Synergistic effects of microbial anaerobic dechlorination of perchloroethene and nano zero-valent iron (nZVI) - A lysimeter experiment

Bioremediation of contaminated soil and groundwater by in situ biostimulation

Bioremediation by in situ biostimulation is an attractive alternative to excavation of contaminated soil. Many in situ remediation methods have been tested with some success; however, due to highly variable results in realistic field conditions, they have not been implemented as widely as they might deserve. To ensure success, methods should be validated under site-analogous conditions before full scale use, which requires expertise and local knowledge by the implementers. The focus here is on indigenous microbial degraders and evaluation of their performance. Identifying and removing biodegradation bottlenecks for degradation of organic pollutants is essential. Limiting factors commonly include: lack of oxygen or alternative electron acceptors, low temperature, and lack of essential nutrients. Additional factors: the bioavailability of the contaminating compound, pH, distribution of the contaminant, and soil structure and moisture, and in some cases, lack of degradation potential which may be amended with bioaugmentation. Methods to remove these bottlenecks are discussed. Implementers should also be prepared to combine methods or use them in sequence. Chemical/physical means may be used to enhance biostimulation. The review also suggests tools for assessing sustainability, life cycle assessment, and risk assessment. To help entrepreneurs, decision makers, and methods developers in the future, we suggest founding a database for otherwise seldom reported unsuccessful interventions, as well as the potential for artificial intelligence (AI) to assist in site evaluation and decision-making.

Adsorption and reduction of trichloroethylene by sulfidated nanoscale zerovalent iron (S-nZVI) supported by Mg(OH)2

Sulfidated nanoscale zerovalent iron (S-nZVI) supported on a flower spherical Mg(OH)₂ with different Mg/Fe ration were successfully synthesized. The synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS). The results showed that S-nZVI particles were well dispersed on the petals of the flower spherical Mg(OH)₂. The influence of factors, including the initial solution pH, Mg/Fe, S/Fe were studied. The trichloroethylene (TCE) adsorption data on Mg(OH)₂ and S-nZVI @Mg(OH)₂ fit well to a Langmuir isotherm model, and the maximum adsorption of S-nZVI @Mg(OH)₂ was 253.55 mg/g, which was 2.6-fold of S-nZVI. Meanwhile, the S-nZVI @Mg(OH)₂ composite expanded the pH selection range of S-nZVI from 2 to 11. Cycling experiments showed that removal rate was 58.3% for the 5th cycle. TCE removal was due to synergistic action of reduction coupled with adsorption. During this process, 65.43% of total remove TCE from ion chromatography data was reduced and 34.57% of total remove TCE was adsorbed finally. At the same time, adsorption favors reduction. These observations indicated that the S-nZVI @Mg(OH)₂ can be considered as potential adsorbents to remove TCE for environment remediation.

Adsorption of arsenic from aqueous solution using a zero-valent iron material modified by the ionic liquid [Hmim]SbF6

The environmental and health impacts caused by arsenic (As) in wastewater make it necessary to carefully manage As wastes. In the present work, a composite of the ionic liquid [Hmim]SbF₆ and nano-iron (H/Fe) was used as an adsorbent to remove As(v) from aqueous solution. To better understand the removal effect of H/Fe on As(v) in aqueous solution, the reaction parameters of pH, reaction temperature, time and H/Fe dosage were systematically analyzed in detail. The results show that H/Fe has significant removal efficiency toward As(v), and that the adsorption of As(v) by 0.5 g H/Fe reaches its maximum adsorption capacity within 2 h. The adsorption of As(v) on H/Fe is a non-linear, time-varying process. The initial adsorption reaction is fast; however, unlike at the beginning, the later reaction involves sustained slow absorption, resulting in a distinct two-phase adsorption characteristic. Redox reaction may be one of the mechanisms responsible for the slow adsorption of As(v) on H/Fe. At the same time, the As(v) removal effect of H/Fe is greatly restricted by the pH. Electrostatic adsorption, adsorption co-precipitation and redox reactions act together on H/Fe in the As(v) removal process. This study provides a basis for further clarifying the adsorption, adsorption rules and mechanism of As(v) on H/Fe and a feasible method for the improvement of As(v) removal efficiency of zero-valent iron materials.

Inhibition and stimulation of two perchloroethene degrading bacterial cultures by nano- and micro-scaled zero-valent iron particles

The pollutant perchloroethene (PCE) can often be found at urban contaminated sites. Thus in-situ clean-up methods, like remediation using zero valent iron (ZVI) or bacterial dechlorination, are preferred. During the remediation with ZVI particles anaerobic corrosion occurs as an unwanted, particle consuming side reaction with water. However, in this reaction H₂ is formed, which is usually scarce during anaerobic microbial dechlorination. Dehalococcoides needs H₂ for cell growth using it as an electron donor to dechlorinate chlorinated hydrocarbons. Combining application of ZVI with bacterial dechlorination can turn ZVI in a H₂ donor leading to a more controllable bacterial dechlorination, a smaller amount of ZVI suspension and decreased remediation costs. In this study nano- and micro scaled ZVI particles (nZVI, mZVI) were combined in microcosms with two dechlorinating bacterial cultures. The two cultures showed different dechlorination behaviors with ethene and cis-DCE as final products. Phospholipid fatty acids (PLFA) associated with Dehalococcoides (18:1w7, 18:1w7c, 10:Me16:0) and Geobacteriaceae (16,1w7c; 15:0; 16:0) have been found in both bacterial cultures, slight differences in their abundance could explain the different dechlorinating behaviors. The combination of both bacterial cultures with mZVI led to a stimulated dechlorination process leading to about two times higher k_obs for PCE dechlorination (0.01-0.05 h^-1). In the otherwise cis-DCE accumulating culture complete dechlorination to ethene was achieved. While addition of nZVI inhibited both cultures. Combined with nZVI the completely dechlorinating culture produced lower amounts of dechlorinated products (3.2 μmol) as compared to the single biotic treatment (5.1 μmol). Combining the incompletely dechlorinating culture with nZVI significantly reduced the k_obs,PCE (single: 8 × 10^-3 ± 3 × 10^-4 h^-1; combination: 5 × 10^-3 ± 2 × 10^-4 h^-1). H₂ produced by nZVI and mZVI was utilized by both bacterial cultures. The particle size, resulting specific surface areas, agglomeration tendencies and reactivity appears to be crucial for the effect on microbial cells.