Preparation Of Functionalized Inorganic Adsorbents And Studies Of Their Phosphate Removal Performances

Phosphorus is the key nutrients in the growth of organisms in ecosystems. However, the excessivepresence of phosphorous in aquatic environment contributes to eutrophication, in which one of examples isalgal bloom, a serious world-wide environmental problem. Various methods have been widely studied toremoval phosphate from aqueous solution, among which the adsorption-based process is considered as one ofthe most efficient routes to remove phosphate, due to its simplicity, high level of efficiency and fast removalrate, especially at low phosphate concentrations. However, many adsorbents reported are far more thansatisfied, adsorbents with a high adsorption capacity and fast adsorption rate for phosphate are highly neededin the fields of water treatment and purification. Considering the above demands, the thesis is conducted basedon the following three areas:1) preparation of functionalized mesoporous adsorbents and study of theiradsorption capacities;2) preparation of functionalized macroporous-mesoporous adsorbents and study of theiradsorption capacities;3) fabrication of hierarchical adsorbents and investigation of their adsorption capacities;4) synthesis of modified clays and study of their adsorption capacities.Firstly, two functionalized mesoporous adsorbents, i.e. Fe（Ш）-coordinated mesoporous silica adsorbentsMCM-41and SBA-15were successfully synthesized and used for phosphate removal study. TheFe（Ш）-coordinated mesoporous SBA-15adsorbents were prepared by a new NH4F-assisted co-condensationmethod and impregnation of Fe³⁺cations. In the batch adsorption tests, the functionalized absorbents withincreasing loadings of diamino groups possessed markedly enhanced adsorption capacities, although there wasa gradual loss of ordered mesostructures accompanied. In particular, for the resulting absorbent prepared with0.5:1molar ratio of AAPTS and TEOS, the maximum phosphate capture capacity calculated from Langmuirmodel is20.7mg P/g. The phosphate adsorption efficiency of prepared absorbent was highly pH-dependentand the high removal of phosphate was achieved within the pH between3.0and6.0. The presence of Cl^-andNO₃^-exhibited small impacts on the phosphate adsorption by using our synthesized absorbent; whereas, therewere significantly negative effects from HCO₃^-and SO₄^2- on the phosphate removal. In0.010M NaOH, morethan90%of the absorbed phosphate anions on the spent adsorbent could be desorbed, suggesting the absorbentwith a capacity of regeneration.Secondly, two macroporous–mesoporous SBA-15phosphate adsorbent was synthesized via adual-templating approach, followed by diamino-functionalization, Fe（III） and Al（III） impregnation, i.e.Fe（III）-coordinated diamino-functionalized macroporous-mesoporous adsorbent and Al（III）-coordinated diamino-functionalized macroporous-mesoporous adsorbent. The resulting Fe（III）-coordinateddiamino-functionalized macroporous-mesoporous adsorbent possessed a maximum adsorption capacity of12.7mg P/g, and92.5%of the final adsorption capacity reached in the first1min. While Al（III）-coordinateddiamino-functionalized macroporous-mesoporous adsorbent had a maximum adsorption capacity of23.59mgP/g. In the kinetic study, over95%of its final adsorption capacity reached in the first1min. Besides, pHranging from3.0to6.0favored the high phosphate adsorption of hierarchically porous adsorbent; however, thecoexistence of other anions, especially F^-, retarded the adsorption.Thirdly, hollow silica microspheres with ordered mesoporous shell （HMS） were impregnated withdifferent loadings of Lanthanum as novel adsorbents for phosphate removal. In batch adsorption tests, theHMS-x adsorbents possessed markedly enhanced adsorption capacities with increasing La amounts, ascompared to HMS which can hardly adsorbed any phosphate in solution. In particular, HMS-1/5possesses amaximum phosphate capture capacity of47.89mg P/g. In the kinetic study, the phosphate adsorption followedpseudo-second-order equation well. High adsorption capacities were achieved by HMS-1/5within the pHbetween3.0and8.0, and high selectivity to phosphate was also observed with the coexisting of0.01M otheranions (e.g. F^-, Cl^-, NO₃^-, SO₄^2- and CO₃^2-). Besides, Hierachical mesoporous La（OH）₃adsorbent wassynthesized by one-pot method and its application in phosphate removal was reported for the first time. Thesynthesized P-La（OH）₃sample exhibited a particle diameter of approximately200nm and possessed irregularmesopores with a pore diameter of8.74nm. In the phosphate adsorption test, the adsorbent had a maximumphosphate adsorption capacity of57.65mg P/g, showing a great potential for use in the practical removal ofphosphate.Finally, two kinds of clays modified by metal hydroxides were successfully synthesized and theirphosphate removal performance are summariezed as following:ⅰ) La（OH）₃-modified exfoliated vermiculites were fabricated, characterized, and investigated for phosphateremoval in batch tests for the first time. The BET surface area of the adsorbent, which was synthesized in thesolution consisting of5.00mmol/g La/exfoliated vermiculite （EV）, was significantly increased, accompaniedwith a larger pore diameter and greater total pore volume, as compared with the unmodified EV. Effects ofinitial phosphate concentration, contact time, temperature, pH, and co-existing ions on the adsorption capacitywere investigated in detail. The experimental equilibrium data were fitted better by using the Langmuir model（maximum adsorption capacity of79.6mg P/g）. The adsorbent exhibited a high adsorption capacity in the pHrange of3.0-7.0. The addition of F, Cl, NO3, and SO42-had neglectable effects on its phosphate removal capacities. The spent adsorbent could be regenerated and reused in phosphate adsorption; that could removemore than70%phosphate in the3rd adsorption-desorption cycle.ⅱ) The phosphate removal performances of a series of ferric-modified laterites （ML） were tested andcompared with raw laterite （RL） in this study. After the modification with0.5MFeCl₃solution, the resultingadsorbent ML-C exhibited90.12%of phosphate removal, which was37.47%higher than that of RL underthe same experimental condition. This may be attributed to the significant increase of BET surface area andtotal pore volume for ML-C, arising from the formation of akaganeite. The effects of contact time, initialphosphate concentration, temperature, pH, and co-existing ions on the adsorption capacity of ML-C wereinvestigated in detail. In the reusability study, the adsorbent showed no significant loss in their adsorptionperformance after four adsorption-desorption cycles, indicating that ML-C was able to be utilized as a potentialcost-effective phosphate adsorbent for practical applications.