Research On Mechanism Of Cu And Zn Migration In Bioreactor Landfill

Landfill site is a long-term and serious secondary pollution because of heavy metal.With respect to protect the environmental quality of the soil, the surface water, and theground water near landfill site, there is a need to evaluate the environmental behaviorof heavy metal in landfill and carry out conservation strategy accordingly. In thisstudy, Cu and Zn were chosen to evaluate the heavy metal behavior in landfill duringdecomposition. Firstly, considering the regional and seasonal variation of MSWcomponents, the characteristic of Cu and Zn distribution in MSW wascomprehensively studied. Based on the insufficient information of heavy metal totalcontent, sequential extraction procedure was also adapted to the heterogeneous MSWto assess the possible migration of Cu and Zn in it more pertinently. Subsequently,three simulated MSW reactor landfill with different operation modes were constructedbasing on the bioreactor landfill. Focusing on the inseparable system includingleachate and landfill refuse, the releasing behavior of Cu and Zn in bioreactor landfillduring decomposition was studied. The long-term leaching behavior of Cu and Zncontent from landfill with different operation modes were modeled. Moreover, themigration of Cu and Zn fractionations in MSW basing on modified BCR sequentialextraction procedure were also modeled. Finally, the phenomenon of in-situbio-immobilization of heavy metal in refuse from bioreactor landfill was presented..The main conclusion of this study was list below.(1)The Cu and Zn contents in MSW exceeded the set standard for "environmentalquality standard for soil" of China with different degree. The MSW has high potentialof heavy metal pollution, especially in spring and summer. The heavy metal source inMSW can be cut by orderly classification. Components including plastic, foodresidual, dust, and paper are the four main contributions for total heavy metal contentin MSW. The controlling of heavy metal pollution source in MSW depends on the cutof the four above MSW components. (2)When it used to MSW with different landfill age among 0-7 yr, the modifiedBCR sequential extraction procedure has high recovery and reproducibility. It is moreexactly to evaluate the environmental behavior of heavy metal in MSW, a veryheterogeneous matrix, at different degradation stage accordingly. Effect of samplepretreatment on speciation of Cu and Zn in different MSW samples illustrated that theshift of sample characteristics such as ammonia nitrogen (NH₄⁺-N), volatile fatty acid(VFA), sulfide were the main reasons. The fresh sample is the ideal pretreatmentmanner of sequential extraction for MSW, and air drying is the other better choice.(3)The contents of Cu and Zn in MSW from bottom layers are always higher thanthat from upper layers due to the migration of leachate. Zn has higher mobility thanCu in MSW. The different releasing behavior of Cu and Zn caused by the differenceon MSW decomposition led to the staggered variation of Cu and Zn contents in MSWaccordingly. The variation degree ranking were CL＞RL＞BL and Cu＞Zn. The Cuand Zn contents in MSW at acidification phase have the highest environmental risk.Cu and Zn in MSW showed behaviors of staggered migration and retention gradually,which corresponded with the degradation process of landfill obviously.(4) The variation on Cu²⁺ and Zn²⁺ concentrations in leachate reflected thereleasing behavior of Cu and Zn in landfill refuse at different stage accordingly. Therewere no significant differences on the concentration of heavy metal in leachate fromlandfills with different operation modes. The leaching amounts of heavy metals fromthe conventional reactor landfill fit for the positive exponential increase model, whilethat of bioreactor landfills fit for negative exponential decrease models. The operationof leachate recirculation can obviously reduce the heavy metal leaching amount fromlandfill into environment than conventional operation, and the introduction ofsequential phase acidification reactor can promote the immobilization of heavy metalin landfill refuse.(5)The leachate recirculation promoted the speciation of Cu and Zn shift fromunsteady fractions (acid soluble fraction and reducible fraction) to steady fractions(oxidable fraction and residual fraction), and attenuated their migration. Zn has higherpotential of migration and pollution than Cu in MSW. There are significant differences on Cu and Zn speciation in MSW from different landfill layers. Theaerobic exposure can promote the migration of Cu and Zn speciation from steadyfractions to unsteady fractions.(6)The regression models of Cu and Zn speciation indicated that characteristics, i.e.ammonia nitrogen, HA, FA, DOC, sulfide, sulfate, are the main factors of heavy metalspeciation variation.Especially, sulfide has significant positive correction and negativecorrection with unsteady fractions (acid soluble fraction and reducible fraction)andsteady fractions (oxidable fraction and residual fraction)of Cu and Zn in MSW,respectively. Simulation computation results from Visual MINTEQ showed that Cu²⁺was mainly associated with humus and migrated from lose association (FA₁-Cu) totight association (FA₂-Cu), while Zn²⁺ was mainly presented with complexation withS^2- which was strengthened as the reducing environmental surrounding in landfillgradually. The long-term leachate recirculation in bioreactor landfill can promote theprecipitation of Cu and Zn as complexation and attenuate their migration. Theenvironmental risk of leachate from landfill with different operation modes depend ontheir accumulation amount rather than their sudden concentrations and fraction.(7)The high SRB metabolism activity and the accumulation of metabolite (sulfide)were the main reasons of in-situ precipitation phenomenon of Cu²⁺ and Zn²⁺ inlandfill refuse in bioreactor landfill. Heavy metals in bioreactor landfill havemechanism of in-situ immobilization controlling by SRB metabolism. Thepredominant SRB in landfill refuse belongs to Desulfovibrio vulgaris sp. with theCu²⁺ and Zn²⁺ bio-precipitation of 1-10 ppm approximately.