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

Eliminating the stabilizer antagonistic effects for efficiently stabilizing Pb and As co-contaminated soil by innovative stepwise steam flash heating

Chemical stabilization is frequently used to stabilize lead (Pb) or arsenate (As), but faces challenges in Pb-As co-contaminated soils because of the antagonistic reactions between chemical stabilizers and contaminants. In this work, we innovated an effective and cost-efficient stepwise steam flash heating (SSFH) strategy to simultaneously immobilize Pb and As, and unraveled the underlying mechanisms. The combination of 1.5% Ca(H2PO4)2 and 2% Fe2(SO4)3 only decreased 1.99% Pb by toxicity characteristic leaching procedure (TCLP-Pb) but increased 17.8% of TCLP-As due to the antagonistic effects. SSFH with Ca(H2PO4)2 in the first step and Fe2(SO4)3 in the second step achieved the minimal TCLP-Pb and TCLP-As of 0.778 and 0.327 mg/L, respectively. It also reduced 69.8% of leachable As in 100-year acid rain simulation, indicating a favorable long-term stabilization performance. Additionally, SSFH approach reduced 43.2% stabilizer dosage and 14.9% cost. X-ray absorption near edge structure (XANES) documented that the stepwise SFH promoted the transformation of Pb(NO3)2 and NaAsO2/NaAsO3/As2O3/As2O5 into stable Pb3(PO4)2 and FeAsO4, preventing the formation of AsO43- and FePO4. Our findings proved the state-of-the-art SSFH approach and unraveled its mechanisms to stabilize Pb and As co-contamination in soils, offering a green and sustainable remediation alternative for the management of heavy metal contaminated sites. ENVIRONMENTAL IMPLICATION: A novel stepwise SFH approach can be applied to overcome the stabilizer antagonist effects by separately immobilizing Pb and As in two sequential steps. It also decreased 43.2% of stabilizer dosage and 14.9% of cost comparing to conventional chemical stabilization. This approach can be used for other metal co-contaminated soils facing similar antagonistic challenges, and our work raises a state-of-the-art solution for cost-effective, green and sustainable remediation practices.

Upcycling textile sludge into magnesium oxychloride cement: Physical properties, microstructure, and leaching behavior

Textile sludge is a by-product produced during the wastewater treatment process in the textile printing and dyeing industry. Textile sludge is rich in heavy metal elements, which makes it a potential risk to the surrounding environment. This study designs a magnesium oxychloride cement (MOC) components to solidify harmful substances in textile sludge and studies the influence of textile sludge ash (TSA) on the mechanical properties and microstructure of MOC samples. The results indicated that adding 5 %-20 % TSA is beneficial for increasing the compressive strength of air-cured MOC paste and improving its water resistance. Meanwhile, the MOC sample shows volume expansion in 168 h, which is related to the further hydration of residual MgO. Incorporating 10 %-20 % TSA substantially increased the volume expansion ratio of the mixture compared to plain MOC sample. In addition, the porosity of TSA-modified MOC after water curing did not change significantly compared to the sample before water curing, while the pore structure of plain MOC after water curing significantly coarsened. This is mainly because TSA reacts with MOC and generates Mg-Al-Cl-Si-H and Mg-Cl-Si-H gels, consequently improving the water stability of MOC sample. At the nanoscale, the 3/5-phase crystal and unreacted MgO content in the 15 % TSA-modified MOC sample is relatively reduced by 7.79 % and 25 %, respectively, compared to the plain sample, but the 13 % gel phase is detected. In addition, the MOC component can effectively solidify heavy metal elements in textile sludge. For the leachate of 20 % TSA-modified MOC paste, the Ni element is not detected, and its solidifying effect on heavy elements such as Zn and Mn exceeded 99 %.

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.

Roles of red mud in remediation of contaminated soil in mining areas: Mechanisms, advances and perspectives

Red mud (RM) is a kind of strong alkaline solid waste produced from the aluminum industry, which contributes significantly to environmental pollution and can cause severe health issues.Currently, RM is widely recognized as a potential material for soil remediation because of its rich metal oxide content, such as Fe/Al oxides. However, there is no comprehensive description on the roles of RM in passivation remediation of contaminated soil in mining areas. This review summarizes the mechanisms of passivation of heavy metals (HMs) in contaminated soil by RM, including precipitation, adsorption and ion exchange. Besides the effects of adding RM on soil physicochemical properties, heavy metal forms and ecological environment are further elaborated. Moreover, using the co-hydrothermal carbonization of RM and biomass for enhancing the efficiency of contaminated soil remediation is proposed as the main prospective research. This paper provides technical references for the resource utilization of RM and the treatment of heavy metal-contaminated soil.

Direct synthesis of Fe-aluminosilicates from red mud for catalytic deoxygenation of waste cooking oil

Conversion of red mud (RM) that contains a high level of silica, alumina and iron minerals into heterogenous catalysts, offers a route for the utilization of abundant toxic by-products of bauxite refining. In this study, the conversion of red mud into mesoporous Fe-aluminosilicate produced selective catalysts for the deoxygenation of waste cooking oil to green diesel hydrocarbons. Direct conversion of red mud in the presence cetyltrimethylammonium bromide into Fe-aluminosilicate (RM-CTA) produced a highly mesoporous structure with oligomeric Fe2O3 clusters within the pores. When red mud was treated with citric acid (RM-CA-CTA), a wide distribution of Fe2O3 particles was obtained on the aluminosilicate external surface. TEM analysis showed a well-defined hexagonal mesoporosity of Fe-aluminosilicate obtained from untreated red mud, while the treated red mud produced lower regularity mesopores. RM-CTA exhibits 60% WCO conversion and 83.72% selectivity towards liquid products with 80.44% diesel hydrocarbon (C11-C18) yield. The high selectivity was due to the high acidity of Fe-aluminosilicate to dissociate the C-O bond and the regularity of mesostructure for efficient hydrocarbon diffusion, preventing a cracking reaction.

Arsenic pollution, geochemical fractions, and leaching characteristics in soil samples from four contaminated sites in the Beijiang River Basin, South China

To remediate historically polluted sites before their land-use changes, it is essential to understand the concentration distribution, geochemical fraction, and migratory behavior of As in soil with varied particle sizes for the use of a sieving procedure. This study investigated the amount and percentage of As in soil with different particle sizes (0.25, 0.25-1, and 1-2 mm) as well as its toxicity characteristic in leaching procedure at four previously contaminated sites in the Beijiang River Basin, South China. The results showed that the total As concentration in the collected soils ranged from 70.1 to 402.8 mg/kg, and only a few percent of soil particle samples had As contents below the local risk threshold value of 60 mg/kg. The amorphous hydrous oxide bound, crystalline hydrous oxide bound, and residual fractions (F3-F5) of the geochemical fraction of As in soil of polluted sites accounted for 82.2-95.7% of the total As distribution. However, the concentration of As in non-specifically bound fractions increased with the mass ratio of soil with coarse particle sizes due to the negative correlation of Fe-bearing minerals concentration with the mass ratio of soil with coarse particle size. According to redundancy analysis, soil with coarse particle sizes and non-specifically bound As were mostly responsible for the As concentration in the leachate. These findings confirmed that a sieving process was not suitable for the remediation of soil As at four historically contaminated sites in the Beijiang River Basin due to the high concentration of As in soil and non-negligible environmental risk of labile extractable As in soil with coarse particle size.

