Effect of soil washing on heavy metal removal and soil quality: A two-sided coin.

Heavy metal contamination in soil due to human activities is a global environmental problem. To find a washing solution that can significantly decontaminate heavy metals and minimize damage to soil quality, six washing solutions (H3PO4, K2CO3, CH3COOK, KH2PO4, HNO3 and KNO3) were used at different concentrations to treat contaminated soil collected from the field. Furthermore, changes in soil physicochemical properties and heavy metal speciation among prewashed, postwashed and neutralized samples were tested. Additionally, soil enzyme activities and soil microbial diversities in contaminated soil among the prewashed, postwashed and neutralized samples were also measured. Finally, a pot experiment was conducted with Mentha haplocalyx to test the efficiency of soil washing. The results revealed that the optimum washing solution was 1% HNO3 and that the removal rates of Cd and Pb were 75.7% and 60.6%, respectively, under treatment conditions of 35 °C, 90 min and a solid-liquid ratio of 1:10. The pH, total phosphorous, available potassium, soil enzyme activities and soil microbial diversity decreased significantly after washing. However, after the neutralization of washed soil with Ca(OH)2, the available phosphorous, total nitrogen and some microorganisms increased significantly compared with those of the soil before washing. After treatment with 1% HNO3, the chemical forms of Cd and Pb in soil mainly existed as F1 (exchangeable) fractions, but the main forms of the two metals changed to F5 (residual) and F3 (bound to Fe-Mn oxides) fractions after neutralization with Ca(OH)2. In addition, the plant height, root length, and fresh and dry weight of M. haplocalyx were not significantly affected by soil neutralization, while the Pb, Cu and As concentrations in the aboveground parts significantly decreased. Therefore, although soil washing could effectively remove Pb and Cd in soil, it also resulted in a significant decline in soil quality, but soil neutralization could effectively alleviate the negative effects during soil washing.

Indole-3-acetic acid promotes cadmium (Cd) accumulation in a Cd hyperaccumulator and a non-hyperaccumulator by different physiological responses.

To study the effects of indole-3-acetic acid (IAA) on cadmium (Cd) accumulation and the physiological responses of the Cd hyperaccumulator Solanum nigrum and non-hyperaccumulator Solanum melongena, a pot experiment was conducted in soil containing 2 mg kg-1 Cd in which different concentrations of IAA (0, 10, 20, or 40 mg L-1) were sprayed on plant leaves. The results showed that Cd accumulation in shoots of S. nigrum was significantly increased by 30% after the addition of 10 mg L-1 IAA under 2 mg kg-1 Cd stress compared to that in the control, but shoot Cd accumulation showed no significant change in S. melongena after this IAA treatment. Additionally, the growth and the proline content in the two species were significantly increased by 20 mg L-1 IAA. The activities of peroxidase and catalase in leaves of S. nigrum and the activity of superoxide dismutase (SOD) in S. melongena were significantly increased and their malondialdehyde content was significantly decreased compared to those in the control. The root activity of S. nigrum was significantly improved after 10 and 20 mg L-1 IAA treatments, but no significant difference was observed in S. melongena. The correlation analysis results showed that the Cd concentration in leaves of S. nigrum was significantly and positively correlated with the carotenoid and proline contents, and there was also a significant positive correlation between the Cd concentration and SOD activity in leaves of S. melongena. Therefore, S. nigrum is an ideal plant for the phytoextraction of Cd-contaminated soil assisted by IAA. IAA promotes Cd accumulation in plant shoots by enhancing the accumulation of carotenoids and proline in S. nigrum and maintaining a high leaf SOD activity in S. melongena.

Re-investigation of cadmium accumulation in Mirabilis jalapa L.: evidences from field and laboratory.

Mirabilis jalapa L. was identified as a cadmium (Cd) hyperaccumulator, but data were mainly from laboratory conditions. The main aim of the present study was to confirm whether M. jalapa is a Cd hyperaccumulator by field survey and laboratory experiment. The field survey was conducted at 3 sites and 66 samples were collected, and the results showed that although M. jalapa did not exhibit any visible damage when growing on soil containing 139 mg Cd kg-1, a low concentration of Cd (11.85 ± 3.45 mg kg-1) in its leaves was observed. Although the translocation factor (TF) was up to 3.24 ± 0.42, the bioconcentration factor (BCF) was only 0.13 ± 0.07. The Cd accumulation in leaves of Lanping (LP, contaminated site) and Kunming (KM, clean site) populations reached 93.88 and 81.76 mg kg-1 when artificially spiked soil Cd was 175 mg kg-1, respectively. The BCFs of LP and KM populations were 0.55 and 0.48, and the TFs of the two populations were 3.98 and 4.15, respectively. Under hydroponic condition, the Cd concentration in young leaves of LP and KM populations was 78.5 ± 0.8 and 46.3 ± 1.2 mg kg-1 at 5 mg L-1 Cd treatment, respectively. Furthermore, a significantly positive correlation between tissue Cd concentration and total Cd, CaCl2-extractable Cd, and TCLP-Cd (toxicity characteristic leaching procedure) in soil was established. Therefore, M. jalapa had constitutional characteristics for Cd tolerance and accumulation, but it was not a Cd hyperaccumulator.

