الزيوليت الطبيعى والصناعى والمنشط فى معالجة المياه
عدد المساهمات : 3533
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: الزيوليت الطبيعى والصناعى والمنشط فى معالجة المياه الجمعة مايو 03, 2013 8:22 am|| |
Natural Zeolites in Water Treatment – How Effective is Their Use
Natural zeolites are environmentally and economically acceptable hydrated aluminosilicate materials with exceptional ion-exchange and sorption properties.
Their effectiveness in different technological processes depends on their physical-chemical properties that are tightly connected to their geological deposits.
The unique tree-dimensional porous structure gives natural zeolites various application possibilities.
Because of the excess of the negative charge on the surface of zeolite, which results from isomorphic replacement of silicon by aluminum in the primary structural units, natural zeolites belong to the group of cationic exchangers.
Numerous studies so far have confirmed their excellent performance on the removal of metal cations from wastewaters.
However, zeolites can be chemically modified by inorganic salts or organic surfactants, which are adsorbed on the surface and lead to the generation of positively charged oxi-hydroxides or surfactant micelles, and which enables the zeolite to bind also anions, like arsenates or chromates, in stable or less stable complexes.
Natural zeolites have advantages over other cation exchange materials such as commonly used organic resins, because they are cheap, they exhibit excellent selectivity for different cations at low temperatures, which is accompanied with a release of non-toxic exchangeable cations (K+, Na+, Ca2+ and Mg2+) to the environment, they are compact in size and they allow simple and cheap maintenance in the full-scale applications.
The efficiency of water treatment by using natural and modified zeolites depends on the type and quantity of the used zeolite, the size distribution of zeolite particles, the initial concentration of contaminants (cation/anion), pH value of solution, ionic strength of solution, temperature, pressure, contact time of system zeolite/solution and the presence of other organic compounds and anions.
For water treatment with natural zeolites, standard procedures are used, usually a procedure in column or batch process.
Ion exchange and adsorption properties of natural zeolites in comparison with other chemical and biological processes have the advantage of removing impurities also at relatively low concentrations and allows conservation of water chemistry, if the treatment is carried out in the column process .
Subject of further academic and industrial research should be to improve the chemical and physical stability of modified zeolites and to explore their catalytic properties, which would allow their use in catalytic degradation of organic pollutants.
More careful consideration of their superb metal removal properties and awareness of possible regeneration or further use of contaminant/metal-loaded forms can considerably increase their environmental application possibilities, with a focus the reduction of high concentrations of cations and anions in drinking water and wastewater, for surface, underground and public municipal water treatment independently or in combination with others physical - chemical methods .
1.1. WATER TREATMENT USING NATURAL ZEOLITES
1.1.1. WASTEWATER TREATMENT
The use of natural zeolites in wastewatertreatment is one of the oldest and the most perspective areas of their application.
The presence of heavy metals (Zn, Cr, Pb, Cd, Cu, Mn, Fe, etc.) in wastewater is a serious environmental problem and their removal by natural zeolites have been extensively studied along with other technologies, including chemical precipitation, ion exchange, adsorption, membrane filtration, coagulation flocculation, flotation and electrochemical methods .
Recent investigations of natural zeolites as adsorbents in water and wastewater treatment, their properties and possiblemodification of natural zeolites have been a subject of many studies.
Various natural zeolites around the world have shown good ion-exchange capacities for cations, such as ammonium and heavy metal ions. Modification of natural zeolites can be performed by several methods, such as acid treatment, ion exchange, and surfactant functionalization.
The modified zeolites can show high adsorption capacity also for organic matter and anions .
1.1.2. SURFACE WATERS, GROUND AND UNDERGROUND WATER TREATMENT
The applicability of natural zeolites for the simultaneous removal of ammonia and humic acid, two of the most encountered current contaminants, from the surface waters was also investigated.
Their removal depends on pH value, initial concentrations of humic acid and ammonia, temperature and contact time.
The obtained results indicated that zeolite showed best performance for simultaneous removal of ammonia and humic acid at the pH close to that of natural waters .
