Xinyan Zhang

Stabilization/solidification (S/S) of mercury-contaminated hazardous wastes using thiol-functionalized zeolite and Portland cement.

Stabilization/solidification (S/S) of mercury-containing solid wastes using thiol-functionalized zeolite and cement was investigated in this study. The thiol-functionalized zeolite (TFZ) used in the study was obtained by grafting the thiol group (-SH) to the natural clinoptilolite zeolites, and the mercury adsorption by TFZ was investigated. TFZ was used to stabilize mercury in solid wastes, and then the stabilized wastes were subjected to cement solidification to test the effectiveness of the whole S/S process. The results show that TFZ has a high level of -SH content (0.562 mmol g(-1)) and the adsorption of mercury by TFZ conform to the Freundlich adsorption isotherm. The mercury adsorption capacity is greatly enhanced upon thiol grafting, the maximum of which is increased from 0.041 mmol Hg g(-1) to 0.445 mmol Hg g(-1). TFZ is found to be effective in stabilizing Hg in the waste surrogate. In the stabilization process, the optimum pH for the stabilization reaction is about 5.0. The optimum TFZ dosage is about 5% and the optimum cement dosage is about 100%. Though Cl(-) and PO(4)(3-) have negative effects on mercury adsorption by TFZ, the Portland cement solidification of TFZ stabilized surrogates containing 1000 mg Hg/kg can successfully pass the TCLP leaching test. It can be concluded that the stabilization/solidification process using TFZ and Portland cement is an effective technology to treat and dispose mercury-containing wastes.

Catalytic behavior and reaction routes of MEK oxidation over Pd/ZSM-5 and Pd-Ce/ZSM-5 catalysts

Catalytic oxidation is a widely used pollution control technology for removing volatile organic compounds. Pd-Ce/ZSM-5 catalysts with different Ce contents were prepared by a co-impregnation method, and their catalytic performance was investigated for the oxidation of methyl ethyl ketone (MEK). The by-products of the reaction were monitored using gas chromatography and collected in thermal desorption tubes, which were further analyzed by gas chromatography/mass spectrometry. The PdCe(9.6)/ZSM-5 catalyst displayed the highest catalytic efficiency, as a consequence of a higher amount of strong acid sites and a superior PdO-Pd redox cycle in the presence of a CeO(2) additive. The introduction of CeO(2) enriched the by-product species. The formation mechanisms and oxidation routes of typical by-products, such as methyl acetate, 1-penten-3-one and 3-buten-2-one, 3-methyl, during MEK oxidation over Pd-Ce/ZSM-5 catalysts was also analyzed. CH(3)* species that formed on the catalyst were identified as reaction intermediaries. Trace amounts of acetic acid and methyl vinyl ketone were also detected and were further oxidized to methyl acetate, 1-penten-3-one and 3-buten-2-one, 3-methyl.

Ligand-assisted preparation of highly active and stable nanometric Pd confined catalysts for deep catalytic oxidation of toluene

In this study, mesoporous SBA-15 supported Pd catalysts were synthesized through impregnation and grafting approaches. Moreover, the influences of different solvents (ethanol, H(2)O, tetrahydrofuran, dimethyl sulphoxide and N,N-dimethylformamide) on the dispersion of supported Pd nanoparticles were also systematically investigated. The prepared materials were comprehensively explored by various techniques, including XRD, EDS, ICP-OES, H(2) chemisorption, N(2) adsorption/desorption, TG-DSC, FT-IR, TEM and STEM. It is found that the traditional impregnation method has some disadvantages in obtaining highly dispersed Pd active phase. Whereas, the grafting method could highly disperse Pd nanoparticles within the mesoporous channels of support material, and the grafting procedure should be promising in designing highly dispersed Pd particles on the silica-based mesoporous materials. The catalyst prepared via the grafting procedure possesses much higher activity and selectivity than that prepared by impregnation method for deep catalytic oxidation of toluene.

Decomposition of nitrous oxide over Co-zeolite catalysts: role of zeolite structure and active site

A series of Co exchanged zeolites with ZSM-5, BEA, MOR and USY structures were prepared and investigated for N2O catalytic decomposition under identical reaction conditions. It is found that Co-zeolites with different structures show dramatically different catalytic activities, which could be attributed to various Co species formed in them. Co-ZSM-5, Co-BEA and Co-MOR exhibit much higher activities than Co-USY catalysts, which is attributed to the predominant formation of active isolated Co2+ ions in the ion exchange positions; while in Co-USY Co mainly exists as less active Co oxides. Moreover, it is observed that the activities of Co2+ ions in ZSM-5, BEA and MOR zeolites are quite different and are related to the specific Co ion sites presented in each zeolite structure. In Co-ZSM-5, the most active sites are the α-type Co ions, which are weakly coordinated to framework oxygens in the straight channel. On the other hand, in Co-BEA and Co-MOR, the most active sites are β-type Co ions, which are coordinated to the framework oxygens of the elongated six-membered ring of BEA and the interconnected small channel of MOR, respectively. The main factors affecting the activities of these individual Co ions are indicated to be their location in the zeolite structure, their chemical coordination and the distances between the Co ions. The highest activity of the α-type Co ions in ZSM-5 could be attributed to their favorite location in the zeolite and weak coordination to framework oxygens, which make them easily accessible and coordinated to reactants. The large number of β-sites and their structural arrangement in MOR allow the formation of two unique adjacent β-Co ions in Co–Co pairs, which cooperate in N2O splitting, consequently yielding the high activity of β-Co ions in MOR.

