Sustainable stabilization/solidification of arsenic-containing soil by blast slag and cement blends

A review of solid wastes-based stabilizers for remediating heavy metals co-contaminated soil: Applications and challenges

The remediation of heavy metals/metalloids (HMs) co-contaminated soil by solid wastes-based stabilizers (SWBS) has received major concern recently. Based on the literature reported in the latest years (2010-2023), this review systematically summarizes the different types of solid wastes (e.g., steel slag, coal fly ash, red mud, and sewage sludge, etc.) employed to stabilize HMs contaminated soil, and presents results from laboratory and field experiments. Firstly, the suitable solid wastes for soil remediation are reviewed, and the pros and cons are presented. Thereafter, the technical feasibility and economic benefit are evaluated for field application. Moreover, evaluation methods for remediation of different types of HMs-contaminated soil and the effects of SWBS on soil properties are summarized. Finally, due to the large specific surface, porous structure, and high reactivity, the SWBS can effectively stabilize HMs via adsorption, complexation, co/precipitation, ion exchange, electrostatic interaction, redox, and hydration process. Importantly, the environmental implications and long-term effectiveness associated with the utilization of solid wastes are highlighted, which are challenges for practical implementation of soil stabilization using SWBS, because the aging of soil/solid wastes has not been thoroughly investigated. Future attention should focus on modifying the SWBS and establishing an integrated long-term stability evaluation method.

Innovative methodology for comprehensive utilization of arsenic-bearing neutralization sludge

Arsenic-bearing neutralization (ABN) sludge is a classical hazardous waste commonly found in nonferrous metallurgy. However, the current storage of these hazardous wastes not only has to pay costly hazardous waste taxes but also poses significant risks to both the environment and human health. To address these issues and achieve the comprehensive utilization and minimization of ABN sludge, this study proposes a new combined process. The process involves selective reduction roasting, leaching, and carbonation, through which, the arsenate and gypsum in the ABN sludge were recovered in the form of As(s), high-purity CaCO3, and H2S. The selective reduction behaviors of arsenate and gypsum were investigated through thermodynamic analysis and roasting experiments. The results indicated that the 95.35 % arsenate and 96.55 % gypsum in the sludge were selectively reduced to As4(g) and CaS at 950 °C by carbothermic reduction. The As4(g) was condensed to As(s) and enriched in the dust (As, 96.78 wt %). In the leaching process, H2S gas was adopted to promote the leaching of CaS, and resulted in 97.41 % of CaS in the roasted product was selectively leached in the form of Ca(HS)2, leading to a 74.11 % reduction in the weight of the ABN sludge. Then, the Ca(HS)2 was subjected to capture CO2 for the separation of Ca2+ and S2-. The result depicted that 99.69 % of Ca2+ and 99.12 % of S2- were separated as high-purity (99.12 wt %) CaCO3 and H2S (24.89 vol %) by controlling the terminal carbonation pH to below 6.55. The generated H2S can be economically converted to sulfur by the Clause process. The whole process realized the comprehensive resource recovery and the minimization of the sludge, which provides an alternative solution for the clean treatment of hazardous ABN waste.

Arsenic species in soil profiles from chemical weapons (CWs) burial sites of China: Contamination characteristics, degradation process and migration mechanism

In this study, soil profiles and pore water from Japanese abandoned arsenic-containing chemical weapons (CWs) burial sites in Dunhua, China were analyzed to understand the distribution of arsenic (As) contamination, degradation, and migration processes. Results of As species analysis showed that the As-containing agents underwent degradation with an average rate of 87.55 ± 0.13%, producing inorganic pentavalent arsenic (As5+) and organic arsenic such as 2-chlorovinylarsonic acid (CVAOA), triphenylarsenic (TPA), and phenylarsine oxide (PAO). Organic arsenic pollutants accounted for 1.27-18.20% of soil As. In the vertical profiles, total As concentrations peaked at about 40-60 cm burial depth, and the surface agricultural soil exhibited moderate to heavy contamination level, whereas the contamination level was insignificant below 1 m, reflecting As migration was relatively limited throughout the soil profile. Sequential extraction showed Fe/Al-bound As was the predominant fraction, and poorly-crystalline Fe minerals adsorbed 33.23-73.13% of soil As. Oxygen-susceptible surface soil formed poorly-crystalline Fe3+ minerals, greatly reducing downward migration of arsenic. However, the reduction of oxidizing conditions below 2 m soil depth may promote As activity and require attention.

