Effects of low-alkalinity binders on stabilization/solidification of geogenic As-containing soils: Spectroscopic investigation and leaching tests

Immobilization of Cr in a heavily Cr(VI)-contaminated soil using alkali-activated blast furnace slag and calcium polysulfide: Remediation efficiency and mechanism

Chromium (Cr)-contaminated soil represents a significant environmental hazard, posing substantial risks to ecological systems. This study investigated the application of Calcium Polysulfide (CPS) and Ground Granulated Blast Furnace Slag (GGBFS) for the stabilization and solidification of Cr-rich soils. The research focused on four key aspects: leachability characteristics, mechanical strength development, hexavalent chromium [Cr(VI)] reduction efficiency, and stabilization mechanisms. Experimental results demonstrated that the treated soil achieved compressive strengths exceeding 2 MPa, indicating its potential suitability as a construction material for roadbeds. Both GGBFS and CPS exhibited strong reducing capabilities, effectively converting highly mobile Cr(VI) to the less mobile trivalent chromium [Cr(III)] species, thereby enhancing Cr stabilization. A reduction ratio of nearly 100 % was achieved with the theoretical dosage of CPS and 30 wt% GGBFS after 56 days of curing. The leached total Cr decreased from 295.6 to 2.1 mg/L, while the leached Cr(VI) concentration decreased from 165.1 mg/L to below the detection limit. The sequential extraction procedure according to Tessier's method demonstrated that chromium was predominantly transformed into more stable fractions, specifically the iron-manganese oxide-bound and residual forms. X-ray diffraction and scanning electron microscopy analyses revealed that the hydration products, predominantly calcium silicate hydrate and ettringite, effectively filled the pores and contributed to the formation of a denser microstructure. The stabilization mechanisms of Cr were identified to involve four key processes: (1) reduction of Cr(VI) to Cr(III), (2) physical encapsulation within the matrix, (3) adsorption onto hydration gels, and (4) ionic substitution of Cr(III) and Cr(VI) into the ettringite structure.

Stabilization of lead (Pb)- and arsenic (As)-contaminated soil using pen shells (Atrina pectinata)

Pen shells (PS), a type of shellfish, are abundantly consumed, and their inedible shell residues are often discarded near the coast without consideration of reutilization. This study sought to investigate the use of natural pen shells (NPS) and calcined pen shells (CPS) to stabilize Pb and As-contaminated soil. During the investigation, NPS and CPS were applied to the contaminated soil in amounts ranging from 1 to 10 wt% and cured for 28 days. After the curing process, the mineral phase was examined through X-ray powder diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) analysis. The XRD and SEM-EDX results revealed the presence of riversideite and ettringite, which contribute to Pb and As stabilization in the CPS-treated soil. The leachability of Pb and As in the treated soil was further examined with three types of chemical extraction methods. Extraction results using 0.1 M HCl displayed a notable pH fluctuation in the extractant due to the residual amendments (NPS and CPS). The fluctuation resulted in a strong correlation of leached Pb and As with the pH of the extractant, which might hinder an accurate assessment of stabilization. In order to minimize the effect of pH, an EDTA-NH4OAc extraction was employed, suggesting its potential as a suitable assessment method. EDTA-NH4OAc extraction showed a higher effectiveness of CPS than NPS at 10 wt% of input amounts. In the SBET extraction, that uses a strongly acidic solution, a higher As leachability was observed by increasing the addition of CPS, which implied a CPS-related chemical fixation mechanism. The comparison of various extraction methods showed a higher CPS effectiveness as compared to NPS. However, it was recommended that CPS-treated soil required caution in strongly acidic conditions, especially for arsenic. This study explores the applicability of PS, which has not been investigated as an amendment for Pb and As-contaminated soil previously. Furthermore, this study revealed that utilization of various extraction methods is beneficial for gaining a comprehensive understanding of the role of CaCO3-based amendment in Pb and As-contaminated soil.

Medical waste incineration fly ash-based magnesium potassium phosphate cement: Calcium-reinforced chlorine solidification/stabilization mechanism and optimized carbon reduction process strategy

The traditional solidification/stabilization (S/S) technology, Ordinary Portland Cement (OPC), has been widely criticized due to its poor resistance to chloride and significant carbon emissions. Herein, a S/S strategy based on magnesium potassium phosphate cement (MKPC) was developed for the medical waste incineration fly ash (MFA) disposal, which harmonized the chlorine stabilization rate and potential carbon emissions. The in-situ XRD results indicated that the Cl- was efficiently immobilized in the MKPC system with coexisting Ca2+ by the formation of stable Ca5(PO4)3Cl through direct precipitation or intermediate transformation (the Cl- immobilization rate was up to 77.29%). Additionally, the MFA-based MKPC also demonstrated a compressive strength of up to 39.6 MPa, along with an immobilization rate exceeding 90% for heavy metals. Notably, despite the deterioration of the aforementioned S/S performances with increasing MFA incorporation, the potential carbon emissions associated with the entire S/S process were significantly reduced. According to the Life Cycle Assessment, the potential carbon emissions decreased to 8.35 × 102 kg CO2-eq when the MFA reached the blending equilibrium point (17.68 wt.%), while the Cl- immobilization rate still remained above 65%, achieving an acceptable equilibrium. This work proposes a low-carbon preparation strategy for MKPC that realizes chlorine stabilization, which is instructive for the design of S/S materials.

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.

