Search

The influence of lime amendment on the absorption and accumulation of lead (Pb) in the grains of three rice cultivars ((Dong Lian 5 (DL-5, Indica), TeYou 009 (TY-009, Hybrid Indica), and YiYou 673 (YY-673, Hybrid Indica)) was investigated and the associated mechanisms were explored. Pot experiment was conducted in heavily Pb-polluted paddy soil. The transformation of Pb species in soil and roots under different levels of lime application was studied by Pb L3-edge X-ray absorption spectroscopy (Pb L3-XAS) and BCR sequential extraction. The results showed that lime amendment significantly increased soil pH and decreased DTPA-extractable Pb in soil. When the additional ratio of Liming was lower than 32 g kg–1, the Pb accumulation in rice plants, particularly in the grains was remarkably reduced. Moreover the biomass of rice was not significantly affected. Among the three cultivars, the lowest Pb content in rice grains occurred in YY-673. The cultivation of rice cultivar with low Pb accumulation will reduce the Pb content in the grains. Liming significantly inhibited the translocation of Pb from the roots to the aboveground parts, and from the stems and leaves to grains. Pb L3-XAS and BCR s sequential extraction method analysis revealed that lime amendment increased Pb2+ association with Fe oxides (Pb-Ferr), the precipitation of 2PbCO3·Pb(OH)2, the residual fraction of paddy soil, and also promoted the transformation of Pb(NO3)2 and Pb-pectin to 2PbCO3·Pb(OH)2 and Pb-Ferr in rice roots. The suitable lime addition (> 4 g kg–1, under this pot experiment) could effectively reduce Pb accumulation in rice grains.

This study aimed to investigate the effects of rice straw biochar and lime (RBL) on the remediation of lead (Pb)-contaminated soil and mitigation of Pb translocation in rice plants by using pot experiments. Lead-contaminated soil collected from a farmland near a Pb-zinc (Zn) mine, biochar, limestone powder, and indica rice (Oryza sativa L.) were used in the present study. The experimental treatments included: (1) control (CK), (2) 2.5% biochar (RB1), (3) 5% biochar (RB2), (4) 0.6% lime (L1), (5) 1.2% lime (L2), (6) 2.5% biochar + 0.6% lime (RBL1), and (7) 2.5% biochar + 1.2% lime (RBL2). The results revealed that the treatment with RBL was more efficient than the treatment with only biochar or lime in decreasing CaCl2-extractable Pb content in the soil by increasing soil pH and soluble sulfate content in the soil. Treatment with RBL reduced in the accumulation of Pb in the shoot of rice plants, this was mainly attributed to the decrease in the concentration of available Pb in the soil. The RBL2 treatment not only decreased the concentration of Pb in brown rice by 84.33% and Pb distribution in rice embryo, but also increased rice yield by 53.38% from that of the control. Further, unlike biochar treatment, RBL and lime treatments decreased the translocation of Pb in rice plants. The RBL treatment increased the proportion of Pb distributed in the cell wall and reduced the mobility of Pb in plant tissues. Thus, application of biochar and lime in combination is more effective than their individual application in reducing the availability of Pb in the soil and Pb accumulation in brown rice.

