Effect of nanoparticle hydroxyapatite on the immobilization of Cu and Zn in polluted soil

An assessment of nanotechnology-based interventions for cleaning up toxic heavy metal/metalloid-contaminated agroecosystems: Potentials and issues

Heavy metals (HMs) are among the most dangerous environmental variables for a variety of life forms, including crops. Accumulation of HMs in consumables and their subsequent transmission to the food web are serious concerns for scientific communities and policy makers. The function of essential plant cellular macromolecules is substantially hampered by HMs, which eventually have a detrimental effect on agricultural yield. Among these HMs, three were considered, i.e., arsenic, cadmium, and chromium, in this review, from agro-ecosystem perspective. Compared with conventional plant growth regulators, the use of nanoparticles (NPs) is a relatively recent, successful, and promising method among the many methods employed to address or alleviate the toxicity of HMs. The ability of NPs to reduce HM mobility in soil, reduce HM availability, enhance the ability of the apoplastic barrier to prevent HM translocation inside the plant, strengthen the plant's antioxidant system by significantly enhancing the activities of many enzymatic and nonenzymatic antioxidants, and increase the generation of specialized metabolites together support the effectiveness of NPs as stress relievers. In this review article, to assess the efficacy of various NP types in ameliorating HM toxicity in plants, we adopted a 'fusion approach', in which a machine learning-based analysis was used to systematically highlight current research trends based on which an extensive literature survey is planned. A holistic assessment of HMs and NMs was subsequently carried out to highlight the future course of action(s).

Arsenic and cadmium simultaneous immobilization in arid calcareous soil amended with iron-oxidizing bacteria and organic fertilizer

Immobilization stands as the most widely adopted remediation technology for addressing heavy metal(loid) contamination in soil. However, it is crucial to acknowledge that this process does not eliminate pollutants; instead, it confines them, potentially leaving room for future mobilization. Presently, our comprehension of the temporal variations in the efficacy of immobilization, particularly in the context of its applicability to arid farmland, remains severely limited. To address this knowledge gap, our research delves deep into the roles of iron-oxidizing bacteria (FeOB) and organic fertilizer (OF) in the simultaneous immobilization of arsenic (As) and cadmium (Cd) in soils. We conducted laboratory incubation and field experiments to investigate these phenomena. When OF was combined with FeOB, a noteworthy transformation of available As and Cd into stable species, such as the residual state and combinations with Fe-Mn/Al oxides, was observed. This transformation coincided with changes in soil properties, including pH, Eh, soluble Fe, and dissolved organic carbon (DOC). Furthermore, we observed synergistic effects between available As and Cd when treated with bacteria and OF individually. The stabilization efficiency of As and Cd, as determined by the Toxicity Characteristic Leaching Procedure, reached its highest values at 33.39 % and 24.67 %, respectively, after 120 days. Nevertheless, the formation of iron‑calcium complexes was disrupted due to pH fluctuations. Hence, long-term monitoring and model development are essential to enhance our understanding of the remediation process. The application of organic fertilizer and the use of FeOB in calcareous soil hold promise for the restoration of polluted soil and the maintenance of soil health by mitigating the instability of heavy metals(loid).

New insights into the environmental application of hybrid nanoparticles in metal contaminated agroecosystem: A review

Heavy metals (HMs) contamination in agricultural soils is a major constraint to provide safe food to society. Cultivation of food crops on these soils, channels the HMs into the food chain and causes serious human health and socioeconomic problems. Multiple conventional and non-conventional remedial options are already in practice with variable success rates, but nanotechnology has proved its success due to higher efficiency. It also led the hypothesis to use hybrid nanoparticles (HNPs) with extended benefits to remediate the HMs and supplement nutrients to enhance the crop yield in the contaminated environments. Hybrid nanoparticles are defined as exclusive chemical conjugates of inorganic and/or organic nanomaterials that are combinations of two or more organic components, two or more inorganic components, or at least one of both types of components. HNPs of different elements like essential nutrients, beneficial nutrients and carbon-based nanoparticles are used for the remediation of metals contaminated soil and the production of metal free crops. Characterizing features of HNPs including particle size, surface area, reactivity, and solubility affect the efficacy of these HNPs in the contaminated environment. Hybrid nanoparticles have great potential to remove the HMs ions from soil solution and restrict their ingress into the root tissues. Furthermore, HNPs of essential nutrients not only compete with heavy metal uptake by plants but also fulfill the need of nutrients. This review provides a comprehensive overview of the challenges associated with application of HNPs in contaminated soils, environmental implications, their remediation ability, and factors affecting their dynamics in environmental matrices.

