Remediation of Cr(VI)-Contaminated Soil by Nano-Zero-Valent Iron in Combination with Biochar or Humic Acid and the Consequences for Plant Performance

Remediation on antimony-contaminated soil from mine area using zero-valent-iron doped biochar and their effect on the bioavailability of antimony

Due to the bioavailability and movement of antimony in trophic web, the overexploitation of antimony mine resulted in antimony contamination that harmed the ecology nearby, raising concerns for public health. Whereas, most researches focused on the removal of antimony in the aqueous instead of the immobilization of antimony in the soil. Herein, the immobilized performance of biochar (BC) loaded with nano zero-valent iron (nZVI-BC) on antimony in the soil near the smelting area was researched through pot experiments for the first time, and its stabilization mechanism on antimony was investigated by valent state variation of antimony. The results demonstrated that BC restricted the cation exchange capacity and catalase activity in the soil, while nZVI-BC had a favorable and negative impact on two variables, respectively. The nZVI-BC showed more stable immobilization capacity on antimony over time than BC, whose exchangeable speciation only marginally rose (2%-6%), although the exchangeable speciation of antimony fell both from 15% to 2% after adding the BC and nZVI-BC, The electron attraction force between nZVI-BC and antimony was also intensified owing to the oxidation-reduction process, which was considered as the stabilizing principle of nZVI-BC on antimony in soil. Furthermore, the decreased bioaccumulation factor for the perennial ryegrass (0.46-0.21) and Galinsoga parviflora Cav. (0.26-0.17) stated that the BC effectively mitigated the bioaccumulation risk of antimony.

A META analysis on the efficacy of functional materials for soil chromium remediation

Metallic chromium pollution in soil is widespread, which aroused intensive research in recent decades. In mainstream research, most studies use materials with a reducing ability to adsorb and reduce hexavalent chromium. However, comprehensive analyses and systematic verifications of these different materials are scarce. Therefore, this study conducted a meta-analysis of relevant papers published from 2013 to October 2024 to compare and analyze the performance and usage conditions of some common materials, such as iron-based materials, mineral inorganic materials, organic materials, and layered double hydroxide materials. We synthesized 31 papers for 186 pairwise comparisons and selected the Standardized Mean Difference (SMD) as the appropriate effect size for mean-to-mean comparisons. Fe-based materials had the most stable performance based on its numerous data support, while organic materials had the worst performance. The difference in performance between inorganic mineral materials was the greatest, which was closely related to the selection of components. The difference in the effectiveness of inorganic materials was the greatest, which was closely related to the selection of components and there was room for further improvement. Through further analysis of the impact of environmental factors on material performance, it can be concluded that the effect of the material was better under alkaline, non-sandy, low organic matter, and high CEC soil conditions.

Nano-biochar as a potential amendment for metal(loid) remediation: Implications for soil quality improvement and stress alleviation

Metal(loid) contamination of agricultural soils has become an alarming issue due to its detrimental impacts on soil health and global agricultural production. Therefore, environmentally sustainable and cost-effective solutions are urgently required for soil remediation. Biochar, particularly nano-biochar, exhibits superior and high-performance capabilities in the remediation of metal(loid)-contaminated soil, owing to its unique structure and large surface area. Current researches on nano-biochar mainly focus on safety design and property improvement, with limited information available regarding the impact of nano-biochar on soil ecosystems and crop defense mechanisms in metal(loid)-contaminated soils. In this review, we systematically summarized recent progress in the application of nano-biochar for remediation of metal(loid)-contaminated soil, with a focus on possible factors influencing metal(loid) uptake and translocation in soil-crop systems. Additionally, we conducted the potential/related mechanisms by which nano-biochar can mitigate the toxic impacts of metal(loid) on crop production and security. Furthermore, the application of nano-biochar in field trials and existing challenges were also outlined. Future studies should integrate agricultural sustainability and ecosystem health targets into biochar design/selection. This review highlighted the potential of nano-biochar as a promising soil amendment for enhancing the remediation of metal(loid)-contaminated agricultural soils, thereby promoting the synthesis and development of highly efficient nano-biochar towards achieving environmental sustainability.

