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

Abstract The thermodynamics has been studied of Pb forms in soil solution for the case of calcareous ordinary chernozem of the Southern Russia, northern zone of the Krasnodar krai for conditions of phosphogypsum utilization into the soil layer of 20–40 cm at doses of 0, 10, 20, and 40 t ha − 1 . The application of phosphogypsum increased the contents of total Pb by 4.2; the content of water-soluble forms – by 17.7. As a result of ion association in soil solution are presented electrically neutral ion pairs СаСО 3 0 ; CaSO 4 0 , MgCO 3 0 , MgSO 4 0 , and charged ion pairs CaHCO 3 + , MgHCO 3 + , NaCO 3 − , NaSO 4 − , CaOH + , MgOH + . The contents of soluble Pb 2 + forms in water extracts were calculated by mathematical chemical-thermodynamic model of ion pairs association in soil solution using algorithm and computer program ION-2. The coefficient of heavy metal ion association k as in soil solution is proposed for calculation of the heavy metal equilibrium concentration in soil solution. The model shows that calculated association coefficient of Pb 2 + is up to 25.0, Pb 2 + is presented mostly in the form of PbOH + and Pb(OH) 2 0 . The quantity of PbCO 3 0  + Pb(CO 3 ) 2 2 − and PbHCO 3 + is lower than content of hydroxide-complexes by 10–15 times. At the phosphogypsum dose of 40 t ha − 1 , the association coefficient of Pb 2 + 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 Pb 2 + ion in soil solution after phosphogypsum apply up to 90.0% of its concentration is revealed.