Geoenvironmental properties of a Cr(VI)-contaminated soil treated by alkali-activated GGBS under freeze-thaw cycles: Insights into Cr species transformation and microscopic mechanism

Stabilization/solidification is the most frequently used method for treating soils contaminated by heavy metals; however, degradation of the treatment will occur under freeze-thaw (F-T) cycles. In this paper, a low-carbon emission by-product, ground granulated blast furnace slag (GGBS), was adopted as a binder to treat Cr(VI)-contaminated soil after alkali excitation. Built on the usage scenarios of subgrade materials, the impact of F-T cycles and initial water content on the geoenvironmental properties of the treated soils, including leaching toxicity, unconfined compressive strength (UCS), pH, Eh, and permeability, were discussed. To investigate the mechanisms of the changing properties, this study analyzed the chemical morphology of Cr, the micromorphology of the reaction products, and the pore characteristics. The results demonstrated that negative impact of F-T cycles on treatment effectiveness was low at the optimal water content. After 28 F-T cycles, the Cr(VI) component increased by 6.4 %, and the leached Cr concentration showed a significant increase, especially for specimens with low water content. A new solid phase with mixed valence Mn(III/IV), mainly composed of birnessite and manganite, was observed via microscopic analysis. During the first 3 F-T cycles, the content of hydration gel increased by 0.18 %, and the cumulative pore volume decreased such that the UCS increased by an average of 1.2 MPa. This study demonstrated that a few F-T cycles would result in a secondary alkali-activated GGBS reaction, enhancing the treatment effect. However, additional F-T cycles would create an oxidizing environment under which the initially precipitated Cr(III) would react with manganese oxide, resulting in more Cr(VI) released. The degree of reoxidation was closely related to the initial water content of the solidified soil.

Management of arsenic-contaminated excavated soils: A review

Ongoing global sustainable development and underground space utilization projects have inadvertently exposed many excavated soils naturally contaminated with geogenic arsenic (As). Recent investigations have revealed that As in certain excavated soils, especially those originating from deep construction projects, has exceeded regulatory limits, threatening the environment and human health. While numerous remediation techniques exist for treating As-contaminated soil, the unique characteristics of geogenic As contamination in excavated soil require specific measures when leachable As content surpasses established regulatory limits. Consequently, several standard leaching tests have been developed globally to assess As leaching from contaminated soil. However, a comprehensive comparative analysis of these methods and their implementation in contaminated excavated soils remains lacking. Furthermore, the suitability and efficacy of most conventional and advanced techniques for remediating As-contaminated excavated soils remained unexplored. Therefore, this study critically reviews relevant literature and summarize recent research findings concerning the management and mitigation of geogenic As in naturally contaminated excavated soil. The objective of this study was to outline present status of excavated soil globally, the extent and mode of As enrichment, management and mitigation approaches for As-contaminated soil, global excavated soil recycling strategies, and relevant soil contamination countermeasure laws. Additionally, the study provides a concise overview and comparison of standard As leaching tests developed across different countries. Furthermore, this review assessed the suitability of prominent and widely accepted As remediation techniques based on their applicability, acceptability, cost-effectiveness, duration, and overall treatment efficiency. This comprehensive review contributes to a more profound comprehension of the challenges linked to geogenic As contamination in excavated soils.

Influence of pH on the leaching behavior of a solidified arsenic contaminated soil

Stabilization/solidification is widely used for treatment of arsenic (As)-contaminated soils. The stability of the soil may deteriorate significantly when exposed to acid or alkaline leachate. In this study, semi-dynamic leaching tests under different pH were carried out to investigate the leaching behavior of As from the solidified soils. Spectroscopic and microscopic analyses were performed to reveal the related mechanisms. The results showed that the leaching of As was closely correlated with the pH of the leachate, because the encapsulation effect of the cementitious matrix and the chemical speciation and valence of As were all significantly influenced by pH. In the strongly acidic leachant (pH 3.0), the leached As concentration increased by an order of magnitude, and the effective diffusion coefficient of As reached 3.71 × 10-13 m2/s. This is because that pores and cracks increased owing to the acidic corrosion of CSH, such that the physical encapsulation effect was reduced and the mobility of As increased. The leachability index showed that the solidified soil was unsuitable for 'controlled utilization' under strongly acidic conditions. The leached As concentration was the lowest in the weakly alkaline leachant (pH 9.0) because under weakly alkaline conditions the hydration process of the cement was facilitated, and more CSH gels were attached to the surface of the soil particles, forming a tighter structure for As encapsulation. However, as pH increased from 9.0-11.0 the leached As concentration increased due to an increased content of As(III)-O in the solidified soil.

