Fly ash and zeolite decrease metal uptake but do not improve rice growth in paddy soils contaminated with Cu and Zn

Bio-based resources: systemic & circular solutions for (agro)environmental services

The global promotion of decarbonisation through the circular solutions and (re)use of bio-based resources (BBR), i.e. waste streams, notably from the agricultural, forest and municipal sectors has steadily increased in recent decades. Among the transformative solutions offered by BBR, biosolids (BS), biochars (BC), and bioashes (BA) specifically attract scientific attention due to their highly complex organo-mineral matrices, which present significant potential for recovery in the agro-/forest-ecosystems. These materials enhance various soil (i) chemical (pH, macro/micro nutrient concentrations, organic matter content), (ii) physical (porosity, water-air relations, compaction) or (iii) microbial (diversity, activity) properties. Furthermore, some of transformed BBR contribute to a multitude of environmental services such as the remediation of contaminated sites and wastewater treatment, employing cost-effective and eco-friendly approaches that align with circular economy/waste management principles, ultimately contributing to climate change mitigation. However, several challenges impede the widespread utilization/transformation of BBR, including technological limitations in processing and application, concerns about contamination (e.g., PAHs, PCBs, micro/nano plastics present in BS), toxicity issues (e.g., heavy metals in BA or nanoparticles in BC), and regulatory constraints (e.g., non-uniform regulations governing the reuse of BA and BS). Addressing these challenges demands an interdisciplinary and intersectoral approach to fully unlock the potential of BBR in sustainable decarbonisation efforts.

Molecular Mechanisms of Plant Responses to Copper: From Deficiency to Excess

Copper (Cu) is an essential nutrient for plant growth and development. This metal serves as a constituent element or enzyme cofactor that participates in many biochemical pathways and plays a key role in photosynthesis, respiration, ethylene sensing, and antioxidant systems. The physiological significance of Cu uptake and compartmentalization in plants has been underestimated, despite the importance of Cu in cellular metabolic processes. As a micronutrient, Cu has low cellular requirements in plants. However, its bioavailability may be significantly reduced in alkaline or organic matter-rich soils. Cu deficiency is a severe and widespread nutritional disorder that affects plants. In contrast, excessive levels of available Cu in soil can inhibit plant photosynthesis and induce cellular oxidative stress. This can affect plant productivity and potentially pose serious health risks to humans via bioaccumulation in the food chain. Plants have evolved mechanisms to strictly regulate Cu uptake, transport, and cellular homeostasis during long-term environmental adaptation. This review provides a comprehensive overview of the diverse functions of Cu chelators, chaperones, and transporters involved in Cu homeostasis and their regulatory mechanisms in plant responses to varying Cu availability conditions. Finally, we identified that future research needs to enhance our understanding of the mechanisms regulating Cu deficiency or stress in plants. This will pave the way for improving the Cu utilization efficiency and/or Cu tolerance of crops grown in alkaline or Cu-contaminated soils.

A field trial for remediation of multi-metal contaminated soils using the combination of fly ash stabilization and Zanthoxylumbungeanum- Lolium perenne intercropping system

Insitu stabilization and phytoextraction are considered as two convenient and effective technologies for the remediation of toxic elements (TEs) in soils. However, the effectiveness of these two remediation technologies together on the bioavailability and phytoextraction of TEs in field trials has not been explored yet. Specifically, the remediation potential of fly ash (FA; as stabilizing agent) and ryegrass (as a TE accumulator) intercropped with a target crop for soil polluted with multiple TEs has not been investigated yet, particularly in long-term field trials. Therefore, in this study, a six-month combined remediation field experiment of FA stabilization and/or ryegrass intercropping (IR) was carried out on the farmland soils contaminated with As, Cd, Cr, Cu, Hg, Ni, Pb and Zn where Zanthoxylumbungeanum (ZB) trees as native crops were grown for years. The treatments include soil cultivated alone with ZB untreated- (control) and treated-with FA (FA), produced by burning lignite in Shaanxi Datong power plant, China, soil cultivated with ZB and ryegrass untreated- (IR) and treated-with FA (FA + IR). This was underpinned by a large-scale survey in Daiziying (China), which showed that the topsoils were polluted by Cd, Cu, Hg and Pb, and that Hg and Pb contents in the Zanthoxylumbungeanum fruits exceeded their allowable limits. The TEs contents in the studied FA were lower than their total element contents in the soil. The DTPA-extractable TEs contents of the remediation modes were as follows: FA < FA + IR < IR < control. Notably, TEs contents in the ZB fruits were lowest under the FA + IR treatment, which were decreased by 27.6% for As, 42.3% for Cd, 16.7% for Cr, 30.5% for Cu, 23.1% for Hg, 15.5% for Ni, 33.2% for Pb and 38.1% for Zn compared with the control treatment. Whereas the FA + IR treatment enhanced TEs contents in ryegrass shoots and roots, and the TEs contents in ryegrass shoots were below their regulatory limits for fodder crops. The findings confirmed that the combined remediation strategy, i.e., FA (with low content of TEs) stabilization effect and intercropping of ZB (target crop) and ryegrass (accumulating plant) could provide a prospective approach to produce target plants within safe TEs thresholds with greater economic benefits, while remediating soils polluted with multiple TEs and mitigating the potential ecological and human health risk. Those results are of great applicable concern.

