An overview of permeable reactive barriers for in situ sustainable groundwater remediation

Enhanced sustainable remediation of co-contaminated soil and groundwater with lead and arsenic utilizing the willow-vetiver-permeable reactive barrier system

Soil and groundwater pollution are closely linked, necessitating integrated remediation strategies. However, research on the combined use of phytoremediation and permeable reactive barriers (PRB) for the synergistic remediation of lead (Pb) and Arsenic (As) co-contaminated soil and groundwater remains limited. This study investigates the sustainable integrated remediation of Pb and As co-contaminated soil and groundwater through laboratory simulations, utilizing a combination of willow, vetiver grass, and PRB. The study demonstrated a significant reduction in groundwater Pb and As concentrations, from 500 μg L-1 to <0.5 μg L-1 and 0.99 to 2.04 μg L-1, respectively. Soil Pb levels decreased by 13.25-33.76 mg kg-1 compared to initial levels. Notably, plant-derived organic carbon significantly influenced the composition and content of organic carbon in groundwater, ensuring a sustainable carbon supply. Transcriptome profiling of willow leaves indicated up-regulation of genes associated with photosynthesis, arsenate reductase activity, and phagosome function, potentially enhancing As and Pb extraction. Dominant soil and groundwater bacteria, including Roseiflexaceae, Intrasporangiaceae, KD4-96, Bacillus_drentensis, and SBR 1031, exhibited Pb and As immobilization capabilities. Furthermore, GC-MS analysis showed that up-regulated carboxyl-containing organics lowered groundwater pH from >9 to 7.5 by day 35, without compromising As and Pb remediation efficiency. This research provides insights into the sustainable integrated remediation of Pb and As contamination in soil and groundwater.

Minimizing byproduct formation in bioelectrochemical denitrification with anammox bacteria

Autotrophic bioelectrochemical denitrification (BED) holds promise for nitrate remediation. However, the accumulation of byproducts such as NO2-, N2O, and NH4+, poses a significant challenge to effluent quality and climate adaptation. This study hypothesized that introducing anaerobic ammonium oxidation bacteria (anammox) to BED could alleviate this issue through synergy: a) anammox can utilize NH4+ and NO2- from BED without producing N2O, as seen in canonical denitrification, and b) BED can recycle NO3- from the anammox anabolic pathway. Results showed that Anammox_BED reduced NO2- accumulation by two-thirds, lowered the relative abundance of N2O by 80 %, and eliminated NO. Metagenomic analysis revealed that the anammox species Ca. Brocadia sapporoensis tripled in abundance in the bulk sludge. Meanwhile, Pseudomonas stutzeri and Bosea robiniae, species capable of reducing nitrate via extracellular electron transfer (EET) and supplying NO2- to anammox, halved in relative abundance, while the abundance of Stenotrophomonas acidaminiphila, a non-EET, ammonia assimilation species, doubled following anammox introduction. Metatranscriptomic analysis found upregulation of denitrification-related functional genes in Anammox_BED biofilm and survival- and motility- related genes in bulk sludge, possibly due to insufficient substrate. Overall, BED-Anammox successfully diverted the rate-limiting EET nitrite reduction towards anammox-driven nitrite utilization thereby mitigating the generation of unwanted intermediates.

Research on the traceability and treatment of nitrate pollution in groundwater: a comprehensive review

The preservation of groundwater quality is essential for maintaining the integrity of the water ecological cycle. The preservation of groundwater quality is crucial for sustaining the integrity of the water ecological cycle. Nitrate (NO3-) has emerged as a pervasive contaminant in groundwater, attracting significant research attention due to its extensive distribution and the potential environmental consequences it poses. The primary sources of NO3- pollution include soil organic nitrogen, atmospheric nitrogen deposition, domestic sewage, industrial wastewater, landfill leachate, as well as organic and inorganic nitrogen fertilizers and manure. A comprehensive understanding of these sources is imperative for devising effective strategies to mitigate NO3- contamination. Technologies for tracing NO3--polluted groundwater include hydrochemical analysis, nitrogen and oxygen isotope techniques, microbial tracers, and numerical simulations. Quantitative isotope analysis frequently necessitates the application of mathematical models such as IsoSource, IsoError, IsoConc, MixSIR, SIAR, and MixSIAR to deduce the origins of pollution. This study provides a summary of the application scenarios, as well as the strengths and limitations of these models. In terms of remediation, pump and treat and permeable reactive barrier are predominant technologies currently employed. These approaches are designed to remove or reduce NO3- concentrations in groundwater, thereby restoring its quality. The study offers a systematic examination of NO3- pollution, encompassing its origins, detection methodologies, and remediation approaches, highlighting the role of numerical simulations and integrating multidisciplinary knowledge. Additionally, this review delves into technological advancements and future trends concerning the detection and treatment of NO3- pollution in groundwater. It proposes methods to control the spread of pollution and acts as a guide for identifying and preventing pollution sources.

Controls of the Nucleation Rate and Advection Rate on Barite Precipitation in Fractured Porous Media

Mineral precipitation is ubiquitous in natural and engineered environments, such as carbon mineralization, contaminant remediation, and oil recovery in unconventional reservoirs. The precipitation process continuously alters the medium permeability, thereby influencing fluid transport and subsequent reaction kinetics. The diversity of preferential precipitation zones controls flow and transport efficiency as well as the capacity of mineral sequestration and immobilization. Taking barite precipitation as an example, previous studies have examined this process in porous and/or fractured media, but pore-scale mechanisms under varying flowing and geochemical conditions remain unexplored. In this study, we conducted real-rock microfluidic experiments to investigate the precipitation dynamics within a fractured porous system. Direct observations of the evolution of the porous structure and flow channel and quantifications of barite precipitation dynamics using X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), revealed two distinct precipitation regimes: precipitation on the fracture surface (regime I) and precipitation in the alteration zone (regime II). Through theoretical analysis of the rate of advection and nucleation, we defined a dimensionless number Da above which regime I occurs and regime II prevails otherwise. At the large Da number, when the precipitation rate is large compared with the flow rate, precipitation on the fracture surface is favored. As the precipitation regimes are expected to impact differently the permeability of the fractured porous media, the mass transfer across matrix and fractures, and the spatial distributions of coprecipitated contaminants, our work sheds light on accurately modeling reactive transport in fractured porous media across diverse applications.

Chemical aging of biochar-zero-valent iron composites in groundwater: Impact on Cd(II) and Cr(VI) co-removal

Biochar (BC), zero-valent iron (ZVI), and their composites are promising materials for use in permeable reactive barriers, although further research is needed to understand how their properties change during long-term aging in groundwater. In this study, BC, ZVI and their composites (4BC-1ZVI) were subjected to the chemical aging tests in five media (deionized water, NaCl, NaHCO3, CaCl2 and a mixture of CaCl2 and NaHCO3 solutions) for 20 days. After treatment, the microscopic analysis and performance tests for the co-removal of Cd(II) and Cr(VI) were carried out. The results indicated that the removal of Cd(II) by aged 4BC-1ZVI followed a pseudo-second-order model, whereas the removal of Cr(VI) was better fitted with a pseudo-first order model. The aging mechanism of 4BC-1ZVI was primarily governed by iron corrosion/passivation, the reduction of soluble components, and the formation of carbonate minerals. Less Fe3O4/ γ-Fe2O3 was formed during aging in deionized water, NaCl and CaCl2 solutions. The corrosion products, Fe3O4/ γ-Fe2O3, FeCO3 and α/γ-FeOOH, were observed after aging in NaHCO3 and a mixture of NaHCO3 and CaCl2 solutions. The decrease in the soluble components of biochar led to a decrease in cation exchange, while carbonate minerals contributed to Cd(II) precipitation. This work provides insights into the aging processes of BC-ZVI composites for long-term groundwater remediation applications.

New insights on zero-valent iron permeable reactive barrier for Cr(VI) removal: The function of FeS reaction zone downstream in-situ generated by sulfate-reducing bacteria

The biogeochemical behavior downstream of the zero-valent iron permeable reactive barrier (ZVI-PRB) plays an enormous positive role in the remediation of contaminated-groundwater, but has been completely neglected for a long time. Therefore, this study conducted a 240-day SRB-enhanced ZVI-PRB column experiment, focusing on what exactly happens downstream of ZVI-PRB. Results show that biosulfidation of SRB inside ZVI-PRB prolonged the complete Cr(VI) removal longevity of ZVI-PRB from 38 days to at least 240 days. More importantly, unlike previous studies that focused on improving the performance of ZVI-PRB itself, this study found an in-situ generated FeS reduction reaction zone downstream of the ZVI-PRB. When the ZVI-PRB fails, the downstream reaction zone can continue to play a role in Cr(VI) removal. The maximum Cr(VI) removal capacity of the aquifer media from the reaction zone reached 155.1 mg/kg, which was 39.7 % of commercial ZVI capacity. The reduction zone was further confirmed to be predominantly FeS rather than FeS2. Biogeochemistry occurring within and downstream of ZVI-PRB leads to the formation of FeS. Gene sequencing revealed significantly higher SRB abundance downstream of ZVI-PRB than within the ZVI-PRB. The understanding of the downstream FeS reaction zone provides new insights for more effective remediation using ZVI-PRB.

