Influence Mechanism Of Water Quality To The Adsorption And Migration Of Heavy Metal In Farmland Soil

The irrigation makes the migration and diffusion of heavy metals in contaminated soil asthe focus of the soil science and environmental scientists.The long-term irrigation process willcontinue to have differentiation of contaminated soil in various pollutants and to furtheraggravate the degree of soil pollution and depth, aiming at the objective facts of farmland soilpollution is serious increasingly. Strong irrigation process is the influence of irrigation waterquality ingredients, so mastering the influence on the mechanism of different water quality tothe pollutants in the soil and pollutant migration and transformation, will help to improve theefficient use of water resources and control the range of soil pollution. At present, theirrigation water quality type is complicated in our country. From water quality componentscan be divided into four categories including organic, inorganic, organic and inorganiccompound and clean type. This study simulated different types of irrigation water ofconfiguration montmorillonite solution, fulvic acid solution, natural water and distilled water,and analyzed effect on the four different types of irrigation water in the process of cropgrowth to the migration of heavy metals in soil using potted plant experiment, and discuss themechanism of different components and soil water conditions on heavy metal migration usingthe migration of saturated soil column experiment about adsorption and desorption ofdifferent soil structures and types of colloid to heavy metals. The following main conclusions:(1) Irrigation water quality components to different heavy metals in the soil have thedifferent migration ability. For the main pollutant of Pb and Cd in the soil, montmorillonitesolution shows great effect of accelerating migration and accumulating. Because crop growthperiod is shorter, different quality has no obvious effect on crops. Once the soil polluted byheavy metals, it is bound to the charged nature of the soil, such as electric properties andcharge quantities, therefore, its function will be changed relatively clean soil. The maincomposition of irrigation water quality change of heavy metals in the soil that exist in theform and quantity, and cause the migration and accumulation. Ir study found types offunctional groups of polluted and clean soil did not change, but absorption of polluted soilpeak intensity weaker than clean soil. The absorptive capacity of soil under certain heavymetal lead pollution was instead higher relatively clean soil, but desorption and desorption rate were correspondingly lower.(2) Micro-aggregate is the basic structure of the soil unit, and have the importantinfluence on distribution and accumulation of heavy metals in soil. Through soil microaggregate adsorption desorption experiments of Pb, we found: With the increase of the Pb2+concentration in equilibrium liquid, the absorptive capacity of different particle size of soilmicro-aggregates was increasing accordingly from the aspects of soil structure. Pb2+accumulated to the minimum sizes under the certain condition of equilibriumconcentration.The results show that adsorption characteristics of Pb2+by different sizefractions of the micro-aggregates fitted well the langmuir isotherm.<0.002mm micro-aggregate has the largest adsorption capacity, while0.2~0.02mm micro-aggregate has thebiggest adsorption energy barrier. The maximum adsorption capacity of Pb2+by the sizefractions was dependent of the specific surface area and correlated well to CEC of thefractions.0.2~0.02mm and2~0.2mm micro-aggregates relatively has a lower adsorptioncapacity, higher bonding strength, faster desorption balance. Secondary desorption rate ofabsorptive lead of different particle size of micro aggregate are bigger than the first desorptionrate. Besides that, With different ion type, soil adsorption of heavy metal desorption ability isdifferent. Na+is more helpful for adsorbing lead to soil micro-aggregates and adsorptiondistribution coefficient than Ca2+. When Na+as accompanying ion, the maximum adsorptionamount of lead on soil micro-aggregates increases with the increasing of specific surface area,CEC and organic matter content, however, it is not affected by soil basic properties with thepresence of calcium. So in the soil environment, the adsorption of Pb2+rate for micro-aggregates from a loess soil is low, and the desorption rate is high when Na+as the co-existing ion, Whereas Ca2+is just the opposite.(3) Colloids are the most active part of the soil, and have important influence onadsorpted ability of heavy metals in soil. Through different types of soil colloid in adsorptionand desorption experiments of Pb, we found: The adsorption of montmorillonite and humicacid to Pb2+increase rapidly and present stage characteristics with the increasing of lead inequilibrium liquid, while kaolinite of Pb2+adsorption quantity is small and has no obviouschange. Adsorptive characteristics of Pb2+by kaolinite, montmorillonite and humic acid fittedwell the langmuir isotherm. Montmorillonite on the adsorption capacity of lead less thanhumic acid, but strong degree of adsorption is far higher than that of humic acid. Three kindof colloids on the desorption rate of lead show: kaolinite>montmorillonite>humic acid, andsecondary desorption rate of absorptive lead is all larger than the first desorption rate. Hightemperature treatment can change montmorillonite structure, So it caused that the adsorption of Pb2+emerge the trend of slowly rising firstly and the sharp rise again, but speed ofreleasing Pb2+rise slowly.(4) Water quality component is the decisive factor on heavy metals in the leachingsolution concentration and distribution of heavy metals in the soil column section under thecondition of irrigation. Migration trend of Pb and Cd were largely consistent under differentirrigation water. Wastewater as complex colloid, has the strongest penetrating power to heavymetal ions in the sand column,while penetrating power of humic acid to the weakest.Themigration ability of Pb and Cd under the humic acid colloid are stronger in, but cd migrativerate is the fastest under the irrigation of montmorillonite colloid and distilled water for Pbmigrative rate is the fastest. Sewage and humic acid to Pb and Cd in soil column surface at2-3cm has accumulated phenomenon,so this may produce toxic to crop root. From theelements itself, the combinative strength of the inorganic colloid with cadmium is better thanthat with lead.