Co-exposure of sulfur nanoparticles and Cu alleviate Cu stress and toxicity to oilseed rape Brassica napus L

Experiments were performed to explore the impact of sulfur nanoparticles (SNPs) on growth, Cu accumulation, and physiological and biochemical responses of oilseed rape (Brassica napus L.) inoculated with 5 mg/L Cu-amended MS medium supplemented with or without 300 mg/L SNPs exposure. Cu exerted severe phytotoxicity and inhibited plant growth. SNPs application enhanced the shoot height, root length, and dry weight of shoot and root by 34.6%, 282%, 41.7% and 37.1%, respectively, over Cu treatment alone, while the shoot and root Cu contents and Cu-induced lipid perodixation as the malondialdehyde (MDA) levels in shoots and roots were decreased by 37.6%, 35%, 28.4% and 26.8%. Further, the increases in superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and glutathione S-transferase (GST) enzyme activities caused by Cu stress were mitigated in shoots (10.9%-37.1%) and roots (14.6%-35.3%) with SNPs addition. SNPs also positively counteracted the negative effects on shoot K, Ca, P, Mg, Mn, Zn and Fe contents and root K, Ca, Mg and Mn contents from Cu exposure alone, and significantly promoted the nutrients accumulation in plant. Additionally, in comparison with common bulk sulfur particles (BSPs) and sulfate, SNPs showed more positive effects on promoting growth in shoots (6.7% and 19.5%) and roots (10.9% and 15.1%), as well as lowering the shoot Cu content (40.1% and 43.3%) under Cu stress. Thus, SNPs application has potential to be a green and sustainable technology for increasing plant productivity and reducing accumulation of toxic metals in heavy metal polluted soils.

Effects of Various Ripening Media on the Mesoporous Structure and Morphology of Hydroxyapatite Powders

Mesoporous hydroxyapatite (HA) materials demonstrate advantages as catalysts and as support systems for catalysis, as adsorbent materials for removing contamination from soil and water, and as nanocarriers of functional agents for bone-related therapies. The present research demonstrates the possibility of the enlargement of the Brunauer-Emmett-Teller specific surface area (SSA), pore volume, and average pore diameter via changing the synthesis medium and ripening the material in the mother solution after the precipitation processes have been completed. HA powders were investigated via chemical analysis, X-ray diffraction analysis, Fourier-transform IR spectroscopy, transmission electron microscopy (TEM), and scanning (SEM) electron microscopy. Their SSA, pore volume, and pore-size distributions were determined via low-temperature nitrogen adsorption measurements, the zeta potential was established, and electron paramagnetic resonance (EPR) spectroscopy was performed. When the materials were synthesized in water-ethanol and water-acetone media, the SSA and total pore volume were 52.1 m²g^-1 and 116.4 m²g^-1, and 0.231 and 0.286 cm³g^-1, respectively. After ripening for 21 days, the particle morphology changed, the length/width aspect ratio decreased, and looser and smaller powder agglomerates were obtained. These changes in their characteristics led to an increase in SSA for the water and water-ethanol samples, while pore volume demonstrated a multiplied increase for all samples, reaching 0.593 cm³g^-1 for the water-acetone sample.

Remediation Agents Drive Bacterial Community in a Cd-Contaminated Soil

Soil remediation agents (SRAs) such as biochar and hydroxyapatite (HAP) have shown a promising prospect in in situ soil remediation programs and safe crop production. However, the effects of SRAs on soil microbial communities still remain unclear, particularly under field conditions. Here, a field case study was conducted to compare the effects of biochar and HAP on soil bacterial communities in a slightly Cd-contaminated farmland grown with sweet sorghum of different planting densities. We found that both biochar and HAP decreased the diversity and richness of soil bacteria, but they differently altered bacterial community structure. Biochar decreased Chao1 (-7.3%), Observed_species (-8.6%), and Shannon indexes (-1.3%), and HAP caused Shannon (-2.0%) and Simpson indexes (-0.1%) to decline. The relative abundance (RA) of some specific taxa and marker species was differently changed by biochar and HAP. Overall, sweet sorghum cultivation did not significantly alter soil bacterial diversity and richness but caused changes in the RA of some taxa. Some significant correlations were observed between soil properties and bacterial abundance. In conclusion, soil remediation with biochar and HAP caused alterations in soil bacterial communities. Our findings help to understand the ecological impacts of SRAs in soil remediation programs.

Inconsistent effects of a composite soil amendment on cadmium accumulation and consumption risk of 14 vegetables

Organic and inorganic mixtures can be developed as immobilizing agents that could reduce heavy metal accumulation in crops and contribute to food safety. Here, inorganic materials (lime, L; zeolite, Z; and sepiolite, S) and organic materials (biochar, B, and compost, C) were selectively mixed to produce six composite soil amendments (LZBC, LSBC, LZC, LZB, LSC, and LSB). Given the fact that LZBC showed the best performance in decreasing soil Cd availability in the incubation experiment, it was further applied in the field condition with 14 vegetables as the test crops to investigate its effects on crop safety production in polluted greenhouse. The results showed that LZBC addition elevated rhizosphere soil pH by 0.1-2.0 units and reduced soil Cd availability by 1.85-37.99%. Both the biomass and the yields of edible parts of all vegetables were improved by LZBC addition. However, LZBC addition differently affected Cd accumulation in edible parts of the experimental vegetables, with the observation that Cd contents were significantly reduced in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., but increased in the three species of Lactuca sativa. Further health risk assessment showed that LZBC application significantly decreased daily intake of metal (DIM), health risk index (HRI), and target hazard quotient (THQ) for Cd in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., whereas increased all the indexes in Lactuca sativa. Our results showed that the effect of a composite amendment on Cd accumulation in different vegetables could be divergent and species-dependent, which suggested that it is essential to conduct a pre-experiment to verify applicable species for a specific soil amendment designed for heavy metal immobilization.

