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

Permeable reactive barriers (PRBs) are one of the innovative technologies widely accepted as an alternative to the 'pump and treat' (P&T) for sustainable in situ remediation of contaminated groundwater. The concept of the technology involves the emplacement of a permeable barrier containing reactive materials across the flow path of the contaminated groundwater to intercept and treat the contaminants as the plume flows through it under the influence of the natural hydraulic gradient. Since the invention of PRBs in the early 1990s, a variety of materials has been employed to remove contaminants including heavy metals, chlorinated solvents, aromatic hydrocarbons, and pesticides. Contaminant removal is usually accomplished via processes such as adsorption, precipitation, denitrification and biodegradation. Despite wide acknowledgment, there are still unresolved issues about long term-performance of PRBs, which have somewhat affected their acceptability and full-scale implementation. The current paper presents an overview of the PRB technology, which includes the state of art, the merits and limitations, the reactive media used so far, and the mechanisms employed to transform or immobilize contaminants. The paper also looks at the design, construction and the long-term performance of PRBs.

Evaluation methods for assessing effectiveness of in situ remediation of soil and sediment contaminated with organic pollutants and heavy metals

Soil and sediment contamination has become a critical issue worldwide due to its great harm to the ecological environment and public health. In recent years, many remediation technologies including physical, chemical, biological, and combined methods have been proposed and adopted for the purpose of solving the problems of soil and sediment contamination. However, current research on evaluation methods for assessing these remediation technologies is scattered and lacks valid and integrated evaluation methods for assessing the remediation effectiveness. This paper provides a comprehensive review with an environmental perspective on the evaluation methods for assessing the effectiveness of in situ remediation of soil and sediment contaminated with organic pollutants and heavy metals. The review systematically summarizes recent exploration and attempts of the remediation effectiveness assessment based on the content of pollutants, soil and sediment characteristics, and ecological risks. Moreover, limitations and future research needs of the practical assessment are discussed. These limitations are not conducive to the implementation of the abatement and control programs for soil and sediment contamination. Therefore, more attention should be paid to the evaluation methods for assessing the remediation effectiveness while developing new in situ remediation technologies in future research.

A review of the environmental implications of in situ remediation by nanoscale zero valent iron (nZVI): Behavior, transport and impacts on microbial communities

The increasing use of strategies incorporating nanoscale zero valent iron (nZVI) for soil and groundwater in situ remediation is raising some concerns regarding the potential adverse effects nZVI could have on indigenous microbial communities and ecosystem functioning. This review provides an overview of the current literature pertaining to the impacts of nZVI applications on microbial communities. Toxicity studies suggest that cell membrane disruption and oxidative stress through the generation of Fe(2+) and reactive oxygen species by nZVI are the main mechanisms contributing to nZVI cytotoxicity. In addition, nZVI has been shown to substantially alter the taxonomic and functional composition of indigenous microbial communities. However, because the physico-chemical conditions encountered in situ highly modulate nZVI toxicity, a better understanding of the environmental factors affecting nZVI toxicity and transport in the environment is of primary importance in evaluating the ecological consequences that could result from a more extensive use of nZVI.

Effect of multi-walled carbon nanotubes on phytotoxicity of sediments contaminated by phenanthrene and cadmium

To implement effective control and abatement programs for contaminants accumulating in sediments, strategies are needed for evaluating the quality of amended sediments. In this study, phytotoxicity of the sediments contaminated by cadmium and phenanthrene was evaluated after in situ remediation with multi-walled carbon nanotubes (MWCNTs) as adsorbents. Adsorption experiments and measurement of aqueous concentrations of the contaminants in overlying water were used to investigate the remediation effectiveness from physical and chemical aspects. The results indicated that MWCNTs showed a much better adsorption performance towards phenanthrene and Cd(II) compared with the sediments. The in situ remediation with MWCNTs could distinctly decrease the aqueous concentrations of phenanthrene and Cd(II) released from the sediments, reducing environmental risk towards overlying water. Influences of MWCNTs dose, MWCNTs diameter, and contact time on phtotoxicity of the contaminated sediments were studied. No significant inhibition of the amended sediments on germination of the test species was observed in the experiments, while the root growth was more sensitive than biomass production to the changes of contaminant concentrations. The analysis of Pearson correlation coefficients between evaluation indicators and associated remediation parameters suggested that phytotoxicity of sediments might inaccurately indicate the changes of pollutant content, but it was significant in reflecting the ecotoxicity of sediments after remediation.

