This review deals with low-cost nanoporous zeolites for the treatment of sanitary landfill leachate. Organic contaminants and ammoniacal nitrogen are significant parameters in landfill leachate treatment. Adsorption processes are regarded as promising alternative treatment options in this respect. Zeolites are aluminosilicate materials that are widely used in separation, ﬁltration, adsorption and catalysis. Natural zeolite is a low-cost and readily available form of zeolite and is a promising candidate to be used as an ion-exchange material for ammonia and other inorganic pollutant removal from landfill leachate. In this review, adsorption isotherms and kinetic models in batch systems are evaluated and adsorption design parameters of the fixed-bed system are presented. Studies on ammonia removal from landfill leachate via zeolites have been thoroughly investigated. Leachate treatment systems combined with zeolites are presented. Cost of zeolites are also reported in comparison with other adsorbents. The investigated studies demonstrate that activated zeolite can improve the removal of chemical oxygen demand, NH3-N and colour significantly compared to the case where raw zeolite is used. Moreover, the composite of activated carbon and zeolite is also favorable for ammonia removal according to reported findings, where best adsorptive removal is attained on the composite media (24.39 mg/g).

• High ammonium contents in leachate is a serious environmental problem.

• Zeolite is an inexpensive material for the ammonia removal.

• Integration of zeolites to landfill leachate treatment systems increased ammonia removal.

• The column efficiency for ammonia adsorption increased after the regeneration process.

• The activated zeolite can improve the removal of COD and NH3-N from landfill leachate.

Sanitary landfilling is the most common municipal solid waste management practice followed throughout the world. The major problems caused by landfilling are related to the generation of highly contaminated leachates that pose long-term environmental problems. The sanitary landfill leachate composition varies depending on the site, season and age of the landfill. As the landfill age increases, this results in the decrease of organic concentration and the increase of ammonia–nitrogen concentration in landfill leachate. There are three types of leachates which have been classified according to the landfill age as shown in Table 1 and the characteristics of landfill leachate used in the literature is presented in Table 2 (Turan et al. 2005a, 2005b; Yalcuk & Ugurlu 2009; Martins et al. 2017; Pauzan et al. 2020; Scandelai et al. 2020). Ammonium is the most significant long-term pollutant present in leachate while its concentration can reach up to 4,000 mg/L (Turan et al. 2005a, 2005b; Lin et al. 2007; Gunay et al. 2008). Conversely, the release of high levels of ammonium into water bodies without proper treatment can have toxic effects on aquatic organisms. Therefore, the treatment of high ammonium contents in the leachate is of great importance to avoid seriously adverse environmental effects.

Table 1

Landfill leachate classification by age (Alvarez-Vazquez et al. 2004)

ParameterOldYoungMedium
Age (year) >5.0 <1 1–5
pH >7.5 <6.5 6.5–7.5
COD (g/L) <3.0 >15 3.0–1.5
BOD5/COD <0.1 0.5–1 0.1–0.5
TOC/COD >0.5 0.3–0.5 0.3–0.5
Ammonium nitrogen (mg/L) >400 <400 400
Heavy metals (mg/L) <2.0 >2.0 <2.0
Organic compound HA+FA 80% VFA 5–30% VFA+HA+FA
ParameterOldYoungMedium
Age (year) >5.0 <1 1–5
pH >7.5 <6.5 6.5–7.5
COD (g/L) <3.0 >15 3.0–1.5
BOD5/COD <0.1 0.5–1 0.1–0.5
TOC/COD >0.5 0.3–0.5 0.3–0.5
Ammonium nitrogen (mg/L) >400 <400 400
Heavy metals (mg/L) <2.0 >2.0 <2.0
Organic compound HA+FA 80% VFA 5–30% VFA+HA+FA

BOD, biological oxygen demand-5 days; COD, chemical oxygen demand; FA, fulvic acid; HA, humic acid; TOC, total organic carbon; VFA, volatile fat acids.

Table 2

Characteristics of landfill leachate used in the literature

Parameter (g/L)Turan et al. (2005a, 2005b)Yalcuk & Ugurlu (2009) Martins et al. (2017) Pauzan et al. (2020) Scandelai et al. (2020)
COD 20–50 2.93–14.65 4.35 1.72–2.08 1.05–2.57
TSS 37.5–46 – – 1.02–11.5 –
TDS 17–35.7 – – 0.88–1.04 –
Alkalinity as CaCO3 12–13 – 10.67 – –
TKN 1.63–2.75 – – 2.2–2.4 –
NH3-N 1.03–2.35 1.7–4.01 2.29 1.10–1.89 0.21–0.51
PO4-P – 0.17–4.01 0.051 – –
NO3-N – 0.058–0.112 – 0.235–0.376 0.01–0.084
pH 7.5–8 – 8.2–8.36 8.3–8.5 7.7–8.3
Parameter (g/L)Turan et al. (2005a, 2005b)Yalcuk & Ugurlu (2009) Martins et al. (2017) Pauzan et al. (2020) Scandelai et al. (2020)
COD 20–50 2.93–14.65 4.35 1.72–2.08 1.05–2.57
TSS 37.5–46 – – 1.02–11.5 –
TDS 17–35.7 – – 0.88–1.04 –
Alkalinity as CaCO3 12–13 – 10.67 – –
TKN 1.63–2.75 – – 2.2–2.4 –
NH3-N 1.03–2.35 1.7–4.01 2.29 1.10–1.89 0.21–0.51
PO4-P – 0.17–4.01 0.051 – –
NO3-N – 0.058–0.112 – 0.235–0.376 0.01–0.084
pH 7.5–8 – 8.2–8.36 8.3–8.5 7.7–8.3

TDS, total dissolved solids; TKN, total Kjeldahl nitrogen; TSS, total suspended solids.