Application of a novel Ca-Fe-Si-S composite for the synchronous stabilization of As, Zn, Cu, and Cd in acidic arsenic slag

The control of multiple heavy metals (HMs) pollution in solid wastes, especially the co-contamination of As and other heavy metal cations, is of great importance to ecological and environmental health. To address this problem, the preparation and application of multifunctional materials have attracted wide attention. In this work, a novel Ca-Fe-Si-S composite (CFSS) was applied to stabilize As, Zn, Cu, and Cd in acid arsenic slag (ASS). The CFSS exhibited synchronous stabilization ability for As, Zn, Cu, Cd and owned strong acid neutralization capacity. Under simulated field conditions, the acid rain extracted HMs in ASS successfully decreased below the emission standard (GB 3838-2002-IV category in China) after incubated by 5% CFSS for 90 days. Meanwhile, the application of CFSS promoted the transformation of leachable HMs into less accessible forms, which was conductive to the long-term stabilization for HMs. There was competitive relation among the three heavy metal cations, following the stabilization sequence of Cu > Zn > Cd during incubation. And the stabilization mechanisms of HMs by CFSS were proposed as chemical precipitation, surface complexation, and ion/anion exchange. The research will be greatly conducive to the remediation and governance of field multiple HMs contaminated sites.

Reutilization of ferro-arsenic waste sludge for the development of concrete blocks through solidification: conservation of natural aggregates with policy suggestion

 In the present study, arsenic sludge and iron sludge extracted from a laboratory scale water treatment plant were aimed to reutilize for the development of concrete blocks. Three different grades (M15, M20 and M25) of concrete blocks were made by blending of arsenic sludge and improved iron sludge (50% sand and 40% iron sludge) in the range of density of 425 to 535 kg/m3 at an optimum ratio of 10:90 (arsenic: iron sludge) followed by mixing of designed quantity cement, coarse aggregates, water and additives. Concrete blocks developed based on this such combination exhibited 26 MPa, 32 MPa and 41 MPa compressive strengths, and 4.68 MPa, 5.92 MPa and 7.78 MPa tensile strengths for M15, M20 and M25, respectively. In comparison with the developed concrete blocks and the blocks made with 10% arsenic sludge and 90% fresh sand, the developed ones (employing 50% sand, 40% iron sludge and 10% arsenic sludge) showed more than 200% higher strength perseverance on average. Successful Toxicity Characteristic Leaching Procedure (TCLP) and compressive strength of the sludge-fixed concrete cubes classified it as a non-hazardous and completely safe to use value-added material. This process involves stabilization of arsenic-rich sludge generated from high-volume long-run laboratory-based arsenic-iron abatement set-up from contaminated water with successful fixation in solid matrix of concrete through complete substitution of natural fine aggregates (river sand) in cement mixture. Techno-economic assessment reveals such concrete block preparation at $0.09 each which is lesser than 1/2 of the present market price of same quality concrete block in India.

Iron-calcium reinforced solidification of arsenic alkali residue in geopolymer composite: Wide pH stabilization and its mechanism

