Elsevier

Chemosphere

Volume 263, January 2021, 128319
Chemosphere

Green rust functionalized geopolymer of composite cementitious materials and its application on treating chromate in a holistic system

https://doi.org/10.1016/j.chemosphere.2020.128319Get rights and content

Highlights

  • Green rust loaded composite cementitious material has been synthesized in specificity.

  • Chromate reduction and immobilization were achieved in a holistic cementation system.

  • Cr(VI) immobilization in the GR-CCM was comprehensively studied in a wide pH range.

  • Crosslinking mechanisms between geopolymer and green rust have been analyzed.

  • This study expands the combination of adsorption and geopolymerization by green rust.

Abstract

Green rust functionalized geopolymer of composite cementitious materials (GR-CCM) was synthesized to improve the adsorption and subsequent stabilization/solidification of chromate in a holistic operating system. The initial pH in solution exhibited the most significant effect on the chromate removal by GR-CCM among three adsorption factors. The maximum monolayer adsorption capacity and theoretical saturation capacity of GR-CCM for Cr(VI) in the acidic condition were 55.01 mg/g and 41.70 mg/g, respectively. Amorphousness brought by loading GR weakened the crystallinity of composite cementitious materials (CCM), which enhanced the adsorption capacity of CCM and boosted the solidification process. The mixed-valent iron species in the GR-CCM not only directly engaged in the adsorption and reduction of chromate also positively strengthened the solidification of Cr species during the whole treatment. This study facilitates the application of GRs on the geopolymer materials and demonstrates the combination of adsorption and immobilization for the treatment of other potential heavy metal contamination.

Introduction

In recent years, the effluent containing excessive Cr species coming from different industrial processes has been unconscionably discharged into the surrounding water, potentially posing ecological harm to the aquatic environment(Aranda-Garcia and Cristiani-Urbina, 2019). Furthermore, the inappropriate handling of the Cr-contaminated wastewater not only endangers human health but also seriously interferes with ecological balance(Nogueira et al., 2019). Cr(III) and Cr(VI) are two of the most stable oxidation states for the Cr existence in environments. Cr(III) has less toxicity due to its lower oxidation potential compared with Cr(VI) and can be easily precipitated in the form of Cr(III) hydroxides and coprecipitations(Huang et al., 2018a). Contrastingly, Cr(VI) known as a potent carcinogen substance is acutely toxic to living organisms and has higher mobility in both rivers and soils. The maximum concentration of Cr(VI) is limited below 0.05 mg/L in the standards for drinking water quality in China (GB 5749–2006)(Sharma et al., 2019).

Some technologies have been particularly developed for achieving the de-toxicity and sequestration of Cr(VI) in wastewater treatment(Banerjee et al., 2019; Hosseinzadeh and Asl, 2019; Islam et al., 2019; Rha et al., 2019; Sethy and Sahoo, 2019). Chemical reduction is commonly essential in the combined process for the efficient removal of Cr(VI) from the wastewater(Sriram et al., 2018; Wang et al., 2019b). Iron-based materials such as zero-valent iron (ZVI), nanoscale zero-valent iron (NZVI), goethite, ferrihydrite, schwertmannite, magnetite, iron sulfides, etc. have been widely used to bring about the chemical reduction as well as simultaneous adsorption for the Cr(VI) contaminants in the water due to their reduction availability, eco-friendliness, and abundance(Shih et al., 2015; Vilardi et al., 2017; Zhao et al., 2019). In recent decades, the application of green rust (GR) among the iron materials on the environmental remediation and engineering for some inorganic contaminants has attracted a significant amount of attention(Freeman et al., 2019; Koeksoy et al., 2019; Perez et al., 2019). GR is one kind of mixed-valent iron oxides, having intense bluish-green colors and brucite-like layered double-hydroxide structures. Leila Alidokht, etc., showed the chromate removal enhanced by graphene-based sulfate and chloride green rust (i.e., Fe(II)4Fe(III)2(OH)12SO4·8H2O GR(SO42) and Fe(II)3Fe(III)(OH)8Cl·∼1.5H2O GR(Cl)) nanocomposites (Alidokht et al., 2016). Graphene-based GR can intensively change the redox transformation and mobility of chromate by reduction of Cr(VI) to Cr(III). From the perspective of mechanism exploration, Abdullah Al Mamun, etc., pointed that the chromate removal achieved by GR was different in pH of 7.5 and 8.75, mainly regarding surface reduction, replacement of intercalated sulfate anions, and bidentate FeCr2O4-like Cr–Fe bonding (Mamun et al., 2018).

