Implications of long term irrigation with wastewater on the contents and retention kinetics of potentially toxic elements in Typic Torripsamment soils

The consequences of irrigation with wastewater (rich in organic matter) on the distribution of PTEs between the soluble and total forms as well as their distribution among the different soil layers of El-Gabal El-Asfar soils was the main target of this work. Eight soil locations were sampled at three different depths to represent different periods of irrigation with wastewater extended beyond 80 years. These samples were analyzed for their contents of SOM, cation exchange capacity (CEC), total and soluble concentrations of PTEs i.e. Ni, Pb, Cd, Co, Zn and Cu. Results revealed that both SOM and CEC increased steadily and significantly in the uppermost 0-30 cm layer with ageing. The corresponding increases within the subsequent soil depths seemed to be slight. Soluble and total concentrations of PTEs also increased progressively within the different soil depths with ageing; however, the depthwise increases seemed less obvious. Based on the Dutch system formula, the soil irrigated for a period extended up to 80 years could be considered slightly polluted with Co, moderately polluted with Pb, severely polluted with Ni, Cu and Zn and very severely polluted with Cd. Soluble PTEs concentrations were correlated significantly with their total concentrations in soil and both were correlated significantly with SOM and CEC. The relationships between amounts of retained PTEs and their corresponding soluble concentrations were best fitted to Langmuir isotherm model. The outcome relations between the amounts of retained PTEs and aging were fitted to five kinetic models, the power function and the parabolic diffusion ones seemed to be the best fitting models. In conclusion, although soil organic matter plays important role in distribution of the PTEs between the soluble and retained forms in soil irrigated with wastewater; yet ageing affects this distribution obviously especially in the uppermost soil layer.