Kawthar Frikha
email: kawthar.frikha@enis.tn

Lionel Limousy

Jamel Bouaziz

Simona Bennici

Kamel

Kamel Chaari

Mejdi Jeguirim

Elaboration of Alumina Based Materials by Solution Combustion Synthesis: A review

Keywords: Alumina, Solution combustion, Alumina-based materials. Contents 1. 3.3. Metal aluminates

Introduction

2.

Solution combustion synthesis of alumina

Introduction

The combination of excellent properties and availability has made alumina (Al 2 O 3 ) an attractive material for various applications. In particular, alumina is widely used in soft abrasive, wear-resistant coating, catalyst support, adsorbent, electrical component, and a large variety of other industrial applications. The synthesis of alumina and related materials is achieved either by traditional ceramic methods or by wet chemical methods like sol-gel, hydrothermal, precipitation, spray drying, spray pyrolysis, etc. Most of these methods generally require numerous synthetic steps, special equipments and energy-demanding protocols. In order to overcome these drawbacks, a technically simpler and less costly effective chemical synthetic route has been developed in the mid-1980s [START_REF] Ravindranathan | Proc. Indian Acad. Sci. Sci[END_REF][START_REF] Patil | [END_REF]: the so-called "solution combustion" (SC) or "solution combustion synthesis" (SCS). This process was accidentally discovered by Kingsley and Patil when they reacted aluminum nitrate hydrate (Al(NO 3 ) 3 •9H 2 O) and urea (CH 4 N 2 O) (as fuel) [3]. Based on this unlooked-for experiment, the SC method was explored and a large number of oxides were prepared [4 -8]. Combustion is an exothermic redox reaction that requires an oxidizing agent and a fuel. Metal nitrates are used at a large scale and considered as essential oxidizing agents for this SC synthesis route.

Other products as glycine and citric acid can also be used as fuels (reducing agents). The SC method involves applying the thermochemical concepts of propellant chemistry developed by Jain et al. [9]. These concepts are based on the estimation of the elemental stoichiometric coefficient (ϕ e ). According to Jain et al. [9], ϕ e is determined by the following equation (Eq. ( 1)).

The combustion reaction is greatly influenced by several parameters such as the chemical precursors, the elemental stoichiometric coefficient (ϕ e ), the pH value, and the method for the reaction initiation. The influence of these parameters on the SC process is explored in terms of combustion sustainability, reaction exothermicity, and propagation rate of the reaction front. The properties of the SC synthesized materials, such as crystalline phase, particle size, morphology, surface area and agglomeration degree are mainly governed by the combustion enthalpy and the volume of gases generated during the combustion process. These two factors operate in a synergetic way on the physicochemical properties of the SCproducts, and they can be varied in order to modulate and obtain the desired material properties. Typically, a high combustion temperature results in a high crystallinity, large particle size, extensive agglomeration, and low surface area, while, the rapid generation of a large volume of gases contributes to the effective dissipation of the heat from the bulk of the material, limiting the inter-particle contact, and enhancing the porosity and the surface area of the products. The possibility of tuning the combustion parameters offers remarkable opportunities for the synthesis of a wide variety of technologically useful materials. However, the exothermic reaction characteristics such as high combustion temperature and fast heating/cooling rate are the key parameters to obtain materials with improved morphological and structural properties.

A wide range of advanced Al 2 O 3 -reinforced materials with interesting magnetic, dielectric, electrical, mechanical, catalytic, luminescent and optical properties were obtained by SC process [10 -14]. In particular, the SC preparation method has been successfully applied for the synthesis of advanced solid catalysts, since the combustion reaction leads to products characterized by a chemical composition homogeneity, high sintering ability, fine-particle size distribution, and enhanced textural properties. Al 2 O 3 -based catalysts can be divided into two categories: bulk and supported catalysts. In the first group, Al 2 O 3 -based bulk catalysts include mixed oxides. In the second group, an active phase (generally a metal or metal oxide) is deposited onto the alumina support. Compared to the conventional catalyst preparation routes, the deposition of the active phase on a support by the SC method is characterized by low temperature and consequently by a slow kinetics, which seems to promote a good dispersion of the metal/metal oxide species on the support surface. Al 2 O 3 -based catalysts are particularly used in gas-phase catalytic reactions, including methane or alcohol reforming, hydrogenation and oxidation reactions, and automotive exhausted gas emissions treatment [13,14].

The interest in combustion synthesis has been on a steady increase and several research papers on the synthesis of alumina-based materials have been published over the last 30 years. However, there are still some issues concerning the management of the reaction products during the synthesis, the request of a calcination step after the synthesis, and the lack of fundamental modelling studies on the existing combustion synthesis processes and their applicability to small-scale synthesis. Many efforts have been made to overcome these encountered limitations and new approaches based on solution combustion synthesis method are currently being developed.

The objective of this work was to help an easy identification of the more adapted modus operandi to prepare alumina-based materials with the required proprieties and composition by the solution combustion method. The remainder of this review article reported on the most relevant research papers related to the combustion synthesis. A brief summary on alumina and related materials synthesized by SC method was presented, along with summaries of their properties and applications. The process details, the reaction parameters and the recent advances of this method were discussed.

Solution combustion synthesis of alumina

SC has been used to prepare Al 2 O 3 nano-powders using aluminum nitrate as an oxidizing agent. The characteristics of the SC-derived products (crystalline phase, particle shape, surface area, and crystallite size) depend on the starting fuels, the elemental stoichiometric coefficient (ϕ e ), the pH of the solution, and the method applied for the reaction initiation. In this section, the solution combustion syntheses of pure alumina performed during the last 30 years are reported in Table 1.

Table 1 here Kingsley and Patil have reported the first investigation on the preparation of fine particles of α-Al 2 O 3 by combustion method. In their synthesis procedure, the stoichiometric amounts of aluminum nitrate hydrate and urea were dissolved in water, then the solution was heated in a muffle furnace at 500 °C for less than 5min [3]. Based on this experiment, a large number of studies were successively performed on the synthesis of the single-phase α-Al 2 O 3 powders using urea [15 -18], hydrazine (N 2 H 4 ) [19] or carbohydrazide (CH 6 N 4 O) [20] as fuels, without any post-synthesis thermal treatments. At the same time, other authors have showed that by using glycine (C 2 H 5 NO 2 ) [21 -24], acetylacetone (C 5 H 8 O 2 ) [25],or citric acid (C 6 H 8 O 7 ) [26,27] an amorphous powder could be produced, and that, after annealing above 1100 °C, it could be transformed into α-Al 2 O 3 . Zhuravlev et al. [18] have used thermal analysis and IR spectroscopy methods to measure the reaction temperature reached during the combustion reactions of the aluminum nitrate-urea mixtures. The temperature in the reaction zone was recorded by a special program using a Pt/Pt-10Rh thermocouple. The formation of αalumina was detected at temperatures ranging between 600 and 800 °C. In particular, the combustion temperature of the aluminum nitrate-urea system is higher than the one for aluminum nitrate-glycine mixtures (Fig. 1), and thus urea seems to be the most convenient fuel to be employed for the combustion synthesis of α-Al 2 O 3 .

Figure 1 here

The morphological and textural properties of SC-derived products (particle shape, surface area, crystalline size) vary depending on the fuels nature (Fig. 2). In the presence of urea, well-crystallized α-Al 2 O 3 powders with a surface area ranging from 5 to 115 m²/g were obtained. α-Al 2 O 3 powders were characterized by nearly hexagonal platelet particles (Fig. 2a) [3,18,19] with sizes ranging from 200 to 800 nm [3,19]. Other fuel-types produce amorphous powders with different nanoparticles morphologies; the aluminum nitrateglycine combustion product presents spherical-shaped particles [23,24] (Fig. 2b) with a diameter size in the range of 35-80 nm [24], and the nitrate-citric acid combustion product displays particles with irregular shapes (Fig. 2c) [27].

As previously reported, a wide variety of fuels are available for the combustion synthesis method. However, the ignition characteristic of various fuels differs in relation to the chemical activity of the ligand groups of the fuel molecule. The ignition character of a fuel can effectively affect the exothermicity of the combustion reaction (enthalpy and flame temperature), the burning rate, and the amount of gases generated during combustion.

Depending upon the fuel used, the combustion reaction will be flaming type (volume combustion), non-flaming type (smoldering or layer-by-layer combustion) or explosive type.

In case of the use of urea, the combustion reaction is flaming type, manifests with a high flame temperature (>1000 °C) and high burning rate. The observed high flame temperature is due to the exothermic reaction between the gaseous decomposition products of aluminum nitrate solutions, as nitrogen oxides (NO x ), and urea, as isocyanic acid (HNCO), ammonia (NH 3 ), and carbon dioxide (CO 2 ) species. Because of the high flame temperature, the particles undergo slight sintering resulting in larger agglomerated particles. Even though the particles are agglomerated, the crystallite sizes will be in the submicron/nanosized range due to the short duration of heating. By changing the fuel from urea to glycine, the combustion reaction is much controlled, follows a smoldering type and propagates slowly from one layer to another. In this case, the combustion reaction is characterized by a reduced flame temperature (<1000 °C), slow burning rate and large amount of gas. Alumina prepared by the combustion of glycine has larger surface areas and smaller particle sizes, mostly in nanosize, compared to the one prepared by the combustion of urea. Other fuels like hydrazine and carbohydrazide that burn at lower temperatures than urea were also found to be suitable for alumina synthesis and to produce finer particles.