The use of natural and modified zeolites has been further investigated for the simultaneous removal of Fe and Mn ions from underground water samples.
In particular, Fe and Mn removal levels are between 22-90% and 61-100% for natural zeolite - clinoptilolite .
The development of new and cost effective methods to remove As from ground waterand drinking water also becomes one of the research priorities.
The occurrence of arsenic in natural ground waters is due to geological composition of soil.
1.1.3. DRINKING AND GREY WATER TREATMENT
Several conventional methods are used for the removal of pollutants from drinking water, such as coagulation followed by filtration, membrane processes and ion exchange.
Adsorption methods proved to be effective, economically efficient, easy to perform and construct.
Some experiments were conducted to study the efficiency of natural zeolite clinoptilolite and of the clinoptilolite-Fe system in removal of Cu, Mn, Zn, which are simultaneously found in water samples.
A very unique property of natural zeolites is their selectivity towards cationic.
The excellent results of adsorption experiments, especially for the modified forms along with the fact that the clinoptilolite–Fe system is inexpensive, easily synthesized and regenerated, harmless for human beings, as well as for the environment, we can consider it as a very promising selective metal adsorbent .
Using iron/aluminum hydroxide to remove arsenic from water is a proven technology.
An alternative method to enhance the performance is to use coarse-grained sorbents to increase the flow rate and throughput of the process.
The removal of arsenic from drinking water was studied by using modified adsorbents (natural zeolite) prepared by the use of different iron solutions.
The arsenic sorption on the Fe-exchanged zeolite could reach up to 100 mg/kg [8,9].
The high surface area of modified natural zeolite (clinoptilolite)-iron oxide system in strongly basic conditions, can also enhance the removal of cations, like Cu from drinking water.
The specific surface area of modified clinoptilolite increased up to 5-times (from 30 to 151m2 /g) and the maximum amount of adsorbed Cu ions was 13.6 mg/g zeolite for natural clinoptilolite and 37.5 mg/g for modified clinoptilolite .
In spite of many scientific evidences of the effectiveness of zeolites in anion removal, not many of them are used on larger scales up to date.
High concentrations of fluoride ions in groundwater up to more than 30 mg/L, occur widely, notably in the United States of America, Africa and Asia. More than 260 million people worldwide consume drinking water with a fluoride content of >1.0 mg/L.
The available techniques for the removal of F- -anions from drinking water are membrane techniques, dialysis, electro-dialysis and finally adsorption techniques.
Clinoptilolite-type natural zeolite was pre-conditioned with nitric acid solution before loading with Al3+, La3+ or ZrO2+.
Aluminium-loaded low-silica zeolites as adsorbents for fluorides showed that modified zeolites were able to defluoridate water to below WHO’s maximum allowable concentration (MAC) of 1.5 mg/L.
The maximum fluoride adsorption was in the pH range of 4–8 .
High nitrateconcentrations in drinking water sources can lead to a potential risk to environment and public health.
Removal efficiency of NO3-ions can be increased by treatment of the clinoptilolite samples with HDTM+ (hexadecyltrimethylammoniumcation) or cetylpyridinium bromide (CPB) .
Grey water is wastewater originated from bathroom and laundry in households.
Ammonium is one of the most significant grey water contaminants.
Natural and modified zeolites are used for their purification and they shows good performance with up to 97% of ammonium removal depending on contact time, zeolite loading, initial ammonium concentration and pH value.
The desorption–regeneration studies demonstrated that the desorption of ammonium on the zeolite is sufficiently high [13,14].
عدد المساهمات : 3533
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: رد: الزيوليت الطبيعى والصناعى والمنشط فى معالجة المياه الجمعة مايو 03, 2013 8:34 am|| |
THE TECHNOLOGICAL APPLICATION OF NATURAL ZEOLITES IN WATER TREATMENT
Numerous and excellent research results in the last 10 years have shown that natural zeolites have practical use, which is confirmed by a large number of patents, especially for the two naturally occurring zeolite minerals: clinoptilolite and modernite
The number of patents is substantial for both zeolite types, which gives a clear notice that the interest of researcher in natural zeolites is strongly encouraged by the commercial sector covering the use in households or in industrial/large-scale processes and treatments.