Negative Impacts of Arsenic on Plants and Mitigation Strategies

Arsenic (As) is a metalloid prevalent mainly in soil and water. The presence of As above permissible levels becomes toxic and detrimental to living organisms, therefore, making it a significant global concern. Humans can absorb As through drinking polluted water and consuming As-contaminated food material grown in soil having As problems. Since human beings are mobile organisms, they can use clean uncontaminated water and food found through various channels or switch from an As-contaminated area to a clean area; but plants are sessile and obtain As along with essential minerals and water through roots that make them more susceptible to arsenic poisoning and consequent stress. Arsenic and phosphorus have many similarities in terms of their physical and chemical characteristics, and they commonly compete to cause physiological anomalies in biological systems that contribute to further stress. Initial indicators of arsenic's propensity to induce toxicity in plants are a decrease in yield and a loss in plant biomass. This is accompanied by considerable physiological alterations; including instant oxidative surge; followed by essential biomolecule oxidation. These variables ultimately result in cell permeability and an electrolyte imbalance. In addition, arsenic disturbs the nucleic acids, the transcription process, and the essential enzymes engaged with the plant system's primary metabolic pathways. To lessen As absorption by plants, a variety of mitigation strategies have been proposed which include agronomic practices, plant breeding, genetic manipulation, computer-aided modeling, biochemical techniques, and the altering of human approaches regarding consumption and pollution, and in these ways, increased awareness may be generated. These mitigation strategies will further help in ensuring good health, food security, and environmental sustainability. This article summarises the nature of the impact of arsenic on plants, the physio-biochemical mechanisms evolved to cope with As stress, and the mitigation measures that can be employed to eliminate the negative effects of As.

Review on stabilization/solidification methods and mechanism of heavy metals based on OPC-based binders

Stabilization/solidification (S/S) with ordinary portland cement (OPC)-based binders is a suitable method to remediate heavy metal (HM)-contaminated soil and reuse resources of industrial wastes. In industrial wastes, alkaline wastes such as red mud (RM), soda residue (SR), pulverized fly ash (PFA), and alkalinity granulated blast furnace slag (GGBS) can immobilize HM ions (Pb²⁺, Zn²⁺, Cd²⁺, Cr³⁺, and Cu²⁺) by precipitation. However, some HM ions (such as AsO₄^3-) would redissolve within the strong alkali environment. In this case, PFA, GGBS, metakaolin (MK), and incinerated sewage sludge ash (ISSA) which have low pH, can be used to immobilize HM ions or added to the OPC-based binders to adjust the pH in the soil products. Moreover, the calcium silicate hydrate (CSH), calcium aluminum silicate hydrate (CASH), ettringite (AFt), and calcium monosulfoalumiante hydrates (AFm) generated during the pozzolanic reaction can also immobilize HM ions by adsorption on the surface, ion exchange, and encapsulation. SR and GGBS can be used to immobilize the HMs (such as CrO₄^2- and AsO₄^3-), which are mainly affected by AFt and AFm. For those not affected by AFt and AFm but related to immobilization by precipitating (such as Mn²⁺), other wastes except SR and GGBS are suitable for treating contaminated soil. Nevertheless, the formation of AFt is also instrumental for soil product strength. There are several factors affecting soil product strength. In the future, the influence of different hydration products on the S/S effects, competitive adsorption of HM ions, effects on long-term HM stabilization, and novel materials are worth being explored by researchers.

Simultaneous stabilization of Pb, Cd, and As in soil by rhamnolipid coated sulfidated nano zero-valent iron: Effects and mechanisms

Sulfidation effectively improves the electron transfer efficiency of nanoscale zero-valent iron (nZVI), but decreases the specific surface area of nZVI. In this study, sulfidated nZVI (S-nZVI) coated with rhamnolipid (RL-S-nZVI) was synthesized and used to stabilize Pb, Cd, and As in combined polluted soil. The stabilization efficiency of 0.3% (wt) RL-S-nZVI to water soluble Pb, Cd, and As in soil reached 88.76%, 72%, and 63%, respectively. Rhamnolipid coating inhibited the reduction of specific surface area and successfully encapsulated nZVI, thus reducing the oxidation of Fe⁰. The types of iron oxides in RL-S-nZVI were reduced compared to S-nZVI, but the content and strength of Fe⁰ iron were obviously enhanced. Furthermore, rhamnolipid functional groups (-COOH and -COO^-) were also involved in the stabilization process. In addition, the stabilization efficiency of RL-S-nZVI to the bioavailable Pb, Cd, and As in soil increased by 41%, 41%, and 50%, respectively, compared with nZVI. The presence of organic acids, especially citric acid, improved the stabilization efficiency of RL-S-nZVI to the three metals. The result of BCR sequential extraction indicated that RL-S-nZVI increased the residual state of Pb, Cd, and As and reduced the acid-soluble and reducible state after 28 days of soil incubation. XRD and XPS analyses showed that the stabilization mechanisms of RL-S-nZVI on heavy metals involved in ion exchange, surface complexation, adsorption, co-precipitation, chemisorption, and redox.

Ternary Ca-Mg-Al layered double-hydroxides for synergistic remediation of As, Cd, and Pb from both contaminated soil and groundwater: Characteristics, effectiveness, and immobilization mechanisms

Layered double hydroxides (LDH) are the cost-effective and high-efficiency materials for remediation of potentially toxic elements (PTEs) in contaminated soil and groundwater. Herein, the effectiveness and mechanisms of a ternary Ca-Mg-Al LDH (CMAL) for the synergistic remediation of As, Cd, and Pb were investigated in contaminated soils and simulative groundwaters for the first time. The immobilization efficiencies of As, Cd, and Pb in both black soil (BS) and red soil (RS) amended by CMAL at 5 wt% were all > 75%. CMAL amendment transferred more mobile As, Cd, and Pb fractions in soils to immobile species than did Ca-Al LDH and Mg-Al LDH treatments. Furthermore, using a pump-and-treat technology, 82-98% of these 3 PTEs from contaminated groundwater were successfully immobilized in both CMAL treated BS and RS top-soils. Meanwhile, leaching of Ca, Mg, and Al from CMAL was minimal indicating the material was stable. The excellent immobilization performance of CMAL for these PTEs was attributed to the coating of soil microparticles by CMAL nanosheets that allowed complexation of Ca-O-As/Cd or Mg-O-As/Cd/Pb formation, co-precipitation of Ca/Fe-As and Cd(OH)₂, and formation of Ca-bridged ternary complex (FeO-Ca-As/Cd). The adverse effect of oppositive pH/Eh-dependence between As and Cd/Pb was overshadowed by these mechanisms and thus allowed As immobilization. Immobilization of As, Cd, and Pb by CMAL amendment was more favorable for RS soil due to its lower reduction potential and more participation of metal-(hydr)oxides for complexation. Overall, the ternary-LDH is a promising synergistic remediation strategy for multi-PTEs contaminated soil and groundwater.

