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Environ Eng Res > Volume 20(1); 2015 > Article
Gupta, Nayak, and Agarwal: Bioadsorbents for remediation of heavy metals: Current status and their future prospects

Abstract

The biosorption process has been established as characteristics of dead biomasses of both cellulosic and microbial origin to bind metal ion pollutants from aqueous suspension. The high effectiveness of this process even at low metal concentration, similarity to ion exchange treatment process, but cheaper and greener alternative to conventional techniques have resulted in a mature biosorption technology. Yet its adoption to large scale industrial wastewaters treatment has still been a distant reality. The purpose of this review is to make in-depth analyses of the various aspects of the biosorption technology, staring from the various biosorbents used till date and the various factors affecting the process. The design of better biosorbents for improving their physico-chemical features as well as enhancing their biosorption characteristics has been discussed. Better economic value of the biosorption technology is related to the repeated reuse of the biosorbent with minimum loss of efficiency. In this context desorption of the metal pollutants as well as regeneration of the biosorbent has been discussed in detail. Various inhibitions including the multi mechanistic role of the biosorption technology has been identified which have played a contributory role to its non-commercialization.

1. Introduction

Technological advances by humans have resulted in the contamination of water bodies resulting in the presence of toxic pollutants at concentrations well above the limits set by World Health Organization (WHO) and the Environmental protection agency (EPA) [1, 2]. The hazards imposed to humans and aquatic life associated with exposure to metals like chromium, lead, mercury, cadmium and arsenic have been well established in the literature [38]. The toxicity of such metal ions arise due to their non-biodegradable nature thereby accumulating in the living cells and impairing the normal functions of various organs of living beings. Technologies like chemical precipitation, electrochemical separation, membrane separation, reverse osmosis, ion exchange and adsorption resins though effective for metal remediation, yet are not competitive in industrial application [922]. Such methods involve either large capital or operational costs, and are not effective in removing metal ions present in ppm levels [12, 18]. Chemical precipitation involves large generation of harmful sludge which requires further treatment thereby increasing the overall cost. Such disadvantages along with the requirement of more effective and economical methods for treatment of metal laden wastewater have resulted in the development of biosorption as an alternative technology. Biosorption is the property exhibited by inactive, non-living substances of biological origin to bind and to accumulate metal ions from aqueous solution [23, 24]. In other words the interaction between the biomass and the metal ions is physico-chemical, metabolism independent process with the underlying mechanism being absorption, adsorption, ion exchange, surface complexation and precipitation [2426]. Bioaccumulation is another technology using living biomass for heavy metal removal. It is an active process depending on the metabolism of the living organism [27, 28]. But the toxicity of metal ions hinders metal uptake by living organisms and hence bioaccumulation has lower kinetics and lesser efficiency as compared to biosorption. The biosorption technique has enjoyed immense success in comparison to conventional techniques in sequestering metal ions from ppm to ppb levels in aqueous suspension [2933]. Also, its metal removal efficiency has been reported to be very high even in very dilute complex solutions having diverse types of pollutants. Its faster kinetics of metal removal has made it beneficial for treating large volume of wastewater. It has been found to be operative over a wide range of temperature, pH conditions along with the presence of other ions. Such advantages along with the lower cost of the biosorbents, potential for regeneration of the biosorbents, metal recovery and minimum generation of toxic sludge have made the biosorption process more competitive for industrial application. Compared to ion exchange and adsorption resins, the use of dead biomass as biosorbent is more environmental friendly because of its easy disposal [31]. The immense potential of this technology has thus led to its evolution from an alternative approach to a powerful technology for the treatment of industrial wastewater for metal removal and recovery [32, 33]. The metal uptake by biosorption is a surface phenomenon and is a function of the surface properties of the biosorbent. Since the ultimate efficiency of the biosorption process hinges on the performance of the biosorbent material, a critical study is required on the different materials of biological origin that have been successfully tested as biosorbents for metal ions. Table 1 and 2 summarizes the findings of an in-depth literature study carried out focusing on not only the different biosorbents used thus far but also on their capacities for metal uptake. Elaborate and detailed comparison of the biosorptive performance of various biomasses has been made by various researchers and scientists [150170]. Researchers have summarized the role of various biomasses in heavy metal removal from aqueous solutions as can be evident from the review works. Vijayaraghavan and Yun [150] demonstrated the biosorption potential of bacterial biosorbents. Bishnoi and Garima [151], Sag [152] and Wang and Chen [153] made extensive studies on fungal biosorbents. McHale AP and McHale S [154] and later Gupta and Mohapatra [155] concluded that microbial biomass were an economical alternative for removal of heavy metals from waste water. The progress and prospects of using various algal species as metal biosorbents were assessed by Wilde and Benemann [156], Mehta and Gaur [157] and Romera et al. [158]. Among the plant based biomass, sawdust of various tree species has been established as an efficient biosorbents by Shukla et al. [159]. Crini [160], Gerente et al. [161] and Suhas and Carrott [162] demonstrated chitosan, polysaccharide and lignin based materials as economical biosorbents. Individual biosorption performances of rice husk and wheat straw/bran were extensively reviewed by Foo and Hameed [163] and Farooq et al. [164] for the removal of metal ions from waters. Various agricultural waste biomasses were compared on the basis of their biosorption performance for the removal of heavy metals as evident from various reviews [165168]. Ngah and Hanafiah [169] reviewed studies on chemically treated plant waste based biosorbents and concluded that the treated biosorbents were better in the removal of metal ions like Cd, Cu, Pb, Zn and Ni from various aqueous streams. Similar conclusion was drawn on modified biosorbents by O’Connell et al. [170]. From such reviews, it is clearly evident that the biosorbents studied till data fall under two major classes; the plant based biomass and microbial based biomass. Secondly the discussion in such reviews is restricted to a particular type of biomass for metal ion remediation from aqueous phase. Infact few researchers have attempted to compile and compare data related to various biomasses a broad range of biomass [171173]. Overall it can be seen that the biosorption performance of a particular biosorbent for specific metal ions has been found to vary widely as can be evident from the compiled literature data as presented in the Tables. This may be because of different process parameters selected by the researchers in their search for optimum biosorbent. Some used biomass in their native form and others used their modified forms so as to improve their metal uptake performance. In spite of the limitations of comparative analyses, overall a major conclusion can be made that microorganisms like micro algae, macro algae, bacteria, fungi, yeast, agricultural wastes and certain industrial wastes have demonstrated good biosorption properties for metal ions.
The overall objective of the study is to make a critical in-depth study on each such class of biosorbents so as to evaluate the various factors affecting the biosorption process along with their underlying mechanism. The findings and analyses are henceforth presented in the present work in the following sections. Current research work on the biosorption is not only summarized but also future directions are suggested. The future prospects pertaining to the establishment of the biosorption technology as a solid foundation for heavy metal remediation from industrial waste streams are discussed.