Engineering and kinetic aspects of bacterial uranium reduction for the remediation of uranium contaminated environments

Biological reduction of soluble uranium from U(VI) to insoluble U(IV) coupled to the oxidation of an electron donor (hydrogen or organic compounds) is a potentially cost-efficient way to reduce the U concentrations in contaminated waters to below regulatory limits. A variety of microorganisms originating from both U contaminated and non-contaminated environments have demonstrated U(VI) reduction capacity under anaerobic conditions. Bioreduction of U(VI) is considered especially promising for in situ remediation, where the activity of indigenous microorganisms is stimulated by supplying a suitable electron donor to the subsurface to contain U contamination to a specific location in a sparingly soluble form. Less studied microbial biofilm-based bioreactors and bioelectrochemical systems have also shown potential for efficient U(VI) reduction to remove U from contaminated water streams. This review compares the advantages and challenges of U(VI)-reducing in situ remediation processes, bioreactors and bioelectrochemical systems. In addition, the current knowledge of U(VI) bioreduction mechanisms and factors affecting U(VI) reduction kinetics (e.g. pH, temperature, and the chemical composition of the contaminated water) are discussed, as both of these aspects are important in designing efficient remediation processes.

In situ immobilization of cadmium in soil by stabilized biochar-supported iron phosphate nanoparticles

The potential for nanoscale phosphate amendments to remediate heavy metal contamination has been widely investigated, but the strong tendency of nanoparticles to form aggregates limits the application of this technique in soil. This study synthesized a composite of biochar-supported iron phosphate nanoparticle (BC@Fe3(PO4)2) stabilized by a sodium carboxymethyl cellulose to improve the stability and mobility of the amendment in soil. The sedimentation test and column test demonstrated that BC@Fe3(PO4)2 exhibited better stability and mobility than iron phosphate nanoparticles. After 28 days of simulated in situ remediation, the immobilization efficiency of Cd was 60.2 %, and the physiological-based extraction test bioaccessibility was reduced by 53.9 %. The results of sequential extraction procedures indicated that the transformation from exchangeable (EX) Cd to organic matter (OM) and residue (RS) was responsible for the decrease in Cd leachability in soil. Accordingly, the pot test indicated that Cd uptake by cabbage mustard was suppressed by 86.8 %. Compared to tests using iron phosphate nanoparticles, the addition of BC@Fe3(PO4)2 to soil could reduce the Fe uptake of cabbage mustard. Overall, this study revealed that BC@Fe3(PO4)2 could provide effective in situ remediation of Cd in soil.

Cadmium and arsenic accumulation during the rice growth period under in situ remediation

Rice (Oryza sativa L.) planted in cadmium (Cd)- and arsenic (As)-contaminated soil is considered the main source of dietary Cd and As intake for humans in Southeast Asia and thereby poses a threat to human health. Minimizing the transfer of these pollutants to rice grain is an urgent task for environmental researchers. The main objective of this study was to investigate the effects and the mechanisms of a combined amendment (hydroxyapatite + zeolite + biochar, HZB) on decreasing Cd and As accumulation in rice. In situ remediation and aqueous solution adsorption experiments were conducted. The results showed that after application of HZB, Cd and As concentrations of the exchangeable fraction and TCLP extraction in soil decreased with the growth of rice plants. Cd concentrations in rice tissues were decreased at the tillering, filling and maturing stages after in situ remediation, while As concentrations in rice tissues were decreased only at the maturing stage. When 8 kg·plot^-1 (9000 kg ha^-1) HZB was applied, concentrations of Cd and inorganic As in brown rice were decreased to 0.18 and 0.16 mg kg^-1, respectively, lower than the levels permissible for grain in China, i.e., 0.2 mg kg^-1. Application of HZB reduced Cd accumulation in rice tissues, and the suppression of Cd accumulation was significantly greater than that of As. Furthermore, HZB significantly increased rice grain yield. An aqueous solution adsorption experiment demonstrated that HZB could adsorb and covalently bind Cd and As (V) via -OH, -COOH, -Si-O-Si and CO₃^2- groups to produce carboxylates, silicates and carbonates, thereby promoting in situ immobilization of Cd and As in soil solution.