Arsenic-alkali residue (AAR) from antimony production can pose significant health and environmental hazards due to the risk of arsenic (As) leaching. In this study, geopolymer composite synthesized from fly ash (FA) was investigated for efficient stabilization of high-arsenic-containing AAR (As₂O₃ of 22.74 wt%). Two industrial wastes, e.g., granulated blast furnace slag (GBFS) with active calcium composition and water-quenched slag (WQS) from lead-zinc smelting with active iron composition, were investigated for the reinforcement of AAR geopolymer solidification. A wide pH stabilization (from pH = 3-pH = 12) of AAR with the geopolymer composite was successfully achieved, and As leaching concentration of geopolymer with the addition of 5 wt% AAR was significantly reduced from 2343.73 mg/L (AAR) to that below 0.18 mg/L, which successfully meet the regulatory limit of Chinese domestic waste landfill (GB, 18598-2019, 1.2 mg/L) and hazardous waste landfill (GB16889-2008, 0.3 mg/L). Johnbaumite (Ca₅(AsO₄)₃(OH)) was formed in geopolymer composite and leached samples with initial pH from 2.6 to 6 (final pH from 5.54 to 13.15). Magnetite and iron hydroxide phases with strong adsorption and/or As co-precipitation capability were also observed. As stabilization was also achieved with iron oxidation from As(III) to As(V). This study solves the problem of unstable As leaching at different pH for the solidification of arsenic-bearing solid waste, and provides a promising and practical strategy for efficient solidification/stabilization of AAR as well as other similar arsenic-bearing solid wastes with geopolymer composite.

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.

Efficient remediation of heavily As(III)-contaminated soil using a pre-oxidation and stabilization/solidification technique

The high mobility of As(III) makes it difficult to remediate heavily As(III)-contaminated soil. A novel remediation technique that combines pre-oxidation and stabilization/solidification (PO + S/S) is proposed in this study to remediate heavily As(III)-contaminated soil. After oxidizing As(III) in the contaminated soil using Fenton's reagent, FeCl₃·6H₂O was used as a chemical stabilizing agent to reduce the toxicity and mobility of As. Finally, Portland cement (PC) was used for solidification. The effects and mechanisms of the proposed technique were studied using unconfined compressive strength tests, leaching tests, sequential extraction procedure (SEP), and a series of spectroscopic/microscopic investigations. The experimental results showed that the addition of FeCl₃·6H₂O increased the strength of the curing body because the hydration degree of PC and pore structure were improved. Portland cement can increase the pH of the curing body. At a 1:1 Fe to As molar ratio and a 15 wt% PC dosage, the leached As concentration decreased to 3.25 mg L^-1, and the remediation efficiency reached 99.54%. The SEP results showed that the PO + S/S treatment converted As into more stable phases and effectively reduced the potential mobile phase risk. The majority of As was bound to hydrated iron oxides; however, the increased pH affected the Fe-As interactions and prompted the release of As from the surface of the hydrated iron oxides. Spectroscopic/microscopic investigations indicated that the PO + S/S treatment converted As(III) to less toxic and less mobile As(V) and then immobilized by the encapsulation of calcium silicate hydrate and ion exchange of ettringite. This study provides a scientific basis and theoretical support for the effective remediation of heavily As(III)-contaminated soil.

Long-Term Stabilization/Solidification of Arsenic-Contaminated Sludge by a Blast Furnace Slag-Based Cementitious Material: Functions of CaO and NaCl

Arsenic is a kind of element widely distributed in the environment that may pose a threat to the ecological environment and human health, while effective remediation and sustainable utilization of arsenic-containing sludge is a challenge. Based on stabilization/solidification blast furnace slag-based cementitious materials (BCMs), this study innovatively proposes to improve the arsenic (As) solidification efficiency and long-term stability by using the activation mode of CaO and NaCl. The effects of different factors on the properties of the BCM were measured by unconfined compressive strength (UCS) tests, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The long-term stability and safety of the BCM were verified by leaching toxicity and improved three stage continuous extraction method (BCR) tests. Experimental results show that the addition of CaO provides conditions for the formation of ettringite (AFt), thus promoting the crystal growth of AFt. The addition of NaCl can promote the formation of Cl-AFt and play a good long-term stabilizing role. When the content of the alkali activator is 10% and the modulus is 1.0, the contents of CaO and NaCl are 10 and 1%, respectively. The BCM has the best efficiency in terms of UCS and As solidification. The UCS at 28 days was 5.4 MPa, and the leaching concentration of As was 0.309 mg/L, and the As solidification efficiency was up to 99.9%. In the improved BCR test, the proportions of residual and oxidizable states of arsenic increased by 19.6 and 13.5%, respectively, and the stability of heavy metals improved. These findings show that the BCM has good long-term stability and safety. Overall, this study shows that CaO and NaCl significantly increase the output of AFt and achieve the purpose of efficient and stable solidification of As by the BCM.

