## 4 Metal Ion Leaching and Solute Transport
### Modified Soil Medium  
The PHREEQC computer code program [193] simulates leaching curves of experimentally modified soil columns (see also Section 4.1). Sewage sludge was amended to experimental calcareous soil. The same soil specimen tested was enriched with injected heavy metals and nanoparticles of ZnO and MgO and zeolite functioning as absorbents. Simulation program inputs comprise four modules, i.e., SOLUTION, EQUILIBRIUM PHASES, EXCHANGE, and SURFACE. The columns of the enriched soil were incubated for 7 days, and leaching experiments that lasted 18 days were conducted by employing two fixed leaching solutions made of CaCl\({}_{2}\) and diethylenetriaminepentaacetic acid (DTPA) in pre-defined conc. The PHREEQC simulation program employs organic surface complexes. Heavy metal Log K\({}_{s}\) values were extracted from SHM and NICADONNAN databases [181]. Modified soil column responses were studied against leaching solution, and more specifically, the leaching tailing of cadmium (Cd), nickel (Ni), copper (Cu) and zinc (Zn). The SOLUTION module received combinations of mixed CaCl\({}_{2}\) and DTPA solution of a determined concentration equilibrated with solid phases received from MINTEQ. The latter is a software program that simulates heavy metal transfer on account of the pre-prepared mixture solutions and solid-phase interaction. Heavy metal propagation was simulated by means of absorption/complexation of MgO, ZnO, and zeolite serving all as reactive sites. Ion exchange sites are simulated by the EXCHANGE module and heavy metal transport by the TRANSPORT component. Monitoring cell (individual grid) and leaching time were properly defined for the running simulation [194]. As a result, the PHREEQC displayed a respectable capacity to simulate the leaching of heavy metals through modified soil columns well prepared to undergo lab-scale testing, though in earlier years [195] there have been experimental mismatches against PHREQC predictions. Input data extracted from databases refer to pure solid phases, which is not our case in the conducted experiments. Therefore, mismatches are to a great extent justified, since leaching rate by simulation was highly dependent on the SI and the solid phase type. The use of different soil types entails deeper study of surface reactions and ion binding. In the bibliography, the PHREEQC code simulates the experimental results from heavy metal leachates derived from various waste forms and sewage sludge [196; 197; 198]. It is suggested that the cation exchange reaction is the predominant mechanism of Cd transport in the soil profile. PHREQC predicts heavy metal simulation leaching well, e.g., Pb, in modified soil, taking into consideration ion exchange mechanisms and surface complexation reactions [199]. Electric power research institute (EPRI), an independent non-profit energy research, development, and deployment organization, adopted the MINTEQ approach model to develop the FASTCHEM geochemical hydrodynamic solute transport model for modeling coal ash leachate dispersion and advection. It demonstrates useful appliances over heavily polluted soils by fly ash dispersion derived from fossil fuel power plant operations [200]. The RZWQM2 (1-D) point scale model was tested over cultivated soils and soils enriched with slurry, mostly sandy and sandy loam quantities. In all cases studied, predictions were made for soil mineralization, ammonia gas and N\({}_{2}\)O emissions, and nitrate leaching regarding testing of each individual soil type. The scope of the experimental effort was to set up integrated strategies over nitrogen gas abatement derived from livestock farming activities and the prevention of nitrate leaching phenomena on the ground [74]. The application of agri-residuals over cultivated soil and inclined terrain farms is a common practice to ensure higher crop yield, to minimize erosion phenomena, and to increase water soil infiltration and retention along with the improved evaporation characteristics.