Removal and recovery of heavy metals from soil with sodium alginate coated FeSSi nanocomposites in a leaching process

https://doi.org/10.1016/j.jhazmat.2020.122732Get rights and content

Highlights

  • Gel beads coated silicon sulfuretted nanoscale zero-valent iron were synthesized.

  • Gel beads performed an excellent heavy metals removal capacity in solution.

  • Gel beads possessed an excellent heavy metals recovery capacity in soil leaching.

  • The effect of leaching factors on recovery rates of heavy metals was investigated.

  • The HOAc-extractable heavy metals were dramatically decreased by the gel beads.

Abstract

Leaching technology has been widely applied to remove heavy metals (HMs) from soil, although the synchronous recovery of multiple HMs during the leaching process was rarely studied. In this study, we synthesized silicon sulfuretted nanoscale zero-valent iron (FeSSi), which was coated with sodium alginate (SA) to form the gel beads (SA-FeSSi). The specific surface area of FeSSi (101.61 m2/g) was significantly increased by adding SiO2 seeding. The SA stabilization reduced the aggregation of FeSSi. The removal efficiency for cadmium (Cd), lead (Pb), nickle (Ni) and chromium (Cr) by SA-FeSSi in solution reached 80.10 %, 99.96 %, 66.80 % and 80.46 %, respectively. The removal kinetics was well fitted with the pseudo-second-order model. Leaching experiments showed that the recovery efficiency of HMs from solution (Rr/w) and soil (Rr/s) reached to the ranges of 59.79 %–98.70 % and 25.94 %–62.67 % with the addition of 0.3 g SA-FeSSi. Moreover, the leaching conditions including pH, temperature, adsorbent dosage, leaching agent concentrations, leaching time and leaching cycles were also investigated. Our results suggested that SA-FeSSi had an excellent HMs removal capacity and the recovery of HMs during the leaching process by SA-FeSSi could be a potential pathway to reuse the metal resources from soil.

Introduction

Due to the intensive mining activities in the past years in China, the surrounding soil was severely contaminated by heavy metals (HMs) (Qing-Ren et al., 2003). Excessive HMs in soil circulation not only threatens ecological safety and human health but also is a waste of HMs resource (Fang et al., 2016). So far, effective HMs recovery from soil is still a challenge because the soil is illiquid and HMs are tightly bound with mineral substances.

Leaching technology has been widely applied to remove HMs by transferring HMs from highly contaminated soil to liquid. However, the difficulty in aftertreatment of wastewater limits its application in large scale and the excessive metal irons in leaching agent can inhibit the removal efficiency (Burckhard et al., 1995; DijkstraJohannes et al., 2004). Hence, we consider that the synergetic recovery of HMs using a combination of a novel adsorption material with the traditional leaching method. This approach could enhance the removal efficiency of HMs from soil and minimize the effort of post-processing of wastewater. In this process, the adsorption material helps to remove HMs from the leaching agent, which delays the saturation of the leaching agent. The leaching agent is only responsible for transferring HMs from soil to the adsorbents, but no longer responsible for the holding of HMs. In recent years, nanoscale zero-valent iron (nZVI), an environment-friendly nanoscale material, has raised the attention in the HMs removal due to its large specific surface area, massive surface adsorption sites, and high reductive activity (Zhu et al., 2017a; Dong et al., 2017; Liu et al., 2015). However, pure nZVI tends to easy aggregation and rapid oxidization, which could significantly decrease the removal efficiency for HMs in wastewater (Kim et al., 2013). In addition, the weak chemical interaction between HMs and nZVI is easy to be affected by some anions such as Cl, NO3 and SO42−, which also could influence the removal efficiency of HMs (Su et al., 2014; Crane et al., 2015). Recent reports indicated that the incorporation of sulfur into nZVI, by synthesizing sulfur-modified nZVI (S-nZVI), could improve the chemical stability of HMs-S-nZVI complex, thus enhancing the removal capacity (Rajajayavel and Ghoshal, 2015; Eun-Ju et al., 2014). Meanwhile, the SiO2 seeding could increase the final Fe° proportion through increasing the reduction of crystalline and amorphous iron oxide, which, in turn, also improves the HMs removal (Su et al., 2016). Hence, in comparison with S-nZVI, the novel FeSSi could be potentially more effective for the application of HMs removal.

To improve the separation of HMs adsorbed on FeSSi nanoparticles from soil, we synthesized the gel beads using sodium alginate (SA) to coat FeSSi nanoparticles. Meanwhile, SA has been reported to decrease the aggregation of nanoparticles and improve the removal efficiency for HMs (Wu et al., 2018a; Huang et al., 2016). In this study, we first compared the characteristics and HMs removal capacity of nZVI, S-nZVI, FeSSi and SA-FeSSi. Then, the HMs recovery efficiency by SA-FeSSi in the leaching process was comprehensively investigated under different leaching conditions. The purpose of this paper is to reveal the effects and mechanisms of sulfur modification, SiO2 coupling and SA beading for the enhancement of HM removal and, to establish the optimal structures of the adsorption materials and their best operation conditions.

Section snippets

Chemicals and soil sample

The main chemicals, including sodium borohydride (NaBH4), sodium dithionite (Na2S2O4), ferrous sulfate (FeSO4·7H2O), colloidal silica, and ferric chloride (FeCl3) were purchased from Kelong Chemical Reagent and Sigma-Aldrich companies. The aged soil was highly contaminated with HMs due to previous studies (Wu et al., 2018b; Peng et al., 2019; Hou et al., 2019). The Cd, Pb, Ni and Cr in soil were the main contaminants and their concentrations were 25.80 mg/kg, 620.52 mg/kg, 734.36 mg/kg, and

Characteristics of composites

SEM and TEM images clearly showed the aggregation of nZVI, S-nZVI and FeSSi particles (Fig. 1). However, FeSSi was more dispersive with respect to nZVI and S-nZVI, which illustrated that the using of SiO2 seeding could reduce the aggregation of S-nZVI. Although, due to the van der Waals interactions, some FeSSi still formed large agglomerate (Wu et al., 2017). SA application offered much more dispersed FeSSi particles. BET analysis showed that the specific surface area of FeSSi (101.61 m2/g)

Conclusions

In this study, the gel beads of SA coated FeSSi nanoparticles (SA-FeSSi) were synthesized. The SA coating reduced the aggregation of FeSSi and increased the density of the HMs reaction sites, which significantly enhanced the HMs removal capacity of FeSSi. The leaching study indicated that SA-FeSSi could simultaneously recycle multiple HMs (Pb, Cr, Ni and Cd) from soil and HMs recovery rates were significantly influenced by leaching pH, leaching temperature, loading of adsorbent, concentrations

CRediT authorship contribution statement

Bin Wu: Conceptualization, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization, Supervision. Ziru Wang: Investigation, Data curation, Software. Dinghua Peng: Investigation, Visualization. Ying Wang: Writing - review & editing. Tingting He: Writing - review & editing. Hao Tang: Data curation. Heng Xu: Resources, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

None.

Acknowledgements

This study was financially supported by the National Key Research and Development Program (2018YFC1802605), the Science and Technology Project of Sichuan Province (2019JDRC0092) and the Key Research and Development Program of Sichuan Province (2017SZ0188). The authors also wish to thank Professor Guanglei Cheng and Hui Wang from Sichuan University for the technical assistance.

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