Hybrid zero valent iron (ZVI)/H₂O₂ oxidation process for landfill leachate treatment with novel nanosize metallic calcium/iron composite

ABSTRACT

A novel nanosize metallic calcium/iron dispersed reagent was synthesized and tested as coagulant/catalyst in a hybrid zero valent iron (ZVI)/H₂O₂ oxidation process to treat leachate. Two different types of leachates, one from municipal solid waste (MSW) tipping hall (MSWIL) and second from an MSW landfill site (MSWLL), were collected and characterized. The morphology, elemental composition, and mineral phases of the nano-Ca/CaO and nano-Fe/Ca/CaO were characterized by scanning electron microscopy–electron dispersive spectroscopy (SEM-EDS) and x-ray powder diffraction (XRD) analysis. The coagulation process with 2.5 g L⁻¹ nano-Ca/CaO attained 64.0, 56.0, and 20.7% removal of color, chemical oxygen demand (COD), and total suspended solids (TSS) in MSWLL. With only 1.0 g L⁻¹ of nano-Fe/Ca/CaO, relatively high color, COD and TSS removal was achieved in MSWLL at 67.5, 60.2, and 37.7%, respectively. The heavy metal removal efficiency reached 91–99% after treatment with nano-Fe/Ca/CaO in both leachate samples. The coupling process, using 1.0 g L⁻¹ of nano-Fe/Ca/CaO and 20 mM H₂O₂ doses, achieved enhancement removal of color, COD, and TSS, up to 95%, 96%, and 66%, respectively, without initial pH control. After this treatment, the color, COD, TSS, and heavy metals were significantly decreased, fitting the Korean discharge regulation limit. A hybrid coupled zero valent iron (ZVI)/H₂O₂ oxidation process with novel nanosized metallic calcium/iron dispersed reagent proved to be a suitable treatment for dealing with leachate samples.

Implications: Conventional treatments (biological or physicochemical) are not sufficient anymore to reach the level of purification needed to fully reduce the negative impact of landfill leachates on the environment. This implies that new treatment alternatives species must be proposed. A coupled zero valent iron (ZVI)/H₂O₂ oxidation process proved to be a suitable treatment for dealing with leachate samples. Coagulation with nFe/Ca/CaO allows 91–99% of heavy metals removal. The coupled coagulation–oxidation process by nFe/Ca/CaO reveals excellent ability to treat leachate. After coupled treatment the color, COD, and TSS were also much lower than the discharge regulation limit.

Introduction

Sanitary landfill for municipal solid waste (MSW) disposal continues to be widely accepted and used in several countries. This method generally offers lower operation and maintenance costs compared to others (Wang et al., 2002; Tatsi et al., 2003). MSW incineration is another waste disposal choice, but only when simpler and less expensive choices are unavailable. However, entering the incineration facility, waste trucks make their way to the tipping hall and the waste is offloaded. Water microspray is employed to suppress dust generation. This process results in the generation of fresh leachate/wastewater (U.S. Environmental Protection Agency (EPA), 1995; Ehring 1982). However, both leachates from MSW landfill sites/tipping halls are often defined as hazardous and heavily polluted wastewaters (Ehring, 1982; EPA, 1995; Wang et al., 2002; Tatsi et al., 2003). Landfill leachate may contain large amounts of organic contaminants, which can be measured in terms of chemical oxygen demand (COD), ammonia, halogenated hydrocarbons, suspended solids, heavy metals, and inorganic salts (Amokrane et al., 1997; Wang et al., 2002; Tatsi et al., 2003). The specific composition of the leachate determines its relative treatability (Mott et al., 1987; Diamadopoulos, 1994; Kang et al., 2002a; Aguilar et al., 2005). With the continuous hardening of the discharge standards in most countries and the aging of landfill sites with more and more stabilized leachates, conventional treatments (biological or physicochemical) are not sufficient to reach the level of purification needed to fully reduce the negative impact of landfill leachates on the environment (Lema et al., 1988; Petruzzelli et al., 2000; Trebouet et al., 2001; Marttinen et al., 2002). Therefore, new treatment alternatives must be proposed (Renou et al., 2008; Nwabanne et al., 2009; Dabhade et al., 2009; Uemura, 2010).

