The present disclosure relates to particles comprising carboxyalkyl starch that are permeated with an acidic gas and their uses as absorbent materials. It was discovered that superabsorbent materials could be obtained from carboxyalkyl starch particles permeated with the acidic gas and heated to a temperature of at least 100° C. until they reach an AUL at 0.7 psi. of at least 14 g/g and a CRC of at least 18 g/g. Moreover, it was discovered that the pH of alkaline starch extrudates can be adjusted by permeating particles of the extrudate with the acidic gas even with treating the particles to temperatures less than 100° C. The carboxyalkyl starch particles obtained by the methods described herein are characterized as having intramolecular ester bonds, which are greater in number at the surface of the particle than in the core, and the particles have a greater concentration of cation of the acidic gas at the surface than a the core.