Electrochemical Removal of Lead (Pb²⁺) from Aqueous Solutions Using Coal-Derived Carbon Quantum Dot-Modified Electrodes

Abstract

Lead contamination of water resources poses significant environmental and public health challenges due to its toxicity, persistence, and bioaccumulative nature. This study investigated the electrochemical removal of lead ions (Pb²⁺) from aqueous solutions using carbon quantum dot (CQD)-modified electrodes synthesized from unutilized low-grade lignite coal. The CQDs, previously synthesized and characterized by Eddy et al. (2026a), were employed as active electrode materials owing to their high surface area, abundant oxygen-containing functional groups, and favorable electron transfer properties. Batch electrochemical experiments were conducted to evaluate the effects of initial Pb²⁺ concentration (10–200 mg L⁻¹), solution pH (2–8), applied potential (0.5–2.5 V), electrolysis time (10–180 min), and supporting electrolyte concentration (0.01–0.50 M Na₂SO₄) on lead removal efficiency. The results revealed that Pb²⁺ removal efficiency decreased slightly from 95.80% to 87.59% as the initial lead concentration increased from 10 to 200 mg L⁻¹, while the adsorption capacity increased from 23.95 to 437.93 mg g⁻¹. The removal efficiency improved significantly with increasing pH, reaching a maximum value of 94.66% at pH 8. Similarly, increasing the applied potential enhanced Pb²⁺ removal from 71.36% at 0.5 V to 96.55% at 2.5 V. Electrolysis time strongly influenced the process, with removal efficiency increasing from 37.55% after 10 min to 96.98% after 180 min. The presence of supporting electrolyte improved solution conductivity and charge transfer, resulting in an increase in removal efficiency from 84.52% at 0.01 M Na₂SO₄ to 96.85% at 0.50 M Na₂SO₄. Under the optimum operating conditions of pH 7.0, applied potential of 2.0 V, electrolysis time of 120 min, initial Pb²⁺ concentration of 100 mg L⁻¹, and 0.10 M Na₂SO₄, the CQD electrode achieved a removal efficiency of 96.08% with an adsorption capacity of 240.20 mg g⁻¹.Kinetic studies demonstrated that the adsorption process was best described by the pseudo-second-order model (R²=0.9977), compared to the pseudo-first-order model (R²=0.942 0.94), indicating that chemisorption was the dominant mechanism controlling Pb²⁺ uptake. The intraparticle diffusion model exhibited a lower correlation coefficient (R²=0.889) and a significant intercept, confirming that diffusion was not the sole rate-controlling step. The overall removal mechanism involved surface complexation of Pb²⁺ ions with oxygen-containing functional groups followed by electrochemical reduction and deposition on the electrode surface. The study demonstrates that lignite coal-derived carbon quantum dots are highly effective electrode materials for the electrochemical remediation of lead-contaminated water. Their high removal efficiency, substantial adsorption capacity, and favorable kinetic behavior highlight their potential as sustainable, low-cost, and environmentally friendly materials for wastewater treatment applications.