Hydrothermal liquefaction accelerates the toxicity and solubility of arsenic in biowaste

Arsenic (As) is one of notorious metalloids due to its high toxicity to human beings and ecological system. Understanding its fate and speciation transformation mechanism during hydrothermal liquefaction (HTL) of microalgae is of crucial importance for the application of its HTL products. 80.0-96.7% of As in raw microalgae was migrated into the liquid phase (aqueous phase and biocrude oil) with the increase of reaction severity from 0.108 to 0.517. HPLC-ICPMS reveals that 67% of the As in microalgae accounted for As(V) with a concentration of 68.4 mg/kg. The other fractions in microalgae were primarily As(III) with a concentration of 36.3 mg/kg. Model compounds experiments illustrate that over 30% of the As(V) in feedstocks was unexpectedly converted into more soluble and toxic As (III). Hydrochar containing O-containing groups (e.g., aliphatic C-OH) was probably contribute to the reduction transformation of As(V) to higher toxic As(III). Meantime, the aqueous phase facilitated the reduction reaction via providing a reducing environment and serving as hydrogen donator. This study firstly revealed the speciation transformation of As(V) to As(III) during HTL of wastewater cultivated microalgae.

Automated summary

Arsenic is a highly toxic metalloid that can migrate from microalgae to the liquid phase during hydrothermal liquefaction, with a majority existing as As(V) and As(III). Model experiments show that a significant portion of As(V) in feedstocks can unexpectedly convert into more toxic As(III), potentially due to the presence of oxygen-containing groups. The aqueous phase plays a role in facilitating the reduction reaction of As(V) to As(III) by providing a reducing environment and hydrogen donation.