CoMOR zeolite catalyst prepared by buffered ion exchange for effective decomposition of nitrous oxide

Co contained MOR zeolite catalysts with high Co loadings were successfully synthesized by buffered ion exchange at pH 8, and were tested for N(2)O catalytic decomposition. The high exchange level of synthesized CoMOR(x)-BIE catalysts probably benefits from the maximizing hydroxycomplexes Co(OH)(+) ion in the buffered solution, which is more preferred for the ion exchange with the zeolites. It has been found that the novel CoMOR(x)-BIE catalysts exhibit excellent catalytic activities, which is attributed to the large population of isolated Co(2+) ions on ion exchange positions. The most active CoMOR(130)-BIE catalyst shows high resistance to the inhibition of oxygen, NO and water vapor. Furthermore, stability tests indicate that the CoMOR(130)-BIE catalyst has no obvious deactivation under simulated emission conditions after reaction for more than 100 h. This extraordinary durability could be related to its high Co(2+) content and low Brönsted acidity sites in the catalyst, which facilitate the stability of active isolated Co(2+) on ion exchange positions. Thus, the CoMOR(130)-BIE catalyst shows a great potential as a cost-effective catalyst for N(2)O elimination in future applications.

Investigation of nitrous oxide decomposition over highly active and stable bimetallic CoFe-MOR zeolite catalyst : effective removal and mechanism study

In this study, monometallic Co-mordenite (MOR) and bimetallic CoFe-MOR catalysts were prepared via simple wet ion exchange and tested for N2O decomposition. Strong promotion effect of Fe on the activity and stability of Co ions in the zeolites was observed. To investigate the origin of this promotion effect, X-ray diffraction, H2-temperature programmed reduction, UV-Vis spectroscopy, extended X-ray absorption fine structure analysis, and N2O temperature-programmed desorption were used to characterize the bimetallic and monometallic catalysts. The characteristic results indicated that higher contents of Co ions located at β sites after Fe addition provided cooperation on N2O splitting by two neighboring Co ions. Consequently, a greater amount of surface NOx species were formed in situ and were more strongly bonded to the catalyst, facilitating the removal of O and increasing the activity. Moreover, extended X-ray absorption fine structure analysis indicated that β-type Co ions exhibited stronger coordination to framework oxygen after Fe addition, and higher exchange level was obtained in the bimetallistic CoFe-MOR. Both of them contribute to prevent the relocation of Co2+ ions to form cobalt oxides, thus, high activity was maintained. Consequently, the CoFe-MOR catalyst demonstrates a superior catalytic activity and a high durability in N2O decomposition, showing great potential as a cost-effective catalyst for N2O elimination in future applications.

Temporal and Spatial Variation of Methylmercury in Sediments in the Second Songhua River, China

The methylmercury (MeHg) content in the Second Songhua River was investigated in this study. Compared with the former data, the following trends in temporal variation were observed. The MeHg content decreased in relation to the distance from the pollution source in 1977; it showed a decline in 1983 after the pollution source had been shut off for one year and continued to decline from 1983 to 1991 when the Second Songhua River was in a cleansing period. The MeHg content in most segments investigated in this study was higher than in 1991. Along the river, sediment samples were collected from the Baishan Segment to the Sifangtai Segment, and from the surface to the underlying layer to check the vertical trend. The MeHg content was high in the segments upstream due to the gold mines existing, with highest content in the Toudaogou Segment (10.0 ng/g). The MeHg content declined from the Jiapigou Segment (6.2 ng/g) to the Hongshi Segment (0.69 ng/g), and it gradually increased from the Hongshi Segment (0.69 ng/g) to the Hadawan Segment (1.8 ng/g). It was still lower than upstream. The MeHg content gradually decreased from the Hadawan Segment to the Zhaoyuan Segment. However, there was no clear trend from the Zhaoyuan Segment to the Sifangtai Segment, where the order of the MeHg content was the Sanzhan Segment > the Sifangtai Segment > the Zhaoyuan Segment > the Laozhou Segment. The vertical variation in sediments showed that the MeHg content in the surface layer was higher than in the underlying layer in all segments with the exception of the Hongshi Segment and the Zhaoyuan Segment. The pollution index of MeHg content in the Second Songhua River was also discussed.