The performance of blended conventional and novel binders in the in-situ stabilisation/solidification of a contaminated site soil

Stabilization of metal and metalloids from contaminated soils using magnesia-based tundish deskulling waste from continuous steel casting

This study presents a groundbreaking exploration into the potential use of refractory tundish deskulling waste (TUN), a magnesium oxide-based by-product from continuous steel casting, as a stabilizing agent for remediating metal and metalloids contaminated soils. Up-flow column horizontal percolation tests were conducted to measure the concentrations of metals and metalloids, pH, and electrical conductivity (EC) in the leachates of two different combinations of contaminated soil and stabilizer (95-5 wt% and 90-10 wt%). The effectiveness of TUN as a soil-stabilizing agent for contaminated soils with metals and metalloids was evaluated by comparing its leachates with those obtained from a sample of a well-established low-grade magnesium oxide (LG-MgO) by-product, which underwent the same testing procedure. The findings revealed a significant correlation between the mobility of the examined metals and metalloids, and the water-soluble or acid phase of the contaminated soil, primarily governed by precipitation-solution reactions. While the stabilizing impact on non-pH-dependent metals, particularly redox-sensitive oxyanions, was less pronounced, both MgO-based stabilizers exhibited a favourable influence on soil pH-dependent metals and metalloids. They achieved this by establishing an optimal pH range of approximately 9.0-10.5, wherein the solubility of metal (hydr)oxides is minimized. Notably, metals like Zn and Cu, which have high leaching potential, experienced a remarkable reduction in leaching - Zn by over 99% and Cu by around 97% - regardless of the stabilizer content. In a broader context, this research champions the principles of the circular economy by offering a technical remedy for treating soils contaminated with pH-dependent metals and metalloids. The proposed solution harnesses industrial waste - currently relegated to landfills - as a resource, aligning with sustainable practices and environmental responsibility.

Arsenic Contamination in Groundwater: Geochemical Basis of Treatment Technologies

Arsenic (As) is abundant in the environment and can be found in both organic (e.g., methylated) and inorganic (e.g., arsenate and arsenite) forms. The source of As in the environment is attributed to both natural reactions and anthropogenic activities. As can also be released naturally to groundwater through As-bearing minerals including arsenopyrites, realgar, and orpiment. Similarly, agricultural and industrial activities have elevated As levels in groundwater. High levels of As in groundwater pose serious health risks and have been regulated in many developed and developing countries. In particular, the presence of inorganic forms of As in drinking water sources gained widespread attention due to their cellular and enzyme disruption activities. The research community has primarily focused on reviewing the natural occurrence and mobilization of As. Yet, As originating from anthropogenic activities, its mobility, and potential treatment techniques have not been covered. This review summarizes the origin, geochemistry, occurrence, mobilization, microbial interaction of natural and anthropogenic-As, and common remediation technologies for As removal from groundwater. In addition, As remediation methods are critically evaluated in terms of practical applicability at drinking water treatment plants, knowledge gaps, and future research needs. Finally, perspectives on As removal technologies and associated implementation limitations in developing countries and small communities are discussed.

Review of the Interactions between Conventional Cementitious Materials and Heavy Metal Ions in Stabilization/Solidification Processing

In the past few decades, solidification/stabilization (S/S) technology has been put forward for the purpose of improving soil strength and inhibiting contaminant migration in the remediation of heavy metal-contaminated sites. Cement, lime, and fly ash are among the most common and effective binders to treat contaminated soils. During S/S processing, the main interactions that are responsible for improving the soil's behaviors can be summarized as gelification, self-hardening, and aggregation. Currently, precipitation, incorporation, and substitution have been commonly accepted as the predominant immobilization mechanisms for heavy metal ions and have been directly verified by some micro-testing techniques. While replacement of Ca²⁺/Si⁴⁺ in the cementitious products and physical encapsulation remain controversial, which is proposed dependent on the indirect results. Lead and zinc can retard both the initial and final setting times of cement hydration, while chromium can accelerate the initial cement hydration. Though cadmium can shorten the initial setting time, further cement hydration will be inhibited. While for mercury, the interference impact is closely associated with its adapted anion. It should be pointed out that obtaining a better understanding of the remediation mechanism involved in S/S processing will contribute to facilitating technical improvement, further extension, and application.

Does Current Knowledge Give a Variety of Possibilities for the Stabilization/Solidification of Soil Contaminated with Heavy Metals?-A Review

Stabilization/solidification of contaminated soil is a process that allows simultaneous strengthening of the soil structure, disposal of contamination and recycling of industrial waste, implemented as substitutes for Portland cement or additives to improve the properties of the final product obtained. Extremely intensive development of studies pertaining to the S/S process prompted the authors to systematize the binders used and the corresponding methods of binding the contamination, and to perform an analysis of the effectiveness expressed in geomechanical properties and leachability. The study pays close attention to the types of additives and binders of waste origin, as well as the ecological and economic benefits of their use. The methods of preparing and caring for the specimens were reviewed, in addition to the methods of testing the effectiveness of the S/S process, including the influence of aging factors on long-term properties. The results of the analyses carried out are presented in the form of diagrams and charts, facilitating individual evaluation of the various solutions for the stabilization/solidification of soils contaminated with heavy metals.

