The post treatment of composting leachate with a combination of ozone and persulfate oxidation processes

Abstract

Due to the presence of refractory substances, biological methods are often not sufficient for leachate treatment to meet the standards of treated wastewater. O₃/persulfate oxidation is a kind of AOP, which has been proven to be an appropriate technology for the final treatment of these types of wastes. In this study, the capability of an O₃/persulfate oxidation process to act as a post treatment method for composting leachate was examined at laboratory scale and in batch mode. The effects of some factors, such as the initial pH, oxidant concentration and reaction time, were investigated on the removal of the organic load and the color of the leachate. Based on the results of the experiments, maximum COD removal (87%) was obtained after 210 minutes of ozonation with a 0.79 g h⁻¹ ozone mass flow rate at pH 9, and in the presence of 4500 mg L⁻¹ sodium persulfate. Under such conditions, the maximum removal of color was also 85%. Based on the results of this study, the combination process showed a higher potential for the removal of contaminants compared with ozonation or persulfate processes alone. In this research, the biodegradability (BOD₅/COD) of the leachate improved from 0.13 to 0.61 and the toxicity reduced by more than 80%. SPE-GC-MS analysis revealed that the composting leachate contained various groups of humic substances, most of which could be degraded into compounds with lower molecular weights, using a combined O₃/persulfate process. Therefore, it is a useful method for the degradation of refractory organic materials in contaminated waters.

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1. Introduction

Leachate may be produced as a result of water penetration in landfills through precipitation and/or through some processes such as size reduction as well as the biodegradation of waste materials in the composting process.

Due to the presence of numerous compounds in solid wastes, leachate is considered as a highly contaminated and toxic liquid, causing adverse effects on the environment.^1–3

Refractory organic compounds and heavy metals are the main components of leachate, which often cause concern due to their unpleasant effects on human health and the environment.⁴

Because the composition of leachate differs from one place to another, no uniform technique has yet been proposed for its treatment.⁵

Conventional leachate treatment techniques can be classified into three major groups: (1) leachate transfer, recycling and combined treatment with domestic sewage in municipal sewage treatment plants; (2) biodegradation techniques, including aerobic and anaerobic processes; and (3) physical and chemical methods, such as adsorption, chemical oxidation, chemical precipitation, sedimentation/flotation, coagulation/flocculation and air stripping.⁶

Due to its reliability, simplicity and high cost-effectiveness, biological processes are among conventional methods used for immature leachate treatment when the BOD₅/COD ratio is high. However, because of the high organic load of leachate, and also the presence of refractory substances (mainly humic and fulvic acids), biological treatment alone cannot remove all of the organic matter from leachate. Therefore, to meet discharge standards, additional treatment is required to remove the remaining materials from biologically treated leachate.^1,7,8

Advanced oxidation processes (AOPs) are commonly used methods for the treatment of effluent containing refractory compounds, where, with the production of hydroxyl radicals, most of the organic substances are degraded into CO₂ and H₂O through mineralization.⁹

A wide variety of AOP applications in wastewater treatment have been reported.^10–12 Among them, applications of AOPs to leachates treatment have been recently considered and different information regarding these processes has been published. Mokhtarani et al. obtained 47%, 86% and 89% removal rates for COD, color and turbidity, respectively, during ozonation of pretreated composting leachate.¹³ Hydrogen peroxide was proved to enhance the treatment process when combined with ozone.¹⁴ In landfill leachate treatment using TiO₂ photocatalytic degradation, the maximum removal efficiencies of COD, DOC and color have been reported as 60%, 72%, and 97%, respectively.¹⁵ During the post-treatment of composting leachate via UV light and TiO₂ nanoparticles immobilized on concrete surfaces, simultaneous COD and color removal efficiencies of 58% and 36% were reported, respectively.¹⁶

Recently, persulfate oxidation processes have drawn attention as an appropriate choice for the chemical oxidation of various organic pollutants.¹⁷ The persulfate anion (S₂O₈²⁻) is one of the oxidants with a high oxidation and reduction potential.¹⁸ As a non-selective anion, it is the strongest oxidant (E⁰ = 2.01 V) in the peroxygen family.^19,20

Although persulfate anions can act as a direct oxidant, their reaction rates are limited in refractory contaminants.^21,22 Moreover, the persulfate anion can be activated to generate an even stronger oxidant known as a sulfate radical (SO₄˙⁻) to initiate sulfate radical (E⁰ = 2.4 V) based advanced oxidation processes.²³ Typically persulfate anions can be activated to generate the intermediate sulfate free radical (SO₄˙⁻) oxidant using transition metals (eqn (1)), heat or ultraviolet light irradiation (eqn (2)).^22,24,25

S₂O₈²⁻ + Mⁿ⁺ → SO₄˙⁻ + SO₄²⁻ + M⁽ⁿ⁺¹⁾⁺ (1)

S₂O₈²⁻ + heat/UV → 2SO₄˙⁻ (2)

where Ag⁺, Co²⁺, Ce³⁺, Ni²⁺, Fe²⁺, Fe³⁺, Mn²⁺, V³⁺ and Ru³⁺ are the most efficient transition metal ions for the first reaction.²⁶

