Management of arsenic-contaminated excavated soils: A review

Molybdenum-contaminated soil stabilization/solidification by mixtures of magnesium oxide and limestone calcined clay cement

Large quantities of soil excavated from civil engineering projects are found to contain leachable contaminants, including metallic oxyanions like molybdate. This study presents a novel treatment approach by identifying a mixture of magnesium oxide (MgO) and hydraulic binders for the stabilization and solidification (S/S) of molybdenum-contaminated limestone tunnel sludge. XRD analysis and geochemical modeling revealed the presence of brucite in all samples, suggesting that the immobilization mechanism was likely molybdate sorption onto brucite. The addition of cement enhanced the geotechnical properties and improved sulfate retention through the formation of ettringite. Considering efficiency and a low CO2 footprint, the S/S formulation consisting of 1 wt% MgO and 3 wt% limestone calcined clay cement (LC3) was selected for this study. Environmental assessments were conducted using leaching test standards, including US EPA 1313 to assess pH sensitivity, US EPA 1311 (TCLP) and EN 12457-2 to evaluate the effects of curing time, at intervals from 6 hours to 90 days. Results showed that molybdenum release remained below 0.05 mg/L (the European inert waste threshold) after just 24 hours of curing, with further improvements over time under alkaline conditions. However, the treatment was ineffective at neutral or acidic pH due to brucite dissolution. The S/S of molybdenum-contaminated sludge using MgO and LC3 proved highly efficient for Mo immobilization and is suitable for industrial applications.

Development of nanoscale zero-valent iron embedded on polyaniline reinforced with sodium alginate hydrogel microbeads for effective adsorption of arsenic from apatite soil leachate water

A novel polymeric nanocomposite hydrogel adsorbent was developed to enhance the efficiency of arsenic removal from apatite soil leachate. Apatite soil aqueous leachate was treated with nanoscale zero-valent iron embedded on polyaniline reinforced with sodium alginate hydrogel beads. Various analytical techniques including attenuated total reflection -Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy were employed to characterize these chemically synthesized hydrogel beads. The influence of different types and ratios of adsorbent materials, solution pH, adsorbent dosage, contact time, temperature, initial arsenic concentration, and the presence of co-existing ions on the adsorption process were investigated. Under optimum operating conditions; a pH range of 4-6, 80 mg of sorbent, 180 min contact time led to a remarkable arsenic removal efficiency of approximately 90.33 %. Thermodynamic, adsorption isotherm, and kinetic models provided a good description of the observed experimental results. Compared to the Freundlich and Temkin models, the Langmuir model was found to be the best fit for the experimental data, with a maximum adsorption capacity of 104.167 mg/g. Physical adsorption is mainly responsible for controlling the adsorption of arsenic ions onto the hydrogel. Thermodynamic studies verified that the adsorption process was endothermic and spontaneous.

Unveiling the potential mobility and geochemical speciation of geogenic arsenic in the deep subsurface soil of the Tokyo metropolitan area

Extensive excavations for urban and subterranean construction often lead to soil and groundwater contamination with geogenic arsenic (As), emphasizing the urgent need for effective management strategies, particularly considering the global excavation of millions of tons of soil annually. This study investigated the chemical speciation and solubility of geogenic As in soil samples collected at 25-cm intervals from boreholes extending up to 16 m deep within the alluvial Yurakucho Formation and the terrestrial Kanto Loam Formation in the Tokyo metropolitan area. Soils from the Yurakucho Formation exhibited significantly higher total As concentrations (10.5 ± 3.26 mg kg-1) compared to those from the Kanto Loam Formation (5.58 ± 1.88 mg kg-1), with notably elevated levels of water-soluble As throughout the profile. The XANES analysis revealed that As-bearing sulfide species, including As2S3 and FeAsS types, were the predominant forms in the Yurakucho Formation, while As(V) species were more prevalent in the Kanto Loam Formation. Micro-XAFS combined with micro-XRF analysis identified framboidal pyrite, characterized by micron-sized grains (∼10 µm), as the primary sink for As sequestration in the Yurakucho Formation, where As occurs mainly in sulfide-associated forms. These findings highlight the importance of characterizing geogenic As speciation to assess its leaching potential and associated environmental risks posed by As in excavated soils.