As general behavior, the low combustion temperature favors small particle sizes and prevents the particles from agglomeration, but leaves behind higher levels of impurities in the final products (carbon particles, hydroxides, carbonates etc.). It is interesting to note that urea is still among the most attractive fuels studied for the combustion synthesis of alumina and related oxides, resulting in the formation of highly thermally stable, pure and homogeneous powders, contrary to others fuels that need further thermal treatments at high temperatures to reach a high purity of the final products.

Bhaduri et al. [15] have examined the effect of fuel/oxidizing agent ratio on the phase formation using different compositions of aluminum nitrate-urea solutions, in stoichiometric and off-stoichiometric proportions. They reported that the molar ratio is crucial in the nucleation of the alumina crystalline phases. Furthermore, Freitas et al. [16] have used an infrared pyrometer to measure the reaction temperature during the combustion of different aluminum nitrate-urea composition mixtures. The authors showed that by diminishing the urea amount a decrease of the combustion temperature is observed, and consequently, particles with smaller size that easily agglomerate are produced.

Figure 2 here

Meng et al. [17] have studied the effect of the fuel composition on phase-stability, morphology and crystallites size of the synthesized alumina. In this investigation, aluminum nitrate and urea were the main reagents used; starch ((C 6 H 10 O 5 ) n ) or ammonium nitrate (NH 4 NO 3 ) was added as a fuel additive. By adding ammonium nitrate, the authors obtained well-crystallized α-Al 2 O 3 powders with fine particles in the size range from 20 to 800 nm). However, the addition of starch led to an amorphous powder, which was transformed into well-crystallized α-alumina with a small crystalline size (~30-50 nm) after heat treatment in a muffle furnace at 1100 °C for 1 h.

Pathak et al. [26] have showed that the morphology of the synthesized powder could be affected by varying the pH of the parental solutions. They suggested that the increase of nitrate ion concentration at low pH (2, 4 and 6) leads to a slow decomposition of the precursors, thus resulting in flaky powders. At a high pH value (10), a rapid decomposition of the precursors takes place and a powder, consisting of well-divided nanoparticles, is produced. The same result was obtained by Peng et al. [28] during their study on the effect of the precursor solution pH for the aluminum nitrate-glycine combustion route.

Ignition is also an important step of the SC synthesis. Different devices, such as muffle oven [3,15,17,23,25], hot plate [16,20,21,28], aluminum open cylindrical reactor [18], and pressure reactor [19] have been used for supplying the required energy for the preparation of SC-alumina-based materials. In recent investigations [29,30], an alternative strategy for SC-reaction ignition based on microwave heating has been developed. Laishram et al. [29] have compared two ignition methods. In the first, a stoichiometric aluminum nitrate-urea solution was heated in a domestic microwave oven working at 900 W, 2.45 GHz while in the second; a similar solution was heated in a preheated muffle furnace at 700 °C. The authors have identified the microwave combusted nanoparticles as α-Al 2 O 3 with fine particles size, ranging from 18 to 20 nm, and a quite uniform spherical morphology (Fig. 3a). However, the well-known furnace combustion method has produced particles of bigger size (50-90 nm), with a polyhedral morphology (Fig. 3b). During this investigation, it was demonstrated that heating by microwaves favors a continuous emission of the evolving gases all along the process. Furthermore, the consequent well controlled dissipation of the heat from the material core generated finer and more uniform particles size, and smaller agglomerates.

Solution combustion synthesis of alumina-based materials

Solution combustion is a very promising method for the preparation of alumina composites, alumina composites/nanocomposites, alumina supported catalysts and metal aluminates (MAl 2 O 4 ). In this section, a list of alumina derived materials prepared by SC was presented. In addition, their properties and applications were described, focusing on the preparation procedures and the influence of the synthesis parameters (composition of initial solutions, ignition method, etc) on the physico-chemical properties of the final materials. Details of various hybrid processes based on combustion synthesis were also presented. Zirconia (Zr 2 O 3 ) exhibits higher mechanical properties compared to alumina. Therefore, zirconia is often added to the alumina matrix in order to enhance certain mechanical properties such as toughness, strength resistance, and hardness. Zirconia-toughened alumina (ZTA) finds applications in heat engines, rocket nozzles cutting tools and biomedical implants [31 -33]. The Al 2 O 3 /ZrO 2 system can be easily prepared by the combustion of aqueous solutions containing stoichiometric amounts of aluminum nitrate, zirconium nitrate (ZrO(NO 3 ) 2 ), and carbohydrazide [34] or urea [35]. The combustion-derived powders, obtained by using carbohydrazide as fuel, showed improved mechanical properties such as toughness and hardness. A modified solution combustion approach has been adopted by Aruna et al. [36] in the preparation of nanosized ZTA composite, using the corresponding metal nitrates as oxidizing agents and a mixture of fuels consisting of ammonium acetate (C 2 H 7 NO 2 ), urea and glycine. Tahmasebi et al. [37,38] have reported different strategies for the preparation of Al 2 O 3 /20 wt.% ZrO 2 nanocomposite powder. In a first approach, different fuel mixtures including urea, ammonium acetate and starch were used. The authors suggested that an excess of starch, used as fuel, can diminish the exothermicity of the combustion reaction carried-on with conventional heating methods, thus producing a nanostructured ZrO 2 /Al 2 O 3 powder with well-dispersed alumina and zirconia nanoparticles and small crystallites size (<20 nm) (Figs. 4a and4b) [37]. In a second approach, a microwave-assisted SC technique, using urea as fuel, was proposed. This process resulted in finer crystallites compared to the previous case [38], with a narrow particle size distribution and a well dispersion of zirconia nanoparticles in the alumina matrix (Fig. 4c). The alumina/ceria (Al 2 O 3 /CeO 2 ) system is currently receiving great attention in the field of catalytic converters for the treatment of vehicle exhaust gases [39,40]. The ability of CeO 2 to reversibly exchange oxygen is a key feature in this application. Moreover, the Al 2 O 3 /CeO 2 system can act as an excellent support system for acid catalysis [41,42]. An active Al 2 O 3 /CeO 2 catalyst for carbon monoxide (CO) oxidation was obtained [43] ) and metal nitrates as starting materials. A conversion of 99.7% was achieved for CO oxidation at 160 °C with a catalyst with Ce/Al molar ratio of 1. This conversion temperature was significantly lower than the reported value for Al 2 O 3 /CeO 2 system prepared by the classical combustion method, using urea as fuel [43], or a fuel mixture containing glycine, urea, hexamine and oxalyldihydrazide [44].

Copper/Alumina (Cu/Al 2 O 3 )

Copper exhibits excellent electrical and thermal conductivities. Pure copper has, however, low tensile strength and elastic modulus. The addition of nano-scale alumina particles to copper matrix can improve the mechanical properties of copper without affecting its electrical conductivity [47 -49]. Therefore, copper/alumina nanocomposites (Cu/Al 2 O 3 ) have attracted attention for their potential use as high-performance electrode materials for industrial spot welding [50]. A Cu/Al 2 O 3 nanocomposite was obtained in one-step SCS from an aqueous solution mixture of copper nitrates (Cu(NO 3 ) 2 ), aluminum nitrates, urea and graphite [51]. Graphite was initially added in order to prevent oxidation of copper into copper oxide after the completion of the combustion reaction. It was noted that the combustion leads to a good dispersion of the nanosized alumina on the copper matrix. Parket al. [52] have prepared bulk Al 2 O 3 with well dispersed Cu-phase, using combustion of metal nitrates and urea solutions, followed by a reduction treatment, in order to reduce CuO to Cu. Also in this case, the combustion route showed good results in the efficient dispersion of fine Al 2 O 3 particles.

