Divalent Heavy Metals and Uranyl Cations Incorporated in Calcite
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www.nature.com/scientificreports OPEN Divalent heavy metals and uranyl cations incorporated in calcite change its dissolution process Xiaohang Zhang1,2, Jianan Guo1,2, Shijun Wu1*, Fanrong Chen1 & Yongqiang Yang1 Due to the high capacity of impurities in its structure, calcite is regarded as one of the most attractive minerals to trap heavy metals (HMs) and radionuclides via substitution during coprecipitation/crystal growth. As a high-reactivity mineral, calcite may release HMs via dissolution. However, the infuence of the incorporated HMs and radionuclides in calcite on its dissolution is unclear. Herein, we reported the dissolution behavior of the synthesized calcite incorporated with cadmium (Cd), cobalt (Co), nickel (Ni), zinc (Zn), and uranium (U). Our fndings indicated that the HMs and U in calcite could signifcantly change the dissolution process of calcite. The results demonstrated that the incorporated HMs and U had both inhibiting and enhancing efects on the solubility of calcite, depending on the type of metals and their content. Furthermore, secondary minerals such as smithsonite (ZnCO3), Co-poor aragonite, and U-rich calcite precipitated during dissolution. Thus, the incorporation of metals into calcite can control the behavior of HMs/uranium, calcite, and even carbon dioxide. Calcite, the most stable polymorph of CaCO3, is the most important and also the most abundant carbonate min- eral on Earth 1–3. Te precipitation of calcite serves as a sink of metals 4–7, organic material 8, and carbon dioxide 9,10. Traditionally, lime materials (including calcite, burnt lime, and dolomite) are used to neutralize acidic soils and to overcome the problems associated with soil acidifcation 11,12. With the application of calcite, heavy metals (HMs) usually become less bioavailable due to the increase in soil pH and formation of metal–carbonate bounded complexes13,14. Furthermore, in situ microbial induced calcite precipitation (MICP) was proposed to remediate soil and underground water contaminated by HMs or radionuclides via substitution/coprecipitation15,16. Nev- ertheless, as much as 30% calcite dissolution was observed, which challenges the long-term sustainability of the calcite formed by MICP17. As a base mineral, the dissolution of calcite can neutralize the acidifcation of soil and water. During the last two centuries, acidifcation of Earth’s air, water, and soil has been accelerated due to anthropogenic activities, such as the combustion of fossil fuels and smelting of ores, mining of coal and metal ores, and application of 18,19 20 nitrogen fertilizer to soils . Due to ocean acidifcation , the dissolution of marine CaCO3 (including sedi- ments and coral reef) has been reported worldwide21–24. On the continent, the concentrations of Ca in freshwater increased due to terrestrial rock dissolution as a result of climate change and anthropogenic acid deposition 25,26. Meanwhile, the carbonate bonded metal will release into the environment, which could make calcite a potential source of heavy metals 27,28. For example, uranium concentrations in river water are primarily determined by the dissolution of limestone (dominated by calcite)29. Strontium (Sr) released from Himalayan carbonate changed the Sr isotope composition in seawater and marine limestones 30. In the Karst area, the weathering of carbonate rock naturally causes HMs to accumulate in soil31–34, resulting in HM pollution in plants35. Te growth and dissolution of calcite have been investigated extensively and reviewed in documents36–38. Generally speaking, the dissolution of calcite is infuenced by the temperature, pH, PCO2 , solution composition and inhibitors36–39. However, the infuence of impurities in calcite on its dissolution is not well understood. Based on atomic force microscopy (AFM) observations, Harstad and Stipp concluded that Fe2+, Mg2+, Mn2+, and Sr2+, which are naturally present in Iceland spar calcites, inhibited the dissolution of calcite 40. However, at least for Mg2+ and Mn 2+, this conclusion conficts with the experimental data obtained from magnesian calcite 41–44 and synthesized Mn2+ containing calcite45. Te macroscopic dissolution experiment of natural inorganic, biogenic, 1CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Guangzhou 510640, China. 2University of Chinese Academy of Science, 19 Yuquan Road, Beijing 100049, China. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:16864 | https://doi.org/10.1038/s41598-020-73555-6 1 Vol.:(0123456789) www.nature.com/scientificreports/ M2+/Ca2+ (mol%) Sample Mineral composition Added Measured BET specifc surface area (m2/g) M0 Cal* – – 0.17 Cd-02 Cal 2.00 1.83 0.38 Cd-04 Cal 4.00 4.05 0.57 Cd-06 Cal 6.00 6.75 1.02 Cd-08 Cal 8.00 9.31 1.57 Cd-10 Cal 10.00 11.73 1.66 Zn-02 Cal 2.00 1.89 0.43 Zn-04 Cal 4.00 3.18 1.65 Zn-06 Cal 6.00 4.83 1.69 Zn-08 Cal 8.00 7.04 2.93 Zn-10 Cal 10.00 12.36 4.49 Co-02 Cal 2.00 1.37 0.25 Co-04 Cal 4.00 3.43 0.53 Co-06 Cal 6.00 6.40 0.80 Co-08 Cal 8.00 7.72 1.39 Co-10 Cal, arg* 10.00 – – Ni-0.04 Cal 0.04 0.05 0.19 Ni-0.08 Cal 0.08 0.07 0.21 Ni-0.2 Cal 0.20 0.28 0.22 Ni-0.4 Cal, vat, arg 0.40 – – U-0.5 Cal 0.50 0.56 0.51 U-01 Cal 1.00 0.95 0.69 U-02 Cal 2.00 1.55 1.46 U-04 Cal 4.00 2.64 4.64 U-08 Cal, vat* 8.00 – – Table 1. Mineral/chemical compositions and BET specifc surface areas of the synthesized samples. *Cal, vat, and arg are abbreviations for calcite, vaterite, and aragonite, respectively. and synthesized samples demonstrated that the dissolution ability of magnesian calcite is positively correlated with the content of Mg in calcite 41–43, which is supported by the AFM observation, according to Davis and his coauthors44. Terefore, high-Mg calcite in tropical continental shelf sediments is more sensitive than low-Mg calcite to ocean acidifcation 46. Recently, we found that the incorporation of Cu 2+ and Mn2+ enhanced the solu- bility of calcite 45. Te above-mentioned studies indicated that the infuences of impurities on calcite dissolution is complex and need further investigation. Because of the multiformity of heavy metal contamination in the feld and the numerous impurities in natural calcite, it is difcult to identify the infuence of a single component on its dissolution. Herein, we provide further 2+ 2+ 2+ 2+ 2+ evidence on the dissolution behavior of synthesized calcite incorporated with Cd , Co , Ni , Zn and UO2 , which are common environmental pollutants. Te results showed that the solubility of calcite was inhibited by 2+ 2+ 2+ 2+ 2+ coprecipitated Cd and Ni but enhanced by Co . Unexpectedly, Zn and UO2 showed both inhibiting and enhancing efects, depending on the mass of the impurities in calcite. Tese observations suggested that the incorporated HMs and radionuclides might control the dissolution of calcite. Inversely, the migration of HMs and radionuclides could be remarkably controlled by the host minerals as well. Results Characterization of calcite incorporated with impurity metals. With the addition of HMs, the XRD patterns of all the Cd and Zn containing products were the same as the pure calcite reference pattern, indicating that no detectable secondary crystalline phases were present (Fig. S1a,b, Table 1). However, traces of aragonite were found in sample Co-10 (Fig. S1c), while vaterite was present in U-08 and U-10 (Fig. S1e). Mean- while, both aragonite and vaterite occurred in Ni-0.4 and Ni-01 (Fig. S1d). To avoid the impact of aragonite and vaterite, we used samples without detectable secondary phases for further experiments. As shown in Fig. 1a, pure calcite showed a typical rhombohedral morphology as a euhedral calcite crystal. With the incorporation of metals, the morphology changed to aggregates of semi-euhedral or anhedral (dumb- bell) phases with small sizes (Fig. 1b–l). Tis trend is supported by the BET surface area data, which are positively correlated with the molar fraction of metals, with R 2 values of 0.65 (Ni), 0.89 (Co), 0.93 (U), and 0.96 (Cd, Zn) (Table 1, Fig. S2). Usually, the incorporation of impurities will decrease the sizes of crystals47,48 due to the inhibi- tion of the crystal growth. Figure S3 showed the spatial distribution of metals in typical polished HM-calcite. Both of the line scan and elemental mapping results showed the presence of Cd/Zn-rich cores in Cd/Zn-calcite (Fig. S3a–h). However, the SCIENTIFIC REPORTS | (2020) 10:16864 | https://doi.org/10.1038/s41598-020-73555-6 2 Vol:.(1234567890) www.nature.com/scientificreports/ Figure 1. SEM images of selected calcite. (a) M0, (b) Cd-02, (c) Cd-10, (d) Zn-02, (e) Zn-10, (f) Co-02, (g) Co-06, (h) Co-08, (i) Ni-0.04, (j) Ni-0.2, (k) U-0.5, and (l) U-04. Co-calcite and Ni-calcite were quite homogenous in general. Interestingly, U-calcite possessed two distributions, as mentioned above, i.e., a U-rich core and homogenous distribution. Release of metals during the dissolution of calcite. Once calcite contacted the solution, dissolution occurred immediately, especially in an acidic solution. For example, when the initial solution pH ranged from 1.0 to 8.9, the proportion of pure calcite that dissolved within 20 min accounted for 83.8–97.4% of the total mass dissolved within 120 min (Fig.