2. Biosorbent Materials

Economic consideration and efficiency of the biosorption technology hinges on the effectiveness of the biosorbents, the easy availability of their precursors and cost effectiveness of the entire process. The easy availability of the biological sources viz. micro-organisms like algae, fungi, weeds, bacteria, yeasts agricultural waste products along with their cost effective processing to biosorbents have resulted in the establishment of a mature and wide spread prevalence of the biosorption technology. The biosorbents have been categorized under microorganism like bacteria, fungi, yeast, algae, agricultural by-products like rice husk, bran of rice, wheat, sugarcane bagasse, fruit wastes, weeds etc. and other polysaccharide materials. The biosorbents, irrespective of their source have demonstrated good metal removal efficiencies. The microbial biosorbents have been either cultured or developed in the laboratory or have been procured from various food processing or pharmacological industries. Potential bacterial biosorbents showing good metal removal capacities have been identified as gram positive bacteria (Bacillus, Corynebacterium, Streptomyces, Staphylococcus sp., etc.), gram negative bacteria (Pseudomonas, Enterobacter, Aeromonassp, etc.) and cyanobacterium (Anabaena sp., etc.) [105121]. Fungal biosorbents which include yeast (Penicillium, Saccharomyces), molds (Aspergillus, Rhizopus) and mushrooms have shown the lowest biosorption potential [122128]. Various species of red algae (Gelidium), blue-green algae (Nostoc, Spirulina sp., etc.), green (Ulva, Oedogonium sp., etc.) and brown algae (Cystoseira, Sargassum sp., etc.) [129149] have also been used as efficient biosorbents. Weeds (Parthenium, Spirodela, Fucusceranoides) [133, 134] are another class of biosorbents which have shown good metal removal properties. Such microbial species have been widely used as efficient biosorbents because of their widespread prevalence in nature and can be grown in large mass at minimal cost. Also various fungal and bacterial species are generated as wastes from different food/pharmaceutical industries. Agricultural wastes like rice/wheat husk, bran, fruit or vegetable, soybean hulls, saw dust of bark of various trees etc. have also shown good metal biosorption properties. The biosorption capacities of the biomass were determined from equilibrium studies by various researchers and such have been tabulated in Table 1, 2. Biosorbents as can be seen in the table show promising biosorption potential for metal removal and have different affinity to different heavy metals. Also some biosorbents can bind onto a wide range of heavy metals with no specific priority, whereas others are specific for certain types of metals [30, 172, 173]. Due to the enormous difference in the nature of the biosorbents used and the differences in the experimental conditions, it is difficult to make a comparison on the efficiency of the biosorbents for the removal of metal ions. Irrespective of such differences, statistics reveals an increased biosorption capacity for the biomass from microbial origin. Data presented in Table 1, 2 shows that the biomass derived from bacteria showed an average metal ion adsorption capacity of 132.2 mg/g ranging from 12.23 to 567.7 mg/g. Biosorption capacity greater than 100 mg/g is demonstrated by the gram positive bacterial species of Bacillus, Corynebacterium, Streptomyces and Staphylococcus sp. [105, 107, 109, 110, 116]. Fungal biomass showed a very low average biosorption capacity of 42.99 mg/g ranging from 6.34 to 204 mg/g. Algal biomass on the other hand had an average metal removal capacity of 113.5mg/g ranging from 10.60 to 357 mg/g. Irrespective of the metal ions, among all species of algal biomass, the brown algae for example Cystoseira baccata, Sargassum sp. have exhibited the highest biosorption capacity [130, 134, 138142, 148]. Cellulosic material showed 69.38 mg/g average metal ion adsorption capacity with a range from 2.18 to 2,000 mg/g. Irrespective of the metal ions, the incidences of biosorbents from microbial and cellulosic origin reporting a biosorption capacity greater than 100 mg/g are 25 (32%) and 15 (12.9%) respectively. But the incidences of the same in the range less than 100 mg/g are 53 (67%) and 116 (87%) Thus biosorbents like the algae and bacteria showed very high adsorption capacity thereby could be used in industrial applications. In fact to qualify a biosorbent for industrial application requires it to have not only high adsorption capacity, but also should have characteristics like wide spread availability, economical viability and capacity to be regenerated [26, 172]. But the fungal biosorbents and certain agricultural residues showed very less biosorption capacity. Various researchers have carried out modification of the biosorbents either by physical or chemical methods with a view to improve its metal removal capacity.
A critical analysis of these two major classes of biomass as biosorbent for metal ions reveals that their chemical and physical characteristics have a contributory role.