Subsequently, in accordance with the following reactions (eqn (3) and (4)), sulfate radicals may initiate the production of other intermediate highly reactive oxygen species, such as hydroxyl radicals. These reactive oxygen species can initiate a series of radical propagation and termination chain reactions, where organics are partially and even fully decomposed.²⁷

All pHs: SO₄˙⁻ + H₂O → SO₄²⁻ + ˙OH + H⁺ (3)

Alkaline pH: SO₄˙⁻ + OH⁻ → SO₄²⁻ + ˙OH (4)

Although both SO₄˙⁻ and ˙OH are possibly responsible for the destruction of organic contaminants, the rate constant of eqn (3) is small by comparison with those for sulfate free radical reactions with organic compounds.²⁸ In addition, SO₄˙⁻ is known for its longer half-life and higher selectivity for the oxidation of target organic compounds than those of hydroxyl radicals.²⁹

Over 90% COD removal from mature landfill leachate (1254 mg L⁻¹ initial COD) using thermal persulfate oxidation at 50 °C has been reported.²⁵ Complete lindane (a carcinogen and a notoriously persistent organic pollutant in the environment) oxidation was achieved when an Fe(II)-activated sodium persulfate process in aqueous solution was employed.²⁴ In another study, the iron(II) activated persulfate oxidation of contaminated soil destroyed 99% of the total BTEX and 92% of the total PAH concentration.³⁰

Recently, persulfate was combined with ozone for the treatment of stabilized landfill leachate.³¹ In this research, after 210 min of ozonation, in the presence of 7 g S₂O₈²⁻ per g COD and a 30 g m⁻³ O₃ dosage, at pH 10, the maximum removal efficiencies of COD, color, and NH₃–N were reported as 72%, 96%, and 76%, respectively. In another study, microwave-enhanced persulfate oxidation using activated carbon (AC) as a catalyst was applied to treat landfill leachate. In this research, under optimal conditions (AC dosage = 10 g L⁻¹, S₂O₈²⁻/COD₀ = 14.4, pH = 9, microwave power = 500 W and radiation time = 10 min) the removal rates of COD and NH₄⁺–N were 78.2% and 67.2%, respectively.³² Electro/Fe²⁺/peroxydisulfate process, as one of the sulfate radical-based AOPs, was also reported to be effective in the degradation of organic pollutants in landfill leachate.³³

Considering the specific characteristics of the sulfate radical, an investigation into the effects of various factors in the post-treatment of composting leachate using a combined O₃/persulfate oxidation process was the main objective of the present study. The effects of pH, ozone mass flow rate, persulfate concentration, and reaction time on the removal of refractory organics and color from biologically treated leachate have been evaluated, and the optimal conditions for conducting this process have been determined. The degradation of different types of organic compounds during the O₃/persulfate oxidation process was also studied using gas chromatography coupled with mass spectrometry (GC/MS) analysis. So far, no report has been found regarding the O₃/persulfate post treatment of composting leachate. Also, to the authors' knowledge, no studies have previously been published to estimate the toxicity of the leachate.

2. Materials and methods

2.1. Experimental system

This investigation was conducted at laboratory scale and in batch mode. A schematic sketch of the reactor is illustrated in Fig. 1.

Fig. 1 Schematic sketch of the used system.

The ozone contact reactor consists of a Plexiglas column with a 20 mm inner diameter and 800 mm height. An ozone generator (ARDA-COG 5S) with a 5 g h⁻¹ nominal capacity was used to produce ozone gas from pure and dry oxygen. Ozone content was measured with a BMT-964 ozone analyser. A rotameter was applied to measure the volume of gas injected into the column. A Varian digital gas flow meter (Dfm-05) with a flow range of 1 to 1000 mL min⁻¹ was also supplied to calibrate the rotameter. The O₃ gas was continuously injected into the column through a diffuser located at the bottom of the reactor. In order to prevent the emission of O₃ into the environment, gas effluent from the reactor was passed through a 2% KI solution.

In each experiment, a specified amount of sodium persulfate solution (ranging from 0 to 7500 mg L⁻¹) was added to 150 mL of leachate in the reactor, at ambient temperature, and a specified amount of ozone gas was blown into it. Samples were taken after different periods of time and filtered through a 0.45 μm filter paper, prior to analysis. The effects of some parameters, such as the concentration of sodium persulfate, ozone mass flow rate, and reaction time, as well as the initial pH of the solution, were examined on the simultaneous COD and color removal efficiency in the leachate. It should be noted that in all of the experiments, the given amount of sodium persulfate was first dissolved in 10 mL of leachate using a magnetic stirrer; thereafter, it was added to the reactor. Likewise, sulfuric acid and sodium hydroxide solution were used to adjust the pH.

2.2. Materials

Biologically pre-treated leachate samples were collected from the effluent of a leachate treatment facility from a composting plant in the north of Iran. In this plant, in addition to leachate derived from the process, polluted runoff is also likely to be produced from contaminated hard surfaces and machinery.³⁴ The treatment system consists of two biological processes followed by disinfection. The characteristics of the used leachate are summarized in Table 1.

