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 Hg 2+ , Pb 2+ , and Cu 2+ 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 Hg 2+ , Pb 2+ , and Cu 2+ , 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 Hg 2+ 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 photocatalyst, demonstrating wide prospects in industrial applications.