2.1. Cellulosic Materials as Metal Biosorbents

Agricultural and plant based by-products have showed good biosorption potential for heavy metal ions like Cd(II), Cu(II), Cr(III), Cr(VI), Pb(II), Hg(II), Zn(II), etc. as is evident from the biosorption capacities in Table 1. Various researchers have demonstrated good biosorption potential in biomass like rice bran, rice husk, wheat bran and husk, saw dust, bark, groundnut shells, coconut shells, hazelnut shells, walnut shells, cotton seed hulls, waste tea leaves, maize corn cob, apple, banana, orange peels, soybean hulls, grapes stalks, water hyacinth, sugar beet pulp, sunflower stalks, coffee beans, cotton stalks, etc. Irrespective of the metal ions, rice and wheat based biomass have demonstrated high biosorption capacity as is seen in Table 1. Research studies have demonstrated high carbon, low ash content and reasonable hardness in such biosorbents. Researchers have also identified the presence of different components and features in such biomass which have been contributory to the uptake of heavy metal ions from aqueous solutions [27, 30, 31]. Cellulose, hemicelluloses and lignin were found to be the major components in rice and wheat based products but the proportion of such components was found to vary in each [163, 164, 165]. Rice based biomass showed a proportion of 32.24% cellulose, 21.34% hemicelluloses, 21.44% lignin whereas wheat based biomass revealed 39% cellulose, 35% hemicelluloses and 14% lignin [173175]. Basso et al and Qaiser et al have proved that cellulose has good metal uptake properties [176, 177]. The presence of cellulose and hemicelluloses in agricultural and plant biomass hasthus improved their biosorption potential. Other agricultural precursors like tea, coffee, shells, nuts and seeds of various fruits, etc. were found to have cellulose, hemicelluloses and lignin [177, 178]. Researchers have made elaborate studies via techniques like Fourier transform infrared (FT-IR) and Raman spectroscopy, electron dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), electron microscopy (scanning and/or transmission), nuclear magnetic resonance (NMR), X-ray diffraction analysis (XRD) on various biosorbents like moringaoleifera bark [95], rhizophora apiculata tannin [83], rice husk ash [90], raw coffee powder [55], hazel nut shell [97], peanut hull pellet [78], mango peel [50], etc. in order to elucidate the mechanism of metal binding onto such biosorbents. Different functional groups like carbonyl (ketone), carboxyl, sulfhydryl (thiol), sulfonate, thioether, amine, alcohols, esters, etc. were identified. Such functional groups were able to bind with metal ions through replacement of hydrogen ions with metal ions in solution or by donation of an electron pair from these groups to form complexes with metal ions in solution. The mechanism identified were thus chemisorption, complexation, ion exchange, chelation, physical adsorption [177, 178]. Sawdust of various tree species in its virgin form revealed a cellulose–lignin polymeric structure as illustrated by Shukla et al. [159]. The analysis of surface properties showed that the metal binding sites were mainly composed of phenolic and alcohol hydroxyl.

2.2. Microbial Materials as Metal Biosorbents

The efficiency and the mechanism of the uptake of metal ions onto the microbes depend on the cellular surface of the microbes [23]. This is because the biosorption is a surface phenomenon. The mechanism of metal binding as documented by various reviews involves the interaction and exchange of metal ions followed by complex formation with the metal ions on the reactive chemical sites on the surface of the microbial cell followed by ion [2931, 171, 172]. This is finally followed by precipitation of excess metal ions on the cell surface. The components of the cell wall are known to vary among the different microbes. Keeping in view the role of microbial cell wall in the biosorption of metal ions, a critical analysis on the cell wall components of different microorganisms would help in assessing and explaining the different metal uptake capacity as observed in different microbial community [172]. All species of bacteria have a cell wall composed of a linear polymer called the peptidoglycan. It accounts for 40–90% of the bacterial cell wall [150]. The core of the peptidoglycan is multilayered containing a peptide cross bridge while the adjacent glycan units are cross linked via amino acids like D-glutamic acid, D-alanine, and meso-di-aminopimelic acid. Cross-bridging between the peptide chains is a common feature of the bacterial cell wall. The frequency of cross bridging is close to 100% as in Staphylococcus aureus whereas it is 30% in a gram negative bacteria like E-Coli. The cross bridging is nearly absent in many gram negative bacteria. The peptidoglycan in gram positive bacteria also contains large amounts of teichoic acids, polymers of glycerol or ribitol joined by phosphate groups. Whereas the peptidoglycan cell wall in gram negative bacteria is composed of phospholipids, lipopolysaccharides, enzymes, glycoproteins and lipoproteins. Such components of the bacterial cell wall were found to be actively involved as metal binding sites [179]. The glycoproteins and lipoproteins were found to play a contributory role in Cd2+ ion uptake of both gram negative and gram positive bacteria [116]. Some of the other potential metal binding sites on the cell wall were found to be lipoproteins, techoic acid, teichouronic acid, peptidoglycan, amino, carboxylate, phosphoryl groups of phospholipids, etc. Extensive studies as reviewed [150] have demonstrated the role of such groups on the bacterial cell in the binding sites of metal cations [150]. Fungal cell wall is rigid and is composed of approximately 80–90% chitin which is a nitrogen containing polysaccharide [151, 152]. Proteins, lipids, polyphosphates and inorganic ions are also constituents of the fungal cell wall [180]. Cellulose is the principal component of all classes of algae [156, 157, 179]. Sulphated polysaccharides are present in the cell wall of both brown and red algae but is absent in green algae. A higher proportion of proteins are bonded to the polysaccharides in the cell wall of green algae. The cell wall of brown algae contains alginic acid and the corresponding salts of sodium, potassium, magnesium and calcium [179]. Thus the potential metal binding sites on the bacterial cell wall are the peptidoglycan, teichoic acids and lipoteichoic acids. Whereas chitin, proteins and phosphates are the principal active sites on the fungal cell wall, cellulose, alginate and glycan in algal cell wall proved to play a very important role in metal binding [179].

