Influence of silicon treatment on antimony uptake and translocation in rice genotypes with different radial oxygen loss

Insights into the biogeochemical transformation, environmental impacts and biochar-based soil decontamination of antimony

Every year, a significant amount of antimony (Sb) enters the environment from natural and anthropogenic sources like mining, smelting, industrial operations, ore processing, vehicle emissions, shooting activities, and coal power plants. Humans, plants, animals, and aquatic life are heavily exposed to hazardous Sb or antimonide by either direct consumption or indirect exposure to Sb in the environment. This review summarizes the current knowledge about Sb global occurrence, its fate, distribution, speciation, associated health hazards, and advanced biochar composites studies used for the remediation of soil contaminated with Sb to lessen Sb bioavailability and toxicity in soil. Anionic metal(loid) like Sb in the soil is significantly immobilized by pristine biochar and its composites, reducing their bioavailability. However, a comprehensive review of the impacts of biochar-based composites on soil Sb remediation is needed. Therefore, the current review focuses on (1) the fundamental aspects of Sb global occurrence, global soil Sb contamination, its transformation in soil, and associated health hazards, (2) the role of different biochar-based composites in the immobilization of Sb from soil to increase biochar applicability toward Sb decontamination. The review aids in developing advanced, efficient, and effective engineered biochar composites for Sb remediation by evaluating novel materials and techniques and through sustainable management of Sb-contaminated soil, ultimately reducing its environmental and health risks.

A review on sources of soil antimony pollution and recent progress on remediation of antimony polluted soils

Antimony (Sb) is a serious toxic and non-essential metalloid for animals, humans, and plants. The rapid increase in anthropogenic inputs from mining and industrial activities, vehicle emissions, and shoot activity increased the Sb concentration in the environment, which has become a serious concern across the globe. Hence, remediation of Sb-contaminated soils needs serious attention to provide safe and healthy foods to humans. Different techniques, including biochar (BC), compost, manures, plant additives, phyto-hormones, nano-particles (NPs), organic acids (OA), silicon (Si), microbial remediation techniques, and phytoremediation are being used globally to remediate the Sb polluted soils. In the present review, we described sources of soil Sb pollution, the environmental impact of antimony pollution, the multi-faceted nature of antimony pollution, recent progress in remediation techniques, and recommendations for the remediation of soil Sb-pollution. We also discussed the success stories and potential of different practices to remediate Sb-polluted soils. In particular, we discussed the various mechanisms, including bio-sorption, bio-accumulation, complexation, and electrostatic attraction, that can reduce the toxicity of Sb by converting Sb-V into Sb-III. Additionally, we also identified the research gaps that need to be filled in future studies. Therefore, the current review will help to develop appropriate and innovative strategies to limit Sb bioavailability and toxicity and sustainably manage Sb polluted soils hence reducing the toxic effects of Sb on the environment and human health.

Effects of cultivar, water condition and their interactions on Cd accumulation in rice grains

Using low Cd accumulation cultivars and managing field water regimes are effective measures to mitigate Cd accumulations in rice grains. However, the effect of the cultivar-water condition interaction (CWI) on grain Cd accumulations has largely been ignored. To solve this problem, pot and hydroponic experiments were conducted using 14 rice cultivars and two contrasting water conditions. The results showed that CWI significantly affected Cd concentrations in rice grains and roots, explaining 8.8% and 22.8% of the total variance, respectively. These CWI effects were derived from cultivar-dependent variations in rhizosphere soil properties [Eh, pH and available Cd associated with root radial oxygen loss (ROL)] and root Cd uptake. In this context, cultivar HH61 exhibited low, stable Cd accumulations, owing to its stably lower translocation rate, root Cd uptake ability and available Cd in its rhizosphere than the other cultivars, which was induced by its lower ROL. Root-to-grain Cd translocation rates were vital in determining Cd accumulations in grain of different cultivars but were independent from CWI. These results indicated that CWI could play an important role in Cd accumulation in rice while stable low-Cd cultivar should possess low ROL under flooding and low root-to-grain Cd translocation rate. The results will provide novel theoretical basis for cultivar selection and hence benefit the extensive use of low-accumulation cultivars and public health.

