The evaluation of in-site remediation feasibility of Cd-contaminated soils with the addition of typical silicate wastes

Hybrid and enhanced electrokinetic system for soil remediation from heavy metals and organic matter

The electrokinetic (EK) process has been proposed for soil decontamination from heavy metals and organic matter. The advantages of the EK process include the low operating energy, suitability for fine-grained soil decontamination, and no need for excavation. During the last three decades, enhanced and hybrid EK systems were developed and tested for improving the efficiency of contaminants removal from soils. Chemically enhanced-EK processes exhibited excellent efficiency in removing contaminants by controlling the soil pH or the chemical reaction of contaminants. EK hybrid systems were tested to overcome environmental hurdles or technical drawbacks of decontamination technologies. Hybridization of the EK process with phytoremediation, bioremediation, or reactive filter media (RFM) improved the remediation process performance by capturing contaminants or facilitating biological agents' movement in the soil. Also, EK process coupling with solar energy was proposed to treat off-grid contaminated soils or reduce the EK energy requirements. This study reviews recent advancements in the enhancement and hybrid EK systems for soil remediation and the type of contaminants targeted by the process. The study also covered the impact of operating parameters, imperfect pollution separation, and differences in the physicochemical characteristics and microstructure of soil/sediment on the EK performance. Finally, a comparison between various remediation processes was presented to highlight the pros and cons of these technologies.

Endogenous silicon-activated rice husk biochar prepared for the remediation of cadmium-contaminated soils: Performance and mechanism

Biochar is widely used for the remediation of heavy metal-contaminated soils. However, pristine biochar generally has limited active functional groups and adsorption sites, thereby exhibiting low immobilization performance for heavy metals. In addition to carbon (C), silicon (Si) is another common macro-element present in rice husk biochar, but it often exists in the form of amorphous oxide and therefore contributes little to the adsorption performance for heavy metals. The transformation of amorphous Si oxide to dissolved silicate through a precipitation effect can significantly improve its heavy metal immobilization capability. Herein, the amorphous Si oxide in rice husk biochar was activated by sodium hydroxide and then the dissolved silicate was immobilized by calcium salt. The as-synthetized Si-activated biochar was used to remediate cadmium (Cd)-contaminated soils. The results indicated that Si-activated rice husk biochar could reduce Cd migration and environmental risks by the transformation from exchangeable Cd into carbonate-bound and residual Cd. With increasing Ca: Si molar ratio, the content of CaCl2 and H2O-extractable Cd exhibited a decreasing trend. Moreover, a higher addition amount of Si-activated biochar improved the Cd immobilization efficiency. The application of 1.0% Ca/Si molar ratio of 2: 2 Si-activated rice husk biochar decreased the CaCl2-Cd and H2O-Cd concentration by a maximum of 83.7% and 90.5% compared with pristine rice husk biochar, respectively. The present work proposes an approach for highly efficient remediation of Cd-polluted soils by biochar.

Development of a Multifaceted Perspective for Systematic Analysis, Assessment, and Performance for Environmental Standards of Contaminated Sites

Contaminated soil and groundwater can pose significant risks to human health and ecological environments, making the remediation of contaminated sites a pressing and sustained challenge. It is significant to identify key performance indicators and advance environmental management standards of contaminated sites. The traditional study currently focuses on the inflexible collection of related files and displays configurable limitations regarding integrated assessment and in-depth analysis of published standards. In addition, there is a relative lack of research focusing on the analysis of different types of standard documents. Herein, we introduce a cross-systematic retrospective and review for the development of standards of the contaminated sites, including the comprehensive framework, multifaceted analysis, and improved suggestion of soil and groundwater standards related to the environment. The classification and structural characteristics of different types of files are systematically analyzed of over 300 national, trade, local, and group standards for the contaminated sites. It exhibits that trade standards are the main types and testing methods are the important format within numerical considerations of soil standards. The guide standard serves as a crucial component in environmental management for investigating, assessing, and remediating of contaminated sites. Future improvement plans and development directions are proposed for advancing robust technical support for effective soil contamination prevention and control. This multidimensional analysis and the accompanying suggestions can provide improved guidance for Chinese environmental management of contaminated sites and sparkle the application of standards in a wide range of countries.

