Mass balance of metals during the phytoremediation process using Noccaea caerulescens: a pot study

Influences of blue and red light irradiations on Cd phytoexcretion using Festuca arundinacea

Although phytoremediation is more economical when compared with traditional physical and chemical soil remediation methods, it remains very expensive when considering the substantial area of the contaminated field. If the quantity of harvested residues can be reduced after each phytoremediation cycle, the practicability and commercial implementation of this environment friendly method can be improved. In this study, cadmium excretion on the leaf surface of Festuca arundinacea was evaluated under various blue and red light conditions. The results indicated that the percentage of decaying and deceased leaves increased by 8.5%, 31.1%, 59.7%, and 35.9% at a blue light ratio of 10%, 50%, 75%, and 100%, respectively, when compared with the control. The highest cadmium concentration was found in decaying and deceased leaves under 75% blue light treatment. Light treatments also altered the excreted cadmium amount on different leaf types. Under all treatments including the control, significantly more cadmium can be washed off from emerging and mature leaves than from decaying and deceased leaves, owing to the detoxification mechanism of the plant (p < 0.05). The differences in cadmium excretion on senescent and dead leaves under all treatments were not statistically significant, but the mass of cadmium excretion on young leaves under 75% and 100% blue light irradiation were significantly higher than that under other treatments (p < 0.05). Herein, a novel phytoremediation method involving the harvesting decaying and deceased leaves and washing emerging and mature leaves was proposed to decrease the costs of plant residue disposal.

Effects of magnetic field on cd subcellular distribution and chemical speciation in Noccaea caerulescens

Implementing an external magnetic field of suitable strength has been reported to increase Cd uptake by Noccaea caerulescence. However, only a few mechanisms promoting this efficiency have been reported. A series of culture experiments was conducted to explore how Cd subcellular distribution and speciation vary within the tissue of N. caerulescens when subjected to external magnetic fields of different intensities. Without a magnetic field, over 80% of the Cd was deposited in the cell wall and cytoplasm, indicating that cell wall retention and cytoplasm isolation are significant mechanisms for the detoxification of Cd. An external magnetic field (120 mT) increased the Cd concentrations deposited in the cytoplasm and water-soluble inorganic Cd in the roots, increasing the cell wall-bound Cd and undissolved Cd phosphate in the shoots. Meanwhile, the magnetic field increased carbonic anhydrase activity in plant shoots, except at 400 mT. These results indicated that an external field can elevate the Cd decontamination capacity of N. caerulescens by changing the subcellular compartmentalization and speciation of Cd in different tissues.

Improving capacity for phytoremediation of Vetiver grass and Indian mustard in heavy metal (Al and Mn) contaminated water through the application of clay minerals

One of the consequences of mining is the release of heavy metals into the environment, especially water bodies. Phytoremediation of areas contaminated by heavy metals using Vetiver grass and Indian mustard is cost-effective and environmentally friendly. This study aimed at enhancing remediation of heavy metal contaminated water through the simultaneous hybrid application of clay minerals (attapulgite and bentonite) and Vetiver grass or Indian mustard. A 21-day greenhouse experiment was carried out to investigate the effectiveness of the clay minerals to improve heavy metal phytoremediation. The highest accumulation of aluminium (Al) by Vetiver grass was 371.8 mg/kg in the BT2.5VT treatment, while for Mn, the highest accumulation of 34.71 mg/kg was observed in the AT1VT treatment. However, Indian mustard showed no significant uptake of heavy metals, but suffered heavy metal toxicity despite the addition of clay minerals. From this study, it was evident that bentonite added at 2.5% (w/v) could improve the phytoremediation capacity of Vetiver grass for Al and Mn polluted water. The current laboratory-scale findings provided a basis for field trials earmarked for remediation in a post-mining coal environment in South Africa. This remediation approach can also be adopted in other places.

