Ulvan dialdehyde-gelatin hydrogels for removal of heavy metals and methylene blue from aqueous solution

Hydrogels based on the polysaccharide ulvan from the green macroalgae Ulva fenestrata were synthesized and evaluated as an adsorbent for heavy metals ions and methylene blue. Ulvan was extracted from Ulva fenestrata using diluted hydrochloric acid and recovered by precipitation with EtOH. The extracted ulvan was converted into ulvan dialdehyde via periodate-oxidation and subsequently combined with gelatin yielding hydrogels. The hydrogels showed good water-uptake capacity with a maximum swelling degree of 2400 % in water and 900 % in PBS buffer. Adsorption tests of methylene blue showed a maximum adsorption capacity of 465 mg/g. The adsorption data of methylene blue followed the pseudo-second order kinetics and agreed with the Langmuir adsorption isotherm. The maximum adsorption capacity of heavy metal ions was 14 mg/g for Cu²⁺, 7 mg/g for Co²⁺and 6 mg/g for Ni²⁺and Zn²⁺ indicating that the hydrogels have a stronger affinity for Cu²⁺ than for Co²⁺, Ni²⁺, and Zn²⁺.