Tangerine peel as a sustainable biomass precursor for carbon quantum dots with photocatalytic application for dye removal from water and wastewater

Carbon quantum dots (CQDs) were synthesized via a green hydrothermal route using tangerine peel as a renewable carbon precursor. Structural characterization confirmed an amorphous carbon network (XRD broad peak at 2θ ≈ 20–25°) with a moderate degree of graphitization ( ID/IG ≈ 0.61–0.84). FT-IR spectroscopy revealed abundant surface functionalities, O–H stretching (~ 3400 cm⁻¹), C–H stretching (2850–2920 cm⁻¹), C = O stretching (~ 1730 cm⁻¹), and C–O–C/C–O stretching (1050–1150 cm⁻¹), alongside a newly formed aromatic C = C band at ~ 1600 cm⁻¹ after carbonization. The CQDs exhibited a mesoporous structure (~ 5 nm diameter) with a near-neutral, slightly negative zeta potential. Strong UV absorption (center ~ 275 nm; shoulder 320–380 nm) corresponded to an optical band gap of ≈ 3.18 eV (absorption edge ~ 389 nm). Under a 400 W Hg lamp, the CQDs displayed excitation-dependent photoluminescence (400–600 nm) and efficiently decolorized four textile dyes (Basic Blue 7, Acid Blue 56, Acid Blue 113, Acid Blue 128). Application to real textile wastewater achieved ≈ 88% removal efficiency, with BOD reduced from 1900 to 19 mg/L and COD from 600 to 27 mg/L.