Hydroxyapatite crystallization in shrimp cephalothorax wastes during subcritical water treatment for chitin extraction

The extraction of calcareous chitin from shrimp cephalothorax was successfully achieved using a subcritical water treatment to attain a deproteinization up to 96%. The treatments also increased the crystalline domain size in the α-chitin fibers. An experimental design of Taguchi allowed the optimization of experiments. The macroelements identified in all samples were Ca, P, S, K, Cl and Al, whereas Cr, Mn, Fe, Ni, Cu, Zn, Br and Sr were also detected as microelements. The assigned crystalline phases by XRD were α-chitin, calcite, HAP and traces of quartz. The presence of these phases was corroborated by ATR-FTIR and SEM-EDS analyses. The highest content of α-chitin (82.2 wt%) was obtained for the 0.17 chitin:dH2O (wt/wt) ratio for 30 min treatment at 260 °C. Noteworthy, this treatment promotes the crystallization of both minerals as microcrystals of calcite and nanocrystals of hydroxyapatite with needle and flake shapes as well as intermediate morphologies.