Assessment and comparison of desorption process for heavy metals from natural soil column due to raw wastewater infiltration and its possible percolation into groundwater

The study of adsorption/desorption process for heavy metals in soil and the evaluation of how these are transferred or accumulated or what happens to them in layers of soil is an important issue, that has attracted the attention of many researchers. Ability of accumulated metal transfer in layers of soil, because of surface water infiltration, precipitation and even infiltration of municipal and industrial wastewater in soil on one hand, and desorption of metals from soil as solution and their transfer and percolation into groundwater, which causes groundwater pollution, on the other hand are among other reasons which add to the importance of these types of research. Flow of raw wastewater in layers of soil due to adsorption/desorption process leads to transport of metals in soil column. This research was carried out as field operations, the aim of which was to assess desorption process for metals Ni, Zn and Pb that exist in soil in Semnan industrial region due to raw wastewater infiltration that are released from soil.
After the region raw wastewater infiltration, changes of concentration of metals in soil layers were measured and a statistical assessment was carried out with data, then for infiltration of wastewater to be applied in soil a pit was excavated. Soil sampling from the lower layers of the infiltration pit was done and Samples depth was from 20 to 300 cm in different layers. Before infiltration of wastewater, samples from the lower depths of the infiltration pit was obtained and used for determining the initial concentration in the soil layers (pre-infiltration) and after infiltration of wastewater, amount of heavy metals concentrations in the same depths are considered as post-infiltration data. A statistical assessment was carried out for evaluating the effect of desorption process (a paired-samples T-test is used for comparison of means) for each metal in the total soil column. This assessment was conducted by using statistical package SPSS18.
The results indicate that, displacement in the center of mass happens from the top to the lower layers for the three metals. The statistical assessment indicated in this soil column, desorption process of Ni from soil to wastewater solution was effective, but for metals Zn and Pb was not. Transport ratio was calculated and for Ni, Zn and Pb were 136, 21 and 10, respectively. Dissolved concentration was calculated for Ni, Zn and Pb at the end of soil column were 4.07, 1.95 and 1.25 (ppm), respectively.
wastewater infiltration leads to transfer of all of metals towards more depths in soil column. Ni percolates into the groundwater much faster than other metals. The order obtained from metals affected by desorption process and amount of percolation into ground water was Ni>Zn>Pb.