OK, in dealing with Mo-99 purification, we have molybdenum in an oxidation state of +VI. At around neutral pH, this means that molybdenum is present as the molybdate ion, MoO<sub>4</sub><sup>2-</sup>. As pH decreases, this oligomerises to a number of different species such as the hexmolybdate and heptamolybdate forms. (I don't remember the formulae off hand, but I can easily check them.) So, knowledge of the pH is important as it provides knowledge of the molybdate speciation. Knowledge of the concnetration is also important, as speciation is also dependant on concentration at low pH, and if the concentration becomes too high the molybdenum is precipitated fron solution as insoluble molybdenum(VI) oxide, MoO<sub>3</sub>. This is bad, as purification requires the molybdenum species remain in solution.
The purification involves sorbing (sticking) the molybdenum onto an alumina column. The column is then washed with various acid and weakly basic solutions to wash off other impurities - under these conditions, the molybdenum stays stuck to the column. It is then desorbed (removed) with a concentrated base wash. (There are a few purification steps, but you get the idea.) The solution is then boiled down, to reduce the volume (concentrating the molybdenum), whilst simultaneously removing the iodine impurities.
The wastes removed under acidic conditions includes both low-level and high-level radioactive wastes, and include unreacted uranium isotopes, as well as other nasty isotopes such as strontium-90. The need for monitoring when dealing with such systems is (hopefully) obvious.
This should be enough to get you started. You will need some sort of reference on molybdenum speciation if you are going to talk about how it changes with pH. I would suggest (from memory - I don't have either of these at home, but they are good sources of information) a textbook like
Chemistry of the Elements by Greenwood and Earnshaw, or Cotton and Wilkinson's
Advanced Inorganic Chemistry. I know there are also some good 'net sources, but I don't have any at hand. I'll have a talk to a colleague of mine that works in the area for some suggestions. Post any further questions you may have.