Other alumina composites

Similarly, a variety of alumina composites, like the Al 2 O 3 -SiO 2 mullite system, has been prepared by the SCS method [53]. Additives like MgO, ZrO 2 , TiO 2 , (0-5 wt.%) and rare earth metal oxides like Y 2 O 3 , La 2 O 3 , and CeO 2 were investigated in order to examine their effect on the mullite sintering [54]. The mullite-cordierite composites (MgO-Al 2 O 3 -SiO 2 ) have also been prepared by the combustion of aluminum nitrate, magnesium nitrate (Mg(NO 3 ) 2 )/magnesia (MgO), fumed silica (SiO 2 ) and urea heterogeneous mixture at 500 °C [55]. Ajamein et al. [56] have recently studied the microwave assisted solution combustion synthesis of various CuO-ZnO-Al 2 O 3 nanocatalysts. These synthesized materials were applied for the steam reforming of methanol, using metal nitrates, and sorbitol (C 6 H 14 O 6 ), ethylene glycol (C 2 H 6 O 2 ) [57] or urea [58], as fuels. The schematic representation of the microwave assisted solution combustion synthesis of CuO-ZnO-Al 2 O 3 nanocatalysts is shown in Fig. 5. Nasiri et al. [59] have studied the effect of the alumina content on the super-paramagnetic properties of a Ni/Al 2 O 3 nanocomposite synthesized starting from a Al(NO 3 ) 3 •9H 2 O, Ni(NO 3 ) 2 •6H 2 O, Graphite, urea gel. Graphite is added to the solution to prevent the oxidation of metallic nickel at high temperature. By increasing the alumina content from 3 to 45 wt.%, smaller nickel particles and higher super-paramagnetic properties could be obtained. Naemi et al. [60] have produced Ag/Al 2 O 3 nanocomposite, containing 1-10 wt.% of metallic Ag, using metal nitrates-ammonium carbonate ((NH 4 ) 2 CO 3 ) solutions, ignited in a pre-heated furnace at 400 °C, followed by an annealing stage at 900 °C for 2 h. For the second series, the Al 2 O 3 support was prepared by the solution combustion synthesis method (SCS). Then, Ni (2, 5, and 10 wt.%) was dispersed on the support by a standard wet impregnation method. Prior to the catalytic run, the free NiO particles on the support surface were reduced under hydrogen flow. The Ni/Al 2 O 3 catalysts synthesized by the combustion method presented a higher metal dispersion and ethanol conversion, compared to those prepared by sol-gel method [61]. In order to study the effect of surface modification, 10 wt.% of Ni was dispersed on both modified and unmodified SC powders using the standard wet impregnation method, and then the samples were thermally treated at 350 °C. The surface modification of Al 2 O 3 carried out by plasma pretreatment, led to a Ni catalyst with enhanced catalytic activity and selectivity [62]. Similarly, Cross et al. [63] have prepared dispersed Ni nanoparticles on ɤ-Al 2 O 3 support for ethanol decomposition reactions. In a typical experiment, ɤ-Al 2 O 3 pellets with a ring geometry were immersed into a reactive solution of nickel nitrate hydrate (Ni(NO 3 ) 2 •6H 2 O) and glycine for 1 s to 30 min, and then placed on a hot plate maintained at 500 °C to initiate the combustion. To keep the metal dispersion, the catalyst pellets were reduced with hydrogen at 300 °C. The authors have reported that short impregnation times lead to a non-uniform distribution of Ni into the pellets, thus negatively affecting the catalyst selectivity to hydrogen, during ethanol decomposition. Zhao et al. [64] have prepared Ni/Al 2 O 3 catalysts with various Ni contents (from 10 to 50 wt.%) by the solution combustion method, and 30 wt.% Ni/Al 2 O 3 catalyst by the impregnation method, for CO methanation. Prior to the reaction, the catalysts were heated under H 2 flow to reduce NiO to metallic Ni. SCS-derived catalysts exhibited a higher catalytic activity than that of 20 wt.% Ni/Al 2 O 3 catalysts obtained by the classical impregnation. The SC-catalysts displayed good temperature stability and total CO conversion already at 320 °C. Zeng et al. [65] succeeded in further improving the catalytic activity of Ni/Al 2 O 3 catalysts up to 100% conversion of CO, in the 300-320 °C range, by adding titanium (Ti) species with loadings varying from 1 to 5 wt.%.

In the same context, Gao et al. [66] have prepared Ni/Al 2 O 3 catalysts (with 20 wt.% Ni loading) by microwave assisted solution combustion using urea, glycine, ethylene glycol, or citric acid as fuels as illustrated in Fig. 6. The catalysts were then tested in CO methanation.

The microstructure and performance of the reduced Ni/Al 2 O 3 catalysts seem to strongly depend on the type of fuel used during the synthesis. Further studies were carried out by Gao et al. aiming to evaluate the influence of the urea quantity [67] and fuel additives (ammonium acetate, starch and polyethylene glycol (PEG)) [68] on the combustion process, the microstructure and the intrinsic catalytic activity of Ni/Al 2 O 3 catalysts. They found that the combustion enthalpy increased with the urea/metal nitrates ratio. The presence of high urea amounts leads to a high combustion temperature, facilitating NiO migration into the Al 2 O 3 matrix to form the NiAl 2 O 4 spinel and that displays a low activity (Fig. 7). If ammonium acetate is used as a fuel additive the nanostructure and activity of the Ni/Al 2 O 3 catalysts are higher than those obtained for the catalysts prepared with starch and PEG, as illustrated in Fig. 8. [69] (Fig. 9). The solution combustion approach was also successfully used to prepare highly dispersed noble metal such as Pt, Pd, Ag and Au on alumina. Bera et al. [71] were the first authors to report on the synthesis of supported Pt, Pd, Ag and Au catalysts by the combustion method. In a typical synthesis, an aqueous solution of aluminum nitrate, noble metal salts and urea was introduced into a furnace preheated at 500 °C. The combustion process brought about spherical nano-metal particles (~7-15 nm in diameter) uniformly dispersed on large surface area α-Al 2 O 3 powder (Fig. 10). These catalytic materials displayed a good activity in CO oxidation and NO reduction. Greca et al. [72] conceived three different methods for the preparation of Pd/Al 2 O 3 catalysts with 0.5 and 1 wt.% Pd for the selective oxidation of ethanol. In a first experiment, they used the conventional wetness impregnation method. In a second experiment, the simultaneous combustion synthesis of the support and active phase was performed. In a third experiment, the independent combustion synthesis of the support, followed by impregnation with the active phase, was applied. In particular, the alumina obtained by combustion synthesis and impregnated with 1 wt.% Pd showed a high ethanol conversion (~90%) and selectivity to acetic acid (>60%), at reaction temperatures between 180 and 200 °C. ) were prepared by a solution combustion process using cobalt nitrate, aluminum nitrate, and urea [75], citricacid [76] or glycine [77] as fuels. Recently, Ragupathi et al. [78] have proposed a novel green SCS to produce cobalt aluminates powders using aloe vera plant extract as fuel. Two reaction ignition methods, microwave heating and conventional furnace heating, were applied. The authors noted that, the microwave ignition provided smaller nanoparticles and a more uniform particle size distribution than those obtained in the furnace (Fig. 11). A similar approach was adopted for the synthesis of NiAl 2 O 4 [79], CuAl 2 O 4 [80], and ZnAl 2 O 4 [81]. Nanoscale powders of nickel aluminates were also produced from solution combustion using nickel and aluminum nitrates as oxidizers and urea [82], glycine [83], sesamum indicum leaves [84] or opuntia dillenii haw plant extract [85] as fuels.

Conclusion

This review showed how the SC synthesis offers a versatile tool for the formulation and preparation of a wide variety of technologically useful alumina and related oxides with improved properties.

However, despite the large number of SC syntheses reported and the wide variety of SCobtained compounds, information on the reaction mechanism of combustion synthesis, the mechanism of nanocrystalline particle formation, and the modeling of combustion synthesis processes is still scarce. Also, it is clear that further research is required to better understand the thermal regime of the combustion reactions.

The rapid combustion reaction and complex nature of the SC-process makes it difficult to control the nucleation and growth steps of products. In the light of these observations, researchers have developed alternative strategies for inhibiting the agglomeration and enabling the tuning of the microstructure properties in a conventional solution combustion synthesis. One solution is to use microwave energy in the so called "microwave-assisted solution combustion synthesis".

In microwave-assisted SC synthesis, the heat is generated by the rapid change in dipole moments or dielectric polarizations of the precursor molecules, which leads to a uniform heating from the core to the surface of the material. This controlled heating brings about a fine particle size distribution, ultimate good phase dispersion, and a uniform morphology of the nanoparticles of the final material.

Another fundamental issue is related to environmental impacts of hazardous (NO 2 , NO, N 2 O, N 2 O 5 ) gases that might be released during large-scale applications of SC processes. Recently, sustainability issues have emerged, urging the need for improving the methodologies employed in the SC synthesis. The sustainability concept requires the consumption of less toxic precursors, in water or more environmentally benign solvents, using the smallest number of reagents and synthetic steps, as well as minimizing the energy consumption and the generated by-products and waste quantities. In this context, researchers have developed an environmentally-friendly approach "Green SC" using plant extract (aloe vera, sesamum indicum leaves, opuntia dillenii haw plant extract), as a bio-reducing agent in the preparation of metal oxide precursor powders.

Following the recent improvement in the SC chemistry, the combination of the microwaveassisted SC and the green SC approaches appears to meet the demands of materials science engineering in making materials with interesting proprieties and environmental friendly.

Thanks to the joint efforts of researchers, the solution combustion has become a promising synthesis route in modern material science and technology. Looking into the future, many promising improvements and technological developments in the NANO-combustion synthesis can be expected.

Figure 3

 3 Figure 3 here

3. 1 .

 1 Alumina composites/nanocomposites 3.1.1. Alumina/Zirconia (Al 2 O 3 /ZrO 2 )

  Figure 4 here

Figure 5 here 3 . 2 . Alumina supported catalysts 3 . 2 . 1 . 3 )

 323213 Figure 5 here

  Figure 6 here Figure 7 here Figure 8 here

Figure 9 here 3 . 2 . 3 .

 323 Figure 9 here

Figure 10 here 3 . 3 .

