Studies On The Adsorption-desorption Of Hg(II) By Two Typical Soils In China

Northeast area of China is a major base of food supplies as well as an important old industrial one.Heavy metal pollution is one of the primary factors in environmental pollution. Mercury pollution in soil has always been an old and big environmental problem in this area. The exacerbating pollution has become a huge potential threat to ecological environment safty,public health and social economy development.The treatment and restoration of soil with heavy metal pollution start with research on mobility of heavy metal in soil which mostly relates to the adsorption-desorption progress in soil.The adsorption-resorption of heavy metal in soil is a key step to effect its concentration,migration,transformation bioavailability and toxicity.This study focused on the mechanism of mercury fixing and releasing based on the unique chemical structure and regional characteristics of black soil in northeast area.Therefore it will be very helpful to supply the academic basis for soil restoration of mercury pollution.In this research,two typical soils in China-northeast black soil and southern red soil(as contrast).We investigated the Hg(â…¡)adsorption-desorption thermodynamics and kinetics characteristics in black soil and red soil through a series of indoor simulating experiments. This study also contained curve data analysis using different fitting equations and a series of experiments under various physical and chemical conditions.Through these works,we hope that we could illustrate the adsorption performances and characteristics of mercury by two soils based on different mechanisms and rules.The main research contents and results are as follows:(1)A series of batch equilibration experiments were conducted in order to assess the adsorption-desorption of Hg(â…¡)in black soil and red soil.The study showed that both of them have high adsorption capacity by fixing over 95%Hg(â…¡).Black soil has nearly triple adsorption capacity(5122 mg kg^-1)for Hg(â…¡)than those of red soil(1781 mg kg^-1).The desorption in both soils were less than 0.5%.Furthermore,red soil has a higher desorption ratio than black soil.(2)Hg(â…¡)adsorption isotherms were well fitted with Langmuir,Freundlich and Temkin equations.Langmuir equation can best describe Hg(â…¡)adsorption isotherm in black soil for higher relative coefficient.So is Freundlich equation for red soil.The fitting curve parameters suggest black soil has higher adsorption capacity and weaker affinity with Hg(â…¡) than red soil.(3)Kinetic experiments were conducted in two soils at 25℃.The results indicated that the adsorption was a bi-phasic process which started with a fast-adsorption phase and a slow-adsorption phase ensued.The adsorption amount and speed in the fast reaction phase were much larger than the slow one.The Elovich equation was well fitted with kinetic adsorption process.And the fitting curve parameters showed black soil had a higher initial instant and average adsorption speed with a shorter reaction time.(4)Initial pH has no impact on Hg(â…¡)adsorption in two soils.(5)A series of batch equilibration experiments were conducted under various temperatures.The thermodynamics calculation revealed that Hg(â…¡)adsorption in both soils was spontaneous exothermic reaction with negativeâ–³G⁰ andâ–³H⁰ values.The values ofâ–³S⁰ suggested the adsorption in black soil was approaching unordered while red soil reversely. Both soils showed strong bonding with Hg(â…¡)for positive Q_m values.(6)The removal of organic matter results in significant declined on Hg(â…¡)adsorption capacity of both soils.Black soil reduced more adsorption(50%loss)than red soil(30%loss) with a higher desorption ratio.(7)The concentration of NaNO₃ as electrolyte has no effect of Hg(â…¡)adsorption in two soils.And so was SO₄^2-.(8)The concentration of Cl^- hugely effected Hg(â…¡)adsorption in two soils.When the concentration was lower than 10^-3mol L^-1,the impact was neglectable.As the concentration added from 10^-3mol L^-1to 10^-1mol L^-1,the amout of adsorption sharply decreased.With over 10^-1mol L^-1Cl^-,both soils turned out to be negative adsorption by releasing their originally fixed mercury.