Recent trends in the use of fly ash for the adsorption of pollutants in contaminated wastewater and soils: Effects on soil quality and plant growth

Fly ash is one of the largest types of industrial wastes produced during the combustion of coal for energy generation. Finding efficient and sustainable solutions for its reuse has been the subject of substantial research worldwide. Here, we review the recent research data related to (i) the use of fly ash as a low-cost adsorbent for pollutants in wastewater and soils and (ii) its implications in soil-plant system. Fly ash showed prominent adsorption capacity for pollutants in water especially when it was activated or applied in composites. In addition to direct pollutant binding in soils, fly ash can enhance the soil pH indirectly increasing metals' immobilization reducing their plant uptake. Its non-selective adsorptive nature may lead to the co-adsorption of nutrients with pollutants which merits to be considered. Owing to its considerable nutrient contents, fly ash can also improve soil fertility and plant growth. The effects of fly ash on soil physico-chemical properties, microbial population and plant growth are critically evaluated. Fly ash can also contain potentially toxic contaminants (toxic metals, hydrocarbons, etc.) which could have harmful impacts on soil health and plant growth. Identifying the levels of inherent pollutants in fly ash is crucial to evaluate its suitability as a soil amendment. Negative effects of fly ash can also be addressed by using co-amendments, biological agents, and most importantly by an adequate calibration (dose and type) of fly ash based on site-specific conditions. Research directions are identified to promote the research regarding its use in wastewater treatment and agriculture.

The preparation of paddy soil amendment using granite and marble waste: Performance and mechanisms

The wastes generated from the mining and processing of granite and marble stone are generally regarded as useless. However, these waste materials were used as the soil amendments for the first time. The functional groups, crystalline structure and micro-morphology of granite and marble wastes amendments (GMWA) were different from the original wastes demonstrated by X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR) and Scanning electron microscope-energy dispersive spectrometer (SEM-EDS) analyses. With the addition of the amendments, the cation exchange capacity, electrical conductivity and nutrient availability of the soil increased, and the extractable heavy metals of the soil reduced significantly. Under the condition of the addition of 3% amendments, 7.0%, 99.9%, 99.7% and 70.5% of Cu, Pb, Zn and Cd in exchangeable fractions in soil were transformed to the more stable Fe-Mn oxides- or carbonates-bounded fractions. Tessier method and correlation analysis showed that the reduction of extractable metals in the acidic paddy soil can be attributed to the adsorption of available SiO₂, the co-precipitation induced by the elevated pH value, the complexation induced by Fe-Mn oxides and the cation exchange induced by mineral nutrients. This study provides a new strategy for resource recovery of waste stones and remediation of heavy metal-contaminated soil.

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

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

Understanding the mechanisms of zeolite in inhibiting Pb accumulation in different rice cultivars (Oryza sativa)

