Effects of biomass types and carbonization conditions on the chemical characteristics of hydrochars.

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.

Genotoxic and phytotoxic risk assessment of fresh and treated hydrochar from hydrothermal carbonization compared to biochar from pyrolysis.

Biochar is discussed as an option for climate change mitigation via C sequestration and may promote sustainable resource efficiency. Large-scale field trials and commercial business with char materials have already started. Therefore char materials have to be assessed for toxic compounds. We tested genotoxic effects of different hydrochars and biochars with the Tradescantia micronucleus test. For this purpose chromosomal aberrations in pollen cells of Tradescantia in the form of micronuclei were evaluated microscopically after defined exposition to extracts from char materials. Hydrochars from hydrothermal carbonization mostly exhibited significantly negative results. Additional germination experiments with hydrochar showed total germination inhibition at additions above five percent v/v in comparison to biochar. However, biological post-treatment of previously toxic hydrochar was successful and toxic effects were eliminated completely. Some post-treated hydrochars even showed growth stimulating effects. Our results clearly demonstrate the necessity of risk assessment with bioindicators. The chosen tests procedures can contribute to biochar and hydrochar characterization for safe application.