Studies Of Adsorption-desorption Properties Of Hg²⁺ On Humic Acid And Fulvic Acid Extracted From Typical Soils In China

Human activities such as sewage irrigation,unreasonable fertilization,landfill disposal and industrial emissions caused heavy metals into soil and resulted in soil. The soil pollution caused by heavy metal has been much more serious in recent years and has become a potential threat to the safety of ecological environment and human health.Humic acids are major components of soil organic matter,which play an important role in adsorbing heavy metals.In that case,it will be significant to study the combining characteristics of mercury on humic acids,and to figure out how mercury was fixed and released on them.Therefore,the study will help to explore effective restoration methods of soil pollution.In this research,two kinds of typical soils in Chinese,northeast black soil and Jiangxi red soil,were used as the sources of humic acids(HAs).The extraction and purification of HAs were performed according to the method recommended by the international humic acid association(IHSS).A series of batch equilibration experiments and kinetic experiments were conducted to investigate the capacity. strength and rates of adsorption/desorption of divalent mercury(Hg²⁺)on humic acid (HA)and fulvic acid(FA).We also charactered the structures of HAs and HAs-Hg samples with a series of instrumental analysis methods including FTIR spectroscopy, NMR technology and SEM.Based on these works,we hope to elucidate combining characteristics of mercury on humic acids,to understand the pollution formation mechanism,to provide theory basis to control mercury harm and to explore effective reparation methods of soil pollution.The main achievements of this paper are as follows:(1)Major physical and chemical properties of soils and element contents of HAs were obtained.Data showed that the content of HAs and the humification level of black soil were all significantly higher than those of red soil;the content of total mercury in soil samples was higher than that of background value. (2)Hg²⁺adsorption isotherms could be well fitted with both Langmuir and Freundlich equations.The isotherms of Hg²⁺on HA conformed better to the Freundlich model,while the isotherms of Hg²⁺on FA conformed better to the Langmuir model.The study showed that HA had higher adsorption capacity for Hg²⁺ than FA.The desorbed percentages of all HAs samples were less than 1%. Furthermore,HA had a lower desorption ratio than FA.All these showed that HA has a higher absorbing capability and binding strength with Hg²⁺in soil environment.(3)Kinetic experiments were conducted in order to assess the adsorption and desorption of Hg²⁺on HA.The results indicated that the adsorption process included the fast-adsorption stage in first and the slow-adsorption stage afterwards.The adsorbed amount and the adsorption speed in the fast reaction stage were far more than those of the following stage.Comparing HA from black soil with HA from red soil,the former had a quicker adsorption speed and shorter reaction time.(4)The effects of pH value on the kinetic adsorption/desorption were researched,and the results showed that the largest amounts adsorbed was under this acidity order:pH 3.50＞pH 5.00＞pH 6.50.This showed that adsorption capacity was higher under a lower pH value.The fitting results of Kinetic equations showed that: adsorption kinetics of Hg²⁺on HA could be best described by Elovich equation, followed by the double-constant equation;Desorption kinetics of black soil HA-Hg could be best described by the first order kinetics equation,and desorption kinetics of red soil HA-Hg could be best described by Parabola Diffusion Equation.(5)The Fourier transform infrared(FTIR)spectroscopy was adopted to characterize the HAs samples and HAs-Hg complexes.The FTIR spectra showed that HAs had similar structures but different in contents,and the adsorption of Hg²⁺ mainly acted on O-H,C-O and C=O groups of HAs.(6)Cross-polarization(CP)with magic-angle spinning(MAS)¹³C nuclear magnetic resonance(NMR)spectroscopy were adopted to characterize the HAs samples.The NMR spectra indicated that HA had higher Paraffin and carbonyl C content compared with corresponding FA,while FA exhibited higher methoxy C, O-alkyl C and carboxyl C contents. (7)Scanning electron microscopy(SEM)was jointly adopted to characterize the HAs samples and HAs-Hg complexes.The SEM images revealed the different surface structures of HA,FA and HAs-Hg complexes,and further explained Hg²⁺ complexation phenomenon on HAs.The structural changes on HAs' surface were caused by complexation.Hg²⁺mailly formed covalent complex or traped internal grid with HA,while the electrostatic adsorption was main form of Hg²⁺adsorbed on surface groups of FA.