Reactive effects of pre-sowing magnetic field exposure on morphological characteristics and antioxidant ability of Brassica juncea in phytoextraction

As magnetic fields constantly act on living and biochemical processes, it is reasonable to hypothesize that magnetic field treatment of plant seeds would enhance the uptake capacity of non-essential elements. To verify this hypothesis, seeds of Brassica juncea were treated with 50, 100, 150, 200, and 400 mT fields, and the dry weight, Cd uptake capacity, ferritin content, antioxidant enzyme activity, and phytoremediation effects of the plant were compared at the end of the experiment. Relative to the control, low- and moderate-intensity fields (50-200 mT) enhanced the dry weight of plant leaves by 15.1%, 24.5%, 35.8%, and 49.1%, respectively, whereas the high-intensity field (400 mT) decreased the biomass yield by 18.9%. The content of Cd in the above-ground tissues of B. juncea enhanced with the increasing field intensity, accompanied by an increase in oxidative damage. The activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased with exposure to low (50 and 100 mT) and moderate (150 and 200 mT) intensities, followed by a reduction at a high intensity (400 mT). Catalase activity (CAT) and ferritin content exhibited an increasing trend with increasing intensity. The Cd decontamination index of B. juncea increased with the increasing magnetic field intensity until it reached a peak at 150 mT, after which the values remained constant. Considering the phytoremediation effect and energy consumption, 150 mT was the optimal scheme for magnetic-field-assisted phytoremediation using B. juncea. This study suggests that a suitable magnetic field can be regarded as an ecologically friendly physical trigger to improve the phytoextraction effect of B. juncea.

Impacts of water deficit and post-drought irrigation on transpiration rate, root activity, and biomass yield of Festuca arundinacea during phytoextraction

Water deficit is a hazardous threat to phytoremediation, while the photosynthetic efficiency of plant leaves can rapidly recover after post-drought irrigation, thereby enhancing the root activity, transpiration rate, and metal accumulation capacity of plants. This study was designed to test whether the phytoextraction effect of drought-stressed Festuca arundinacea could recover to normal levels after post-drought irrigation. Two drought stress levels (D₁, slight stress and D₂, moderate stress) were carried out at one of five plant growth stages (G₁, germinating; G₂, tillering; G₃, jointing; G₄, booting; and G₅, flowering). The results showed that drought stress, regardless of level, significantly decreased the transpiration rate of F. arundinacea by 38.9%-85.7%. The degree of reduction of this physiological index was significantly higher in D₁G₁ and D₂G₁ than in other treatments. The biomass yield and root activity in D₁G₃, D₁G₄, D₁G₅, D₂G₃, and D₂G₄ recovered and even surpassed the normal values after rewatering, suggesting that the detrimental effects of drought stress on F. arundinacea at certain growth stages can be compensated by post-drought irrigation. Drought stress also decreased the Cd uptake capacity of F. arundinacea, and the degree of reduction depended on the stress level and growth stage. Overcompensation for Cd accumulation was observed in D₁G₃, D₁G₄, D₂G₃, and D₂G₄ after post-drought irrigation. The results indicated that suitable irrigation strategies can improve the phytoextraction effect of F. arundinacea and conserve water resources in practice.

Effects of magnetically treated Sedum alfredii seeds on the dissolved organic matter characteristics of Cd-contaminated soil during phytoextraction

The effects of magnetic field treatments on the two determining factors of phytoremediation, growth status and element uptake capacity, of Sedum alfredii Hance. have been thoroughly studied; however, minimal studies have been performed to determine the influence of the Cd hyperaccumulator S. alfredii, grown from magnetically treated seeds, on the dissolved organic matter (DOM) characteristics in its rhizosphere. A series of pot experiments were conducted to evaluate the variations in the DOM concentration and fractionations in the rhizosphere of S. alfredii treated with external magnetic fields. Compared with the untreated seeds, S. alfredii grown from magnetically treated seeds excreted more DOM in its rhizosphere. Additionally, the hydrophilic DOM fractionation proportion, which presented a greater capacity to mobilize Cd in the soil, increased from 42.7 % in the control sample to 47.2 % in the 150 mT magnetically treated S. alfredii sample. The water-soluble and exchangeable forms of Cd in the rhizosphere of the magnetically treated S. alfredii were significantly lower than those of the control sample. Furthermore, the Cd extraction capacity of DOM from the rhizosphere of the magnetically treated S. alfredii was greater than that of the control sample, thereby increasing the Cd uptake ability of the magnetically treated species. This study proves that a suitable magnetic field treatment can enhance the phytoremediation effect of S. alfredii, and reveals the mechanism of the phenomenon from the perspective of changes in soil DOM.