Enhanced removal of Cr(VI) in the Fe(III)/natural polyphenols system: role of the in situ generated Fe(II)

This study developed a cost-effective and eco-friendly method by coupling plant extracts (take green tea for example) and Fe(III) to reduce Cr(VI) and precipitate Cr(III). At acidic pH, 1.43 mM Fe(III) combined with 1.33 g/L green tea extracts could reduce 93% of Cr(VI) in 180 min, which was much larger than ˜50% by green tea extracts alone. Moreover, 52% of Cr(III) could automatically precipitate out as mixed Fe(III)-Cr(III) (oxy)-hydroxide solids. In the viewpoint of mechanism, polyphenols in green tea extracts were the reactive constituents and transformed Fe(III) to Fe(II), by which step the aqueous Fe(II) level was maintained to continuously reduce Cr(VI) to Cr(III), and thus accelerating Cr(VI) reduction. The generated Fe(III) partially participated in the reaction with polyphenols again and some Fe(III) formed precipitates with Cr(III). Overall, the electron transfers in the polyphenol-Fe-Cr cyclic reactions made Fe(III) used for multiple times, thus accelerated Cr(VI) reduction. The applicability of the combined process was further verified by removing 100% and 70% of Cr(VI) from electroplating wastewater and contaminated soil, respectively. As polyphenols can be derived from plant wastes and Fe(III) is naturally abundant, this study provides a promising method for in situ remediation of Cr(VI)-contaminated sites.

Sulfidated nano-scale zerovalent iron is able to effectively reduce in situ hexavalent chromium in a contaminated aquifer

In a number of laboratory studies, sulfidated nanoscale zero-valent iron (S-nZVI) particles showed increased reactivity, reducing capacity, and electron selectivity for Cr(VI) removal from contaminated waters. In our study, core-shell S-nZVI particles were successfully injected into an aquifer contaminated with Cr(VI) at a former chrome plating facility. S-nZVI migrated towards monitoring wells, resulting in a rapid decrease in Cr(VI) and Cr_tot concentrations and a long-term decrease in groundwater redox potential observed even 35 m downstream the nearest injection well. Characterization of materials recovered from the injection and monitoring wells confirmed the presence of nZVI particles, together with iron corrosion products. Chromium was identified on the surface of the recovered iron particles as Cr(III), and its occurrence was linked to the formation of insoluble chromium-iron (oxyhydr)oxides such as Cr_xFe_(1-x)(OH)_3(s). Injected S-nZVI particles formed aggregates, which were slowly transformed into iron (oxyhydr)oxides and carbonate green rust. Elevated contents of Fe⁰ were detected even several months after injection, indicating good S-nZVI longevity. The sulfide shell was gradually disintegrated and/or dissolved. Geochemical modelling confirmed the overall stability of the resulting Cr(III) phase at field conditions. This study demonstrates the applicability of S-nZVI for the remediation of a Cr(VI)-contaminated aquifer.

Sulfidated nano zerovalent iron (S-nZVI) for in situ treatment of chlorinated solvents: A field study

Sulfidated nano zerovalent iron (S-nZVI), stabilized with carboxymethyl cellulose (CMC), was successfully synthesized on site and injected into the subsurface at a site contaminated with a broad range of chlorinated volatile organic compounds (cVOCs). Transport of CMC-S-nZVI to the monitoring wells, both downgradient and upgradient, resulted in a significant decrease in concentrations of aqueous-phase cVOCs. Short-term (0-17 days) total boron and chloride measurements indicated dilution and displacement in these wells. Importantly however, compound specific isotope analysis (CSIA), changes in concentrations of intermediates, and increase in ethene concentrations confirmed dechlorination of cVOCs. Dissolution from the DNAPL pool into the aqueous phase at the deepest levels (4.0-4.5 m bgs) was identifiable from the increased cVOCs concentrations during long-term monitoring. However, at the uppermost levels (∼1.5 m above the source zone) a contrasting trend was observed indicating successful dechlorination. Changes in cVOCs concentrations and CSIA data suggest both sequential hydrogenolysis as well as reductive β-elimination as the possible transformation mechanisms during the short-term abiotic and long-term biotic dechlorination. One of the most positive outcomes of this CMC-S-nZVI field treatment is the non-accumulation of lower chlorinated VOCs, particularly vinyl chloride. Post-treatment soil cores also revealed significant decreases in cVOCs concentrations throughout the targeted treatment zones. Results from this field study show that sulfidation is a suitable amendment for developing more efficient nZVI-based in situ remediation technologies.

