PROTECTION AND RESTORATION OF THE ENVIRONMENT VII – MYKONOS2004

SORBENT MATERIALS FOR METAL IONS REMOVAL FROM AQUEOUS SOLUTIONS: A REVIEW D.D. Asouhidou, N.K. Lazaridis, K.A. Matis Section of Chemical Technology & Industrial Chemistry, School of Chemistry, Aristotle University, Thessaloniki, GR-54124, Greece E-mail: [email protected]

ABSTRACT This mini-review article provides a comprehensive source of knowledge for sorbent materials, except natural occurring sorbent materials. It presents their sorption properties as well as their potential applications for metal ions removal from aqueous solutions. Emphasis is given to the new class of sorbents as chelating resins and mesoporous materials.

ΡΟΦΗΤΙΚΑ ΥΛΙΚΑ ΓΙΑ ΤΗΝ ΑΠΟΜΑΚΡΥΝΣΗ ΙΟΝΤΩΝ ΜΕΤΑΛΛΩΝ ΑΠΟ Υ∆ΑΤΙΚΑ ∆ΙΑΛΥΜΑΤΑ: ΑΝΑΣΚΟΠΗΣΗ ∆.∆. Ασουχίδου, Ν.K. Λαζαρίδης, Κ.Α. Μάτης Εργαστήριο Γενικής και Ανόργανης Χηµικής Τεχνολογίας, Τµήµα Χηµείας, Θεσσαλονίκη, GR54124, Eλλάδα

ΠΕΡΙΛΗΨΗ Το άρθρο αυτό ανασκόπησης, αποτελεί µια απλή και συγκριτική πηγή των εµπορικών και νέων ροφητικών υλικών, εκτός από τα φυσικά απαντώµενα υλικά. Παρουσιάζει τις ροφητικές τους ικανότητες και τις εφαρµογές τους για την αποµάκρυνση ιόντων από υδατικά διαλύµατα. Έµφαση δίνεται στις χηλικές ρητίνες και στα µεσοπορώδη υλικά.

PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

1. INTRODUCTION The use of solids for removing substances from either gaseous or liquid solutions has been widely used since biblical times. This process known as of sorption involves nothing more than the preferential portioning of substances from gaseous or liquid phase onto the surface of the solid substrate. Sorption phenomena are operative in most natural physical, biological and chemical system and sorption operations employing solids such as activated carbon and synthetic resins are used widely in industrial applications and for purification\s of waters and wastewaters. Because this process takes place into the sorbent and involves circular processes the sorption constitutes an important method of separation for the industry. The sorption is usually applied in columns packed with sorbent particles or fixed-beds. The high separation power of chromatography that is achieved in a column is the main advantage of adsorption against the other methods of separation. The high separating power is caused by continuous contact and equilibration between the fluid and sorbent phases. Under conditions free of diffusion limitations each contact is equivalent to an equilibrium stage or theoretical plate. Usually several hundred to several thousand such equilibrium stages can be achieved within a short column. For this reason sorption is ideally suited for purification applications as well as difficult separations. [1,2] 2. SORBENT MATERIALS Several researchers have tried in the past to classify the sorbent materials [3,4]. In this work we propose the following classification. Carbon sorbents (Class Ι) Active carbons Activated carbon fibers Molecular carbon sieves Fullerenes Heterofullerenes Nanomaterials

SORBENT MATERIALS Mineral sorbents (Class ΙΙ) Silica gel Activated alumina Oxides of metals Hydroxides of metals Zeolites Clay minerals Inorganic nanomaterials

