated that the role of structure and microporosity in Phen sorption is dependent on the sources of sorbents.

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