The biological treatment of landfill leachates has been shown to be very effective in removing organic matter in early stages when the BOD5/COD ratio of the leachate is high. As a landfill stabilizes in the course of time, the biodegradable organic content of the leachate tends to decrease, which result in the loss of the effectiveness of the biological process. In such cases, physico-chemical processes may become one of the most favorable options to be implemented. Several investigators have reported studies on such leachate treatment methods including coagulation–flocculation (Amokrane et al. 1997), electro-Fenton method (Gau & Chang 1996), membrane processes (Chianese et al. 1999; Visvanathan et al. 2006), biological processes (Inanc et al. 2000; Loukidou & Zouboulis 2001), fixed and fluidized bed reactors (Gulsen & Turan 2004a, 2004b, 2004c; Turan et al. 2005a, 2005b; Karadag et al. 2008b) and sequencing batch reactors (Aziz et al. 2011). The adsorption process provides an attractive alternative for the treatment of sanitary landfill leachate and low-cost and readily available adsorbents have been used for various applications. Many non-conventional low-cost adsorbents, including natural materials, biosorbents and waste materials from industry and agriculture, have been proposed by several workers. Some of the reported sorbents include clay materials (sepiolite, bentonite, kaolinite), zeolites, siliceous materials (silica beads, alunite, perlite), biosorbents (chitosan, peat, biomass), agricultural wastes and industrial waste products.

Zeolites are microporous (pore size <2 nm) crystalline aluminosilicates that are widely used in separation, ﬁltration, adsorption and catalysis (Corma 1995; Danyliuk et al. 2020; Soltys et al. 2020). The catalytic performance of zeolites is generally attributed to the existence of a network of micropores with uniform size and shape. However, micropores can also be detrimental to catalytic reactions by limiting the diffusion of reagents and/or products throughout the crystals. One possibility to minimize diffusion limitations is to use zeolite nanocrystals, with a size typically smaller than 0.5 μm (Jacobsen et al. 2000; Tosheva & Valtchev 2005). Nanoporous zeolites have negative charges that arise due to isomorphous substitution of Al3+ for Si4+ and its typical unit cell formula is given either as Na6[(AlO2)6(SiO2)30].24H2O or (Na2,K2,Ca,Mg)3[(AlO2)6(SiO2)30].24H2O (Breck 1974). The framework structure may contain linked cages, cavities or channels which are of the right size to allow small molecules to enter as shown in Figure 1. The three-dimensional crystal structure of zeolite contains two-dimensional channels (Mortier & Pearce 1981; Ackley & Yang 1991) which embody some ion-exchangeable cations such as Na+, K+, Ca2+ and Mg2+. These cations can be exchanged with organic and inorganic cations (Ames 1960; Barrer et al. 1967; Blanchard et al. 1984). Such sorptive properties have been utilized for a variety of purposes such as removal of ammonia (Gaspard et al. 1983; Turan & Celik 2003), heavy metals (Semmens & Martin, 1988; Mier et al. 2001; Turan et al. 2005a, 2005b) and dyes (Meshko et al. 2001; Faki et al. 2008; Ozdemir et al. 2009).

Figure 1

Scheme of zeolite structure (Bell 2001).

Figure 1

Scheme of zeolite structure (Bell 2001).

Among many other treatment alternatives, ammonium removal by ion exchange is very attractive especially in case a low-cost exchanger is used. Many researchers have reported that this process is promising due to its low cost and relatively simple application (Baykal & Guven 1997; Nguyen & Tanner 1998; Karadag et al. 2006). Clinoptilolite is a natural aluminosilicate zeolite mineral that contains exchangeable cations, such as Na+, K+, Ca2+ and Mg2+ in its structural framework. Ammonium ions in wastewater can replace these cations during the treatment process (Colella 1999; Townshend et al. 2003). Ammonium removal by clinoptilolite can be performed in either batch- or column-wise. Batch studies have been widely preferred by researchers to examine the effects of parameters such as pH, contact time, ammonium concentration, temperature and competitive ions on ammonium removal (Chen et al. 2002; Jorgensen & Weatherley 2003). Batch experiments are more convenient to be performed at laboratory scale, while column studies are more suitable for practical applications of continuous pollutant treatment at higher scales. In this paper, the technical feasibility of zeolite material as a low-cost adsorbent for the treatment of sanitary landfill leachate has been reviewed. The review discusses ammonia adsorption capacities and other parameters for nanoporous zeolites, describes ammonia adsorption onto nanoporous zeolite in batch and fixed bed systems, investigates ammonia removal efficiency for landfill leachate treatment systems combined with zeolites and analyses the application costs of zeolites and other low-cost materials.

### Equilibrium isotherms

Ammonium sorption studies are performed batch-wise to generate rate and equilibrium data. The removal efficiency (%) and the adsorption capacity at equilibrium qe (mg/g) of adsorbent (zeolite) are as follows, respectively:
(1)
(2)
where Ci and Ce are the initial and equilibrium solution concentrations (mg/L), respectively, V is the volume of the solutions (L) and W is the weight of the adsorbent used (g). The widely used Langmuir isotherm (Langmuir 1918) has been found to represent adsorption behavior well in many real sorption processes and is expressed as:
(3)
where Q (mg/g) is the maximum amount of the adsorbate per unit weight of adsorbent to form a complete monolayer coverage on the surface bound at high equilibrium adsorbate concentration Ce, and b is the Langmuir constant related to the affinity of binding sites (L/mg). The well known Freundlich isotherm (Freundlich 1906) used for isothermal adsorption is a special case utilized for heterogeneous surface energy systems, where the energy term in the Langmuir equation is substituted with a function of surface coverage strictly due to variation of the sorption, and the Freundlich equation is given as:
(4)
where Qf is roughly an indicator of the adsorption capacity and 1/n of the adsorption intensity.
The magnitude of the exponent 1/n gives an indication of the favorability of adsorption. Values, n > 1 represent favorable adsorption condition (McKay et al. 1982). The three-parameter Koble–Corrigan (KeC) model is the combination of Langmuir and Freundlich models and is given by Equation (5) while this model is valid provided that m>1 (Aksu & Isoglu 2005):
(5)
Redlich–Peterson (ReP) is a three-constant model, which has been proposed to improve the fit by Langmuir and Freundlich models. Equation (6) reduces to a linear isotherm at low surface coverage, to the Freundlich isotherm at high adsorbate concentration and to the Langmuir isotherm when β=1 (Stephen et al. 2003):
(6)
where KRP, αRP and β are the ReP parameters and β is between 0 and 1. The derivation of the Temkin isotherm assumes that the fall in the heat of adsorption is linear rather than logarithmic, as implied in the Freundlich equation and the Temkin isotherm is given as follows (Fu et al. 1994):
(7)
where A and B are isotherm constants.

