Quantitative monitoring and potential mechanism of the secondary corrosion risk of PAH-contaminated soil remediated by persulfate oxidation.

The proportion of activated persulfate (PS) oxidation technology in the remediation of domestic organic contaminated sites has increased every year, and the potential corrosion risk of site reuse caused by residual oxidants and by-products has also attracted the attention of researchers. In this work, the potential corrosion degree such as the mass reduction rate and surface crack width of standard iron flakes under different conditions, including with different PS dosages and release times, was monitored quantitatively over a long period, and the corresponding corrosion risk was quantitatively assessed. The results showed that when n (Na2S2O8):n (PAHs) increased from 5:1 to 100:1, the higher the oxidizer dosage, the more severe the corrosion weight loss and surface crack width, indicating that the oxidizer dosage was positively correlated with the potential corrosion risk. In addition, the corrosion crack width of the standard iron flake had a significant positive correlation with the reaction time and a significant negative correlation with the mass change. According to the changes in the standard iron flake, the corrosion process could be divided into three stages, in which the corrosion risk from high to low followed the order of oxidant corrosion stage > oxidant and salt corrosion stage > salt and microbial corrosion stage. Therefore, the dosage of chemicals should be controlled, the molar ratio of oxidizer to contaminant should not exceed 25:1, and a natural recovery period of at least one year should be left post remediation. During the reuse of the remediation sites in the future, the potential corrosion risks should also be calculated based on the dosage and time, to avoid redevelopment and use of the restoration site in the high corrosion risk stage.

Characteristic study of biological CaCO3-supported nanoscale zero-valent iron: stability and migration performance.

nZVI has attracted much attention in the remediation of contaminated soil and groundwater, but the application is limited due to its aggregation, poor stability, and weak migration performance. The biological CaCO3 was used as the carrier material to support nZVI and solved the nZVI agglomeration, which had the advantages of biological carbon fixation and green environmental protection. Meanwhile, the distribution of nZVI was characterised by SEM-EDS and TEM carefully. Subsequently, the dispersion stability of bare nZVI and CaCO3@nZVI composite was studied by the settlement experiment and Zeta potential. Sand column and elution experiments were conducted to study the migration performance of different materials in porous media, and the adhesion coefficient and maximum migration distances of different materials in sand columns were explored. SEM-EDS and TEM results showed that nZVI could be uniformly distributed on the surface of biological CaCO3. Compared with bare nZVI, CaCO3@nZVI composite suspension had better stability and higher absolute value of Zeta potential. The migration performance of nZVI was poor, while CaCO3@nZVI composite could penetrate the sand column and have good migration performance. What's more, the elution rates of bare nZVI and CaCO3@nZVI composite in quartz sand columns were 5.8% and 51.6%, and the maximum migration distances were 0.193 and 0.885 m, respectively. In summary, this paper studies the stability and migration performance of bare nZVI and CaCO3@nZVI composite, providing the experimental and theoretical support for the application of CaCO3@nZVI composite, which is conducive to promoting the development of green remediation functional materials.

Differential and mechanism analysis of sulfate influence on the degradation of 1,1,2- trichloroethane by nano- and micron-size zero-valent iron.

The in-situ reduction of zero-valent iron (ZVI) is an effective method for removing chlorinated aliphatic hydrocarbons (CAHs) from groundwater. The heterogeneity of environmental conditions is also crucial in affecting dechlorination efficiency. Until now, the effect of Sulfate (SO42-) on ZVI activity has been debated, and the related mechanism research on SO42- behaviour during the abiotic reduction process of chlorinated alkanes is still lacking. In this study, the impacts of SO42- concentrations (0, 2, 4, 8, 80 mM) on the degradation of 1,1,2-trichloroethane (1,1,2-TCA) by micron-size ZVI (mZVI) and nano-size ZVI (nZVI) were systematically investigated. For mZVI, Kobs increased by 0.6 (2 mM), 0.5 (4 mM), 1.1 (8 mM), and 1.6 times (80 mM). For nZVI, Kobs decreased by 32% (2 mM), 39% (4 mM), 45% (8 mM), and 9% (80 mM). The results showed that SO42- increased the rate of 1,1,2-TCA degradation by mZVI but weakened the reduction performance of nZVI; however, this inhibition was reduced when the concentration reached 80 mM. SO42- controlled the degradation of 1,1,2-TCA mainly through the formation of different iron-sulfate complexes on the ZVI surface: water-soluble bidentate iron-sulfate complexes formed on the mZVI surface promoted the corrosion of the oxide layer and accelerated the reduction of 1,1,2-TCA, monodentate complexes mainly formed on the nZVI surface inhibited the reduction of 1,1,2-TCA by blocking surface sites. These results demonstrate the proof of concept to assist land managers in the field application of ZVI technology for the remediation of CAHs contaminated sites with different background concentrations of SO42-.

[Application Case of Accurate Site Investigation with Life-Cycle Conceptual Site Model Development].

A life cycle conceptual site model (LC-CSM) can represent the actual spatial distribution and migration of pollution of a site very accurately and be beneficial in supporting decisions for accurate site remediation or risk management. A volatile chlorinated hydrocarbon contaminated site in the Beijing-Tianjin-Hebei region was chosen as the study case. LC-CSMs were established following the site assessment, preliminary investigation, detailed investigation, and supplementary investigation of each stage. The application of field screening tests such as a membrane interface probe and the multi-electrode resistivity method assisted in identifying potential pollution sources and hot points. Concurrently, a large amount of vinyl chloride, the end product of chlorinated hydrocarbon degradation, was detected in some boreholes, indicating that pollutant biodegradation had occurred at this site. Some typical boreholes and cross-sections were chosen to analyze the biodegradation indicators and chemical fingerprints, combining the results of the comprehensive score of chlorinated hydrocarbon anaerobic biodegradability in groundwater reaching 22. It is judged that the site has strong anaerobic biodegradability. This step-by-step optimization forms an LC-CSM for site investigation, which provides scientific support for accurate site characterization.

[High Efficiency Removal of 1,2-Dichloroethane from Groundwater by Microscale Zero-valent Iron Combined with Biological Carbon Source].

In order to explore the degradation mechanism of 1,2-dichloroethane (1,2-DCA), which is one of the refractory saturated chlorinated hydrocarbons, the groundwater and aquifer soil from a chlorinated hydrocarbon contaminated site in Beijing were collected to carry out microcosm experiments under anaerobic conditions using zero-valent iron (ZVI) coupled with biological action. The removal rate of 1,2-DCA under different conditions was investigated by adding a composite agent consisting of micron zero-valent iron (mZVI), a biological carbon source, and a few nutrients. Changes of the groundwater physical and chemical parameters were monitored. The results showed that, when the dosage of the composite agent reached 3%, the concentration of 1,2-dichloroethane could be reduced to below the detection limit within 15 days. Neutral pH and the presence of SO42- were more conducive to the dechlorination of 1,2-DCA. After 30 days, an obvious increase in the ethene concentration was detected (0.17-0.52 mg·L-1) and no significant vinyl chloride, chloroethane, or ethane was observed in the microcosms, illustrating that the dihaloelimination was the main degradation pathway of 1,2-DCA in the system. In addition, the groundwater could maintain a low oxidation-reduction potential (-100 to -300 mV), dissolved oxygen (<0.5 mg·L-1), and a suitable pH value (6.5-7.5) for a long time under the synergy of mZVI and a biological carbon source. This was beneficial to the activity of anaerobic microorganisms and to the dechlorination reaction.