Properties of natural zeolites
The structure of natural zeolite is very interesting and complex.
The primary building units (PBU) of zeolites are the SiO4 and AlO4tetrahedra.
They connect via oxygen ions into secondary building units (SBU), which are then linked into a three-dimensional crystalline structure of zeolite.
Substitution of Si by Al defines the negative charge of the zeolite framework, which is compensated by alkaline and earth alkaline metal cations.
Therefore natural zeolites appear as cation exchangers because they have negative charge on the surface.
In the zeolite lattice, substitution is not limited to Si-Al substitution.
Atoms of iron, boron, chromium, germanium, and titanium may also substitute silicon.
Water molecules can be present in voids of large cavities and bonded to framework ions and exchangeable ions via aqueous bridges.
One of the most investigated zeolite in basic and applied research is clinoptilolite.
The characteristic way of linking of PBUs and the formation of unique structural units ultimately results in the fact that these materials are highly porous with channels and cavities in the structure that have characteristic pore sizes and shapes.
In the structure of clinoptilolite, there are three types of channels, of which two are parallel, and made of ten and eight-membered rings of Si/AlO4, while one, defined by eight-membered rings, is vertical.
In these channels the hydrated cations can occupy the following places:
I - cation (Na- and Ca-ions) is located in the 10-member ring channels (free diameters 0.44 x 0.72 nm);
II - cation (Na- and Ca-ions) is located in the 8-member ring channels (free diameters 0.41 x 0.47 nm);
III - cation (K- ion) is located in the 8-member ring vertical channels (free diameters 0.40 x 0.55 nm);
IV - cation (Mg-ion) is located in the channel of 10-member rings and it is located in the center of the channel
Usually the number of water molecules in the zeolite structure does not exceed the number of oxygen atoms. Ratio (Si + Al): O is 1:2, and the number of aluminum atoms in the tetrahedrons is equal to the sum of the positive charges (x + 2y) of exchangeable cations.
Replacement of silicon and aluminum atoms in zeolites ranges from a minimum ratio of 1:5 (mordenit), up to a maximum 1:1 (erionit) .
CATEGORIZATION AND CHARACTERIZATION OF NATURAL ZEOLITES
Natural zeolites are divided into seven main groups according to their crystal structure, based on morphology, their physically properties, different ways of binding secondary units in the three-dimensional framework, the free pore volume and types of exchangeable cations in zeolite structure.
These diverse types of zeolite are a reflection of the fascinating structures of these microporous materials.
Each time a new zeolite framework structure is reported, it is examined by the Structure Commission of the International Zeolite Association (IZA-SC), and if it is found to be unique, it is assigned a 3-letter framework type code, like CLI, MOR, ANA etc.
This code is part of the official IUPAC nomenclature for microporous materials . Characterizations of natural zeolites include chemical and instrumental analyses of the samples and are crucial for their further application in water treatment.
The chemical composition, usually determined by several different methods:
classical chemical analysis – gravimetric method, Atomic Absorption Spectrometry or X-ray Fluorescence Spectrometry, is very important for the efficiency of the water treatment processes and provides insight into the main amount of basic oxide components (SiO2 and Al2O3), exchangeable cations (Na+, K+, Ca2+, Mg2+, Ba2+, Sr2+) and other elements present in smaller concentrations (like Ti atoms).
According to the proportion of exchangeable cations, we can sometimes already determine the type of zeolite.
The proportion of the oxide components in natural zeolite materials depends on the geological deposits.
Because of the complex mineralogical composition of natural zeolites and consequently uneven distribution of different phases and elements in the zeolite tuff, a combination of microscopic, spectroscopic, and other instrumental techniques must be used to fully characterize the material.
The basic information about the accessibility of the pores for different ions and molecules can be obtained from BET Analysis based on nitrogen physisorption, which gives information about the BET surface and accessibility of the pores for different ions and molecules.
The BET values usually range from 15 to 40 m2/g .