Other sorbents (Class III) Synthetic polymers Composite sorbents Mixed sorbents

2.1 Class I 2.1.1 Activated Carbon It is the most widespread sorbent which is widely used for potable water and wastewater treatments. A large compilation of equilibrium sorption data is available for organic compounds in dilute aqueous solutions such as phenolic compounds, aromatic and chlorinated aromatic compounds, chloroethylenes and other volatile organic compounds, carbon tetrachloride and organic pesticides. It is also a good source for sorption of inorganic compounds such as those of As, Ba, Cd, Cr, Cu, Pb, Se, Hg, F and Cl [5,6]. Its usefulness derives mainly from its large micropore and mesopore volumes and the resulting high surface area (300 – 4000m2/g). The unique attributes of surface of activated carbon contrary to other major sorbents are focused on the fact that its surface is nonpolar or slightly polar as a result of the surface oxide groups and inorganic impurities. This unique property gives activated carbon the following advantages: It is the only commercial sorbent used to perform separation and purification processes without required prior stringent moisture removal. For this reason it is used widely as sorbent for processes treating aqueous solutions.

PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

Because of its large accessible internal surface it sorbs more nonpolar and weakly polar organic molecules than other sorbents do. For example the amount of methane sorbed by activated carbon in P=1atm and room temperature is approximately twice that sorbed by an equal weight of molecular sieve 5A. The heat of sorption or bond strength is generally lower on activated carbon than on other sorbents. This is because only non-specific Van der Waals forces are available as the main forces for sorption. Consequently, stripping of the sorbed molecules is relatively easier and results in relatively lower energy requirement for regeneration of the sorbent. 2.1.2 Activated Carbon Fibers (ACF) Is the most universal sorbent for volatile organic compounds control, which is produced, commercial from 1960 [7]. The precursors for carbon fibers were polymeric fibers, cellulose (and rayon derived from cellulose) and pitches. The carbon fibers have high tensile strength and high elasticity and are considerably more graphitic than activated carbon because a mesophase is usually formed during the carbonization process of the fibers. Gas activation (usually with CO2) of these carbon fibers results in activated carbon fibers. They have high surface areas, ranging from 1000 - > 2000 m2/gr and the following unique properties: • Narrow and uniform pore size distribution, with result stronger interactions with sorbates. • Small and uniform diameter of fiber, with result faster uptake and desorption. • Graphitic, with result more conductive and more heat resistant properties. • High strength and elasticity (allowing greater flexibility in the shapes and forms of the sorbent). 2.2 Class II 2.2.1 Silica gel is the most widely used desiccant because of its large capacity for water (~40% by weight) and easy in regenaration (~1500C compared with 3500C for regenerating zeolites). Silica gel was successfully applied in the removal of heavy metals in the rinsing wastewater from plating factory. Silica gel has much higher capacity for uranium (VI) than for the heavy metal ions Pb2+, Cu2+, Ni2+, Zn2+, Cd2+ of which the uptake of Pb2+ was the highest [8]. The commercial silica gel sorbents are strong mesoporous, with pores mostly larger than 2nm, bearing an abundant in surface hydroxyl groups. Two common types of silica gel are known as regular-density and low-density silica gels, although they have the same densities. The regular-density gel has a surface area of 750850 m2/gr and an average pore diameter of 2.2-2.6 nm whereas the respective values for the lowdensity gel are 300-350 m2/gr and 10-15 nm. The silica gel is amorphous. By use of high-resolution electron microscopy, it is known that its amorphous framework is composed of small globular particles having sizes of 1-2 nm. [3] 2.2.2 Zeolites They are crystalline aluminosilicates of alkali or alkali earth elements, such as sodium, potassium, calcium, and are depicted as follows: Mx/n[(AlO2)x(SiO2)y].zH2O where x and y are integers with y/x equal to or greater than 1, n is the valance of cation M and z is the number of water molecules in each unit cell. The primary structural units of zeolites are tetrahedral of silicon and aluminum, SiO4 and AlO4. These units are assembled into secondary polyhedral building units such as cubes, hexagonal prisms, octahedral, and truncated octahedral. At least 40 different types of naturally occurring zeolites have been found, beginning with the discovery of stilbite by the Swedish mineralogist Cronstedt in 1756. The principle natural zeolites are chabazite, gmelinite, mordenite, heulandite, clinoptilolite, levynite and faujasite. More than 150 types of zeolites have been synthesized and designated by a letter or groups of letters, as adopted by the International Zeolite Association (type A, type X, type Y, type ZSM, etc.). Synthetic zeolites that were prepared in 1959 are the newest types of sorbents, than all mentioned before.

PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

Types A, X, Y remain the dominant zeolites and moleculer sieves that are in commercial use for sorption. They used mainly for their sorptive properties, which is due to their pore crystalline structure, for the removal of NH4+ from municipal and industrial wastes, for the removal of heavy metals such as Cd from industrial wastes and for the removal of radio-isotope of Cs and Sr from radioactive wastes [9]. Due to the presence of aluminium in their structure, many zeolites have strong acid sites at their surface, making them superior cracking catalysts. They are used as sorbents for the selective separation of gaseous mixtures or for the condensation of harmful compounds. Zeolites are characterized by high selectivity, which is supposed to be due, among others, to steric effects (steric separation mechanism). Sorption processes based on molecular sieving and selectivity is always reversible. This allows the zeolite to be reused many times, cycling between sorption and desorption step of actual processes. This accounts for the considerable economic value of zeolites in sorption applications. 2.2.3 Metal Oxides Iron-based adsorption media include granular ferric hydroxide, ferric hydroxidecoated newspaper pulp, iron oxide-coated sand and iron filings mixed with sand. These media primarily have been used to remove arsenic from drinking water. Processes that use these media typically remove arsenic using adsorption in combination with oxidation, precipitation/co precipitation, ion exchange or filtration. The media require periodic regeneration or disposal and replacement with new media. The regeneration process is similar to that used for Activated Alumina and consists of rinsing the media with a regenerating solution containing excess sodium hydroxide, flushing with water and neutralizing with a strong acid [10]. Quartz is very poor at removing arsenic under most environmental conditions, because the mineral surface is negatively charged above a pH of 2. However, quartz sand, or indeed any other granular media, can be made highly sorptive by coating the grains with metal oxides. In recent years many researchers have used this principle to develop low-cost arsenic removal methods using locally available materials. A similar coated sand material can be prepared using manganese dioxide instead of iron. Since MnO2 is a good oxidant, this material can remove arsenite as well as arsenate. [11] 2.2.4 Alumina The commercial production of activated alumina is performed exclusively by thermal dehydration or activation of Al(OH)3 or gibbsite. The oldest form, which is still widely used, is made from Bayer α-trihydrate, which is a by-product of the Bayer process for aqueous caustic extraction of alumina from bauxite. Depending on the thermal treatment is produced crystalline γ/η alumina with a surface area of ~250m2/gr or amorphous alumina with surface area of 300-350m2/gr. Among the aluminas the γ- alumina is the most commonly used form for both sorption and catalysis [12, 13]. The pore structure of the activated alumina depends strongly from the conditions of the heat treatment. The desiccation remains to be a major application for activated alumina. The pore structure can be modified by acid, base or controlled thermal treatment, in order to be more easily applicable in relation with silica gel. The activated alumina is a versatile sorbent that can be tailored for many special applications as [3]: 1. removal of HCl and HF from gases and liquids 2. removal of acidic gases (COS, CO2, H2S, CS2) from hydrocarbons 3. removal of oxygenates and Lewis Bases 4. removal of polar organic compounds 5. removal of As5+, PO43-, Cl-,F- from water 6. scavenger for organic process liquids 7. alkalized alumina for SO2 removal 2.2.5 Pillared Clays Are classes of porous, high surface area, two-dimensional materials that have been studied extensively for application as catalysts and to a much lesser degree as sorbents for gas separation. There have been only a few studies on the sorption properties of pillared clays since PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