Effects of Amendments and Indigenous Microorganisms on the Growth and Cd and Pb Uptake of Coriander (Coriandrum sativum L.) in Heavy Metal-Contaminated Soils

Heavy metal (HM) contamination of soils is a worldwide problem with adverse consequences to the environment and human health. For the safe production of vegetables in contaminated soil, efficient soil amendments need to be applied such as nano-hydroxyapatite (n-HAP) and poly γ-glutamic acid (γ-PGA), which can mitigate heavy metal uptake and enhance crop yield. However, the combined effects of soil amendments and indigenous microorganisms (IMOs) on HMs immobilisation and accumulation by crops have received little attention. We established a pot experiment to investigate the effects of IMOs combined with n-HAP and γ-PGA on coriander (Coriandrum sativum L.) growth and its Cd and Pb uptake in two acidic soils contaminated with HMs. The study demonstrated that applying n-HAP, with and without IMOs, significantly increased shoot dry biomass and reduced plant Cd and Pb uptake and diethylenetriaminepentaacetic acid (DTPA) extractable Cd and Pb concentrations in most cases. However, γ-PGA, with and without IMOs, only reduced soil DTPA-extractable Pb concentrations in slightly contaminated soil with 0.29 mg/kg Cd and 50.9 mg/kg Pb. Regardless of amendments, IMOs independently increased shoot dry biomass and soil DTPA-extractable Cd concentrations in moderately contaminated soil with 1.08 mg/kg Cd and 100.0 mg/kg Pb. A synergistic effect was observed with a combined IMOs and n-HAP treatment, where DTPA-extractable Cd and Pb concentrations decreased in slightly contaminated soil compared with the independent IMOs and n-HAP treatments. The combined treatment of γ-PGA and IMOs substantially increased shoot dry biomass in moderately contaminated soil. These results indicate that solo n-HAP enhanced plant growth and soil Cd and Pb immobilisation, and mitigated Cd and Pb accumulation in shoots. However, the combination of n-HAP and IMOs was optimal for stabilising and reducing HMs' uptake and promoting plant growth in contaminated soil, suggesting its potential for safe crop production.

Oxalic acid activated bone meal for immobilization of Pb and Cd contaminated soils

Bone meal (BM) is a cost-effective and low-carbon material to remediate heavy metal contaminated soils. Moreover, its immobilization efficiency for heavy metals still requires improvement. This study aimed to assess the activation effect of oxalic acid on the BM to develop an oxalic acid-activated bone meal (ABM) for improving immobilization efficiency. Several series of tests, including the available phosphorus content test, toxicity characteristic leaching procedure (TCLP), modified European Community Bureau of Reference (BCR) sequential extraction procedure, and X-ray diffraction (XRD) analysis, are used to investigate the effect of activation on the immobilization ability and chemical speciation of lead (Pb) and cadmium (Cd) in soils and the different mechanisms of Pb/Cd immobilization using the ABM and BM. The results indicate that the ABM possesses a higher solubility than the BM. The activation of BM achieves optimal effect when using 1 mol/L oxalic acid solution with a liquid-solid ratio of 2:1. The TCLP and BCR test results show that the ABM significantly outperforms the BM in terms of Pb immobilization. The leaching concentration of Pb from ABM immobilized soils can meet regulatory limits in China and the USA, and it is also 30 to 75% lower than that from BM immobilized soils. Regarding Cd immobilization, ABM outperforms BM after 90 days of curing. The XRD analysis shows that heavy metal phosphates are the primary products of Pb and Cd immobilized by ABM, whereas heavy metal carbonates are the main products after the immobilization by BM.

Application of Phosphate Materials as Constructed Wetland Fillers for Efficient Removal of Heavy Metals from Wastewater

Constructed wetlands are an environmentally friendly and economically efficient sewage treatment technology. Heavy metals (HMs) removal is always regarded as one of the most important tasks in constructed wetlands, which have aroused increasing concern in the field of contamination control in recent times. The fillers of constructed wetlands play an important role in HMs removal. However, traditional wetland fillers (e.g., zeolite, sand, and gravel) are known to be imperfect because of their low adsorption capacity. Regarding HMs removal, our work involved the selection of prominent absorbents, the evaluation of adsorption stability for various treatments, and then the possibility of applying this HM removal technology to constructed wetlands. For this purpose, several phosphate materials were tested to remove the heavy metals Cu and Zn. Three good phosphates including hydroxyapatite (HAP), calcium phosphate (CP), and physic acid sodium salt hydrate (PAS) demonstrated fast removal efficiency of HMs (Cu²⁺, Zn²⁺) from aqueous solution. The maximum removal rates of Cu²⁺ and Zn²⁺ by HAP, CP, and PAS reached 81.6% and 95.8%; 66.9% and 70.4%; 98.8% and 1.99%, respectively. In addition, better adsorption stability of these heavy metals was found to occur with a wide variation of desorption time and pH range. The most remarkable efficiency for heavy metal removal among tested phosphates was PAS, followed by HAP and CP. This study can provide a basis for the application of HMs removal in manmade wetland systems.

