Yanshan Cui

Soil Contamination and Plant Uptake of Heavy Metals at Polluted Sites in China

Abstract We investigated heavy metal contamination in soils and plants at polluted sites in China including some with heavy industries, metal mining, smelting and untreated wastewater irrigation areas. We report our main findings in this paper. The concentrations of heavy metals, including Cd and Zn, in the soils at the investigated sites were above the background levels, and generally exceeded the Government guidelines for metals in soil. The concentrations of metals in plants served to indicate the metal contamination status of the site, and also revealed the abilities of various plant species to take up and accumulate the metals from the soil. Substantial differences in the accumulation of heavy metals were observed among the plant species investigated. Polygonum hydropiper growing on contaminated soils in a sewage pond had accumulated 1061 mg kg−1 of Zn in its shoots. Rumex acetosa L. growing near a smelter had accumulated more than 900 mg kg−1 of Zn both in its shoots and roots. Therefore these species have potential for phytoremediation of metal-contaminated sites. Our results indicate the need to elucidate the dynamics of soil metal contamination of plants and the onward movement of metal contaminants into the food chain. Also our results indicate that the consumption of rice grown in paddy soils contaminated with Cd, Cr or Zn may pose a serious risk to human health, because from 24 to 22% of the total metal content in the rice biomass was concentrated in the rice grain. Platanus acerifolia growing on heavily contaminated soil accumulated only very low levels of heavy metals, and this mechanism for excluding metal uptake may have value in crop improvement. Sources of metal entering the environmental matrices studied included untreated wastewater, tailings or slurries and dust depositions from metal ore mining, and sewage sludge. Pb, Zn or Cd concentrations declined with the distance from metal smelter in accordance with a good exponential correlation (R 2 >0.9), and this shows that metal dust deposition is an important contributor to metal contamination of soils.

Instances of Soil and Crop Heavy Metal Contamination in China

Both general and specific investigations of soil and crop heavy metal contamination were carried out across China. The former was focused mainly on Cd, Hg, As, Pb, and Cr in soils and vegetables in suburbs of four large cities; the latter investigated Cd levels in both soils and rice or wheat in contaminated areas throughout 15 provinces of the country. The results indicated that levels of Cd, Hg, and Pb in soils and some in crops were greater than the Governmental Standards (Chinese government limits for soil and crop heavy metal contents). Soil Cd ranged from 0.46 to 1.04 mg kg−1, on average, in the four cities and was as high as 145 mg kg−1 in soil and 7 mg kg−1 in rice in the wide area of the country. Among different species, tuberous vegetables seemed to accumulate a larger portion of heavy metals than leafy and fruit vegetables, except celery. For both rice and wheat, two staple food crops, the latter seemed to have much higher concentrations of Cd and Pb than the former grown in the same area. Furthermore, the endosperm of both wheat and rice crops had the highest portion of Cd and Cr. Rice endosperm and wheat chaff accumulated the highest Pb, although the concentrations of all three metals were variable in different parts of the grains. For example, 8.3, 6.9, 1.4, and 0.6 mg kg−1 of Pb were found in chaff, cortex, embryo, and endosperm of wheat compared with 0.11, 0.65, 0.71, and 0.19 mg kg−1 in the same parts of rice, respectively. Untreated sewage water irrigation was the major cause of increasing soil and crop metals. Short periods of the sewage water irrigation increased individual metals in soils by 2 to 80% and increased metals in crops by 14 to 209%. Atmospheric deposition, industrial or municipal wastes, sewage sludge improperly used as fertilizers, and metal-containing phosphate fertilizers played an important role as well in some specific areas.