Advanced oxidation processes (AOPs) involving the production of highly reactive hydroxyl radicals (•OH) have shown great potential for organic pollutant treatment in wastewater. On the other hand, the catalytic decomposition of hydrogen peroxide by ferrous ions (i.e., Fenton’s reagent) was first described by Fenton and is one of the most commonly used AOPs for wastewater treatment (Huang et al., 1993; Trujillo et al., 2006; Hermosilla et al., 2009). Recently there have been numerous reports about landfill leachate treatment by the classical Fenton process (H₂O₂/ferrous salt) (Trujillo et al., 2006; Deng et al., 2007; Zhang et al., 2005; Zhang et al., 2009). However, its major disadvantage is that when the chloride or sulfate salt of iron (i.e., FeCl₃ and FeSO₄) is used, the production of Fe(OH)₃ sludge requires further separation and proper disposal, with a consequent increase in operation costs (Benatti et al., 2006). In addition to the classical Fenton reaction, recent studies have focused on the Fenton process with alternative iron sources (Barbusinski and Majewski, 2003) such as elemental iron (Fe°), which is one of the most widely used zero-valent metals (such as Fe°, Zn°, Sn°, and Al°) because it is readily available, inexpensive, and nontoxic (Joo and Cheng, 2006). These results have opened promising perspectives since their conception, as fast and economical alternative iron sources for Fenton reactions. Some authors document information on zero-valent iron (ZVI) effectiveness in Fenton oxidation in a variety of contaminants, such as treatment of methyl tert-butyl ether (MTBE) (Bergendahla and Thies, 2004), discoloration of azo dye compounds (Barbusinski and Majewski, 2003), dichlorodiphenyltrichloroethane (DDT) degradation (Boussahel et al., 2007), phenolic compound removal (Barbusinski and Majewski, 2003; Kallel et al., 2009a), and COD reduction (Barreto et al., 2008; Kallel et al., 2009b). On the other hand, not many studies consider heavy metal treatment in landfill leachates (Jung et al., 2009).

Recently, utilization of nanorange (1–200 nm) catalysts in water purification has become more popular (Kallel et al., 2009a; Kallel et al., 2009b). In terms of catalysts or reagents for water purification, nanoparticles offer a high specific surface area (SSA), and in some cases (e.g., for nano-zero-valent iron [nZVI]) different properties in comparison to their bulk phase might positively influence the reaction efficiency (Barreto et al., 2008). This can be an advantage for the treatment of large contaminant molecules such as dyes, organic loads, and heavy metals, which are considerably limited in diffusion through microporous catalysts.

On the other hand, we have previously reported that treatment of waste containing multipollutants (i.e., persistent organic pollutants [POPs], heavy metals, and radioactive cesium) with a nanometallic calcium/iron dispersion mixture is most effective (Mallampati et al., 2012; Mitoma et al., 2013; Mallampati et al., 2014a; Mallampati et al., 2014b; Mallampati et al., 2015). Nanometallic calcium/iron has been never used as a catalyst for the purpose of multicontaminant treatment in landfill leachate. The nanometallic iron/calcium is a promising catalyst for oxidative processes using H₂O₂. Its catalytic properties are often attributed to its Fe° content, which is considered to be the key parameter in Fenton-like reactions (Hanna et al., 2008; Klara et al., 2012; Xue et al., 2009). Although for larger particles the formation of a passivating shell can prevent complete oxidation of iron, nanosized iron may be more sensitive in this process. Furthermore, the magnetic properties of nanosized iron allow easy and complete separation from the treated medium by means of magnetic separation (Sun et al., 1998).

The aim of this investigation is to synthesize and characterize a novel nanosize metallic calcium/iron dispersed reagent as a coagulant/catalyst in the hybrid zero valent iron (ZVI)/H₂O₂ oxidation process to treat two different type leachates, one from an MSW tipping hall (MSWIL) and the second from an MSW landfill site (MSWLL). Contrary to other studies in which H₂O₂ is added to the raw leachate together with the coagulant (Hanna et al., 2008; Klara et al., 2012; Xue et al., 2009), the idea developed here was to use in a first step nano-Fe/Ca/CaO as coagulant in order then to use the nonprecipitated Fe remaining in dissolution as a catalyst in the oxidation process. By the nanocoagulation process, the colloidal particles’ electrostatic surface potential is neutralized. The resulting destabilized particles stick together into larger floes. These floes are able to aggregate with suspended polluting matter. Therefore, nanocoagulantion initiates organic compounds encapsulation through the formation of immobile salts, for example, by reacting with water, yielding a Ca-associated (CaCO₃/Ca(OH)₂) precipitate that encloses/binds organic particle surfaces (Mallampati et al., 2012; Mallampati et al., 2014b) (eq 1). On the other hand, with nFe/Ca/CaO, the iron particle addition in the aqueous phase dissolves ferrous ions and electrons transfer to react with the organic molecules on the iron’s surface. This has proved to be a fast and successful process to reduce a variety of organic compounds (eq 2).

(1)

(2)

In the zero valent iron (ZVI)/H₂O₂ oxidation process, ferrous ion reacts with hydrogen peroxide, producing a hydroxyl radical. (OH). The oxidation reaction is characterized by the catalytic decomposition of H₂O₂ as described in the following (eqs 3–5). Hydroxyl radicals created by the process can be scavenged by excess Fe²⁺ (eq 4). The major reactions for the generation of oxidizing radicals may be represented in the following (eq 5). The nano-iron comes from the excess of coagulant (nFe/Ca/CaO), rendering unnecessary further additions. Furthermore, treated compounds that do not precipitate in the coagulation process may be removed in the subsequent zero valent iron (ZVI)/H₂O₂ oxidation process. Therefore, this hybrid coupled coagulation–Fenton process with nFe/Ca/CaO and H₂O₂ appears to be a suitable treatment of leachate.

