Nanocellulose from Spinifex as an Effective Adsorbent to Remove Cadmium(II) from Water

Nanocelluloses, in the form of carboxycellulose nanofibers, with low crystallinity (CI ∼ 50%), high surface charge (-68 mV), and hydrophilicity (static contact angle 38°), were prepared from an untreated (raw) Australian spinifex grass using a nitro-oxidation method employing nitric acid and sodium nitrite. The resulting nanofibers (NOCNF) were found to be an effective medium to remove Cd²⁺ ions (cadmium(II)) from water. For example, a low concentration of NOCNF suspension (0.20 wt %) could remove Cd²⁺ ions over a large concentration range (50-5000 ppm) in a relatively short time period (≤5 min). The results showed that at low Cd²⁺ concentrations (below 500 ppm), the remediation mechanism was dominated by interactions between carboxylate groups on the NOCNF surface and Cd²⁺ ions, which also acted as a cross-linking agent to gel the NOCNF suspension. At high Cd²⁺ concentrations (above 1000 ppm), the remediation mechanism was dominated by the mineralization process of forming Cd(OH)₂ nanocrystals, which was verified by TEM and WAXD. Based on the Langmuir isotherm model, the maximum Cd²⁺ removal capacity of NOCNF was around 2550 mg/g, significantly higher than those of any adsorbents reported in the literature. NOCNF exhibited the highest removal efficiency of 84%, when the Cd²⁺ concentration was 250 ppm. This study demonstrated a simple pathway to convert underutilized biomass into valuable absorbent nanomaterials that can effectively remove cadmium(II) ions from water.