The role of citric acid on the phytoremediation of heavy metal contaminated soil

Role of Root Exudates on the Soil Microbial Diversity and Biogeochemistry of Heavy Metals

Due to extensive industrialization and escalation in pollution, the world is facing problems related to soil heavy metal pollution. The traditional ways of soil remediation are neither feasible nor cost-effective in most of the real-world scenarios, where metal concentration is relatively low in soil. Therefore, phytoremediation using plants and plant secretions to remediate heavy metal-contaminated soil is recently getting more attention. The plant root exudates act as an ecological driver in the rhizospheric region where they influence and guide the microbial community to function in such a way that can be advantageous for plant growth. They also promote phytoremediation process by altering the bioavailability of pollutants in soil. Root exudates affect the biogeochemical properties of heavy metals as well. In this review, existing literature on the role of root exudates (natural as well as artificial) on the phytoremediation of heavy metal-contaminated (particularly lead) soil is reviewed. The effect of root exudates on the biogeochemistry of lead in soil is also discussed.

Geochemical properties of blue carbon sediments through an elevation gradient: study of an anthropogenically impacted coastal lagoon

Global research is showing that coastal blue carbon ecosystems are vulnerable to climate change driven threats including accelerated sea-level rise and prolonged periods of drought. Furthermore, direct anthropogenic impacts present immediate threats through deterioration of coastal water quality, land reclamation, long-term impact to sediment biogeochemical cycling. These threats will invariably alter the future efficacy of carbon (C) sequestration processes and it is imperative that currently existing blue carbon habitats be protected. Knowledge of underlying biogeochemical, physical and hydrological interactions occurring in functioning blue carbon habitats is essential for developing strategies to mitigate threats, and promote conditions to optimise C sequestration/storage. In this current work, we investigated how sediment geochemistry (0-10 cm depth) responds to elevation, an edaphic factor driven by long-term hydrological regimes consequently exerting control over particle sedimentation rates and vegetation succession. This study was performed in an anthropogenically impacted blue carbon habitat along a coastal ecotone encompassing an elevation gradient transect from intertidal sediments (un-vegetated and covered daily by tidal water), through vegetated salt marsh sediments (periodically covered by spring tides and flooding events), on Bull Island, Dublin Bay. We determined the quantity and distributions of bulk geochemical characteristics in sediments through the elevation gradient, including total organic carbon (TOC), total nitrogen (TN), total metals, silt, clay, and also, 16 individual polyaromatic hydrocarbon's (PAH's) as an indication of anthropogenic input. Elevation measurements for sample sites were determined on this gradient using a LiDAR scanner accompanied by an IGI inertial measurement unit (IMU) on board a light aircraft. Considering the gradient from the Tidal mud zone (T), through the low-mid marsh (M) to the most elevated upper marsh (H), there were significant differences between all zones for many measured environmental variables. The results of significance testing using Kruskal-Wallis analysis revealed, that %C, %N, PAH (µg/g), Mn (mg/kg), TOC:NH₄ ⁺ and pH are significantly different between all zones on the elevation gradient. The highest values for all these variables exists (excluding pH which followed a reverse trend) in zone H, decreasing in zone M and lowest in the un-vegetated zone T. TC content is 16 fold higher overall in vegetated (3.43 -21.84%) than uninhabited (0.21-0.56%) sediments. TN was over 50 times higher (0.24-1.76%), more specifically increasing in % mass on approach to the upper salt marsh with distance from the tidal flats sediments zone T (0.002-0.05%). Clay and silt distributions were greatest in vegetated sediments, increasing in % content towards upper marsh zones The retention of water, metals, PAHs, mud, chloride ions, NH₄ ⁺, PO₄ ^3- and SO₄ ^2- increased with elevated C concentrations, concurrently where pH significantly decreased. Sediments were categorized with respect to PAH contamination where all SM samples were placed in the high polluted category. The results highlight the ability of Blue C sediments to immobilise increasing levels of C, N, and metals, and PAH with over time and with both lateral and vertical expansion. This study provides a valuable data set for an anthropogenically impacted blue carbon habitat predicted to suffer from sea-level rise and exponential urban development.

GRAPHICAL ABSTRACT

Summarized results from this study demonstrating the geochemical changes through an elevation gradient, with a transect encompassing intertidal sediments through supratidal salt marsh sediments within Bull Island's blue carbon lagoon zones.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10533-022-00974-0.

Lead accumulation and biochemical responses in Rhus chinensis Mill to the addition of organic acids in lead contaminated soils

Adding organic acid is an effective approach to assist phytoremediation. The effects of organic acids on phytoremediation efficiency are unknown in Rhus chinensis. This study aimed to evaluate the effect of citric acid (CA) and oxalic acid (OA) on the lead phytoremediation potential of R. chinensis with significantly inhibited growth in Pb-contaminated soil. The experimental pot culture study evaluated the long-term physiological response and metal accumulation patterns of R. chinensis grown in varying Pb-treated soil, and examined the effects of 0.5 and 1.0 mmol L^-1 CA and OA on the growth, oxidative stress, antioxidant system, and Pb subcellular distribution of R. chinensis grown in pots with 1000 mg kg^-1 Pb. Compared with the control, the biomass, leaf area, root morphological parameters, and chlorophyll concentration of R. chinensis decreased, whereas the carotenoid, malondialdehyde, H₂O₂, and O₂˙^- concentrations, and superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activity increased under Pb stress. A copious amount of Pb was taken up and mainly stored in the cell walls of the roots. The application of CA and OA increased plant growth. The highest shoots and roots biomass increase recorded was 44.4 and 61.2% in 1.0 mmol L^-1 OA and 0.5 mmol L^-1 CA treatment, respectively. The presence of CA and OA increased SOD, POD, and CAT activities and decreased the H₂O₂, O₂˙^- and malondialdehyde content. A concentration of 0.5 mmol L^-1 CA significantly increased the Pb concentration in the organs. The other organic acid treatments changed root Pb concentrations slightly while increasing shoot Pb concentrations. The translocation factor values from organic acid treatments were increased by 38.8-134.1%. Our results confirmed that organic acid could alleviate the toxicity of stunted R. chinensis and improve phytoremediation efficiency.

Rhizosphere microbial community composition and survival strategies in oligotrophic and metal(loid) contaminated iron tailings areas

In this study, the metal(loid) fractions in two alkaline iron tailings areas with similar physico-chemical properties and the enrichment ability of dominant plants in these areas were investigated. Additionally, high-throughput sequencing and metagenome analysis were used to examine the rhizosphere microbial community structures and their strategies and potential for carbon fixation, nitrogen metabolism, and metal(loid) resistance in mining areas. Results showed that Salsola collina, Setaria viridis, and Xanthium sibiricum have strong enrichment capacity for As, and the maximum transport factor for Mn can reach 4.01. The richness and diversity of bacteria were the highest in rhizosphere tailings, and the dominant phyla were Proteobacteria, Actinobacteria, Ascomycota, and Thaumarchaeota. The key taxa present in rhizosphere tailings were generally metal(loid) resistant, especially Sphingomonas, Pseudomonas, Nocardioides, and Microbacterium. The reductive citrate cycle was the main carbon fixation pathway of microorganisms in tailings. Rhizosphere microorganisms have evolved a series of survival strategies and can adapt to oligotrophic and metal(loid) polluted mining environments. The results of this study provide a basis for the potential application of plant-microbial in situ remediation of alkaline tailings.