### Kinetics and thermodynamics

In order to investigate the mechanism of adsorption, the pseudo-first-order adsorption, the pseudo-second-order adsorption and the intraparticle diffusion models are used to test dynamical experimental data. The first-order rate expression of Lagergren (Lagergren 1898) is given as:
(8)
where qe and q are the amounts of solute adsorbed on adsorbent (mg/g) at equilibrium and at time t, respectively and k1 is the rate constant of first-order adsorption (1/min). The slopes and intercepts of plots of log(qe–q) vs. t are used to determine the first-order rate constant k1. In many cases the first-order equation of Lagergren does not fit well to the whole range of contact time and is generally applicable over the initial stage of the adsorption processes (McKay & Ho 1999a). The second-order kinetic model (McKay & Ho 1999b) is expressed as:
(9)
where k2 (g/mg min) is the rate constant of second-order adsorption and h = k2qe2 is the initial adsorption rate (mg/g min). The slopes and intercepts of plots of t/q vs. t are used to calculate the second-order rate constant k2 and qe. This model is more likely to predict the behavior over the whole range of adsorption. The intraparticle diffusion equation can be described as suggested by Weber & Morris (1963):
(10)
where qt is the adsorption capacity (mg/g) of the adsorbent at adsorption time t, qt is the desorption capacity (mg/g) of the adsorbent at desorption time t, kd is the rate constant of intraparticle diffusion (mg/(g min0.5)) and I is a constant related to the thickness of the boundary layer. The pseudo-second-order rate constant of solute adsorption is expressed as a function of temperature by the Arrhenius type relationship:
(11)
where Ea is the Arrhenius activation energy of sorption, representing the minimum energy that reactants must have for the reaction to proceed, A is the Arrhenius factor, R is the gas constant and (8.314 J/(mol K)) and T is the solution temperature. The thermodynamic parameters such as change in free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) are determined using the following equations (Catena & Bright 1989):
(12)
(13)
(14)
where KC is the equilibrium constant, CAe is the amount of adsorbate (g) adsorbed on the adsorbent per L of the solution at equilibrium, Ce is the equilibrium concentration (g/L) of the adsorbate in the solution, T is the solution temperature (K) and R is the gas constant. ΔHo and ΔSo are calculated from the slope and intercept of van 't Hoff plots of logKC vs. 1/T. The magnitude of activation energy gives an idea about the type of adsorption i.e., whether it is physical or chemical. Generally, the magnitude of the change in free energy for physisorption is between −20 and 0 kJ/mol; chemisorption has a range of −80 to −400 kJ/mol (Nollet et al. 2003). The negative values of the standard enthalpy change (ΔHo) indicate that the interaction of the adsorbate with adsorbent is exothermic in nature.
Adsorption performance of fixed bed systems can be evaluated via by calculating bed volumes (BV) at breakthrough point (C/Co=0.1). The breakthrough curves are constructed by plotting the normalized effluent concentration (C/Co) versus time (t) or BV. The BV and the empty bed contact time (EBCT) at the fixed-bed column are defined as follows, respectively:
(15)
(16)
where VF is the total volume of wastewater treated during the adsorption process at time t (L), VR is the fixed-bed volume of zeolite (L), Co is the influent concentration (mg/L), C is the effluent concentration at time t (mg/L), Q is the feed flow rate (L/min) and t is the service time (min). The formation and the movement of the adsorption zone can be evaluated numerically (Benefield et al. 1982; Kundu & Gupta 2005). The time required for the adsorption zone to become established and move completely out of the bed at exhaust time is:
(17)
The rate at which the adsorption zone (Uz, cm/min) is moving up or down through the bed is:
(18)
From Equation (18), the height of the adsorption zone (hz) is obtained:
(19)
where VExh is the total volume of wastewater treated in the zeolite column at exhaust time (L), hz is the height of adsorption zone (cm), h is the total bed height (cm), and tf is the time required for the adsorption zone to initially form (min). The tf value can be found as follows:
(20)
At breakthrough, the fraction (F) of adsorbent present in the adsorption zone still possessing the ability to remove solute is:
(21)
where Vb is the total volume of the wastewater treated to the breakthrough point (L), Sz is the amount of solute that has been removed by the adsorption zone from breakthrough to exhaustion and Smax is the amount of solute removed by the adsorption zone when completely exhausted. The percentage of the saturation of total column at breakthrough point is:
(22)

The use of zeolite minerals in environmental applications is gaining new research interests mainly due to their physico-chemical properties, wide distribution all over the world and the convenience of its modification (Li et al. 2011). Several modification methods can be applied to enhance the ammonium adsorption capacity of zeolite. Some modification processes reported in the literature are given as follows:

Heat-activated zeolite (HAZ): Zeolite was activated at 150 for 3 h. Batch study experiments were performed to identify the optimum zeolite dosage and the optimum pH of leachate (Aziz et al. 2020). The granular zeolite was filtered to a final working size between 2 and 4 mm. Zeolite was weighed, then cleaned with distilled water and dried in an oven at 105 °C for 2 h. After that, it was heated in a muffle furnace at 150 °C for 3 h and afterwards conditioned in a desiccator for 2 h to obtain the HAZ. The activation process was repeated at two different temperatures namely 200 °C and 250 °C for other batch studies.

Sodium-natural zeolite (SNZ): Natural zeolite (NZ) was crushed and sieved through 200–230 mesh sieve. The zeolite powder was washed with distilled water to remove undesirable materials and dried at 100 °C for 24 h and then modified with sodium chloride. The suspension was stirred in a 500 ml conical flask at 90 °C using a magnetic stirrer water bath at a rate of 120 rpm for 2 h. Subsequently, the suspension was filtered and washed with distilled water. The wet modified material was dried at 100 °C in an oven for 24 h (Alshameri et al. 2014).