1991 [14, 15]. The ammonia treatment can be accomplished by pillared interlayered clays. Like zeolites, also show selectivity for N2 over O2. However, their capacities were substantially lower than that of zeolites [3]. Pillared clays development started in the mid 1950`s by Barrer and coworkers. They synthesized high surface area materials by pillaring montmorillonite clay with cations. However, such materials have low thermal and hydrothermal stabilities and therefore have limited use as sorbents and catalysts. Much interest and research have been directed toward the synthesis of pillared clays with high thermal and hydrothermal stabilities. The most promising ones for use as sorbents and catalysts are as follows: Al-PILC, Zr-PILC, Cr-PILC, Fe-PILC, Ti-PILC. The choices for hydroxy cations are not limited to those mentioned. In fact, any metal oxide or salt that forms polynuclear species upon hydrolysis can be inserted as pillars. An important current trend in new materials developments is the modification of classical supports to produce new, more versatile, low-priced materials with well-defined characteristics Among the most used is silica gel, a material of well-established particle sizes and well-defined porosity, high surface area and high mechanical, chemical and thermal stability as well as low tendency to swell in solvents [15]. Because of silicas properties, its surface can be readily modified by reacting or grafting with a monomolecular layer of organic ligand, and these modified silica gels are being applied in an increasing number of applications in chromatography. The grafted silicas are also promising sorbents for selective sorption [3]. The sorption properties of these functionalized silicas have given rise to great interest for analytical purposes since several evaluations have demonstrated that most of these materials show strong sorption capacities and mainly selective characteristics, important for speciation. However, more emphasis has been given to silica modified with organic groups that are able to provide a great number of different materials to sorb metal ions from aqueous and nonaqueous solvents [17-24]. For inorganic functionalized silica, few studies of metal ion sorption properties have been described in the literature [16]. 2.3 Class III 2.3.1Polymeric resins, a broad range of synthetic non-ionic polymers, are available for use as sorbents and chromatographic column packing. The technology of designing and building porosity into polymers was accomplished in the late 1950`s and early 1960`s. Building porosity can be accomplished by emulsion polymerization of the monomers in the presence of a solvent which dissolves the monomers but which is a poor swelling agent for the polymer. Although macroreticular polymers of acrylates and methacrylates are available, most commercial macroreticular polymers are available, most commercial macroreticular polymers are based on styrene cross-linked by divinylbenzene (DVB). Polymeric resins have been widely used for water treatment. In the past two decades, resin sorption technology has been eventually used for removing aromatic pollutants such as phenols, humic acids from organic wastewaters and polymeric sorbents have been considered as a practical alternative for treatment and resourse reuse of wastewaters in chemical industry [25]. The main advantage of the polymeric resins lies in its ease of regenaration. An additional advantage is that the resins can be tailored for special applications such as that in pharmaceutical and semiconductor industries. The polymer resins are in the form of spherical beads, usually in the size range of 20-60 mesh. Each spherical bead consists of an agglomeration of a large number of very small microspheres. These microspheres are clusters of micro-gel particles ranging in size between 0.01-15µm. Thus, the pore structure is comprised of inter-microsphere mesopores and the micropores within the microspheres. The latter depends directly on the degree of cross-linking, i.e., the amount of DVB. Cross-linking provides the high surface area as well as the rigidity and mechanical strength. The aromatic surfaces of the resins make them excellent sorbents for removal of organic compounds from aqueous solution, particularly those with low solubilities. The macroporous polymeric resins can be further reacted to attach functional groups to the benzene rings to generate functionalities for ion exchange. The resulting polymers are ion exchange resins. PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

