Combined effects of aquifer heterogeneity and subsurface dam on nitrate contamination in coastal aquifers.

Subsurface dams are effective for seawater intrusion mitigation, yet they can cause upstream nitrate accumulation. This research examines the interplay between subsurface dam construction and aquifer layering on nitrate pollution in coastal settings, employing numerical models to simulate density-driven flow and reactive transport. The study reveals that while subsurface dams are adept at curbing seawater intrusion, they inadvertently broaden the nitrate accumulation zone, especially when a low-K layer is present. Heterogeneous aquifers see more pronounced nitrate accumulation from subsurface dams. This effect is pronounced as it influences dissolved organic carbon dynamics, with a notable retreat inland correlating with the expansion of the nitrate pollution plume. A critical finding is that controlling seawater intrusion via dam height adjustment within the Effective Damming Region effectively reduces nitrate levels and bolsters freshwater output. However, exceeding the critical threshold-where the dam surpasses the low-K layer's bottom-results in a substantial shift in nitrate concentration, underscoring the need for precise dam height calibration to avoid aggravating nitrate pollution. This study's innovative contribution lies in its quantification of the nuanced effects of subsurface dams in stratified aquifers, providing an empirical basis for dam design that considers the layered complexities of coastal aquifers. The insights offer a valuable framework for managing nitrate contamination, thus informing sustainable coastal groundwater management and protection strategies.

Hydrodynamic behavior of freshwater-saltwater mixing zone in the context of subsurface physical barriers.

The seawater intrusion (SWI) process lasts for decades in real world, thus the research on dynamic process of SWI is essential. The freshwater-saltwater mixing zone plays a crucial role in governing the groundwater movement and the solute transport in coastal aquifers. To date, there has been a lack of research on the hydrodynamic behavior of the mixing zone in the presence of subsurface physical barriers. In this work, we employed laboratory experiments and numerical simulations to investigate the dynamics of the mixing zone, comparing scenarios with and without subsurface physical barriers. The findings indicate that the construction of a subsurface physical barrier will not immediately slow down the seawater intrusion velocity and change the salinity distribution of mixing zone. The block effect of subsurface physical barriers with different heights or bottom opening sizes became apparent only when the wedge toe approached the physical barriers. The widening effect of increasing longitudinal dispersivity on the mixing zone width was more pronounced during the dynamic process compared to the steady state. Furthermore, the widening effect of increasing longitudinal dispersivity on the mixing zone was more significant compared to transverse dispersivity in both the SWI and subsurface dam scenarios throughout the intrusion process. However, in the cutoff wall scenarios, the widening effect of increasing transverse dispersivity became more obvious during the later intrusion period. Our conclusions provide a reference for the groundwater management in coastal aquifers. According to the current seawater intrusion situation, the local water bureau can predict the seawater intrusion velocity and the temporal changes of mixing zone after the construction of physical barriers.

A critical review of the central role of microbial regulation in the nitrogen biogeochemical process: New insights for controlling groundwater nitrogen contamination.

With the increase of nitrogen (N) input in vadose zones-groundwater systems, N contamination in groundwater has become a global environmental and geological issue that has a profound impact on the ecological environment and human health. N migration in the vadose zone is the most significant means of contaminating the groundwater aquifer. However, the current research on the control of groundwater N contamination focuses solely on the content change of certain indicators and is unable to comprehend the cause and subsequent development of groundwater N contamination. These factors pose significant environmental management challenges in areas where groundwater is contaminated with nitrate. In recent years, research on the migration and transformation behavior of various N forms in vadose zones-groundwater systems has yielded some breakthroughs but also encountered some roadblocks. The biogeochemical behavior of nitrogen consists of a series of intricate chain reaction cycles (called N-cycle). The crucial role of microorganisms in the N biogeochemical process has attracted the interest of soil carbon- and N-cycle researchers and become a hot topic of study. Nonetheless, the role of microbial regulation in groundwater systems has been largely neglected and needs to be summarized immediately. Consequently, this review summarizes recent advancements, mechanisms, and challenges, and proposes a dynamic perspective on microbial regulation. On the basis of these findings, we propose a dynamic and comprehensive groundwater N system centered on microbial regulation. In addition, we critically summarized the migration and transformation behavior of the most recent N indicators, the impact of global environmental change on each N component, and the non-negligible effects of these factors on the control of groundwater N contamination. Future research must focus on the migration and transformation behavior of nitrogen in the deep vadose zone, based on the dynamic regulation of microorganisms, and complete the missing pieces of the developed N-cycle index system. These are essential for providing scientific guidance for global N management and effectively mitigating N contamination in groundwater.

