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

Nanocelluloses, in the form of carboxycellulose nanofibers, with low crystallinity (CI similar to 50%), high surface charge (-68 mV), and hydrophilicity (static contact angle 38 degrees), 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.