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.

Reduction, stabilization, and solidification of Cr(VI) in contaminated soils with a sustainable by-product-based binder

This study evaluated the use of a sustainable GFD binder for the stabilization/solidification (S/S) of chromium VI (Cr(VI))-contaminated soil. The GFD binder was composed of ground granulated blast furnace slag (GGBFS), fly ash and desulfurization ash, named after the initials of the three materials. The effects of curing time and binder dosage on soil unconfined compressive strength (UCS), Cr leachability, soil pH, and reduction ratio of Cr (VI) were tested. The immobilization mechanisms of Cr(VI) in contaminated soil were further explored using X-ray diffraction (XRD), scanning electron microscopy (SEM), and sequential extraction procedure (SEP). The results showed that the UCS and pH of the soil increased substantially after the GFD binder was added. After 28 days of curing with a 20% binder dosage, the leached total Cr concentration decreased from 34.4 mg/L in the contaminated soil to 1.44 mg/L in the treated soil, and the leached Cr(VI) concentration decreased from 28.0 mg/L to 0.45 mg/L. A Cr(VI) reduction ratio of 96.2% was achieved, indicating the strong reducibility of GGBFS. XRD revealed that the main hydration products of the GFD binder were hydrated calcium silicate (C-S-H) and ettringite. SEM results showed that the formation of hydration products and Cr-bearing precipitates filled the soil pores, resulting in a dense soil structure. The SEP results demonstrated that the levels of the unstable fraction F1 decreased considerably, and that the levels of the stable fractions F3 and F5 increased after treatment. Encapsulation by C-S-H, reduction by sulfides, adsorption of C-S-H, and precipitation of Cr-bearing hydroxides were the main mechanisms involved in Cr immobilization using the GFD binder.

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.

Simultaneous stabilization/solidification of arsenic in acidic wastewater and tin mine tailings with synthetic multiple solid waste base geopolymer

Stabilization/Solidification (S/S) is considered as a feasible technology for the treatment of arsenic (As) in acidic wastewater (AW) and tin mine tailings (TMTs); however, high cost, high carbon footprint, and strict reaction conditions are the main limitations. Herein, a novel alkali-activated geopolymer material (AAGM) for S/S As was synthesized by combining AW, TMT, gypsum (GP), and metakaolin (MK). At room temperature, an initial As concentration of 3914 mg/L, a NaOH content of 4.98%, and an MK content of 20% decreased the As leaching concentration to 1.55 mg/L (<5 mg/L). The main S/S mechanisms of As included physical encapsulation of C-(A)-S-H and geopolymer structures, ion exchange of ettringite, and formation of Fe-As and Ca-As precipitates. Further studies showed that increasing initial As concentration and MK content facilitated the formation of Ca-As precipitates and C-(A)-S-H gels. The semi-dynamic leaching tests revealed that the leaching mechanism of As was surface wash-off. The effective diffusion coefficients of the samples were less than 10^-13 cm²/s, and the respective leachability indexes were greater than 9, indicating that AAGM was effective in preventing the leaching of As. Therefore, this study provides a green and low cost solution for the synergistic utilization of AW, TMT, GP, and MK.

In-situ stabilization of potentially toxic elements in two industrial polluted soils ameliorated with rock phosphate-modified biochars

The present study was aimed at determining the efficacy of rock phosphate (RP) 3% loaded in a green coconut shell, chicken manure, and vegetable waste to make green coconut-modified biochar (GMB), chicken manure modified-biochar (CMB), and vegetable waste-modified biochar (VMB) in the fixation of Cr, Pb, Cu, Zn, Ni, and Cd in Sharafi goth and Malir polluted soils. The impact of RP impregnated with organic waste material to produce modified biochars (MBs) on stabilizing PTEs from polluted soils and reducing their uptake by mustard plant has not yet been thoroughly investigated. All modified BCs in 0.5, 1, and 2% doses were used to stabilize Cr, Pb, Cu, Zn, Ni, and Cd in two polluted soils and to reduce their uptake by the mustard plant. The obtained results revealed that the maximum mustard fresh biomass was 17.8% higher with GMB 1% in Sharafi goth polluted soil and 25% higher with VMB 0.5% in Malir polluted soil than in the control treatment. After applying modified BCs, immobilization of Cr, Pb, Cu, Ni, and Cd was observed in both soils and it reduced the uptake of these elements by mustard plants. On the other hand, although Zn mobilization increased by 0.38% for CMB 0.5% and by 5.9% for VMB 0.5% in Sharafi goth polluted soil, as well as by 3.15% for GMB 1%, 6.34% for GMB 2%, and 4.78% for VMB 0.5% in Malir polluted soil, this was due to changes in soil pH and OM. It was found that GMB 1%, CMB 0.5%, and VMB 0.5% have the potential to increase Zn uptake by mustard, while VMB 2% can reduce the element uptake by the plant. Redundancy analysis showed that soil chemical parameters were negatively correlated with PTEs in both soils and reduced their uptake by mustard. The present study revealed that MBs can stabilize PTEs in industrial and wastewater soils polluted with multiple metals and reduce their uptake by plants.

