| Landfills are the predominant form of treatment of municipal solid waste (MSW) in the vast majority of the available case studies, because it is cost-effective and can accommodate different types as well as large fluctuations in the amount of waste. However, it is still a hot topic that how to find a feasible way to solve the secondary pollution of nitrogen during the process of municipal solid waste treatment by landfill. In order to protect the soil, ground water and suface water environment around landfill, achieve the purpose of in situ nitrogen removal and reduce the cost and degree of difficulty for landfill leachate treatment, theoretical and technical problems on in situ nitrogen removal must be resolved as soon as possible. Hereupon, iron, the most abundant element with variable valence in the landfill and leachate, was chosen to carry out the following research. First of all, in view of environmental behavior of iron can not be assessed accurately only by the total content of iron, the extraction conditions for extractable iron speciation (ferric and ferrous) in MSW by hydrochloric acid and acid ammonium oxalate solution (Tamm's reagent) were investigated and optimized. Subsequently, the redox state of iron in municipal solid waste (MSW) with different deposit ages was evaluated. Secondly, two simulated MSW landfill with different operational modes were constructed based on the model of bioreactor landfill, the leaching behavior for the total content of iron and variation of iron in different oxidation state was investigated, and the correlation between particulate bound iron and Fe(Ⅲ) in the leachates of the simulated landfills was obtained. Finally, according to the nitrogen removal efficiency and the variation of iron in different oxidation state in batch experiment, the potential effect and mechanism of iron redox on nitrogen removal in the landfill was presented. The main experiments and conclusions are as follows:(1) According to the orthogonal experiments results, the optimal extraction conditions for HC1 were determined as follows:the liquid-to-solid ratio was set at 100, and then the samples were extracted at the shaking speed of 200 rpm at 35℃for 60 min by 1mol.L-1 HC1. For Tamm's reagent, the optimal extraction conditions were extracted at the shaking speed of 175 rpm at 30℃for 12 h with the same liquid-to-solid ratio. Under the optimal extraction conditions, both extraction reagents perform very well to evaluate the redox state of iron in MSW with different deposit ages.(2) MSW samples with different deposit ages exhibited decidedly different basic physicochemical characteristics, and moisture content, organic matter content determined as LOI and BDM decreased with the increasing of deposit age. After the burial of 2 years, the organic matter content and BDM decreased from 72.40±1.28% and 54.90±0.17% to 15.20±2.93% and 3.34±0.74%, respectively. The results also showed that the yield of extractable iron increased with deposited age. In the fresh refuse, no significant difference of extractable ferrous iron was found extracted with both HC1 and Tamm's reagent, while statistically significant differences were found for the yield extractable ferrous iron extracted by the two extractants in the other four aged refuse samples (SI to S8).(3) The results depicted that there was large amount of variability for the iron contents in different MSW. Both the contents of iron in the brick and dust were higer than 30000 mg kg-1, which was significantly higher than that in the fractions of food and fruit waste, papers and residue (10000-30000 mg kg-1) and cellulose textile, wood and plastics (<10000 mg kg-1). The food and fruit waste contributed a large amount of iron (45.5%) due to their high proportion in the MSW on dry weight basis.(4) The leaching behavior and accumulated amounts of iron leached out of the refuse by leachate from CL and RL exhibited decidedly different trends except for the initial stage. After 200 days operation, the accumulated amounts of iron leached out from CL increased with the passage of time (from 3710 to 7270 mg), while the iron leached out from RL decresed sharply from 3320 mg to 991 mg. The percentage of iron leached from CL and RL accounted 1.00% and 0.14% for the total amount in landfills, respectively. (5) No significant correlations were found between iron leaching and basic physicochemical characteristics of leachate, such as chemical oxygen demand (CODCr), dissolved organic carbon (DOC), volatile fatty acid (VFA), pH, SO42- and S2- concentrations in the leachate of the simulated landfills. Variation of ferrous and ferric iron percentage in the leachate of RL and CL exhibited similar trends during the whole operating period, ferrous iron percentages in the leachate of RL and CL increased with the passage of time. During the succeeding 200 days, the ferrous iron percentages in the leachate of RL and CL kept within the range of 63.0-92.7% with an average of 79.7% and 71.3-98.8% with an average of 89.4%, respectively. Significant positive correlations between particulate bound iron and ferric iron were found in the leachates from RL (R2=0.748) and CL (R2=0.833).(6) With the addition of organic carbon, nitrate nitrogen, ferrous iron and aged refuse, repeated anaerobic of microbial redox cycling of iron concomitant with nitrate reduction was found. During this process, nitrate was reduced to nitrogen gas and ammonium nitrogen. Without addition of organic carbon, only anaerobic ferrous iron oxidation nitrate reduction was found, redox cycling of iron did not happen and iron was always presented as ferric iron in the system.
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