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Over the past three decades or so the discovery and further development of biosorption phenomena has gained momentum and has transformed the methods by the means of which waste water effluent is treated to remove pollutants and recover valuable resources present in these aqueous systems like dyes. Biosorption is becoming a promising alternative to replace or supplement the present dye removal processes from textile industries effluent. This technology has drawn the attention of industries as it is economically viable and environmentally friendly. The status of scientific development of a technology can be reflected through analyses of the literatures pertaining to it, in this review, we qualitatively examine almost all aspects of biosorption research through research articles and other review papers. We have basically focused on biosorption of textile dyes using dead fungal biomass obtained from autoclaved or inactivated Aspergillus Niger. Materials used, methodologies used and data obtained has been assimilated from literature cited below. Finally, we summarized the important considerations of the current research on biosorption, the results and conclusions obtained from the data, as well as the suggestions and our thoughts and ideas for its future directions.
Rapid industrialization and urbanization all over the globe has resulted in the generation of large quantities of aqueous effluents, many of which contain high levels of toxic pollutants. Various physical, chemical and biological processes are being employed to remove pollutants from industrial wastewaters before discharge into the environment as in the case of treatment of adsorptive pollutants like heavy metals and ionic dyes, however, most of the conventional treatment processes, especially chemical precipitation, coagulation, activated carbons and the use of ion-exchange resins become less effective and more expensive when the adsorbates are in a low concentration range and their high cost and low efficiency and lack of practicality have limited their commercial use in the field . Since any type of solid material has the capacity to absorb pollutants to some degree, a number of industrial inorganic wastes, such as ash, or natural inorganic materials like clay, synthetic materials, as well as, living or nonliving biomass/biomaterials, have been investigated as cheap adsorbents capable of replacing the well-known, but more expressive ones as their cost is low and efficiency is higher and the biosorbants can be regenerated, and the possibility of dye recovery following adsorption biomass-based adsorbents or biosorbents as they are commonly called, are the most attractive alternatives to physical and chemical processes. The use of biosorbents for the removal of toxic pollutants or for the recovery of valuable resources from aqueous waste waters is one of the most recent developments in environmental or bioresource technology. Biosorption of dyes has become a popular environmentally driven research topic, and is one of the most sought after processes in the modern day where bioremediation is key in preserving the environment for future generations. Bohumil Volesky, a pioneer in the field, defined 'biosorption' as the property of certain biomolecules (or types of biomass) to bind and concentrate selected ions or other molecules from aqueous solutions. Biosorption by dead biomass (or by some molecules and/or their active groups) is passive and occurs primarily due to the 'affinity' between the biosorbent and adsorbate.
Types of Biomass or Biomaterials: Pollutants like metals and dyes can be removed by adsorption by living microorganisms, but can also be removed by dead biomass. Studies on practicality in the field for large-scale applications have demonstrated that biosorptive processes using dead biomass is much more viable option than the processes that use living biomass, since the latter require a nutrient supply and complicated bioreactor systems. Plus the use of dead biomass eliminates the maintenance of a healthy microbial population, and the other environmental factors like temperature and pH of the solution being treated. Dye recovery is also limited in living cells since these may be bound intracellularly. Therefore keeping these factors in mind, attention has been focused on the use of dead biomass as biosorbents. As mentioned above, dead biomass has advantages over living microorganisms. A hybrid process can also be employed which uses both dead and living biomass so as to increase the efficiency of biosorption. However, we have chosen to focus on single biosorption processes in this review and to avoid discussion of hybrid processes combined with biosorption. The first major challenge faced is to select the most promising types of biomass from an extremely large pool of readily available and inexpensive biomaterials. To streamline this when choosing biomass, for on field or industrial uses, the main factor to be taken into account is its availability and cheapness. Therefore keeping these factors in mind, native biomass can come from (i) industrial wastes free of charge; (ii) organisms easily obtainable in large amounts in nature; and (iii) organisms that can be grown quickly and which can be cultivated easily. A broad range of biomass types have been tested for their biosorptive capacities under various conditions at this point in time, but there are no limits to exploration of new biomass types having low cost and high efficiency. Biosorptive capacities of various biomass types have been quantitatively compared in many review papers. Biosorbents primarily fall into the following categories: bacteria, fungi, algae, industrial wastes, agricultural wastes, natural residues, and other biomaterials. Quantitative comparison of the hundreds of biosorbents reported thus far is not possible therefore data from various papers that have done these types of comparisons of biosorptive capacities of various biosorbents for various pollutants were used. It should be noted that the biosorptive capacity of a certain type of biosorbent depends on its pretreatment methods, as well as, on experimental conditions like pH and temperature. When comparing biosorptive capacities of biosorbents we consider it for a target pollutant, therefore, the experimental data should be carefully considered in light of these factors. After choosing a form of cheap and abundant biomass, the biosorbent capability for removing a target pollutant can be derived through simple chemical and/or physical method(s). New biosorbents can be manipulated for better efficiency and for multiple reuses to increase their economic attractiveness, compared with conventional adsorbents like ion-exchange resins or activated carbons.