 33 Figure 10 here

Figure 11 here

 11 Figure 11 here

Ni/Al 2 O 3 ) 3.2.2. Cobalt Oxide/Alumina (Co 3 O 4 /Al 2 O 3 ) 3.2.3. Pt, Pd, Ag or Au/Al 2 O 3

	3.	Solution combustion synthesis of alumina-based materials
	3.1.	Alumina composites/nanocomposites
	3.1.1.	Alumina/Zirconia (Al 2 O 3 /ZrO 2 )
	3.1.2.	Alumina/Ceria (Al 2 O 3 /CeO 2 )
	3.1.3.	Copper/Alumina (Cu/Al 2 O 3 )
	3.2.	Alumina supported catalysts
	3.2.1.	Nickel/Alumina (

  in a one-step microwave induced solution combustion from a metal nitrate mixtures with urea as fuel. By utilizing the fuel mixing approach and tuning the ratio between glycine, urea, hexamine (C 6 H 12 N 4 ), and oxalyldihydrazide (C 2 H 6 N 4 O 2 ), Deshpande et al.[44] were able to selectively produce CeO 2 -Al 2 O 3 -CeAlO 3 mixed oxide for CO oxidation. Similarly, the CeO 2 -Al 2 O 3 -CeAlO 3 system was prepared by Venâncio et al.[45] as a solid oxide fuel cell functional anode material. The CeO 2 -Al 2 O 3 -CeAlO 3 electrocatalyst was obtained starting from metal nitrates

	mixed with citric acid, as fuel. Ngoc Nhiem et al. [46] have prepared nanostructured
	Al 2 O 3 /CeO 2 catalyst with Ce/Al molar ratio of 1/9, 2/8, 3/7, 4/6, and 5/5, then calcined at 850
	°C, for the oxidation of CO, by gel combustion method, using polyvinyl alcohol ((C 2 H 4 O) n

3.2.2. Cobalt Oxide/Alumina (Co 3 O 4 /Al 2 O 3 )

  Highly active supported Co oxide catalysts for methane oxidation were prepared by Zavyalova et al.[69] using a combination of wetness impregnation and combustion method. ɤ-Al 2 O 3 supports impregnated with aqueous solutions of various reactive mixtures (cobalt acetate (Co(CH 3 COO) 2 ), cobalt nitrate (Co(NO 3 ) 2 )/cobalt acetate, and mixtures of cobalt nitrate with urea, citric acid, glycine, and glycerine (C 3 H 5 (OH) 3 )) were dried at 110 °C and ignited on a hot plate preheated in the 230-250 °C range. For comparison purposes, a 10wt.% Co 3 O 4 /ɤ-Al 2 O 3 catalyst was prepared by the conventional impregnation method. The catalysts obtained with the combination of impregnation and combustion method from equimolar redox mixtures of cobalt nitrate/glycine or cobalt nitrate/cobalt acetate were found to have the highest activity in methane oxidation (complete conversion of methane at 400-425 °C). Another example of Co 3 O 4 /Al 2 O 3 catalyst was obtained by SC using α-Al 2 O 3 supports instead of ɤ-Al 2 O 3 supports, and tested for phenol decomposition. Activated α-Al 2 O 3 supports were mixed with nitrate-urea mixtures, stirred for 30 min and heated in furnace at 500 °C for 30 min[70]. The physicochemical characterizations showed that crystallites of Co 3 O 4 and α-Al 2 O 3 were present as two distinct chemical phases, with bigger particle size than those obtained with ɤ-Al 2 O 3 supports, as reported in previous works

  Kingsley et al.[73] reported for the first time the preparation of metal aluminates MAl 2 O 4 (where M = Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu and Zn), using the corresponding metal nitrate and urea/carbohydrazide redox mixtures.Metal aluminates MAl 2 O 4 (where M=Mn, Cu and Zn) have been produced from solution combustion using aluminum nitrate, ammonium nitrate, metal acetate precursors and different fuels such as urea, carbohydrazide, oxalyldihydrazide, hexamine or glycine. The spinels obtained were nanosized (15-65 nm) oxides with a surface area varying from 40 to 180 m 2 /g, depending on the kind of fuel[74]. Cobalt aluminates (CoAl 2 O 4


Ioannis Vardopoulos

Ioannis Konstantopoulos

Lionel Limousy

Simona Bennici

Vassilis Inglezakis

Antonis A Zorpas
email: antonis.zorpas@ouc.ac.cy

Sustainable urban perspectives through waste indicators evaluation

Keywords: sustainability indicators, solid waste management, DPSIR, waste strategies, circular economy, sustainable urban development Highlights

Human Activities are consider among the main production of any kind of pollution. The paper focus on the evaluation and the assessment through DPSIR model the existing practices, procedures and results obtained in order to determine whether the Municipal Solid Waste (MSW) management implemented in three major Municipalities in the Urban Area of Greek in Attica Prefecture and consist from Municipalities of Nea Smirni, Vyronas and Piraeus, could be considered viable and sustainable. The evaluation indicated that MSW in Greek cities have reduced over the last years, also suggesting a steady downward trend, which could be considered consistent with that of that of the per capita incomes in Greece due to the extended economic austerity, while at the same time the recycling indicator seems to optimize. The results are very useful for policy makers, local authorities to take actions related with the target set from circular economy strategy as well as the targets set from United Nation Development Program

The technological development and improvement of living standards through the global economic prosperity of the past decades have developed massive needs for goods and products [START_REF] Amvrosiou | The Study of the Attitudes and Perceptions of Modern Greeks during the Economic Crisis on the Consumption of Organic Products[END_REF][START_REF] Theodoropoulou | The Green Market In Greece After The Outbreak Of The Financial Crisis:Case Study Of 10 Enterprises[END_REF], resulting in several environmental impacts such as the greenhouse effect, climate change, reduction of natural resources, etc. [START_REF] Baumgartner | Critical perspectives of sustainable development research and practice[END_REF][START_REF] Kyramargiou | Use and sustainability of the ships' waste reception facilities. evidence from the Port of Corinth, Greece[END_REF]. Those damages arose from the lack of a proper institutional framework as, until recently, our society ignored the dangers of prosperity and did not hold the necessary ecological consciousness [START_REF] Zorpas | Sustainability behind Sustainability[END_REF], as well as the need to satisfy the increasing products and services consumption [START_REF] Lebel | Enabling Sustainable Production-Consumption Systems[END_REF][START_REF] Lawson | Freedom from ownership: An exploration of access-based consumption[END_REF] of the growing urban influx [START_REF] Kammen | City-integrated renewable energy for urban sustainability[END_REF].

In addition, more than 245 billion tonnes of waste per year are produced from commercial activities in Europe (Eurostat 2019a). Moreover according to the projections of the World Bank (2018) the average waste production per citizen in the developed countries may reach 2.1 kg/day by 2025. Thus, waste management nowadays, and in particular sustainable municipal waste management is considered one of the most social and significant issues, which is often an element of conflict between social and political aspects especially in local level [START_REF] Miller | Living in the environment: principles, connections, and solutions[END_REF]. Due to its particular nature, managing most of the matters associated, lies in the cooperation of all social partners and the broader scientific consensus [START_REF] Grodzińska-Jurczak | Hellenic Chamber of Hotels Hellenic Chamber of Hotels Hellenic Official Gazette A' 174 (2015) Εθνικό Σχέδιο Διαχείρισης Αποβλήτων Hellenic Official Gazette B' 1572 (2002) Μέτρα και όροι για την υγειονομική ταφή των αποβλήτων Hellenic Official Gazette B' 4326 (2016) Έγκριση Εθνικού Σχεδίου Διαχείρισης Επικίνδυνων Αποβλήτων (ΕΣΔΕΑ) Hellenic Official Gazette B' 698[END_REF]. The use of a rational and sustainable waste management strategies (covering prevention, reuse, recycling, energy recovery, etc.) involves the co-operation of the local and regional competent bodies while at the same time, it should be governed by the requirements of the national environmental policies and legislation [START_REF] Eriksson | Municipal solid waste management from a systems perspective[END_REF]Zorpas et al. 2015a[START_REF] Zorpas | A prevention strategy plan concerning the Waste Framework Directive in Cyprus[END_REF][START_REF] Zorpas | Monitoring and evaluation of prevention activities regarding household organics waste from insular communities[END_REF][START_REF] Poulos | Inconsistencies and deficiencies in the waste management practices in Athens. An intersectional approach[END_REF]. Therefore, sustainable solid waste management is a multidimensional and multidisciplinary subject to be addressed by the local and regional authorities.

Waste in simple terms result from any kind of consumption and any kind of production process, while any time a simple item is disposed of then a whole new set of impact is involved treating it. Taking into account the sources of waste, those can be divided into several groups: municipal solid waste (MSW) (including household and commercial), industrial waste (including numerous activities such as manufacturing -light and heavy-, refinery, chemicals mining, etc.), hazardous, and nonhazardous waste, medical waste, and other not classified as municipal, industrial, medical etc.. Solid waste are substances or objects which appear mainly in solid state from which the holder needs or is required to be exempted and are not considered hazardous according the European Union Hazardous Waste Catalogue [START_REF] Karagiannidis | Full cost accounting as a tool for the financial assessment of Pay-As-You-Throw schemes: A case study for the Panorama municipality, Greece[END_REF]Inglezakis and Moustakas 2015;[START_REF] Malamis | Study and assessment of segregated biowaste composting: The case study of Attica municipalities[END_REF]. MSW include the heterogeneous solid waste mass deriving from local authorities (including municipalities and communities) as well as the homogeneous mass of agricultural, industrial and debris waste (Zorpas et al. 2013b;[START_REF] Inglezakis | Current municipal solid waste management in the cities of Astana and Almaty of Kazakhstan and evaluation of alternative management scenarios[END_REF], namely solid waste produced by households, shops, street and other public spaces cleaning procedures, but also those produced by all kinds of businesses and which by nature can be considered similar to household solid waste. In particular, within the classification of MSW, all waste other than those in liquid form are included, without measuring a significant percentage of particulate pollutants (i.e. liquid waste) and gaseous pollutants [START_REF] Gidarakos | Συσκευασίες και εναλλακτική διαχείριση των συσκευασιών και άλλων προϊόντων -Ίδρυση Εθνικού Οργανισμού Εναλλακτικής Διαχείρισης Συσκευασιών και Άλλων Προϊόντων[END_REF]. MSW are divided into two main categories, municipal waste and special waste, and these are classified into many other sub-categories. Zorpas, et al. (2015) divide MSW into 13 main streams and those in more than 50 sub categories, while according to [START_REF] Misra | Hazardous waste, impact on health and environment for development of better waste management strategies in future in India[END_REF] MSW is divided into 970 types, 406 of which pose a risk to the environment and public health due to either high toxicity, or mobility, or carcinogenicity effects, or to some extent persistence in the environment, etc. like chlorinated solvents, waste from metal degreasing, cyanide waste, dioxin-based waste, and polychloro biphenyl waste.

