Sustainable biorefining: Hydrothermal liquefaction of diatom biomass for bio-crude and nano-biosilica recovery

Diatom, a unique species of microalgae is a highly promising resource for the production of valuable bio-crude and silica nanomaterial. However, the complex nature of diatom cell structure presents significant downstream processing challenges and leads to inefficient bio-product recovery. To overcome this challenge, this study aimed to develop a one-step approach for the valorization of Denticula sp. dominated diatom consortium using hydrothermal liquefaction (HTL) process with the recovery of three major products. The first output, referred to as biocrude, constituted 22% of the total yield, consisting of 9.5% unbound bio-crude and 12.5% cell wall-bound organic fraction. The bio-crude consisted primarily of oxidized and desirable organic compounds like alcohols (24.4%), long-chain saturated hydrocarbons (23.6%), and esters (17.6%), with only a few undesirable nitrogencontaining compounds (18.9%). The second resultant product comprised nanoporous bio-silica structures, yielding 56%. These silica structures exhibited enhanced quality in comparison to the conventional thermal extraction technique employed for their retrieval and were found to be three-dimensional, highly porous, intact silica structures with low trace elements and a high surface area (53.7 m2 g-1). The third acquired product, the aqueous fraction, was observed to possess elevated levels of nitrate-nitrogen (440 mg L-1) and total dissolved phosphate (975 mg L- 1). Overall, the study highlights the potential of HTL as an efficient diatom downstream processing method that allows for the complete valorization of intermediates, resulting in a sustainable biomass conversion and product recovery technology.