Water is the most important natural resource in the world; its importance cannot be over emphasized. It is the most abundant liquid on earth as it covers three quarters of the earth surface. While 71% of the surface of the Earth is covered by oceans, the freshwater resources contained in aquifers, lakes, rivers and glaciers only account for 3% of the total earth water. Freshwater is a renewable resource; this is done via the hydrological cycle through the process of evaporation, condensation, precipitation, run off and transportation. The rapid growth of industries in the society has led to an increase in the pollution of the environment especially through improper disposal of waste such as heavy metals, organics, and bacteria have posed a danger to our society. Every day, 2 million tons of sewage and industrial and agricultural waste are discharged into the world’s water (UN WWAP 2003), the equivalent of the weight of the entire human population of 6.8 billion people. The UN estimates that the amount of wastewater produced annually is about 1,500 km3, six times more water than exists in all the rivers of the world. (UN WWAP 2003). These problems require new methods of purifying water that is cost effective, environmentally friendly and require less energy than the traditional methods.
HEAVY METALS IN WATER
Heavy metals refer to metallic elements that are relatively dense and toxic even at low concentrations. Heavy metals such as lead, arsenic, nickel, zinc, mercury, chromium, and cadmium are non-biodegradable and therefore accumulate in the environment. They are also very toxic and known to be carcinogenic. The issue of heavy metal ions in water is one of great concern in the society. Heavy metals are one of the most common pollutants in source and treated water. The increased use of heavy metals industrially resulted in an increase in the availability of metallic substances in natural source water. Wastewater from leather industries, petroleum refineries, mining, chemical industries, metallurgical, and paint industries, as well as run off from agriculture and forest lands being disposed to the environment without proper treatment are one the reasons for the presence of heavy metals in water, these are not only harmful to the aquatic lives but also to humans in terms of bioaccumulation, bioconcentration and biomagnification. Lead causes damage to the kidney, liver and brain disease, also known as Plumbism. Cadmium causes Itai-Itai disease which leads to softening of bones, body shrinkage and painful death. Mercury, when retained for a long time in the cells of a pregnant woman, can move the placenta barrier exposing the child to dangers. Cyanide causes seizures, cardiac arrest, and death when inhaled. Hence, the removal of heavy metals is utmost importance. There are various methods being employed in the removal of heavy metals such as reverse osmosis, chemical precipitation, membrane filtration, ion exchange, electrodialysis, coagulation, adsorption, among others. The best method being adsorption, because it is cost effective, very efficient, removes even trace amount of metals and it is environmentally friendly. The common adsorbent used is activated charcoal, chelating agents and clay materials. These methods have low adsorption capacities and are not effective in removing heavy metals at high concentrations. This resulted in the development of nanomaterials in the removal of heavy metal ions from water and in the treatment of waste water.
Nanomaterials are applied in numerous ways to tackle technological and environmental challenges in solar energy conversion, catalysis, medicine, and water treatments. Nanomaterials are materials with particle size between 1nm to 100nm. As a result of their small size, nanoparticles can penetrate etremely small contaminants zone as opposed to micro particles which makes them very valuable in the removal of heavy metals. The recent development of nanoscience and nanotechnology has shown remarkable potential for the remediation of environmental problems. Nanomaterials are the driving force in the development of nanotechnology. In comparison with traditional materials, nanostructure adsorbents have shown extreme efficiency and faster rates in water treatment. Several type of research has been carried out and still on-going regarding the use of nanoadsorbents in water purification.
Nanomaterials as Adsorbents
Nanomaterials used as adsorbents in removal of heavy metal ions in wastewater should be nontoxic, have relatively high adsorption capacities which is due to a much larger surface area, be sensitive to the high and low concentration of pollutants, easily remove adsorbed pollutant from the surface of the nanoadsorbent, and be reusable. These properties make them very useful in water purification, remediation and treatment. A variety of nanomaterials such as carbon nanotubes, Graphiteoxide/Silica composites, graphene, nano metal or metal oxides, and polymeric sorbents have been studied in the removal of heavy metal ions from aqueous solutions, and the results indicate that these nanomaterials show high adsorption capacities.
