A　series　of　BUC-21/N-K2Ti4O9　composites　(B1NX)　were　facilely　fabricated　from　BUC-21　and　N-K2Ti4O9　via　ball-milling,　and　they　were　fully　characterized　using　various　techniques.　The　photocatalytic　reduction　of　Cr(VI)　over　the　B1NX　composites　was　investigated　systematically　under　various　conditions,　including　different　light　sources,　pH　values,　hole　scavengers,　and　inorganic　ions,　in　both　real　lake　water　and　tap　water.　The　BUC-21/N-K2Ti4O9　composites　demonstrated　remarkable　photocatalytic　Cr(VI)　reduction　performance,　good　reusability,　and　stability　under　both　UV　and　white　light　irradiation.　The　introduction　of　N-K2Ti4O9　into　BUC-21　not　only　broadened　its　light　absorption　region　to　white　light,　but　also　strongly　inhibited　the　recombination　of　the　photo-generated　electrons　and　holes.　Mechanisms　of　photocatalytic　Cr(VI)　reduction　under　both　UV　light　and　white　light　were　proposed　and　verified　by　electrochemical　measurements,　active　species　capture　experiments,　and　ESR　measurements.　(C)　2021,　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.　All　rights　reserved.