Low-molecular-weight organic acid-mediated tolerance and Pb accumulation in centipedegrass under Pb stress

Lead (Pb) is one of the most harmful, toxic pollutants to the ecological environment and humans. Centipedegrass, a fast-growing warm-season turfgrass, is excellent for Pb pollution remediation. Exogenous low-molecular-weight organic acid (LMWOA) treatment is a promising approach for assisted phytoremediation. However, the effects of this treatment on the tolerance and Pb accumulation of centipedegrass are unclear. This study investigated these effects on the physiological growth response and Pb accumulation distribution characteristics of centipedegrass. Applications of 400 μM citric acid (CA), malic acid (MA) and tartaric acid (TA) significantly reduced membrane lipid peroxidation levels of leaves and improved biomass production of Pb-stressed plants. These treatments mainly increased peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) activities and enhanced free protein (Pro), ascorbic acid (AsA) and phytochelatins (PCs) contents, ultimately improving the Pb tolerance of centipedegrass. Their promoting effects decreased as follows: TA>CA>MA. All the treatments decreased root Pb concentrations and increased stem and leaf Pb concentrations, thus increasing total Pb accumulation and TF values. MA had the best and worst effects on Pb accumulation and Pb transportation, respectively. CA had the best and worst effects on Pb transportation and Pb accumulation, respectively. TA exhibited strong effects on both Pb accumulation and transport. Furthermore, all treatments changed the subcellular Pb distribution patterns and distribution models of the chemical forms of Pb in each tissue. The root Pb concentration was more highly correlated with the Pb subcellular fraction distribution pattern, while the stem and leaf Pb concentrations were more highly correlated with the distribution models of the chemical forms of Pb. Overall, TA improved plant Pb tolerance best and promoted both Pb absorption and transportation well and is considered the best candidate for Pb-contaminated soil remediation with centipedegrass. This study provides a new idea for Pb-contaminated soil remediation with centipedegrass combined with LMWOAs.

LncRNA PMAT-PtoMYB46 module represses PtoMATE and PtoARF2 promoting Pb2+ uptake and plant growth in poplar

Lead (Pb²⁺) is one of the most toxic heavy-metal contaminants. Fast-growing woody plants with substantial biomass are ideal for bioremediation. However, the transcriptional regulation of Pb²⁺ uptake in woody plants remains unclear. Here, we identified 226 Pb²⁺-induced, differentially expressed long non-coding RNAs (DELs) in Populus tomentosa. Functional annotation revealed that these DELs mainly regulate carbon metabolism, biosynthesis of secondary metabolites, energy metabolism, and signal transduction through their potential target genes. Association and epistasis analysis showed that the lncRNA PMAT (Pb²⁺-induced multidrug and toxic compound extrusion (MATE) antisense lncRNA) interacts epistatically with PtoMYB46 to regulate leaf dry weight, photosynthesis rate, and transketolase activity. Genetic transformation and molecular assays showed that PtoMYB46 reduces the expression of PtoMATE directly or indirectly through PMAT, thereby reducing the secretion of citric acid (CA) and ultimately promoting Pb²⁺ uptake. Meanwhile, PtoMYB46 targets auxin response factor 2 (ARF2) and reduces its expression, thus positively regulating plant growth. We concluded that the PMAT-PtoMYB46-PtoMATE-PtoARF2 regulatory module control Pb²⁺ tolerance, uptake, and plant growth. This study demonstrates the involvement of lncRNAs in response to Pb²⁺ in poplar, yielding new insight into the potential for developing genetically improved woody plant varieties for phytoremediating lead-contaminated soils.

Lead speciation and availability affected by plants in a contaminated soil

Changes in lead (Pb) speciation in the rhizosphere may be plant species-dependent and dictate Pb fate and behavior in the soil-plant system. X-ray absorption near edge structure (XANES) spectroscopy can explain how these changes affect Pb availability in soils and its uptake by plants. We investigated the changes in Pb speciation and availability in the rhizosphere of eucalypt (Eucalyptus urophylla x Eucalyptus grandis), palisade grass (Urochloa brizantha cv. Marandu), and Indian mustard (Brassica juncea L.) using XANES spectroscopy. A greenhouse experiment was performed in a complete randomized design, with three plant species and a no plant control treatment. After three months, rhizosphere and bulk soil samples were collected, Pb speciation was assessed by Pb L₃-edge XANES spectroscopy, and Pb concentration was determined in plant tissue. In bulk soil, we found Pb primarily as Pb-Il (Pb sorbed to illite; 48%) and lead monoxide (PbO; 36%). In the rhizosphere, Pb-Ka (Pb sorbed to kaolinite; 33-56%) and (CH₃COO)₂Pb (25-41%) were the main Pb forms, the latter suggesting Pb complexation by low molecular weight organic acids (LMWOAs). Palisade grass rhizospheric soil had a lower abundance of Pb-Ka and a higher abundance of (CH₃COO)₂Pb than eucalypt and mustard, which led to low Pb concentration in plant tissue. LMWOAs exudation followed by Pb²⁺ complexation is the apparent mechanism used by palisade grass to detoxify the rhizosphere and control Pb uptake. Given its low Pb uptake and potential to complex Pb in organic forms, palisade grass may be a promising species for Pb phytostabilization in contaminated soils.

Improved Mineral Acquisition, Sugars Metabolism and Redox Status after Mycorrhizal Inoculation Are the Basis for Tolerance to Vanadium Stress in C3 and C4 Grasses

Vanadium (V) can be beneficial or toxic to plant growth and the interaction between arbuscular mycorrhizal fungi (AMF) and V stress was rarely investigated at physiological and biochemical levels of plant groups (C3 and C4) and organs (roots and shoots). We tested the potential of AMF to alleviate the negative effects of V (350 mg V/Kg soil) on shoots and roots of rye and sorghum. Relative to sorghum (C4), rye (C3) showed higher levels of V and lower levels of key elements under V stress conditions. V inhibited growth, photosynthesis, and induced photorespiration (increased HDR & GO activities) and oxidative damage in both plants. AMF colonization reduced V stress by differently mitigating the oxidative stress in rye and sorghum. This mitigation was accompanied with increases in acid and alkaline phosphatase activities in plant roots and increased organic acids and polyphenols exudation into the soil, thus reduced V accumulation (29% and 58% in rye and sorghum shoot, respectively) and improved absorption of mineral nutrients including Ca, Mg and P. AMF colonization improved photosynthesis and increased the sugar accumulation and metabolism. Sugars also acted as a supplier of C skeletons for producing of antioxidants metabolite such as ascorbate. At the antioxidant level, rye was more responsive to the mitigating impact of AMF. Higher antioxidants and detoxification defence system (MTC, GST, phenolics, tocopherols and activities of CAT, SOD and POX) was recorded for rye, while sorghum (C4) improved its GR activity. The C3/C4-specificity was supported by principal component analysis. Together, this study provided both fundamental and applied insights into practical strategies to mitigate the phytotoxicity hazards of V in C3 and C4 grasses. Moreover, our results emphasize the importance of AMF as an environment-friendly factor to alleviate stress effects on plants and to improve growth and yield of unstressed plants.

Soil bacteria, genes, and metabolites stimulated during sulfur cycling and cadmium mobilization under sodium sulfate stress

Sodium sulfate stress is known to improve cadmium (Cd) mobilization in soil and microbial sulfur oxidation, Cd resistance, and the accumulation of stress tolerance-associated metabolites has been correlated with increased soil Cd availability and toxicity. In this study, aerobic soil microcosms with Cd-contamination were stimulated with sodium sulfate to investigate its effects on soil microbial community structure, functional genes, and associated metabolite profiles. Metagenomic analysis revealed that sulfur oxidizing and Cd-resistant bacteria carried gene clusters encoding sox, dsr, and sqr genes, and znt, czc, and cad genes, respectively. Exposure to sodium sulfate resulted in the reprogram of soil metabolites. In particular, intensification of sulfur metabolism triggered an up-regulation in the tricarboxylic acid (TCA) cycle, which promoted the secretion of carboxylic acids and their precursors by soil bacteria. The accumulation of organic acids induced in response to high sodium sulfate dosages potentially drove an observed increase in Cd mobility. Pseudomonas and Erythrobacter spp. exhibited a high capacity for adaptation to heavy metal- or sulfur-induced stress, evident by an increased abundance of genes and metabolites for sulfur cycling and Cd resistance. These results provide valuable insights towards understanding the microbial mechanisms of sulfur transformation and Cd dissolution under saline stress.

Evaluation of some chelating agents on phytoremediation efficiency of Amaranthus caudatus L. and Tagetes patula L. in soils contaminated with lead

PURPOSE

This study was designed to evaluate the possible effects of some chelating agents on phytoremediation efficiency and plant growth parameters of Amaranthus caudatus L. and Tagetes patula L. in soils contaminated with lead.

METHOD

The plant species were grown in pots and treated with lead nitrate and in combination with 2.5, 2.0 and 2.5 mmol/kg of EDTA, SA and CA, respectively.