Porous media transport of iron nanoparticles for site remediation application: A review of lab scale column study, transport modelling and field-scale application

Injection of surface modified zero valent iron nanoparticles for in situ remediation of soil, contaminated with an array of pollutants has attracted great attention due to the high reactivity of zero valent iron towards a broad range of contaminants, its cost effectiveness, minimal physical disruption and low toxicity. The effectiveness of this technology relies on the stability and mobility of injected iron nanoparticles. Hence the development of a modelling tool capable of predicting nZVI transport is indispensable. This review provides state of the art knowledge on the mobility of iron nanoparticles in porous media, mechanisms involved in subsurface retention of nZVI based on continuum models and field scale application. Special attention is given to the identification of the influential parameters controlling the transport potential of iron nanoparticles and the available numerical models for the simulation of laboratory scale transport data.

Activated Carbon and Biochar Reduce Mercury Methylation Potentials in Aquatic Sediments

Much of the toxic methylmercury (MeHg) that biomagnifies in the aquatic food chain and accumulates in fish and seafood is believed to originate from microbial methylation of inorganic Hg(+2) in anoxic sediments. We examined the effect amending wetland sediments with activated carbon and biochar on Hg methylation potentials using microcosms and Hg stable isotope tracers. The inorganic (200)Hg(+2) spike was methylated at ~0.37 %/day in the untreated sediment, but that rate decreased to <0.08 %/day for the amended sediments, with 80 % and 88 % reductions in methylation rates for activated carbon and biochar amendments, respectively. Demethylation rates were relatively unchanged. Our key finding is that amending contaminated sediment with activated carbon and biochar decreases bioavailable Hg, and thus may also decrease Hg transfer into food webs. However, further research is needed to evaluate exactly how the sorbents impact Hg methylation rates and for related field studies.

Relationship between Pb relative bioavailability and bioaccessibility in phosphate amended soil: Uncertainty associated with predicting Pb immobilization efficacy using in vitro assays

In this study, an in vitro in vivo correlation (IVIVC) between Pb in vitro bioaccessibility (IVBA) and relative bioavailability (RBA) was explored to determine whether the efficacy of Pb immobilization in phosphate amended soils could be predicted using an in vitro approach. Mining/smelting impacted soil from Broken Hill, Australia (582-3536 mg/kg of Pb in the <250 μm soil particle fraction) was amended with Phosphoric Acid (PA), Mono Ammonium Phosphate (MAP) or Triple Super Phosphate (TSP) at Pb:P molar ratios of 1:1-1:5. Pb speciation in pre- and post-treated soil was assessed using X-ray Absorption Spectroscopy (XAS), Pb IVBA was measured using the Solubility Bioaccessibility Research Consortium (SBRC) assay (gastric and intestinal phases), and Pb RBA was determined in mice using blood Pb concentration as the bioavailability endpoint. XAS analysis revealed a 3.75-6.00 fold increase in the weighted % of Pb phosphates in soil containing >1000 mg/kg Pb while treatment effect ratios of 0.89-0.99 (SBRC-G), 0.09-0.71 (SBRC-I) and 0.27-0.80 (RBA) were observed in PA amended soil (Pb:P = 1:5). Although significant (p < 0.05) correlation were obtained between Pb RBA and IVBA (%) determined using SBRC-G (r = 0.64) and SBRC-I (r = 0.67), the strengths of the relationships were weak (r2 = 0.41-0.45). This research highlights the complexities associated with the prediction of Pb RBA in phosphate amended soil.

Characteristics and mechanisms of controlled-release KMnO4 for groundwater remediation: Experimental and modeling investigations

