Comparative study of the quick action effect of multiple enzyme-based nano-emulsified oils in enhancing nitrate contamination remediation in groundwater.

Emulsified vegetable oil (EVO), as a novel green slow-releasing substrate, has performed great potential in subsurface bioremediation due to its slow release and longevity. Nevertheless, the long time it takes to initiate this process still exposed some limitations. Herein, multiple enzyme-based EVOs (EN-EVOs) were developed to enhance the quick-acting effect in nitrate-contaminated bioremediation. This study demonstrated that EN-EVOs loaded with cellulose (c-EVO) and protein enzymes (p-EVO) performed best, not only did not change the advantages of traditional EVO, but also optimized the stability and particle size to the level of 0.8‒0.9 and 247.95‒252.25 nm, respectively. Nitrate (NO3-N) degradation further confirmed the superiority of c-EVO in rapidly initiating degradation and achieving stable denitrification. Compared with traditional EVO, the maximum start-up efficiency and the rapid achieving stable denitrification efficiency were improved by 37.6% and 1.71 times, respectively. In such situation, the corresponding NO3-N removal efficiency, kinetics rate constant (k1), and half-life period (t1/2) reached as high as 85.39%, as quick as 1.079 d-1, and as short as 0.64 d after 30-day cultivation. Meanwhile, the rapid conversion efficiency of NO2-N was observed (k2 = 0.083 d-1). High-throughput 16S rRNA gene sequencing indicated that the quick-acting process of NO3-N reduction coupled to c-EVO was mediated by microbial reducers (e.g., Ralstonia, Gulbenkiania, and Sphingobacterium) with regulations of narG, nirS and norB genes. Microorganisms with these genes could achieve quick-acting not only by enhancing microbial activity and the synthesis and metabolism of volatile fatty acids, but also by reducing the production and accumulation of loosely bound-extracellular polymeric substances (LB-EPS). These findings advance our understanding on fast-acting of NO3-N degradation supported by c-EVO and also offer a promising direction for groundwater remediation.

Revealing the drivers and genesis of NO3-N pollution classification in shallow groundwater of the Shaying River Basin by explainable machine learning and pathway analysis method.

Nitrate (NO3-N), as one of the ubiquitous contaminants in groundwater worldwide, has posed a serious threat to public health and the ecological environment. Despite extensive research on its genesis, little is known about the differences in the genesis of NO3-N pollution across different concentrations. Herein, a study of NO3-N pollution concentration classification was conducted using the Shaying River Basin as a typical area, followed by examining the genesis differences across different pollution classifications. Results demonstrated that three classifications (0-9.98 mg/L, 10.14-27.44 mg/L, and 28.34-136.30 mg/L) were effectively identified for NO3-N pollution using Jenks natural breaks method. Random forest exhibited superior performance in describing NO3-N pollution and was thereby affirmed as the optimal explanatory method. With this method coupling SEMs, the genesis of different NO3-N pollution classifications was proven to be significantly different. Specifically, strongly reducing conditions represented by Mn2+, Eh, and NO2-N played a dominant role in causing residual NO3-N at low levels. Manure and sewage (represented by Cl-) leaching into groundwater through precipitation is mainly responsible for NO3-N in the 10-30 mg/L classification, with a cumulative contribution rate exceeding 80 %. NO3-N concentrations >30 mg/L are primarily caused by the anthropogenic loads stemming from manure, sewage, and agricultural fertilization (represented by Cl- and K+) infiltrating under precipitation in vulnerable hydrogeological conditions. Pathway analysis based on standardized effect and significance further confirmed the rationality and reliability of the above results. The findings will provide more accurate information for policymakers in groundwater resource management to implement effective strategies to mitigate NO3-N pollution.

Effect and genesis of soil nitrogen loading and hydrogeological conditions on the distribution of shallow groundwater nitrogen pollution in the North China Plain.