Table 1 Characteristics of the used leachate

Parameter	Average	Unit
pH	8.9	—
COD	750	mg L^−1
BOD5	95	mg L^−1
TDS	8150	mg L^−1
TS	8200	mg L^−1
Ag	<0.05	mg L^−1
Al	0.52	mg L^−1
Cr	<0.05	mg L^−1
Cd	<0.05	mg L^−1
Co	<0.05	mg L^−1
Cu	<0.05	mg L^−1
Mn	0.10	mg L^−1
Ni	0.14	mg L^−1
Pb	<0.05	mg L^−1
Zn	0.40	mg L^−1
Alkalinity	605	mg L^−1 as CaCO3
EC	13	mS cm^−1
Color	7	Gardner

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The leachate samples were collected in 20 L plastic containers and transported to the laboratory, and immediately stored in a refrigerator at 4 °C to minimize any changes in the physical, chemical, and biological properties until the experiments were carried out.

All chemicals employed for analysis were of analytical grade and obtained from reliable companies.

2.3. Toxicity estimation

A Washington University laboratory safety method was used to determine the toxicity of the leachate.³⁵ For this purpose, based upon fish LC₅₀ values (the concentration of a substance required to kill 50% of the tested population), the constituents of the leachate are divided into five categories as listed in Table 2. Then, the equivalent toxicity of the leachate was calculated using eqn (5).where X, A, B, C and D are the percentages of leachate constituent associated with each toxicity category. The LC₅₀ of each constituent of the leachate was also calculated using TEST (Toxicity Estimation Software Tool) software.³⁶

Table 2 Chemical waste toxicity categories²⁹

Toxic category	Fish LC50 (ppm)
X	<0.01
A	0.01–<0.1
B	0.1–<1.0
C	1.0–<10.0
D	10.0–100.0

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2.4. Analytical procedures

A GC-7890A Agilent gas chromatograph, coupled to an MS-5975C Agilent mass spectrometer, was used to identify the intermediate products. The GC-MS applied to this purpose was equipped with a capillary column (Chrompak CP-Sil 8 CB, 50 m length, 250 μm internal diameter, 0.12 μm film thickness) and a liquid & headspace analyser.

The initial temperature of the column was set to 60 °C for 2 min. Afterwards, it was increased to 200 °C at a rate of 15 °C min⁻¹, and immediately the temperature was further increased to 280 °C at a rate of 5 °C min⁻¹. One microliter of sample was split injected into the GC column (the split ratio was 1 : 5).

A Metrohm 691 pH meter with a glass combination electrode was used to measure the pH. Color and COD measurements were assayed at 780 nm and 640 nm, respectively, using a DR 4000 Hach spectrophotometer (Method 10105 and 8000). A WTW OxiTop BOD measurement system was used to measure BOD₅. The ozone gas concentration was measured using an ultraviolet gas ozone analyser (BMT 964). A Varian 735 inductively coupled plasma/optical emission spectrometer (ICP-OES) was also used to quantify heavy metals. All other parameters were analysed according to standard methods for the examination of water and wastewater.³⁷

3. Results and discussion

3.1. pH effect

The type and quantity of radicals produced as a result of pH variation are paramount causes for organic compound degradation within advanced oxidation processes.^18,38 In a combined O₃/persulfate oxidation process, like other advanced oxidation processes, different radicals are produced depending on the operating conditions. Therefore, changes in pH can affect the removal efficiency due to different radical production.¹⁸ In order to determine the effect of pH on the removal rate of COD and color, the process was examined by means of 150 mL of leachate with a 0.79 g h⁻¹ ozone mass flow rate in the presence of 3000 mg L⁻¹ sodium persulfate and at different pH values (3, 4, 5, 7, 9, and 11). As shown in Fig. 2, after 60 min and at pH 9, a maximum COD removal efficiency of 60% was obtained.

Fig. 2 Effect of pH on the COD removal efficiency (150 mL of leachate with 750 mg L⁻¹ initial COD, a 0.79 g h⁻¹ ozone mass flow rate and 3000 mg L⁻¹ Na-persulfate).

As shown, by increasing pH, the removal efficiencies were also increased. The increase in removal efficiency can be related to the ability of O₃ to initiate hydroxyl radical formation at high pH values.³⁹ Thereafter, according to eqn (6), under the effect of hydroxyl radicals, persulfate can be activated to initiate sulfate radical creation. Due to its high oxidation and reduction potentials, the sulfate radical plays an important role in the degradation of organic compounds.