Repeated inoculation of antimony resistant bacterium reduces antimony accumulation in rice plants

Applying beneficial bacteria in rice rhizosphere to manage heavy metal behaviour in soil-plant system is a promising strategy. However, colonization/domination of exogenous bacteria in rhizosphere soils remains a challenge. In this study, a bacterium Ochrobactrum anthropi, which showed the potential of transforming soluble Sb^III into Sb₂O₃ mineral, was repeatedly inoculated into the rice rhizosphere weekly throughout the rice growth period, and the colonization of this bacterium in rice rhizosphere soils and its effect on Sb accumulation in rice plants were investigated. Results showed that repeated inoculants changed the native bacterial community in rhizosphere soils in comparison with the control, but the inoculated O. anthropi was not identified as an abundant species. With weekly inoculation, the decrease in Sb in rice roots and straws was maintained throughout the rice growth period, with decrease percentages ranging from 36 to 49% and 33-35%. In addition, decrease percentages of Sb in husks and grains at the maturing stage obtained 34 and 37%, respectively. Furthermore, the XRD identified the formation of valentinite (Sb₂O₃) on rice root in inoculation treatment, and the decrease percentages in aqueous Sb^III in rhizosphere were 53-100% through the growth period. It demonstrated that weekly inoculants performed their temporary activity of valentinite formation, and reduced Sb accumulation in rice plants efficiently. This study suggests that regardless of successful colonization, repeated inoculation of beneficial bacteria is an option to facilitate the positive effects of inoculated bacteria in the management of heavy metal behaviour.

Co-incorporation of Chinese milk vetch (Astragalus sinicus L.), rice straw, and biochar strengthens the mitigation of Cd uptake by rice (Oryza sativa L.)

Soil cadmium (Cd) contamination is becoming a widespread concern because of its threat to global ecosystem health and food security. Co-incorporation of Chinese milk vetch (MV) and rice straw (RS) is a common agricultural practice in Southern China; however, the effects of combining these two materials with biochar on Cd bioavailability remain unclear. This study investigated the effects of MV, RS, rape straw biochar (RB), iron-modified biochar (FB), and their combinations on Cd uptake by rice through incubation and field experiments. The results showed that compared with the control without material input (CK), MV + RS (MR), MV + RS + RB (MRRB), and MV + RS + FB (MRFB) considerably reduced the Cd concentration in brown rice by 61.20 %, 65.38 %, and 62.65 %, respectively. Furthermore, the treatments increased the formation of iron‑manganese plaque (IMP) at different growth stages; MRRB and MRFB exhibited the highest increase rates among the treatments. Quantitatively, the Fe plaque and Mn plaque were increased by 20.61 %-47.23 % and 80.18 %-172.74 %, respectively. Compared with CK, the MRRB and MRFB treatments reduced the soil available Cd by 35.09 %-54.45 % and 38.20 %-50.20 %, respectively, at all stages. This decrease was substantially lower than that observed in the MV, RS, and MR treatments. Similar trends were observed in the incubation experiment. Additionally, the Community Bureau of Reference Sequential Extraction Analysis indicated that the MRRB and MRFB treatments converted the bioavailable Cd fractions into a stable form. Partial least squares path model and redundancy analysis revealed that pH was the major factor influencing Cd bioavailability. This study emphasized that the dual impact factors from the enhancement of Cd passivation capability and IMP formation jointly result in the reduction of Cd uptake by rice. Consequently, the co-incorporation of MV, RS, and biochar is promising for remediating Cd-contaminated paddy soils in Southern China.

Impact of physiochemical properties, microbes and biochar on bioavailability of toxic elements in the soil: a review

The pollution of toxic elements (TEs) in the ecosystem exhibits detrimental effects on the human health. In this paper, we debated remediation approaches for TEs polluted soils via immobilization methods employing numerous amendments with reverence to type of soil and metals, and amendment, immobilization competence, fundamental processes and field applicability. We argued the influence of pH, soil organic matter, textural properties, microbes, speciation and biochar on the bioavailability of TEs. All these properties of soil, microbes and biochar are imperative for effective and safe application of these methods in remediation of TEs contamination in the ecosystem. Further, the application of physiochemical properties, microbes and biochar as amendments has significant synergistic impacts not only on absorption of elements but also on diminution of toxic elements.

