Heavy metal contamination of agricultural soils poses risks and hazards to humans. The remediation of heavy metal-polluted soils has become a hot topic in environmental science and engineering. In ...this review, the application of clay minerals for the remediation of heavy metal-polluted agricultural soils is summarized, in terms of their remediation effects and mechanisms, influencing factors, and future focus. Typical clay minerals, natural sepiolite, palygorskite, and bentonite, have been widely utilized for the in-situ immobilization of heavy metals in soils, especially Cd-polluted paddy soils and wastewater-irrigated farmland soils. Clay minerals are able to increase soil pH, decrease the chemical-extractable fractions and bioavailability of heavy metals in soils, and reduce the heavy metal contents in edible parts of plants. The immobilization effects have been confirmed in field-scale demonstrations and pot trials. Clay minerals can improve the environmental quality of soils and alleviate the hazards of heavy metals to plants. As main factors affecting the immobilization effects, the pH and water condition of soils have drawn academic attention. The remediation mechanisms mainly include liming, precipitation, and sorption effects. However, the molecular mechanisms of microscopic immobilization are unclear. F~ture studies should focus on the long-term stability and improvement of clay minerals in order to obtain a better remediation effect.
At present, the remediation of heavy-metal polluted soil is a considerable problem. In this study, in situ immobilization field experiments were conducted by planting rice (Oryza sativa L.) in Cd ...contaminated paddy soil to determine the optimal remediation mode that would reduce the accumulation of Cd in brown grain. Sepiolite and palygorskite were utilized as amendments, while ordinary and low Cd-accumulating rice cultivars were selected as model plants. The remediation effects of the amendments on the soil and rice were investigated and the potential immobilization mechanisms were analyzed using X-ray diffraction and X-ray photoelectron spectroscopy. The application of sepiolite and palygorskite was found to increase the pH value of the paddy soil, by varying amounts, and significantly reduce the HCl, TCLP, CaCl2, and NH4OAc-extractable Cd concentrations resulting in a notable decrease in the Cd concentration in brown rice. Both sepiolite and palygorskite were found to reduce exchangeable Cd concentrations and increase the carbonate-bound and residual fractions in paddy soil. Precipitation of Cd as carbonates or hydroxides and surface complexation were the main immobilization mechanisms for sepiolite and palygorskite. The concentration of Cd in brown rice of the low Cd-accumulating cultivar from the control plot was 0.72mg·kg−1. Sepiolite treatment reduced the value to 0.18mg·kg−1 below the maximum levels reported by Codex Stan 193-1995 and the national standard of China. It was concluded that chemical immobilization combined with crop cultivar selection could be a suitable proposal for crop safety regulation and sustainable agriculture.
Combination of chemical immobilization and low Cd accumulating rice cultivar.
In situ chemical immobilization can significantly decrease the bioavailability and bioaccessibility of cadmium in paddy soil. Meanwhile, selection and breeding of low Cd accumulating cultivars is an environmentally friendly and cost effective method for reducing the risk of contamination from Cd in food. The results of this study suggested that safer food production was more likely to be achievable by combining low Cd accumulating cultivars with in situ immobilizing agent amendment of soils. Display omitted
•Sepiolite and palygorskite significantly reduced phytoavailable Cd in paddy soils.•Precipitation and surface complexation were main immobilization mechanisms.•Combination of chemical immobilization and low accumulating cultivar was recommended.
Long-term effectiveness and persistence are two important criterias to evaluate alternative remediation technology of heavy metal polluted soils. Pot and field studies showed addition of sepiolite ...was effective in immobilizing Cd in polluted soils, with significant reduction in TCLP extracts (0.6%–49.6% and 4.0%–32.5% reduction in pot and field experiments, respectively) and plant uptake (14.4%–84.1% and 22.8%–61.4% declines in pot and field studies, correspondingly). However, the applications of sepiolite offered a limited guarantee for the safety of edible vegetables in Cd-polluted soils, depending on the soil type, the Cd pollution type and level, and the dose and application frequency of chemical amendments. Bioassays, such as plant growth, soil enzymatic activities and microbial community diversity, indicated a certain degree of recovery of soil metabolic function. Therefore, sepiolite-assisted in situ remediation is cost-effective, environmentally friendly, and technically applicable, and can be successfully used to reduce Cd enter into the food chain on field scale.
•Sepiolite has reliability and stability for remediation of contaminated Cd.•Sepiolite significantly decreases Cd bioavailability in soil and uptake in plant.•The treatment of sepiolite improves soil quality.
Sepiolite not only decreased soil Cd bioavailability and plant Cd uptake, but also improved soil quality.