(3)

(4)

(5)

The advantages of this coupled process are threefold: (i) Coagulation considerably reduces leachate COD and color, thus leading to a lower requirement for H₂O₂, (ii) nanosize iron (Fe°) comes from the excess of coagulant (nano-Fe/Ca/CaO), rendering unnecessary further Fe addition, and (iii) unprecipitated/untreated compounds that do not precipitate/are not treated in the coagulation process may be removed in the subsequent Fenton process. By this treatment method the production of residual sludge can be minimized/eliminated. The coagulation with nano-Fe/Ca/CaO was optimized to achieve the best starting conditions for the Fenton process by generating a low COD and high Fe concentration. The morphology, elemental composition, and mineral phases of the nano-Fe/Ca/CaO were characterized. The Fenton process was also optimized in terms of the amount of nano-Fe/Ca/CaO and H₂O₂ doses.

Material and methods

Leachate sample collection and characterization

Two leachate samples were taken by polyethylene bottles from the municipal sanitary landfill, located in Ulsan, South Korea, in July 2015. First, fresh leachate was collected from the municipal solid waste reception area/tipping hall and named MSWIL. Next, raw leachate was collected from the influent end of the detention pond near the entrance of the landfill and named MSWLL. Both leachate samples were stored in a cold room in the laboratory at 4ºC to minimize biological and chemical reactions (American Public Health Association [APHA], American Water Works Association [AWWA], and Water Pollution Control Federation [WPCF], 1995). The leachate quality main parameters (pH, total suspended solids [TSS] color, COD, and heavy metals) were measured according to standard methods (APHA, 1992; APHA, AWWA, and WPCF, 1992, 1995). The pH measurement was performed using a pH meter (HACH sension 4) and a pH probe (HACH platinum series pH electrode, model 51910, HACH Co., USA). Color measurements were reported as true color (filtered using 0.45-µm filter paper) assayed at 400 nm using a DR 2800 HACH spectrophotometer adapted from the Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, and WPCF, 1992, 1995). The effects of filtration on color removal were corrected by control sample (pure water) comparison. COD was estimated by the standard dichromate method ASTM D1252 (ASTM, 2012). Heavy metal concentrations were measured using inductively coupled plasma–optical emission spectrometry (ICP-OES; Varian, 720-ES). Removal efficiency of color, COD, TSS and heavy metals was obtained using the following equation:

(6)

where C_i and C_f are the initial and final concentrations of leachate before and after treatment, respectively.

Nanometallic calcium/iron dispersed reagent synthesis

Solvent-free production of the coagulant/catalyst reagent, a dispersed mixture of nanometallic Ca/CaO (nCa/CaO) and nano-Fe/Ca/CaO (nFe/Ca/CaO), was through a planetary ball-milling process. Unlike conventional methods such as chemical synthesis and vapor-phase condensation, which typically involve toxic chemicals, sophisticated equipment, and extensive labor, the precision ball-milling method relies solely on mechanical impact forces generated by stainless-steel beads in a high-speed rotary chamber to break down the (nCa/CaO and nFe/Ca/CaO) particles (Mallampati et al., 2012; Mallampati et al., 2014a; Mallampati et al., 2014b; Mallampati et al., 2015). The system uses no toxic solvents and is scalable to large-scale manufacturing.

Granular particles of metallic calcium (Ca) (99%, 2–2.5 mm particle size distribution; 0.43–0.48 m² g⁻¹ surface area) and iron (Fe) (0.15 mm diameter) were purchased from Kishida Chemical Co. Ltd. The nCa/CaO and nFe/Ca/CaO (dry system) systems were prepared by combining metallic Ca and CaO, and Fe, Ca, and CaO, respectively, through a mechanochemical process. At room temperature under Ar, dry CaO (preheated at 825°C for 2 hr), and dry metallic Ca/iron (Fe) (Ca/CaO = 2/5; Fe/Ca/CaO = 2/2/5 ratios) were introduced into a planetary ball mill (20 pieces SUS, 32 g/ball). Milling was conducted for 1 hr at 600 rpm to achieve a rotation-to-revolution ratio of 1 to –2. The conditions for the dispersed nanosystems preparation were established after performing several experiments to ascertain optimal conditions (Mallampati et al., 2012; Mallampati et al., 2014a; Mallampati et al., 2014b; Mallampati et al., 2015). After preparation, the nCa/CaO and nFe/Ca/CaO systems were sampled for additional leachate treatment studies.

Nanometallic calcium/iron dispersed reagent characterization

To characterize the nCa/CaO and nFe/Ca/CaO composites, we employed scanning electron microscopy combined with electron dispersive spectroscopy (SEM-EDS), microanalysis, and elemental map and semiquantitative analyses (JSM6510A equipped with a Si(Li) probe with 138 eV resolution; JEOL). The SEM images were obtained at a magnification of 1,000× and 10,000×. Transmission electron microscopy (TEM) analysis was also carried out to characterize single particle of nFe/Ca/CaO composites (H-8100, Hitachi).