Immobilization and recycling of contaminated marine sediments in cement-based materials incorporating iron-biochar composites

Sustainable stabilization/solidification (S/S) incorporating biochar for hazardous wastes has attracted increasing attention. In this study, contaminated marine sediments were remediated and recycled as useful materials via cement-based S/S process incorporating iron-biochar composites derived from incinerated sewage sludge ash (ISSA) and peanut shell. Results showed that incorporation of 20% iron-biochar composites notably increased the Cr immobilization (52.8% vs 92.1-99.7%), while attained similar As (70%) and Cu (95%) immobilization efficiencies compared to the control group (CK) prepared with plain cement as the binder based on the Toxicity Characteristic Leaching Procedure. S/S products with the addition of ISSA derived iron-biochar composite had a mechanical strength of 5.0 MPa, which was significantly higher than its counterparts derived from pure iron oxide or pristine biochar (< 4.5 MPa). Microstructural and spectroscopic characterizations and chemical leaching experiments demonstrated that reduction of Cr(VI) to Cr(III) followed by formation of Cr-Fe precipitates by zero valent iron in iron-biochar composites contributed to the enhanced immobilization efficacy of Cr(VI) compared to CK. Overall, these results demonstrated the potential of applying ISSA and peanut shell derived iron-biochar composites as additives in the cement-based S/S treatment for contaminated sediments.

Combined Treatment of Cr(VI)-Contaminated Soils by Reduction, Adsorption, and Solidification

Remediation of Cr(VI)-contaminated soil usually includes reducing Cr(VI) to Cr(III) with sub-sequent solidification. In this paper, a treatment technique that combines reduction, adsorption, and solidification was proposed. By introducing an adsorbent into the reduction process, the remediation effectiveness was improved and the amount of reducing and solidified agent was decreased. Synthetic precipitation leaching procedure (SPLP), unconfined compressive strength (UCS) test, and scanning electron microscope (SEM) analysis were carried out to evaluate the remediation effect under different agent combinations and different agent-adding procedures. The results of SPLP showed that the reduction/adsorption/solidification treatment significantly reduced the leachability of Cr. UCS increased with increasing dosage of cement and CaS5, and decreased with an increasing dosage of vermiculite. The best agent dosage was CaS5 of 2 times molar stoichiometric ratio of Cr(VI), 15% of vermiculite, and 20% of cement. Orthogonal test showed that for soil with low Cr(VI) content, CaS5 dosage was the most important factor that affected the leachability of Cr. Cement and vermiculite have greater impact in limiting the leachability of Cr when Cr(VI) content in soil increased.

Enhanced remediation of arsenic-contaminated excavated soil using a binary blend of biodegradable surfactant and chelator

The reclamation of geogenic As-contaminated excavated soils as construction additives can reduce the post-disposal impact on the ecosystem and space. Although retaining soil characteristics while reducing contaminant load is a challenging task, washing remediation with biodegradable surfactants or chelators is a promising alternative to non-biodegradable counterparts. In this study, newly synthesized biodegradable surfactants (SDG: sodium N-dodecanoyl-glycinate, SDBA: sodium N-dodecanoyl-β-alaninate, SDGBH: sodium N-dodecanoyl-α,γ-glutamyl-bis-hydroxyprolinate, SDT: sodium N-dodecanoyl-taurinate, and DCPC: N-dodecyl-3-carbamoyl-pyridinium-chloride) and biodegradable chelators (EDDS: ethylenediamine N,N'-disuccinic acid, GLDA: L-glutamate-N, N'-diacetic acid, and HIDS: 3-hydroxy-2,2'-imino disuccinic acid) are evaluated for the remediation of As-contaminated soil. The operating variables, such as washing duration, solution pH, and surfactant or chelator concentration, are optimized for maximum As extraction. SDT shows the highest As-extraction efficiency irrespective of solution pH and surfactant variants, while HIDS is the superior chelator under acidic or alkaline conditions. A binary blend of SDT and HIDS is evaluated for As extraction under varying operating conditions. The SDT-HIDS binary blend demonstrates 6.9 and 1.6-times higher As-extraction rates than the SDT and HIDS-only washing, respectively, under acidic conditions. The proposed approach with a binary blend of a biodegradable surfactant and chelator is a green solution for recycling As-contaminated excavated soils for geotechnical applications.

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.

Performance of biochar-supported nanoscale zero-valent iron for cadmium and arsenic co-contaminated soil remediation: Insights on availability, bioaccumulation and health risk

Simultaneous stabilization of cadmium (Cd) and arsenic (As) in co-contaminated soil is challenging in environmental remediation because of their opposite properties. In this study, biochar-supported nanoscale zero-valent iron (nZVI-BC) was designed for simultaneously decreasing the soil availability of Cd and As and their bioaccumulation in vegetables. It was found that nZVI-BC exhibited remarkable performance for the stabilization of Cd and As in soil, and their availability decreased by 34.93% and 32.64% compared to the control sample, respectively, under 1.00% nZVI-BC treatment. The increase of soil pH and complexation dominated the Cd remediation process, while the formation of precipitation together and surface complexes transformed labile As into stable forms. Pot experiments showed that nZVI-BC application inhibited the bioaccumulation of Cd and As in vegetables by 23.63-36.48% and 43.09-45.10%, respectively, and hence effectively decreased the cancer risks by 38.19-42.93% related with vegetable consumption (P < 0.05). This study revealed that nZVI-BC is a promising amendment for achieving the simultaneous remediation of Cd and As co-contaminated farmland soil.

Immobilization of high-Pb contaminated soil by oxalic acid activated incinerated sewage sludge ash

Identifying effective and low-cost agents for the remediation of Pb-contaminated soil is of great importance for field-scale applications. In this study, the feasibility of reusing incinerated sewage sludge ash (ISSA), a waste rich in phosphorus, under activation by oxalic acid (OA) for the remediation of high-Pb contaminated soil was investigated. ISSA and OA were mixed at different proportions for the treatment of the high-Pb contaminated soil (5000 mg/kg). The Pb immobilization efficacy was further examined by both the standard deionized water leaching test and the toxicity characteristic leaching procedure (TCLP). The overall results showed that the use of the ISSA alone and an appropriate mixture of the ISSA and OA could effectively reduce the leachability of Pb from soil. 20% ISSA together with 30% OA (0.2 mol/L) reduced leached Pb concentration by 99%. The main stabilization mechanisms were then explored by different microstructural and spectroscopic analytical techniques including SEM, XRD and FTIR. Apparently, OA released phosphate from the ISSA and Pb from soil via acid attack, which combined and precipitated as stable lead phosphate minerals. However, excessive OA could cause high leaching of phosphate and zinc from the ISSA. Overall, this study indicates that ISSA could be used together with OA to remediate high-Pb contaminated soil, but careful design of mix proportions is necessary before practical application to avoid excessive leaching of phosphate and zinc from the ISSA.