RESULTS

The results showed that the highest accumulations of Pb (mg/kg) with 0.74 and 0.13 were found in the roots and stems of A. caudatus exposed to 400 mg/kg Pb containing EDTA and SA, respectively. Moreover, the highest accumulation of Pb in the roots and stems of T. patula with 0.87 and 1.5 mg/kg were observed in 400 mg/kg Pb- contaminated soil containing SA.

CONCLUSIONS

Although the results obtained showed that T. patula would have a better phytoextraction potential than A. caudatus, it should be noted that due to the Pb behavior in the soil and/or leaching of Pb from the soil columns during the irrigation period the low amounts of Pb absorption by the root and aerial parts of the plants compared to the added doses of Pb(NO₃)₂ solution to the soil samples, imply the studied plants haven't the adequate potential for phytoextraction of Pb from contaminated soils.

In situ plant bionic remediation of cadmium-contaminated soil caused by a high geological background in Kaihua, Zhejiang Province, China

A plant bionic in situ soil remediation system was designed to rehabilitate acidic cadmium (Cd)-contaminated soil in a high geological background area, Kaihua County of Zhejiang Province in China. In this system, citric acid, an environmental-friendly organic compound, was adopted to activate soil Cd. The soil solution was driven into the plant bionic root using a solar powered simulated transpiration system. Activated Cd in the soil solution was adsorbed by the modified polyurethane foam (DTC-LPEI-PUF) in the bionic root. Under the acidic conditions caused by citric acid (pH = 4.5), DTC-LPEI-PUF could effectively adsorb Cd, and the adsorption rate reached equilibrium after 5 h. Theoretical calculations suggested that the absorption behavior followed pseudo -second order kinetics, and the saturated adsorption capacity of Cd by DTC-LPEI-PUF was 89.05 mg/g, obeying Langmuir isothermal adsorption models. In addition, the main ions in soil, such as calcium (Ca) and magnesium (Mg), had little effect on the adsorption by DTC-LPEI-PUF. However, iron ions (Fe³⁺) significantly influenced the adsorption of Cd by DTC-LPEI-PUF. After 28 d of an in situ remediation, the total contents of Cd in contaminated soil declined from 3.63 mg/kg to 2.69 mg/kg, i.e., 26% of the total Cd was removed. In addition, after remediation, the removal of available Cd reached 47%. Our results demonstrate that the proposed plant bionic in situ remediation system has a promising prospect for application to rehabilitate Cd-contaminated soil in a high geological background area, although the technology needs further improvement.

Biofertilizer-induced response to cadmium accumulation in Oryza sativa L. grains involving exogenous organic matter and soil bacterial community structure

The accumulation of cadmium (Cd) in Oryza sativa L., the world's most significant staple crop, is a health threat to millions of people. The objective of this study was to evaluate the effectiveness of commercially available biofertilizers (with high (BF2) and low organic matter (OM) content (BF1)) on Cd accumulation in two types of soils and to determine the bacterial community responses by high-throughput sequencing. The study was conducted in the form of pot experiment in greenhouse in 2018. Four treatments were set: BF1, BF2, organic fertilizer (OF), and control (CK) and the amendments were applied before the rice cultivation. The results showed that the addition of biofertilizers immobilized or mobilized Cd in soils, depending on the soil type and the OM content in biofertilizers. The exogenous OM in biofertilizers was the driving factor for the difference in pH and Cd accumulation in rice grains. The application of biofertilizers with high OM content was effective in reducing Cd accumulation in the rice grains (19.7% lower than CK) by significantly increasing soil pH (from 6.02 to 6.67) in acid silt loam soil (TZ). The consumption of acid fermentation products by soil chemoorganotrophs and the complexation of organic anions in the biofertilizer treatment tended to buffer the pH drop in the drainage and decrease the Cd availability. However, in the weak acid silty clay loam soil (SX), the addition of biofertilizer with high OM significantly increased Cd accumulation in rice grains (21.9% higher than CK), probably owing to the release of acid substances, resulting from the significant increase of the predominant bacteria Chloroflexi. The addition of biofertilizer with low OM content did not significantly change Cd accumulation in rice grains or affect the soil microbial structures in both soils. In conclusion, the effects of biofertilizer on rice Cd accumulation were related to the OM content and soil bacterial community. Biofertilizers with high organic matter may not be suitable for amendments in the paddy soils with high clay content to reduce Cd accumulation in rice grains.

The effect of EDTA and citric acid on biochemical processes and changes in phenolic compounds profile of okra (Abelmoschus esculentus L.) under mercury stress

The present study aimed to explore the effect of synthetic and naturally occurring chelators, EDTA and citric acid (CA), respectively, on changes in physiological and biochemical factors including cell death, level of mercury ions accumulation, malondialdehyde (MDA) content, total phenol and total flavonoids, anthocyanins and DPPH free radical scavenging activity, in the leaves of okra (Abelmoschus esculentus L.) plants exposed to mercury stress. In addition, polyphenolic compounds profile was assessed by high-performance liquid chromatography. The okras were planted in completely controlled hydroponic conditions (Hoagland solution). After they reached the four-leaf stage, they were treated simultaneously with different concentrations of HgCl₂, EDTA and CA chelators, and their combination for one month. At the stage of maturity, the physiological and biochemical factors of the plant leaves were measured. The results showed that with the application of higher concentration of HgCl₂, cell death, level of shoot and root Hg²⁺ content and root MDA, total phenols and total flavonoids, anthocyanin content, and DPPH free radical scavenging activity were increased. Also, the results indicated that okra plants have high biomass and a high rate of Hg mobilization and accumulation in the shoot versus the roots (TF=2.152 for the plants treated with 60 mg L^-1 Hg²⁺), hence, can be considered as Hg hyperaccumulator plant for the phytoremediation of Hg-polluted soils and waters. In the Hg-treated plants changes in their phenolic profile were induced, and the increase of chlorogenic acid, rosmaric acid, apigenin, quercetin and rutin content was observed. The application of EDTA and CA improved the toxic effects of Hg²⁺, by modifying phenolic compounds, chelating Hg²⁺, and its proper compartmentation, while EDTA outperformed CA in this respect. Based on the results, it could be concluded that due to the high biomass and growth of okra in the presence of Hg²⁺, this plant is suitable for phytoremediation of soil and water contaminated with mercury. In addition, EDTA and CA can play a significant role in removing this toxic metal through transferring it from the culture medium to the plant.

Effect of soil cadmium on root organic acid secretion by forage crops

The two forage species used in New Zealand pastoral agricultural systems, chicory (Cichorium intybus) and plantain (Plantago lanceolata) show differential ability to absorb and translocate cadmium (Cd) from roots to shoots. Chicory can accumulate Cd from even low Cd soils to levels that might exceed regulatory guidelines for Cd in fodder crops and food. Chicory and plantain were grown in soil-filled rhizocolumns under increasing Cd levels (0 (Control), 0.4, 0.8 and 1.6 mg Cd/kg soil) for 60 days and showed variable secretion of oxalic, fumaric, malic and acetic acids as a function of Cd treatment. Plant roots secrete such Low Molecular Weight Organic Acids into the rhizosphere soil, which can influence Cd uptake. Chicory showed significantly (P < 0.05) lower secretion of fumaric acid, and higher secretion of acetic acid than plantain at all Cd treatments. We propose that the significant secretion differences between the two species can explain the significantly (P < 0.05) higher shoot Cd concentration in chicory for all Cd treatments. Understanding the mechanism for increased uptake in chicory may lead to breeding or genetic modification which yield low Cd uptake cultivars needed to mitigate the risk of Cd accumulation in pastoral agricultural food chains from this increasingly important fodder crop.

Removal and recovery of heavy metals from soil with sodium alginate coated FeSSi nanocomposites in a leaching process

Leaching technology has been widely applied to remove heavy metals (HMs) from soil, although the synchronous recovery of multiple HMs during the leaching process was rarely studied. In this study, we synthesized silicon sulfuretted nanoscale zero-valent iron (FeSSi), which was coated with sodium alginate (SA) to form the gel beads (SA-FeSSi). The specific surface area of FeSSi (101.61 m²/g) was significantly increased by adding SiO₂ seeding. The SA stabilization reduced the aggregation of FeSSi. The removal efficiency for cadmium (Cd), lead (Pb), nickle (Ni) and chromium (Cr) by SA-FeSSi in solution reached 80.10 %, 99.96 %, 66.80 % and 80.46 %, respectively. The removal kinetics was well fitted with the pseudo-second-order model. Leaching experiments showed that the recovery efficiency of HMs from solution (Rr/w) and soil (Rr/s) reached to the ranges of 59.79 %-98.70 % and 25.94 %-62.67 % with the addition of 0.3 g SA-FeSSi. Moreover, the leaching conditions including pH, temperature, adsorbent dosage, leaching agent concentrations, leaching time and leaching cycles were also investigated. Our results suggested that SA-FeSSi had an excellent HMs removal capacity and the recovery of HMs during the leaching process by SA-FeSSi could be a potential pathway to reuse the metal resources from soil.