In-situ release and sequestration of CO2 in cement composites using LTA zeolites

This paper investigates the impact of in-situ release and sequestration of CO₂ on the compressive strength, volume of permeable voids, phase change, hydration reaction, and micro-morphology of cement mortars. Two Linde Type A (LTA) zeolites with micro-pore dimensions of 5 Å and 4 Å (i.e., LTA 5A and 4A zeolites) were employed as CO₂ carriers herein. The incorporation of 312 wt% plain LTA 5A and 4A zeolites increases the 1-day compressive strength of the mortars. However, the use of plain LTA 5A zeolite shows marginal contributions to the 7 and 28-day compressive strengths of the mortars, whilst using plain LTA 4A zeolite even deteriorates their 7 and 28-day compressive strengths. The micro-structural analyses reveal that the addition of LTA zeolites promotes the cement hydration and improves the mean chain length (MCL) of calcium aluminosilicate hydrates (C-A-S-H). Nevertheless, this introduces numerous weak points or even a porous structure to the cement matrix. In contrast, in-situ release of CO₂ via LTA zeolites significantly enhances the compressive strengths of the mortars at various ages, as this can further facilitate the hydration evolution and improve the MCL of C-A-S-H. Moreover, in-situ release of CO₂ brings an incremental content of calcium carbonates. The calcium carbonate contents in the specimens containing 12 wt% LTA 5A and 4A zeolites are increased by 5.3 wt% and 4.8 wt%, respectively. This leads to homogenous distributions of calcite with a grain size of 150600 nm. Thus, LTA 5A zeolite outperforms LTA 4A zeolite with regard to CO₂ uptake and the corresponding mechanical properties. This work presents in initial exploration into the application of porous pozzolanic materials in conjunction of CO₂ in cement-based materials.

Soil Amendments and Foliar Melatonin Reduced Pb Uptake, and Oxidative Stress, and Improved Spinach Quality in Pb-Contaminated Soil

Amending Pb-affected soil with biochar (BH) and magnesium potassium phosphate cement (MKC) reduces Pb uptake in plants. Moreover, foliar applications of melatonin and proline are also known to reduce plant oxidative stress and Pb uptake. However, little is known about combining both techniques, i.e., adding a combo immobilizing dose (CIA = mixture of BH and MKC at 50:50 ratio) in Pb-polluted soil and foliar application of proline and melatonin for reducing Pb uptake and oxidative stress in spinach. Control, proline, melatonin, CIA, CIA+proline, and CIA+melatonin were the treatments utilized in this pot study to see their effects on reducing plant oxidative stress, Pb uptake, and improving spinach quality in Pb-polluted soil. Moreover, Pb bioavailability, enzymatic activities, and numbers of bacteria, fungi, and actinomycetes in the soil were also evaluated. The effect of CIA on reducing Pb in the soil-plant system and improving soil enzymes and microbial numbers was more pronounced than melatonin alone. The most effective treatment was CIA+melatonin reducing Pb availability in soil (77%), shoots (95%), and roots (84%), alleviating oxidative stress, and improving plant biomass (98%) and nutrients. Soil enzymatic activities and the number of microorganisms in the rhizosphere were also highest with CIA+melatonin. Results highlight the significance of CIA+melatonin, as an inexpensive approach, in remediating Pb-polluted soil and improving spinach quality. However, further research is needed to understand the significance of CIA+melatonin on different crops and various soil Pb concentrations before employing this technique commercially in agriculture and environment sectors.

Stabilization and strengthening of chromium(VI)-contaminated soil via magnesium ascorbyl phosphate (MAP) and phytase addition

Cr(VI) contamination of soil threatens the environment and reduces soil strength. Therefore, both Cr(VI) stabilization and soil reinforcement should be considered in site remediation for future construction. This study investigated a biochemical treatment process using magnesium ascorbyl phosphate (MAP) and phytase. MAP was hydrolyzed via phytase catalysis to produce ascorbic acid (AA) and MgHPO₄·3H₂O precipitation. The AA reduced Cr(VI) into low-toxic Cr(III), which precipitated as Cr(OH)₃ and CrPO₄. More than 90% of the 500 mg/kg Cr(VI) in soil was reduced by 5% MAP (wt% of soil) and 1% phytase (vol/vol of soil water) doses at the geotechnically optimal soil moisture content of 16.8%. The MgHPO₄·3H₂O precipitates filled soil pores and enhanced the unconfined compression strength of treated soil by more than two times. This research reports a novel and practical enzymatically induced phosphate precipitation process for the remediation of Cr(VI)-contaminated soil.

Immobilization of Zn(Ⅱ) and Cu(Ⅱ) in basic magnesium-sulfate-cementitious material system: Properties and mechanism

To solve the environmental problems caused by heavy metal pollution, a new cementitious material (basic magnesium sulfate cement, BMSC) was developed for the solidification of Cu²⁺/Zn²⁺. First, the effects of different amounts of Cu²⁺/Zn²⁺ on the properties (compressive strength, setting time, pH, and leaching toxicity) of the BMSC were investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the effects of different amounts of Cu²⁺/Zn²⁺ on the phase and microstructure of BMSC. The results showed that Cu²⁺/Zn²⁺ inhibited the hydration of BMSC, reduced compressive strength, and prolonged the setting time. The results of the leaching tests showed that the BMSC system exhibited high immobilization efficiency (up to 99%) for Cu²⁺/Zn²⁺. Further, the BMSC solidification matrix exhibited excellent acid resistance (compressive strength >40 MPa after 28 days of immersion). The physical phase analysis showed that the main phases of BMSC were the 5Mg(OH)₂-MgSO₄-7 H₂O (5-1-7) phase and Mg(OH)₂, and the crystal structure refinement analysis suggested that Cu²⁺/Zn²⁺ ions were substituted with Mg²⁺ in the 5-1-7 phase. It was confirmed that the solidification mechanism of BMSC on Cu²⁺/Zn²⁺ is mainly performed by chemical complexation and ionic substitution.

Lead-chlorine synergistic immobilization mechanism in municipal solid waste incineration fly ash (MSWIFA)-based magnesium potassium phosphate cement

The high chlorine (Cl) and lead (Pb) content characteristics of municipal solid waste incineration fly ash (MSWIFA) pose environmental risks and hinder resource utilization. Herein, an MSWIFA-based magnesium potassium phosphate cement (MKPC) preparation strategy was developed, which allowed the MSWIFA recycling and the Pb-Cl synergistic immobilization without the washing pretreatment. The compressive strength of the resulting 10 wt% MSWIFA-based MKPC was 28.44 MPa, with over 99.2% reduction in leaching toxicity of Pb and Cl. The high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) and X-ray absorption spectroscopy (XAS) analyzes showed that Pb, phosphate and Cl^- formed Pb₅(PO₄)₃Cl in MKPC. In-situ X-ray diffraction (XRD) tests showed that Pb₃(PO₄)₂ was gradually transformed to Pb₅(PO₄)₃Cl through a dissolution-precipitation process. The formation energy, Bader charge, charge density difference and density of states (DOS) of Pb₅(PO₄)₃Cl were analyzed by first-principles calculations, confirming that Pb₅(PO₄)₃Cl was more thermodynamically stable than Pb₃(PO₄)₂ and PbCl₂ and that electronic interactions between Pb-p, O-p, P-p and Cl-p orbits were the origin of Pb-Cl synergistic immobilization. This work provides a new strategy for the resource utilization of MSWIFA without washing pretreatment, and provides an in-depth understanding of the Pb-Cl synergistic immobilization mechanism.