Experimental Study on Solidification and Stabilization of Heavy-Metal-Contaminated Soil Using Cementitious Materials

In order to solve the shortcomings of the traditional curing agent in the treatment of composite heavy-metal-contaminated soil with the solidification and stabilization method, a new type of cementing material A was used as a curing agent, and the Pb, Cd, Cu composite heavy-metal-contaminated soil was artificially prepared to carry out an experimental study on solidification and stabilization (SS) restoration by the mechanical properties test, leaching performance test, and microscopic test. The results show that in the range of test dosage, with the increase in the curing agent content, the unconfined compressive strength of the solidified body increased, and the resistance to deformation was enhanced. From the perspective of leaching characteristics, the new curing agent A had an excellent curing effect on the composite heavy-metal-contaminated soil. To achieve safe disposal, a curing agent content of 10% applies only for the soil heavily contaminated by heavy metals. The curing agent A could significantly reduce the content of acid-extractable heavy metals after solidifying the heavy metal Pb, Cd, and Cu composite contaminated soil and effectively converted it into a residue state. The solidified phase contained hydrated products such as calcium silicate hydrate (CSH) and ettringite (AFt). These hydrated products can inhibit the leaching performance of heavy metal ions through adsorption, encapsulation, and ion exchange. The study provides a feasible method and reference for the solidification, restoration, and resource utilization of heavy-metal-contaminated soil in the subgrade.

Immobilization properties and adsorption mechanism of nickel(II) in soil by biochar combined with humic acid-wood vinegar

Biochar (BC) combined with humic acid (HA) and wood vinegar (WV) was designed and prepared as an inexpensive, effective, and environmentally friendly immobilization material (BHW) for metal-polluted soil. The influences of the wood vinegar and humic acid on the immobilization properties and adsorption mechanism of this new material were also investigated. The remediation performance was evaluated using a laboratory-made, nickel-contaminated soil with a Ni²⁺ concentration of 200 mg per kg surface soil (top 20 cm from agricultural land). The results indicated that the immobilization ratio sequence of nickel (II) in the soil was BC< BH< BHW. The maximum adsorption capacity increased in the same order: BC< BH< BHW. All three adsorption isotherms were better fitted by the Freundlich model, which were consistent with the surface heterogeneity of the remediation materials. The cause of this surface heterogeneous migration may be due to the increase in oxygen-containing groups in the BC introduced by the HA and WV. The WV can increase the number of the oxygen-containing groups in the BC combined with HA, which enhanced the adsorption and immobilization of Ni²⁺ ions. The results suggested that BHW is recommended for the remediation of metal-contaminated soils, because of its high efficacy, economic feasibility, environmental and food safety.

Stabilization mechanism of arsenic-sulfide slag by density functional theory calculation of arsenic-sulfide clusters

Stabilization of arsenic sulfur slag (As‒S slag) is of high importance to prevent the release of deadly As pollutants into environment. However, the molecular understanding on the stability of As‒S slag is missing, which in turn restricts the development of robust approach to solve the challenge. In this work, we investigated the structure-stability relationship of As‒S slag with adopting various As‒S clusters as prototypes by density functional theory (DFT). Results showed that the configuration of S multimers-covering-(As₂S₃)_n is the most stable structure amongst the candidates by the analysis of energies and bonding characteristics. The high stability is explained by orbital composition that the 4p-orbital (As) binding with 3p-orbital (S) decreases energy level of highest occupied molecular orbital (HOMO). Inspired from the calculations, an excess-S-based hydrothermal method was successfully proposed and achieved to promote the stabilization of As‒S slag. Typically, the As concentration from the leaching test of stabilized As‒S slag is only 0.8 mg/L, which is much lower than the value from other stabilized slag.

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

Arsenic (As) is a naturally occurring trace element that may pose a threat to human health and the ecosystem, while effective remediation and sustainable reuse of As-containing soil is a challenge. This study investigated the geoenvironmental characteristics of a geogenic As-rich soil, and green binders (ground granulated blast slag (GGBS) and cement blends) were employed for the stabilization/solidification (S/S) of the soil under field-relevant conditions. Results indicate that the use of 10% binder could effectively immobilize As and chemical stabilization/physical encapsulation jointly determined the leaching characteristics of the S/S soils. The geogenic As could be effectively immobilized at the pH range of 5.5-6.5. The increasing use of GGBS enhanced the strength of the 28-d cured S/S soils because of long-term pozzolanic reaction, but also slightly improved the As leachability. Besides, the moisture content of the contaminated soils should be suitably adjusted to allow for desirable compaction of S/S soils, which resulted in high compressive strength and low of As leachability. Results show that soil moisture content of 20% was the most appropriate, which resulted in the highest strength and relatively lower As leaching. In summary, this study presents a sustainable S/S binder for recycling As-contaminated soil by using a combination of cement and GGBS.