Uptake, translocation and toxicity of chlorinated polyfluoroalkyl ether potassium sulfonate (F53B) and chromium co-contamination in water spinach (Ipomoea aquatica Forsk)

Bioaccumulation and toxicity of per-and polyfluoroalkyl substances and metal in plants have been confirmed, however their contamination in soil and plants still requires extensive investigation. In this study the combined effects of chlorinated polyfluoroalkyl ether potassium sulfonate (F53B) and chromium (Cr) on water spinach (Ipomoea aquatica Forsk) were investigated. Compared with each single stress, the combination of F53B and Cr (VI) reduced the biomass and height and increasingly accumulated in the roots and destroyed the cell structure. Besides, the co-contamination led to the immobilization of F53B and Cr (VI) in soil, which affected their migration in soil and transfer to plants. The antioxidant response and photosynthesis of the plant weakened under the single Cr (VI) and enhanced under the single F53B treatment; however the contamination of F53B and Cr (VI) could also reduce this effect, as confirmed by the gene expression of MTa, psbA and psbcL genes. This study provides an evidence of the environmental risks resulting from the coexistence of F53B and Cr (VI).

Excess sulfur and Fe elements drive changes in soil and vegetation at abandoned coal gangues, Guizhou China

Coal gangue piles accumulate outside mines and can persist for years, negatively impacting the regional environment. To determine the main cause of soil pollution at coal gangues, several coal gangues in Guizhou Province, China that had undergone natural recovery via native plants for 8 years were investigated in summer 2019. Three plots (2 m × 2 m) from the coal gangue area were selected for the treatment (GP). Control plots that were 100 m away from GP were also investigated in contrast (CK-near). In addition, plots from forest, farmland and lake land that were far from GP and largely undisturbed were also investigated as more extreme contrasts (CK-far). A series of soil indicators that can be affected by coal-gangue, such as heavy metals (Mn, Cr, Cd, Ni, Zn, Cu, Pb), As, pH, cation exchange capacity (CEC), sulfur (S) and iron (Fe), were tested for in the plots. Plant species, coverage and height were also analyzed to uncover biodiversity and dominant species information. The results suggested that coal gangue significantly influences soil S, pH and plant species after 8 years of natural recovery. The CK-far plots contained relatively low soil sulfur content, normal pH (close to 7) and abundant plant biodiversity. Generally, pH related positively with both the Patrick (R = 0.79, n = 22, p < 0.001) and Shannon indices (R = 0.67, n = 22, p < 0.001); the soil S related negatively with both the Patrick (R = 0.85, n = 22, p < 0.001) and Shannon indices (R = − 0.79, n = 22, p < 0.001). S content was highest (S = 1.0%) in GP plots, was lower in CK-near plots (S = 0.3%) and was the lowest of all in the plots distant from the coal mine (S = 0.1%, CK-far). S content was negatively correlated with pH. Soil pH decreased significantly, from 7.0 in CK-far, to 5.9 in CK-near, to 4.2 in GP. Soil Fe was 3.4 times higher in GP and CK-near than in CK-far. The excess sulfur and Fe elements and the acidified soil drove changes in soil and vegetation in the coal gangue areas. After 8 years of natural recovery, only a few plants, like Miscanthus floridulus, were able to live near the coal gangue in the area where the soil was still acidic and high in S and Fe.

Phytoremediation—From Environment Cleaning to Energy Generation—Current Status and Future Perspectives

Phytoremediation is a technology based on the use of green plants to remove, relocate, deactivate, or destroy harmful environmental pollutants such as heavy metals, radionuclides, hydrocarbons, and pharmaceuticals. Under the general term of phytoremediation, several processes with distinctively different mechanisms of action are hidden. In this paper, the most popular modes of phytoremediation are described and discussed. A broad but concise review of available literature research with respect to the dominant process mechanism is provided. Moreover, methods of plant biomass utilization after harvesting, with particular regard to possibilities of “bio-ore” processing for metal recovery, or using energy crops as a valuable source for bio-energy production (bio-gas, bio-ethanol, bio-oil) are analyzed. Additionally, obstacles hindering the commercialization of phytoremediation are presented and discussed together with an indication of future research trends.

Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils

Accumulation of heavy metals in agricultural soils due to human production activities-mining, fossil fuel combustion, and application of chemical fertilizers/pesticides-results in severe environmental pollution. As the transmission of heavy metals through the food chain and their accumulation pose a serious risk to human health and safety, there has been increasing attention in the investigation of heavy metal pollution and search for effective soil remediation technologies. Here, we summarized and discussed the basic principles, strengths and weaknesses, and limitations of common standalone approaches such as those based on physics, chemistry, and biology, emphasizing their incompatibility with large-scale applications. Moreover, we explained the effects, advantages, and disadvantages of the combinations of common single repair approaches. We highlighted the latest research advances and prospects in phytoremediation-chemical, phytoremediation-microbe, and phytoremediation-genetic engineering combined with remediation approaches by changing metal availability, improving plant tolerance, promoting plant growth, improving phytoextraction and phytostabilization, etc. We then explained the improved safety and applicability of phytoremediation combined with other repair approaches compared to common standalone approaches. Finally, we established a prospective research direction of phytoremediation combined with multi-technology repair strategy.

Effects of EDTA, citric acid, and tartaric acid application on growth, phytoremediation potential, and antioxidant response of Calendula officinalis L. in a cadmium-spiked calcareous soil

The improved efficiency of cadmium (Cd) phytoextraction potential of Calendula officinalis L. was evaluated in Cd-spiked calcareous soil, using various chelating agents. In a greenhouse study, three chelating agents, including EDTA, citric acid (CA), and tartaric acid (TA), were applied to Cd-spiked soils (50 and 100 mg kg^-1) under C. officinalis L. cultivation. According to the results, C. officinalis grew normally without any toxicity signs at various Cd levels of the soil; however, with increasing the Cd levels, the plant dry weight biomass decreased, and activities of antioxidant enzymes (AOEs) increased. The application of CA and TA in Cd-spiked soils improved the physiologic traits of plants and mitigated the Cd stress since the activities of AOEs decreased. Oppositely, due to increasing the Cd excessive permeability to the root of the plant, EDTA application diminished the physiologic traits and increased the activities of AOEs. The results also showed that all the chelators, especially EDTA, markedly increased the Cd mobility factor (from 58.80% to 65.20-89.60%) in Cd-spiked soils. The bioconcentration factor (BCF = 1.3-2.90) and translocation factor (TF = 1.28-1.58) of Cd, which were >1 in all treated and untreated plant samples, as well as the accumulated Cd >100 mg kg^-1, demonstrated that C. officinalis is a Cd-hyperaccumulator plant which could remediate Cd by the phytoextraction process. Regarding the biodegradation of CA, as well as the increased TF efficiency of Cd and plant biomass of CA treatments (by decreasing oxidative stress), compared to EDTA and TA treatments, it is recommended that CA be used as a superior chelating agent to enhance the efficiency of Cd phytoremediation in C. officinalis.

Acceleration and centralization of a back-diffusion process: Effects of EDTA-2Na on cadmium migration in high- and low-permeability systems

Pollutant accumulation in the low-permeability zones (LPZs) in groundwater systems is regarded as a secondary source, and its consequent back-diffusion can extend the timeframe of pump-and-treat remediation. However, the bioavailability and mobility of heavy metals and the medium characteristics can be changed during the process. This study investigated the accumulation and back-diffusion law of toxic metals and the effects of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) on them by implementing a series of tank experiments. In these experiments, a cadmium solution was injected first, and deionized water or EDTA-2Na constantly washed the system consisting of different medium layers. The experimental results showed that the cadmium breakthrough curves had some concentration gradient reverse points where the curves fluctuated with elution by deionized water, which did not exist when EDTA-2Na was the eluent. In these scenarios, the mass of accumulated cadmium in the media before elution was large, with a value of 931 mg (153 mg/kg), when the low-permeability medium was clay. However, when EDTA-2Na was injected together with cadmium, the value dropped to 319 mg (52.3 mg/kg), greatly reducing the cadmium accumulation. Additionally, the use of EDTA-2Na as an eluent resulted in the appearance of a secondary peak in the breakthrough curve, showing that EDTA-2Na accelerated and centralized the back-diffusion. Notably, the reduced cadmium accumulation in LPZs with the elution by EDTA-2Na was partly due to a reduced adsorption capacity of the clay minerals. The above results can advance the technology related to pump-and-treat remediation.