MSW sustainable management, must take under consideration all the relevant economic, social and environmental constraints as well as the carrying capacities of the ecosystem, aiming towards autonomy [START_REF] Tatsi | A field investigation of the quantity and quality of leachate from a municipal solid waste landfill in a Mediterranean climate (Thessaloniki, Greece)[END_REF][START_REF] Skordilis | Modelling of integrated solid waste management systems in an island[END_REF][START_REF] Seadon | Integrated waste management -Looking beyond the solid waste horizon[END_REF][START_REF] Bossel | Alternative scenarios to meet the demands of sustainable waste management[END_REF][START_REF] Asase | Comparison of municipal solid waste management systems in Canada and Ghana: A case study of the cities of London, Ontario, and Kumasi, Ghana[END_REF][START_REF] Abeliotis | Life Cycle Assessment in Municipal Solid Waste Management[END_REF]). More specifically it is considered necessary to conduct relative qualitative and quantitative analysis of the municipal waste produced [START_REF] Fragkou | A general methodology for calculating the MSW management self-sufficiency indicator: Application to the wider Barcelona area[END_REF]. Zorpas et al. (2015) suggests that using waste compositional analysis could define the origin and the scale of the waste produced. The heterogeneity of the materials and their quantities is a function of the geographic and demographic characteristics of each region, the GDP, the technology used, the seasoning, the consumption patterns, ethics, daily routines, educational level etc. [START_REF] Tchobanoglous | Integrated Solid Waste Management[END_REF]Zorpas et al. 2015b).

Sustainability is a matrix looking towards an ideal economic outcome for both human and natural environment, both in present and the indefinite future. Principal element is the harmony among raw materials and the production of goods (WCED, 1987). Hence, sustainable actions objective is to achieve high production using as fewer raw material as possible. Sustainability implies that natural resources should be exploited at a rate proportionately lower than the rate of which are recycled, otherwise the environment is degraded which at the long-term could lead to the inadequacy of the earth's ecosystem to sustain human existence (ecological crisis) [START_REF] Zorpas | Sustainability behind Sustainability[END_REF]) and as result sustainability is divided into three interrelated pillars, namely the environment, the economy and the society [START_REF] Duić | Components and structures of the pillars of sustainability[END_REF][START_REF] Vardopoulos | Multi-criteria analysis for energy independence from renewable energy sources case study Zakynthos Island, Greece[END_REF]. Furthermore, sustainability is a dynamically improving process with long-term objectives and varying goals [START_REF] Bagheri | Planning for sustainable development: a paradigm shift towards a process-based approach[END_REF] and is linked to political thinking and participation of society in decision-making [START_REF] Bagheri | Planning for sustainable development: a paradigm shift towards a process-based approach[END_REF][START_REF] Zorpas | Multi-criteria analysis of sustainable environmental clean technologies for the treatment of winery's wastewater[END_REF]. Sustainable development is addressing economic growth, as studied and implemented, considering the environment and the society [START_REF] Lélé | The concept of circular economy strategy in food waste management for the optimization of energy production through anaerobic digestion[END_REF][START_REF] Kostakis | Tourism, culture, education and economic growth nexus: Evidence from the Greek Regions, 1998-2016[END_REF]. The criterion for sustainable development is the maximum possible raw materials use without however interrupting their natural production rates and their endless sufficient quantity [START_REF] Borowy | Defining sustainable development for our common future: A history of the world commission on environment and development[END_REF]. Possible decision, planning and activation of the mechanisms towards sustainable development require both qualitative and quantitative assessment (Mitoula et al. 2012;[START_REF] Vardopoulos | Multi-criteria decision-making approach for the sustainable autonomous energy generation through renewable sources. Studying Zakynthos Island in Greece[END_REF]. For this reason it is best advised to use an indicators system which is able to highlight the most important aspects of sustainability [START_REF] Farrell | What Does Sustainability Really Mean?: The Search for Useful Indicators[END_REF][START_REF] Tscherning | Does research applying the DPSIR framework support decision making?[END_REF][START_REF] Inglezakis | Environmental indicators[END_REF][START_REF] Vardopoulos | Sustainability assessment of the agri-environmental practices in Greece. Indicators' comparative study[END_REF].

Indicators are computational sets seeking at simplifying, quantifying and transmitting certain information contributing towards identifying information that are not easily perceptible [START_REF] Gamberini | Municipal solid waste management: Identification and analysis of engineering indexes representing demand and costs generated in virtuous Italian communities[END_REF][START_REF] Vardopoulos | Sustainability assessment of the agri-environmental practices in Greece. Indicators' comparative study[END_REF][START_REF] Vardopoulos | Commission on Environment and Development (1987) Our Common Future Zavali M, Theodoropoulou E (2018) Investigating determinants of green consumption: evidence from Greece[END_REF]. Sustainability indicators are developed for measuring the environmental performance of actions and/or procedures implemented at an area or by an organization [START_REF] Mori | Review of sustainability indices and indicators: Towards a new City Sustainability Index (CSI)[END_REF][START_REF] Ravetz | Sustainability indicators[END_REF]. Although mainly used at national inventories (eg. national greenhouse gas emissions inventory), there is a huge interest from corporations for the calculation and reporting of their environmental performance [START_REF] Bossel | Alternative scenarios to meet the demands of sustainable waste management[END_REF]). An environmental and sustainability indicators system constitutes the framework for monitoring and recording the environmental condition and contributes towards the compensation of established needs and requirements for assessing and comparing the situation of an area to another, while it seeks to support competent bodies in decision making and developing action plans (Victor 1991;Zorpas et al. 2013a;[START_REF] Inglezakis | Environmental indicators[END_REF].

Sustainability indicators development for managing MSW depends almost entirely on the competent bodied reported quantities. However, there is a significant uncertainty about the reliability of the data inputs delivered. Some researchers try to determine the "fault" range using mathematical models (Chowdhury 2009; [START_REF] Kollikkathara | A system dynamic modeling approach for evaluating municipal solid waste generation, landfill capacity and related cost management issues[END_REF][START_REF] Eleyan | A System Dynamics Model to Predict Municipal Waste Generation and Management Costs in Developing Areas[END_REF], while others are addressing the municipal waste management issue via the economics discipline (Victor 1991;[START_REF] Pearce | Capital theory and the measurement of sustainable development: an indicator of "weak" sustainability[END_REF][START_REF] Olewiler | Environmental sustainability for urban areas: The role of natural capital indicators[END_REF].

The current research proceeds comparing practices, procedures and results obtained in order to determine whether the MSW management implemented in Greek urban areas, could be considered viable and sustainable. In doing so, three major municipalities in Attica Prefecture of Greece are used as case study; Municipalities of Nea Smirni, Vyronas and Piraeus.

Methodology

Area Selection

According to Eurostat (2019), the per capital production of MSW in Greece was 504 kg for 2017, which is 60% highest compared with the 1995 baseline. Since the first bailout agreement early 2010, Greece has come through a nearly ten year actual economic, political and social crisis; economic adjustment programs followed austerity packages amid elections and successive change of power, Greece-wide strikes and riots and the European migrant crisis. The consequences were far from negligible and no-one currently disputes the fact that the crisis triggered particular social and consequently urban transformation; unemployment, migration, gentrification, etc. (Mavridis 2018) (Table 1). As a result, and since waste production, collection and sustainable management depends on the social and economic system [START_REF] Koumparou | Circular economy and social sustainability[END_REF], it is anticipated that MSW issue in Greece has also been affected.

[Table 1]

The legal framework for waste management in Greece closely follows the European developments as well as the corresponding directives. The main legislation on waste management governing Greek authorities is Greek Law 4042/2012. In full compliance with the waste framework directive 2008/98/EC, the basic concepts and the management framework laid down in Greek Law 4042/2012 are applicable to the scope, definitions, waste hierarchy, permits, liability, etc.. In addition, Greek Law 2939/2001 regulates the packaging waste and other special waste types management and reuse. Further legislative measures, such as presidential decrees and ministerial decisions, complete the Greek legislative scheme with the Joint Hellenic Ministerial Decision 50910/2727/2003 on measures and conditions for the management of solid waste, being one of the most significant.