Dynamics of Strength Gain in Sandy Soil Stabilised with Mixed Binders Evaluated by Elastic P-Waves during Compressive Loading

This paper addresses the problem of stabilisation of poor subgrade soil for improving its engineering properties and stiffness. The study aim is to evaluate the effects from single and mixed binders on the gain of strength in sandy soil over the period of curing. We propose an effective non-destructive approach of using P-waves for identifying soil strength upon stabilisation. The growth of strength and stiffness is strongly dependent on time of curing and type of the stabilising agents which can include both single binders and their blended mixtures. The diverse effects from mixed binders on the properties of soil were evaluated, compared and analysed. We performed the experimental trials of five different binders for stabilisation of sandy soil using cement, lime, Ground Granulated Blast Furnace Slag (GGBFS), energy fly ash and bio fly ash. The methodology included soil stabilisation by binders during a total period of 90 days, strength test for the Unconfined Compressive Strength (UCS) and seismic tests on the stabilised samples. The dynamics of soil behaviour stabilised by different binders for days 7, 14, 28 and 90 was statistically analysed and compared. The optimisation of binder blending has been performed using mixture simplex lattice design with three binders in each case as independent variables. Using P-waves naturally exploited strength characteristics of soil samples and allowed us to compare the effects from the individual and blended binders over the complete period of curing with dominating mixes. The results indicate that strength growth in stabilised soil samples is nonlinear in both time and content of binders with dominating effects from slag which contributed the most to the compressive strength development, followed by cement.

Stabilization of arsenic and antimony Co-contaminated soil with an iron-based stabilizer: Assessment of strength, leaching and hydraulic properties and immobilization mechanisms

Soils with relatively high concentrations of arsenic (As) and antimony (Sb) in mining areas would impose significant risks to human health and ecosystem. A new stabilizer PFSC composed of polymerized ferric sulfate (PFS) and calcium hydroxide (Ca(OH)₂) is proposed to stabilize the soil with co-existed As and Sb sampled at an abandoned arsenic factory site. The effects of stabilizer dosage on the properties of the stabilized soil including leached concentrations of As and Sb, unconfined compressive strength (UCS), and hydraulic conductivity (k_w) were investigated. The mechanisms of As and Sb immobilization in the soils were interpreted by Tessier's sequential extraction procedure (SEP), scanning electron microscope (SEM), and X-ray diffraction (XRD) results. The results showed increasing PFSC dosage was effective for reducing leached concentrations of As and Sb. When the PFSC dosage increased from 2% to 10%, the UCS and k_w increased from 84 to 206 kPa and decreased from 6.48 × 10^-8 to 6.33 × 10^-9 m s^-1, respectively. Tessier's SEP results showed that the leachable As and Sb fractions decreased from 12% to 5.6% and 7.5% to 3.8%, while the Fe-Mn oxides bound fractions increased from 22.3% to 29.4% and 13.2% to 19.5%. The SEM images and XRD patterns of untreated and PFSC stabilized contaminated soils indicated that hematite and calcite (CaCO₃) were the main products of PFSC stabilization processes. Adsorption on ferrihydrite, entrapment in hematite lattices, and co-precipitate with calcite might were the main mechanisms of As and Sb immobilization.

Pyrolysis of Chromated Copper Arsenate-Treated Wood: Investigation of Temperature, Granulometry, Biochar Yield, and Metal Pathways

Chromated copper arsenate-treated (cca) wood disposal faces environmental restrictions due to its toxicity, heavy metal leaching in storage sites, and greenhouse gas emissions during incineration. Thus, finding new management methods for this contaminated wood at the end of life is crucial. This study evaluated the effect of pyrolysis temperature (300, 400, and 500 °C), particle size, biochar yield, and the behavior of arsenic (As), chromium (Cr), and copper (Cu) during treated-wood pyrolysis. The highest biochar yield was obtained at 300 °C for fine particles. The biochar retention of heavy metals decreased with increasing pyrolysis temperature. At 300 °C, the highest biochar As, Cr, and Cu retentions were 76, 91, and 83%. At 500 °C, biochar only retained 43% of the As. Additionally, heavy metal leaching from the biochar exceeded the Environmental Protection Agency’s (EPA) maximum concentration limit of 5 mg/L. High-density polyethylene encapsulation of contaminated biochar reduced the leaching of As, Cr, and Cu by 96, 95, and 91%, respectively. Thus, combining pyrolysis and plastic encapsulation to produce a composite material could be a solution for reducing waste (conversion of CCA-wood into biochar) and for the safe disposal of contaminated wood.

Suppression of Sulfate-Induced Expansion with Lime-Silica Fume Blends

Sulfate-induced expansion resulting from the formation of ettringite in sulfate-bearing soil stabilised with calcium-based stabilisers is a problematic issue with technical and economic implications. Thus, this research examines the viability of the co-addition of lime (L) and silica fume (S) at varying binder dosages (4, 6, and 10 wt%), with a view of establishing the optimum blend of L–S for suppressing the ettringite-induced expansion of artificially high sulfate-dosed soil (kaolinite-K and gypsum-G). To do so, a series of laboratory specimens, designed using different gypsum and lime concentrations, were investigated using unconfined compression strength (UCS), linear expansion, and derivative thermo-gravimetric analysis (DTG) as the main criteria for the examination. The research outcomes indicated that the increasing substitution of L with S induces a gradual reduction on the UCS and linear expansion at binder levels of 4 and 6 wt%, while its usage in a high binder level (10 wt%), can yield an expansion reduction, with no compromise on the UCS performance. Therefore, silica fume has the potential for restricting ettringite formation and suppressing the expansion, of which 3L7S is the optimum blending ratio for suppressing the expansion.

Stabilization/Solidification of Heavy Metals and PHe Contaminated Soil with β-Cyclodextrin Modified Biochar (β-CD-BC) and Portland Cement

Conventional stabilization/solidification materials have defects in the simultaneous treatment of heavy metals (HMs) and phenanthrene (PHe). In order to solve this problem, a new functional material β-cyclodextrin modified biochar (β-CD-BC) was prepared by integrating the properties of biochar (BC) and the hydrophilic and hydrophobic properties of the β-CD surface and combined with Portland cement (PC) to cure and stabilize HMs and PHe. The effect of key parameters on the treatment effect was discussed by response surface method. The results showed that the minimum leaching concentration if HMs was 16.81 mg·L^-1, and the leaching concentration of PHe can be as low as 0.059 μg/kg under the conditions of β-CD-BC and Portland cement ratio of 9.75% and 11.4%, curing for 22.85 d. The weak acid soluble state reduced from 9~13% to 0.5~6%, the residual state was increased from 37~61% to 77~87%. The unconfined compressive strength of sample is more than 50 kPa. The results of this study can provide a new technical scheme for long-term curing and stabilization of HMs and PHe.