Red mud-enhanced magnesium phosphate cement for remediation of Pb and As contaminated soil

Lead (Pb) and arsenic (As) contaminated soil poses severe threats to human health. This study proposes a novel approach for synchronous stabilisation/solidification (S/S) of Pb and As contaminated soil and explains the immobilisation mechanisms in red mud-modified magnesium phosphate cement (MPC). Experimental results show that incorporation of red mud in MPC binder retarded over-rapid reaction and enhanced compressive strength via the formation of (Al,Fe,K)PO₄·nH₂O compounds as indicated by X-ray diffractometer (XRD) and elemental mapping. The presence of Pb had a marginal effect on the MPC reaction; however, the presence of As suppressed the generation of MgKPO₄·6H₂O, leading to a significant delay of setting time and a reduction of compressive strength. Extended X-ray absorption fine structure (EXAFS) analysis proved that Pb²⁺ strongly coordinated with the PO₄^3-, whereas AsO₂- gently coordinated with K⁺. The MPC binder displayed an excellent immobilisation efficiency for Pb (99.9%), but was less effective for As. The use of red mud enhanced the As immobilisation efficacy to 80.5% due to strong complexation between AsO₂- and Fe³⁺. The treated soils fulfilled requirements of metal(loid) leachability and mechanical strength for on-site reuse. Therefore, red mud-modified MPC can be an effective binder for sustainable remediation of Pb and As contaminated soil.

The factors influencing sludge incineration residue (SIR)-based magnesium potassium phosphate cement and the solidification/stabilization characteristics and mechanisms of heavy metals

Magnesium potassium phosphate cement (MKPC) is prepared from MgO and KH₂PO₄ through an acid-base reaction and has been widely used in the rapid repairs of building structures and the solidification/stabilization (S/S) of heavy metals (HMs). The use of sludge incineration residue (SIR) rich in phosphorus resources to prepare SIR-based MKPC can achieve the reclamation of SIR and efficient HM S/S. Herein, based on the exploration of the optimal MKPC magnesia/phosphate ratio (M/P), the effects of SIR and HMs on the performance of the matrix and its interaction mechanism were comprehensively investigated. The results indicated that the compressive strength of the SIR-based MKPC increased first and then decreased with the gradual increase of SIR incorporation; the optimal was reached at 40.31 MPa when the SIR incorporation was 5 wt%. The peak signal and crystal lattice of Pb₂(PO₄)₃ indicated that there is a mixed effect between HMs (in SIR) and KH₂PO₄. The Visual MINTEQ analysis results also indicated that HMs are precipitated as HM phosphates. The formation of HM phosphates not only increases the M/P (with 30 wt% SIR, M/P increased by 0.019), affecting the microstructure and changing the compressive strength of the matrix, but also promotes the transformation of HMs from the bioavailable to the more stable residual forms. The residual forms of the six HMs were all above 84% after S/S. Therefore, the SIR-based MKPC preparation significantly immobilized the HMs; particularly, the leaching toxicities of Cu (96.6%) and Zn (96.3%) were alleviated.

Reduction of bioaccessibility of As in soil through in situ formation of amorphous Fe oxides and its long-term stability

The bioaccessibility of As in soil, rather than its total concentration, is closely related to its potential risk. In this study, the in situ formation of amorphous Fe oxides was applied to As-contaminated soil to induce As-Fe coprecipitates that can withstand the gastric digestion condition of human beings. To promote the formation of Fe oxides, 2% ferric nitrate (w/w) and 30% water (v/w) were introduced, and the pH was adjusted to ~7. The chemical extractability of As in soil was determined using the solubility/bioavailability research consortium method and five-step sequential extraction. In situ formation of Fe oxides resulted in a remarkable increase in the As associated with amorphous Fe oxides, decreasing most of the exchangeable As (i.e., the sum of SO₄^2- and PO₄^3- extractable As), and thereby reducing the bioaccessibility of As. The types of association between As and Fe oxides were investigated using X-ray absorption spectroscopy analysis. Linear combination fit (LCF) analysis demonstrated that As bound to amorphous Fe oxides could exist as coprecipitates with ferrihydrite and schwertmannite after stabilization. The bioaccessibility of the coprecipitated As in soil further decreased as amorphous Fe oxides transformed to crystalline form with time, which was supported by the LCF results showing an increase of goethite in aged soil.

Stabilization/Solidification of Zinc- and Lead-Contaminated Soil Using Limestone Calcined Clay Cement (LC3): An Environmentally Friendly Alternative

Due to increased carbon emissions, the use of low-carbon and low-cost cementitious materials that are sustainable and effective are gaining considerable attention recently for the stabilization/solidification (S/S) of contaminated soils. The current study presents the laboratory investigation of low-carbon/cost cementitious material known as limestone-calcined clay cement (LC3) for the potential S/S of Zn- and Pb-contaminated soils. The S/S performance of the LC3 binder on Zn- and Pb-contaminated soil was determined via pH, compressive strength, toxicity leaching, chemical speciation, and X-ray powder diffraction (XRPD) analyses. The results indicate that immobilization efficiency of Zn and Pb was solely dependent on the pH of the soil. In fact, with the increase in the pH values after 14 days, the compressive strength was increased to 2.5–3 times compared to untreated soil. The S/S efficiency was approximately 88% and 99%, with increase in the residual phases up to 67% and 58% for Zn and Pb, respectively, after 28 days of curing. The increase in the immobilization efficiency and strength was supported by the XRPD analysis in forming insoluble metals hydroxides such as zincwoodwardite, shannonite, portlandite, haturite, anorthite, ettringite (Aft), and calcite. Therefore, LC3 was shown to offer green and sustainable remediation of Zn- and Pb-contaminated soils, while the treated soil can also be used as safe and environmentally friendly construction material.