Rhamnolipid-induced alleviation of bioclogging in Managed Aquifer Recharge (MAR): Interactions with bacteria and porous media.

The prevention and treatment of bioclogging is of great significance to the application of Managed Aquifer Recharge (MAR). This study investigated the alleviating effect of biosurfactant rhamnolipid (RL) on bioclogging by laboratory-scale percolation experiments. The results show that the addition of RL greatly reduced bioclogging. Compared with the group without RL, the relative hydraulic conductivity (K') of the 100 mg/L RL group increased 5 times at the end of the experiment (23 h), while the bacterial cell amount and extracellular polymeric substances (EPS) content on the sand column surface (0-2 cm) decreased by 60.8% and 85.7%, respectively. In addition, the richness and diversity of the microbial communities within the clogging matter decreased after the addition of RL. A variety of bacterial phyla were found, among which Proteobacteria were predominant in all groups. At the genus level, RL reduced the relative abundance of Acinetobacter, Bacillus, Klebsiella, and Pseudomonas. These microbes are known as strong adhesion, large size, and easy to form biofilms, therefore playing a critical role during MAR bioclogging. Moreover, RL changed the surface properties of bacteria and porous media, which results in the increase of electrostatic repulsion and decrease of hydrophobic interaction between them. Therefore, RL mediated the bacteria-porous media interaction to reduce biomass in porous media, thereby alleviating bioclogging. This study implies that RL's addition is an environmentally friendly and effective method to alleviate the bioclogging in MAR.

APCS-MLR model: A convenient and fast method for quantitative identification of nitrate pollution sources in groundwater.

Nitrate (NO3-) contamination in groundwater has diverse sources and complicated transformation processes. To effectively control NO3- pollution in groundwater systems, quantitative and accurate identification of NO3- sources is critical. In this work, we applied hydrochemical characteristics and isotope analysis to determine NO3- source apportionment. For the first time, the NO3- source contributions were calculated using hydrochemical indicators combined with multivariate statistical model (PCA-APCS-MLR). The results interpret that chemical fertilizers (58.11%) and natural sources (22.69%) were the primary NO3- sources in the vegetable cultivation area (VCA) which were rather close to the estimation by Bayesian isotope mixing model (SIAR). In particular, the contributions of chemical fertilizers in the VCA differed by only 3.79% between the two methods. Compared with previous approaches e.g. SIAR, the key advantage of the proposed PCA-APCS-MLR model is that it only requires the hydrochemical indicators which can be easily measured. A series of complicated experiments including measurement of isotope data of NO3- in groundwater, monitoring of in-situ pollution source information and calculation of isotopic enrichment factor can be simply avoided. The PCA-APCS-MLR model offers a much more convenient and faster method to determine the contribution rates of NO3- pollution sources in groundwater.

Influence of velocity distribution conditions of coastal aquifer on nitrate contamination and seawater intrusion in the presence of subsurface physical barriers.

