چكيده فارسي :
Research interests for the designing of alternative adsorbents to replace costly commercial grade activated carbon and synthetic ion exchange resins, which are derived from petroleum-based raw materials using the not environmentally friendly processing chemistry, have intensified in recent years. Attention has been focused on the various adsorbents rich on functional groups able to bind a broad range of pollutants from contaminated streams at low cost. Based on their economic feasibility and local availability, natural materials such as chitosan, zeolites, clay, certain industrial or agricultural waste products, i.e. fly ash, low rank coal, lignite, natural metal oxides, peat, sawdust, waste slurry, lignine, shungite, coconut shell charcoal, waste biomass, cork (Quercus suber L.) seem to be perspective and sustainable adsorption materials. Among the current commercial prices, natural zeolites are classified far as the most inexpensive alternative adsorbents in regard to other commercial-trade materials (e.g. the price of chitosan, a biodegradable linear polysaccharide obtained upon partial alkaline deacetylation of chitin, a structural element in the exoskeleton of crustaceans and insects, is on the market approximately 15-times higher).
Traditional zeolites are the only existing crystalline materials with a well defined pore structure in a microporous range. Their narrow pore size and tunable affinity to certain molecules make them ideal adsorbents for selective purification of gases in multicomponent mixtures or in encapsulation of hazardous compounds from other contaminated media. Traditional zeolite frameworks consist of AlO4 and SiO4 tetrahedra, set in a repeating pattern to form a microporous three-dimensional structure (micropores are defined by IUPAC as being less or equal to 2 nm in diameter). Oxygen atoms form the link between adjacent silicon and aluminium atoms. The final framework structure is therefore composed of SiO2 and AlO2- units and hence carries a net negative charge. This charge is balanced by cations that reside as guests within the pores alongside neutral species, such as solvents, that are also able to fit inside the cavities. The general formula for the composition of a zeolite is as follows:
Am+y/m [(SiO2)x . (AlO2-)y] . zG
where A is the cation (of charge m) and G is the neutral guest. The Si/Al ratio (x/y) must be greater than or equal to 1. It is possible for T-atoms other than silicon and aluminium to be incorporated in a zeolite-type framework (such as Ge, Ga, P and As) and these can significantly alter its properties. Such compounds have been referred to as zeotypes. Zeolites can differ both in terms of geometry (internal pore structure) and connectivity (bonding network). The difference between zeolite topologies and zeolite structures is an important feature. The topology of a zeolite describes the spatial connectivity of the nodes, although the exact chemical make-up of the compound describes the structure. Different structures can be of same topology. There are currently about 200 unique zeolite topologies recognized and according to Coombs et al. over 82 naturally occurring zeolite species are found. Approximately 15 novel structures are estimated to be synthesised annually, however only few of them are also decoded.
Simultaneously in recent years, there has been pronounced tendency to utilize mechanically stable synthetic or natural solid matrices in many applications, such as chemically bonded phase in chromatography, extraction of cations from aqueous media, ion preconcentration, electronics, ceramics, environmental control and also bioengineering. The solvent extractants or chelating groups used to be either impregnated into the pores of the porous solids or covalently bonded to the surfaces to improve their specific properties. Surface modification via functional group immobilization provides thus unique opportunity to engineer the interfacial properties of various solid matrices, while retaining their basic geometry and mechanical strength. By other words, such surface engineering used to be achieved either by physically adsorbing or chemically grafting functional groups onto a suitable matrix. In order to promote the removal performance of natural zeolites, i.e. native cation exchangers, towards the broadest range and versatility of environmental pollutants, their surface has been chemically modified, too (Fig.1).
Fig. 1: Adsorption isotherms for antimony onto some Slovakian and Hungarian (green curve) clinoptilolite tuff modifications (left) and cefazoline removal onto some natural and commercial products (right)
Nowadays, some update or other zeolitic applications related to water treatment and purification processes are available by hundreds. However, recent literature reports the state of the art mainly in zeolite surface modification using the hydrophobization (sol-gel technique for coating the zeolitic surface by different surfactants) or peletization of zeolite matrices with some biopolymeric eco-friendly carbohydrates. For water purification, beside the metal-containing nanoparticles, carbonaceous materials and dendrimers, the zeolites are being evaluated as the most progressive functional and nanosized (their pore size is in nanometer range) materials in the last few decades. Since that time, zeolite´s unique market position has been carried out by continual development of their ion exchange and adsorption properties and especially through their surface treatment using carbonization, hydrophobization with surfactants or pelletization with natural biopolymers. Currently, a development of new hybrid organic and inorganic materials is one of the principal direction of modern material sciences. Usually, the inorganic component of such hybrids provide mechanical, thermal or structural stability of the new product (adsorbent) and the organic component functionalizes the adsorbent´s surface. Such composed (hybridized) natural zeolites also potentially improve or broaden their adsorption performance towards the selected, even hydrophobic or anionic pollutants (thus upgraded to amphiphilic properties) and remain still cost-effective. As proven so far, surface charging or templating with carbonaceous substances including application of sol-gel methods had the major influence for broadening of zeolite adsorption properties. Recently, numerous approaches have been studied for the development of economically feasible and more effective environmental adsorbents containing natural biopolymers derived from chitin, chitosan, starch or cyclodextran. Moreover, natural fibres, biopolymers and biocomposites integrate the principles of sustainability, industrial ecology, green chemistry and engineering in the development of the new, challenging generation of materials, products and processes. Environmental requirements are becoming of great importance in today´s society, since there is an increased interest in the industrial use of renewable resources. Considerable efforts are now being made in the research and development of polysaccharide derivatives as the basic materials for new environmentally sound applications.