Effects of nanoparticles on trace element uptake and toxicity in plants: A review

Agricultural soils are receiving higher inputs of trace elements (TEs) from anthropogenic activities. Application of nanoparticles (NPs) in agriculture as nano-pesticides and nano-fertilizers has gained rapid momentum worldwide. The NPs-based fertilizers can facilitate controlled-release of nutrients which may be absorbed by plants more efficiently than conventional fertilizers. Due to their large surface area with high sorption capacity, NPs can be used to reduce excess TEs uptake by plants. The present review summarizes the effects of NPs on plant growth, photosynthesis, mineral nutrients uptake and TEs concentrations. It also highlights the possible mechanisms underlying NPs-mediated reduction of TEs toxicity at the soil and plant interphase. Nanoparticles are effective in immobilization of TEs in soil through alteration of their speciation and improving soil physical, chemical, and biological properties. At the plant level, NPs reduce TEs translocation from roots to shoots by promoting structural alterations, modifying gene expression, and improving antioxidant defense systems. However, the mechanisms underlying NPs-mediated TEs uptake and toxicity reduction vary with NPs type, mode of application, time of NPs exposure, and plant conditions (e.g., species, cultivars, and growth rate). The review emphasizes that NPs may provide new perspectives to resolve the problem of TEs toxicity in crop plants which may also reduce the food security risks. However, the potential of NPs in metal-contaminated soils is only just starting to be realized, and additional studies are required to explore the mechanisms of NPs-mediated TEs immobilization in soil and uptake by plants. Such future knowledge gap has been highlighted and discussed.

Effect of a low-cost and highly efficient passivator synthesized by alkali-fused fly ash and swine manure on the leachability of heavy metals in a multi-metal contaminated soil

Soil pollution, caused by heavy metals, is an environmental problem that requires an urgent solution in China. Chemical passivation is a technology that uses various passivators to reduce the availability of heavy metals in soil and realize the remediation of contaminated soil. In this study, we examined the effects of fly ash (FA), alkali-fused fly ash (AFFA), swine manure biochar (SB), and modifying biochar (MB) on the leachability of Cu, Zn, Pb, and Cd via soil culture experiments. The results showed that the addition of AFFA, SB, and MB significantly reduced the extractable contents of Cu, Pb, and Cd in the soil. AFFA and MB had the best passivation effect, followed by SB and FA. The passivation effect on Pb was the best, followed by that on Cu and Cd. AFFA modification significantly improved the passivation effect of MB on Cu, Pb, and Cd in composite contaminated soil. With the addition of 3% MB, the Pb, Cu, and Cd extracted by TCLP decreased by 95.7, 74.1, and 59.1%, respectively. Correlation analysis, Fourier transform infrared spectrometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy showed that the passivation mechanism is mainly due to an increase in the soil pH, silicate content, and cohesiveness. The soil culture experiments in this study proved that MB is a low-cost and highly efficient organic-inorganic composite passivator for multi-metal contaminated soils.

Zinc oxide nanoparticles alleviates the adverse effects of cadmium stress on Oryza sativa via modulation of the photosynthesis and antioxidant defense system

Cadmium (Cd) is a trace element causing severe toxicity symptoms in plants, besides posing hazardous fitness issue due to its buildup in the human body through food chain. Nanoparticles (NPs) are recently employed as a novel strategy to directly ameliorate the Cd stress and acted as nano-fertilizers. The intend of the current study was to explore the effects of zinc oxide nanoparticles (ZnO-NPs; 50 mg/L) on plant growth, photosynthetic activity, elemental status and antioxidant activity in Oryza sativa (rice) under Cd (0.8 mM) stress. To this end, the rice plants are treated by Cd stress at 15 days after sowing (DAS), and the treatment was given directly into the soil. Supply of ZnO-NPs as foliar spray was given for five consecutive days from 30 to 35 DAS, and sampling was done at 45 DAS. However, rice plants supplemented with ZnO-NPs under the Cd toxicity revealed significantly increased shoot length (SL; 34.0%), root fresh weight (RFW; 30.0%), shoot dry weight (SDW; 23.07%), and root dry weight (RDW; 12.24%). Moreover, the ZnO-NPs supplement has also positive effects on photosynthesis related parameters, SPAD value (40%), chloroplast structure, and qualitatively high fluorescence observed by confocal microscopy even under Cd stress. ZnO-NPs also substantially prevented the increases of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) triggered by Cd. Physiological and biochemical analysis showed that ZnO-NPs increased enzymatic activities of superoxide dismutase (SOD; 59%), catalase (CAT; 52%), and proline (17%) that metabolize reactive oxygen species (ROS); these increases coincided with the changes observed in the H₂O₂ and MDA accumulation after ZnO-NPs application. In conclusion, ZnO-NPs application to foliage has great efficiency to improve biomass, photosynthesis, protein, antioxidant enzymes activity, mineral nutrient contents and reducing Cd levels in rice. This can be attributed mainly from reduced oxidative damage resulted due to the ZnO-NPs application.

Nanoremediation technologies for sustainable remediation of contaminated environments: Recent advances and challenges

A major and growing concern within society is the lack of innovative and effective solutions to mitigate the challenge of environmental pollution. Uncontrolled release of pollutants into the environment as a result of urbanisation and industrialisation is a staggering problem of global concern. Although, the eco-toxicity of nanotechnology is still an issue of debate, however, nanoremediation is a promising emerging technology to tackle environmental contamination, especially dealing with recalcitrant contaminants. Nanoremediation represents an innovative approach for safe and sustainable remediation of persistent organic compounds such as pesticides, chlorinated solvents, brominated or halogenated chemicals, perfluoroalkyl and polyfluoroalkyl substances (PFAS), and heavy metals. This comprehensive review article provides a critical outlook on the recent advances and future perspectives of nanoremediation technologies such as photocatalysis, nano-sensing etc., applied for environmental decontamination. Moreover, sustainability assessment of nanoremediation technologies was taken into consideration for tackling legacy contamination with special focus on health and environmental impacts. The review further outlines the ecological implications of nanotechnology and provides consensus recommendations on the use of nanotechnology for a better present and sustainable future.