3. Biosorption Mechanism

Due to the complex nature of the cell wall of the microbial biomass, the mechanism of the biosorption process is not well understood. The process can take place via many mechanisms depending on the speciation of the metal ion, the source of biomass and its processing to biosorbent [2931, 171, 172]. Various reviews have highlighted that metal binding to the microbial cell wall follows complex mechanisms like the ion exchange, chelation and adsorption. This can be followed by the deposition of metal ion in the inter- and intra-fibrillar capillaries and spaces of the structural polysaccharide or peptidoglycan network as a result of the concentration gradient and diffusion through cell walls and membranes. With the help of sophisticated techniques like the Fourier transform infrared (FT-IR) and the Raman spectroscopy, nuclear magnetic resonance spectroscopy (NMR), electron spin resonance (ESR) spectroscopy, and X-ray absorption spectroscopy (XAS), which includes X-ray absorption near-edge (XANES) and extended X-ray fine structure (EXAFS) spectroscopy, and X-ray reflectivity, researchers have identified the key functional groups present in the biomass cell surface in the biosorption process [50, 51]. These are the hydroxyl groups of polysaccharides, sulphated polysaccharides, uronic acids and the amino acids, the ketone group in peptide bonds, carboxyl groups in uronic acid and amino acid, sulphydryl (thiol) group in sulphated polysaccharides, thio-ethers and amines in amino acids, imine and immidazole in amino acids, phosphonate in phospholipids, and phosphordiester in techoic acid and lipopolysaccharides [30, 179181]. A review of the acidity constants of such functional groups reveals that the biomass surface charge is predominantly negative at pH 3–10. This has been verified by Volesky and Holan [30]. Over this entire pH range, the ligands of each such group identified as the O, S, N and P were known to easily bind with the cationic metals ions facilitating electrostatic interactions. The biosorbent behavior for metallic ions is thus a function of the chemical make-up of the microbial cells of which it consists as verified by the works of Volesky and Holan [30]. But the presence of favourable functional groups on the microbial cell wall does not guarantee the binding of metal ions because stearic, conformational and other external factors could be operative. The pH of the aqueous medium is known to affect the biosorption process and the operative underlying mechanisms; thereby affecting the capacity of the biomass for metal uptake [158]. The metal adsorbates undergo ionization as well as there is a change in solubility as a result of the pH of the aqueous medium. The functional groups on the biomass surface too undergo protonation and deprotonation as a result of the increased acidity or basicity of the aqueous medium. Thus at higher solution pH, the solubility of metal ions decreases leading to precipitation and complication of the biosorption process. Also, as a result of the protonation of the functional groups on the biomass surface, more protons are released resulting in lowering of pH in aqueous medium. Excess protons too results in increased competition with the metal cations and subsequent lesser biosorption. It is thus essential to maintain a constant pH during the biosorption process as maximum and faster uptake takes place in the initial phases. An exhaustive review of literature reveals that irrespective of the biomass type and source, the pH at which maximum biosorption occurs depends on the speciation of metal ion at that pH. Ni(II), Cd(II), Pb(II) and Zn(II) are known to exist in their bivalent form at pH values of ≤ 6.5, 8, 7 and 8 respectively. Perusal of literature revealed that maximum biosorption of such metal ions occurred for Ni(II), Cd(II), Pb(II) and Zn(II) at pH values of 5–7, 6–7, 5–7 and 4–8 respectively [182]. Optimum pH for Cr(VI) occurred at pH 1–5, due to the strong electrostatic interaction with the HCrO4 species.

