Mitigation effects of silicon rich amendments on heavy metal accumulation in rice (Oryza sativa L.) planted on multi-metal contaminated acidic soil
Highlights
► The application of fly ash and steel slag ameliorated the extremely acidic soil. ► Heavy metal diffusion flux from amended soil to solution phase sharply decreased. ► Metal silicates, phosphates and hydroxides deposited in amended soil. ► Amendment addition dramatically restrained metal translocation from stem to leaf. ► Heavy metal amount in polished rice complied with the China’s food safety standards.
Introduction
The industry of mining and processing metals is a major source of farmland heavy metal contamination (Navarro et al., 2008). Despite strict regulations restricting the illegal discharge, the acidic mine drainage (AMD) with a low pH and high heavy metal concentrations still emerged in the nearby rivers (Zhuang et al., 2009). Large areas of farmlands contaminated by irrigation with the AMD contaminated water, which not only leads to soil degradation, but also poses a public health threat to humans by food chain (Pruvot et al., 2006). The contaminated farmlands by heavy metals have reached 20 Mha in China, and most of them are still cultivated due to serious cropland shortage (Chen, 2007).
Popular and widely used techniques to remediate heavy metal contaminated soil include chemical washing, electroremediation, phytoremediation and immobilization (or phytostabilization) (Mulligan et al., 2001, Dermont et al., 2010), of which in situ immobilization by using nontoxic amendments into soil is a credible choice for contaminated farmlands because of its cost-effective and environmentally compatible characteristics (Zhao and Masaihiko, 2007b). Many different amendments, such as limestone, zeolite or other organic and inorganic amendments have been applied for heavy metal stabilization (Chen et al., 2000, Geebelen et al., 2006, Zhao and Masaihiko, 2007a). However, the cost of some pure amendments is relatively high. Therefore substituting low cost natural/industrial by-products for expensive commercial amendments seems to be more feasible. Fly ash (FA) and steel slag (SS) are by-products of coal-fired thermal power plants and steel mills, respectively. The application of FA to soil has been reported to increase pH and decrease heavy metal phytoavailability (Rautaray et al., 2003, Dwivedi et al., 2007, Rijkenberg and Depree, 2010). During recent decades, a few research groups have used SS in wastewater treatments as a low cost absorbent to remove heavy metals (Ortiz et al., 2001, Jha et al., 2008). Navarro et al. (2010) reported SS reduced the release of the harmful trace elements Cr (VI) and V. However, the mechanisms involved in remediation of metal contaminated soils and their effects on metal accumulation in plants remain poorly understood.
High silicate is an important characteristic of FA and SS. Silicon has not been considered as an essential mineral element for all higher plants (Epstein, 1999), but there is increasing evidence that Si is beneficial and likely essential for the rice healthy growth (Liang et al., 1994, Ma and Yamaji, 2006). Silicon is also suggested to help to increase the resistance to toxic metals, which might owe to stimulation of antioxidant systems, alleviation of inhibition to photosynthesis and complexation of heavy metals with Si (Neumann and zur Nieden, 2001, Shi et al., 2005, Liang et al., 2007). Whereas the mechanisms involved in Si-mediated reduced metal accumulation and detoxification in rice are still not clear.
In situ immobilization with amendments has been investigated as a promising method for remediation of heavy metal contaminated soil (Geebelen et al., 2006, Kumpiene et al., 2007). However, lack of appropriate tools for estimating the fixing efficiency has become a limiting factor in acceptance of this technology (Basta and McGowen, 2004). Conventional chemical extractions may be appropriate for the total accumulation of metals by roots for long-term studies; however, extractants usually cannot explain the redistribution and resupply of metals from the soil solid phase (Ernstberger et al., 2005). In contrast, the diffusive gradient in thin films (DGT) technique, which is composed of a membrane filter, a diffusive gel, and a resin gel, can estimate the dynamics of these processes (Zhang et al., 1998). Therefore DGT technique might provide a reliable surrogate for estimating the stabilization efficiency of amendments.
The aims of this study are (1) to demonstrate the effects and possible mechanisms involved in FA and SS assisted soil heavy metal stabilization and assess the stabilization efficiency, (2) to illustrate the effects of FA and SS on metal accumulation and translocation in rice and (3) to provide both theoretical and practical bases for performing field-scale remediation aiming at remediating multi-metal contaminated acidic soils and ensuring food safety.
Section snippets
Soil and amendments
The tested soil was collected from a paddy field (24°32′N and 113°42′E) in Shangba village which is 6 km from the Dabao Mountain mining area located in Guangdong province, south China. Dabao Mountain mine is a cluster of opencast mines with a multi-metal sulfide deposit. The studied soil both for pot experiment and field experiment were from the same paddy field polluted by the AMD of Dabao Mountain mine. The total concentrations of Cd, Zn, Cu and Pb in soil (Table 1) significantly exceeded the
Plant growth
The effects of amendments on rice growth are shown in Fig. 1. The addition amount of SS at both 3 and 6 g kg−1 significantly increased the dry weights of roots, stems and leaves, and the increment was higher in SS3 treatment. Compared with SS, there were smaller increases treated with FA at both 20 and 40 g kg−1.
Effects of amendments on soil pH, the unstable pools and forms of heavy metals in soil
After application of FA and SS, soil pH significantly increased from that of the control (4.0) with the increasing amendment dosages (Table 2). The treatment of FA40 had the highest pH
Discussion
In this study, the contaminated soil was acidic and very high in Cd, Zn, Cu and Pb, and the rice production on the soil was very poor. Thus in order to reduce the phytotoxicity of the soil metals and toxic metal accumulation in rice grain, remediation measure needed to be carried out. Use of industrial byproducts to remediate soil contamination problems has been increasing as a measure of in situ inactivation of metals (Adriano et al., 2004). FA and SS are rich in alkaline substances and might
Conclusions
The present results demonstrated that, in FA and SS amended contaminated soils, the reduction in Cd, Zn, Cu and Pb uptake by rice were attributed to two processes including in situ immobilization of heavy metals in soil and Si-mediated effects in rice. The application of FA and SS decreased the heavy metal DGT pools and the fluxes from the soil solid phase to solution by transforming the soluble metals to the less soluble and slower exchanging forms such as metal silicates, phosphates and
Acknowledgements
The Project was supported by the NSFC-Guangdong Joint Foundation of China (No. U0833004), Guangdong Provincial Natural Science Foundation (No. 06202438), National High Technology Research and Development Program of China (863 Program) (Nos. 2007AA06Z305 and 2007AA061001) and the Fundamental Research Funds for the Central Universities (No. 09lgpy23, 101gzd10).
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