Silicate-carbon modified zeolite (SCMZ): SCMZ was used for ammonium removal from drinking water (Li et al. 2011). The modification process applied in the preparation of SCMZ consists of the following steps: In the first step; the clinoptilolite powder (74 μm) was repeatedly washed with tap water and then dried at 100 °C for 4 h. In the second step; zeolite was dried and 1,000 g of the dry zeolite was dispersed into 1 L sodium chloride solution (2 mol/L) that was prepared using tap water and stirring for 12 h. The sample was repeatedly washed with tap water and then dried at 100 °C for 4 h. Finally, the dried sample was ground and sieved to 74 μm again, and designated as sample A. In the final step; sample A, Na2SiO3 and powdered activated carbon were mixed at weight/weight ratio of 100:9:2, and mixed evenly; then, 10% tap water (weight/weight ratio) was added and stirred again. The mixture was shaped into a cylinder (D = 4mm, H = 8mm) by an extrusion method and then dried at 100 °C for 2 h and calcined in a muffled furnace at 500 °C for 2 h and the SCMZ filter was obtained.

Calcium formed clinoptilolite (CaY): A modified clinoptilolite zeolite-Ca2+-formed material was prepared for the removal of ammonium ions from aqueous solutions (Ji et al. 2007). The chosen clinoptilolite was ground and sieved into 0.425–0.970 mm mesh (between 20 and 40 mesh) particle size and then washed twice with distilled water (volume ratio of liquid/solid 3:1) to remove any non-adhesive impurities and small particles. NaY was gained by treating the samples three times with saturated NaCl solution at boiling point for 2 h, and then changed into NH4+-formed clinoptilolite (NH4Y) by washing it with 1 mol/l NH4Cl solution for 20 min at room temperature. Finally, CaY was prepared through washing the NH4Y with Ca(OH)2 solution at boiling point for 2–3 h.

An overview of the nanoporous zeolites utilized for the removal of ammonia from sanitary landfill leachate is presented along with significant parameters like adsorption capacities and the experimental conditions used in the batch adsorption systems (Table 3). The feasibility of using natural raw zeolite to remove ammonia in the treatment of leachate from a pilot-scale composting plant for vegetable greenhouse solid waste was studied using batch (Liu 2000; Liu & Lo 2001a) and column (Liu 2000; Liu & Lo 2001b, 2001c) systems. It was observed that ammonium adsorption increased significantly with decreasing zeolite particle size for all tests and the adsorption capacities ranged from 14.35–17.81 mg N/g. Otal et al. (2002) used two synthetic zeolites for the decontamination of a highly contaminated municipal waste landfill leachate. Zeolites, especially CV-Z, clearly led to a strong reduction in the leachate nitrogen content. In addition, biological treatment of landfill leachate usually resulted in low COD removals because of high COD levels, high ammonium-N content and the presence of toxic compounds. Pretreated leachate was subjected to adsorbent supplemented biological treatment in an aeration tank operated in fed-batch mode. COD and NH4-N removal performances of powdered activated carbon (PAC) and powdered zeolite (PZ) were compared during biological treatment. Ammonium-N removals were 30 and 40% with PAC and zeolite concentrations of 5 g/L, respectively at the end of 30 h of fed-batch operation (Kargi & Pamukoglu 2003, 2004).

Table 3

Ammonia adsorption capacities and other parameters for zeolites in the batch systems

Canadian zeolite 25–200 8.75 14.4–17.8 Liu & Lo (2001a)
Powdered zeolite 290–340 12 – Kargi & Pamukoglu (2004)
Chinese clinoptilolite 11–115 7–8 1.74 Wang et al. (2006)
Turkish clinoptilolite 200–3,750 4–10 20.37 Karadag et al. (2008a)
Zeolite-carbon (Z-C) 1,890 8.29 24.39 Halim et al. (2006, 2010a)
Malaysian zeolite 500–1,100 2–12 21.01 Aziz et al. (2010)
Heat-activated zeolite 1,080 8.09 32.8 Aziz et al. (2020)
Turkish clinoptilolite 263–1,364 2–8 7.8–9.1 Temel & Kuleyin (2016)
Turkish clinoptilolite 263–1,364 2–8 8.68 Temel et al. (2021)
Synt.zeolite (geopolymer) 10–1,000 4–8 21.07 Luukkonen et al. (2016)
Cuban zeolite 2,292 7–8.2 10.8 Martins et al. (2017)
Ca, Na-treated zeolite 78–2,805 8–8.5 12.1 Vollprecht et al. (2019)
Natural/synthetic zeolite 1,104–1,890 8–8.5 6.9–17.45 Pauzan et al. (2020)
Malaysian zeolite 3,125–3,782 8–10 – Hamid et al. (2020)
Greek zeolite 100–600 6–10 3.59 Genethliou et al. (2021)
Malaysian zeolite 406.68 – Detho et al.(2021)
Canadian zeolite 25–200 8.75 14.4–17.8 Liu & Lo (2001a)
Powdered zeolite 290–340 12 – Kargi & Pamukoglu (2004)
Chinese clinoptilolite 11–115 7–8 1.74 Wang et al. (2006)
Turkish clinoptilolite 200–3,750 4–10 20.37 Karadag et al. (2008a)
Zeolite-carbon (Z-C) 1,890 8.29 24.39 Halim et al. (2006, 2010a)
Malaysian zeolite 500–1,100 2–12 21.01 Aziz et al. (2010)
Heat-activated zeolite 1,080 8.09 32.8 Aziz et al. (2020)
Turkish clinoptilolite 263–1,364 2–8 7.8–9.1 Temel & Kuleyin (2016)
Turkish clinoptilolite 263–1,364 2–8 8.68 Temel et al. (2021)
Synt.zeolite (geopolymer) 10–1,000 4–8 21.07 Luukkonen et al. (2016)
Cuban zeolite 2,292 7–8.2 10.8 Martins et al. (2017)
Ca, Na-treated zeolite 78–2,805 8–8.5 12.1 Vollprecht et al. (2019)
Natural/synthetic zeolite 1,104–1,890 8–8.5 6.9–17.45 Pauzan et al. (2020)
Malaysian zeolite 3,125–3,782 8–10 – Hamid et al. (2020)
Greek zeolite 100–600 6–10 3.59 Genethliou et al. (2021)
Malaysian zeolite 406.68 – Detho et al.(2021)