Zeolite is one of the potential passivating amendments for the immobilization of lead (Pb) in contaminated farmland soils. In this study, pot experiments were carried out to investigate the effects and the mechanisms of zeolite on Pb accumulation in two rice cultivars grown in a slightly Pb-contaminated soil. Results showed that Pb content in grains of Zheyou 18 (ZY-18) decreased by the addition of 6 g zeolite kg-1 soil (E6), which can be attributed to the reduction in soil Pb availability, dissolved organic carbon (DOC), water-soluble iron (Fe) and manganese (Mn), and the transfer factor from soil to grain (TFsoil-grain). These reductions were mainly resulting from the significant increase in soil pH, glutathione (GSH), phytochelatins (PCs), and non-protein (NPT) content in rice root, and the decrease in soil redox potential (Eh), due to zeolite addition. Pb content in brown rice of DL-5 was not significantly affected with E6 treatment, whereas it was raised by applying 12 g zeolite kg-1 soil (E12). The increase of Pb content of Donglian 5 (DL-5) grains with E12 treatment can be attributed to more Pb uptake by the root, higher Pb transfer factors (TFs) between various parts of rice, and significant decrease in GSH, PCs, and NPT contents in the root. It is concluded that a suitable rate of zeolite addition can immobilize Pb in slightly Pb-contaminated acidic soil. However, the final immobilization effect also depends on rice cultivars.

A microscopic and quantitative analysis on the separation of chloride ion by fly ash washing: Effect of liquid-to-solid ratio, washing time and temperature

In order to understand the removal characteristics of Cl^- (chloride ion) in the fly ash from municipal solid waste incineration, so as to realize the harmless treatment of fly ash. The fly ash washing treatment experimental system was designed and built. The single factor experiments were used to explore the effect of liquid-to-solid ratio, washing time and temperature on the leaching rate of Cl^- in the fly ash. The best experimental parameters of fly ash washing were obtained. At the same time, the microscopic morphology and crystal phase composition of fly ash before and after washing were explored. The results showed that the maximum removal rate of Cl^- in the fly ash was 88.72% when the liquid-to-solid ratio was 8:1, the washing time was 5 min and the washing temperature was 70 ℃. It can be seen from the scanning electron microscope (SEM) image that most of the irregularly shaped square fly ash particles gradually transformed into regular spherical or elliptical shapes after washing. The X-ray diffractometer (XRD) results proved that the chlorine salt content in the fly ash after washing was significantly reduced, and the CaSO₄•2(H₂O), SiO₂, CaCO₃ and other substances were formed.

Biodegradation and effects of EDDS and NTA on Zn in soil solutions during phytoextraction by alfalfa in soils with three Zn levels

In chelator-enhanced Zn phytoremediation studies, it is crucial to understand how the degradable chelators and the competition from other ions influence the concentration of Zn in soil solutions. This study investigated the biodegradability of two chelators (EDDS: Ethylenediamine-N,N'-disuccinic acid, and NTA: Nitrilotriacetic acid) and their effects on the Zn concentration in the soil solution during the growth of alfalfa (Medicago Sativa L.). The chelators were added at four doses (0, 0.5, 2 and 5 mmol kg^-1) in soils with varying Zn levels (189, 265 and 1496 mg kg^-1). The results showed that the lag phase before EDDS and NTA biodegradation varied from 0 to 7 days in the three soils. EDDS and NTA were completely decomposed within the assessed 57 days regardless of the applied dosage, with a half-life of 1.3-3.0 days in highly Zn-contaminated soil and 4.2-10.8 days in the two other soils. In soil solutions, the change in solubilized Zn was in line with EDDS and NTA degradation kinetics. Cu, Al, Fe and Mn were the main metal ions that competed against Zn for chelation. Besides, Ni competed with Zn in the whole process. Ca did not compete effectively in the three soils, while Mg was a competitor only at the initial stage. Our results show the importance of considering both the biodegradation rate and the competition between the target cation and other elements present in the soil when using chelators to enhance phytoremediation. A 30-day explorative incubation experiment is recommended to evaluate the appropriate application time of chelators and the target Zn exposure time for plants during phytoremediation.

Effect of Combined Soil Amendment on Immobilization of Bioavailable As and Pb in Paddy Soil

Heavy metal pollution in soil can have detrimental effects on soil ecosystems and human health. In situ remediation techniques are widely used to reduce the bioavailable fractions of heavy metals in soil. The main objective of this study was to examine the reduction of the bioavailable fractions of As and Pb in paddy soil with artificial lightweight material (ALM) manufactured from recycled materials. A total of four treatments, including a control (no amendment), ALM10 (10% of ALM in soil), ALM10+L (10% ALM combined with 0.5% lime), and ALM10+FeO (10% ALM combined with 0.5% FeO), were applied to paddy fields, and rice (Oryza sativa L.) was cultivated after 32 weeks. The highest reduction efficiencies for the bioavailable fractions of As and Pb in soil were observed in the ALM10+FeO (52.8%) and ALM10+L treatments (65.7%), respectively. The uptake of As decreased by 52.1% when ALM10+FeO was applied to paddy soil, and that of Pb decreased by 79.7% when ALM10+L was applied. Correlation analysis between bioavailable heavy metals in soil and soil chemical properties showed that soil pH, electrical conductivity (EC), P₂O_5, and soil organic matter (SOM) were the main factors controlling the mobility and bioavailability of As and Pb. Overall, the efficiencies of As and Pb reduction increased synergistically in both soil and plants when FeO and lime were combined with the ALM. In future studies, long-term monitoring is necessary to examine the longevity of soil amendments.