The North China Plain (NCP) has experienced increasingly severe groundwater nitrogen (TN) pollution. However, the factors influencing TN distribution are still poorly understood. Previous studies have identified surface soil nitrogen (TSN) loading and intrinsic groundwater vulnerability (Inv) as the main factors controlling groundwater TN pollution. However, in this study, based on 3245 shallow groundwater samples in the NCP, the multiple regression analysis results(R2=0.105, p<0.001) revealed that the TN was not mainly controlled by TSN and Inv. The lower prediction accuracy indicated the large data dispersion of TN, which might be affected by nitrogen attenuation or accumulation. Thus, the NCP was divided into balance, attenuation, and accumulation zones according to the regression equation. The attenuation zone was mainly distributed in the inter-fan and fan edge parts of the pre-mountain alluvial floodplain, as well as the west and south of the runoff area, while the accumulation zone was mainly distributed in the top part of the pre-mountain alluvial floodplain and the east of discharge area. Multi-indicators comparative analysis showed that compared to the balance (Eh= 76.2 mV) and accumulation (Eh=126.7 mV) zones, the attenuation zone has a stronger reducing environment (Eh=30.8 mV) favorable to denitrification, which can reduce the TN pollution (0.49 mg/L) caused by surface nitrogen input and consume more electron donors. Conversely, the stronger oxidizing environment in the accumulation zone limited denitrification, resulting in higher TN concentrations (19.14 mg/L) in the aquifers under the same TSN and Inv conditions as the other two zones. The standardized effects and significance on each path of the structural equation model (SEMs) fully confirmed the reliability of the above zonal analysis. Importantly, the feature importance (23.6%) of random forest and standardized effects (0.455, p<0.001) of SEMs showed that the Eh had the strongest influence on TN. Thus, the redox conditions of the aquifer, in addition to TSN and Inv, played a crucial role in controlling the TN pollution in the groundwater of a large region. The zoning work and the analysis of influencing factors are important to guide scientific prevention and control of groundwater nitrogen pollution.

Hydrochemical evolution characteristics and genesis of groundwater under long-term infiltration (2007-2018) of reclaimed water in Chaobai River, Beijing.

The reuse of reclaimed water (RW) for river ecological restoration in global water-shortage regions has inevitably brought some potential risks for groundwater. However, little is known about the effects of reclaimed water on the hydrochemical evolution of groundwater especially under long-term infiltration conditions. Herein, 11-years monitoring data (2007-2018) of reclaimed water and groundwater were adopted to analyze the characteristics and genesis of groundwater hydrochemical evolution under long-term infiltration of reclaimed water from Jian River to Chaobai River in Beijing. The results showed that the hydrochemical components in groundwater totally performed a significant increase in Na+, Cl-, and K+and decrease in Ca2+, Mg2+, and HCO3- concentration after long-term infiltration of reclaimed water. Meanwhile, a significant hydrochemical evolution difference between the groundwater of Jian River and Chaobai River was observed. In Jian River, the hydrochemical type in groundwater shifted gradually from HCO3-Ca·Mg to the type of HCO3·Cl-Na·Ca approaching reclaimed water. In contrast, the hydrochemical evolution in the Chaobai River shows an obvious opposite trend from HCO3-Ca·Mg to HCO3·Cl-Na·Mg and finally deviating reclaimed water type of Cl·HCO3·SO4-Na. PHREEQC simulation indicated that the differences in hydrochemical evolution were mediated synergically by sediment thickness and geochemical processes (e.g. mixing and sulfate reduction). In such mediators, thinner sediment and strong mixing in the Jian River were confirmed to be the genesis of groundwater hydrochemical evolution progressively approaching reclaimed water. Different from the Jian River, multiple regression analyses revealed that the genesis of groundwater hydrochemical evolution in the Chaobai River was divided into two stages according to the increase of sediment thickness. Reclaimed water quality and infiltration amount are the leading proposed cause in the initial stage (2007-2008) due to thinner sediment formation, contributing 53.5% and 29.8% within the 95% confidence interval, respectively. Subsequently, the rise in sediment thickness is proved to play a crucial role in groundwater hydrochemical evolution trend away from reclaimed water (2009-2018), with a contribution of 41.6% within the 95% confidence interval. It is mainly attributed to the reduced reclaimed water infiltration rate and favorable sulfate reduction conditions. These findings advance our understanding on groundwater hydrochemical evolution under long-term infiltration of reclaimed water and also guide future prediction of evolution trends.

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.

Coupling of multi-hydrochemical and statistical methods for identifying apparent background levels of major components and anthropogenic anomalous activities in shallow groundwater of the Liujiang Basin, China.