Mixed solvent fabrication of tobermorite and the fixation of heavy metals in water and soil

Here, tobermorite was prepared by a solvothermal technology using calcite and quartz with a mixed solvent of ethanol and water. Factors including reaction temperature, time and KOH content were studied to optimize the preparation procedure. To study the relationship between ethanol content-material structural characteristics-adsorption capacity, a series of materials were prepared in different mixed solvent proportions of ethanol and water, and their structural characteristics and adsorption capacity were compared. We found that the adsorption capacity of different samples for Pb2+ and Cd2+ was positively correlated with negatively correlated with the surface area and negatively correlated with the crystallinity of materials. Then, the material prepared by 30% ethanol solution (30-T) with the best adsorption performance was used for further research; the results were fitted by kinetic and thermodynamic models, and adsorbed materials were analyzed by various characterizations, suggesting that the adsorption process was ascribed to comprehensive pathways including ion exchange, chemical precipitation, and surface-complexation. Then, the 30-T was further used to remediate heavy metals contaminated soil, and the remediation effect was examined by the DTPA-extractable method and the European Community Bureau of Reference (BCR) sequential extraction method. The DTPA-extractable results showed that tobermorite observably reduced the bioavailability of Pb and Cd, and the BCR results suggested that the acid-soluble and reducible fractions of Pb and Cd were transformed to the oxidizable and residual fractions after remediation. In summary, tobermorite has great potential in the remediation of heavy metal polluted-aquatic environment/system and soil.

Effects of blast furnace slag on the immobilization, plant uptake and translocation of Cd in a contaminated paddy soil

The potential toxicity of Cd to soil and rice plant severely threaten human health. This study was conducted to investigate the remediation effects of blast furnace slag (BFS) on the bioavailability of Cd in a contaminated paddy soil from a perspective of soil solution chemistry. Batch experiments, pot culture experiments, and principal analysis (PCA) were used to study the effects and mechanisms of BFS addition changing Cd chemical behavior and Cd toxicity. Results indicated that BFS facilitated Cd adsorption in soils, increased pH, Eh, and EC values in soil solution, whereas reduced dissolved Cd content. BFS amendment was efficient in decreasing root Cd intake and Cd upward transport in rice plant, with the Cd translocation factor in brown rice decreased by ∼ 75% (BFS treatment, 6‰ wt) relative to Cd treatment, which in turn increased rice biomass and grain yield. PCA indicated that the dissolved Cd concentration had a close relationship with soil pH and metal concentration in soil solution. Results from this study indicated that BFS had potential ability for either immobilization or remobilization of Cd in soils, and the findings have important implications for Cd-polluted soil remediation or other resource utilization with slag-based materials.

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

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

Roasted modified lead-zinc tailings using alkali as activator and its mitigation of Cd contaminated: Characteristics and mechanisms

To comprehensively reuse lead-zinc tailings, leaching residue (LR) of solid by-products was produced after the recovery of valuable metals. This study provided a "waste-ecology" strategy by a simple, inexpensive method of roasting prepared highly active silicon modified tailing (HAST) to eliminate the environment risk of LR, and investigates performance and mechanism of HAST as sorbents and passivators. The results indicated that HAST possesses high pH, abundant mineral content, microporous structure and high stability. The adsorption kinetic experiment revealed that chemisorption is the main reaction and the Q_m of Cd via Langmuir model is 72.75 mg/g. As further demonstrated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis, the Cd was adsorbed onto the HAST surface successfully, with the main interaction mechanisms involving ion exchange, complexation, precipitation and electrostatic interaction. Besides, the soil incubation experiment results showed that HAST had positive effects on exchange fractions (Cd) converting to stable fractions in soil, which modifies Cd migration and transformation, HAST added into soil decreased the DTPA-Cd by 4.7%-8.1%, 5.9-9.8% and 9.1%-13.4%, respectively, in different stages, as compared with the control. Therefore, this study provides a novel strategy to address LR recycling, and the relevant, wastewater and soil treatment, which has high practicability for industrial applications.

Vertical fate of Cd in soil under phytoremediation by Indian mustard and tall fescue

Soil columns were designed to investigate the vertical migration of Cd in Indian mustard (IM) and tall fescue (TF). The TF biomass was greater than the IM biomass, and the Cd content in IM was higher in the shoots but lower in the roots than that in TF. Both IM and TF released N and absorbed P and K during outdoor growth, differing from the results of the previous experiment in which plants were grown in greenhouses. TF was more absorbent and had less upward attraction than IM. The IM soil was more favorable for Cd precipitation than was the TF soil. Leaching remained the dominant effect, with only 2.28-3.40% and 2.65-3.90% of Cd absorbed by IM and TF, respectively. The present study on the vertical migration of Cd provides new insights into the phytoremediation mechanisms of IM and TF. HIGHLIGHTSVertical migration rate of Cd in soil was calculated.Cd precipitation in IM soil was greater and more excellence than TF soil.TF was more absorbent and had less upward attraction than IM.Leaching remained the dominant effect with only small absorb.

