Analysis and evaluation of heavy metals in Rubia tinctorum (L.) as eco-friendly dyes by inductively coupled plasma emission spectrometry and infrared spectroscopy

Abstract

This study investigates the heavy metal composition (Cd, Pb, Fe, Li, Cu, Zn, K, Na, Ca) of Rubia tinctorum (L.) roots collected from Souk Attarine in the historic medina of Fez, Morocco. Thermal analysis (DTA/TGA) revealed significant mass losses within specific temperature ranges. Samples were calcined at 110 °C, 325 °C, 450 °C, and 600 °C. FTIR spectroscopy confirmed the degradation of organic matter and the formation of calcite and silica at higher temperatures. UV–Vis spectroscopy revealed π–π* transitions in the 250–280 nm range and n–π* transitions between 420–550 nm, indicative of alizarin-type chromophores. For elemental quantification, samples were acid-digested and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The ICP-AES method exhibited excellent linearity (R² > 0.998) over a concentration range of 0–50 mg L-1, depending on the element. Limits of detection (LOD) ranged from 0.003 to 0.066 mg/L, and limits of quantification (LOQ) from 0.010 to 0.083 mg L-1, demonstrating high sensitivity and precision. X-ray diffraction (XRD) confirmed the presence of calcite (major peak at 2θ = 29.4°) and secondary quartz in the calcined samples. High levels of essential elements (Ca, K, Fe) and Cd and Pb concentrations below WHO toxicity limits confirm the ecological safety and analytical value of Rubia tinctorum as a natural dye source.