Phytoremediation of polluted waters potentials and prospects of wetland plants

To investigate the possible use of plants to remediate polluted waters, a pot experiment was carried out in the laboratory with five wetland plant species, i.e., sharp dock (Polygonum amphibium L.), duckweed (Lemna minor L.), water hyacinth (Eichhornia crassipes), water dropwort [Oenathe javanica (BL.) DC.] and calamus [Lepironia articulata (RETZ.) DOMIN]. Nitrogen (N), phosphorus (P) and three heavy metals, cadmium (Cd), mercury (Hg) and lead (Pb), were the objects of remediation. Sharp dock was found to be a good accumulator of N and P. Indeed, on a dry weight basis the shoots of sharp dock accumulated up to 6.4% of N and 1.1 % of P with BCF (bioconcentration factor) values of 2235 and 1568, respectively. Water hyacinth and duckweed strongly accumulated Cd with concentrations of 462 and 14200 mg/kg, respectively, and BCF values of 1225 and 2567, respectively. Water dropwort achieved the highest concentrations of Hg, i.e., 1.2 mg/kg with a BCF value of 807, whereas calamus achieved the highest concentrations of Pb, i.e., 512.4 mg/kg in its roots with a BCF value of 1217. Thus, it could be concluded that the above plant species are good candidates for phytoremediation of polluted waters, as follows: sharp dock through accumulation of N and P in its shoots, water hyacinth and duckweed as hyperaccumulators of Cd, water dropwort as an hyperaccumulator of Hg and calamus as an hyperaccumulator of Pb.

Enhanced uptake of soil Pb and Zn by Indian mustard and winter wheat following combined soil application of elemental sulphur and EDTA

Enhancement of Pb and Zn uptake by Indian mustard (Brassica juncea (L.) Czern.) and winter wheat (Triticum aestivumL.) grown for 50 days in pots of contaminated soil was studied with application of elemental sulphur (S) and EDTA. Sulphur was added to the soil at 5 rates (0–160 mmol kg−1) before planting, and EDTA was added in solution at 4 rates (0–8 mmol kg−1) after 40 days of plant growth. Additional pots were established with the same rates of S and EDTA but without plants to monitor soil pH and CaCl2-extractable heavy metals. The highest application rate of S acidified the soil from pH 7.1 to 6.0. Soil extractable Pb and Zn and shoot uptake of Pb and Zn increased as soil pH decreased. Both S and EDTA increased soil extractable Pb and Zn and shoot Pb and Zn uptake. EDTA was more effective than S in increasing soil extractable Pb and Zn, and the two amendments combined had a synergistic effect, raising extractable Pb to ¿1000 and Zn to ¿6 times their concentrations in unamended control soil. Wheat had higher shoot yields than Indian mustard and increasing application rates of both S and EDTA reduced the shoot dry matter yields of both plant species to as low as about half those of unamended controls. However, Indian mustard hyperaccumulated Pb in all EDTA treatments tested except the treatment with no S applied, and the maximum shoot Pb concentration was 7100 mg kg−1 under the highest application rates of S and EDTA combined. Wheat showed similar trends, but hyperaccumulation (1095 mg kg−1) occurred only at the highest rates of S and EDTA combined. Similar trends in shoot Zn were found, but with lower concentrations than Pb and far below hyperaccumulation, with maxima of 777 and 480 mg kg−1 in Indian mustard and wheat. Despite their lower yields, Indian mustard shoots extracted more Pb and Zn from the soil (up to 4.1 and 0.45 mg pot−1) than did winter wheat (up to 0.72 and 0.28 mg pot−1), indicating that the effects of S and EDTA on shoot metal concentration were more important than yield effects in determining rates of metal removal over the growth period of 50 days. Phytoextraction of Pb from this highly contaminated soil would require the growth of Indian mustard for nearly 100 years and is therefore impractical.

