Incorporation Of Zero-valent Iron Into The Channels Of Mesoporous Structure For The Groundwater Remediation

As a chemical reduction, nanoscale zero-valent iron (NZVI) has been widely used to in situ remediation of groundwater pollution. But such drawbacks of NZVI as aggregation and oxidation limit its application in scale. To overcome the above problems, immobilizing of nanoscale zero-valent iron particles (NZVIs) inside the channels of the ordered mesoporous structures provides an alternative solution. So far, the way to synthesize these composites includes a conventional liquid phase reduction method and a hydrogen reduction process. However, NZVIs composites prepared by the conventional liquid phase reduction method are dispersed in random with larger size on the surfaces of the supports, which can not make full use of the advantages of the ordered mesopores. As to the hydrogen reduction process, though it can confine NZVIs in the mesopores, the reduction reaction must proceed under a harsh and dangerous condition including the explosive hydrogen and a higher temperature. Therefore, in this paper, a new reduction process named a "two solvents" reduction technique was developed to confine NZVIs inside the channels of SBA-15 under a mild condition. The resulting NZVIs/SBA-15 composites were used to remove Cr(VI) and nitrobenzene (NB) from the groundwater. Results showed that the composites had superior reduction ability. The main research work and results are summarized as follows:(1) A "two solvents" reduction technique was found up and used to incorporate ultrosmall zero-valent iron nanoparticles inside the channels of SBA-15 at room temperature and pressure. The synthesized samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption isotherms, transmission electron microscopy (TEM) and all-direct-reading plasma atomic emission spectrometry (ICP-AES), which helping to analyze the mechanism of the new technique. Results revealed that, the new reduction technique can effectly confine NZVIs in the mesopoes of SBA-15, which preventing NZVIs from being aggregated and decreasing the iron particle’s size (c.a.5.9nm). In the "two solvents" reduction technique, Fe3+ precursor was first immobilized inside the mesoporous channels by the "two solvents" impregnation method, then cyclohexane acted as the first solvent to augment the hydrophilicity of the mesopores while minute quantities of water served as a second solvent to dissolve the reducing agent. Reducing agent aqueous could be impregnated into the mesopores due to the hydrophobic environment, the combined action of capillaries and the hydrophilic behavior of the internal surface, which inducing the reduction reaction accomplished directly inside the channels of the silicas.(2) In batch experiments, Cr(VI) and nitrobenzene (NB) were designed as target contaminants to investigate the reactivity of NZVIs/SBA-15 in the reduction of heavy metals and aromatic compounds. Results presented that NZVIs confined in the ordered mesopores had superior reactivity for the removal of Cr(VI) and NB from the model groundwater than the ones supported on the surfaces. Reduction reaction followed a pseudo-first order kinetics model. When the initial pH was 5.5,20 mg/L Cr(VI) could be removed 99.7% in the first 10 min at 0.17 g/L (equivalent quality of Fe0) with the reduction constant kobs 0.600 min-1, and 20 mg/L NB was removed 97.2% in the first 15 min at 0.25 g/L with kobs 0.215 min-1. The reduction process mainly proceeded in the pores of SBA-15, in which Cr(VI) and NB in aqueous solution were first absorbed by the composites, impregnated into the silica pores by capillaries and then reduced by the incorporated NZVIs. In the mesopores, NZVIs donated electrons to reduce the contaminations, and at the same time, a very small quality of free Fe2+/Fe3+ ions from iron corrosion may move out of silica pores under an oscillating condition.(3) The effect of geochemistry contents such as hardness, carbonate, bicarbonate and humic acid (HA) on the reactivity of NZVIs/SBA-15 was evaluated. Batch experiments showed that an inhibitory effect was observed in the presence of hardness or carbonate. Bicarbonate would be an enhancement at a high concentration for its buffer effect during the initial time but an inhibitor as the reaction went on. HA had little influence on the reactivity of NZVIs/SBA-15 but could decrease the reduction rate. Under a coexistence of calcium hardness, bicarbonate and HA, the presence of HA could weaken the inhibiting effects of calcium and bicarbonate on the reactivity of NZVIs/SBA-15. However, it could not be ignored that the sedimentation of carbonate will have a long-lasting negative effect on the service life of NZVIs/SBA-15. A superior reactivity, good stability and better resistance to HA of NZVIs/SBA-15 suggested that the new composites offer a promising alternative nanomaterial to remo ve heavy metals and aromatic compounds from the groundwater.