The velocity distribution is an important factor that affects seawater intrusion (SI) and nitrate (NO3-) pollution. However, there are few studies on the impact of subsurface physical barriers (SPBs) on the velocity distribution of the whole aquifer and the impact of velocity distribution on SI and NO3- pollution. Especially, the quantitative method of velocity distribution has not been studied. By the methods of laboratory experiments and numerical simulations, effects of the NO3- concentrations of the pollution source, hydraulic gradients (HGs), the location of the SPB and relative heights of SPBs (HP') on the SI, NO3- pollution levels and velocity in the presence of SI and SPBs were investigated. The velocity distribution was first quantified to better describe the relationships between the velocity and degrees of SI and NO3- pollution. The results showed that the HG and HP' were the main factors that affected the velocity, NO3- pollution and SI. The higher the HG, the smaller the HP', and the decreased SI inferred a more serious NO3- pollution. The influence of SPBs on NO3- pollution and SI was mainly affected by the changes in the velocity distribution in the aquifer. With increasing HGs, for the region with flow rate less than 0.5 m/d (A0.5), the smaller its distribution area is, the smaller the relative area of SI (TLs') is. With an increase in the HG or decrease in the HP', the relative area of NO3- pollution (Ns') is proportional to the distribution area where the flow velocity is greater than 1 m/d (A1). When the flow velocity distribution condition was A'1 (the relative area of A1) > A'0.5-1 (A'0.5-1 is the ratio of the area where the flow velocities are greater than 0.5 m/d and less than 1 m/d to the total area of the aquifer) > A'0.5 (the relative area of A0.5), NO3- pollution was serious; when the flow velocity distribution condition was A'0.5 > A'0.5-1 > A'1, the levels of NO3- pollution were the lowest.

Quantitative evaluation of the synergistic effect of biochar and plants on immobilization of Pb.

Biochar and plant cooperation in remediation of heavy metal contaminated soil is effective and important, but there still have knowledge gaps of synergistic effect between the two and the synergistic pathway has not been clarified. We prepared the Enteromorpha prolifera biochar at 400 °C and 600 °C (denoted as BC400 and BC600). The Pb fractions changes in soil and Pb toxicity in Brassica juncea were investigated by adding 30 g kg-1 biochar to soil containing 1200 mg kg-1 Pb in a pot experiment. There was a significant synergistic effect between biochar and plants on Pb immobilization in soil, according to the "E > 0" of Pb fractions in the interaction equation. Pb immobilization rates of biochar-plant treatments (BJBC4 and BJBC6) were 12.47% and 11.38% higher than biochar treatment (BC4, BC6), and 17.66% and 16.28% plant treatment (BJ). BJBC4 had a better immobilization effect than BJBC6. Biochar alleviated the phytotoxicity of Pb by increasing the antioxidant enzymes activities of plants. These results indicated two synergistic pathways: (1) The high pH and oxygen-containing functional groups of biochar could immobilize Pb through ion exchange, precipitation, or complexation. (2) Biochar enhanced the activity of the antioxidant enzyme system in plants thus improving the Pb tolerance of plants. Statistical analysis methods such as the partial least squares path modeling (PLS-PM) also confirmed the pathways. In a word, clear synergistic effects and pathways could guide the application of biochar and plants in Pb-contaminated soil.

Impact of a Hydrocarbon Surfactant on the Retention and Transport of Perfluorooctanoic Acid in Saturated and Unsaturated Porous Media.

