ABSTRACT bound up in the clay through the

ABSTRACT

 

To
produce a new modified clay as a sorbent for removing heavy metals and other
toxicants from low quality water, two Egyptian natural clay sediments dominated
mainly by kaolinite were selected. Unusual
treatments depend on thermal transformation and acid activation to increase the
exchangeability properties of kaolinite were
used. This was proved by different methods, among them XRD
analysis, I.R spectroscopy Scanning Electron Micorscope and C.E.C measurements.
The explanation was focused on release of some octahedral Al-ions without
disturbing the structure itself. This helps in sucking the pollutants out of
low quality water.

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Key words: Heavy metals removal, I.R ,Modified kaolinite,S.E.M,
Wastewater treatment  .

Introduction

 As the world
population continues to increase accompanied by growth in food needs,
urbanization and industrialization; limited available  easily and clean surface and ground water
resources are being not only limited but also exhausted. Currently, there is a
general agreement by the global community on the seven challenges facing World
Water Resources and their management. The third one is deterioration of water
quality,(1).

       In the field of wastewater treatment,
many techniques are used to remove the contaminants and eliminate or reduce the
hazardous nature of the effluent and prepare the water for release into the
environment. Most of these techniques are multi-step and require time as well as
extensive handling to accomplish the task of removing solids, oils and metal
ions from the water.

     
Many materials used to adsorb wastewater contaminants ions; among them,
clays which have remarkable affinity for metals particularly heavy ones. These metals
become bound up in the clay through the process of ion exchange which is driven
by electrostatic attractive forces between the metal ion in solution and the
anionic surfaces on the clay particles.

      
Although kaolinite exhibits the least exchangeability among clay
minerals, several studies have confirmed the potential of natural kaolinite for
the adsorption of metal ions from solution (2) 
and  (3).

A modified kaolinite amorphous derivative (metakaolinite Al2(OH)4Si2O5
) which produced from calcining  the
kaolinite at 600oC, (4) and (5), absorbs heavy metals and could be used in the
treatment of toxic metal pollution in water. Definitely, (2) and (6) reported
that the porous of collapsed crystalline structure of this modified kaolinite
can adsorb a large amount of metallic ions such as Pb, Cd and Cu from
wastewater.

      The present work reports the
development of some local clay sediments in Egypt to produce modified clay
mineral by thermal and chemical treatments which could improve the removeability  of Cu and Zn from industrial wastewater.

2-Materials
and Methods

2-1 Sampling and preparation

2-1-1 Clay sediments

Two kaolinitic clay samples were
collected from both Aswan
, South of western desert  and El Tih
plateau , Sinai, Eastern desert, Egypt.

