Synthesis, Characterization and Hexavalent Chromium Adsorption Characteristics of Aluminum- and Sucrose-Incorporated Tobermorite

Abstract

Tobermorites were synthesized from the lime-quartz slurries with incorporations of aluminum and sucrose under hydrothermal conditions, and then used for adsorption of Cr(VI). The chemical components, and structural and morphological properties of tobermorite were characterized by X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetry (TG-DSC), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopic (XPS) and N2 adsorption–desorption measurements. The formation and crystallinity of tobermorite could be largely enhanced by adding 2.3 wt.% aluminum hydroxide or 13.3 wt.% sucrose. Sucrose also played a significantly positive role in increasing the surface area. The adsorption performances for Cr(VI) were tested using a batch method taking into account the effects of pH, the adsorption kinetics, and the adsorption isotherms. The adsorption capacities of the aluminum- and sucrose-incorporated tobermorites reached up to 31.65 mg/g and 28.92 mg/g, respectively. Thus, the synthesized tobermorites showed good adsorption properties for removal of Cr(VI), making this material a promising candidate for efficient bulk wastewater treatment.

Removal of heavy metals (Co, Cr, and Zn) during calcium–aluminium–silicate–hydrate and trioctahedral smectite formation

“…C-A-S-H precipitated at moderately high Me ion concentrations (* 18.9 mM) exhibited much higher q e values in the range from 27.5 to 40.4 mg/g. This is in the narrow range of the maximum adsorption capacities of Cr 6þ by Al-and sucrose-incorporated tobermorite (* 32 mg/g and * 29 mg/g, respectively), according to Zhao et al [29]. However, chromitite formed as a reaction by-product in their work, which suggests that the reported adsorption capacities for Cr 6þ by these tobermorite materials may be slightly lower.…”

“…Several studies have been carried out in order to describe and to quantify ion exchange and adsorption properties of natural and synthetic C-A-S-H [13,14,42] and clay minerals [33,43]. Especially, smectite minerals have been shown to be good adsorbent materials for the removal of Me ions from aqueous solutions, owning to their worldwide Figure 1 Schematic structure of C-A-S-H (modified from [2] with potential immobilization effects indicated in red (adapted from [29]). occurrence, low mining costs, and unique physicochemical and surface (charge) properties, as indicated before.…”

“…For C-A-S-H phases basically the same uptake mechanisms of Me ions have been considered, but experimental studies are still scarce and/or lack control of the nature of the precipitates. Zhao et al [29] have recently shown that synthetic aluminium-and sucrose-substituted tobermorite (Ca 5 [44] have presented removal efficiencies for Cd 2þ , Zn 2þ , Pb 2þ , and Cr 3þ ions of 99.96% or higher after reaction with freshly prepared C-S-H, and Giergiczny and Król [45] have reported very high immobilization degrees for Pb 2þ , Cu 2þ , Zn 2þ , Cr 6þ , Cd 2þ , and Mn 2þ ions, from * 86 to 99.99%, after reaction with mortars made of OPC and different levels of GGBFS and fly ash. The principle mechanism proposed for the removal of these Me ions by C-A-S-H or C-S-H was structural incorporation, though co-precipitation of discrete Me-oxyhydrates was frequently observed [29,44,45], suggesting that the removal mechanism(s) are more complex than previously thought.…”

“…Thus, with increasing substitution of Al for Si in C-A-S-H, a negative charge is generated [27,28], which has to be compensated through a coupled substitution of type: h þ OH À ! Ca 2þ þ O 2À , where h represents an ''empty'' position or a defect site in the C-A-S-H structure [29]. This defect site is occupied mainly by Na þ , K þ and Ca 2þ ions in ''traditional'' C-A-S-H, or by other metal ions, such as Pb 2þ , Cd 2þ , Co 2þ , Ni 2þ , and Zn 2þ (hereafter called ''Me'' ions) in synthetic C-A-S-H gels, which serve as charge compensators.…”

Hexavalent Chromium Adsorption Removal From Aqueous Solution by Fe-Modified Biochar Derived From Rice Straw

“…Thus, the Cr(VI) adsorption removal efficiency of Fe-R-BC increased as pH decreased. Furthermore, the surface charge of adsorbent significantly influences the Cr(VI) adsorption removal efficiency (Zhao et al, 2017). For Fe-R-BC, Figure 5 also displays the change of zeta potential with the variation of pH.…”