Engineered MnO_x-biochar from sugarcane bagasse enables sensitive electrochemical detection of Cd²⁺ in water

Cadmium (Cd) is a toxic metal widely used in batteries, pigments, and coatings, and its presence in natural environments poses a serious risk to human health. This study aimed to develop an electrochemical sensor based on a carbon paste electrode modified with manganese oxide-functionalized sugarcane bagasse biochar for Cd²⁺ determination using differential pulse adsorptive anodic stripping voltammetry (DPAdASV), providing a sustainable strategy that combines biomass valorization with enhanced electrochemical performance. Manganese precursors with different oxidation states (MnCl₂ or KMnO₄) were employed to produce biochars with distinct MnO_x phase compositions, enabling the investigation of the relationship between the nature of these phases and the electroanalytical performance of the resulting materials. The biochars produced via manganese pretreatment of sugarcane bagasse using different precursor salts were characterized by Fourier-transform infrared, Raman spectroscopy, nitrogen adsorption-desorption analysis, determination of the pH at the point of zero charge (pH_PZC), quantification of acidic and basic surface functions, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Cd²⁺ ions were adsorbed onto the electrode surface and subsequently determined electrochemically. The electrode containing biochar obtained from KMnO₄ exhibited superior performance, evidenced by a more intense analytical signal. Adsorption assays using Cd²⁺ on the biochars confirmed that the nature of the oxides formed governs the electrochemical activity of the electrode. Experimental parameters such as biochar content (30% w w^-1), pH (4.5), reduction potential (-1.0  V), pre-accumulation time (10  min), and reduction time (60  s) were optimized. Under optimized conditions, the sensor displayed a linear response in the concentration range of 0.498-4.76 µmol L^-1 and a sensitivity of 1.178 µA L µmol^-1. The limits of detection (LOD) and quantification (LOQ) were 0.02 µmol L^-1 and 0.06 µmol L^-1, respectively. Recovery tests in river and tap water samples yielded recoveries above 80%, confirming the sensor's applicability for environmental monitoring. Interference studies demonstrated satisfactory selectivity toward Cd²⁺ in the presence of common inorganic ions and organic species found in environmental waters.