Effects and mechanisms of Cd remediation with zeolite in brown rice (Oryza sativa)

The effects of zeolite amendment on the Cd absorption and accumulation of two varieties of rice, ZY-18 (Zheyou 18, hybrid indica) and DL-5 (Donglian 5, conventional indica), planted in a slightly Cd-contaminated paddy soil, and the associated mechanisms were studied based on a pot experiment. The results showed that zeolite addition significantly reduced the Cd availability in the soil by the increasing of soil pH and the Cd sorption capacity of soil, as well as the reduction of Eh, dissolved organic carbon (DOC) and the water-soluble Fe/Mn in the soil. The decrease in the amount of Fe and Cd in iron plaques on rice roots treated with zeolite resulted in the reduction of Cd uptake by rice roots. Zeolite application decreased the Cd contents in the roots, stems and leaves, and finally the brown rice through the inhibition of uptake and translocation of Cd by rice plant. The Cd concentration in rice grains of ZY-18 was more decreased than that of DL-5, which was attributed to its higher pH and lower Eh, DOC, and Cd availability in the soil, higher mounts of iron plaque and Cd in plaques on rice roots, and lower root-to-grain transfer factor (TF_root-grain) of ZY-18.

Multi-element Interactive Improvement Mechanism of Coal Fly Ash-Based Soil Conditioner on Wheat

Globally, coal fly ash (CFA) is a bulk industrial solid waste that is difficult to be disposed of, which posed serious environmental risks to the atmosphere, water, and soil. Besides, the food crisis outbreaks worldwide. In this case, the utilization of CFA to produce soil amendments is expected to improve the soil quality and to increase the grain yield. This paper took the soil conditioner prepared by chemical activation method as the research object, analyzed, and found out its mechanism when increasing the yield and improving the quality of crops. First, the simulated hydroponics method was used to identify the key yield-increasing factors in the soil conditioner as well as the effects of those factors by taking the plant height, stem thickness, dry weight, and fresh weight of wheat as indicators at the early stage of growth. Then, SPSS was used to analyze the interaction among K, P, and other four middle trace elements in the stem and the leaf of wheat. The results showed that for wheat seedlings, there were strong interactions between Fe and Mg, Mg and Ca, and Ca and Si. Fe had a significant enhancement effect on the fresh weight of wheat seedlings. Mg had a significant enhancement effect on both the fresh weight and dry weight of wheat seedlings. Si can greatly enhance the dry weight and plant height, and Ca can greatly increase the stem thickness. It was also found that the soil conditioner and the basic N, P, and K fertilizer had a good mutual promotion effect. Among the four elements, Mg and Si are the key growth factors. When the nutrient elements were relatively poor, the increase of Mg by 50% would lead to the growth of the fresh weight of wheat seedlings by 65%; when the content of active Si increased by 50%, the fresh weight would increase by 52%. Therefore, the soil conditioner prepared by modified treatment of CFA owns a good application prospect to increase the yield and quality of crops.

Combined passivators regulate the heavy metal accumulation and antioxidant response of Brassica chinensis grown in multi-metal contaminated soils

