Upcycling of heavy metal adsorbents into sulfide semiconductors for photocatalytic CO2 reduction

Although adsorption is regarded as a facile and efficient method to remove heavy metals from polluted water, the disposal of the spent adsorbents remains a great challenge. Here, an "adsorbent-to-photocatalyst" conversion strategy is reported. One-unit-cell calcium silicate hydrate (CSH) nanosheets (similar to 2.8 nm) are used as an ideal adsorbent, and four typical heavy metals including Cu2+, Zn2+, Co2+, and Cd2+ ions are selected for studies. CSH nanosheets show superiority in the ultrahigh specific surface area (577.8 m(2) g(-1)) and chemical stability. After the heavy metal removal, the CSH nanosheets containing heavy metal ions are transformed into metal sulfides through in situ sulfurization treatment. Interestingly, in the case of Cd2+ ions, CdS nanoparticles are produced and well dispersed on the surface of CSH nanosheets. CSH-CdS has a narrow bandgap of 2.34 eV and shows the photoabsorption edge up to visible light (550 nm). Besides, CSH-CdS also possesses a suitable energy band structure, making itself an ideal photocatalyst for CO2 reduction under visible light (lambda > 420 nm). The "adsorbent-to-photocatalyst" conversion strategy demonstrated here not only ameliorates the water and air pollution but also produces the valuable chemical feedstock (CO and H-2).