Enhanced Removal Of Heavy Metals From Wastewater By Using The Mixture Of Zero-valent Iron And Clay Minerals

Permeable reactive barriers containing zero valent iron(Fe0-PRB) is considered as the most potential technology for the in situ treatment of groundwater, which can be a substitute for the traditional pump and treat process. However, the decrease of reactivity and permeability of Fe0-PRB during the remediation can affect its longevity and limit its practical application. In this thesis, two kinds of heavy metal ions(Se(VI), Pb2+) were selected as the target pollutants, column experiments were used to simulate the Fe0-PRB. The positively-charged modified bentonite(hydroxy aluminum pillared bentonite, Al-bent) or the negatively-charged natural clay minerals(i.e., sepiolite, calcium bentonite(Ca-bent), kaoline) were uniformly mixed with ZVI as the reactive materials of PRB. The enhanced removal of heavy metals by Fe0-PRB in the presence of these clays and the influence of common coexisting components in groundwater(Humic acid(HA) and anions(Cl-, NO3-, HCO3- and SO42-)) were investigated and compared. The synergy effect of clay on the removal of heavy metals by Fe0-PRB were studied in detail by combing the modern analytical methods such as synchrotron radiation X-ray absorption fine structure spectroscopy(XAFS) and X-ray diffraction(XRD). The main contents and results are as follows:(1) The results shows that the reductive removal of Se(VI) by the Al-bent/Fe0/sand reaction system, which was obtained by the mixture of Al-bent with good adsorption to Se(VI), Fe0, and sand, was not only higher than that of Fe0/sand system, but also much higher than the sum of the removal efficiency of Se(VI) by Fe0/sand and Al-bent/sand system, This illustrates that an obvious synergetic effect exists between Al-bent adsorption and Fe0 reduction. The synergy effect is mainly reflected as follow: Al-bent can effectively enrich Se(VI) to Fe0 surface that promotes the electron transformation from Fe0 to Se(VI), thus increased the reduction capacity of Se(VI) by Fe0. The SiOH and Al-OH on Al-bent surface can buffer medium p H to a lower level, reducing the corrosion products on iron surface. Al-bent can also transfer iron corrosion products to bentonite effectively, so that more reactive sites on iron surface can be used for Se(VI) reduction, that thus extend the longevity of Fe0. The results of XAFS showed that the addition of Al-bent into the Fe0-PRB system can promote the reduction of Se(VI) into Se(-II) or Se(0) with lower solubility and toxicity.(2) Humic acid(HA) and the coexisting anions(Cl-, NO3-, HCO3- and SO42-) with certain concentrations can decrease the removal of Se(VI) by both Fe0/sand and Al-bent/Fe0/sand reaction system. However, this inhibitory effect can be significantly weakened by the introduction of Albent due to its adsorption to these coexisting constituents.(3) When three natural clay minerals were uniformly mixed with Fe0, and sand, the removal of Pb2+ from synthetic wastewater could be enhanced to some extent, which was positive to the adsorption of Pb2+ by the clay minerals, namely, sepiolite/Fe0/sand >Ca-bent/Fe0/sand >kaoline/ Fe0/sand >Fe0/sand.