Phytoremediation potential of castor (Ricinus communis L.) in the soils of the abandoned copper mine in Northern Oman: implications for arid regions

Assessment of the Cu phytoremediation potential of Chrysanthemum indicum L. and Tagetes erecta L. using analysis of growth and physiological characteristics

In the current study, the Cu phytoremediation ability of two ornamental plants, Chrysanthemum indicum L. and Tagetes erecta L., was tracked concerning the growth and physiological responses. Plants were subjected to varying concentrations of Cu (0, 100, 200, and 400 mg/kg) under the pot experiment for 8 weeks. The results showed that the measured growth and physiological characteristics declined in T. erecta shoots and roots at all tested treatments compared with the control. However, in C. indicum at 100 mg/kg, shoot biomass, shoot total soluble protein, and leaves number remained equal to that of the control and then reduced by rising Cu concentrations, compared with the control. Also, results indicated that in C. indicum, after 56 days of exposure to Cu, the chlorophyll pigments content markedly increased and reached a maximum level at 100 mg/kg dose and gradually declined with enhancing Cu concentrations, compared with the control. Other measured growth and physiological parameters decreased in both tissues of C. indicum in response to Cu usage in the growth medium. The carotenoid content of T. erecta decreased in all studied Cu levels in comparison to the control, but in C. indicum remained unaffected up to 200 mg/kg Cu in comparison to the control and then enhanced with increasing Cu level. The augmentation of antioxidant enzyme activity in two species, especially in roots, reflected the incident of Cu stress as demonstrated by elevated MDA and ion leakage levels. Data concerning copper accumulation in tissues, TF, and BAF showed T. erecta is a weak Cu accumulator and seems not to be an appropriate candidate for Cu phytoremediation. However, the Cu content in shoots and roots of C. indicum increased significantly with an increment in applied Cu level. Also, C. indicum accumulated higher Cu concentrations in the roots than in shoots and exhibited TF < 1, 0.1 < BAF root < 1, and can be considered as a Cu excluder by the phytostabilization mechanism.

Bio/phyremediation potential of Leptospirillum ferrooxidans and Ricinus communis on metal contaminated mine sludge

The heavy metal pollution is a serious environmental pollution around the globe and threatens the ecosystem. The physicochemical traits (pH, Electrical conductivity, hardness, NPK, Al, Fe, Cd, Cr, Pb, Mg, and Mn) of soil sample collected from the polluted site were analyzed and found that the most of the metal contents were beyond the acceptable limits of national standards. The metals such as Mn (1859.37 ± 11.25 mg kg-1), Cd (24.86 ± 1.85 mg kg-1), Zn (795.64 ± 9.24 mg kg-1), Pb (318.62 ± 5.85 mg kg-1), Cr (186.84 ± 6.84 mg kg-1), and Al (105.84 ± 5.42 mg kg-1) were crossing the permissible limits. The pre-isolated L. ferrooxidans showed considerable metal tolerance to metals such as Al, Cd, Cr, Pb, Mg, and Mn at up to the concentration of 750 μg mL-1 and also have remediation potential on polluted soil in a short duration of treatment. The greenhouse study demonstrated that the bio/phytoremediation potential of metal tolerant L. ferrooxidans and R. communis under various remediation (A, B, and C) groups. Surprisingly, remediation group C demonstrated greater phytoextraction potential than the other remediation groups (A and B). These results strongly suggest that coexistence of L. ferrooxidans and R. communis had a significant positive effect on phytoextraction on metal-contaminated soil.

The root-associated Fusarium isolated based on fungal community analysis improves phytoremediation efficiency of Ricinus communis L. in multi metal-contaminated soils

Phytoremediation is a widely accepted bioremediation method of treating heavy metal contaminated soils. Nevertheless, the remediation efficiency in multi-metal contaminated soils is still unsatisfactory attributable to susceptibility to different metals. To isolate root-associated fungi for improving phytoremediation efficiency in multi-metal contaminated soils, the fungal flora in root endosphere, rhizoplane, rhizosphere of Ricinus communis L. in heavy metal contaminated soils and non-heavy metal contaminated soils were compared by ITS amplicon sequencing, and then the critical fungal strains were isolated and inoculated into host plants to improve phytoremediation efficiency in Cd, Pb, and Zn-contaminated soils. The fungal ITS amplicon sequencing analysis indicated that the fungal community in root endosphere was more susceptible to heavy metals than those in rhizoplane and rhizosphere soils and Fusarium dominated the endophytic fungal community of R. communis L. roots under heavy metal stress. Three endophytic strains (Fusarium sp. F2, Fusarium sp. F8, and Fusarium sp. F14) isolated from Ricinus communis L. roots showed high resistances to multi-metals and possessed growth-promoting characteristics. Biomass and metal extraction amount of R. communis L. with Fusarium sp. F2, Fusarium sp. F8, and Fusarium sp. F14 inoculation in Cd-, Pb- and Zn-contaminated soils were significantly higher than those without the inoculation. The results suggested that fungal community analysis-guided isolation could be employed to obtain desired root-associated fungi for enhancing phytoremediation of multi-metal contaminated soils.

