Research On Preparation Of Diatomite Sorbents And Performance Of Phosphate Removel From Aqueous Solution

Water pollution is becoming the most important environmental problem in addition to the atmospheric pollution. The amounts of soluble phosphate in water body from households, agriculture, and industries are the main cause for eutrophication; thus controlling the discharge of wastewater containing phosphate can be an effective way to control eutrophication. Various methods have been applied to remove phosphate from water and wastewater, including chemical precipitation, biological processes, ion exchange, and adsorption. Among them, adsorption is considered to be one of the most attractive approaches with the advantage of high removal efficiency, simple operation, and economy in application. Development of new adsorption materials, which is cheap, efficient, and environmentally is the key to promote the development of technology of phosphorus removal by adsorption, meanwhile raw material source should be sustainable.Due to massive, cheap accessible, pore structure and surface reactivity, mineral materials are becoming the current hot topics in the study of adsorption material. Diatomite, one of the representative, is a natural material formed from the remains of diatoms, which grew and were historically deposited in sediments of seas or lakes. It is abundant in many areas of the world and has unique physical characteristics, such as low cost, porosity, small particle size, low thermal conductivity and density and it has been approved as a food-grade material by the U.S. Food and Drug Administration. It has been widely research and used, being regarded as a kind of natural green environmental protection material. With unique pore structure and excellent specific surface area, diatomite is also regarded as a kind of ideal adsorbent substrate. Natural diatomite regardless of the type and shape, however, the adsorption capacity is limited, cannot be directly used as adsorbents, especially the anion adsorbent, by which research is focused on the surface modification means to increase its adsorption capacity. At present, the modifications of diatomite for strengthening of the phosphorus removal performance are based on metal oxide or hydroxide loaded, such as Fe, Mg, Al etc., and performance improvement effect is limited. On the other hand, diatomite, like other powder materials, is not convenient to separate because of powder form, which limits its wide use, thus to improve the separation performance is also an important research content. Based on the predecessors’ research, lanthanum ion, which has stronger affinity ability with phosphate, was joint to further improve the adsorption ability of diatomite composite material. Meanwhile, magnetic components were introduced by coprecipitation of Fe3+ and Fe2+. The Modified diatomite, named DE_FeLa, not only has an excellent adsorption ability, but also has the characteristics of magnetic separation. In addition, functionalization modification of silicon-based materials by organic grafting, has being got more and more attention. Organic grafting be widely applied in modification of Silicon-based materials, however its application in surface modification of diatomite was rarely reported. In this paper, quaternary ammonium groups were introducted onto surface of the diatomite by two organic grafted technical route （"grafting to" and "grafting from"）. The resultant composite adsorbents, which has the advantage of high adsorption capacity, rapid adsorption rate, wide scope of pH and good reusability, were perpared.The innovations of this paper are shown as follow:1. Lanthanum has the largest affinity among the different metal oxide, an ultrasonic assisted coprecipitation method was introduced to the synchronous deposition of amorphous La₂O₃ and Fe₃O₄ nanoparticle on the surface of diatomite, the prepared mineral based phosphate adsorbent has the dual characters of high adsorption capacity and magnetic separation.2. The macromolecules of polyethyleneimine was grafted on the surface of diatomite through "grafting to" method. Moreover, the introduced amino groups were transformed to the quaternary ammonium for the improvement of adsorption performance, the resultant composite adsorbent has the advantage of high adsorption capacity, rapid adsorption rate, wide scope of pH and good reusability.3. Based on "grafting from" approach, by adding the Lewis acid as catalyst, the amino modification of the surface of diatomite can be carried out by an esterification reaction with aminosalicylic acid at normal temperature, which replace the traditional silane coupling agent modification technology, hence the SI-ATRP technology was successfully applied on the silicon substrate to achieve the graft of polymer brush, the prepared anionic composite adsorbent （denoted as DE_PDMC） has the advantage of high adsorption capacity, rapid adsorption rate, wide scope of pH and good reusability.The achievements and main conclusions of this paper are as follow:（1） The mineralogical analysis of diatomite was studied in the laboratory. The results show that the main chemical component were SiO₂2 （65.531%）, Al₂O₃ （14.866%）, Fe2O3（5.858%）. Its loss on ignition values was up to 7.96%. It has many associated mineral impurities and XRD analysis shows that mainly phases are mica, kaolinite, siderite, etc. Particle size are mainly ranging from 30 to 100 um. As the dominant algae in shengzhou diatomite, the melosira granulata have many characteristics such as cylindrical shell, porous side wall, uniform aperture size and distribution. But a large number of pores in the shell wall is blocked for the coating and filling by associated minerals and organic impurities. It caused the spatial structure of diatomite can not be fully revealed, and seriously affect the surface activity. So before further utilized as a substrate, it is necessary to purify and activate the diatomite. In addition, As a result of surface adsorption effect, the surface energy of diatomite was decreased during the process of geological deposition. Therefore, in order to enhance the surface energy and improve reactive activity, it is necessary to removal of adsorbed species on surfaces.