Low cost materials for fluoride removal from groundwater

In several parts of the world, high fluoride concentrations in groundwater have been reported.Fluoride concentrations above the World Health Organization's (WHO) threshold level of 1.5 mg/L in drinkable water pose a health concern for communities and the environment. The distribution of fluoride is mainly related to the geological environment: rocks that contain fluorine, for example basalt, shale, and granite, release their respective minerals containing fluoride to the groundwater by dissolution. Excessive fluoride intake leads to dental and skeletal fluorosis, fragile bones, cancer, infertility, damage to the brain function, Alzheimer syndrome, and thyroid disorder. Cheap, abundant, and locally available fluoride removal techniques are needed to meet the requirement for fluoride-free drinking water in developing countries, especially in rural communities. Different conventional methods, such as membrane technologies, ion exchange, coagulation and precipitation techniques, are employed to remove fluoride from drinking water. However, only a few of these techniques can be applied at large-scale in developing countries due to their high investment costs, high maintenance and operating costs, and the possibility of producing toxic intermediates during the treatment process. Unlike conventional methods, adsorption is a promising technology due to its simple operation in a batch or continuous systems, simple design, low-cost of operation and wide range of locally available adsorbents. Adsorption is widely applied for removing fluoride from groundwater and wastewater, effectively maintaining water quality and taste. Based on the review, adsorption stands out as the best method for fluoride removal, considering surface modification and regeneration to increase the efficiency of adsorbent materials. This makes it an ideal solution for ensuring safe drinking water in resource-limited settings.

Practical Remediation of Hg-Contaminated Groundwater by MoS2: Batch and Column Tests

Trace mercury contamination in groundwater poses a serious threat to ecological systems and human health. The kinetics and isotherms of MoS2 (MS) for Hg removal were studied in batch tests under an unfavorable high salinity and low mercury environment. Flower-like MS with nanosheets can effectively remove Hg in the groundwater matrix, with a shorter equilibrium time (3 h), superior removal efficiency (94.26%), excellent distribution coefficient (5.69 × 106 mL g-1), and higher maximum adsorption capacity (926.10 ± 165.25 mg g-1). Furthermore, the Adams-Bohart model (R2 = 0.9052-0.9416) can accurately describe the dynamic interception process of the initial stage (≤40 PVs), and the Yan model (R2 = 0.9765-0.9941) depicts the whole process (140 PVs) of MS in a fixed column well. A higher dosage of m, but lower C0 and νp facilitate the interception efficiency in column tests. Based on the characterizations of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), which were used to simultaneously consider the species of Hg and the groundwater matrix, surface complexation, electrostatic attraction, ion exchange, and precipitation is a plausible interfacial adsorption mechanism of MS for mercury. The excellent performance demonstrates that MS with nanosheets is a promising candidate for the PRB remediation of trace Hg in saline groundwater.

Effect of carbonaceous materials on phosphorus removal in flow-through packed column systems

Phosphorus (P) overloading in aquatic environments has long-been recognized as the leading cause of water quality deterioration, harmful algal bloom, and eutrophication. This study investigated P removal performance by five cost-effective carbonaceous materials (CMs) in flow-through packed column systems. These CMs include biochars pyrolyzed from feedstocks of Eucalyptus (E-biochar) and Douglas fir (D-biochar), commercial biochar (C-biochar), iron oxide-coated biochar (Fe-biochar), and commercial activated carbon (AC). The physicochemical properties of CMs, such as specific surface area (SSA), pore volume, pore diameter, elemental composition, and surface charge, were characterized. The packed column experimental results showed that P removal performance followed the order: E-biochar < D-biochar < C-biochar < Fe-biochar < AC. Specifically, the sorption capacity of 1 mg/L of P in packed columns was 0.0036 mg P/g E-biochar, 0.0111 mg P/g D-biochar, 0.0369 mg P/g D-biochar, 0.077 mg P/g Fe-biochar, and 0.088 mg P/g AC, respectively. The largest SSA (1012 m2/g) and pore volume (0.57 cm3/g) of AC accounted for the most outstanding P removal efficiency mainly by physical sorption, while electrostatic interaction explained the high P removal by Fe-biochar (SSA as low as 32.4 m2/g). Our findings provide direct practical implications for effectively removing P in water by cost-effective CMs.

Removal of cadmium and zinc by calcium polysulfide in acidic groundwater: Injection ratio and precipitation mechanism

In this study, we conducted lab-scale experiments to assess the applicability of calcium polysulfide (CPS) for the removal of cadmium (Cd2+) and zinc (Zn2+) from acidic groundwater, with an emphasis on the injection ratio and removal mechanism. For the experiments, CPS was used to treat Cd2⁺ and Zn2⁺ contaminated solution. Solutions were purged with nitrogen gas, and CPS was injected in an anaerobic chamber. After 18 h, solutions were filtered, and heavy metal concentrations were measured using ICP-MS and ICP-OES. Total polysulfide (Sx2⁻) concentration in CPS was determined by converting it to bisulfide (HS⁻) at pH 8.20 and quantifying via UV-Vis spectrophotometry. Precipitates were analyzed using XRD and SEM-EDS after centrifugation. The findings revealed that more than 99.5% of the heavy metals were removed when CPS/Cd2+ (w/w) = 1.45 and CPS/Zn2+ (w/w) = 2.50, determined as the injection ratios for maximum efficiency. These ratios were applicable when Cd2+ and Zn2+ coexisted. From the XRD and SEM-EDS analyses, it was clarified that Cd2+ and Zn2+ precipitated in sulfide forms, consistently showing the preferential precipitation of Cd2+ because of the lower solubility of cadmium sulfide compared to zinc sulfide. In addition, the concentration of Sx2- in the 29% CPS solution was determined to be approximately 2.442 M. Finally, by comparing the injected Sx2- concentration with the concentration of heavy metals removed accordingly, it was concluded that CPS and heavy metals react with a 1:1 M ratio. Based on the above results and precise quantification method, our study suggests that CPS can be effectively utilized to address heavy metal contamination issues and may serve as a valuable tool for the remediation and management of contaminated groundwater globally.

Defining quality assurance guidance for effective selection of technical grade zero-valent iron production batch for groundwater remediation using permeable reactive barrier

Zero-valent iron (ZVI) applied to the remediation of contaminated groundwater (GW) in situ, especially using engineered permeable reactive barriers (PRBs), has been proven to be an effective reactive material. However, many of ZVI brands do not represent tailored reagents specifically regarding destroying pollutants in GW. Thus, their reactivity towards certain contaminants in GW may vary significantly in a wide range even with different production batches of the same ZVI brand. This issue has rarely been known and consequently not addressed to a higher extend so far. Therefore, this study implemented extensive, long-term column experiments followed by short-term batch experiments for chlorinated volatile organic compounds (cVOCs) degradation for developing a semi-empirical test methodology to thoroughly resolve this pivotal issue by achieving an improved quality assurance guidance regarding proper field-scale emplacement of different ZVI brands and their production batches. The results showed that during column experiments perchloroethylene (PCE) led to a significant degradation up to a certain period but sulfate-reducing microorganisms enhanced the dehalogenation and led approximately to 100 % PCE removal. However, the efficacy varied for different ZVI brands, i.e., Gotthart Maier (GM) and Sponge Iron (Responge®). Furthermore, it could be shown that it might even vary among different production batches of the same ZVI brand. It was also observed that evolution of sulfate-reducing microorganisms may improve the efficacy of PCE degradation vastly that occur at different intensities with different ZVI brands and their respective production batches over time. Further, comparing comprehensive long-term column (kobs = 0.0488 1/h) and short-term batch experiments (kobs = 0.07794 1/h) as well as refined kinetic analyses (kobs = 0.0424 1/h) clearly prove that an appropriate guidance protocol for successful full-scale in situ remediation is required for properly select the right ZVI brand and production batch before it is loaded to a PRB in the field.