Yield advantage and cadmium decreasing of rice in intercropping with water spinach under moisture management

Rice (Oryza sativa L.) intercropping with water spinach (Ipomoea aquatica Forsk) is an effective agricultural practice for safe crop production and for phytoremediation in cadmium-contaminated soil. A field and pot experiment were conducted to investigate the growth and cadmium absorption of rice intercropped with water spinach under different moisture management schemes (continuous flooding, interval flooding, and 75% field capacity). In the field experiment, the concentration of Cd in the grain of rice was significantly lower in the intercropping system than that permitted by the National Food Safety Standard of China (GB 2762-2017). Furthermore, the land equivalent ratio (1.42) was higher in the rice-water spinach intercropping system, indicating a significant advantage of the intercropping system in yield. At the same time, the bio-concentration amount (BCA) of Cd of rice and water spinach in intercropping system significantly increased by 17.99% and 31.98%, respectively (P＜0.05). However, the metal removal equivalent ratio (MRER) of Cd was 1.34, which showed the intercropping system of rice-water spinach had advantage in Cd removal. In the pot experiment, the total iron plaque concentration on the root surface of rice and the pH of the rhizosphere soil were higher under continuous flooding (T_CF) than under the control conditions (75% field capacity, T_CK), which could significantly decrease the available Cd in the rhizosphere soil and the accumulation of Cd in rice organs. So, this study demonstrated that iron plaque can obstruct and decrease the Cd absorbed by rice in a rice-water spinach intercropping system combined with water management. The intercropping rice with water spinach can achieve the goal of remediation while producing for farmland contaminated by Cd.

Phytoremediation of cadmium-contaminated wetland soil with Typha latifolia L. and the underlying mechanisms involved in the heavy-metal uptake and removal

The effects of Typha latifolia L. on the remediation of cadmium (Cd) in wetland soil were studied using greenhouse pot culture, with soil Cd concentrations of 0, 1, and 30 mg/kg. The T. latifolia showed excellent tolerance to the low and high concentrations of Cd in soil. A higher bioaccumulation of Cd was observed in roots, with bioconcentration factor values of 51.6 and 9.30 at 1 and 30 mg/kg of Cd stress, respectively; Cd concentration in T. latifolia was 77.0 and 410.7 mg/kg, and Cd content was 0.11 and 0.22 mg/plant at the end of the test period. The soil enzyme activities (urease, alkaline phosphatase, and dehydrogenase) exposed to 0, 1, and 30 mg/kg Cd were measured after 0-, 30-, 60-, and 90-day cultivation period and showed an increasing trend with exposure time. Metabolite changes were analyzed using liquid chromatography-mass spectrometry, combined with principal component analysis and orthogonal partial least squares discrimination analysis. Among 102 metabolites, 21 compounds were found and identified, in response to treatment of T. latifolia with different Cd concentrations. The results showed that T. latifolia had a good remedial effect on Cd-contaminated soil. The metabolites of T. latifolia changed with different Cd concentration exposures, as a result of metabolic response of plants to Cd-contaminated soils. Analysis of metabolites could better reveal the pollution remediation mechanism involved in different Cd uptake and accumulate properties.

Effect of plant root exudates on the desorption of hexachlorocyclohexane isomers from contaminated soils

Plants and their associated microbiota can have a significant impact on the behaviour of soil contaminants. Particularly, root exudation is one of the most important plant-associated processes in this respect, as it may have a substantial effect on the bioavailability of soil contaminants, specially of hydrophobic contaminants strongly sorbed by soil. The aim of the present study was to evaluate the effect of root exudates (natural and artificial) on the desorption of α-, β-, δ- and γ-isomers of hexachlorocyclohexane (HCH) from contaminated soil, using batch experiments. Natural root exudates were obtained from Holcus lanatus plants growing in the same (contaminated) area. Fifteen compounds (mainly organic acids and phenolic compounds) usually found in root exudates were also tested, individually or as mixtures (1 and 10 mM). Both natural and artificial exudates favoured the mobilization of sorbed HCH in soil. The effect was highly significant for α-, β- and γ-HCH isomers, for which the desorption rates increased by 23.0, 26.8 and 15.5% in the presence of natural root exudates and by 40.1, 25.9 and 25.6% in the presence of the artificial mixture (at 10 mM). The δ-HCH desorption rates increased by less than 10%. The effect of individual exudate components was very variable, but increased with the carbon content, reflecting the significance of hydrophobic interactions between the exudates and HCH molecules in the desorption of these last from soil. These findings indicate that plants may significantly influence the bioavailability of persistent contaminants, with major implications for improving phyto- and bioremediation procedures.

Distribution characteristics of Cd in different types of leaves of Festuca arundinacea intercropped with Cicer arietinum L.: A new strategy to remove pollutants by harvesting senescent and dead leaves

Although cost-effective, phytoremediation is too expensive when considering the large-scale pollution. Relative to harvesting the whole plant, it is more practicable to remove and dispose of senescent and dead leaves after phytoremediation. The phytoremediation efficiency of Festuca arundinacea for Cd was evaluated in this study, because over about 7% of the land area in China was contaminated with Cd. The accumulation, redistribution, and extraction of Cd were evaluated in different leaves of F. arundinacea intercropped with N-fixing species at different densities (Cicer arietinum L). The results showed that coordinate and malposed intercropping systems increased the dry weight of the senescent and dead leaves of F. arundinacea by 30-41% and 103-168% compared to the monoculture system, respectively. More Cd was redistributed to the senescent and dead leaves of F. arundinacea under both intercropping systems. Occupying only 22-30% of the total leaf biomass, senescent and dead leaves accumulated 74-88% of leaf Cd under different cultivation conditions. Relative to the monoculture system, intercropping decreased the amount of time needed to reduce soil Cd by 44-53%. The biomass production and Cd accumulation of F. arundinacea were higher in the malposed intercropping system, and it had higher remediation efficiency than the coordinate intercropping system. This study demonstrated that intercropping, especially malposed intercropping of F. arundinacea and C. arietinum L., is a practicable technology for leaf harvesting phytoremediation.

Effect of pH and citric acid on the growth, arsenic accumulation, and phytochelatin synthesis in Eupatorium cannabinum L., a promising plant for phytostabilization

Heavy-metal contamination of soils has increased in the last decades due to anthropogenic and industrial activities. Arsenic is one of the pollutants that is commonly found in industrial soils and is toxic for both plants and humans. The pH of the soil or the culture medium is one of the most important factors that interferes with the bioavailability of this metalloid to the plant. The addition of chelating agents, such as citric acid (CA), can increase the absorption of As by plants. Therefore, the objective of this work is to study the effect of the pH and the exogenous addition of citric acid on the growth, As accumulation, and thiol compounds in Eupatorium cannabinum; this plant grows naturally in contaminated soils in Asturias, Spain, and has a potential use in phytoremediation. The results showed that E. cannabinum was able to tolerate As stress even at extreme pH values and accumulated a high amounts of As in its roots, which makes it a promising species for the phytostabilization of soils polluted with this metalloid. An addition of 20 mg CA L^-1 led to increased biomass and As accumulation at acidic pH. In order to determine if thiolic compounds, such as phytochelatins, are involved in As accumulation and detoxification in E. cannabinum, we analyzed the synthesis of these compounds in the presence and absence of As and/or citric acid. Our results suggest that these thiolic compounds play a major role in As detoxification, since the presence of CA as a chelating agent reduced the amount of thiols necessary to cope with the toxicity caused by As.