Passivation of heavy metals is one of the most efficient techniques to remediate soil pollution. However, passivators with single component are usually unsatisfactory in the case of multi-metal contaminated soils. To resolve this problem, a series of combined passivators containing different ratios of Fe-Mn ore, Fe powder, zeolite, bentonite, etc. were designed and used to study their effects on the growth, heavy metal accumulation, and the antioxidant response of Chinese cabbage (Brassica chinensis L.) as well as the soil available forms of heavy metals in a copper refinery's multi-metal (As, Cd, Pb, Cu) contaminated yellow-brown soil and an artificially contaminated (As, Cd, Pb, Cu) calcareous alluvial soil. The results showed that compared with the control, the addition of combined passivators significantly promoted cabbage growth, with the biomass increase up to 1.77 and 3.54 times in yellow-brown soil and calcareous alluvial soil, respectively. The activity of antioxidant enzymes (SOD, CAT, POD) and the content of malondialdehyde (MDA) and glutathione (GSH) decreased, while the chlorophyll content increased significantly, as compared with no passivators. In addition, passivator application decreased As, Cd, Pb, and Cu contents in shoots and roots by 34.8%, 45.6%, 34.9%, and 11.1% and 49.2%, 63.8%, 38.6%, and 46.4% in yellow-brown soil and by 29.8%, 27.3%, 26.8%, and 25.5% and 45.8%, 55.2%, 61.8%, and 5.7% in calcareous alluvial soil, respectively. Besides, the content of soil available heavy metals was reduced by 8.0-17.1% in yellow-brown soil and 3.3-19.1% in calcareous alluvial soil after the application of passivators. The results indicated that the combined passivators formulated in this experiment could efficiently reduce the content of the multi-metals in cabbage and relieve the oxidant stress and could be used as a way to remediate multi-metal polluted soils.

Effect of inorganic additives (rock phosphate, PR and boron waste, BW) on the passivation of Cu, Zn during pig manure composting

The bioavailability of heavy metals in compost is critical for their agronomic value. The effect of inorganic additives (rock phosphate, PR and boron waste, BW) on Copper (Cu) and Zinc (Zn) bioavailability during co-compost of swine manure and rice straw was assessed using sequential extraction procedure (European Community Bureau of Reference). The result showed that both additives, applied at rates of 2.5%-7.5% (w/w) could promote the change of exchangeable Cu and reducible Cu into oxidizable Cu, thereby reducing their bioavailability factor (BF) by 15.5%-47.2%. While additives provided no significant reduction in BF of Zn, the shift from exchangeable Zn into reducible Zn can still reduce the mobility of Zn. Based on redundancy analysis (RDA), organic matter (OM) and electrical conductivity (EC) were identified as the most important controlling factors for redistribution of Cu and Zn fractions during composting. The inorganic additives strengthened the passivation of Cu and Zn bioavailability by stimulating OM degradation. The 7.5% (w/w) rock phosphate showed best passivating effect on the bioavailability of Cu.

Biomass bottom ash & dolomite similarly ameliorate an acidic low-nutrient soil, improve phytonutrition and growth, but increase Cd accumulation in radish

One of negative side-effects of usage of bio-renewables might be generation of mineral (ash) material, potential source of environmental pollution. A hypothesis was that bottom ash (BA; from biomass cogeneration facility) could be efficiently (re) used in soil chemical conditioning similarly to widely-used dolomite-based soil conditioner (DO; from Croatian Dinaric-coastal region) which we tested by: i) physicochemical characterisation of BA and DO, and ii) bioassay with Raphanus sativus cultivated in acidic soil amended with BA or DO. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) confirmed complex chemical/physical structures and morphology between amendments, X-ray diffraction (XRD) showed their distinctive mineralogy with predominantly dolomite (in DO) vs. quartz and calcite (in BA), while secondary ion mass spectrometry (SIMS) revealed their diverse elemental/isotopic composition. The BA or DO amendments ameliorated soil acidity, increased available P, K and most other nutrients, but not Cd. The BA or DO amendments improved vegetative growth and edible hypocotyl yield. However, both amendments also increased Cd accumulation in all radish tissues, which was unexpected given the alkaline matrix of bio-ash and dolomite that would be likely to facilitate retention and immobilisation of toxic Cd. Thus, thorough characterisation and evaluation of BA- and/or DO-based materials and relevant soils (with an emphasis on metal sorption/immobilisation) prior to application in (agro) ecosystems is crucial for producing food clean of toxic metals.