4. Design of Better Biosorbents

The biosorption process is typically a surface phenomenon in which the cell wall components of the biomass have a direct involvement, as discussed earlier. The use of biosorbents in their native form has demonstrated various shortfalls on account of their low biosorption capacity as well as variable physical stability [169, 170, 183]. Various researchers have focused on modifying the biomass surface via chemicals so as to achieve both structural durability as well as efficient biosorption capacity for heavy metal ions. Results with respect to the maximum biosorption capacity have been promising not only with the plant based biomass but also with microbial biomass. This is revealed from data presented in Table 1 [3438, 49, 50, 54, 58, 59, 63, 74, 75, 87, 88, 92, 93, 98] and Table 2 [131, 143, 144, 148, 149]. Analyses and reviews have revealed that surface modification of biomass have brought about significant changes in their hydrophobicity, water sorbency, ion exchange capability, resistance to microbiological attack and thermal resistance. Methods employed for modification of cell surface are the physical pre-treatments which include heating/boiling, freezing/thawing, drying, autoclaving and lyophilization. The chemical treatments used for surface modification by various workers have been identified as washing with detergents, alkaline solutions (sodium hydroxide, calcium hydroxide, sodium carbonate) mineral and organic acid solutions (hydrochloric acid, nitric acid, sulphuric acid, tartaric acid, citric acid, thioglycollic acid), organic compounds (ethylenediamine, formaldehyde, epichlorohydrin, methanol), oxidizing agent (hydrogen peroxide), etc. Such pre-treatments helped in modifying the surface functional groups either by removing or masking the groups or by exposing more metal binding sites. Amination of hydroxyl and carboxyl group, carboxylation and phosphorylation of hydroxyl group, carboxylation of amine group, saponification of ester group, sulfonation, xanthanation, thiolation, halogenation, oxidation, etc. have resulted in enhancement of biosorptive efficiency. This has been corroborated by earlier works of Vieira and Volesky [184]. Various biomasses irrespective of their source like pine bark, Spirogyra, rose petals, rubber leaves, walnut shell and sawdust showed promising biosorption capacities with alkaline pre-treatment. [185190]. Calcium oxide pre-treatment provided strong basic sites on the surface of date pit (Phoenix dactylifera) causing higher biosorption for positively charged Cu(II) and Ni(II) from aqueous suspension [191]. Another factor responsible for higher biosorption capacity of the pretreated biosorbents was its greater mesoporous surface area of 645.5 cm3/g. KOH proved to be the most promising alkaline activating agent in producing efficient biosorbents for Ni(II) and Zn(II) from bamboo as investigated by Lalhruaitluang et al. [192]. KOH pre-treatment have resulted in the incorporation of favourable functional groups but also have helped in the increase in micropore volume and increase in surface area. In yet another example, alkali modified rice husk showed faster kinetics as well as higher bio-sorption for Cd(II) than the virgin rice husk [35]. The higher bio-sorption of modified rice husk (125.94 mg/g) was attributed to the surface structural changes of the biosorbent. Acid treatment too has resulted in the better biosorption behaviors of the biosorbents. For example, rice husk was modified by various organic acids like citric acid, salicylic acid, tartaric acid, oxalic acid, mandelic acid, maleic and nitrilotriacetic acid and were tested for biosorption of lead and copper. Tartaric acid modified rice husk demonstrated highest binding capacity but when the same biosorbent was esterified, it resulted in lesser biosorption for the same metal ions. The maximum adsorption capacities for Pb(II) and Cu(II) were reported to be 108.0 and 29.0 mg/g, respectively [74]. But some researchers have observed a change in surface texture due to acid pre-treatment which in turn has helped in improving the biosorption performance. H2SO4 treated wheat bran [72] showed better biosorption for Cu(II) (51.5 mg/g at pH 5 and contact time 30 mins) due to increase in its surface area. It was postulated that there was a significant conversion of macropores to micropores resulting in higher surface area as a result of the acid treatment. In yet another example, acid pre-treatment resulted in oxidation of existing functional groups on corncob based biosorbent resulting in decrease of surface area and surface volume [193]. Improved biosorption performance due to acid pre-treatment is also observed due to incorporation of acidic functional groups [189, 194]. Irrespective of the behavior to metal ions, various biomasses have shown better biosorption tendency as a result of chemical modification. Sha et al. [49] demonstrated that under identical conditions of pH and contact time, chemically modified orange peel was a better biosorbent showing a capacity of 70.67 mg/g and 136.05 mg/g for Cu(II) and Cd(II) removals than its unmodified counterpart. Sawdust of Pinus sylvestris pre-treated with formaldehyde showed enhanced biosorption for the removal of Cd(II) and Pb(II) from aqueous solutions [195]. HCl treated sawdust of Mulberry showed a maximum biosorption capacity of 403.73 mg/g for Cd(II) ions at pH of 6 after a 30 min shaking [196]. Different complexing groups like the aminoalkyl [197], 2,2-diaminoethyl, and amidoxime [198], or an ionic moiety such asphosphate [199], thiolate [200], carboxy [201], and carboxymethyl [202, 203] were also used for modification of various biomass so as to enhance their biosorption for heavy metal ion. Observations of improved biosorption capacity are seen in the case of chemically treated algal species of Ulva onoi [131], U. lactuca [143], Oedogonium h. [144] and Sargassum [148]. Fungal species have shown better metal uptake capacity by alkali treatment whereas acid treatment of the same biomass almost had no influence on metal biosorption [183, 203]. Biomass of yeast treated with ethanol recorded highest biosorption for Cd(II) and Pb(II) as compared to that treated with NaOH [204]. Similarly, calcium chloride treated biomass of brown alga F. vesiculosus demonstrated highest biosorption for the removal of copper, cadmium, leadand nickel [205]. Thus it can be concluded that the functional groups present on the biomass have favoured the formation of hydrophilic and polar surface thereby facilitated the uptake or binding of the cationic metal ions.
Besides the direct chemical modification, literature reveals the grafting of various monomeric units like acrylic acid, acrylamide, acrylonitrile, ethylenediamine, hydroxylamine, glycidyl monomers, urea, etc. followed by functionalization of the biomass surface so as to develop a better biosorbent [206209]. Graft co-polymerization is a process in which an additional polymer is introduced into the backbone of the main polymeric chain. Literature reveals that the long polymeric chain of the biomass has been activated via high energy radiation, photochemical or by chemical so as to initiate the process of polymerization or grafting. Various studies have already been carried out in this direction. Details on the preparation of efficient grafted biosorbents and their effects on the metal binding capacity are outlined by O’Connell et al. [170]. In a typical example, Grey et al. [210] developed an efficient biosorbent via succinic anhydride grafted wood pulp which showed a biosorption capacity of 169 mg/g for Cd(II). Wood sawdust was grafted via acrylonitrile and hydroxylamine and the resulting biosorbents brought about a binding capacity of 246 mg/g for Cu(II) and 188 mg/g for Ni(II) respectively [211]. Acrylamide, ethylenediamine and succinic anhydride used as grafting agents helped in designing a better biosorbent from banana stalk by Shibi and Anirudhan [212] for removal of Hg(II) from waste streams. Grafting of acrylic acid was also done on the surface of ozone pretreated P. chrysogenum. The developed biosorbent showed significant increase in the binding of for copper and cadmium. It was postulated that the higher biosorption was due to the presence of a large number of carboxyl groups present on the biomass surface and such groups were converted tocarboxylate ions using NaOH [213].
Metal biosorption has also been enhanced by heat or chemical sterilization or by crushing. The use of such processes have helped in increasing the available surface area, thereby resulting in greater exposure of the biomass surface and more surface binding sites [214]. The use of microwave assisted chemical activation of various agricultural biomasses like orange peel, sunflower seed etc. [215] has resulted in development of refined porous properties and better surface chemistry [216]. Such biosorbents showed significantly higher biosorption for metal ions [217, 218].

5. Regeneration of the Biosorbent

The recycling and reuse of the biosorbent for subsequent removal of metal ions from aqueous medium contributes to the economic viability of the biosorption process. A desirable factor is that the desorbing medium used for regeneration of the biomass should not damage the biosorbent. Besides, the loaded metals onto the surface of the biomass after biosorption are recovered. Literature reveals that chemicals like acids like HCl, bases like NaOH and chelating agents like EDTA have been used for desorption of metal ions along with simultaneous regeneration of the loaded biosorbent (Table 3). HCl is widely used as a desorbing chemical bringing about greater than 90% recovery of metal ions, yet various researches has proved that it brings about simultaneous hydrolysis of the functional groups present on the biomass. This would result in loss of biosorption efficiency of the regenerated biosorbent [220]. A detailed observation of Table 3 also highlights the fact that metal ions like Cd(II), Pb(II), Cu(II) Hg(II) can be eluted via acidic medium thereby proving beyond doubt that a basic medium would enhance the biosorption potential of the biomass. This corroborates our earlier conclusion about the effect of pH on the biosorption potential. Cr(VI) and Ni(II), on the other hand could be desorbed using a basic medium as evident from the Table 3 thereby indicating the requirement of an acidic pH for maximizing biosorption. Table 3 thus highlights the fact that the desorbing medium required for maximum recovery of metals is dependent on the metal ions to be recovered but is independent of the biosorbent type and its source. From such observations, it can be concluded that the mechanism underlying the biosorption of metal ions is same irrespective of the nature, type and source of biosorbent. An ion exchange mechanism is operative during the metal uptake process as has been discussed in our earlier work [182]. More than 70% recovery of metal ions along with the use of biosorbent for a continuous sorption/desorption for removal of metal ions from waste streams as evident from Table 3 highlights the economy of the biosorption process. But although desorption via chemicals results in regeneration of the biosorbent along with recovery of the metal ions, yet it results in higher investment cost.