The effect of co-pyrolysis temperature for iron-biochar composites on their adsorption behavior of antimonite and antimonate in aqueous solution

Iron-biochar is an efficient adsorbent for contaminants, whereas the role of prepared temperature on removal of antimony (Sb) is unacquainted. In this study, the iron-biochar composites (FBC) were fabricated by co-pyrolysis at 500°C and 800°C and applied to remove antimonite (Sb(III)) and antimonate (Sb(V)) in aqueous. The results showed Fe₃O₄ was loaded on biochar prepared at 500°C (FBC500), while FeOOH with zero-valent iron (ZVI) was formed on biochar pyrolyzed at 800°C (FBC800). However, FBC500 showed the maximum absorbance for Sb(V) (30.47 mg/g), and FBC800 had optimal removal efficiency for Sb(III) (52.30 mg/g). The sorption of Sb(III) and Sb(V) on FBC was multilayer heterogeneous chemisorption (complexation and ligand exchange). Sb(III) was oxidized to Sb(V) with less toxicity during the corrosion of ZVI on FBC800, leading to the co-precipitation of Sb₂O₅. The electrostatic interaction affected the adsorption of Sb(V) on FBC500 and FBC800. The FBC800 showed superior reusability and resistance than FBC500.

Silicon alleviates antimony phytotoxicity in giant reed (Arundo donax L.)

Silicon enhances photosynthetic efficiency, biomass, and lignification of root structures possibly limiting antimony translocation and mitigating phytotoxicity in giant reed plants. Antimony (Sb) is a non-essential metalloid causing toxic effects in plants. Silicon has been reported to impart tolerance against biotic and abiotic stress in plants. Fast-growing plant, giant reed (Arundo donax L.) is a promising energy crop, can be a suitable plant for phytoremediation. However, information regarding the tolerance capacity with respect to Sb toxicity and potential of Si to mitigate the Sb phytotoxicity in giant reed are very scarce. Rhizomes of giant reed were grown for ten weeks in hydroponics exposed to Sb, Si, and their combination wherein treatment without Sb/Si served as control. Effect of these treatments on rate of net photosynthesis and photosynthetic pigments, phytoextraction ability of Sb, Si and Sb uptake, plant biomass, and lignification and suberization of roots along with localization of Sb and Si were analysed. We found that Si considerably improved the growth and biomass of giant reed under Sb toxicity. Antimony reduced the photosynthesis and decreased the content of photosynthetic pigments, which was completely alleviated by Si. Silicon amendment to Sb treated plants enhanced root lignification. Silicon enhanced lignification of root structures probably restricted the Sb translocation. However, co-localization of Sb with Si has not been observed neither at the shoot nor at the root levels. Similarly, Sb was also not detected in leaf phytoliths. These findings suggest that Si treatment promotes overall plant growth by improving photosynthetic parameters and decreasing Sb translocation from root to shoot in giant reed by improving root lignification.

Aerenchyma, gas diffusion, and catalase activity in Typha domingensis: a complementary model for radial oxygen loss

Radial oxygen loss is a physical phenomenon that occurs naturally in aquatic plants. Typha domingensis was chosen as a model plant because it possesses basic morphological characteristics, such as a stem (rhizome) that produces leaves and adventitious roots, which are present in many aquatic plants. This study aimed to evaluate the following: the relevance of the anatomy of T. domingensis on gas diffusion among organs; the influence of plant parts on radial oxygen loss; the role of catalase in radial oxygen loss; and the proposition of a novel explanation for the downward diffusion of oxygen through the organs of this aquatic macrophyte and into the environment. Typha domingensis plants were cultivated in a greenhouse under different conditions: plants with intact leaves, plants with leaves cut in half, and plants without leaves. Furthermore, we evaluated the percentage of aerenchyma in different vegetative organs, the minimum pressure required for radial oxygen loss, the daily variations of dissolved oxygen, and the roots' catalase activity. The results demonstrated that certain cellular features contributed to decreased oxygen diffusion among the organs, specifically, those found in the leaf-rhizome and root-rhizome interfaces as well as the suberin and lignin layers in these regions. Additionally, our experiments with a catalase activator and inhibitor validated that a significant amount of the oxygen released in radial oxygen loss could not, in fact, be exclusively supplied by the atmosphere. Thus, a complementary model is proposed in which catalase activity is an important component of radial oxygen loss.