A pot trial was conducted to evaluate the effectiveness of bentonite in immobilizing Cd and Pb in soils using a set of parameters such as biochemical and physiological characteristics, metal ...fractions, plant growth and heavy metal concentrations, and soil enzyme activities and microbial community. Results showed that applying bentonite to soil increased activities of superoxide dismutase (SOD) in roots and peroxidase (POD) in leaves and soluble protein (SP) contents in roots of rice seedling by 2.19–2.46, 0.18–0.34, and 0.06–2.20 times, but decreased malondialdehyde (MDA) contents in leaves and roots by up to 74.1% and 62.1%, respectively, as compared with the control. The additions of bentonite reduced the exchangeable fraction of Cd by 11.1%–42.5% and that of Pb by 20.3%–49.3%, most of which was converted into unavailable forms, resulting in 3.0%–54.3% and 6.7%–10.0% increases in residual portions, respectively. Treatments with bentonite inhibited Cd and Pb translocation from soil to the aerial parts of Oryza sativa L. Compared with the control, bentonite applications reduced concentrations of Cd and Pb in the roots by 9.4%–31.3% and 5.1%–26.7%, and by 17.4%–44.3% and 3.7%–7.8% in the shoots, respectively. The decreased catalase and increased values of invertase and urease activities and bacteria, fungi, and actinomycete population in the soil implied certain metabolic recovery and soil quality improvements by bentonite additions. These results indicate that bentonite has the potential to reduce heavy metal bioavailability in paddy soils.
•Bentonite can alleviate heavy metal phytotoxicity to the plant.•Bentonite significantly decreased Cd bioavailability in soil and uptake in plant.•The addition of bentonite improves soil quality.
A pot trial was conducted to assess the effectiveness of sepiolite, bentonite, and phosphate on the immobilization remediation of cadmium (Cd)-contaminated soils using a set of variables, namely, ...physiological traits, sequential extraction procedure, plant growth and Cd concentration, and soil enzymatic activities and microbial population. Results showed that superoxide dismutase and peroxidase activities in the leaves of Oryza sativa L. and catalase activities in soils were stimulated after applying the amendments. However, soluble protein contents in leaves and urease and invertase activities in soils were reduced from 7.1% to 31.7%, 1.0%–23.3%, and 21.1%–62.5%, respectively, compared with the control. Results of the sequence extraction procedures revealed that the exchangeable fraction of Cd in soils was mostly converted into carbonated-associated forms. The water soluble plus exchangeable fraction (SE) of Cd in soil decreased when treated with single and compound materials of sepiolite, bentonite and phosphate, which resulted in 13.2%–69.2% reduction compared with that of CK (control test). The amendments led to decreased Cd concentrations in roots, stems, leaves, brown rice, and rice hull by 16.2%–54.5%, 16.6%–42.8%, 19.6%–59.6%, 5.0%–68.2%, and 6.2%–20.4%, respectively. Higher bacterial and actinomycete amount indicated that remediation measures improved soil environmental quality. Composite amendments could be more efficiently used for the stabilization remediation of Cd contaminated soils with low Cd uptake and translocation in the plants and available contents of Cd in soil.
•The effect of sepiolite, bentonite and phosphate on Cd immobilisation is studied.•The amendments are effective in immobilizing Cd in soils.•The remediation measures improve soil quality.
Cadmium (Cd) is toxic throughout the food chain. Selenium (Se) can mitigate Cd accumulation in rice plants, although it is unclear why it has such effect. The objectives of this study were to ...investigate the effects of Se application on Cd-induced oxidative stress and antioxidant activities, and the combined effects of Se application and water management on the formation of iron plaque on the rice surface and Fe, Cd, and Se accumulation in rice plants. Rice seedlings were grown in Cd-contaminated soil with or without the addition of Se, and in aerobic or flooded conditions. Exogenous Se reduced Cd-induced oxidative stress. In the flooded treatment, exogenous selenite significantly decreased Cd concentrations in rice tissues, whereas it noticeably enhanced Cd concentrations in rice tissues in the aerobic treatment. Furthermore, selenite addition and flooding promote the formation of iron plaque and increase Fe concentrations in rice tissues. Pearson correlation analysis shows that plant Cd was significantly negatively correlated with Fe concentrations in rice tissues, and plant Fe was significantly positively correlated with Se concentrations in rice tissues, but no significant correlation was found between Cd and Se concentrations Thus, exogenous selenite may indirectly affect Cd uptake by influencing the formation of iron plaque on rice root surface, Fe uptake and Fe levels in rice.
•Selenite addition significantly alleviated Cd-induced oxidative stress.•Selenite addition and water management significantly affect Cd accumulation in rice tissues.•Selenite may indirectly affect Cd uptake by influencing the formation of iron plaque on rice root surface, Fe uptake and Fe levels in rice.
The heavy metal ions may be removed by LDHs via (i) precipitation of metal hydroxides onto the surface of LDHs. (ii) adsorption through the bonding with surface hydroxyl groups of LDHs. (iii) ...isomorphic substitution and (iv) chelation with the functional ligand in the interlayers.