6. Future Prospects

Biosorption of metal ions has been extensively studied as evident from innumerable research papers. The performance of the several biosorbents were reviewed and summarized; the major features are high versatility for wide-range of operational conditions, high selectivity for metal and not influenced by alkaline earth and common light metals, independent of concentration (for ≤ 10 ppm or ≥ 100 ppm), high tolerance to organics, and effective regeneration. According to Wang and Chen [179], biosorption is a cost-effective technology for the treatment of complex industrial wastewater containing high volume and low-concentration heavy metals. Many natural biosorbents both from cellulosic based and microbial origin having efficient biosorption characteristics have been identified. But many of such biosorbents have shown poor performance. Surface modifications carried out on such biosorbents helped improve their metal binding properties as evident from vast studies carried out but the modifications increase the overall cost of the process bringing it closer to the price of commercial ion-exchange resins. Also, there are instances, when the incorporation of functional groups as a result of chemical modification does not result in enhanced biosorption which might be due to steric, conformational or other effects. In spite of such short comings, both native as well as modified biosorbents have demonstrated their compatibility when tested with real industrial effluents. Faster kinetics and better performance under both laboratory as well as real effluent conditions as compared to commercial ion exchange resins and adsorbents are the major advantages of the biosorbents for removal of heavy metals, but the application of such biosorbents for industrial scale has not yet become a reality. It is well known that the biosorption process does not have competition with any of the other metal removal technologies like ion exchange, reverse osmosis, precipitation, etc. [30, 221]. Also, a huge market exists for cheap and efficient biosorbents due to the continuous discharge of complex metal effluents from industrial activities and simultaneous ever increasing environmental regulations. Earlier works have indicated the development of some commercial biosorbents by immobilization techniques. For example, AlgaSORB™ was developed by immobilization of C. vulgaris in silica and poly-acryamide gels [222] AMT-BIOCLAIM™ similarly comprised of Bacillus subtilis immobilized onto polyethyleneimine and glutaraldehyde. The biosorbent Bio-fix was made up of a variety of biomass including Sphagnum peat moss, algae, yeast, bacteria, and/or aquatic flora and immobilized onto high density polysulphone. Similarly a series ofbiosorbents were produced based on different types of biomaterial, including the algae S. natans, A. nodosum, Halimeda opuntia, Palmyra pamata, Chondruscrispus and C. vulgaris. The preparation of such commercial biosorbents and their performance has been adequately reviewed by eminent researchers [31, 171, 222]. Such biosorbents showed effective metal removal over a wide pH range and solution conditions and could remove a wide range of metal ions. The biosorption process was not affected by the presence of calcium, magnesium and organics. Moreover the biosorbents resembled ion exchange resins. Very high biosorption capacity was demonstrated by such commercial biosorbents [222]. Although extensive efforts were made for the commercialization of the biosorption technology, yet it did not attract widespread industrial adoption. Certain factors inhibiting the widespread industrial application of the biosorption technology as identified by various researchers [30, 31, 150, 171, 179, 220, 222] working in this field include:
  1. difficulty in obtaining a reliable supply of inexpensive raw biomass

  2. Lower robustness of the biosorbents

  3. Non specificity and non selectivity of the biosorbents to the metal mixture solutions

  4. difficulty in regeneration and reuse of the biomass

  5. negative effects of co-existing ions on biosorptive capacity

Thus the following features need to be assessed prior to industrial application:
  1. Effluent characteristics :

    Biosorption of metal ions is strongly affected by the properties of the water to be treated, such as pH, ionic strength, coexisting ions, and suspended solids. An optimal pH of 7 is required for biosorption of metal cations. The biosorption decreases on lowering the pH, as acidic media tend to cause protonation of negatively charged sites on the biosorbents. pH of the aqueous media play a definite role in affecting the biosorption behavior to metal cations as the hydrogen ion itself is a tough competitor [223]. Industrial wastewater contains more than one type of metal ions and the competition between such metal ions for the limited binding sites on biosorbents tends to decrease the biosorption. The presence of Ca, Mg in hard water and Al, Fe in industrial effluents too may retard biosorption of the target heavy metal pollutants [220]. Also, the industrial waste water tends to have many organic moieties which may hinder the biosorption process. Thus, the performance of the biosorbents needs to be assessed in not only a single-metal solution system but alsomulti-metal and multi-pollutant solution systems prior to their industrial applications [30]. This evaluation would clarify at the same time the selectivity of the biosorbents in metal binding.
  2. Biosorbent characteristics (like availability, overall manufacturing cost, regenerability and reusability, pollutant specificity, biosorptive capacity and rate, mechanical stability, etc.) :

    The large scale availability of the biosorbent at one particular location, collection, development and finally transportation to the wastewater treatment site are some of the important criteria for economical viability and usability. Agricultural waste by-products like rice husk etc have shown good metal biosorption capacity. But their usage as biosorbent in industries would warrant not only their availability in tones per day but also their continuous supply. Food processing industries could be a possible alternative for production of agricultural waste by-products. Although microbial biomass like algae, fungi and bacterial strains showing modest biosorption capacity are naturally available but their large scale production is expensive. Continuous supply of filamentous fungi and yeast for industrial application as biosorbent can be made possible from large-scale fermentation industries. Irrespective of their origin, the biosorbents require some modifications before their application into the real wastewater treatment systems. Modification of the virgin biosorbent materials not only helps to impart mechanical strength and resistance to chemical and microbial degradation but also improves biosorption capacity and selectivity for target metal pollutants. Physical or chemical modifications, grafting techniques as well as immobilization techniques as demonstrated by various research groups have been successful in enhancing the physico-chemical characteristics of the biosorbents. But such techniques will obviously increase operational costs making it unrealistic for industrial applications. Regeneration and reuse of the biosorbents for further biosorption of metal ions will definitely make the process more cost effective on an industrial scale. Dilute acid and alkaline solutions, salt solutions, andchelating agents such as ethylene-diammine-tetra-acetic acid (EDTA) solution are reported to be effective in eluting metals from the metal-laden biosorbents.
Keeping in focus the inhibitions of the biosorption technology for its adoption in large scale wastewater treatment, the future prospects look promising on account of two considerations. Firstly, hybrid technologies comprising various processes like biosorption, bioreduction, bioprecipitation, electrochemical processes, membrane technology etc may be helpful for treating complex industrial wastewater in a large-scale and also for simultaneous removal of organic substances and heavy metal ions in solution. Novel biosorbents having increased robustness and increased specificity need to be developed. This could be achieved by immobilization technique and by optimization of process parameters. Various researchers have also suggested the development of a better biosorbent from various biomasses [220, 223]. More efforts should be made in the direction of reuse and recycling of the biosorbent.

