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

PO

4

3-

Diclofenac sodium

Methylchlorophenoxy-propionic acid

Phenols

CrO

4

2-

SeO

3

2-

and AsO

4

-

). 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·g

-1

. The significantly high adsorption capacity of UiO-66 towards ARS adds further evidence to the recently reported exceptional performance of MOFs in pollutants removal from water.