Effect of slag composition on the distribution and separation behavior of arsenic between CaO-based slag and liquid copper

To determine the effects of slag basicity and oxygen potential on the distribution of arsenic (As) between slag and liquid copper, high-temperature experiments were conducted using a Kanthal-Super electric furnace. The thermodynamic driving force of As removal from liquid copper was found to increase as the slag basicity increased because the excess free energy of As₂O₃ decreased with increasing basic oxide content in the slag. The addition of a small amount of Na₂O improved As removal efficiency from the liquid copper. In the case of acidic (i.e., low basicity) slag, the As was minimally distributed to slag phase due to the low activity of CaO in the slag. As a result, the basicity of slag should be carefully controlled for the efficient removal of As. In addition, As was segregated and stabilized in the (Mg,Fe)O monoxide as well as in crystalline phases (magnesium arsenate, Mg₃(AsO₄)₂ and ferric arsenate, FeAsO₄) formed due to the high contents of MgO and FeO in the slag, from which it was concluded that As enrichment and separation can be achieved during cooling process.

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.

An all-in-one strategy for resource recovery and immobilization of arsenic from arsenic-bearing gypsum sludge

Arsenic (As)-bearing gypsum sludge, one of the most prominent hazardous wastes, has created a myriad of critical problems in human health, waters, soils, and sediments at the global scale. Unfortunately, the reclamation and disposal of As-bearing gypsum sludge have been rarely investigated. This paper aims to explore a novel technology for simultaneous value-added utilization and harmless exploitation of As-bearing gypsum sludge. In the experiment, As-bearing gypsum sludge and anthracite were mixed, granulated, and then roasted in Ar atmosphere. Based on the thermodynamic analysis and experimental results, the As migration mechanism in the As-bearing gypsum sludge was determined during the roasting process. Under optimal conditions, 90% of As phase was volatilized and then recovered in the form of elemental As_99.5, and it could act as a chemical product. In addition, As_99.5 could be further processed into high-purity As and As₂O₃ using existing chlorination-rectification-reduction process and oxidation process, respectively, which can be widely used in the treatments of semiconductor material, pigment, and wood. Residual As primarily occurred as Fe-As compounds, but the leached As concentration in the toxicity characteristic leaching procedure was only 0.008 mg/L. Correspondingly, a new As immobilization method that generates Fe-As compounds (α-Fe and AsFe₂) is first proposed and then verified, which may be widely used for simultaneous As-bearing solid wastes reduction and improved harmlessness. This paper is significant for development of the metallurgical, mining, acid, and thermal power industries, minimizing its environmental risk.

Study on Solidification and Stabilization of Antimony-Containing Tailings with Metallurgical Slag-Based Binders

Blast furnace slag (BFS), steel slag (SS), and flue gas desulfurized gypsum (FGDG) were used to prepare metallurgical slag-based binder (MSB), which was afterwards mixed with high-antimony-containing mine tailings to form green mining fill samples (MBTs) for Sb solidification/stabilization (S/S). Results showed that all MBT samples met the requirement for mining backfills. In particular, the unconfined compressive strength of MBTs increased with the curing time, exceeding that of ordinary Portland cement (OPC). Moreover, MBTs exhibited the better antimony solidifying properties, and their immobilization efficiency could reach 99%, as compared to that of OPC. KSb(OH)₆ was used to prepare pure MSB paste for solidifying mechanism analysis. Characteristics of metallurgical slag-based binder (MSB) solidified/stabilized antimony (Sb) were investigated via X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). According to the results, the main hydration products of MSB were C-S-H gel and ettringite. Among them, C-S-H gel had an obvious adsorption and physical sealing effect on Sb, and the incorporation of Sb would reduce the degree of C-S-H gel polymerization. Besides, ettringite was found to exert little impact on the solidification and stabilization of Sb. However, due to the complex composition of MSB, it was hard to conclude whether Sb entered the ettringite lattice.

