Temporal effect of MgO reactivity on the stabilization of lead contaminated soil

Comparison and mechanism analysis of MgO, CaO, and Portland cement for immobilization of heavy metals in MSWI fly ash

The significant production of municipal solid waste incineration fly ash (MSWI FA) underscores the importance of developing efficient solidification materials. This study employed MgO and CaO for immobilizing MSWI FA (with a 70% fly ash incorporation), and the immobilization effect was compared with that of Portland cement (PC). Experimental findings revealed that MgO exhibited the most effective stabilization for heavy metals (Cd, Cu, Pb, and Zn) compared to CaO and PC. XRD, FTIR, TG, and SEM analysis indicated that the principal hydration products in MSWI FA binders solidified with MgO, CaO, and PC were Mg(OH)2, CaCO3, and C-S-H gel, respectively. Mg(OH)2 efficiently immobilized heavy metals through chemical complexation and surface adsorption mechanisms. The MgO-treated MSWI FA demonstrated the highest residual fractions and the lowest easily leachable fractions. Moreover, the leaching characteristics of heavy metals were significantly influenced by the pH level, so MgO-treated MSWI FA with a leachate pH of 9.18 achieved the precipitation and stabilization of most heavy metals. In summary, this study provided an effective material selection for MSWI FA immobilization and presented a novel strategy for MSWI FA management.

Mechanisms of enhancing MgO for stabilization/solidification of Pb-contaminated red clay through CO2 sequestration

Pb-contaminated soil poses significant environmental and health risks as well as soil stability issues. Research on sandy soils highlights CO2-enhanced reactive MgO as a promising solution for improving the solidification of Pb-contaminated soils. However, carbonation effects can differ markedly between soil types owing to varying soil properties. In this study, we evaluated the effects of CO2-enhanced reactive MgO on the engineering and environmental characteristics of Pb-contaminated red clay and explored its mechanism of carbonation solidification. The results showed that CO2-enhanced reactive MgO increased the strength of Pb-contaminated red clay to over 3 MPa within 1 h, which was approximately 25 times the strength of untreated soil (0.2 MPa) and significantly higher than that of reactive MgO-treated, uncarbonated soil (0.8 MPa). The pH of the carbonated soil (9-10) facilitated Pb2+ immobilization, and the increase over the initial parameter elevated the electrical conductivity value. Moreover, CO2-enhanced reactive MgO reduced the Pb2+ leaching concentration to below 0.1 mg/L, even at high Pb concentrations (10,000 mg/kg). Pb2+ transformed into lead carbonates during the carbonation process, with the hydrated magnesium carbonates forming a dense internal structure. This solidification mechanism included chemical precipitation, physical adsorption, and encapsulation. Notably, the carbonation time should be controlled within 1 h to prevent soil expansion. Together, these findings support the potential of CO2-enhanced reactive MgO for efficient and low-carbon application in the solidification of Pb-contaminated red clay.

The longevity evaluation of multi-metal stabilization by MgO in Pb/Zn smelter-contaminated soils

The re-mobilization risks of potentially toxic elements (PTEs) during stabilization deserve to be considered. In this study, artificial simulation evaluation methods based on the environmental stress of freeze-thaw (F-T), acidification and variable pH were conducted to assess the long-term effectiveness of PTEs stabilized by MgO in Pb/Zn smelter contaminated soils. Among common stabilizing materials, MgO was considered as the best remediation material, since PTEs bioavailability reduced by 55.48% for As, 19.58% for Cd, 10.57% for Cu, and 26.33% for Mn, respectively. The stabilization effects of PTEs by MgO were best at the dosage of 5 wt%, but these studied PTEs would re-mobilize after 30 times F-T cycles. Acid and base buffering capacity results indicated that the basicity of contaminated soils with MgO treatment reduced under F-T action, and the leached PTEs concentrations would exceed the safety limits of surface water quality standard in China (GB3838-2002) after acidification of 2325 years. No significant changes were found in the pH-dependent patterns of PTEs before and after F-T cycles. However, after F-T cycles, the leaching concentrations of PTEs increased due to the destruction of soil microstructure and the functionality of hydration products formed by MgO, as indicated by scanning electron microscopy (SEM) coupled with energydispersive Xray spectroscopy (EDS) results. Hence, these findings would provide beneficial references for soil remediation assessments of contaminated soils under multi-environmental stress.