Physico-mechanical and microstructural behaviour of high-water content zinc-contaminated dredged sediment treated with integrated approach PHDVPSS

This paper uses a new integrated method, namely PHDVPSS, which utilizes vacuum pressure (VP) coupled with prefabricated horizontal drain along with solidification/stabilization (SS) for the effective treatment of high-water content dredged contaminated sediment (DCS). This study sought to evaluate the physico-mechanical and microstructural behaviour of high-water content DCS treated with MgO-GGBS (MG) and Portland cement (PC) as PHDVPSS binders and compared to the traditional Portland cement solidification/stabilization (SS-PC) method. Physico-mechanical and microstructural characteristics of the DCS treated with the PHDVPSS method were evaluated by performing a number of tests such as unconfined compressive strength (UCS), toxicity characteristics of the leaching process (TCLP), pH, X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). Treatment results showed that the DCS treated with the MG binder in the PHDVPSS method showed superior performance in terms of a significant reduction in the water content and leachability of zinc (Zn) along with higher mechanical strength and dry density of the samples compared to the traditional SS-PC method. After 56-day curing time, VP-MG cases showed 17.6 % and 50 % higher dry density values, resulting in 2.5 and 17.3 times higher UCS values than VP-PC and SS-PC cases, respectively. In contrast, VP-MG cases showed lower pH values than those of VP-PC and SS-PC cases. Moreover, VP-MG cases exhibited 37.5 % and 44.3 % lower leached Zn concentration during a TCLP test than VP-PC cases and SS-PC cases, respectively. XRD and SEM-EDS tests showed that more voluminous hydration products were produced in the VP-MG cases, which in turn produced a dense stabilized matrix and significantly reduced the leachability of zinc.

Strength and Leaching Behavior of Contaminated Mining Sludge at High Water Content Stabilized with Lime Activated GGBS

Sludge management is one of the major challenges in mining activities. The direct disposal of contaminated mining sludge can bring severe damages to the environment and community. Solidification/stabilization (S/S) is a very efficient technology for the treatment of contaminated mining sludge because it improves the stability of sludge dumping sites and reduces the leachability of contaminants. Very few studies investigate the S/S of mining sludge, especially with high water content. This paper investigated the effectiveness of S/S for the treatment of mining sludge at high water content by using quick lime (CaO) activated ground granulated blast furnace slag (GGBS) in comparison to ordinary Portland cement (OPC). To evaluate the mechanical, leaching, and microstructural behavior of CMS at high water content stabilized by lime-activated GGBS and OPC, a series of laboratory experimental tests were performed. Experimental results indicated that increasing the dosage of binder led to increased strength and decreased leachability of the heavy metal. In contrast, an increase in the water content of the mixture resulted in a decrease in compressive strength and an increase in the leachability of heavy metals. On the other hand, lime-activated GGBS mixes had substantially better performance than OPC mixes in the aspect of strength development of treated mining sludge and showed comparable capability of heavy metal stabilization compared to OPC. The microstructural tests revealed the formation of different hydration products such as calcium silicate hydrate, calcium aluminum silicate hydrate, ettringite, hydrotalcite, and heavy metal complexes in CG and OPC mixes.

Laboratory Study on the Effectiveness of Limestone and Cementitious Industrial Products for Acid Mine Drainage Remediation

Acid mine tailings may affect several environmental matrices. Here, we aimed to stabilize acid-generated mine tailings using several alkaline and cementitious amendments, which were tested in columns for 361 days. The alkaline amendments consisted of 10 and 20 wt.% limestone, while the cementitious amendments consisted of different binders at a total dosage of 5 wt.% binder. The different formulations for the cementitious amendments were: 50% Kruger fly ash and 50% class F fly ash; 20% ordinary Portland cement, 40% Kruger fly ash, and 40% class F fly ash; 80% ordinary Portland cement and 20% Kruger fly ash; and 20% ordinary Portland cement, 40% Kruger fly ash, and 40% fly ash. Kinetic testing on the amendment formulations showed that the pH values increased from <2.5 to circumneutral values (~7.5). The mobility of various chemical species was greatly reduced. Cumulative Fe released from the unamended tailings was ~342.5 mg/kg, and was <22 mg/kg for the amended tailings. The main mechanisms responsible for metal(loid) immobilization were the precipitation of secondary phases, such as Fe-oxyhydroxides, physical trapping, and tailing impermeabilization.

Acid washing of incineration bottom ash of municipal solid waste: Effects of pH on removal and leaching of heavy metals

This study systematically investigated the acid washing of incineration bottom ash (IBA) of municipal solid waste, focusing on the removal and leaching of heavy metals (Pb, Zn, Cr, Cd, Cu, and Ni), as well as their pH-dependent behavior. A series of small-scale laboratory acid washing tests with different nitric acid concentrations and washing periods were conducted. The concentrations of metals in the washing water were measured to evaluate the metal removal efficiency. Then, one stage batch leaching test was conducted for washed IBA to evaluate the leaching reduction efficiency of washing. The results showed that the maximum metal removal efficiencies for Zn, Cu, and Ni (62-76%) were higher than those for Pb, Cr, and Cd (17-25%), which were reached at the highest acid addition for most of the metals. Increasing the washing period did not always increase the metal removal efficiency. The maximum leaching reduction efficiencies were higher for Zn, Cr, and Cu (93-98%) than those for Pb, Ni, and Cd (73-79%). Both washing and leaching processes showed a similar metal concentration-pH profile for each metal. For Pb, Zn, Cr, and Cd, the metal concentration-pH profile generally followed the metal hydroxide solubility versus pH curves. For Cu and Ni, the concentration of metal decreased with the increasing pH first and then kept at a stable concentration higher than the solubility of the hydroxide, indicating that Cu and Ni in the IBA washing water and leachates did not exist dominantly as their hydroxides.