Impact of H2O on the Microscopic Oxidation Mechanism of Lollingite: Experimental and Theoretical Analyses

Lollingite (FeAs2) is considered an arsenic-bearing mineral that is oxidized faster than arsenopyrite. The geometric configuration, chemical valence bond, and microscopic reaction of the oxidation on the surface of lollingite were systematically studied, which are of great significance for understanding the mechanism of oxidative dissolution. X-ray photoelectron spectroscopy (XPS) measurements and density functional theory (DFT) calculations were carried out to characterize the (101) surface oxidation process of lollingite under the O2/O2 + H2O conditions. XPS results confirmed that the participation of water molecules can promote the formation of abundant OH structures on the surface of lollingite, while the relative concentration of O, As(III), and Fe(III) increased. Moreover, the DFT results demonstrated that the (101) As-terminal plane of FeAs2 was the most stable surface with the lowest surface energy. H2O molecules were physically adsorbed onto the Fe atoms of the lollingite surface, while oxygen molecules can readily be adsorbed on the Fe-As2 site by chemical adsorption processes. The oxidation process of the lollingite surface with water includes the following mechanisms: adsorption, dissociation, formation of the hydrogen bond, and desorption. The dissociation of the H2O molecule into OH and H led to the hydroxylation of both Fe and As atoms and the formation of hydrogen bonding. The participation of H2O molecules can also reduce the reaction energy barrier and accelerate the oxidation reaction of the lollingite surface, especially as far as the water dissociation and formation of hydrogen bonds are concerned. According to PDOS data, there is considerable hybridization between the d orbitals of bonded Fe atoms and the p orbitals of O atoms, as well as between the p orbitals of bonded As atoms and the p orbitals of O atoms. Due to a strong propensity for orbital hybridization and bonding between the s orbitals of the H atoms in H2O molecules and the p orbitals of the O atoms on the (101) surface, water molecules have the ability to speed up the oxidation on the surface.

Arsenic(V) immobilization in fly ash and mine tailing-based geopolymers: Performance and mechanism insight

Global mining activities produce thousands of millions of toxic-bearing mine tailing (MT) wastes each year. Storage of the mine tailings not only encroaches upon large areas of cropland but also arouses additional ecological and environmental risks. Herein we demonstrate that geopolymerization of a mixture of the toxic-bearing mine tailings and the coal fly ash (FA) can effectively immobilize exogenous arsenic (As) species in addition to inherent As from the raw materials. The geopolymers also possess high compressive strengths (e.g., >25 MPa for specimens with 54 wt% FA and activated with 10 M sodium hydroxide (NaOH)), allowing them to be further used as low-carbon, cement-free building materials. The geopolymer strength was found to depend clearly upon the NaOH concentration, the FA content, and the curing time, with the maximum being 37.07 MPa for a specimen with 54 wt% FA, 0.03 wt% As, activated with 10 M NaOH and cured for 28 days. Leaching tests showed that all specimens achieved an immobilization efficiency as high as 95.4% toward As, and that both the short-term and long-term leachabilities of all toxic elements are far below the standard maximum contaminant levels. Microstructural analyses indicate that calcite, calcium silicate, and calcium silicate hydroxide are likely to play a crucial role in immobilizing As species and heavy metals of concern in the geopolymer matrixes. Given the superior mechanical strengths and long-term stabilities, the FA/MT-based geopolymers demonstrate a promising low-carbon material for both the remediation of As-bearing lands and the construction industry.

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.

Preparation of Fe-filled MOF-Al-based hydrogel for efficient reclaim of phosphate from wastewater and reusing as a slow-release fertilizer

In this study, a novel Fe-filled MOF-Al-based hydrogel (SA@Fe@MOF-Al) was prepared and characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis. The SA@Fe@MOF-Al hydrogel bead was used as an adsorbent to adsorb H2PO4- from wastewater. The effects on adsorption were investigated, including pH and coexist ion. The adsorption reached equilibrium within 30 min. The maximum H2PO4- adsorption capacity of SA@Fe@MOF-Al was 103.09 mg g^-1 at 298K with pH 7.0. Meanwhile, thermodynamic results confirmed that adsorption is exothermic and spontaneous. The adsorption kinetics displayed that SA@Fe@MOF-Al adsorption process was suitable to the pseudo-first-order and Langmuir model. Moreover, the feasibility of reusing the P-laden carrier material as a slow-release fertilizer was determined. The study results indicated that the product demonstrated excellent slow-release and water-retention properties. Thus, it has potential applications in improving soil moisture content and reducing soil moisture evaporation rate.

Designing Magnesium Phosphate Cement for Stabilization/Solidification of Zn-Rich Electroplating Sludge

Electroplating sludge is a hazardous waste due to its high potential to leach toxic elements into the natural environment. To alleviate this issue, we tailored magnesium phosphate cement (MPC) as a low-carbon material for stabilization/solidification (S/S) of Zn-rich electroplating sludge. The interaction between MPC and ZnO was investigated to clarify the precipitate chemistry, microstructure transition, and chemical environment of Zn species in the MPC-treated Zn sludge system. Comprehensive characterization (by X-ray diffraction (XRD), ³¹P nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure spectroscopy (EXAFS)) and thermodynamic modeling results revealed that the incorporated ZnO preferentially reacted with phosphate to form Zn₃(PO₄)₂·2H₂O/Zn₃(PO₄)₂·4H₂O, changing the orthophosphate environment in the MPC system. Stronger chemical bonding between Zn and phosphate in comparison to the bonding between Mg and phosphate also resulted in the formation of amorphous Zn₃(PO₄)₂·2H₂O/Zn₃(PO₄)₂·4H₂O. Zn₃(PO₄)₂·4H₂O precipitate appears to predominate at high {K⁺}{H⁺}{HPO₄^2-} values, and the formation of Zn₃(PO₄)₂·2H₂O/Zn₃(PO₄)₂·4H₂O competed for the Mg sites in the MPC system, leading to the inhibition of formation of Mg-phosphate precipitates. Overall, this work uncovers the precipitate chemistry and microstructure transition of Zn species in the MPC system, providing new insights into the sustainable S/S of Zn-contaminated wastes by adopting MPC.

Rhizobacteria communities reshaped by red mud based passivators is vital for reducing soil Cd accumulation in edible amaranth

Red mud (RM) was constantly reported to immobilize soil cadmium (Cd) and reduce Cd uptake by crops, but few studies investigated whether and how RM influenced rhizobacteria communities, which was a vital factor determining Cd bioavailability and plant growth. To address this concern, high-throughput sequencing and bioinformatics were used to analyze microbiological mechanisms underlying RM application reducing Cd accumulation in edible amaranth. Based on multiple statistical models (Detrended correspondence analysis, Bray-Curtis, weighted UniFrac, and Phylogenetic tree), this study found that RM reduced Cd content in plants not only through increasing rhizosphere soil pH, but by reshaping rhizobacteria communities. Special taxa (Alphaproteobacteria, Gammaproteobacteria, Actinobacteriota, and Gemmatimonadota) associated with growth promotion, anti-disease ability, and Cd resistance of plants preferentially colonized in the rhizosphere. Moreover, RM distinctly facilitated soil microbes' proliferation and microbial biofilm formation by up-regulating intracellular organic metabolism pathways and down-regulating cell motility metabolic pathways, and these microbial metabolites/microbial biofilm (e.g., organic acid, carbohydrates, proteins, S^2-, and PO₄^3-) and microbial cells immobilized rhizosphere soil Cd via the biosorption and chemical chelation. This study revealed an important role of reshaped rhizobacteria communities acting in reducing Cd content in plants after RM application.

Biogeochemistry of Iron Enrichment in Groundwater: An Indicator of Environmental Pollution and Its Management

Iron (Fe) is one of the most biochemically active and widely distributed elements and one of the most important elements for biota and human activities. Fe plays important roles in biological and chemical processes. Fe redox reactions in groundwater have been attracting increasing attention in the geochemistry and biogeochemistry fields. This study reviews recent research into Fe redox reactions and biogeochemical Fe enrichment processes, including reduction, biotic and abiotic oxidation, adsorption, and precipitation in groundwater. Fe biogeochemistry in groundwater and the water-bearing medium (aquifer) often involves transformation between Fe(II) and Fe(III) caused by the biochemical conditions of the groundwater system. Human activities and anthropogenic pollutants strongly affect these conditions. Generally speaking, acidification, anoxia and warming of groundwater environments, as well as the inputs of reducing pollutants, are beneficial to the migration of Fe into groundwater (Fe(III)→Fe(II)); conversely, it is beneficial to the migration of it into the media (Fe(II)→Fe(III)). This study describes recent progress and breakthroughs and assesses the biogeochemistry of Fe enrichment in groundwater, factors controlling Fe reactivity, and Fe biogeochemistry effects on the environment. This study also describes the implications of Fe biogeochemistry for managing Fe in groundwater, including the importance of Fe in groundwater monitoring and evaluation, and early groundwater pollution warnings.

