At work I have got about 12000 litres of water in IBC containers that I need to bring the pH levels down from 12ish to between 7 and 9. What is the easiest option/additive that I can just go and get today please?
The extra alkalinity is from concrete cutting slurry (not un-cured cement which is totally different). We have a very expensive machine that squeezes all of the solids out which is usually fine for direct disposal but we're on a private technology park and disposing of it into a private water system. My requirement is that I have to achieve a PH of 7 to 9 and its current 12.4. I tested what's in the gully already and it's 8.4.
You can try adding some form of acid but it sounds like the solution won't have much buffering capacity so it will be easy to overshoot and end up with the opposite position. Even citric would cause a very low pH solution if you get is wrong.
If you had enough time bubbling air through it would have the effect you need as the CO2 in the atmosphere is slightly acidic in solution. This is the same carbonation process that affects mass concrete and stops the surface being highly alkaline.
Look at the Siltbuster concrete wash water treatment thing they sell. It bubbles CO2 through the water and the carbonic acid thus produced lowers the pH to acceptable levels.