Distribution characteristics and toxicity evaluation of polycyclic aromatic hydrocarbons in macroalgal biochars

Biochar has been prevalently recognized as a readily available and environmentally friendly material in recent years. The excellent properties can be a developed pore structure, abundant functional groups, and outstanding cation exchange capacity. Therefore, biochar is often used for the fertilization and/or remediation of water and soil, as well as the long-term sequestration of carbon. Notably, the persistent organic pollutants (e.g. Polycyclic Aromatic Hydrocarbons (PAHs)) are inevitably generated to stagnate in the biochar during the pyrolysis stage. The concentrations and characteristics of these PAHs in the biochar vary significantly, according to the biomass feedstock, pyrolysis temperature, and pyrolysis conditions. Macroalgae plays crucial roles in carbon cycling to slow down eutrophication in the coastal sea ecosystems. Macroalgae can be expected to serve as the precursors for deriving biochars, due to the short growth cycle, abundance, and accessibility. Moreover, the conversion of macroalgae biomass to biochar is beneficial to the waste management and resource usage of macroalgae. However, it is still lacking on the content and toxicity of PAHs in the macroalgal biochars. In this study, the macroalgal biochars were produced from the Sagassum vachellianum, Sargassum fusiforme, Sargassum thunbergii, Grateloupia turuturu, Chondria crassicaulis, and Ulva pertusa at different pyrolysis temperatures (200, 300, 400, 500, and 600 °C) under oxygen-limited conditions. Sixteen typical PAHs in the macroalgal biochars were extracted and determined using the Soxhlet extraction combined with gas chromatography-mass spectrometry (GC-MS). Their toxicities were evaluated in this case. The results showed that the PAHs were widely distributed in all tested macroalgal biochars. Specifically, the abundance of PAHs in the biochars first increased and then decreased, as the pyrolysis temperature increased. There was the lowest (78.2 μg/kg) total concentration of PAHs in the C. crassicaulis biochar that was prepared at 600 °C among the macroalgal biochars. By contrast, the highest (2 244.2 μg/kg) was achieved in the G. turuturu biochar prepared at 300 °C, indicating the most abundant naphthalene and phenanthrene. The redundancy analysis revealed that there were different effects of pyrolysis temperature on the concentration and proportion of each PAH in the macroalgal biochar. The contents of PAHs in the macroalgal biochars were all lower than the limit value of EBC-AgroOrganic grade (4±2 mg/kg) stipulated in the European Biochar Certificate (EBC, Version 10.1). There were mainly composed of 2 and 3 rings for the PAHs in the macroalgal biochars that were prepared at the pyrolysis temperatures of 200°C-600°C. The 4-ring PAHs were presented in all the macroalgal biochars, whereas the 5- and 6-ring PAHs were detected only in some macroalgal biochars, in which the proportion was very low. In addition, the macroalgal biochars exhibited various toxic equivalence quantity of benzo[a]pyrene (TEQBaP) at different pyrolysis temperatures. This change was attributed to the content, ring number, and type distribution of PAHs in the macroalgal biochars. There was the lowest (0.196 μg/kg) TEQBaP of the C. crassicaulis biochar that derived at 600 °C among the tested macroalgal biochars. By contrast, the highest (46.151 μg/kg) was also achieved in the S. thunbergii biochar that was derived at 400 °C. The TEQBaP of the macroalgal biochars was lower than that of biochars reported previously. The energy consumption of pyrolysis temperature and yield were combined to determine the biochar remediation effect and similar potential environmental risks. Biochar materials with a lower pyrolysis temperature can be selected to provide important guidance for the production and application of macroalgal biochars, thereby improving the utilization of macroalgae. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.