Preparation Of Quaternary Ammonium-functionalized SiO₂ For Selective Adsorption Of Vanadium

Vanadium is a strategic metal resource with significant applications in alloy manufacturing,chemical industry,and new energy fields.China generates a large amount of industrial waste containing vanadium annually.Traditional disposal methods such as landfilling not only waste precious resources but also pose environmental safety risks.To address this issue,we prepared five different quaternary ammonium-functionalized SiO₂ adsorbents using inexpensive and easily available silica（SiO₂）to selectively adsorb and recover vanadium resources.We first examined the vanadium adsorption capacity of five different types of quaternaryammonium-functionalizedSiO₂andfoundthat dimethyloctadecylammonium-functionalized SiO₂ had the best vanadium adsorption performance.Then,at room temperature,we conducted adsorption kinetics and isotherm studies on dimethyloctadecylammonium-functionalized SiO₂,specifically investigating the effects of pH value and temperature of the vanadium（V）solution on its vanadium adsorption performance.The results showed that at a vanadium（V）solution concentration of 2.00×10^-3 mol/L and a pH of 3.3,with a dosage of 100 mg/20m L of dimethyloctadecylammonium-functionalized SiO₂ as the adsorbent and an adsorption time of 2 hours,the vanadium adsorption performance of this SiO₂ was optimal under these conditions,with a maximum vanadium adsorption rate of 99.8%.In regard to the aforementioned adsorption process,we conducted fitting for isotherm adsorption and kinetic adsorption models,and the results showed that the vanadium adsorption behavior of dimethyloctadecylammonium-functionalized SiO₂ was more consistent with Langmuir isotherm adsorption and conformed to pseudo-second-order adsorption kinetics.The correlation coefficients（R²）for both models were 0.996 and0.998,respectively,indicating that the adsorption behavior of dimethyloctadecylammonium-functionalized SiO₂ is single-molecule layer chemical adsorption,with a theoretical maximum adsorption capacity of 361.17 mg/L.In response to the problem of the difficult separation of vanadium and chromium,we investigated the selective adsorption performance of dimethyloctadecylammonium-functionalized SiO₂ towards vanadium and chromium.The results showed that the selective adsorption ability for vanadium and chromium was better when the pH value of the mixed solution was 2.7,with an n V/n Cr ratio of 12.4.In addition,we compared the desorption efficiency of three desorption agents:NaOH,HCl,and H₂O.The results showed that NaOH had a higher desorption capacity,with a desorption rate of 81.4%.After repeating the desorption process three times,the desorption rate of vanadium increased to 92.5%.To gain a deeper understanding of the adsorption mechanism of quaternary ammonium-functionalized SiO₂,we conducted structural analysis on the prepared dimethyloctadecylammonium-functionalized SiO₂ using scanning electron microscopy-energy dispersive spectroscopy,thermogravimetric analysis,and Fourier transform infrared spectroscopy.The results showed that the functional groups were successfully chemically bonded to the surface of SiO₂.We investigated the speciation of vanadium in solutions with different pH values using a time-of-flight mass spectrometer and found that at pH=1-2,vanadium mainly existed in the form of dimers（V₂）and tetramers（V₄）,while at pH 3-6,it mainly existed in the form of decamers(V₁₀),with a possible molecular formula of V₁₀O₂₈^6-.Based on these experimental and analytical results,we propose that the adsorption of vanadium onto dimethyloctadecylammonium-functionalized SiO₂ is achieved through a combination of physical and chemical adsorption,with the ion exchange mechanism playing a dominant role,and the steric hindrance effect and the synergistic constraints of adsorbent/adsorbate softness and hardness influencing the adsorption efficiency and characteristics.This article develops a novel dimethyloctadecylammonium-functionalized SiO₂,which can achieve efficient and low-cost enrichment and recovery of vanadium resources,and has potential industrial application value.It provides an important option for the resource utilization of vanadium-containing residues and wastewater.