The transport and retention behavior of perfluorooctanoic acid (PFOA) in the presence of a hydrocarbon surfactant under saturated and unsaturated conditions was investigated. Miscible-displacement transport experiments were conducted at different PFOA and sodium dodecyl sulfate (SDS) input ratios to determine the impact of SDS on PFOA adsorption at solid-water and air-water interfaces. A numerical flow and transport model was employed to simulate the experiments. The PFOA breakthrough curves for unsaturated conditions exhibited greater retardation compared to those for saturated conditions in all cases, owing to air-water interfacial adsorption. The retardation factor for PFOA with a low concentration of SDS (PFOA-SDS ratio of 10:1) was similar to that for PFOA without SDS under unsaturated conditions. Conversely, retardation was greater in the presence of higher levels of SDS (1:1 and 1:10) with retardation factors increasing from 2.4 to 2.9 and 3.6 under unsaturated conditions due to enhanced adsorption at the solid-water and air-water interfaces. The low concentration of SDS had no measurable impact on PFOA air-water interfacial adsorption coefficients (Kia) determined from the transport experiments. The presence of SDS at the higher PFOA-SDS concentration ratios increased the surface activity of PFOA, with transport-determined Kia values increased by 27 and 139%, respectively. The model provided very good independently predicted simulations of the measured breakthrough curves and showed that PFOA and SDS experienced various degrees of differential transport during the experiments. These results have implications for the characterization and modeling of poly-fluoroalkyl substances (PFAS) migration potential at sites wherein PFAS and hydrocarbon surfactants co-occur.

Determination of the nitrogen isotope enrichment factor associated with ammonification and nitrification in unsaturated soil at different temperatures.

For nitrogen (N) migration and transformation from unsaturated soil to groundwater, the N stable isotope (δ15N) was modified due to the isotope fractionation effect. To quantitatively evaluate the N cycle in groundwater systems, the determination of isotope fractionation is decisive. In this research, for the first time, incubation experiments were conducted to quantitatively investigate the N isotope enrichment factor (ϵp/s) associated with ammonification in unsaturated soil. Under weak isotopic fractionation, the Rayleigh function cannot be directly applied during ammonification. Thus, we proposed a different method calculating the ϵp/s values during ammonification, which were -0.03‰ for 15 °C and -2.34‰ for 30 °C. Moreover, for the first time, experimental equipment is presented to explore the isotopic fractionation effects under the co-occurrence of nitrification and volatilization. The results indicated that the isotope effect of volatilization during nitrification can be ignored in this study, and the ϵp/s values during nitrification were -10.59 and -6.81‰ at 15 and 30 °C, respectively. This work provides a novel arrangement determining the crucial parameters for identifying nitrate pollution sources in groundwater systems.

An improved method of recharge sources analysis and its application in an unconfined aquifer.

Groundwater recharge sources analysis, including identification of the recharge sources and calculation of the mixing ratios, is of great importance for hydrogeological research and water resources management. In this research, a new approach, multivariate mixing and mass-balance calculations (M3) model combined with MIX calculations (M3-MIX calculations), was proposed to overcome shortcomings and limitations of existing methods and to accurately describe aquifer systems with more than three groundwater sources and get more accurate mixing ratios. A synthetic case with random sources were applied to evaluate the effectiveness of M3-MIX calculations. The results of both mixing ratios and composition of recharge sources show that M3-MIX calculations is superior to traditional methods such as least squares, and is also superior to the results obtained by using M3 model or MIX calculations alone. The approach is then applied to analyze groundwater recharge sources of the Huangshui River groundwater reservoir, China. Three recharge sources were calculated based on M3-MIX calculations: brackish groundwater affected by seawater intrusion, atmospheric precipitation, and groundwater from upstream affected by agricultural activities. The mixing ratios of the three recharge sources are 3.3%, 19.3%, and 77.4%, respectively. In addition, ion concentrations deviate from the mixing line indicates that hydrochemical processes, such as mineral dissolution/precipitation and ion exchange processes, may occur in the study area. The results of this work indicate that M3-MIX calculations has the potential to provide the accurate understanding of groundwater recharge, and thus providing useful information for the exploitation, utilization, and protection of groundwater in unsaturated and saturated zones.

Transport of GenX in Saturated and Unsaturated Porous Media.

