In order to decisively determine the adsorption selectivity of zirconium MOF (UiO-66) towards anionic versus cationic species, the adsorptive removal of the anionic dyes (Alizarin Red S. (ARS), Eosin (E). Fuchsin Acid (FA) and Methyl Orange (MO)) and the cationic dyes (Neutral Red (NR), Fuchsin Basic (FB), Methylene Blue (MB), and Safranine T (ST)) has been evaluated. The results clearly reveal a significant selectivity towards anionic dyes. Such an observation agrees with a plethora of reports of UiO-66 superior affinity towards other anionic species (Floride, PO43-, Diclofenac sodium, Methylchlorophenoxy-propionic acid, Phenols. CrO42-, SeO32-, and AsO4-). The adsorption process of ARS as an example has been optimized using the central composite design (CCD). The resultant statistical model indicates a crucial effect of both pH and sorbent mass. The optimum conditions were determined to be initial dye concentration 11.82 mg.L-1, adsorbent amount 0.0248 g, shaking time of 36 min and pH(2). The adsorption process proceeds via pseudo-second order kinetics (R-2 = 0.999). The equilibrium data were fit to Langmuir and Tempkin models (R-2 = 0.999 and 0.997 respectively). The results reveal an exceptional removal for the anionic dye (Alizarin Red S.) with a record adsorption capacity of 400 mg center dot g(-1). The significantly high adsorption capacity of UiO-66 towards ARS adds further evidence to the recently reported exceptional performance of MOB in pollutants removal from water. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.