S₂O₈²⁻ + OH˙ → HSO₄˙⁻ + SO₄˙⁻ + 1/2O₂ (6)

However, as shown in Fig. 2, increasing the pH to more than 9 decreases the removal rate of COD. The initial pH of the leachate was 8.9 which indicates that the major alkaline component of the leachate was bicarbonate ions (HCO₃⁻). Under alkaline conditions (pH > 8.3), bicarbonate ions tend to convert into carbonate ions (CO₃²⁻). Under such conditions (alkaline conditions) both carbonate and bicarbonate ions are known as hydroxyl radical scavengers.⁴⁰ These scavengers consume OH radicals and reduce the oxidation potential of the system. However, carbonate ions are much stronger scavengers than bicarbonate ions (reaction rate CO₃²⁻: k = 4.2 × 10⁸ M⁻¹ s⁻¹, and reaction rate HCO₃⁻: k = 1.5 × 10⁷ M⁻¹ s⁻¹). That is why in this process at pH < 9, the bicarbonate concentration is less important.⁴¹

In the pre-treatment of stabilized landfill leachate using integrated O₃/persulfate oxidation processes, similar results have been reported.³¹ Therefore, in the present study, pH 9 was selected to conduct the following experiments.

3.2. Effect of ozone dose

In order to determine the optimum dose of ozone to achieve maximum COD removal, experiments were conducted with 0.0, 0.34, 0.6, 0.79, 1.00 and 1.09 g h⁻¹ ozone, in the presence of 3000 mg L⁻¹ sodium persulfate at pH 9, for 60 minutes.

As is seen in Fig. 3, ozone has a significant effect on COD removal from the leachate. By increasing the ozone mass flow rate to 0.79 g h⁻¹, the COD removal rate also increased simultaneously and reached 65%, after 60 minutes. Afterwards, by increasing the ozone dose (to more than 0.79 g h⁻¹), the removal rate of COD remained relatively constant.

Fig. 3 Effect of the ozone mass flow rate on the COD removal efficiency (150 mL of leachate with 750 ppm initial COD, 3000 ppm Na-persulfate, and at pH 9).

Since a mass flow rate higher than 0.79 g h⁻¹ has less effect on the removal rate of COD, and considering the energy consumption and economic issues, the optimum mass flow rate of ozone for this process is considered to be 0.79 g h⁻¹.

As noted, in an O₃/persulfate system, ozone has a significant role in the production of hydroxyl radicals. With increasing the ozone mass flow rate, the COD removal efficiency is increased as a result of a higher production of hydroxyl radicals.

But, as shown in Fig. 3, increasing the ozone mass flow rate to above 0.79 g h⁻¹ has no significant effect on COD removal. According to eqn (7), with an excessive increase in hydroxyl radicals followed by an increase in the ozone gas concentration, hydroxyl radicals may react with sulfate radicals to produce oxygen gas and sulfate ions.¹⁴

SO₄˙⁻ + OH˙ → SO₄²⁻ + 1/2O₂ (7)

Since in this situation some hydroxyl and sulfate radicals are consumed, the increase in the COD removal rate with an increase in ozone mass flow rate is not significant.

3.3. Effect of persulfate dosage

The concentration of the oxidant is one of the effective factors in conducting chemical oxidation processes.²⁵ To investigate the effect of persulfate concentration (0, 1500, 3000, 4500, 6000, and 7500 mg L⁻¹) on the organic load removal from the leachate, some experiments were carried out with a 0.79 g h⁻¹ ozone mass flow rate, at pH 9 for 60 min. As seen in Fig. 4, persulfate has a significant effect on the COD removal rate from the leachate.

Fig. 4 Effect of sodium persulfate concentration on the COD removal rate (150 mL of leachate with 750 mg L⁻¹ initial COD, a 0.79 g h⁻¹ ozone mass flow rate, and at pH 9).

In this study, by increasing the persulfate concentration to 4500 mg L⁻¹, the COD removal efficiency was also increased. However, by subsequently increasing the oxidant concentration to more than 4500 mg L⁻¹, no significant change was observed in the removal efficiency.

As noted, the persulfate anion can be activated to generate an even stronger oxidant known as the sulfate radical. Subsequently, the sulfate radical may initiate a series of radical propagation and termination chain reactions where organics are partially and even fully decomposed. In addition to direct reactions, under alkaline conditions, according to eqn (4), the sulfate radical may react with OH⁻ to form more hydroxyl radicals (OH˙), which are an important factor in the decomposition of organic matter.⁴²

As shown in Fig. 4, an increase in the persulfate concentration to more than 6 times the leachate COD (i.e., more than 4500 mg L⁻¹) had no significant effect on the COD removal rate.

According to eqn (8), with an excessive increase in persulfate concentration followed by an increase in sulfate radical concentration, sulfate radicals may react with each other and convert to persulfate anions again. Differently, according to eqn (9), sulfate radicals may react with persulfate anions directly and convert into sulfate ions.

SO₄˙ + SO₄˙ → S₂O₈²⁻ (8)

SO₄˙⁻ + S₂O₈²⁻ → SO₄²⁻ + S₂O₈⁻ (9)

However, in both cases, due to the consumption of sulfate radicals, the removal efficiency does not increase considerably.^18,42 Therefore, a sodium persulfate concentration of 4500 mg L⁻¹ was selected to carry out subsequent experiments.