Natural background levels (NBLs) is a prerequisite for distinguishing anthropogenic groundwater pollution and judging the evolution of groundwater quality. However, due to regional differences of hydrogeochemitry and water-rock interaction, coupled with long-term anthropogenic activities, it is no longer accurate to assess NBLs with only statistical methods or without considering human impact. Herein, multi-hydrochemical and statistical methods were examined to identify apparent background levels and anthropogenic anomalous activities of shallow groundwater by selecting Liujiang Basin as a study area. The results showed that the differences in hydrochemical characteristics among each hydrogeological unit (HU) fully illustrated the necessity of rationally dividing HU for background value identification. The application of the concept of apparent background levels (ABLs), that is, incorporating normal human activities into the background levels, efficiently solved the problem of being unable to obtain pristine NBLs due to long-term human activities. The coupling of Hydrochemistry and Grubbs' test (Hydro-Grubbs) was confirmed as the optimal method in identifying and eliminating anthropogenic groundwater anomalies, performing sufficiently superiority when compared with purely statistical methods. It is mainly because the Hydro-Grubbs method not only considers the discreteness of the data itself, but also considers the internal connection and evolution process of the hydrochemical compositions. For the eliminated abnormal points, 91.0-93.6% of which have been effectively explained by pollution percentage index and the impact of coal mining, industrial activities, residents, agricultural activities, and septic tanks leakage, proving the rationality and reliability of Hydro-Grubbs method and ABLs evaluation result. This finding will assist in accurately identifying anthropogenic pollution on a regional scale and guiding future efforts to protect groundwater resources.

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.

Assessment, formation mechanism, and different source contributions of dissolved salt pollution in the shallow groundwater of Hutuo River alluvial-pluvial fan in the North China Plain.

With rapid urbanization and industrialization processes, the problem of groundwater pollution under the influence of various human activities has become increasingly severe in most developed areas of China. However, the problem of dissolved salt pollution caused by increasing concentrations of conventional ions is often overlooked and easier to be seen as a high background of natural formation rather than pollution. The Hutuo River alluvial fan in North China was selected as the study area; dissolved salt pollution is evaluated based on the factor analysis method (FA); groundwater exploitation, pollutant input, and the attenuation of the vadose zone were discussed to explain the salt pollution; the formation mechanism and different source contributions were also explored. The results show that the total hardness (TH) and nitrate are the main contributing indicators of salt pollution in the Hutuo River alluvial fan. The long-term overexploitation of groundwater promoted the leaching and nitrification reactions, resulting in a large area of moderate to strong salt pollution in the top unit of the alluvial fan, which accounted for 51.6% of the salt pollution according to the multivariate regression model. In addition, the input pollution generated by various types of pollution sources along with rainfall infiltration is also an important driving factor. The surface pollution load and hydrogeological conditions affected the cation exchange and leaching, resulting in a point distribution of strong salt pollution, with a contribution rate of 37.6%. The analysis of the factors that affect salt pollution and the specific contributions in different regions cannot only help decision-makers understand the causes of water quality deterioration but also propose solutions in a targeted manner.

Sensitivity assessment of denitrifying bacteria against typical antibiotics in groundwater.

The effects of antibiotics on nitrate denitrification in groundwater have acquired growing concern. Denitrification is a microbially mediated process. The effects of antibiotics on denitrification were mainly reflected in denitrifying bacteria. However, little is known about the relationship between antibiotics and denitrifying bacteria. Based on this, both direct antimicrobial susceptibility testing and microbial batch-culture experiments were conducted to assess the influences of typical antibiotics on denitrifying groundwater bacteria, mainly Pseudomonas (46.17%). Denitrifying bacteria, screened from a long-term groundwater denitrification environment, were tested for sensitivity to five typical antibiotics in groundwater: sulfamethoxazole (SMX), erythromycin (ERY), enrofloxacin (ENR), clindamycin (CLI), and tetracycline (TCY). The results showed that the sensitivity of denitrifying bacteria to antibiotics is mainly related to the type and concentration of antibiotics. For antibiotic types, the order of sensitivity by quantitative assessment is ENR > TCY > SMX > ERY > CLI. Fluoroquinolones (FQs) represented by ENR were selected to explore their concentration effects. The influences on denitrifying bacteria were divided into the high concentration effect (500 µg L-1 to 100 mg L-1) and the low concentration effect (100 ng L-1 to 10 µg L-1) with about 100 µg L-1 as a boundary. Exposure to high concentrations had significant inhibitory effects on bacterial growth and exhibited dose dependency, especially for ciprofloxacin (CIP). The low concentration effect was independent of concentration, which may be stimulation or inhibition. The stimulation mainly occurred due to ENR-exposure. For inhibitory effects, Lomefloxacin (LOM) was more effective than other FQs. Especially for inhibition at ng-level exposure, LOM and norfloxacin (NOR) exposures led to the highest and lowest inhibition rates, respectively.

Effect of hydrogeological conditions and surface loads on shallow groundwater nitrate pollution in the Shaying River Basin: Based on least squares surface fitting model.