Waste-derived nanobiochar: A new avenue towards sustainable agriculture, environment, and circular bioeconomy

The greatest challenge for the agriculture sector in the twenty-first century is to increase agricultural production to feed the burgeoning global population while maintaining soil health and the integrity of the agroecosystem. Currently, the application of biochar is widely implemented as an effective means for boosting sustainable agriculture while having a negligible influence on ecosystems and the environment. In comparison to traditional biochar, nano-biochar (nano-BC) boasts enhanced specific surface area, adsorption capacity, and mobility properties within soil, allowing it to promote soil properties, crop growth, and environmental remediation. Additionally, carbon sequestration and reduction of methane and nitrous oxide emissions from agriculture can be achieved with nano-BC applications, contributing to climate change mitigation. Nonetheless, due to cost-effectiveness, sustainability, and environmental friendliness, waste-derived nano-BC may emerge as the most viable alternative to conventional waste management strategies, contributing to the circular bioeconomy and the broader goal of achieving the Sustainable Development Goals (SDGs). However, it's important to note that research on nano-BC is still in its nascent stages. Potential risks, including toxicity in aquatic and terrestrial environments, necessitate extensive field investigations. This review delineates the potential of waste-derived nano-BC for sustainable agriculture and environmental applications, outlining current advancements, challenges, and possibilities in the realms from a sustainability and circular bioeconomy standpoint.

Integrative application of biochar and arbuscular mycorrhizal fungi for enhanced chromium resistance in Medicago sativa

Soil chromium (Cr) contamination has become an environmental problem of global concern. However, the joint effects of combined utilization of biochar and arbuscular mycorrhizal (AM) fungal inoculum, which are considered as two promising remediation strategies of soil heavy metal pollutions, on plant Cr resistance are still poorly understood. In this study, a two-factor pot experiment was conducted to investigate how biochar and AM fungus Rhizophagus irregularis regulate Medicago sativa growth, physiological trait, nutrient and Cr uptake, relevant gene expressions, soil properties, and Cr speciation, independently or synergistically. The results showed that biochar notably decreased AM colonization, while biochar and AM fungus could simultaneously increase plant dry biomass. The greatest growth promotion was observed in mycorrhizal shoots at the highest biochar level (50 g kg-1 soil) by 91 times. Both biochar application and AM fungal inoculation enhanced plant photosynthesis and P nutrition, but the promoting effects of AM fungus on them were significantly greater than that of biochar. In addition, the combined application of biochar and AM fungus dramatically reduced shoot and root Cr concentrations by up to 92 % and 78 %, respectively, compared to the non-amended treatment. Meanwhile, down-regulated expressions were observed for metal chelating-related genes. Furthermore, Cr translocation from roots to shoots was reduced by both two soil amendments. Transcriptional levels of genes involved in reactive oxygen species and proline metabolisms were also regulated by biochar application and AM fungal colonization, leading to alleviation of Cr phytotoxicity. Furthermore, AM fungal inoculation slightly elevated soil pH but decreased plant-available soil P, which was, by contrast, lifted by biochar addition. The combined application reduced soil acid-extractable Cr concentration by 40 %. This study provides new insights into comprehensively understanding of the mechanisms of biochar and AM fungi combination on improving plant Cr tolerance.