Effect of elemental sulphur on solubility of soil heavy metals and their uptake by maize

A pot experiment was conducted to study the influence of elemental sulphur (S) on solubility of soil Pb, Zn and Cd and uptake by maize (Zea mays L.). Two rates of elemental sulphur (S) applied at 0 (S0) and 200 (S200) mmol kg(-1) soil with three rates of each heavy metal at Pb, 0 (Pb0), 200 (Pb200), 400 (Pb400) mg kg(-1) soil, Zn, 0 (Zn0), 100 (Zn100), 200 (Zn200) mg kg(-1) soil and Cd, 0 (Cd0), 50 (Cd50), 100 (Cd100) mg kg(-1) soil, respectively. The result showed that with S application at 200 mmol S kg(-1), soil pH decreased about 0.3 unit and the solubility of the Zn and Cd was significantly increased, but the solubility of Pb had no significant influence. The concentration of Pb, Zn and Cd in maize shoots and roots were increased with increasing rates of heavy metals. However, the concentration of Zn and Cd in shoots and roots were higher with application of S rather than without S but no significant difference was found for Pb. The highest concentration of Zn in the shoots was 2.3 times higher with application of S rather than without at the same rate of Zn, 200 mg kg(-1). Plant biomass was also significantly affected by the application of S and of heavy metals. With heavy metal addition, the shoot and root biomass were decreased with the rates of those of heavy metals increased either with or without application of S. However, the shoot biomass was significantly decreased with S application at the same rate of heavy metals except that with Zn addition. The removal of Cd and Pb by maize uptake and accumulation with application of S had no significant increase compared to that without, but the removal Zn by maize uptake from the soil increased by application of S, 90.9 microg plant(-1) contrast to 25.7 microg plant(-1) at Zn200 within a growth period of only 40 days.

RESPONSES OF LEGUME AND NON-LEGUME CROP SPECIES TO HEAVY METALS IN SOILS WITH MULTIPLE METAL CONTAMINATION

ABSTRACT Field and glasshouse investigations were conducted on the responses of two legumes (field pea and fodder vetch) and three non-leguminous crops (maize, wheat and rapeseed) to the heavy metals Cd, Cr, Zn, Pb, Cu and Mn in soil with multiple metal contamination. In general, the results indicate that the two legumes and wheat were more susceptible to soil metals than were rapeseed and maize. The dry matter yields of field pea, wheat, fodder vetch, rapeseed and maize decreased by up to 169, 123, 113, 93 and 68%, respectively, in metal-contaminated soil. Among the crops, maize had the highest concentrations of Mn, Zn and Cd, rapeseed had the highest concentrations of Cr, the concentration of Cu was highest in fodder vetch, and wheat was the highest accumulator of Pb. The bioconcentration factors (BCF) of the metals decreased as the soil metal loading rates increased except for Cr in fodder vetch and Cd in wheat, whose BCF increased as the metal loading rate increased. Significant linear correlations were found between plant and soil metal concentrations. Patterns of metal distribution in plant parts varied with different crops and metals, with more Cd and Cu accumulating in the grain of wheat than of maize, suggesting that growing wheat would represent a higher risk of food contamination than growing maize in Cd- or Cu-contaminated soil. The results suggest that on sites with multiple metal contamination, growing maize and rapeseed would be safer than growing wheat or legumes. However, maize could perhaps be used for phytoremediation of lightly contaminated soils, providing that the crop residues were safely disposed of.

Arsenate and phosphate adsorption in relation to oxides composition in soils: LCD modeling.

The pH dependent solid-solution distribution of arsenate and phosphate in five Dutch agricultural soil samples was measured in the pH range 4-8, and the results were interpreted using the LCD (ligand and charge distribution) adsorption modeling. The pH dependency is similar for both oxyanions, with a minimum soluble concentration observed around pH 6-8. This pH dependency can be successfully described with the LCD model and it is attributed mainly to the synergistic effects from Ca adsorption. The solubility of phosphate is much lower than that of arsenate. This big difference cannot be sufficiently explained by the reduction of small amount of As(V) into As(III), neither by slow desorption/adsorption. The difference between phosphate and arsenate in their solid-solution distribution becomes larger with the increase of aluminum (hydr)oxides (Al-oxides) contribution to the total amount of metal (Al and Fe) (hydr)oxides. The influence of Al-oxides is much larger than its relative amount extracted from the soils. When Al-oxides account for >40% of the soil oxides, the whole adsorbents behave apparently similarly to that of pure Al-oxides. These results indicated that surface coating and substitution may have modified significantly oxyanion adsorption to Fe-oxides in soils, and how to account for this complexity is a challenge for geochemical modeling.