Laboratory-scale and pilot-scale stabilization and solidification (S/S) remediation of soil contaminated with per- and polyfluoroalkyl substances (PFASs)

Remediation of soil contaminated with per- and polyfluoroalkyl substances (PFAS) is critical due to the high persistence and mobility of these compounds. In this study, stabilization and solidification (S/S) treatment was evaluated at pilot-scale using 6 tons of soil contaminated with PFAS-containing aqueous film-forming foam. At pilot-scale, long-term PFAS removal over 6 years of precipitation (simulated using irrigation) in leachate from non-treated contaminated reference soil and S/S-treated soil with 15 % binder and 0.2 % GAC was compared. PFAS removal rate from leachate, corresponding to reduction in leaching potential after 6 years, was >97 % for four dominant PFASs (perfluorohexanoic acid (PFHxA), perfluorooctanoic acid (PFOA), perfluorohexanesulfonic acid (PFHxS) and perfluorooctanesulfonic acid (PFOS)), but low (3%) for short-chain perfluoropentanoic acid (PFPeA). During the pilot-scale experiment, PFAS sorption strength (i.e., soil-water partitioning coefficient (K_d)) increased 2- to 40-fold for both reference and S/S-treated soil, to much higher levels than in laboratory-scale tests. However, PFAS behavior in pilot-scale and laboratory-scale tests was generally well-correlated (p < 0.001), which will help in future S/S recipe optimization. In addition, seven PFASs were tentatively identified using an automated suspect screening approach. Among these, perfluorohexanesulfonamide and 3:2 fluorotelomer alcohol were tentatively identified and the latter had low removal rates from leachate (<12 %) in S/S treatment.

Lead Toxicity: Health Hazards, Influence on Food Chain, and Sustainable Remediation Approaches

Lead (Pb) toxicity has been a subject of interest for environmental scientists due to its toxic effect on plants, animals, and humans. An increase in several Pb related industrial activities and use of Pb containing products such as agrochemicals, oil and paint, mining, etc. can lead to Pb contamination in the environment and thereby, can enter the food chain. Being one of the most toxic heavy metals, Pb ingestion via the food chain has proven to be a potential health hazard for plants and humans. The current review aims to summarize the research updates on Pb toxicity and its effects on plants, soil, and human health. Relevant literature from the past 20 years encompassing comprehensive details on Pb toxicity has been considered with key issues such as i) Pb bioavailability in soil, ii) Pb biomagnification, and iii) Pb- remediation, which has been addressed in detail through physical, chemical, and biological lenses. In the review, among different Pb-remediation approaches, we have highlighted certain advanced approaches such as microbial assisted phytoremediation which could possibly minimize the Pb load from the resources in a sustainable manner and would be a viable option to ensure a safe food production system.

Immobilization of Radionuclide 133Cs by Magnesium Silicate Hydrate Cement

The radionuclide cesium (Cs) was solidified using magnesium silicate hydrate (M–S–H) cement. The influence of Cs⁺ on the reaction of the M–S–H gel system was evaluated by measuring the compressive strength and microscopic properties of the solidified body. By testing the impact resistance, leaching resistance and freeze–thaw resistance of the solidified body, the immobilizing ability of Cs⁺ by the M–S–H cement was analyzed. Results indicate that Cs⁺ only slightly affects the reaction process of the M–S–H gel system, and only slows down the transformation rate of Mg(OH)₂ into the M–S–H gel to a certain extent. The M–S–H cement exhibits superior performance in solidifying Cs⁺. Both the leaching rate and cumulative leach fraction at 42 d were considerably lower than the national requirements and better than the ordinary Portland cement-solidified body. The curing effect of the M–S–H cement on Cs⁺ is mainly physical encapsulation and chemisorption of hydration products.

GMCs stabilized/solidified Pb/Zn contaminated soil under different curing temperature: leachability and durability

In this study, the impact of the curing temperature on leaching behaviour and durability of GGBS-MgO-CaO (GMC)-stabilized/GMC-solidified Pb/Zn-contaminated clay soils was investigated. Toxicity characteristic leaching procedure (TCLP) test, wetting-drying cycles, freeze-thaw cycles and unconfined compression strength (UCS) test were carried out. The influence of curing temperature, binder dosage and curing time on the performance of these samples was investigated. The results show that the leachability and the durability of all samples were improved by increasing curing temperature, curing time and binder dosage. GMCs are more functional in immobilizing Pb compared with Zn, especially in immobilizing high Pb-contaminated soils. The mass loss and Pb/Zn leachability of all samples increased, while their strength decreased after cyclic wetting-drying and cyclic freeze-thaw. Furthermore, curing at 21 °C and 45 °C, the freeze-thaw resistance of 10% GMC-treated soil (GMC10) was found better than that of 10% Portland cement-treated soil (PC10). After 10 cycles of wetting-drying, GMC10 is more chemically stable than PC10.