A practical evaluation on integrated role of biochar and nanomaterials in soil remediation processes

Soil decontamination and restoration continue to be a key environmental concern around the globe. The degradation of soil resources due to the presence of potentially toxic elements (PTEs) has a substantial influence on agricultural production, food security, and human well-being, and as a result, urgent action is required. PTEs pollution is not a threat to the agroecosystems but also a serious concern to human health; thereby, it needs to be addressed timely and effectively. Hence, the development of improved and cost-effective procedures to remove PTEs from polluted soils is imperative. With this context in mind, current review is designed to distinctly envisage the PTEs removal potential by the single and binary applications of biochar (BC) and nanomaterials (NMs).2 Recently, BC, a product of high-temperature biomass pyrolysis with high specific surface area, porosity, and distinctive physical and chemical properties has become one of the most used and economic adsorbent materials. Also, biochar's application has generated interest in a variety of fields and environments as a modern approach against the era of urbanization, industrialization, and climate change. Likewise, several NMs including metals and their oxides, carbon materials, zeolites, and bimetallic-based NMs have been documented as having the potential to remediate PTEs-polluted environments. However, both techniques have their own set of advantages and disadvantages, therefore combining them can be a more effective strategy to address the growing concern over the rapid accumulation and release of PTEs into the environment.

Biochar Decreases Cr Toxicity and Accumulation in Sunflower Grown in Cr(VI)-Polluted Soil

Biochar is preferentially recommended for the remediation of heavy metal-polluted soils. Sunflower is an important high-biomass oil crop with a promising potential for phytoremediation of Cr(VI)-polluted soil. However, how biochar affects sunflower growth and Cr accumulation in Cr(VI)-polluted soil needs to be elucidated. Here, a pot culture experiment was conducted to study whether soil amendment with biochar (0, 0.1%, 1%, and 5%, w/w) can mitigate Cr toxicity and accumulation in sunflower seedlings grown in soils artificially polluted with different levels of Cr(VI) (0, 50, and 250 mg Cr(VI)/kg soil). The addition of Cr(VI) exhibited significant phytotoxicity, as evidenced by inhibited plant growth and even the death of seedlings at 250 mg/kg Cr(VI). Overall, biochar amendment showed positive effects on plant growth and Cr immobilization, dependent on both the biochar dose and Cr addition level. When 50 mg/kg Cr(VI) was added, 1% biochar showed positive effects similar to 5% biochar on improving plant growth and mineral nutrition (particularly K), reducing Cr content in shoots and roots, and decreasing Cr availability and Cr(VI) content in the soil. In comparison with non-amendment, 1% and 5% biochar caused 85% and 100% increase in shoot dry weights, and 75% and 86% reduction in shoot Cr concentrations, respectively. When 250 mg/kg Cr(VI) was added, a 5% dose produced much better benefits than 1%, while a 0.1% dose did not help plants to survive. Overall, an appropriate dose of biochar enhanced Cr(VI) immobilization and subsequently decreased its toxicity and accumulation in sunflower seedlings. Our findings confirm that biochar can be used as an efficient amendment for the remediation of Cr(VI)-polluted soils and cleaner production of sunflower oil and biomass.

Remediation of soil polluted with Pb and Cd and alleviation of oxidative stress in Brassica rapa plant using nanoscale zerovalent iron supported with coconut-husk biochar

Accumulation of toxic elements by plants from polluted soil can induce the excessive formation of reactive oxygen species (ROS), thereby causing retarded plants' physiological attributes. Several researchers have remediated soil using various forms of zerovalent iron; however, their residual impacts on oxidative stress indicators and health risks in leafy vegetables have not yet been investigated. In this research, nanoscale zerovalent iron supported with coconut-husk biochar (nZVI-CHB) was synthesized through carbothermal reduction process using Fe2O3 and coconut-husk. The stabilization effects of varying concentrations of nZVI-CHB and CHB (250 and 500 mg/kg) on cadmium (Cd) and lead (Pb) in soil were analyzed, and their effects on toxic metals induced oxidative stress, physiological properties, and antioxidant defence systems of the Brassica rapa plant were also checked. The results revealed that the immobilization of Pb and Cd in soil treated with CHB was low, leading to a higher accumulation of metals in plants grown. However, nZVI-CHB could significantly immobilize Pb (57.5-62.12%) and Cd (64.1-75.9%) in the soil, leading to their lower accumulation in plants below recommended safe limits and eventually reduced carcinogenic risk (CR) and hazard quotient (HQ) for both Pb and Cd in children and adults below the recommended tolerable range of <1 for HQ and 10-6 - 10-4 for CR. Also, a low dose of nZVI-CHB significantly mitigated toxic metal-induced oxidative stress in the vegetable plant by inhibiting the toxic metals uptake and increasing antioxidant enzyme activities. Thus, this study provided another insightful way of converting environmental wastes to sustainable adsorbents for soil remediation and proved that a low-dose of nZVI-CHB can effectively improve soil quality, plant physiological attributes and reduce the toxic metals exposure health risk below the tolerable range.