Enhanced fluoride extraction from contaminated soil combining chelator and surfactant: Insights into adsorptive controlment of soil surface charge

Biodegradable chelators and surfactants are promising alternatives to conventional washing agents for remediating soil contaminated with toxic elements, owing to their excellent extractability and environmental compatibility. Most previous studies have primarily aimed at maximizing removal efficiency. However, understanding their underlying extraction mechanism is essential to expand the application potential of chelator- or surfactant-assisted washing systems. This study evaluated the effectiveness of chelators and surfactants in remediating fluoride (F)-contaminated soil and explored their associated extraction mechanisms. Our findings highlight a biodegradable chelator, HIDS (3-hydroxy-2,2'-imino disuccinic acid) as uniquely effective in F extraction with minimal F-bearing minerals dissolution (Ca, Fe, and Al). Chelator recovery rates and zeta potential measurements in post-washed solutions suggests that HIDS adsorbs onto soil surfaces, displacing the originally adsorbed F and enhancing the negative surface charge to inhibit F re-adsorption. Additionally, applying an anionic surfactant to enhance F extraction from soil showed promising results. Notably, a binary blend of HIDS and in-lab designed anionic surfactant, SDT (sodium N-dodecanoyl-taurinate), achieved the highest F removal rate (132 mg kg-1) under optimized washing conditions (HIDS: 10 mmol L-1, SDT: 10 mmol L-1, solution pH: 3, and washing time: 1 h), enhancing F extraction by 22% compared to HIDS-only washing (108 mg kg-1; washing time: 3 h). The FT-IR and zeta potential measurements suggested that SDT adsorbed onto the soil surface. The action of the HIDS-SDT blend towards F extraction involves the complexation and acid dissolution of F-bearing soil minerals, followed by F replacement through chelator and surfactant adsorption. This process mitigated F back-adsorption and enhanced F extraction by generating a negatively charged soil surface.

Synergy between Fenton reagent and solid waste-based solidifying agents during the solidification/stabilization of lead(II) and arsenic(III) contaminated soils

Due to the different physicochemical properties of lead (Pb) and arsenic (As), coupled remediation processes of contaminated soils containing Pb and As have always been a technical challenge. In the present study, a novel solidifying agent (BER) was synthesized using alkaline oxygen furnace slag (BOFS), modified electrolytic manganese residue (EMR) and red mud (RM). The solidifying agent was synergistically used with Fenton reagent for solidifying/stabilizing Pb- and As-contaminated soils. The experimental results indicated that the low stability of Ca-As(III) complexes, which serve as the primary fixation mechanism for As in BER-solidified soil, led to inadequate stabilization of As by BER. As a result, the leaching concentration of As in the BER-solidified soil reached 2.61 mg/L. The incorporation of Fenton reagent had minimal effect on the strength of the solidified soil and Pb stabilization, but it substantially improved As stabilization. The immobilization efficiency of the BER-Fenton for lead and arsenic reached 99% and 98%, respectively. The strength of solidified/stabilized soil using BER-Fenton was close to that of cement-based solidified/stabilized soil. Microscopic analysis showed that the incorporation of Fenton reagent increased the content of goethite and ettringite, which effectively filled the soil pores and enhanced the adsorption of Pb and As. The use of BER and Fenton reagent increased the content of As(V) to 88%, which promoted the complexation between As and Fe(III)/Mn(IV). This study provides a novel method for efficient remediation of heavy metal-contaminated sites, as well as a useful reference for ensuring the safe reuse of hazardous wastes.

Remediation of fluoride-contaminated wastes: Chelator-assisted washing and subsequent immobilization using CaO and H3PO4

Fluoride (F) contamination in industrial waste is a significant challenge for sustainable materials recycling. Existing techniques for mitigating F contamination focus on immobilization, converting F compounds to insoluble forms while leaving the total F content untreated. Chelator-assisted washing is considered a promising alternative remediation strategy that can indirectly release F by entrapping and dissolving F-bearing minerals. This study evaluates the effectiveness of chelator-assisted washing in removing F from three real F-contaminated waste samples (TCS-49, TCS-51, and TCS-52) by treating with four different chelators, ethylenediaminetetraacetic acid (EDTA), ethylenediamine N,N'-disuccinic acid (EDDS), diethylenetriaminepentaacetic acid (DTPA), and 3-hydroxy-2,2'-imino disuccinic acid (HIDS). The influence of key washing variables, including chelator type, solution pH, chelator concentration, washing time, and liquid-to-solid (L/S) ratio toward F extraction was assessed and optimized for attaining the maximum extraction. All chelators exhibited the highest F extraction from TCS-49 and TCS-51 at pH 11, whereas in TCS-52 it showed a discrete extraction pattern. Under optimized conditions (concentration, 10 mmol L-1; pH, 11; washing duration, 3 h; and L/S ratio, 10:1), EDTA outperformed the other chelators, enhancing F extraction by 2.1 and 1.2 times for TCS-49 and TCS-51, respectively, compared with those of control treatments. However, for TCS-52, the efficiency of EDTA was analogous to that of the control under the same washing conditions. The linear correlation between the extracted F and F-containing minerals suggests that the chelator-induced F removal from contaminated waste involves the entrapment and dissolution of F-bearing minerals, especially Ca and Fe. The subsequent post-washing immobilization of chelator-washed waste residues using CaO or H3PO4 significantly reduced the content of leachable F under the regulatory limit.