3.4. Effect of reaction time

In order to determine the optimum reaction time to achieve maximum COD and color removal, experiments were conducted with a 0.79 g h⁻¹ ozone mass flow rate and 4500 mg L⁻¹ of sodium persulfate, for 240 min at pH 9. As shown in Fig. 5, after nearly 210 min, 84% removal of COD was obtained. Afterwards, this remained approximately constant. In other words, after 210 min, the primary degradable pollutants were degraded, and the accumulation of refractory compounds and resulting by-products from sodium persulfate reactions led to a fixing of the rate of COD removal. As is seen, the rate of color removal was also stabilized and reached 85% during the given period. Thus, a reaction time of 210 min was considered as the optimal time for this process. Based on the results of this study, and considering the initial organic load of the leachate, the combined O₃/persulfate oxidation process can reduce the COD of the leachate to about 120 mg L⁻¹.

Fig. 5 Effect of the reaction time on the COD and color removal efficiency (150 mL of leachate with 750 mg L⁻¹ initial COD, a 0.79 g h⁻¹ ozone mass flow, at pH 9 and with 4500 mg L⁻¹ of Na-persulfate).

The results of the experiments also revealed that the applied process had no significant effect on the initial pH of the leachate (data not shown in this paper).

3.5. Ozone consumption rate

The ozone consumption (OC) rate is defined as the ozone mass required for the removal of a certain amount of COD. In order to investigate the OC rate for the post-treatment of biologically treated leachate, experiments were repeated at pH 9, with 0.79 g h⁻¹ of ozone gas, in the presence of 4500 mg L⁻¹ sodium persulfate, for 210 minutes. The ozone content of the exhaust gas from the ozone contact reactor and the COD of the degraded leachate were measured at different time intervals. The ozone consumption rate was then investigated using eqn (10).

In this equation, Q_G is the inlet gas flow rate (L min⁻¹), V is the sample volume (L), C_in and C_out are the ozone input and output gas concentration (mg L⁻¹), t is time (min), and COD₀ and COD_t are the initial COD of the solution and the COD of the solution at a specified time, respectively (mg L⁻¹).

As shown in Fig. 6, the OC rate decreases over time. This means an increase in the ozone content of the output gas. A gradual reduction in the OC can be attributed to a decrease in the concentration of degradable contaminants in the reactor. In the present study after 210 minutes, an ozone consumption rate of 0.35 mg O₃ per mg COD was achieved.

Fig. 6 Ozone consumption (150 mL of leachate with 750 mg L⁻¹ initial COD, at pH 9, with a 0.79 g h⁻¹ O₃ mass flow rate and 4500 mg L⁻¹ of Na-persulfate).

In the present study, during a single ozonation process, an average OC rate of 1.16 mg O₃ per mg COD was obtained. The difference between ozone consumed through an integrated process with a single ozonation process indicates that during the integrated process, the rate of ozone degradation increases; thus, ozone will be consumed more effectively.

An ozone consumption rate of 0.60 g O₃ per g COD during the pretreatment of stabilized landfill leachate via an O₃/persulfate oxidation process was reported by Abu-Amr et al.³¹ An ozone consumption rate of 1.5 g O₃ per g COD for an integrated O₃/H₂O₂ process during landfill leachate treatment has been achieved.⁴¹ In another study, an OC rate of 5.3 g O₃ per g COD for an ozonation process and 3.2 g O₃ per g COD for an integrated O₃/granular activated carbon process was reported, during the post treatment of composting leachate.⁴³ The variations in ozone consumption shown in the literature are mainly due to a wide variation in the characteristics of the studied leachates such as leachate age, pH, organic composition, and ˙OH scavengers, and also the type of oxidant used.^14,41

3.6. Degradation of contaminants during the process

During biological degradation processes some of the organic materials are transformed into refractory residual compounds. After biological and/or abiological oxidation, these compounds polymerize into so called “waste-humic-substances”.⁴⁴ As humic substances can participate in various chemical transformation reactions, they are important from an environmental perspective. O₃/persulfate oxidation processes have been reported as useful methods for the degradation of persistent organic pollutants, and for the production of more biodegradable substances.²⁷

Since humic substances are one of the main refractory constituents of biologically treated leachates, the fate of humic acids (HA) and fulvic acids (FA) throughout the oxidation process were estimated in this research.

SPE-GC-mass spectra of the leachate before and after sulfate radical-based advanced oxidation processes are compared in Fig. 7. Tables 3 and 4 list the main products identified in the chromatograms using the NIST library.

Fig. 7 GC-Mass spectra of the leachate: (a) before the process, and (b) after the process.