Pilot study of temporal variations in lead bioaccessibility and chemical fractionation in some Chinese soils

The effect of ageing, following the addition of approximately 400mgkg(-1) lead (Pb) as Pb(NO(3))(2), on Pb bioaccessibility was examined in five typical Chinese soils using a physiologically based extraction test. Sequential extraction was employed to identify the source fraction(s) of bioaccessible Pb in the soils. Pb bioaccessibility decreased exponentially to nearly steady levels in mildly acidic or alkali (pH 6.09-7.43) soils, for both gastric (69.91-71.75%) and small intestinal (7.53-9.63%) phases within the first 2-4 weeks and 1-2 months of incubation, respectively; however, it took only 1-2 weeks for strongly acidic ( approximately pH 4.5) soils to reach nearly steady levels of Pb bioaccessibility (73.01-74.46% and 10.30-10.98% in the gastric and small intestinal phases, respectively). In addition to the water-soluble and exchangeable fractions, the carbonate fraction of mildly acidic or alkali soils appeared to be a third main source of bioaccessible Pb in the small intestinal phase; however, bioaccessible Pb was likely to derive principally from Pb in the water-soluble and exchangeable fractions of strongly acidic soils. Bioaccessible Pb in the gastric phase appeared to derive from all the fractions in all five studied soils, even the residual fraction.

In vitro digestion/Caco-2 cell model to estimate cadmium and lead bioaccessibility/bioavailability in two vegetables: the influence of cooking and additives.

The estimation of heavy metal bioaccessibility and bioavailability in vegetables is helpful for human health risk assessment. Using an in vitro digestion/Caco-2 cell model, the bioaccessibility and bioavailability of cadmium (Cd) and lead (Pb) in raw/cooked pakchoi (Brassica rapa L., Chinensis Group) and Malabar spinach (Basella rubra L.) were studied. The effect of the addition of iron, calcium and acetic acid to the samples was also determined. The results indicated that Cd bioaccessibility was higher in the gastric phase and Pb bioaccessibility was higher in the small intestinal phase. Cadmium and Pb bioavailability were 11.2% and 9.4% in the raw vegetables, respectively, and found to be higher significantly than the cooked vegetables with 6.1% for Cd and 3.2% for Pb. The results showed that it will be overestimating the risk of Pb and Cd based on the data of raw vegetables ingestion. Using bioavailability values, average Cd and Pb daily intake by adult were 23% and 28% respectively, of the base bioaccessibility values. Our study will be better understanding the possible health risks of some vegetables base on the bioaccessibility or bioavailability.

Lead (Pb) and arsenic (As) bioaccessibility in various soils from south China

Seventeen soil samples with various concentrations of lead (Pb) and arsenic (As) were collected from five provinces of south China, and bioaccessibility of Pb and As in the soils were examined using a physiologically based extraction test. The results showed that the bioaccessibility ranged from 24.6% to 82.5% and 2.3% to 57.5% for Pb, 2.5% to 65.5% and 1.2% to 31.8% for As in the gastric and small intestinal phases, respectively. The effect of soil properties on the bioaccessible of Pb/As was evaluated. Path analysis showed that coefficients of determination (R2) for the bioaccessible Pb were 0.93 in the both gastric and small intestinal phases and for the bioaccessible As were 0.98 and 0.99 in the gastric and small intestinal phases, respectively. Among all the soil characteristics, OM, DCBFe, CEC, and WAs were significant for controlling PbD, and CEC, CaCl2–Pb, and WAs were important for controlling PbI. Partitioning by path analysis also showed significant direct effects by CaCl2–Pb and WAs on AsD, and OM, CEC, CaCl2–Pb, and WAs on AsI.