Despite GR equipping with some advantages, its applications on the environmental remediation of chromate are still limited(Usman et al., 2018). Heterogeneous reduction reactions are induced based on the excellent binding of substrate ions to oxide surfaces, which also determines the adsorption of target pollutants to some extent. Theoretically, GR can sorb both anionic and cationic metals from the wastewater due to the existence of amphoteric hydroxyl groups on surfaces (Bhave and Shejwalkar, 2018; Mamun et al., 2018). However, the adsorption capability of GR aiming at specific inorganic contaminants is rarely investigated to date, especially in regardless of the redox reaction due to its poor stability and high activity. The increase in aggregation and the compression in the diffused double layer further dwarf the employment of GR for the chromate contamination with high ionic strength (Rogers et al., 2014; Alidokht et al., 2016). Moreover, the reduction of Cr(VI) to Cr(III) caused by aqueous and solid-bound Fe(II) rapidly reacts in water. The poor immobilization of Cr(III) realized by the anion exchange, surface complexation, and (co)precipitation will not only reverse the removal performance of GR materials for the total Cr and also increase the risk of secondary pollution over time once the released Cr(III) is re-oxidized. The stability and adsorption capacities of GR towards chromate and the immobilization of Cr(III) after adsorption experiments both must be strengthened(Refait et al., 2017).

In this study, the mixtures of granulated blast furnace slag (GBFS) and fly ash (FA) are used as cementitious materials for the loading of GRs and the preparation of geopolymers to achieve the above-mentioned strengthening goals. GR was loaded onto the composite cementitious materials (CCM) in a strongly alkaline environment to operate the removal, reduction, and immobilization in a holistic operating system or process. BFS and FA both have been widely used in the stabilization/solidification (S/S) technology(Huang et al., 2019b, 2020). The abundant aluminosilicate minerals in the CCM supply the potential loading points for the stability of GR and adsorption sites for chromate. Moreover, the formation of alkali-activated FA and GFBS binders also provides a possible way for the effective immobilization of Cr. Therefore, to quantitatively test the adsorption performance and stability of GR loaded CCM (GR-CCM), the equilibrium absorption experiments including the effects of pH and the competing cations, the adsorption kinetics and isotherms, and the parametrical optimization were conducted and correspondingly analyzed in this work. The immobilization of Cr(VI) in the GR-CCM was comprehensively studied with the changes in two indicators including compressive strengths and toxicity leaching in a wide pH range. The removal mechanisms of chromate and the subsequent immobilization pathways were investigated and discussed by different characterization strategies. This study will directly facilitate the application of GR materials on the environmental contaminations of heavy metals (HMs) and expand the combination process of adsorption and immobilization.

Section snippets

Synthesis of green rust loaded composite cementitious materials (GR-CCM)

The chemicals and materials used for the synthesis of GR-CCM were stored in a vacuum chamber (FZG-20, NANJING SHENWEI, China) and have been specified in the supporting information (SI). All containers have been rinsed by deionized (DI) water, air-dried, and prepared for the synthesis, adsorption, and immobilization experiments. The chemical compositions of GBFS and FA are shown in Table S1in the SI. GR-CCM was synthesized according to a coprecipitation method in an anaerobic environment(Usman

Effect of synthesis parameters on the adsorption of GR-CCM

The influence of synthesis parameters (Table S2) on the adsorption capacities (qe, mg/g) of GR-CCM towards Cr(VI) and total Cr is shown in Fig. 1. For Cr(VI), qe gradually increased with the increase in the molar ratios of Fe(II) to Fe(III), regarding GR-CCM-1 to GR-CCM-3. Fe(II) concentration of 0.75 M was most conducive to the maximum qe of GR-CCM (i.e., GR-CCM-4) in the concentration range. The heterogeneous chemical reduction of Cr(VI) to Cr(III) was not only controlled by the absolute

Conclusion

GR-CCM-11 exhibited higher adsorption capacities towards the total Cr at the pH of 9. pH influence verified that anion exchange and surface complexation reaction on external sites may have also anticipated the adsorption processes of GR-CCM towards Cr species at different aqueous environments. The inhibition effect brought by the cations on the adsorption capacities of GR-CCM was followed as Na+ > Ca2+ > Al3+, Na+ > Al3+ > Ca2+, and Na+ > Ca2+ > Al3+ at the pH of 3, 7, and 11, respectively. Na+

Credit author statement

Tao Huang, Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Shu-wen Zhang, Validation, Resources, Supervision. Long-fei Liu, Resources, Project administration. Lulu Zhou, Data curation, Writing - review & editing.

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.

Acknowledgment

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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