Table 3 Leachate identified constituents (before the process)

Peak no.	Compound	Retention time (min)	Fish LC50 (ppm)	Area%
1	Propionic acid, 2-methyl-, tert-butyldimethylsilyl ester	4.838	294.06	0.81
2	Ammonia	10.712	3.49	0.24
3	Pyrrolidine-2-one, 5-(perhydroazepin-1-ylcarbonyl)-	13.447	44.11	0.11
4	4-Methyl-benzaldehyde	16.028	7.95	0.21
5	1H-Pyrrole-2-acetonitrile, 1-methyl	17.236	137.25	0.16
6	4-Cyclopentene-1,3-dione, 4-methoxy-5-methyl-	21.397	480.14	0.40
7	4H-1,2,4-Triazole-3-thiol, 4-allyl-5-(1-naphthylmethyl)-	21.607	290.12	0.21
8	Benz[b]-1,4-oxazepine-4(5H)-thione, 2,3-dihydro-2,8-dimethyl-	23.249	10.25	0.13
9	(3-Methoxyphenyl)-6-methyl-4-phenyl-quinazolin2-	23.954	0.03	0.25
10	Dodecane, 1-chloro-	27.836	0.18	0.29
11	1-Pentadecanol	27.968	0.36	0.27
12	Octahydro-3(2H)-isoquinolinone	30.719	23.98	0.11
13	4-(1,1,3,3-Tetramethylbutyl)-phenol	31.564	2.17	0.53
14	Butanamide, N,N-dihexyl-	32.641	1.3	0.15
15	2,3,6,7-Tetramethylquinoxaline	33.028	17.81	0.11
16	2,4-Di-t-butyl-6-nitro-phenol	33.129	0.28	0.26
17	1-Cyclohexyl 2-(4-methylpentyl) benzene-1,2-dicarboxylate	36.918	0.63	2.26
18	1-Butyl 2-cyclohexyl benzene-1,2-dicarboxylate	37.863	1.46	0.23
19	1-Butyl 2-cyclohexyl benzene-1,2-dicarboxylate	38.824	1.46	3.23
20	Cyclohexaneethanol, 4-methyl-beta-methylene-	41.459	12.74	0.15
21	2-[2-[4-(1,1,3,3-Tetramethylbutyl)phenoxy]ethoxy]-ethanol	42.536	5.45	0.4
22	3-trans-(1,1-Dimethylethyl)-4-cis-methoxycyclohexan-1-ol	42.954	41.34	0.22
23	Cyclopentadecanone, 2-methyl-	46.108	0.33	0.58
24	N-Acetyl-2-aminovaleric acid	46.495	169.16	0.75
25-1	Propyl 2-[ethyl(3-methylphenyl)carbamoyl]benzoate	50.137	1.28	0.13
25-2	Pentyl 2-[(propan-2-yl)carbamoyl]benzoate	50.215	2.67	0.15
25-3	1-Cyclohexylmethyl 2-pentan-2-yl benzene-1,2-dicarboxylate	50.416	0.93	0.3
25-4	1,2-Ditridecyl benzene-1,2-dicarboxylate	50.625	0.007	0.6
25-5	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	50.741	0.43	0.32
25-6	1-Butyl 2-(8-methylnonyl) benzene-1,2-dicarboxylate	50.842	1.11	0.54
25-7	1,2-Diundecyl benzene-1,2-dicarboxylate	50.943	0.08	0.45
25-8	1-(4-Methylpentyl) 2-pentan-2-yl benzene-1,2-dicarboxylate	51.183	1.8	2.04
25-9	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	51.330	0.43	0.81
25-10	1-Cyclohexyl 2-(2,2-dimethylpropyl) benzene-1,2-dicarboxylate	51.462	0.60	1.47
25-11	1-(4-Methylpentyl) 2-pentadecyl benzene-1,2-dicarboxylate	51.563	0.13	1.73
25-12	1-Decyl 2-[2-(2-methoxyethyl)hexyl] benzene-1,2-dicarboxylate	51.694	0.94	1.23
25-13	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	51.811	0.09	1.98
25-14	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	51.942	0.43	3.48
25-15	1-(5-Methoxy-3-methylpentan-2-yl) 2-nonyl benzene-1,2-dicarboxylate	52.028	1.23	3.25
25-16	1-Decyl 2-[2-(2-methoxyethyl)hexyl] benzene-1,2-dicarboxylate	52.152	0.94	1.49
25-17	1-Nonyl 2-pentan-2-yl benzene-1,2-dicarboxylate	52.276	0.93	2.37
25-18	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	52.384	0.43	3.66
25-19	1,2-Dinonyl benzene-1,2-dicarboxylate	52.508	0.11	8.37
25-20	1,2-Dinonyl benzene-1,2-dicarboxylate	52.826	0.11	5.18
25-21	1-(2-Methylpropyl) 2-pentadecyl benzene-1,2-dicarboxylate	52.981	0.05	7.09
25-22	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	53.151	0.01	2.35
25-23	1-Dodecyl 2-octyl benzene-1,2-dicarboxylate	53.314	0.06	3.86
25-24	1-Nonyl 2-tridec-2-yn-1-yl benzene-1,2-dicarboxylate	53.446	0.002	1.76
25-25	1-(4-Methylpentyl) 2-pentadecyl benzene-1,2-dicarboxylate	53.500	0.13	1.42
25-26	1-Decyl 2-hexyl benzene-1,2-dicarboxylate	53.616	0.34	3.49
25-27	1-Dodecyl 2-nonyl benzene-1,2-dicarboxylate	53.825	0.08	3.63
25-28	1,2-Dinonyl benzene-1,2-dicarboxylate	54.019	0.11	2.30
25-29	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	54.128	0.01	1.74
25-30	1-[3-(2-Methoxyethyl)heptyl] 2-nonyl benzene-1,2-dicarboxylate	54.345	0.45	2.19
25-31	1-[3-(2-Methoxyethyl)octyl] 2-nonyl benzene-1,2-dicarboxylate	54.685	0.17	0.50

