Effects of Gamma-Synthesized Chitosan on Morphological, Thermal, Mechanical, and Heavy-Metal Removal Properties in Natural Rubber Foam as Sustainable and Eco-Friendly Heavy Metal Sorbents

The properties of natural rubber foam (NRF) containing gamma-synthesized chitosan (CS)
powder were investigated to address the growing demand for efficient methods to treat industrial
wastewater contaminated with heavy metals. The CS powder was prepared by irradiating chitin (CT)
powder with varying doses of gamma rays (0–100 kGy), followed by deacetylation using 40% sodium
hydroxide (NaOH) at 100 ◦C for 1 h. The resulting CS powders were then mixed with natural rubber
latex (NRL) at different contents (0, 3, 6, and 9 parts per hundred parts of rubber by weight; phr) and
processed using Dunlop techniques to prepare the foam samples. The experimental findings indicated
that the degree of deacetylation (%DD) of the CS powder increased initially with gamma doses up to
60 kGy but then decreased at 80 and 100 kGy. In addition, when the CS powder was incorporated into
the NRF samples, there were increases in total surface area, density, compression set, and hardness
(shore OO), with increasing gamma doses and CS contents. Furthermore, the determination of heavy
metal adsorption properties for Cu, Pb, Zn, and Cd showed that the developed NRF sample exhibited
high adsorption capacities. For instance, their removal efficiencies reached 94.9%, 82.5%, 91.4%, and
97.0%, respectively, in NRF containing 9 phr of 60 kGy CS. Notably, all adsorption measurements were
determined using 3 cm × 3 cm × 2.5 cm specimens submerged in respective metal solutions, with an
initial concentration of 25 mg/L. However, the removal capacity per unit mass of the sample (mg/g)
showed less dependencies on CS contents, probably due to the higher density of CS/NRF composites
in comparison to pristine NRF, resulting in a smaller volume of the former being submerged in
the solution, subsequently suppressing the effects from CS in the adsorption. Lastly, tests on the
reusability of the developed NRF indicated that the samples could be reused for up to three cycles,
with the Cu removal capacity remaining relatively high (83%) in the sample containing 9 phr of 60 kGy
CS. The overall outcomes implied that the developed NRF with the addition of gamma-synthesized
CS not only offered effective and eco-friendly heavy metal adsorption capacity to improve public
health safety and the environment from industrial wastewater but also promoted greener and safer
procedures for the synthesis/modification of similar substances through radiation technologies.