Ions association in soil solution as the cause of lead mobility and availability after application of phosphogypsum to chernozem

The model shows that calculated association coefficient of Pb2 + is up to 25.0, Pb2 + is presented mostly in the form of PbOH+ and Pb(OH)20. The quantity of PbCO30 + Pb(CO3)22 − and PbHCO3+ is lower than content of hydroxide-complexes by 10-15 times. At the phosphogypsum dose of 40 t ha− 1, the association coefficient of Pb2 + ion decreases by 2.0 times comparing to initial soil, and its activity increases by 60.0%. The molar fraction of hydroxo complexes decrease for 4.0, carbonate for 5.0, and sulfate associates - increase for 4.5%. The dangerous increase of activity of Pb2 + ion in soil solution after phosphogypsum apply up to 90.0% of its concentration is revealed.