The activation of persulfate proceeds through its reaction with a catalyst to form the highly reactive persulfate anion radical and an oxidized catalyst. The oxidized catalyst oxidizes a substrate to regenerate the reduced form of a catalyst.

"what is the activation order?" Not clear what do you mean.

"poisoning experiment" could be an addition of a molecule, which selectively binds to the reactive site. The active site is likely to include V.

Thanks a lot for your kind response. Sir, can you please suggest some specific poisoning experiments to find out the active site in BiVO4? As for transition metals like iron, copper, etc., potassium thiocyanate is used, but I am concerned about bismuth and vanadium. Any reference for this will be helpful.

The poisoning experiments are tricky and difficult to interpret. Think about a technique which allows observing the change of Vanadium oxidation state in the course of the reaction.

Thank you for your suggestion about the poising experiment but what other most reliable technique for finding an active site? Should be experimental instead of DFT or theoretical calculation. As some studies suggest that Bi playsa role as an active site and others suggest the stable structure of VO4 but I am concerned about finding the active site of a material and what are the possible ways to determination. Your valuable suggestions will be helpful in this regard.

A simple method to identify the reactive (catalytic) site does not exist. As commonly written "this line of evidence is consistent with our hypothesis." The best would be to to look at the surface in operando. For this you need sophisticated equipment. Poisoning is a very tricky experiment and difficult interpreting. As an example, read this paper

https://www.nature.com/articles/s41467-017-01100-7