Application of an emulsified polycolloid substrate biobarrier to remediate petroleum-hydrocarbon contaminated groundwater

Establishment of evaluation system for biological remediation on organic pollution in groundwater using slow-release agents

In situ bioremediation through slow-release agents can continuously degrade organic pollutants for a long time and have high application potential in solving problems such as tailing and rebound. However, the existing evaluation system is difficult to reflect the performance of bioremediation through slow-release agents, which is not conducive to the promotion of technology. It is urgent to establish a targeted evaluation system. Therefore, based on the multi-criteria decision-making method (MCDA), a comprehensive evaluation model was established. The evaluation index system was constructed for bioremediation through slow-release agents consisting of 16 indicators including pollutant degradation rate, agent preparation cost, engineering operation and maintenance cost, secondary pollution, long-term degradation stability, slow release time, slow release stability, increase in functional microbial flora, increase in total DNA content, agent particle size, solid agent morphology, liquid agent viscosity, dispersibility in aqueous phase, zeta potential, operability of agent preparation, and engineering operation management difficulty. Then, the weight of the indicators was determined by using the best-worst method (BWM), and evaluation criteria was established based on relevant norms and literature. Both and the indicators aggregation simple additive weighting (SAW) method constitute a quantitative evaluation model. The above content together constitutes a new evaluation system for biological remediation on organic pollution in groundwater using slow-release agents, which was defined as AOBS evaluation system. In order to verify the rationality and scientificity of the evaluation system, a typical bioremediation slow-release agent was evaluated using the established AOBS evaluation system. The results showed that the evaluation system could reasonably and comprehensively evaluate bioremediation through slow-release agents and provide suggestions for agent improvement.

Anaerobic mineralization of toluene by enriched soil-free consortia with solid-phase humin as a terminal electron acceptor

The anaerobic biodegradation of toluene proceeds very slowly owing to limited electron acceptors in contaminated aquifer. The liquid reagents traditionally used to enhance this process readily migrate away from the contaminated site, and continuous addition would cause secondary pollution. In our previous study, the reduced solid-phase humic substances (humin), which are redox active, were found to act as electron donors to promote the microbial reactions. Here, we provide new evidence that humin can promote the anaerobic biodegradation of toluene as a terminal electron acceptor. When inoculating nitrate-reducing (NR) and iron-reducing (IR) consortia with toluene degradation activities, the average toluene degradation rates reached 21.20 ± 1.18 μmol/(L·d) and 15.43 ± 0.41 μmol/(L·d) in the presence of a sediment humin (HMcj), and 94.69% ± 4.26% and 93.20% ± 3.73% of the electrons released from toluene oxidation to CO₂ could be recovered by the reduction of HMcj, respectively. Spectroscopy analyses revealed that quinone moieties and nitrogen-containing moieties may be the electron-accepting groups of HMcj. Based on 16S rRNA sequencing, Cellulomonas spp. were the possible functional bacteria in the culture with NR consortium as the inoculum, while Azospira spp., Cellulomonas spp. and Bacillus spp. were the possible functional bacteria in the culture with IR consortium as the inoculum. Further Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analyses indicated that toluene oxidation and extracellular electron transfer functions were more abundant in HMcj amended cultures, suggesting a possible enhancement mechanism by HMcj. Additionally, experiments using natural groundwater illustrated that toluene degradation was highly dependent on its concentration, HMcj dosage, pH, and salinity. The study of a column filled with HMcj-coated quartz sand demonstrated a desirable level of toluene degradation in a continuous-flow mode without the presence of other electron acceptors. This study provided an effective and green approach for the remediation of the toluene-contaminated groundwater.

Transport and retention of nano emulsified vegetable oil in porous media: Effect of pore straining, roughness wedging, and interfacial effects

Emulsified vegetable oil (EVO), as one of the novel green substrates, has been widely used in subsurface remediation. In these applications, the retention behavior of EVO presents a challenge to remediation efficiency as mechanism insights into the retention of EVO is limited. Herein, Brinell funnels experiments with X-ray microtomography (XMT) were conducted to examine the drainage and retention of nanoscale EVO in porous media, with a specific focus on investigating the impact of pore straining, grain surface roughness, and interfacial effects on Nano-EVO (NEVO) retention. This study demonstrated that the retention of NEVO in porous media is the synergistic result of pore straining, roughness wedging, and interface attachment. With the action of these effects, three residual states of NEVO, incorporating retention at porous ganglia, grain-grain contacts, and grain surface, were identified by XMT in porous media. After multiple periods of drainage and imbibition, the NEVO arrived at stable retention proportions of 46.3%, 72.2%, and 85.9% in three independent systems with coarse, medium, and fine sand as porous media, respectively. The interfacial effects, including the attachment of solid-phase and air-liquid interface, are confirmed as the dominant factors for the retention of NEVO in porous media, which contributed 35.63-47.33% of total retention for the conditions employed. Correspondingly, the contributions of pore straining and roughness wedging only ranged 3.78-24.06% and 3.87-9.94%, respectively. The consistency of the contributions between the actual measurement of XMT and computational evaluation further confirmed the rationality and reliability of the results. In such the dominant factor, interfacial tension, contact angle, and capillary radius play an essential role in NEVO retention, which could be reflected by capillary rise height. These findings advance our understanding on NEVO retention caused by substrate-media interaction and also offer a promising direction for subsurface remediation.

