Studies On Electrochemistry Properties And Lead Adsorption-desorption Behavior Of Meadow Brown Soil In Northeast Of China

Northeast area is the major grain-producing base, meanwhile, is the important old industrial base in China. But pollution of heavy metals in soil has been more and more serious during recent several decades and has become a potential threat to eco-environmental security and human health. So understand the pollution formation mechanism of northest soil scientificly and explore effective restoration ways are of great significance.Electrochemistry properties of clay-water interface and the adsorption-desorption behavior of pollutants on clay-water interface are the important fators which influence the speciation distribution, migration, transformation and fate of pollutants in soil, therefore, they are the main issue that people have been studying all the time. In this paper we taking the meadow brown soil in northeast of China as the sample, studied its interfacial electrochemistry, the adsorption-desorption properties and the speciation distribution of lead. Moreover, we investigated effects of some environment factors especially the influence of low temperature (freezing). Through these works we hope we can provide the theoretical basis to understanding the pollution formation mechanism of northest soil scientificly and exploring effective restoration ways. The main research work and achievements of this paper are as follows:(1) The point of zero net charge (PZNC) for the meadow brown soil in northeast of China with structural negative charges was measured by three methods: potentiometric titration, mass titration and inert electrolyte titration, showing that for meadow brown soil possesses structural negative charges the PZNC independent of electrolyte concentration also exists, and the values of pH_PZNC obtained by the above three methods are 2.9, 5.0 and 3.3, respectively. It was found that the values of pH_PZNC determined by potentiometric titration and inert electrolyte titration were very close while that determined by mass titration was relatively high.(2) The estimating method of the total density of surface proton-active sites (Ns) in literatures was improved to obtain the Ns value of the meadow brown soil in northeast of China, which is 2.5 mmol·g^-1. According to the values of pH_PZNC, Ns and structural negative charge density (σ_st) measured, the intrinsic surface reaction equilibrium constants, pK of 1-pK model, pK_al^int and pK_a2^int of 2-pK, model, were directly calculated to be 3.37, 2.42 and 4.32, respectively, for the meadow brown soil in northeast of China.(3) Adsorption properties of Pb²⁺ on meadow brown soil in northeast of China were studied, and the sorption mechanism was discussed. The results reveal that the sorption kinetic and the sorption isotherm of Pb²⁺ on the soil sample can be described by the pseudo-second order kinetic and Langmuir isotherm, respectively. With increasing pH, the sorption amount (q_e) increases rapidly in the pH range of 2～4 and doesn't change on the whole in the pH range of 4～ 6. The presence of inert electrolyte (NaNO₃) may obviously restrain the sorption of Pb²⁺ on the soil sample, and with the increase of NaNO₃ concentration, the initial rapid decrease and subsequent gentle decrease of qe were observed. The sorption mechanism of Pb²⁺ on the soil sample may be described as the ion-exchange sorption and the surface functional group binding sorption. Furthermore, the surface functional group binding sorption may be divided into the chemical binding sorption to form the inner-sphere surface complexes and the electrostatic binding sorption to form the outer-sphere surface complexes.(4) Desorption properties of Pb²⁺ on meadow brown soil in northeast of China were studied. The results reveal that the desorption kinetics of metal ions from the soil sample showed a biphasic type behavior, that is, an initial fast release was followed by a slow release. With increasing pH, the desorption amount decreases rapidly in the pH range of 2～5 and doesn't change on the whole in the pH range of 4～10. With the increase of NaNO₃ concentration, the initial rapid increase and subsequent gentle decrease of desorption amount were observed, and the sorption and desorption process were not totally reversible. Freezing witn -70℃ can decrease the desorption amounts of Pb²⁺appreciably. (5) The speciation characteristics of lead adsorbed in the meadow brown soil were investigated by means of Tessier continuous chemistry extraction.The results showed that most speciation is ion exchange and the least speciation is residua, that is ion exchange > carbonate fractions >Fe-Mn oxides >organic fractions. Under the discussing conditions, freezing had severe influence on the speciations of lead in the meadow brown soil.