The clay was washed with distilled
water to remove soluble impurities and wet sieved through 350 mesh sieve ( 45 ?m)
, homogenized and <45  ?m fraction was collected. The clay then was reacted with 0.5 N Ca(NO3)2 and rinsed with distilled water to eliminate excess salts, ( 12).     Each clay sample was divided into two portions, the former was retained as a raw sample and the later portion was modified as follows : Heating the clay sample to 600 oC for 2 hrs. before subjecting to acid treatment by refluxing with 2M HCl at 80 oC for 20 min. The sample was washed until free of chloride ions using double distilled water. Thus generating a set of 4 samples was established (two raw and two modified) as follows: Aswan kaolinite raw (Ar)                        Aswan modified kaolinite (Am) Sinai kaolinite raw (Sr)                             Sinai modified kaolinite (Sm) 2-1-2 wastewater Industrial non-treated wastewater was collected from Helwan Iron and Steel factory, Cairo, Egypt. Wastewater samples were  prepared according to (7) for both chemical analysis and heavy metals determination.  2-2 Methods of analyses:      2-2-1 physical and chemical analyses: According to (8) the different analyses were carried out as follows: for clay sediments, particle size analyses (pipette method), calcium carbonate (volumetric calcimeter method), organic carbon (walkely and Black method), cation exchange capacity( sodium saturation method).  Heavy metals were determined in water by  ICP-OES. While pH and EC were determined in both water and clay sediments by glass electrode and electrical conductivity, respectively. 2-2-2 Mineralogical analyses: 2-2-1 XRD analysis A thin slurry of the sample solution was aliquot into a glass slide, air dried at room temperature and subjected for X- ray diffraction analysis. XRD analysis was done using Ni filter and Cu-Ka? radiation. Constant criteria for all samples x-rayed used were as follows: KV=40, MA=20, Gain=60, Range=1000, time constant during scans were 2, chart drive 2 cm/min. -– S.E.M-– S.E.M 2-2-2 I.R spectroscopy Infra red absorption spectra in region of 400 cm-1 to 4000 cm-1  were recorded  using KBr sample pellets. The samples prepared as pellet method according to (9). 2-2-3 Scanning electron microscope (S.E.M)  Scanning electron microscope was used in scanning Ar and Am clays after samples preparation according to (10). 2-3 Procedure for Heavy metals uptake: Fifty milliliters of Industrial wastewater was equilibrated with 0.5 gm clay sample for 5,30,60,90,120,180 and 240 min. intervals. Initial pH was adjusted at 5.5 before starting the experiments. After centrifugation, the solution was analyzed for Cu and Zn using ICP-OES. Removed amounts by clay mineral were calculated by the difference between initial concentration of the heavy metals and their concentration at equilibrium.  3. Results and Discussion 3.1 Chemical characterization of used wastewater: Used wastewater is alkaline, saline and sodium is the dominant cation while chloride is the dominant anion as shown in table (1).  Cu, Cd and Zn are the most contaminants pollutants in water. 3.2 Characterization of clay sediments: 3.2.1 Physical and chemical characteristics Some physical and chemical characteristics of the sediments are presented in table (2). Both locations are non-saline, non-calcareous, moderate alkaline and almost weak in organic carbon. Values of Cation Exchange Capacity (C.E.C) are related to clay content. Clayey textural classes are shown in both sources. 3.2.2 Mineralogical characterization of raw and modified clays 3.2.2.1 XRD analysis X- ray diffraction was done to identify the mineralogical composition of the used sediments as well as to follow the different changes after the modification treatments take place. Obtained diffraction patterns was illustrated in fig.(1). The results revealed that the kaolinite is the predominant mineral in both used sediments as indicated by the diffraction patterns at   7.13, 4.64, 3.83, 3.56, 2.73, 1.66 and 1.48 Å.        Besides, quartz is also presents in the two sediments as indicated by diffraction patterns at 3.34, 1.82 and 1.54  Å.  After modification process had been done the following changes were noticed: ý  All the peaks in the diffractograms  due to kaolinite disappeared indicating its transformation. ý  Certain diffraction patterns due to quartz remained stable indicating do not change by the treatments.  3.2.2.2 Infra- red absorption spectrometry To ensure the mineralogical composition of raw and modified clays, infra-red analysis was adopted and shown in fig. (2). Complete band assignments were provided in table (3). Obtained data introduced a congruency with the same obtained in XRD analysis. Raw sediments exhibited all the characteristic bands of both kaolinite and quartz mineral as indicated by  ~3696, 3621, 913, 754 cm-1 and 793, 694  cm-1 , respectively. Band at 3654 cm-1 indicated that kaolinite is in disorder form (9). Spectrums of modified clays show some differences as follows:           The Absence of vibration band at 913 cm-1 indicates the breakage of Al-OH linkage and increasing the availability of –OH groups in the structure.           The persistence of 1008, 1034, 1114 cm-1 band explains the existence of Si-O bonds in the structure.           The presence of a broad band with little change in the intensity in the region together with broad Si-O and Si-O-Al bands explains the extent of structural disorder resulted in as a response of modification treatment.         From a mineral structural viewpoint, destruction through heat treatment exposes directed –OH bonds located between the tetrahedral and octahedral layers (amorphization) figure (3). After acidification treatment, the –OH groups become less stable. The newly formed vacant sites during the modification treatments accommodate extra structural water; thereby; broadening the –OH bands in the I.R spectrum. 