Ion Association in Soil Solution as a Cause of Pb Mobility and Availability to Plants

Abstract

A soils restoration important condition in different regions of the world is a soil solution correct management. Soil solution chemical equilibrium determines dynamics of its material composition, migration and accumulation of salts. In the soil solution are formed electrically neutral ion pairs СаСО3 0; CaSO4 0, MgCO3 0, MgSO4 0, and charged ion pairs CaHCO3 +, MgHCO3 +, NaCO3 −, NaSO4 −, CaOH+, MgOH+. A method was proposed of real ion forms in the soil solution quantitative assessment taking into account the ions association. Depending on the concentration and composition of soil solution, in Kastanozem of the Southern Russia in the south-east part of Rostov region are presented the ionic pairs: 15– 45% Ca2+; 16–49% Mg2+; 0.6–8.4% Na+; 2.2–17.6% HCO3 − 16–51% SO4 2− and up to 88% CO3 2−. The thermodynamics has been studied of Pb forms in soil solution of Chernozem in the standard conditions and after phosphogypsum application to a 20–40 cm soil layer at the doses of 0, 10, 20, and 40 t ha−1. A soluble Pb2+ form content in a water extract was calculated by the mathematical chemical-thermodynamic model of ion pairs association in soil solution using the algorithm and computer programs ION–2 and ION–3. A heavy metal (HMs) ions association in the soil solution coefficient kas was proposed for calculation of HMs equilibrium concentration. The model showed that the calculated Pb2+ association coefficient is up to 25.0, and a Pb2+ ion is presented in the soil solution mostly in a PbOH+ and Pb(OH)2 0 form. A chemical soil engineering device for the intra-soil mechanical processing and intra-soil application of amendments and other substances was developed. Intrasoil milling provides a close contact between the soil fine particles and the fine particles of ameliorative substance into a treated soil layer ensuring the HMs passivation, high ameliorative effect, soil fertility and environmental safety.