Assisted by low molecular weight organic acids (LOAs), photocatalytic reduction of Cr-VI was studied in the presence of TiO2 nanowire arrays (NWs) under the visible light. Theoretical calculations indicates that the decisive factors for this conversion are the stable adsorption behavior of LOA on TiO2 and orbital overlapping between the LOA HOMO and TiO2 LUMO. The orbital configuration interprets that covalent bonds are formed between the LOA and TiO2. Direct electron transfer occurs from LOA to TiO2 surface which facilitates the reducing of Cr-VI. Tartaric acid (TA) is proved to be the optimum organic acid for the surface-catalyzed reduction of Cr-VI. The TA can be easily excited by the light with wavelength higher than 1033 nm and be broken into two same molecular segments. The molecular segments accelerate the Cr-VI reduction which follows a two-stage pseudo-first-order kinetics that is slow at initial stage and fast at the second stage. (C) 2016 Elsevier B.V. All rights reserved.