7. Conclusions

Biosorption process is thus identified as basically an exchange of ions where the metal species in aqueous medium is exchanged for a counter ion attached to the biomass. The promising potential of the biosorption technology undoubtedly relies on the efficiency of the various microbial and plant based biomass. Various biomasses which are available in plenty and exhibiting good metal binding characteristics have been identified. A detailed study of literature has revealed the contributory role of active binding sites on the peptidoglycan and polysaccharides components of the cell wall of the biomasses. Besides, the higher metal uptake, the technology has numerous other advantages like faster kinetics, high metal binding over a broad range of pH, temperature and low capital and operation cost. Various publications have amply proposed it as a cost effective, green and effective technology which can be used complimentarily with other traditional metal removal technologies like chemical precipitation, reverse osmosis, membrane technologies etc. Despite the market requirement for cheaper and greener treatment technologies and despite the major advantages of the biosorption technology, it has extremely limited industrial adoption even when used in conjunction with other conventional treatment approaches. The presence of multi functional groups on the biomass surface is responsible for its non selectivity to a particular metal ion. This non selective, non specific nature of the biosorbent and lower robustness of the technology is a major hindrance to its commercialization. Although immobilization and granulation has helped in increasing the robustness and further problems related to separation, yet the non-specific nature of the biosorbent is not yet been addressed. Future research directions of the biosorption technology relies on the identification and designing of better and moreselective biosorbents, more development of biosorption modelsand identification of biosorption mechanisms, and further assessments of the costs of development.

Acknowledgement

Authors are thankful to the Department of Science and Technology, New Delhi, India, for the financial support under project grant DST/INT/South Africa/P-03/2014.