Biomimetics and bioinspiration used to be defined as the science of nature and living systems for the development of sustainable life, as one of the most revolutionary scientific field of the 21st century. With a better understanding of biological systems, the ability to observe nature has increased dramatically, and rational nature-driven solutions have transformed current technological problems in extended manner. The study of biomineralization offers valuable insights also into the scope of current material chemistry, specifically into the inorganic-organic (hybridized) adsorbent fabrication. Therefore, it is supposed that nature´s pattern will indicate also the synthesis development of functional gradient adsorbents in near future more intensely.
In contrast to synthetic methods, nature´s way of processing the materials, such as bone and shells, is not by traditional methods, but by growing hierarchically structured organic-inorganic composites in which soft materials organized in nanometric size are used as frameworks for the growth of specifically oriented and shaped inorganic crystals. By other words, inorganic phase formed by precipitation from aqueous solution, is directed into domains defined by the soft tissue matrix. Such biogenic hierarchical composites posses complex structural designs, while displaying unique properties. Thereby, soft matter found in biogenic-inorganic materials enables the processing of products using water-based processes, and thus eliminating concerns about environmental pollution associated with the use of petrochemical derivatives.
Another unique example from the nature, the S-layer, is the outermost layer of the cell wall and is thought to serve various functions in different bacteria. It serves as a filtering device, blocking the passage of large molecules and allowing small metabolites to be excreted out of the cell. Another possible function is as mechanical barrier against osmotic shock. The S-layer is approximately 10 nm thick, its three-dimensional structure is porous, with the protein occupying approx. 30% of the volume. Protein molecules in periodically arranged structure of such layer create regularly holes which may better match as metal nucleation sites also during the common adsorption process. Indeed, such S-layer, which is directly exposed to the external environment exhibits long-term stability and resistance to acids, high temperature and degradation, all of which usually destroy the integrity of conventional membranes. Thus, such onto external surface immobilized biomembrane in new adsorbent processing may act as reinforcing component.
Another interesting remarks of living cells base on excretion of biogenic surfactants or specific biopolymeric acids like alginic acid and their salt alginate. Alginate is a copolymer of the isomers mannuronic and guluronic acids enabling the dense packing of submicrometer-sized particles in suspensions in order to enhance their colloidal stability. To promote the native zeolite adsorption performance and prepare more efficient amphoteric product for specific water decontamination, flexible component, i.e. alginate biopolymer with a rigid component (powderized) zeolite was crosslinked using Fe(III) and Ca(II) salts, as some of bellow papers report. While Fe(III) cationic sites were responsible for electrostatic forces between oxyanionic pollutants and pelletized biosorbent in the process studied, Ca ions were responsible for exchange of metal cations from multi ionic solutions. Thus, such biopolymeric alginate indeed enhanced the uptake performance of the adsorbent prepared and stabilized its overall technological properties. Using the hydrophilic, expandable and permeable hydrogels with low interfacial tension for the novel hybridized adsorbent synthesis, substances which resemble to soft living tissues, is another excellent example for advanced surface treatment strategy to broaden the native zeolite properties for a wide variety of applications. Various aspects regarding above mentioned octadecylammonium (ODA) zeolitic adsorbents have been reviewed and highlighted in papers introduced under Literature.
Metal oxides based adsorbents are effective, low cost adsorption materials for heavy metals and other pollutants together with pathogen detection. Their sorption process is mainly controlled by complexation. When their particle size is reduced to below 20 nm, the specific surface area of normalized adsorption capacity increases 10– 100 times, suggesting a „nanoscale effect“. They may be combined with other carriers or pelletized or enriched with a broaden range of functional groups and thus separated magnetically. Current immobilization techniques usually result in significant loss of treatment efficiency. Therefore, research is needed to develop simple, low-cost methods to immobilize nanomaterial without significantly impacting its performance. Nevertheless, to overcome a potential human risk from environmental spreading, nanomaterials need to be embedded in a solid matrix, respectively, to have minimum release until they are disposed of. We used zeolite (clinoptilolite tuff) from the inland deposit Nižný Hrabovec as template in more effective biomimetic sol-gel synthesis and thus prepared another new, economically feasible adsorbent FeO(OH)-zeolite, respectivelly (Fig.1).
Although numerous adsorbents have been developed and examined in water treatment, their potential needs to be further assessed on pilot scale with real surface/ground water or/and wastewater. Development of some synthetic, hybrid and nano-scale adsorbents show high efficiency towards specific pollutants removal, but more research is needed prior to their use in full-scale application in water and wastewater treatment.