Nitrate pollution in groundwater has become a widespread problem worldwide, but understanding of the factors influencing groundwater nitrate pollution remains limited. Numerous studies have attributed nitrate pollution mostly to surface conditions and have neglected the role of hydrogeology. Therefore, this study used the Shaying River Basin as the study area and developed a least squares surface fitting (LSSF) model to systematically analyze the effect of hydrogeological conditions and surface pollution loads on groundwater nitrate pollution. Intrinsic vulnerability and total soil nitrogen (TSN) were used to represent hydrogeological conditions and surface pollution loads, respectively. The results showed that the concentrations of NO3-N in shallow groundwater ranged from 0.002 to 256.29 mg/L (with an average of 14.38 mg/L). The concentration had an overall decreasing trend along the flow path. The water chemistry tended to change from HCO3-Ca to HCO3·Cl-Ca as the NO3-N concentration increased. Groundwater nitrate pollution was simultaneously controlled by intrinsic vulnerability and TSN, and the LSSF model explained 83.5% of the result within a 95% confidence interval. These findings explained the phenomenon by which some areas had high surface loads but no serious groundwater nitrate pollution and some areas had nitrate pollution but no high surface loads. Nitrate accumulated in high levels in areas with a high intrinsic vulnerability due to hydrogeological conditions. TSN, which was the main source of NO3-N in groundwater, came mainly from agricultural nitrogen fertilizer inputs and livestock manure. These findings provide helpful information for those tasked with managing and controlling groundwater quality.

Formation process and mechanism of humic acid-kaolin complex determined by carbamazepine sorption experiments and various characterization methods.

To explore the formation process and mechanism of organic matter and organic-mineral complex under humification and mineralization conditions, a series of samples including humic acid, kaolin, and humic acid-kaolin complex were prepared using a subcritical water treatment method (SWT) under specific temperature, pressure and reaction time conditions. HA was used as a surrogate for natural organic matter because it has a similar abundant pore structure, variety of carbon types, and chemical components. These samples were used in carbamazepine (CBZ) sorption experiments and characterized by a variety of techniques. The polymerization of humic acid under the conditions of increased temperature and pressure resulted in an increase in specific surface area and molecular quantity. In addition, the degree of aromaticity rose from 59.52% to 70.90%. These changes were consistent with the transformation from 'soft carbon' to 'hard carbon' that occurs in nature. The results of sorption experiments confirmed the interaction between humic acid and kaolin from the difference between the predicted and actual Qe values. The conceptual model of humic acid-kaolin complex could be deduced and described as follows. Firstly, the aromatic components of humic acid preferentially combine with kaolin through the intercalation effect, which protects them from the treatment effects. Next, the free carboxyl groups and small aliphatic components of humic acid interact on the surface of kaolin, and these soft species transform into dense carbon through cyclization and polymerization. As a result, humic acid-kaolin complex with a mineral core and dense outer carbonaceous patches were formed.

Abnormal pH elevation in the Chaobai River, a reclaimed water intake area.

The pH is a primary index reflecting water quality in rivers. The Jian River and Chaobai River are two reclaimed water intake areas which have elevated pH. This elevated pH has a marked effect on both the phytoplankton, species in water and vegetation on the shore. Understanding the main reasons causing pH elevation in river water has important implications for river ecosystem management and the improvement of water quality and can provide a theoretical basis for the direction of water quality improvement. For this reason, each biogeochemical and physical process influencing pH changes in water was quantified along the flow direction in the Wenyu to Chaobai reclaimed water diversion project, in which proton consumption and production by such processes were monitored and calculated at five monitoring sections. The calculations indicated that photosynthesis and denitrification were the primary reasons for the increase of pH in the Jian River and Chaobai River. Oppositely, carbonate precipitation and sediment decomposition restricted the increase of pH in both rivers. In addition, CO2 emission to the air also promoted a increase of pH in the Jian River, while CO2 absorption from the air restricted the increase of pH in the Chaobai River. NO3- nitrogen in reclaimed water was not efficiently removed and the reclaimed water flow condition in the intake area created favorable conditions for photosynthesis of algae breeding and denitrification by microorganisms. Therefore, biogeochemical and physical processes that promoted the increase of pH were greater than inhibiting processes and the pH gradually increased along the flow direction. The contribution rates of photosynthesis and denitrification for the increase of pH were 55.48 and 27.09%, respectively, in the Jian River and 78.08 and 21.92%, respectively, in the Chaobai River. In addition, CO2 emission contributed 17.43% of the increase in pH in the Jian River.