Leaching behavior and comprehensive toxicity evaluation of heavy metals in MSWI fly ash from grate and fluidized bed incinerators using various leaching methods: A comparative study

Municipal solid waste incineration fly ash (MSWI FA) is a kind of hazardous waste that contains a substantial amount of heavy metals. To facilitate the appropriate treatment of MSWI FA, the leaching behavior of heavy metals was evaluated in MSWI FA from various sources using different leaching methods. Nine kinds of MSWI FA were investigated using three kinds of batch leaching tests (TCLP, HJ/T 300, and EN12457-2). The chemical form distributions of heavy metals in MSWI FA were obtained by sequential extraction procedures (SEPs) and the environmental risk posed by MSWI FA was comprehensively evaluated. The results showed that the grate and fluidized bed MSWI FA performed differently in various leaching methods, which was mainly dependent on the leachate pH and the chemical form distributions of the heavy metals. In addition, the BCR SEP was more suitable for the fractionation of heavy metals and the environmental risk assessment of MSWI FA when compared with Tessier's SEP. The overall pollution toxicity index allowed a comprehensive risk assessment specific to the leaching environment, thereby offering valuable guidelines for the stabilization or resource-based treatment of MSWI FA.

Dynamic response of cadmium immobilization to a Ca-Mg-Si soil conditioner in the contaminated paddy soil

Soil conditioners are often used to immobilize soil heavy metals. Understanding the transfer of Cd in soil-plant system to different application rates and modes of soil conditioners application is essential for food safety. The stabilization persistence of soil conditioners in immobilizing Cd, to date however, is still limited. In this study, the stabilization persistence of a Ca-Mg-Si soil conditioner (SC) was assessed based on a six-year Cd-contaminated paddy field study with growth of two rice local main varieties (Yongyou17-YY and Xiushui14-XS) and four application rates (1500 kg ha-1 (low), and 3000 kg ha-1 (high) for the first year only, and 1500 kg ha-1 and 3000 kg ha-1 every year). Results showed that continuous SC application with high rate increased soil pH, simultaneously with more water soluble and exchangeable Cd was transferred to Fe-Mn oxides bound and carbonate-bound Cd in the first 3-4 years; while the low rate was only effective with growth of YY that were applied for a shorter period of time. Statistical analysis indicated that the stability effect of SC was integratedly affected by soil pH, SC application rate, and meteorological factors (precipitation and temperature). Especially, soil fractionation contributed the most changes of Cd availability in soil, while meteorological factors, SC application rate and crop varieties altogether exhibited the great effect on Cd accumulation in grain. Our finding demonstrated the potential long-term stabilization of SC in soil Cd immobilization, with the performance needed for further verification on the basis of different soil types.

Unraveling the aging dynamics in the simultaneous immobilization of soil metal(loid)s using oxides

Immobilization represents the most extensively utilized technique for the remediation of soils contaminated by heavy metals and metalloids. However, it is crucial to acknowledge that contaminants are not removed during this process, thereby leaving room for potential mobilization over time. Currently, our comprehension of the temporal variations in immobilization efficacy, specifically in relation to amendments suitable for industrial sites, remains very limited. To address this knowledge gap, our research delved into the aging characteristics of diverse oxides, hydroxides, and hydroxy-oxides (collectively referred to as oxides) for the simultaneous immobilization of arsenic (As), cadmium (Cd), and antimony (Sb) in soils procured from 16 contaminated industrial sites. Our findings unveiled that Ca-oxides initially showed excellent immobilization performance for As and Sb within 7 days but experienced substantial mobilization by up to 71 and 13 times within 1 year, respectively. In contrast, the efficacy of Cd immobilization by Ca-oxides was enhanced with the passage of time. Fe- and Mg-oxides, which primarily operate through encapsulation or surface complexation, exhibited steady immobilization performances over time. This reliable and commendable immobilization effect was observed across distinct soils characterized by varying physicochemical properties, including pH, texture, CEC, TOC, and EC, underscoring the suitability of such amendments for immobilizing metal(loid)s in diverse soil types. MgO, in particular, displayed even superior immobilization performance over time, owing primarily to gradual hydration and physical entrapment effects. Remarkably, Mg-Al LDHs emerged as the most effective candidate for the simultaneous immobilization of As, Cd, and Sb. The results obtained from this study furnish valuable data for future investigations on the immobilization of metals and metalloids in industrial soils. They enable the projection of immobilization performance and offer practical guidance in selecting suitable amendments for the immobilization of metal(loid)s.

