Novel Nanocomposites and Biopolymer-Based Nanocomposites for Hexavalent Chromium Removal from Aqueous Media

Designing new engineered materials derived from waste is essential for effective environmental remediation and reducing anthropogenic pollution in our economy. This study introduces an innovative method for remediating metal-contaminated water, using two distinct waste types: one biowaste (eggshell) and one industrial waste (fly ash). We synthesized three novel, cost-effective nanoadsorbent types, including two new tertiary composites and two biopolymer-based composites (specifically k-carrageenan and chitosan), which targeted chromium removal from aqueous solutions. SEM analysis reveals that in the first composite, EMZ, zeolite, and magnetite nanoparticles are successfully integrated into the porous structure of the eggshell. In the second composite (FMZ), fly ash and magnetite particles are similarly loaded within the zeolite pores. Each biopolymer-based composite is derived by incorporating the corresponding tertiary composite (FMZ or EMZ) into the biopolymer framework. Structural modifications of the eggshell, zeolite, chitosan, and k-carrageenan resulted in notable increases in specific surface area, as confirmed by BET analysis. These enhancements significantly improve chromium adsorption efficiency for each adsorbent type developed. The adsorption performances achieved are as follows: EMZ (89.76%), FMZ (84.83%), EMZCa (96.64%), FMZCa (94.87%), EMZC (99.64%), and FMZC (97.67%). The findings indicate that chromium adsorption across all adsorbent types occurs via a multimolecular layer mechanism, which is characterized as spontaneous and endothermic. Desorption studies further demonstrate the high reusability of these nanomaterials. Overall, this research underscores the potential of utilizing waste materials for new performant engineered low-cost composites and biocomposites for environmental bioremediation applications.