Speciation characterization of arsenic-bearing phase in arsenic sulfide sludge and the sequential leaching mechanisms

Arsenic sulfide sludge (ASS) is a kind of deleterious waste which contains various valuable metallic elements, such as Re and Pb, which are always associated with arsenic-bearing phases in ASS. The leaching speed and efficiency of valuable elements may depend on the phase constitution. Here, we proposed a sequential leaching method to thoroughly understand the constitution of arsenic-bearing phase and the distribution of valuable elements in ASS. The results show that five major arsenic-bearing phases exist in ASS: amorphous As₂S₃, crystalline As₄S₄, As₂O₃, and As atoms dissolved into the lattice of PbS and PbSO₄ phases. Re is mainly distributed in As₂S₃ and As₄S₄ phases. During the leaching process, the dissolution of As₂O₃ particles and As₂O₃ layers on the surface of As₂S₃/As₄S₄ particles occurs first. Then, the reaction between As₂S₃/As₄S₄ particles and copper sulfate happens. The order of leaching sequence is As₂O₃, amorphous As₂S₃ and crystalline As₄S₄. The majority of Re element exists in the solution while almost all Pb element remains in the solid residues, which is beneficial for the separation and purification valuable elements individually. This work not only detailed determines the arsenic-bearing species, but also provides significant theoretical bases for extracting valuable elements from ASS.

Red mud-metakaolin based cementitious material for remediation of arsenic pollution: Stabilization mechanism and leaching behavior of arsenic in lollingite

The proper treatment of lollingite is of great significance due to its rapid oxidation leading to release of arsenic into the environment. Herein, a green multi-solid waste geopolymer, consisting of red mud, metakaolin, blast furnace slag, and flue gas desulfurization gypsum, was developed. The obtained red mud-metakaolin-based (RMM) geopolymer demonstrated good arsenic retention capability. The results showed that the replacement of SO₄^2- in ettringite with AsO₄^2- via ion exchange, formation of Ca-As and Fe-As precipitates, and physical encapsulation with aluminosilicate gel were the main mechanisms that prevented the release of arsenic. Further dissolution of ettringite in RMM was alleviated by adding a suitable amount of Ca(OH)₂ and controlling the pH of the leachate. TCLP results verified that RMM materials possessed an outstanding ability to stabilize arsenic, with a leaching rate below the permitted value of 5 mg/L for safe disposal. The low leachability of the RMM geopolymers (<0.50 mg/L) is potentially related to the pH buffering capacity of the hydration products at a pH range of 2-5. RMM geopolymers showed a high compressive strength (>15 MPa) and low arsenic leaching concentration (<2.66 mg/L) after 28 days of curing. These results demonstrate the potential of RMM geopolymers to be utilized as an environmentally friendly backfilling cementitious material for sustainable remediation of arsenic pollution.

Immobilization of molybdenum by alternative cementitious binders and synthetic C-S-H: An experimental and numerical study

Excavation operations during construction produce millions of tons of soil sometimes with high leachable molybdenum (Mo) contents, that can lead to risks for both human health and the environment. It is therefore necessary to immobilize the Mo in excavated soils to reduce pollution and lower the costs of soil disposal. This paper studies the immobilization of Mo by three cementitious binders. To this end, one Ordinary Portland cement (OPC), one binder composed of 90% ground granulated blast furnace slag (GGBS) and 10% OPC, and one supersulfated GGBS binder were spiked with sodium molybdate at six different Mo concentrations from 0.005 wt% to 10 wt% before curing. In addition, to gain mechanistic insights, the capacity of synthetic calcium silicate hydrates (C-S-H) to immobilize Mo was studied. This study was completed by thermodynamic modeling to predict the immobilization of Mo at low Mo concentrations (<0.005 wt%). Paste leaching tests results showed that more than 74% of the initial Mo spike was immobilized by the three binders. The supersulfated GGBS binder consistently showed the highest retention levels (92.0 to 99.7%). The precipitation of powellite (CaMoO4) was the dominant mechanism of Mo retention in all binders and most leaching solutions were oversaturated with respect to powellite. Also, in C-S-H syntheses, Mo was largely immobilized (>95%) by the coprecipitation of powellite. Thermodynamic modeling was in good agreement with measured values when the equilibrium constant of powellite was modified to LogK = -7.2. This suggested that powellite is less stable in cementitious environments than would be expected from thermodynamic databases. Moreover, modeling showed that, for a solution at equilibrium with portlandite or C-S-H, the Mo concentration is limited to 1.7 mg/L by powellite precipitation. In contrast, for a solution saturated with respect to ettringite, the threshold concentration for powellite precipitation is 6.5 mg/L.