Effects of aging on surface properties and endogenous copper and zinc leachability of swine manure biochar and its composite with alkali-fused fly ash

Biochar aging is a key factor leading to the decline of biochar stability and the release of endogenous pollutants. This study investigated the effects of five artificial and simulated aging processes on the surface properties and endogenous copper (Cu) and zinc (Zn) leachability of swine manure biochar and its composite with alkali-fused fly ash. Aging obviously reduced carbon (C) content on the surface of swine manure biochar and increased oxygen (O) content. Among all the aging treatments, high-temperature aging had the greatest effect on C content. Following the aging treatments, the C-C bond contents on the surfaces of swine manure biochar decreased significantly, whereas the C-O bonds increased significantly; however, there were less changes in the amounts of C-C and C-O bonds on the surfaces of modified biochar than on swine manure biochar. Aging significantly enhanced the leaching toxicity of Cu and Zn, and Zn availability and bioaccessibility in swine manure biochar and modified biochar. However, it minimized Cu availability and bioaccessibility, especially under high-temperature aging. Greater amounts of Zn than Cu were extracted from swine manure biochar and modified biochar. However, under all the aging treatments, the leaching toxicity, availability, and bioaccessibility of Cu and Zn in modified biochar were significantly lower than in swine manure biochar. This implies that modified biochar application poses lower environmental risks than swine manure biochar.

Effects of an additive (hydroxyapatite-bentonite-biochar) on Cd and Pb stabilization and microbial community composition in contaminated vegetable soil

A two-year pot experiment was conducted with a pimiento-celery cabbage (Capsicum annuum L.-Brassica pekinensis) rotation in acidic soil contaminated with Cd and Pb, which was amended with 0.0, 1.0, 2.5, 5.0 and 10.0% (w/w) premixtures of hydroxyapatite, bentonite and biochar combinations (HTB, in a ratio of 1 : 2 : 2). The results showed that the application of HTB at 2.5-10.0% significantly increased soil pH and organic carbon by an average of 10.38-17.60% and 35.60-55.34% during the two years, respectively. Compared to the control treatment, 1.0-10.0% HTB decreased the available Cd and Pb concentrations by 40.92-77.53% and 41.60-82.79% on average, respectively. In addition, the diversity and richness of the soil bacterial community improved after the two-year application of HTB. The relative abundances of Acidobacteria, Bacteroidetes and Chloroflexi increased under the HTB treatments, while those of Proteobacteria and Actinobacteria decreased. Redundancy analysis (RDA) and regression analysis indicated that soil pH and Cd and Pb availability were important factors shaping the soil bacterial community. The Cd and Pb concentrations in the edible parts of pimiento and celery cabbage decreased as the HTB application rate increased and met the Food Quality Standard in each season when the HTB application rate was 5.0% or higher. Higher rates of HTB (5.0% and 10.0%) not only ensured the quality of vegetables, but also significantly promoted pimiento and celery cabbage growth. Overall, these results indicated that the application of HTB, especially at a rate of 5.0%, could be an effective way to immobilize Cd and Pb, improve soil quality and ensure vegetables produced in acidic contaminated soil are safe for human consumption.

Application of Nanoparticles Alleviates Heavy Metals Stress and Promotes Plant Growth: An Overview

Nanotechnology is playing a significant role in addressing a vast range of environmental challenges by providing innovative and effective solutions. Heavy metal (HM) contamination has gained considerable attention in recent years due their rapidly increasing concentrations in agricultural soil. Due to their unique physiochemical properties, nanoparticles (NPs) can be effectively applied for stress alleviation. In this review, we explore the current status of the literature regarding nano-enabled agriculture retrieved from the Web of Science databases and published from January 2010 to November 2020, with most of our sources spanning the past five years. We briefly discuss uptake and transport mechanisms, application methods (soil, hydroponic and foliar), exposure concentrations, and their impact on plant growth and development. The current literature contained sufficient information about NPs behavior in plants in the presence of pollutants, highlighting the alleviation mechanism to overcome the HM stress. Furthermore, we present a broad overview of recent advances regarding HM stress and the possible mechanism of interaction between NPs and HM in the agricultural system. Additionally, this review article will be supportive for the understanding of phytoremediation and micro-remediation of contaminated soils and also highlights the future research needs for the combined application of NPs in the soil for sustainable agriculture.

Effects of Soil Amendments on Heavy Metal Immobilization and Accumulation by Maize Grown in a Multiple-Metal-Contaminated Soil and Their Potential for Safe Crop Production