Combined application of strong alkaline materials and specific organic fertilizer accelerates nitrification process of a rare earth mining soil

The extensive usage of ammonium sulfate as the leaching agent to extract rare earth elements led to widespread ammonia nitrogen (NH₄⁺-N) pollution in the tailing soils of ion-adsorbed rare earth deposits in southern China. However, the cost-effective technologies to tackle with the long-term retention of NH₄⁺-N in the rare earth mining soil have been largely unresolved. In this study, we developed a cost-effective approach to activate soil nitrification by the co-application of alkaline materials and organic fertilizer. The co-application of 0.3 % of organic fertilizer and 0.1 % ∼ 0.2 % of CaO or MgO or Mg(OH)₂ stimulated a soil NH₄⁺-N decrease rate of 2.01-7.58 mg kg^-1 d^-1 and a soil NO₃^--N accumulation rate of 1.56-7.09 mg kg^-1 d^-1. Noting that only if the soil pH was elevated to 7.81-9.00, the NH₄⁺-N decrease rate and NO₃^--N accumulation rate were dependent on the proton consumption capacity of the alkaline materials. The application of CaCO₃ could not stimulate soil nitrification possibly due to the soil pH was uncapable to be elevated to above 7.68. The qPCR, amplicon sequencing, and nitrification inhibitor batch incubation results demonstrated that organic fertilizer supplied active ammonia-oxidizing bacteria Nitrosomonas europaea. The proliferation of Nitrosomonas europaea in the alkaline materials and organic fertilizer co-applied soil was responsible for the soil nitrification. Furthermore, the application of commercial denitrifying bacteria inoculum promoted the removal of accumulated NO₃^--N. The findings of this study provide a lost-cost technology to remove NH₄⁺-N from the rare earth mining soil.

Remediation methods of heavy metal contaminated soils from environmental and geotechnical standpoints

Heavy metal contaminated soil (HMCS) threatens world health and sustainable growth, owing to which numerous remediation methods have been devised. Meanwhile, environmental sustainability and geotechnical serviceability of remediated HMCS are important considerations for reusing such soils and achieving sustainable development goals; therefore, these considerations are critically reviewed in this article. For this purpose, different onsite and offsite remediation methods are evaluated from environmental and geotechnical standpoints. It was found that each remediation method has its own merits and limitations in terms of environmental sustainability and geotechnical serviceability; generally, sustainable green remediation (SGR) and cementation are regarded as effective solutions for the problems related to the former and latter, respectively. Overall, the impact of remediation techniques on the environment and geotechnical serviceability is a developing area of study that calls for increased efforts to improve the serviceability, sustainability, reusability and environmental friendliness of the remediated HMCS.

Film mulching alters soil properties and increases Cd uptake in Sedum alfredii Hance-oil crop rotation systems

Film mulching (FM) is an agronomic measure worldwide, yet its effect on cadmium (Cd) accumulation in plants is unknown. This study investigated the potential for phytoremediation with FM treatment of Cyperus esculentus L. (chufa) and Sedum alfredii Hance (S. alfredii)-oil crop rotation system. The FM increased the biomass and Cd content of the chufa, resulting in an increase of 65.0-193.5% in the Cd accumulation. S. alfredii also was planted using non-film mulching and film mulching (FMSA), followed by rotation oil plants using non-film mulching. Soil pH and dissolved organic carbon content were significantly reduced, and the Cd grain size fraction of macro-aggregates was significantly increased by FMSA, which increased the uptake of available Cd by S. alfredii. This phenomenon further promoted the accumulation of Cd in S. alfredii and reduced the Cd content of aboveground tissues and seeds in subsequent oil crops. Vegetable oils were safely produced in all treatments due to their low Cd content. Compared with non-film mulching, FM increased the Cd accumulation of rotation systems by 66.8-96.4%, and the Cd remediation efficiency reached 11.8-12.9%. Collectively, the FM treatment effectively improved the remediation efficiency of Cd in the rotation system and ensured the safe production of vegetable oil.

Manganese stabilization in mine tailings by MgO-loaded rice husk biochar: Performance and mechanisms

Leachable metal in abandoned mine tailings may be toxic to vegetation, affecting effective ecological restoration. In this study, MRB was synthesized through MgCl₂·6H₂O wet impregnation followed by duplicate slow pyrolysis. Manganese tailings were mixed with MRB, rice husk biochar (RB), and MgO at a dosage of 0-5%, followed by 90-day incubation. Toxicity characteristic leaching procedure and sequential leaching were used to analyze the leachability and species of Mn in tailings, while a stabilization mechanism was proposed with the support of the characterization of the tailings before and after amendment. Results suggested MRB addition significantly decreased leachable Mn by 63.8%, reducing from 59.88 mg/L to 21.68 mg/L, while only a 14.39% reduction was achieved by rice husk biochar (RB). The sharp decline of leachable Mn after 90-day mixing was contributed by the transformation from labile to stable fractions. A microporous biochar matrix along with the uniform dispersion of MgO active component were both responsible for the better Mn stabilization. Only less than 10% of the variation in substrate pH was observed with the increase of MgO loading or incubation time. Linear correlation analyses indicated substrate pH's strongl negative relationship with leachable Mn and moderately positive relationship with residual fraction. Characterization results revealed that MRB exhibited different stabilization mechanisms in mine tailings, where Mn was likely to be stabilized by direct interaction with active MgO or indirect alkaline precipitation to form stable MgMn₂O₄, Mn(CH₃COO)_2, and MnO(OH)₂. This work validated the promoting potential of recycling agricultural biomass waste for the amendment of manganese mine tailings.

