Fractionations And Plant Availability Of Germanium In Soil

Germanium(Ge)is classified as“critical raw material”and predominantly used in diverse fields such as microelectronics,fiber optics,new energy,infrared detectors and biomedicine.Organic germanium derivatives(particularly Ge-132)can enhance the production of interferon and can be used in the treatment of malignant tumors and AIDS.Low concentrations of Ge in soil have been proved to improve crop quality,yield,and salt stress tolerance,while high levels of soil Ge exhibits toxicity in the environment.Ge phytomining and Ge-rich agriculture have contributed to a growing interest in the plant availability of germanium in geochemical studies.However,our understanding of the plant availability of germanium in soil is still limited and has been evaluated mainly through the total germanium concentration in soil.As with any other element present in soils,the plant availability of Ge rather than its total concentrations is considered to be the key factor controlling the uptake and accumulation of this element in plants.Different Ge fractionations in soil have different geochemical properties and environmental behaviors.Studying the geochemical behaviors of Ge from a perspective of its fractions in soil is essential for understanding the migration and transformation,plant availability and Ge cycling in the soil.From a standpoint of environmental geochemistry,a four-step sequential extraction method was first set up in this dissertation,and soil aging experiment and complexation experiment were then carried out to explore the effect of organic matter on germanium in soil.The concentration of germanium fractionations in soil from high germanium area of Zhejiang Province was analyzed to explores the distribution,migration,transformation,and plant availability of Ge in soil.The main results are achieved as follows:1.A four-step sequential extraction method of soil germanium and the extraction method of soil available germanium were established.According to the geochemical properties and environmental ecological significance of germanium,the soil germanium were divided into four operationally defined fractionations,namely mobile Ge(F1),organic matter bound Ge(F2),non-crystalline components bound Ge(F3)and residual Ge(F4),and the extraction methods were optimized and verified.Based on the DGT technology,the iron membrane was used as the adsorption membrane and the1 mol/L HNO₃ solution was used as eluent.The extraction and determination methods of plant available germanium were established,and the diffusion coefficient of germanium of the diffusion membrane(polyacrylamide gel)was also determined.2.The effects of organic matters in soil on germanium activity were explored.Based on the soil aging experiment,it was found that exogenous germanium could transform rapidly into organic matter bound Ge,non-crystalline components bound Ge and residual Ge.Under the environment of low organic matter content,Ge was dominantly adsorbed by amorphous iron oxide,while Ge mainly bonded with organic matters under a high organic matter condition.The binding of Ge to organic matter may be affected by the redox conditions.The conditional stability constant of humic acid(HA)-Ge complex was larger than that of fulvic acid(FA)-Ge complex.Humic substances played a dual role in affecting the behavior of dissolved Ge in soil.HA formed insoluble and stable complexes with Ge and hence tended to sequester Ge and limit its activity,while FA formed soluble and unstable complexes with Ge and tended to act as a Ge carrier and improved the activity and migration ability of Ge in soil-plant system.3.The distribution characteristics,transformation mechanism and plant availability of germanium in Ge-rich soils of Zhejiang Province were studied.The soil samples of four typical Ge-rich of Zhejiang Province are acidic with rich organic matter.The soil total Ge concentrations of typical Ge-rich areas were in the following order:1.86 mg/kg(Pujiang)>1.77 mg/kg(Longyou)>1.65 mg/kg(Jiangshan)>1.49 mg/kg(Changxing).The soil total Ge contents in Changxing and Jiangshan were mainly affected by soil physiochemical properties,while those of Longyou and Pujiang were mainly affected by the parent material.The soil Ge fraction distribution characteristics were similar in the four high Ge areas,which were residual Ge(91.85%)>non-crystalline components bound Ge(3.83%)>mobile Ge(2.69%)>organic matter bound Ge(1.63%).The residual Ge was the dominant Ge fraction in soil(more than90%),and the proportion of unstable Ge(mobile Ge,organic matter bound Ge and non-crystalline components bound Ge)was less than 10%of the total Ge content in soil.In the process of soil weathering,the proportion of unstable Ge gradually increased with the loss of total Ge in soil.p H and Eh values of soil may be the key factors affecting the dynamic balance of unstable Ge in soil.Organic matter bound Ge in the soil had high plant availability and can promote the migration and accumulation of Ge from soil to plant,which may also be the main source of Ge in soil solution.Compared with the traditional chemical extraction method,the DGT method can better characterize the plant availability of germanium in soil.