Soil amendments have been proposed for immobilizing metallic contaminants, thus reducing their uptake by plants. For the safe production of crops in contaminated soil, there is a need to select suitable amendments that can mitigate heavy metal uptake and enhance crop yield. The present experiment compared the effects of three amendments, hydroxyapatite (HAP), organic manure (OM), and biochar (BC), on plant growth and heavy metal accumulation by maize in an acidic soil contaminated with Cd, Pb, and Zn, and their potential for safe crop production. Toxicity characteristic leaching procedure (TCLP) tests, energy dispersive X-ray spectroscopy (EDS) analysis, and X-ray diffraction (XRD) analysis were used to evaluate the effectiveness and mechanisms of heavy metal immobilization by the amendments. The results showed that shoot and root biomass was significantly increased by HAP and 1% OM, with an order of 1% HAP > 0.1% HAP > 1% OM, but not changed by 0.1% OM and BC (0.1% and 1%). HAP significantly decreased Cd, Pb, and Zn concentrations in both shoots and roots, and the effects were more pronounced at the higher doses. OM decreased the shoot Cd and Pb concentrations and root Zn concentrations, but only 1% OM decreased the shoot Zn and root Pb concentrations. BC decreased the shoot Cd and Pb concentrations, but decreased the shoot Zn and root Pb concentrations only at 1%. HAP decreased the translocation factors (TFs) of Cd, Pb, and Zn (except at the 0.1% dose). OM and BC decreased the TFs of Cd and Zn, respectively, at the 1% dose but showed no significant effects in other cases. Overall, plant P, K, Fe, and Cu nutrition was improved by HAP and 1% OM, but not by 0.1 OM and BC. Soil pH was significantly increased by HAP, 1% OM, and 1% BC, following an order of 1% HAP > 1% OM > 0.1% HAP > 1% BC. The TCLP levels for Cd, Pb, and Zn were significantly reduced by HAP, which can be partly attributed to its liming effects and the formation of sparingly soluble Cd-, Pb-, and Zn-P-containing minerals in the HAP-amended soils. To some extent, all the amendments positively influenced plant and soil traits, but HAP was the optimal one for stabilizing heavy metals, reducing heavy metal uptake, and promoting plant growth in the contaminated soil, suggesting its potential for safe crop production.

Effects of hematite on the stabilization of copper, cadmium and phosphorus in a contaminated red soil amended with hydroxyapatite

Iron (Fe) oxides are intimately coupled with phosphorus and closely associated with the bioavailability of potential toxic elements (PTEs) in soil. Thus, Fe oxides may influence the stabilization of PTEs in contaminated soils amended by phosphorus. To evaluate the effects of hematite (HMT) on the stabilization of PTEs, 1-5% (by weight) of HMT was added into a contaminated red soil amended with hydroxyapatite (HAP) to simulate naturally occurring Fe oxides. The stabilization efficiencies of soil copper (Cu) and cadmium (Cd) amended with HAP in soils with low, moderate, and high content of HMT were assessed after a 60-day incubation. HAP treated the soil with high rate HMT decreased the CaCl₂-extractable and acid-soluble fractions of Cu and Cd than that of HAP alone. In particular, CaCl₂-extactable Cu and Cd in the soil with 5% HMT amended by HAP were 91-95% and 41-68% lower than those amended with only HAP. High content of HMT in soil could decrease the concentration of labile phosphorus in the presence of HAP, but it did not increase the concentration of NaOH-extractable inorganic phosphorus (the fraction bound to Fe oxides). The concentrations of free and crystalline Fe oxides were significantly increased by adding high dosages of HMT with or without HAP. High content of HMT in soil amended by HAP reduced metal phytotoxicity and uptake by wheat shoots than the soil containing HAP without HMT. The results indicate that HMT can promote Cu and Cd stabilization while decrease labile phosphorus in red soil amended with HAP, suggesting that phosphorus-based amendments combined with Fe oxides can be used to stabilize PTEs in contaminated red soils.

Nanotechnology in soil remediation - applications vs. implications

Engineered nanomaterials (ENMs) and nanotechnology have shown great potential in addressing complex problems and creating innovative approaches in soil remediation due to their unique features of high reactivity, selectivity and versatility. Meanwhile, valid concerns exist with regard to their implications towards the terrestrial environment and the ecosystem. This review summarizes: (i) the applications and the corresponding mechanisms of various types of ENMs for soil remediation; (ii) the environmental behavior of ENMs in soils and their interactions with the soil content; (iii) the environmental implications of ENMs during remedial applications. The overall objective is to promote responsible innovations so as to take optimal advantage of ENMs and nanotechnology while minimizing their adverse effects to the ecological system. It is critical to establish sustainable remediation methods that ensure a healthy and safe environment without bringing additional risk.

Poultry litter ash characterisation and recovery

This paper reports a complete characterisation of poultry litter ash and its potential use as a heavy metal stabiliser. We propose a novel approach, in which the ashes deriving from municipal solid waste incineration (MSWI) are combined with poultry litter ash, rather than with coal combustion flue gas desulfurisation (FGD) residues. Heavy metals stabilisation was demonstrated by comparing the elemental concentrations in the leaching solutions of the starting raw and stabilised materials: leachable Pb and Zn showed a reduced solubility. The characterisation was conducted by total reflection X-ray fluorescence (TXRF), X-ray diffraction (XRD), micro-Raman spectroscopy and scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM-EDX). The results showed that the poultry litter ash was Ca-, P-, K- and S-rich (>29 g/kg). It contained amorphous materials (i.e. fly ash economiser (FAECO) 73% and fly ash cyclone (FACYC) 61%) and soluble phases (e.g. arkanite and sylvite; up to 13% FAECO and 28% FACYC), as well as resilient crystalline (up to 2% of FAECO and FACYC) and amorphous phases (e.g. hydroxyapatite). After two months, the Pb and Zn concentrations in the leachate solutions were below the limit set by the European regulations for waste disposal (<0.2 mg/L and 1.5 mg/L, respectively). We propose a mechanism for the heavy metals stabilisation based on the carbonation process and high amounts of P, Ca and reactive amorphous phases. In conclusion, it is demonstrated that poultry litter ash can be an effective secondary source of heavy metals, allowing their immobilisation through P- and Ca-based reactive amorphous phases.