Stabilisation/solidification of landfill leachate concentrate and its residue obtained by partial evaporation

Landfilling of waste is inseparably linked to the production of landfill leachate, which is treated and processed by different procedures. One of the options according to technical and economic development is the application of pressure-driven membrane processes, where landfill leachate concentrate (LLC) is produced. This may be further subjected to a stabilisation/solidification process (S/S) as one of its possible processing methods that leads to limited re-introduction of undesirable substances into the landfill body. This paper presents the research of the S/S of LLC, investigates the effect of the waste/binder ratio, the influence of Portland cement substitution, the influence of the additional concentration of the concentrate by evaporation at different levels from the original LLC, and the use of an innovative special highly absorbing binder based on specifically treated fly ash for selected leachate characteristics and compressive strength of the test specimen. The S/S process in most cases met the legislative requirements for water leachate characteristics for non-hazardous waste. Additionally, the comparison of indicative expense for selected solidificate compositions and scenarios is involved. The results of the study serve as necessary basement for further development of treatment of LLC.

In Situ Effectiveness of Alkaline and Cementitious Amendments to Stabilize Oxidized Acid-Generating Tailings

This study investigates the effectiveness of alkaline and cementitious additives in the in situ stabilization of localized acid-generating tailings from a closed gold mine in Abitibi–Témiscamingue, Québec (Eagle/Telbel mine site). Five field cells (including one control) were constructed and equipped with mechanisms for collecting vertical water infiltration and surface runoff. The five cells included: (C1) Control cell; (C2) 5 wt % limestone amendment; (C3) 10 wt % limestone amendment; (C4) 5 wt % half ordinary Portland cement and half fly ash amendment; and (C5) 5 wt % ordinary Portland cement amendment. The control cell showed an acidic behavior (pH < 4.5) with variable concentrations of Fe, Al, Zn, and Cu. The amendments were used to neutralize the acidic leachates and decrease dissolved metal concentrations. Leachates from surface runoff samples of amended cells were less loaded with metals compared to samples of vertical infiltration. All amendment formulations increased the pH of the leachates from approximately 4 to circumneutral values. Furthermore, metal and metalloid concentrations were greatly limited, except for Cr and As for the carbonate-based amendments. Metal(-oid) stabilization was successfully achieved using the different amendment formulations, with the exception of C2, which still released As.

In-situ solidification/stabilization of heavy metals contaminated site soil using a dry jet mixing method and new hydroxyapatite based binder

In this study, in-situ treatment using dry jet mixing construction method and SPC (single superphosphate and calcium oxide) new binder are used to solidify/stabilize a heavy metal contaminated site soil with relatively high content of organic matters. Time-dependent field performance of the soils at 41 and 326 days after treatment is evaluated, which includes electrical conductivity (EC), leachability of heavy metals and chemical oxygen demand (COD), soil penetration resistance, acid neutralization capacity (ANC), and chemical speciation of heavy metals. The results indicate that the stabilized soils exhibit satisfactory performance which is comparable with the laboratory study. In-situ SPC treatment significantly decreases EC values and increases penetration resistance values of the soils. Leachability of lead, zinc, cadmium and COD decreases with increasing SPC content or curing time. Large percentages of heave metals in the soils are transformed from exchangeable fractions to residual fractions after treatment. These, coupled with the improved ANC, result in low heavy metal leachability in stabilized soils.

Stabilization and solidification remediation of soil contaminated with poly- and perfluoroalkyl substances (PFASs)

Remediation methods for soils contaminated with poly- and perfluoroalkyl substances (PFASs) are urgently needed to protect the surrounding environment and drinking water source areas from pollution. In this study, the stabilization and solidification (S/S) technique was tested on aged PFAS-contaminated soil that were artificially spiked with 14 PFAS. To further reduce leaching of PFASs in S/S-treated soil, seven different additives were tested at 2% concentration: powdered activated carbon (PAC), Rembind®, pulverized zeolite, chitosan, hydrotalcite, bentonite, and calcium chloride. Standardized leaching tests on S/S-treated soil revealed that leaching of 13 out of 14 target PFASs (excluding perfluorobutane sulfonate (PFBA)) was reduced by, on average, 70% and 94% by adding PAC and Rembind®. Longer-chained PFASs such as perfluorooctane sulfonate (PFOS), which is considered persistent, bioaccumulative and toxic, were stabilized by 99.9% in all S/S treatments when PAC or Rembind® was used as an additive. The S/S stabilization efficiency depended on PFAS perfluorocarbon chain length and functional group, e.g., it increased on average by 11-15 % per CF₃-moeity and was on average 49% higher for the perfluorosulfonates (PFSAs) than the perfluorocarboxylates (PFCAs). Overall, the S/S treatment with active carbon-based additives showed excellent performance in reducing leaching of PFASs, without marked loss of physical matrix stability.

Gas thermal remediation of an organic contaminated site: field trial

This paper investigated the performance of in situ gas thermal remediation (GTR) technique for the remediation of organic contaminated soil and groundwater. The pollutants of the site are mainly benzene, chlorobenzene, and petroleum. To evaluate the effectiveness of GTR, 100-m² site soil with a depth of up to 18 m were installed with heating wells and extraction wells. The temperature in the GTR system was 100~200 °C, and the pressure was 0.5~2.0 kPa. It ran for 34 days. Later, soil and groundwater were sampled 7 days after treatment. The results show that the temperatures of the central remediation area and the heating well surrounding area were higher than that of the edge position. The average temperature of the surface soil was the lowest. The highest temperatures were found at depths of - 3 m and - 6 m. In addition, the removal rates of benzene, chlorobenzene, and petroleum in soil were up to 99.81%, 99.72%, and 98.23%, respectively. Meanwhile, those for groundwater were up to 98.77%, 97.70%, and 99.99%, respectively. The removal rate of pollutants in soil is affected by temperature and the nature of the soil. Although many GTR projects have been applied in the USA, this project is the first engineering application project of GTR technology in China. The results present the good in situ organic removal efficiency of GTR system in this study and therefore provide a guidance and technical support for similar projects in China.

