LITERATURE REVIEW

2.1 Biosorption

Biosorption can be defined as the removal of metallic ions by means of passive adsorption, or complexion by living biomass or organic waste (Davis and Volesky, 2003). Other definition of biosorption is the ability of biological materials to accumulate heavy metals from wastewater through metabolically mediated or physico-chemical pathways of uptake (Fourest and Roux, 1992). Biosorption is a process in which solids of natural origin are employed for binding heavy metals/radionuclide species in biomass (Saravanan and Brindha, 2009). More specifically, the metal binding in biosorption may be due to a combination of several sequestering mechanisms such as complexing, coordination, chelation, adsorption ion exchange or micro-precipitation (as metal or metal salt) (Eneida and Ravagnani, 2002). This process is less disruptive and can be often carried out on site, eliminating the need to transport the toxic materials to treatment site (Lokeshwariand Josh, 2009). Biosorption is proven to be quite effective for removing metal ions from contaminated solutions in a low cost and environment-friendly manner (Jamila and Hussan, 2009). Biosorption of toxic metal is based on non-enzymatic processes such as adsorption. Adsorption is due to the non-specific binding of ionic species to cell surface associated or extracellular polysaccharide (cellulose and hemicellulose) (Volesky 1990).

The biosorption process involves a solid phase (sorbent or biosorbent; biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (sorbate, metal ions). Due to higher affinity of the sorbent for the sorbate species, the latter is attracted and bound there by different mechanisms. The process continues till equilibrium is established between the amount of solid-bound sorbate species and its portion remaining in the solution. The degree of sorbent affinity for the sorbate determines its distribution between the solid and liquid phases (Parket al., 2006). Bacterial cell walls and envelopes, and the polysaccharides present in the maize cob are efficient biosorbent that bind charge groups (Volesky 1990).Biosorption technology is based on the interaction between toxic metals and the binding structural group on the cell structure of microorganisms and plants.

2.1.1 Mechanisms of Biosorption

The biosorptionmechanisms are various and are not fully understood. Based on the adsorption by a biomass they may be classified according to various criteria.

Physical adsorption: In this category, physical adsorptiontakes place with the help of vanderWaals' forces. Kuyucakand Volesky(1988) hypothesized that uranium, cadmium, zinc, copper and cobalt biosorption by dead biomasses ofalgae, fungi and yeasts takes place through electrostaticinteractions between the metal ions in solutions and cell wallsof microbial cells. Electrostatic interactions have beendemonstrated to be responsible for copper biosorption bybacterium Zoogloearamigeraand alga Chiarella vulgaris(Aksu et al. 1992), for chromium biosorption by fungiGanodermalucidumand Aspergillusniger.

Ion Exchange: Cell walls of Agricultural biomass and microorganism

containpolysaccharides and bivalent metal ions exchange with thecounter ions of the polysaccharides. For example, thealginates of marine algae occur as salts of K+,Na+,Ca2+,andMg2+.These ions can exchange with counter ions such

asC02+,Cu2+,Cd2+and Zn2+resulting in the biosorptive uptakeof heavy metals (Kuyucak

and Volesky 1988). Thebiosorption of copper by fungi

Ganodermalucidium(Muraleedharan and Venkobachr, 1990) and Aspergillusnigerwas also uptaken by ion exchange mechanism.

Complexation: The metal removal from solution may alsotake place by complex

formation on the cell surface afterthe interaction between the metal and the active groups. Aksuet al. 1992 hypothesized that biosorption of copper by C.vulgaris and Z. ramigeratakes place through both adsorptionand formation of coordination bonds

between metals andamino and carboxyl groups of cell wall

polysaccharides.Complexation was found to be the only mechanismresponsible for calcium, magnesium, cadmium, zinc, copperand mercury accumulation by Pseudomonas syringae. Microorganismsmay also produce organic acids (e.g., citric, oxalic,gluonic, fumaric, lactic and malic acids), which may chelatetoxic metals resulting in the formation of metallo-organicmolecules. These organic acids help in the solubilisation of metal compounds and their leaching from their surfaces.Metals may be biosorbed or complexed by carboxyl groupsfound in microbial polysaccharides and other polymers.