Effects of Temperature on the Leaching Behavior of Pb from Cement Stabilization/Solidification-Treated Contaminated Soil

Solidification/stabilization (S/S) is one of the most widely used techniques in the disposal of heavy-metal-contaminated soil, though the long-term effectiveness of S/S technology remains implicit. Temperature is an important factor affecting the leaching behavior of heavy metals and the long-term effectiveness of S/S treatment. This study systematically explored the influence of temperature on the leaching behavior of lead in an S/S monolith through semi-dynamic leaching test at different temperatures. The results showed that an increase in temperature could accelerate the leaching concentration and cumulative leaching amount of lead ions in the S/S monolith. The cumulative leaching amount of lead ions in the S/S monolith after 11 days at 55 °C was about 5.8 times that at 25 °C. The leaching rate of lead ions in the S/S monolith increased with the increase in temperature. The leaching index of lead ions was larger than 9, which met the requirements for “controlled utilization” in the environment. The leaching mechanism of lead ions was diffusion control and did not change in the temperature range of 25–55 °C. These findings indicate that temperature affects the leaching behavior and the long-term effectiveness of S/S treatment, and temperature variation should be considered in the effectiveness evaluation of S/S treatment.

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

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

Reuse of lead glass sludge in the fabrication of thermally insulating foamed glass with outstanding properties and high Pb-stabilization

This study represents the sustainable/safe consumption of lead glass sludge (LGS) in the fabrication of thermally insulating foamed glass via sintering (750-950º C) and chlorination processes. The impact of selected additives including calcium chloride (CaCl₂) and sodium hydroxide (NaOH) on the foaming efficiency and Pb-stabilization has been deeply investigated. LGS is mainly lead silicate material with considerable content of calcium carbonate, which acts as foaming agent during sintering process. The newly developed foamed-materials exhibited thermal conductivity of 0.054-0.136 W/m.K, density of 0.23-1.10 g/cm³, porosity of 63.3-92.6%, and compressive strength of 0.10-2.69 MPa. X-ray diffraction proved that the immobilization mechanism was attributed to the transformation of free Pb within LGS into insoluble ganomalite Pb₉Ca₅MnSi₉O₃₃ phase. Adding NaOH enhanced the foaming process accompanied by a significant reduction in Pb-leaching. Incorporating CaCl₂ has resulted in a retardation in Pb-leaching, which associated with Pb-stabilization and Pb-vaporization. In an attempt to reduce CO₂-emission, the potential use of alkali-rich-wastewater (AW) as eco-friendly alkali source in lieu of NaOH was studied. Regardless of the variation in Pb-concentrations in leachates, all samples recorded Pb-concentrations lower than the safe limit (≤ 5 mg/l), achieving Pb-immobilization of 95.98-99.87%. The significantly reduced thermal conductivity and enhanced Pb-immobilization efficiency along with the reasonable compressive strength summarize the major innovation presented in this study.

Immobilization of Sb in a smelting residue by micro-sized zero-valent iron: Long-term performance under accelerated exposure to strong acid rain

This study investigated the long-term leachability of antimony (Sb) in a smelting residue immobilized by three commercial micro-sized zero-valent iron (ZVI) products. Effect of oxic incubation time (14 days and 120 days) on the immobilization efficiency of Sb were compared, and the long-term leaching risk was evaluated by an accelerated exposure test, in which the slag was consecutively extracted by simulated strong acid rain (SSAR, HNO₃: H₂SO₄ = 1:2, pH = 3.20). Notably, all ZVI treatments efficiently immobilized the Sb in this slag in a short term (14 days); the one-step SSAR-leached Sb was reduced by 89%-91% compared to the original slag (5.9 mg/L) and was far below the environmental standard (0.6 mg/L) established by the US EPA. The sequential SSAR leaching results reflected that the 14-d incubated slags after ZVI treatments had strong H⁺ resistance, and the immobilized Sb was not easily activated by continuous SSAR corrosion. The binding of Sb with amorphous phase Fe oxyhydroxides (e.g. ferrihydrite) derived from ZVI corrosion played a dominant role in the Sb immobilization efficiency. However, the longer aging process (120 days) easily resulted in the reduction of Sb immobilization by ZVI treatments. The changes in crystallinity of Fe oxyhydroxides (transformation from poorly-crystalline to crystalline ones) and the pH elevation to alkaline range might explain the weakening of the immobilization of Sb in ZVI-amended slags with 120 days of incubation. In total, the effectiveness of Sb immobilization in smelting residue greatly depended on the type of ZVI and the aging process. Our work has demonstrated that the ZVI treatment was potentially feasible to mitigate the Sb leaching risk from smelting slags; however, the ZVI type needs to be carefully selected and its long-term performance should be adequately verified before practical application.

