Sorption of HAsO₄^2- anions on modified layered double hydroxides

Arsenate(V) ions occur naturally in the environment as a component of the lithosphere and, due to their relatively easy penetration into groundwater, also in the hydrosphere. However, in recent decades, their content has increased significantly due to intensive human activity, primarily related to the development of the mining and metallurgical industries. Arsenic compounds are characterized by high toxicity and proven carcinogenic properties. Therefore, it is necessary to search for increasingly effective methods for their removal from the natural environment. The most popular method is sorption. The aim of the presented research was to remove HAsO42- anions from aqueous solutions using layered double hydroxides (LDHs) as adsorbents. Their structure consists of positively charged layers of mixed hydroxides of metal cations, in this case Cu²⁺, Mg²⁺, Zn²⁺, and Al³⁺, arranged alternately with charge-compensating interlayers of Cl⁻ or CO₃²⁻ anions. LDH, both before and after the sorption process, was analyzed using the following analytical techniques: (i) thermal analysis using thermogravimetry (TG) and differential scanning calorimetry (DSC) methods (SETSYS16/18 analyzer, Setaram); (ii) Fourier transform infrared spectroscopy (FTIR) (Alpha spectrometer, Bruker Inc., Germany); and (iii) powder X-ray diffraction (XRD) (MiniFlex II diffractometer, Rigaku). The concentration of HAsO₄^2- ions in the solutions was determined by a colorimetric method based on ammonium molybdate using a JASCO V-660 UV-Vis spectrophotometer. The effect of contact time, initial concentration and pH of the solutions on the sorption efficiency of As(V) ions on LDH materials was determined. Layered double hydroxides, particularly in the chloride form, have proven effective in removing arsenic contaminants from aqueous systems. Depending on the LDH form, different mechanisms of As ion sorption were observed: surface adsorption or mixed adsorption, with a significant contribution from ion exchange.

This research is funded by the Lithuanian Research Council under the postdoctoral fellowship project no. S-PD-24-145.