Enhanced edible plant production using nano-manganese and nano-iron fertilizers: Current status, detection methods and risk assessment

BACKGROUND

Nanotechnology offers many benefits in the globally important field of food production and human nutrition, particularly by implementing agricultural nanoproducts. Of these, edible plant fertilizers enriched with nanosized forms of essential metals, Mn and Fe, are growing in importance with the advantages of enhanced action on plant roots.

SCOPE AND APPROACH

This review focuses on the importance of tracking the bioaccumulation and biodistribution of these pertinent nanofertilizers. An emphasis is given to the critical analysis of the state-of-the-art analytical strategies to examine the Mn and Fe nanoparticles in edible plant systems as well as to shedding light on the vast gap in the methodologies dedicated to the speciation, in vitro simulation, and safety testing of these promising nanomaterials. Also provided are guidances for the food chemists and technologists on the lights and shadows of particular analytical approaches as a matter of authors' expertise as analytical chemists.

KEY FINDINGS AND CONCLUSIONS

While the use of nanotechnology in agriculture seems to be growing increasingly, there is still a lack of analytical methodologies capable of investigating novel Mn- and Fe-based nanomaterials as potential fertilizers. Only the advent of reliable analytical tools in the field could bridge the gaps in our knowledge about processes in which those materials participate in the plant systems and their effects on crop production and quality of the produced food.

Hexavalent chromium removal by a resistant strain Bacillus cereus ZY-2009

Bioreduction of Cr(VI) to Cr(III) by reducing microbes has attracted increasing concern. Here, Cr(VI) removal capacity of a Cr(VI)-resistant bacterium isolated from activated sludge was investigated. Based on its physio-biochemical attributes and 16S rDNA sequence analysis, the strain was identified as Bacillus cereus ZY-2009. It grew normally in the media containing 10-100 mg/L Cr(VI), indicating its high resistance to Cr(VI). Under the optimal conditions of pH 7.0, inoculation amount 10%, and temperature 30°C, Cr(VI) was effectively removed, with a removal rate of ∼80%. Co-existing Fe³⁺ and Cu²⁺ greatly increased Cr(VI) removal, but Cd²⁺ showed significant inhibition. Cr(VI) was removed mainly via enzyme-mediated bioreduction but not biosorption. Cr(VI) was reduced by different cell fractions (i.e. extracellular secretions, cytoplasm, and cell envelope), implying that Cr(VI) can be reduced both extracellularly and intracellularly. This strain can be used in the bioremediation of Cr(VI)-containing wastewater, with Fe³⁺ and Cu²⁺ as stimulators.

A review on the clean-up technologies for heavy metal ions contaminated soil samples

The soil contamination with heavy metal ions is one of the grave intricacies faced worldwide over the last few decades by the virtue of rapid industrialization, human negligence and greed. Heavy metal ions are quite toxic even at low concentration a swell as non-biodegradable in nature. Their bioaccumulation in the human body leads to several chronic and persistent diseases such as lung cancer, nervous system break down, respiratory problems and renal damage etc. In addition to this, the increased concentration of these metal ions in soil, beyond the permissible limits, makes the soil unfit for further agricultural use. Hence it is our necessity, to monitor the concentration of these metal ions in the soil and water bodies and adopt some better technologies to eradicate them fully. From the literature survey, it was observed that three main types of techniques viz. physical, chemical, and biological were employed to harness the heavy metal ions from metal-polluted soil samples. The main goal of these techniques was the complete removal of the metal ions or the transformation of them into less hazardous and toxic forms. Further the selection of the remediation technology depends upon different factors such as process feasibility/mechanism of the process applied, nature and type of contaminants, type and content of the soil, etc. In this review article, we have studied in detail all the three technologies viz. physical, chemical and biological with their sub-parts, mechanism, pictures, advantages and disadvantages.