Potential of using a new aluminosilicate amendment for the remediation of paddy soil co-contaminated with Cd and Pb

Cadmium (Cd) and lead (Pb) are toxic heavy metals that impact human health and biodiversity. Removal of Cd/Pb from contaminated soils is a means for maintaining environmental sustainability and biodiversity. In this study, we applied a newly modified material fly ash (NA), zeolite (ZE), and fly ash (FA) to the paddy soils and evaluated the effects of Cd/Pb accumulation in rice via a one-year field experiment. The results showed that the application of NA and ZE enhanced the soil pH and nutrients to a large extent and reduced the availability of Cd/Pb in soil. The Cd and Pb concentrations in rice grains decreased by 32.8% and 62.9%, respectively, with the NA treatments. Similarly, the application of ZE reduced the Cd and Pb concentrations in rice grains by a factor of 27.9% and 63.5%, respectively, which indicates that the amendments can promote the transfer of Cd and Pb from acid-exchangeable fraction to oxidizable and residual fractions. The Cd/Pb showed a significant positive correlation to other metal ions and a negative correlation to the nutrients. Generally, the application of NA and ZE was effective in reducing Cd/Pb accumulation and improving rice yield. Moreover, the NA was more cost-effective than ZE. Hence, this study proves that NA may be a better amendment for remediation of Cd/Pb contaminated soils.

The influences of fly ash on stabilization for Cd in contaminated soils

Soil contaminated with potentially toxic metals (PTMs) has being a global environmental issue, which needs to be addressed on the priority basis. Fly ash (FA) is a kind of low-cost alkaline materials, which has been widely used in remediation of soil contaminated by PTMs, while the effects of FA on the stability for PTMs in contaminated farmland soil are still not clearly evaluated. In this study, cadmium (Cd) contaminated soil samples, collected from Shaanxi (SX), Hubei (HB), and Zhejiang (ZJ) province of China, were amended with FA addition (0, 1%, 2.5%, 5%, and 10% dose), and 1-year changes of Cd availability in soil samples were focused on. In addition, biological assessment method through pot culture was carried out to evaluate the reuse potential of Cd contaminated soils amended by FA. The result indicated that FA had a notable impact on decreasing the Cd mobility of SX soil (sand type), with 18.2~52.1% reduction in the DTPA extractable solution, followed by HB soil with 5.9~16.7% reduction, but no obvious effect of FA on ZJ soil (clay type) was observed. Furthermore, the results of pot experiment revealed that FA application could increase the biomass of Chinese cabbage. However, the DTPA extractable Cd in soils after planation and the Cd accumulation of plant increased. The results revealed that FA was not a promising soil stabilizer to immobilize HMs in Cd contaminated soil, and careful consideration should be given to Cd contaminated soils with FA restoration especially in their using for farmland productive due to the remaining risk of Cd bioavailability. These results also contributed to provide references for similar soil pollution remediation.

Cadmium mobility in three contaminated soils amended with different additives as evaluated by dynamic flow-through experiments

As arable land has become an important sink for cadmium (Cd), soil is being recognized as a major source of metals to the food chain. It becomes, therefore, essential to investigate metal mobility in contaminated soils and to identify suitable remediation strategies. For this, immobilization of Cd was evaluated in contaminated stagnic anthrosol: S1, gleysol: S2 and fluvisol: S3 under flow through conditions. Ten treatments including control were tested alone or in composite form firstly at natural Cd contents (0.58-0.69 mg kg^-1). Here, T2 (lime), T5 (biochar) and T10 (composite amendment) were found better in reducing the Cd concentration in the soils' leachates, so, their efficacy was further investigated in the same soils of higher Cd contents (1 and 2 mg kg^-1 imposed by soil spiking). Amendments significantly reduced the leachate metal contents especially in 1 mg kg^-1 spiked soils. Characterization of T2, T5 and T10 revealed their structural transformations in all the studied soil types, while active functional groups e.g. C-O, CO, O-H, Si-O-Si, ester and alcoholic groups were notably involved in Cd precipitation or adsorption on amendments surface. Variations in Cd speciation in these soils exhibited the exchange of Cd to more stable fractions with tested amendments. These continuous-flow experiments confirmed the strong efficiency of T2, T5 and T10 in reducing the Cd concentration in the leachate of three soils. This study has strong implications in understanding the role of different amendments in controlling the fate, leaching behavior and immobilization of Cd in diverse soil types.

