Intra-soil waste recycling provides safety of environment

Amelioration and remediation technology was developed for phosphogypsum utilization in Haplic Chernozem of South-European facies (Rostov Region). The technology comprises phosphogypsum dispersed application into the soil layer of 20–45 cm during intra-soil milling. In the model experiment, the phosphogypsum doses 0 (control), 10, 20, and 40 t ha−1 were studied. The Cd thermodynamic forms in soil solution were calculated via the developed mathematical chemical-thermodynamic model and program ION–3. The form of ion in soil solution (or water extract) was considered accounting the calcium-carbonate equilibrium (CCE) and association of ion pairs CaCO30; CaSO40, MgCO30, MgSO40, CaHCO3+, MgHCO3+, NaCO3−, NaSO4−, CaOH+, MgOH+. For calculation of the equilibrium of microelements concentration in soil solution ion including heavy metals (HMs), the coefficient of microelement association kas was proposed. According to calculations, Cd2+ ion in soil solution was mostly bounded to associates CdOH+, partly to associates CdCO30 and CdHCO3+. The calculated kas of Cd was 1.24 units in the control option of experiment and decreased to 0.95 units at phosphogypsum dose 40 t ha−1. The ratio of “active [Cd2+] to total Cd” reduced from 33.5% in control option to 28.0% in the option of phosphogypsum dose 40 t ha−1. The biogeochemical barrier for penetration of HMs from soil to plant roots was high after application of phosphogypsum. According to calculation by ION–3, the standard soil environmental limitations overestimate the toxicity of Cd in soil solution. New decision for intra-soil milling and simultaneous application of phosphogypsum was developed to provide the environmentally safe waste recycling.