Long-term immobilization of soil metalloids under simulated aging: Experimental and modeling approach

Aging is an inevitable natural process, leading to faded performances of soil amendments. Understanding long-term aging features is crucial for the risk management of contaminated soil. In this study, a novel quantitative aging method, namely, the "soil coin" method, was developed, which can simulate the effects of natural aging on metal(loid) immobilization performances. To better depict the aging features, two models on the basis of conditional probability-induced failure were developed. To effectively immobilize soil arsenic (As) and antimony (Sb), magnesium (Mg) and iron (Fe) oxides were simultaneously introduced to either fresh or pre-oxidized biochar via a facile method. Although post-application aging is harmful, pre-aging (i.e., pre-oxidation using H₂O₂) in turn served as an effective means to introduce more metal oxides, thereby rendering better short-term and long-term effectiveness for metalloid immobilization. Experimental and modeling approaches suggested that precipitation accounted for long-term immobilization, while a constant aging rate is the key feature for a promising soil amendment. It is suggested that to further calibrate this method and better understand the immobilization performances in the long run, more evidence from the field is needed.

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.

Combined amendment reduces soil Cd availability and rice Cd accumulation in three consecutive rice planting seasons

The scientific application of stabilized materials has been considered an effective method for the in situ remediation of Cd-contaminated soil. This study aimed to investigate the persistence of the effect of a combined amendment of limestone and sepiolite (LS) on soil Cd availability and accumulation in rice grown in a mildly Cd-contaminated paddy field (0.45 mg/kg of Cd) over three consecutive rice seasons. 1125-4500 kg/ha of LS was applied to the soil before the first rice planting season and 562.5-2250 kg/ha of LS was supplemented before the third rice planting season. The application of LS (1125-4500 kg/ha) increased the soil pH by 0.44-1.09, 0.18-0.53, and 0.42-0.68 in the first, second, and third season, respectively, and decreased the soil acid-extractable Cd content by 18.2-36.4%, 17.7-33.5%, and 9.6-17.6%. LS application significantly decreased the Cd contents in the rice tissues. The application of 4500 kg/ha of LS decreased the Cd content in brown rice to below the National Food Limit Standard of 0.2 mg/kg (GB 2762-2017) in the three consecutive rice seasons. However, the effect of LS on the soil-rice system was significantly weakened in the third season. The supplementary application of 562.5-2250 kg/ha of LS further decreased the Cd content in brown rice by 26.1-56.5% and decreased the health risk index by 23.7-43.8%. Therefore, it was recommended to apply 4500 kg/ha of LS in the first season and to supplement 2250 kg/ha of LS in the third season to effectively guarantee the clean production of rice in three consecutive rice seasons.

Comparison of the effectiveness of biochar vs. magnesite amendments to immobilize metals and restore a polluted soil

Here we addressed the remediation of a soil severely contaminated by Cu, Cd, Pb and Zn. In this regard, we tested the capacity of magnesite and biochar, inorganic and organic soil amendments, respectively, to reduce metal availability and improve soil properties. To this end, 1-kg pots containing the polluted soil were amended with either magnesite or biochar. Metal availability and soil properties were then measured at days 15 and 75. Also, to evaluate the impact of the two treatments on plant growth, we conducted experimental trials with Brassica juncea L. and compost addition. Both amendments, but particularly magnesite, markedly decreased metal availability. Soil properties were also enhanced, as reflected by increases in the cation exchangeable capacity. However, plant growth was inhibited by magnesite amendment. This observation could be attributable to an increase in soil pH and cation exchange capacity as well as a high Mg concentration. In contrast, biochar increased biomass production but decreased the quantity of metals recovered when the plants are harvested. In conclusion, on the basis of our results, we propose magnesite as a suitable approach for stabilizing contaminated soils (or even spoil heaps) where revegetation is not a priority. In contrast, although biochar has a lower, but still significant, capacity to immobilize metals, it can be used to restore natural soil properties and thus favor plant growth.