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As shown in Table 3 the SPE-GC-mass products of biologically treated leachate are composed mainly of aromatic compounds like alkyl-benzenes, alkyl-naphthalene, phthalates, and higher condensed aromatic substances, which reveal the presence of humic substances in the sample.⁴⁴

In this study, the presence of 2-methyl propionic acid (peak 1), 4-methyl-benzaldehyde (peak 4), 4H-1,2,4-triazole-3-thiol 4-allyl-5-(1-naphthylmethyl) (peak 7), benz[b]-1,4-oxazepine-4(5H)-thione, 2,3-dihydro-2,8-dimethyl (peak 8), and (3-methoxyphenyl)-6-methyl-4-phenyl-2-quinazolinamine (peak 9) confirms the presence of HA, and pyrrolidine-2-one (peak 3), 4-methoxy-5-methyl-4-cyclopentene-1,3-dione (peak 6), and phthalic acids (peaks 17, 18, 19, & 25) revealed the presence of FA in the biologically treated leachate.⁴⁵

As can be seen in Fig. 7(b) and Table 4, none of the HA family compounds were found in the treated leachate, which represents the break-down of its molecules into other simpler compounds during the process. Also, from the FA family, only for the phthalic acids (peaks 29 & 30) was it observed that their amounts were greatly reduced compared with those from the initial leachate (peaks 17, 18, 19 & 25 in Fig. 7(a)).

Table 4 Leachate identified constituents (after the process)

Peak no.	Compound	Retention time (min)	Fish LC50 (ppm)	Area%
1	Chloro-benzene	9.364	19.01	1.58
2	Oxime-, methoxy-phenyl-	12.463	296.20	1.24
3	Bicyclo[3.1.0]hex-3-en-2-ol, 2-methyl-5-(1-methylethyl)-, (1alpha,2alpha,5alpha)-	12.727	16.08	1.36
4	2-Mercapto-4-phenylthiazole	13.246	9.72	0.36
5	2,6-Dimethyl-1,3,5,7-octatetraene,ee-	13.967	0.48	0.82
6	1,3,8-p-Menthatriene	14.4	0.97	1.29
7	1-Methyl-3-(1-methylethyl)-benzene	15.059	9.35	3.71
8	Tricyclo[2.2.1.0(2,6)]heptane	16.593	6.55	1.5
9	2-Methyl-2,3-dihydro-1-benzofuran	17.461	33.97	1.23
10	Cyclohexane-1,2-dimethanol, diacetate	18.313	8.18	0.46
11	Bicyclo[3.1.1]hept-3-en-2-ol, 4,6,6-trimethyl	18.724	12.75	1.00
12	3-Cyclohexene-1-ethanol,beta,4-dimethyl	19.693	18.84	1.21
13	3,5-Dihydro pyrrolo(3,2-d)pyrimidin-4-one	20.289	4.84	1.91
14	alpha-Thujenal	20.460	4.80	0.95
15	5H-Naphtho[2,3-c]carbazole, 5-methyl-	21.421	0.04	0.81
16	Propanal, 2-methyl-3-phenyl-	21.847	11.74	0.75
17	Bicyclo[2.2.1]heptan-2-one, 4-hydroxy-1,7,7-trimethyl-	22.033	36.26	1.18
18	3-Nonen-5-one	22.691	9.67	4.01
19	Benzenemethanol, 4-(1-methylethyl)	23.42	22.99	0.84
20	Phenol, m-tert-butyl-	23.931	15.37	3.66
21	5-Isopropyl-6-methyl-hepta-3,5-dien-2-ol	24.295	4.60	1.73
22	Acetaldehyde, (3,3-dimethylcyclohexylidene)-, (E)-	25.884	4.70	6.95
23	3-Cyclopentene-1-acetaldehyde, 2,2,3-trimethyl-	26.868	12.27	1.34
24	Cyclohexene, 1,2-dimethyl-	27.054	3.70	1.51
25	2-Butyl-3-methyl-5-(2-methylprop-2-enyl)cyclohexanone	27.286	0.96	2.18
26	Oxacyclododecan-2-one	27.968	34.5	1.33
27	3-Buten-2-one, 4-(5,5-dimethyl-1-oxaspiro[2.5]oct-4-yl)	28.983	4.70	0.95
28	1,2-Cyclohexanediol, 3-methyl-6-(1-methylethyl)-, (1alpha,2beta, 3beta,6alpha)-	29.851	180.53	2.01
29	1-(2-Ethylhexyl) 2-(4-methylpentyl) benzene-1,2-dicarboxylate	48.975	1.95	0.55
30-1	1-Hexyl 2-[3-(2-methoxyethyl)octyl] benzene-1,2-dicarboxylate	50.540	0.8	0.54
30-2	1-Decyl 2-(2,2-dimethylpropyl) benzene-1,2-dicarboxylate	50.850	0.61	0.36
30-3	1,2-Bis(8-methylnonyl) benzene-1,2-dicarboxylate	51.199	0.2	0.53
30-4	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	51.447	0.43	0.31
30-5	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	51.563	0.43	0.88
30-6	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	51.795	0.43	0.55
30-7	1-Nonyl 2-pentan-2-yl benzene-1,2-dicarboxylate	51.904	0.93	0.85
30-8	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	52.012	0.43	0.84
30-9	1-(2,2-Dimethylpropyl) 2-octyl benzene-1,2-dicarboxylate	52.136	0.6	0.61
30-10	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	52.245	0.43	0.72
30-11	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	52.353	0.43	0.84
30-12	1,2-Bis(7-methyloctyl) benzene-1,2-dicarboxylate	52.477	0.43	2.18
30-13	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	52.779	0.01	1.40
30-14	1,2-Dinonyl benzene-1,2-dicarboxylate	52.942	0.11	1.99
30-15	1-Dodecyl 2-nonyl benzene-1,2-dicarboxylate	53.128	0.08	0.72
30-16	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	53.283	0.01	0.72
30-17	1,2-Dinonyl benzene-1,2-dicarboxylate	53.415	0.11	0.78
30-18	1-Hexyl 2-[2-(2-methoxyethyl)hexyl] benzene-1,2-dicarboxylate	53.616	1.32	1.09
30-19	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	53.864	0.01	2.09
30-20	1-Dodecyl 2-nonyl benzene-1,2-dicarboxylate	54.120	0.08	0.62
30-21	1,2-Diundecyl benzene-1,2-dicarboxylate	54.329	0.08	0.59
30-22	1-Nonyl 2-pentadecyl benzene-1,2-dicarboxylate	54.678	0.01	0.25