Alleviation mechanisms of metal(loid) stress in plants by silicon: a review

Silicon (Si), although not considered as an essential element for plants in general, can ameliorate the phytotoxicity induced by excess metal(loid)s whether non-essential (e.g. Cd, Pb, Cr, Al, As, and Sb) or essential (e.g. Cu, Ni, and Zn). The Si-enhanced resistance allowing plants to cope with this type of abiotic stress has been developed at multiple levels in plants. Restriction of root uptake and immobilization of metal(loid)s in the rhizosphere by Si is probably one of the first defence mechanism. Further, retention of elements in the root apoplasm might enhance the resistance and vigour of plants. At the cellular level, the formation of insoluble complexes between Si and metal(loid)s and their storage within cell walls help plants to decrease available element concentration and restrict symplasmic uptake. Moreover, Si influences the oxidative status of plants by modifying the activity of various antioxidants, improves membrane stability, and acts on gene expression, although its exact role in these processes is still not well understood. This review focuses on all currently known plant-based mechanisms related to Si supply and involved in amelioration of stress caused by excess metal(loid)s.

The effect of straw-returning on antimony and arsenic volatilization from paddy soil and accumulation in rice grains

Pollution by antimony (Sb) and arsenic (As) in soil can pose a great threat to human health. Straw-returning is widely applied to paddy fields for improving and remediating soil. A pot experiment was conducted to investigate the effect of straw-returning on Sb and As transformation and translocation in a soil-rice system. In this study, Sb and As co-contaminated soil was thoroughly mixed with different proportions (0, 0.5, 1, and 2%) of straw and used for growing rice plants through the entire growing stage in a pot experiment and 4 weeks in a microcosm experiment. The straw application significantly increased Sb and As mobility. The concentrations of total Sb and As in soil-pore water increased after the application of straw in most growing stages. The Sb volatilization in the pot and microcosm experiments was also stimulated by straw application. With the high dose of straw application (2%), the concentration of Sb in brown grain was reduced by 72% compared with the control, but As concentrations increased by around 77%. These findings provide a new perspective in that straw-returning could affect the behavior of both Sb and As in soil and reduce the Sb accumulation in brown grain and some guidance in the use of straw-returning in Sb-contaminated paddy soil.

The effect of an antimony resistant bacterium on the iron plaque fraction and antimony uptake by rice seedlings

Iron plaque (IP) is crucial in mitigating antimony (Sb) uptake and accumulation in rice plants, while, few studies focused on the effect of the iron plaque-associated Sb resistant bacteria on IP and Sb uptake into rice plants. Here, the effect of a Sb resistant bacterium (GenBank accession No. MH345840, with potential of conversion soluble Sb(III) into insoluble Sb₂O₃) on IP and Sb(III)/Sb(V) uptake under hydroponic condition was investigated. The results showed that in the presence of Sb(III), a large quantity of bacterial cells consorted with IP on rice roots, the bacterial inoculum altered the IP fraction distribution without enhancing its amount. However, it reduced Sb(III) uptake into rice roots. On contrary, seldom bacterial cells associated with the IP on rice roots in the presence of the Sb(V), the bacterial inoculum increased the IP amount slightly, and did not decline the Sb(V) uptake into rice roots. It also showed that the bacterial inoculum decreased Sb concentrations in rice shoots greatly in both Sb(III) and Sb(V) supplied treatments.

Assembly of root-associated microbiomes of typical rice cultivars in response to lindane pollution

Organochlorine pesticides have been extensively used for many years to prevent insect diseases of rice (Oryza sativa L.), but little is known about their residual impacts on the underground micro-ecology in anaerobic environment. In this glasshouse study, we characterized the lindane effects on the assembly of root-associated microbiomes of commonly used indica, japonica and hybrid rice cultivars, and their feedback in turn, in modifying lindane anaerobic dissipation during 60 days' rice production. The results showed that rice growth inhibited the anaerobic dissipation of lindane, but was not affected apparently by lindane at initial spiked concentration of 4.62 and 18.54 mg kg^-1 soil. Suppressed removal of lindane in rice planted treatments as compared with that in unplanted control was likely due to inhibited reductive dechlorination induced by a comprehensive effect of radial O₂ secretion of rice root and co-occurring Fe(III) reduction that consumed electron competitively in rice rhizosphere. However, the hybrid cultivar exhibited a less suppression than the conventional cultivars in high polluted soils. Bacteria was more sensitively responded to lindane pollution than fungal taxa, and Actinobacteria, Chloroflexi, Verrucomicrobia and Proteobacteria were the main different phyla between hybrid and conventional cultivars, with a more stable community structure exhibited in the hybrid rice under lindane stress. Our study highlights the assembly and variation of root-associated microbiomes in responses of lindane pollution, and suggests that hybrid rice cultivar might be most competent for cultivation in paddy fields polluted by lindane and other organochlorine pesticides, especially in the area with high residual levels.