Heavy metal pollution and potential health risks of commercially available Chinese herbal medicines.

A survey was conducted to investigate the pollution and health risks of copper (Cu), cadmium (Cd), lead (Pb), arsenic (As), mercury (Hg) and zinc (Zn) in 60 Chinese herbal medicines (CHMs) collected from a market in Kunming City, Yunnan Province, China. Furthermore, eight CHMs (Cyathulae radix, Drynariae rhizoma, Peucedani radix, Homalomenae rhizoma, Dryopteris setosa, Polygonati rhizoma, Lilii bulbus, and Linderae radix) containing high Cd concentrations were selected to further analyse their Cd chemical forms. Additionally, the dissolution rates of six heavy metals in decoction liquid were also analysed for four CHMs (Typhonii rhizoma, Linderae radix, Homalomenae rhizoma, and Cyathulae radix), and the health risks of heavy metals in CHMs were evaluated. The results showed that the Cd, Hg and Cu concentrations in the 60 CHMs exceeded the limiting values of the "Green Trade Standards of Importing & Exporting Medicinal Plants & Preparations" (WM2-2001), with exceedance ratios of 38.8%, 8.3% and 1.7%, respectively. The majority of Cd was integrated with pectates and protein in CHMs, and the other five Cd chemical forms followed the order of water-soluble > insoluble heavy metal phosphates > oxalate > residual > inorganic form, indicating that Cd had relatively low bioactivity and toxicity. The average dissolution rates of Zn, Cu, Cd, Hg, As and Pb in the four CHMs were 47.4%, 33.8%, 20.5%, 6.1%, 5.4% and 4.8%, respectively. The calculation results of hazard quotients (HQs) for Cd and Hg showed that the CHMs did not pose a threat to human health.

Effect of arsenate on endogenous levels of cytokinins with different existing forms in two Pteris species.

Our previous results showed that content of trans-zeatin (tZ) increases in leaves of heavy metal hyperaccumulators but decreases in non-hyperaccumulators growing in multiple heavy metal polluted soils. However, the relationship between arsenic (As) accumulation and endogenous forms of cytokinins (CTKs) in As hyperaccumulators remains unknown. Here a hydroponic experiment was conducted to compare the CTK forms in the As hyperaccumulator Pteris cretica var. nervosa and non-hyperaccumulator Pteris ensiformis under arsenate stress (0, 2, 5, and 10 mg L-1). A simple and cost-effective procedure for the determination of CTK forms in plants was established, and a stepwise regression analysis was used to study the relationship among total As contents and different forms of endogenous CTKs in fronds of two plants. The results showed that the optimized chromatographic parameters were Zobax SB-C18 column (5 µm × 4.6 mm × 250 mm), UV detection detector at 269 nm, a flow rate of 0.6 mL min-1, constant temperature of 45 °C and gradient elution with methanol-acetonitrile-1% acetic acid. Contents of chlorophylls in the fronds of P. ensiformis were significantly decreased with addition of As compared to P. cretica var. nervosa. Furthermore, the total As content in fronds of P. cretica var. nervosa was positively correlated to the contents of N6-(2-isopentenyl) adenine-7-ß-D-glucoside (iP7G) and N6-(2-isopentenyl) adenosine (iPR). However, the total As content in fronds of P. ensiformis was negatively correlated to its trans-zeatin riboside (ZR) content. Therefore, iP7G and iPR could positively improve As accumulation by P. cretica var. nervosa.

Effects of kinetin on plant growth and chloroplast ultrastructure of two Pteris species under arsenate stress.

Cytokinins (CTKs) are effective in alleviating abiotic stresses on plants, but little information is available regarding the effects of CTKs on arsenic (As) accumulation and changes of chloroplast ultrastructure in plants with different As-accumulating ability. Here a hydroponic experiment was designed to evaluate the effects of different concentration of kinetin (KT, 0-40 mg/L) on growth and chloroplast ultrastructure of As hyperaccumulator Pteris cretica var. nervosa and non-hyperaccumulator Pteris ensiformis treated by 5 mg/L arsenate for 14 days. The growth parameters, As accumulation, contents of photosynthetic pigments and chloroplast ultrastructure were examined. The results showed that KT promoted the growth of two plants, and significantly increased As accumulation and translocation in P. cretica var. nervosa and P. ensiformis at 5 and 20 mg/L, respectively. Additionally, the contents of chlorophyll a and carotenoid in two plants showed no significant difference at 20 mg/L KT compared to the control. Chloroplast ultrastructure of P. cretica var. nervosa was integral with KT application. Comparatively, the swollen chloroplasts were increased, plasmolysis appeared, and chloroplast grana slice layers and stroma lamellas were clearly separated or distorted at 5 mg/L KT in P. ensiformis. The length and width of chloroplasts in P. cretica var. nervosa were significantly increased with KT addition compared to the control. However, the length of chloroplasts in P. ensiformis was significantly decreased but their width showed no significant change. Furthermore, the deterioration of chloroplast ultrastructure in P. ensiformis was ameliorated by 40 mg/L KT. These results suggested that KT increased As accumulation and was beneficial to maintain the photosynthetic pigments for a good growth of plants. Therefore, KT could maintain and reorganize the ultrastructure integrality of As-stressed chloroplasts to some extent for the two plants, especially at high concentration.

