Chemical Soil-Biological Engineering Theoretical Foundations, Technical Means, and Technology for Safe Intrasoil Waste Recycling and Long-Term Higher Soil Productivity

The amelioration and remediation technology was developed on the basis of research of phosphogypsum and utilization in the Haplic Chernozem of South-European facies (Rostov Region). Phosphogypsum was utilized via dispersed application into a soil layer of 20–45 cm with intrasoil milling of this layer. The phosphogypsum utilization doses were 0, 10, 20, and 40 t ha–1. The Pb analytical content in soil solution was studied in the model experiment. The soil solution Pb thermodynamic forms were calculated. The mathematical chemical–thermodynamic model ION-2 was developed to calculate the real soil solution (water extract) calcium-carbonate equilibrium (CCE) ion forms, considering the ion pair association. The associated ion pairs CaCO30, CaSO40, MgCO30, MgSO40, CaHCO3+, MgHCO3+, NaCO3–, NaSO4–, CaOH+, and MgOH+ were accounted for in soil solution equilibrium macroion form calculation. The procedure for the microelement ion [including heavy metals (HMs)] equilibrium concentration in the soil solution coefficient kas calculation was proposed to account for the real soil solution CCE, macroions, and HM (including Pb) association. The Pb2+ ion in soil solution was mostly bound to associates PbOH+, Pb(OH)20, PbCO30, Pb(CO3)22–, and PbHCO3+. The calculation of CCE and ion association in soil solution revealed 14.5–21.5 times HM passivation compared to HM water-soluble values. The calculated HM activity in the soil solution in the example of the Pb2+ ion was less than 4% after phosphogypsum application in the target amelioration layer of 20–45 cm. The studied phosphogypsum doses were substantiated as environmentally safe. This was because the real soil solution CCE provided HM ion form association and consequent passivation. The dry steppe soil remediation after phosphogypsum application was justified as highly probable. The intrasoil milling chemical soil-biological engineering technology was developed for simultaneous soil amelioration and remediation on the basis of the biogeosystem technique (BGT*) transcendental methodology. The BGT*-based technology was tested in the long-term field experiments and is capable of ensuring the priority geophysical micro- and macroaggregate structure via intrasoil milling and mixing of soil illuvial and transitional horizons. This helps synthesize soil multilevel architecture, providing intrasoil-dispersed environmentally safe recycling of wastes of different origin. Addressing the environment safety concerns, a new decision of the intrasoil milling device was proposed for phosphogypsum and other substance application to soil.