Removal of Cr(VI) from wastewater using acid-washed zero-valent iron catalyzed by polyoxometalate under acid conditions: Efficacy, reaction mechanism and influencing factors

Strategies to enhance the reactivity of zero-valent iron for environmental remediation: A review

Application of zero-valent iron (ZVI) has become one of the most promising innovative technologies for the remediation of environmental pollutants. However, ZVI may suffer from the low intrinsic reactivity toward refractory pollutants, which seriously restricts its practical application in fields. Therefore, strategies have been developing to enhance the reactivity of ZVI. Until now, the most commonly used strategies include pretreatment of ZVI, synthesis of highly-active ZVI-based materials and additional auxiliary measures. In this review, a systematic and comprehensive summary of these commonly used strategies has been conducted for the following purposes: (1) to understand the fundamental mechanisms of the selected approaches; (2) to point out their advantages and shortcomings; (3) to illustrate the main problems of their large-scale application; (4) to forecast the future trend of developing ZVI technologies. Overall, this review is devoted to providing a fundamental understanding on the mechanism for enhancing the reactivity of ZVI and facilitating the practical application of ZVI technologies in fields.

The sequestration of Cr(VI) by zero valent iron under a non-uniform magnetic field: An interfacial dynamic reaction

A non-uniform magnetic field was applied to sequester Cr(VI) with microscale zero-valent iron (ZVI). When the non-uniform magnetic field was applied, the average removal rate of Cr(VI) was increased and the lag phase was shortened with the increasing of magnetic field intensity. The instantaneous rate was fast at the beginning and about 40% of the Cr(VI) was sequestered rapidly when ZVI was added into the magnetic field system. Later, the sequestration rate of Cr(VI) was reduced and remained stable with time until Cr(VI) was removed completely. The instantaneous removal rate was positively correlated with ZVI dosage and the rate per unit mass of ZVI was 0.455 mg/(L·min·gZVI). The constant rate stage was not affected by the initial and the residual concentration of Cr(VI). In the case where no magnetic field was applied, the removal of Cr(VI) is a process in which ZVI is depassivated and its reactivity is restored continuously. The promotion of a magnetic field on the removal of Cr(VI) is mainly due to increasing the role of adsorbed reducing species of Fe²⁺ or Fe⁰ on the ZVI surface. Aging of ZVI under a magnetic field could enhance the release rate of Fe²⁺ in the initial 5 min though the remanence of this kind of ZVI had little effect on the enhancement of the sequestration of Cr(VI).

Simulated reactive zone with emulsified vegetable oil for the long-term remediation of Cr(VI)-contaminated aquifer: dynamic evolution of geological parameters and groundwater microbial community

Cr(VI), which is highly toxic and soluble, is one of the most challenging groundwater contaminants. Previous work has indicated that emulsified vegetable oil (EVO) is an effective in situ amendment for removing Cr(VI) from groundwater. However, the spatial and temporal changes in geological parameters and microbial community structures throughout the remediation period are poorly understood. In this study, a large laboratory-scale sand-packed chamber (reactive zone of 100 × 50 × 30 cm) was used to simulate the bioremediation of Cr(VI)-contaminated aquifer by EVO over a 512-day period. Various geological parameters and microbial communities were monitored during both the establishment and remediation stages. The results indicate that several biogeochemical reactions occurred in a specific sequence following the injection of EVO, creating an acidic and reducing environment. A shift in the community structure and a decrease in the community diversity were observed. The abundance of microbes involved in the degradation of EVO and reduction of electron acceptors significantly increased. Then, the EVO reactive zone was flushed with Cr(VI)-contaminated groundwater. Biogeochemical reactions were inhibited after the inflow of Cr(VI) and subsequently recovered a month later. The pH of the aquifer returned to the initial neutral condition (approximately 7.2). The EVO reactive zone could remediate Cr(VI)-contaminated groundwater at an efficiency exceeding 97% over 480 days. Biogeochemistry played a major role in the early period (0~75 days). In the later period (240~480 days), the remediation of Cr(VI) in the reactive zone depended mostly on bio-reduction by Cr(VI)-reducing bacteria.

Biosynthesis of Iron Nanoparticles Using Tie Guanyin Tea Extract for Degradation of Bromothymol Blue

Facile synthesis of zero-valent iron nanoparticles has been developed using Tie Guanyin tea extract as reducing and stabilizing agent. The characterization carried out by UV-Vis, SEM, TEM, XRD, and FTIR techniques has identified the successful synthesis of the zero-valent iron nanoparticles. It is evident from the TEM result that spherical zero-valent iron nanoparticles with average size of 6.58±0.76 nm have been obtained through biological method in this study. FTIR spectrum demonstrates that the polyphenols play an important role in the synthetic process. Diffraction peak at 2θ of 44.9° and 49.1° in XRD spectrum explains the existence of the iron nanoparticles. Additionally, effect of concentration of iron nanoparticles and concentration of bromothymol blue on the kinetic rate constants during the degradation process was studied.