Risk evaluation of biochars with indicator organisms

With increasing numbers of biochar applications to soil, there is an urgent need for quick and cost-efficient environmental risk assessment. The pyrolysis and hydrothermal carbonization (HTC) developed to the most important way to produce biochar and hydrochar in Germany. Beside the positive aspects of biochar applications it has to be clarified if the materials (which are produced industrially) contain toxic compounds. We had chosen plant bioassays for evaluation of different biochars (which differ from process and feedstock) in its effects on plants. The focus of our study was the verification of possible genotoxic effects with Tradescantia pollen cells which present genotoxic effects in DNA damage and can be evaluated by microcopy. Plant stipes from Tradescantia (clone #4430) were exposed to filtrated biochar extracts or liquid phases from hydrochar for 24h. The tests were modified for testing and comparing biochars and hydrochars with its different material properties. Some results indicate genotoxic potentials in both char materials in similar intensity like the positive control (Maleic hydrazide acid). Some hydrochars showed high significant negative results, which were less significantly higher than damages in pollen cells exposed in biochar filtrates. Further experiments with samples from each production process in germination tests showed total germination inhibition in hydochar soil mixtures above applications of 5% v/v. The effects seemed to be dependent on feedstock as well as process temperature. The samples from char materials from hydrothermal carbonization were more genotoxic than samples from biochars. Despite low concentrations of PAH´s, PCDD or PCDF´s our results indicate the presence of other compounds with genotoxic potential which should be identified and eliminated prior to the large scale application of hydrochars in the environment.
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