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ABSTRACT Three reaction media (zero valent iron, activated carbon and modified bentonite) were used to design four kinds of permeable reactive barriers (PRB) and study the feasibility and the efficiency of the PRB technology in the remediation of landfill leachate. The results indicated that PRB is highly effective for the treatment of landfill leachate. The ammonium removal rates in the two reaction media with zero valent iron and modified bentonite was 97.58% at maximum, the removal rate of COD in reactors with zero valent iron and activated carbon was 84.87% at maximum, and the removal rate of total phosphorus in PRB reactors was 80% at maximum
Journal of Water and Environment Technology, Vol. 9, No.2, 2011 Address correspondence to Wen-xin Xu, Guilin Research Institute of Geology for Mineral Resources, Email: kdy_tws@163.com, Received September 25 2010, Accepted December 25, 2010. - 209 - Effects of the Use of Permeable Barrier for Landfill Leachate Treatment Jing-jing LIU*, Nan-shi ZENG**, Wen-xin XU* *Guilin Research Institute of Geology for Mineral Resources, Guilin 541004, China **Guilin University of Technology, Guilin 541004, China ABSTRACT Three reaction media (zero valent iron, activated carbon and modified bentonite) were used to design four kinds of permeable reactive barriers (PRB) and study the feasibility and the efficiency of the PRB technology in the remediation of landfill leachate. The results indicated that PRB is highly effective for the treatment of landfill leachate. The ammonium removal rates in the two reaction media with zero valent iron and modified bentonite was 97.58% at maximum, the removal rate of COD in reactors with zero valent iron and activated carbon was 84.87% at maximum, and the removal rate of total phosphorus in PRB reactors was 80% at maximum. Keywords: landfill leachate, modified bentonite, PRB INTRODUCTION Permeable Reactive Barrier (PRB) is a new method used for the removal of pollutants in underground water and soil in the western part of the world at present. Now, it is in experimental stage in China. Compared with traditional methods, PRB has many advantages, such as continued removal of pollutants, wiping off various pollutants, high effectivity, convenient construction, high cost-performance ratio and a lot more. MATERIALS AND METHODS Experimental Device The four glass reactors (length is 100 cm and inner diameter is 3.5 cm), used were labeled I, II, III and IV, respectively. Figs. 1, 2 are the schematic diagrams of the reactors. Water inlet and water oulet were stuffed with polyethylene nets to avoid the overflow of silica sand. Sampling was done in the water outlet. water inlet water outlet Fig.1 - Schematic diagram of reactors I, II and III water inlet water outlet Fig.2 - Schematic diagram of reactor IV Journal of Water and Environment Technology, Vol. 9, No.2, 2011 - 210 - Experiment Materials The landfill leachate used in this study was obtained from Guilin Chongkou landfill treatment plant. It was diluted five times during the experiments and the pH was maintained in the range of 7.5 - 8.0. Table 1 shows the properties of landfill leachate. Zero valent iron, and calcareous stone were both passed through a 100 mesh screen, modified bentonite was passed through a 120 mesh screen, while silica sand and activated carbon were both passed through a 60 - 100 sized mesh screen. Experimental Methods The water used in the experiment was the landfill leachate from Guilin Chongkou landfill treatment plant. Influent speed was controlled between 50 and 100 cm·d -1 and sampling was done at 9 am after devices in motion. The four reactors were in synchronized operation. Table 2 shows the composition of reaction media in the reactors. Table 1 - Properties of landfill leachate (20%) Table 2 - Composition of reaction media in the reactors Parameter COD BOD 5 NH 4 + TP Concentration (mg/L) 304 146 82.9 1.15 Reactor Components Composition (%) I zero valent iron modified bentonite silica sand 40 20 40 II zero valent iron activated carbon silica sand 40 20 40 III zero valent iron modified bentonite silica sand 60 20 20 IV zero valent iron silica sand 40 60 Journal of Water and Environment Technology, Vol. 9, No.2, 2011 - 211 - RESULTS AND DISCUSSION Reactor Efficiency for Ammonium Removal Fig. 3 illustrates the efficiency of the reactors for the removal of ammonium The efficiency of the reactors in removing ammonium is clearly seen in the graph. The most efficient in ammonium removal is reactor III and the maximum removal rate is 97.85%. In reactor I, the maximum removal rate is 85.76%. The least efficient in ammonium removal is reactor IV, and the maximum removal rate is 51.75%. The principles of removal involved in the experiment are ion exchange and adsorption. There are certain adsorption for ammonium in zero valent iron, activated carbon and modified bentonite. The reactors I and III which contain some modified bentonite have high removal efficiency. Compared with others, modified bentonite and activated carbon have special capacities for ion exchange and adsorption. Furthermore, there are some magnesium ions and phosphate ions in landfill leachate, which can generate ammonium magnesium phosphate with ammonium. Reactor Efficiency for COD Removal Fig. 4 illustrates the efficiency of the rectors for the removal of COD. 0 10 20 30 40 50 60 70 80 