3.2.2.3 Scanning Electron Microscope (S.E.M) To support the previous results, scanning electron microscope was carried out for both raw and modified Aswan clay samples, figure (4). As general view, rounded particles in a hexagonal shape with clear edges highly confirmed the presence of kaolinite mineral in raw clay sample photo (a). After modification treatments, transformation in kaolinite structure was clearly noticed from hexagonal edgeless shape photo (b). The micrograph is more pronounced when duplicating the magnification power takes place, photo (c). 3.2.3 Cation Exchange Capacity: In order to assess the exchangeability of clay samples after modification treatments, C.E.C values were measured for raw and modified clay samples, table (4).     Modification treatments nearly, duplicated C.E.C values in the clay samples from 8.2 to 18.41 Meq/100g and from 12.66 to 28.53 Meq/100g in both Sinai and Aswan sediment, respectively.   In this respect, obtained results were relatively similar to those published by (12), who reported that the thermal treatment at 600oC followed by acid activation treatment increased C.E.C values of kaolinite  four times. The observed increase in C.E.C after modification treatments is indicative of the increasing number of exchangeable sites caused during the used modification treatments. Acid's attack cleans up the mineral through leaching out metal ions from collapsed lattice structure due to the disorder caused by the transition of kaolinite to metakaolinite by using thermal treatment. In this respect, (13) mentioned that little surface change was observed on calcining kaolinite to less than 550oC, otherwise a marked change in the exchange kinetics can not be expected.   Metakaolinization Increased the exchange sites, the partial collapse of the lattice structure takes place and octahedral aluminum changes to tetrahedral co-ordination making the material more susceptible to acid attack, thus leaving more number of exchange sites. 3.3 heavy metals uptake The removing kinetics of Cu and Zn ions by raw and modified clay samples were monitored as a function of equilibration time and plotted in figure (5). All metal ions attain a near equilibrium condition within the first 90 min. and nearly maximum removing at 2 hrs. Obtained results indicate that the modified clay sample have rendered maximum ion removing {2.78(Cu) and 2.00 (Zn) mg/g} for Sm clay, representing about {27.25% and 18.18%} from initial wastewater concentration, respectively. Beside, {3.38(Cu) and 2.44(Zn) mg/g} for Am clay representing about {33.14% and 22.18%} from initial wastewater concentration, respectively. Observed increase of metal ions removed by modified clays are due to the increasing of exchange sites produced by the acid leaching on a collapsed kaolinite framework.      A comparison of relative removing values in figures (5) revealed that the removing kinetics of Cu are more favored under identical molar concentrations than Zn. This may be due to the formation of Cu(OH)+  and Cu(H2O4)2+ which can stabilize copper ions. It was also suggested that Cu2+ ions largely occupy the planar external surface rather than the edges of platelets. Moreover, zinc can form comparatively stable chloride complex in solution which will not get re-adsorbed once it happened to desorb into the solution due to the prevailing conditions. This explains the observed lower sorption kinetics for Zn as compared to Cu.       Several studies confirmed the previous finding such as (14) and (15). The later showed that the higher the surface charge is the more selective the exchange process is for Cu with respect to Zn. Clay sediments interact with heavy metals through a variety of mechanisms. The dominating type of bond depends on the metal, the type of binding sites, the access to these binding sites and a variety of different geochemical conditions. The main mechanisms by which heavy metals are retained in clay mineral are adsorption including, ion exchange, precipitation and co-precipitation, (3). The present study revealed that the uptake of Cu and Zn ions by modified clays is an ion exchange phenomenon beside great possibility that the metal ions getting entrapped in the pores cannot be ruled out. Conclusion   To produce new modified clay as a sorbent for removing heavy metals and other toxicants from low quality water, two Egyptian natural clay sediments dominated mainly by kaolinite were selected. Unusual treatments depend on thermal transformation and acid activation to increase the exchangeability properties of kaolinite were used. This was proved by different methods, among them XRD analysis, I.R spectroscopy Scanning Electron Microscope and C.E.C measurements. Industrial non-treated wastewater was collected from Helwan Iron and Steel factory, Cairo, Egypt and treated with the sediment before and after modification. Obtained results reported that the porous of collapsed crystalline structure of this modified kaolinite can adsorb a large amount of pollutants metallic ions such as Zn and Cu from wastewater. Observed increase of metal ions removed by modified clays are due to the increasing of exchange sites produced by the acid leaching on a collapsed kaolinite framework.      A comparison of relative removing values revealed that the removing kinetics of Cu is more favored under identical molar concentrations than Zn.   References 1 Abu-zeid, M. (2002) Water in Africa and future challenges. The 18th congress of the international commission an irrigation and drainage, Montreal, Canada, July 21-28. 2 Miranda-Trevino, J.C. and Coles, C.A. (2003). Kaolinite properties, structure and influence of  metal retention on PH. Appl. Clay sci. 23:133-139. 3  O'Day,P.A.; Parks, G.A. and Brown,G.E. 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