Monitoring carbon-based remediation of DNAPL-contaminated groundwater via spectral induced polarization

Colloidal activated carbon (CAC) is an emerging remedial enhancement fluid that is injected into the subsurface to adsorb hazardous industrial compounds for subsequent removal. CAC-enhanced remediation relies on accurate subsurface characterization and monitoring to ensure CAC reaches intended treatment locations. The objective of this study was to assess the effectiveness of the spectral induced polarization (SIP) technique to track CAC migration within porous media and its adsorption of the chlorinated solvent, tetrachloroethylene (PCE). Dynamic column experiments were performed with cyclic injection and flow of groundwater, CAC, and PCE within porous media, and simultaneous measurements of SIP and effluent quality. Results showed an increase in both the real and imaginary conductivities of the SIP response during injection/flow of CAC within porous media. Real conductivity returned to pre-CAC levels during subsequent flushing of CAC with groundwater, which had left behind only carbon-coated soil grains; however, imaginary conductivity identified the change in polarizability due to the alterations on the grain surface. The subsequent adsorption of aqueous phase PCE did not generate a distinctive change in SIP response, mainly due to the low 50 mg/L concentrations used. Overall, this study suggests that SIP can be a valuable tool to effectively and non-invasively track the migration of injected CAC within porous media for contaminant adsorption, suggesting it can be used to enhance the implementation and management of environmental remediation programs.

Ironstone and red mud barriers to reduce subsurface movement of soil phosphorus

Loss of phosphorus in seepage may contribute to eutrophication of downstream water bodies. This study examined the potential use of pedogenic ironstone and untreated red mud (bauxite refining residue) as P sorbents in a permeable reactive barrier (PRB) to mitigate such loss. Effects of ironstone and red mud on P sorption (batch), transport (columns), saturated hydraulic conductivity (KS), and growth of common bermudagrass (Cynodon dactylon; greenhouse) were examined. Both materials had sorption maxima of ∼30 mmol P kg-1 or about five times that of a P-enriched sandy soil; however, sorption by red mud greatly increased with decreasing pH. Transport of P through columns of ironstone and red mud (diluted with nonreactive sand) was similar and slower compared to soil + sand. However, when red mud was mixed with soil, increased sorption at lower pH resulted in greater P retention compared to ironstone + soil (76% vs. 13%). Although addition of ironstone to soil up to 20% did not reduce KS, red mud at even 5% did. Soil amendment with red mud increased bermudagrass growth and P uptake. Given long-term neutralization of red mud in an acidic soil and increased P sorption, it may be suitable in a PRB if incorporated at a low rate and/or co-incorporated with a coarser material.

Application of low-density polyethylene (LDPE) passive samplers for monitoring PAHs in groundwater

Equilibrium passive sampling continues to find increasing use for performing in situ assessments and monitoring of hydrophobic organic compounds (HOCs). Although this method has been successfully used in several field studies including open surface waters and sediments, comparatively, their use in groundwater has been very limited. In this study, low-density polyethylene (LDPE) passive samplers were deployed for 80 days in three groundwater wells contaminated with polycyclic aromatic hydrocarbons (PAHs). Prior to deployment, LDPE was loaded with performance reference compounds (PRCs) consisting of deuterated PAHs and their release used to ascertain system equilibrium. Within the 80-day deployment period, LDPE-groundwater equilibrium was confirmed for PAHs with molecular weights (MWs) in the range of 178 to 228 (i.e. anthracene, chrysene). Measured freely dissolved concentrations (Cw) were between one to three orders of magnitude lower than the total filtered concentrations (Ctotal) in the studied wells. The sum of PAHs (ΣPAHs) measured based on Cw and Ctotal were 2.05, 0.07 and 29.2 μg L-1 and 197, 59.7 and 1010 μg L-1, at wells 1, 2 and 3, respectively. A separate dataset, comprising long-term (2010 to 2022) concentrations of PAHs in total (i.e., unfiltered) groundwater, is also presented to provide insight into PAH contamination levels at the assessed groundwater wells based on conventional measurement. Estimated in situ LDPE daily clearance volumes (2.34 to 27.56 Ld-1) for the target analytes were far less than the daily turnover of ground water (144 to 348 Ld-1) encountered in the wells eliminating the possibility of depletive sampling of the groundwater by the passive samplers. These results represent the first published study on the practical application of equilibrium passive sampling using LDPE for monitoring and quantitatively assessing PAHs in groundwater. Also, this work demonstrates that LDPEs are a useful tool for measuring the Cw of PAHs in groundwater, a critical contaminant in many ecological and human health risk assessments.

Biochar/Biopolymer Composites for Potential In Situ Groundwater Remediation

This study explores the use of pine wood biochar (BC) waste gasified at 950 °C as fillers in polymer matrices to create BC@biopolymer composites with perspectives in groundwater remediation. Four biochar samples underwent different sieving and grinding processes and were extensively characterized via UV-Vis, FTIR, and FESEM-EDS, highlighting the fact that that BCs are essentially graphitic in nature with a sponge-like morphology. The grinding process influences the particle size, reducing the specific surface area by about 30% (evaluated by BET). The adsorption performances of raw BC were validated via an adsorption isotherm using trichloroethylene (TCE) as a model contaminant. A selected BC sample was used to produce hydrophilic, stable polymer composites with chitosan (CS), alginate (ALG), potato starch (PST), and sodium carboxymethylcellulose (CMC) via a simple blending approach. Pilot sedimentation tests over 7 days in water identified BC@PST and BC@CMC as the most stable suspensions due to a combination of both hydrogen bonds and physical entrapment, as studied by FTIR. BC@CMC showed optimal distribution and retention properties without clogging in breakthrough tests. The study concludes that biopolymer-based biochar composites with improved stability in aqueous environments hold significant promise for addressing various groundwater pollution challenges.

Theoretical assessment of influential factors and application in chlorinated hydrocarbon detection with membrane interface probe

The membrane interface probe (MIP) is an efficient and economical in-situ tool for chlorinated hydrocarbon (CH) contaminated site investigation. Given that the interpretation of MIP test is currently limited to a qualitative level, a theoretical model considering multiphase flow and multifield coupling is firstly proposed to simulate MIP test process. This model can consider phase change, membrane effect, adsorption and dissolution of the CH liquid, gas diffusion, and evaporation. Then, the model is used to study the changes in soil temperature and soil CH concentration during MIP test, as well as the influences of soil CH concentration and soil properties (initial water saturation, soil intrinsic permeability, and thermal properties) on MIP response. Finally, a simplified MIP interpretation model is developed based on parametric analysis results and verified against field and laboratory test data. It is found that the soil CH concentration, rather than soil properties, dominates the MIP response. The simplified interpretation model can deliver practical prediction of the CH concentration through the detected results by MIP, which may improve the applicability of MIP.

A review of passive acid mine drainage treatment by PRB and LPB: From design, testing, to construction

An extensive volume of acid mine drainage (AMD) generated throughout the mining process has been widely regarded as one of the most catastrophic environmental problems. Surface water and groundwater impacted by pollution exhibit extreme low pH values and elevated sulfate and metal/metalloid concentrations, posing a serious threat to the production efficiency of enterprises, domestic water safety, and the ecological health of the basin. Over the recent years, a plethora of techniques has been developed to address the issue of AMD, encompassing nanofiltration membranes, lime neutralization, and carrier-microencapsulation. Nonetheless, these approaches often come with substantial financial implications and exhibit restricted long-term sustainability. Among the array of choices, the permeable reactive barrier (PRB) system emerges as a noteworthy passive remediation method for AMD. Distinguished by its modest construction expenses and enduring stability, this approach proves particularly well-suited for addressing the environmental challenges posed by abandoned mines. This study undertook a comprehensive evaluation of the PRB systems utilized in the remediation of AMD. Furthermore, it introduced the concept of low permeability barrier, derived from the realm of site-contaminated groundwater management. The strategies pertaining to the selection of materials, the physicochemical aspects influencing long-term efficacy, the intricacies of design and construction, as well as the challenges and prospects inherent in barrier technology, are elaborated upon in this discourse.

Key role of hydrogen atoms in the preparation of sulfidated zero valent iron

Sulfidated zero valent iron (ZVI) is a popular material for the reductive degradation of halogenated organic pollutants. Simple and economic synthesis of this material is highly demanded. In this study, sulfidated micro/nanostructured ZVI (MNZVI) particles were prepared by simply heating MNZVI particles and sulfur elements (S0) in pure water (50℃). The iron oxides on the surface of MNZVI particles were conducive to sulfidation reaction, indicating the formation of iron-sulphide minerals (FeSx) on the surface of MNZVI particles might not be from the direct reaction of Fe0 with S0 (Fe0 and S0 acted as reductant and oxidant, respectively). As an important reductant, hydrogen atom (H•) can be generated from the reduction of H+ by MNZVI particles and participate in the formation of FeSx. Quenching experiment and cyclic voltammetry analysis proved the existence of H• on the surface of MNZVI particles. DFT calculation found that the potential barrier of H•/S0 and Fe0/S0 were 1.91 and 7.24 eV, respectively, indicating that S0 would preferentially react with H• instead of Fe0. The formed H• can quickly react with S0 to generate hydrogen sulfide (H2S), which can further react with iron oxides such as α-Fe2O3 on the surface of MNZVI particles to form FeSx. In addition, the H2 partial pressure in water significantly affected the amount of H• generated, thereby affecting the sulfidation efficiency. For TCE degradation, as the sulfur loading of sulfidated MNZVI particles increased, the contribution of H• significantly decreased while the contribution of direct electron transfer increased. This study provided new insights into the synthesis mechanism of sulfidated ZVI in water.

