INTRODUCTION 

Industrial  operations handle large volumes of  process water, which are  contaminated with  organic chemicals, metal ions, oils  and  other  materials. Aqueous streams containing  heavy metals  are frequently encountered  in  industrial effluents and sources of copper, lead, zinc  and cadmium  are very common in  the electroplating  facilities, electrolytic refining  plants  and acid  mine waters, among  others.  Dyes as organic chemicals are  commonly found  is  waste streams from  food and textile industries. Due to  high  level  of  industrial activities in  South Africa, organic contaminants  and  heavy  metals  such as chromium,  vanadium  and dyes have  been  found  to  gain entry into  receiving water streams and  groundwater sources such  as was the case with  groundwater  outside Bayer chemical factory in  Durban. 

MAIN TEXT

Clays are hydrous aluminosilicates broadly defined as those minerals that make up the colloid fraction of soils, sediments, rocks, and water and may be composed of mixtures of fine grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate, and metal oxides. Clays play an important role in the environment by acting as a natural scavenger of pollutants by taking up cations and anions either through ion exchange or adsorption or both. Thus, clays invariably contain exchangeable cations and anions held to the surface.

Composites can be defined as natural or synthesized materials made from two or more materials with significantly different physical and chemical properties which remain separate and distinct at the microscopic or macroscopic scale within the material. Composites are synthesized to combine the desired properties of the materials in the composite. In nanocomposite, nanoparticles (clay, metal, carbon nanotubes, etc.) act as fillers in a matrix. Nanoparticles are particles of less than 100 nm in diameter that exhibit new or enhanced size-dependent properties compared with larger particles of the same material. Clay composite or nanocomposites are the materials in which major component of the material is clay in combination with other materials like metals, polymer, and so forth. Recently, the development and characterization of nanostructured polymer-clay composites has received special attention because of their advantages in comparison to the traditional polymer composites. Minimal additions of nanoclay enhance mechanical, thermal, and dimensional and barrier performance properties significantly because of the large contact area between polymer and clay on a nanoscale.

However,  organics and  heavy  metals  are toxic  and environmentally harmful substances. Contaminants  that accumulate within the living  organisms as well  as in human body  are especially  more dangerous.  The presence  of  these contaminants in the environment  has been  of  great concern because  of  their increased  discharge, toxic nature and other  adverse effects  on receiving waters. It  is  well  known that  heavy metals  can damage  nerves, liver and  bone and  block  functional  groups of essential enzymes. The  large  volume  of  contaminated exert enormous pressure to the  existing water  handling mains. Moreover, to  protect  human health, groundwater sources and other receiving water bodies from  contamination, industrial discharges must be  treated  sufficiently and where  permitted, recycled. There are several treatment  options that  have been developed  or  show  potential  for remedying contaminated waters. These techniques  include:  membrane bioreactor technology, electrodialysis, reverse  osmosis, adsorption  and hybrid  processes combining two  or  more  of  the  above techniques, among  others. The choice of a treatment technique for a given  utility  depends  on  the nature and  concentration  of the ions, chemical species in source water, existing  treatment processes,  treatment  costs, handling  of  residuals  and  versatility of a  given  technique. Because  of  limitations in  terms of cost, production of enormous  waste and difficulty  in  end-use applications  of  some  of the  above treatment techniques, an environmentally  benign, robust  and low-cost  a technique  has to  be implemented for remedying contaminated wastewater.  Proponents of  adsorption technology argue that  the technique is economical, efficient and produces high  quality water.  In  the recent  past,  adsorption technique  has been arguably one  of  the most versatile techniques in  removing pollutants  from  water. Moreover, it attracts wider acceptance, is environmentally benign and is simple. 

