Synthesis And Adsorption Performance Of Functionalized Mesoporous Silicates

The treatment of heavy metal and dye wastewater is attracting more and more public attention for their wide source and serious impact on the environment.The utilization of adsorption technology can realize the unification of pollution control and resource reuse;satisfy the demands of circular economy and sustainable development.Since the early 1990s,functionalized mesoporous silicates have shown wide application prospects in the fields of adsorption and separation;however,the deficiencies existing in the synthetic process,such as complex steps,toxic reagents and lowly modifying stability of organic ingredients,limit the extensive application of the mesoporous adsorbents.Guided by principles of green chemistry,two aminated mesoporous silicates with different aperture were synthesized for the adsorption of heavy metal ions and dyes,respectively.And then,the adsorption action of two adsorbents was systemically investigated according to experimental results.Firstly,a new approach,which was simple,efficient and atoxic,was provided for preparing mesoporous adsorbents.With tetraethyl orthosilicate（TEOS）and production-grade of 3-Aminopropyltriethoxysilane（KH-550）as hybrid silica source, the aminated mesoporous adsorbent was synthesized in the presence of cetyltrimethylammonium（CTAB）via the integration of co-condensation and evaporation-induced self-assembly（EISA）procedure.During the preparation,atoxic ethanol was the organic solvent and tetramethylammonium hydroxide（TMAOH） served as the subsidiary template.The optimum reactant ratio was ascertained by the orthogonal test:n（CTAB）:n（TMAOH）:n（Si）:n（ethanol）= 0.15:0.25:1:32.5, n（KH-550）:n（TEOS）= 1:4.The synthesized adsorbent exhibited high modification amount（2.584 mmol/g）,BET pore size（5.27 nm）and pore volume （0.556 cm³/g）.These natural parameters demonstrate that the exploratory preparation method can enhance the modification ratio of amido and pore-forming performance of the templating reagent to improve the adsorption capacity of mesoporous silicates for adsorbates with small pore size（e.g.heavy metal ions）.Secondly,the adsorption isotherms,kinetics,thermodynamics and adsorption selectivity of Cu²⁺,Pb²⁺,Zn²⁺and Cd²⁺onto the mesoporous adsorbent were studied, the adsorption mechanisms and its regeneration capacity were then proposed in the paper.The adsorption isotherms were studied using Redlich-Peterson,Langmuir and Freundlich models;the first one was best fit for the nonlinear regression of adsorption data.The saturated adsorption capacity decreased in the order of Cu²⁺>Pb²⁺>Zn²⁺> Cd²⁺,which was calculated by Langmuir isotherm.The kinetic data indicate that the adsorption process of heavy metal ions can be better described by the pseudo-second-order model than the pseudo-first-order model.And the intra-particle diffusion accompanied by film diffusion is testified the rate-controlling step.The thermodynamic results show that the adsorption process is endothermic（E_a>0,△H >0）so that the rise in temperature is helpful to the adsorption behaviour.Moreover, the spontaneous process（△G<0）is considered to be driven by the increase in adsorption entropy（△S>0）.The adsorption mechanisms of heavy metal ions were also proposed as:under the condition of pH value 5,amido can react with heavy metal ions to produce chelate complex,which results in the removal of heavy metals. With experiments of adsorption selectivity,the mesoporous adsorbent revealed different selectivity among the metal ions,resulting from the bonding force of coordination link and stability of chelate complex.And the adsorption selectivity decreased in the same order as that of saturated adsorption capacity.In addition,the mesoporous adsorbent can be regenerated and used repeatedly by the elution of HC1 and neutralization of NaHCO₃ solution.Thirdly,directly pore-size expansion of mesoporous silicates was developed, which could be explained as:N,N-dimethyldecylamine（DMDA）is added directly in the established nonaqueous system,and then aminated mesoporous silicates with expanded pores were prepared by one-step process.The addition of DMDA can not only enlarge the pore size,but also increase pore volume.The results of the contrast test exhibit that DMDA,as a micelle expander,is propitious to improve adsorbents’ decolorization performance.The optimal adsorbent was also selected(n_DMDA:n_ctAb= 1.5);its modification amount,BET pore size and pore volume were 2.139 mmol/g, 18.69 nm and 1.136 cm³/g,respectively.Finally,the DMDA-expanded adsorbent was used for the disposal of four simulated wastewater contained acid gold yellow G,acid orangeⅡ,reactive brilliant red X-3B or reactive brilliant blue KN-R.Related coefficients of three isotherm models reduced in the order of Redlich-Peterson>Langmuir>Freundlich;and the saturated adsorption capacity decreased in the order of acid gold yellow G>acid orangeⅡ>reactive brilliant red X-3B>reactive brilliant blue KN-R.The kinetic data of the decolourization process closely follow the pseudo-second-order model; and the data also prove that the process is mainly controlled by the combination of intra-particle diffusion and film diffusion.In the range of test temperature,the adsorbance and adsorption rate of acid gold yellow G increased early but decreased late;while that of the other three dyes kept increasing with the rise in temperature. The different results were related to dyes’ molecular structure.The thermodynamic analysis indicates that the decolourization process is spontaneous.The decolourization reaction of acid gold yellow G includes two stages:the former is endothermic and driven by the increase in entropy,which is similar to that of the other three dyes;the latter is exothermic（E_a<0,AH<0）and mainly driven by enthalpy difference.The decolourization mechanisms of dyes were discussed in the paper.Under the condition of pH value 2,a protonated amido and a sulfonic group can link up by the chemical bond;dye moleculars deprived of hydrophilicity are then isolated from wastewater because sulfonic groups have been blocked.Moreover,the effect of decolourization is related to the number of sulfonic group and structure of aromatic ring in a dye molecular.In addition,the expanded adsorbent can be regenerated and used repeatedly by the elution of NaOH solution.In this paper,a preparation method with simple,efficient and atoxic properties was exploited to synthesize functionalized mesoporous silicates.The synthetic product has the good adsorption and regeneration performance and exhibits promising applications in the treatment of wastewater contained various heavy metals and dyes. Therefore,it should be a novel adsorbent with great potential.