Controlled release materials (CRMs) are emerging oxidant delivery techniques for in-situ chemical oxidation (ISCO) for groundwater remediation. Successful implementation of CRM relies on good understandings of the kinetics and mechanism of controlled release of reactive agents. In this study, batch experiments and model simulations were conducted to explore the impacts of CRM properties (composition and size) and environmental conditions (temperature, pH, water volume and anions) on KMnO₄ release from KMnO₄ -paraffin controlled release beads. Experimental results indicated that higher KMnO₄: paraffin mass ratio resulted in shorter release longevities and higher release rate. Larger bead resulted in lower release rate, longer release longevity, and more KMnO₄ released. Higher incubation temperature resulted in higher release rate and shorter release longevity, but did not affect the total mass of KMnO₄ released. Acidic pH decreased the total mass of KMnO₄ released while alkaline pH did not affect KMnO₄ release. The presence of SO₄^2-, CO₃^2-, Cl^- and Br^- had negligible impacts on KMnO₄ release. A dissolution-diffusion conceptual model was developed. The above experimental observation and the associated controlled release mechanisms can be qualitatively explained by the conceptual model. A more detailed two-film boundary mathematical model was developed to simulate KMnO₄ release process. Comparison of modeling results with experimental data suggest that the new mathematical model gave a good quantitatively predication. Overall, this study shows that properly designed CRM can sustain release for years, thus representing a cost-effective and low-maintenance groundwater remediation technology. Both CRM properties and environmental conditions significantly affect the release kinetics and longevity, therefore these factors should be considered in the design and maintenance of CRM-based ISCO system.

Ecotoxicity and environmental safety related to nano-scale zerovalent iron remediation applications

This mini-review summarizes the current information that has been published on the various effects of nano-scale zerovalent iron (nZVI) on microbial biota, with an emphasis on reports that highlight the positive aspects of its application or its stimulatory effects on microbiota. By nature, nZVI is a highly reactive substance; thus, the possibility of nZVI being toxic is commonly suspected. Accordingly, the cytotoxicity of nZVI and the toxicity of nZVI-related products have been detected by laboratory tests and documented in the literature. However, there are numerous other published studies on its useful nature, which are usually skipped in reviews that deal only with the phenomenon of toxicity. Therefore, the objective of this article is to review both recent publications reporting the toxic effects of nZVI on microbiota and studies documenting the positive effects of nZVI on various environmental remediation processes. Although cytotoxicity is an issue of general importance and relevance, nZVI can reduce the overall toxicity of a contaminated site, which ultimately results in the creation of better living conditions for the autochthonous microflora. Moreover, nZVI changes the properties of the site in a manner such that it can also be used as a tool in a tailor-made approach to support a specific microbial community for the decontamination of a particular polluted site.

Removal of ammonium-nitrogen from groundwater using a fully passive permeable reactive barrier with oxygen-releasing compound and clinoptilolite

A novel fully passive permeable reactive barrier (PRB) with oxygen-releasing compound (ORC) and clinoptilolite was proposed for the removal of ammonium-nitrogen from groundwater. The PRB involves a combination of oxygen release, biological nitrification, ion exchange, and bioregeneration. A pilot-scale performance comparison experiment was carried out employing three parallel columns to assess the proposed PRB. The results showed that the PRB achieved nearly complete [Formula: see text] depletion (>99%). [Formula: see text] of 5.23-10.88 mg/L was removed, and [Formula: see text] of <1.93 mg/L and [Formula: see text] of 2.03-19.67 mg/L were generated. Ion exchange and biological nitrification both contributed to [Formula: see text] removal, and the latter played a dominant role under the condition of sufficient oxygen. Biological nitrification favored a delay in sorption saturation and a release of exchange sites. The ORC could sufficiently, efficiently supply oxygen for approximately 120 pore volumes. The clinoptilolite ensured a robust [Formula: see text] removal in case of temporary insufficient biological activities. No external alkalinity sources had to be supplied and no inhibition of aerobic metabolism occurred. The ceramicite had a negligible effect on the biomass growth. Based on the research findings, a full-scale continuous wall PRB was installed in Shenyang, China in 2012.

Effects of oxidants on in situ treatment of a DNAPL source by nanoscale zero-valent iron: A field study