The algorithm  of  adsorption  technology  process development  starts  with  selection of  an appropriate media for a given process problem;  in  the foregoing  water  quality improvement.  A number of  established  and potential  media exist in  open  literature for ameliorating  the quality of  water. However, most  of these media have  poor  capacity, show  slow kinetics, have  only  been  synthesized in  the  laboratory, are  not regenerable and in  some  cases expensive and  hence  not desirable for water treatment applications. 

This study  proposes to  explore the application  of  polymernatural clay composites as  robust materials for adsorptive treatment of waste streams. This  line of thinking  arises  due  to fact that:

(i) most adsorption materials have very  low capacity for contaminants  

(ii) Nigeria is a  water scarce  country and as such contaminated water resources should  be treated sufficiently for re-use and  

(iii) a number of  existing  treatment technologies are expensive.  We  have  previously  shown  the application  of  polymers in  fluoride  and  chromium  removal from  water. These  new  kind  of  materials exhibited superior  quality compared to  the conventional adsorbents. However, polymers have  very low densities rendering  them unsuitable for  fixed bed operations. Adsorbent-based water treatment  mostly  occur in  fixed-bed  operations  which  produce high  quality treated  water.  Dense media are required  in  fixed beds to avoid having extremely large (in volume terms) beds.

 To extend  the  knowledge  gained  in the  previous  works, we evaluate the feasibility of  using  low-cost  locally available natural clay as a matrix  to  hold  polymers in  removing chromium  as an example of a  pollutant  from  water. Natural  clays  have sufficient  density  and are suited for fixed bed  operation.  The study  focuses on  exploring the effect  of several  water quality parameters and  process  variables on chromium  removal  efficiency in  both  batch  and  column modes of  operation. II.  

POLYMER-NATURAL CLAY COMPOSITES FOR WATER TREATMENT 

Clays are alumino-silicates and  their  performance properties are related  to their weak  silica-alumina bond. Examples of clays widely used  in  water and  water treatment  are kaolinite  and  bentonite. Charge  development  on  silicate clays is mainly due  to  isomorphous substitution.  This is the substitution  of  one element for another  in  ionic crystals without change of the structure.  It takes  places only between ions  differing  by  less than  about 10%  to  15%  in  crystal  radii. In tetrahedral  coordination,  Al3+ for  Si4+  and in  octahedral coordination Mg2+, Fe2+, Fe3+ for  Al3+. Charges developed as a result of  isomorphous substitution  are permanent, responsible for sorption features  of the clays and not  pHdependent.

 Clays in  the natural form  have very low capacity for (waste) water contaminants.  On  the  other hand,  polymeric  resins have a wide  range  of pore  structures, good surface  areas, and a  good selectivity  toward  aromatic solutes and  little or  no  adsorbent loss  on  regeneration  [11].  However, their  performance is dependent  on  the type of resin used  (extent  of chemical activation and modifications, nature  and  degree  of functionality  grafted  on  the polymeric surface), physicochemical characteristics (i.e.  porosity, specific surface area and  particle size of adsorbents) and  they are not effective for all  organic  solutes.  Another drawback is  their  pHdependence, poor water wettability and  sensitivity  to  particle size.  Furthermore, they  are  very  expensive  which  have rendered  them unattractive for  use  by many industries  and water treatment  plants  especially  in  the developing  world. Consequently,  they  have  been  modified with  various  polymers to  improve their  performance in  inorganic and  organic contaminants  removal  from  aqueous stream. For instance, Unuabonah showed  the potential  of a polymer–clay based composite  adsorbent  prepared  from  locally  obtained kaolinite  clay  and  polyvinyl  alcohol  for the  removal  of Pb(II) ions from  aqueous solution.  However, the  material showed significant  capacity  reduction during  regeneration  and therefore from  economic point  of view not  suitable for water treatment  operations.

