Simultaneous removal of organic matter and nitrate from bio-treated leachate via iron–carbon internal micro-electrolysis

Abstract

The removal of nitrate and organic matter from landfill leachate has been an arduously difficult challenge in recent years. Iron–carbon internal micro-electrolysis combined with ammonia stripping has been employed for the simultaneous removal of nitrate and organic matter from the biochemically treated landfill leachate. Compared with synthetic wastewater, nitrate removal from practical leachate is much more difficult, with a good removal efficiency only at low pH value. The removal efficiency of organic matter would quickly reach the maximum in 20 minutes, but in 100 minutes for that of nitrate. The higher the initial pH was, the more ammonia could be removed. pH = 11 is a reasonable choice based on total cost and nitrate removal efficiency. The mechanisms of nitrate and organic matter removal by iron–carbon internal micro-electrolysis were investigated. Nitrate reduction to ammonia was the main way for nitrate removal, while adsorption by activated carbon and coagulation by iron ions were the two main ways for organic matter removal (with contribution of about 70% of the total COD removal). These results proved that iron–carbon internal micro-electrolysis followed with ammonia stripping could be a promising option for further nitrate and organic matter removal from biochemically treated leachate.

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1. Introduction

Landfill leachate is a complex, high strength wastewater, in which there is plenty of organic matter, ammonia, heavy metals, and other toxic matter. So reasonable and effective disposal of the leachate is very necessary.¹ From the perspective of cost and reliability, biological methods (including anaerobic treatment and aerobic treatment) are the most widely used in landfill leachate treatment all over the world.^2,3 However, the organic matter and nitrogen (nitrate and ammonia) in the biochemically treated leachate both need to be further removed by advanced treatment processes to meet the discharge standard.⁴

The traditional methods for advanced treatment of organic matter in landfill leachate include: membrane separation,^5,6 coagulating sedimentation,^7–9 adsorption,^10,11 and advanced oxidation processes (AOP).¹² Although these methods have great advantages on removal of organic matter, their high cost, unmanageable by-products and uncontrollable secondary pollution limit the large-scale application.¹³

Chemical reduction method can be implemented for fast and efficient removal of nitrate.^14,15 There are widespread reports revealing that zero-valent iron (Fe⁰) can be used as a reductant to remove nitrate from contaminated water.^16,17 Our previous study showed that, with the addition of activated carbon (AC, forming iron–carbon internal micro-electrolysis system), the reducing ability of Fe⁰ was promoted.¹⁸ Besides, various studies on using iron–carbon internal micro-electrolysis to remove organic matter were reported.^19–24 The main mechanisms of this process include coagulation,²⁰ adsorption,²¹ oxidation reduction^22,23 and electrochemical action.^19,24 Among which, coagulation and adsorption have effective performances on organic matter removal of landfill leachate,^13,25 and could be improved by oxidation and electrochemical action.²³

This study was focused on the simultaneous removal of nitrate and organic matter from biochemical treated landfill leachate using iron–carbon internal micro-electrolysis. In the first step, nitrate could be reduced to ammonia, and organic matter could be removed by combined effect of coagulation, adsorption, reduction and electrochemical action. In the second step, ammonia produced in previous step was supposed to be stripped out by aeration.

Thus, the objectives of this study were: (1) to examine the simultaneous removal performances of organic matter and nitrate from landfill leachate using iron–carbon internal micro-electrolysis followed with ammonia stripping; (2) to study the main mechanisms of organic matter and nitrate removal.

2. Materials and methods

2.1. Materials and chemicals

Commercial iron powder was bought from Sinopharm Chemical Reagent Co., Ltd, China. Iron powder was pretreated firstly with 0.1 M HCl for 10 min in order to remove oxides covered on its surface. Before dried in a vacuum drying oven (105 °C), iron powder was washed 15 times with distilled water. The same to the iron powder, activated carbon was washed for 10 times and dried at 105 °C. Both iron powder and AC were sieved through a 100 mesh sieve. The wastewater was taken from the effluent of the mineralized garbage biological reaction bed, which locates in Laogang refuse landfill, Shanghai, China. Table 1 shows the characteristics of the biochemical treated landfill leachate. The synthetic wastewater (49.4 mg N L⁻¹) was prepared with sodium nitrate (0.3 g) and deionized water (1 L) for studying the differences of nitrate reduction.