A significant number of waste management plans have been established during the last ten years by local authorities and central administration in Greece, in line with the European Union planning requirements. The Hellenic National Waste Management Plan (NWMP) and the Hellenic National Waste Prevention Program (NWPP) (Hellenic Official Gazette A' 174/2015) describe domestic waste management strategies, policies, and objectives, indicating the overall requirements and suitable steps for waste management, addressing the problem of particular waste types or even regional planning. Particularly, included in the NWMP are adequate data on location identifying requirements and the capability of potential landfill or significant waste disposal facilities, current waste collection systems and large waste disposal and recovery installations and waste prevention programs, etc.. In addition to the Hellenic National Hazardous Waste Management Plan (NHWM) (Hellenic Official Gazette B' 4326 2016), Regional Management Plans (RMP) have been released towards ensuring a sufficient and integrated network of waste disposal systems, as well as addressing the present waste management state. In the RMP, the landfill sites or major waste treatment site (Hellenic Official Gazette B' 1572[START_REF] Tatsi | A field investigation of the quantity and quality of leachate from a municipal solid waste landfill in a Mediterranean climate (Thessaloniki, Greece)[END_REF] have to be included, but unfortunately most often they are not, arising local conflicts.

The Hellenic Ministry for the Environment is accountable for the development of strategies, domestic planning, technical engineering issues, licensing, regulating and funding the waste and disposal systems in Greece (www.ypeka.gr). The collection, the transportation as well as the storage of MSW is, by default, the responsibility of the Greek Municipalities. It is the duty of each municipal's Waste Management Authorities to operate transmission facilities, process and disposal of waste. Greece lacks waste prevention practice on separate collection, re-use, recycling and recovery. Hellenic Decentralized Administrative Authorities are the competent officials in economic issues, environmental matters and permits, whereby Hellenic Regional Authorities are the competent bodies on industrial permits and health and safety provisions. The private sector has recently become more involved in urban waste treatment, primarily through the public-private partnership system, regulated by the Greek Law 3389/2005. Despite the fact that the local authorities' power is limited in terms of intervention throughout the waste management stages, as previously described, it is clear that their role is of particular importance for a number of reasons. Waste collection (Pressure), environmental protection (Impact), recycling (Response), ensuring public health (Driver) and maintaining of public spaces cleanliness (Pressure), etc., fall within the competence of all the local authorities. Thus, local authorities have the opportunity to elaborate their own adapted to local needs operational and management plan.

All municipal authorities, by developing environmental strategies (Response), look forward to the sustainable development of the region. The protection (Impacts) and improvement of the environment as well as the improvement of the quality of life of the residents along with the improvement of their activities regarding the environment, constitute the fundamentals under which the local competent bodies operate. The identity and the physiognomy of each municipality (Drivers) are of particular importance in shaping the sustainable local development agenda. The specific characteristics of the municipalities act as a reference point since they shape the residents way of life and environmental identity. The importance of the urban environment constitutes a collective principle since the environment is not only a physical space but also an organic tier that connects humans to the physical world and culture. The three municipalities of the current research were chosen for case study by population density. Municipality of Piraeus representing Attica Prefecture urban areas with population approximately 150000 inhabitants, Municipality of Nea Smirni of about 100000, and Municipality of Vyronas of about 50000 residents. In addition, it is estimated that this three Municipalities share mutual characteristics and combine certain features to a number of cities in Greece and Europe, such as the temperate Mediterranean climate, the scale and geometry of the urban layout, consumption patterns, etc..

In particular, Municipality of Nea Smirni occupies an area of approximately 3500 km 2 . According to the Hellenic Statistical Authority latest census (Hellenic Official Gazette B' 698 2014), Nea Smirni counts approximately 73500 residents, however, according to Municipal Authority calculations, the actual number of residents is estimated at around 97000. Morphologically, the municipality of Nea Smirni has significant slopes from the West to the East. Population is purely urban and the neighborhoods are almost homogenized. Currently, there is no city space left for horizontal expansion. The affinity with the Municipality of Athens, in addition to housing development inducement, is considered the main reason for the development of the services market, mainly around Syngrou Avenue, and has been the main reason for not witnessing any noisy activities market. The -services-market expands around the municipality but is focusing to the city center [START_REF] Mitoula | The physiognomy of Asia Minor in the Municipality of Nea Smyrni[END_REF]. In recent years, Municipality of Nea Smirni has witnessed an intense construction outbreak and a considerable population increase that gradually stretched the municipal infrastructure to the limits, while a number of environmental issues have been raised [START_REF] Malindretos | City Solid Waste Logistic: The Case of Nea Smirni, Greece[END_REF]. Minimization of the unstructured land, constantly burdensome traffic volume, lack of parking spaces, considerable waste quantities, etc..

[Figure 1]
According to the Municipality of Nea Smirni Regional Waste Management Plan (2015) 7.6% w/w of the total MSW produced end up to recycling or other recovery, while the remaining 92.4% w/w is led for disposal in the Sanitary Landfill of Fili (Attica region), whereas the average quantity of MSW generated per capita is shaped at 1.01 kg/day, which is 19% lower than the national average (1,25 kg/day) (Table 1). The municipality for the collection of the MSW (including recycling) offers 2440 waste collecting containers or bins, and holds 18 waste collection heavy vehicles and a number of other smaller supporting equipped vehicles. MSW management costs approximately € 85/ inhabitant/ y. The main objectives set by the Municipality of Nea Smirni Regional Waste Management Plan (2015) for 2020 are presented in Table 4.

[Table 2] [Table 3] [Table 4]
Municipality of Piraeus covers an area of 10.9 square kilometers and hosts approximately 164.000 inhabitants, a number which classifies Piraeus as the fourth largest city in Greece. Also, Piraeus is the most important industrial center of the country, the largest commercial center of the Greek economy, and the most significant commercial and passengers port of Greece (the largest in Europe with regard to passenger traffic), connecting the main land with the almost all of the Aegean Islands. Thus given the tourist traffic, according to the Hellenic Chamber of Hotels, 41 hotels are located in Piraeus, holding 1645 rooms or 2967 hotel beds. In addition, in Piraeus are located 114 public schools, 62 churches, 6 public hospitals and 5 public factories, while according to the Piraeus Rrofessional Chamber approximately 2200 businesses of all types and size are currently operating. Solid waste production quantities are presented in Table 5 according to the Municipality of Piraeus Regional Waste Management Plan (2015). Piraeus has a fleet of motor-driven equipment including 54 waste collection heavy vehicles, 22 several utilities heavy vehicles, and 14 other supporting equipped heavy vehicles. Within the municipality of Piraeus approximately 12000 waste collecting containers or bins are currently serving the waste collection needs. The main objectives set by the Municipality of Piraeus Regional Waste Management Plan (2015) for 2020 follow the objectives set by the NWMP, as presented in Table 4, without further in-depth investigation or assessment of the handling capacity.

[Figure 2]

[Table 5]
The municipality of Vyronas is located at the foothill of mount Hymettus in the northeast of the Attica Prefecture. It owns a privileged position due to its proximity to the city center of Athens, a fact that justifies the urbanization of recent years. The municipality area located close to mount Hymettus is a forest, while the southwest part is urban. The total area covered by municipality of Vyronas is approximately 9500 square kilometers. According to the latest census the population counts approximately 61000 residents, without any considerable population fluctuations over the last years. However, anthropogenic pressures are intensifying due to the increase of immigrant and students (proximity to University campus). Hence, the management of the natural and anthropogenic environment along with the municipal waste has been identified as of first priority by the municipal authorities. For 2014 MSW production is estimated to 327.75 kg / person, with the total MSW management cost accounting to € 2536340. However, it should be noted that the recyclable materials collected are directly sold (Concerns secondary materials absorption industries by recycling and incorporating (recyclers) into their production process. These are mainly the metal industry (Fe, Cu, Al) and secondarily the plastics, glass, tires, oils, polyurethane industry and generally all the industry is able to utilize the secondary materials in the production process).

The municipality for the collection, reception, and recycling of the MSW holds 35 waste collection heavy vehicles including a number of some smaller supporting equipped vehicles, and offers 1751 waste collecting containers or bins, located at 103 points around the Vyronas district. The main objectives set by the Municipality of Vyronas Regional Waste Management Plan (2015) for 2020 follow the objectives set by the NWMP, as presented in Table 4, without further in-depth investigation or assessment of the handling capacity.

[Figure 3]

[Table 6]
At this point it should be noted that all three selected municipalities, as most of the municipalities in Attica Prefecture for the collection and transportation of the MSW use the same pattern, id est heavy vehicles collecting waste arrive at the transfer station and empty their load to container vehicles, which will end up at the Sanitary Landfill of Fili (Attica region). No compaction of the municipal waste occurs in the containers.

DPSIR framework

The evaluation of the most significant aspects of sustainability is done by creating a series of appropriate indicators. Selection and use of indicators, which reflect and identify the connection between human activities and their environmental effects, and consequently, the appropriate strategies to respond to these effects. The most typical method for reporting the connection between environmental causes and effects is the DPSIR model (Fig. 1) [START_REF] Vardopoulos | Sustainability assessment of the agri-environmental practices in Greece. Indicators' comparative study[END_REF].

[Figure 4]

The DPSIR indicators development model is based on the relative actions taken in order to assess the negative consequences deriving from human economic activity, and is associated with the quantitative results of the MSW management procedures. Thus, using the appropriate indicators, the human-and-environment cause-and-effect relationship and interaction is captured [START_REF] Jacob | Institutions and Instruments for Government Self-Regulation: Environmental Policy Integration in a Cross-Country Perspective[END_REF]. Thus in order to highlight the most important aspects of sustainability, methodologically, an indicators system is applied. Sustainability indicators for the scope of the current research are developed, using the latest official data available for three major municipalities in Attica Prefecture, Greece.