•The sorption mechanisms of metal cations on LDHs are discussed in detail.•The affecting factors such as pH, temperature and ionic strength are discussed.•Precipitation, surface adsorption and chelation are main sorption mechanisms.•The sorption behaviors of metal cations on LDHs are well summarized for comparison.
Layered double hydroxides (LDHs) are a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules. In recent years, many studies have focused on the application of LDHs in environmental remediation of anion pollutant. Meanwhile many reports are devoted to investigating the ability of LDHs to remove metal cations such as Pb2+, Cd2+ and Cu2+ from aqueous solutions. This paper provides an overview of the recent advances achieved in sorption of metal cations on LDHs. Sorption behaviors of various cations on LDHs including sorption isotherms and kinetics have been comprehensively reviewed. Several essential factors such as pH, temperature, ionic strength that influence the sorption are discussed. The sorption mechanisms including precipitation, surface complexation, isomorphic substitution and chelation with the anions in the interlayer of LDHs are discussed in detail. Possible improvement and potential applications of LDHs in the future are also proposed.
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Empirical evidence has shown that there is an ideal arrangement of facial features (ideal ratios) that can optimize the attractiveness of a person’s face. These putative ratios define ...facial attractiveness in terms of spatial relations and provide important rules for measuring the attractiveness of a face. In this paper, we show that a deep neural network (DNN) model can learn putative ratios from face images based only on categorical annotation when no annotated facial features for attractiveness are explicitly given. To this end, we conducted three experiments. In Experiment 1, we trained a DNN model to recognize the attractiveness (female/male × high/low attractiveness) of face in the images using four category-specific neurons (CSNs). In Experiment 2, face-like images were generated by reversing the DNN model (e.g., deconvolution). These images depict the intuitive attributes encoded in CSNs of the four categories of facial attractiveness and reveal certain consistencies with reported evidence on the putative ratios. In Experiment 3, simulated psychophysical experiments on face images with varying putative ratios reveal changes in the activity of the CSNs that are remarkably similar to those of human judgements reported in a previous study. These results show that the trained DNN model can learn putative ratios as key features for the representation of facial attractiveness. This finding advances our understanding of facial attractiveness via DNN-based perspective approaches.
Field experiments was conducted to evaluate the effectiveness of sepiolite (S), sepiolite + fungi residues (SFR) and sepiolite + vermicompost (SVC) on in situ immobilization remediation of Cd ...contaminated soils. The results showed that treatments of S, SFR and SVC decreased soil Cd availability by 15.2–47.8%, 17.5–44.9% and 13.2–44.9%, respectively, when compared with the control groups. Moreover, the content of Cd in edible parts of Lactuca sativa L., Cichorium endivia L. and Brassica campestris L. was experienced a decrease of 15.9–41.9%, 1.6–38.0% and 29.0–37.4% reduction, respectively, under the amended soil. The improvement of soil fertility was obtained under addition of SVC and SFR, while the amounts of available P, K, organic matter, microbial carbon, microbial nitrogen and dehydrogenase activity were increased by 9.6–68.2%, 1.2–28.3%, 37.5–70.5%, 4.1–121.0%, 220–640% and 6.8–56.8%, respectively, in contrast to CK. Moreover, high-throughput sequencing analysis showed that the combined treated soils got higher values of alpha diversity indices, Chao1, ACE and Shannon. The number of dominant phyla (Proteobacteria, Acidobacteria, Gemmatimonadetes, Crenarchaeota) and genera (Aquicella, Lysobacter, Candidatus Nitrososphaera, Sphingopyxis, Mesorhizobium) were enhanced. Therefore, the use of sepiolite and organic amendments could be an adequate strategy to immobilization remediation of Cd–contaminated soils.
•Effect of in situ remediation of Cd was conducted using field experiment.•Sepiolite co-applied with organic amendments efficiently decreased Cd bioavailability in soils.•Combined amendments inhibited Cd uptake in vegetables.•Combined amendments could increase soil fertility and improve microbial community composition.
For the deterioration model of a material, it is crucial to design a validation experiment to determine the ability of the deterioration model to simulate the actual deterioration process. In this ...paper, a design method of a validation experiment for a deterioration model is proposed to obtain the experiment scheme with low cost and satisfactory credibility. First, a normalized area metric based on probability density functions for the deterioration model is developed for validation results quantification. Normalized area metrics of different state variables in an engineering system can be applied to a unified evaluation standard. In particular, kernel density estimation is used to obtain smooth probability density functions from discrete experimental data, which can reduce the systematic error of the validation metric. Furthermore, a design method for the validation experiment for the deterioration model is proposed, in which the number of experimental samples and observation moments in each experimental sample are design variables, while the credibility of the validation experiment is the constraint. For the experiment design, the problem with varying dimensions of design variables occurred in the optimal design. Thus, a collaborative optimization method using the Latin hypercube sampling was developed to solve this problem. Finally, the results of the two examples showed the characteristics of the proposed metric and also reflected the correlation between the design variables and experimental credibility.
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