Table 1 The characteristics of the biochemical treated landfill leachate^a

Parameter	Value
a Remark: the units of index are mg L^−1 except pH.
pH	7.5–8.2
CODcr	237.3–262.6
NO3^−-N	69.19–71.8
NO2^−-N	2.38–2.7
NH4^+-N	217.9–234.0

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2.2. Experimental procedures

Wastewater (200 mL) was added to headspace bottle (250 mL) for batch tests of landfill leachate reduction. Certain initial pH was controlled by dilute NaOH or HCl solution. The initial pH was adjusted to 3 except for discussing the influence of initial pH. After purging the solution in headspace bottle with highly purified nitrogen gas for 15 min, iron (12 g) and AC (4 g) were added. And then, take out the sample from every bottle after the reaction, which have an individual reaction time, pH, NO₃⁻-N, NH₄⁺-N and COD were analyzed. NaOH was added to regulate pH to 9, 10, 11, 12. Then NH₄⁺-N was blown out through aeration (1.5 L min⁻¹). AC and ferrous chloride were added and then the removal efficiency of organic matter/nitrate was measured for the independent experiment.

2.3. Analytical methods

On the basis of Standard Methods for the Examination of Water and Wastewater (American Public Health Association), concentrations of Fe²⁺ (with phenanthroline) and NH₄⁺-N (with Nesster's reagent) were determined colorimetrically using a UV-vis spectrophotometer (UV-4802, UNICO, USA). The pH value was measured using a Mettler-toledo pH-meter. COD was determined using potassium dichromate method. Concentrations of NO₃⁻-N and NO₂⁻-N were tested using ion chromatography (MIC, METROHM CHINA LTD, Switzerland).

3. Results and discussion

3.1 Nitrogen removal

3.1.1 Nitrate reduction. As shown in Fig. 1, the iron–carbon internal micro-electrolysis system had a significant difference between synthetic wastewater and leachate. When the initial pH was 3, the removal efficiency of NO₃⁻-N in synthetic wastewater and leachate could both reach as high as 80%. When the initial pH was 5, the NO₃⁻-N removal efficiency was higher than 70% in synthetic wastewater, which was only 2.7% in leachate. When the initial pH was 6, the NO₃⁻-N removal efficiency in synthetic wastewater reduced to 49.9%, while little NO₃⁻-N was removed in landfill leachate. Our previous work¹⁸ has also proved that the main product was NH₄⁺-N (accounting for about 88–95% of removed NO₃⁻-N).

Fig. 1 Nitrate reduction by iron–carbon internal micro-electrolysis: contrast of synthetic wastewater and leachate.

The different behavior of practical leachate and synthetic wastewater towards NO₃⁻-N removal efficiency could be attributed to the competition usage of electrons for nitrate and organic matter reduction. In this iron–carbon internal micro-electronic system, organic matter also can be reduced by electrons from Fe.²⁶ When the initial pH = 3, the activity of iron was more reactive, and the oxidation reduction potential was higher, so there were more electrons produced by Fe,^18,27 and they were enough to reduce nitrate and organic matter together. When the initial pH = 5/6, few electrons was produced. In competition with organic matter, the action of nitrate reduction was limited. In order to achieve higher nitrate reduction efficiency, it is necessary to control the pH = 3.

3.1.2 Kinetics study. Fig. 2a shows that the nitrate reaction rate reduced gradually with time. In the first 60 minutes, nitrate maintained very high removal efficiency, achieving almost 80% at 60 minutes. But the nitrate reaction rate decreased gradually to zero after another 60 minutes. The same tendency also has been gotten during the preliminary experiment.

Fig. 2 Kinetic study of nitrate reduction: (a) the relationship between time and the nitrate removal efficiency; and (b) a plot of ln(1 − R) vs. reduction time (R: the removal efficiency of nitrate).

The nitrate reaction rate (named as r in the following equation) can be seen as a pseudo-first order reaction related to nitrate concentration as given in eqn (1).

r = −dR_NO3⁻/dt = KC_NO3⁻ (1)

where, K is observed as first-order reaction rate constant, and it is the slope of the regression lines by plotting a natural log graph related to nitrate concentration changes along with reaction time according to eqn (2). As shown in Fig. 2b.

ln(1 − R) = −Kt (R: the nitrate remove efficiency) (2)

It can be seen, the degradation is pseudo-first order with strong correlation to nitrate concentration, which was also observed by Fan Xiaomeng.²⁸ The electron transfer rate is determined by the concentration of nitrate under such a solid–liquid contact interface reaction. As the reaction continued, the nitrate was not removed anymore after 120 min because of the corrosion of iron is stopped.

3.1.3 Ammonia stripping. Some relations between the completion time of ammonia stripping and initial pH were depicted in Fig. 3. Ammonia stripping could be completed within about 80 minutes. The stripping efficiency of ammonia could be obviously improved with the increase of the initial pH. When the initial pH were 9, 10, 11 and 12, the eventually removal efficiencies of ammonia were 20%, 43%, 59% and 65%, respectively. The same trend also has been gotten during the preliminary experiments. NH₄⁺ was the main form of ammonium existed in the biochemical treated landfill leachate, and the improvement of pH would destruct its ionization balance, thus, the production of NH₃·H₂O will increased. When the initial pH is lower than 9, the ionization balance in the solution is relatively stable, but the stripping effect of NH₃-N is not obvious. With pH increasing, ionization balance was broken and ammonia dissociation rate increased rapidly, thus, realizing the effective stripping of NH₃-N. So, pH = 11 is a reasonable choice based on total cost and nitrate removal efficiency.

