Hydrothermal synthesis of needle-shaped manganese oxide nanoparticle for superior adsorption of thallium(I): characterization, performance, and mechanism study

Thallium as a highly toxic metal element has been listed as one of priority drinking water contaminants. In this study, manganese oxide nanoparticles were synthesized through a simple hydrothermal method and applied for the removal of thallium(I). The adsorbent was composed of numerous needle-like nanorods and had an average volume diameter of 230 nm after heat-drying procedure. The crystal form of adsorbent was determined as α-MnO₂. The adsorbent exhibited a much faster adsorption rate than most of previously reported adsorbent, achieving over 66.4% of equilibrium adsorption capacity in the first 10 min. The adsorption process was found to be highly affected by solution pH and higher than 100 mg/g of adsorption capacity could be obtained in a wide pH range of 6.0–10.0. The isotherm study indicated that the adsorption of Tl(I) on the adsorbent was favorable and governed by a chemisorption process, with the maximum adsorption capacity of 505.5 mg/g at pH 7.0. The adsorption process was confirmed to be thermodynamically spontaneous and endothermic. The presence of Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cu²⁺ cations had certain negative effects on the uptake of Tl(I). Based on the batch experiments and XPS analysis, the deprotonated hydroxyl groups that bonded to manganese atoms worked as the binding sites for the effective removal of Tl(I) ions and no redox reaction occurred during the adsorption process.