90 0123456789101112 time/d ammonium /mg/L I II III IV 0 50 100 150 200 250 300 350 0123456789101112 time/d COD/mg/L I II III IV Fig. 4 - Changes of COD in effluent of different reactors Fig.3 - Changes in the concentration of ammonium in the effluent of different reactors COD mg/L ammonium mg/L Journal of Water and Environment Technology, Vol. 9, No.2, 2011 - 212 - The efficiencies obtained from different reactors for COD removal indicate that, reactor II, is the most efficient with a maximum removal rate of 84.87%. This is followed by reactor III with a maximum removal rate of 67.10%. The removal rate in reactor II is lower than that in reactor III. The least efficient for COD removal is reactor IV with a maximum removal rate of 60.86%. These results could be due to the following reasons: (1) Deoxidation of zero valent iron. All reactors have certain zero valent iron, which can generate deoxidation by corrosion for pollutants. When iron is in water that contains electrolytes, tiny carbon particles act as cathode and iron acts as anode, which form countless microcells, then the iron could be reduced. The reaction can reduce the toxicity of pollutants, promote growth of microorganism in reactors especially biological degradation for pollutants, then, lower the COD in the water outlet. Iron oxide hydrated, which engender corrosion, has strong activity for adsorption-flocculation, which can adsorb organic molecules and decrease pollutants in water outlet. (2) Adsorption of activated carbon It is indicated from Fig. 4 that reactors with zero valent iron and activated carbon are the most efficient.The main reason may be the abundant acidic groups or alkaline groups existing in the surface of activated carbon. The groups not only have adsorption capacity, but also have catalytic action for oxidation-reduction reactions. (3) Ion exchange and adsorption of modified bentonite The efficiencies obtained in using reactor I and III are higher than reactor IV that only contain zero valent iron, which indicates ion exchange and adsorption of modified bentonite. Exchangeable positive ion K + , Na + , Ca 2+ in modified bentonite can exchange with H + acid solution to a certain extent, because the radius of H + is less than the radius of K + , Na + and Ca 2+ . The H + ion replaces the positive ion K + , Na + , Ca 2+ hole volume is enlarged, force of chemical bond between layers is weakened, so the hole can be dredged. Impurities in the structural channel of modified bentonite can also be removed at the same time in the acidification process. Therefore, modified bentonite has features for great specific surface, great volume for ion exchange, fine adsorption, and special adsorption for COD. The use of reactor IV gives similar removal efficiency as reactor I. After 9 days, results indicated that modified bentonite was easily saturated and disabled as a reaction medium. Reactor Efficiency for Total Phosphorus (TP) Removal Fig. 5 illustrates the efficiency of the reactors for the removal of TP. The highest efficiency for the removal of TP was obtained upon using reactor II, and the maximum removal rate is 80%. The least efficient for TP removal is reactor IV with a maximum removal rate of 46.96%. These indicatethat the efficiencies for TP removal vary in different reactors due to the adsorption of zero valent iron, activated carbon and modified bentonite. The Fe 2+ and Fe 3+ ions generated from iron electrolysis are fine flocculants. Removal efficiency of the reactor that contains activated carbon or modified bentonite is better than that of the reactor that only contains zero valent iron. Journal of Water and Environment Technology, Vol. 9, No.2, 2011 - 213 - CONCLUSIONS This study indicates that when the reaction medium ratio of zero valent iron to modified bentonite to silica sand is 60:20:20, the highest efficiency for ammonium removal can be obtainedwith a removal rate of 97.58%. When the reaction medium ratio of zero valent iron to activated carbon to silica sand is 40:20:40, the highest efficiency for COD and TP removal can be acquired at a rate of 84.87% and 80% for COD and TP, respectively. The experiment indicated that the use of PRB is highly effective for landfill leachate treatment. REFERENCES Gao W., He S., Feng Y and Li H. (2005). The application of bentonite in the disposal of heavy metal waste water, Mining Engineering, 3(3), 52-54. Lan J. and Wang Y. (2002). 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EPA-542-r-01-004. 0 0.2 0.4 0.6 0.8 1 1.2 0123456789101112 time/ d TP/mg/L I II III IV Fig. 5 - Changes in the concentration of total phosphorous in the effluent of different reactors Journal of Water and Environment Technology, Vol. 9, No.2, 2011 - 214 - Zhao M., Shi H. and Xu G. (2002). Study on the pretreatment of p-fluoronitrobenzene wastewater by microelectrolysis, Environmental Protection of Chemical Industry, 22(1), 15-18. Zhou Q. and Lin H. (2001). Study on permeable reactive barrier for the remediation of contaminated soils and groundwater, Techniques and Equipment for Environmental Pollution Control, 10(2), 48-53. . COD. 0 10 20 30 40 50 60 70 80 90 01234567 891 01112 time/d ammonium /mg/L I II III IV 0 50 100 150 200 250 300 350 01234567 891 01112 time/d COD/mg/L I II III. kdy_tws@163.com, Received September 25 2010, Accepted December 25, 2010. - 2 09 - Effects of the Use of Permeable Barrier for Landfill Leachate Treatment