Investigation on Effects of Aging on the Formation and Physicochemical Properties of Hydrothermally Synthesized Magnesium Aluminum–Layered Double Hydroxide/Rice Husk Hydrochar Nanocomposites for Effective Remediation of Arsenic-Contaminated Soil

This study presents the synthesis and characterization of MgAl-layered double hydroxide (LDH)/rice husk hydrochar (RHH) nanocomposites (MgAl-LDH/RHHs) via an in situ one-pot hydrothermal route at 150 °C, utilizing Mg:Al molar ratio of 2:1 for arsenic remediation. The formation of MgAl-LDH/RHHs and their physicochemical properties were evaluated under varying hydrothermal aging times systematically. Prolonging the aging period to 12 hrs significantly enhanced the crystallinity and crystal size of the LDHs, resulting in a 3D hierarchical structure with the highest specific surface area (27.98 m^2/g) formed on the hydrochar surface. The hexagonal crystal structure (d₀₀₃ = 0.8246 nm) was characterized by a rhombohedral unit cell with lattice parameters a = 0.3049 nm and c = 2.4738 nm, and a high positive charge density of 4.284 e/nm^2. These properties were found to be favorable for the sorption of arsenic oxyanions. Batch adsorption experiments were conducted to assess the potential of MgAl-LDH/RHHs-12h for the remediation of arsenic-contaminated soils. The original soil sample (CY) was mechanically sieved into fine-grained (CYF, < 75 μm) and coarse-grained (CYC, 75 μm–2 mm) fractions. When these soil samples were reacted with deionized water, arsenate was identified as the dissolved arsenic species, with concentrations of 2.85 mg/L for CY, 4.02 mg/L for CYF, and 2.55 mg/L for CYC, respectively. Kinetic sorption experiments, conducted at pH 5.0 and 8.0 in the presence and absence of 0.1 M NaCl as a background electrolyte, revealed that arsenic sorption onto MgAl-LDH/RHHs-12h was inhibited at pH 8 in the presence of NaCl. These findings suggest that effective arsenic sorption requires low pH conditions with minimal background electrolytes in soils.