Effects of pH variations caused by redox reactions and pH buffering capacity on Cd(II) speciation in paddy soils during submerging/draining alternation

Incubation experiments were conducted to investigate the influencing factors of pH variation in different paddy soils during submerging/draining alternation and the relationship between pH buffering capacity (pHBC) and Cd speciation in ten paddy soils developed from different parent materials (including 8 acid paddy soils and 2 alkaline paddy soils). The soil pHBC and the changes in soil pH, Eh, Fe2+, Mn2+, SO42- and Cd speciation were determined. The results showed that there was a significant positive correlation between cation exchange capacity (CEC) and pHBC of these paddy soils, indicating that soil CEC is a key factor affecting the pHBC of paddy soils. The contribution of Fe(III) oxide reduction to H+ consumption is far greater than the reduction of Mn(IV)/Mn(III) oxides and SO42- during the submerging. For example, the contribution of the reduction of manganese oxides, SO42- and iron oxides to H+ consumption in the paddy soils from Anthrosol at 15 d submerging was 1.2%, 11.6% and 87.2%, respectively. This confirms that the reduction of Fe(III) oxides plays a leading role in increasing soil pH. Importantly, we noticed that during submerging, soil pH was increased and resulted in the content of available Cd in soils being reduced. This was due to the transformation of Cd to less active forms. Also, there was a significant positive correlation between the change rate of available Cd, the percentage of acid extractable Cd and pH variation. This suggests that the variation in soil pH was responsible for the transformation of Cd speciation. In addition, the change rate of available Cd and the percentage of acid extractable Cd concentration were significantly negatively correlated with soil pHBC. The soil with higher pHBC experienced less pH change, and thus the change rate of available Cd and the percentage of acid extractable Cd concentration were less for the soil. The results of this study can provide a basis for the remediation of Cd-contaminated acidic paddy soils.

Remediation of cadmium contaminated soil by modified gangue material: Characterization, performance and mechanisms

Nowadays, remediation of soil contaminated with potentially toxic metal is a great international concern. In this study, a novel modified gangue material (MGE) is synthesized from coal gangue (GE) through a low-temperature assisted with alkali roasting method, and is applied to the immobilization of cadmium (Cd²⁺) in contaminated soil. The various instruments (SEM-EDS, FTIR, XRD, TGA, and XPS) are employed to investigate the changes of microstructure and function of GE before and after the modification. The results showed that a large number of active groups (Si-O, Al-O, Fe-O, -OH, -CO, and -COOH) are observed on the surface of MGE, which is conducive to the removal of Cd²⁺. Besides, the adsorption kinetics, and isotherm models are introduced to analyze the potential adsorption mechanism, which suggesting that the adsorption behavior can be well fitted by pseudo-second-order and Langmuir models. The potential mechanisms of MGE include the ion exchange, complexation, electronic attraction, and precipitation. According to the pot experiment, the application of MGE can significantly improve the growth of pakchoi, and increase the pH of soils. Meanwhile, the content of available Cd²⁺ is reduced in the treatment with MGE, by a factor of 14.2%-29.8%. Correspondingly, the content of Cd²⁺ in different parts of pakchoi is also decreased. The study shows that the MGE can be strongly recommended as an efficient and safe amendment to stabilize Cd²⁺ in contaminated soil.

Abatement of Cd in rice grain and toxic risks to human health by the split application of silicon at transplanting and jointing period

Silicon (Si) has the potential to ameliorate the toxic effects of cadmium (Cd) on rice growth and mitigate Cd-uptake by rice under Cd-contaminated soil. However, it is not completely clear whether there are differences in the impacts of different Si management on the chemical behavior of Cd in soil-rice system under Cd-contaminated paddy field. Here, pot trials were conducted to explore the effects of three modes of Si application (T-applying Si at transplanting stage, J-applying Si at jointing stage, TJ-applying Si at transplanting stage and jointing stage with a ratio of 50% to 50%) on the accumulation of Cd in rice grain and the toxic risk of Cd on human health in rice consumption under Cd-polluted soil (4.21 mg·kg^-1), and that without Si application was used as control (CK). Results showed that rice growth and Cd-retention in root were enhanced by Si application, and the retention of Cd in TJ root was the highest, reaching 82.36%∼84.06% of total Cd absorbed by rice plant. TJ also elevated soil pH and CEC value significantly during the whole growth period, diminished Cd availability and converted exchangeable-Cd into residual-Cd in soil. Moreover, Si application reduced Cd concentration in iron plaque, while TJ had the lowest concentration of DCB-Cd and the highest molar ratios of Fe/Cd and Mn/Cd. The bioaccessibility of Cd from grains and cooked rice were decreased by Si application. Compared with T and J, the hazard quotient of digestion from cooked white rice of TJ in gastric phase was reduced by 19.61% and 21.94%, respectively. In brief, TJ had more efficiency on reducing the Cd availability in soil during the rice growing period, promoting the retention of Cd in root, decreasing Cd uptake by rice plant and distribution to grains, as well as the bioaccessibility of Cd from cooked rice. These results also provide a novel strategy of Si application to decrease the risk of Cd migration in the soil-rice-humans system and simultaneously promote rice yields.

