Investigation On Soil Humic Acids Electron Transfer Coefficient And Their Functional Groups

Organic and inorganic pollutant problems become serious in soil environment, and the degradation of them influence the definition of soil environmental capacity. The processes of contaminants removal contain a series of physics, chemistry, biology, physical chemistry and biochemistry methods, in the most of which redox reactions are invovled. Humic substances(HS) are widespread in soil enviroment and have strong redox activities to affect the pollutants degradation in soil environmental capacity. HS constitute the main fraction of soil organic materials. They are high molecular weight polymer and composed of benzoquinonyl, carbonyl and carboxyl functional groups. HS could function as a recyclable electron shuttle between microorganisms and many inorganic as well as organic compounds, through which can accelerate the redox rates. Consequently, this property could have great impact(s) on the transformation of redox-active contaminants, and also contribute to the solution of some insoluble materials. Furthermore, they have significant effects on the biogeochemical circle.According to the International Humic Substances Society(IHSS) standard method extracted humic acid(HA), five chosen soil types were studied with different concentrations and characterized with functional groups. There are different forest mountains(Jiufeng, JF1~JF3 and Shangfang, SF1~SF3), woodland(Mangshan, MS1 ~ MS3), artificial wetland(Olympic Park, OW1 ~ OW3), natural wetland(Daoxianghu, DW1~DW2) and arable soil environments(Changping arable soil, CG1~CG3). HA extraction concentrations of different samples are much variable, however, functional groups species are almost same with C≡C and C=O.Soil HA significantly accelerated ferrihydrite reduction, and the reduction rates were plot against concentrations of HA to illustrate electron transfer properties of JFHA, SFHA, MSHA, OWHA, DWHA and CGHA. All of them match certain power equations y=a·xb, in which x shows the total organic carbon(TOC) concentration of soil HA(mg/L), y shows the acceleration rates(mmol/h), and a and b are constant with a certain range, a=0.02~0.05, b=0.3~0.6. The electron transfer coefficient, ket, is defined as the reduction rate with increasing unit additional HA and time. Same HA samples have distinct ket value with different TOC concentrations, and different HA samples have similar ket value with same TOC concentrations, which shows great similarity with different HA samples accelerate Shewanella oneidensis MR-1 reduce ferrihydrite. Finally, acceleration electron transfer coefficient in soil environmental capacity will be defined as: ket=a·b·TOC(b-1)(a=0.02~0.05, b=0.3~0.6), in which ket is a predictable in a certain situation. Taking ket value into account the soil environmental capacity will be more accurate and close to the natural environment.Furthermore, with the different TOC concentrations of JF1 HA, SF1 HA, MS1 HA, OW1 HA, DW1 HA and CG1 HA, the three-dimensional excitation-emission fluorescence matrix spectroscopy(3DEEM) plots show that their peak intensities in the range of visible humic-like range(350-440 nm Ex and 430-510 nm Em) have a positive linear relationship with their iron reduction rates or/and TOC concentrations. Moreover, their dominating functional groups all are quinonoid nâ†'Ï€*, this could illustrate HA electron transfer capacity are close correlate with quinonoid group.According to the three-dimensional character of p-benzoquinone, the different TOC concentrations of JF1 HA, SF1 HA, MS1 HA, OW1 HA, DW1 HA and CG1 HA are variable with their main functional groups, which indicate that the molecular structures of soil HA have changed with varying TOC concentrations. Each 3DEEM plot revealed a fluorescence peak with different excitation/emission wavelengths. With the increasing of concentration, the fluorescence peak had a red shift and variable main functional groups. Meanwhile, the benzenoid Ï€â†'Ï€* represented as the main functional group with the concentrations of 2-10 mg C/L of JF1 HA, SF1 HA and MS1 HA, and the main functional group with the rest TOC contents range was showed quinonoid nâ†'Ï€*. However, benzenoid Ï€â†'Ï€* and quinonoid nâ†'Ï€* were altered for wetland and arable soils.