Recently, the potential of natural Chinese clinoptilolite for ammonia removal from the leachate solution of sewage sludge was investigated (Wang et al. 2006) and the maximum adsorption capacity of the clinoptilolite, for ammonium concentration ranging from 11.12 to 115.16 mg/L NH4+ N in leachate solution, was reported as 1.74 mg/g NH4+ N. Karadag et al. (2008a) studied the removal of ammonium ion (NH4+) from landfill leachate (Odayeri sanitary landfill, Istanbul) using Gordes (Turkish) clinoptilolite in both batch and column experiments and the chemical characteristics of clinoptilolite is given in Table 4. The equilibrium was reached at 3 h and the highest amount of the ammonium exchanged was 20.37 mg/g for the concentration of 3,750 mg/L. At C/Co value of 0.1 (ammonia removal of 90%), the operating time was 20 and 28 h for natural and preconditioned clinoptilolite, respectively. COD and ammoniacal nitrogen have always been the crucially problematic parameters in landfill leachate treatment. Halim et al. (2006, 2010a) investigated the adsorption properties of ammoniacal nitrogen and COD in semi-aerobic leachate on zeolite, activated carbon and a new composite media. This composite adsorbent contained 45.94% zeolite, 15.31% limestone, 4.38% activated carbon and rice husk carbon and 30% of ordinary Portland cement (OPC) which was used as a binder. Best ammonia adsorption was obtained on composite media (24.39 mg/g) (Table 3), followed by zeolite (17.45 mg/g) and activated carbon (6.08 mg/g).

Table 4

Chemical composition of nanoporous zeolites in the literature

Component (wt %)Karadag et al. (2008a) Martins et al. (2017) Hamid et al. (2020) Scandelai et al. (2020) Genethliou et al. (2021)
SiO2 74.4 68.0 71.8 74.68 69.62
Al2O3 11.5 12.0 12.63 11.97 13.62
Fe2O3 1.1 1.11 1.43 2.03 0.75
K25.0 1.40 2.56 2.73 2.94
MgO 0.5 0.80 0.61 0.96 0.90
Na2O 0.6 2.67 0.93 1.39 0.55
CaO 2.0 0.98 2.11 4.31 3.28
TiO2 0.1 0.37 0.13 0.34 0.11
MnO <0.001 – 0.03 – –
P2O5 0.02 0.03 0.02 0.06 –
LOIa 5.85 12.64 – – 8.23
Component (wt %)Karadag et al. (2008a) Martins et al. (2017) Hamid et al. (2020) Scandelai et al. (2020) Genethliou et al. (2021)
SiO2 74.4 68.0 71.8 74.68 69.62
Al2O3 11.5 12.0 12.63 11.97 13.62
Fe2O3 1.1 1.11 1.43 2.03 0.75
K25.0 1.40 2.56 2.73 2.94
MgO 0.5 0.80 0.61 0.96 0.90
Na2O 0.6 2.67 0.93 1.39 0.55
CaO 2.0 0.98 2.11 4.31 3.28
TiO2 0.1 0.37 0.13 0.34 0.11
MnO <0.001 – 0.03 – –
P2O5 0.02 0.03 0.02 0.06 –
LOIa 5.85 12.64 – – 8.23

aLoss of ignition.

Leachate generated from one of the old landfills in Malaysia contains high concentrations of colour, COD, iron and ammoniacal species. The ability of activated carbon-zeolite mixtures as an adsorbent was investigated and 21.01 mg/g of ammoniacal nitrogen was removed by a mixture of activated carbon and zeolite (Aziz et al. 2010). In addition, the potential use of raw zeolite and heated activated zeolite in the abatement of COD, NH3-N and colour from leachate was examined (Aziz et al. 2020). Zeolite was activated using different temperatures, namely at 150 °C, 200 °C and 250 °C for 3 h. The optimum pH for NH3-N was 7 with a percentage removal of 55.8% while better abatement of COD and colour was obtained at pH 4 with a percentage removal of 24.3% and 73.8%, respectively. The capacity of the zeolite before and after heat activation was reported as 41.30 cmol/kg and 181.90 cmol/kg, respectively. SEM analysis of zeolite after heat activation showed an increase in the displacement between the crystal structures with enlarged pore size, which is marked by the arrows in Figure 2.

Figure 2

SEM results for (a) raw zeolite, (b) activated zeolite heated at 150 °C (Aziz et al. 2020).

Figure 2

SEM results for (a) raw zeolite, (b) activated zeolite heated at 150 °C (Aziz et al. 2020).

The removal of NH4+-N from landfill leachate using natural Turkish zeolite by adsorption process was investigated (Temel & Kuleyin 2016). The optimum conditions in the adsorption process were found to be as follows: 60 min contact time, 100 g/L adsorbent dosage, 200 rpm agitation speed, 263.2 mg/L initial concentration and −20/+35 mesh particle size. Also, the values of Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy of activation (ΔS°) were 5.7113–6.5018 kJ/mol, −8.5415 and 8.8209 J/(mol K), respectively. In addition, multilayer perceptron (MLP) artificial neural network was utilized to predict the adsorption rate of ammonium on zeolite (Temel et al. 2021). Recently, Aydın & Kuleyin (2011) studied the capacity of natural Turkish zeolite for NH4+ N removal from landfill leachate and Langmuir isotherm model was found to best represent the data for NH4+ N. Ammonium ion-exchange performance of the NZ was investigated in both batch and column studies and ammonium removal increased with increasing zeolite dosage from 25 to 150 g/L while optimum pH was found as 7.1. In the column studies, the total ammonium removal percentage during 180 min operation time decreased with the flow rate ranging from 4 to 9 mL/min (Ye et al. 2015).