However, this paper reviews the recent use of natural clay and its composites as an ecofriendly efficient adsorbent for removal of organic, inorganic, and pathogenic contaminants from drinking water and its sources. The major goals of the paper are

(1) to conduct comprehensive review of the literature to emphasize the importance of using clay and its modified forms as versatile, environmentally friendly adsorbents for contaminant removal from drinking water and its sources,

(2) to emphasize on the types of modification on the natural clays and the benefits of these modification on removal of major emerging contaminants of the present time,

(3) to analyze the quantitative efficiency of the individual clay and its composites in removing the various contaminants and study the effects of variable like pH, temperature, and other conditions limiting or enhancing the adsorbent efficiencies of the clay materials.

The review section is divided into following four headings on the basis of the type of contaminant removed by the clay and its composites: (1) heavy metals (2) inorganic contaminants (3) organic contaminant, and (4) pathogens.

(1) Heavy Metals

Heavy metal contamination in drinking water resources has serious effects on the health of human beings, animals, and plants. Currently, many researchers are working in this field to find an appropriate solution for removing various metals present in the water. Application and efficiency of different type of natural clay and their composites in removing various metals like arsenic, iron, manganese, lead cadmium, uranium, chromium, selenium tungsten, and zinc are reviewed in the following sections. Clays and their modified forms have received wide attention recently for use as adsorbents of metal ions from aqueous medium because of their easy availability and comparatively less cost.

Removal of heavy metals by natural clays and their modified forms, kaolinite and montmorillonite in particular, has been reviewed by Bhattacharyya and Sen Gupta. Their review reports the modification of the above mentioned clays by pillaring with various polyoxy cations of Zr , Al , Si , Ti , Fe , Cr or Ga , and so forth. The adsorption of toxic metals, namely, As, Cd, Cr, Co, Cu, Fe, Pb, Mn, Ni, Zn, and so forth, has been studied predominantly. They found montmorillonite and its modified forms have much higher metal adsorption capacity compared to that of kaolinite and its modified forms. Their work reports the successful and improved adsorption of metals likezin CO, Cr, CO, Cu, Fe, Pb, Mn, Ni, and zinc by kaolinite, montmorillonite, and their modified forms. They found that montmorillonite and its modified forms have higher metal adsorbing capacity as compared to their counterparts.

Oliveira prepared clay iron oxide composite for adsorption of metal ions Ni , Cu , Cd , and Zn from aqueous solution. They compared the metal adsorption capacity of bentonite clay and its magnetic composite. They showed that the presence of iron oxide increased the adsorption capacity of the bentonite. These adsorbents showed the advantage to be easily removed from the medium by a simple magnetic separation procedure after saturation is reached.

(2) Inorganic Contaminant

Inorganic contamination of drinking water and its sources is caused by natural and anthropogenic factors. Fluorosis is endemic in at least 25 countries across the globe and has affected millions of people. (Ndagba, 1992). It is caused by high concentration of fluoride above 1.5 mg L in drinking water. Fluoride is beneficial when present within the permissible limit of 1.0–1.5 mg L for calcification of dental enamels. Similarly, excess of nitrates in drinking water causes methemoglobinemia or blue baby disease. Thus, management of these inorganic contaminants in water is of prime importance.

(3) Organic Contaminants

Clays offer an attractive and inexpensive option for the removal of organic and inorganic contaminants. The adsorption of several organic contaminants in water such as pesticides, phenols, and chlorophenols has been reported recently in the literature. (Afore, 1997).

Chloroacetic acids, such as trichloroacetic acid (TCAA), dichloroacetic acid (DCAA), and monochloroacetic acid (MCAA) are receiving increasing attention in the literature. These chloroacetic acids, commonly formed during the reactions between chlorine and natural organic matter (NOM) during prechlorination or disinfection in potable water production, have been shown to be carcinogenic and may potentially pose a risk to human health.

Chloroacetic acids, formed during the disinfection process in potable water production, are considered to pose a potential risk to human health. In this work, Guetal (1990) investigated dichloroacetic acid (DCAA) removal from drinking water by using a process of bentonite-based adsorptive ozonation. This process is formed by combined addition of ozone, bentonite, and Fe . During the reaction, DCAA is removed by the joint effect of adsorption, ozonation, and catalytic oxidation.

