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　m2　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　CO2　reduction.　The　CO　yield　is　up　to　1.71　×　104　μmol　g−1　h−1　with　99.2%　selectivity　under　visible　light.　©　2020