This study aimed to evaluate the efficiency of a nanoscale zero-valent iron (NZVI)-based treatment process for an aquifer contaminated with trichloroethylene (TCE) in which TCE in dense non-aqueous phase liquid (DNAPL) form was also present. The study further investigated the effects of site oxidants on the reactivity and lifetime of NZVI. The injection of 30 kg of NZVI into the site successfully removed 95.7% of TCE in the groundwater within the first 60 days without producing chlorinated intermediates. The chloride balance analysis estimated that 2214 g of TCE was removed and confirmed the presence of DNAPL TCE. The oxidation of NZVI particles by nitrate, dissolved oxygen (DO), and TCE consumed 29.5%, 13.5%, and 14.3% of the Fe(0) initially present, respectively, over 60 days. Thus, nitrate was identified as the priority among groundwater oxidants. The reactive lifetime of NZVI at the site was found to be at least 103 days, based on the monitoring of TCE, DO, and nitrate concentrations, oxidation-reduction potential (ORP), and the residual Fe(0) content of the NZVI particles. Solid samples that were retrieved from the site on the 165th day still contained substantial amounts of Fe(0), occupying up to 21.9% of the total mass, and retained considerable reactivities towards TCE. This indicates that the NZVI particles aged more than 5 months at the site can potentially be reused for TCE reduction even after extensive corrosion of Fe(0) has occurred.

Citric acid modified ultrasmall copper peroxide nanozyme for in situ remediation of environmental sulfonylurea herbicide contamination

Herbicide residues in the environment threaten high-quality agriculture and human health. Consequently, in situ remediation of herbicide contamination is vital. We synthesized a novel self-catalyzed nanozyme, ultrasmall (2-3 nm) copper peroxide nanodots modified by citric acid (CP@CA) for this purpose, which can break down into H₂O₂ and Cu²⁺ in water or soil. Ubiquitous glutathione reduces Cu²⁺ into Cu⁺, which promotes the decomposition of H₂O₂ into •OH through a Fenton-like reaction under mild acid conditions created by the presence of citric acid. The generated •OH efficiently degrade nicosulfuron in water and soil, and the maximum degradation efficiency could be achieved at 97.58% in water at 56 min. The possible degradation mechanisms of nicosulfuron were proposed through the 25 intermediates detected. The overall ecotoxicity of the nicosulfuron system was significantly reduced after CP@CA treatment. Furthermore, CP@CA had little impact on active components of soil bacterial community. Moreover, CP@CA nanozyme could effectively remove seven other sulfonylurea herbicides from the water. In this paper, a high-efficiency method for herbicide degradation was proposed, which provides a new reference for the in situ remediation of herbicide pollution.

Dynamic responses of soil enzymes at key growth stages in rice after the in situ remediation of paddy soil contaminated with cadmium and arsenic

The practical application of in situ remediation techniques requires an understanding of the dynamic changes in soil enzyme activity as indicators of soil fertility and health. Experiments were carried out in paddy soils co-contaminated with cadmium (Cd) and arsenic (As) at low (L) and high (H) levels. A calcium and iron (CaFe)-based amendment (limestone + iron powder + silicon fertilizer + calcium‑magnesium-phosphate fertilizer) was applied to the soil at concentrations of 0, 450, and 900 g·m^-2 (labeled CK, T1, and T2, respectively), and sampling was conducted at the tillering (TS), booting (BS), filling (FS), and mature (MS) stages. In soil L, urease activity increased significantly by 15.8% under T1 treatment at the MS, catalase activity increased significantly under T2 treatment by 52.4% at the FS and 25.9% at the MS, and acid phosphatase activity increased significantly by 50.1%-65.9% at the TS. For soil H, urease activity increased by maximum values of 101.6% and 28.6% at the FS and MS, respectively. Catalase activity increased by 29.0% at the MS under T2 treatment, and acid phosphatase activity increased by maximum values of 40.5%, 16.0%, and 53.9% at the BS, FS, and MS, respectively. The results indicate that the changes in soil enzyme activity were mainly related to the rice growth stage, soil pH, and available Cd and As after the application of Ca-Fe-based amendment. Overall, at the FS and MS, the amendment increased the soil pH, soil enzyme activity, and cation exchange capacity and reduced the available Cd and As, which reduced the Cd and As contents in brown rice.

Field-scale demonstration of in situ immobilization of heavy metals by injecting iron oxide nanoparticle adsorption barriers in groundwater