2.1.2 Factors Affecting Biosorption

The following factors affect the biosorption process:

Temperature seems not to influence the biosorption performances in the range of 20-35 CC (Aksuet al., 1992)

pH seems to be the most important parameter in the biosorptive process, it affects the solution chemistry of the metals, the activity of the functional groups in the biomass and the competition of metallic ions (Friis and Myers-Keith, 1986, Galunet al., 1987)

Biomass concentration in solution seems to influence the specific uptake: for lower values of biomass concentrations there is an increase in the specific uptake(Gaddet al., 1988; Fourest and Roux, 1992). Suggesting that an increase in biomass concentration leads to interference between the binding sites. Fourest and Roux, 1992 invalidated this hypothesis attributing the responsibility of the specific uptake decrease to metal concentration shortage in solution. Hence this factor needs to be taken into consideration in any application of microbial biomass as biosorbent.

Biosorption is mainly used to treat wastewater where more than one type of metal ions would be present; the removal of one metal ion may be influenced by the presence of other metal ions. For example: Uranium uptake by biomass of bacteria, fungi and yeasts was not affected by the presence of manganese, cobalt, copper, cadmium, mercury and lead in solution (Sakaguchi and Nakajima, 1991). In contrast, the presence of Fe2+ Zn2+wasfound to influence uranium uptake by Rhizopusarrhizus(Tsezos and Volesky, 1982) and cobalt uptake by different microorganisms seemed to be completely inhibited by the presence of uranium, lead, mercury and copper (Sakaguchi and Nakajima, 1991).

2.1.3 Advantages of Biosorption

The major advantages of biosorption over conventional treatment method include lowcost, high efficiency of metal removal from dilute solution, biosorbent regeneration and metal recovery, Potentiality, minimization of chemical and/ or biological sludge, no additional nutrient requirement and regeneration of biosorbent and the possibility of metal recovery.(Sara et al., 2009).

These advantages of biosorption are seen over conventional treatment method such as the physical and chemical processes available for removal of heavy metals. These method include chemical coagulation using aluminum and ferric salts (Fatoki and Ogunfowokan, 2002), and cationic (Evans and Li, 2003), electro-chemical precipitation, ultrafiltration, ion exchange and reverse osmosis (Nomanbhay and palanisamy, 2005).

2.2 History of Corn

Corn is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times.

Its main classification is vegetable, yet it is a technically a fruit. Between 1700 and 1250

BCE, the crop spread through much of the Americas after the European contact with the

America’s, in the late 15th and early 16th centuries, explorers and traders carried corn back to Europe and introduced it to other countries through trade. Spread corn to the rest of the world due to its popularity and ability to grow in diverse climates. (Lance andGarren, 2002). In Nigeria corn is a staple food that is mainly grown in the northern part, therefore it produces large volume of waste (corn cob)( Woranart , 2008).

2.2.1 Classification of Corn

Kingdom – Plantae

Sub-kingdom – Tracheobionta

Super-division – Spermatophyta

Division – Magnoliophyta

Class – liliopsida

Sub-class - Commelinidae

Order – Cyperales

Family – poaceae

Genus – Zea

Species – Zeamays

(Gibson and Benson, 2002).

2.2.2 Biosorbent Properties of Corncob

Biosorption of toxic metal is based on non-enzymatic processes such as adsorption. Adsorption is due to the non-specific binding of ionic species to cell surface associated or extracellular polysaccharide (cellulose and hemicellulose) (Mullen et al., 1989; Volesky 1990).The polysaccharides present in the maize cob are efficient biosorbent that bind charge groups. Corn cobs are rich in cellulose and hemicellulose which comprise about 80% of the dry matter which are converted into metal ion adsorbent for waste water treatment. (Sharma and Forster, 1994).Maize cobs are recyclable, organic and natural, virtually dust free and non-sparkling. (Raoand Venkobachar, 1993). Corn cobs are rich in cellulose and hemicellulose which comprises 80% of the dry matter, which are converted into metal ion adsorbent for waste treatment. (Patterson and Fendorf, 1997).Corn cob powder possesses various functional groups predominantly large proportions of polysaccharides (OH group) on the surface after drying process (Khemaniet al., 2011), hence can be used as a biosorbent.