Carbon nanotubes (CNT) are a common nanoadsorbent. They are thin, hollow cylinders composed of carbon atoms with diameter as small as 1nm. They are highly porous, have a large surface area, oxygen-containing functional groups and hydrophobic surfaces, these properties of CNT have increased their preference as an adsorbent in removal of organic, inorganic and heavy metal ions from waste water. They have high affinity for divalent metal ions. Carbon nanotubes can be Single-walled (SWCNT) which are made of a single layer of graphene or Multi-walled (MWCNT) that possess numerous concentric layers. Numerous researches have been carried out in the use of nanomaterial to purify water. Li et al. (2003) carried out a study on the the adsorption of Pb(II), Cu(II) and Cd(II) onto multi-walled carbon nanotubes (MWCNTs) and reported excellent adsorption capacities of 97.08 mg/g for Pb(II), 24.49 mg/g for Cu(II) and 10.86 mg/g for Cd(II) at room temperature, pH 5.0 and metal ion equilibrium concentration of 10 mg/l. He also reported that the metal–ion adsorption capacities of the MWCNTs were 3 to 4 times greater than those of powder activated carbon and granular activated carbon, which are the two commonly used adsorbents in water purification. A research in which Graphite Oxide/silica was used as an adsorbent in removal of lead, nickel, and chromium. There was a decrease in the concentration of heavy metals after treatment with the nanoadsorbent. This was also corroborated with a positive shift in the wave number from pure metal and adsorbed metal.
Graphene is an allotrope of carbon with one or carbon atoms arranged in a hexagonal lattice. It has excellent electrical, mechanical, thermal, optical and transport properties, all of which made it an area of interest as a nanoadsorbent. They possess strong large electronic surface which allows for a strong intermolecular attraction of adsorbents. Their open layer structure and abundant oxygen containing functional group enhances their adsorption capacity relative to CNTs. As study on Magnetite–graphene composite adsorbent with a particle size of 10 nm reported that they have a high binding capacity for As(III) and As(V). The large surface area, large delocalized pi (?) electrons and tunable chemical properties of graphene make them a riveting choice as adsorbents for environmental decontamination applications.
Zeolites are micro porous, crystalline aluminosilicates with infinite open three-dimensional structures. They have spherical shape and exchange sites where ions bind to the zeolite. Adsorption is dependent on the intensity of interactions between the cations and the zeolite surface which is due to steric effects. Preliminary tests on preferential adsorption demonstrated that Zn2+ and Cu2+ had a higher affinity than Ca2+ for the zeolite types (Moreno et al., 2001). NaP1 zeolites have been evaluated as ion exchange media for the removal of heavy metals from acid mine wastewaters (Moreno et al., 2001)
Dendrimers possess a centre core, interior layer (generation), and an exterior with a functional surface. Dendritic polymers exhibit many features that make them particularly attractive as functional materials for water purification. They bind and retain contaminants in their branched structures which are the interior layer or surface groups. These ‘soft’ nanoparticles, with sizes in the range of 1–20 nm, can be used as high capacity and recyclable water soluble ligands for toxic metal ions, radionuclides and inorganic anions (Ottaviani et al., 2000; Birnbaum et al., 2003; Diallo et al., 2004).
Adsorption occurs when the adsorbate accumulates on the surface of the adsorbent. The adsorption isotherm is a curve that describes the amount of adsorbate on the adsorbent with pressure at constant temperature. Adsorption is the process in which heavy metals are adsorbed on the solid surface, and the equilibrium is established when the concentrations of heavy metal adsorbed and in water become constant. From these isotherms, several adsorption parameters could be calculated. The most widely used adsorption isotherms are Langmuir model and Freundlich model. At low metal concentration, the isotherms are linear. However, the Freundlich isotherm becomes a curve (non-linear) at a higher metal concentration indicating low adsorption capacities as the adsorption sites have been filled. In Langmuir isotherm, when the adsorption sites are completely filled, adsorption stops and continual increase in metal ion concentration or species remains in the solution.