The objective of this research was to investigate the retention and transport behavior of GenX in five natural porous media with similar median grain diameters but different geochemical properties. Surface tensions were measured to characterize surface activity. Miscible-displacement experiments were conducted under saturated conditions to characterize the magnitude of solid-phase adsorption, while unsaturated-flow experiments were conducted to examine the impact of air-water interfacial adsorption on retention and transport. The results from surface-tension measurements showed that the impact of solution composition is greater for the ammonium form of GenX than for the acid form, due to the presence of the NH4 counterion. The breakthrough curves for the experiments conducted under saturated conditions were asymmetrical, and a solute-transport model employing a two-domain representation of nonlinear, rate-limited sorption provided reasonable simulations of the measured data. The magnitudes of solid-phase adsorption were relatively small, with the highest adsorption associated with the medium containing the greatest amount of metal oxides. The breakthrough curves for the experiments conducted under unsaturated conditions exhibited greater retardation due to the impact of adsorption at the air-water interface. The contributions of air-water interfacial adsorption to GenX retention ranged from ∼24% to ∼100%. The overall magnitudes of retardation were relatively low, with retardation factors < ∼3, indicating that GenX has significant migration potential in soil and the vadose zone. To our knowledge, the results presented herein represent the first reported data for solid-water and air-water interfacial adsorption of GenX by soil. These data should prove useful for assessing the transport and fate behavior of GenX in soil and groundwater.

Assessment of the hydrodynamics role for groundwater quality using an integration of GIS, water quality index and multivariate statistical techniques.

To explore the impact of groundwater hydrodynamics on water quality, a cost-effective geospatial model was developed using geographic information system (GIS) technology and the Dupuit assumption. Meanwhile, the groundwater quality in the Dagu River Basin was evaluated based on the water quality index (WQI) and multivariate statistical analyses. In April (dry season) and September (rainy season) 2017, the groundwater level was automatically monitored from 115 wells, and the water quality including 21 hydrochemical parameters was sampled from 37 wells. Results reveal that the WQI values varied from 35.01 to 64.74, with mean values of 51.89 and 47.87 in the rainy and dry seasons. Approximately 80% of the samples exhibited moderate water quality, with no significant difference between the rainy and dry seasons. Nitrate pollution and the integrated water quality in the central and northern regions were generally worse than that in the southern region. The Darcy velocity in the central and northern regions was relatively high with a maximum rate of 0.56 m/d, compared with the southern region. This correlation illustrates the effect of groundwater hydrodynamics on quality. The sowing of greater chemical fertilizers combined with faster groundwater movement is likely responsible for the large-scale nitrate pollution in the central and northern regions. Results also proved the accuracy of the geospatial model with a valid uncertainty. The geospatial model provides a valuable alternative for the spatial analysis of the effect of groundwater hydrodynamics on water quality.

An index system constructed for ecological stress assessment of the coastal zone: A case study of Shandong, China.

Coastal zones, which have high ecological value and environmental function and play a key role in human development, face intense ecological stress from human activities. This study constructed an assessment index system for coastal zones and proposed a coastal ecological stress index (CESI) model. This method was then applied to the Shandong coastal zone. The results showed an increase in ecological stress from 2001 to 2016 and implied a further growth trend. The stress caused by terrigenous pollution was the most prominent, with the ecological stress index showing significant spatial difference. Qingdao exerted the highest ecological stress on the population and economy, while Yantai showed the highest stress on the coastal index. The CESI model effectively reflects the temporal and spatial characteristics of the coastal ecological stress and provides a theoretical basis for the management of different regions.

Effects of carbon availability in a woody carbon source on its nitrate removal behavior in solid-phase denitrification.