Machine learning vs. statistical model for prediction modeling and experimental validation: Application in groundwater permeable reactive barrier width design

Permeable reactive barrier (PRB) is an effective in-situ technology for groundwater remediation. The important factors in PRB design are the width and reactive material. In this study, the beaded coal mine drainage sludge (BCMDS) was employed as the filling material to adsorb arsenic pollutants in groundwater, aiming to design the width of PRB. The design methods involving traditional continue column experiments and empirical formulas, as well as machine learning (ML) predictions and statistical methods, which are compared with each other. Traditional methods are determined based on breakthrough curves under several conditions. ML method has advantages in predicting the width of mass transfer zone (WMTZ), which simultaneously consider the characteristics of material, pollutant, and environmental conditions, with data collected from articles. After data preprocessing and model optimizing, selected the XGBoost algorithm based on the high accuracy, which shows good prediction for WMTZ (R2 = 0.97, RMSE = 0.15). The experimentally derived WMTZ values were also used to validate the predictions, demonstrating the ML low error rate of 7.04 % and the feasibility. Subsequent statistical analysis of multiple linear regression (MLR) showed the error rate of 39.43 %, interpret superiority of ML due to the complexity of influencing factors and the insufficient precision of math regression. Compared to traditional width design methods, ML can improve design efficiency and save experimental time and manpower. Further expansion of the dataset and optimization of algorithms could enhance the accuracy of ML, overcoming existing limitations and gaining broader applications.

Efficient remediation of mercury-contaminated groundwater using MoS2 nanosheets in an in situ reactive zone

Mercury contamination in groundwater is a serious global environmental issue that poses threats to human and environmental health. While MoS2 nanosheets have been proven promising in removing Hg from groundwater, an effective tool for in situ groundwater remediation is still needed. In this study, we investigated the transport and retention behavior of MoS2 nanosheets in sand column, and employed the formed MoS2in situ reactive zone (IRZ) for the remediation of Hg-contaminated groundwater. Breakthrough test revealed that high flow velocity and MoS2 initial concentration promoted the transport of MoS2 in sand column, while the addition of Ca ions increased the retention of MoS2. In Hg removal experiments, the groundwater flow velocity did not influence the Hg removal capacity due to the fast reaction rate between MoS2 and Hg. With an optimized MoS2 loading, MoS2IRZ effectively reduced the Hg effluent concentration down to <1 μg/L without apparent Hg remobilization. Additionally, flake-like MoS2 employed in this study showed much better Hg removal performance than flower-like and bulk MoS2, as well as other reported materials, with the Hg removal capacity a few to tens of times higher than those materials. These results suggest that MoS2 nanosheets have the potential to be an efficient IRZ reactive material for in situ remediation of Hg in contaminated groundwater.

Stabilization of PFAS-contaminated soil with sewage sludge- and wood-based biochar sorbents

Sustainable and effective remediation technologies for the treatment of soil contaminated with per- and polyfluoroalkyl substances (PFAS) are greatly needed. This study investigated the effects of waste-based biochars on the leaching of PFAS from a sandy soil with a low total organic carbon content (TOC) of 0.57 ± 0.04 % impacted by PFAS from aqueous film forming foam (AFFF) dispersed at a former fire-fighting facility. Six different biochars (pyrolyzed at 700-900 °C) were tested, made from clean wood chips (CWC), waste timber (WT), activated waste timber (aWT), two digested sewage sludges (DSS-1 and DSS-2) and de-watered raw sewage sludge (DWSS). Up-flow column percolation tests (15 days and 16 pore volume replacements) with 1 % biochar indicated that the dominant congener in the soil, perfluorooctane sulphonic acid (PFOS) was retained best by the aWT biochar with a 99.9 % reduction in the leachate concentration, followed by sludge-based DWSS (98.9 %) and DSS-2 and DSS-1 (97.8 % and 91.6 %, respectively). The non-activated wood-based biochars (CWC and WT) on the other hand, reduced leaching by <42.4 %. Extrapolating this to field conditions, 90 % leaching of PFOS would occur after 15 y for unamended soil, and after 1200 y and 12,000 y, respectively, for soil amended with 1 % DWSS-amended and aWT biochar. The high effectiveness of aWT and the three sludge-based biochars in reducing PFAS leaching from the soil was attributed largely to high porosity in a pore size range (>1.5 nm) that can accommodate the large PFAS molecules (>1.02-2.20 nm) combined with a high affinity to the biochar matrix. Other factors like anionic exchange capacity could play a contributing role. Sorbent effectiveness was better for long-chain than for short-chain PFAS, due to weaker, apolar interactions between the biochar and the latter's shorter hydrophobic CF2-tails. The findings were the first to demonstrate that locally sourced activated wood-waste biochars and non-activated sewage sludge biochars could be suitable sorbents for the ex situ stabilization and in situ remediation of PFAS-contaminated soil, bringing this technology one step closer to full-scale field testing.

An innovative permeable reactive bio-barrier to remediate trichloroethene-contaminated groundwater: A field study

Permeable reactive bio-barrier (PRBB), an innovative technology, could treat many contaminants via the natural gradient flow of groundwater based on immobilization or transformation of pollutants into less toxic and harmful forms. In this field study, we developed an innovative PRBB system comprising immobilized Dehalococcoides mccartyi (Dhc) and Clostridium butyricum embedded into the silica gel for long-term treatment of trichloroethene (TCE) polluted groundwater. Four injection wells and two monitoring wells were installed at the downstream of the TCE plume. Without PRBB, results showed that the TCE (6.23 ± 0.43 μmole/L) was converted to cis-dichloroethene (0.52 ± 0.63 μmole/L), and ethene was not detected, whereas TCE was completely converted to ethene (3.31 μmole/L) with PRBB treatment, indicating that PRBB could promote complete dechlorination of TCE. Noticeably, PRBB showed the long-term capability to maintain a high dechlorinating efficiency for TCE removal during the 300-day operational period. Furthermore, with qPCR analysis, the PRBB application could stably maintain the populations of Dhc and functional genes (bvcA, tceA, and vcrA) at >108 copies/L within the remediation course and change the bacterial communities in the contaminated groundwater. We concluded that our PRBB was first set up for cleaning up TCE-contaminated groundwater in a field trial.

Longevity prediction of reactive media in permeable reactive barriers considering the contamination level and groundwater velocity at the planning site, with a focus on cadmium removal by zeolite

Zeolite is a versatile and effective reactive material used in permeable reactive barriers (PRBs) for remediating groundwater contaminated with heavy metals. In this study, we evaluated the influence of subsurface environmental conditions, namely contamination level (C0) and groundwater velocity (v), on predicting the longevity of zeolite for cadmium (Cd) removal. Batch experiments were performed to investigate the effect of C0 on Cd removal, and column experiments were performed to examine how Cd transportation through zeolite varies at different C0 and v. Breakthrough curves (BTCs) were analyzed with an advection-dispersion equation (ADE) coupled with nonequilibrium sorption rate models. The reaction parameters indicating the performance metrics of zeolite were determined using an iterative fitting approach-retardation factor (R), partitioning coefficient (β), and mass transfer coefficient (ω). R exhibited dependence on C0, but was unrelated to v; its rapid increase at lower C0 was explained by Langmuir sorption isotherms. β and ω, integral to sorption dynamics and mass transfer, respectively, showcased functional relationships with v. β decreased gradually as v increased, described by the nonequilibrium sorption model, whereas ω increased steadily with v, guided by the Monod function. Using the relationship of these parameters, the fate and transport of Cd within zeolite was simulated under various subsurface environmental conditions to construct the longevity prediction function. Thus, this study introduces a method for predicting the longevity of reactive materials, which can be valuable for designing PRBs with high longevity in the future.