X-ray powder diffraction (XRD) analyses were performed to identify crystalline phases on the surface of nCa/CaO and nFe/Ca/CaO composites, and were conducted using XRD (RINT, Rigaku) using Cu Kα Ni-filtered radiation (λ = 15.418 Å). XRD data were collected for values of 2θ from 10 to 90º. In the cases of freshly prepared samples, a drying step under inert atmosphere was performed and the samples were subjected directly to analysis. Particle size distributions of nCa/CaO and nFe/Ca/CaO were performed by a ZETA-SIZER, nano-series, Malvern (nano-ZS) in ethanol.

Leachate treatment with nanometallic calcium/iron dispersed reagents

Prior to the experiments, the leachate-containing bottles were sufficiently mixed in order to make homogeneous conditions for all samples. A preliminary set of blank assays was performed in order to characterize the evolution of pH, COD, TSS, color, and heavy metal removal without addition of any coagulants. Next, the two types of coagulants used were nCa/CaO and/or nFe/Ca/CaO. Different doses, such as adding 0.5, 1.0, and 2.5 g L⁻¹ of coagulant (nCa/CaO or nFe/Ca/CaO), were slowly added to a 1000-mL leachate sample in Erlenmeyer flasks kept in cool water. The coagulant and the leachate were dynamically mixed for 10 min at 500 rpm, followed by slow mixing for 30 min at 60 rpm by a magnetic stir bar and allowed to settle for 10 min. To study the effects of coagulant (nCa/CaO or nFe/Ca/CaO) doses on the coagulation process, supernatant was withdrawn using a pipette from a point located ~2 cm below the liquid level for pH, COD, TSS, color, and heavy metal measurement. For the purpose of coagulation, the initial pH of leachate was not adjusted, directly treated. The laboratory procedures met all quality assurance/quality control (QA/QC) requirements, including the use of triplicate experimental replication and matrix spikes.

The zero valent iron (ZVI)/H₂O₂ oxidation treatment

In order to improve the nFe/Ca/CaO efficiency on color, COD, and TSS removal we explored hydrogen peroxide (H₂O₂) addition, creating the Fenton treatment. In the zero valent iron (ZVI)/H₂O₂ treatment experiments, a fixed dose of H₂O₂ (20 mM) was added to the coagulation process. Leachate samples were treated in precisely the same manner described in the preceding, and supernatant was sampled to determine COD, color, and TSS so that H₂O₂ addition effects on Fenton coagulation (nFe/Ca/CaO) could be studied. The treatment efficiency was measured with respect to nFe/Ca/CaO dose and addition of H₂O₂ solution, respectively. For COD analysis, all tests were made after the total removal of residual H₂O₂ from all samples, because the residual H₂O₂ increases the COD value since it acts as a reductant, especially in chromate-based analysis of COD (Barbusinski and Filipek, 2001). Deng’s method was used to eliminate the remaining H₂O₂ and stop the oxidation reaction (Deng, 2007).

Results and discussion

Leachate characterization

The main characteristics of the leachate are presented in Table 1. All values presented in the table are the average values of three replications. Table 1 suggests that the both leachates have moderate concentrations of color, organic matter, and heavy metals. However, the heavy metals concentrations are high in MSWIL. The high levels of pH and heavy metals in MSWIL may be due to the presence of some type of heavy-metal-contaminated waste or chemical agent (cleaning, deodorizing), or due to some very alkaline waste having recently been freshly tipped. Further, the values of color, COD, and heavy metals exceed the standard discharge limits of the Korean Water Quality Standards (Bae et al., 2012) (Table 1). The presence of high concentrations of color and COD was supplemented by the presence of dissolved organics. These organic compounds may present in the form of recalcitrant humic acid and carbohydrate materials (Artiola and Fuller, 1982; Langlais et al., 1991). This implies that there is a need to treat the leachate before it can be discharged.

Table 1. Properties of the two different leachates used for this study.

	MSWIL (mg L^−1)	MSWLL (mg L^−1)	Discharge limit (mg L^−1)
pH	12.37 ± 0.12	7.01 ± 0.36	6.5–9
Color (nTu)	240 ± 2.89	222 ± 1.15	20
TSS	53 ± 1.73	70 ± 1.15	20
COD	472 ± 4.62	352 ± 4.68	50
As	1.149 ± 0.01	1.109 ± 0.01	0.05
Cd	1.134 ± 0.01	0.104 ± 0.02	0.005
Cr	1.17 ± 0.01	0.11 ± 0.01	0.001
Co	1.18 ± 0.01	0.13 ± 0.01	0.05
Fe	1.153 ± 0.05	1.123 ± 0.07	1.0
Pb	1.487 ± 0.04	1.087 ± 0.08	0.05
Mn	0.189 ± 0.02	0.139 ± 0.03	0.05
Ni	0.113 ± 0.02	0.103 ± 0.02	0.05
Zn	1.425 ± 0.07	1.125 ± 0.08	0.05

