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

Transforming restored heavy metal-contaminated soil into eco-friendly bricks: An insight into heavy metal stabilization and environmental safety

Materialization is currently the primary method for utilizing restored heavy metal-contaminated soil (RHMCS). However, compared to ordinary building materials, the migration and transformation mechanisms of heavy metals (HMs) while preparing these materials remain unclear. To bridge these gaps, this study investigated the migration and transformation mechanisms of As and Pb during the sintering of RHMCS into bricks. This study is the first to conduct a systematic study from the perspectives of both the inner and outer brick layers on the patterns and mechanisms of HM migration and transformation during the sintering process, along with the safety of product utilization. Approximately 90% of As and 36% of Pb migrated out of the RHMCS, with significant transformations observed after sintering. Adjusting the sintering parameters increased migration at long dwell times and high temperatures. These findings indicate different migration behaviors and transformations of HMs within the brick layers, emphasizing the need for cautious application and potential secondary pollution risks. A potential ecological risk index confirmed the safety of the bricks in accordance with construction material standards. Overall, this study provides crucial insights into safe and effective RHMCS utilization, contributing significantly to environmental remediation and sustainable construction practices.

Stabilization effect of nano-SiO2@iron-phosphorus on ferrallisols, calcareous soil and organic soil heavily polluted by heavy metals: A fast reaction curing stabilization process

The remediation of soil pollution by heavy metals (HMs) presents a significant challenge in environmental restoration. Stabilization remediation technology has proven effective in treating HMs contaminated soil. However, its development is constrained by drawbacks such as slow reaction kinetics and low adsorption capacity. This research synthesized a nano-SiO2@iron‑phosphorus (FPOH) material by SiO32- encapsulating the iron-phosphate precipitate obtained from Fe ion and phosphate. In addition, this research applied this material to ferrallisols, calcareous soils and organic soils with three different levels of high pollution by Cd, Pb, Cu and Zn. The experimental results indicate that all experimental soils stabilized rapidly within 1 day and met the requirements of remediation engineering standards (ChinaMEE HJ 1282-2023). Analysis of the possible mechanisms suggests that the FPOH material effectively fills voids with phosphate mineral formation, preventing the secondary release of HMs. During the stabilization process, FPOH involves the adsorption of free ions and small organic molecules in the soil, which does not affect its high reactivity. The development and utilization of FPOH offer valuable insights for soil stabilization remediation.

PFOA-contaminated soil remediation: a comprehensive review

Soil and groundwater contamination has been raised as a concern due to the capability of posing a risk to human health and ecology, especially in facing highly toxic and emerging pollutants. Because of the prevalent usage of perfluorooctanoic acid (PFOA), in industrial and production processes, and subsequently the extent of sites contaminated with these pollutants, cleaning up PFOA polluted sites is paramount. This research provides a review of remediation approaches that have been used, and nine remediation techniques were reviewed under physical, chemical, and biological approaches categorization. As the pollutant specifications, environmental implications, and adverse ecological effects of remediation procedures should be considered in the analysis and evaluation of remediation approaches, unlike previous research that considered a couple of PFAS pollutants and generally dealt with technical issues, in this study, the benefits, drawbacks, and possible environmental and ecological adverse effects of PFOA-contaminated site remediation also were discussed. In the end, in addition to providing sufficient and applicable understanding by comprehensively considering all aspects and field-scale challenges and obstacles, knowledge gaps have been found and discussed.

Biochar-nanoparticle combinations enhance the biogeochemical recovery of a post-mining soil

Here we addressed the capacity of distinct amendments to reduce arsenic (As), copper (Cu), selenium (Se) and zinc (Zn) associated risks and improve the biogeochemical functions of post-mining soil. To this, we examined nanoparticles (NPs) and/or biochar effects, combined with phytostabilization using Lolium perenne L. Soil samples were taken in a former metal mine surroundings. Ryegrass seeds were sown in pots containing different combinations of NPs (zero-valent iron (nZVI) or hydroxyapatite (nH)) (0 and 2 %), and biochar (0, 3 and 5 %). Plants were grown for 45 days and the plant yield and element accumulation were evaluated, also soil properties (element distribution within the soil fractions, fertility, and enzymatic activities associated with microbiota functionality and nutrient cycling) were determined. Results showed biochar-treated soil had a higher pH, and much higher organic carbon (C) content than control soil and NP-treated soils, and it revealed increased labile C, total N, and available P concentrations. Soil treatment with NP-biochar combinations increased exchangeable non-acid cation concentrations and reduced exchangeable Na%, improved soil fertility, reduced sodicity risk, and increased ryegrass biomass. Enzymatic activities, particularly dehydrogenase and glucosidase, increased upon the addition of biochar, and this effect was fostered by NPs. Most treatments led to a significant reduction of metal(loid)s contents in biomass, mitigating contamination risks. The two different NPs had similar effects in many parameters, nH outperformed nZVI in terms of increased nutrients, C content, and enzymatic activities. On the basis of our results, combined biochar-NP amendments use, specially nH, emerges as a potential post-mining soil restoration strategy.

Enhanced multi-metals stabilization: Synergistic insights from hydroxyapatite and peroxide dosing strategies

Sediment contamination by heavy metals is a pressing environmental concern. While in situ metal stabilization techniques have shown promise, a great challenge remains in the simultaneous immobilization of multi-metals co-existing in contaminated sediments. This study aims to address this challenge by developing a practical method for stabilizing multi-metals by hydroxyapatite and calcium peroxide (HAP/CaO2) dosing strategies. Results showed that dosing 15.12 g of HAP/CaO2 at a ratio of 3:1 effectively transformed labile metals into stable fractions, reaching reaction kinetic equilibrium within one month with a pseudo-second-order kinetic (R2 > 0.98). The stable fractions of Nickel (Ni), Chromium (Cr), and lead (Pb) increased by approximately 16.9 %, 26.7 %, and 21.9 %, respectively, reducing heavy metal mobility and ensuring leachable concentrations complied with the stringent environmental Class I standard. Mechanistic analysis indicated that HAP played a crucial role in Pb stabilization, exhibiting a high rate of 0.0176 d-1, while Cr and Ni stabilization primarily occurred through the formation of hydroxide precipitates, as well as the slowly elevated pH (>8.5). Importantly, the proposed strategy poses a minimal environmental risk to benthic organisms exhibits almost negligible toxicity towards Vibrio fischeri and the Chironomus riparius, and saves about 71 % of costs compared to kaolinite. These advantages suggest the feasibility of HAP/CaO2 dosing strategies in multi-metal stabilization in contaminated sediments.