Immobilization is widely used to decrease the availability of heavy metals, such as Cd and Pb, in contaminated soils. However, the spatial and temporal changes in the immobilization of soil by amendments combined with planting effects have not been studied well. In this study, unplanted and planted (with rice plants) pot experiments were used to assess the spatial and temporal changes in water-soluble Cd, Fe, Mn, and Ca. Soil properties, such as pH, redox potential (Eh), and dissolved organic carbon (DOC), were continuously recorded in both the rhizosphere and bulk soil using non-invasive rhizon samplers and a microelectrode system (Unisense). In unplanted soil, pH and Eh varied with time, but showed little radial variation from the rhizosphere to the bulk soil. The addition of hydrated lime (Ca(OH)2) sharply increased the pH, DOC, and Ca content; decreased the Eh, Fe content, and Mn content; and gradually decreased the water-soluble Cd content in the soil profile. Hydroxyapatite showed no obvious effects in reducing Cd concentrations in different soil zones. The water-soluble Fe, Mn, Ca, and DOC content did not differ significantly between soil zones over time and a non-significant correlation with water-soluble Cd was shown. In planted soil, the pH increased while the Eh value decreased with an increase in the distance from the roots, regardless of the soil amendments used during the rice growth period. Hydroxyapatite gradually increased but hydrated lime decreased the water-soluble Cd in the rhizosphere. The concentration of water-soluble Cd in the rhizosphere was higher than that of the other soil zones during rice growth. These changes lead to more Cd uptake by roots and induced Cd accumulation in rice tissues. In addition, Cd and Fe concentration in iron plaque showed a significant positive correlation with Cd in rice, indicating that iron plaque promotes the uptake and accumulation of Cd in rice with soil amendments. Compared with the control, hydroxyapatite did not significantly affect the Cd content, while Ca(OH)2 significantly reduced the Cd content in iron plaque and rice tissues. In conclusion, the application of hydrated lime can significantly reduce the risk of Cd accumulation by rice in Cd-contaminated soils under flooding conditions.

Methylmercury (MeHg) in rice grains grown in Hg-contaminated areas has raised environmental health concerns. Pot experiments found that selenium (Se) could reduce MeHg levels in rice grains. However, relatively high levels of Se (up to 6 mg/kg) were applied in these pot experiments, which may have adverse effects on the soil ecology due to the toxicity of Se. The aims of this work were thus to study 1) the effect of low levels of Se on the accumulation and distribution of Hg, especially MeHg, in rice plants grown in a real Hg-contaminated paddy field and 2) the effect of Se treatment on Se and other nutritional elements (e.g., Cu, Fe, Zn) in grains. A field study amended with different levels of Se was carried out in Hg-contaminated paddy soil in Qingzhen, Guizhou, China. The levels of MeHg and total Hg were studied using cold vapor atomic fluorescence spectrometry (CVAFS) and inductively coupled plasma mass spectrometry (ICP-MS). The distribution and relative quantification of elements in grains were examined by synchrotron radiation X-ray fluorescence analysis (SR-XRF). This field study showed that low levels of Se (0.5 μg/mL, corresponding to 0.15 mg Se/kg soils) could significantly reduce total Hg and MeHg in rice tissues. Se treatment also reduced Hg distribution in the embryo and endosperm and increased the levels of Fe, Cu, Zn and Se in grains and especially embryos. This field study implied that treatment with an appropriate level of Se is an effective approach to not only decrease the level of MeHg but to also increase the levels of nutritional elements such as Fe, Cu, Zn and Se in rice grains, which could bring beneficial effects for rice-dependent residents living in Hg-contaminated areas.

Mercury (Hg) pollution seriously threatens food safety and has attracted global attention. Phytoextraction, due to its low cost, applicability, and environmental friendliness, is considered a new technology for clean-up of heavy metal contamination in the environment. However, the low bioavailability of Hg in polluted areas greatly limits the applicability of phytoextraction. Here, we compared the effects of six common chelating ligands on the absorption and transport of Hg in maize (Zea mays L.), which has a high biomass and short growth cycle. The results showed that the root length and biomass of maize seedlings of the groups treated with the six chelating ligands (EDTA, iodide, ammonium, thiosulfate, thiocyanate, and thiocarbamide) did not change compared with those of the non-treated groups. Co-exposure to Hg and each chelating ligand markedly alleviated the inhibitory effect induced by Hg. Iodide treatment resulted in the lowest root Hg content and highest translocation factor (TF) value, while ammonium treatment gave rise to the highest shoot Hg concentration and lowest TF. Compared with other chelating ligands, thiosulfate exhibited the maximum alleviation of Hg toxicity and achieved the highest concentration of Hg in the roots and aerial parts. Moreover, the TF and Hg accumulation in the thiosulfate and Hg co-exposed group were much higher than those in the group exposed to Hg alone. This finding suggests that, among these common chelating ligands, thiosulfate compounds have great potential for Hg phytoextraction, while the others can immobilize Hg in polluted areas.