Geopolymer, which is an amorphous analogue of zeolite and thus possesses similar cation-exchange properties, was synthesized from metakaolin and applied to remove NH4+N from model solutions and landfill leachate (Luukkonen et al. 2016). The synthesis procedure of geopolymer is illustrated in Figure 3. The maximum NH4+-N removal capacity of the geopolymer was found to be 21.07 mg/g (Table 3). A small-scale column experiment with landfill leachate was performed at a flow rate of 3 mL/min (4 BV/h) and 4 h, respectively (Table 5). The saturation–regeneration cycle was repeated three times. The process consisted of ammonium nitrogen adsorption from raw leachate followed by zeolite regeneration via nitrification and next, initial adsorptive capacity of zeolite was evaluated (Martins et al. 2017). The adsorptive capacity (q) was reduced by only 4.55% after regeneration from q = 10.80±2.14 mg NH4+-N/gzeolite to 10.32±0.74 mg NH4+-N/gzeolite. Regeneration by nitrification was performed in 72 h, where main product was nitrite. de Paula Couto et al. studied the ammonia–nitrogen removal by aluminosilicates from pretreated landfill leachate using three types of commercial clays and one commercial zeolite (de Paula Couto et al. 2017). The Langmuir model was adequate to describe the ion-exchange equilibrium and the sorption kinetics fit the pseudo-second-order kinetic model.

Table 5

Ammonia removal efficiency and other parameters for zeolites in the fixed bed systems

AdsorbentConcentration range (mg/L)Flow rate (mL/min)Removal efficiency (%)References
Canadian zeolite 25–200 8.3 98 Liu & Lo (2001b)
Canadian zeolite 25–200 8.3 95 Liu & Lo (2001c)
Turkish clinoptilolite 410–830 6.9 >90 Turan et al. (2005a, 2005b)
Turkish clinoptilolite 100–400 21 >90 Karadag et al. (2008a)
Turkish clinoptilolite 400–3,000 21 >90 Karadag et al. (2008b)
Turkish zeolite 122±27 6.94 62–49 Yalcuk & Ugurlu (2009)
Zeolite-carbon (Z-C) 1,640 93.7 Halim et al. (2010b)
Activated carbon-zeolite 500 20 70 Aziz et al. (2010)
Synt. zeolite (geopolymer) 700 65 Luukkonen et al. (2016)
Czechoslovakian zeolite 1,800±50 – 96 Lim et al. (2016)
Zeolite–slag hybrid subst. 97±27.6 20.8 60–70 He et al. (2017)
Natural zeolite 25–500 90 Scandelai et al. (2020)
Turkish clinoptilolite 1,384 2.5–10 99 Temel et al. (2021)
AdsorbentConcentration range (mg/L)Flow rate (mL/min)Removal efficiency (%)References
Canadian zeolite 25–200 8.3 98 Liu & Lo (2001b)
Canadian zeolite 25–200 8.3 95 Liu & Lo (2001c)
Turkish clinoptilolite 410–830 6.9 >90 Turan et al. (2005a, 2005b)
Turkish clinoptilolite 100–400 21 >90 Karadag et al. (2008a)
Turkish clinoptilolite 400–3,000 21 >90 Karadag et al. (2008b)
Turkish zeolite 122±27 6.94 62–49 Yalcuk & Ugurlu (2009)
Zeolite-carbon (Z-C) 1,640 93.7 Halim et al. (2010b)
Activated carbon-zeolite 500 20 70 Aziz et al. (2010)
Synt. zeolite (geopolymer) 700 65 Luukkonen et al. (2016)
Czechoslovakian zeolite 1,800±50 – 96 Lim et al. (2016)
Zeolite–slag hybrid subst. 97±27.6 20.8 60–70 He et al. (2017)
Natural zeolite 25–500 90 Scandelai et al. (2020)
Turkish clinoptilolite 1,384 2.5–10 99 Temel et al. (2021)
Figure 3

A pictorial presentation of the synthesis procedure of geopolymer (Luukkonen et al. 2016).

Figure 3

A pictorial presentation of the synthesis procedure of geopolymer (Luukkonen et al. 2016).

Vollprecht et al. (2019) conducted laboratory-scale experiments regarding the sorption of NH4+ from landfill leachates using natural and modified clinoptilolite to assess the applicability of the innovative ion-exchanger loop stripping (ILS) process for ammonium recovery. Between 13 and 61% of the dissolved NH4+ was adsorbed to the clinoptilolite. Recently, Morris et al. (2019) employed four low-cost materials, oyster shells, pumice stone, sand and zeolite as adsorbents in a batch system for the removal of ammonia, phosphate and nitrate from landfill leachate. In addition, oyster shells and pumice stone, were also employed as adsorbents in a fixed-bed column using bed height of 20 cm with a flow rate of 5 mL/min.

Advanced oxidation processes based on ozonation, UV solar radiation, hydrogen peroxide and persulfate for the complete treatment of a specific landfill leachate was tested (Poblete et al. 2019). As a post-treatment of the advanced oxidation process, an additional adsorption process was carried out using an NZ and 36, 99 and 18% reductions were observed for COD, ammonium and chloride, respectively. Ammonia removal from landfill leachate using natural and synthetic zeolites was investigated, where response surface methodology approach based on a three-factor three-level central composite design was applied to compare and optimize the removal of NH3-N from landfill leachate (Pauzan et al. 2020). Under optimized conditions, clinoptilolite (2 g/L, 50 μm, and 50%) and Sigma 96,096 (4 g/L, 150 μm, and 50%) effectively removed 58.2% and 37.8% of NH3-N, respectively. The behavior of a novel zeolite augmented on the electrocoagulation process (ZAEP) using an aluminum electrode in the removal of high-strength concentration ammonia (3,471 mg/L) from landfill leachate was examined (Hamid et al. 2020). A response surfaces methodology (RSM) through central composite designs (CCD) was used to optimize the treatment process and the following operation conditions were found to be optimum: Zeolite dosage of 105 g/L, the current density of 600 A/m2, electrolysis duration of 60 min, and pH 8.20.

