The Characteristic And Influence Factors Of Ammonia-nitrogen Adsorption Desorption In The Loess

Study on the adsorption of ammonia-nitrogen and the effect mechanism of soil is moreextensive, and the systematical research of adsorption/desorption kinetics mechanism isrelatively weak, the material composition in Loess on ammonia-nitrogen adsorption/desorption effect is unclear. Taking Xi’an Lantian section of the South Loess plateau slopeloess section as the research object, using ammonia nitrogen in Loess adsorption/desorptionbehavior as the main line, we systematically study dynamic mechanism of ammonia-nitrogenadsorption/desorption system in loess, and reveal organic matter, calcium carbonate andoxide on the mechanism of the effect of ammonia-nitrogen adsorption.This work providescientific support for the study of pollution controlling.Some conclusions are obtained as follows:1. The kinetics process of loess and paleosol on nitrogen adsorption/desorptioncontrolled by ion exchange reaction rate, quasi-two-level models fits best, the equation is:Qt=k2Q2et/1+k2Qet.Ammonia-nitrogen desorption is about60times slower than the adsorption.2. Ammonia-nitrogen isothermal adsorption curves accord with the Langmuir EXT1model, the equation is: S=Smaxkc1-x/1+kc1-x.The saturated adsorption capacity of paleosol is largerthan the loess. Ammonia desorption curves can be divided into two types, namely lineardesorption model and Langmuir (or LangmuirEXT1) model, in general the desorptioncapacity of loess is larger than paleosol.3. The saturated adsorption capacity of ammonia-nitrogen in soil medium is about5times of desorption, it exists desorption hysteresis phenomenon obviously, and the strongerthe ability of soil desorption, desorption hysteresis phenomenon is not clear. The desorptionrate of Loess rate is higher than paleosol, so the ability of loess to block the migration ofammonia-nitrogen is weaker, and the probability of shallow groundwater contaminated byammonia-nitrogen is higher.4. In the ammonia-nitrogen isothermal adsorption equilibrium process, pH is in6.7~8.1,substantially decreasing. Eh is in180mv~270mv, shallow soil (depth <20m), Eh decreaseswith increasing the NH4+equilibrium concentration; the middle layer, Eh basically unchangein the adsorption process; deep soil (depth>40m), Eh increases with the increasingconcentration of NH4+equilibrium. Electrical conductivity has a positive linear relation toNH4+equilibrium concentration. The cation contents and Cl-increase with the increasingconcentration of NH4+equilibrium concentration, other anion content is constant with increasing NH4+equilibrium concentration. The nitrification in the nitrogen adsorptionprocess can be ignored.5. Ammonia-nitrogen in vadose zone adsorption in soil influenced by externalenvironmental factors (pH, temperature, salinity) and its composition (organic matter, CaCO3,Fe2O3, Al2O3). The pH is higher, the adsorbing capacity is more, and in alkaline conditions pHmakes a big difference to the adsorption behavior; pH has little effect on desorption. Thetemperature is lower, the adsorption/desorption quantity is greater, only when theconcentration of ammonia-nitrogen in the external environment is higher, the influence oftemperature is more obvious. Salt promotes the adsorption process of ammonia-nitrogen, andrestrain the desorption process. Pollution load is higher, the effect of salinity is more.Influenceof NaCl is the weakest, and the t impact of CaCl2is the stronges. Low concentrations (<5%)of organic matter will promote the adsorption/desorption process, while high concentrationof organic matter has a weak inhibition. After removal of CaCO3, adsorption/desorptionquantity decreased, so CaCO3has a strong role in promoting the adsorption/desorptionprocess. Fe2O3restrains the adsorption process, and promotes desorption. Ammonia-nitrogenpollution load is larger, and the effect of Fe2O3is more obvious. Al2O3with lowconcentrations promotes the adsorption/desorption of loess, and restrain the adsorption/desorption of paleosol; Al2O3with high concentrations almost has no impact.Ammonia-nitrogen pollution load is larger, and the influence of Al2O3on adsorption/desorption behavior is more obvious. Through the contrast of the factors’influence, we foundthe favorable factors include alkali environment, low temperature, a certain amount of organicmatter and CaCO3, and pH has the most obvious impact among these factors. The adversefactors include salinity and Fe2O3.6. In the process of adsorption, water rock interaction mainly contains dissolution ofplagioclase, K-feldspar, gypsum, halite and dolomite; precipitation of quartz, illite, and calcite;and cation exchange interaction. The cation exchange interaction makes a significantcontribution. The NH4+ion exchange capacity in the simulation is close to adsorbing capacityin the experiment, showing that the absorption of ammonia-nitrogen is dominated by ionexchange.When the ammonia-nitrogen pollution load is large(100mg/L), NH4+ion exchangecapacity obtained by simulating is larger than the adsorbing capacity in the experiment, andthe difference is near to the amount of ammonia volatilization.The data indicates that theadsorbing capacity in the traditional experiment does not consider volatilization of ammonia,resulting in the larger adsorbing capacity.