Recent developments and prospects of sustainable remediation treatments for major contaminants in soil: A review

Rapid industrialisation and urbanisation are contributing to the entry of emerging contaminants into the environment, posing a significant threat to soil health and quality. Therefore, several remediation technologies have been investigated and tested at a field scale to address the issue. However, these remediation technologies face challenges related to cost-effectiveness, environmental concerns, secondary pollution due to the generation of by-products, long-term pollution leaching risks, and social acceptance. Overcoming these constraints necessitates the implementation of sustainable remediation methodologies that prioritise approaches with minimal environmental ramifications and the most substantial net social and economic advantages. Hence, this review delves into diverse contaminants that threaten soil health and quality. Moreover, it outlines the research imperatives for advancing innovative remediation techniques and effective management strategies to tackle this concern. The review discusses a remediation treatment train approach that encourages resource recovery, strengthens the circular economy, and employs a Life Cycle Assessment (LCA) framework to assess the environmental impacts of different remediation strategies. Additionally, the study explores mechanisms to integrate sustainability principles into soil remediation practices. It underscores the necessity for a comprehensive and systematic approach that takes into account the economic, social, and environmental consequences of remediation methodologies in the development of sustainable solutions.

Mixing uniformity effect on leaching behaviour of cement-based solidified contaminated clay

Mixing uniformity is essential for the quality control of the contaminated clay's solidification. To investigate the effect of the mixing uniformity on the leaching behaviour of the cement-based solidified contaminated clay, this study proposed a quantitative method to characterize the mixing uniformity and investigated the leaching behaviour by the leaching toxicity tests and semi-dynamic leaching tests. X-ray computed tomography (X-CT) was employed to reveal the internal mesoscopic structure. In this case, Pb2+ was selected as a tagged pollutant because of the widespread attention at heavy metal-contaminated sites. The leaching toxicity indicates the significant Pb2+ concentration deviation among the parallel specimens and non-association with the mixing uniformity. However, the Pb2+ cumulative leaching mass and observed diffusion coefficient by the semi-dynamic leaching tests both decrease with the mixing uniformity. X-CT image analysis reveals that the high cement zones wrap the low cement zones with different dimensions in the heterogeneous solidified matrix. Moreover, the specimen pretreatment method in the existing leaching toxicity standards may be inadequate because of the overall encapsulation destruction by the crushing process and representativeness uncertainty when sampling. However, for semi-dynamic leaching, the Pb2+ migration depends on the uniformity, reflecting the continuous distribution of high cement zones.

Application of apatite particles for remediation of contaminated soil and groundwater: A review and perspectives

With rapid industrial development and population growth, the pollution of soil and groundwater has become a critical concern all over the world. Yet, remediation of contaminated soil and water remains a major challenge. In recent years, apatite has gained a surging interest in environmental remediation because of its high treatment efficiency, low cost, and environmental benignity. This review summarizes recent advances in: (1) natural apatite of phosphate ores and biological source; (2) synthesis of engineered apatite particles (including stabilized or surface-modified apatite nanoparticles); (3) treatment effectiveness of apatite towards various environmental pollutants in soil and groundwater, including heavy metals (e.g., Pb, Zn, Cu, Cd, and Ni), inorganic anions (e.g., As oxyanions and F-), radionuclides (e.g., thorium (Th), strontium (Sr), and uranium (U)), and organic pollutants (e.g., antibiotics, dyes, and pesticides); and (4) the removal and/or interaction mechanisms of apatite towards the different contaminants. Lastly, the knowledge or technology gaps are identified and future research needs are proposed.

Impacts of the steel industry on sediment pollution by heavy metals in urban water system

The impact of the steel industry on sediment heavy metal (HM) pollution in urban aquatic environments was investigated in a major iron ore-producing area (Ma'anshan) in China. The concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were 9.68 ± 3.56, 170.31 ± 82.40, 90.62 ± 19.54, 30.61 ± 6.72, 125.43 ± 63.60, and 1276.59 ± 701.90 mg/kg in the steel industry intruded upon sediments and 4.63 ± 1.41, 87.60 ± 10.96, 52.67 ± 19.99, 37.49 ± 6.17, 35.84 ± 11.41, and 189.02 ± 95.57 mg/kg in the control area, respectively. Comparing with the local soil background (0.08 mg/kg for Cd, 62.6 mg/kg for Cr, 19.3 mg/kg for Cu, 28.1 mg/kg for Ni, 26.0 mg/kg for Pb, and 58.0 mg/kg for Zn), significantly higher levels of Cd, Cr, Cu, Pb, and Zn were detected in the steel industry affected sediments. The enrichment factor and principal component analysis indicated that the heavy metals (HMs), except for Ni, were primarily derived from anthropogenic inputs, particularly from steel industrial activities. Multiple risk assessment models suggested that the sediments affected by industrial activities showed significant toxic effects for Cd, Cr, Pb, and Zn, with Cd being the main contributor to sediment toxicity. However, the alkaline nature of the sediments (pH = 7.85 ± 0.57) and the high proportion of residual fraction Cd (61.09% ± 26.64%) may help to reduce the toxic risks in the sediments. Effective measures to eliminate tinuous thethe continous input of Cd and Zn via surface runoff are crucial.

Testing of a novel multi-branch horizontal well remediation technology for in situ remediation of contaminated soil and groundwater

Soil and groundwater contamination has become a key issue in urban redevelopment. It is particularly difficult to use heavy equipment for the remediation of restricted sites or areas contaminated by factories that are still in operation. In this case, horizontal wells are considered a potentially useful technology as they can potentially remediate contamination areas located below buildings and other surface/subsurface obstacles. This research first introduces the principles and advantages and disadvantages of direct push injection, improved slant well, high-pressure rotary jet technology, horizontal reactive media treatment wells, and horizontal directional drilling well. The key aspects for promising in-situ remediation techniques were summarized as remediation well design, remediation agent injection technology and drill pipe and well wall sealing technology. Based on the requirements for key technologies, a novel multi-branch horizontal well in-situ remediation process was proposed, which integrates vertical/horizontal directional drilling, rotary injection, and expansion sealing techniques, and relevant supporting drilling and injecting equipment were developed. A bench test and field test were conducted to test drilling tool performance, drilling accuracy, and injection radius of influence. The results showed that the developed supporting drilling tool met the process requirements and could complete multi-branch horizontal well remediation engineering construction. The deviation between the measured depth and the design depth of the multi-branch horizontal well constructed using this technology was less than 9%, and the deviation between the depth displayed by the guidance instrument and the measured depth was less than 1%. The injection radius of influence in the test field measured from the monitoring wells was greater than or equal to 5 m. The results of this research can provide an effective method for the remediation of contaminated sites.

Mechanistic insights into Pb and sulfates retention in ordinary Portland cement and aluminous cement: Assessing the contributions from binders and solid waste

Identifying immobilization mechanisms of potentially toxic elements (PTEs) is of paramount importance in the field application of solidification/stabilization. Traditionally, demanding and extensive experiments are required to better access the underlying retention mechanisms, which are usually challenging to quantify and clarify precisely. Herein, we present a geochemical model with parametric fitting techniques to reveal the solidification/stabilization of Pb-rich pyrite ash through conventional (ordinary Portland cement) and alternative (calcium aluminate cement) binders. We found that ettringite and calcium silicate hydrates exhibit strong affinities for Pb at alkaline conditions. When the hydration products are unable to stabilize all the soluble Pb in the system, part of the soluble Pb may be immobilized as Pb(OH)₂. At acidic and neutral conditions, hematite from pyrite ash and newly-formed ferrihydrite are the main controlling factors of Pb, coupled with anglesite and cerussite precipitation. Thus, this work provides a much-needed complement to this widely-applied solid waste remediation technique for the development of more sustainable mixture formulations.