Natural zeolite with maximum adsorption capacity of 3.59 mg/g was used for the simultaneous removal of ammonium nitrogen (NH4+-N), dissolved COD (d-COD) and colour from raw sanitary landfill leachate (SLL) (Genethliou et al. 2021). Optimum adsorption results were obtained for particle size of 0.930 μm, stirring rate of 1.18 m/s, zeolite dosage of 133 g/L and pH 8. NH4+-N removal efficiency reached 51.63±0.80% within 2.5 min of contact time. NH4+-N release from the saturated zeolite was not completely reversible, suggesting that the zeolite may be used as a slow NH4+-N releasing fertilizer and an attractive low-cost material for the treatment of SLL. NH4+-N removal with the regenerated zeolite exceeded 40% of the initial concentration in the fluid within 2.5 min. Ammonium and phosphate in leachate are potential contaminants for both surface and groundwater. Suprihatin et al. (2019) aimed to eliminate these pollutants simultaneously by using activated zeolite. While a physically activated zeolite dosage of 120 g/L resulted in the smallest concentration of ammonium residue of 72.6 mg/L and a phosphate residue of 0.37 mg/L, the chemically–physically activated zeolite (dose of 45 g/L) produced an ammonium residue of 198 mg/L and phosphate residue of 0.74 mg/L. Detho et al. (2021) studied to find an alternative treatment by combining low-cost adsorbent such as green mussel waste (Perna viridis) and ordinary adsorbent media, granular activated carbon and zeolite. The best ratio for hydrophobic (granular activated carbon and green mussel) and hydrophilic (zeolite) media ratio was selected as 7:3 and the leachate concentration of COD was 310 mg/L with 83% reduction and ammonia–nitrogen was 150 mg/L with 63% reduction.

The removal of ammonia from sanitary landfill leachate in fixed bed columns has been also investigated in the literature. Bench-scale packed zeolite columns were set up and operated to investigate the continuous removal of ammonium ions from compost leachate. Over 98% of the ammonia input was consistently removed for over five BV of compost leachate flowing through the zeolite column (Liu & Lo 2001b) and more than 95% of adsorbed ammonium ions were recovered after using 7–8 BV of the regenerating solution (Liu & Lo 2001c) (Table 5). The adsorption–regeneration time ratio was approximately 5:1. High concentration of potassium ions in the composting leachate competed with NH4+ ions for the exchange sites, resulting in a reduction in the efficiencies of ammonia removal and zeolite column regeneration.

The use of a combined anaerobic fluidized bed and zeolite fixed bed system in sanitary landfill leachate treatment was investigated (Turan et al. 2005a, 2005b). The landfill leachate used in the experiments was obtained once every week from a municipal landfill site which had been under operation since 1995 and characterized by young leachates (Table 2). COD removal was attained up to 90% with increasing organic loading rates (OLRs) as high as 18 g COD/L day after 80 days of operation. Biogas production yield (Ygas) of 0.53 Lgas/gCODrem was obtained along with methane (CH4) content of 75%. The anaerobically treated landfill leachate was further treated by a zeolite fixed bed reactor and the ammonia removal was obtained as 90% (Table 5). Karadag et al. (2008b) studied the removal of ammonium from municipal landfill leachate using clinoptilolite zeolite in upflow fixed-bed and fluidized-bed columns with different ammonium concentrations. Higher effluent volumes and removal rates were obtained at lower ammonium concentrations and increased expansion ratios in the fluidized-bed column reduced the treatment efficiency.

Treatment of organic pollution, ammonia and heavy metals present in landfill leachate by the use of constructed wetland systems was studied (Yalcuk & Ugurlu 2009). The effect of different bed material (gravel and zeolite surface) was also investigated on subsurface flow constructed wetland systems operated in vertical and horizontal modes. Concentration-based average removal efficiencies for two different vertical flow systems (VF1, VF2) and one horizontal flow system (HF) were 62.3%, 48.9% and 38.3% for NH4+-N; 27.3%, 30.6% and 35.7% for COD; 52.6%, 51.9% and 46.7% for PO4-P; 21, 40 and 17% for Fe(III), respectively. Better NH4+–N removal performance was observed in the vertical system with a zeolite layer. The removal efficiencies of two horizontal subsurface flow constructed wetlands (HSSF CWs, down-flow (F1) and up-flow (F2)) filled with the zeolite–slag hybrid substrate for the rural landfill leachate treatment was investigated (He et al. 2017). The constructed wetland operated in horizontal subsurface flow mode is shown in Figure 4. The constructed wetlands were capable of removing COD at a level of 20.5–48.2% (F1) and 18.6–61.2% (F2); NH3-N at a level of 84.0–99.9% (F1) and 93.5–99.2% (F2); TN at a level of 80.3–92.1% (F1) and 80.3–91.2% (F2); and heavy metals at a level of about 90% (F1 and F2). The up-flow constructed wetland (F2) has a higher removal efficiency for the PAH compounds.

Figure 4

Schematic diagram of subsurface flow constructed wetland (F2) (He et al. 2017).

Figure 4

Schematic diagram of subsurface flow constructed wetland (F2) (He et al. 2017).

The performance of a carbon-mineral composite adsorbent used in a fixed bed column for the removal of ammoniacal nitrogen and aggregate organic pollutant (COD), which are commonly found in landfill leachate, was evaluated (Halim et al. 2010b). The breakthrough capacities for ammoniacal nitrogen and COD adsorption were 4.46 and 3.23 mg/g, respectively. The column efficiency for ammoniacal nitrogen adsorption was 86.4 and 90% using fresh and regenerated adsorbents, respectively. An aerobic sequencing batch reactor (ASBR) was proposed for the treatment of locally obtained real landfill leachate with initial ammoniacal nitrogen and COD concentration of 1,800 and 3,200 mg/L, respectively (Lim et al. 2016). ASBR could remove 65% of ammoniacal nitrogen and 30% of COD during 7 days of treatment time. After that, an effective adsorbent, i.e., zeolite was used as a secondary treatment step for polishing the ammoniacal nitrogen and COD content that is present in leachate while the removal of ammoniacal nitrogen and COD were up to 96 and 43%, respectively.