Gram-positive bacteria (Bacillussp. Corynebacteriumsp.,etc) gram-negative bacteria(Es-cherichia sp., Pseudomonas sp) cyanobacteria.
Micro-algae (Clorella sp., Chlamydomonas sp., etc) macro-algae (green seaweed (Enteromorpha sp.) brown seaweed (Sargassum sp.)and red seaweed )
Fermentation wastes, food/beverage wastes, activated
sludges, anaerobic sludges, etc.
Molds (Aspergillus sp., Rhizopus sp. Etc.) mushrooms (Agaricus sp., Trichaptum sp. Etc.)And Yeast.
Fruit/vegetable wastes, rice straws, wheat bran,
soybean hulls, etc.
Plant residues, sawdust, tree barks, weeds, etc.
Chitosan-driven materials, cellulose-driven materials,etc.
Table 1: Different type of biosorbents.
Mechanisms of Pollutants Removal by Biosorbents: There are many types of biosorbents derived from bacteria, fungi, yeasts, and algae (Table 1). The complex structure of these implies that there are many ways, by which these biosorbents remove various pollutants, but these are yet to be fully understood. Thus, there are many chemical/functional groups that can attract and sequester pollutants, depending on the choice of biosorbent. These can consist of amide, amine, carbonyl, carboxyl, hydroxyl, imine, imidazole, sulfonate, sulfhydryl, thioether, phenolic, phosphate, and phosphodiester groups. However, the presence of some functional groups does not guarantee successful biosorption of pollutants, as steric, conformational, or other barriers may also be present. The importance of any given group for biosorption of a certain pollutant by a certain biomass depends on various factors, including the number of reactive sites in the biosorbent, accessibility of the sites, chemical state of the sites (i.e. availability), and affinity between the sites and the particular pollutant of interest (i.e. binding strength). The understanding of the mechanisms by which biosorbents remove pollutants is very important for the development of biosorption processes for the concentration, removal, and recovery of the pollutants from aqueous solutions, also on the basis of these mechanisms modifications can be made on the biomass so as to increase the adsorption-desorption capacity of it. When the chemical or physiological reactions occurring during biosorption are known, the rate, quantity, and specificity of the pollutant uptake can be manipulated through the specification and control of process parameters. Biosorption of metals or dyes occurs mainly through interactions such as ion exchange, complexation, and adsorption by physical forces, precipitation and entrapment in inner spaces.
Schematic diagram for processing different Biosorption mechanisms
types of native biomass into biosorbents.
Recovery and Regeneration: One of the important reasons why biosorption is favoured over conventional processes is due to the recovery of pollutant from the biosorbent and simultaneous regeneration of the biosorbent for reuse which makes it economically viable for industries. In fact, the usefulness of a specific biomass as a biosorbent depends not only on its biosorptive capacity, but also on the ease of its regeneration and reuse. However, most researchers have tended to focus only on the biosorptive capacity of biosorbent tested, without consideration of the regeneration required for industrial applications. The adsorbate bound onto the surface of a biosorbent through metabolism-independent biosorption may be easily desorbed by simple non-destructive physical/chemical methods using chemical eluants, but intracellularly bound adsorbate through metabolism-dependent bioaccumulation can be only released by destructive methods like incineration or dissolution into strong acids or alkalis. If cheap biomass is used as a biosorbent for recovering a certain pollutant, then destructive recovery would be economically feasible. However, most attention to date has focused on non-destructive desorption from the loaded biosorbent. For this reason, the choice between living or dead biomass systems is important because of the implication for recovery. In many cases, dilute mineral acids or alkalis allow efficient desorption from the biosorbent, but they also cause serious structural damage to the biosorbent itself, resulting in a drop in the biosorptive capacity of the biosorbent following regeneration. Organic solvents such as ethanol can be also used for desorbing organic pollutants such as dyes from the biosorbent. Sometimes heating or microwaving can aid desorption with an eluant or mixture solution. As well, as previously mentioned, the solution pH will have a strong influence on biosorption of a target pollutant; thus, simple manipulation of the pH of the desorbing solution should theoretically be a good method for regeneration of the biosorbent and recovery of the pollutant.
