Tetrahydrazide-EDTA Cross-Linked Cellulose Hydrogels for Water Treatment by Heavy Metal Chelation

Polluting heavy metals persist in the environment, leading to bioaccumulation and toxicity, which is a growing problem in developing countries. Various water filtration systems for heavy metal removal have been developed, with sorption being the simplest and most economically viable. However, many commercial sorbents are powders, leading to inefficient sorbent removal and secondary pollution. The goal of our research was to develop renewable, biodegradable, and cost-effective hydrogel sorbents able to bind heavy metals. This was accomplished using hydroxyethyl cellulose (HEC) and carboxymethyl cellulose (CMC) functionalized with aromatic aldehydes (aa-HEC and aa-CMC), and an ethylenediaminetetraacetic acid (EDTA)-based cross-linker modified with four hydrazide groups (4h-EDTA). By varying the ratio of aldehyde-to-hydrazide (a:h) groups in the aa-HEC/4h-EDTA hydrogel, a ratio of 1:2 a:h was found to have the maximum storage modulus (G'). This was used to make 2 wt % hydrogels with a composition of 25/75 aa-HEC/aa-CMC cross-linked with 4h-EDTA (aa-HEC/aa-CMC/4h-EDTA), with a G' of 200 Pa and a maximum sorption capacity of 102 mg of Cu²⁺ per gram of hydrogel. The sorption capacity of the hydrogels was tested for Cu²⁺, Ni²⁺, Zn²⁺, Co²⁺, and Mg²⁺ individually and as a mixture, with Cu²⁺ showing the highest affinity. This work shows that cellulose-based hydrogels can be used as a green alternative for the removal of heavy metal pollutants from water.