Study Of Landfill Leachate Redox Zones And Degradation Mechanisms Of Pollutants In The Subsurface Environment

In recent years, soil and groundwater pollution caused by random dumpingor poor management of municipal solid waste has become global environmentproblem. Transportation, transformation and attenuation of landfill leachatepollutants such as organics, heavy metals in underground environment couldchange soil environment and groundwater quality;this poses a serious threat toour agriculture production and drinking water supplying. Therefore, it isnecessary to further understand natural attenuation process and mechanisms oflandfill leachate pollutants in underground environment.Landfill leachate is rich in nitrogen and organics;if they infiltrate into soil,it could lead to increase in nutrient content of soil and enhance activity ofmicroboes. Microbes utilize different terminal electron acceptors such as O2,NO3-, iron and manganese oxides, SO42-, CO2 etc. and degrade contaminantsthrough different metabolizing pathways, namely aerobic respiration, nitratereduction, iron and manganese reduction, sulfate reduction and methanogenesisetc. These biogeochemical processes could control the behavior of organics andinorganics through change of redox condition, pH and alkalinity of subsurfaceenvironment. In a word, biogeochemical processes occurring in the subsurfaceenvironment plays an important role in degradation of landfill leachatepollutants.According to the foreign and domestic research situation, a seriesexperiments were conducted to study transportation and transformation rules andattenuation mechanisms of landfill leachate pollutants, formation, developmentand dynamic evolution process of different redox environments in leachateplume, distribution rules and characteristics of different contaminants and redoxsensitive compounds in different redox environments, mechanisms anddegradation efficiencies of biogeochemical processes of different pollutants indifferent redox environment. This was done in order to comprehensively analyzeand evaluate variation of redox buffering capacity of sediments in different redoxenvironments;and to establish regression equations and distribution models ofpollutants and redox sensitive compounds varying in different environment.The innovations of this work are: (1) A strategy of separation andenhancement was used, and the continuous redox zones in nature were dividedinto several individual redox zones and enhanced depending on theircharacteristics. Two operational modes, redox zones operated in parallel and inseries, were used to study degradiation mechanisms and efficiencies of pollutantsin different sequential redox zones. (2) Other studies have established overalldegradiation models for pollutants in subsurface. This work establisheddegradiation models of pollutants and variation models of redox sensitivecompounds in different redox zones.Results of experiment 1 indicated that: (1) Attenuation rate of TOC has adirect relationship with biomass, that is increasing with increase of biomass. Inthe initial growth phase, the rate of biomass increase is high, reaching andaverage of 1.7×105/d and an average decrease of 6.4×103/d because of change inenvironmental conditions and inhibition of reaction products over time in thesubsequent phases. Therefore, growth rate of microbes had great impact on TOCattenuation. (2) Attenuation rate of NH4+-N had no clear relation with microbes'growth in short term where the attenuation of NH4+-N mainly depended onbiological processes. (3) Migration rate of the TOC is higher than that of NH4+-Nin subsurface environment;migration rates were about 3.0cm/d and 2.4 cm/d forTOC and NH4+-N respectively.Although some conclusions on the migration and attenuation rules of TOCand NH4+-N and contribution of microbes to the degradation of pollutants havebeen drawn, the mechanism of contaminant degradation in subsurfaceenvironment have not been well understood. Based on the theory ofthermodynamics and biogeochemistry, microorganisms can utilize differentterminal electron acceptors to degrade contaminants through differentmetabolizing pathways in subsurface environment. In addition, the amount of theenergy released in these pathways was different. Based on these theories, a seriesof redox gradient or different redox environment, viz. sequential redox zones areexpected to appear in pollution plume. Was it true? It was confirmed from theresults of experiment 2;that different redox zones appear in pollution plumeduring the course of the degradation of pollutants in subsurface environment.Four sequent redox zones appeared namely methanogenic zone/sulfate reductionzone, iron reduction