Sorption–spectrophotometric determination of Cd(II) using SNPTSB immobilised on silk fibroin

The present study reports a novel sorption-spectrophotometric method for the selective detection of Cd(II) ions in wastewater using the sodium 5-nitro-2-(3-(4-((4-sulfonatophenyl)diazenyl)phenyl)triaz-1-en-1-yl)benzenesulfonate (SNPTSB) immobilized on silk fibroin as a biodegradable and eco-friendly sorbent. The method relies on the formation of a Cd(II) – SNPTSB complex, confirmed by UV-Vis spectroscopy, Raman analysis, conductometry, and SEM-EDS techniques. In solution, a bathochromic shift in the absorption spectrum from 415 nm (free reagent) to 490 nm (complex) was observed, indicating complex formation. In the solid phase, a more pronounced shift from 420 nm to 510 nm (Δλ = 90 nm) suggested stronger interactions, likely due to matrix effects. Raman spectroscopy further confirmed metal – ligand coordination, as evidenced by characteristic shifts in the N=N and N–O vibrational modes. Conductometric measurements supported complex formation via a drop in conductivity from 65.8 μS/cm (Cd2+) and 24.4 μS/cm (reagent) to 15.2 μS/cm (complex). The method exhibited excellent linearity (R2 = 0.9951) within the range of 0.16–2.08 µg/mL, with a molar absorptivity of 2.2 × 104 L·mol− 1·cm− 1 and Sandell sensitivity of 0.00122 µg/cm2. EDS analysis confirmed 5.85% Cd content in the complexed fibre. The method demonstrated high selectivity, with minimal interference from Cu2 +, Fe3 +, Zn2 +, and others (Sr < 0.018). Recovery from complex model mixtures and real industrial wastewater (Sr < 0.052) validated the method’s practical applicability. Comparative analysis with inversion voltammetry showed no significant difference (tobs = 1.62 < ttab = 3.17; Fobs = 1.58 < Ftab = 4.46), confirming the method’s reliability and competitive performance.