Effects of aqueous phase recycling on the bio-oil characteristics and arsenic enrichment during hydrothermal liquefaction of Pteris vittata L. in ethanol-water system

The ecologically friendly disposal of arsenic-enriched plants is the key link to achieve the industrial development of phytoremediation, and plays a restricting role in the high-quality development of soil heavy metal pollution control. Conventional treatment methods of hyperaccumulators mainly include incineration and landfill, but these methods may lead to secondary pollution of heavy metals. Hydrothermal liquefaction can transform agricultural and forestry waste with low energy density into liquid biofuel with high energy density. It has the advantages of low reaction temperature, low energy consumption, water as solvent and no need for drying as raw material. As a typical representative of arsenic contaminated soil phytoremediation, the effects of ethanol ratio and water cycle times on hydrothermal oil production and heavy metal arsenic distribution of centipede grass were investigated in this study using Pteris vittata L. as raw material and ethanol-water reaction co-solvent system. The results showed that under the conditions of 60% ethanol concentration and 3 cycles of aqueous phase, the highest yield of liquefied bio-oil was 51.08%, and its calorific value was 29.15 MJ/kg. The reasons for the increase of bio-oil after water phase circulation in the co-solvent system are as follows: 1) the organic acids (acetic acid, etc.) in the recycled water phase have an acid catalytic effect to promote biomass liquefaction; 2) Compared with pure water, ethanol in cosolvent has lower liquefaction critical point and dielectric constant, which promotes biomass degradation and hydrolysis; 3) Ethanol can esterify the acid in the aqueous phase to improve the yield of bio-oil. Bio-oil is mainly composed of esters (>80%), phenols, alcohols, ketones, hydrocarbons, etc. With the increase of water cycle times, the esters and phenols in bio-oil increased and the high boiling point compounds decreased, the bio-oil yield and energy recovery increased by 58% and 32.23%, respectively. This may be affected by esterification of ethanol with fatty acids. Moreover, aqueous cycle can promote the degradation of high boiling point compounds to low boiling point compounds, which may be related to the low boiling point compounds produced after the reuse of liquefied water phase with the addition of 60% ethanol-water cosolvent. After water phase circulation, the content of bio-oil distillate (boiling point <538 ℃) increased from 65.57% to 71.70%, which show that ethanol-water co-solvent can produce high quality bio-oil in aqueous phase recycling. Finally, the migration and transformation of arsenic in enriched plants during liquefaction were also investigated. After aqueous phase circulation, more than 80% of heavy metal arsenic was mainly distributed in the water phase and the concentration of arsenic increased to 215.05 mg/kg. The enrichment and removal of arsenic during liquefaction plays an important role in the post-disposal of arsenic-enriched plants, which greatly reduces the secondary pollution of heavy metals. Therefore, water phase recycling is an effective way to obtain high-yield bio-oil and arsenic enrichment from liquefaction disposal of arsenic-enriched plants, which provides a new idea for ecologically friendly disposal of arsenic-enriched plants, promotes the industrial development of phytoremediation, and speeds up the treatment of soil heavy metal pollution. To provide theoretical support for green, low-carbon and new quality development in the process of heavy metal pollution control in our country.