Stabilization treatment of arsenic-alkali residue (AAR): Effect of the coexisting soluble carbonate on arsenic stabilization

Arsenic-alkali residue (AAR) from antimony smelting is highly hazardous due to its ready leachability of As, seeking for proper disposal such as stabilization treatment. However, As stabilization in AAR would be challenging due to the high content of coexisting soluble carbonate. This study conducted the stabilization treatments of AAR by ferrous sulfate and lime, respectively, and revealed the significant influence of coexisting carbonate. It was found that ferrous sulfate was more efficient than lime, which required only one-tenth of dosages of lime to reduce the As leaching concentration from 915 mg/L to a level below 2.5 mg/L to meet the Chinese regulatory limit. The combining qualitative and quantitative analyses based on XRD, SEM-EDS, and thermodynamic modeling suggested that the formation of insoluble arsenate minerals, ferrous arsenate or calcium arsenate, was the predominant mechanism for As stabilization in the two treatment systems, and their efficiency difference was primarily attributed to the coexisting carbonate, which had a slight effect on ferrous arsenate but severely obstructed calcium arsenate formation. Moreover, the examination of As leaching concentrations in 1-year-cured samples indicated that the long-term stability of ferrous sulfate treatment was far superior to that of lime treatment. This study provides ferrous salts as a promising and green scheme for stabilization treatment of AAR as well as other similar As-bearing solid wastes with coexisting soluble carbonate.

Soil amendments for immobilization of potentially toxic elements in contaminated soils: A critical review

Soil contamination by potentially toxic elements (PTEs) has led to adverse environmental impacts. In this review, we discussed remediation of PTEs contaminated soils through immobilization techniques using different soil amendments with respect to type of element, soil, and amendment, immobilization efficiency, underlying mechanisms, and field applicability. Soil amendments such as manure, compost, biochar, clay minerals, phosphate compounds, coal fly ash, and liming materials are widely used as immobilizing agents for PTEs. Among these soil amendments, biochar has attracted increased interest over the past few years because of its promising surface properties. Integrated application of appropriate amendments is also recommended to maximize their use efficiency. These amendments can reduce PTE bioavailability in soils through diverse mechanisms such as precipitation, complexation, redox reactions, ion exchange, and electrostatic interaction. However, soil properties such as soil pH, and clay, sesquioxides and organic matter content, and processes, such as sorption/desorption and redox processes, are the key factors governing the amendments' efficacy for PTEs immobilization in soils. Selecting proper immobilizing agents can yield cost-effective remediation techniques and fulfill green and sustainable remediation principles. Furthermore, long-term stability of immobilized PTE compounds and the environmental impacts and cost effectiveness of the amendments should be considered before application.

Effect of cement incorporation on the leaching characteristics of elements from fly ash and slag treated soils

Inclusion of cement in fly ash and slag mixed soils could potentially alter the leaching behavior of elements. This study investigated the leaching characteristics of calcium (Ca), magnesium (Mg), sulfur (S), manganese (Mn), barium (Ba) and chromium (Cr) from cement activated soil-fly ash, soil-slag mixtures and soil, fly ash, steel slag and cement alone. Batch water leach tests, acid neutralization capacity and pH-dependent leach tests were performed. Test results indicated that, effluent concentrations of Ca and Ba increased, while Mg concentrations decreased with cement additions. No consistent trend was observed between S concentrations and cement content. The leaching of Cr and Mn remained unaffected by cement incorporation. Results of this study showed that the solution pH had the greatest influence on the leaching behaviors of the elements. Ca, Mg, S and Mn followed cationic leaching patterns, whereas Ba showed both cationic and amphoteric leaching patterns. The highest concentrations of Cr were observed at extreme acidic conditions, followed by a concentration plateau at the pH range of 5.5-10, and subsequent decrease and increase in concentrations at pH of 11.5 and 13, respectively. Geochemical modeling results suggested that except for Cr, the leaching mechanisms of the elements were controlled by their sulfate and (hydr)oxide minerals. The leaching of Cr was possibly controlled by BaCrO₄ and CaCrO₄. It was observed that the presence of carbonate minerals did not play a significant role on the leaching mechanisms of the elements, when cement was used as an activator.

Geochemical fractionation of thallium in contaminated soils near a large-scale Hg-Tl mineralised area

Enriched levels of thallium (Tl) in the environment are not only derived from anthropogenic sources but also have potential natural origins owing to Tl-rich sulphide mineralization. However, little is known regarding the geochemical fractionations of Tl in contaminated soils from geogenic sources. This study aims to reveal the Tl geochemical fractionations in different types of soils from a large-scale independent Tl mine in southwestern China, via a modified Institute for Reference Materials and Measurement (IRMM) sequential extraction (four-step) scheme. The results revealed that a large percentage of Tl was related to the labile portions (including reducible, weak-acid-exchangeable, and oxidizable fraction) of the soils (68.8-367 mg kg^-1). Further analyses by Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectrometer (STEM-EDS) found that Tl mainly existed in the Fe-containing minerals (such as jarosite and hematite) with fine particles (∼1 μm). These results highlight that, apart from the anthropogenically induced Tl pollution, the naturally occurring Tl contamination in soils may also pose significant risks to human health and ecological safety. Owing to the relatively high mobility and bioavailability of Tl in the labile fractions, it is important to understand geochemical fractionations of this element for alleviating Tl pollution and effective management of naturally occurring Tl contaminated soils.

Mechanistic insights into red mud, blast furnace slag, or metakaolin-assisted stabilization/solidification of arsenic-contaminated sediment

Elevated level of arsenic (As) in marine sediment via deposition and accumulation presents long-term ecological risks. This study proposed a sustainable stabilization/solidification (S/S) of As-contaminated sediment via novel valorization of red mud waste, blast furnace slag and calcined clay mineral, which were selected to mitigate the increased leaching of As under alkaline environment of S/S treatment. Quantitative X-ray diffraction and thermogravimetric analyses illustrated that stable Ca-As complexes (e.g., Ca₅(AsO₄)₃OH) could be formed at the expense of Ca(OH)₂ consumption, which inevitably hindered the hydration process and S/S efficiency. The ²⁹Si nuclear magnetic resonance analysis revealed that incorporation of metakaolin for As immobilization resulted in a low degree of hydration and polymerization, whereas addition of red mud promoted Fe-As complexation and demonstrated excellent compatibility with As. Transmission electron microscopy and elemental mapping further confirmed the precipitation of crystalline Ca-As and amorphous Fe-As compounds. Therefore, red mud-incorporated S/S binder achieved the highest efficiency of As immobilization (99.9%), which proved to be applicable for both in-situ and ex-situ S/S of As-contaminated sediment. These results advance our mechanistic understanding for the design of green and sustainable remediation approach for effective As immobilization.