Remediation of heavy metal-contaminated aquifers is a challenging process because they cannot be degraded by microorganisms. Together with the usually limited effectiveness of technologies applied today for treatment of heavy metal contaminated groundwater, this creates a need for new remediation technologies. We therefore developed a new treatment, in which permeable adsorption barriers are established in situ in aquifers by the injection of colloidal iron oxides. These adsorption barriers aim at the immobilization of heavy metals in aquifers groundwater, which was assessed in a large-scale field study in a brownfield site. Colloidal iron oxide (goethite) nanoparticles were used to install an in situ adsorption barrier in a very heterogeneous, contaminated aquifer of a brownfield in Asturias, Spain. The groundwater contained high concentrations of heavy metals with up to 25 mg/L zinc, 1.3 mg/L lead, 40 mg/L copper, 0.1 mg/L nickel and other minor heavy metal pollutants below 1 mg/L. High amounts of zinc (>900 mg/kg), lead (>2000 mg/kg), nickel (>190 mg/kg) were also present in the sediment. Ca. 1500 kg of goethite nanoparticles of 461 ± 266 nm diameter were injected at low pressure (< 0.6 bar) into the aquifer through nine screened injection wells. For each injection well, a radius of influence of at least 2.5 m was achieved within 8 h, creating an in situ barrier of 22 × 3 × 9 m. Despite the extremely high heavy metal contamination and the strong heterogeneity of the aquifer, successful immobilization of contaminants was observed in the tested area. The contaminant concentrations were strongly reduced immediately after the injection and the abatement of the heavy metals continued for a total post-injection monitoring period of 189 days. The iron oxide particles were found to adsorb heavy metals even at pH-values between 4 and 6, where low adsorption would have been expected. The study demonstrated the applicability of iron oxide nanoparticles for installing adsorption barriers for containment of heavy metals in contaminated groundwater under real conditions.

Resuspension of sediment, a new approach for remediation of contaminated sediment

Natural events and anthropogenic activities are the reasons of undesirable resuspension of contaminated sediments in aquatic environment. Uncontrolled resuspension could remobilize weakly bound heavy metals into overlying water and pose a potential risk to aquatic ecosystem. Shallow harbours, with contaminated sediments are subjected to the risk of uncontrolled resuspension. Remediation of sediments in these areas cannot be performed by conventional in situ methods (e.g. capping with or without reactive amendment). Ex situ remediation also requires dredging of sediment, which could increase the risk of spreading contaminants. Alternatively, the resuspension technique was introduced to address these issues. The concept of the resuspension method is that finer sediments have a greater tendency to adsorb the contamination. Therefore, finer sediments, believed carry more concentration of contaminants, were targeted for removal from aquatic environment by a suspension mechanism in a confined water column. The objective of this study was to evaluate the feasibility of the resuspension technique as a new approach for remediation of contaminated sediment and a viable option to reduce the risk of remobilization of contaminants in harbours due to an undesirable resuspension event. Unlike the common in situ techniques, the resuspension method could successfully reduce the total concentration of contaminants in almost all samples below the probable effect level (PEL) with no significant change in the quality of overlying water. The results indicated that removal efficiency could be drastically enhanced for metals in sediment with a higher enrichment factor. Moreover, availability of metals (e.g. Cd and Pb) with a high concentration in labile fractions was higher in finer sediments with a high enrichment factor. Consequently, removal of contaminants from sediment through the resuspension method could reduce the risk of mobility and availability of metals under changing environmental conditions. Potential dredging in harbours could be performed safer and more cost-effective afterward.

Remediation of arsenic-contaminated groundwater using media-injected permeable reactive barriers with a modified montmorillonite: sand tank studies

A modified montmorillonite (MMT) was prepared using an acid activation-sodium activation-iron oxide coating method to improve the adsorption capacities of natural MMTs. For MMT, its interlamellar distance increased from 12.29 to 13.36 Å, and goethite (α-FeOOH) was intercalated into its clay layers. Two novel media-injected permeable reactive barrier (MI-PRB) configurations were proposed for removing arsenic from groundwater. Sand tank experiments were conducted to investigate the performance of the two MI-PRBs: Tank A was filled with quartz sand. Tank B was packed with quartz sand and zero-valent iron (ZVI) in series, and the MMT slurry was respectively injected into them to form reactive zones. The results showed that for tank A, total arsenic (TA) removal of 98.57% was attained within the first 60 mm and subsequently descended slowly to 88.84% at the outlet. For tank B, a similar spatial variation trend was observed in the quartz sand layer, and subsequently, TA removal increased to ≥99.80% in the ZVI layer. TA removal by MMT mainly depended on both surface adsorption and electrostatic adhesion. TA removal by ZVI mainly relied on coagulation/precipitation and adsorption during the iron corrosion. The two MI-PRBs are feasible alternatives for in situ remediation of groundwater with elevated As levels.

