Abstract
This research focused on the investigation of layered double hydroxide (LDH)/halloysite materials’ adsorption efficiency and mechanisms in reactions with aqueous As(V) and Cr(VI) in a broad pH range. The materials consisting of Mg/Al LDH and halloysite were synthesized using both direct precipitation and physical mixing methods. The XRD, FTIR, DTA, SEM and XPS methods were used to evaluate the quality of the obtained materials and get insight into removal mechanisms. The XRD, FTIR and DTA confirmed LDH formation and showed the dominating presence of intercalated carbonates in the LDH structure. The SEM of the materials revealed characteristic agglomerates of layered LDH particles deposited on halloysite tubular forms. The raw LDH phases showed high removal efficiency of both As(V) and Cr (VI) for initial pH in the range of 3–7. In the studied concentration range the materials containing 25 wt % of LDH exhibited a removal efficiency very similar to the raw LDH. In particular, the halloysite presence in the materials’ mass had a positive effect in the reactions with As(V), which was removed by chemisorption. At a low pH the LDH component underwent partial dissolution, which lowered the adsorption efficiency. Apart from the anion exchange mechanism at a low pH the Cr(VI) was removed via formation of MgCrO4 with Mg (II) being released from the LDH structure. The XPS spectra for As(V) did not show changes in oxidation state in the reactions. In turn, a partial reduction of Cr(VI) to Cr(III) was observed, especially at a high pH. The use of materials composed of two different minerals is promising due to reduction of costs as well as prevention of adsorbent swelling. This opens the possibility of its use in dynamic adsorption flow through systems.
Highlights
Along with the intense and rapid development of industry, one of the major concerns is the generation of large volumes of wastewater that would affect the surrounding environments and subsequently human health
In our recent work we have summarized findings on Mg-Fe layered double hydroxide (LDH) derived from chemicals or magnesite and its composites with halloysite [38]
The obtained materials were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and differential thermal analysis (DTA)
Summary
Along with the intense and rapid development of industry, one of the major concerns is the generation of large volumes of wastewater that would affect the surrounding environments and subsequently human health. Due to poor dispersity and low chemical stability in acidic conditions it is essential to develop LDH-based hybrids [36,37,38] Such an approach can reduce production costs of the adsorbent where the active LDH component is in sufficient amount for the efficient removal of the pollutant, and at the same time it is stabilized by the support material. The application of the halloysite support for the LDH should result in increased physical and chemical stability as well as a reduction of undesirable swelling properties. The latter poses a limitation of using pure LDH phases in dynamic adsorption conditions [37]. The removal mechanisms were investigated using solid-state analytical techniques, in particular including X-ray photoelectron spectroscopy for oxidation state determination of As and Cr
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