Effects of acid and phosphate on arsenic solidification in a phosphogypsum-based cement backfill process

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects. Cement backfill technology is an effective management technique that is used to store PG to prevent such problems. The goal of this paper is to study the influencing factors and mechanism of arsenic stabilization in a PG-based cement backfill process. First, a leaching toxicity test was conducted, which showed that the arsenic concentration in PG batches ranged from 129.1 μg L⁻¹ to 407.1 μg L⁻¹, which were all far above the standard limit (10 μg L⁻¹) set by GB/T 14848-93. In addition, the arsenic content was higher in samples with larger PG particles. Secondly, hydrogen and phosphate ions were added to the backfill to investigate how they influenced arsenic solidification, and the results indicated that phosphate ions, rather than hydrogen ions, delayed the arsenic solidification process. This suggests that controlling the soluble phosphate in PG will help reduce arsenic pollution during backfilling. A toxicity leaching test was carried out after backfill samples were cured for 28 d. All arsenic concentrations were below the standard limit, indicating that the cement backfill technology ensured the long-term solidification and stabilization of arsenic.

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects.

Green remediation of As and Pb contaminated soil using cement-free clay-based stabilization/solidification

Stabilization/solidification (S/S) is a low-cost and high-efficiency remediation method for contaminated soils, however, conventional cement-based S/S method has environmental constraints and sustainability concerns. This study proposes a low-carbon, cement-free, clay-based approach for simultaneous S/S of As and Pb in the contaminated soil, and accordingly elucidates the chemical interactions between alkali-activated clay binders and potentially toxic elements. Quantitative X-ray diffraction and ²⁷Al nuclear magnetic resonance analyses indicated that the addition of lime effectively activated the hydration of kaolinite clay, and the presence of limestone further enhanced the polymerization of hydrates. X-ray photoelectron spectroscopy showed that approximately 19% of As^[III] was oxidized to As^[V] in the alkali-activated clay system, which reduced toxicity and facilitated immobilization of As. During the cement-free S/S process, As and Pb consumed Ca(OH)₂ and precipitated as Ca₃(AsO₄)₂·4H₂O and Pb₃(NO₃)(OH)₅, respectively, accounting for the low leachability of As (7.0%) and Pb (5.4%). However, the reduced amount of Ca(OH)₂ decreased the degree of hydration of clay minerals, and the pH buffering capacity of the contaminated soil hindered the pH increase. Sufficient dosage of lime was required for ensuring satisfactory solidification and contaminant immobilization of the clay-based S/S products. The leachability of As and Pb in high-Ca S/S treated soil samples was reduced by 96.2% and 98.8%, respectively. This is the first study developing a green and cement-free S/S of As- and Pb-contaminated soil using clay minerals as an environmentally compatible binding material.

Novel synergy of Si-rich minerals and reactive MgO for stabilisation/solidification of contaminated sediment

Disposal of significant amounts of dredged contaminated sediment poses an economic and environmental problem worldwide. Transforming contaminated sediment into value-added construction materials using low-carbon MgO cement is a sustainable option; however, the weak mechanical strength and unreliable water-solubility of MgO cement restrict its practical engineering applications. This study elucidates the potential role of industrial Si-rich minerals in the performance enhancement of MgO-based products via the promotion of magnesium silicate hydrate (M-S-H) gel formation. Quantitative X-ray diffraction and ²⁹Si nuclear magnetic resonance analyses indicated that compositions and crystallinities of the Si-rich minerals significantly influence the formation and polymerisation of the M-S-H gel. Pulverised fly ash was found to be a promising Si-rich mineral for generating polymeric M-S-H gel, whereas incinerated sewage sludge ash samples demonstrated a low degree of polymerisation, and the use of glass powder samples gave a low yield of M-S-H. The formation of M-S-H gel enhanced the compressive strength and water resistance (strength retention after water immersion). Further experiments demonstrated that Si-modified MgO cement can transform dredged sediment into fill materials with satisfactory mechanical properties and contaminant immobilisation. Therefore, the synergy between reactive MgO and Si-rich industrial waste is a novel option for sustainable remediation and environmental applications.

Mitigating arsenic accumulation in rice (Oryza sativa L.) from typical arsenic contaminated paddy soil of southern China using nanostructured α-MnO2: Pot experiment and field application

Manganese oxides are naturally occurring powerful oxidants and scavengers, which can control the mobility and bioavailability of arsenic (As). However, the effect of synthetic nanostructured manganese oxides on the mobilization and transportation of As at actual paddy soils are poorly understood, especially in soils with low or medium background Mn concentration. In the present study, a novel nano manganese oxide with superior reactivity and surface area has been synthesized. A 90-d soil incubation experiment combined with pot and field rice cultivation trials were designed to evaluate the effectiveness of exogenous α-MnO₂ nanorods on the mobilization and transportation of As in soil-rice systems. Our results proved that the addition of α-MnO₂ nanorods can effectively control the soil-to-solution partitioning of As under anaerobic conditions. After treatment with different amounts of α-MnO₂ nanorods, the content of effective As decreased, offset by an increase in residual As and insoluble binding As (Ca-As and Fe-As). Enhancing the oxidation of As(III) into As(V), the α-MnO₂ nanorods increased the adsorption of As onto indigenous iron (hydr)oxides which greatly reduced the soil porewater As content. In addition, pot experiments and field applications revealed that the influx of As into the aerial parts of rice plants (stems, husk and leaves) was strictly prohibited after treatments with different amount of α-MnO₂ nanorods; more interestingly, significantly negative correlations have been observed between As and Mn in rice, which indicated that as Mn is increased in soil, As in brown rice decreases. Our results demonstrated that the use of α-MnO₂ nanorods in As polluted paddy soil containing low levels of background Mn oxides can be a promising remediation strategy.