Chromium Toxicity in Plants: Signaling, Mitigation, and Future Perspectives

Plants are very often confronted by different heavy metal (HM) stressors that adversely impair their growth and productivity. Among HMs, chromium (Cr) is one of the most prevalent toxic trace metals found in agricultural soils because of anthropogenic activities, lack of efficient treatment, and unregulated disposal. It has a huge detrimental impact on the physiological, biochemical, and molecular traits of crops, in addition to being carcinogenic to humans. In soil, Cr exists in different forms, including Cr (III) "trivalent" and Cr (VI) "hexavalent", but the most pervasive and severely hazardous form to the biota is Cr (VI). Despite extensive research on the effects of Cr stress, the exact molecular mechanisms of Cr sensing, uptake, translocation, phytotoxicity, transcript processing, translation, post-translational protein modifications, as well as plant defensive responses are still largely unknown. Even though plants lack a Cr transporter system, it is efficiently accumulated and transported by other essential ion transporters, hence posing a serious challenge to the development of Cr-tolerant cultivars. In this review, we discuss Cr toxicity in plants, signaling perception, and transduction. Further, we highlight various mitigation processes for Cr toxicity in plants, such as microbial, chemical, and nano-based priming. We also discuss the biotechnological advancements in mitigating Cr toxicity in plants using plant and microbiome engineering approaches. Additionally, we also highlight the role of molecular breeding in mitigating Cr toxicity in sustainable agriculture. Finally, some conclusions are drawn along with potential directions for future research in order to better comprehend Cr signaling pathways and its mitigation in sustainable agriculture.

Lead Responses and Tolerance Mechanisms of Koelreuteria paniculata: A Newly Potential Plant for Sustainable Phytoremediation of Pb-Contaminated Soil

Phytoremediation could be an alternative strategy for lead (Pb) contamination. K. paniculata has been reported as a newly potential plant for sustainable phytoremediation of Pb-contaminated soil. Physiological indexes, enrichment accumulation characteristics, Pb subcellular distribution and microstructure of K. paniculata were carefully studied at different levels of Pb stress (0-1200 mg/L). The results showed that plant growth increased up to 123.8% and 112.7%, relative to the control group when Pb stress was 200 mg/L and 400 mg/L, respectively. However, the average height and biomass of K. paniculata decrease when the Pb stress continues to increase. In all treatment groups, the accumulation of Pb in plant organs showed a trend of root > stem > leaf, and Pb accumulation reached 81.31%~86.69% in the root. Chlorophyll content and chlorophyll a/b showed a rising trend and then fell with increasing Pb stress. Catalase (CAT) and peroxidase (POD) activity showed a positive trend followed by a negative decline, while superoxide dismutase (SOD) activity significantly increased with increasing levels of Pb exposure stress. Transmission electron microscopy (TEM) showed that Pb accumulates in the inactive metabolic regions (cell walls and vesicles) in roots and stems, which may be the main mechanism for plants to reduce Pb biotoxicity. Fourier transform infrared spectroscopy (FTIR) showed that Pb stress increased the content of intracellular -OH and -COOH functional groups. Through organic acids, polysaccharides, proteins and other compounds bound to Pb, the adaptation and tolerance of K. paniculata to Pb were enhanced. K. paniculata showed good phytoremediation potential and has broad application prospects for heavy metal-contaminated soil.