zone, nitrate reduction zone and oxygen reduction zonerespectively;redox zones were not clearly resolved out, and there was overlapzone or transitional zone between neighboring redox zones. This conclusion issignificant to the control and remediation of landfill leachate polluted sites.In order to evaluate the change of physical and chemical characteristics ofleachate polluted soil and biogeochemical process in pollution plume, a conceptof redox buffering capacity which was evaluated quantitatively by twoparameters, viz. oxidation capacity (OXC) and reduction capacity (RDC) wasused. Results indicated that (1) in methanogenic zone/sulfate reduction zone, ironreduction zone, nitrate reduction zone and oxygen reduction zone, OXC ofsediments increased gradually while the RDC of sediments decreased gradually.(2) Iron played an important role on pollutants degradation in underground.Content of total iron, Fe3+ and Fe2+ was different in different redox zones. Thecontent of total iron in methanogenic zone/sulfate reduction zone sediments waslower and Fe2+ was significantly higher than that in unpolluted zone. In ironreduction zone, nitrate reduction zone and oxygen reduction zone, content oftotal iron was lower slightly than that in unpolluted soil, in addition, content ofFe3+ increased while that of Fe2+ decreased. (3) Fe3+ was an importantcomponent of OXC;it accounted for 70.5% of OXC in unpolluted soil;Fe3+ wasmajor electron acceptor in sediments, it could play an important role in thedegradation of pollutants. TOC was a major component of RDC accounted for98.7% of RDC in unpolluted soil, and was a major electron donor in sediments.(4) In methanogenic zone/sulfate reduction zone, iron reduction zone, the contentof ion changeable Fe2+ increased from 0.5% of unpolluted soil to 3% and 1.84%respectively because of Fe2+ precipitation. Fe2+ mainly precipitated as FeCO3 andFeS which account for 555.1%and 377.7% of total Fe2+ contained in unpollutedsoil respectively. Most of amorphous and part of crystal Fe3+ were reduced. Withenhancing of oxidation, the content of Fe3+ increased while Fe2+ decreased.Therefore , the study of redox buffering capacity was significant forcomprehensive evaluation of self-cleaning ability and natural attenuation ofsubsurface environment.Results of experiment 2 demonstrated that the sequential redox zones ofpollution plume had great impact on subsurface environment. What aredegradation mechanisms and efficiency of different contaminants in sequentialredox zones was not clear yet. There were no clear boundaries between redoxzones, which made it very difficult to study and come up with a conclusiveanswer to this question. Therefore, a strategy of separation and enhancement wasused in experiment 3, continuous redox zones in nature were divided into severalindividual redox zones, and enhanced depending on their characteristics;twooperational modes that are redox zones connected in parallel and in series wereused and studied accordingly. Experimental results showed that:(1) Parallel operational mode ① naphthalene, cyclohexanone and part ofcyclo-alkyl hydrocarbon etc. were difficult to biodegrade, and their content washigh in 5 redox zones. However, benzoxazole, antipyrine, indene etc. were notfound in sulfate reduction zone;they could be found in oxygen reduction zoneand nitrate reduction zone. Therefore, anaerobic reducing environment couldhave played an important role on degradation of these contaminants. ② Ironreduction zone may be important for the degradation of organics;thedegradation rate was as high as 86.24% on average. The reduction zonesfor nitrate, sulfate and methanogenic degradation rate were on average75.93%, 79.81%, 74.02% and 65.09% respectively. Maxium degradationefficiency of pollutants was different in different redox zones, for instance, thehighest degradation efficiencies of 1, 1, 1-trichloroethane and 1,2-dimethylbenzene were in sulfate reduction zone, and were 80% and 71.43%respectively. The highest degradation efficiencies of TCE and MTBE were innitrate reduction zone, and reached 81.25% and 92.58% respectively;Benzenewas degraded completely in sulfate reduction zone and iron reduction zone. ③Removal efficiency of different heavy metals was different in different redoxzones. The highest removal efficiency of Cr, Ni, Zn, As, Cd and Pb in sulfatereduction zone was reached 93.27%, 73.33%, 81.52%, 76.83%, 100% and100% respectively, and the lowest removal efficiency in oxygen reduction zonewas 36.13%, 20.45%, 7.45%, 25.86%, 13.04% and 37.36% respectively.Heavy metals deposited in sediments of different redox zones were different;heavy metal content of sediments in methanogenic zone, sulfate reduction zoneand nitrate reduction zone were high, and the highest content was in sulfatereduction zone;the content of Cr, Ni, Zn, As, Cd and Pb increased 8.5%, 11.3%,19.9%, 75.6%, 14.8% and 14.3% respectively. Heavy metals contained insediments of iron reduction zone, sulfate reduction zone and oxygen reductionzone were low relatively, and the content of Cr, Ni, Zn, As, Cd and Pb decreasedto 1.86%, 8.6%, 22.1%, 26.4%, 5.8% and 25.6% respectively.