X-ray powder diffraction analysis allows quantitative determination of mineralogical composition of zeolitic tuffs, including the type of zeolite, and is crucial for any further application of natural zeolites.
The size and the morphology of the crystals in the zeolite samples is usually studied by scanning electron microscopy and accompanied Energy Dispersive X-ray Spectroscopy (EDXS) analysis system, that is attached to the scanning electron microscope.
The obvious advantage of EDXS elemental analysis over conventional chemical analysis is that we can obtain elemental composition of selected phase in the tuff, not only in the bulk sample.
An average elemental composition of the sample using EDXS is usually obtained by a data collection at three or more different mm2- sized windows on the sample surface.
SEM of zeolite tuffs from DonjeJesenje (a), VranjskaBanja (b), Brus (c) and Strezovce (d).
Magnification of app. 20.000x was used in the measurements.
Beside the typical plate-like morphology of clinoptilolite crystals, some fiber-like particles of mordenite could be also observed in all four samples.
The X-ray Photoelectron Spectroscopy (XPS or ESCA) analysis is used to determine the type and oxidation state of elements on the surface of natural zeolite by X-ray radiation from anode, e.g. Al anode, mostly by putting the sample in the form of 1 mm thick pressed pallet.
XPS depth profiling can be performed by alternating cycles of ion sputtering to remove surface layers of zeolite and acquisition of photoelectron spectra (ion sputtering can be performed with 1 keVAr+ beam rastering over 3 x 3 mm2 area).
In this way a depth distribution of elements can be obtained. The relative error for calculated concentrations of metals is estimated to be about 20%.
The X-ray absorption spectroscopy (XAS) is a particularly useful method for the characterization of metal-loaded zeolite samples.
Information on the oxidation state of metals, like copper, zinc, chromium, arsenic and many others, in natural zeolites can be obtained by XANES (X-ray Absorption Near Edge Structure).
The energy position of the metal absorption edge is shifted to higher energies with increasing oxidation state.
The relation can be calibrated with XANES spectra of reference compounds with the same type of metal ligands as in the investigated sample.
More information on the local environment of metal atoms in the zeolite samples can be obtained by EXAFS (Extended X-ray Absorption Fine Structure) analysis.
The EXAFS spectra can be quantitatively analyzed for the coordination number, distance, and Debye- Waller factor of the nearest coordination shells of the metal.
عدد المساهمات : 3533
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: رد: الزيوليت الطبيعى والصناعى والمنشط فى معالجة المياه الجمعة مايو 03, 2013 8:41 am|| |
Modifications of natural zeolite
Natural zeolite can be modified by single or combined treatment such as heating and chemical modification (acids, bases and inorganic salts).
Chemical and thermal treatment of zeolite may result in cation migration and thus affect the cation location and pore opening.
”Pore engineering” is a popular term for methods used in zeolite modification in which some of its sorbent properties are manipulated.
The processes of ion exchange and adsorption in zeolite/solution contact occur concurrently.
3.1. MODIFICATION WITH SOLUTION OF INORGANIC SALTS
Chemical modification with inorganic salts (NaCl, CaCl2, BaCl2, NH4Cl, FeCl3) or a cationic surfactant(hexsadecyltri-methylammonium (HDTMA) - bromide) give to improve zeolite properties and increase its efficiency in water treatment [19, 31-38].
Under normal conditions, large cavities and entries to the channels inside the zeolite framework are filled with water molecules forming hydration spheres around exchangeable cationic
After the contactof zeolite with an inorganic salt solution such as NaCl, exchange of cations (H+ or Na+) from solution with exchangeable cations (Na+, K+, Ca2+, Mg2+) from the zeolite framework occurs
To remove anions from the water, zeolite surface has to be modified with a solution of inorganic salts (for example FeCl3) whose adsorption on the zeolite surface leads to the formation of oxi-hydroxides, which then form stable complexes with anions in solution.
This modification can result in – to a smaller or greater extent – the creation of an adsorption layer on zeolite surface and modification of surface charge on zeolite surface (from negative to positive)