Woody biomass is the most common natural carbon source applied in solid-phase denitrification (SPD). However, its denitrification ability is low in the SPD process due to its poor carbon availability. In this study, sawdust samples were pretreated to various degrees, and then filled into SPD bioreactors to reveal the relationship between carbon availability and denitrification behaviors. The behaviors include the denitrification process, internal effects of major factors (carbon availability, pH and temperature), and the presence of bacterial communities. Results shown that the long-term denitrification rate of pretreated sawdust was increased by 4.5-4.8 times over that of untreated sawdust (29.3 mg N L-1 sawdust d-1). However, despite improving the pretreatment degree of the sawdust in the bioreactor, the long-term denitrification rate shown no further increase. The denitrification rate was most influenced by the temperature, followed by the pH, and then the sawdust pretreatment degree. The denitrification rate increased with decreasing pH and rising temperature of the pretreated sawdust. The removed nitrate was rarely converted into nitrite or nitrous oxide, but ammonium was produced at high pH and temperature for the pretreated sawdust. The adverse effects of ammonium and dissolved organic carbon (DOC) reduced when the pH of the pretreated sawdust was lowered to 6.5. Hydrolytic and denitrifying bacteria formed the main SPD bioreactor bacteria, whose abundances increased with increasing sawdust pretreatment degree. The results were beneficial to reduce the hydrolytic retention time and adverse products for the SPD system using woody carbon source.

Identifying key factors of the seawater intrusion model of Dagu river basin, Jiaozhou Bay.

Seawater intrusion is a complex groundwater - seawater interaction process, and it is influenced by many factors from ground surface to underground, from groundwater to seawater. Generally, for seawater intrusion model, some model parameters and boundary conditions are always specified by model users' personal experiences or literature's reference value. The defective model would damage the groundwater management for controlling and preventing seawater intrusion when making decisions are based on this model. In order to improve the reliability of seawater intrusion model, the influences of model inputs on output should be identified prior at optimizing model inputs. Dagu river basin, Jiaozhou Bay is one of the most serious areas of seawater intrusion in China, and it is chosen as the study area in this study. The seawater intrusion model of Dagu river basin is built based on a general program SEAWAT4. The key influence factors of model output are analyzed by two sensitivity analysis methods, i.e., stepwise regression and mutual entropy. The results demonstrated that the most important influence factors which have largest sensitivities to groundwater Cl- concentration are the precipitation rate and groundwater pumping in agriculture area. In addition, the hydraulic conductivity of zone 1 has a non-negligible influence on seawater intrusion process. Stepwise regression analysis is capable of identifying most important influence factor, and it can't handle complicated nonlinear input-output relationship. Mutual entropy analysis is reliable for identifying the influence factors for complex seawater intrusion model.

Effects of alkali-treated agricultural residues on nitrate removal and N2O reduction of denitrification in unsaturated soil.

Lignocellulosic agricultural residues were utilized as denitrification carbon substrates to improve the purification capacity of unsaturated soil and alleviate nitrate pollution of groundwater. In this study, corncob and wheat straw were treated by calcium hydroxide to improve biodegradability and enhance denitrification potential. Calcium hydroxide treatment decreased the contents of lignin (i.e., from 16.7 wt% to 15.2 wt% in corncob and from 21.9 wt% to 20.6 wt% in wheat straw), increased potential biodegradable carbon by 4.4-5.3 times, reached complete nitrate removal 7-14 days earlier and decreased N2O/(N2O+N2) ratios by 85-99%. The results provide an insight into the application of alkali-treated agricultural residues as denitrification carbon sources to alleviate nitrate transport to groundwater and reduce potential greenhouse effect.

Mechanism insights into enhanced trichloroethylene removal using xanthan gum-modified microscale zero-valent iron particles.

