Composite of Organo-LDH and biochar for diclofenac sodium removal from aqueous solutions

Abstract

Classic layered double hydroxides (LDHs) and their composites with biochar are promising adsorbents for removing emerging contaminants like diclofenac sodium (DFS) from water. However, the use of organo-LDHs in these composites remains unexplored and could enhance their adsorption properties for organic molecules. This study investigates the synthesis, characterization, and application of an organo-LDH based on sodium dodecyl sulfate as intercalating agent (SDS-LDH) and its composite with tangerine peel biochar (SDS-LDH/BC) for the removal of DFS from water. The adsorption performance of these materials was compared with those of a composite of SDS-LDH and tangerine peel biomass (SDS-LDH/Bio) and the layered double oxides (LDOs) obtained from the pyrolysis of SDS-LDH and SDS-LDH/Bio, referred to as SDS-LDO and SDS-LDO/Bio, respectively. The materials were characterized by FTIR, XRD, SEM, TG, and pH point of zero charge analyses, and the adsorption of DFS was evaluated in aqueous medium under different conditions. XRD and FTIR demonstrated the successful incorporation of sodium dodecyl sulfate into the interlamellar space of the SDS-LDH, which was preserved in the SDS-LDH/BC and SDS-LDH/Bio composites. SEM analysis revealed that the SDS-LDH structures, with varied shapes and sizes, were dispersed on the matrix of the biochar, while TG analysis showed that the SDS-LDH/BC had similar thermal stability to SDS-LDH. SDS-LDH achieved 90 % DFS removal at pH 8.0 and 25 °C, while SDS-LDH/BC reached 81 % removal under the same conditions, with maximum DFS adsorption capacities of 247 and 169 mg g⁻¹, respectively, outperforming pristine biochar (no removal). Under the same conditions, SDS-LDH/Bio, SDS-LDO/Bio, and SDS-LDO removed 55 %, 36 %, and 36 % of DFS, respectively. These results revealed the pivotal role of the interlamellar structure of SDS-LDH in DFS adsorption, as confirmed by XRD and FTIR, which indicated that DFS interacted with dodecyl sulfate anion in the SDS-LDH interlamellar space, specially through hydrophobic interaction. Kinetics studies revealed that DFS adsorption on SDS-LDH/BC followed the pseudo-first-order model, while adsorption on SDS-LDH followed the pseudo-second-order model, with equilibrium reached in approximately 30 min. The Redlich-Peterson model satisfactorily fitted the adsorption equilibrium isotherms for both SDS-LDH and SDS-LDH/BC. Although no synergistic effect was observed between biochar and SDS-LDH/BC regarding the adsorption capacity, the formation of the SDS-LDH/BC composite improved the preservation of the SDS-LDH structures, allowing for a lower loss of adsorption capacity upon reuse. These findings suggest that the study of such advanced material can contribute to the development of more effective biochar-LDH composite-based adsorbents, specifically for the removal of organic pollutants.