Simultaneous reduction and immobilization of Cr(VI) in seasonally frozen areas: Remediation mechanisms and the role of ageing

Among the toxic metals, hexavalent chromium [Cr(VI)] has attracted much attention due to its high mobility and toxicity, rendering considerable challenges for long-term remediation. In this study, the soil was collected from a dichromate contaminated industrial site in Liaoning Province, a seasonally frozen area in northern China, and subjected to frequent freeze-thaw cycles. Three additives, including (i) ferrous sulfate; (ii) calcium polysulfide; and (iii) combined biochar and calcium polysulfide were applied to reduce and immobilize Cr(VI) in the soils. The samples underwent 28 days of incubation followed by 16 freeze-thaw cycles. The toxicity characteristic leaching procedure (TCLP) and simulated acid rain leaching were adopted to test the remediation performances. It was observed that all three treatments can significantly reduce and immobilize Cr(VI) after short-term incubation, while biochar with abundant functional groups could adsorb and reduce Cr(VI) effectively. Notably, the concentration of Cr(VI) in TCLP leachates after incubation in combined treatment decreased by 67.87% and 37.27%, respectively, compared with the application of ferrous sulfate or calcium polysulfide alone. Freeze-thaw cycles induced the disintegration of soil particles and increased the risk of contaminant mobilization. Conversely, biochar particles has become finer and even produced nanoparticles with ageing, accompanied by the increase in oxygen-containing surface functional groups. Additionally, the specific surface area increased with the pyrolysis of biochar, which further enhanced the retention of soil colloidal particles and suppressed the migration of contaminants. Therefore, the cumulative release of Cr(VI) in the combined treatment (i.e., 10.97 ~ 32.97 mg/kg) was much lower than that of the other two treatments after freeze-thaw ageing. Overall, the combination of biochar and calcium polysulfide displayed advantages in the reduction and immobilization of Cr(VI), and offered a long-term, effective strategy for the remediation of Cr(VI) contaminated soils in cold regions.

Enhancement of exchangeable Cd and Pb immobilization in contaminated soil using Mg/Al LDH-zeolite as an effective adsorbent

In the present study, experiments using zeolite and Mg/Al LDH-zeolite for immobilization of Cd and Pb ions in artificial soil were conducted. The conditions which affect Cd and Pb ion immobilization in soil were evaluated, namely soil pH (5-7), the mass ratio of adsorbents (1%, 3% and 5%), incubation time (15 days, 30 days and 45 days) and soil moisture (30%, 50% and 70%). The results indicated that the optimal soil pH, mass ratio of adsorbents, incubation time and soil moisture for immobilization of Cd and Pb ions by the adsorbent were, respectively, 7.0, 3%, 30 days and 70%. The exchangeable Cd ion content in the contaminated soil dropped from 22.17 mg kg-1 (87.65%) to 11.03 mg kg-1 (43.48%) and 6.47 mg kg-1 (26.36%) on incubation with zeolite and Mg/Al LDH-zeolite, respectively, while the exchangeable Pb content fell from 23.28 mg kg-1 (90.02%) to 14.12 mg kg-1 (54.04%) and 9.47 mg kg-1 (35.24%) using zeolite and Mg/Al LDH-zeolite as absorbents in contaminated soil, respectively. Fe-Mn oxide occluded (F2), carbonate bound (F3) and organically complexed (F4) were the main forms for immobilization of the exchangeable Cd and Pb when the zeolite and Mg/Al LDH-zeolite absorbents were separately cultivated into soil. Precipitation, co-precipitation and electrostatic attraction were the main mechanisms of exchangeable Cd and Pb immobilization onto the Mg/Al LDH-zeolite to form carbonate metals (CdCO3 and PbCO3). This was due to the surface functional groups of the adsorbent and the presence of Fe and Al oxyhydroxides, Mn oxides, and Si and O elements in the Mg/Al LDH-zeolite's constituents. The efficiency of Cd and Pb immobilization by the Mg/Al LDH-zeolite was higher than that by zeolite from 1.5 to 1.6 times. The Mg/Al LDH-zeolite showed an enhanced ability of exchangeable Cd and Pb immobilization in contaminated soil.

Stabilization of hazardous lead glass sludge using reactive magnesia via the fabrication of lightweight building bricks

This study focused on the stabilization of lead glass sludge (LGS) using reactive magnesia (MgO) via the fabrication of lightweight building bricks. Two types of MgO with different reactivities were prepared by the thermal treatment of magnesium carbonate at 800 °C and 1200 °C (MgO-800 and MgO-1200, respectively). The fabrication of bricks and Pb stabilization were performed by wet mixing LGS with MgO followed by humidity incubation. Results showed that the Pb immobilization and performance of the produced bricks were strongly affected by MgO reactivity, curing time, and LGS-MgO weight ratios. Pb immobilization was performed by the transformation of soluble lead into an insoluble hydrocerussite phase, particularly in hydrated mixtures with high MgO content (> 25 wt%). Pb immobilization inside a magnesium silicate hydrate skeleton is the main mechanism in the hydrated samples containing 25 wt% MgO. To achieve "sustainability," we recommend the use of a hydrated mixture containing 75 wt% of LGS and 25 wt% of MgO-800 in the production of building bricks because this mixture exhibits high compressive strength, high Pb immobilization, low energy demand, and low environmental pollution.

