Growing demands for sustainable agricultural practices and environmental protection have intensified interest in valorizing lignocellulosic residues through thermochemical and biochemical pathways. This dissertation investigates two independent, yet conceptually connected biomass conversion systems based on poplar and grass feedstocks, each transformed into multifunctional carbon materials with distinct agronomic, environmental, and techno-functional applications. The research demonstrates how process selection, pyrolysis for poplar and hydrothermal conversion for grass, determines product chemistry, stability, and downstream functionality, ultimately establishing integrated strategies for carbon sequestration, nutrient recovery, pollutant adsorption, and humified carbon generation. The first section examines poplar-based biochar (PB) produced via slow pyrolysis under varying temperatures and residence times. Increasing thermal severity led to progressive carbon enrichment, reduced hydrogen and oxygen contents, and lower H/C and O/C ratios, confirming intensified devolatilization and aromatization. Biochars produced at 500 °C exhibited a balanced combination of structural development and mass retention, with PB500C-20m selected for further study due to its optimal compromise between surface functionality and biochar yield. This material displayed high thermal stability (R50 = 0.57), a substantial specific surface area (291 m²/g at the highest severity), and notable adsorption capabilities. PB500C-20m removed 4.44 mg/g phosphate, 116.83 mg/g ammonium, and 157.37 mg/g potassium from digestate, outperforming isolated ion systems and demonstrating synergistic adsorption behaviors in nutrient-rich matrices. Germination tests revealed temporarily reduced Seedling Vigor Index (SVI = 8.9 vs. 12.5 for control), consistent with delayed nutrient release characteristic of carbonaceous amendments. These findings position PB as a multifunctional material capable of carbon sequestration and nutrient recycling, with the potential to treat manure equivalent to that produced by two dairy cows per hectare annually. The grass-based pathway followed a contrasting hydrothermal route, generating hydrochars and process liquids enriched with humic-like substances. Torrefaction (GTK230), pyrolysis (GP500, GPK500), and hydrothermal treatments including carbonization (GHTC), humification (GHTH), and fulvification (GHTF) produced solids with distinct compositions, ranging from carbon-dense hydrochars (GHTC: 60.6% C; HHV = 25.67 MJ/kg) to highly oxidized residues (GHTF: H/C = 1.89; O/C = 0.49). Fulvification yielded the lowest solid mass (13.18%) but produced the highest levels of dissolved organic carbon (TOC = 58.53 g/L), demonstrating that alkalinity promotes solubilization and artificial humic acid formation. Artificial humic acids exhibited C contents of 40.38-46.60% and C/N ratios near the agronomically significant 24:1 threshold, indicating a balance between nutrient immobilization and long-term soil carbon stabilization. The integration of biochemical and thermochemical conversion was achieved by coupling lactic acid fermentation with hydrothermal fulvification. Acid-pretreated hydrolysates yielded the highest lactic acid concentration (36 g/L) but also produced inhibitory furans, whereas base-washed hydrolysates provided a favorable compromise, generating 29.7 g/L lactic acid with markedly reduced acetic acid levels (1.34 g/L). Residual solids from fermentation were converted into humified carbon under HTF conditions (230 °C, 4 h), producing artificial humic substances with controlled molecular partitioning into fulvic-enriched liquids and humic-rich solids. Particle-size and spectroscopic analyses confirmed that the liquid fraction consisted of small fulvic particles (100-200 nm, E4/E6 of 8.0-8.5), while the solid fraction contained aggregated humic macromolecules (≥400 nm, E4/E6 of 2.7-4.0; zeta potential −41.25 mV), demonstrating clear structural differentiation. Together, these findings present a scientifically validated biorefinery concept in which biomass is converted into stable carbon sinks, nutrient-capturing chars, organic acid platforms, and humified biostimulants. The innovations described here advance the circular bioeconomy by demonstrating that carbon-nutrient valorization can be achieved without generating waste streams, providing scalable pathways to climate-resilient agriculture and sustainable residue management.
Saman Ghobadian (Thu,) studied this question.