For example, polystyrene can be sulfonated by sulfuric acid resulting in an -SO3-H+ group attached to the benzene ring, and the proton can be easily exchanged with other cations. Likewise, attaching ammonium or amine groups results in anion exchange resins. [3] More recently, carbonaceous polymeric sorbents have been developed by partially pyrolyzing the styrene/DVB polymers and their sulfonated forms. These sorbents are particularly of interest for water purification, because they have shown 5 to 10 times the capacity of granulated activated carbon for low concentrations of volatile organic compounds. [3] The major use for both polymeric sorbents and ion exchange resins involves water treatment (75% of the resins were used for this purpose since 1987). Commercial applications include removal of halogenated organic compounds, phenols and pesticides from water, decolorization of effluents and dye wastes, removal of VOCs from air and bioseparations. Ion-exchange resins are used mainly for industrial and domestic water softening and deionization. As sorbents, they are also used for demineralization, dealkalization, desilicazation and sorption of ionic constituents from dilute solutions. [3] A chelating resin is a polymeric solid substance with organic mainframe containing active functional groups, capable of interacting with metal ions forming coordinate bonds. The sorption of metal ions on chelating resins is mainly due to complex formation within the resin, which distinguishes it from conventional ion exchange. The versatility of these polymers is attributed to the triple function of ion exchange, chelate formation and physical adsorption. A chelating sorbent essentially consists of two components: the chelate forming functional group and the polymeric matrix or support. The properties of both components determine the features and the applications of the respective material. The selectivity of the sorbents is determined mainly by the chelating group (the nature of the functional group). The analytical properties of the sorbents (capacity, kinetic features, mechanical and chemical strength and regeneration depend on the polymeric substrate. Chelating resins compared to other separation methods (solvent extraction, co-precipitation, ion exchange resins) exhibit good selectivity, high enrichment factor, better chemical and mechanical stability, efficient clean-up sorbents, greater versatility to chemical modifications. They are used widely in industry for metal ion removal [26-36]. Incorporation of complexing (more often chelating) functional groups into polymeric support can be accomplished by three methods: 1. by formation of a chemical covalent bond between the organic reagent and support (chelating sorbents with grafted groups). It occurs by direct polycondensation or polymerization of monomers containing chelating groups, or by chemical transformation of a preformed polymer. Although the polycondensation method is extensively used because of its simplicity, condensation polymers have a poor chemical and mechanical stability. The main synthesis method of chelating sorbents proves to be the incorporation of an active functional group into polymeric matrix by reactions on polymers (the grafting procedure). In this way have been prepared starting from linear low molecular polystyrene, chelating sorbents with high capacity and selective retention properties. Sorbents with a three dimensional structure have a better stability in aggressive medium and can be easily regenerated and are thus used for sorption under dynamic conditions. The sorbents with macroporous structure have a superior capacity and kinetic properties. Among the naturally organic matrices, cellulose is the most extensively used support for grafting of suitable functional groups because of its typical properties: availability and low price, mechanical strength, high porosity, hydrophilicity and chemical reactivity in functionalizations. Chelating sorbents with functional groups immobilized by covalent bonds on inorganic polymers, usually silica, have been synthesized by chemical PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

transformation of the matrix. Although inorganic supports have high mechanical strength, thermal and chemical stability, chelating sorbents based on inorganic matrix have poor degree of functionalization and relatively low sorptive capacity. 2. by formation of an ionic bond between the chelating reagent and the functional group of an ion exchange resin(modified resins). The chelating reagents must be organic molecules with strongly dissociated anionic or cationic group. The disadvantages of chelating sorbents with grafted functional groups determined by synthesis difficulty, such as: low reversibility of sorption-desorption processes and sometimes unsatisfactory kinetic features can be eliminated by sorption of organic reagents on ion exchangers. In this case, a conventional ionic resin that keeps its properties represents the polymer matrix. 3. by impregnation and physical sorption of the organic reagent on support(impregnated sorbents). A simple and rapid technique of chelating sorbent preparation is based on the mechanical impregnation of the inert matrix with complexing reagents. For this reason, the support is treated with the solution of the complexing reagent in an organic solvent, which is then removed by filtration or evaporation [28]. A new family of mesoporous materials the so-called MCM materials (Mobil Composition of Matter) were developed in 1992 by Beck et al. by Mobil Oil Corporation. Τhese materials have very high surface area (>1000m2/gr), ordered pore structure (mostly hexagonal packed cylindrical pore channels),and extremely narrow pore size distribution. The pore diameter can be adjusted from 2-15 nm. For these reasons, MCM materials have applications in sorption and catalysis. The preparation methods involve mixing ceramic precursors (such as silica) in a surfactant solution and reacting the agents at temperatures below 1500C. An organic surfactant in an aqueous medium forms rod-like micelles which are used as templates to form two or three monolayers silica or alumina particles encapsulating the micelles external surface. By removing the organic species from well-ordered organic-inorganic condensed phase (by calcination at high temperature e.g.4500C), a porous silicate or alumina material with uniformly porous structure remains. Several structures can be prepared in this way, but two types of them have gained widespread interest: MCM-41 with hexagonal array of uniform cylindrical channels with pore size 2-10nm and large pore volume >0.7cm3/g and the cubic MCM-48 species which has a random pore structure. In order to obtain useful sorption properties, MCM-41 needs to be modified in either surface chemistry or pore structure. The reported modifications have been based mainly on the use of reactive silanes that contain organic groups, (such as alkyls) and chloride. The reported applications of MCM-41 material are: i) VOC removal, ii) SO2 removal, iii) heavy metal removal such as Hg, Fe, Pb from wastewaters, by grafted the MCM-41 with silane containing thiol (-SH) group. The sorbent is also regenerable with HCl. Because of the large arrays of functionalities and pore sizes that can be achieved with the MCM-41 material, unique sorption properties will no doubt be obtained. However, due to the cost, its potential use appears to be limited to specialized applications [3,4,37-46].

PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

Figure 1. a) hexagonal MCM-41 cubic MCM-48

b) Figure 2. Figural preparation of mesoporous material

The synthesis of mesoporous silica has greatly expanded the possibilities for the design of open pore structures. Because of their large surface area and well-defined pore size and pore shape, these materials have great potential in environmental and industrial processes. However, many applications (such as sorption, catalysis and sensing) require the materials to have specific attributes such as binding sites, stereochemical configuration, charge density and acidity [47]. The grafting of organic units onto the walls of the mesoporous molecular sieve MCM-41 has been reported resulting in a lining of the pore with chemical functionalities while retaining the mesoporosity of the material [47-55]. Recently have been reported the preparation of highly effective heavy metal ion adsorbents by grafting thiol moieties to the pore channel walls of mesoporous silica molecular sieves [52]. The milestone achievement in the preparation of novel porous inorganic materials like glasses and ceramics is presented by sol-gel technology. In this new technique, the high temperature heating procedures were placed by polymerization and condensation of inorganic and organometallic compounds at room temperature. At present, almost all of the important inorganic oxides, optical fiber glasses, ceramic membranes, can be prepared via sol-gel processes. In adsorption processes a significant role is also played by microporous glasses sorptive properties of which are similar to those of silica gels and zeolites, however, the area of their application is wider. Microporous glasses may be used as semi-permeable membranes for the separation of liquid and gaseous mixtures, as well as gel filling in chromatography. Because of their high silica content, they may find application in chemical, metallurgical, electrotechnical and other industries. On account of their sorptive properties, microporous glasses represent an excellent material for storing highenergy radioactive waste products in nuclear power engineering and for bounding toxins in natural environmental [4,55-58]. As we can see from the following table, the functionalized silica materials have been shown to have uptake capacity (qmax) higher than those other materials found in the literature.

PROTECTION AND RESTORATION OF THE ENVIRONMENT VII HAZARDOUS WASTE MANAGEMENT

TABLE 1: Comparison of sorption capacity(qmax)of adsorbents for metal ion removal Adsorbent Organically Modified (a) Silica Gel Functionalized (b) Silica (MCM) Functionalized (c) Silica (LCT mechanism:MCM) Functionalized (b) Silica (LCT mechanism:MCM) Functionalized (b) Silica (Sol-gel:Organo-ceramic) Activated Carbon Clinoptilolite Zeolite

qmax mmole Hg/g 0.15

Ref

1.5

51,52

5.0

53

2.5 6.4 0.94 0.75

17

Adsorbent Organically Modified (a)Silica Gel Silica gel

qmax mmole Cd/g 0.15

Ref

0.02

8

Functionalized (b) Silica 1.9 (Sol-gel:Organo-ceramic) 47 Functionalized (d) Silica 0.17 (covalent attachment) 57 Functionalized (e) Silica 0.32 (LCT mechanism:MCM) 6 Activated Carbon 1.37 9 Clinoptilolite Zeolite 0.75 Chelating Resin(f)