Ash formation and the inherent heavy metal partitioning behavior in a 100 t/d hazardous waste incineration plant

Hazardous waste incineration (HWI) ash is also defined as hazardous waste and its disposal performance depends largely on the ash compositions as well as the potential environmental risk of heavy metals. In this work, HWI ashes of four sampling sites were collected in a 100 t/d hazardous waste incineration plant with rotary kiln over three consecutive days. The formation characteristics of ash samples including heavy metal partitioning were given, with further discussions on the melting disposal of HWI ash mixtures. Results showed significant differences in the ash compositions among the sampling sites. Caused by NaHCO₃ injection as de-acidizing adsorbent, the sum of Na, S and Cl content in bag filter ash even exceeded 70%. Cu/Mn/Cr tended to transfer into the bottom ash due to low volatilities, while Zn/Pb/Cd/Se/As were more likely to be enriched in the ash particles. In particular, chemical adsorption at medium- to high- temperature range was dominant for As enrichment in the waste heat boiler ash. Despite the complexity and diversity of raw hazardous wastes, little difference was found in the melting temperature of bottom ash during the sampling period. However, it could vary by more than 200 °C for fly ash due to the fluctuation of alkali components in raw wastes. Moreover, slagging medium was encouraged in order to achieve rapid and complete melting of ash mixtures. The objective of this study is to gain knowledge on the HWI ash formation and inherent heavy metal partitioning behavior, expecting to provide guidelines on the deep harmless disposal of HWI ash in future.

Recycling phosphogypsum as the sole calcium oxide source in calcium sulfoaluminate cement production and solidification of phosphorus

Because the disposal of phosphogypsum (PG) can lead to serious contamination of the air, soil, and water, recycling of PG has attracted wide attention. This study investigated the effect and solidification of phosphorus in the production of calcium sulfoaluminate (CSA) cement using PG as the sole CaO source. The effects of three phosphorus impurities (Ca₃(PO₄)₂, CaHPO₄, Ca(H₂PO₄)₂) on the decomposition of CaSO₄, formation of minerals, microstructure of the clinker, and the hydration and mechanical properties of the cement were studied. Experimental results show that Ca₃(PO₄)₂ and Ca(H₂PO₄)₂ promoted the decomposition of CaSO₄ and the formation of clinker minerals with the increase in P₂O₅ content, whereas CaHPO₄ showed a promoting effect only when the P₂O₅ content was more than 1.5 wt%. The increase in phosphorus incorporation in Ca₂SiO₄ leads to the transformation of β-Ca₂SiO₄ to α'-Ca₂SiO₄ and then to Ca₇Si₂P₂O₁₆. The presence of three phosphates in the clinker enhanced the growth of crystal grains and the generation of a liquid phase. Compared with Ca₄Al₆SO₁₆ without phosphorus, the hydration reaction of phosphorus-bearing Ca₄Al₆SO₁₆ started later and ended earlier, and the reaction time was shorter. The presence of phosphorus impurities reduces the 1-day strength of CSA cement but does not affect the development of the 3-day and 28-day strengths. Considering environmental aspects, the solidification of phosphorus in the production of CSA clinker were quantified by measuring the distribution of elements. The results indicated that phosphorus is solidified by Ca₄Al₆SO₁₆, Ca₂SiO₄, and Ca₄Al₂Fe₂O₁₀, and Ca₂SiO₄ has a stronger ability to solidify phosphorus than the other two minerals. Ca₃(PO₄)₂ is more difficult to solidify than CaHPO₄ and Ca(H₂PO₄)₂. This study is of great significant to guide the large-scale clean utilization of PG in the production of CSA cement.

Transferring waste red mud into ferric oxide decorated ANA-type zeolite for multiple heavy metals polluted soil remediation

Herein, a novel "waste reclamation for soil remediation" strategy has been developed for both alkaline waste red mud (RM) recycling and heavy metal (HM) polluted soil remediation. Through a direct one-pot hydrothermal reaction process, the Al, Si, alkali, and Fe₂O₃ components in waste RM have been transferred into ferric oxide decorated ANA-type zeolite (Fe₂O₃-ANA). As tested by the HMs polluted soil remediation and oilseed rape planting experiment, when 25 g/kg Fe₂O₃-ANA is added into the Pb²⁺, Cu²⁺, Cr³⁺ and anionic AsO₂^- polluted soil (HM concentration: 100-200 mg/kg), it can effectively suppress the HMs mobility in soil and reduce the bio-accumulation concentrations of HMs in the harvested oilseed rape (reduce ratio: 37.9-69.5%). The detailed mechanism study using energy dispersive X-ray energy spectroscopy, in-depth X-ray photoelectron spectroscopy and density function theory calculation concludes that the Cu²⁺, Pb²⁺ and Cr³⁺ in soil have been adsorbed and trapped in the framework structure of ANA in Fe₂O₃-ANA mainly via the cation exchange process. While the anionic AsO₂^- species are mainly caught by the Fe₂O₃ component in Fe₂O₃-ANA via surface adsorption. Overall, this work firstly transforms waste RM into Fe₂O₃-ANA for soil remediation, which is valuable to waste resource recycling and environmental conservation.

Designing novel magnesium oxysulfate cement for stabilization/solidification of municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is a typical hazardous waste worldwide. In this study, an innovative magnesium oxysulfate cement (MOSC) binder was designed for stabilization/solidification (S/S) of MSWI fly ash, focusing on the interactions between MOSC binder and typical metallic cations (Pb²⁺)/oxyanions (AsO₃^3-). Experimental results showed that Pb and As slightly inhibited the reaction of high-sulfate 5MS system but significantly suppressed the reaction process of low-sulfate 10MS system. The 5MS binder system exhibited excellent immobilization efficiencies (99.8%) for both Pb and As. The extended X-ray absorption fine structure spectra suggested that Pb²⁺ coordinated with SO₄^2-/OH^- in the MOSC system and substituted Mg²⁺ ion sites in the internal structure of 5Mg(OH)₂·MgSO₄.7H₂O (5-1-7) phase. In contrast, the AsO₃^3- substituted SO₄^2- sites with the formation of inner-sphere complexes with Mg²⁺ in the large interlayer space of the 5-1-7 structure. Subsequent MSWI fly ash S/S experiments showed that a small amount of reactive Si in MSWI fly ash interfered with the MOSC reaction and adversely influenced the immobilization efficiencies of Pb, As, and other elements. Through the use of 33 wt% tailored MOSC binder for MSWI fly ash treatment, a satisfying S/S performance could be achieved.

Long-term leachability of Sb in smelting residue stabilized by reactive magnesia under accelerated exposure to strong acid rain

This study investigated the long-term leachability of antimony (Sb) in a smelting residue (39519 mg/kg) solidified/stabilized by reactive magnesia (MgO). Different dosages of MgO (0% as control, 2%, 5%, and 10% on a dry basis) were compared, and the long-term performance was evaluated by an accelerated exposure test consist of 20 consecutive leaching steps with simulated strong acid rain (SAR, HNO₃: H₂SO₄ = 1:2, pH = 3.20) as the extractant. Notably, the MgO treatments efficiently reduced the Sb leachability. Compared to the original slag (8.3 mg/L), the leaching concentrations based on a Chinese standard HJ/T299-2007 were reduced by 58%, 79%, 85%, and 86% at MgO dosages of 0%, 2%, 5%, and 10%, respectively. Because the studied slag was rich in oxides like SiO₂, CaO, and MgO, the hydration reactions probably happened during the aging processes with oxic water. It was inferred that the formed hydration products have a self-solidification/stabilization function to suppress the Sb leaching from the solid phase. The mineralogical characterization results proved that the hydrated Mg(OH)₂ played an essential role in the decrease of Sb leachability. Besides, the MgO addition promoted the hydration of this smelting slag and formed new hydrate gels that immobilize Sb in this slag. Our results confirmed that MgO-amended slags were resistant to continuous SAR corrosion. Compared to the control, the dosage of 5% MgO could effectively reduce the cumulatively released Sb by 57%, with only 0.46% of total Sb could be leached. The decomposition of Mg(OH)₂ and hydrate gels determined the re-release of Sb in a long term. Our work has demonstrated that reactive MgO amendment could be potentially selected as an effective strategy for the treatment of Sb-containing smelting residues in field conditions.

