Efficient phosphate removal utilizing N, Zn-doped carbon dots as an innovative nanoadsorbent

The primary goal of this study is to examine PO₄ ^3- adsorption from aqueous solutions using zinc-doped carbon dots (Zn-N-CDs) as a new adsorbent and cost-effective technique. Zn-N-CDs were produced through a hydrothermal process and subsequently identified using various techniques. The effect of reaction time, temperature, pH, ionic strength, adsorbent dosage, initial PO₄ ^3- concentration, and anion competition (NO₃ ^-, Cl^-, HCO₃ ^-, and SO₄ ^2-) on PO₄ ^3- adsorption using Zn-N-CDs were investigated. The characterization results depicted that Zn-N-CDs have a spherical structure without obvious aggregation and revealed the amorphous nature of carbon dots with many pores. Zn-N-CDs had a high affinity for adsorbing PO₄ ^3-, reaching equilibrium within 5 minutes. While PO₄ ^3- adsorption reduced with an increase in temperature, it increased with pH and ionic strength. The optimal conditions for PO₄ ^3- adsorption were determined to be pH 8, 100 mM KCl as an ionic strength, and 5 g L^-1 of Zn-N-CDs. The presence of SO₄ ^2- and HCO₃ ^- as competing anions slightly decreased PO₄ ^3- adsorption. Thermodynamic studies revealed that PO₄ ^3- adsorption onto Zn-N-CDs was endothermic, spontaneous, and disordered, as evidenced by ΔG° < 0, ΔH° > 0, and ΔS° > 0. The experimental data fit well with a pseudo-second-order kinetic model (R ² = 0.999) and the Freundlich isotherm model (R ² = 0.9754), signifying that PO₄ ^3- adsorption onto Zn-N-CDs occurs through multi-layer and chemi-sorption mechanisms. Overall, Zn-N-CDs indicated a great capability to adsorb high concentrations of PO₄ ^3- across a wide range of pH values, indicating their potential for environmental remediation.