(4) Pathogens

The microcystins are potent mammalian liver toxins, (Dagtel, 2001) known to be potent and specific in vitro inhibitors of the catalytic subunits of protein phosphatases-1 and 2A and are extremely potent tumor promoters. Since it is widely suspected that many conventional water treatment methods are ineffective at reducing human exposure to microcystins, investigations into economic and practical methods of remedial water treatment are important. Although removal of cyanobacterial cells and toxins from drinking water using domestic water filters resulted in marginal success, it is following the termination of cyanobacterial growth that the majority of microcystins are considered to enter into the surrounding water after lysis and cell death. To date, perhaps photoirradiation is the most promising new method for detoxifying microcystins in raw water. In a recent report Harada and Tsuji [ 1998] looked at the persistence and decomposition of these hepatotoxins in the natural environment. Five pathways were considered as contributing to natural routes of detoxification. Of relevance to the work presented here is that it was ascertained that microcystins are absorbed strongly on sediment and that they are difficult to recover. The results of microcystin-LR scavenged by naturally occurring clay minerals are reported by Harada and Tsuji. The microcystin cyanobacterial hepatotoxins represent an increasingly severe global health hazard. Since microcystins are found worldwide in drinking water reservoirs concern about the impact on human health has prompted investigations into remedial water treatment methods. The preliminary study by Morris et al. [1984] investigated the scavenging from water of microcystin-LR by fine-grained particles known to have a high concentration of the clay minerals kaolinite and montmorillonite. The results show that more than 81% of microcystin-LR can be removed from water by clay material. Thus, microcystin-LR is indeed scavenged from water bodies by fine-grained particles and that this property may offer an effective method of stripping these toxins from drinking water supplies.

CONCLUSION

 Polymer–clay composite  is a  robust adsorption media  for water treatment  applications. The composite combines  both the features of clay and polymer  to give a material expected to have a lot  of  applications in  water treatment. It has  been shown by  various  researches  that  the composite  is  suited for both organic and inorganic  contaminants  in  water. For  our study  the focus was  inclined  towards  hexavalent  chromium removal  using  polypyrrole  modified montmorillonite  (PpyOMMT NC3).  

The paper summarizes the variety of pollutants treated with different types of clays, their efficiency, and the effect of different variables on their adsorption capacity. It is clear that natural clay and its composites are capable of removing contaminants ranging from metals to priority pollutants from contaminated drinking water and its sources. Results from the recent advances in using natural clay and its modified composites show the flexible nature of the clay and its ecofriendly nature. The results are capable of removing organic and inorganic contaminants from drinking water with very high removal ratios of toxic trace metals, nutrients, and organic matter. In most of the cases, it proved to be better or comparable with the existing commercial filter materials, adsorbents, and conventional methods used for decontamination of drinking water. Being natural and their abundance presence makes them a low-cost green, nontoxic adsorbent which can be used for removal of different contaminants from water and making clean and pure drinking water available for developed and developing nations.

REFERENCES   

[1]  D.  Feng,  C.  Aldrich and H.  Tan,   “Removal of heavy  metal ions by  carrier magnetic separation of adsorptive  particulate.  Hydrometallurgy.  vol.  56,  pp. 359-368,  2000. 

[2]  K.  Pietersen, “Groundwater  crucial to rural development.  The Water Wheel, pp.  26-27,  2000. 

[3]  U.  Muradiye  and I.  Ar,  .  Removal of Cr(VI)  from  industrial wastewater  by adsorption part I: Determination of  optimum  condition. J. Hazard.  Mater. vol. 149,  pp.482-491,  2007. 

[4]  Demil,  A.  and Arisoy,  M.  Biological  and chemical  removal of Cr(VI)  from wastewater: Cost  and benefit analysis. J. Hazard. Mater. vol. 147,  pp. 275280,  2007. 