Using restored heavy metal contaminated soil as brick making material: Risk analysis upon different scenarios, considering the completeness of bricks

Restored heavy metal contaminated soil (RHMCS) can be utilized as building material, but the risks of heavy metal dissolution (HMD) under different scenarios are not clear. This study focused on sintered bricks made from RHMCS and assessed the HMD process and utilization risks of whole bricks (WB) and broken bricks (BB) under two simulated utilization scenarios of leaching and freeze-thaw. Part of the studied bricks were crushed, which increased the surface area (SSA) 3.43-fold and exposed the inner heavy metals, increasing the HMD in BB. However, the HMD in sintered bricks did not exceed the "Groundwater Quality Standard" and "Integrated Wastewater Discharge Standard" under different utilization scenarios, although the dissolution processes were different. In the leaching scenario, the release rate of HMs (As, Cr, Pb) changed from fast to slow over time; the maximum concentration was 17% of the standard limits. In the freeze-thaw scenario, there was no significant correlation between the release of HMs and freeze-thaw time, and the HMD of As was the highest, reaching 37% of the standard limits. Further analysis of health risks of bricks in the two scenarios found that the carcinogenic risks (CR) and the non-carcinogenic risks (NCR) were below 9.56 × 10^-7 and 3.21 × 10^-2, respectively, which are both lower than the Guidelines for Health Risk Assessment of Groundwater Pollution issued by Ministry of Ecology and Environment of China. These findings suggest that the utilization risks of RHMCS sintered bricks analyzed in this study are low in both scenarios, and higher completeness of bricks leads to higher safety in product utilization. The results provide a reference for the engineering utilization and disposal of building materials made from RHMCS.

Research and Application of High-Pressure Rotary Jet Method in the Seepage Treatment of Heavy Metal Tailing Ponds of Southwest China

The developed karst caves may become the seepage channels of heavy metal to the soil and underground water in Southwest China. Therefore, it is necessary to apply effective seepage treatments to the base of heavy metal tailing reservoirs. This paper addressed the high-pressure rotary jet technology and slurry systems used in the seepage treatment of the deep tailing sand of the Shenxiandong tailing pond located in Southwest China. In this study, the factors of fluidity, initial and final setting times, compressive strength, and permeability coefficient of the slurry were conducted. The mechanism analysis was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and inductively coupled plasma-mass spectrometry (ICP-MS). Three different types of slurry systems were proposed, and the permeability coefficients of the solidification body following 28 days of curing were less than 1 × 10^-7 cm/s. The concentrations of Pb and Zn in the slurry system containing bentonite were reduced by 26.2% and 45.7%, respectively. In the presence of slaked lime and fly ash, the concentrations of Pb and Zn could be reduced by 26.8% and 30%, respectively. A total of 2142 high-pressure rotary jet piles were completed by the high-pressure rotary jet method in the field trial. The diffusion radius of these piles was over 1 m. Following 28 days of curing, the solidification body's compressive strength was 7.45 MPa and the permeability coefficient was 6.27 × 10^-8 cm/s. Both the laboratory and on-site trials showed that this method produced a good pollution barrier effect, which could prevent the diffusion of heavy metal into the adjacent underground water through the karst caves. It is also an effective way of engineering technology concerning heavy metal pollution control that occurs in tailing ponds.

Immobilizing lead and copper in aqueous solution using microbial- and enzyme-induced carbonate precipitation

Inappropriate irrigation could trigger migration of heavy metals into surrounding environments, causing their accumulation and a serious threat to human central nervous system. Traditional site remediation technologies are criticized because they are time-consuming and featured with high risk of secondary pollution. In the past few years, the microbial-induced carbonate precipitation (MICP) is considered as an alternative to traditional technologies due to its easy maneuverability. The enzyme-induced carbonate precipitate (EICP) has attracted attention because bacterial cultivation is not required prior to catalyzing urea hydrolysis. This study compared the performance of lead (Pb) and copper (Cu) remediation using MICP and EICP respectively. The effect of the degree of urea hydrolysis, mass and species of carbonate precipitation, and chemical and thermodynamic properties of carbonates on the remediation efficiency was investigated. Results indicated that ammonium ion (NH4+) concentration reduced with the increase in lead ion (Pb2+) or copper ion (Cu2+) concentration, and for a given Pb2+ or Cu2+ concentration, it was much higher under MICP than EICP. Further, the remediation efficiency against Cu2+ is approximately zero, which is way below that against Pb2+ (approximately 100%). The Cu2+ toxicity denatured and even inactivated the urease, reducing the degree of urea hydrolysis and the remediation efficiency. Moreover, the reduction in the remediation efficiency against Pb2+ and Cu2+ appeared to be due to the precipitations of cotunnite and atacamite respectively. Their chemical and thermodynamic properties were not as good as calcite, cerussite, phosgenite, and malachite. The findings shed light on the underlying mechanism affecting the remediation efficiency against Pb2+ and Cu2+.

Heavy metal stabilization remediation in polluted soils with stabilizing materials: a review

The remediation of soil contaminated by heavy metals has long been a concern of academics. This is due to the fact that heavy metals discharged into the environment as a result of natural and anthropogenic activities may have detrimental consequences for human health, the ecological environment, the economy, and society. Metal stabilization has received considerable attention and has shown to be a promising soil remediation option among the several techniques for the remediation of heavy metal-contaminated soils. This review discusses various stabilizing materials, including inorganic materials like clay minerals, phosphorus-containing materials, calcium silicon materials, metals, and metal oxides, as well as organic materials like manure, municipal solid waste, and biochar, for the remediation of heavy metal-contaminated soils. Through diverse remediation processes such as adsorption, complexation, precipitation, and redox reactions, these additives efficiently limit the biological effectiveness of heavy metals in soils. It should also be emphasized that the effectiveness of metal stabilization is influenced by soil pH, organic matter content, amendment type and dosage, heavy metal species and contamination level, and plant variety. Furthermore, a comprehensive overview of the methods for evaluating the effectiveness of heavy metal stabilization based on soil physicochemical properties, heavy metal morphology, and bioactivity has also been provided. At the same time, it is critical to assess the stability and timeliness of the heavy metals' long-term remedial effect. Finally, the priority should be on developing novel, efficient, environmentally friendly, and economically feasible stabilizing agents, as well as establishing a systematic assessment method and criteria for analyzing their long-term effects.

Application of microbial-induced carbonate precipitation (MICP) techniques to remove heavy metal in the natural environment: A critical review

The occurrence of imbalanced heavy metals concentration due to anthropogenic hindrances in the aquatic and terrestrial environment has become a potential risk to life after circulating through different food chains. The microbial-induced carbonate precipitation (MICP) method has gradually received great attention from global researchers but the underlying mechanism of heavy metal mineralization is not well-understood and challenging, limiting the applications in wastewater engineering. This paper reviews the metabolic pathways, mechanisms, operational factors, and mathematical/modeling approaches in the MICP process. Subsequently, the recent advancement in MICP for the remediation of heavy metal pollution is being discussed. In the follow-up, the key challenges and prospective associated with technical bottlenecks of MICP method are elaborated. The prospective study reveals that MICP technology could be efficiently used to remediate heavy metal contaminants from the natural environment in a cost-effective way and has the potential to improve soil properties while remediating heavy metal contaminated soil.

