Applying sludge to agricultural land is reasonable as it improves some soil properties such as pH, organic matter content and nutrients. But the use of sludge and biosolid on forested and agricultural land is restricted by heavy metal content. Long-term use of sludge leads to the accumulation of heavy metals in the soil. Even after a short-term application, the content of heavy metals can increase considerably. The application of sludge affects the salinity of the soil and consequently the availability of metals to plants increases [1]. To evaluate the potential impact of sludge application on agricultural land it is necessary to know the mobility and bioavailability of heavy metals in soil. The solubility of metals is controlled by adsorption/desorption, precipitation/dissolution and complex formation reactions. Although metal solubility is initially reduced by sorption reactions, long-term solubility is controlled by chemical forms/species that may vary over time. Knowledge of metal distribution and speciation in solution is essential to understanding the chemistry of the soil solution–metal ion system. The thermodynamic evaluation of soluble and insoluble species of Cu (II) in soils treated with sludge from domestic wastewater treatment processes was carried out, based on experimental data regarding the typical chemical composition of soil solutions from the Republic of Moldova. The choice of copper is conditioned by its high content in native soils. Thermodynamic calculations of Cu (II) ion speciation in sludge-treated soil solutions were performed. The developed thermodynamic model was used for chemical speciation in homogeneous monophasic and heterogeneous biphasic systems as a function of a series of thermodynamic parameters [2]. As a result, expressions to calculate the domains of thermodynamic stability of Cu (II) species in the analyzed systems were deduced. One of the most important advantages of using the developed thermodynamic model, which is based on computer calculations, is the possibility to predict the distribution of metal ion species in sludge. The results of this study suggest that as soil pH decreases, the availability and mobility of copper ions increases due to the soluble chemical species in which this ion is present in soil solutions.