An immobilized composite microbial material combined with slow release agents enhances oil-contaminated groundwater remediation

Microbial remediation of oil-contaminated groundwater is often limited by the low temperature and lack of nutrients in the groundwater environment, resulting in low degradation efficiency and a short duration of effectiveness. In order to overcome this problem, an immobilized composite microbial material and two types of slow release agents (SRA) were creatively prepared. Three oil-degrading bacteria, Serratia marcescens X, Serratia sp. BZ-L I1 and Klebsiella pneumoniae M3, were isolated from oil-contaminated groundwater, enriched and compounded, after which the biodegradation rate of the Venezuelan crude oil and diesel in groundwater at 15 °C reached 63 % and 79 %, respectively. The composite microbial agent was immobilized on a mixed material of silver nitrate-modified zeolite and activated carbon with a mass ratio of 1:5, which achieved excellent oil adsorption and water permeability performance. The slow release processes of spherical and tablet SRAs (SSRA, TSRA) all fit well with the Korsmeyer-Peppas kinetic model, and the nitrogen release mechanism of SSRA N2 followed Fick's law of diffusion. The highest oil removal rates by the immobilized microbial material combined with SSRA N2 and oxygen SRA reached 94.9 % (sand column experiment) and 75.1 % (sand tank experiment) during the 45 days of remediation. Moreover, the addition of SRAs promoted the growth of oil-degrading bacteria based on microbial community analysis. This study demonstrates the effectiveness of using immobilized microbial material combined with SRAs to achieve a high efficiency and long-term microbial remediation of oil contaminated shallow groundwater.

Utilizing machine learning for reactive material selection and width design in permeable reactive barrier (PRB)

Permeable reactive barrier (PRB) is an important groundwater treatment technology. However, selecting the optimal reactive material and estimating the width remain critical and challenging problems in PRB design. Machine learning (ML) has advantages in predicting evolution and tracing contaminants in temporal and spatial distribution. In this study, ML was developed to design PRB, and its feasibility was validated through experiments and a case study. ML algorithm showed a good prediction about the Freundlich equilibrium parameter (R2 0.94 for KF, R2 0.96 for n). After SHapley Additive exPlanation (SHAP) analysis, redefining the range of the significant impact factors (initial concentration and pH) can further improve the prediction accuracy (R2 0.99 for KF, R2 0.99 for n). To mitigate model bias and ensure comprehensiveness, evaluation index with expert opinions was used to determine the optimal material from candidate materials. Meanwhile, the ML algorithm was also applied to predict the width of the mass transport zone in the adsorption column. This procedure showed excellent accuracy with R2 and root-mean-square-error (RMSE) of 0.98 and 1.2, respectively. Compared with the traditional width design methodology, ML can enhance design efficiency and save experiment time. The novel approach is based on traditional design principles, and the limitations and challenges are highlighted. After further expanding the data set and optimizing the algorithm, the accuracy of ML can make up for the existing limitations and obtain wider applications.

Efficient adsorption of Cu2+ and Cd2+ from groundwater by MgO-modified sludge biochar in single and binary systems

In this study, MgO-modified sludge biochar (1MBC) prepared from sewage sludge was successfully used as an efficient adsorbent to remove heavy metals from groundwater. The adsorption performance and mechanism of 1MBC on Cu2+ and Cd2+ were investigated in single and binary systems, and the contribution of different mechanisms was quantified. Adsorption kinetics and isotherms analysis revealed that the adsorption processes of Cu2+ and Cd2+ by 1MBC followed the pseudo-second-order kinetic and Langmuir isotherm model in both systems, indicating that Cu2+ and Cd2+ were mainly controlled by chemisorption, and their theoretical maximum adsorption capacities were 240.36 and 219.06 mg·g-1, respectively. The results of the binary system showed that due to the competitive adsorption, the adsorption capacity of 1MBC for both heavy metals was lower than that of the single system, and the selective adsorption of Cu2+ was higher. The influencing variable experiments revealed that the adsorption of Cu2+ and Cd2+ by 1MBC had a wide pH adaption range and strong anti-interference ability to coexisting organics and ions. The adsorption mechanisms involved ion exchange (Cu: 47.39%, Cd: 53.17%), mineral precipitation (Cu: 35.31%, Cd: 24.18%), functional group complexation (Cu: 10.44%, Cd: 14.53%), and other possible mechanisms (Cu: 6.87%, Cd: 8.12%). Furthermore, 1MBC demonstrated excellent regeneration potential after five cycle times. Overall, the results have significant reference value for the practical application of removing heavy metals.

An overview of in situ remediation for groundwater co-contaminated with heavy metals and petroleum hydrocarbons

Groundwater is an important component of water resources. Mixed pollutants comprising heavy metals (HMs) and petroleum hydrocarbons (PHs) from industrial activities can contaminate groundwater through such processes as rainfall infiltration, runoff and discharge, which pose direct threats to human health through the food chain or drinking water. In situ remediation of contaminated groundwater is an important way to improve the quality of a water environment, develop water resources and ensure the safety of drinking water. Bioremediation and permeable reactive barriers (PRBs) were discussed in this paper as they were effective and affordable for in situ remediation of complex contaminated groundwater. In addition, media types, technology combinations and factors for the PRBs were highlighted. Finally, insights and outlooks were presented for in situ remediation technologies for complex groundwater contaminated with HMs and PHs. The selection of an in situ remediation technology should be site specific. The remediation of complex contaminated groundwater can be approached from various perspectives, including the development of economical materials, the production of slow-release and encapsulated materials, and a combination of multiple technologies. This review is expected to provide technical guidance and assistance for in situ remediation of complex contaminated groundwater.

Current and emerging technologies for the remediation of difficult-to-measure radionuclides at nuclear sites

Difficult-to-measure radionuclides (DTMRs), defined by an absence of high energy gamma emissions during decay, are problematic in groundwaters at nuclear sites. DTMRs are common contaminants at many nuclear facilities, with (often) long half-lives and high radiotoxicities within the human body. Effective remediation is, therefore, essential if nuclear site end-state targets are to be met. However, due to a lack of techniques for in situ DTMR detection, technologies designed to remediate these nuclides are underdeveloped and tend to be environmentally invasive. With a growing agenda for sustainable remediation and reduction in nuclear decommissioning costs, there is renewed international focus on the development of less invasive technologies for DTMR clean-up. Here, we review recent developments for remediation of selected problem DTMRs (129I, 99Tc, 90Sr and 3H), with a focus on industrial and site-scale applications. We find that pump and treat (P&T) is the most used technique despite efficacy issues for 129I and 3H. Permeable reactive barriers (PRBs) are a less invasive alternative but have only been demonstrated for removal of 99Tc and 90Sr at scale. Phytoremediation shows promise for site-scale removal of 3H but is unsuitable for 129I and 99Tc due to biotoxicity and bioavailability hazards, respectively. No single technique can remediate all DTMRs of focus. Likewise, there has been no successful site-applied technology with high removal efficiencies for iodine species typically present in groundwaters (iodide/I-, iodate/IO3- and organoiodine). Further work is needed to adapt and improve current techniques to field scales, as well as further research into targeted application of emerging technologies.

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.

Monitoring the remediation of groundwater polluted by MSW landfill leachates by activated carbon and zeolite with spectral induced polarization technique

The municipal solid waste (MSW) landfill in Hangzhou, China utilized zeolite and activated carbon (AC) as permeable reactive barrier (PRB) fill materials to remediate groundwater contaminated with MSW leachates containing ammonium, chemical oxygen demand (COD), and heavy metals. The spectral induced polarization (SIP) technique was chosen for monitoring the PRB because of its sensitivity to pore fluid chemistry and mineral-fluid interface composition. During the experiment, authentic groundwater collected from the landfill site was used to permeate two columns filled with zeolite and AC, and the SIP responses were measured at the inlet and outlet over a frequency range of 0.01-1000 Hz. The results showed that zeolite had a higher adsorption capacity for COD (7.08 mg/g) and ammonium (9.15 mg/g) compared to AC (COD: 2.75 mg/g, ammonium: 1.68 mg/g). Cation exchange was found to be the mechanism of ammonium adsorption for both zeolite and AC, while FTIR results indicated that π-complexation, π-π interaction, and electrostatic attraction were the main mechanisms of COD adsorption. The Cole-Cole model was used to fit the SIP responses and determine the relaxation time (τ) and normalized chargeability (mn). The calculated characteristic diameters of zeolite and AC based on the Schwarz equation and relaxation time (τ) matched the pore sizes observed from SEM and MIP, providing valuable information on contaminant distribution. The mn of zeolite was positively linear with adsorbed ammonium (R2 = 0.9074) and COD (R2 = 0.8877), while the mn of AC was negatively linear with adsorbed ammonium (R2 = 0.8192) and COD (R2 = 0.7916), suggesting that mn could serve as a surrogate for contaminant saturation. The laboratory-based real-time non-invasive SIP results showed good performance in monitoring saturation and provide a strong foundation for future field PRB monitoring.

