Production of carbon dots, biofuels, bio-adsorbents, and biological nutrients via hydrothermal conversion of Chlorella pyrenoidosa and oilseed rape straw

Abstract Biomass hydrothermal conversion has received extensive attention due to its advantages of strong adaptability of raw materials, no need to dry feedstock, and relatively mild conditions. Chlorella pyrenoidosa (CP) and oilseed rape straw (OS), two kinds of typical biomass, were hydrothermally treated at 230 °C for 6 h to produce carbon dots (CDs), hydrochar (HC), bio-oil (OR), aqueous product (AQ), and volatile product (VO). The CP hydrothermal process generated the highest yield of CDs (16.3%), and the OS hydrothermal process produced the most HC (26.3%). The co-hydrothermal treatment of CP/OS promoted the production of HC and VO via carbonization, decarboxylation, and dehydration reactions between CP and OS degradation products. The CP, OS, and CP/OS based CDs (CD-CP, CD-OS, and CD-CP/OS) in the size of 1.5‒26.5 nm emitted blue light and displayed 3.3‒11.1% of fluorescence quantum yield. More than 42.3% of methylene blue could be photodegraded by CD-CP, which was 2.1 and 1.4 times higher than that by CD-OS and CD-CP/OS. The higher heating values of HCs and ORs were 23.0‒27.8 MJ kg−1 and 25.5‒38.5 MJ kg−1, showing potential to apply as biofuels. The HCs were confirmed to be outstanding bio-adsorbents that could remove 15.4‒68.9% methylene blue with an absorption capacity of up to 275.6 mg g−1. Moreover, the AQs were verified to be potentially used as biological nutrients for microalgae cultivation. This study co-produced CDs, BO, HC, and AQ through the hydrothermal conversion of CP and OS, efficiently utilizing them as photocatalysts, biofuels, bio-adsorbents, and biological nutrients. Graphical abstract