The sustainability indicators are developed under the following principles [START_REF] Jacob | Institutions and Instruments for Government Self-Regulation: Environmental Policy Integration in a Cross-Country Perspective[END_REF]):

 The social and economic development (driving force) is considered the major source of pressure on the environment.  Driving forces act pressure on the environment.  Therefore, the state of the environment is altered thus the ability to support demand is differentiated.  Alterations on the environmental state will most likely have impact on human health, ecosystems, etc., which may be reflected by the volume of those environmental state alterations.  Thereby, impact leads to necessary mitigation measures (response), which accordingly with their kind and nature act directly on the driving forces, the pressure sources the environment and its state.

The developed sustainability indicators were: i) the Municipal Solid Waste Production (MSW-P) indicator representing the ratio of the amount of MSW produced to the municipal population at a given time. This indicator provides a measure of the average waste quantity, a figure that is quite usable and comparable, ii) the Municipal Solid Waste Composition (MSW-C) indicator recording the ratio of the most known materials included in the reported amount of MSW (Table 3) to the total amount of MSW produced at a given time. The indicator provides information on the economic and ecological utility of the waste as well as the disposal methods provided, and iii) the Municipal Solid Waste Recycling (MSW-R) indicator measuring the recycled municipal solid waste compared with the total amount of MSW produced at a given time.

By putting the focus on quantifiable parameters, the developed sustainability indicators identify differences among similar activities and efficiency-reduced points, providing insights for optimizing MSW management. The developed sustainability indicators address certain aspects for the assessment of the MSW management efficiency and effectiveness, and have been drawn up by the authors following a significant number of years in research on the matter subject and after several conservations and debates with different stakeholders in the field. However, the efficiency and effectiveness of the MSW management appears to be affected by a number of crucial non-quantifiable (at least necessarily) parameters, namely cultural, social, economic, and political conditions. Hence, although it is clear for the authors that the developed sustainability indicators do not cover the entire spectrum for the assessment of the MSW management efficiency and effectiveness, they have chosen to further proceed with the current research, aiming towards paving the way for a future deeper assessment. In addition, the information and data accessible to the study group did not permit for a more advanced analysis during the course of this study.

For the current methodology setup commercially available data were collected from; (a) Eurostat (www.ec.europa.eu/eurostat), the Director General of the European Commission responsible to provide statistical information data, (b) The Hellenic Statistical Authority (ELSTAT) (www.statistics.gr) the on regular basis updated official national statistical provider in Greece, (c) The Hellenic Ministry of Environment and Energy (www.ypeka.gr) the Hellenic government department responsible for industrial, environmental and energy policy, (d) the Municipality of Nea Smirni Regional Waste Management Plan ( 2015), (e) the Municipality of Piraeus Regional Waste Management Plan (2015) (f) the Municipality of Vyronas Regional Waste Management Plan (2015).

Results and Discussion

MSW management by sustainability indicators assessment, from an environmental point of view, is a common structured approach for the thorough examination and comparison of the procedures and activities followed by various municipalities [START_REF] Zotos | Developing a holistic strategy for integrated waste management within municipal planning: Challenges, policies, solutions and perspectives for Hellenic municipalities in the zero-waste, low-cost direction[END_REF]). The DPSIR model based sustainability indicators developed stand on the relative procedures and activities followed by various municipalities, as actions taken towards addressing social activities. Table 7 lists the developed sustainability indicators and their function in the DPSIR causal framework.

[Table 7]

Table 8, providing a measure of the average waste quantity, presents the ratio of amount of MSW produced to the municipal population of the three municipalities under study during 2010-2015 (based on the data of the 2011 Hellenic National Census). Using data listed in Table 8 the MSW-P sustainability indicator is developed as illustrated in Fig. 5.

[Table 8]

[Figure 5]

The annual amount of MSW attributable to each resident presents a sharp fall in 2011, which stabilized around 2013. This was somewhat anticipated reflecting the first signs of the actual economic, political and social crisis occurred in Greece. Ranging from 178 kg MSW per capita in Romania to 789 kg MSW per capita in Denmark, the corresponding value for Greece in 2015 was somewhere in the European in-between, id est 488 kg MSW per capita as recently reported by Eurostat (2019b). Thus, the values reported in Table 8 and illustrated in Fig. 5 concerning the three Greek municipalities under study are placed below the Greek average, while the best performance is over time held by Vyronas. The indicator clearly reflects a consumer patterns change tendency [START_REF] Theodoropoulou | The Green Market In Greece After The Outbreak Of The Financial Crisis:Case Study Of 10 Enterprises[END_REF], and perhaps reflects on population growth trends. Of particular importance is the fact that the indicator trend is convergent with the socio-economic dimensions presented in Table 1.

The MSW-C sustainability indicator, recording the ratio of the most known materials included in the reported amount of MSW (Table 3) across the three municipalities of the Attica Prefecture is presented in Table 9 and Fig. 6 accordingly.

[Table 9]

[Figure 6]
Moreover, in an attempt to capture the recycling status among the under study municipalities of the Attica Prefecture, the MSW-R sustainability indicator is developed measuring the total recycled MSW compared with the total amount of MSW as presented in Table 10 and illustrated in Fig. 7 accordingly.

[Table 10]

[Figure 7]
The above analysis suggests that MSW management has evolved over the last two decades in certain municipalities in Greece. Properly treating MSW can provide social and economic benefits by dealing waste as a resource (suitable waste management can provide vital resources), reducing environmental pressures, create new jobs and boost competitiveness, in the course towards circular economy aiming at achieving a desirable urban sustainable development. However, although progress has evidently been made, the sustainability indicators developed suggest that MSW management is far from been considered sustainable, while progress -statically wise-is depended to the economic differences among districts (usually most affluent produce more MSW per capita), and the recent economic crisis. Despite the fact that urban waste account for only about 10% of all waste, waste composition patterns are directly related to the consumption patterns. The current prevailing EU waste policy rationale is the waste hierarchy which gives priority to waste prevention, followed by reuse, recycling, other forms of recovery and, finally disposal or landfill as the less desirable option [START_REF] Zorpas | Monitoring and evaluation of prevention activities regarding household organics waste from insular communities[END_REF]. Although the main driving forces behind sustainable MSW management are the European policies and guidelines, policymaking at national, regional and local level is of great importance [START_REF] Karagiannidis | Assessing the Collection of Urban Solid Wastes: A Step Towards Municipality Benchmarking[END_REF]).

This research in particular indicates that there is currently room for improvement in the MSW management plans implemented by the three municipalities, and consequently in other municipalities in Greece, thus it cannot be considered to be viable and sustainable. Through examination and comparison of the three local municipal waste plans it is evident that the Greek municipalities operate independently. Moreover, the plans are mostly developed with estimation methodssubject to a large margin of errors-, and they tend to present similar projections, while it is noted that they are quite similar at both their estimations and future projections. Additionally, although these plans have been drawn up by scientific groups (either inhouse or outsourced) it is noted that the huge ever-growing body of scholarly literature dealing with MSW, which is found to be consisted with the findings of the current research, and which would greatly contribute towards achieving the main sustainability objective, is flagrantly ignored. More particularly, [START_REF] Abeliotis | Decision support systems in solid waste management: A case study at the national and local level in Greece[END_REF], studying Greece, developed a novel decision support system for MSW, while [START_REF] Zotos | Developing a holistic strategy for integrated waste management within municipal planning: Challenges, policies, solutions and perspectives for Hellenic municipalities in the zero-waste, low-cost direction[END_REF] developed a systemic approach for MSW summarizing the stateof-the art available tools while compiling a set of guidelines for developing waste management master plans at the municipal level. Economopoulos in 2010 suggested a methodology for developing strategic municipal solid waste management plans. Chalkias and Lasaridi already since 2011 stressed the benefits from GIS-based modelling for MSW management, while [START_REF] Economopoulou | A methodology for optimal MSW management, with an application in the waste transportation of Attica Region, Greece[END_REF], discussed about a system capable of formulating alternative MSW management plans meeting certain constraints set by local communities and authorities. In 2015 Lasaridi et al. presented the design, development and main features of a web-based tool capable of enabling local authorities to select and implement the optimum MSW strategies considering their local conditions and to prepare their local municipal waste plans. Later, [START_REF] Panaretou | Implementation and Evaluation of an Integrated Management Scheme for MSW in Selected Communities in Tinos Island, Greece[END_REF] presented an integrated methodology on the development, implementation and evaluation of the MSW scheme. Lately, [START_REF] Poulos | Inconsistencies and deficiencies in the waste management practices in Athens. An intersectional approach[END_REF] recorded the inconsistencies and deficiencies in the MSW management practices in the municipality of Athens, Attica, pointing out certain major issues affecting the overall efficiency and effectiveness of the strategy as a whole such as the working hours of the landfill site, the stealing of the recyclable waste from the available bins (for black-market sales), traffic jam, the lack of cooperation by the inhabitants and among authorities, the economic crisis and the lack of political will. Scientifically proven and tested perspectives, analyses and solutions that are neither encountered nor adopted across the three municipalities' local waste plans, which would provide after all adequate solutions in specific aspects of the MSW management issue.

As follows the prospects for achieving the 2020 objectives although mixed and somewhat complicated are particularly bleak. However, most Greek cities will have to step up their efforts to reach the target. Although bottlenecks preventing a more integrated approach are perfectly understandable (political orientations, budgets, etc), a cooperative, measurable, applicable and effective MSW system shall be applied in full compliance with the requirements set by the EU, reflecting contemporary scientific standards.

