Migration And Transformation Of Cr(?) And Ammonia-nitrogen From Tannery Sludge And The Corresponding Microbial Response Mechanisms In The Vadose Zone

Due to the expensive treatment costs and the lack of landfill sites,illegal disposing of tannery sludge above the soil around the tanneries has been becoming a common problem and causes serious environmental pollution in China.In this study,an investigation was made into the effects of tannery sludge on soil chemical properties and microbial community in a typical soil profile with long-term tannery sludge contamination.Indoor soil culture experiments,soil leaching column experiments,microbial fluorescence quantitative PCR and high-throughput sequencing testing methods were conducted to explore the releasing mechanism of Cr(?)and NH4+-N from tannery sludge,and reveal the influence of environment factors on the form and valence changes of Cr(?)and the mechanism of microbial function on NH4+-N transformation.The aim of this study was to explore the migration and transformation of Cr(?)and NH4+-N from tannery sludge and the corresponding microbial response mechanisms in vadose zone.The main results and conclusions as follows.The shallow soils were seriously contaminated due to the high concentrations of salinity,dissolved organic matter(DOM),Cr(?)and NH4+-N released from tannery sludge.A good correlation was observed between total organic carbon(TOC)and Cr(?)content in the soil profile,indicating that the small molecular DOM from sludge leachate promoted the vertical mobility of Cr(?)via forming Cr(?)-organic complexes.The distribution characteristics of microbial community were related to the soil contamination level owing to the different adaptability of microorganisms to contaminants.The halotolerant bacteria and Tepidimicrobium)and organic degrading bacteria(Flavobacteriaceae and Tissierella)became the dominant microflora in the soil profile.The release process of Cr(?)and NH4+-N from tannery sludge were affected by the solid-liquid ratio,temperature,and pH value of the solution.A larger solid-liquid ratio,a higher temperature,and a low pH value of the solution were beneficial for the Cr(?)and NH4+-N release from tannery sludge to solution.The release amount of NH4+-N reached to 1.69 mg/g under the leaching condition(pH 7),while the release amount of Cr(?)was only 4.42 ?g/g.The release process of Cr(?)and NH4+-N from tannery sludge was controlled by the adsorption and desorption reactions.The release of NH4+-N was mainly controlled by the diffusion effect in solid-liquid phase,and the release rate of Cr(?)was enhanced with the decrease of pH value in solution.The aging rate and aging degree of Cr(?)were decreased in the soil under the condition of a higher contamination load,higher soil moisture and larger particle size.The iron oxide and humic acid can effectively decrease the content of bioavailable Cr(?)in the contaminated soil.The iron oxide promoted the transformation of Cr(?)to the reduction state and residual state,while the humic acid promoted the transformation of Cr(?)to the oxidation state.The dissolved Cr(?)can be quickly oxidized by the ?-MnO2 in soil,causing the content of bioavailable Cr increased.The oxidation process of Cr(?)was well fitted by first order kinetics and could be divided into two sections.The adsorption process of Cr(?)to ?-MnO2 and react with active sites are the key process which affects the oxidation rate of Cr(?).Fe or A1 coated ?-MnO2 can inhibit the Cr(?)oxidation process by influencing the contact reaction between Cr(?)and reactive sites in ?-MnO2.The nitrification was promoted under the lower contamination load,finer particle size and oxidizing environment.The abundance changes of amoA and nxrA functional genes can be used to explain the functions of microbe on NH4+-N transformation.The silt had a stronger nitrification due to owing 2 times larger amounts of amoA and nxrA genes than fine sand.The amounts of amoA and nxrA genes in aerobic environment are 2.7 times and 2.2 times larger than that in an anaerobic environment.The degree of inhibition order on the transformation of NH4+N by the tannery contaminants in soils was Cr(?)>salinity>DOM.The existence of Cr(?)in soil has an obvious inhibition on the nitrification process.The abundance of main nitrification bacteria(Alcanivoraxt Bacteria,Arenimonas,Nitrosomonas and Nitrosospira)had a negative relationship with Cr(?)content.The Cr(?)controlled the NH4+-N transformation by inhibiting the activity of related nitrifying bacteria.The released Cr(?)from tannery sludge was easily immobilized in shallow soils due to the adsorption and precipitation,while high content of NH4+-N could flow through the 40 cm silt and fine sand column.The nitrification occurred with the decrease of salinity and DOM concentration under the leaching condition.Compared to the results of field pollution site,the microbial community structure was more diverse and the abundance of nitrifying bacteria increased obviously in the columns with low pollution load.However,the organic degrading bacteria were still the dominant microflora,and the abundance of autotrophic nitrifying bacteria was low due to the competition of heterotrophic bacteria.