This report focuses on the enhancement in trichloroethylene (TCE) removal from contaminated groundwater using xanthan gum (XG)-modified, microscale, zero-valent iron (mZVI). Compared with bare mZVI, XG-coated mZVI increased the TCE removal efficiency by 30.37% over a 480-h experimental period. Because the TCE removal is attributed to both sorption and reduction processes, the contributions from sorption and reduction were separately investigated to determine the mechanism of XG on TCE removal using mZVI. The results showed that the TCE sorption capacity of mZVI was lower in the presence of XG, whereas the TCE reduction capacity was significantly increased. The FTIR spectra confirmed that XG, which is rich in hydrophilic functional groups, was adsorbed onto the iron surface through intermolecular hydrogen bonds, which competitively repelled the sorption and mass transfer of TCE toward reactive sites. The variations in the pH, Eh, and Fe(2+) concentration as functions of the reaction time were recorded and indicated that XG buffered the solution pH, inhibited surface passivation, and promoted TCE reduction by mZVI. Overall, the XG-modified mZVI was considered to be potentially effective for the in-situ remediation of TCE contaminated groundwater due to its high stability and dechlorination reactivity.

Effects of biochar-BDE-47 interactions on BDE-47 bioaccessibility and biodegradation by Pseudomonas putida TZ-1.

With biochar amendments widely accepted as efficient POPs contamination remediation methods, the post-remediation risk assessment and effectiveness evaluation were urgently needed. So in the study, the effects of biochar -2,2',4,4'-tetrabromodiphenyl ether (BDE-47) interactions on the bioaccessibility and biodegradation of BDE-47 were systematically examined. Biodegradation was monitored over 7 day incubation time with strain Pseudomonas putida TZ-1 and it was revealed that the presence of three model biochars dramatically decreased the biodegradation rate by 87.50-92.19%. The desorption rate gradually decreased to eventually make it a rate-limiting process for BDE-47 biodegradation. To further explore the impact of biochar-BDE-47 sorption on its bioaccessibility, chemical extraction and biosurfactant facilitated desorption experiments were conducted. Both results suggested that almost all the molecules sorbed onto non-porous biochars could be completely desorbed, whereas BDE-47 molecules sequestered within deep micropores were more persistent on the solid phase, and resulted in lower bioaccessibility.

Aerobic transformation of BDE-47 by a Pseudomonas putida sp. strain TZ-1 isolated from PBDEs-contaminated sediment.

A bacterial isolate, TZ-1, was isolated from contaminated sediment near electronic waste dismantling workshops, Taizhou, China that degraded 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47). The isolate was identified as Pseudomonas putida sp. with respect to its morphology, biochemical characteristics and 16SrDNA sequence analysis. TZ-1 can use BDE-47 as the sole carbon and energy source for growth in mineral salt medium. The isolate degraded BDE-47 up to 49.96 % of the initially applied concentration of 50 µg L(-1) after 7 days of incubation at 150 rpm, 30°C. Static conditions with pH 6.5 and temperature 30°C were considered to be optimum for BDE-47 biodegradation. Addition of co-substrates promoted cell growth, but decreased the degradation rate for BDE-47.

Effective control of modified palygorskite to NH4+-N release from sediment.

Sediment capping is an in situ treatment technology that can effectively restrain nutrient and pollutant release from the sediment in lakes and reservoirs. Research on sediment capping has focused on the search for effective, non-polluting and affordable capping materials. The efficiency and mechanism of sediment capping with modified palygorskite in preventing sediment ammonia nitrogen (NH4+-N) release to surface water were investigated through a series of batch and sediment capping experiments. Purified palygorskite and different types of modified palygorskite (i.e. heated, acid-modified and NaCI-modified palygorskite) were used in this investigation. Factors affecting control efficiency, including the temperature, thickness and grain size of the capping layer, were also analysed. The batch tests showed that the adsorption of NH4+-N on modified palygorskite achieved an equilibration in the initial 45 min, and the adsorption isotherm followed the Freundlich equation. Sediment capping experiments showed that compared with non-capped condition, covering the sediment with modified palygorskite and sand both inhibited NH4+-N release to the overlying water. Given its excellent chemical stability and strong adsorption, heated palygorskite, which has a NH4+-N release inhibition ratio of 41.3%, is a more effective sediment capping material compared with sand. The controlling effectiveness of the modified palygorskite increases with thicker capping layer, lower temperature and smaller grain size of the capping material.