[5]Chen,  H. Frey, M.M., Clifford,  D., McNeill, S.L. and  M. Edwards, “Arsenic treatment considerations. JAWWA 92, 74-90. 

[6]  Onyango,  M.S.,  Kojima,  Y.,  Ochieng,  A.,  Bernado,  E.C.,  and H.  Matsuda, “Adsorption  equilibrium  modeling and solution chemistry  dependence of fluoride removal  from  water by trivalent-cation-exchanged zeolite,” J  Colloid Inter.  Sci.  vol.  279,  pp.  341-350,  2004. 

[7]  M.S.  Onyango,Y.  Kojima,  A.  Kumar,  D.  Kuchar, K.  Mitsuhiro  and  H. Matsuda,  “Uptake of fluoride by  Al3+  pretreated low-silica synthetic zeolite: Adsorption equilibria and rate studies. J.  Separ. Sci. and Technol. vol. 41,  pp. 683-704,  2006. 

[8]  K.K.H.  Chay,  J.F.  Porter, J.F. and  G.  Mckay,  “Film-pore  diffusion  modelsanalytical and numerical solutions,”  Chem. Eng.  Sci.  vol.  59,  pp.  501-512, 2004. 

[9]  Bhaumik,  M.  Maity,  A.,  Srinivasu,  V.V.  and Onyango,  M.S.  “Enhanced removal of Cr(VI)  from  aqueous solution  using  polypyrrole/Fe3O4  magnetic nanocomposite,  ”    J.  Hazard.  Mater.  vol.  190(1),  pp.  381-390,  2011.

 [10]  Bhaumik,  M.,  Leswifi,  T.Y.,  Maity,  A.,  Srinivasu,  V.V.  and Onyango, M.S. “Removal of  fluoride from  aqueous solution  by  polypyrrole/Fe3O4 magnetic nanocomposite.  J.  Hazard.  Mater.  vol 186(1)  (2011)150-159,  2011. 

[11] G. Crini, “Recent developments  in polysaccharide-based  materials used as adsorbents in wastewater treatment,” Prog.  Polym.  Sci.  vol.  30,  pp.  38–70, 2005. 

[12]  E.I,  Unuabinah,  M.I.  El-Khaiary,  B.I.  Olu-Owolabi  and K.O. Adebowale, “Predicting the  dynamics and performance of a polymer–clay  based composite in  a fixed bed system  for  the removal of lead (II)  ion,” Chem.  Eng. Res.  Des.  vol.  90,  1105–1115,  2012.

 [13]  Y.-F.  He,  L.  Zhang,  R.-M,  Wang,  H.-R.  Yan &  Y.  Wang,  “Loess clay based copolymer for removing Pb(II) ions,”   J.  Hazard. Mater. vol. 227–228, pp.  334–340,  2012. 

[14]  T.S.  Anirudhan &  P.S.  Suchithra,  “Heavy  metals  uptake  from  aqueous solutions  and  industrial wastewaters by  humic  acid-immobilized polymer/bentonite  composite:  Kinetics and equilibrium  modelling,”  Chem. Eng.  J.  vol.  156,  pp.  146–156,  2010. 

[15]  A.  A.  El-Zahhar,  N.  S.  Awwad &  E.  E.  El-Kator,  “Removal of bromophenol blue dye from  industrial  waste water  by  synthesizing polymerclay  composite,”   J.  Mol. Liq.  vol.  199,  pp.  454-461,  2014. 

 [16]  T.S.  Anirudhan ,  P.S.  Suchithra &  P.G.  Radhakrishnan,  “Synthesis  and characterization of humic acid immobilized-polymer/bentonite composites and their ability to adsorb basic  dyes  from  aqueous solutions,”  Appl.  Clay Sci.  vol.  43,  pp.  336–342,  2009.

Get this material NGN 5,000

Message Us on WhatsApp  »