Development of new materials for the adsorption of arsenic (V)

CARINA LUENGO; NICOLÁS LOPEZ; MARCELO AVENA

Wuhan

Conferencia; The 11th International Conference Interfaces Against Pollution (IAP2021); 2021

Huazhong Agricultural University

The Chaco-Pampean plain is a large area in Argentina, whose groundwater has high As concentrations exceeding the WHO guideline values for drinking water.1 This is an important problem especially in locations where people depend on groundwater for drinking. The development of new materials for environmental remediation is a topic of high priority due to the increasing contamination of water. Arsenic is highly carcinogenic after long-term or high-dose exposure thus remediation techniques are continuously investigated to remove it from aqueous media. Although several methods are used to removing As from water, the adsorption method is considered to be one of the most promising technologies due to its low cost, high uptake capacity, great selectivity, and easy equipment handling. Layer Double Hydroxides (LDHs) are promising materials because they have a very high sorption capacity. LDH, also known as hydrotalcite-like-compounds or anionic clays, have been widely used as adsorbents. The general formula of LDHs is [M2+1-x M3+x(OH)2]x+[An-]x/n.yH2O, where M2+ and M3+ are divalent and trivalent metal cations, respectively, and An− is an anion incorporated in the interlayer space along with water molecules for charge neutrality. The structure of LDH presents partial isomorphic substitution of Mg2+ by trivalent cations. This replacement leads to an excess of positive charge and to a high anion exchange capacity. They are promising materials for the removal of ecologically undesirable anions.The main objective of this work was to evaluate the ability to adsorb arsenic (V) (arsenate and its protonated species) of an iron-rich LDL (Fe-LDH). This solid was selected because it is not harmful for the environment. Fe-LDH was prepared by the coprecipitation method at constant pH. The solid was characterized by XRD, FTIR, SEM, TEM, DSC, electrophoretic mobility, stability and elemental analysis. To investigate the adsorption kinetics of arsenate, the batch technique for adsorption was used.2 The effects of various experimental conditions such as contact time, pH, temperature, stirring rate and arsenate concentration were investigated. The characterization of the sample revealed that Fe-HDL had a high crystallinity and a laminar structure, which was formed by particles of size 30-50nm. Fe-HDL had positive zeta potentials throughout the studied pH range (3-10). Dissolution kinetic studies performed in the pH 3-8 range revealed that the sample is relatively stable at pH greater than 5. It was observed that the arsenate adsorption as a function of time consists of two well-differentiated stages: a rapid one that is completed before 5 minutes of reaction, and a slow one that reaches an apparent equilibrium of the system between 4 and 8 hours of reaction. The amount of arsenate adsorbed over time increased as the initial concentration of the anion increased. The increase in pH produced a decrease in the amount of arsenate adsorbed at any reaction time, probably as a consequence of the decrease in the positive charge of LDH, which translates into a lower electrostatic attraction between the surface and the anions. The increase in temperature and the variation of the stirring rate did not produce significant changes in the adsorption of arsenate. It was also observed that the adsorption of arsenate on Fe-LDH caused charge reversal, as detected by zeta potential masurements, probably due to the formation of inner-sphere complexes between the anions and the surface of the solid. The results of this study show that Fe-HDL is a very effective arsenate adsorbent, and could be used in environmental remediation. Translated into percentages of adsorption, as an example, for a system containing 1x10-4 M of As and 2 g/L of Fe-LDH content, the studied LDH could adsorb more than 93% of the arsenic.