A Comparison among Synthetic Layered Double Hydroxides (LDHs) as Effective Adsorbents of Inorganic Arsenic from Contaminated Soil–Water Systems

• Main Authors: Massimo Pigna, Antonio Violante, Antonio Giandonato Caporale
• Format: Article
• Language: English
• Published: MDPI AG 2020-06-01
• Series: Soil Systems
• Subjects: inorganic arsenic contamination; layered double hydroxides; arsenate; arsenite; adsorption; Langmuir isotherm
• Online Access: https://www.mdpi.com/2571-8789/4/2/37

The need for cost-effective adsorbents of inorganic arsenic (As(III) and As(V)) stimulates the academia to synthesize and test novel materials that can be profitably applied at large-scale in most affected areas worldwide. In this study, four different layered double hydroxides (Cu-Al-, Mg-Al-, Mg-Fe- and Zn-Al-LDH), previously synthesized and studied for As(III) removal capacity, were evaluated as potential adsorbents of As(V) from contaminated systems, in absence or presence of common inorganic anions (Cl⁻, F⁻, SO₄²⁻, HCO₃⁻ and H₂PO₄⁻). The As(V) desorption by H₂PO₄⁻ was also assessed. Lastly, the As(V) adsorption capacities of the four layered double hydroxides (LDHs) were compared with those observed with As(III) in a complementary paper. All the LDHs adsorbed higher amounts of As(V) than As(III). Fe-Mg-LDH and Cu-Al-LDH showed higher adsorption capacities in comparison to Mg-Al-LDH and Zn-Al-LDH. The presence of competing anions inhibited the adsorption of two toxic anions according to the sequence: Cl⁻ < F⁻ < SO₄²⁻ < HCO₃⁻ < < H₂PO₄⁻, in particular on Mg-Al-LDH and Zn-Al-LDH. The kinetics of As(V) desorption by H₂PO₄⁻ indicated a higher occurrence of more easily desorbable As(V) on Zn-Al-LDH vs. Cu-Al-LDH. In conclusion, synthetic Cu- and Fe-based LDHs can be good candidates for an efficient removal of inorganic As, however, further studies are necessary to prove their real feasibility and safety.