Physicochemical properties, metal availability and bacterial community structure in heavy metal-polluted soil remediated by montmorillonite-based amendments

Clay materials are commonly used in remediation techniques for heavy metal contaminated soil. In this study, a magnesium (Mg(OH)₂/MgO)-montmorillonite was proposed to be utilized for heavy metals immobilization in contaminated soil, with the remediation efficiency evaluated through the toxicity characteristic leaching procedure (TCLP) and the community bureau of reference sequential extraction procedure (BCR). The addition of magnesium-montmorillonite resulted in lower TCLP extractability for the heavy metals (Cu, Pb, Zn and Cd) in soil as it promoted their conversion from acid soluble fraction to residual fraction. Meanwhile, MM raised the soil pH and water-soluble organic carbon (WSOC). It was demonstrated that the immobilization of heavy metal in the presence of magnesium-montmorillonite was primarily induced by electrostatic attraction, precipitation and chelation with water-soluble organic carbon. Interestingly, a decreased bacterial community diversity was observed in soil treated by magnesium-montmorillonite (MM). The presence of pure magnesium-montmorillonite promoted the relative abundance of Proteobacteria, Actinobacteria and Firmicutes but reduced that of Bacteroides and Acidobacteria. Our results suggest that integrating the biochar into montmorillonite-based amendments can alleviate the damage to soil microorganisms by weakening the negative correlation between the two factors (content clay and WSOC in soil) and soil bacteria.

Iron-modified rice husk hydrochar and its immobilization effect for Pb and Sb in contaminated soil

Cationic and anionic heavy metal contamination sometimes co-exists in soil systems, such as mining areas and shooting ranges, seriously threatens human health and ecological stability. In this study, iron-modified rice husk hydrochar showed commendable ability to immobilize both heavy metal cation (Pb) and anion (Sb) simultaneously in soils. Iron-modified rice husk hydrochar (HC12.5-180) (5%) amendment reduced the bioavailability (EX- and CB-fraction) of Pb and Sb by 25 and 40%, respectively, which were 8 and 5 times higher than that of pristine rice husk hydrochar (HC0-180) (5%) amendment. The cation (Pb) immobilization mainly depends on cation exchange with mineral components (K⁺, Ca²⁺, Na⁺, Mg²⁺), precipitation with nonmetallic anions (Cl- and SO₄^2-), and complexation. Meanwhile, the iron oxides (FeO, Fe₂O₃, Fe₃O₄), formed during hydrothermal process, can be easily combined with anion (Sb) to form geochemically stable minerals. In conclusion, this work offered a practical and cost-effective technology based on the iron modification rice husk hydrochar for the immobilization of both anionic and cationic heavy metal contaminants in soils.

Effect of immobilizing reagents on soil Cd and Pb lability under freeze-thaw cycles: Implications for sustainable agricultural management in seasonally frozen land

Agricultural soil contamination in seasonally frozen land threatens food security. It is necessary to investigate the effects of freeze-thaw cycles on heavy metal bioavailability so as to select suitable immobilization agents. In this study, the soil was collected from a mid-latitude agricultural site in Liaoning Province, China, which was spiked with cadmium (Cd²⁺) and lead (Pb²⁺). Four immobilization treatments were set up, including (i) corn stover biochar, (ii) organic fertilizer, (iii) combined biochar and organic fertilizer, and (iv) the control group. The immobilized soils were subjected to 16 freeze-thaw cycles to temperatures of -10 °C, -20 °C, and -30 °C. It was found that freeze-thaw cycling increased the labile cadmium (Cd) and lead (Pb) content in the soil (i.e., exchangeable). The organic fertilizer treatment performed best in short-term immobilization, which was demonstrated by the amount of diethylenetriaminepentaacetic acid (DTPA) extractable lead (Pb) being 17.3-53.3% lower than that of the other treatments, and 7.2-31.5% lower for cadmium (Cd). Biochar, on the other hand, displayed better long-term performance under freeze-thaw cycling. This is probably because the biochar's organic carbon content is relatively stable, and therefore, releases relatively little dissolved organic carbon (DOC) which could re-mobilize heavy metals. Furthermore, additional sorption sites are formed and the abundance of oxygen-containing functional groups increased when biochar breaks down during freeze-thaw cycles. Overall, the joint application of biochar and organic fertilizer had the greatest immobilization effect, which inhibited the cracking of soil aggregates, reduced the labile metal content, and displayed both short- and long-term immobilization effectiveness. It is suggested that combined biochar and organic fertilizer may offer an effective strategy for the sustainable agricultural management of cadmium (Cd) and lead (Pb) contaminated in seasonally frozen land.