Fig. 3 The influence of different initial pHs on ammonia stripping.

3.2 Organic matter removal

The effect of reaction time on the internal micro-electrolysis process was investigated. As can be seen in Fig. 4, organic matters were quickly removed from the leachate during the internal micro-electrolysis and 46% of COD removal was achieved after 20 min, and it had a slight change when the treatment time was extended to 40 min. With the reaction continued, more and more ferrous and ferric hydroxides were generated. The hydroxides precipitated on the cast iron surface and blocked the electron transfer between iron and wastewater.²⁴ This resulted in deactivation of the cast iron surface and termination of internal micro-electrolysis. pH has a great influence on the effects of flocculation electrophoresis and redox action on contaminant removal.²⁹ As a consequence, the iron could maintain a high activity via adjusting pH.

Fig. 4 COD removal efficiency by iron–carbon internal micro-electrolysis.

3.3 Mechanism study

3.3.1 Nitrate removal mechanism. In this system, nitrate could be removed by activated carbon adsorption^30,31 or reduction of iron.¹⁸ From the adsorption tests (Fig. 5), it can be found that nitrate adsorption by AC was more effective at lower pH. One possible reason may be that the increased protons in solutions led to the decrease of electro negativity on the AC surface. Meanwhile, the increasing positive charged sites on AC surface also made contributions to the nitrate adsorption via enhancing the electrostatic attraction to nitrate.³¹ Batch tests of nitrate adsorption by AC showed that the nitrate adsorption capacity of AC was weak (less than 5%). Therefore, the main mechanism of nitrate removal was nitrate reduction to ammonia followed by ammonia stripping.

Fig. 5 Nitrate reduction by iron–carbon internal micro-electrolysis with different pHs.

In iron–carbon internal micro-electrolysis system, electrons were transferred from iron to carbon. Hydrogen ion or H₂O molecule adsorbed on the carbon surface accepted the electrons, and then they were converted to adsorbed atomic hydrogens (denoted as H_ad) under acidic condition (as shown in Fig. 6). Ultimately, neighboring adsorbed nitrate were reduced into ammonium by H_ad, represented as follow (eqn (3)).¹⁸

NO_3,ad⁻ + 8H_ad → NH_4,ad⁺ + 2OH⁻ + H₂O (3)

Fig. 6 Diagram of iron–carbon internal micro-electrolysis followed with ammonia stripping for deep treatment of biochemical treated leachate.

3.3.2 Organic matter removal mechanism. Internal micro-electrolysis for organic matter removal mechanisms mainly includes: iron coagulation, adsorption, oxidation–reduction reaction, and so on.¹⁸ The possible mechanisms of organic matter removal (Table 2) were analyzed by independent experiments. The result of adsorption tests showed that only about 11.5% of COD was removed by adsorption under the same AC amount. To separate the COD removal efficiency contributed by coagulation, coagulation tests were performed with iron ion concentration of 200 mg L⁻¹ (similar to iron ion concentration in the iron–carbon internal micro-electrolysis system at initial pH of 3). The results are shown in Table 2. It can be found that about 20% of the total COD was removed by coagulation. With the difference of water quality and AC amount, the adsorption effect of organic matter has a wide range reported by literatures.^27,32 This fully illustrated that coagulation and adsorption were both important to organic matter removal (with contribution of about 70% of the total COD removal) in this test.

Table 2 Contributions of coagulation and adsorption to the total COD removal

COD removal rate	This study	Ref. 27	Ref. 32
Coagulation	18.0–23.0%	18.5%	—
Adsorption	11.5%	—	19.1%
Total COD removal rate	43.0–47.0%	47.1–50.7%	—

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The mechanisms of nitrate removal and organic matter removal were summed up as shown in Fig. 6. Adsorption and coagulation played dominant roles on organic matter removal. Organic matter and nitrate would compete for electrons on the carbon particles surface. Ammonia could be blown off by aeration at pH value of 11.

4. Conclusions

The results of this study showed that iron–carbon internal micro-electrolysis system followed with ammonia stripping could be used as a possible option for deep treatment of biochemical treated leachate. The total COD removal efficiency was about 45% and the total nitrogen removal efficiency was about 54% (with nitrate reduction efficiency of about 90% and ammonia stripping efficiency of about 60% at pH 11). Both nitrate and organic matter could be reduced by iron–carbon internal micro-electrolysis by electron competition. The main mechanisms of COD removal include adsorption and coagulation (with contribution of about 70% to the total COD removal).

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21377079), the key research project from Science and Technology Commission of Shanghai Municipality (13DZ0511600) and the Program for Innovative Research Team in University (No. IRT13078).

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