Strategy for enhancing Cr(VI)-contaminated soil remediation and safe utilization by microbial-humic acid-vermiculite-alginate immobilized biocomposite

Bioreduction is an efficient approach to in-situ remediate Cr(VI)-contaminated soil, but further strengthening methods are still urgently needed. Herein, a novel immobilized biocomposite (B-HA-VE-SA) was successfully synthesized by embedding a efficient strain Bacillus sp. CRB-7 with humic acid (HA) combined vermiculite (VE) and sodium alginate (SA). The performance and enhancement mechanism of the immobilized biocomposite on remediating Cr(VI)-contaminated soil were also investigated by analyzing the whole-genome of CRB-7, Cr(VI) detoxification, soil microecological regulation, and subsequent crop growth response. Genomic annotation demonstrated that CRB-7 contains multiple genes contributed to Cr(VI) tolerance, Cr(VI) reduction and other metals resistance. Results showed that embedded CRB-7 biocomposites exhibited more effective reduction of Cr(VI) in soil compared with control and free CRB-7 treatment, especially B-HA-VE-SA achieved the highest Cr(VI) removal efficiency (96.18%) and the residual Cr proportion (49.04%) via multiple mechanisms including carrier effects, nutrient sustained-release, and electron-shuttle effect enhanced the bioremediation process. Furthermore, the synergies of CRB-7 and immobilizers (HA, VE and SA) significantly improved soil microecology (soil enzyme activities, microbial quantity and diversity), and engendered the evolution of microbial community composition and functional pathways. Consequently, pot experiments (Brassica napus L.) verified the plant-growth-promoting (12.00-18.00% and 43.82-69.00% higher in emergence rate and biomass) and Cr-accumulation-reducing effects (19.47-91.09% and 29.11-89.80% lower in root and aerial parts) of free and immobilized CRB-7. Taken together, these findings highlighted the superiority of B-HA-VE-SA in simultaneous remediation, microecological improvement and safe utilization of Cr(VI)-contaminated soil.

Immobilization of hexavalent chromium in contaminated soil by nano-sized layered double hydroxide intercalated with diethyldithiocarbamate: Fraction distribution, plant growth, and microbial evolution

Soil contamination by hexavalent chromium (Cr(VI)) poses great risks to human health and ecosystem safety. We introduced a new cheap and efficient layered double hydroxide intercalated with diethyldithiocarbamate (DDTC-LDH) for in-situ remediation of Cr(VI)-contaminated soil. The content of Cr(VI) in contaminated soil (134.26 mg kg^-1) was rapidly reduced to 1.39 mg kg^-1 within 10 days by 0.5% of DDTC-LDH. This result attains to or even exceeds the effectiveness of most of reported soil amendments for Cr(VI) removal in soils. The production cost of DDTC-LDH ($4.02 kg^-1) was relatively low than some common materials, such as nano zero-valent iron ($22.80-140.84 kg^-1). The growth of water spinach became better with the increase of DDTC-LDH dose from 0% to 0.5%, suggesting the recovery of soil function. DDTC-LDH significantly altered the structure and function of soil microbial communities. The species that have Cr(VI)-resistant or Cr(VI)-reductive ability were enriched in DDTC-LDH remediated soils. Network analysis revealed a significant functional niche differentiation of soil microbial communities. In addition to the enhancement of Cr(VI) reduction, the stimulation of plant growth promoting traits, including siderophore biosynthesis, oxidation resistance to reactive oxygen species, and phosphorus availability by DDTC-LDH was another essential mechanism for the immediate remediation of Cr(VI)-contaminated soil.

Method and mechanism of chromium removal from soil: a systematic review

Heavy metal pollution has increasingly affected human life, and the treatment of heavy metal pollution, especially chromium pollution, is still a major problem in the field of environmental governance. As a commonly used industrial metal, chromium can easily enter the environment with improperly treated industrial waste or wastewater, then pollute soil and water sources, and eventually accumulate in the human body through the food chain. Many countries and regions in the world are threatened by soil chromium pollution, resulting in the occurrence of cancer and a variety of metabolic diseases. However, as a serious threat to agriculture, food, and human health. Notwithstanding, there are limited latest and systematic review on the removal methods, mechanisms, and effects of soil chromium pollution in recent years. Hence, this article outlines some of the methods and mechanisms for the removal of chromium in soil, including physical, chemical, biological, and biochar methods, which provide a reference for the treatment and research on soil chromium pollution drawn from existing publications.