Review on heavy metal contaminants in freshwater fish in South India: current situation and future perspective

The primary natural resource we use in our daily lives for a variety of activities is freshwater for drinking and various developmental goals. Furthermore, the pace of human population increase worldwide is rising rapidly and has a great impact on the Earth's natural resources. Natural water quality has diminished owing to various anthropogenic activities. Water is crucial to the life cycle. On the other hand, chemical and agricultural industries pollute heavy metals. Acute and chronic diseases caused by heavy metals, such as slow metabolism and damage to the gills and epithelial layer of fish species, are divided into two categories. Pollutants can also harm liver tissues and result in ulceration as well as diseases such as fin rot, tail rot, and gill disease. The most prevalent heavy metals are As, Cr, Pb, and Hg, which are systemic toxicants that affect human health. These metals are categorized as carcinogens by the US Environmental Protection Agency and the worldwide agency for cancer research because they cause organ damage even at low exposure levels. The focus of the current study is to review various freshwater sources of heavy metal pollution.

Remediation of lead-contaminated groundwater by oyster shell powder-peanut shell biochar mixture

Groundwater pollution caused by lead ions has become a widespread issue worldwide due to the ever-increasing development of industrial activities. Such pollution poses significant threats to both humans and the environment. Oyster shell powder-peanut shell biochar mixture (OSP-PSB mixture) was used for lead-contaminated groundwater treatment by permeable reactive barrier (PRB) technology. Basic characteristics of materials proved that OSP-PSB mixture has good adsorption properties; OSP with particle sizes ranging from 0.85 to 1.18 mm was used in this research; according to engineering and adsorption characteristics, OSP-PSB mixture (5:1) showed excellent permeability (4.35 × 10-4 cm/s) and lead adsorption capacity(27 mg/g); long-term permeability of the OSP-PSB mixture slightly decreased over time and met the permeability requirements for PRB; the removal mechanisms of lead ions by OSP-PSB mixture include precipitation, surface complexation, ion exchange, and physical adsorption. The experiment results showed that the OSP-PSB mixture fulfills the actual project requirements of PRB.

Cork barriers for the remediation of soils polluted with lindane

The in-situ removal of lindane from spiked soil was studied using cork barriers combined with electrokinetic and ohmic heating soil remediation processes. Both vertical and horizontal cork barriers have been evaluated to retain pollutants mobilized by electro-osmotic flow or volatilized by ohmic heating. Moreover, the addition of surfactant solutions in electrolyte wells has been evaluated to promote the dragging of lindane by electrokinetic fluxes. Results indicated that the drag of lindane by liquid flows is not as important as expected, opposite to what happened with the dragging by gaseous flows. The retention of gaseous lindane was also confirmed in adsorption tests carried out in a column packed with cork granules. The addition of surfactant had a very limited effect on the mobility of lindane, and dragging of this species to the electrode wells or to a permeable reactive barrier. On the contrary, the reactivity of lindane during the electrochemical treatments is relevant due to the electrokinetic basic front promoting the in-situ conversion of lindane into less chlorinated pollutants.

Low-cost treatment method for organic matter and nutrients in landfill leachate

In this study, the ability of low-cost composite adsorbents to treat organic compounds in terms of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) was investigated. The composite adsorbents were composed of washed sea sand (WSS), dewatered alum sludge (DAS), zero-valent iron (ZVI), and granular activated carbon (GAC). The removal efficiency of COD in landfill leachate by a composite adsorbent (composed of WSS (40%), DAS (40%), ZVI (10%), and GAC (10%) in weight) was 79.93 ± 1.95%. The corresponding adsorption capacity was 8.5 mg/g. During batch sorption experiments, the maximum COD removal efficiencies given by DAS, WSS, ZVI, and GAC were 16, 51.3, 42, and 100.0%, respectively. The maximum removal efficiencies of the above composite adsorbent for TN and TP were 84.9 and 97.4%, respectively, and the adsorption capacities were 1.85 and 0.55 mg/g, respectively. The Elovich isotherm model gave the best fit for COD, TN, and TP adsorption. This composite adsorbent can treat more than one contaminant simultaneously. The application of DAS and ZVI to make an efficient adsorbent for wastewater treatment would be a good re-use application for them, which would otherwise be landfilled directly after their generation.

A passive sink-zeolite permeable reactive barrier to control NH4+-N pollution plume within groundwater: Conceptual design and numerical modeling

Ammonium nitrogen (NH₄⁺-N) is a typical inorganic pollutant in the groundwater at landfill sites, and high-concentration NH₄⁺-N is toxic to humans and organisms. Zeolite can effectively remove NH₄⁺-N in water by adsorption, and it is suitable to be used as a type of reactive materials for permeable reactive barriers (PRBs). A passive sink-zeolite PRB (PS-zPRB) with higher capture efficiency than a continuous permeable reactive barrier (C-PRB) was proposed. And a passive sink configuration was integrated with PRB in the PS-zPRB, this configuration enabled the high hydraulic gradient of groundwater at the treated sites to be fully utilized. In order to explore treatment efficiency for groundwater NH₄⁺-N using the PS-zPRB, numerical modeling on decontamination of NH₄⁺-N plumes at a landfill site was performed. The results indicated that the NH₄⁺-N concentrations of PRB effluent gradually decreased from 21.0 mg/L to 0.5 mg/L within 5 y, and met the drinking water standards after treatment for 900 d. The decontamination efficiency index of PS-zPRB was consistently higher than 95% within 5 y, and the service life of PS-zPRB would be over 5 y. The capture width of PS-zPRB effectively exceeded the PRB length by around 50%. Compared with C-PRB, the capture efficiency of PS-zPRB was increased by around 28%, and the reactive material of PS-zPRB was saved by approximately 23% in volume.

Contaminant containment for sustainable remediation of persistent contaminants in soil and groundwater

Contaminant containment measures are often necessary to prevent or minimize offsite movement of contaminated materials for disposal or other purposes when they can be buried or left in place due to extensive subsurface contamination. These measures can include physical, chemical, and biological technologies such as impermeable and permeable barriers, stabilization and solidification, and phytostabilization. Contaminant containment is advantageous because it can stop contaminant plumes from migrating further and allow for pollutant reduction at sites where the source is inaccessible or cannot be removed. Moreover, unlike other options, contaminant containment measures do not require the excavation of contaminated substrates. However, contaminant containment measures require regular inspections to monitor for contaminant mobilization and migration. This review critically evaluates the sources of persistent contaminants, the different approaches to contaminant remediation, and the various physical-chemical-biological processes of contaminant containment. Additionally, the review provides case studies of contaminant containment operations under real or simulated field conditions. In summary, contaminant containment measures are essential for preventing further contamination and reducing risks to public health and the environment. While periodic monitoring is necessary, the benefits of contaminant containment make it a valuable remediation option when other methods are not feasible.

Contribution of non-point source pollution that migrated with underground runoff process based on the SWAT model and a digital filter algorithm

Non-point source (NPS) pollution has always been the focus of research worldwide, and understanding the migration process is the basis for effective control of NPS pollution. In this study, the SWAT model and digital filtering algorithm were combined to explore the contribution of NPS pollution that migrated with underground runoff (UR) process to the Xiangxi River watershed. The results showed that the surface runoff (SR) was the main migration process of NPS pollution, while the contribution of NPS pollution that migrated with the UR process only accounted for 30.9%. With the decrease in annual precipitation among the three selected hydrological years, the proportion of NPS pollution that migrated with the UR process for TN decreased, whereas the proportion for TP increased. The contribution of NPS pollution migrated with UR process varied remarkably during different months. Although the maximum total load and the load of NPS pollution that migrated with the UR process for TN and TP all appeared in the wet season, due to the hysteresis effect, the load of NPS pollution that migrated with the UR process for TP appeared 1 month later than the total load of NPS pollution. With an increase in precipitation from the dry season to the wet season, the proportion of NPS pollution that migrated with the UR process for TN and TP decreased gradually, and the degree of decrease in NPS pollution that migrated with the UR process for TP was more evident than that for TN. Besides, being affected by topography, land use, and other factors, the proportion of NPS pollution that migrated with the UR process for TN decreased from 80% in upstream areas to 9% in downstream areas, while that for TP reached a maximum of 20% in downstream areas. Based on the research results, the contribution of soil and groundwater cumulative nitrogen and phosphorus should be considered, and different managements and control measures for different migration routes should be adopted in controlling pollution.