Cadmium accumulation in rice straws and derived biochars as affected by metal exposure, soil types and rice genotypes

Straw residues, as one of the conservation farming practices, are being strongly encouraged in China, including some cadmium-polluted areas. Nowadays, a portion of this plant residue is promoted to be removed for reducing excess metal(loid) in the soil and to be used for bioenergy production. Nevertheless, the possible influences of contaminated straw or the burial of its derived biochars on Cd accumulation in soil and data based on health risk assessment associated with different status and extent of soil contamination were relatively unknown. Thus it is important to provide a more systematical understanding of contaminated straw burial at specific contamination zones, which may provide useful guidance for straw utilization. In this study, we harvested two genotypes of rice straw from 6 contaminated levels among three soil types to comprehensively study the total Cd contents in straws and its derived biochars and correlate the sets of straw characteristics and Cd contents in three different contamination zones. The total Cd concentration in straws grew at a steady rate relatively with increasing soil Cd contamination levels, compared to those in biochars which performed more fluctuate due to the strong burning. According to correlational analyses, three-way ANOVA showed that the moisture, ash, volatile and fixed carbon were all significantly affected by straw Cd_Total contents (p < 0.001). Such relationships were attributed to guide straw removal portions for gasification. Meanwhile, there was a significant correlation between straw Cd concentration and soil types (p < 0.001), confirming that it might be worth determining soil remediation by straw removal according to site-specific farmland conditions. This work will help to assess efforts toward predicting Cd concentration in the paddy soils related to kinds of contamination status and would also give useful guidance to make sustainable management strategies for crop straws in polluted regions.Novelty statement This work provided data on how much rice straw is needed to remove to ensure the minimal amount to control soil contamination and reduce costs according to site-specific conditions and soil Cd contamination status. It also explains the correlations between straw characteristics related to bioenergy use and soil conditions which would give guidance to balance using crop straw for increased bioenergy production and the need to also protect, preserve, and enhance soil resources.

Identification of a plant endophytic growth-promoting bacteria capable of inhibiting cadmium uptake in rice

AIMS

The study aims to identify a novel plant growth-promoting bacteria (PGPB), which contributes to promoting growth and reducing cadmium (Cd) concentration in rice under Cd-contaminated conditions.

METHODS AND RESULTS

Nine bacterial strains were isolated from plants grown in Cd-contaminated soil. These bacteria were tolerant to 1000 μmol/L CdCl₂ , capable of producing indole-3-acetic acid, fixing nitrogen and solubilizing phosphate. The result of hydroponic experiment showed that under the control and Cd stress conditions, the dry weight of the Tm02-inoculated rice seedlings increased significantly. Furthermore, under Cd stress, the concentration of Cd in the shoot of the Tm02-inoculated seedlings decreased significantly, while there was no significant difference in Cd concentration between treatment with other eight strains and noninoculated seedlings. The same results were observed in the pot experiment as well, where there was a significantly reduced Cd concentration in rice grains of the Tm02-inoculated rice plants. Tm02 was classified as Pantoea agglomerans through 16S rDNA sequencing.

CONCLUSIONS

A novel PGPB strain Tm02 was identified and confirmed that it has the function of promoting rice growth and reducing Cd concentration in rice grain under Cd-contaminated conditions. This strain has the potential to improve rice yield in Cd-contaminated paddy fields.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides a new example of using PGPB to improve the tolerance of rice to Cd pollution.

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

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

Cadmium accumulation in wheat and maize grains from China: Interaction of soil properties, novel enrichment models and soil thresholds

The cadmium (Cd) activity in soil has been widely studied. However, the interactive effects of soil properties (e.g. soil pH, CEC, and SOM) on Cd transfer from soil to grain are generally overlooked. In total 325 datasets including soil pH, CEC, SOM, and soil Cd content were used in this study. The descriptive statistics indicated that Cd content in wheat and maize soils ranged from 0.05 to 10.31 mg/kg and 0.02-13.68 mg/kg, with mean values of 0.87 and 1.14 mg/kg, respectively. Cd contents in wheat and maize grains were 0.01-1.36 mg/kg and 0.001-1.08 mg/kg with average values of 0.15 and 0.10 mg/kg, respectively. The results of SEM demonstrated that the interactive effects of soil properties contributed more to Cd transfer from soil to wheat grain than the soil Cd content. Subsequently, CITs-MLR indicated that the critical factors, including soil pH and total soil Cd content, could mask the contribution of other soil properties on Cd accumulation in grain; soil CEC may prevent Cd from leaching and therefore improve grain Cd level of wheat especially at acidic soil condition. The result of derived Cd thresholds revealed that current Cd thresholds are not completely suitable to wheat and maize grain at different soil conditions. This study provides a new model for further investigation on relationships between soil properties, soil Cd content and grain Cd level.