2.3 Chemistry and Characteristics of Chromium

In nature, chromium generally occurs in small quantities associated with other metals, particularly iron. Its atomic weight is 51.996.Chromium has six oxidation states. Most chromate (Cr(VI)) results from man-made production, as the form is rare in nature (Barceloux, 1999). Hexavalent chromium reduces readily to Cr(III); the rate increases with decreasing pH (Barceloux, 1999).The hexavalent state is one of the three most stable forms in which chromium is found in the environment (U.S. EPA, 1988). The other two of these forms are the 0 (metal and alloys), and the +3 (trivalent chromium, Cr(III)) valence states. . Hexavalent chromium, in contrast to the trivalent form, exists as highlyoxidizing species (IARC, 1990). As noted by NTP (2008), Cr(VI) is usually “present in complexes with halide (chromyl chloride) and oxygen ligands (chromium trioxide, chromate, dichromate).” There are numerous Cr(VI) compounds. Some examples are potassium chromate, dichromate, sodium chromate, chromium trioxide, and lead chromate. Hexavalent chromium compounds can vary considerably in their water

solubility and other physical properties.

2.4 Hexavalent Chromium (Chromium VI)

Chromium VI as one of the major pollutants of the environment is available in nature as an odorless, steel grey hard metallic element. It is the seventh most abundant element on the earth and twenty first most abundant elements in the rocks (McGrath and Smith, 1990). Elemental chromium is not usually found pure in nature and principally occurs as the mineral chromite FeOCr2O3or chrome iron stone in which form it is extremely stable.

Chromium exists in nature as stable hexavalent and trivalent forms (Sen and Ghosh, 2010). The hexavalent form of chromium is more toxic than trivalent chromium and is

often present in wastewater as chromate (CrO42-) and dichromate (Cr2O72-). This is of serious environmental concern as Cr (VI) persists indefinitely in the environment complicating its removal (Sen and Ghosh, 2010 ).

Hexavalent chromium (chromium VI) refers to Chromate, a chemical compound that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately 136,000,000 kilograms (300,000,000 lb) of hexavalent chromium were produced in 1985 (Gerdet al., 2005.) Other hexavalent chromium compounds are chromium trioxide and various salts of chromate and dichromate. Hexavalent chromium is used for the production of stainless steel, textile dyes, wood preservation, leather tanning, and as anti-corrosion and conversion coatings as well as a variety of niche uses. (David and Volesky, 2003).

Hexavalent chromium is recognized as a human carcinogen via inhalation. Workers in many different occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products as well as those who arc weld stainless steel. Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive (Kadirveluet al., 2001).

2.4.1 Toxicity of Chromium (VI)

Relying on chemical, toxicological, and epidemiological evidence, Chromium (VI) is both a powerful epithelial irritant and a confirmed human carcinogen. Additionally, chromium (VI) is toxic to many plants, aquatic animals, and bacteria. (Gregory and Rudolph, 2002).

Chromium (VI) can act as an oxidant directly on the skin surface or it can be absorbed through the skin, especially if the skin surface is damaged. Interestingly enough, irritation of the skin a most frequently reported human health effect from exposure to Cr (VI), taking the form of skin ulceration (dermatosis) and allergic sensitization (dermatitis) ( Khitrov and Jaeger,2002).