A 3-year field study on lead immobilisation in paddy soil by a novel active silicate amendment

Lead (Pb) is a toxic metal in industrial production, which can seriously threat to human health and food safety. Thus, it is particularly crucial to reduce the content of Pb in the environment. In this study, raw fly ash (FA) was used to synthesise a new active silicate materials (IM) employing the low-temperature-assisted alkali (NaOH) roasting approach. The IM was further synthesised to form zeolite-A (ZA) using the hydrothermal method. The physicochemical characteristics of IM and ZA amendments before and after Pb²⁺ adsorption were analysed using the Scanning electron microscope-Energy Dispersive Spectrometer (SEM-EDS), Fourier Transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) apparatuses. The results revealed the considerably change in the microstructure and functional groups of IM and ZA amendments, conducive to Pb²⁺ removal. Moreover, a 3-year field experiment revealed that the IM and ZA significantly improved the growth of rice and reduced available Pb by 21%-26.8% and 9.7%-16.9%, respectively. After 3 years of remediation, the Pb concentration of the rice grain reached the national edible standard (≤0.2 mg kg^-1) of 0.171 mg kg^-1 and 0.179 mg kg^-1, respectively. Meanwhile, the concentration of acid-exchangeable Pb reduced, while those of reducible and residual fractions of Pb increased. There was no significant difference between the IM and ZA treatments. The potential mechanisms of remediation by the amendments were ion-exchange, complexation, precipitation, and electrostatic attraction. Overall, the results indicate that IM is suitable for the remediation of contaminated soil and promotes safe food production, and develops an environmentally friendly and cost-effective amendment for the remediation of polluted soil.

Effect of phosphogypsum on the properties of magnesium phosphate cement paste with low magnesium-to-phosphate ratio

The incorporation of phosphogypsum (PG) in magnesium potassium phosphate cement (MPPC) can promote the utilization of PG not only by utilising the phosphate impurity in PG, but also by immobilising the heavy metals with MPPC. This paper investigates the feasibility of the incorporation of PG in preparing MPPC. Both early age properties, including workability and setting time, and hardened properties of compressive strength and microstructure, of PG-incorporated MPPC paste were investigated, and the hydration mechanism was explored. The results indicated that the addition of PG increased the workability of MPPC and extended the setting time of MPPC. However, incorporation of 20% PG slightly reduced the compressive strength because higher PG content led to the loose microstructure. Moreover, the addition of PG did not change the formation of hydration product, while it only reduced the hydration heat. Finally, compared to PG, the concentration of leached heavy metals of MPPC with PG was significantly reduced after 28 days curing.

Solidification/Stabilization of Arsenic-Containing Tailings by Steel Slag-Based Binders with High Efficiency and Low Carbon Footprint

The disposal of nonferrous metal tailings poses a global economic and environmental problem. After employing a clinker-free steel slag-based binder (SSB) for the solidification/stabilization (S/S) of arsenic-containing tailings (AT), the effectiveness, leaching risk, and leaching mechanism of the SSB S/S treated AT (SST) were investigated via the Chinese leaching tests HJ/T299-2007 and HJ557-2010 and the leaching tests series of the multi-process Leaching Environmental Assessment Framework (LEAF). The test results were compared with those of ordinary Portland cement S/S treated AT (PST) and showed that the arsenic (As) curing rates for SST and PST samples were in the range of 96.80–98.89% and 99.52–99.2%, respectively, whereby the leached-As concentration was strongly dependent on the pH of the leachate. The LEAF test results showed that the liquid–solid partitioning limit of As leaching from AT, SST, and PST was controlled by solubility, and the highest concentrations of leached As were 7.56, 0.34, and 0.33 mg/L, respectively. The As leaching mechanism of monolithic SST was controlled by diffusion, and the mean observed diffusion coefficient of 9.35 × 10⁻¹⁵ cm²/s was higher than that of PST (1.55 × 10⁻¹⁶ cm²/s). The findings of this study could facilitate the utilization of SSB in S/S processes, replacing cement to reduce CO₂ emissions.

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.

Sustainable stabilization/solidification of the Pb, Zn, and Cd contaminated soil by red mud-derived binders

Red mud and phosphogypsum are voluminous industrial by-products worldwide. They have long been disposed of in landfills or open storage, leading to a waste of resource and environmental pollution. This study provides a novel approach to recycle these industrial by-products as sustainable red mud-phosphogypsum-Portland cement (RPPC) binders for stabilization/solidification (S/S) of multimetal-contaminated soil. The physical strength, metal leachability and microstructure of S/S soil were investigated after 7-day and 28-day curing, as well as freezing-thawing (F-T) cycle and wetting-drying (W-D) cycle. The results show that the strength of soil treated by all binders fulfilled the uniaxial compressive strength requirement (350 kPa) of S/S waste in landfills. Microstructural analyses show that the main hydration products of the RPPC S/S soil are ilmenite, ettringite, anhydrite and hydrated calcium silicate. The 10% and 15% RPPC binders have a competitive metal immobilization ability compared with 10% PC, but the immobilization priority is different: Pb > Zn > Cd in RPPC system and Zn > Cd > Pb in PC system, respectively, probably due to the precipiataion of Pb²⁺ with the abundant SO₄^2- in phosphogypsum in RPPC system. The strength of RPPC and PC treated soil was still higher than 350 kPa except for RPPC7.5 after 10 freeze-thaw or 10 wetting-drying cycles. The RPPC binder performed worse than PC binder after both freeze-thaw and wetting-drying cycles, especially at a lower dosage. Only the metal leaching concentrations of samples treated by RPPC15 and PC10 could fulfil the Chinese standards for hazardous wastes.

Thallium geochemical fractionation and migration in Tl-As rich soils: The key controls

Thallium (Tl) pollution caused by mining and processing of Tl-enriched ores has become an increasing concern. This study explored the geochemical fractionation and vertical transfer of Tl in a soil profile (200 cm) from a representative Tl-As mineralized area, Southwest China. The results showed that the soils were heavily enriched by Tl and As, with concentration ranging from 3.91-17.3 and 1830-8840 mg/kg (6.79 and 2973 mg/kg in average), respectively. Approximately 50% of Tl occurred in geochemically mobile fractions in the topsoil, wherein the reducible fraction was the most enriched fraction. Further characterization using LA-ICP-MS and TEM revealed that enriched Tl and As in soils were mainly inherited from the weathering of mine tailing piles upstream. XPS characterization indicated that Fe oxides herein may play a critical role in the oxidation of Tl(I) to Tl(III) which provoked further adsorption of Tl onto Fe oxides, thereby facilitating Tl enrichment in the reducible fraction. The findings highlight that the pivotal role of Fe oxides from mineralized area in the co-mobility and migration of Tl and As in the depth profile.

