Synthesis of Mg–Al layered double hydroxides-functionalized hydrochar composite via an in situ one-pot hydrothermal method for arsenate and phosphate removal: Structural characterization and adsorption performance

Abstract

• Mg–Al LDHs-FHC composite was synthesized via an in situ one-pot hydrothermal route. • The Mg:Al molar ratio and hydrothermal temperature were systematically optimized. • Synthetic conditions play significant roles in arsenate and phosphate removal. • Mechanism involves intercalation, inner-sphere, and outer-sphere surface complexes. • Competitive adsorption behavior mainly occurred at the binding sites on Mg(OH) 2 . In recent years, hydrochar has been recognized as an ideal and eco-friendly adsorbent for the removal of various pollutants, while it has a relatively low adsorption affinity toward negatively charged contaminants, such as arsenate and phosphate. Thus, to address serious worldwide water pollution caused by arsenate and phosphate, the Mg–Al layered double hydroxides-functionalized hydrochar (Mg–Al LDHs-FHC) composite was synthesized via an in situ one-pot hydrothermal method. The Mg:Al molar ratios (2:1–4:1) and hydrothermal temperatures (150–210 °C) played significant roles in influencing the structural properties of the Mg–Al LDHs-FHC composites. Specifically, the Mg–Al LDHs-FHC composite prepared at Mg:Al molar ratio of 2:1 and hydrothermal temperature of 150 °C showed the best arsenate (100%) and phosphate (99.7%) removal due to the highest charge density (4.132 e/nm 2 ) and the largest interlayer d -spacing (0.8371 nm), facilitating strong electrostatic adsorbent–adsorbate interaction and easy intercalation into the interlayer. Single-component isotherm data fit Langmuir and Freundlich models, while binary-component data used extended and ideal adsorbed solution theory combining these models. Overall, the adsorption affinity of the composite followed the order arsenate > phosphate (single-component system) and arsenate < phosphate (binary-component system). Spectroscopic analyses demonstrated that the different physicochemical characteristics of arsenate and phosphate, such as partial charge density, ionic radius, and the coordinate bonding characteristics were responsible for the competitive adsorption behavior. Stable reusability at least five times indicates that the Mg–Al LDHs-FHC composite could be a promising adsorbent for simultaneous arsenate and phosphate removal from water.