A two-in-one thiosemicarbazide and whole pine needle-based adsorbent for rapid and efficient adsorption of methylene blue dye and mercuric ions

Abstract

Herein, we report the synthesis of an oxidized pine needle-thiosemicarbazone Schiff base (OPN-TSC) from whole pine needles (WPN) as a dual-purpose adsorbent to remove a cationic dye, methylene blue (MB), and Hg²⁺ ions in separate processes. The adsorbent was synthesized by periodate oxidation of WPN followed by a reaction with thiosemicarbazide. The syntheses of OPN and OPN-TSC were confirmed by FTIR, XRD, FESEM, EDS, BET, and surface charge analysis. The emergence of new peaks at 1729 cm^-1 (-CHO stretching) and 1639 cm^-1 (-COO^- stretching) in the FTIR spectrum of OPN confirmed the oxidation of WPN to OPN. FTIR spectrum of OPN-TSC has a peak at 1604 cm^-1 (C = N stretching), confirming the functionalization of OPN to OPN-TSC. XRD studies revealed an increase in the crystallinity of OPN and a decrease in the crystallinity of OPN-TSC because of the attachment of thiosemicarbazide to OPN. The values of %removal for MB and Hg²⁺ ions by OPN-TSC were found to be 87.36% and 98.2% with maximum adsorption capacity of 279.3 mg/g and 196 mg/g for MB and Hg²⁺ ions, respectively. The adsorption of MB followed pseudo-second-order kinetics with correlation coefficient (R² of 0.99383) and Freundlich isotherm (R² = 0.97239), whereas Hg²⁺ ion removal demonstrated the Elovich (R² = 0.97076) and Langmuir isotherm (R² = 0.95110). OPN-TSC is regenerable with significant recyclability up to 10 cycles for both the adsorbates. The studies established OPN-TSC as a low-cost, sustainable, biodegradable, environmentally benign, and promising adsorbent for the removal of hazardous cationic dyes and toxic metal ions from wastewater and industrial effluents, especially the textile effluents.

Keywords: Adsorption; Biowaste; Oxidized pine needles; Reusability; Wastewater treatment.