Mechanisms for potential Pb immobilization by hydroxyapatite in a soil-rice system
Graphical abstract
Introduction
Among remediation techniques for heavy metal contaminated soils, immobilization is a generally accepted method, especially for agricultural soils, because it has the less impact on soil fertility and is easy and low-cost. Several amendments have been commonly used in soil immobilization, including phosphorous-, calcium-, silicon-containing, organic, and other synthesized amendments (Udeigwe et al., 2011; Li et al., 2019). Phosphorous-containing materials have attracted significant attention as an important type of amendment, because they are effective in immobilizing heavy metals, particularly Pb in soils. (Chrysochoou et al., 2007; Miretzky and Fernandez-Cirelli, 2008; Fang et al., 2011; Mignardi et al., 2012; Ogawa et al., 2020). Chen et al. (1997) reported that apatite (a sedimentary phosphate rock, Ca9.53Na0.34Mg0.13 (PO4)0.77(CO3)l.23 F2.49) could efficiently adsorb Pb and showed moderate capability for Cd and Zn retention at pH 4–5. The combination of hydroxyapatite and KCl decreased leachable Pb and Cd of soils by 83.3–97.27% and 57.82–35.96%, respectively (Li et al., 2014). Cao et al. (2009) also indicated that phosphoric acid and/or phosphate rock were more effective in reducing the availability of Pb than Cu and Zn in soils. In addition, the effects of H3PO4 on Pb immobilization could last for at least 8–9 years (Tang et al., 2009). Application of phosphate could transform non-residual Pb into residual fractions, thereby decreasing the bioavailability of soil Pb (Melamed et al., 2003; Cao et al., 2004). The major mechanisms of Pb immobilization with phosphate have been considered to be the formation of insoluble pyromorphite-like minerals (Melamed et al., 2003; Cao et al., 2004; Shibao et al., 2007; Xinde et al., 2008; Wacławska and Szumera, 2009), and the bonding between Pb2+and phosphate through ion exchange or complexation (Prasad et al., 2008; Mignardi et al., 2012).
Phosphorus treatments not only decrease the availability of Pb in soils, but also influence the uptake of Pb by roots, and the translocation of Pb from roots to shoots. In most cases, the application of phosphate reduces Pb uptake and Pb concentrations in shoots or edible parts of plants. Mechanisms for the transfer of Pb from roots to shoots as affected by phosphates are intricate (Fang et al., 2011; Wang et al., 2012). Cao et al. (2004) demonstrated that phosphate significantly reduced root-to-shoot transfer of Pb in St Augustine grass and suggested that this reduction was due to the formation of pyromorphite-like minerals on the membrane surface of the root. Wang et al. (2012) also reported that Ca(H2PO4)2.H2O treatment led to a reduction in root-to-rice grain transfer of Pb. Previous studies reported that applications of phosphate amendments (hydroxyapatite, phosphate rock, single-superphosphate) either decreased or increased the R/S ratio of Pb (the ratio of Pb concentration in roots to that in shoots) of cauliflower, suggesting an non-systematic impact on Pb translocation (Chen et al., 2009). Similarly, non-systematic changes in the R/S ratio of B. campetris (Chinese cabbage) were also found when soils were amended with hydroxyapatite, phosphate rock, or single superphosphate. However, the application of phosphate-related materials decreased the R/S values of Brassica oleracea (Cabbage) (Zhu et al., 2004). These findings implied that the effects of phosphates on Pb translocation is related to the category of crop.
The contamination of Pb in rice, a staple food in China and Asia, is of great concern. A recent survey of six provinces in southeastern China demonstrated that, 41 out of 269 rice grain samples contained Pb concentration exceeding the maximum allowable Pb concentration of 0.2 mg kg−1 Pb in rice grains, implying that Pb contamination in rice is a common problem (Cheng et al., 2006). It was also reported that among 13 countries examined, Pb concentrations in rice grains were the highest in China, and it was much higher in mine-impacted regions than areas not impacted by mining (Norton et al., 2014). Therefore, it is necessary to develop effective remediation methods for Pb contamination in rice.
Limited studies have shown that phosphate amendments are effective in immobilizing and reducing Pb concentrations in rice grains (Wang et al., 2012; Gao et al., 2015). However, the mechanisms underlying the mitigation of Pb translocation from roots to shoots following phosphate amendments remain obscure. There are only few studies provide reliable evidence of Pb retention in rice roots after soil amendment with hydroxyapatite. Thus, this study aimed to (1) clarify the effects of hydroxyapatite on Pb immobilization in soil-rice systems; (2) determine the related mechanisms in terms of Pb availability in soils, Pb uptake by rice roots, and Pb translocation in rice plants.
Section snippets
Materials
The soil used for the pot experiments was collected from the surface layer (0–20 cm) of an agricultural field near a PbZn mine in central Fujian Province, southeast China. The soil was air-dried and crushed to pass through a 1 cm sieve. The basic properties of the soil were summarized in Table S1. The soil was been heavily contaminated with Pb (1602 mg kg−1). The rice cultivar used in this study was Donglian 5, a conventional variety of indica rice. Hydroxyapatite (HAP) was supplied by a
pH, available P and Pb fractions in the soil
The pH of the soils increased with the addition of HAP addition (Fig. 1). The pH of P5 soil (32 g HAP kg−1 soil, the highest rate of HAP added) was 1.16 and 1.08 higher than that of the controls in tillering and in maturity stage, respectively. While HAP addition reduced the weakly acid-extractable Pb (WAEPb), the reducible and residual Pb increased upon the addition of HAP (Fig. 2). The addition of HAP exerted a trivial impact on the amount of oxidizable Pb. The contents of weakly
Discussion
The soil in this study was severely contaminated by Pb, with a peak concentration of 1602 mg kg−1. This concentration was much higher than the risk screening value of 80 mg kg−1 and risk intervention value of 500 mg kg−1 for agricultural paddy soils with pH values between 5.5 and 6.5 in China (GB 15618-2018). With increasing HAP application, DTPA-extractable Pb decreased (Fig. 1). This could be attributed to a combination of effects. First, the application of HAP increased soil pH (Fig. 1),
Conclusions
Application of HAP increased soil pH and promoted the formation of Pb-phosphate complexes in soils, leading to a reduction in Pb bioavailability and soil acid soluble Pb. HAP reduced the retention of Pb in the iron plaque on the root surface at maturity stage, resulting in a decrease of Pb uptake by rice roots. Furthermore, the translocation of Pb from roots to stems decreased with increasing HAP application due to the formation of Pb5PO4Cl in rice roots. The transfer factor of Pb from the stem
CRediT authorship contribution statement
Honghong Li: Data curation, Writing – original draft, Visualization. Yuting Liu: Formal analysis, Writing – review & editing. Shouyin Tang: Investigation, Visualization. Zuchen Yu: Visualization. Xuezhi Cai: Investigation. Shupeng Xu: Investigation. Yanhui Chen: Supervision, Project administration. Mingkuang Wang: Writing – review & editing. Guo Wang: Conceptualization, Methodology, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was funded by the National Natural Science Foundation of China (grant no. U1305232). The XAS measurements were performed at Spring-8. We thank all the staff at BL12B for their assistance during the XAS measurement and data processing. The authors thank Haixia Dong and Mingliu Zhao for their experimental cooperation.
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Co-first authors: Honghong Li, Yuting Liu, Shouyin Tang.