Influence of Drought Stress and Post-Drought Rewatering on Phytoremediation Effect of Arabidopsis thaliana

Globally, plants face large amounts of environmental stresses, which can inhibit their growth rate and element uptake capacity. Droughts are a critical limitation to plant growth in arid and semi-arid areas. Effects of drought stress and post-drought rewatering on the compensatory growth and Cd phytoremediation efficiency of Arabidopsis thaliana were estimated using slight and moderate drought conditions. Results showed compensatory growth can be induced by post-drought rewatering, as manifested by the increased dry weight and photosynthetic efficiency of the species under drought stress (particularly slight stress) as compared to those of the control. Slight stress increased concentrations of Cd in roots and leaves of A. thaliana by elevating its transpiration rate, whereas moderate stress induced the opposite effect. When the species was subjected to slight stress, Cd concentrations in plant tissues surpassed those in the control after rewatering, indicating that post-drought rewatering can compensate for the detrimental impacts caused by slight drought in A. thaliana. At the end of the experiment, slight and moderate drought stresses increased the Cd extraction ability of the species by 48.9% and 12.7%, respectively, compared to the control. This study demonstrates compensatory effects of post-drought rewatering on the Cd phytoextraction capacity of A. thaliana and suggests that suitable water deficit irrigation practices can enhance soil remediation efficiency and simultaneously save water in the field.

Enhancement of the Cd phytoremediation efficiency of Festuca arundinacea by sonic seed treatment

It has been reported that both naturally occurring and artificially created sounds can alter the physiological parameters of various plants. A series of experiments were designed in the present study to estimate the physiological responses and the variation in the Cd decontamination capacity of Festuca arundinacea under sonic wave treatments. Plant seeds were treated by sound waves of frequency 200, 300, 400, 500, and 1000 Hz, and the germinated seedlings were transplanted to Cd-polluted soil. The results showed that all the sonic treatments increased the whole plant dry weight of F. arundinacea compared with that of the control, and the highest value was observed in the 200 Hz treatment. The Cd content in below-ground and aerial tissues of the species increased with increasing frequency till 400 Hz, after which they became constant. A higher proportion of senescent and dead leaf tissues was observed in the high-frequency treatment (1000 Hz), and more Cd was transferred to these failing tissues. Therefore, in the 1000 Hz treatment, a significantly greater amount of Cd could be eliminated by harvesting the senescent and dead leaf tissues of the species compared with that of the other treatments. The concentrations of dissolved organic matter (DOM) and the proportions of hydrophilic fractions which have a strong Cd affinity, in the rhizosphere soil of F. arundinacea increased with the increase in sound frequency. Cd extraction ability of DOM also increased with increasing frequency. This study indicated that a suitable sonic treatment can improve the phytoextraction efficiency of F. arundinacea, and also explained the mechanism from the perspective of the variations in soil DOM.

Effects of decapitated and root-pruned Sedum alfredii on the characterization of dissolved organic matter and enzymatic activity in rhizosphere soil during Cd phytoremediation

Decapitation and root pruning, can impact plant morphological and physiological characteristics, which may determine the efficiency of phytoremediation. However, the effects of decapitated and root-pruned plants on the characterization of dissolved organic matter (DOM) and enzymatic activity, which determine the bioavailability of soil pollutants, have rarely been reported. This study aims to characterize DOM and enzymatic activity in the rhizosphere soil of Sedum alfredii when treated by decapitation and root pruning. Decapitation, slight pruning (10% root cutting), and their combination stimulated S. alfredii to secrete more DOM in the rhizosphere soil compared with the control. Furthermore, the proportions of hydrophilic increased from 42.7% in the control to 57.1% in the decapitation and slight pruning combination. Soil urease, invertase, and neutral phosphatase activities were higher in the rhizosphere soil of decapitated and root-pruned S. alfredii, and the highest values were observed with their combination. DOM from the soils of decapitated and root-pruned S. alfredii had significantly higher Cd extraction ability compared with that of the untreated species. Based on the findings of this study, we suggest that decapitation and root pruning can improve the phytoremediation efficiency of S. alfredii by increasing the bioavailability of Cd in its rhizosphere.