Phytomanagement of iron mine soil by Ricinus communis L. and garden soil

Soil pollution and heavy metals (HMs) contamination caused by the improper management of mine soil is a major concern for the environment and the associated living beings. The present study was carried out for 90 days with iron mine soil (MS) amendment with different ratios of garden soil (GS) (0, 25, 50, 75 and 100%). The study investigates the growth performance, metal tolerance, metal accumulation (Fe, Pb, Cu and Ni) ability of R. communis L. and the improvement in soil health after harvesting the plants. The MS had a high level of Fe, Pb, Cu and Ni (2017.17, 65.34, 34.02 and 69.15 mg kg^-1 respectively) with significantly low pH, water holding capacity (WHC), organic carbon (OC), organic matter (OM) and nutrients along with microbial biomass carbon and nitrogen (Cmic and Nmic). The study found that there are higher growth rates and biomass for plants grown in all GS treatments compared to 100% MS. The relative water content (%), tolerance index and carotenoid content exhibit upwards trends with the increasing growing period. The HMs accumulation in shoot and root was found highest for Fe (1354.44 and 3989.61 mg kg^-1) and Pb (31.88 and 34.83 mg kg^-1). The metal extraction ratio for all studied metals was found maximum in 50 and 75% GS treatment plants. Further, the HMs removal percentage was recorded between 14.82 and 54.86%. The soil physicochemical and biological properties like electrical conductivity, total nitrogen, Cmic and Nmic increased up to 50% and the OC and OM improved manyfold in 100% MS. Based on the findings, it is concluded that R. communis L. has the potential to easily cultivate in mine abandoned soil and tolerate high concentrations of HMs.

Cu and Pb accumulation and removal from aqueous medium by Enydra fluctuans Lour. (Asteraceae) - a medicinal plant with potential for phytoremediation

Enydra fluctuans Lour. (Asteraceae) is an edible semi-aquatic floating or trailing herbaceous plant widely distributed in tropical Africa, South and South East Asia, and Australia. Its leaves, which are consumed as a vegetable, are also used in traditional medicine to treat several diseases. The efficacy of this plant in removal of copper and lead from aqueous medium was tested in the present study. The plants were exposed to graded measured concentrations of 0.55-10.2 mg Cu L^-1 and 11.5-50.2 mg Pb L^-1 in hydroponic systems. Controls without added Cu and Pb were maintained under identical conditions. Cu and Pb were estimated by atomic absorption spectrometry, and the bioconcentration factor (BCF) and the translocation factor (TF) were calculated for each element at the different concentrations. Accumulation of both Cu and Pb was significantly higher in root than that in leaf and stem. Though all the bioconcentration factor (BCF) values were greater than unity, none of the translocation factor (TF) values was greater than unity, indicating that this plant could not be considered a hyperaccumulator of these metals. Nevertheless, E. fluctuans could remove Cu from aqueous medium at rates ranging from 98.8 to 99.7%, with a mean reduction of 99.2% after 96-h exposure at various concentrations. The removal of Pb ranged from 97.1 to 99.1%, with a mean reduction of 98.2%. Thus, E. fluctuans showed high potential for removal of Cu and Pb from aqueous medium and has the prospect of being used in phytoremediation of these metals.