Pore-scale identification of residual morphology and genetic mechanisms of nano emulsified vegetable oil in porous media using 3D X-ray microtomography

The application of emulsified vegetable oil (EVO) has attracted widespread attention in environmental remediation. Residual morphology is an important factor affecting its migration and mass transfer. However, proper identification of the EVO residual morphology at pore-scale has still remained a challenging task. Hence, this study aimed to identify the residual morphology of nanoscale EVO (NEVO) through developing a method combining natural breaks with 3D X-ray microtomography, then further explore the genetic mechanism of each residual morphology to verify the rationality of this method. The results showed that the natural breaks method can effectively classify the residual morphology of NEVO. Four morphologies including cluster, throat, corner, and membrane state were obtained from coarse, medium, and fine sands with a total proportion of 18.3%, 26.2%, and 30.8%. The cluster state was the main residual morphology, accounting for 10.0- 16.2%, then followed by corner-throat state and membrane state. Pore radius, throat radius, and length were confirmed providing sufficient evidences for cluster residues, because these factors determined the connectivity of porous media for the trapping of droplets. Comparison of the theoretical and actual results implied that capillarity coupling pore-throat shape jointly controlled corner and throat residues. Grain surface roughness and specific surface area were the main factors of membrane residue. The different residual morphologies of NEVO identified by the natural breaks method can reasonably explain their magnitude and controlling mechanisms, which in turn confirms the rationality of this method. Although the proportions of each form are related to the experimental conditions, the classification method and mechanism are of great significance for understanding NEVO residues.

Integrated remediation for organic-contaminated site by forcing running-water to modify alkali-heat/persulfate via oxidation process transfer

As organic pollution of soil and groundwater increases, the effective and economical remediation of contaminated sites has drawn growing attention. In this study, running-water (RW) was designed to modify alkali-heat/persulfate (MAH/PS) for integrated remediation of an actual organic-contaminated site. The degradation efficiency mainly reached 60%-99% for Benz[a]anthracene, Benzo[a]pyrene and total petroleum hydrocarbons (TPHs). MAH/PS was more effective in degrading Benzene and 1,2-Dichloroethane with simple molecular configurations. The pollutant degradation efficiencies decreased with increasing site depth and increased with increasing pollutant concentrations. Migration with RW enhanced site remediation. By monitoring the groundwater after remediation, it was found that residual TPHs presented anomalous diffusion; SO₄^2- ranged from 8.00 to 237.00 mg L^-1 to 8.00-290.00 mg L^-1 and pH presented alkalescence (7.00-8.20). Mathematical models were established to describe the reaction process including the solubility equilibrium of calcium hydroxide, temperature equilibrium, and reaction kinetics. Moreover, MAH/PS provided a cost-saving approach for site remediation.

A multi-path chain kinetic reaction model to predict the evolution of 1,1,1-trichloroethane and its daughter products contaminant-plume in permeable reactive bio-barriers

Permeable reactive bio-barriers (Bio-PRBs) are a new and developing technique for in situ remediation of groundwater contamination. Some remediation technologies have often been impeded by insufficient understanding of contaminant transport and transformation in the subsurface environment. Therefore, advanced knowledge in contaminant transport and reactions in Bio-PRBs will be crucial to the successful practical application of this technique. A two-dimensional reaction model C1 was developed for predicting the multi-path chain kinetic reaction of 1,1,1-trichloroethane (1,1,1-TCA) in Bio-PRBs. This study demonstrates that model C1 is able to predict the 1,1,1-TCA breakthrough time and rapidly evaluate the Bio-PRBs retardation performance. The results show that microbial growth and immobilization are the key factors that affect the retardation and remediation performance of Bio-PRBs. The free growth of microorganisms had significant negative effects on hydraulic conductivity (K) in the zero-valent iron (ZVI) region of free microorganism Bio-PRBs (FM-PRBs). The total head loss in the FM-PRB was 9.0 cm, which was significantly greater than the head loss (6.5 cm) of immobilized microorganism Bio-PRBs (IM-PRBs). Compared to ZVI-PRBs and FM-PRBs, the numerical simulation results reveal that microbial immobilization significantly improves the remediation performance of IM-PRBs by 550.9% and 32.7%, respectively. The dual effect of microorganisms leads to significant differences in the 1,1,1-TCA and daughter products (1,1-dichloroethane, 1,1-dichloroethene, chloroethane and vinyl chloride) contaminant-plume evolution between FM-PRBs and IM-PRBs. In addition, model C1 can be utilized to design standard Bio-PRBs for real site of 1,1,1-TCA contanminated groundwater. To meet the safety standard of groundwater as potable water, the width of IM-PRBs needs to be increased by 24 cm. However, in FM-PRBs, the width needs to be increased by 42 cm. Therefore, IM-PRBs save costs significantly. This work has successfully used a model to optimize Bio-PRBs and to predict 1,1,1-TCA and daughter products contaminant-plume evolution in different Bio-PRBs.