Titanium Dioxide-Layered Double Hydroxide Composite Material for Adsorption-Photocatalysis of Water Pollutants

Although adsorption has gained favor among numerous water treatment technologies as an effective pollutant removal method, its application is often hindered by challenges with its resource- and energy-intensive regeneration procedure once the available adsorption sites are exhausted. Herein, we present adsorption-photocatalysis composite materials combining layered double hydroxides (LDHs) and titanium dioxide (TiO2) for water treatment. Incorporation of the photocatalyst into the material opens opportunities to harness light from the sun or lamps for oxidative degradation of the adsorbed contaminants on the material surface, to free adsorption sites for material reuse. In addition to allowing photocatalytic regeneration, the addition of TiO2 to colloidal suspensions of delaminated LDH enabled the formation of TiO2 LDH composites with far superior adsorptive performances compared to their parent LDH compounds. During the material synthesis, positively charged LDH layers and negatively charged TiO2 particles combine through electrostatic attraction to yield composites with dramatically enhanced adsorption capacities toward model contaminants, methyl orange and 2,4-dichlorophenoxyacetic acid, by 16.0 and 76.7 times, respectively. Combining delaminated LDH with TiO2 allowed us to maximize the exposure of positively charged surfaces to the contaminants, in a form that can be used as a solid adsorbent. After regeneration, the material regained up to 92% of its adsorption efficiency toward model contaminants. In light of our findings showing significantly different kinetics of adsorption and photocatalytic regeneration, we propose a new scheme to utilize adsorption-photocatalysis systems, in which the two processes are separated to better utilize their unique strengths.