In this model, adsorption occurs on the active sites of the adsorbent, and once the active sites are fully occupied by the metal ions, the adsorption is naturally terminated at this site. The non-linear Langmuir equation is:
qe = qmaxKLCe
1 + KLCe
Where qe is the amount of metals adsorbed at equilibrium (mg/g), KL is the adsorption equilibrium constant (L/mg), qmax is the maximum adsorption capacity of adsorbent (mg/g), Ce (mg/ L) is the equilibrium concentration of metals remaining in solution when amount adsorbed equals qe.
The linear Langmuir model is given by following equation
Ce = Ce + 1
qe qe bqm.
Where qm and b are the saturated monolayer adsorption capacity and the adsorption equilibrium constant. A plot of Ce /qe versus Ce would result in a straight line. From the slope and intercept, the maximum adsorption capacity of metals can be calculated.
The Freundlich equation is an empirical model allowing for multilayer adsorption on sorbent. The non-linear equation of the Freundlich model is given as:
The linear form of the Freundlich model can be expressed as:
Log qe = log KF + log Ce
where qe is amount of metals on adsorbent at equilibrium (mg/.kg); KF is indicator of adsorption capacity (mg1?n Ln g?1), 1/n is adsorption intensity and Ce is aqueous concentration of metals at equilibrium (mg/L). The value of n and KF are specific for a system. The higher the value of KF, the higher the maximum adsorption capacity.
Adsorption Kinetic Model
In evaluating the kinetic mechanism of the adsorption process, the proposed model in evaluating the kinetic mechanics of the adsorption process are the pseudo first order by Lagergren in 1898, pseudo second order by Ho. Y. S. and McKay. G., Elovich model, and intra particle diffusion model.
Pseudo first order
It is a model used in the adsorption in solid/liquid phase. The equation is given as:
Log(qe-qt) = log qe – k1 t
Where k1 is the pseudo-first-order rate constant for the adsorption process (min-1), qe and qt are the amounts of metal ions adsorbed per gram of sorbents (mg/g) at equilibrium and at time t (min) respectively.
The plot (Log(qe-qt) against times gives a straight line from which k1 can be calculated from the slope.
The equation for the pseudo-second order of the adsorption kinetic model is
t = 1 + t
Qt k2qe2 qe
Where k2 (g /mg.min) is the pseudo-second – order adsorption kinetic parameter.
Challenges in Using Nanotechnology
Every technology that is of benefit to humans has its own risks and nanotechnology is no exception. Although the use of nanomaterials is continually expanding in the area of water treatment, there hasn’t been enough research on the adverse effect of nanomaterials. Are nanomaterials completely removed from water after treatment is completed? Instruments that can detect even the slightest amount of nanomaterials are needed to ensure that they are completely removed before distribution of water or disposal of waste water into the environment. The extremely small size of nanomaterials makes it easy for them to penetrate the body when in contact which could be harmful. Health practitioners raise concerns about nanomaterials crossing the blood barrier in the brain (which is a membrane that protects the brain from harmful chemicals in the bloodstream).
The increasing scarcity clean water is a cause of concern in the world today. The rapid growth of industries, poorly planned urbanization, and overpopulation has led to a great decline in the quality of water. Traditional methods such as ion exchange, reverse osmosis, membrane filtration, adsorption, among others. Of all the methods used, adsorption is gaining an enormous preference in water treatment due of its versatility, wide applicability and economic feasibility. Nanomaterials are revolutionary in the purification of water, their effectiveness in removing large and even trace amount of heavy metals have made them a growing preference. Their large surface area and very small size, and high number of adsorption sites allow them to adsorb more metal ions and more effectively than the traditional counterpart. Various nanomaterials such as Carbon nanotubes, graphene, dendrimers, zeolites are being employed in the removal of heavy metals ions from water. The use of nanomaterials for water purification is a fast-growing area of research as they are cost effective and most importantly environmentally friendly.
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