Quantitative Analysis And Mechanistic Insight Of Heavy Metals Adsorbed By Functionalized Silica

With the rapid development of the industry in human society,the environmental problems of water pollution have become increasingly serious.Among them,heavy metal pollution has long-lasting harm to living things and human body due to its non-degradable nature and enrichment with food chains.The development of heavy metal recycle technology is an urgent task.The adsorption of heavy metal ions in water by adsorbent is a simple,low-cost,high-cyclability treatment method,and organic functionalized SiO₂ nanospheres are considered to as prospective materials to capture heavy metal ions from wastewater.However,the relationship between the adsorption capacity and a specific functional group on the surface of SiO₂ is still unknown,so quantitative analysis and mechanism study on the improvement of adsorption performance caused by surface modification functional groups are required.The main research contents of this paper are as follows:1.Silica nanospheres with good monodispersity were successfully synthesized by the classical St?ber method,and the surface functionalization of SiO₂ was realized by simple silylation reaction.The synthesis of-EDTA,-COOH,-SO₃H,-SH and-NH₂functionalized SiO₂ was successfully carried out.Scanning electron microscopy?SEM?,X-ray diffraction?XRD?and specific surface area?BET?indicate that the size and morphology of the five organic functionalized SiO₂ are consistent with those of the pristine silica nanospheres.Fourier Transform infrared spectroscopy?FTIR?indicates that the substrate material itself is not destroyed after synthesis.Finally,energy dispersive spectroscopy?EDS?and X-ray photoelectron spectroscopy?XPS?analysis showed that the characteristic elements of the corresponding functional groups were detected on the functionalized SiO₂,further indicating that different organic functional groups were successfully grafted on the surface of SiO₂.2.Bulk adsorption experiments show that the adsorption capacity of Pb²⁺,Cu²⁺,Ni²⁺,Zn²⁺and Cd²⁺in the original silica nanospheres is negligible,which means the adsorption capacity of SiO₂ itself is negligible.The contents of organofunctional groups of-EDTA,-SO₃H,-COOH,-SH and-NH₂ functionalized SiO₂ were calculated by thermogravimetric analysis.Next,the adsorption experiments of of Pb²⁺,Cu²⁺,Ni²⁺,Zn²⁺and Cd²⁺by-EDTA,-SO₃H,-COOH,-SH and-NH₂ functionalized SiO₂ showed that the adsorption capacities of these heavy metal ions by-EDTA functionalized SiO₂were 0.387,0.380,0.312,0.349 and 0.362 mmol/g.respectively.Normalized by the number of moles of graft functional groups,the adsorption capacities by-EDTA functionalized SiO₂ per unit millimolar group were 1.511,1.497,1.228,1.366 and 1.423mmol/mmol,which were larger than others'.This also indicates that the modifying EDTA group on the SiO₂ nanospheres has the largest increase in its adsorption capacity.At the same time,the results of adsorption experiments with different pH values,kinetic experiments and isothermal adsorption experiments show that adsorption capacity of the-EDTA functionalized SiO₂ increased most compared with those of-COOH,-SO₃H,-SH and-NH₂ functionalized SiO₂.This is the first time to evaluate the adsorption performance of functionalized SiO₂ based on the number of moles of effective functional groups,which can make up for the shortcomings of conventional evaluation methods based on unit mass calculation.3.In the further mechanism investigation,Zeta potential analysis showed that the adsorption amount of NH₂ functionalized SiO₂ was very low,and its surface was positively charged during adsorption.The surfaces of-EDTA,-COOH and-SO₃H functionalized SiO₂ with good adsorption effects are all negatively charged.The full-scan and fine-scan spectrum analysis in X-ray photoelectron spectroscopy?XPS?revealed that the adsorption sites of-EDTA,-SO₃H,-COOH,-SH and-NH₂functionalized SiO₂ adsorbing Pb²⁺ions were derived in turn.Their adsorption configurations were built as 3D models on Material Studio.The structure was optimized and the binding energy was calculated with the Dmol³ module.The calculated adsorption energies?Ead?for the adsorption of Pb²⁺ions on-EDTA,-COOH,-SO₃H,-SH and-NH₂ functionalized SiO₂ are-23.51 eV,-20.57 eV,-19.95 eV,-17.55 eV and-7.57 eV?arranged from large to small?and the order of magnitude is exactly the same as the theoretical maximum adsorption capacity per unit millimolar,which means there is a certain positive correlation between the two.Finally,by comparing the models of adsorption models f Pb²⁺ions by-EDTA,-SO₃H,-COOH,-SH and-NH₂ silylation reagents and the adsorption models of Pb²⁺ions by their corresponding molecules,it was found that a single EDTA group can interact with the Pb²⁺ion,which has the advantage of not causing a positional resistance problem between the functional groups.Comprehensive analysis shows that EDTA not only attracts the positive charged Pb²⁺ions on the surface,but also has higher adsorption energy with Pb²⁺ions,and has less steric hindrance.Therefore,it is further speculated that the EDTA group modified on SiO₂ has a larger expression of its adsorption force and adsorption capacity than other groups.4.The presence of cations such as Na⁺,K⁺,Ca²⁺and Mg²⁺in the actual water body has little effect on the adsorption of Pb²⁺by EDTA-SiO₂.When five heavy metal ions including Cu²⁺,Pb²⁺,Ni²⁺,Cd²⁺and Zn²⁺coexist,EDTA exhibits broad-spectrum adsorption at low concentrations,and has adsorption priority at high concentrations.EDTA has a good adsorption capacity at pH>3,and a small adsorption amount at pH<2,whereby a pH-driven adsorption-desorption cycle can be realized to achieve material recycling.As the good performance of EDTA-SiO₂ under these conditions,the stability of SiO₂ as matrix and the wide adaptability of EDTA in various environments make the application prospect of EDTA-SiO₂ material in actual wastewater environment.This work not only developed an effective SiO₂ adsorbent,but also provided valuable insights for the evaluation and design of new SiO₂ materials with abundant adsorption sites.