Facile synthesis of layered double hydroxide/superabsorbent polymer composites for water and wastewater treatment

Abstract

Layered double hydroxides (LDH) possess high anion adsorptivity and are attractive for water and wastewater treatment, but are available mainly in a powder form that has low hydraulic conductivity and difficulties in solid-liquid separation and handling. Incorporation of LDH into a polymer gel produces composite granules that are suitable for use in water and wastewater treatment. This study demonstrates a simple and new method for synthesizing an LDH-containing hydrogel that exhibits high anion adsorption, using SO42− as a model anion. Chloride-containing LDH particles were obtained by coprecipitation, and were subsequently either treated hydrothermally or left untreated. The superabsorbent polymer poly(acrylic acid) was mixed with aqueous suspensions of LDH particles and was swollen in the suspensions to form LDH-containing hydrogels, followed by rinsing with water and vacuum drying. LDH particles appeared to enter gels via macroscopic features—gaps like cracks, slits, and fractures—that may have widened during gel swelling, and larger, hydrothermally-treated LDH particles got stuck in the gaps within gels to a greater extent than smaller, non-hydrothermally treated particles. When the superabsorbent polymer was added to a 5 mM Na2SO4 solution in the absence of LDH particles, the SO42− concentration outside the resulting gel increased by 18%, likely due to electrostatic repulsion between SO42− ions and carboxylate groups in the polymer. Interference of anionic polymer carboxylates groups with SO42− adsorption was assessed. Despite this negative effect of SO42− adsorption by the polymer, 86% of LDH in gel with hydrothermally-treated LDH particles actively adsorbed SO42−, indicating that the easily-synthesized LDH-containing gel is highly effective anion adsorbents.