Respiratory cancer is the health effect of most concern and is the basis for the regulation of chromium (VI). There is also some indication that chromium (VI) may cause cancer of the upper airways and upper gastrointestinal tract, such as the esophagus, larynx, trachea, and stomach. Chromium (VI) appears to be a contact carcinogen to the respiratory system. It has not been implicated in skin cancer, where there is far more frequent and intense contact than for any other part of body (Gregory and Rudolph, 2002).

Autopsy studies of human lungs show that inhaled chromium builds up in the lungs. Chromium concentration in the lungs was found to increase with age for both occupationally and environmentally exposed individuals. The upper lobes tended to have higher concentrations than lower lobes and cancerous portions of lungs showed the highest chromium concentrations. The study on environmental exposure in Hudson County, New Jersey, showed elevated urine chromium levels among residents living on or near landfills containing chromium; the chromium levels in urine also correlated with chromium content of household dust, but the exposure appears to have resulted from ingestion (Stern et al., 1992). Likewise, a similar study in Lecheria, Estado de Mexico, Mexico found elevated concentrations of chromium in the urine of residents around a chromate manufacturing plant. Respirable airborne particles, contaminated water, and contaminated soil were all possible exposure routes (Rosas et al., 1989).

In several epidemiological studies slightly elevated incidence of stomach cancer were reported. In these cases, the route of exposure was inhalation, not ingestion. This may indicate that the chromium reaches the stomach via clearance of the mucous membranes lining the airways. However, these results are not definitive, and it is not widely accepted that chromium (VI) is a carcinogen in the stomach (Gregory and Ruldolph, 2002).

There is vast literature documenting the mutagenic and cytogenic effects of various chromium compounds, which are reviewed by (Levis and Bianchi, 1982; Cohen et al., 1993). Various studies have shown various chromium (VI) compounds cause many kinds of genetic damage in the laboratory. However, in a field study (Gaoet al., 1994) found no increased DNA damage to lymphocytes among exposed chromium workers when compared with an unexposed group, even though the exposed group had elevated blood, plasma, and urine chromium concentrations. On the other hand (Taioliet al., 1995) found slightly elevated numbers of chromate-specific DNA-protein cross links among residents of Hudson County (New Jersey) with elevated chromium urine concentrations when compared with a control population. Both of these studies used white blood cells (WBCs) for DNA testing; however, because chromates are not associated with any white blood cells or even circulatory disorders. Another theory holds that chromium (VI)-induced damage to cells causes the release of hydrolytic enzymes from the lysosomes(Khitrov and Jaeger,2002).

2.4.2 Chromium (VI) and the Environment

Chromium VI can be found in air, soil, and water after release from the manufacture, use, and disposal of chromium-based products.Chromium VI does not usually remain in the atmosphere, but is deposited into the soil and water.Chromium VI can easily change from one form to another in water and soil, depending on the conditions present (Kratochvil and Volesky, 1998):

2.4.3 Health Hazards of Chromium (VI)

Breathing high levels of chromium (VI) can cause irritation to the lining of the nose, nose ulcers, runny nose, and breathing problems, such as asthma, cough, shortness of breath, or wheezing. The concentrations of chromium in air that can cause these effects may be different for different types of chromium compounds, with effects occurring at much lower concentrations for chromium (VI) compared to chromium (III) (Sen and Dastidar, 2010).

The main health problems seen in animals following ingestion of chromium (VI) compounds are irritation and ulcers in the stomach and small intestine and anemia. Chromium (III) compounds are much less toxic and do not appear to cause these problems (Gregory and Jaeger, 2002).

Sperm damage and damage to the male reproductive system have also been seen in laboratory animals exposed to chromium (VI).Skin contact with certain chromium (VI) compounds can cause skin ulcers. Some people are extremely sensitive to chromium (VI) or chromium (III). Allergic reactions consisting of severe redness and swelling of the skin have been noted(Khitrovet al., 2000).

