Sustainable sensor from coconut waste for sensitive and simultaneous detection of heavy metals

Heavy metal detection is essential for both environmental monitoring and public health protection. Electrochemical sensors are a cost-effective and overly sensitive alternative to conventional analytical techniques, particularly when applied to complex matrices. In this study, biochars derived from green coconut mesocarp were synthesized via phosphoric acid activation followed by pyrolysis at four different temperatures. These materials were structurally and morphologically characterized and after used to fabricate chemically modified electrodes. Among the prepared materials, the electrode based on CMA300 exhibited the most favorable performance and was applied to the simultaneous detection of Cd²⁺, Pb²⁺, and Hg²⁺ using differential pulse voltammetry. Electrochemical studies revealed competitive adsorption phenomena, with Pb²⁺ showing prolonged retention on the electrode surface, for up to 1080 min, writing down favorable kinetic interactions. The CMA300 sensor demonstrated high sensitivities for Cd²⁺ (42.75 μA L μmol^-1), Pb²⁺ (38.96 μA L μmol^-1), and Hg²⁺ (8.07 μA L μmol^-1), along with low detection limits, particularly for Cd²⁺ (LOD = 0.0014 μM), surpassing the performance of several reported sensors. Its low cost and straightforward preparation further underscore its potential for sustainable applications. Validation in six real samples, including sewage, cosmetic products, and tap water, yielded recovery rates between 89 % and 106 %, confirming the sensor's accuracy and selectivity in interference-rich matrices. Beyond its analytical merits, this work introduces a sustainable sensing platform derived from agro-industrial waste and offers new mechanistic insights into metal-electrode interactions through kinetic evaluation and surface adsorption analysis.