Environmental Safety Analysis of Red Mud-Based Cemented Backfill on Groundwater

As one of the main industrial solid wastes, there are a large number of free alkaloids, chemically bound alkaloids, fluoride, and heavy metal ions in Bayer process red mud (BRM), which are difficult to remove and easily pollute groundwater as a result of open storage. In order to realize the large-scale industrial application of BRM as a backfilling aggregate for underground mining and simultaneously avoid polluting groundwater, the material characteristics of BRM were analyzed through physical, mechanical, and chemical composition tests. The optimum cement–sand ratio and solid mass concentration of the backfilling were obtained based on several mixture proportion tests. According to the results of bleeding, soaking, and toxic leaching experiments, the fuzzy comprehensive evaluation method was used to evaluate the environmental impact of BRM on groundwater. The results show that chemically bound alkaloids that remained in BRM reacted with Ca²⁺ in PO 42.5 cement, slowed down the solidification speed, and reduced the early strength of red mud-based cemented backfill (RMCB). The hydration products in RMCB, such as AFT and C-S-H gel, had significant encapsulation, solidification, and precipitation inhibition effects on contaminants, which could reduce the contents of inorganic contaminants in soaking water by 26.8% to 93.8% and the leaching of toxic heavy metal ions by 57.1% to 73.3%. As shown by the results of the fuzzy comprehensive evaluation, the degree of pollution of the RMCB in bleeding water belonged to a medium grade Ⅲ, while that in the soaking water belonged to a low grade II. The bleeding water was diluted by 50–100 times to reach grade I after flowing into the water sump and could be totally recycled for drilling and backfilling, thus causing negligible effects on the groundwater environment.

Effects of Long-Term Freeze-Thaw Cycles on the Properties of Stabilized/Solidified Lead-Zinc-Cadmium Composite-Contaminated Soil

Lead, zinc, and cadmium were used to prepare a composite-contaminated soil to replicate common situations, in which soil is usually simultaneously contaminated by multiple metals. To examine the long-term durability of stabilized/solidified (S/S) contaminated soil, specimens were subjected to a series of freeze–thaw (F-T) cycles, up to ninety times (one day per cycle), prior to testing. Triaxial compression tests, soil column leaching tests, and X-ray diffraction analysis were then employed to study the mechanical properties, environmental influences, and micro-mechanisms of the S/S lead-zinc-cadmium composite-contaminated soils after long-term F-T. The results showed that triaxial compressive strength increases within three F-T cycles, then decreases before slightly increasing or stabilizing after thirty F-T cycles. The stage of decreased cohesion thus occurs between three and fourteen F-T cycles, with variation in other factors similar to that of the triaxial compressive strength. The cohesion mainly increases between three and seven cycles. The soil column leaching test showed that the permeability of soil is more than four times higher than that of soil not subject to freeze–thaw cycles after ninety F-T cycles. XRD tests further revealed that the chemical composition of S/S contaminated soil and the occurrence of each heavy metal (HM) remained unchanged under F-T treatment.

Treatment of municipal solid waste incineration fly ash: State-of-the-art technologies and future perspectives

Municipal solid waste incineration (MSWI) fly ash is considered as a hazardous waste that requires specific treatment before disposal. The principal treatments encompass thermal treatment, stabilization/solidification, and resource recovery. To maximize environmental, social, and economic benefits, the development of low-carbon and sustainable treatment technologies for MSWI fly ash has attracted extensive interests in recent years. This paper critically reviewed the state-of-the-art treatment technologies and novel resource utilization approaches for the MSWI fly ash. Innovative technologies and future perspectives of MSWI fly ash management were highlighted. Moreover, the latest understanding of immobilization mechanisms and the use of advanced characterization technologies were elaborated to foster future design of treatment technologies and the actualization of sustainable management for MSWI fly ash.

Bioaugmentation with Acidithiobacillus species accelerates mineral weathering and formation of secondary mineral cements for hardpan development in sulfidic Pb-Zn tailings

The development of hardpan caps has great potential in rehabilitating sulfidic and metallic tailings, which may be accelerated by using exogenous Acidithiobacillus species. The present study aims to establish a bioaugmentation process with exogenous Acidithiobacillus species for accelerating the weathering of sulfidic minerals and formation of secondary mineral gels as precursors for hardpan structure development in a microcosm experiment. Exogenous Acidithiobacillus thiooxidans (ATCC 19377) and A. ferrooxidans (DSM 14882) were inoculated in a sulfidic Pb-Zn tailing containing negligible indigenous Acidithiobacillus species for accelerating the weathering of pyrite and metal sulfides. Microspectroscopic analysis revealed that the weathering of pyrite and biotite-like minerals was rapidly accelerated by exogenous Acidithiobacillus species, leading to the formation of secondary jarosite-like mineral gels and cemented profile in the tailings. Meanwhile, approximately 28% Zn liberated from Zn-rich minerals undergoing weathering was observed to be re-immobilized by Fe-rich secondary minerals such as jarosite-like mineral. Moreover, Pb-bearing minerals mostly remained undissolved, but approximately 30% Pb was immobilized by secondary Fe-rich minerals. The present findings revealed the critical role of exogenous Acidithiobacillus species in accelerating the precursory process of mineral weathering and secondary mineral formation for hardpan structure development in sulfidic Pb-Zn tailings.

Potential of a novel modified gangue amendment to reduce cadmium uptake in lettuce (Lactuca sativa L.)

In this study, the modified gangue (GE) was prepared by calcination at lower temperatures using potassium hydroxide (KOH) as the activating agent. The field emission scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray fluorescence (XRF) methods were employed to analyze the physicochemical characteristics of GE before and after the modification. Besides, the GE and commercial zeolite (ZE) were compared in the remediation of Cd-contaminated soil in field experiments. The results showed that both the GE and ZE had positive effects on the stabilization of Cd, decreasing the available Cd by 21.2-33.9% and 22.1-28.2%, respectively, while no significant difference was observed between the two amendments, indicating that the modification of GE was successful. Moreover, the application of GE decreased the Cd mobilization and uptake in lettuce shoot and root by 54.9-61.5% and 9.3-13.2%, respectively, and at the same time, the bio-available Cd decreased by 20.9-34.5%. Moreover, with the addition of GE, activities of urease and alkaline phosphatase increased in soil, while the peroxidase and superoxide dismutase activities were notably reduced in plants. Therefore, GE could be used as an effective amendment for the alleviation of Cd accumulation and toxicity, and thereby improve food safety.

Arsenic immobilization and removal in contaminated soil using zero-valent iron or magnetic biochar amendment followed by dry magnetic separation

The potential of using zero-valent iron (ZVI) or a Fe₃O₄-loaded magnetic biochar to stabilize arsenic (As) in contaminated soil was investigated in the processes of incubation trial, chemical extraction, pot experiments with ryegrass growth. Additionally, a dry magnetic separation technique was applied to verify the possible permanent removal of As from the bulk soil. Results showed the ZVI amendment greatly reduced the As leaching, and the leached concentration became much lower than the Japanese environment standard (10 μg/L) after 180 days of incubation. Contrarily, the magnetic biochar amendment readily increased the As leachability due to the changes in pH, dissolved organic carbon, and soluble P and Si. The ZVI had a greater effect over the magnetic biochar, supported by the significantly reduced As leachability in the combined amendments. Furthermore, results from sequential extraction analysis indicate that both amendments significantly decreased the available As in (NH₄)₂SO₄ and NH₄H₂PO₄ extraction and increased the As bound to amorphous Fe oxides. But ZVI amendment alone performed better than magnetic biochar amendment alone. Plant growth experiment showed that the ZVI amendment enhanced ryegrass growth and significantly increased the ryegrass biomass. However, the magnetic biochar amendment resulted in an adverse effect on the ryegrass root growth, probably due to a marked enhancement of salinity. Meanwhile, the As uptake by ryegrass was significantly reduced in both ZVI and magnetic biochar-amended soils. Results of dry magnetic separation showed that averaged 20% and 25% of total As could be retrieved from ZVI and magnetic biochar amended soil, respectively; and the As bound to amorphous Fe oxides was the main retrieved fraction. This study indicated that ZVI or magnetic biochar could be applied as a promising amendment for reducing (phyto)availability of As in soil, and dry magnetic separation could be served as an alternative option for permanently removing As.