Green immobilization of toxic metals using alkaline enhanced rice husk biochar: Effects of pyrolysis temperature and KOH concentration

Biochar is a "green" material that has been widely used in environmental applications for its capability to remove or immobilize contaminants in different environmental media (i.e. soil, water and air) and mitigate climate change. In this study, the feasibility of using KOH enhanced biochar for soil Cd and Pb stabilization was investigated, and the effects of pyrolysis temperature and alkaline concentrations for modification were explored. Field-emission scanning electron microscopy (FESEM), N₂ adsorption-desorption, and Fourier Transform Infrared Spectroscopy (FTIR) analyses were conducted to reveal the influence on biochar physiochemical properties. The immobilization performances were examined through Toxicity Characteristics Leaching Procedure (TCLP), and Response Surface Methodology (RSM) was adopted to visualize the results from leaching tests. The stabilization mechanisms of alkaline enhanced biochars were investigated using Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS), Tessier sequential extraction method and X-ray diffraction (XRD) analyses. The results indicated that rice husk biochar pyrolyzed at a relatively low temperature (i.e., 300 °C) and activated by moderate alkaline concentrations (i.e., 1 M or 3 M KOH) rendered optimum stabilization performance. KOH activation was a double-edged sword, with high alkaline concentrations destroying biochar's cell structures. Moreover, the integration of TOF-SIMS, XRD and sequential leaching method shed lights on the underlying mechanisms involved in metal stabilization. Surface complexation between toxic metals and oxygen-containing functional groups rather than liming or precipitation was proven to be the fundamental stabilization mechanism.

Exogenous phosphorus treatment facilitates chelation-mediated cadmium detoxification in perennial ryegrass (Lolium perenne L.)

Cadmium (Cd) is an on-going environmental pollutant associated with hindered plant growth. In response, plants possess various strategies to alleviate Cd stress, including reactive oxygen species (ROS) scavenging and chelation-mediated Cd detoxification. The present study examined the Cd defense mechanism of perennial ryegrass (Lolium perenne L.), taking into account the effect of exogenous phosphorus (P) input. It was found that despite triggering antioxidant enzyme activity, Cd stress heightened lipid peroxidation levels. Exogenous P input partially mitigated the lipid peroxidation impact and decreased the levels of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) antioxidant enzymes, revealing reduced ROS-scavenging activity. Importantly, notable relationships were determined between the amount of Cd uptake in the root and the amount of non-protein thiols (R² = 0.914), glutathione (R² = 0.805) and phytochelatins (R² = 0.904) in proportion to the amount of exogenous P applied. The levels of amino acids proline and cysteine were also enhanced by exogenous P input showing their influence in alleviating Cd stress. Overall, it is reported that Cd detoxification in ryegrass plants can be stimulated by exogenous P input, which facilitates chelation-mediated Cd detoxification processes.

Green remediation of Cd and Hg contaminated soil using humic acid modified montmorillonite: Immobilization performance under accelerated ageing conditions

Solidification/Stabilization (S/S) is an effective way to immobilize toxic metals in contaminated soil. However, utilization of ordinary Portland cement (PC) in this process has raised environmental concerns owing to the high carbon footprint from PC manufacturing and the risk of toxic element leaching in the long term. Hence there is an urgent need to seek for "green" immobilization approaches with long-term stability. In this study, a clay-based material, humic acid modified montmorillonite (HA-Mont) was applied to a Cd and Hg contaminated soil. Field emission scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (FESEM/EDS), N₂ adsorption-desorption, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analyses were performed to investigate the characteristics of this material. Compared to the soil without any treatment, dosage of 5 % HA-Mont could effectively reduce Cd and Hg concentrations by 94.1 % and 93.0 %, respectively and to below the regulatory limits in the TCLP (Toxicity Characteristic Leaching Procedure) leachates. Compared to the soil treated with virgin montmorillonite, HA modification resulted in the reduction of leachate concentrations of Cd and Hg by 69.5 % and 65.9 %, respectively. Long-term immobilization performance of the HA-Mont treatment was examined using a quantitative accelerated ageing method. In order to examine the ageing features, a novel method based on conditional probability was developed, and the reliability of HA-Mont immobilization was found to fit the Weibull model well, as the ageing rate of immobilization effect increased with time. After 120 years of ageing, reliability of both metals could still remain above 0.95. Cd concentration in TCLP leachates at 120^th year could still remain below the regulatory limit (294 μg/L vs 1000 μg/L), while Hg concentration reached the regulatory limit of 200 μg/L in 96^th year. This is the first attempt developing a green S/S method of Cd and Hg contaminated soil using HA-Mont and examining the long-term ageing characteristics of the stabilized soil using a probability-based approach.