A global meta-analysis of heavy metal(loid)s pollution in soils near copper mines: Evaluation of pollution level and probabilistic health risks

With the rapid development of the mining industry, the pollution of heavy metal(loid)s in soils near copper (Cu) mining sites is a significant concern worldwide. However, the pollution status and probabilistic health risks of heavy metal(loid)s of soils associated with Cu mines, have rarely been studied on a global scale. In this study, eight heavy metal(loid) concentrations in soil samples taken near 102 Cu mining sites worldwide were obtained through a literature review. Based on this database, the heavy metal(loid) pollution and ecological risk in soils near Cu mines were evaluated. Most of the study sites exceeded the moderately to heavily polluted levels of Cu and Cd; compared to other regions, higher pollution levels were observed at sites in Oman, China, Australia, and the United Kingdom. Soil pollution by Cd, Pb, and Zn at agricultural sites was higher than that in non-agricultural sites. In addition, these heavy metal(loid)s produced a high ecological risk to soils around Cu mining sites in which the contribution of Cd, Cu, and As reached up to 46.5%, 21.7%, and 18.4%, respectively. The mean hazard indices of the eight heavy metal(loid)s were 0.209 and 0.979 for adults and children, respectively. The Monte Carlo simulation further predicted that 1.40% and 29.9% of non-carcinogenic risk values for adults and children, respectively, exceeded the safe level of 1.0. Moreover, 84.5% and 91.0% of the total cancer risk values for adults and children, respectively, exceeded the threshold of 1E-04. Arsenic was the main contributor to non-carcinogenic risk, while Cu had the highest exceedance of carcinogenic risk. Our findings indicate that the control of Cu, Cd, and As should be prioritized because of their high incidence and significant risks in soils near Cu mines. These results provide valuable inputs for policymakers in designing effective strategies for reducing the exposure of heavy metal(loid)s in this area worldwide.

Plant growth promoting strain Bacillus cereus (RCS-4 MZ520573.1) enhances phytoremediation potential of Cynodon dactylon L. in distillery sludge

Elevated levels of physico-chemical pollution including organic pollutants, metals and metalloids were detected in distillery sludges despite of the anaerobic digestion treatment prior to disposal. The concentrations of the metals were (in mg kg^-1): Fe (400.98 ± 3.11), Zn (17.21 ± 0.54), Mn (8.32 ± 0.42), Ni (8.00 ± 0.98), Pb (5.09 ± 0.43), Cr (4.00 ± 0.98), and Cu (3.00 ± 0.10). An invasive grass species, Cynodon dactylon L., demonstrated its ability to remediate the distillery waste sludge (DWS) in the field study. All the physico-chemical parameters of the sludge significantly improved (up to 70-75%) in the presence of Cynodon dactylon L. (p < 0.001) than the control with no plant growth. The highest phytoremediation capacity was associated with the uptake of Fe in the root and shoot. Sludge samples collected near the rhizosphere also showed lower amount of organic compounds compared to control sludge samples. Metal resistant Bacillus cereus (RCS-4 MZ520573.1) was isolated from the rhizosphere of Cynodon dactylon L. and showed potential to enhance the process of phytoremediation via plant growth promoting activities such as production of high level of ligninolytic enzymes: manganese peroxidase (35.98 U), lignin peroxidase (23.98 U) and laccase (12.78 U), indole acetic acid (45.87(mgL^-1), phosphatase activity (25.76 mg L^-1) and siderophore production (23.09 mg L^-1). This study presents information on the performance of Cynodon dactylon L., an abundant invasive perennial grass species and its associated plant growth promoting rhizobacteria demonstrated good capacity to remediate and restore contaminated soil contained complex organic and inorganic pollutants, they could be integrated into the disposal system of distillery sludge to improve the treatment efficiency.

Identification of heavy metals tolerant Brevundimonas sp. from rhizospheric zone of Saccharum munja L. and their efficacy in in-situ phytoremediation

Heavy metals phytoremediation from pulp and paper industry (PPI) sludge was conducted by employing root-associated Brevundimonas sp (PS-4 MN238722.1) in rhizospheric zone of Saccharum munja L. for its detoxification. The study was aimed to investigate the efficiency of Saccharum munja L. for the removal of heavy metals along with physico-chemical parameters through bacterial interactions. Physico-chemical examination of PPI sludge showed biochemical oxygen demand (8357 ± 94 mg kg^-1), electrical conductivity (2264 ± 49 μmhoscm^-1), total phenol (521 ± 24 mg kg^-1), total dissolve solid (1547 ± 23 mg kg^-1), total nitrogen (264 ± 2.13 mg kg^-1), pH (8.2 ± 0.11), chemical oxygen demand (34756 ± 214 mg kg^-1), color (2434 ± 45 Co-Pt), total suspended solid (76 ± 0.67 mg kg^-1), sulphate (2462 ± 13 mg kg^-1), chlorolignin (597 ± 13.01 mg kg^-1), K⁺ (21.04 ± 0.26 mg kg^-1), total solid (1740 ± 54 mg kg^-1), phosphorous, Cl^-, and Na⁺. Heavy metals, such as Fe followed by Zn, Mn, Cd, Cu, Ni, Pb, As, Cr and Hg were above the permissible limit. Root and shoot of Saccharum munja L. revealed highest concentrations of Cd followed by Mn, Ni, Fe, Zn, Cu, As, Cr, Hg, and Pb. Tested metals (Fe, Mn, Pb, Cd, Cr, Cu, Zn, Ni, As, and Hg) bioaccumulation and translocation factors were also revealed to be < 1 and >1, respectively, demonstrating that these plants have considerable absorption and translocation abilities. Plant growth-promoting activity, such as ligninolytic enzymes, hydrolytic enzymes, indole acetic acid, and siderophore production activity of Brevundimonas sp. (PS-4 MN238722.1) were also noted to be higher. These findings support the use of Brevundimonas sp (PS-4 MN238722.1) in combination with Saccharum munja L. plant as interdisciplinary management of industrial sludge at polluted areas for the prevention of soils near the industrial site.