Modified oyster shell powder with iron (II) sulfate heptahydrate to improve arsenic uptake in solution and in contaminated soils

Arsenic is a metalloid whose presence can be due to natural or anthropological causes. It is considered as a toxic chemical that puts human health at high risk. In this study, we evaluated a novel modified oyster shell (MOS) that was coated with iron (II) sulfate heptahydrate using two different proportions through batch sorption experiments in an arsenic solution and in arsenic-contaminated soils. The arsenic solution was prepared using As(III)-standard solution. The arsenic contaminated soils were extracted from a contaminated site in Cheonan, South Korea, where the average arsenic concentration of the soil was reported as 136.28 mg/kg. Different doses of oyster shell and modified oyster were used to understand the effect of the addition of iron (II) sulfate heptahydrate via sorption batch experiments in solution and sorption tests in soils. The sorption tests were conducted with 50 g of contaminated soil; then, 150 g of soils was used for the pot cultivation tests, and finally, 150 g of contaminated soils was used for column percolation test. Through the experiments, the authors observed a comparable improvement of arsenic stabilization from 10 to 60% with the addition of iron (II) sulfate heptahydrate to oyster shell.

Co-application of combined amendment (limestone and sepiolite) and Si fertilizer reduces rice Cd uptake and transport through Cd immobilization and Si-Cd antagonism

Limestone and sepiolite combined amendment (LS) and silicon (Si) fertilizers are commonly applied for the remediation of Cd-polluted paddy soil. However, it is difficult to further decrease cadmium (Cd) accumulation in rice grains by the individual application of LS or Si fertilizer to heavily Cd-polluted paddy fields. Two seasons of continuous field experiments were conducted in heavily Cd-polluted soil to study how the co-application of LS and Si fertilizer (namely soil-applied Si and foliar-sprayed Si) influences Cd and Si bioavailability in soil and Cd uptake and transport in rice. The results indicated that LS co-applied with soil-applied Si fertilizer treatments can enhance pH, cation exchange capacity (CEC), and available Si content in soil by 0.56-1.26 units, 19.3%-57.2%, and 14.7%-58.9% (p < 0.05), respectively, and reduce the toxicity characteristic leaching procedure (TCLP) extractable Cd content in soil by 26.5%-49.8% (p < 0.05) relative to the control. Furthermore, the co-application of LS and soil and foliar-sprayed Si fertilizer treatments reduced the Cd content in brown rice by 18.8%-70.6% (p < 0.05) compared with the control. Particularly, the brown rice Cd content under the co-application treatment (4500 kg/ha of soil applied LS, 90 kg/ha of Si fertilizer, and 0.4 g/L of foliar-sprayed Si fertilizer) was below 0.20 mg/kg in both seasons. Meanwhile, the Si content in rice was considerably enhanced by LS co-applied with Si fertilizer and negatively (p < 0.05) correlated with the rice Cd content. Therefore, the reduction of Cd bioavailability in soil and the antagonistic effect between Cd and Si in rice might be the key factors regulating Cd accumulation in rice via the co-application of LS and Si fertilizer.

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

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

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

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

Evaluation of the effectiveness of ex-situ stabilization for arsenic and antimony contaminated soil: Short-term and long-term leaching characteristics

Ex-situ stabilization for As and Sb co-contaminated soil was conducted through an iron-based stabilizer, PFSC (a mixture of polymerized ferric sulfate (PFS) and hydrated lime (Ca(OH₂)) with a dry mass ratio of 2:1). After field aging for one week, the stabilized contaminated soil was subjected to a horizontal vibration leaching test (HJ 557), Wenzel's sequential extraction, and a semi-dynamic leaching test (ANS 16.1). By assessing the cumulative fractions of As and Sb, the observed diffusion coefficients (D^obs) and leachability indices (LX) of metalloids released from the soil specimens were calculated. The PFSC ex-situ stabilization was effective to immobilize metalloids, and the As and Sb leached concentrations of stabilized contaminated soil samples were lower than remediation targets. Nonspecifically bound As and Sb in the stabilized contaminated soil samples decreased from 4.5 - 9.2 % to 1.5-2.5 % and from 2.2 - 5.8 % to 1.1-1.5 %, respectively. The mechanisms controlling the leaching behaviors of As and Sb included wash-off and diffusion and they were changed with the leaching interval. The mean D^obs of As and Sb released from stabilized contaminated soil specimen were 3.46 × 10^-12 and 2.99 × 10^-13 cm² s^-1, in the which were two orders of magnitude lower than that of untreated contaminated soil specimen. The mean LX of stabilized contaminated soil specimen for As and Sb releases were 11.40 and 12.83, respectively, indicating that the stabilized contaminated soil was acceptable for "controlled utilization".

Electrochemical response of solidification Cu2+ contaminated soil influenced by red mud/fly ash ratio

The main purpose of this work was to study a new method for evaluating the solidification of contaminated soil based on electrochemical impedance spectroscopy (EIS). To explore how the EIS parameters were affected by the pore structure and mesostructure of the cured system, the physical and mechanical properties, leaching toxicity, microstructure, and EIS of the stabilized contaminated soil were tested after 7, 28, 60, and 90 days of curing. Based on the EIS results, a physical and equivalent circuit model of the stabilized contaminated soil's impedance response was established to reveal the mechanism of binder-heavy metal ion-soil interaction. The results showed that as the red mud (RM)-fly ash (FA) mass ratio and curing age increased, the strength and structural compactness of the solidified body also increased. The best curing effect was achieved with an RM-FA mass ratio of 7:3 after curing for 90 days. The equivalent circuit model of the solidified body obtained by EIS was R_s (Q₁ (R_ct1W) Q₂R_ct2). The pore solution resistance R_s, solid-liquid interface ion transfer resistance Rct 1, and unconfined compressive strength (UCS) q_u all showed an increasing trend with increasing RM-FA mass ratio and increasing curing time. Fitting the model demonstrated that both R_s and R_ct1 were closely correlated with the strength of the solidified bodies. These conclusions were further verified by scanning electron microscope (SEM) experiments. Overall, this work demonstrates that the strength characteristics of solidified bodies can be evaluated by EIS and reveals the microscopic mechanism of the solidification of Cu²⁺-contaminated soil.

Study on the migration mechanisms of water-soluble agents in high-pressure rotary jetting remediation

High-pressure rotary jetting (HPRJ) remediation is a recent-applied technology for in situ remediation of contaminated soils. The effectiveness of remediation depends upon the migration and distribution of the injected agents in the soil. However, the corresponding migration mechanisms have received little attention. In this study, laboratory HPRJ tests and numerical simulations were performed using chlorine (Cl^-) as a tracer to investigate the transport during HPRJ and the subsequent advection and diffusion. The test results showed that the HPRJ transported Cl^- into the mixing zone by eroding the sand, and the radius of the mixing zone could be reasonably predicted by the erosion model. The Cl^- concentration decreased linearly along the radial direction in the mixing zone. In addition, the Cl^- transport distance increased with the increase in nozzle diameter, jetting times, especially injection pressure, and decreased with an increasing rotation speed. The Cl^- concentration and radial uniformity were correlated positively with rotation speed, particularly nozzle diameter and jetting times. Numerical simulation showed that part of Cl^- migrated from the mixing zone to diffusion zone by advection-diffusion after rotary jetting, which contributed positively to the agent distribution distance and uniformity. The Cl^- migration was dominated by advection in the initial stage (30 days), while diffusion became more important thereafter.

