Recycling heavy metals from wastewater for photocatalytic CO2 reduction

Polluted water and exhaust gas released from industrial activities cause a series of environmental issues such as heavy metals accumulation and greenhouse effect. Here, we have proposed an "adsorbent-to-photocatalyst" conversion strategy to bridge water remediation with photocatalytic CO2 reduction. Harmful heavy metals in polluted water are removed and collected by adsorbents, which are converted into valuable photocatalysts for CO2 reduction without secondary treatment. Calcium silicate hydrate (CSH) nanosheets are prepared as an ideal "bridge". Their ultrathin thickness (2.8 nm), ultrahigh surface area (637.2 m(2) g(-1)), and abundant surface hydroxyls are much favorable for both heavy metals removal and photocatalysis processes. Four typical heavy metals including Cu2+, Zn2+, Ni2+, and Pb2+ are selected for studies. Interestingly enough, in the case of Ni2+ removal, CSH nanosheets undergo phase change and they are spontaneously converted into a new semiconductor nickel silicate hydroxide. The nickel silicate hydroxide has a suitable energy level for reducing CO2 into CO. And its strong CO2 adsorption and abundant exposed Ni2+ sites contribute to efficient and selective photocatalytic CO(2 )reduction. The CO yield is up to 1.71 x 10(4) mu mol g(-1)l(-1) with 99.2% selectivity under visible light.