A novel remediation method of cadmium (Cd) contaminated soil: Dynamic equilibrium of Cd2+ rapid release from soil to water and selective adsorption by PP-g-AA fibers-ball at low concentration
Graphical Abstract
Introduction
Cd(II) is considered as a primary heavy metal contamination of soil with high persistence and high toxicity (Liu et al., 2020a, Wiggenhauser et al., 2016, He et al., 2019). This is mainly due to the acid-extractable fraction Cd(II) migrates easily in the ecological environment, tends to be uptaken by plants, especially rice, and then can be bioaccumulated in the human body through the food chain (Zhai et al., 2018, Wang et al., 2018, Sebastien and Murray, 2000).
Methods such as immobilization/stabilization, etc have been widely employed for remediation of Cd(II) contaminated soil by transforming the acid-extractable fraction Cd(II) into more stable forms (Brown et al., 2013, Liu et al., 2018). Zhang et al. investigated the immobilization performance and mechanisms of modified biochars to the soil heavy metal (Zhang et al., 2019). Soil washing (Xia et al., 2019, Cao et al., 2017), phytoremediation (Xing et al., 2013, Gong et al., 2018) and electrokinetic treatment (Xu et al., 2019, Rosestolato et al., 2015), etc have been applied for remediation of Cd(II) contaminated soil by removing Cd(II) from the soil (Cui et al., 2020, Cui et al., 2019, Delil and Koleli, 2019). Arwidsson et al. (2010) evaluated the effects of two biodegradable chelating agents [S, S]-ethylenediaminedisuccinic acid (EDDS) and methylglycinediacetic acid (MGDA) on the removal of Cu, Pb, and Zn from soil. Li et al. (2020b) investigated the phytoremediation effect of Cd(II) and PYR (pyrene) contaminated agricultural soil with willows (Li et al., 2020b). The activity and bioavailability of Cd(II) in soil are increasingly mentioned to assess the remediation effect instead of just highlighting the total Cd(II) content (Zhang et al., 2010, Cui et al., 2017, Bolan et al., 2014).
One preferable approach for the removal of Cd2+ from water is adsorption. It has been commonly praised due to its advantages of economy, efficiency, and easy operation (Pourbeyram, 2016, Boix et al., 2020, Weidman et al., 2017). However, when used in the Cd(II) contaminated soil remediation, adsorption usually faces some challenges: (1) It is difficult for Cd(II) in contaminated soil to release into water rapidly and continuously. The acid-extractable fraction Cd(II) in soil could be released to water, but the release rate and the equilibrium concentration are low (Khaokaew et al., 2011, Tsang and Lo, 2006, Ernstberger et al., 2005, Andre, 2008). Although adding some chemicals, such as EDTA (Wang et al., 2017), citric acid (Gu and Yeung, 2011), CaCl2 (Makino et al., 2006), etc to the soil can effectively increase the release of Cd(II). These chemicals remaining in soil have potential environmental risk (Schneider, 2008, Xu and Zhao, 2005); (2) Adsorption is difficult to apply to the remediation of Cd(II) contaminated soil because the porous adsorption materials (such as activated carbon, zeolite) can be blocked by soil particles in the soil-water system (Oen et al., 2012, Tamura et al., 2019, Sormo et al., 2021). And these adsorption materials are also difficult to separate from the soil-water system (Zhou et al., 2018); (3) Coexisting interfering metal ions, such as K, Na, Ca, Mg, etc are much higher than that of Cd(II) content in the soil-water system. Therefore, the high selectivity of the adsorption materials to Cd(II) in the soil-water system is essential.
This paper focuses on these challenges by using the PP-g-AA fibers-balls as adsorption materials, which are composed of a large number of carboxylated polypropylene fibers radially. PP-g-AA fibers-ball could adsorb trace Cd2+ from water rapidly and selectively, thereby promoting the continuous and rapid release of the acid-extractable fraction Cd(II) from soil to water. This established a migratory pathway for the acid-extractable fraction Cd(II) to be released from soil to water and simultaneously adsorbed on the surface of PP-g-AA fibers-ball. When used in the soil-water system, PP-g-AA fibers-ball cannot be blocked due to its radial structure. The densities are kept at about 0.95–0.98 g/cm3, thus PP-g-AA fibers-balls are easily separated from the soil-water system. The objectives of this work were to: (1) Determine the adsorption selectivity of PP-g-AA fibers-ball; (2) Study the processes of Cd(II) desorption and adsorption in the soil-water system during the remediation process; (3) Investigate the content change of the acid-extractable fraction Cd(II) before and after remediation.
Section snippets
Chemical reagents
Concentrated sulfuric acid (H2SO4, AR), acrylic acid (AA, AR), hydrofluoric acid (HF, AR), concentrated nitric acid (HNO3, GR) and perchloric acid (HClO4, GR) were purchased from Tianjin Guangfu Fine Chemical Research Institute (China). Benzophenone (C13H10O, AR) was bought from Shanghai Macklin Biochemical Co. Ltd (China). Ammonium ferrous sulfate ((NH4)2Fe(SO4)2.6H2O, AR) was bought from Comiou Chemical Reagent Co. Ltd (Tianjin, China). Anhydrous ethanol (C2H5OH, AR) was bought from Tianjin
Soil properties
The properties of studied soils were summarized in Table S1. Soil S2 (pH = 6.85) was a typical acidic soil. The contents of acid-extractable fraction Cd(II), total Cd(II) in soil S2 were 10.17, 18.14 mg/kg, respectively. This contributed that the soil S2 was Cd(II) contaminated soil. Compared with soil S1 and soil S3, the content of K, Ca, Na, Mg, Fe in soil S2 was lower which is due to the scouring of the Xiangjiang river water. The sampling point of soil S2 is located by the Xiangjiang river
Conclusions
PP-g-AA fibers-ball with high selectivity was successfully prepared by irradiation grafting methods. After PP-g-AA fibers-balls were added to the contaminated soil-water system, a migration pathway of the acid-extractable fraction Cd(II) to be released from soil to water and simultaneously adsorbed on the surface of PP-g-AA fibers-ball was established and a dynamic equilibrium of rapid release- selective adsorption toward the acid-extractable fraction Cd(II) was formed. More than 90% the
Supporting Information
Physicochemical properties of three contaminated soil; Density of PP and PP-g-AA fibers-ball; Surface chemistry composition of PP, PP-g-AA fibers-ball; Kinetic parameters of Cd2+ release from soil in water; Adsorption kinetics parameters of Cd2+ by PP-g-AA fibers-ball; XPS analysis of PP and PP-g-AA fibers-ball; The release of interfering ions; The adsorption of interfering ions in soil desorption solutions.
CRediT authorship contribution statement
Kaijian Zou: Investigation, Methodology, Writing - original draft, Formal analysis, Data curation. Junfu Wei: Supervision, Resources, Writing - review & editing, Funding acquisition. Di Wang: Investigation, Methodology. Zhiyun Kong: Conceptualization. Huan Zhang: Resources, Project administration. Huicai Wang: Methodology.
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.
Acknowledgement
Funding for this project was provided by National Natural Science Foundation of China (51678409), Major Science and Technology Projects of Inner Mongolia Autonomous Region: (zdzx2018029) and Science and Technology Project, Tianjin, China (18ZXJMTG00120).
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