Copper uptake, essentiality, toxicity, detoxification and risk assessment in soil-plant environment

Copper (Cu) is an essential metal for human, animals and plants, although it is also potentially toxic above supra-optimal levels. In plants, Cu is an essential cofactor of numerous metalloproteins and is involved in several biochemical and physiological processes. However, excess of Cu induces oxidative stress inside plants via enhanced production of reactive oxygen species (ROS). Owing to its dual nature (essential and a potential toxicity), this metal involves a complex network of uptake, sequestration and transport, essentiality, toxicity and detoxification inside the plants. Therefore, it is vital to monitor the biogeo-physiochemical behavior of Cu in soil-plant-human systems keeping in view its possible essential and toxic roles. This review critically highlights the latest understanding of (i) Cu adsorption/desorption in soil (ii) accumulation in plants, (iii) phytotoxicity, (iv) tolerance mechanisms inside plants and (v) health risk assessment. The Cu-mediated oxidative stress and resulting up-regulation of several enzymatic and non-enzymatic antioxidants have been deliberated at molecular and cellular levels. Moreover, the role of various transporter proteins in Cu uptake and its proper transportation to target metalloproteins is critically discussed. The review also delineates Cu build-up in plant food and accompanying health disorders. Finally, this review proposes some future perspectives regarding Cu biochemistry inside plants. The review, to a large extent, presents a complete picture of the biogeo-physiochemical behavior of Cu in soil-plant-human systems supported with up-to-date 10 tables and 5 figures. It can be of great interest for post-graduate level students, scientists, industrialists, policymakers and regulatory authorities.

The Effect of a Fly Ash-Based Soil Conditioner on Corn and Wheat Yield and Risk Analysis of Heavy Metal Contamination

The utilization of coal fly ash (CFA) as a soil conditioner has been a research hotspot in recent years. In this paper, the continuity and stability of the yield-increasing effect of a fly ash-based soil conditioner were investigated through field trials for three consecutive years. The yield-increasing effect and applicability were also investigated by planting a variety of crops in different types of soil. Field test results revealed that the new fly ash-based soil conditioner had wide adaptability and stability under the condition of reducing the application of traditional fertilizers by 20%. For corns grown in castano-cinnamon soil and dark brown soil, the application of the new fly ash-based soil conditioner for three consecutive years has steadily increased the yield by more than 10%. The yield of wheat planted in castano-cinnamon soil also increased by 15%. Additionally, considering that heavy metals (HMs) contained in the fly ash-based soil conditioner may cause environmental contamination, this paper calculated and analyzed the bio-concentration factor (BCF) and the translocation factor (TF) of Cr, As, Cd, Hg and Pb in the corn experimental fields. The results showed that the five heavy metals were not significantly enriched in various parts of the corn stalk after application of the soil conditioner, and there was temporarily no environmental risk in terms of returning straws to the field or raising livestock.

Simultaneous reduction of arsenic and cadmium bioavailability in agriculture soil and their accumulation in Brassica chinensis L. by using minerals

In situ immobilization of heavy metal cations in contaminated soil using natural minerals is an attractive remediation technique. However, little research has focused on the remediation of arsenic (As) and cadmium (Cd) co-contaminated. In this work, three different crystal structures and chemical compositions minerals, zeolite; bentonite; and dolomite, were applied to simultaneously reduce the uptake of As and Cd in Brassica chinensis L., and the mechanism on reducing As and Cd bioavailability in soil were also investigated. The results showed that the three minerals decreased the bioavailability of As and Cd and restrained their uptake by Brassica chinensis L. with the order followed bentonite > zeolite > dolomite. Particularly, bentonite decreased the exchangeable As and Cd by 4.05% and 32.5% and the concentrations of As and Cd in shoots of Brassica chinensis L. by 36.2% and 64.6%, as compared with the controls. Moreover, with the addition of minerals increased, the dry biomass of Brassica chinensis L. and the rhizosphere microbial functional diversity increased significantly, and the highest biomass increased by 289% at 4.0% addition of bentonite. Correlation analysis indicated that the uptake of As and Cd was positive with the available Cd and As in soil, and was negative with soil pH and available N. Furthermore, the Scanning Electron Microscopy-Energy Dispersive Spectroscopy and Fourier Transform Infrared Spectroscopy analysis illustrated the interaction between minerals and Cd mainly involved ion-exchange and adsorption, while As was mainly immobilized by calcium and magnesium through forming precipitation. In conclusion, this present study implied that the bentonite can be recommended as the more effective amendment to immobilize metal (loid)s in soil and thereby reduce the exposure risk of metal (loid)s associated with grains consumption.