Enhancing lead extraction efficiency from contaminated soil: A synergistic approach combining biodegradable chelators and surfactants

Lead (Pb), a persistent and bio-accumulative contaminant, poses threats to the environment and human health. The effective removal of Pb from contaminated soil proves challenging due to its tendency to form stable complexes with soil components. Chelators have been extensively studied for their ability to extract metal contaminants, including Pb, from soil environment. However, the prolonged environmental persistence of traditional chelators and the high cost of biodegradable alternatives have hindered their practical application in remediation efforts. This study investigated a novel synergistic approach that combined a biodegradable chelator, [S,S]-ethylenediamine succinic acid (EDDS), with cationic and anionic surfactants to enhance Pb extraction efficiency. The study revealed that cationic surfactants, such as cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB), significantly enhanced Pb extraction efficiency when combined with EDDS, whereas anionic surfactants, like sodium N-dodecanoyl-taurinate (SDT) and sodium dodecyl sulfate (SDS), inhibited the extraction process. Specifically, blending 5 mmol L-1 EDDS with 20 mmol L-1 CPC resulted in a 72.6% enhancement in Pb extraction efficiency. The proposed synergistic strategy offers a promising avenue for soil remediation, mitigating Pb contamination while preserving essential soil minerals. By addressing chelator limitations and improving efficiency, this approach presents a viable solution for enhancing soil remediation practices.

Long-term performance of the adsorption layer system for the recycling and repurposing of arsenic-bearing mudstone as road embankment

The adsorption layer system has shown great potential as a cost-effective and practical strategy for the recycling and management of excavated rocks containing potentially toxic elements (PTEs). Although this system has been employed in various civil engineering projects throughout Japan, its long-term performance to immobilize PTEs has rarely been investigated. This study aims to evaluate the effectiveness of the adsorption layer system applied in an actual road embankment approximately 11 years after construction. The embankment system is comprised of a layer of excavated arsenic (As)-bearing mudstone built on top of a bottom adsorption layer mixed with an iron (Fe)-based adsorbent. Collection of undisturbed sample was carried out by implementing borehole drilling surveys on the embankment. Batch leaching experiments using deionized water and hydrochloric acid were conducted to evaluate the water-soluble and acid-leachable concentrations of As, Fe, and other coexisting ions. The leaching of As from the mudstone layer was likely induced by As desorption from Fe-oxides/oxyhydroxides naturally present under alkaline conditions, including the oxidation of framboidal pyrite, which was identified as a potential source of As. This was supported by electron probe microanalyzer (EPMA) observations showing the presence of trace amounts of As in framboidal pyrite crystals. Arsenic leached from the mudstone layer was then immobilized by Fe oxyhydroxides found in the adsorption layer. Based on geochemical modeling and X-ray photoelectron spectroscopy (XPS) results, leached As predominantly existed as the negatively charged HAsO42- oxyanion, which is readily sequestered by Fe oxyhydroxides. Moreover, the effectiveness of the adsorption layer was assessed and its lifetime was estimated, and the results revealed it still possessed enough capacity to adsorb As released from mudstone in the foreseeable future. This prediction utilized the maximum potential amount of As that could leach from the excavated rock layer with time.

Recent trends in the phytoremediation of radionuclide contamination of soil by cesium and strontium: Sources, mechanisms and methods: A comprehensive review

