| Arsenic (As) contamination in groundwater has become one of major environmental and social concern in many countries around the world, and investigating the remediation technology for removing As from groundwater and its mechanism is of great significance in protecting ecological environment, enhancing the people's health standard and constructing a harmonious society. The aim of this project was to work out low-cost, reliable and efficient in situ remediation technology of "Permeable Reactive Barrier" (PRBs) for arsenic-contaminated shallow groudwaters contaminated by realgar mine (AsS) in Chenzhou, Hunan province. The purpose of this project, concretely, was to select highly effective and stable PRB materials that was suitable for our country economic by comparing the effects of As removal from As-spiked water of the materials widely used in China and the abroad. Batch experiments combined with instrumental analysis of X-ray Diffraction (XRD), Fourier-transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS) were carried out to investigate the effects, influential factors and mechanisms of As removal from water by two kinds of nano TiO2 crystals (Anatase and Rutile).The removal of arsenic from water by using the ferric modified rutile minerals were also studied in batch experiment. The main results were as follows:1.Among the 26 kinds of materials, P25 nanometer titanium dioxide was the best for As(V) and As (Ⅲ) removal, followed by VK-TA18H nanometer titanium dioxide.The Clinoptilolite removed the least As (Ⅲ), and rutile mineral removed the least As(V).2.Based on the elements'content, the 26 materials were clustered into 6 groups by using the least distance method at the distance of 0.5.The Group 1 consisted of 16 materials, including nonmetallic minerals, metallic minerals, abandoned wastes, and chemical compounds.The same type of nonmetallic mineral could contained different concentrations of elements. For instances, the content of arsenic, potassium, sodium, zinc in bentonite ZJL ore was much higher than bentonite 16pu red and bentonite 16pu white. The manganese(93.22 mg kg-1) and copper (7.46 g kg-1) in Yichang manganese ore were much higher than that in other minerals.Red mud contained titanium of 70.25 g kg-1 which was just second to titanium dioxides, and sodium of 34.62 g kg-1 which was in the third place of 26 materials tested. The sodium content of ion exchange resin (43.64 g kg-1) was the top of 26 minerals, indicating that it was a Na+ ion exchange resin. The Group 2 was closely related to iron, including iron ore,primary iron ore, and iron slag, which contained iron of 571.80 g kg-1,447 g kg-1,and 313.05 g kg-1,respectively. It was noteworthy that the arsenic content in iron slag was the highest among 26 materials, reaching 478 mg kg-1.The Group 3 was made up of 4 kinds of titanium dioxide. The titanium content of P25 titanium dioxide was 738.84g kg-1,which was much higher than that of VK-TA18H titanium dioxide (540.43 g kg-1),VK-T25H titanium dioxide (522.20 g kg-1) and VK-T01 titanium dioxide (479.22 g kg-1).However, these minerals contained some arsenic of 3 mg kg-1,6mg kg-1,4 mg kg-1,and 93 mg kg-1,respectively. Other three Groups were humic acid, activated alumina, steel slag.Activated alumina contained the highest Al (496.65 mg kg-1),Humic acid contained the highest calcium (834.42 g kg-1), and steel slag contained Mn of 1179 mg kg-1 and P of 51.14 g kg-1.3.The results showed that the removing percentage of As(V) and As(III) from water spiked by As of 0.2 mg L-1 under light condition in 1 h was 99.55% and 100% by Anatase (20 g mineral L-1),and was 98.54% and 58.02% by Rutile (20 g mineral L-1),respectively. The adsorption behavior of Anatase and Rutile for As(V) and As(III) could be well described by Freundlich equations. Anatase had greater As removal efficiency and adsorption capacity than Rutile.When the temperature was 25℃,Solution pH had no influences on the As adsorption of Anatase TiO2;whereas, the As removal by Rutile TiO2 were increased with the increase of pH from 4 to 10.The adsorption dynamics of As(V) and As(Ⅲ) by Anatase and Rutile were well fit pseudo-second-order equation. Presence of accompanying anions, such as phosphate, silicate, nitrate, and sulfate decreased the As(V) and As(Ⅲ) removal by both crystals significantly, of which phosphate was the most strong influencing ions.Removal of As by Rutile TiO2 was substantially enhanced in the presence of divalent cations (i.e.,Ca+ and Mg2+).4.Shading of light decreased the removal of As(V) and As(Ⅲ) of Anatase TiO2 by 7.52% and 12.00%, respectively, while there was even a slight increase of As removeal in the case of Rutile TiO2. FT-IR characterization of As(Ⅴ) or As(Ⅲ)-treated nano TiO2 crystals indicated that both As-O-Ti and As-0 groups participated in As adsorption. Both FT-IR and XPS analysis demonstrated that photooxidation of As(Ⅲ) into As(Ⅴ) could happen when adsorption of As(Ⅲ) by anatase under the light condition. These results suggested that the effect of crystal types and light condition on As removal should be taken into consideration when nano TiO2 was applied for As removal.5.The results showed that the absorption capacity of As(Ⅴ) and As(Ⅲ) from water spiked with As of 200μg L-1 under light condition for 1 h by ferric modified rutile mineral was more 10 times than that by rutile mineral (20 g mineral L-1).The adsorption behavior of the two minerals for As(V) and As(Ⅲ) could be best described by Freundlich equations.6.The effect of As removal from As-spiked water by rutile mineral obviously presented the characteristic of parabola at 25℃,but to ferric modified rutile mineral, when pH was low, the change was slow, when pH>4, the precentage of As-removeing suddenly decreased. Desorption of As(V) or As(Ⅲ)-treated ferric modified rutile mineral for 24h by 0.1 mol/L NaOH, the desorption percentages reached 86.48%,79.28%, respectively, and the removed percentages of As (Ⅴ) and As (Ⅲ) were 95.43% and 94.86% by the regenerated ferric modified rutile mineral, respectively. In a column test using ferric modified rutile ore as PRBs material, when the flow of water spiked with As (Ⅴ) and As (Ⅲ) of 0.5 mg L-1 reached 20.35 L and 24.62 L, the As concentrations of effluent water became stable and maintained 50.10μg L-1 to 55.5lμg L-1,and the removal percentages of As (Ⅴ) and As (Ⅲ) were about 90%.7.FT-IR characterization of arsenic treated ferric modified rutile ore indicated the presence of both As-O-Fe and As-O groups and supported the concept of surface complex formation.X-ray photoelectron spectroscopy (XPS) indicated that As(Ⅲ) was oxidized and adsorbed in the form of As(Ⅴ) on the surface of ferric modified rutile mineral. |
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