Citric acid as soil amendment in cadmium removal by Salix viminalis L., alterations on biometric attributes and photosynthesis

During the past decade, the target in cleaning polluted sites is an application of chelating agents, such as citric acid (CA), which is proposed as a good candidate in the promotion of phytoremediation. Among heavy metals, cadmium (Cd) is one of the most common and dangerous elements, which strongly disturbs morphophysiological properties in plants. A pot experiment was assessed to evaluate the influence of CA in Cd phytoremediation in alkaline soil by Salix viminalis (clone SV068). The effects of CA on Cd bioavailability, mobility, and distribution in plants, various morphometric measurements, and physiological performances as photosynthesis, transpiration, water use efficiency, and pigment content were tested. The highest Cd accumulation was evident after 60 days of growing, in plants subjected to combined treatment of CA with a higher dose of Cd. Application of CA showed a beneficial effect in maintaining the photosynthetic rate as well as gas exchange capacity in willows grown in Cd-contaminated soil. Furthermore, CA slightly increased plant growth and biomass production, depending on applied Cd dose and harvest period. A chelating agent like CA showed strong influence in plant response to combat Cd toxicity.

Enhancement of phytoextraction of Pb by compounded activation agent derived from fruit residue

Chelate-assisted phytoextraction is an attractive strategy to remove toxic metals from soil. However, there is lack of an effective and sustainable chelating agent. In this study, 11 kinds of fruit residue were extracted and selected to combine with N, N-bis (carboxymethyl) glutamic acid (GLDA) (0.7%) and tea saponin (4%) for the compounded activation agent (CAA), and its enhancement on Pb phytoextraction by Sedum alfredii was further evaluated by pot experiment. Among 11 fruit residue extracts, lemon residue showed the highest ability (34.7%) to extract Pb from soil. Through combining with GLDA (0.7%) and tea saponin (4%) at the optimal volume ratio of 15:2.5:2.5, the CAA removed Pb most effectively (57.1%) from soil and increased the solubility of three Pb mineral (PbS, PbCO₃ and PbSO₄) by 8.7-56.4 times. In pot experiment, the addition of high dosage (15 mL) CAA increased the biomass of S. alfredii by 52% and doubled the Pb accumulation. In addition, CAA-assisted phytoextraction also increased both water-soluble and acid-soluble Pb in soil, while reduced the proportion of the immobile Pb (oxidizable and residual). Generally, the compounded activation agent derived from lemon residue could be considered as-a promising enhancer for Pb phytoextraction.

Opportunities and challenges in the remediation of metal-contaminated soils by using tobacco (Nicotiana tabacum L.): a critical review

The successful phytoextraction of potentially toxic elements (PTEs) from polluted soils can be achieved by growing non-food and industrial crops. Tobacco (Nicotiana tabacum L.) is one of the main industrial crops and is widely grown in many countries. Tobacco can uptake high concentrations of PTEs especially in aboveground biomass without suffering from toxicity. This review highlighted the potential of tobacco for the phytoextraction of heavy metals and tolerance mechanisms under metal stress. Different management practices have been discussed which can enhance the potential of this plant for metal extraction. Finally, suitable options for the management/disposal of biomass enriched in excess metal have been elaborated to prevent secondary pollution.

Comparative effect of organic amendments on physio-biochemical traits of young and old bean leaves grown under cadmium stress: a multivariate analysis

The current study investigated the influence of organic amendments on cadmium (Cd) uptake and its effects on biochemical attributes of young and old leaves of bean. Bean seedlings were exposed to two levels of Cd (25 and 100 μM) in the presence and absence of different levels of ethylenediaminetetraacetic acid (EDTA) and citric acid (CA). An increase in Cd concentration in growth medium significantly enhanced Cd accumulation in bean roots and shoot. Cadmium stress increased the production of H₂O₂ which resulted in lipid peroxidation and decreased chlorophyll contents. The presence of organic amendments significantly affected Cd accumulation and toxicity to bean plants. Application of EDTA alleviated Cd toxicity in terms of chlorophyll contents, H₂O₂ contents, and lipid peroxidation possibly by chelating toxic Cd ions, and as such forming Cd-EDTA complexes. The presence of CA decreased Cd toxicity by decreasing its uptake. The biochemical responses (H₂O₂ contents, lipid peroxidation, and chlorophyll contents) of bean plants were more severely affected by Cd treatments in old leaves compared to young leaves. This study shows that the effect of CA and EDTA on biochemical behavior of Cd varies greatly with applied levels of Cd and amendments as well as the age of leaves. Based on the results, it is proposed that the presence of organic amendments can greatly affect biogeochemical behavior of Cd in the soil-plant system (ecosystem).

A multivariate analysis of health risk assessment, phytoremediation potential, and biochemical attributes of Spinacia oleracea exposed to cadmium in the presence of organic amendments under hydroponic conditions

Cadmium (Cd) phytoremediation potential and its accumulation in edible and nonedible plant tissues is the function of various biochemical processes taking place inside plants. This study assessed the impact of organic ligands on Cd phyto uptake and different biophysiochemical processes of Spinacia oleracea L., and associated health hazards. Plants were exposed to Cd alone and chelated with citric acid (CA) and ethylenediaminetetraacetic acid (EDTA). Results revealed that the effect of Cd on lipid peroxidation, H₂O₂ production and pigment contents varied greatly with its applied level and the type of organic ligand. Moreover, the effect was more prominent in root tissues than leaf tissues and for high concentrations of Cd and organic ligands. Cadmium accumulation increased by 90 and 74% in roots and leaves, respectively, with increasing Cd levels (25-100 µM). Cadmium exposure at high levels caused lipid peroxidation in roots only. Application of both CA and EDTA slightly diminished Cd toxicity with respect to pigment contents, lipid peroxidation and hydrogen peroxide (H₂O₂) contents. Hazard quotient (HQ) of Cd was <1.00 for all the treatments. Under nonlinear effect of treatments, multivariate analysis can be an effective tool to trace overall effects/trends.

Biochar and crushed straw additions affect cadmium absorption in cassava-peanut intercropping system

Cassava (Manihot esculenta Crantz) intercropped with peanut (Arachis hypogaea) has good complementary effects in time and space. In the field plot test, the land equivalent ratio (LER) of cassava-peanut intercropping system was 1.43, showing obvious intercropping yield advantage. Compared with monocropping, Cd contents in the roots of cassava and seeds of peanut were significantly reduced by 20.00% and 31.67%, respectively (p < 0.05). Under the unit area of hectare, compared with monocropping of cassava and peanut, the bioconcentration amount (BCA) of Cd in the intercropping system increased significantly by 24.98% and 25.59%, respectively (p < 0.05), and the metal removal equivalent ratio (MRER) of Cd was 1.25, indicating that the intercropping pattern had advantage in Cd removal. In the cement pool plot test, compared with the control, cassava intercropped with peanut under biochar and crushed straw additions did not only enhance the available nutrients and organic matter contents in rhizosphere soil but also promoted the crop growth and increased the content of chlorophyll (SPAD values) of plant leaves. The peanut seeds biomass under biochar and straw additions were significantly increased by 112.34% and 59.38% (p < 0.05), respectively, while the cassava roots biomass under biochar addition was significantly increased by 63.54% (p < 0.05). Applying biochar significantly decreased the content of Cd which extracted by diethylenetriaminepentaacetic acid (DTPA-Cd) in soil and reduced Cd uptake as well as translocation into plant tissues. The BCA of Cd of cassava under biochar addition decreased significantly by 53.87% in maturity stage (p < 0.05), thus reduced the ecological risk of Cd to crops and was of great significance to produce high quality and safe agricultural products. Besides, the crushed straw enhanced the biomass of crops, reduced Cd content in all tissues and maintained Cd uptake in the intercropping system. Therefore, it can realize the integration of ecological remediation and economic benefit of two energy plants in Cd contaminated soil after applied crushed straw in cassava-peanut intercropping system.