This comprehensive review examines recent trends in phytoremediation strategies to address soil radionuclide contamination by cesium (Cs) and strontium (Sr). Radionuclide contamination, resulting from natural processes and nuclear-related activities such as accidents and the operation of nuclear facilities, poses significant risks to the environment and human health. Cs and Sr, prominent radionuclides involved in nuclear accidents, exhibit chemical properties that contribute to their toxicity, including easy uptake, high solubility, and long half-lives. Phytoremediation is emerging as a promising and environmentally friendly approach to mitigate radionuclide contamination by exploiting the ability of plants to extract toxic elements from soil and water. This review focuses specifically on the removal of 90Sr and 137Cs, addressing their health risks and environmental implications. Understanding the mechanisms governing plant uptake of radionuclides is critical and is influenced by factors such as plant species, soil texture, and physicochemical properties. Phytoremediation not only addresses immediate contamination challenges but also provides long-term benefits for ecosystem restoration and sustainable development. By improving soil health, biodiversity, and ecosystem resilience, phytoremediation is in line with global sustainability goals and environmental protection initiatives. This review aims to provide insights into effective strategies for mitigating environmental hazards associated with radionuclide contamination and to highlight the importance of phytoremediation in environmental remediation efforts.

Remediation of cadmium-contaminated soil: GLDA-assisted extraction and sequential FeCl3-CaO-based post-stabilization

Cadmium (Cd) contamination of farmland soils is a growing concern because of its highly toxic impact on ecosystems and human health. Chelator-assisted washing and chemical immobilization are effective remediation strategies for Cd-contaminated soils. Ethylenediaminetetraacetic acid (EDTA) has traditionally been used for soil washing, but its persistence in the environment and subsequent toxicity have raised significant ecological concerns. Consequently, biodegradable chelators have gained increasing attention as eco-friendly alternatives to the persistent chelator, EDTA. Therefore, this study evaluated the performance and efficacy of three biodegradable chelators: L-glutamate-N,N'-diacetic acid (GLDA), methylglycine-diacetic acid (MGDA), and 3-hydroxy-2,2'-iminodisuccinic acid (HIDS) in comparison to EDTA for remediating a real Cd-contaminated agricultural soil. The influence of treatment parameters, including chelator variants, washing time, chelator concentration, solution pH, and liquid-to-soil ratio (L/S) on Cd extraction was studied and optimized to attain the maximum removal rate. Following chelator-assisted washing, the efficacy of a stabilization preference combining FeCl3 and CaO in reducing the leaching potential of residual Cd in chelator-washed soil residues was also investigated. GLDA demonstrated comparable Cd extraction efficiency to EDTA, and the Cd extraction efficiency was found to be positively correlated with the soil washing parameters. However, under the optimized conditions (chelator concentration: 10 mmol L-1; washing time: 3 h; solution pH: 3; L/S ratio: 10:1), GLDA exhibited a higher Cd extraction rate than EDTA or the other chelators. Furthermore, a post-treatment process incorporating FeCl3 and CaO substantially diminished the water-leachable Cd content in the resultant soil residues. The proposed remediation strategy, which combines chemically assisted washing and stabilization, could be a practical option for extracting bulk Cd from soil and reducing the leaching potential of residual Cd.

Evaluation of newly designed flushing techniques for on-site remediation of arsenic-contaminated excavated debris

Excavated debris (soil and rock) contaminated with geogenic arsenic (As) is an increasing concern for regulatory organizations and construction stakeholders. Chelator-assisted soil flushing is a promising method for practical on-site remediation of As-contaminated soil, offering technical, economic, and environmental benefits. Ethylenediaminetetraacetic acid (EDTA) is the most prevalent chelator used for remediating As-contaminated soil. However, the extensive environmental persistence and potential toxicity of EDTA necessitate the exploration of eco-compliant alternatives. In this study, the feasibility of the conventional flushing method pump-and-treat and two newly designed immersion and sprinkling techniques were evaluated at the laboratory scale (small-scale laboratory experiments) for the on-site treatment of As-contaminated excavated debris. Two biodegradable chelators, L-glutamic acid-N,N'-diacetic acid (GLDA) and 3-hydroxy-2,2'-iminodisuccinic acid (HIDS), were examined as eco-friendly substitutes for EDTA. Additionally, this study highlights a useful post-treatment measure to ensure minimal mobility of residual As in the chelator-treated debris residues. The pump-and-treat method displayed rapid As-remediation (t, 3 h), but it required a substantial volume of washing solution (100 mL g-1). Conversely, the immersion technique demonstrated an excellent As-extraction rate using a relatively smaller washing solution (0.33 mL g-1) and shorter immersion time (t, 3 h). In contrast, the sprinkling technique showed an increased As-extraction rate over an extended period (t, 48 h). Among the chelators employed, the biodegradable chelator HIDS (10 mmol L-1; pH, 3) exhibited the highest As-extraction efficiency. Furthermore, the post-treatment of chelator-treated debris with FeCl3 and CaO successfully reduced the leachable As content below the permissible limit.