Identification of nitrate sources of groundwater and rivers in complex urban environments based on isotopic and hydro-chemical evidence

Groundwater and rivers in Chinese cities suffer from severe nitrate pollution. The accurate identification of nitrate sources throughout aquatic systems is key to the water nitrate pollution management. This study investigated nitrogen components of groundwater for twelve years and analyzed the sources of nitrate in the aquatic system based on dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in the city of Nanjing, a core city of the Yangtze River Delta region, China. Our results showed that the ratio of nitrate to the sum of ammonia and nitrate in groundwater show an increasing trend during 2010-2021. The nitrate concentration was positively correlated with the proportion of cultivated land and negatively correlated with the proportion of forest land in the buffer zone. The relationship between Cl^- and NO₃^-/ Cl^- showed that agriculture and sewage sources increased during 2010-2015, sewage sources increased during 2016-2018, agriculture sources increased during 2019-2021. Manure and sewage were the primary sources of groundwater nitrate (72 %). There was no significant difference between the developed land (78 %), cultivated land (69 %), and aquaculture area (72 %). This indicates that dense population and intensive aquaculture in the suburbs have a significant impact on nitrate pollution. The contributions of manure and sewage to the fluvial nitrate sources in the lower reaches of the Qinhuai River Basin were 61 %. The non-point sources, including groundwater N (39 %) and soil N (35 %), were 74 % over the upper reaches. This study highlights the necessity of developing different N pollution management strategies for different parts of highly urbanized watersheds and considers groundwater restoration and soil nitrogen management as momentous, long-term tasks.

Groundwater arsenic contamination: impacts on human health and agriculture, ex situ treatment techniques and alleviation

Groundwater is consumed by a large number of people as their primary source of drinking water globally. Among all the countries worldwide, nations in South Asia, particularly India and Bangladesh, have severe problem of groundwater arsenic (As) contamination so are on our primary focus in this study. The objective of this review study is to provide a viewpoint about the source of As, the effect of As on human health and agriculture, and available treatment technologies for the removal of As from water. The source of As can be either natural or anthropogenic and exposure mediums can either be air, drinking water, or food. As-polluted groundwater may lead to a reduction in crop yield and quality as As enters the food chain and disrupts it. Chronic As exposure through drinking water is highly associated with the disruption of many internal systems and organs in the human body including cardiovascular, respiratory, nervous, and endocrine systems, soft organs, and skin. We have critically reviewed a complete spectrum of the available ex situ technologies for As removal including oxidation, coagulation-flocculation, adsorption, ion exchange, and membrane process. Along with that, pros and cons of different techniques have also been scrutinized on the basis of past literatures reported. Among all the conventional techniques, coagulation is the most efficient technique, and considering the advanced and emerging techniques, electrocoagulation is the most prominent option to be adopted. At last, we have proposed some mitigation strategies to be followed with few long and short-term ideas which can be adopted to overcome this epidemic.

A comprehensive review on permeable reactive barrier for the remediation of groundwater contamination

Groundwater quality is deteriorating due to contamination from both natural and anthropogenic sources. Traditional "Pump and Treat" techniques of treating the groundwater suffer from the disadvantages of a small-scale and energy-intensive approach. Permeable reactive barriers (PRBs), owing to their passive operation, offer a more sustainable strategy for remediation. This promising technique focuses on eliminating heavy metal pollutants and hazardous aromatic compounds by physisorption, chemisorption, precipitation, denitrification, and/or biodegradation. Researchers have utilized ZVI, activated carbon, natural and manufactured zeolites, and other by-products as reactive media barriers. Environmental parameters, i.e., pH, initial pollutant concentration, organic substance, dissolved oxygen, and reactive media by-products, all influence a PRB's performance. Although their long-term impact and performance are uncertain, PRBs are still evolving as viable alternatives to pump-and-treat techniques. The use of PRBs to remove anionic contaminants (e.g., Fluoride, Nitrate, etc.) has received less attention since precipitates can clog the reactive barrier and hinder groundwater flow. In this paper, we present an insight into this approach and the tremendous implications for future scientific study that integrates this strategy using sustainability and explores the viability of PRBs for anionic pollutants.

Effects of injection directions and boundary exchange times on adaptive pumping in heterogeneous porous media: Pore-scale simulation

Adaptive pumping, changing pumping rates or exchanging injection and extraction wells, is an enhancement of traditional Pump-and-Treat (P&T) technology. Since most previous studies on adaptive pumping are conducted through field-scale simulations, the mechanism behind it is not fully understood. An in-depth investigation of the pore-scale remediation mechanism of adaptive pumping is undoubtedly helpful in combining it with other decontamination methods to further enhance the remediation efficiency. In this study, coupling the Cahn-Hilliard phase field method and the Navier-Stokes equations, the dynamic displacement process in a heterogeneous porous medium is obtained. The effects of initial injection direction, boundary exchange times, and displacement regimes on the interface evolution and the remediation efficiency are systematically investigated. The results present that a significant increase in phase interface area is the most critical remediation mechanism for adaptive pumping. The effects of injection directions and boundary exchange times on remediation performance are mainly determined by the differences in pore connectivity and flow parameters. Higher pore connectivity under high and low viscosity ratios inhibits and promotes remediation performance, respectively. At high viscosity ratios, the residual oil morphology in the matrix after adaptive pumping is similar to that obtained by positive pumping with the opposite initial injection direction. The improvement in remediation performance of adaptive pumping is more significant under low viscosity ratio conditions. These results provide new pore-scale insights into the remediation mechanism of adaptive pumping, which contribute to the design and application of innovative remediation methods.

Solution, exchangeable and fixed ammonium in natural diatomite as a simulated PRB material: effects of adsorption and bioregeneration processes

Ammonia nitrogen (NH₄⁺-N) is widely found in aquifers with strong reducibility or poor adsorptivity as a dissolved inorganic nitrogen pollutant. The application of adsorbents with effective long-term in situ bioregeneration as permeable reactive barrier (PRB) media for nitrogen removal has raised concern. In this study, the advantage of natural diatomite as a PRB material was investigated by exploring its NH₄⁺-N adsorption and desorption characteristics, and the ability of diatomite and zeolite to be loaded nitrifying bacteria was also compared. The results showed that the exchangeable ammonium from chemical-monolayer adsorption was the main form of NH₄⁺-N and was adsorbed by diatomite. Moreover, the adsorption process was limited with a maximum adsorption capacity of 0.677 mg g^-1. However, diatomite demonstrated an excellent loading of aerobic-heterotrophic microorganisms, even stronger than zeolite. Compared with zeolite reactors, a higher OD₆₀₀ value of nitrifiers, a faster NH₄⁺-N degradation rate and more abundant functional genes were observed during the bioregeneration process of diatomite. Both the solution and exchangeable ammonium forms were bioavailable, and the regeneration of diatomite was more than 80.0% after two days. Moreover, desorption-biodegradation was systematically analysed to determine the bioregeneration mechanism of diatomite. Diatomite with good regeneration ability can be used as a competitive alternative to address sudden nitrogen pollution.

Groundwater contamination status in Malaysia: level of heavy metal, source, health impact, and remediation technologies

Groundwater is defined as water that exists underground in voids or gaps in sediments and is extracted for human consumption from aquifers. It is critical to our daily lives because it contributes to the sustainability of our natural ecosystem while also providing economic benefits. Heavy metals are metallic compounds with a relatively high atomic weight and density compared to water. In Malaysia, heavy metal contamination of groundwater has become a concern due to rapid population growth, economic development, and a lack of environmental awareness. Environmental factors or their behaviors, such as density, viscosity, or volume, affect the distribution and transportation of heavy metals. The article discusses the difficulties created by the presence of heavy metals in groundwater supplies and the resulting health problems. Additionally, remediation methods are discussed for managing contaminated water to preserve the ecological environment for current and future generations, as well as their advantages and disadvantages.

Long-term performance evaluation of zero-valent iron amended permeable reactive barriers for groundwater remediation - A mechanistic approach

Permeable reactive barriers (PRBs) are used for groundwater remediation at contaminated sites worldwide. This technology has been efficient at appropriate sites for treating organic and inorganic contaminants using zero-valent iron (ZVI) as a reductant and as a reactive material. Continued development of the technology over the years suggests that a robust understanding of PRB performance and the mechanisms involved is still lacking. Conflicting information in the scientific literature downplays the critical role of ZVI corrosion in the remediation of various organic and inorganic pollutants. Additionally, there is a lack of information on how different mechanisms act in tandem to affect ZVI-groundwater systems through time. In this review paper, we describe the underlying mechanisms of PRB performance and remove isolated misconceptions. We discuss the primary mechanisms of ZVI transformation and aging in PRBs and the role of iron corrosion products. We review numerous sites to reinforce our understanding of the interactions between groundwater contaminants and ZVI and the authigenic minerals that form within PRBs. Our findings show that ZVI corrosion products and mineral precipitates play critical roles in the long-term performance of PRBs by influencing the reactivity of ZVI. Pore occlusion by mineral precipitates occurs at the influent side of PRBs and is enhanced by dissolved oxygen and groundwater rich in dissolved solids and high alkalinity, which negatively impacts hydraulic conductivity, allowing contaminants to potentially bypass the treatment zone. Further development of site characterization tools and models is needed to support effective PRB designs for groundwater remediation.