Accumulation of cadmium in potential hyperaccumulators Chlorophytum comosum and Callisia fragrans and role of organic acids under stress conditions

Cadmium (Cd) accumulation, antioxidant activity (AOA), chlorophyll fluorescence (F) and organic acid distribution in Chlorophytum comosum and Callisia fragrans plants exposed to artificially added Cd (40, 160 and 320 mg kg^-1) were examined in pot experiment. At the highest Cd concentration, C. comosum accumulated in roots and the aboveground parts up to 1331 and 1054 mg Cd kg^-1 DW, and C. fragrans up to 1427 and 1263 mg Cd kg^-1 DW, respectively, which are quite near at the level of hyperaccumulator. Cd accumulation in both plant species increased significantly with the increment of soil Cd dosage, and the distribution was roots > shoots > stolons. Values of BC showed rising trend indicating an accumulation potential of both species. The root AOA was positively correlated to Cd addition, especially in C. comosum. Higher values of free SA were found in roots with a significant enhancement at concentrations of 40 and 160 mg kg^-1 Cd. It was observed that citric acid significantly reacted in both species, while fumaric acid only in C. comosum in response to Cd which may contribute to Cd chelation. Our data indicate that both species are suitable for phytoextraction of Cd from contaminated soils which increases their value as ornamentals.

Response of edible amaranth cultivar to salt stress led to Cd mobilization in rhizosphere soil: A metabolomic analysis

The present study aimed to investigate the metabolic response of edible amaranth cultivars to salt stress and the induced rhizosphere effects on Cd mobilization in soil. Two edible amaranth cultivars (Amaranthus mangostanus L.), Quanhong (low-Cd accumulator; LC) and Liuye (high-Cd accumulator; HC), were subject to salinity treatment in both soil and hydroponic cultures. The total amount of mobilized Cd in rhizosphere soil under salinity treatment increased by 2.78-fold in LC cultivar and 4.36-fold in HC cultivar compared with controls, with 51.2% in LC cultivar and 80.5% in HC cultivar being attributed to biological mobilization of salinity. Multivariate statistical analysis generated from metabolite profiles in both rhizosphere soil and root revealed clear discrimination between control and salt treated samples. Tricarboxylic acid cycle in root was up-regulated to cope with salinity treatment, which promoted release of organic acids from root. The increased accumulation of organic acids in rhizosphere under salt stress obviously promoted soil Cd mobility. These results suggested that salinity promoted release of organic acids from root and enhanced soil Cd mobilization and accumulation in edible amaranth cultivar in soil culture.

The chelating effect of citric acid, oxalic acid, amino acids and Pseudomonas fluorescens bacteria on phytoremediation of Cu, Zn, and Cr from soil using Suaeda vera

Phytoextraction is a green technique for the removal of soil contaminants by plants uptake with the subsequent elimination of the generated biomass. The halophytic plant Suaeda vera Forssk. ex J.F.Gmel. is an native Mediterranean species able to tolerate and accumulate salts and heavy metals in their tissues. The objective of this study was to explore the potential use of S. vera for soil metal phytoextraction and to assess the impact of different chelating agents such as natural organic acids (oxalic acid [OA], citric acid [CA]), amino acids (AA) and Pseudomonas fluorescens bacteria (PFB) on the metal uptake and translocation. After 12 months, the highest accumulation of Cu was observed in the root/stem of PFB plots (17.62/8.19 mg/kg), in the root/stem of CA plots for Zn (31.16/23.52 mg/kg) and in the root of OA plots for Cr (10.53 mg/kg). The highest accumulation of metals occurred in the roots (27.33-50.76 mg/kg). Zn was the metal that accumulated at the highest rates in most cases. The phytoextraction percentages were higher for Cu and Zn (∼2%) with respect to Cr (∼1%). The percentages of metal removal from soil indicate the need to monitor soil properties, to recognize the influence of each treatment and to increase the concentration of bioavailable metals by the use of agricultural management practices aimed at promoting plant growth.

Complete Genome Sequence of Cd(II)-Resistant Arthrobacter sp. PGP41, a Plant Growth-Promoting Bacterium with Potential in Microbe-Assisted Phytoremediation

Microbe-assisted phytoremediation has great potential for practical applications. Plant growth-promoting bacteria (PGPB) with heavy metal (HM) resistance are important for the implementation of PGPB-assisted phytoremediation of HM-contaminated environments. Arthrobacter sp. PGP41 is a Cd(II)-resistant bacterium isolated from the rhizosphere soils of a Cd(II) hyperaccumulator plant, Solanum nigrum. Strain PGP41 can significantly improve plant seedling and root growth under Cd(II) stress conditions. This bacterium exhibited the ability to produce high levels of indole-3-acetic acid (IAA), as well as the ability to fix nitrogen and solubilize phosphate, and it possessed 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. Here, we present the complete genome sequence of strain PGP41. The genome consists of a single chromosome with a G+C content of 65.38% and no plasmids. The genome encodes 3898 genes and contains 49 tRNA and 12 rRNA genes. Multiple genes associated with plant growth promotion were identified in the genome. The whole genome sequence of PGP41 provides information useful for further clarifying the molecular mechanisms behind plant growth promotion by PGPB and facilitates its potential use as an inoculum in the bioremediation of HM-contaminated environments.

Phosphorus-loaded biochar changes soil heavy metals availability and uptake potential of maize (Zea mays L.) plants

Biochar (BC) was produced by pyrolyzing the date palm leaf waste at 600 °C and then loaded with phosphorus (P) via sorption process. Greenhouse pot experiment was conducted to investigate the application effects of BC and P-loaded biochar (BCP) on growth and availability of P and heavy metals to maize (Zea mays L.) plants grown in contaminated mining soil. The treatments consisted of BC and BCP (at application rates of 5, 10, 20, and 30 g kg^-1 of soil), recommended NK and NPK, and a control (no amendment). Sorption experiment showed that Langmuir predicted maximum P sorption capacity of BC was 13.71 mg g^-1. Applying BCP increased the soil available P, while BC and BCP significantly decreased the soil labile heavy metals compared to control. Likewise, heavy metals in exchangeable and reducible fractions were transformed to more stable fraction with BC and BCP applications. The highest application rate of BCP (3%) was most effective treatment in enhancing plant growth parameters (shoot and root lengths and dry matter) and uptake of P and heavy metals by 2-3 folds. However, based on metal uptake and phytoextraction indices, total heavy metals extraction by maize plants was very small for practical application. It could be concluded that using P-loaded biochar as a soil additive may be considered a promising tool to immobilize heavy metals in contaminated mining areas, while positive effects on the biomass growth of plants may assist the stabilization of contaminated areas affected by wind and water erosion.

Phytoextraction of heavy metals from a multiply contaminated dredged sediment by chicory (Cichorium intybus L.) and castor bean (Ricinus communis L.) enhanced with EDTA, NTA, and citric acid application

The remediation of contaminated dredged sediments is necessary to eliminate the risk towards human beings or the environment when there is disposal on land. A greenhouse experiment was carried out to evaluate the chemically assisted phytoextraction to clean up dredged sediment contaminated with Cr, Cu, Pb, and Zn. The ability of castor bean and chicory to absorb, translocate, and accumulate metals from sediment to root and shoot was evaluated by applying EDTA (5 mM), NTA (5 mM), and citric acid (60 mM) to sediment, before the harvest. Citric acid 60 mM was the most effective treatment in increasing Cr, Cu, and Pb in castor bean and chicory shoot. Chicory could accumulate 1730 mg Cr kg-1 in shoot, and had greater values than one for the bioaccumulation and translocation factors when citric acid was added to the sediment. But, the Cr percentages removed per harvest of chicory were 0.05% and were lower for Cu, Pb, and Zn due to low biomass obtained. Citric acid-assisted phytoextraction with chicory can be a promising short time solution to reduce Cr concentration in sediment and reach the Cr level guide for industrial land use only if suitable agronomic practices could be implemented to increase crop yield.

Inhibitory effects of rice residues amendment on Cd phytoavailability: A matter of Cd-organic matter interactions?

Incorporating crop residues into soils, a most common way of organic input into farmland soils, is being encouraged in many parts of the world, while its potential impacts on Cd phytoavailability are not well understood. Here, a Cd-contaminated soil was amended with rice residues (RR, i.e., straw + root mixture) or not (Control) and incubated for 81 days under laboratory-controlled conditions. During the incubation, key soil parameters (e.g., dissolved organic carbon and pH), Cd geochemical fractionation (by BCR sequential extraction), and CaCl₂ extracted Cd in soils (by 0.01 M CaCl₂ extraction) were quantified to explain the effects of RR amendment on Cd phytoavailability (assessed by 7 day-cultivation of rice seedling in soils). Besides, hydroponic experiments were designed to explore the effects of D-RR-OM (dissolved-RR-organic matter) on the uptake of Cd by rice seedlings (quantified by uptake constant rate, k_u, using stable isotope tracing technique). Our results demonstrated that RR amendment reduced Cd phytoavailability by 17-92% compared with Control during incubation, which might be explained by the interactions between Cd and RR-OM (RR-organic matter) in soil or porewater: (1) Cd immobilization due to its association with solid-RR-OM in soils, (2) Cd mobilization by D-RR-OM, and (3) Cd complexation with D-RR-OM in porewater, and thus reduced k_u of Cd. Our results suggested that dynamics of RR-OM (e.g., dissolution, decomposition and transformation) in soils, and thus interactions between Cd and solid/dissolved-RR-OM may control Cd phytoavailability under RR amendment. Information gained in this study would further our understanding about Cd phytoavailability in farming soils.