Superior elimination of Cr(VI) using polydopamine functionalized attapulgite supported nZVI composite: Behavior and mechanism

In this study, a polydopamine (PDA) modified attapulgite (ATP) supported nano sized zero-valent iron (nZVI) composite (PDA/ATP-nZVI) was rapidly synthesized under acidic conditions, and employed to alleviate Cr(VI) toxicity from an aqueous solution. Kinetic studies revealed that Cr(VI) adsorption process followed the pseudo-second order model, suggesting chemisorption was the dominant adsorption mechanism. Liu isotherm adsorption model was able to better describe the Cr(VI) adsorption isotherm with the maximum adsorption capacity of 134.05 mg/g. The thermodynamic study demonstrated that the adsorption process occurred spontaneously, accompanied by the increase in entropy and endothermic reaction. Low concentrations of coexisting ions had negligible effects on the removal of Cr(VI), while high concentrations of interfering ions were able to facilitate the removal of Cr(VI). Reactive species test revealed that Fe²⁺ played a key role in Cr(VI) reduction by PDA/ATP-nZVI. PDA enhanced the elimination of Cr(VI) via donation of electrons to Cr(VI) and acceleration of Fe³⁺ transformation to Fe²⁺. Furthermore, PDA was able to effectively inhibit the leaching of iron species and generation of ferric hydroxide sludge. Mechanistic study revealed that 72% of Cr(VI) elimination was attributed to reduction/precipitation, while 28% of Cr(VI) elimination was due to the surface adsorption.

Low levels of TiO2-nanoparticles interact antagonistically with Al and Pb alleviating their toxicity

The contamination and bioavailability of deleterious metals in arable soils significantly limits crop development and yield. Aiming at mitigating Pb- and Al-induced phytotoxicity, this work explores the use of P25 titanium dioxide nanoparticles (nTiO₂) in soil amendments. For that, Lactuca sativa L. plants were germinated and grown in the presence of 10 ppm Pb or 50 ppm Al, combined or not with 5 ppm nTiO₂. Growth parameters, as well as endpoints of the redox state [cell relative membrane permeability (RMP), thiobarbituric acid reactive substances content, total phenolic content and photosynthesis (sugars and pigments levels, chlorophyll a fluorescence and gas exchange), were evaluated. Concerning Al, nTiO₂ treatment alleviated the impairments induced in germination rate, seedling length, water content, RMP, stomatal conductance (g_s), intercellular CO₂ (C_i), and net CO₂ assimilation rate (P_N). It increased anthocyanins contents and effective efficiency of photosystem II (Φ_PSII). In Pb-exposed plants, nTiO₂ amendment mitigated the effects in RMP, P_N, g_s, and C_i. It also increased the pigment contents and the transpiration rate (E) comparatively to the control without nTiO₂. These results clearly highlight the high potential of low doses of nTiO₂ in alleviating metal phytotoxicity, particularly the one of Pb. Additionally, further research should explore the use of these nanoparticles in agricultural soil amendments.

Contribution of Nano-Zero-Valent Iron and Arbuscular Mycorrhizal Fungi to Phytoremediation of Heavy Metal-Contaminated Soil