Phosphorus immobilization in water and sediment using iron-based materials: A review

As an essential element for life, phosphorus (P) is very important for organisms. However, excessive P in water and sediment can cause eutrophication, which poses a potential risk to drinking water safety and the sustainability of aquatic ecosystems. Therefore, effective phosphorus-control in water and sediment is the key strategy to control eutrophication. Iron-based materials exhibit high efficiency for P immobilization due to their strong affinity with P, low cost, easy availability, and environmentally friendliness. They are promising materials for controlling P in application. This work comprehensively summarizes the recent advances on P immobilization in water and sediment by different iron-based materials, including iron (hydr)oxides, iron salts, zero-valent iron and iron-loaded materials. This review is focused on the mechanism of the processes and how they are impacted by major influencing factors. The combination of iron-containing materials with other assisting materials is a good strategy to enhance P-fixation efficiency and selectivity. Finally, the current challenges and prospects of P-control technologies based on iron-containing materials are proposed. This review provides a systemic theoretical and experimental foundation for P-immobilization in water and sediment using iron-based materials.

Effects of fine fractions of soil organic, semi-organic, and inorganic amendments on the mitigation of heavy metal(loid)s leaching and bioavailability in a post-mining area

This study investigated the effects of soil amendments including biomasses (rice husk, RRH and maple leaf, RML), biochar (rice husk biochar, RHB and maple leaf biochar, MLB), and industrial by-products (red mud, RM and steel slag, SS), at two application rates (0, 1, and 2% w/w) on leaching and bioavailability of heavy metal(loid)s (HMs) (As, Cd, Cu, Pb, and Zn) in the presence of an Asteraceae (i.e., lettuce). Physicochemical properties of the soil (i.e., pH, EC, CEC, and HMs leaching) and plants were examined before and after amending. The addition of amendments significantly (p < 0.05) increased soil EC (from 100 to 180 μScm^-1) and CEC (from 7.6 to 15 meq100 g^-1). Soil pH from 6.7 ± 0.05 increased about 2 units with increasing in the application rate of MLB, RM, and SS, while it decreased about 0.8 units in RML amended soil. Soil amendments reduced the easily leachable fractions (exchangeable and carbonate) of HMs in the order of MLB > SS > RM > RHB. The average concentration of Cd, Cu, Pb, and Zn in plant roots and shoots decreased >30 wt% in biochars and industrial by-products amended soils, while biomasses mitigated As uptake in lettuce. Results demonstrated that adding maple-derived biochar combined with revegetation effectively immobilized HMs in a post-mining area beside an induce in plant growth parameters.

High-efficiency and low-carbon remediation of zinc contaminated sludge by magnesium oxysulfate cement

Electroplating sludge is classified as a hazardous waste due to its extremely high leachability of potentially toxic elements. This study concerns the use of magnesium oxysulfate cement (MOSC) for the stabilisation/solidification (S/S) of Zn-rich electroplating sludge. According to X-ray diffraction and thermogravimetric analyses, Zn was mainly immobilised through both chemical interaction and physical encapsulation in the MOSC hydrates of 5Mg(OH)₂·MgSO₄.7H₂O (5-1-7) phase. The crystal size analysis, elemental mapping, and extended X-ray absorption fine structure (EXAFS) analysis proved that the Zn²⁺ was also incorporated in the structure of 5-1-7 phase. Unlike Portland cement system, hydration kinetics, setting time, and compressive strength of the MOSC system were only negligibly modified by the presence of Zn, indicating its superior compatibility. Subsequent S/S experiments demonstrated that the MOSC binder exhibited an excellent performance on immobilisation efficiency of Zn (up to 99.9%), as well as satisfying the requirements of setting time and mechanical strength of sludge S/S products. Therefore, MOSC could be an effective and sustainable binder for the treatment of the Zn-rich industrial wastes.

Effect of Si/Al molar ratio on the immobilization of selenium and arsenic oxyanions in geopolymer

The effect of Si/Al molar ratio of geopolymer on the immobilization of Se and As oxyanions was studied through leaching test and solid characterizations including XRD, FTIR, TG, NMR, XAFS, and N₂ adsorption-desorption isotherm. As a whole, the leaching percentages of Se and As oxyanions increased with the increase of the Si/Al molar ratio of geopolymer. Linear combination fitting confirmed that most of selenite, selenate and arsenate ions existed in geopolymers through electrostatic interaction. Thus, Al tetrahedrons in geopolymer structure control the charge stability for these oxyanions to a large extent. Differently, as for arsenate ions, they were recrystallized into an arsenate compound (Na_3.25(OH)_0.25(H₂O)₁₂)(AsO₄) in geopolymers. The additive of these pollutants has an adverse effect on the compactness of geopolymer, then influencing the leaching performance in turn. However, the changes in leaching results did not follow the variation trend of specific surface areas and pore volumes of geopolymers with different Si/Al ratios. The number and distribution of Al tetrahedron and compactness of geopolymer have a synergistic effect on the immobilization of these oxyanions. Besides, the compressive strengths of geopolymer samples are always higher than 20 MPa, which meets the requirement of safe disposal of hazardous waste.

Remediation of Cu-polluted soil with analcime synthesized from engineering abandoned soils through green chemistry approaches

Due to the large output and potential ecological risks, disposal of engineering abandoned soils (EAS) has become an enormous challenge for human society. Herein, EAS has been transformed into microporous analcime (ANA) zeolite material through a fast, energy-efficient, and straightforward conversion process. The as-synthesized ANA has been employed to remediate Cu-polluted soil, which shows a significant ecological restoration function in a vegetable pot experiment. With 25 g/kg ANA into Cu contaminated soil (total Cu concentration: 200 ppm), the Cu accumulation concentration in vegetables has been decreased from 5.60 down to 1.80 mg/kg (approaching the background Cu level 1.70 mg/kg in vegetables). Detailed mechanism study combining with DFT calculations reveals that the Cu²⁺ in soil has been captured both inside the ANA pore channels and on the surface via ion-exchange and surface adsorption mechanism. The whole process, including ANA synthesis and Cu polluted soil remediation, has been optimized to show a valuable conceptual model to recycle EAS resource and in-situ remediate Cu polluted soil.

Protonation stabilized high As/F mobility red mud for Pb/As polluted soil remediation

The hazardous red mud (RM) with high As/F mobility and heavy metal contaminated soil have constituted severe environmental threats. This work demonstrates a "waste to eco-material" strategy through a reliable and low-cost protonation approach to eliminate the As/F leaching risk of RM, and then recycle it as heavy metal passivators for Pb/As polluted soil remediation. The As/F anions have been immobilized by the protonated Fe/Al (hydr)oxides within RM via the formation of stable As/F compounds during the protonation process, which satisfies the requirement by the World Health Organization (As leaching <0.01 mg/L; F leaching <0.8 mg/L). Moreover, in the oilseed rape pot experiments, by adding 30 g/kg stabilized RM into Pb/As polluted soils (100 ~ 300 ppm), benefited from its large adsorption capacity, approximately 40.9 ~ 49.7% Pb and 40.8 ~ 54.8% As concentrations in the plant are reduced without adverse effects. The whole process for RM treatment and soil remediation is cost-effective, straightforward and eco-friendly without secondary pollution or soil degradation. This research provides a green chemical strategy to address both RM recycling and heavy metal contaminated soil remediation problems, which shows high economic feasibility and ecological benefits.