Nanometallic calcium/iron dispersed reagent characterization

SEM images of the nCa/CaO and nFe/Ca/CaO composite were taken at two different magnifications, 1,000× and 10,000×. Microstructure images are presented in Figure 1. The nCa/CaO and nFe/Ca/CaO particles exhibit porous surfaces, as shown in Figures 1a and 1c. We also observe that the nCa/CaO and nFe/Ca/CaO microstructures include solid spherical and irregularly shaped amorphous particles that formed chain-like aggregates, which are apparent at higher magnifications (Figure 1b and 1d). The maximum particle size distribution observed was 263 and 206 nm for nCa/CaO and nFe/Ca/CaO, respectively. Through XRD analysis, the broad peaks at 2θ of 45º and 85º confirmed the presence of elemental iron (Fe) in the nFe/Ca/CaO sample. A similar result was obtained by Giasuddin (2007). This peak is referred to as the XRD spectrum, as shown in Figure 2. However, the surface compositions of the nCa/CaO and nFe/Ca/CaO particles were slightly different chemically.

Figure 1. SEM and TEM microstructure of nano-Fe/Ca/CaO at different magnifications: (a) nFe/Ca/CaO at 1,000× magnification and (b) nFe/Ca/CaO at 10,000× magnification, (c) TEM analysis of single particle at 200nm, and (d) TEM analysis of single particle at 20 nm.

Figure 2. X-ray diffraction (XRD) pattern of nCa/CaO and nFe/Ca/CaO.

The surface composition was analyzed using an EDS attached to the SEM. Figure 3 illustrates the EDS elemental maps of the nCa/CaO and nFe/Ca/CaO. Table 2 shows the percentage of elements present. All values presented in the table are the average values of three experimental replications. The EDS elemental mapping image of nCa/CaO shows that the surface is primarily comprised of calcium (Ca) and oxygen (O₂). The nFe/Ca/CaO surface is primarily comprised of calcium (Ca), oxygen (O₂), and iron (Fe) (Figure 3). As presented in Table 2, Ca, at 65.8 mass percent, was the most abundant element present on the surface of nCa/CaO, followed by O at 34.2 mass percent. In nFe/Ca/CaO, Fe at 6.2 mass percent was observed, in addition to Ca at 62.4 and O at 31.4 mass percent, respectively.

Table 2. Surface chemical compositions of nCa/CaO and nFe/Ca/CaO.

	nCa/CaO		nFe/Ca/CaO
Elements	Mass (%)	Atom (%)	Mass (%)	Atom (%)
O	34.2 ± 1.3	56.4 ± 1.6	31.4 ± 1.2	54.1 ± 1.3
Ca	65.8 ± 1.8	43.6 ± 1.4	62.4 ± 1.5	42.8 ± 1.1
Fe	0.1 ± 0.01	0.1 ± 0.01	6.2 ± 0.61	3.1 ± 0.26

Figure 3. SEM-EDS elemental mapping of nCa/CaO and nFe/Ca/CaO surfaces.

Leachate treatments with nanometallic calcium/iron dispersed reagents

A preliminary set of blank assays was performed in order to compare the efficiency of removal of color, COD and TSS with different doses of nCa/CaO and/or nFe/Ca/CaO coagulation in both MSWIL and MSWLL samples. Table 3 shows the averages values of three experimental replications. As presented in Table 3, nCa/CaO treatment revealed low color, COD, and TSS removal capacity, although by adding 2.5 g L⁻¹, it attained 29.2, 12.7, and 5.7% of color, COD, and TSS, respectively in MSWIL. Noticeably, nCa/CaO showed better removal efficiency, that is, 64.0, 56.0, and 20.7% for color, COD, and TSS, respectively, in MSWLL. On the other hand, only with 1.0 g L⁻¹ of nFe/Ca/CaO dose there was relative enhancement removal of color of about 45.8 and 67.5% from 240 to 130 nTu and 222 to 72 nTu achieved in both MSWIL and MSWLL samples, compared with nCa/CaO treatment (Table 3). Similarly, the COD concentration was reduced (33.9 and 60.2%) from 472 to 312 mg L⁻¹ and 352 to 130 mg L⁻¹ in both MSWIL and MSWLL leachate samples, respectively, with 1.0 g L⁻¹ nFe/Ca/CaO. We observed more evidence of this removal trend in TSS, with 1.0 g L⁻¹ nFe/Ca/CaO (17.0 and 37.7%) in MSWIL and MSWLL, respectively. Color removal efficiency, COD, and TSS were higher than when other conventional coagulation methods are applied (Amokrane et al., 1997; Petruzzelli et al., 2000; Trebouet et al., 2001; Tatsi et al., 2003).

Table 3. Evolution of landfill leachate characteristics during varying coagulation concentrations (0.5, 1.0, and 2.5 g L⁻¹; nCa/CaO and nFe/Ca/CaO) and treatment processes.