Differential Effects of Low-Molecular-Weight Organic Acids on the Mobilization of Soil-Borne Arsenic and Trace Metals

A batch experiment was conducted to examine the effects of six low-molecular-weight organic acids on the mobilization of arsenic and trace metals from a range of contaminated soils. The results showed that the organic acids behaved differently when reacting with soil-borne As and trace metals. Oxalic acid and acetic acid had the strongest and weakest capacity to mobilize the investigated elements, respectively. The solubilisation of iron oxides by the organic acids appears to play a critical role in mobilizing other trace metals and As. Apart from acidification and complexation, reductive dissolution played a dominant role in the dissolution of iron oxides in the presence of oxalic acid, while acidification tended to be more important for dissolving iron oxides in the presence of other organic acids. The unique capacity of oxalic acid to solubilize iron oxides tended to affect the mobilization of other elements in different ways. For Cu, Mn, and Zn, acidification-driven mobilization was likely to be dominant while complexation might play a major role in Pb mobilization. The formation of soluble Fe and Pb oxalate complexes could effectively prevent arsenate or arsenite from combining with these metals to form solid phases of Fe or Pb arsenate or arsenite.

Removal of toxic metals from vanadium-contaminated soils using a washing method: Reagent selection and parameter optimization

Vanadium (V) contamination in soils is an increasing worldwide concern facing human health and environmental conservation. The fractionation of a metal influences its mobility and biological toxicity. We analyzed the fractionations of V and several other metals using the BCR three-step sequential extraction procedure. Among methods for removing metal contamination, soil washing is an effective permanent treatment. We conducted experiments to select the proper reagents and to optimize extraction conditions. Citric acid, tartaric acid, oxalic acid, and Na₂EDTA all exhibited high removal rates of the extractable state of V. With a liquid-to-solid ratio of 10, washing with 0.4 mol/L citric acid, 0.4 mol/L tartaric acid, 0.4 mol/L oxalic acid, and 0.12 mol/L Na₂EDTA led to removal rates of 91%, 88%, 88%, and 61%, respectively. The effect of multiple washing on removal rate was also explored. According to the changes observed in metal fractionations, differences in removal rates among reagents is likely associated with their pK_a value, pH in solution, and chemical structure. We concluded that treating with appropriate washing reagents under optimal conditions can greatly enhance the remediation of vanadium-contaminated soils.

Phytoextraction of contaminated urban soils by Panicum virgatum L. enhanced with application of a plant growth regulator (BAP) and citric acid

Lead (Pb) contamination in soil represents a threat to human health. Phytoextraction has gained attention as a potential alternative to traditional remediation methods because of lower cost and minimal soil disruption. The North American native switchgrass (Panicum virgatum L.) was targeted due to its ability to produce high biomass and grow across a variety of ecozones. In this study switchgrass was chemically enhanced with applications of the soil-fungicide benomyl, chelates (EDTA and citric acid), and PGR to optimize phytoextraction of Pb and zinc (Zn) from contaminated urban soils in Atlanta, GA. Exogenous application of two plant hormones was compared in multiple concentrations to determine effects on switchgrass growth: indole-3-acetic acid (IAA), and Gibberellic Acid (GA₃), and one PGR benzylaminopurine (BAP), The PGR BAP (1.0 μM) was found to generate a 48% increase in biomass compared to Control plants. Chemical application of citric acid, EDTA, benomyl, and BAP were tested separately and in combination in a pot experiment in an environmentally controlled greenhouse to determine the efficacy of phtyoextraction by switchgrass. Soil acidification by citric acid application resulted in highest level of aluminum (Al) and iron (Fe) in plants foliage resulting in severe phytotoxic effects. Total Pb phytoextraction was significantly highest in plants treated with combined chemical application of B + C and B + C + H. Suppression of AMF activities by benomyl application significantly increased concentrations of Al and Fe in roots. Application of benomyl reduced AMF colonization but was also shown to dramatically increase levels of septa fungi infection as compared to Control plants.

Citric acid: emerging applications of key biotechnology industrial product

Owing to new biotechnological production units mostly located in China, global supply of citric acid in the course of the last two decades rose from less than 0.5 to more than 2 million tonnes becoming the single largest chemical obtained via biomass fermentation and the most widely employed organic acid. Critically reviewing selected research achievements and production trends, we identify the reasons for which this polycarboxylic acid will become a key chemical in the emerging bioeconomy.Graphical abstract

Malate secretion from the root system is an important reason for higher resistance of Miscanthus sacchariflorus to cadmium

Miscanthus is a vigorous perennial Gramineae genus grown throughout the world as a promising bioenergy crop and generally regarded as heavy metal tolerant due to its ability to absorb heavy metals. However, little is known about the mechanism for heavy metal tolerance in Miscanthus. In this study, two Miscanthus species (Miscanthus sacchariflorus and Miscanthus floridulus) exhibiting different cadmium (Cd) sensitivity were used to address the mechanisms of Cd tolerance. Under the same Cd stress, M. sacchariflorus showed higher Cd tolerance with better growth and lower Cd accumulation in both shoots and roots than M. floridulus. The malate (MA) content significantly increased in root exudates of M. sacchariflorus following Cd treatment while it was almost unchanged in M. floridulus. Cellular Cd analysis and flux data showed that exogenous MA application markedly restricted Cd influx and accumulation while an anion-channel inhibitor (phenylglyoxal) effectively blocked Cd-induced MA secretion and increased Cd influx in M. sacchariflorus, indicating that MA secretion could alleviate Cd toxicity by reducing Cd uptake. The genes of malate dehydrogenases (MsMDHs) and Al-activated malate transporter 1 (MsALMT1) in M. sacchariflorus were highly upregulated under Cd stress, compared with that in M. floridulus. The results indicate that Cd-induced MA synthesis and secretion efficiently alleviate Cd toxicity by reducing Cd influx in M. sacchariflorus.

The effect of biochar and crop straws on heavy metal bioavailability and plant accumulation in a Cd and Pb polluted soil

Biochar derived from various materials has been investigated with regard to its ability to decrease the bioavailability of heavy metals in contaminated soils, and thus reduce their potential to enter the food chain. However, little attention has been given to the adsorption capacity of untreated crop straws, which are commonly used as a biochar feedstock, especially in soils. Hence, this study was conducted to investigate the effect of crop straws on heavy metal immobilization and subsequent heavy metal uptake by maize and ryegrass in a soil artificially polluted by Cd and Pb. Bamboo biochar, rice straw, and wheat straw were mixed into soil four weeks before the experiment, enabling them to reach equilibrium at 2% (w/w), 1% (w/w), and 1% (w/w), respectively. The results showed that soil pH for both species was significantly increased by all treatments, except when wheat straw was used for ryegrass cultivation. Soil organic carbon was only improved in the rice straw treatment and the soil alkali-hydrolyzable N content was significantly decreased with all of the amendments, which may have contributed to the lack of an effect on plant biomass. Soil available Cd was significantly lower in the rice straw treatment than in the control soil, while Pb levels clearly decreased in wheat straw treatment. The Cd concentration in shoots of maize was reduced by 50.9%, 69.5%, and 66.9% with biochar, rice straw, and wheat straw, respectively. In addition, shoot Cd accumulation was decreased by 47.3%, 67.1%, and 66.4%, respectively. Shoot Pb concentration and accumulation were only reduced with the rice straw treatment for both species. However, metal uptake in plant roots was more complex, with increased metal concentrations also detected. Overall, the direct application of crop straw could be considered a feasible way to immobilize selected metals in soil, once the long-term effects are confirmed.