Recent advances in soil remediation technology for heavy metal contaminated sites: A critical review

With the increasing development of industry and urbanization, heavy metal contaminated sites have become progressively conspicuous, particularly by unreasonable emissions from electroplating, nonferrous metals smelting, mine tailing, etc. In recent years, soil remediation technologies for heavy metal contaminated sites have developed rapidly. New and effective remediation technologies have emerged successively, and more successful practical applications have appeared. Therefore, systematical summarization of the current progress is essential. As a result, in this paper, some mainstream soil remediation technologies for heavy metal contaminated sites, including physical remediation (soil thermal desorption and soil replacement), bioremediation (phytoremediation and microbial remediation), chemical remediation (chemical leaching, chemical stabilization, electrokinetic remediation-permeable reactive barrier, and chemical oxidation/reduction), as well as various combined remediation are comprehensively reviewed. The influencing factors, advantages, disadvantages, remediation mechanism, and practical applications are also deeply discussed. Besides, the corresponding remediation strategies are put forward for the remediation of heavily polluted sites such as the chemical industry, smelting, and tailing areas. Overall, this review will be beneficial for the in-depth understanding and provide references for the reasonable selection and development of soil remediation technology for heavy metal contaminated sites.

Natural field freeze-thaw process leads to different performances of soil amendments towards Cd immobilization and enrichment

Cadmium (Cd) soil pollution is a global issue affecting crop production and food safety. Remediation methods involving in-situ Cd immobilization have been developed, but their effectiveness can diminish under seasonal freeze-thaw aging processes. In this study, we assessed the field performance of four soil treatments at a seasonally frozen rice paddy. Amendments were applied at 2 wt%, including: (i) sepiolite (a 2:1 clay mineral), (ii) superphosphate, (iii) biochar (produced by rice husk at 500 °C for 2 h), and (iv) joint application of biochar & superphosphate (1:1 mixture by weight). Immobilization performance was determined as DTPA extractable Cd and plant uptake in various organs. Overall, the four treatments significantly reduced Cd bioavailability during the plant growth period, with average DTPA-extractable concentrations decreasing by 43%, 34%, 39% and 45% for the four treatments, respectively, relative to untreated soil (control). Rice grain yields from the superphosphate and the joint application treatments increased by 8.0% and 11.8%, respectively, and Cd accumulation within those grains reduced by 14.3% and 48.9%, respectively. During the winter non-growth period, freeze-thaw aging facilitated Cd mobilization, with DTPA-extractable Cd increasing by 16.9% in the control soil, relative to the initial period. However, this reduced to 10.9%, 14.4%, 7.6% and 5.0%, for the sepiolite, superphosphate, biochar and joint application treatments, respectively. Overall, the joint application of biochar and superphosphate provided the best performance in terms of both long-term Cd immobilization and rice production enhancement, offering a green remediation option for risk management at Cd contaminated rice paddies in seasonally frozen regions.

The application of machine learning methods for prediction of metal immobilization remediation by biochar amendment in soil

Biochar has been used widely in heavy metal contaminated sites as a soil remediation agent. However, due to the diversity of soils, biochars, and heavy metal contamination status, the remediation efficiency is difficult to measure, owing to a variety of parameters such as soil, biochar properties, and remediation procedure. Thus, an appropriate method to predict the remediation results and to select the appropriate biochar for the remediation is required. We initially created a database on soil remediation by biochars, which has 930 datasets with 74 biochars and 43 soils in it, based on collecting and organizing data from published literatures. Then, using data from the database, we modeled the remediation of five heavy metals and metalloids (lead, cadmium, arsenic, copper, and zinc) by biochars using machine learning (ML) methods such as artificial neural network (ANN) and random forest (RF) to predict remediation efficiency based on biochar characteristics, soil physiochemical properties, incubation conditions (e.g., water holding capacity and remediation time), and the initial state of heavy metal. The ANN and RF models outperform the lineal model in terms of accuracy and predictive performance (R² > 0.84). Meanwhile, model tolerance of the missing data and reliability of the interpolation were studied by the predicted outputs of the models. The results showed that both ANN and RF have excellent performances, with the RF model having a higher tolerance for missing data. Finally, through the interpretability of ML models, the contribution of factors used in the model were analyzed and the findings revealed that the most influential elements of remediation were the type of heavy metals, the pH value of biochar, and the dosage and remediation time. The relative importance of variables could provide the right direction for better remediation of heavy metals in soil.

Pollution characteristics and vertical cutoff wall optimization at an industrial contaminated site in China

Vertical cutoff walls have been widely used in the remediation of contaminated sites. However, determining the best method for evaluating the long-term barrier performance of vertical cutoff walls presents a major difficulty in actual projects. Here, a case study is presented for a typical electroplating, medical, and chemical industrial park in China. Based on the analysis of groundwater pollution characteristics at the site, pollutants included metals (Ni, Al), ammonia nitrogen, and 1,2-dichloroethane. Finite element model simulations of Ni transport at the site showed that a vertical cutoff wall with a thickness of 60 cm and a hydraulic conductivity of 1.0 × 10^-8 cm/s could significantly attenuate pollutant transport in the horizontal direction. Compared with other methods such as reducing the hydraulic conductivity or increasing the adsorption retardation factor of the vertical cutoff wall, increasing the thickness was more effective in controlling pollutant transport at the study site. Doubling the thickness would cause the Ni leakage concentration to decrease by more than 98% and the breakthrough time to increase by more than 47 years. It is recommended that the thickness of cutoff walls be maximized to optimize their effects on pollutant transport.

Propositure of maximum permissible limits for environmental assessment and classification of materials stabilized by solidification incorporated with oil waste

In the present work, oil sludge was subjected to the process of stabilization by solidification, the objective was to indicate maximum permissible limits of chemical demand for oxygen and oils and greases to evaluate the resulting material. A factorial design was used with the addition of three repetitions at the central point, to evaluate the performance of different percentage of residue and different curing times. The factors adopted were the percentage of oil sludge and the curing time. This material was evaluated using contaminants immobilization tests. From the leached and solubilized liquid, the concentration of the contaminants was determined and the environmental assessment was also carried out through the analysis of chemical demand for oxygen and oils and greases. One of the contributions of this work was to propose the maximum allowable limit for chemical oxygen demand, which is 1,000 (one thousand) milligrams per litre and, for oil and grease content, 100 (one hundred) milligrams per litre, both for the leaching test. For the solubilization test, 10 (ten) milligrams per litre were proposed for chemical oxygen demand and 1 (one) milligrams per litre for the oil and grease content. It was observed that the best results are obtained, when lower values of percentage of oil sludge were used and longer cure time. This work allows to affirm that the proposal of limits of environmental evaluation contributes to assure an adequate disposition and use of the cement matrix, that is the final product of the oily residue stabilized by solidification.