	MSWIL							MSWLL
	nCa/CaO				nFe/Ca/CaO			nCa/CaO				nFe/Ca/CaO
	0.0^#	0.5	1	2.5	0.5	1	2.5	0.0^#	0.5	1	2.5	0.5	1	2.5
Color (nTu)	240 ± 2.8	216 ± 2.4	193 ± 2.1	170 ± 1.8	148 ± 1.6	130 ± 1.4	125 ± 1.2	222 ± 1.1	84 ± 1.0	81 ± 1.2	80 ± 1.1	73 ±1.3	72 ± 1.0	70 ± 1.1
COD (mg L^−1)	472 ± 4.6	408 ± 4.8	400 ± 2.9	412 ± 2.8	332 ± 2.4	312 ± 2.3	313 ± 2.1	352 ± 4.6	173 ± 1.9	163 ± 1.0	155± 1.2	160 ± 1.4	140 ± 1.4	135 ± 1.4
TSS(mg L^−1)	53 ± 1.7	52 ± 1.3	51 ± 1.4	50 ± 1.1	49 ± 1.1	44 ± 1.3	40 ± 1.1	70 ± 1.5	45±1.3	44 ±1.0	42± 1.0	35±1.0	33 ± 1.0	30 ± 1.0
pH	12.4± 0.1	12.6± 0.1	12.8± 0.2	12.9± 0.2	12.4± 0.1	12.5± 0.1	12.6± 0.1	7.01± 0.4	7.81± 0.5	7.9± 0.6	8.1± 0.7	7.6± 0.4	7.8± 0.2	7.9± 0.1

^#Blank assay test.

On the other hand, we do not observe further color, COD, and TSS reduction by coagulant dose increase to 2.5 g L⁻¹ in leachate samples with nFe/Ca/CaO (Table 3). This behavior may be explained by the charge neutralization theory (Melia, 1972; Sincero and Sincero, 2003; Wang et al., 2002). When the coagulant nCa/CaO was added to the leachate, cations and hydrolyzed products interacted with negative colloids to neutralize their charges, which promoted colloid destabilization. In the appropriate dosage, colloids can absorb cations and become positively charged, and hence may stablize again as a result of electrical repulsion. This process diminishes the capacity for organic matter removal (Melia, 1972; Sincero and Sincero, 2003; Wang et al., 2002).

However, the highest removal of color and COD was achieved at a dose of 1.0 g L⁻¹ of nFe/Ca/CaO in the MSWLL sample compared with the MSWIL (Table 3). This further may be because, at pH 7, nFe/Ca/CaO can effectively react with organic substances in the MSWLL leachate. In general, organic colloids are hydrophilic while inorganic colloids are hydrophobic. The primary charges on hydrophilic colloids are due to polar groups such as carboxylic (-COOH) and amine (-NH₂) groups (Sincero and Sincero, 2003). Under acidic conditions the H⁺ counterions neutralize the primary charges, reducing the zeta potential and the force of repulsion between particles (Melia, 1972). At pH 7, the highest removal of COD, TSS, and color occurred, resulting from agglomeration and settling of the colloids. Wang et al. (2002) observed that within the pH range 3–8, coagulation–flocculation was most effective in the lower pH range for landfill leachate. On the other hand, under the alkaline condition of pH 12.3, MSWIL nFe/Ca/CaO reactivity with organic substance is less, because of the large amount of nonbiodegradable organic matter. In contrast, in both leachate samples, TSS removal was comparatively decreased in terms of color and COD removal (Table 3). This may be due to the precipitation of presented metallic Ca and CaO to CaCO₃/Ca(OH)₂ (Mallampati et al., 2014a; Mallampati et al., 2015).

Heavy metal removal efficiency reached about 91–99% after treatment with 1.0 g L⁻¹ of nFe/Ca/CaO (Figures 4a and 4b). Of note, after treatment with nFe/Ca/CaO, the heavy metal concentrations were lower than the discharge limit of the Korean Water Quality Standards (Bae et al., 2012). Furthermore, the effects of nCa/CaO and nFe/Ca/CaO dosage on heavy metal removal in MSWIL and MSWLL were studied and the results are presented in Figure 5. With 1.0 g L⁻¹ of nFe/Ca/CaO, dose enhancement of heavy metal removal was achieved in comparison with nCa/CaO in both leachate MSWIL and MSWLL samples.

Figure 4. The effects of nCa/CaO and nFe/Ca/CaO dosage (0.5, 1.0, and 2.5 g L⁻¹) on heavy metal removal from (a) MSWIL and (b) MSWLL.

Figure 5. The effects of H₂O₂ addition, at a dose of nFe/Ca/CaO (0.5, 1.0, and 2.5 g L⁻¹), on (a) color, (b) COD, and (c) TSS removal in the preliminary treatment test for MSWIL and MSWLL. Zero indicates the blank assays test.