Radiation Shielding Concrete with alternate constituents: An approach to address multiple hazards

Radiation Shielding Concrete (RSC) is a superior alternative to many conventional and modern shields against gamma and neutron radiation hazards. The present work is the first comprehensive review on utilization of alternate materials, emphasizing hazardous industrial byproducts, as constituents of RSC. Such usage enhances the performance, sustainability, and affordability of RSC. Added advantages are the immobilization of wastes and the conservation of natural resources for RSC. The review analyses incorporation of ferrous and non-ferrous slags, mines wastes, plastics, red mud, cathode ray tube's glass, metallic wastes, fly ash, silica fume, and miscellaneous residues. Besides, utilization of fibers, nanoparticles, and calcined clay is investigated. The influence on shielding efficiency is adjudged by scrutinizing changes in parameters such as half-value layer and linear attenuation coefficients. Similarly, variations in mechanical and durability properties are investigated and compared. The underlying responsible factors related to the physical, chemical and morphological characteristics of materials and their consequences on RSC's behavior are correlated. In association with alternatives, the advantages, disadvantages, and possible treatment methods are discussed. The country-wise, material-specific, and progressive research trends are revealed to facilitate future work in this upcoming field. Finally, conclusions are drawn with exposition of current bottlenecks and scope of future research.

Insights into the effect of NaOH on the hydration products of solidified cement-NaNO3 matrices and leaching behavior of Sr2

The low-level and intermediate-level radioactive waste is generally an alkaline solution with high concentration of sodium salts. Moreover, extra NaOH will be added into the matrices when the high content of mineral admixtures is used in the preparation of solidified cement matrices. This study mainly researches the influence of NaOH on the structure of C-S-H gels and the leaching rate of Sr²⁺ in the solidified cement matrices by isothermal conduction calorimetry, X-ray diffractometry, thermogravimetry, ²⁹Si nuclear magnetic resonance, mercury intrusion porosimetry and inductively coupled plasma. The results show that the addition of NaOH accelerated the hydration of cement in the early age but decreased the normalized hydration heat until 28 days. Also, the fraction of total Ca(OH)₂ decreased with the increasing content of NaOH. Moreover, NaOH promoted the formation of C-S-H gels along the main chain by increasing the mean chain length but decreased the Q³ sites. However, the addition of NaOH has optimized the pore structure through reducing the porosity and median pore diameter. At last, the cumulative leaching fraction of Sr²⁺ at the NaOH content of 10% decreased by 17.5% compared to the NaOH content of 0.82%. Based on the results, it can be concluded that NaOH enhanced the adsorption capacity of C-S-H gels to Sr²⁺ and improved the immobilization performance of solidified cement matrices.

Prospective usage of magnesium potassium phosphate cement combined with Bougainvillea alba derived biochar to reduce Pb bioavailability in soil and its uptake by Spinacia oleracea L

Combining biochar (BR) with other immobilizing amendments has additive effects on Pb immobilization and been recognized to be effective for the restoration of Pb polluted soils. However, the impacts of different proportions between BR and a highly efficient Pb immobilizing agent called "magnesium potassium phosphate cement (MC)" have never been earlier investigated. This work aimed to investigate the consequences of BR and MC alone and their mixtures of 25:75, 50:50, and 75:25 ratios on Pb bioavailability, Pb immobilization index (Pb-IMMi), and enzymatic activities in Pb polluted soil. Furthermore, amendments effects on Pb distribution in spinach, growth, antioxidant capacity, biochemical, and nutritional spectrum were also investigated. We found that MC alone performed well to immobilize Pb in soil and reducing its distribution in shoots, but was less efficient to improve soil enzymatic activities and plant attributes. Conversely, the application of BR alone stimulated soil enzymatic activities, plant growth, and quality but was less effective to immobilize Pb in soil and reducing shoot Pb concentrations. The combinations of BR and MC of various ratios showed variable results. Interestingly, the most promising outcomes were obtained with BR50%+MC50% treatment which resulted in enhanced Pb-IMMi (73%), activities of soil enzymes, plant growth and quality, and antioxidant capacity, compared to control. Likewise, significant reductions in Pb concentrations in shoots (85%), roots (78%), extractable Pb (73%) were also obtained with BR50%+MC50% treatment, compared to control. Such outcomes point towards a cost-effective approach for reducing Pb uptake by the plants via using MC and BR at a 50:50 ratio.

Recycling drinking water treatment sludge into eco-concrete blocks with CO2 curing: Durability and leachability

Drinking water treatment sludge (DWTS) can be recycled into low-strength concrete blocks for construction use. The sodium sulfate resistance and leaching behaviours of the DWTS-derived blocks are investigated in this study. The experimental results show that the addition of DWTS degrades the sodium sulfate resistance of the concrete blocks, however CO₂ curing compensates for such property, especially in the case of blocks incorporating 30% DWTS. The improvement can be attributed to the formation of crystalline CaCO₃ during CO₂ curing for microstructure refinement evidenced by X-ray Computed Tomography and Scanning Electron Microscopy. Leaching analyses show that Cu and Al concentrations increased with increasing DWTS content, and CO₂ curing adversely increased the leachability of metals due to the decrease of pH, especially at early leaching stage. Nevertheless, the total leaching concentrations of Cu and Al after 60-day test is far below the prescribed limitations, regardless of samples subject to air curing or CO₂ curing. In summary, sludge-derived blocks exposed to CO₂ curing are safe and behave well in aggressive environments. Therefore, this study showcases a green technology that successfully recycling DWTS into value-added and durable concrete blocks with low environmental impacts.

Evaluating the environmental impact of contaminated sediment column stabilized by deep cement mixing

Deep cement mixing (DCM) method is a widely used geotechnical technique for increasing ground stabilization before construction works. However, the environmental influence of stabilized ground on the surrounding area remains a concern. A physical model experiment of DCM-treated sediment column was conducted to investigate both geotechnical and environmental effects on the surrounding sediment. The DCM column contained the cement-stabilized contaminated sediment and surrounded by uncontaminated sediment. The physical behaviour, including settlement, pore water pressure, and total pressure were measured under different loadings. Simultaneously, the migration of the major ions into seawater, and leaching of potentially toxic elements into the surrounding sediment were evaluated. The results revealed that the leaching of major ions from the DCM column followed the dissipation of excess pore water and migrated to the seawater above the sediment surface. Nevertheless, the leaching behaviour of potentially toxic elements into the surrounding sediment and variation of pH value after the DCM treatment were within an acceptable level. Therefore, the contaminated marine sediment could be effectively stabilized and solidified by in-situ remediation with minimal secondary pollution to the surrounding environment.

Evaluating the environmental impacts of stabilization and solidification technologies for managing hazardous wastes through life cycle assessment: A case study of Hong Kong

Proper management of hazardous materials arouses widespread environmental concerns due to its enormous ecological and health impacts. The development of green stabilization/solidification (S/S) technology for resourceful utilization of hazardous materials, as well as the immobilization of potentially toxic elements is of great scientific interests. Cement-based S/S is often considered a low-cost and highly efficient technology, but the environmental sustainability of a broad spectrum of S/S technologies has yet to be evaluated. Therefore, this study assessed the environmental sustainability of S/S technologies for managing two common types of hazardous wastes, i.e., contaminated marine sediment and municipal solid waste incineration fly ash (MIFA) by using life cycle assessment (LCA). A total of 17 scenarios under three strategies for sediment and two strategies for MIFA S/S technologies were comprehensively evaluated. The LCA results identified the most preferable S/S technology in each strategy. In particular, Scenario 1 (mixture of sediment with a small percentage of ordinary Portland cement and incinerated sewage sludge ash) of Strategy 1 (use as fill materials) would be the preferred option, as it reduces about 54% and 70% global warming potential compared to those of Scenarios 2 and 3, respectively. This is the first initiative for evaluating the environmental impacts of a wide range of recently developed S/S technologies using green/alternative binders for diverting hazardous wastes from disposal. The results can serve as a decision support for the practical application of the environmentally friendly S/S technology for sustainable remediation.