Thus, in view of the above, efficiently visualizing the complex aspects surrounding waste management in the inner-city areas, offering a solid ground for rethinking the entire waste process and perhaps policy, further actions and strategies are suggested in order to achieve the goals of sustainable MSW management. For start the promotion of the most-clean and environmentally friendly processes such as prevention, reuse and zero waste recycling. Subsequently separation of the biodegradable waste in order to make them available for compost and energy production. Moreover, development of innovative, cost-effective and environmentally friendly solid waste management methods and tools, through transferring the know-how and experience among Greek cities. Next, alteration of the pricing policy by introducing a 'pay as you pollute' system. And finally, public awareness raising and awakening of the ecological consciousness thought environmental education.

The most important limitation in this particular study lies in data. The three municipalities' incomplete collection and availability of statistical data for the longitudinal assessment of appropriate sustainability indicators makes it difficult to assess the situation in the long term and to provide a comprehensive solution to the complex and multidimensional problem of the municipal waste management.

Future work should focus on the assessment of further Attica Prefecture municipalities in order for a complete argument to be drawn. Subsequently, future work is required to raise the number of sustainability indicators seeking evidence of interconnection among society, culture, environments and economy. Furthermore, and since indicators adequacy is not addressed, future research in order to avoid possible unnecessary duplication of facts and improve the accuracy of the study, should first use an interview analysis system to better understand the waste management system pathogenesis and particularities of each municipality.

Conclusion

This study provides an urban sustainability assessment through indices of the MSW management. In particular, following the DPSIR model, three indicators were developed for three municipalities of the Attica Prefecture, namely Piraeus, Vyronas and Nea Smirni in order to assess the urban sustainability perspectives by means of addressing in an appropriate way offering adequate response to the matter of the waste -global crisis-with the urgency and seriousness that it deserves.

The sustainable indicators developed were i) the Municipal Solid Waste Production Indicator representing the ratio of the amount of MSW produced to the municipal population at a given time, providing a measure of the average waste quantity, ii) the Municipal Solid Waste Composition Indicator reporting the weight of the most known materials included in the total amount of MSW, providing information on the economic and ecological utility of the waste as well as the disposal methods provided, and iii) the Municipal Solid Waste Recycling Indicator measuring the recycled MSW compared with the total amount of MSW.

The results indicated that MSW in Greek cities have reduced over the last years, also suggesting a steady downward trend, which could be considered consistent with that of that of the per capita incomes in Greece due to the extended economic austerity.

Recycling has also become effective in the last decade in the municipalities of Attica Prefecture, which is to be found well below the targets set by the EU and enacted in Greek Law 4042/2012., and which could also partially explain the decline in generated waste. The findings from this study also support that the impact of the economic crisis is notably negatively expressed in the sustainability indicators, evidently apart from the significant reduction in the amount of waste, which is of course, a critical parameter for the urban sustainable development. However, it should not be directly related to recycling increase, or better waste management, but better to income loss and an unemployment increase effecting consumer habits. After all, sustainability is directly connected with municipal policies in waste treatment and tends towards the so called circular economy [START_REF] Lélé | The concept of circular economy strategy in food waste management for the optimization of energy production through anaerobic digestion[END_REF]. The main MSW management method is disposal in the Sanitary Landfill of Fili (Attica region), while recyclable materials are subject to undertakings entrusted by the municipal authorities with the operation of recycling services, expect the case of Vyronas as previously indicated. The MSW composition does not differ considerably from municipality to municipality, but it certainly changes with time. However, MSW composition data as subject to estimations calculated by reference to the national composition. Comparing the three local municipal waste plans, which by definition indicate, that the Greek municipalities operated independently, it is noted that they are -strangely enoughquite similar, they are mostly developed with estimation methods -subject to a large margin of errors-, and they tend to present similar projections. The regional objectives set for 2020 follow the objectives set by the Hellenic National Waste Management Plan, without further in-depth investigation or assessment of the handling capacity. Exception is the case of Nea Smirni, where a more realistic approach is presented in accordance with the pathogens and management capacity of the Municipality. It should be stressed that because these Regional Waste Management Plans lack of a mandatory character, they do not strive for any kind of efficient, effective and sustainable waste management, and should merely be viewed upon as an actual guide or plan but rather as (perhaps political oriented) wish list in the run-up to the forthcoming European regulation(s). In addition, a centralized national waste data system evidently does not yet exist, clearly making the collection of reliable figures and information a challenging task, thus setting the solid waste management objectives, assessment and results -conveniently-into question.

Figure 1 Figure

 1 Figure 1 Streep map by Google displaying area covered by Nea Smirni

  Total unemployment percentage rate of active population in Attica Region 5. Percentage of people at risk of poverty or social exclusion Source: Eurostat, Hellenic Statistical Authority Table 2 Municipality of Nea Smirni solid waste generated amounts Years MSW (tn) Per Capita (kg/day) Separate Collection Rate (%) MSW rate of change Source: Municipality of Nea Smirni Regional Waste Management Plan (2015)

	1. Net national disposable income
	2. Total employment in number of persons
	3. Total unemployment percentage rate of active population
	4. 2011 32234,32	1.21		10,2
	2012 29917,26	1.12		9.2		-7.2%
	2013 26973,29	1.01		7.6		-9.8%
	2014 26976,32	1.01		7.6
	0,09
	2010	2011	2012	2013	2014	2015
		Piraeus	Vyronas		Nea Smirni

Table 3

 3 Municipaltiy of Nea Smirni generated solid waste qualitative compositionSource: Municipality of Nea Smirni Regional Waste ManagementPlan (2015), Municipality of Piraeus Regional Waste ManagementPlan (2015), Municipality of Vyronas Regional Waste ManagementPlan (2015)

			Nea Smirmi	Piraeus	Vyronas
			Generated	Composition	Composition	Composition	Quantity
			MSW	(%)	(%)	(%)	(tn/2014)
			(tn/2015)
	Organic		11870	44	44.3	43.6	8760.97
	Fraction
	Paper	&	39535	22	22.2	28.1	5646.40
	Cardboard
	Plastic		3777	12	13.9	13	2612.21
	Metal		1079	4	3.9	3.3	663.10
	Glass		1079	4	4.3	3.4	683.19
	Wood		1349	5	4.6	-	-
	Other (WEEE,	1888	7	6.8	8.6	1728.08
	Bateries, etc.)
	Total		26976	100	100	100	20093.96

Table 4

 4 Municipality of Nea Smirni Regional Waste Management Plan main objectives set for 2020 and 2025 compared with the National Objectives Regional (2020) National (2020) Regional (2025)

	Separate collection of waste	45%	50%	57%
	Separate collection of 4 recyclable materials	53%	65%	68%
	(glass, paper, metal, plastic)
	Separate collection of bio-waste	40%	40%	44%
	Source: Municipality of Nea Smirni Regional Waste Management Plan (2015)

Table 5

 5

		Municipality of Piraeus solid waste generated amounts
			2011	2012	2013	2014	2015
	MSW (tn)	74928.16 57654.311 71389.255 70559.88	-
	Recyclable & packaging materials (tn) 6672.79	4896.09	3737.31	3657	-
	Total (tn)	81600.95	62550.4	75126.57 74216.88 75757
	Source: Municipality of Piraeus Regional Waste Management Plan (2015)
	Table 6 Municipality of Vyronas solid waste generated amounts
		2010	2011	2012	2013	2014
	Total (tn)	2855.83 23509.57 22388.33 19662.77 20093.96
	Recycled
	Paper & Cardboard (tn) 465.58	314.55	189.75	153.82	205.46
	Plastic (tn)	126.58	91.12	117.15	106.18	113.38
	Metal (tn)	10.73	8.31	8.31	6.04	0
	Glass (tn)	64.52	57.10	29.26	25.16	3.2

Source: Municipality of Vyronas Regional Waste Management

Plan (2015)

Table 7

 7 The sustainability indicators developed and their function in the DPSIR cause framework

	Indicator	DPSIR
	Municipal Solid Waste Production (MSW-P) indicator	Driving Force
	Municipal Solid Waste Composition (MSW-C) indicator	State
	Municipal Solid Waste Recycling (MSW-R) indicator	Response

Table 8

 8 The ratio of the amount of MSW produced to the municipal population

	Year Piraeus Vyronas Nea Smirni
	2010	-	0.47	-
	2011	0.5	0.38	0.44
	2012	0.38	0.37	0.41
	2013	0.46	0.32	0.37
	2014	0.45	0.33	0.37
	2015	0.46	-	-
	Source: Hellenic Statistical Authority

Table 9

 9 The weight of the most known materials included in the reported amount of MSW

	Year	Piraeus		Vyronas	Nea Smirni
	2010	-		0.024	-
	2011	0.081		0.02	0.063
	2012	0.078		0.015	0.052
	2013	0.049		0.014	0.029
	2014	0.049		0.016	0.029
	2015	0.05		-	-
	Source: Hellenic Statistical Authority
	The most known materials included in the reported amount of MSW are;(1)Organic fraction, (2) Paper &
	Cardboard, (3) Plastic, (4) Metal, (5) Glass, (6) Wood
		Table 10 The recycled MSW compared with the total amount of MSW
		Year Piraeus Vyronas Nea Smirni
		2010	-	0.011	-
		2011 0.041	0.0077	0.028
		2012	0.03	0.0056	0.022
		2013 0.021	0.0048	0.11
		2014 0.022	0.0053	0.011
		2015 0.021	-	-

Source: Hellenic Statistical Authority