Direct and residual impacts of zeolite on the remediation of harmful elements in multiple contaminated soils using cabbage in rotation with corn

In the present work, in-situ two pot trials were conducted to explore the direct and residual influences of zeolite (ZL) on plant height, dry biomass and bioavailability of Pb, Cd, Cu, and Zn by growing cabbage followed by corn in goldmine-contaminated (GM-C), smelter factory-contaminated (SF-C), and farmland-contaminated (FL-C) soils. Initially, a single treatment of ZL was applied at 20 t/ha, and cabbage was grown under greenhouse pot conditions. After cabbage harvesting, corn was grown in the same pots without additional application of ZL. The results indicated that ZL as an amendment evidently promoted the cabbage and corn yields, whereas the residual influence of ZL did not promote corn dry matter yield in SF-C and FL-C soils compared to CK. Incorporation of ZL potentially decreased the mobility of Pb, Cd, Cu and Zn in contaminated soils after harvesting cabbage and corn compared with CK. In both crops, the Pb, Cd, Cu and Zn contents in plants root and shoot biomasses were dramatically reduced by the direct and residual impacts of ZL rather than CK. This study highlights that the direct and residual influences of ZL at a 20 t/ha application rate have the possibility to support the reclamation of soils polluted with harmful elements and that, by itself, ZL can promote plant growth and increase the value of field crops. The detailed studied regarding residual influence of ZL for restoration of multi-metal polluted soils would be confirmed at the ex-situ condition.

Prospects and applications of plant growth promoting rhizobacteria to mitigate soil metal contamination: A review

The rapid increase in world population has generated the issues of hunger, poverty, food insecurity and malnutrition. To meet the challenge of increased food production of better quality, the farmers were compelled to use more chemical fertilizers, especially in developing countries. The higher use of chemical fertilizers interrupts the food chain through eutrophication, the polluting air and soil by incorporating metals. Trace metals have a deleterious effect on soil microbial and plant growth. To minimize metal toxicity and maximize the production of food, there are different approaches that can lead to lessen the use of chemical fertilizers. Plant growth promoting rhizobacteria (PGPR) are capable to enhance the plant growth and can remediate metal contaminated soils. PGPR has the ability to improve food production with diverse attributes e.g. producing siderophores that promote rhizosphere trace metal sequestration and production of organic and inorganic acids thus affecting trace metal bioavailability and plant induced systemic tolerance (IST) to limit the crop metal accumulation. In this review paper, we have discussed the biological approach which is environmentally friendly and cost-effective mean for metal polluted soils and gives some new insights for safety use of PGPR in trace metal contaminated fields.

Carboxylated graphene oxide-chitosan spheres immobilize Cu2+ in soil and reduce its bioaccumulation in wheat plants

Due to the strong interaction with pollutants and the huge adsorption capacity, graphene adsorbents are widely applied in water decontamination. However, graphene adsorbents are seldom used in soil remediation, because the adsorptive sites on graphene would be occupied by soil components. In this study, we prepared carboxylated graphene oxide-chitosan (GO-COOH/CS) spheres for the immobilization of Cu²⁺ from water and soil. The pores in GO-COOH/CS allowed the internal diffusion of Cu²⁺ solution, while they blocked the direct contact between the solid soil and the adsorptive sites on graphene sheets. Therefore, the high adsorption capacity of GO-COOH/CS spheres (78 mg/g) was largely retained for the soil Cu²⁺ fixation. The partition coefficient (PC) for Cu²⁺ adsorption onto GO-COOH/CS spheres was 4.2 mg/g/μM at C_e of 0.48 mg/L and q_e of 31 mg/g, while the PC value decreased to 0.096 mg/g/μM at C_e of 91.4 mg/L and q_e of 78 mg/g. At initial Cu²⁺ concentrations of 120 mg/L and lower, the fixation efficiencies were all higher than 99% and the corresponding free Cu²⁺ concentrations in leachates were lower than 1.0 mg/L. The Cu²⁺ fixation on GO-COOH/CS spheres largely reduced its bioaccumulation in wheat roots from 127.8 μg/g to 51.2 μg/g. The toxicity evaluations suggested that GO-COOH/CS spheres were of low toxicity to wheat seedlings and did not amplify the toxicity of Cu²⁺. The implications to the design of graphene adsorbents for soil remediation are discussed. Overall, our results collectively indicated that porous GO-COOH/CS spheres were high-performance adsorbents for the immobilization of Cu²⁺ to reduce Cu²⁺ bioaccumulation in plants.