Performance of Halloysite-Mg/Al LDH Materials for Aqueous As(V) and Cr(VI) Removal

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.