The zero valent iron (ZVI)/H₂O₂ oxidation treatment

The highest amounts of removal of color, COD, TSS, and heavy metals were achieved at a dosage of 1.0–2.5 g L⁻¹ of nFe/Ca/CaO in the coagulation process for both MSWIL and MSWLL samples. However, the color, COD, and TSS concentrations in both MSWIL and MSWLL samples still exceeded the standard discharge limits of the Korean Water Quality Standards (Bae et al., 2012). Generally, the coagulation stage is essential to reduce total suspended solids, organic content, and color in order to improve the later treatment’s efficiency. Leachate’s brown color is mainly due to the presence of high concentrations of humic substances, which represent the majority of organic compounds in the leachate (Petruzzelli et al., 2000; Renou et al., 2008). Additionally, these substances are considered hydroxyl radical scavengers. Lower turbidity values favor oxidation performance. Further, in order to improve the nFe/Ca/CaO effiency of color, COD and TSS removal, we studied the influence of hydrogen peroxide (H₂O₂) (coagulation–oxidation process) on leachate treatment with the same manner as for the nFe/Ca/Cao process. Blank assays were also done in order to compare the efficiency of these systems and the classical Fenton process. The removal effect on color, COD, and TSS with and without H₂O₂ addition on different dosages of nFe/Ca/CaO, during the coagulation–oxidation process, in both MSWIL and MSWLL samples is as shown in Figure 5. Color, COD, and TSS removal increased with nFe/Ca/CaO dosage in the 0.5–2.5 g L⁻¹ range. Addition of H₂O₂ enhanced color, COD, and TSS removal, at 95%, 96%, and 66%, respectively, with the 1.0 g L⁻¹ nFe/Ca/CaO dose, compared to no H₂O₂ addition, in both MSWIL and MSWLL samples (Figures 5a–5c). As described in the preceding, the increase of nFe/Ca/CaO dosage to 2.5 g L⁻¹ did not improve removals significantly.

However, Fenton oxidation was hindered at pH values above 2 because the absence of H⁺ can inhibit H₂O₂ decomposition and reduce the production of .OH radicals. Therefore, the oxidation potential of .OH decreased as pH increased (Zhang et al., 2005; Zhang et al., 2009). pH and the H₂O₂/Fe molar ratio are the most important variables in Fenton process. Several authors have demonstrated that optimal H₂O₂/Fe molar ratio is independent of initial COD and ferrous iron dosage (Zhang et al., 2005; Deng 2007). Furthermore, Zhang et al. (2005, 2009), report that the optimal H₂O₂/Fe molar ratio is related to initial pH. However, in the present work, we achieved perfect COD and color removal without pH control (i.e., we directly used our nFe/Ca/CaO composite).

Generally, during the Fenton oxidation process with the chloride or sulfate salt of iron (i.e., FeCl₃ and FeSO₄) and, mainly during the pH adjustment to 8, a large amount of flocs of various sizes in the wastewater/leachate were observed. According to Walling and Kato (1971), the small flocs were ferric hydroxo complexes formed by complex chain reactions of ferrous and hydroxide ions. Therefore, the major disadvantage is the production of Fe(OH)₃ sludge that requires further separation and proper disposal, with a consequent increase in operation costs (Benatti et al., 2006). However in the present study using nFe/Ca/CaO, the generation volume of the residual chemical/catalytic sludge was very low compared to other reported studies with chloride or sulfate salt of the iron (i.e., chemical/catalytic sludge generation was about 40 mL) (Benatti and Tavares, 2012; Miled et al., 2015). However, in the present study, the volume is very low, only about 8–10 mL. This may be due to the fact that the nanosize iron (Fe°) comes from the excess of coagulant (nano-Fe/Ca/CaO), rendering unnecessary further Fe addition, and unprecipitated/untreated compounds that do not precipitate/are not treated in the coagulation process may be removed in the subsequent Fenton process. These results indicate that the coupled coagulation–Fenton process with nFe/Ca/CaO and H₂O₂ appears to be a suitable treatment of the leachate without any initial pH control.

The mechanism of the Zero valent iron (ZVI)/H₂O₂ process with a nanometallic calcium/iron dispersed reagent

Figure 6 portrays a schematic diagram of a possible pathway for the enhanced coupled H₂O₂ oxidation process for the treatment of landfill leachate by the novel nanosized metallic calcium/iron dispersal reagent. Colloidal particles generally carry a negative electrical charge. They are surrounded by an electrical double layer (due to attachment of positively charged ions from the ambient solution) and thus inhibit the particles getting too close. They remain finely divided and don’t agglomerate. Due to their low specific gravity, they do not settle out (Figure 6a). When the double layer is compressed by the nanocoagulant (nFe/Ca/CaO) (generally positively charged in this case, Ca²⁺/Fe²⁺), the particles’ electrostatic surface potential is neutralized. The resulting destabilized particles stick together sufficiently when contact is made. Subsequent gentle and prolonged (several minutes) mixing cements the microscopic coagulated particles into larger floes. These floes are able to aggregate with suspended polluting matter. When increased sufficiently in size and weight, the particles settle to the bottom (Figure 6b). Consequently, by the nanocoagulation process, the particle surface area increases and the heavy metal mobility decreases because of the higher adsorption rate (Mallampati et al., 2012; Mallampati et al., 2014a). Therefore, nanocoagulantion initiates heavy metals/organic compounds encapsulation through the formation of immobile salts, for example, by reacting with water, yielding a Ca-associated (CaCO₃/Ca(OH)₂) precipitate that encloses/binds heavy metal/organic particle surfaces (Figure 6b) (eq 1) (Mallampati et al., 2012; Mallampati et al., 2014b).

