Effects of the Use of Permeable Barrier for Landfill Leachate Treatment

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

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

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

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

silica sand

40

60

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

time/d

0 50 100 150 200 250 300 350

Fig.3 - Changes in the concentration of ammonium in the effluent of different reactors

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+, Ca2+ 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 Ca2+ The H+ ion replaces the positive ion K+, Na+, Ca2+ 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 Fe2+ and Fe3+ 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

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

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