A significant positive correlation between endogenous trans-zeatin content and total arsenic in arsenic hyperaccumulator Pteris cretica var. nervosa.

A pot experiment was conducted to compare the content of endogenous trans-zeatin (Z), plant arsenic (As) uptake and physiological indices in the fronds of As-hyperaccumulator (Pteris cretica var. nervosa) and non-hyperaccumulator (Pteris ensiformis). Furthermore, a stepwise regression method was used to study the relationship among determined indices, and the time-course effect of main indices was also investigated under 100mg/kg As stress with time extension. In the 100-200mg/kg As treatments, plant height showed no significant difference and endogenous Z content significantly increased in P. cretica var. nervosa compared to the control, but a significant decrease of height and endogenous Z was observed in P. ensiformis. The concentrations of As (III) and As (V) increased significantly in the fronds of two plants, but this increase was much higher in P. cretica var. nervosa. Compared to the control, the contents of chlorophyll and soluble protein were significantly increased in P. cretica var. nervosa but decreased in P. ensiformis in the 200mg/kg As treatment, respectively. A significant positive correlation was found between the contents of endogenous Z and total As in P. cretica var. nervosa, but such a correlation was not found in P. ensiformis. Additionally, in the time-course effect experiment, a peak value of each index was appeared in the 43rd day in two plants, except for chlorophyll in P. ensiformis, but this value was significantly higher in P. cretica var. nervosa than that in P. ensiformis. In conclusion, a higher endogenous Z content contributed to As accumulation of P. cretica var. nervosa under As stress.

Effects of indole-3-acetic acid on arsenic uptake and antioxidative enzymes in Pteris cretica var. nervosa and Pteris ensiformis.

A hydroponic experiment was conducted to investigate the effects of indole-3-acetic acid (IAA) on arsenic (As) uptake and antioxidative enzymes in fronds of Pteris cretica var. nervosa (As hyperaccumulator) and Pteris ensiformis (non-hyperaccumulator). Plants were exposed to 2 mg L-1 As(III), As(V) or dimethylarsinic acid (DMA) and IAA concentrations for 14 d. The biomass and total As in the plants significantly increased at 30 mg L-1 IAA. Superoxide dismutase (SOD) activities significantly increased with IAA addition. Catalase (CAT) activities showed a significant increase in P. ensiformis exposed to three As species at 30 or 50 mg L-1 IAA but varied in P. cretica var. nervosa. Peroxidase (POD) activities were unchanged in P. ensiformis except for a significant decrease at 50 mg L-1 IAA under As(III) treatment. However, a significant increase was observed in P. cretica var. nervosa at 10 mg L-1 IAA under As(III) or DMA treatment and at 50 mg L-1 IAA under As(V) treatment. Under DMA stress, malondialdehyde contents in fronds of P. cretica var. nervosa showed a significant decrease at 10 mg L-1 IAA but remained unchanged in P. ensiformis. Therefore, IAA enhanced As uptake and frond POD activity in P. cretica var. nervosa under As stress.

Endogenous trans-zeatin content in plants with different metal-accumulating ability: a field survey.

A field survey was conducted to evaluate soil metal pollution and endogenous trans-zeatin content in the leaves of plants growing at six sites in a metal-polluted area located in Gejiu, Yunnan, China. Five plant species were collected, and the physicochemical properties and concentrations of five metals in the soil were analyzed. The trans-zeatin content in plant leaves was measured by high-performance liquid chromatography. Based on the Nemerow pollution index, the six sites were classified into four levels of pollution (i.e., low, medium, high, and severely high). The degree of soil metal pollution was cadmium (Cd) > arsenic (As) > lead (Pb) > zinc (Zn) > copper (Cu). The leaf trans-zeatin content in Pteris vittata (an arsenic hyperaccumulator) increased significantly by 98.6 % in soil with a severely high level of pollution compared with soil at a low level of pollution. However, in non-hyperaccumulators Bidens pilosa var. radiata and Ageratina adenophora, a significant decrease in leaf trans-zeatin content of 35.6 and 87.6 %, respectively, was observed. The leaf trans-zeatin content in Artemisia argyi also decreased significantly by 73.6 % in high metal-polluted soil compared with that in medium metal-polluted soil. Furthermore, significant correlations were observed between leaf trans-zeatin content in Pteris vittata and As, Pb, and Cd concentrations in the soil; however, either no correlation or a negative one was observed in the other plant species. Therefore, a high content of trans-zeatin in the leaves of Pteris vittata may play an important role in its normal growth and tolerance to metals.