Comparison of limestone calcined clay cement and ordinary Portland cement for stabilization/solidification of Pb-Zn smelter residue

Decreasing carbon emissions by replacing Portland cement (PC) with supplementary cementitious materials (SCMs), such as low-grade limestone (LS) and calcined clays (CC), has tremendous potential for stabilization/solidification (S/S) of industrial hazardous waste primarily with heavy metals. Recently, a low-carbon-based cementitious binder, namely, limestone calcined clay cement (LC), has emerged as an alternative for S/S treatment of wastes. However, comprehensive comparison between LC and PC application in solidifying/stabilizing wastes has not been conducted. This study aims to investigate the S/S efficiency of Pb-Zn smelter residue (LZSR) comprising heavy metals lead (Pb), zinc (Zn), and cadmium (Cd) at higher concentrations. LZSR is treated with LC and PC for capturing strength and leaching toxicity. The test results indicate that low-grade CC and LS in the LC binder can promote the alkaline environment, and act as fillers in solidifying heavy metals. The toxicity characteristic leaching procedure leaching concentrations of untreated (UT) LZSR were 503 mg/kg, 1266 mg/kg, and 251 mg/kg for Pb, Zn, and Cd, respectively. After a 28-day curing, the leaching concentrations in LC-treated LZSR reduced to 4.33 mg/kg, 189.68 mg/kg, and 0.46 mg/kg, while the leaching concentrations of PC-treated LZSR reduced to 29 mg/kg, 338 mg/kg, and 6 mg/kg for Pb, Zn, and Cd, respectively. The maximum immobilization efficiencies for Pb, Zn, and Cd reached 85%, 99%, and 99%, respectively. Moreover, the insoluble phases for Pb, Zn, and Cd obtained from the sequential extraction test results were 63.5%, 72.1%, and 42.4% for LC-treated LZSR and 35.7%, 38%, and 43% for PC-treated LZSR with binder content of 8% binder and curing time of 28 days. Increasing curing time and binder content reduced leaching concentrations, and the underneath mechanisms were interpreted by XRD, SEM-EDS, and FTIR analyses. Overall, the results indicate that Pb, Zn, and Cd can be successfully immobilized using 8% LC binder by transforming soluble heavy metals to insoluble hydroxides and their complexes.

Preparation of Fe2O3-coated vermiculite composite by hydrophobic agglomeration and its application in As/Cd co-contaminated soil

Exploring an economic and efficient method for simultaneous passivation of As and Cd in soils is of great current significance. In this study, a low-cost composite material, Fe₂O₃-vermiculite (Fe-V), for effectively passivating As/Cd was synthesized successfully based on hydrophobic aggregation method. The reaction products were characterized by XRD, SEM, EDS and FTIR, results showed that Fe₂O₃ was successfully loaded onto the surface of vermiculite by the connection with sodium stearate and employed this composite material to passivate the co-contaminated soil with As/Cd. All the percentage of toxicity reduction (P_d value) was higher than the control group, which indicated the passivation was effective. In soil A (As 45 mg/kg and Cd 6 mg/kg), the P_d of As were higher than 90%, the P_d of Cd were 80-100%. And in soil B (As 80 mg/kg and Cd 10 mg/kg), the P_d of As were more than 84.68%, the P_ds of Cd were about 99%. In the meantime, the application of Fe-V could apparently increase the residual fraction of As and Cd in soil A and soil B. Moreover, the passivation of As and Cd in soils by Fe-V composite materials was a combined physical and chemical action system. This research shows that Fe-V could play a good role in the passivation of As/Cd in different pollution levels of soils.

Hydraulic conductivity of soil-bentonite backfill comprised of SHMP-amended Ca-bentonite to Cr(VI)-impacted groundwater

Hexavalent chromium (Cr(VI)) in groundwater impose serious health problems for human society. This study investigates the potential of using calcium (Ca) bentonite amended with sodium hexametaphosphate (SHMP) as a backfill constituent material in the soil-bentonite slurry trench wall to envelop the Cr(VI) impacted groundwater. The hydraulic conductivity (K) and consolidation of backfill comprising of 80 wt% sand and 20 wt% SHMP-amended Ca-bentonite were determined via flexible-wall permeameter tests and oedometer tests, respectively. Microstructure characterizations of the amended bentonites before and after contamination were also explored. The results indicated that when the permeated liquid changed from tap water to Cr(VI) solution, the tested specimens exhibited a 1.0 to 1.2-fold variation in short-term K, with all K values fall in range of 2.1 × 10^-10 to 2.5 × 10^-10 m/s. This mild variation may be attributed to terminate the tests without achieving chemical equilibrium. On the other hand, the Cr(VI) solution had insignificant effect on consolidation of the amended backfill, which is attributed to the dominated incompressible sand matrix skeleton in the backfill that withstood the consolidation pressure and shield the negative effects of the contaminated solution. The microstructure images revealed that the Cr(VI) resulted in relatively strong interlink between particles. Overall, the SHMP-amended bentonite is promising for enhancing Cr(VI) containment performance of the soil-bentonite slurry trench wall backfills.

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

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

Stabilization of lead in polluted sediment based on an eco-friendly amendment strategy: Microenvironment response mechanism

Stabilization is the most important remediation mechanisms for sediment polluted heavy metals. However, little research has been done on the identification of microenvironmental response and internal correlation, as well as synergistic mechanisms during heavy metal remediation. This study aims to investigate the inner response mechanisms of microenvironment after the lead (Pb) are gradually stabilized in sediment. An eco-friendly amendment strategy which firstly used 100% biodegradable sophorolipids (SOP) to modify chlorapatite (ClAP) for the fabrication of SOP@nClAP was applied in this study. The stabilization efficiency of Pb was significantly improved by SOP@nClAP compared with ClAP. Most importantly, the high-throughput sequencing showed that the dominant species in the sediment changed with the stabilization of Pb. The decrease of Proteobacteria and increase of Firmicutes, especially the Sedimentibacter within the phylum Firmicute directly suggested that large amounts of Pb were stabilized. This research is not only devoted to stabilize Pb in sediment by eco-friendly amendment strategy, but also keep a watchful eye on microenvironment response mechanisms during the Pb stabilization in sediment. Therefore, this study lays a foundation for the future application of more heavy metal amendment strategies in the sediment environment and improves the possibility of large-scale site amendment.

Changes in the Structures and Directions of Heavy Metal-Contaminated Soil Remediation Research from 1999 to 2020: A Bibliometric & Scientometric Study

The pollution of heavy metals in soil is a problem of great concern to international scholars today. This research investigates the current research activities in the field of soil heavy metal pollution remediation and discusses the current areas of research focus and development trends. We conducted a bibliometric analysis of the literature on soil heavy metal pollution remediation from 1999 to 2020. CiteSpace and Vosviewer were used to conduct document co-citation and cluster analyses on the collected data. The research was mainly carried out based on the following factors: chronological distribution, country and institution distribution, source journal analysis, keyword co-occurrence analysis, and reference co-citation analysis. China (2173, 28.64%) and the United States (946, 12.47%) are the top two countries in terms of the number of articles published, and Environmental Science and Pollution Research (384, 5.06%) and Science of the Total Environment (345, 4.55%) published the most articles. The Chinese Academy of Science (485) is the organization that has contributed the most to the total number of publications. Furthermore, based on a keyword co-word analysis with Vosviewer and CitesSpace, it was concluded that the applications of phytoremediation and biochar in the remediation of heavy metals in soil are current research hotspots. Additionally, future research should focus on repair mechanisms, the development of new repair technology and joint repair systems.