An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade

Soil contamination by heavy metals and metalloids has been a major concern to human health and environmental quality. While many remediation technologies have been tested at the bench scale, there have been only limited reports at the field scale. This paper aimed to provide a comprehensive overview on the field applications of various soil remediation technologies performed over the last decade or so. Under the general categories of physical, chemical, and biological approaches, ten remediation techniques were critically reviewed. The technical feasibility and economic effectiveness were evaluated, and the pros and cons were appraised. In addition, attention was placed to the environmental impacts of the remediation practices and long-term stability of the contaminants, which should be taken into account in the establishment of remediation goals and environmental criteria. Moreover, key knowledge gaps and practical challenges are identified.

Fly Ash Modified Coalmine Solid Wastes for Stabilization of Trace Metals in Mining Damaged Land Reclamation: A Case Study in Xuzhou Coalmine Area

In China, coalmine wastes, such as gangues, are used for reclamation of mining subsided land. However, as waste rocks, gangues contain several trace metal elements, which could be released under natural weathering and hydrodynamic leaching effects and then migrate into the reclamed soil layer. However, it is very difficult to find adequate other backfill materials for substitution of gangues. In this paper, we present a novel method and case study to restrict the migration ability of trace metal elements in gangues by using another kind of coalmine solid waste—fly ashes from coal combustion. In this study, fly ashes were mixed with gangues in different mass proportions 1:0.2, 1:0.4, 1:0.6 and 1:0.8 as new designed backfill materials. Due to the help of fly ash, the occurrence states of studied trace metal elements were greatly changed, and their releasing and migration ability under hydrodynamic leaching effect were also significantly restricted. In this research seven trace metal elements in gangues Cu, Zn, Pb, Cd, Cr, Mn and Ni were studied by using soil column hydrodynamical leaching method and simulated precipitation for one year. The results show that under the driving of natural precipitation trace metal elements were generally transported deep inside the reconstructed land base, i.e., far away from soil layer and most of the trace metal elements were transformed into a bonded state, or combined in inert occurrence states, especially the residual state. With this method, the migration activities of tested trace metal elements were greatly restricted and the environmental potential risk could be significantly reduced.

Evaluation and determination of soil remediation schemes using a modified AHP model and its application in a contaminated coking plant

Soil remediation has been considered as one of the most difficult pollution treatment tasks due to its high complexity in contaminants, geological conditions, usage, urgency, etc. The diversity in remediation technologies further makes quick selection of suitable remediation schemes much tougher even the site investigation has been done. Herein, a sustainable decision support hierarchical model has been developed to select, evaluate and determine preferred soil remediation schemes comprehensively based on modified analytic hierarchy process (MAHP). This MAHP method combines competence model and the Grubbs criteria with the conventional AHP. It not only considers the competence differences among experts in group decision, but also adjusts the big deviation caused by different experts' preference through sample analysis. This conversion allows the final remediation decision more reasonable. In this model, different evaluation criteria, including economic effect, environmental effect and technological effect, are employed to evaluate the integrated performance of remediation schemes followed by a strict computation using above MAHP. To confirm the feasibility of this developed model, it has been tested by a benzene workshop contaminated site in Beijing coking plant. Beyond soil remediation, this MAHP model would also be applied in other fields referring to multi-criteria group decision making.

Physicochemical properties of stabilized sewage sludge admixtures by modified steel slag

Cement stabilization is used extensively to solidify sludge. However, previous studies suggest that the organic acids in sludge can restrain the hydration processes of cementitious materials. This study presents a new binder (steel slag based materials) whose effectiveness of stabilization was investigated on sewage sludge. Several series of tests for solidified sludge were conducted including the moisture content, compressive strength, shear force, permeability, and toxicity characteristic leaching (TCLP). The result shows that the stabilization effect of physicochemical properties of stabilized sewage by S1* (content 90% steel slag) were better than that of cement. With 20% S1, the moisture content of the sludge could be reduced to 30% at 20+1°C for 7 days; The compressive strength was 74.5 kappa and the permeability coefficient was 5.03 × 10–5 (cm/s); The TCLP was below the limit value. The hydration products of paste and solidified sludge at 1, 3, 14, 28, and 90 days were analyzed by XRD, SEM, and energy spectroscopy to distinguish ettringite/iron ettringite in S1 solidified sludge. the result shows the Needle-like ettringite of S1 possess high iron content and may be Fe–Aft.

An evaluation of stabilised/solidified contaminated model soil using PC-based and MgO-based binders under semi-dynamic leaching conditions

The leaching performance of stabilised/solidified contaminated model soil was studied to investigate the benefit of stabilisation/solidification treatment using novel binders over conventional binders. Different combinations of Portland cement (PC), ground granulated blast-furnace slag (GGBS), pulverised fly ash (PFA), and magnesia (MgO) were used and grouped into PC-based and MgO-based binders. A semi-dynamic leaching test was used, where the cumulative releases of Zn, Cu, Ni, Pb, Ca, and Mg were measured and the effective diffusion coefficients (De) and the leachability indices (LX) were calculated. The effects of different binders and water/cement ratios (w/c) on the migration of different metals after treatment were also discussed. The results showed that w/c ratio has a significant impact on the cumulative leachability of heavy metals. The diffusion coefficients of Pb and Zn are higher than those of Cu and Ni. In addition, mixes (w/c at 0.5:1) showed better performance in immobilising heavy metals than mixes (w/c at 1:1), especially in the case of Cu, Ni, and Pb.