Multi-generational effects and variations of stress response by hexabromocyclododecane (HBCD) exposure in the nematode Caenorhabditis elegans

In order to understand multi-generational effects and changes of stress response by hexabromocyclododecane (HBCD) exposure, the animal model Caenorhabditis elegans was chosen for toxicity study. Multiple endpoints, including the physiological levels (growth, reproduction, and locomotion behaviors), stress-related gene expressions, reactive oxygen species (ROS) production and degree of cell apoptosis, were evaluated on exposed nematodes and their progeny. Prolonged exposure to HBCD at concentrations of 2 nM-200 nM caused adverse physiological effects in the parental generation (F0), and these effects were also observed in the offspring under HBCD-free conditions (F1). HBCD-induced toxicities could be transferred from parent to offspring. The integrated gene expressions profiles showed that exposure to HBCD at concentrations of 20-200 nM resulted in obvious changes in stress-related gene expressions, which were more increased in F0 generation than in F1 generation. The increased expressions were pronounced in several genes related to oxidative stress and cell apoptosis, e.g., hsp-16.2, hsp-16.48, sod-1, sod-3 and cep-1 genes. Exposure to 200 nM of HBCD could significantly increase ROS production and degree of cell apoptosis in the F0 and F1 generations. Therefore, it was speculated that HBCD exposure induced oxidative stress and cell apoptosis, which resulted in the adverse physiological effects. This finding is helpful for understanding the multi-generational effects and evaluating the potential risk of HBCD.

Antimony speciation in the environment: Recent advances in understanding the biogeochemical processes and ecological effects

Antimony (Sb) is a toxic metalloid, and its pollution has become a global environmental problem as a result of its extensive use and corresponding Sb-mining activities. The toxicity and mobility of Sb strongly depend on its chemical speciation. In this review, we summarize the current knowledge on the biogeochemical processes (including emission, distribution, speciation, redox, metabolism and toxicity) that trigger the mobilization and transformation of Sb from pollution sources to the surrounding environment. Natural phenomena such as weathering, biological activity and volcanic activity, together with anthropogenic inputs, are responsible for the emission of Sb into the environment. Sb emitted in the environment can adsorb and undergo redox reactions on organic or inorganic environmental media, thus changing its existing form and exerting toxic effects on the ecosystem. This review is based on a careful and systematic collection of the latest papers during 2010-2017 and our research results, and it illustrates the fate and ecological effects of Sb in the environment.

Chronic toxicity of hexabromocyclododecane(HBCD) induced by oxidative stress and cell apoptosis on nematode Caenorhabditis elegans

In order to gain insights into the chronic effects and mechanisms of hexabromocyclododecane (HBCD), the animal model Caenorhabditis elegans (C. elegans) was chosen for toxicity study. Multiple endpoints, including the physiological (growth and locomotion behaviors), biochemical (reactive oxygen species (ROS) production, lipofuscin accumulation, and cell apoptosis), and molecular (stress-related gene expressions) levels, were tested by chronic exposure for 10 d to low concentrations of HBCD (0.2 nM-200 nM). The results revealed that chronic exposure to HBCD at concentrations more than 20 nM would significantly influence the growth, locomotion behaviors, ROS formation, lipofuscin accumulation, and cell apoptosis of nematodes. Treatment with antioxidants of ascorbate and N-acetyl-l-cysteine (NAC) suppressed the toxicity induced by HBCD. The integrated gene expression profiles showed that the chronic exposure to 200 nM of HBCD significantly increased the expression levels of stress-related genes (e.g., hsp-16.2, hsp-16.48, sod-1, sod-3, and cep-1 genes). Among these genes, the sod-1, sod-3, and cep-1 gene expressions were significantly correlated with HBCD-induced physiological effects by the Pearson correlation test. The mutations of sod-3 and cep-1 induced more severe toxicity compared to wild-type nematodes. Therefore, HBCD exposure induced oxidative stress by ROS accumulation and cell apoptosis, which resulted in HBCD-induced toxicity on nematodes, and sod-3 and cep-1 played important roles in protecting nematodes against HBCD-induced toxicity.