How is the textile effluents treated today?
It is not easy to treat the effluents by the conventional biological and physico-chemical processes, e.g. light, heat, wash and oxidizing agents, used in regular treatment plants. That is because of the complexicity of the dyes aromatic molecular structures. Adsorption is the most helpful physical process in the treating these dye waste waters. Today activated carbon is normally used for adsorption in many treatment plants. But the producing costs for activated carbon is very high, there is a need of an alternative material that is more cost capable. A low costs adsorbent is defined as one which is rich in nature or one that is produces as a byproduct in another industry. There have been studies on lots of different natural materials as adsorbents in treating textile effluents, for example saw dust and agricultural wastes like wheat straw and corn cob. Now biosorption is investigated as a method to absorb the effluents and different organisms treating different kinds of dyes are tested.
Synthetic dyes are widely used in textile industries. As a result, about 10-20% of the dyes are lost during the built-up and dyeing process, producing large amounts of dye-containing wastewater. Mostly dyes used are azo, anthraquinone and triphenylmethane dyes, classes is based on its chromophore .The white rot fungi are known to be very efficient for azo dye decolorization as various Aspergillus species, have been reported to decolorize various dyes.
The dye solution will be treated with inactivated Aspergillus niger. A. niger is a fungi which has already been used industrially in producing citric acid. Citric acid used to be produced by extraction from lemons and other citrus fruits, but today microbial fermentation is a broadly spread technique and nearly all citric acid is produced this way. In these fermentation industries A. niger also comes out as a waste product which makes it suitable for investigations of the biosorption ability. A. niger is a dark colored fungi (see Figure a and b) that could be seen at moldering food and is then called black mold. It is mostly fruits and vegetables that are affected by the mold, for example grape fruits, onions and peanuts. One should not forget when dealing with the fungi that it could cause fungus diseases on both humans and animals. Aspergillus niger is a common saprophytic fungus in terrestrial environments. If the cells of the fungi are active they are easily affected by toxic compounds and chemicals in the waste water and they may then pollute the environment by releasing toxins or propagules.
Figure a: Aspergillus niger growing Figure b: Onion with black mold
on Czapek dox agar in a Petri dish.
On the whole a large many number of dyes have been used by different researchers but it is not possible to present the data for all the dyes which were tested hence in this review we have concentrated on a few dyes which are most commonly used by the textile industries.
Direct Blue 199
Acid Blue 29
Basic Blue 9
Dispersed red 1
Table 2: Different types of dyes.
Culture Conditions and Microorganism: Aspergillus niger pellets were used to obtain the paramorphic forms of A. oryzae. Pure culture was maintained on nutrient beef agar medium at 4°C or were grown in potato-dextrose broth at pH 5.6, 29 ± 1 C on the shaker . After seven days, when sporulation occurred, the biomass was autoclaved at 121 C, 103.42 kPa for 45 min in order to kill the fungal biomass (figure c). The biomass was separated by filtering the growth medium through Whatman No. 1 paper after washing the fungal biomasses it will dried at 80 C for 20 h. The quantification of fungal biomass was carried out using a linear calibration between volumes of fungal pelletized culture and its respective dry weight. The concentration found may have suffered minor modifications, consequently to the procedures made during its paramorphogenesis.
Figure c: Biosorbent powdered
Experiments were conducted 30 ml of the dye solution at an orbital shaking of 120 cycles/min. The temperature and pH conditions were varied for the different experiments The estimative biomass (autoclaved )for total removal of the dyes were calculated at three different pH values (2.50; 4.50, and 6.50) After the selection of the better pH (2.50), the dye solutions were equipped with the same dye concentration. Therefore, the solutions were inoculated with A. niger pellets (mg mL−1) getting through different biomass concentration. Samples were withdrawn at specified interval of time to monitor dye adsorption by UV-VIS (Scanning was performed between 300 and 800 nm) spectrophotometer at the absorbance maximum of the respective dye.