# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 3 Prospects for Joint Processing of Macroalgae Biomass and Marine Debris
### Joint Hydrothermal Liquefaction of Polymers and Biomass  
Joint processing of pistachio shells and a mixture of the four most common polymers (PET, polyethylene, polypropylene, and nylon) caused an increase of solid yields compared to HTL of pure pistachio shells. Co-processing of polyethylene and polypropylene decreased oil yield from 34.1% to 20.4% and from 34.1% to 30.7%. The yield of bio-oil during processing with PET remained practically unchanged, while the yield of char increased by a multiple of the number of added polymers [88]. Unlike other plastics, the increase in biochar yield was much less when pistachio shells were co-liquefied with nylon (the maximum solids increase was 9% with the addition of 20% polymer). The biofuel yield for nylon mixes was almost the same and amounted to 30.0%, while a significant change in the composition of the aqueous phase was observed. This behavior of nylon has been confirmed by other studies [88] and is associated with the transformation of nylon during HTL processing into water-soluble caprolactam. The co-processing of pistachio shells with a mixture of plastics in equal proportions led to a significant increase in bio-char production, while the bio-oil yield decreased by 20%. This fact, according to the authors, confirmed the lack of expediency with the joint processing of biomass and polymers regarding bio-oil production. [PERSON] et al. [90] studied the synergistic effect of the combined processing of cellulose, starch, soy protein isolate, lignin, stearic acid, glucose, and various polymers, including polypropylene, polystyrene, polycarbonate, and polyethylene terephthalate. They found that the combined processing of cellulose, starch, and lignin with plastics provided an increase in the fuel yield by 13.9 wt% (at 300 \({}^{\circ}\)C, 30 min). It was also established that polyolefins were activated during the joint processing of these components, which ensured a decrease in the temperature of their decomposition. When processing the biomass of shrubs and a mixture of polyolefin waste, it was found that there was a synergistic effect that provided an increase in the average fuel yield from 40% to 50%. The addition of 3 wt% catalyst (bentonite clay activated with hydrochloric acid) provided the maximum bio-oil yield--61.23 wt% (temperature 420 \({}^{\circ}\)C for a mixture of three parts of biomass: one part polymer, holding time 60 min) [89]. There is little research on the co-processing of macroalgae with polymers typical of marine debris. Detailed studies assessing the effect of several of the most common polymer contaminants (polyethylene, PET, polypropylene, nylon) on the hydrothermal liquefaction (HTL) process of four species of macroalgae (_L. digitata, U. lactuca, F. serratus, and S. moticum_) were carried out by [PERSON] et al. [51]. The biomass to polymer ratios were 100:0, 90:10, 75:25, 50:50, and 0:100. The process was carried out at a temperature of (340 \(\pm\) 3) \({}^{\circ}\)C, the reaction time was 12-27 min, and the pressure inside the reactor was 15 MPa. It was found that polyethylene and propylene practically did not decompose under the influence of subcritical water. The presence of all types of plastics in the processed biomass resulted in a moderate synergistic effect which led to an increase in the amount of carbon in biofuels, a decrease in nitrogen content, and an increase in heating value. Nylon (fishing nets) was depolymerized almost completely under the HTL process to form caprolactam, which entered mainly into the aqueous phase. Disadvantageous for biofuel production, the disposal of marine nylon debris to produce monomers still could be a promising source of income for future biofuel plants [49]. To summarize, it can be seen from the results of joint processing of polymers and various types of biomasses that polymers transformed at higher temperatures than biomass [47, 89, 90]. The addition of polymers does not increase, but rather decreases the yield of biofuels and increases the yield of char, which is undesirable.