## 2 Materials and Methods  
### Preparation of Biocarbon from Fish Waste (Blue Shark Chondroitin Sulfate and Blue Shark Gelatine)  
By-products (heads and skins) generated in the industrial filleting of blue shark (_Prionace glauc_) were kindly provided by Propegal S.L. (Vigo, Spain) and stored at \(-\)20 \({}^{\circ}\)C. Chondroitin sulfate (CS) was isolated from head cartilage according to the procedure and characterization reported in the works of [PERSON] et al. [20] and [PERSON] et al. [21], with slight modifications. Briefly, to achieve this isolation, (a) the heads were heated in a water bath (80 \({}^{\circ}\)C/30 min), and the cartilage was manually separated from the muscle after cooking, and then milled; (b) this cartilaginous substrate was enzymatically digested by catalase, and the resulting hydrolysate after centrifugation (supernatant) was chemically treated with the alkaline solution to improve protein proteolysis; (c) this impurified CS was neutralized with HCl and purified by a combination of concentration-daifiltration stages using a 30 kDa ultrafiltration membrane; (d) the concentrates of CS were finally dried in an air-forced convection oven. CS reached a purity value of 90% (\(w/w\)) concerning protein content, a richness of 75% (\(w/w\)) of CS regarding other glycosaminoglycans, a molecular weight of 60 kDa, and a sulfation ratio (4S/6S) of 0.17.  
Gelatin was recovered from the skins of blue shark using various chemical washes and thermal extraction, as described in the work of [PERSON] et al. [22]. To summarize, frozen skins were cut into 5 \(\times\) 5 cm portions, washed with water to remove impurities on the skin surface, and processed as follows. (a) Sequential alkaline (using NaOH) and two acids (with sulphuric and citric acids) treatments were used, including washing between them; (b) gelatin was extracted from treated skins by soaking them in slightly warm water; (c) the gelatin solution was deodorized and purified via adsorption with active charcoal; (d) dried gelatin was produced in an air-forced convection oven. The gelatin sample was composed of 93% (\(w/w\)) of protein, 1% (\(w/w\)) fat, 4% (\(w/w\)) of moisture, and 2% (\(w/w\)) ash (salts and minerals). The contents of amino acids in gelating were (%, \(w/w\) of total amino acids): 32,10.3, and 8.5 of glycine, proline, and hydroxyproline, respectively. The strength of the blue shark gelatin gel was 189 g [22].  
Commercial chondro sulfate (chondroitin sulfate C sodium salt, N-acetyl galactosamine sulfate linked \(\beta\)1,4 to glucuronic acid, Biosynth, Compton, UK) was used as a comparison.  
The raw biomass/commercial chondro sulfate was placed in a tube-shaped furnace and subjected to a controlled environment with a flow of N\({}_{2}\) gas at a rate of 0.3 L h\({}^{-1}\). Different combinations of time and temperature were tested to determine the optimal conditions for carbonization. Among these, the best outcomes were observed at a temperature of 1000 \({}^{\circ}\)C and a duration of 1 h, resulting in the formation of carbonized materials, which accounted for approximately 39% of the original weight. These specific parameters were therefore selected for further investigation in this study. The detailed findings are in Table S1, which is included in the Supplementary Materials.