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

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

Geoenvironmental properties of industrially contaminated site soil solidified/stabilized with a sustainable by-product-based binder

This paper presents a study on utilizing a novel BCP binder, basic oxygen furnace slag (BOFS) activated with mixed calcium carbide residue (CCR) and phosphogypsum (PG), to solidify/stabilize heavy metals in industrial contaminated site soil. The effects of curing time and binder dosage on the geoenvironmental properties of the solidified/stabilized soil including soil pH, electrical conductivity, unconfined compressive strength, and leachability were tested and discussed. Chemical speciation of target heavy metals, pore-size distribution of treated soil, and phase identification of reaction products were analyzed to understand the mechanisms leading to the change of geoenvironmental properties. The results demonstrated that the addition of the BCP binder yielded remarkable increase in soil pH, unconfined compressive strength, and relative binding intensity index (I_R) of target heavy metals including nickel (Ni) and zinc (Zn), while significantly decreased the electrical conductivity and leachability of contaminated soil. The I_R value of heavy metals had a good linear relationship with the leached concentrations on a semi-logarithmic scale. The formation of heavy metal-bearing precipitates, absorptivity of calcium silicate hydrate (C-S-H), heavy metals encapsulation by C-S-H, and ion-exchange of heavy metals with ettringite (AFt) contributed to the immobilization of heavy metals in the solidified/stabilized soil.

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

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

A review on the thermal treatment of heavy metal hyperaccumulator: Fates of heavy metals and generation of products

Phytoremediation is perceived as a promising technique for remediation of heavy metal (HM) contaminated soils, while the harvested HM-enriched hyperaccumulator biomass should be appropriately disposed. Recently, various thermal treatments of hyperaccumulator have drawn increasing attention. After thermal treatment, the hyperaccumulator was converted to bio-oil, bio-gas, biochar, or ash in accordance with the corresponding conditions, and the HMs were separated, immobilized, or trapped. The migration and transformation of HMs during the thermochemical conversion processes are critical for the safe disposal and further utilization of HM hyperaccumulator. This paper provides a systematic review on the migration and transformation of typical HMs (Cd, Ni, Mn, As, and Zn) in hyperaccumulator during various thermochemical conversion processes, and special emphasis is given to the production and application of targeted products (e.g. biochar, hydrochar, bio-oil, and syngas). Besides, future challenges and perspectives in the thermal treatment of hyperaccumulator are presented as well. The distribution and speciation of HMs were influenced by thermal technique type and reaction conditions, thereby affecting the utilization of the derived products. This review suggests that speciation and availability of HMs in hyperaccumulator are tunable by varying treatment techniques and reaction conditions. This information should be useful for the selective conversion of hyperaccumulator into green and valuable products.

Oyster Shell Powder, Zeolite and Red Mud as Binders for Immobilising Toxic Metals in Fine Granular Contaminated Soils (from Industrial Zones in South Korea)

Low-cost absorbent materials have elicited the attention of researchers as binders for the stabilisation/solidification technique. As, there is a no comprehensive study, the authors of this paper investigated the performance of Oyster shell powder (OS), zeolite (Z), and red mud (RM) in stabilising heavy metals in three types of heavy metal-contaminated soils by using toxicity characteristic leaching procedure (TCLP). Samples were collected from surroundings of an abandoned metal mine site and from military service zone. Furthermore, a Pb-contaminated soil was artificially prepared to evaluate each binder (100× regulatory level for Pb). OS bound approximately 82% of Pb and 78% of Cu in real cases scenario. While Z was highly effective in stabilizing Pb in highly polluted artificial soil (>50% of Pb) at lower dosages than OS and RM, it was not effective in stabilising those metals in the soils obtained from the contaminated sites. RM did not perform consistently stabilising toxic metals in soils from contaminated sites, but it demonstrated a remarkable Pb-immobilisation under dosages over than 5% in the artificial soil. Further, authors observed that OS removal efficiency reached up to 94% after 10 days. The results suggest that OS is the best low-cost adsorbent material to stabilize soils contaminated with toxic metals considered in the study.

Enhancement of cement-based solidification/stabilization of a lead-contaminated smectite clay

The cement-based solidification/stabilization is commonly used to remediate heavy-metal-contaminated clayey soils. The major problem associated with this method is heavy-metal precipitation, which retards cement hydration. The objectives of this paper are to study the influence of pH-dependent lead solubility patterns on the solidification/stabilization of contaminated smectite and to overcome the problems associated with cement hydration in this process through NaOH treatment. A series of physicochemical experiments were performed on untreated and NaOH-enhanced samples. Contaminated smectite with 5-100 cmol/kg-soil of lead nitrate was solidified/stabilized by 10-50% cement. This research demonstrates that solidification/stabilization is a pH-dependent phenomenon. Enhancement increases the pH of contaminated soil in which lead components transfer to a soluble form. Hereafter, as the results of XRD reveal, a decrease in lead precipitation on cement components is observed. Consequently, a noticeable increase in CSH formation is detected. The capsulation of lead ions by CSH improves the setting-time and unconfined compressive strength of solidified/stabilized samples. Furthermore, the TCLP results show a significant reduction in samples' lead-leaching abilities. Therefore, enhancement has changed the governing retention phenomena from precipitation/stabilization in lead carbonate form to mainly capsulation/solidification by CSH. Moreover, the results show a noticeable reduction in the required cement content.

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

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

Approach based on TOPSIS and Monte Carlo simulation methods to evaluate lake eutrophication levels

This study presents an approach for eutrophication evaluation based on the technique for order preference by similarity to an ideal solution (TOPSIS) method and Monte Carlo simulation (MCS). The MCS is employed to produce a normally distributed dataset based on the observed data while the TOPSIS method and membership function are used to evaluate the level of eutrophication. Herein, a eutrophication problem in Lake Erhai is evaluated to check the performance of the proposed approach. The evaluation results were consistent with the real situation when the coefficient P in the membership function is equal to 1. Moreover, the developed approach is able to (i) deal with evaluation items with inherent fuzziness and uncertainties, (ii) improve the reliability of evaluation results via MCS, and (iii) raise the tolerance to errors in measured data. A global sensitivity analysis indicated that the potassium permanganate index (COD_Mn) and Secchi disc (SD) are the most sensitive factors in the developed approach. Finally, a range for the coefficient P value in the membership function was recommended.

