Mercaptocarboxylic acid intercalated MgAl layered double hydroxide adsorbents for removal of heavy metal ions and recycling of spent adsorbents for photocatalytic degradation of organic dyes

Adsorption is considered an effective way for the removal of heavy metal ions in aqueous solutions; however, recycling of spent adsorbents is a formidable challenge. We synthesized mercaptocarboxylic acid/layered double hydroxide (MPA/LDH) adsorbents for the removal of heavy metal ions from wastewater, and it demonstrated excellent adsorption capacities; the maximum adsorption capacities for Hg2+, Pb2+, and Cu2+ were 250.30, 122.10, and 105.33 mg/g at adsorbent dosage of 0.2 g/L and pH of 5.5. The excellent adsorption performance was attributed to the inner-sphere surface complexation between the heavy metal ions and the S-S and COO- groups in MPA/LDH. Moreover, host-guest interactions between MPA and the LDH laminate may have enhanced the adsorption efficiency and selectivity of MPA/LDH. After the adsorption of Hg2+, Pb2+, and Cu2+, highly dispersed metal sulfide NPs were spontaneously formed in the LDH interlayer via confined base catalysis, providing excellent photocatalytic activity for the degradation of organic dyes. High photocatalytic activity was attributed to the suitable energy band and high dispersity of metal sulfides. MPA/LDH-absorbed Hg2+ showed the best photodegradation performance of 99.2% for methylene blue. Notably, MPA/LDH can be used for the simultaneous removal of heavy metals and organic dyes under visible light irradiation. The findings of this study provide insights for developing an environmentally friendly method for conversion from adsorbent to photo catalyst, demonstrating wide prospects in industrial applications.