Hydrothermally-altered feldspar as an environmentally-friendly technology to promote heavy metals immobilization: Batch studies and application in smelting-affected soils
Graphical abstract
Introduction
Mining and smelting activities are crucial for the population and stimulation of economic development, but industrial wastes generated from these operations may directly or indirectly impact soil and water quality in areas nearby mining and smelting regions (Bernhardt and Palmer, 2011; Hudson-Edwards, 2016; Zipper et al., 2016). One of the major concerns in this context is the high heavy metal levels in soil and water surrounding mining/smelting sites (Chen et al., 2018; Hoaghia et al., 2019; Kalyvas et al., 2018; Shen et al., 2019; Zhao et al., 2020). These heavy metals, such as cadmium (Cd), zinc (Zn), and lead (Pb), pose short- and long-term risks to the ecosystem and human health (Ali et al., 2019; ATSDR, 2012).
Many strategies for remediating contaminated soils have been studied worldwide in recent years, including electrokinetic (Sun et al., 2019), vitrification of contaminant sources (Ballesteros et al., 2017), phytoremediation (Zeng et al., 2019), and bioremediation (Zhao et al., 2019). Another promising technique is the immobilization of heavy metals by adding amendments to contaminated soils (Palansooriya et al., 2020). These amendments may reduce metal mobilities in soil by adsorption, precipitation, and ion exchange, thus preventing the leaching of these elements in water or their uptake by plants (Lwin et al., 2018; Mahar et al., 2015). Besides being the use of amendments relevant to decrease the mobility of contaminants, it can also be a source of plant nutrients, which may be released to plants, enhancing plant growth and, as a result, the remediation process (Lopes et al., 2016; Teodoro et al., 2020).
Many amendments such as biochar (Fellet et al., 2011; Yuan et al., 2019), phosphates (Mignardi et al., 2012), zero-valent iron (Tang et al., 2019), limestone (Bade et al., 2012), and industrial by-products (Costa et al., 2020; Martins et al., 2018) have been studied for reducing bioavailability and toxicity of metals in contaminated soils. Due to their abundance and availability, silicate minerals, such as montmorillonite (Ijagbemi et al., 2009), bentonite (Anna et al., 2015; Kumararaja et al., 2017; Sun et al., 2015), and zeolite (Hong et al., 2019; Yuna, 2016) have also been used as adsorbent materials of heavy metals. Of over 40 types of natural zeolites, clinoptilolite is the most commonly used for agronomic purposes, as soil conditioning and soil remediation, immobilizing metals in contaminated soils (Reháková et al., 2004; Yuna, 2016).
Hydrothermally altered feldspar called HydroPotash (HYP), patented and licensed by APT - Advanced Potash Technologies Ltd., is a silicate mineral synthesized through hydrothermal alteration of potassium (K) feldspars plus Ca(OH)2 (Ciceri et al., 2017a, b). It has been marketed as a potassium fertilizer and its efficiency for promoting plant growth has been reported by Ciceri et al. (2019). Feldspar, altered by a hydrothermal process, is considered a green technology since it is produced with globally abundant raw materials (Ciceri et al., 2017a, b). Because of its high pH buffering and cation exchange capacity (CEC), it may have the efficiency to immobilize heavy metals from contaminated sites. Therefore, considering that little is known about the HYP use in such context, studies to assess its effectiveness as heavy metals adsorbent and as an amendment in contaminated soil are of great relevance and required. This study aimed to: 1) evaluate the sorption capacity of Cd, Zn, and Pb in HYPs (HYP-1 and HYP-2) and zeolite in natural suspension pH (without altering pH); 2) assess the adsorption-desorption of Cd, Zn, and Pb on HYPs and zeolite at pH 5.5; and, 3) evaluate the efficiency of these products in immobilizing metals in contaminated soils collected from a smelting site. From that, we intended to evaluate the potential use of these products as immobilization agents in soil-remediation projects and learn how they may act (i.e., only due to an increasing-pH-effect causing precipitation or thru additional adsorption processes when the pH is controlled).
Section snippets
Adsorbent materials description and characterization
HydroPotash (HYPs) samples were provided by Advanced Potash Technologies Ltd. The detailed preparation of these materials was described by Ciceri et al. (2017a). Briefly, milled feldspars (>80% pure KAlSi3O8) and Ca(OH)2 were mixed under a hydrothermal process (200-230 °C for 1–2.5 h) and subsequently dried. HydroPotash-1 and HYP-2 were the two types of HYPs used for this study. They were prepared using the same raw material with different temperatures (200 °C for HYP-1 and 230 °C for HYP-2)
Characterization of adsorbents/amendments
The pH of materials (HYP-1, HYP-2, and zeolite) varied from 8 to 12.6, and their CEC from 530 to 1971 mmolc kg−1 (Table 1). Based on SEM images with EDS, the main composition of HYP, an aluminosilicate after the hydrothermal process, was still silicon (Si), aluminum (Al), and oxygen (O) (Fig. S2 and S3). Peaks of potassium (K) and calcium (Ca) were observed in all materials.
Mineralogical analysis of X-ray diffraction (XRD) confirmed the presence of clinoptilolite in the zeolite sample (Yang et
Conclusions
Hydrothermally-altered feldspar (HydroPotash) showed the ability to immobilize heavy metals in both aqueous solution and smelting-affected soils. The mechanisms for immobilizing metals by HydroPotash materials were precipitation by increasing soil/solution pH and adsorption, which was proven by the batch test with adjusted pH (5.5). The zeolite tested in the present study had significant lower efficiency for immobilizing heavy metals compared with the HYPs in the soil incubation experiment.
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.
Acknowledgments
The authors are grateful to the Advanced Potash Technologies Ltd. for producing and providing the HydroPotash patented amendments for the present research. The authors also would like to thank the funding agencies National Council for Scientific and Technological Development (CNPq Grant # 141228/2018–0), Coordination for the Improvement of Higher Education Personnel (CAPES-PRINT - 88887.371138/2019–00) and Minas Gerais State Research Foundation (FAPEMIG) for support. The authors want to thank
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