Struvite precipitation - What conditions must be satisfied to favor phosphate precipitation by Mg2+ than Ca2+ provided that both ions are present in a solution?

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in the above studies, specific equations were added to Minteq to improve its prediction capability concerning struvite precipitation 

In general, Ca will precipitate first.
Best

As a general response, Ca will precipitate first as hydroxyapatite and then struvite can precipitate, but it depends on the specific pH and concentration of Ca, Mg, NH4 ... You have to consider all the possible species able to precipitate. I would use Visual Minteq (or other similar tools) to calculate the possible precipitate competitors for your specific conditions.  (https://vminteq.lwr.kth.se/) 

Perhaps the attached Pastor et al., 2008 will help.