Development and application of a Schiff-base colorimetric sensor for lead (Pb²⁺) detection in borehole water from...

Lead contamination of groundwater remains a serious environmental and public health concern, particularly in urban regions where borehole water serves as a major domestic water source. Conventional analytical techniques such as atomic absorption spectroscopy (AAS) and inductively coupled plasma-based methods provide excellent sensitivity for trace metal determination but are often limited by high cost, technical expertise requirements, and infrastructural constraints in many developing regions. In this study, a simple Schiff-base ligand derived from salicylaldehyde (0.01 mol) and ethylenediamine (0.01 mol) was synthesized under reflux for 2 h and evaluated as a low-cost colorimetric sensor for Pb²⁺ detection in borehole water collected from Kaduna metropolis, Nigeria. The ligand was obtained as a yellow crystalline product with a percentage yield of 78.4% and melting point of 189-191 °C. FT-IR analysis confirmed successful azomethine formation with ν(C=N) band at 1624 cm^-1, while 1H NMR spectroscopy showed characteristic azomethine proton resonance at δ 8.31 ppm, aromatic proton signals at δ 6.82-7.45 ppm, methylene linker protons at δ 3.76 ppm, and phenolic OH resonance at δ 12.94 ppm, confirming ligand formation. Coordination of Pb²⁺ through azomethine nitrogen and phenolic oxygen donor sites was accompanied by a shift of the ν(C=N) band to 1598 cm^-1 and formation of a probable distorted hemidirected Pb(II) coordination geometry, producing a visible color change from pale-yellow to orange with the emergence of an absorption band at 445 nm. Good linearity was obtained over the investigated concentration range with correlation coefficient (R² = 0.995), slope-based sensitivity of 0.126 AU mg^-1 L, limit of detection (LOD) of 0.82 mg/L, and limit of quantification (LOQ) of 2.49 mg/L. Selectivity studies showed stronger response toward Pb²⁺ relative to competing ions such as Cu²⁺, Zn²⁺, Ni²⁺, Fe³⁺, and Cd²⁺ tested at 50 mg/L under identical conditions. Application to twelve borehole water samples produced recovery values of 92-98% with relative standard deviations below 3.0%, indicating satisfactory precision and analytical applicability. Although not intended to replace ultra-trace instrumental methods, the developed sensor offers a simple, rapid, and affordable approach for preliminary groundwater lead monitoring and may serve as an early-warning screening tool for potentially contaminated water sources in resource-limited settings.