Soil pollution with heavy metals has attracted increasing concern, which calls for the development of new remediation strategies. The combination of physical, chemical, and biological techniques can achieve more efficient remediation. However, few studies have focused on whether nanomaterials and beneficial microbes can be jointly used to facilitate phytoremediation. Therefore, we studied the role of nano-zero-valent iron (nZVI) and arbuscular mycorrhizal (AM) fungi in the phytoremediation of an acidic soil polluted with Cd, Pb and Zn, using sweet sorghum. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and mapping analyses were conducted to explore the mechanisms of metal immobilization by nZVI. The results showed that although both bare nZVI (B-nZVI) and starch-stabilized nZVI (S-nZVI) inhibited root mycorrhizal colonization, Acaulospora mellea ZZ successfully colonized the plant roots. AM inoculation significantly reduced the concentrations of DTPA-Cd, -Pb, and -Zn in soil, and the concentrations of Cd, Pb, and Zn in plants, indicating that AM fungi substantially facilitated heavy metal immobilization. Both B-nZVI and S-nZVI, ranging from 50 mg/kg to 1000 mg/kg, did not impede plant growth, and generally enhanced the phytoextraction of heavy metals. XRD, EDS and mapping analyses showed that S-nZVI was more susceptible to oxidation than B-nZVI, and thus had more effective immobilization effects on heavy metals. Low concentrations of nZVI (e.g., 100 mg/kg) and AM inoculation had synergistic effects on heavy metal immobilization, reducing the concentrations of Pb and Cd in roots and enhancing root Zn accumulation. In conclusion, our results showed that AM inoculation was effective in immobilizing heavy metals, whereas nZVI had a low phytotoxicity, and they could jointly contribute to the phytoremediation of heavy metal-contaminated soils with sweet sorghum.

Remediation of Metal/Metalloid-Polluted Soils: A Short Review

The contamination of soil by heavy metals and metalloids is a worldwide problem due to the accumulation of these compounds in the environment, endangering human health, plants, and animals. Heavy metals and metalloids are normally present in nature, but the rise of industrialization has led to concentrations higher than the admissible ones. They are non-biodegradable and toxic, even at very low concentrations. Residues accumulate in living beings and become dangerous every time they are assimilated and stored faster than they are metabolized. Thus, the potentially harmful effects are due to persistence in the environment, bioaccumulation in the organisms, and toxicity. The severity of the effect depends on the type of heavy metal or metalloid. Indeed, some heavy metals (e.g., Mn, Fe, Co, Ni) at very low concentrations are essential for living organisms, while others (e.g., Cd, Pb, and Hg) are nonessential and are toxic even in trace amounts. It is important to monitor the concentration of heavy metals and metalloids in the environment and adopt methods to remove them. For this purpose, various techniques have been developed over the years: physical remediation (e.g., washing, thermal desorption, solidification), chemical remediation (e.g., adsorption, catalysis, precipitation/solubilization, electrokinetic methods), biological remediation (e.g., biodegradation, phytoremediation, bioventing), and combined remediation (e.g., electrokinetic–microbial remediation; washing–microbial degradation). Some of these are well known and used on a large scale, while others are still at the research level. The main evaluation factors for the choice are contaminated site geology, contamination characteristics, cost, feasibility, and sustainability of the applied process, as well as the technology readiness level. This review aims to give a picture of the main techniques of heavy metal removal, also giving elements to assess their potential hazardousness due to their concentrations.

Nano-remediation of toxic heavy metal contamination: Hexavalent chromium [Cr(VI)]

The inexorable industrialization and modern agricultural practices to meet the needs of the increasing population have polluted the environment with toxic heavy metals such as Cr(VI), Cu²⁺, Cd²⁺, Pb^2+, and Zn²⁺. Among the hazardous heavy metal(loid)s contamination in agricultural soil, water, and air, hexavalent chromium [Cr(VI)] is the most virulent carcinogen. The metallurgic industries, tanneries, paint manufacturing, petroleum refineries are among various such human activities that discharge Cr(VI) into the environment. Various methods have been employed to reduce the concentration of Cr(VI) contamination with nano and bioremediation being the recent advancement to achieve recovery at low cost and higher efficiency. Bioremediation is the process of using biological sources such as plant extracts, microorganisms, and algae to reduce the heavy metals while the nano-remediation uses nanoparticles to adsorb heavy metals. In this review, we discuss the various activities that liberate Cr(VI). We then discuss the various conventional, nano-remediation, and bioremediation methods to keep Cr(VI) concentration in check and further discuss their efficiencies. We also discuss the mechanism of nano-remediation techniques for better insight into the process.

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.