Energy plants as biofuel source and as accumulators of heavy metals

Fossil fuel depletion and soil and water pollution gave impetus to the development of a novel perspective of sustainable development. In addition to the use of plant biomass for ethanol production, plants can be used to reduce the concentration of heavy metals in soil and water. Due to tolerance to high levels of metals, many plant species, crops, non-crops, medicinal, and pharmaceutical energy plants are well-known metal hyperaccumulators. This paper focuses on studies investigating the potential of Miscanthus sp., Beta vulgaris L., Saccharum sp., Ricinus communis L. Prosopis sp. and Arundo donax L. in heavy metal removal and biofuel production. Phytoremediation employing these plants showed great potential for bioaccumulation of Co, Cr, Cu, Al, Pb, Ni, Fe, Cd, Zn, Hg, Se, etc. This review presents the potential of lignocellulose plants to remove pollutants being a valuable substrate for biofuel production. Also, pretreat-ments, dealing with toxic biomass, and biofuel production are discussed.

Recent Developments in Microbe-Plant-Based Bioremediation for Tackling Heavy Metal-Polluted Soils

Soil contamination with heavy metals (HMs) is a serious concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Rapid industrialization and activities such as mining, manufacturing, and construction are generating a huge quantity of toxic waste which causes environmental hazards. There are various traditional physicochemical techniques such as electro-remediation, immobilization, stabilization, and chemical reduction to clean the contaminants from the soil. However, these methods require high energy, trained manpower, and hazardous chemicals make these techniques costly and non-environment friendly. Bioremediation, which includes microorganism-based, plant-based, microorganism-plant associated, and other innovative methods, is employed to restore the contaminated soils. This review covers some new aspects and dimensions of bioremediation of heavy metal-polluted soils. The bioremediation potential of bacteria and fungi individually and in association with plants has been reviewed and critically examined. It is reported that microbes such as Pseudomonas spp., Bacillus spp., and Aspergillus spp., have high metal tolerance, and bioremediation potential up to 98% both individually and when associated with plants such as Trifolium repens, Helianthus annuus, and Vallisneria denseserrulata. The mechanism of microbe's detoxification of metals depends upon various aspects which include the internal structure, cell surface properties of microorganisms, and the surrounding environmental conditions have been covered. Further, factors affecting the bioremediation efficiency and their possible solution, along with challenges and future prospects, are also discussed.

Evaluation of two wild castor (Ricinus communis L.) accessions for cadmium tolerance in relation to antioxidant systems and lipid peroxidation

The present study was conducted to assess the effect of toxicity of cadmium (Cd) on growth, tolerance index (TI), antioxidant activities, and malondialdehyde (MDA) content in two contrasting wild castor accessions (16-024 and S2-4) via hydroponic experiment (0 and 100 mg/L Cd). The results showed that Cd significantly reduced the growth rate, seedling height, root length, and shoot length of the castor accessions compared to the control, with the Cd effect being more severe in S2-4 than in 16-024. In addition, biomass response including the root and shoot fresh weight and root dry weight decreased in both accessions compared to the control. Compared to the control group, the shoot dry weight of accession S2-4 declined by 21.7%, whereas there was no change in 16-024, suggesting a level of tolerance in 16-024. Analysis of TI on all the growth parameters and biomass content revealed that accession 16-024 was highly tolerant to Cd stress than S2-4. The results further revealed that the expression of the antioxidant enzymes, viz., superoxide dismutase (SOD), catalase (CAT), non-enzymatic antioxidant, glutathione, and MDA content, was influenced by genotype. S2-4 exhibited a higher antioxidant activity (SOD, CAT) and lipid peroxidation activity than 16-024, indicative of oxidative damage from Cd stress.