Phosphate: Coupling the functions of fertilization and passivation in phytoremediation of manganese-contaminated soil by Polygonum pubescens blume

Phosphate (P) fertilization is a commonly used agronomic practice. However, research on bioremediation is very limited. This study's principal objective was to evaluate the role of P in the growth and heavy metals (HMs) accumulation of Polygonum pubescens Blume cultured in Mn-contaminated soil. To this end, the effects of sodium dihydrogen phosphate (SDP) and single superphosphate (SSP) on the growth, Mn bioremediation efficiency, organ HMs, and physiological parameters related to antioxidant stress of P. pubescens were examined. The results showed that both SDP and SSP increased soil pH and available P but decreased available HMs. Phosphate significantly (P < 0.05) promoted P. pubescens height and biomass. Average height increased by 36.1% and 32.6% with SDP and SSP, respectively, with corresponding biomass increases of 71.8% and 135%. Phosphate significantly (P < 0.05) reduced Mn concentrations, especially in leaves, where the values decreased by >50.0% for DSP and SSP. Total Mn significantly (P < 0.05) decreased with DSP amendment but significantly (P < 0.05) increased by 38.5% with SSP (200 mg kg^-1) through an increase in biomass. Phosphate significantly (P < 0.05) decreased all organ HM concentrations and translocation, indicating that less HM stress occurred with P amendment. The changes in reactive oxygen species, antioxidants and non-antioxidant materials further supported these results. Pearson correlation analysis revealed negative relationships between soil available P and HMs, indicating a novel role of P in HM passivation. The uncommonly high Ca concentrations in leaves suggested that Ca plays a vital role in promoting growth and alleviating HM stress in P. pubescens, which warrants further study.

Effects of organic matter components and incubation on the cement-based stabilization/solidification characteristics of lead-contaminated soil

Stabilization/solidification (S/S) has been studied since 1950s and widely used for the treatment of potentially toxic elements (PTEs). The coexistence of organic matter (OM) and PTEs can cause a very complicated mechanism for cement-based S/S applications and bring challenges from both scientific and engineering perspectives. To fill in the knowledge gap, this paper investigates for the first time the effects on S/S characteristics of OM components and incubation, which are the two main factors that result in the inconsistency in the leaching characteristics from the available studies. OM samples with different components (humic acid (HA) and fulvic acid (FA)) and contents were mixed into lead-contaminated soil and incubated for different durations of up to 90 days. The experimental results show that the strength of stabilized soils increases with increasing incubation duration and the lead leaching concentration of stabilized soils is decreased by 60.7%-83.6% from zero to 90 days. The lead leaching concentration of the HA group, which is 144.0% higher with no incubation than the non-OM group, becomes 58.3% lower with 90 days of incubation. The leaching concentration of the FA group remains much higher than those of the other groups. Finally, a competing mechanism of HA-weakening cement hydration reactions and stabilizing lead with a critical incubation duration of 14 days-is proposed, together with a cooperating mechanism of FA-weakening cement hydration reactions and releasing lead.

A low cost of phosphate-based binder for Mn2+ and NH4+-N simultaneous stabilization in electrolytic manganese residue

Electrolytic manganese residue (EMR) is a solid waste remained in filters after using sulfuric acid to leaching manganese carbonate ore. EMR contains high concentration of soluble manganese (Mn²⁺) and ammonia nitrogen (NH₄⁺-N), which seriously pollutes the environment. In this study, a low cost of phosphate based binder for Mn²⁺ and NH₄⁺-N stabilization in EMR by low grade-MgO (LG-MgO) and superphosphate was studied. The effects of different types of stabilizing agent on the concentrations of NH₄⁺-N and Mn²⁺, the pH of the EMR leaching solution, stabilizing mechanisms of NH₄⁺-N and Mn²⁺, leaching test and economic analysis were investigated. The results shown that the pH of the EMR leaching solution was 8.07, and the concentration of Mn²⁺ was 1.58 mg/L, both of which met the integrated wastewater discharge standard (GB8978-1996), as well as the concentration of NH₄⁺-N decreased from 523.46 mg/L to 32 mg/L, when 4.5 wt.% LG-MgO and 8 wt.% superphosphate dosage were simultaneously used for the stabilization of EMR for 50 d Mn²⁺ and NH₄⁺-N were mainly stabilized by Mn₃(PO₄)₂·2H₂O, MnOOH, Mn₃O₄, Mn(H₂PO₄)₂·2H₂O and NH₄MgPO₄·6H₂O. Economic evaluation revealed that the treatment cost of EMR was $ 11.89/t. This study provides a low-cost materials for NH₄⁺-N and Mn²⁺ stabilization in EMR.

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.

Effects of different water management strategies on the stability of cadmium and copper immobilization by biochar in rice-wheat rotation system

Chemical immobilization of heavy metals is a simple, low-cost, and environment-friendly technology for remediation of heavy metals contaminated soils. However, changes in environmental conditions, such as water management, acid deposition, temperature fluctuation, etc., might result in release of metal ions from the fixation sites, and the long-term stability of immobilization remediation is unclear. This study attempted to investigate the impact of water management strategies (wetting-drying cycle and dry cycle) on the stability of heavy metal immobilization by one-time application of biochar during 3 consecutive years of rice-wheat crop in Cu/Cd-contaminated soil. The transformation and accumulation of Cd and Cu in soil-crop system and the morphololgy and composition of biochar were analyzed. The results revealed that wetting-drying cycle and drying treatments reduced the contents of available Cd and Cu in soil by 15.9%-17.7% and 23.9%-31.5% and by 19.8%-62.7% and 16.1%-65.0%, as well as increased soil pH by 0.11-0.31 and 0.17-0.56, respectively. In the wetting-drying cycle treatment, biochar was more favorable for decrease in Cd and Cu accumulation in crop, when compared with that in dry treatment; however, the differences were insignificant in the subsequent years. Although the different water management strategies had no obvious effect on the soil total C, physicochemical analysis of the biochar collected after pot experiments indicated that the obvious structural decomposition of biochar in the drying treatment may have resulted in the release of heavy metals immobilized in biochar. These findings help in better understanding of the long-term immobilization mechanism of biochar in soil-plant system.

Evaluation of red mud based binder for the immobilization of copper, lead and zinc

In this study, by products such as red mud, phosphorus gypsum and fly ash were used as binders, and are compared with Portland cement (PC) in immobilizing Cu, Pb and Zn. Cu, Pb and Zn -doped pastes and mortars were prepared with a metal to binder ratio at 1%. Samples were cured for 7d, 14d and 28d. The unconfined compressive strength (UCS) test, a batch leaching test along with scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were applied for the testing of Cu, Pb and Zn-doped pastes and mortars. The UCS results show that red mud-phosphorus gypsum treated samples produce higher strength than these treated by red mud-fly ash, or PC. The results of leaching test revealed that the immobilization degree of heavy metals from these pastes depends on the leachate pH of these pastes. With the aid of the cement, red mud-phosphorus gypsum-cement pastes leached less metals compared to that of red mud-phosphorus gypsum pastes. The leachate concentrations of Cu, Pb and Zn from red mud-phosphorus gypsum-cement pastes are 1.5 mg/L, 1 mg/L, and 3 mg/L respectively. They are able to meet the China Ministry of Environment Protection (MEP) regulatory limit. With the increase of the curing time, the unconfined compressive strength and the leaching concentrations of these pastes showed a slightly increasing trend. In addition, SEM and TGA analyses show that the major hydration product is ettringite.