Hematite enhances the immobilization of copper, cadmium and phosphorus in soil amended with hydroxyapatite under flooded conditions

Hydroxyapatite (HA) is often applied as chemical amendment in soils contaminated with trace metals such as copper (Cu) and cadmium (Cd). Large amounts of iron oxides in red soil may interacts with phosphate released from HA and influence trace metal immobilization of HA. Here we simulated a red paddy soil with 1-5% wt iron oxides by adding hematite and evaluated the Cu and Cd availability in soil amended with HA under flooded conditions. Changes in phosphorus and iron oxide fractions were also evaluated after a 42-day flooding incubation experiment. Results showed that the addition of HA-only and hematite-only decreased soil redox potential and increased pore water pH compared to the control. HA combined with hematite could effectively decrease phosphate, Cu and Cd in soil pore water compared to HA-only. Additionally, HA combined with hematite could also increase soil pH and decrease soil CaCl₂-extractable Cu and Cd. In particular, HA combined with 5% hematite was most effective in reducing soil exchangeable fractions of Cu and Cd by 53.7% and 65.6% compared to the control, respectively. The addition of HA-only increased water-soluble phosphorus, NaHCO₃-extractable inorganic phosphorus, NaOH-extractable inorganic phosphorus, and HCl-extractable phosphorus. Conversely, HA combined with hematite treatments decreased NaHCO₃-extractable inorganic phosphorus by 11.3-43.0% compared to HA-only. Vivianite and metal-phosphate precipitates were not observed using the Visual MINTEQ model, X-ray diffraction, and chemical analysis. The addition of hematite with or without HA increased free and crystal iron oxide fractions, while it substantially enhanced amorphous iron oxides in the soil. Thus, this study indicates that soil with high hematite content could enhance Cu and Cd immobilization while decreasing phosphorus availability in the red paddy soil amended with HA under the flooded conditions.

Bioavailability of cadmium to celery (Apium graveolens L.) grown in acidic and Cd-contaminated greenhouse soil as affected by the application of hydroxyapatite with different particle sizes

Cadmium contamination in greenhouse vegetable fields greatly limited the sustainable production especially of leafy vegetables. Hydroxyapatite (HAP), as a common soil amendment, has been widely used in the remediation of Cd-contaminated soils, while its remediation efficiency greatly depends on its particle sizes. In this study, a rhizobag pot experiment was conducted to investigate the effects of HAP (<60 nm, <12 μm and <80 μm) on bioavailability of Cd to celery grown in acidic and slightly Cd-contaminated greenhouse soil. The results suggested that HAP with the largest particle size (<80 μm) had the best effectiveness in reducing Cd uptake especially by the edible part of celery. Specifically, the increase in HAP (<80 μm) addition from 0.5% to 3% prominently reduced Cd concentrations in celery shoot by 19.6%-76.8% as compared with the untreated group. Also, adding HAP (<80 μm) especially at 3% significantly decreased translocation factor (TF) of Cd from celery root to shoot by 30.6% and reduced bioconcentration factor (BCF) of Cd from rhizosphere soil to celery shoot by 76.4%. These were predominantly associated with the significantly increased soil pH and the subsequently decreased soil CaCl₂-Cd concentration after adding HAP (<80 μm). Overall, although rhizosphere soil pH was the key factor in controlling Cd uptake by edible celery and regulating BCF and TF of Cd, insignificant root-induced acidification had limited effect on the immobilization efficiency of Cd by HAP (<80 μm). In conclusion, HAP (<80 μm) has good potential for the remediation of Cd-contaminated greenhouse soils.

Oxalic acid activated phosphate rock and bone meal to immobilize Cu and Pb in mine soils

The contamination of soil by copper (Cu) and lead (Pb) is a serious concern because of its high health risk via the food chain. Oxalic acid-activated phosphate rock (APR) and bone meal (BM) were applied to Cu and Pb co-contaminated soil to investigate their efficacy in the immobilization of Cu and Pb. APR and BM were applied into the contaminated soil (158.8 mg/kg total Pb and 573.2 mg/kg Cu) at four levels of dosages (0.1%, 0.5%, 2%, and 4%) and incubated for one year. The results demonstrated that the acid exchangeable Pb fraction in the soil treated with APR and BM decreased compared to the control, while there was no noticeable change in the acid-exchangeable Cu fraction in the soil treated with either APR or BM. Meanwhile, the application of BM and APR increased the fraction of residual Cu and Pb in the polluted soils. Moreover, the addition of either APR or BM at the dose of 4% decreased the concentrations of CaCl₂-extractable Cu and Pb in the amended soil, and the percentages of that reduction in the APR amended soils were 56% and 91% and in BM amended soils were 67% and 64%, respectively. The immobilization of Cu and Pb by APR and BM might be induced by the increased soil pH and soluble P contents in the amended soils. In general, BM is more effective than APR on the immobilization of Cu in polluted soil, while APR had greater efficiency than BM on the immobilization of Pb when the levels of amendments were above 2%.

