Comparative Activation Process of Pb, Cd and Tl Using Chelating Agents from Contaminated Red Soils

Adding chelating agents is a critical technique of heavy metal activation for enhancing phytoextraction through the formation of soluble metal complexes which will be more readily available for extraction. The preliminary, dynamic, equilibrium activation experiments and speciation analysis of Pb, Cd and Tl in contaminated red soils were used to select six chelates with relatively good activation performance from nine chelates, and the effects of dosage and pH on the heavy metals activation were studied systematically. Results showed that the activation of Pb, Cd and Tl by chelates reached equilibrium within 2 h, and the activation process showed three stages. Under neutral conditions, chelates had better activation performance on Pb- and Cd-contaminated soils. Except for S,S-ethylenediamine disuccinic acid (S,S-EDDS) and citric acid (CA), the maximum equilibrium activation effect (MEAE) of ethylenediaminetetraacetic acid (EDTA), N,N-bis (carboxymethyl) glutamic acid (GLDA), diethylenetriaminepentaacetic acid (DTPA) and aminotriacetic acid (NTA) was over 81%. The MEAE of Tl-contaminated soil was less than 15%. The decreasing order of the dosage of chelating agents corresponding to MEAE for three types of contaminated soils was Pb-, Cd- and Tl-contaminated soil, relating to the forms of heavy metals, the stability constants of metal–chelates and the activation of non-target elements Fe in red soil. Under acidic conditions, the activation efficiencies of chelates decreased to differing degrees in Pb- and Cd-contaminated soils, whereas the activation efficiencies of chelating agents in Tl-contaminated soils were slightly enhanced

Insights into Heavy Metals Leakage in Chelator-Induced Phytoextraction of Pb- and Tl-Contaminated Soil

Chelators including DTPA (diethylene triamine pentaacetic acid) and oxalic acid were selected for inducing phytoextraction of heavy metals (HMs) from Pb-, Tl-, and Pb-Tl- contaminated soil, in which heavy metals leakage was highly remarkable. Results showed that compared with the control group without chelating agent under planting conditions, the extraction efficiency (i.e., uptake coefficient) of Pb, Tl increased by 86%, 43% from Pb-Tl- contaminated soil in the presence of oxalic acid, and there was no significant change in heavy metal leakage under rainfall conditions. It was the best phytoremediation scheme in this work. Under rainfall conditions, the HMs concentration in the leachate showed a linear decreasing trend. Acid rain promoted the leakage of heavy metals, and the average leached amount of Tl increased by 1.47 times under acid rain conditions. However, for Pb, DTPA was the main influencing factor, followed by acid rain

Patterns of Plant Biomass Allocation in Temperate Grasslands across a 2500-km Transect in Northern China

<div><p>Plant biomass allocation between below- and above-ground parts can actively adapt to the ambient growth conditions and is a key parameter for estimating terrestrial ecosystem carbon (C) stocks. To investigate how climatic variations affect patterns of plant biomass allocation, we sampled 548 plants belonging to four dominant genera (<i>Stipa spp.</i>, <i>Cleistogenes spp.</i>, <i>Agropyron spp.</i>, and <i>Leymus spp.</i>) along a large-scale (2500 km) climatic gradient across the temperate grasslands from west to east in northern China. Our results showed that <i>Leymus spp.</i> had the lowest root/shoot ratios among the each genus. Root/shoot ratios of each genera were positively correlated with mean annual temperature (MAT), and negatively correlated with mean annual precipitation (MAP) across the transect. Temperature contributed more to the variation of root/shoot ratios than precipitation for <i>Cleistogenes spp</i>. (C4 plants), whereas precipitation exerted a stronger influence than temperature on their variations for the other three genera (C3 plants). From east to west, investment of C into the belowground parts increased as precipitation decreased while temperature increased. Such changes in biomass allocation patterns in response to climatic factors may alter the competition regimes among co-existing plants, resulting in changes in community composition, structure and ecosystem functions. Our results suggested that future climate change would have great impact on C allocation and storage, as well as C turnover in the grassland ecosystems in northern China.</p></div

Effects of MAT and MAP on root/shoot ratios of four dominant genera (<i>Stipa, Cleistogenes, Agropyron</i>, and <i>Leymus</i>) across a 2500-km long transect in northern China’s temperature grassland.

<p>MAT, mean annual temperature; MAP, mean annual precipitation.</p><p>A linear mixed model was employed, using sample plots as the random factor and climate variables as the fixed factors. R is the correlation coefficients between climate factors and root/shoot ratios. P-values are estimated using restricted maximum likelihood (REML) estimates and are reported for significant (<i>p</i><0.05) model terms.</p

Semi-variogram test of spatial autocorrelation of mean annual temperature (MAT) and mean annual precipitation (MAP) among sampling sites across a 2500-km long transect in northern China.

<p>Semi-variogram test of spatial autocorrelation of mean annual temperature (MAT) and mean annual precipitation (MAP) among sampling sites across a 2500-km long transect in northern China.</p

Allometric relationship between root biomass and shoot biomass.

<p><i>Stipa spp.</i> (A), <i>Cleistogenes spp.</i> (B), <i>Agropyron spp.</i> (C), and <i>Leymus spp.</i> (D). Red lines are the standardized major axis regression curves (for a summary of regression statistics, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071749#pone-0071749-t005" target="_blank">Table 5</a>). Green lines are isometric lines with slope equal to 1 and y-intercept equal to that of the corresponding red lines.</p

Path coefficients between climate factors (MAT and MAP) and plant root/shoot ratios of four dominant genera (<i>Stipa</i>, <i>Cleistogenes</i>, <i>Agropyron</i>, and <i>Leymus</i>) across a 2500-km long transect in northern China’s temperature grassland.

<p>MAT, mean annual temperature; MAP, mean annual precipitation; R/S, root/shoot ratio.</p><p>Indirect path coefficient means the indirect dependency of root/shoot ratios on certain climate factor and R is the correlation coefficient between the climate factors and the root/shoot ratios.</p

Root/shoot ratios (mean ±1 SE) of four grasses genera (<i>Stipa</i>, <i>Cleistogenes</i>, <i>Agropyron</i>, and <i>Leymus</i>).

<p>Bars with dissimilar letters denote signiffcant difference (<i>p</i><0.01).</p

Locations of the sampling sites along the transect across the temperate grasslands of northern China.

<p>Locations of the sampling sites along the transect across the temperate grasslands of northern China.</p