Assessment the copper-induced changes in antioxidant defense mechanisms and copper phytoremediation potential of common mullein (Verbascum thapsus L.)

In this research, the Cu phytoremediation capacity of common mullein (Verbascum thapsus L.) was evaluated concerning plant growth, antioxidant enzymes, and photosynthetic activities. Plants were subjected to five Cu concentrations (0, 125, 250, 375, and 500 mg/L) under the hydroponic conditions for 2 weeks. The results showed that at 125 mg/L, root and shoot biomass and chlorophylls remained the same as that of the control and then declined with increasing concentrations of Cu, when compared with control. The carotenoid contents remained unchanged up to 250 mg/L compared with control and then dropped with raising Cu dose. The raising of antioxidant enzymes activity reflected the occurrence of stress due to Cu exposure as manifested by increased MDA and ion leakage level. However, increased antioxidant enzymes may be associated with the tolerance capacity of V. thapsus to protect the plant from oxidative damage. Except for the highest concentration (500 mg/L), Cu accumulation in the roots and shoots all increased significantly with increasing Cu concentration, and the Cu accumulation in shoots was greater than roots. The Cu accumulation reached its maximum level at 375 mg/L Cu concentration, with 492.8 and 447.3 mg/kg DW in shoots and roots, respectively, which is highly greater than the threshold value for a Cu (hyper)accumulator plant. The extraction coefficient (EC) close to 1, and translocation factor (TF) > 1 from 125 to 375 mg/L Cu, suggested that V. thapsus could be used as a viable plant species for Cu phytoextraction.

Role of temperature, wind, and precipitation in heavy metal contamination at copper mines: a review

The increasing demand for minerals pressurizing the mining authorities to extract low-grade ore results in more mining waste and degradation of the environment. The main aim of review was to understand the role of climatic factors (temperature, wind, and precipitation) in dispersal and mobility of heavy metals in soil, water, and vegetation in Cu mining region. The major source of contamination in the mining sector is tailings, overburden rocks, and abandoned mines. The contaminates or fine particles of sulfide-rich mining waste follow two major pathways for the dispersal: aerial and leaching. Sulfides on exposure to oxygen and water generate acid mine drainage which results in leaching of heavy metals. The pit water of abandoned mines is also a cause of concern which contaminates the groundwater resources. Climatic factors such as temperature, precipitation, and wind significantly influence the paths of contaminate dispersal. In arid/semi-arid regions, high temperature forms fine-grained efflorescence salts on tailings or exposed surficial mines which are carried away by strong winds/water and contaminates the surroundings. In wet regions, the leaching of heavy metals from both tailings and overburden rocks sulfides results in environmental contamination. The application of impermeable layers is highly recommended. The climatic factors (temperature, wind, and precipitation) significantly control the dispersal and mobility of heavy metals in Cu mining region. The implementation of waste management policies and pollution control technologies is recommended after considering the climatic factors.

Cotyledonary leaves effectively shield the true leaves in Ricinus communis L. from copper toxicity

The stress-buffering effects of cotyledonary leaves of Ricinus communis and the protection thus offered to the true leaves upon exposure to copper stress was performed by analyzing bioaccumulation of Cu and associated metabolic processes in the presence and absence of cotyledonary leaves. One-month-old seedlings of R. communis were treated with various concentrations of CuSO₄ for 6 d under hydroponics with quarter strength modified Hoagland medium. Even though the photosynthetic pigments showed a decreasing trend with an increase in CuSO₄ concentration and days of exposure in cotyledonary and true leaves, it was significant in true leaves with excised cotyledonary leaves. The results of chlorophyll a fluorescence parameters indicated that toxic levels of CuSO₄ do not impart any major negative effect on the photochemistry of true leaves along with cotyledonary leaves. The analysis of osmolality, malondialdehyde, and metabolites showed a significantly increasing trend in true leaves with excised cotyledonary leaves. The above observations were strongly supported by higher Cu bioaccumulation in true leaves with excised cotyledonary leaves. The results suggest that the cotyledonary leaves have got the potential to evade Cu toxicity and thereby R. communis can be effectively used for the phytoremediation of Cu contaminated lands.