Characterisation of iron, nickel, and vanadium porphyrins in HTL bio-crude of Chlorella minutissima : insights into metal speciation and refining challenges

This research is the first to characterise iron (Fe), nickel (Ni), and vanadium (V) porphyrins in Chlorella minutissima hydrothermal liquefaction (HTL) bio-crude, revealing species-specific metal retention critical for refining. Using ICP-MS, EPR, and FTICR-MS, bio-crude yields of 35.0-42.0% (w/w, dry basis) were achieved, alongside aqueous phase yields of 40.0-45.0%, gaseous products of 12.0-15.0%, and solid residues of 6.0-8.0% (300-350°C, 15 MPa, 45 min, no catalyst). Relative to raw biomass (Fe: 75 ± 5 ppm, Ni: 50 ± 5 ppm, V: 45 ± 4 ppm), Fe accumulated in bio-crude (56.7-90%, 42.5-60.4 ppm at 300-350°C, 80.5 ± 4.0% at 350°C), while Ni (81.6 ± 5.4%, 40.8 ± 2.7 ppm) and V (74.9 ± 5.1%, 33.7 ± 2.3 ppm) concentrated in solid residues. FTICR-MS identified Fe(III), Ni(II), and V(IV) porphyrins at m/z 537.207, 543.191, and 521.183, with trace Cu and Zn porphyrins at m/z 541.198 and 542.201, and EPR confirmed oxidation states. Compared to C. vulgaris (Fe: ∼35 ppm), C. minutissima showed higher Fe retention (60.4 ± 3.0 ppm, 80.5 ± 4.0% at 350°C), increasing catalyst deactivation risks. Adsorption reduced metals by 74-75% (Fe: 60.4 ± 3.0 ppm to 15.2 ± 1.8 ppm; Ni: 11.5 ± 0.9 ppm to 3.0 ± 0.4 ppm; V: 7.6 ± 0.5 ppm to 2.0 ± 0.3 ppm), offering a viable refining strategy. Statistical analysis (ANOVA, p < 0.05) confirmed temperature's role in Fe partitioning. These findings underscore the need for tailored refining processes, combining adsorption and hydroprocessing, to enhance C. minutissima bio-crude quality for sustainable biofuels.