Over the past few years, supercritical water oxidation (ScWO) has shown great potential for application to landfill leachate treatment, providing substantial organic matter degradation in terms of biochemical oxygen demand (BOD), COD and total organic carbon (TOC). Scandelai et al. (2020) evaluates the intensification of the supercritical water oxidation (ScWO) process through ion exchange with zeolite. The zeolite (clinoptilolite) was used without any modification inside a glass column. The ScWO (600 oC)/zeolite system removed 90% of ammoniacal nitrogen (NH3-N), 100% of nitrite (NO2-N), 98% of nitrate (NO3-N), colour and turbidity, 81% of TOC and 74% of COD, suggesting that this system is a promising alternative for leachate treatment.

Zeolites consist of a wide variety of species such as clinoptilolite and chabazite. Clinoptilolite is quite abundant in nature and is readily available from more than 40 NZ species. High ion-exchange capacity and relatively high specific surface areas and more importantly relatively inexpensive prices, make zeolites attractive adsorbents for wastewater treatment. Currently their price range is within ca. US$0.03– 0.12/kg while the actual price depends on the quality of the mineral (Babel & Kurniawan 2003). Comparatively, the cost of the most inexpensive carbon which is commercially available is about US$2,000/ton. NZ costs about US$70/ton, including the cost of its purchase, transport and processing (chemicals, electrical energy and labor required in the process), whereas modified zeolite costs about US$420/ton (Table 6). That is why, modified zeolite can also be considered as a good alternative to commercially available activated carbon (Bowman 2006; Ozdemir 2007; Faki et al. 2008).

Table 6

Comparative evaluation of costs of zeolite and other adsorbents

AdsorbentsUnit price (US$/ton)References Natural zeolite 70 Babel & Kurniawan (2003) Surfactant modified zeolite 420 Bowman (2006), Ozdemir (2007) Bentonite ≤100 Gupta & Suhas (2009) Montmorillonite clay 40–120 Babel & Kurniawan (2003) Fuller's earth 40 Atun et al. (2003) Chitosan 15,430 Babel & Kurniawan (2003) Peat moss 23 Babel & Kurniawan (2003) Blast-furnace slag 38 Babel & Kurniawan (2003) Starch xanthates 1,000 Babel & Kurniawan (2003) Commercial activated carbon 2,000 Babel & Kurniawan (2003) Commercial activated carbon 1,500 Gupta & Suhas (2009) Commercial activated carbon 20,000 Atun et al. (2003) AdsorbentsUnit price (US$/ton)References
Natural zeolite 70 Babel & Kurniawan (2003)
Surfactant modified zeolite 420 Bowman (2006), Ozdemir (2007)
Bentonite ≤100 Gupta & Suhas (2009)
Montmorillonite clay 40–120 Babel & Kurniawan (2003)
Fuller's earth 40 Atun et al. (2003)
Chitosan 15,430 Babel & Kurniawan (2003)
Peat moss 23 Babel & Kurniawan (2003)
Blast-furnace slag 38 Babel & Kurniawan (2003)
Starch xanthates 1,000 Babel & Kurniawan (2003)
Commercial activated carbon 2,000 Babel & Kurniawan (2003)
Commercial activated carbon 1,500 Gupta & Suhas (2009)
Commercial activated carbon 20,000 Atun et al. (2003)

Bagasse fly ash, peat, sphagnum moss peat, Fuller's earth, BF slag, bentonite, manganese oxide, Fuller's earth, carbonaceous adsorbent (fertilizer industry waste) are materials costing ≤ US$0.1/kg making them useful low-cost materials as compared to commercial activated carbons (CAC) which normally costs more than US$1.5/kg (Gupta & Suhas 2009). Among low-cost materials, montmorillonite clay has the largest surface area and the highest cation-exchange capacity. Its current market price (about US$0.04–0.12/kg) is considered to be 20 times cheaper than that of activated carbon (Babel & Kurniawan 2003). The removal performances of Fuller's earth and CAC for basic blue 9 were compared by Atun et al. (2003). Moreover, Fuller's earth is an interesting sorbent since its average price is US$0.04/kg whereas CAC costs US$20/kg. Chitosan is produced by alkaline N-deacetylation of chitin, which is widely found in the exoskeleton of shellfish and crustaceans. It was estimated that chitosan could be produced from fish and crustaceans at a market price of US$15.43/kg (Babel & Kurniawan 2003). Among low-cost materials, zeolite is undoubtedly the most inexpensive alternative adsorbent and is 15 times cheaper than chitosan. Peat moss is a relatively inexpensive material and commercially sold at US$0.023/kg and blast-furnace slag is sold at US$38/ton. Besides, the cost of the cheapest CAC is about US$1,000/ton (Babel & Kurniawan 2003). Nanoporous zeolites as low-cost adsorbents have been studied worldwide for the removal of ammonia from sanitary landfill leachate. Zeolite is an inexpensive and locally available material and could be used effectively utilized in place of commercial activated carbon for the ammonia removal from landfill leachate. Synthetic and NZ are important alternatives as adsorbents due to their high ion-exchange and adsorption capacities as well as good thermal and mechanical stabilities. The capacity of the zeolite before and after heat activation was 7.50 mg/g and 32.8 mg/g, respectively. If zeolite performs well in removing ammonia from landfill leachate at low cost, it can be adopted and widely used in sanitary landfill deposits not only to minimize cost inefficiency, but also to improve profitability. Sorptive properties of zeolites have been utilized in previous works for ammonia removal from landfill leachate in batch and fixed-bed modes. The price of zeolites itself is considered very inexpensive, namely about US$0.03–0.12/kg, depending on the quality of the zeolite itself. Among other low-cost material alternatives, zeolites are the most inexpensive adsorbents. Undoubtedly zeolite as a low-cost adsorbent offers a lot of promising benefits for commercial purpose in the future.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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