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Benzoic acids, phenols and straight chain acids, some of which could be observed in Fig. 7(b) (peaks 1, 4, 6, 8, 13 and 18), are also reported as intermediate decomposition products of fulvic and humic acids.⁴⁵

The diversity of identified hydrocarbons in the treated leachate can be related to defunctionalisation and dehydration reactions of aromatic acids, which are building blocks of fulvic and humic acids.^44,46 The nitrogen-containing heterocycle products might originate from proteins as structural subunits of HA and FA.⁴⁵ The most probable sources of phenols and aromatic acids could be lignin or partly degraded lignin.^47,48

In view of the above, the used composting leachate contained various groups of humic substances, most of which could be degraded into compounds with lower molecular weights, through the O₃/persulfate oxidation process. During this process (210 minutes ozonation with a 0.79 g h⁻¹ ozone mass flow rate, at pH 9 and with 4500 mg L⁻¹ sodium persulfate), the biodegradability (BOD₅/COD) of the leachate improved from 0.13 to 0.61, which confirmed the conversion of complex and refractory substances into simpler compounds. A significant improvement in the biodegradability of pretreated landfill leachate during advanced oxidation processes by means of a solar photo-Fenton process was also reported.²

In this study, the toxicity of the leachate before and after the sulfate radical-based advanced oxidation process was also calculated to be 4.946% and 0.908%, respectively. Thus, the O₃/persulfate oxidation process not only removes the color and COD of the leachate, but also greatly converts complicated compounds into simpler substances and reduces the toxicity of the leachate considerably.

4. Conclusion

The main objective of this work was to investigate the capability of an O₃/persulfate oxidation process to act as a post treatment method for composting leachate on the laboratory scale and in batch mode. The effect of some factors, such as the initial pH, oxidant concentration, ozone mass flow rate and reaction time, was investigated on the removal rate of the organic load and color of the leachate. In this study, maximum COD and color removal rates of 87% and 85% were obtained, after 210 minutes ozonation with a 0.79 g h⁻¹ ozone mass flow rate, at pH 9 and with 4500 mg L⁻¹ sodium persulfate. Based on the results, the O₃/persulfate oxidation process can reduce the COD of the leachate to near 120 mg L⁻¹. According to the results, ozone consumption rates of 1.16 and 0.35 mg O₃ per mg COD for ozonation alone and for the integrated process were obtained, respectively. SPE-GC-MS analysis shows that during the integrated process, most of the persistent organic pollutants and refractory substances can be converted into simpler materials. In this process the biodegradability (BOD₅/COD) of the leachate was also improved from 0.13 to 0.61, and the toxicity was reduced considerably. The results suggest that the O₃/persulfate oxidation process is an effective chemical oxidation process for the simultaneous reduction of COD and color from biologically treated composting leachates.

Acknowledgements

The authors would like to acknowledge the Tarbiat Modares University (TMU) research fund for financial support, Miss Paniz Attarian and Miss Fatemeh Barancheshmeh from the civil and environmental engineering faculty of TMU for interpretation of GC-Mass spectra, and Jahesh Kimia Company, Tehran (Iran), for their help.

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