2.5 Packed Bed Column Reactor

The most effective apparatus for continuous operation is a column reactor, which handles product as a flowing stream, much like that used for ion exchange. A column reactor is also a kind of bioreactor which is a vessel in which a chemical process is carried out, it usually involves organisms or a biochemically active substance. Thus, many researchers have used a column packed with various biomasses capable of removing heavy metals (Voleskyet al., 2003). Meanwhile, many mathematical models have been used to study column systems, and their dynamic behavior has also been well established (Figueiraet al., 2000). All these models have been mainly originated from research on activated carbon sorption, ion exchange or chromatographic applications. However, in the case of Cr (VI) removal in the column, a few studies have been reported, but no theoretical model has been proposed to predict the experimental breakthrough data (Parket al., 2006).Biosorbent packed in a column appears to be the most appropriate device for effective and continuous removal of heavy metals (Kratochvil and Volesky, 1998). Various parameters affect the biosorption process in column such as, mass of biosorbent, flow rate of influent, temperature, initial concentration of influent and pH.

2.5.1 Benefits of Continuous Reactors

The rate of many chemical reactions is dependent on reactant concentration. Continuous reactors are generally able to cope with much higher reactant concentrations due to their superior heat transfer capacities. Plug flow reactors have the additional advantage of greater separation between reactants and products giving a better concentration profile.

The small size of continuous reactors makes higher mixing rates possible.

The output from a continuous reactor can be altered by varying the run time. This increases operating flexibility for manufacturers.

2.6 The Adsorption Isotherm of Metal Removal

The adsorption isotherm indicates how the adsorption molecules distribute between the liquid phase and solid phase, when the adsorption process reaches equilibrium state. The equilibrium of the biosorption process of heavy metals is often described by fitting the experimental points with the model (Gaddet al., 1988). The Freundlich and Langmuir models were used to describe the biosorption equilibrium.

2.6.1Freundlich Adsorption Study

The empirical Freundlich equation based on sorption on a heterogeneous surface is given below by the equation.

qe = KFCe1/n qe

WhereKF and n are Freundlich constants characteristic of the system.KF and n are indicators of adsorption capacity and intensity, respectively. The equation can be linearized in logarithmic form and Freundlich constants can be determined.

The Freundlich isotherm is also more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model(Donmezet al., 1999). The Freundlich model was chosen to estimate the adsorption intensity of the sorbent towards the powder and the linear form is represented by equation :

lnqe= ln Kf + 1/nln Ce

Where; qe (mg/g) is the metal ion uptake per unit weight of maize cob powder; Ce is the concentration of metal ions in solution at equilibrium (mg/dm3); Kfand n are the

Freundlich constants. The value of Kfis a measure of the degree of adsorption and n indicates the affinity of the sorbent toward the powder. For 1/n less than unity, adsorption is the predominant process taking place otherwise desorption becomes predominant (Matakaet al, 2010).

A plot of lnqe against lnCe in equation above, yielding a straight line indicates the confirmation of Freundlich adsorption isotherm. The constants, n and Kfcan be obtained from the slope and intercept respectively.

2.6.2Langmuir Adsorption Study

The Langmuir equation which is valid for monolayer sorption onto a surface a finite number of identical sites and is given by below: qe= KL1/qmaxCe/1+1/qmaxCe where KL is the maximum amount of the metal ion per unit weight of cell to form a complete monolayer on the surface bound at high Ce (mg g−1) and1/qmaxis the constant related to the affinity of the binding sites, KL represents a practical limiting adsorption capacity when the surface is fully covered with metal ions and assists in the comparison of adsorption performance, particularly in cases where the sorbent did not reach its full saturation in experiments. KLand1/qmaxcan be determined from the linear plot of Ce/qeversus Ce (Matakaet al, 2010).

The linearized form of the equation after arrangement is given by:

Ce/qe =1/qmax KL + Ce/qmax

Where KL (dm3 g-1) is a constant related to the adsorption energy and qmaxis the maximum sorption upon complete monolayer saturation of the powder surface. The experimental data were fitted to equation 1 for linearization by plotting Ce/qeagainst Ce to obtain a straight line graph.