Simultaneous stabilization/solidification of Mn2+ and NH4+-N from electrolytic manganese residue using MgO and different phosphate resource

This study examined simultaneous stabilization and solidification (S/S) of Mn²⁺ and NH₄⁺-N from electrolytic manganese residue (EMR) using MgO and different phosphate resource. The characteristics of EMR NH₄⁺-N and Mn²⁺ S/S behavior, S/S mechanisms, leaching test and economic analysis, were investigated. The results show that the S/S efficiency of Mn²⁺ and NH₄⁺-N could reach 91.58% and 99.98%, respectively, and the pH value is 8.75 when the molar ratio of Mg:P is 3:1 and the dose of PM (MgO and Na₃PO₄·12H₂O) is 8wt%. In this process, Mn²⁺ could mainly be stabilized in the forms of Mn(H₂PO₄)₂·2H₂O, Mn₃(PO₄)₂·3H₂O, Mn(OH)₂, and MnOOH, and NH₄⁺-N in the form of NH₄MgPO₄·6H₂O. Economic evaluation indicates that using PM process has a lower cost than HPM and HOM process for the S/S of Mn²⁺ and NH₄⁺-N from EMR at the same stabilization agent dose. Leaching test values of all the measured metals are within the permitted level for the GB8978-1996 test suggested when the dose of PM, HPM and HOM is 8wt%.

Application of a γRS index-based method and techno-economic analysis for in situ treatment of 137Cs-contaminated soils by cement-barite based stabilisation/solidification

This paper examines the application of cement (C)-barite (Ba) based-Stabilisation/Solidification (S/S) for the remediation of ¹³⁷Cs-contaminated soils, investigating the influence of soil: grout and C: Ba ratios on the shielding performance of the S/S mix assessed as gamma radiation shielding (γRS) index variation. Results from experiments were used to perform a novel approach and an economic analysis in order to calculate the effective dose reduction achievable by S/S and to assess the optimum quantities and costs of selected mixes, respectively. Gamma ray spectrometer measurements indicate that γRS index increases with increasing barite percentage up to a maximum level of 50%; however a further increase results in a worsening of the shielding performances. A maximum γRS variation of 46.5% was recorded with grout percentage increasing from 16.6 to 50%. At the photon energy of 662 keV (¹³⁷Cs), the maximum grout amount results in the possibility to shield up to 24.1% of γ-rays emitted. The effective dose reduction achievable by the investigated S/S allows a maximum ¹³⁷Cs-soil contamination in the range 2.94-14.55 kBq kg^-1 successfully treatable employing a soil: grout ratio of 1: 1 (C: Ba = 1:1). Technical data, jointly with economic analysis findings, make cement-barite based-S/S very competitive in cost-effectiveness and could provide a basis for decision-making of ¹³⁷Cs-contaminated site remediation.

Stabilisation/solidification of 137Cs-contaminated soils using novel high-density grouts: γ-ray shielding properties, contaminant immobilisation and a γRS index-based approach for in situ applicability

In this work, γ-ray shielding and Cs immobilisation performances of high-density magnetite (MG) and iron powder (IP) in Portland cement (PC) based-S/S treatment were investigated. Experimental results were elaborated using a γ-radiation shielding (γRS) index-based approach for assessing the effectiveness of in situ S/S of ¹³⁷Cs-contaminated soils. Main results reveal that the replacement of PC by MG or IP (up to 50%) leads to a marked increase (up to about 4-fold) in the γ-ray shielding performance, whereas a further material addition decreases the S/S shielding performance. The highest γRS index of ∼26% (662 keV) was found in the case of IP addition (33.3%). The use of MG-mixes allows reaching slightly slower γRS index jointly with the highest Cs-immobilisation of 97.8%. In this case, calculation shows a maximum ¹³⁷Cs-contamination level successfully treatable by in situ S/S up to ∼2.9 or ∼14.5 kBq kg^-1 for the realistic or low probability scenario, respectively, highlighting the possibility to remediate a very wide range of real contamination. Findings show MG - PC S/S as the best choice and could provide a basis for decision-making of S/S remediation of ¹³⁷Cs-contaminated sites.

A comparative study for the stabilisation of heavy metal contaminated sediment by limestone, MnO2 and natural zeolite

Three amendments including limestone, MnO₂ and natural zeolite were assessed for their stabilisation effects on Zn, Cu, Pb and Cd in sediment of east Dongting Lake, mid-south China. The metals were first subject to pollution status and potential ecological risk assessment to identify the current and potential hazards to ecology. Speciation of the metals in the sediment treated with amendments was then carried out by BCR sequential extraction procedure in order to evaluate metal contents and their potential mobility. The total concentrations of Zn, Cu, Pb and Cd in the east Dongting Lake were all above the means of national stream sediment and soil guidelines. The sediments were currently unpolluted to moderately pollute with Zn, Cu and Pb and strongly polluted with Cd. Zinc, Cu and Pb posed low risks to the lake ecosystem, and Cd had the highest potential risk that caused the overall risk of the sediment to be high or very high. Limestone stabilised Cu and Zn effectively, yet mobilisation of Cd and Pb by such amendment was also observed. MnO₂ effectively reduced the extractable Cd and Pb. The effect of natural zeolite on metal stabilisation in the sediment was least notable. Limestone and MnO₂ showed higher metal stabilisation efficiencies than zeolite. However, a single amendment is unable to achieve the goal of stabilisation for all metals. A combination use of the advantageous amendments or a search of a stronger stabiliser should be the interest of our future study.

Three-year performance of in-situ mass stabilised contaminated site soils using MgO-bearing binders

This paper provides physical and chemical performances of mass stabilised organic and inorganic contaminated site soils using a new group of MgO-bearing binders over 3 years and evaluated the time-dependent performance during the 3 years. This study took place at a contaminated site in Castleford, UK in 2011, where MgO, ground granulated blastfurnace slag (GGBS) and Portland cement (PC) were mixed with the contaminated soils in a dry form using the ALLU mass mixing equipment. Soil cores were retrieved 40-day, 1-year and 3-year after the treatment. The core quality, strength, and the leaching properties were determined via physical observation, unconfined compressive strength (UCS) and batch leaching tests. After 3-year treatment, the UCS values of ALLU mixes were in the range of 50-250kPa; the leachate concentrations of Cd, Pb, Cu and Zn (except Ni) in all mixes were lower than their drinking water standards; and the leachability of total organics was in the range of 10-105mg/L. No apparent degradation of the mass stabilised materials after 3 years' exposure to the field conditions was found. MgO-GGBS blends were found able to provide higher strength and less leachability of contaminants compared to PC and MgO-only mixes in mass stabilised soils.

