The Coexistence Of Ammonium Nitrogen And Heavy Metals In The Soil Migration Experiment And Simulation Study

Soil is an important component of the ecological environment, it is a basic part of the human society. With the development of industry, a large number of sewage discharge, mining, smelting, leaching of garbage and waste mine tailings discharge, atmospheric deposition, agriculture irrigation, pesticide and fertilizer application make such a large number of heavy metals pollution and nitrogen pollution in soil, which resulting in that heavy metals(Cu, Pb, Zn, Cd) and NH4+ coexisted in soil. Typically, it is concerned that these elements in soil are difficult to move, which means that they tend to accumulate in the soil surface. However, as the soil is a very complex system, which will have a series of physical, chemical and biological reactions, in some cases, heavy metals(Cu, Pb, Zn, Cd and so on) and NH4+ will transport into deep soil, so it would increase the scope of soil contamination, and even pollute the groundwater. Therefore, the study of soil heavy metals and ammonium in different physical and chemical conditions of their migration and transformation rules has important theoretical significance and practical significance to the prevention of environmental pollution and restoration of contaminated soils.This paper studyed the common contamination of heavy metals (such as copper, zinc and cadmium) and ammonium nitrogen, analyzed the adsorption properties with the coexistence of heavy metals and ammonium nitrogen in the brown soil in Qingdao; revealed the migration rules of heavy metals and ammonium nitrogen under different pH values and ionic concentration conditions; furtherly explores the impact of the competitive adsorption mechanism and its migration rules with the coexistence of two kinds of heavy metals and ammonium. CXTFIT2.1 package was used to determine model parameters and predict the outflow dynamics, and predicted the migration curves in different soil depths.The conclusions had been drawn as follows:1) In the static adsorption isotherm experiments, the adsorption of heavy metals and ammonium increased with increasing of pH. Kd decreased with the increase of the initial concentration, indicating that the adsorption rate is decreasing. The the order of adsorption capacity of three heavy metals: Cd>Cu>Zn. The fitting results of Langmuir and Freundlich equation are good (except Langmuir fitting of the Cu addition), the correlation coefficient reached more than 0.9. The fitting results of the NH4+ are good, R2 are greater than 0.99.2) Compared with the BTCs of Br-, its shape and peak are affected by pH and ionic concentration of heavy metals and ammonium. The BTCs of heavy metals from transport experiment were exceedingly asymmetric. Low pH makes the transport of heavy metals and ammonium faster, this is because:for the heavy metal, the adsorption in the soil particles mainly by electrostatic adsorption in low pH. As pH increased, heavy metal ions are difficult to desorb, then it will precipitate, and the dissolved heavy metals in the soil will rapidly decrease. For the ammonium, OH- will increase when pH increased, and it reacted with NH4+ to form NH3, reducing the adsorption in the soil, so that the percentage of adsorption decreased rapidly.3) With the increase of ionic strength, heavy metals and ammonia adsorption decreased, which may be due to the results of competitive adsorption of Na+, heavy metals and ammonium nitrogen. With the increase in ionic strength, Na+ occupy medium the surface negative charge, reducing the electrostatic adsorption, and make electrostatic adsorption point saturated. Furthermore, the increase in ionic strength also led to increased interaction between ions, decreases activity coefficient, accelerated migration speed of heavy metals and ammonium nitrogen in soil.4) In the competition experiment, the order of adsorption capacity:NH4+>Cu2+>Cd2+>Zn2+. This result is different with single heavy metal, which maybe caused by differences of the ionic radius of metal element and the hydrolysis constants. NH4+ adsorption capacity is the largest in the competition experiment, and the smallest in the single metal adsorption, which may be due to the competitive of heavy metal ions, which increased the NH4+ adsorption.5) The BTCs of Br- were fitted by LEA model (r2> 0.9, MSE<0.0094). It is fitted by TRM that the dynamic percentage of water is close to 1, so it can ignore the impact of physical non-equilibrium. Using the data for the fitting process, it is found that BTCs trailing part generally are underestimated, which shows the parameters obtained by static adsorption method for the migration experiment has some limitations.6) The BTCs of Br- can be fitted by model LEA with high goodness of fit (r2>0.9, MSE<0.010). By these model parameters acquired by the model TSM, we can predict the BTCs of heavy metals and ammonium nitrogen at different depth of quartz sand column, as the depth of quartz sand column increased from 5cm to 15cm.