（2） Diatomite was purified and activated by roasting and acid soaking. The results show that organic impurities of doatomite were almost completely removed, clay mineral impurities that attach to the doatomite were stripped after purification, the purity of diatomite was improved and the channels were dredged. In addition, the number of surface hydroxyl is increased from 4.68×10-5 mol/g to 7.34×10-5 mol/g. Besides, the reserch show that using 70% H2SO4 solution to tosoak the diatomite makes the comprehensive performance of the SiO₂2 content, specific surface area and the number of hydroxyl to be the best.（3） Amorphous La₂O₃ and Fe₃O₄ nanoparticle were synchronously deposited on the surface of diatomite by an ultrasonic assisted coprecipitation method. The prepared diatomite based phosphate adsorbent, named DE_FeLa, has the dual characters of high adsorption capacity and magnetic separation. the success of diatomite modified was confirmed by SEM, EDS and FTIR. In addition, XRD analysis indicated La₂O₃ and Fe₃O₄ nanoparticles on the surface of diatomite were amorphous. The experimental results showed that the most optimal proportion is lanthanum to iron equals 3:7, and the product of DE_FeLa can obtain the biggest adsorption performance with the character of magnetic separation at this ratio. The best total ion concentration of the supported liquid is 0.08mol/L.（4） Diatomite based anion adsorbent, which the surface is modified by grafting organic functional polymer to grafted with the PEI quaternary amination positive ion polymer, has been prepared in the way of "grafting to", expressed as DE_QPEI. The grafting rate of PEI is the most essential factor for the adsorption performance of DE_QPEI phosphate radical, the phosphorus adsorptive capacity of the end product of the DE_QPEI is in proportion to the grafting ratio of PEI without quaternary amination. The single factor experiments had been carried out for the grafting to the PEI in the three preparation conditions, which are reaction time, concentration of the PEI aqueous solution and molecular weight of the PEI. The experiment indicated that the best condition of three single factor were 6 h,35%,4000.The max adsorption amount was 21.22 mg/g. EDS analysis indicated that there are lanthanum and iron element on the surface of Diatomite. However, there was not signhtly change of surface appearance. In FT-IR spectra of DE_QPEI, new peaks at 1662 cm-1、1454 cm-1 and 698 cm-1 were related to benzene ring structure of benzyl chloride. TG analysis indicated that grafting rate of DE_QPEI was 14.76%.（5） Surface modification of diatomite was carried on "grafting from" model. （2-Methacryloyloxyethyl） trimethyl ammonium chloride （DMC） was chosed to use as a modified agent for diatomite. It has prepared the long chain PDMC, which was using surface initiated atom transfer radical polymerization （SI-ATRP） technique, by grafting of DMC onto reactive sites on the surface of diatomite and with a process of the chain growth. It can form functional polymer layer on the surface of diatomite and achieve a quaternary ammoniated modification of diatomite surface, expressed as DE_PDMC.It is successful to establish the response surface model which is about the effect of phosphorus adsorption capacity on three preparation conditions （monomer concentration, reaction time, liquid solid ratio） by applying response surface method to optimization of preparation condition for DE_PDMC, and the response surface equation which has been established is effective to the relationship between the above three preparation conditions and the absorption property of phosphorous by-product through ANOVA analysis and accurate prediction of the three model validation points. The optimal conditions for the preparation of DE_PDMC by response surface model were as follows:the monomer concentration was 32.3%, the reaction time was 8.2h, and the liquid to solid ratio was 7.1:1. The max adsorption amount was 40.61 mg/g. EDS analysis indicated that there are lanthanum and iron element on the surface of Diatomite. FTIR analysis:After bromination, peaks at 1658 cm-1 and 1548 cm-1 were comfired the exist of amido bond. It indicated that BBiBr was fixed onto surface of diatomite. TG analysis indicated that grafting rate of DE_PDMC was 30.6%（6） Adsorption performance of three adsorbents （DE_FeLa, DE_QPEI, DE_PDMC） were tested. The maximum adsorption capacity of them were 34.39 mg/g,21.22 mg/g, 41.34 mg/g, respectively. The coexistence of other anions in solutions has an adverse effect on phosphate adsorption with the extent following the order:SO42->NO3-> Cl-. Interference of DE_FeLa is better than DE_QPEI and DE_PDMC. Result of pH effect experments indicated that optimal pH for DE_FeLa was 4, and adsorption capacity descreasing rapidly. The adsorption capacity of DE_QPEI and DE_PDMC had a slight effect with the change of pH value.80% adsorption capacity can be retained at pH 4～10. In similar initial concentration, the adsorption equilibrium time of DE_FeLa, DE_QPEI and DE_PDMC were 240 min、 90 min and 60 min.（7） After eight experiments of regeneration reaction, the adsorption capacity of DE_QPEI and DE_PDMC can maintain over 87% and 79% of their initial levels, respectively. Hower, the adsorption capacity of DE_FeLa can only maintain 22% after one cycle of desorption and regeneration experiments.（8） Adsorption mechanisms of three absorbents are followed:The adsorption performance of DE_FeLa is mainly provided by active metal component load on the surface. There are three mechanisms:electrostatic, ion exchange and Lewis acid-base interactions. The main mechanism involved in adsorption process varied with the change of solution pH. The effect of electrostatic, ion exchange mechanisms became weaker while the Lewis acid-base interaction mechanism gradually dominated with increase of pH value. The adsorption performance of DE_FeLa is mainly provided by Quaternary ammonium groups on the surface of diatomite. There are two mechanisms:electrostatic and ion exchange. The pseudo second-order model and the Langmuir isotherm fitted the experimental data quite well, indicating that the adsorption process was mainly through single-molecule sorption on the surface. Thermodynamic parameters show that adsorption of phosphate onto DE_FeLa is a Endothermic process, but onto DE_QPEI or DE_PDMC is a exothermic process.