(2) In the series operational mode, the removal efficiency of1,3-dimethylbenzene, 1,4-dimethylbenzene, cymene, 1,1,1-trichloroethane,1,1,1-trichloroethane, MTBE,trichloromethane etc. was relatively high in ironreduction zone or oxygen reduction zone, ethylbenzene and 1,2-dichloroethanewas also high in nitrate reduction zone and it was as high as 60.38% and57.65 % respectively. The relative removal efficiency of organics inmethanogenic, sulfate reduction, iron reduction, nitrate reduction and oxygenreduction zones were as high as 26.56%, 22.7%, 38.41%, 35.36% and 39.7%respectively. Results of the heavy metals were similar to those of paralleloperational mode and needn't discuss in detail here.(3) Different reducing microbes growing in their corresponding redox zonesunderwent four phases, namely adaptation phase, logarithm growth stage,stationary phase and decline phase. The length of their growth time in differentredox environments mainly depended on the amount and availability of suppliedterminal electron acceptors and electron donors, which led to different redoxenvironment performing different contribution to natural attenuation (intrinsicremediation) of contaminants differently in natural state.(4) The appearance of peak concentration times for different reductionproductions was related to the ability of terminal electron acceptors to competefor terminal electrons. The most competitive ones showed sharp concentrationincrease earlier for example, the peak concentration of NO2-appeared earlier thanthat of Fe2+;degradation rate of contaminants by different microbes mainlydepended on the amount and quantity of terminal electron acceptors. Thereaction rate was related to ability of terminal electron acceptors to compete forterminal electron, that is the stronger the ability of terminal electron acceptorsto compete for terminal electrons, the easier could be used and consumed bymicroorganisms and the more sensitive to reflect on environmental pollution.(5) From the intrinsic remediation point of view, all kinds of iron species inaquifer could supply enough terminal electrons for biogeochemical processes.Therefore, iron reduction zone was significant to contaminants degradation andintrinsic remediation of pollution site, but the reaction rate was slower than thatof nitrate or oxygen used as terminal electron acceptors. From the point of viewof enhanced in situ bioremediation, biogeochemical degradation rate ofcontaminants could be accelerated through addition of terminal electronacceptors such as nitrate, oxygen. Pollution sites could be remediated throughstimulating growth of indigenous microorganisms and enhancing biodegradationprocesses.Mathematic regression model and 2-dimension physical model ofcontaminants and redox sensitive compounds were established depending on theresults of experiment 3 and experiment 4 respectively.Finally, based on results of experiment 2 and experiment 4 andcomprehensive analysis of dynamitic evolution of redox zones in pollutionplume, following conclusions could be drawn: (1) During the course of thedynamitic development and evolvement of redox zones, with time elapsing andpollution degree aggravating, redox zones moved ahead slowly. Methanogeniczone/sulfate reduction zone extended gradually, and then inert zone appearedwith further pollution. (2) During the full course of the development andevolution of redox zones, concentration of TOC, sulfide, NH4+-N, HCO3-, CO2and Fe2+ increased with time, and decreased with distance;concentration of NO3-while the DO decreased with time and increased with distance;SO42-concentration decreased first and then increased over time;but decreased withdistance. In conclusion, content of reducing compounds were higher in anaerobicreducing environment and content of oxidizing compounds was high in aerobicoxidizing environment. (3) In general, pollution degree aggravates over time,pollution plume environment changed from oxidizing to reducing,correspondingly, redox zones transformed from oxygen reduction zone tomethanogenic zone, and methanogenic zone and part of iron reduction zoneclosed to the fringe of methanogenic zone evolved to inert environment whichhave no redox buffering capacity and self-cleaning ability for environment.