Solidification/stabilization of electrolytic manganese residue using phosphate resource and low-grade MgO/CaO

In this study, P-LGMgO (low-grade MgO and NaH2PO4·2H2O), P-CaO (CaO and NaH2PO4·2H2O), and P-MgCa (low-grade MgO, CaO and NaH2PO4·2H2O) were used for the solidification/stabilization (S/S) of electrolytic manganese residue (EMR). Relevant characteristics such as ammonia nitrogen and manganese stabilization behavior, unconfined compressive strength (UCS), probable S/S mechanisms, and EMR leaching test were investigated. The results demonstrate that using P-LGMgO had higher stabilization efficiency than P-CaO and P-MgCa for the S/S of EMR at the same stabilization agent dose. The stabilization efficiency of ammonia nitrogen and manganese in the EMR were 84.0% and 99.9%, respectively, and the UCS of EMR was 5.1MPa using P-LGMgO process after curing for 28 days when the molar ratio of Mg:P was 5:1 and dose of stabilization agent was 12wt%. In this process, ammonia nitrogen was stabilized by struvite (NH4MgPO4·6H2O), and manganese by bermanite (Mn3(PO4)2(OH)2·4H2O) and pyrochroite (Mn(OH)2). The leaching test results show that the values of all the measured metals on the 28th day were within the permitted level for the GB8978-1996 test suggested by China's environmental protection law and the concentration of ammonia nitrogen can be reduced from 504.0mgL(-1) to 76.6mgL(-1).

Effects of modified zeolite on the removal and stabilization of heavy metals in contaminated lake sediment using BCR sequential extraction

Sediment can be applied on land as a soil conditioner. However, toxic substances such as heavy metals within the sediment often lead to soil contamination if no proper management is conducted prior to land application. In order to reduce the bioavailable portion of heavy metals such as Pb, Cu, Zn and Cd, zeolite as a kind of stabilizer was investigated on the effect of metal stabilization in sediment. Zeolite was firstly modified and screened to get the best condition for removal of heavy metals. Results showed that the granulated zeolite with NaCl conditioning had the highest CEC and metal sorption. Using BCR sequential extraction, the selected modified zeolite effectively stabilized Pb, Cu, Zn and Cd in sediment to different extents. It was most suitable for Cd stabilization by reducing its acid exchangeable fraction while increasing the contents of the reducible and residual fractions. Modified zeolite also immobilized Cu, Zn and Pb in sediment by enhancing one stable fraction while decreasing the acid exchangeable fraction.

Three-year performance of in-situ solidified/stabilised soil using novel MgO-bearing binders

A new group of MgO-bearing binders has been developed recently which showed improved sustainability and technical performance compared to Portland cement (PC). However, the application of these MgO-bearing binders in the Solidification/Stabilisation (S/S) techniques is very limited. This study investigates the three-year performance of a highly contaminated soil treated by in-situ S/S using MgO-bearing binders and PC. The core quality, strength, permeability and the leaching properties of the S/S materials were evaluated. The effects of binder composition, addition of inorgano-organo-clay (IOC) and the grout content on the properties of the 3-y S/S materials are discussed. It is found that although MgO alone provided negligible strength to the soil, it is superior in immobilising both inorganic and organic contaminants. Replacing MgO by ground granulated blast-furnace slag (GGBS) significantly enhanced the strength while also performed well in immobilising the contaminants. The improved pH buffering capacity was attributed to the low solubilities of brucite and hydrotalcite-like phases formed in the MgO-bearing binders, and was also the reason for the improved performance in stabilising contaminants. The addition of IOC slightly decreased the strength and the permeability of the S/S materials but inconsistent effect on the contaminant immobilisation was found depending on the binder composition. This study showed no degradation of the S/S materials after 3 y exposure to field conditions and has proved the applicability and the advantages of MgO-bearing binders over PC in S/S.

Time-dependent performance of soil mix technology stabilized/solidified contaminated site soils

This paper presents the strength and leaching performance of stabilized/solidified organic and inorganic contaminated site soil as a function of time and the effectiveness of modified clays applied in this project. Field trials of deep soil mixing application of stabilization/solidification (S/S) were performed at a site in Castleford in 2011. A number of binders and addictives were applied in this project including Portland cement (PC), ground granulated blastfurnace slag (GGBS), pulverised fuel ash (PFA), MgO and modified clays. Field trial samples were subjected to unconfined compressive strength (UCS), BS CN 12457 batch leaching test and the extraction of total organics at 28 days and 1.5 years after treatment. The results of UCS test show that the average strength values of mixes increased from 0-3250 kPa at 28 days to 250-4250 kPa at 1.5 years curing time. The BS EN 12457 leachate concentrations of all metals were well below their drinking water standard, except Ni in some mixes exceed its drinking water standard at 0.02 mg/l, suggesting that due to varied nature of binders, not all of them have the same efficiency in treating contaminated soil. The average leachate concentrations of total organics were in the range of 20-160 mg/l at 28 days after treatment and reduced to 18-140 mg/l at 1.5 years. In addition, organo clay (OC)/inorgano-organo clay (IOC) slurries used in this field trial were found to have a negative effect on the strength development, but were very effective in immobilizing heavy metals. The study also illustrates that the surfactants used to modify bentonite in this field trail were not suitable for the major organic pollutants exist in the site soil in this project.