Abstract
Access to safe drinking water is a basic human need and a necessity for excellent health. Unfortunately, over the past years, the quality of water has decreased as a result of the presence of various pollutants, including trace metal(loid)s (such as Cd, Pb, As, Tl and Hg, among others) that get introduced into water bodies such as rivers, lakes, seas, and groundwater. Therefore, there is a need for innovative and cost-effective methods for decontamination of water. Most researchers have previously used several materials to remove these contaminants from water. Hydrotalcites-like materials, also known as layer double hydroxides (LDHs) and Layer double oxides (LDO), have gained lots of attention as adsorbents for the removal of trace metal(loid)s from water. This is due to their special properties, such as low toxicity, ion exchange capacity, and reusable, highly selective, and tuneable composition. The main aim of this study was to synthesise Fe-Zn-Al LDH and Fe-Zn-Al LDO using the co-precipitation method as potential sorbents for the adsorption of As, Cd and Pb from surface water and groundwater samples. The Fe-Zn-Al LDH and Fe-Zn-Al LDO materials were characterised by N2 adsorption-desorption isotherms, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS) and zeta potential analyser. These analytical instruments confirmed the successful synthesis of the materials.
The TEM images of Fe-Zn-Al LDH showed that the particles were platelet plate-like structures. After calcination, Fe-Zn-Al LDO showed that the particles were spherically shaped, suggesting the destruction of the lamellar structure of parent Fe-Zn-Al LDH. The SEM-EDS confirmed the elemental composition of the materials. The FTIR was utilised to verify the successful formation of Fe-Zn-Al LDO from Fe-Zn-Al LDH using the functional groups. From BET, it was observed that both materials had mesopores and micropores characteristics with specific surface areas of 171 m2/g and 88.1 m²/g for Fe-Zn-Al LDH and Fe-Zn-Al LDO, respectively, with pore size distribution ranging 0.35-18.6 nm for both materials. The removal of As, Cd and Pb was investigated using a batch adoption experiment. The percentage removal (%) was enhanced by optimising the pH of the solution and mass of the adsorbent (MA).
Under optimum conditions, the Fe-Zn-Al LDH and Fe-Zn-Al LDO adsorbents demonstrated enhanced adsorption capabilities towards target analytes owing to their high
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surface functionality and adsorption active sites. The Elovich and Freundlich models described the adsorption of Cd and Pb onto Fe-Zn-Al LDH, suggesting that the adsorption process was dominated by chemisorption on a heterogeneous surface and was favourable. The Langmuir isotherm showed that the maximum adsorption capacities of Fe-Zn-Al LDH for Cd and Pb were 11.9 mg/g and 280 mg/g respectively. The kinetics and isotherm data for the adsorption of As, Cd and Pb on the surface of Fe-Zn-Al LDO adsorbent was explained by pseudo-second-order and Freundlich model. The Langmuir model results showed that Fe-Zn-Al LDO exhibited better adsorption capacity for Pb (256 mg/g), than As (233 mg/g) and Cd (204 mg/g). The plausible adsorption mechanisms of the analytes onto Fe-Zn-Al LDH and Fe-Zn-Al LDO adsorbents involved surface complexation, ion sharing, ion exchange, precipitation reaction and electrostatic interactions. The application of Fe-Zn-Al LDH and Fe-Zn-Al LDO adsorbents in surface water and groundwater demonstrated the potential of these adsorbents for practical application.