Sorption of humic acid to organo layered double hydroxides in aqueous solution

Compared with inorganic LDHs, the surfaces of organo layered double hydroxides (LDHs) are less hydrophilic and more organophilic, thus their ability to remove organic contaminants from water can be significantly enhanced. Organo-LDHs with different Mg/Al molar ratios (2:1, 3:1 and 4:1) containing dodecylsulfonate (DSO) as the interlamellar anions were prepared through coprecipitation. Their ability to remove humic acid (HA) from an aqueous medium was studied as a function of pH, sorbent dosage, initial HA concentration, contact time and ionic strength. Under the experimental conditions, the maximum HA removal capacities for 2:1, 3:1 and 4:1 type LDH-DSO were 594.3, 646.7, 428.4 mg/g, respectively, while a carbonate-based LDH only removed 23.77 mg/g. Isotherm studies showed that the sorption of HA to organo-LDH fits well with a “dual-mode model” equation as well as the Freundlich isotherm equation. The sorption was a combination of both partition and specific adsorption. For 3:1 LDH-DSO, the HA sorption process is well described by a pseudo-second order model. LDH-DSO shows a buffering capacity in the pH range from 5.0 to 9.0. The organo-LDHs could be powerful sorbents for removing both high and low concentrations of HA from wastewater.

► LDH-DSO showed great adsorption capacity for humic acid.
► The sorption was a combination of both partitioning and specific adsorption.
► The sorption fits well with a dual-mode model and the Freundlich equation.
► LDH-DSO has buffering capacity over pH 5–9.