Response of environmental variables and microbial community to sodium percarbonate addition to contaminated sediment

Sodium percarbonate (SPC) is a common reagent used for in situ remediation of contaminated soil. Current studies focus on the effects of SPC on pollutant removal; however, a knowledge gap exists for the biochemical process following SPC addition. In this study, a microcosm batch experiment was conducted to investigate the residual effect caused by different doses of SPC addition on native microbial communities, as well as on the environmental variables of contaminated sediments. The obtained results showed that the more SPC was added, the more dissolved matters were generated and the oxidation-reduction potential was lowered. Furthermore, the metabolic activities of the microbial community were enhanced and the microbial community structure responded differently to different SPC doses: the microbes that increased at high SPC dose mainly belonged to the phylum Firmicutes, the class Clostridia, and the genera Petrimonas and Proteiniclasticum. The microbes that increased at medium SPC dose mainly belonged to the class Alphaproteobacteria and the genus Brevundimonas. In contrast, vulnerable microbes mainly belonged to the phylum Acidobacteria, the class Caldisericia, Holophagae, and the genus Sulfuricurvum. Microbes capable of fermentation, ureolysis, and chemohetrotrophy increased. These results indicate that SPC addition could indirectly provide both electron acceptors and donors, thus improving the metabolic activities of the microorganisms in the contaminated sediment. Furthermore, the utilized SPC dose should be considered to achieve the optimal benefit for in situ remediation. This study forms a valuable reference for the application of SPC in ecological engineering.

Comparative Life Cycle Assessment of possible methods for the treatment of contaminated soil at an environmentally degraded site

This study reports on the assessment of the environmental sustainability of different management practices for an environmentally degraded site in Slovenia: the Old Zinc-Works in the town of Celje. Life Cycle Assessments (LCAs) were applied in order to evaluate possible trade-offs by comparing a proposed in situ remediation scenario with two other reclamation scenarios (scenario 2: incineration, metal extraction, underground disposal and reclamation of the site by refilling it with replacement material, and scenario 3: underground disposal and reclamation of the site by refilling it with replacement material) and with a no-action scenario. The results of the comparisons performed show that in the case of the in situ remediation scenario, the consumption of resources is smaller by a factor of 51 compared to that in the second scenario and by a factor of 7 compared to that in the third scenario. The impacts on human health and ecosystem quality are approximately 30 and 3.5 times less in the first scenario than in the second and third scenarios, respectively. Compared to the impact of the no-action scenario, the impact on human health of the in situ soil remediation scenario is approximately 6 times less, whereas its impact on the ecosystem is approximately 4 times less. The results confirmed that the in situ soil remediation scenario is the most sustainable practice from an environmental point of view. Its main advantage lies in the achieved conservation of natural resources. Despite the recovery of valuable metals (Zn, Pb, Cu, and Ni) from the bottom ash, the second scenario is significantly more environmentally burdensome compared to both the first and third scenarios. This outcome is due to the significantly high impacts related to the consumption of fuels needed to support the incineration of low-calorific contaminated soil and to electricity consumption. The present study demonstrates that the results of LCA studies, in addition to technological, economic and social indicators, yield important information about the sustainability of different management practices and therefore should be an important part of decision-making when approaching the reclamation of environmentally degraded sites.

Remediation of a historically Pb contaminated soil using a model natural Mn oxide waste

A natural Mn oxide (NMO) waste was assessed as an in situ remediation amendment for Pb contaminated sites. The viability of this was investigated using a 10 month lysimeter trial, wherein a historically Pb contaminated soil was amended with a 10% by weight model NMO. The model NMO was found to have a large Pb adsorption capacity (qmax 346±14 mg g(-1)). However, due to the heterogeneous nature of the Pb contamination in the soils (3650.54-9299.79 mg kg(-1)), no treatment related difference in Pb via geochemistry could be detected. To overcome difficulties in traditional geochemical techniques due to pollutant heterogeneity we present a new method for unequivocally proving metal sorption to in situ remediation amendments. The method combines two spectroscopic techniques; namely electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). Using this we showed Pb immobilisation on NMO, which were Pb free prior to their addition to the soils. Amendment of the soil with exogenous Mn oxide had no effect on microbial functioning, nor did it perturb the composition of the dominant phyla. We conclude that NMOs show excellent potential as remediation amendments.