Efficient visual adsorption of Pb²⁺ by nanocellulose/sodium alginate microspheres with fluorescence sensitivity

In this study, carbon dots (CDs) were prepared by a one-pot hydrothermal method using tempo-oxidized cellulose nanocrystals (TOCNC) and polyethylenimine (PEI). The CDs were self-assembled with a microsphere adsorbent prepared using TOCNC and sodium alginate (SA). CDs-TOCNC/SA-an environmentally friendly, fluorescent-sensitive, and recyclable microsphere adsorbent-was obtained. FTIR analysis showed that PEI successfully modified the CDs. In addition, the fluorescence quenching of CDs was observed when the concentration of Pb²⁺ was 0.0001-100 mg/L, indicating that CDs can dynamically monitor Pb²⁺. CDs-TOCNC/SA can produce blue fluorescence under 365 nm UV light and selectively and efficiently adsorb Pb²⁺. When the concentration of Pb²⁺ was 0.0001-100 mg/L, fluorescence quenching of the adsorbent was observed, indicating that CDs-TOCNC/SA could visually adsorb Pb²⁺. The adsorption isotherm and kinetic parameters show that the adsorption process conforms to the Langmuir isotherm model at 298 K, and the maximum adsorption capacity of Pb²⁺ was 190.1 mg/g at pH = 5. Moreover, CDs-TOCNC/SA could still obtain 78.99 % Pb²⁺ after five sorption-desorption cycles. The adsorption mechanism may involve ion exchange, electrostatic attraction, intra-particle diffusion, and chemical complexation.