Figure 6. A possible pathway of coupled zero valent iron (ZVI)/H₂O₂ oxidation process for the treatment of landfill leachate: (a) nFe/Ca/CaO acts as coagulant, (b) heavy metals and organic compound encapsulation through the formation of immobile salts, and (c) nonprecipitated Fe remaining in dissolution was used as catalyst in the further Fenton process.

On the other hand, with nFe/Ca/CaO, the iron particle addition in the aqueous phase dissolves ferrous ions and electrons transfer to react with the organic molecules on the iron’s surface (eq 2). This has proved to be a fast and successful process to reduce a variety of organic compounds. It is intended that toxic contaminants (e.g., heavy metals and organic contaminants) would be able to degrade (Kang et al., 2002b; Vedrenne et al., 2012; Taha and Ibrahim, 2014). Nanosized iron particles with high specific surface area can decrease the color/COD concentration. Removal efficiencies of color/COD incremented promptly during the treatment (Figure 6b). Furthermore, the control mechanism is a contaminant metal function of the standard redox potential in the presence of nano-iron (Fe).

Heavy metals and organic compounds are adsorbed directly to the nano-Fe surface, where they are rendered immobile—for example, Zn²⁺, Cd²⁺, Cr⁶⁺, Cu²⁺, Pb²⁺, As³⁺, and more (Kanel et al., 2005; Uzum et al., 2009). The standard redox potential of zerovalent iron is −0.41 V, which is similar to that of Cd²⁺ (−0.40 V), Cr³⁺ (−0.42 V), Pb²⁺ (−0.13 V), and As³⁺ (−0.23 V). Therefore, the removal of As, Cd, Cr, and Pb ions by nano-Fe results from sorption (Li and Zhang, 2007). Consequently, the treatment of leachate with nano-Fe/Ca/CaO can reduce heavy metal leachability while minimizing their solubility by the reduction of heavy metals, fixing by adsorption, or heavy metal precipitation (Figure 6b).

The amount of organic compounds and heavy metals exposed to leaching is reduced. In order to achieve the best conditions for Fenton oxidation, the coagulation process was intended to reduce COD as much as possible to avoid interference from other organic compounds and to reduce the dose of H₂O₂ required. This was accomplished by maintaining a high concentration of dissolved iron (Perdigon-Melon et al., 2010; Carlos et al., 2015). In the zero valent iron (ZVI)/H₂O₂ oxidation process, ferrous ion reacts with hydrogen peroxide, producing a hydroxyl radical. (OH). The oxidation reaction is characterized by the catalytic decomposition of H₂O₂ as described earlier (eqs 3 to 5) Figure 6c. Hydroxyl radicals created by the process can be scavenged by excess Fe²⁺ (eq 4). The major reactions for the generation of oxidizing radicals may be represented by eq). The nano-iron comes from the excess of coagulant (nFe/Ca/CaO), rendering unnecessary further additions. Furthermore, treated compounds that do not precipitate in the coagulation process may be removed in the subsequent zero valent iron (ZVI)/H₂O₂ oxidation process (Figure 6c). Therefore, this hybrid coupled coagulation–Fenton process with nFe/Ca/CaO and H₂O₂ appears to be a suitable treatment of leachate.

Conclusion

A coupled zero valent iron (ZVI)/H₂O₂ oxidation process with novel nanosize metallic calcium/iron dispersed reagent proved to be a suitable treatment for dealing with leachate samples. The coupling process, using a 1.0 g L⁻¹ of nFe/Ca/CaO and 20 mM of H₂O₂, achieved enhancement of color, COD, and TSS removal, at 95%, 96%, and 66%, respectively, without initial pH control. The heavy metal removal efficiency also reached about 91–99% after treatment nFe/Ca/CaO in both leachate samples. Further, the color, COD, TSS, and heavy metals concentrations were lower than the discharge limits of the Korean Water Quality Standards. These results encourage the idea that the hybrid coagulation–Fenton process with nFe/Ca/CaO appears to be a suitable treatment of the leachate.

Funding

This work was supported by the 2017 Research Fund of University of Ulsan.

Additional information

Funding

This work was supported by the 2017 Research Fund of University of Ulsan.

Notes on contributors

Son Dong Lee

Son Dong Lee is a Ph.D. student in the Department of Civil and Environmental Engineering, University of Ulsan, Ulsan, Republic of Korea.

Srinivasa Reddy Mallampati

Srinivasa Reddy Mallampati is an assistant professor in the Department of Civil and Environmental Engineering, University of Ulsan, Ulsan, Republic of Korea.

Byoung Ho Lee

Byoung Ho Lee is a professor in the Department of Civil and Environmental Engineering, University of Ulsan, Ulsan, Republic of Korea.