Chemical modification of lettuce leaves using NaOH and EDTA: A brilliant biosorbent for the adsorption of heavy metal ions from aqueous solution

Abstract

Recently, the use of natural materials to treat pollutants, especially heavy metals in water, has received significant attention in research. In this report, lettuce leaves (LC), an environmentally friendly and cost-effective biosorbent, were chemically modified with NaOH and ethylenediaminetetraacetic acid (EDTA) for the first time in two steps to remove Pb²⁺. The compositional and structural properties of the LC/NaOH/EDTA (LCNE) biosorbent were ascertained by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The effects of diverse operating conditions including working environment pH, sorbate concentration, sorbent mass, modifier content, and working temperature were tested. The experimental results estimated that at pH 6, 91.22 % of Pb²⁺ at 80 mg/L concentration was adsorbed by 1.0 g/L sorbent. The second-order kinetic provided better linearity (R² = 0.999) to depict the chemisorption, whereas the biosorption isotherm closely followed the Langmuir model, revealing a maximum uptake capacity of 117.51 mg/g. In the multi-metal biosorption system, the biosorption capacity of Pb²⁺ was 81.98 mg/g compared to only 26.63 mg/g of Ni²⁺. Our report revealed that Pb²⁺ adsorption was moderately stable after many regeneration cycles, i.e., it decreased by almost 20 % after 3 consecutive cycles of immersion in harsh environments. In addition, the efficiency of LCNE in removing Pb²⁺ from urban wastewater sample was tested, with 72.48 % of Pb²⁺ being adsorbed by the biosorbent. These results underline that LNCE is a biosorbent with excellent application potential for the on-site sequestration of Pb²⁺ ions.