Assessment of bacterial communities in Cu-contaminated soil immobilized by a one-time application of micro-/nano-hydroxyapatite and phytoremediation for 3 years

Heavy metals contamination of soil has been considered as a global environmental problem, and consequently various soil amendments have been widely used in immobilization. Previous studies have reported that micro-/nano-hydroxyapatite (MHA/NHA) as a novel chemical material could alleviate soil acidity and reduce the bioavailability of heavy metals. However, the mechanism of soil microorganism responding to the application of MHA/NHA is little studied. Presently, an in-situ field experiment was conducted to determine the effects of MHA/NHA and the other three traditional amendments including alkali slag (AS), lime (L) and apatite (AP) on soil copper (Cu) bioavailability and dominate bacterial population. The results showed that the application of MHA/NHA effectively increased soil pH and decreased soil available Cu content, and showed the highest increasing effects on the activities of urease, catalase and acid phosphatase. Compared with the control, MHA/NHA significantly changed the soil bacterial community structure and increased the bacterial abundance and diversity. Besides, analysis of the dominate population showed that the application of MHA/NHA decreased the relative abundance of acidophiles and the indicator of soil degradation. Additionally, the relative abundance of potential plant growth promoting bacteria increased with the addition of MHA/NHA, which was confirmed by the characteristics (the ability of producing indole acetic acid and siderophore) of bacterial strains. These results suggested that these dominate bacterial populations with significant changes may be regarded as the biomarkers for the recovery of soil ecological environment, which provides a theoretical basis for the ecological evaluation of MHA/NHA.

An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade

Soil contamination by heavy metals and metalloids has been a major concern to human health and environmental quality. While many remediation technologies have been tested at the bench scale, there have been only limited reports at the field scale. This paper aimed to provide a comprehensive overview on the field applications of various soil remediation technologies performed over the last decade or so. Under the general categories of physical, chemical, and biological approaches, ten remediation techniques were critically reviewed. The technical feasibility and economic effectiveness were evaluated, and the pros and cons were appraised. In addition, attention was placed to the environmental impacts of the remediation practices and long-term stability of the contaminants, which should be taken into account in the establishment of remediation goals and environmental criteria. Moreover, key knowledge gaps and practical challenges are identified.

Availability and vertical distribution of Cu, Cd, Ca, and P in soil as influenced by lime and apatite with different dosages: a 7-year field study

It is well known that alkaline amendments could effectively decrease the bioavailability of heavy metals in soils. However, the vertical distribution of heavy metals and the nutrients enriching in amendments are little concerned during long-term field remediation. Thus, the objective of the present study was to investigate the vertical distribution and availability of Cu, Cd, Ca, and P after a 7-year field experiment. In this study, a single application of lime and apatite was conducted with the rates of 1.71-6.84 and 6.84-19.8 tons/ha, respectively. Soil pH and immobilization efficiency of Cu and Cd were both increased with increasing dosages of lime and apatite (0-50 cm). Applications of lime and apatite decreased the mobility of Cu and Cd although soil Cu and Cd in the surface soil were increased due to the input by atmospheric dry and wet deposition. Moreover, concentrations of Cu and Cd in lime- and apatite-amended soils (0-13 cm) were higher than those in the control group. However, applications of lime and apatite decreased the downward eluviations of heavy metals in soils (13-50 cm). For soil nutrients, the Ca concentrations at 0-13 and 13-30 cm were both enhanced with increasing amendment dosages, while only soil P concentration at 0-13 cm was increased in apatite-treated soils and majority of them presented in stable-P. In addition, resin-P was increased with increasing dosages of the apatite, which suggested that high eutrophication risk was induced by excessive P loss. Thus, more attention should be paid to the nutrients (phosphorus) and pollutants enriching in the amendments during in-situ remediation of heavy metal-contaminated soils.

Stability of immobilization remediation of several amendments on cadmium contaminated soils as affected by simulated soil acidification

Chemical immobilization is a practical approach to remediate heavy metal contamination in agricultural soils. However, the potential remobilization risks of immobilized metals are a major environmental concern, especially in acid rain zones. In the present study, changes in the immobilization efficiency of several amendments as affected by simulated soil acidification were investigated to evaluate the immobilization remediation stability of several amendments on two cadmium (Cd) contaminated soils. Amendments (hydrated lime, hydroxyapatite and biochar) effectively immobilized Cd, except for organic fertilizer, and their immobilizations were strongly decreased by the simulated soil acidification. The ratio of changes in CaCl₂-extractable Cd: pH (△CaCl₂-Cd/△pH) can represent the Cd remobilization risk of different amended soils. Hydroxyapatite and biochar had a stronger durable immobilizing effect than did hydrated lime, particularly in soil with a lower pH buffering capacity, which was further confirmed by the Cd concentration and accumulation in lettuce. These results can be attributed to that hydroxyapatite and biochar transformed greater proportions of exchangeable Cd to other more stable fractions than lime. After 48 weeks of incubation, in soil with a lower pH buffering capacity, the immobilization efficiencies of lime, hydroxyapatite, biochar and organic fertilizer in the deionized water group (pH 6.5) were 71.7%, 52.7%, 38.6% and 23.9%, respectively, and changed to 19.1%, 33.6%, 26.5% and 5.0%, respectively, in the simulated acid rain group (pH 2.5). The present study provides a simple method to preliminarily estimate the immobilization efficiency of amendments and predict their stability in acid rain regions before large-scale field application. In addition, hydrated lime is recommended to be combined with other acid-stable amendments (such as hydroxyapatite or biochar) to remediate heavy metal-contaminated agricultural soils in acid precipitation zones.