Nitrate removal rates, isotopic fractionation, and denitrifying bacteria in a woodchip-based permeable reactive barrier system: a long-term column experiment

This study evaluated the effectiveness of using organic carbon materials (i.e., woodchips) to remove nitrate from groundwater. The results of our flow-through column experiment, which was conducted over 1.6 years, suggested that denitrifying bacteria reduce nitrate by using it as an electron acceptor and woodchips as an electron donor. The nitrate removal rates were sufficiently high (0.39-1.04 mmol L-1 day-1) during the operation of the column. Denitrification process was supported by fractionation of nitrogen and oxygen isotopes (δ15N and δ18O), with the δ15N and δ18O values enriched from 7.4‰ and 22.3‰ to 21.2‰ and 30.4‰, respectively. Enrichment factors ([Formula: see text]) for 15 N and 18O were calculated using the Rayleigh fractionation model, with values of - 13.2‰ for ε15N and - 7.1‰ for ε18O. The fractionation ratio of 15 N to 18O was 1.9:1, confirming denitrification. The most abundant bacterial genera in the soil used for inoculation were Enterobacter (86.7%), Nitrospira (1.8%), and Arthrobacter (1.5%), while those in the column effluent were Macrococcus (37.1%), Escherichia (14.7%), and Shigella (14.6%), indicating that bacterial communities changed in response to geochemical conditions in the column. This study suggests that nitrate in groundwater can be effectively removed using woodchip-based passive treatment systems and that information on isotopic fractionation and denitrifying bacteria can be key tools to understand denitrification.

Performance of field-scale permeable reactive barriers: An overview on potentials and possible implications for in-situ groundwater remediation applications

Permeable reactive barriers (PRBs) are significant among all the promising remediation technologies for treating contaminated groundwater. Since the first commercial full field-scale PRB emplacement in Sunnyvale, California, in 1994-1995, >200 PRB systems have been installed worldwide. The main working principle of a PRB is to treat a variety of contaminants downstream from the contaminated source zone ("hot spot"). However, to accurately assess the longevity of PRBs, it is essential to know the total contaminant mass in the source area and its approximate geometry. PRBs are regarded as both a safeguarding and an advanced decontamination technique, depending on the contamination scenario and its outcome during the operational lifetime of the barrier. In the last three decades, many PRBs have performed very well, that is, met expected clean-up goals at a variety of contaminated sites. However, there is still the necessity of thoroughly evaluating the implications of the performance of different PRB designs and reactive or adsorptive materials worldwide. Therefore, this study presents a comprehensive overview of field-scale PRBs applications and their long-term performance after on-site emplacements. This paper provides in-depth insight into this passive in-situ remediation technology for treating and even eliminating a contaminated plume over a long time in the subsurface. The overview will help all stakeholders worldwide understand the implications of PRBs and guide them to take all the required measures before its on-site application to avoid any potential failure.

Assessment of sulfidated nanoscale zerovalent iron for in-situ remediation of cadmium-contaminated acidic groundwater at a zinc smelter

Unprecedented high concentrations of heavy metals have been detected in the groundwater at a zinc smelter in Seokpo, South Korea. The outflow of the contaminated groundwater into the nearby Nakdong River must be prevented by some means such as permeable reactive barrier (PRB). As a reactive material for injection-type PRB, we have tested sulfidated nanoscale zerovalent iron (S-nZVI) to assess its efficacy in remediating the groundwater from the smelter. The S-nZVI efficiently removed Zn, Ni, and Al in the groundwater, and neutralized the groundwater to pH > 6. Sulfidation of nZVI greatly increased the removal of Cd (99.8%) compared to that by nZVI (7.2%). MINEQL+ modeling and particle characterization were performed to elucidate the forms of heavy metals in the solution and on the surface of S-nZVI. Raman and XPS results suggested that FeS on the surface of S-nZVI reacted with Cd(II) and Zn(II), forming more-stable CdS and ZnS. Sequential application of NaHCO₃ after S-nZVI treatment in a column setup was suited for the removal of remaining Zn and Fe as well as the reduction of microbial toxicity. This study guides to use of S-nZVI for in-situ remediation of cadmium-contaminated groundwater with other coexisting heavy metals from a zinc smelter.

Efficient and selective capture of uranium by polyethyleneimine-modified chitosan composite microspheres from radioactive nuclear waste

Uranium extraction from radioactive nuclear waste is vital for sustainable energy supply and ecological security. Herein, a polyethyleneimine-chitosan composite microspheres n-PEI/ECH-CTS (n = 0.1, 0.2, 0.3, 0.4, 0.5) were synthetized for efficient and selective uranium adsorption. The prepared chitosan microspheres with uniform size, uniform dispersion and good mechanical strength combine cost-effectiveness and environmental benefits. The 0.4-PEI/ECH-CTS exhibits the highest adsorption capacity of 380.65 mg g^-1 within only 4 h due to high nitrogen content of 6.57 mol kg^-1. The DFT calculations confirms that the optimal coordination mode of UO₂²⁺ and 0.4-PEI/ECH-CTS is one UO₂²⁺ chelated with two -NH₂ from two adsorption units, respectively. Adsorption efficiency of U(VI) from simulated nuclear wastewater achieves to 100%, and the K_d value is up to 1.1 × 10⁴ mL g^-1, which is 1.7 × 10⁴-6.1 × 10⁴ times that of coexisting ions. The C_U(VI) reduces in simulated wastewater from 10.98 mg L^-1 to 1 μg L^-1, which is well below the US Environmental Protection Agency uranium limits for drinking water (30 μg L^-1). Besides, 0.4-PEI/ECH-CTS still maintains above 95% adsorption efficiency after seven cycles. In short, the 0.4-PEI/ECH-CTS microspheres integrate high performance, practicality and cost-effectiveness, which has great advantages in practical industrial applications.

Preparation of a sulfonated coal@ZVI@chitosan-acrylic acid composite and study of its removal of groundwater Cr(VI)

In this research, a new composite adsorbent (SC@ZVI@CS-AA) was designed and synthesized, and its application for the removal of Cr(VI) in groundwater was investigated. The interaction between SC@ZVI@CS-AA and Cr(VI) conformed to a pseudo-second-order model, and the adsorption process was dominated by chemisorption. The effects of material ratios, pH, temperature, SC@ZVI@CS-AA dosage, and coexisting ions on the removal of Cr(VI) were investigated. The removal efficiency of Cr(VI) by SC@ZVI@CS-AA reached 95%, and the reaction was significantly inhibited when SO₄^2- was present. Thermodynamically, the adsorption of Cr(VI) proceeded spontaneously above 35 °C (ΔG^θ < 0). According to scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometry, and synchronous thermal analysis, the removal mechanism of Cr(VI) by SC@ZVI@CS-AA was attributed to electrostatic attraction and reduction. In addition, SC@ZVI@CS-AA had good cyclic adsorption performance. Overall, the SC@ZVI@CS-AA composite showed great potential in the remediation of Cr(VI)-contaminated groundwater.

Evaluation the feasibility of using clinoptilolite as a gravel pack in water wells for removal of lead from contaminated groundwater

The ability of clinoptilolite zeolite as a filter in water wells to remove lead from polluted groundwater was tested in batch and fixed-bed column experiments. XRF, XRD, SEM, and BET were used to characterize the zeolite. Because of the pH variation in groundwater, batch experiments were performed at pH = 6, 7, and 8, with the highest removal efficiency (84.2%) at pH = 6 and 298 K within 90 min. The Freundlich model accurately predicted metal ion adsorption behavior and indicated a multilayer adsorption of Pb(II) molecules on the inhomogeneous surface of clinoptilolite. The best-fitting kinetic model for clinoptilolite is the pseudo-second order equation, highlighting that the rate of adsorption is dependent on absorbent capacity. Next, the effect of flow rate, bed depth, and grain size of clinoptilolite on lead removal was investigated in column experiments at an initial concentration of 450 mg pb/L. The highest removal efficiency was achieved in column experiments with a flow rate of 1 mL/min, a bed height of 10 cm, and a grain size of 0.6 to 0.8 mm. Breakthrough curves were predicted by the Thomas and Yoon-Nelson models, with excellent agreement with the corresponding experimental data. This research will be used to develop a new in situ remedial approach for removing lead from polluted groundwater.