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Table 1
Biosorption Capacities of Various Biomass of Plant Origin for Removal of Toxic Metal Ions from Waste Water
Toxic metal ions Biomass source Biomass Adsorption capacity (mg/g) Reference
Cd(II) Rice
Cd(II) Rice waste Rice husk (phosphate treated) 2,000.00 [34]
Cd(II) Rice waste Rice husk (alkali treated) 125.94 [35]
Cd(II) Rice Husk Natural rice husk 73.96 [35]
Cd(II) Rice waste Rice husk (H3PO4 treated) 102.00 [36]
Cd(II) Rice waste NaOH treated rice husk (NRH) 20.24 [37]
Cd(II) Rice waste NaHCO3 treated rice husk (NCRH) 16.18 [37]
Cd(II) Rice Epichlorohydrin treated rice husk (ERH) 11.12 [37]
Cd(II) Rice waste Rice husk (sulfuric acid treatment) 40.92 [38]
Cd(II) Rice waste Rice husk ash 39.87 [39]
Cd(II) Wheat waste Wheat bran (ultrasonic treatment) 51.58 [40]
Cd(II) Wheat waste Wheat bran 22.78 [40]
Cd(II) Wheat waste Wheat straw 39.22 [41]
Cd(II) Wheat Wheat straw (urea treated) 4.25 [41]
Cd(II) Wheat waste Wheat straw 21.00 [42]
Cd(II) Wheat waste Wheat bran 21.00 [43]
Cd(II) Wheat waste Wheat bran 15.82 [43]
Cd(II) Wheat waste Wheat straw 14.56 [44]
Cd(II) Wheat waste Wheat straw 11.60 [45]
Cd(II) Coconut waste Puresorbe 285.70 [46]
Cd(II) Coconut waste Coir pith 93.40 [47]
Cd(II) Coconut Copra meal 4.99 [48]
Cd(II) Peels Orange peel (chem mod) 136.05 [49]
Cd(II) Peel Orange peel 47.60 [49]
Cd(II) Peel Mango peel 68.92 [50]
Cd(II) Peels Banana peel 35.52 [51]
Cd(II) Peel Banana peel 5.71 [52]
Cd(II) Peel Pomelo peel 21.83 [53]
Cd(II) Seeds Raw date pit 35.90 [54]
Cd(II) Coffee waste Raw coffee powder 15.65 [55]
Cd(II) Tea Tea waste 11.29 [56]
Cd(II) Bark Pinus roxburghii bark 3.01 [57]
Cr(VI) Wheat Wheat straw (chem mod) 322.58 [58]
Cr(VI) Wheat Wheat bran (chem mod) 93.00 [59]
Cr(VI) Wheat Wheat bran 310.58 [60]
Cr(VI) Wheat Wheat straw 21.34 [61]
Cr(VI) Peel Banana peel 131.56 [62]
Cr(VI) Seed Sapotaceae seed (chitosan+acid) 84.31 [54]
Cr(VI) Seed Sapotaceae seed (chitosan coated) 76.23 [54]
Cr(VI) Seed Sapotaceae seed (acid coated) 59.63 [54]
Cr(VI) Coconuts CSC (chitosan+HNO3) 10.88 [63]
Cr(VI) Coconut CSC (chitosan+H2SO4) 4.05 [63]
Cr(VI) Coconut CSCCC (chitosan) 3.65 [63]
Cr(VI) Fruit Bael fruit 17.27 [64]
Cr(VI) Husk Groundnut husk (Ag coated) 11.40 [65]
Cr(VI) Husk Groundnut husk 7.00 [65]
Cr(VI) Shell Almond shell 3.40 [66]
Cr(VI) Shells Hazelnut shell 8.28 [66]
Cr(VI) Shells Walnut shell 8.01 [66]
Cr(VI) Bark Pinus roxburghii bark 4.15 [67]
Cu(II) Wheat Wheat bran 8.34 [68]
Cu(II) Wheat Wheat bran 6.85 [69]
Cu(II) Wheat Wheat straw 11.43 [44]
Cu(II) Wheat Wheat bran 12.70 [70]
Cu(II) Wheat Wheat bran 17.42 [71]
Cu(II) Wheat Wheat bran (dehydrated) 51.50 [72]
Cu(II) Sago Sago husk ash 12.40 [73]
Cu(II) Rice Rice husk (acid treated) 29.00 [74]
Cu(II) Peel Potato peel (ZnCl2 treatment) 74.00 [75]
Cu(II) Peel Orange peel (chem mod) 70.67 [49]
Cu(II) Peel Orange peel 50.94 [49]
Cu(II) Peel Mango peel 46.09 [76]
Cu(II) Hull Peanut hull 21.25 [77]
Cu(II) Hull Peanut hull pellet 12.00 [78]
Cu(II) Hull Peanut hull 9.00 [78]
Cu(II) Seed Cicerarientinum 18.00 [79]
Cu(II) Shell Chestnut shell 12.56 [80]
Cu(II) Shell Chestnut shell (acid treated) 5.48 [81]
Cu(II) Bark Casuarina equisetifolia bark 16.58 [82]
Cu(II) Bark Rhizophoraapiculata tannin 8.78 [83]
Cu(II) Bark Pinus roxburghii bark 3.81 [67]
Cu(II) Tea Tea waste 8.64 [56]
Cu(II) Tea Tea waste 48.00 [84]
Co(II) Coconut Coir pith 12.82 [85]
Co(II) Peel Lemon peel 22.00 [86]
Hg(II) Rice Rice husk (sulphuric acid treatment) 384.62 [87]
Hg(II) Shell Walnut shell (ZnCl2 mod) 151.50 [88]
Hg(II) Wheat Wheat bran (chem mod) 70.00 [59]
Hg(II) Coconut Chem mod coir pith (PGCP-COOH) 13.73 [89]
Pb(II) Rice Rice husk (acid treated) 108.00 [74]
Pb(II) Rice Rice husk ash 91.74 [90]
Pb(II) Rice Rice husk ash 39.74 [73]
Pb(II) Wheat Wheat bran 87.00 [91]
Pb(II) Wheat Wheat bran (chem mod) 62.00 [59]
Pb(II) Coconut Coir pith waste 263.00 [92]
Pb(II) Tea Spent black tea 129.90 [93]
Pb(II) Tea Spent green tea 90.10 [93]
Pb(II) Tea Tea waste 65.00 [84]
Pb(II) Coffee Coffee (ZnCl2 mod) 63.00 [95]
Pb(II) Peel Mango peel 99.05 [50]
Pb(II) Peel Banana peel 2.18 [52]
Pb(II) Bark Moringa oleifera bark 34.60 [95]
Pb(II) Bark Rhizophoraapiculata tannin 31.32 [83]
Pb(II) Shell Shell carbon 30.00 [96]
Pb(II) Shel Hazelnut shell 28.18 [97]
Pb(II) Seed Cicerarientinum 20.00 [79]
Pb(II) Shell Chestnut shell (acid treated) 8.50 [81]
Pb(II) Shell Almond shell 8.08 [97]
Ni(II) Bark Pinus roxburghii bark 3.53 [67]
Ni(II) Bark Acacia leucocephala bark 294.10 [98]
Ni(II) Peel Orange peel 158.00 [99]
Ni(II) Peel Pomegranate peel 52.00 [100]
Ni(II) Peel Mango peel 39.75 [76]
Ni(II) Seed Guava seed (chem mod) 32.05 [101]
Ni(II) Seed Guava seed 18.05 [101]
Ni(II) Coconut Coir pith 15.95 [85]
Ni(II) Tea Tea waste 73.00 [102]
Zn(II) Shell Shell carbon (H3PO4+chitosan) 60.41 [103]
Zn(II) Shell Shell carbon (chitosan mod) 50.93 [103]
Zn(II) Shell Chestnut shell (acid treated) 2.41 [81]
Zn(II) Seed Cicerarientinum 20.00 [79]
Zn(II) Rice Rice husk ash 39.17 [73]
Zn(II) Rice Rice husk (sulphuric acid treatment) 19.38 [87]
Zn(II) Wheat Wheat bran 16.40 [70]
Zn(II) Tea Tea waste 8.90 [104]
Zn(II) Peel Mango peel 28.21 [76]
Se(IV) Rice Rice husk (sulphuric acid treatment) 41.15 [38]
Table 2
Biosorption Capacities of Various Biomass of Microbial Origin for Removal of Toxic Metal Ions from Waste Water
Metal ions Biomass source Maximum capacity (mg/g) Reference
Biomass type : Bacteria
Pb(II) Corynebacterium glutamicum 567.7 [105]
Pb(II) Enterobacter sp. 50.9 [106]
Pb(II) Pseudomonas putida 270.4 [107]
Pb(II) Streptomyces rimosus 135 [108]
Pb(II) Thiobacillus ferrodoxins 443.00 [109]
Zn(II) Thiobacillus ferrooxidans 172.4 [110]
Zn(II) Cyanobacterium 71.42 [111]
Cu(II) Enterobacter sp. 32.5 [106]
Cu(II) Pseudomonas putida 96.9 [107]
Cu(II) Streptomyces coelicolor 66.7 [112]
Cu(II) Thiobacillus ferrooxidans 39.8 [110]
Cu(II) Pseudomonas p. 89.60 [113]
Cu(II) Sphigomonas p. 50.10 [114]
Cd(II) Aeromonas caviae 155.3 [115]
Cd(II) Enterobacter sp. 46.2 [106]
Cd(II) Pseudomonas sp. 278.0 [116]
Cd(II) Staphylococcus xylosus 250.0 [116]
Cd(II) Streptomyces rimosus 64.9 [108]
Cd(II) Pseudomonas f. 66.25 [117]
Cr(IV) Aeromonas caviae 284.4 [115]
Cr(IV) Bacillus thuringiensis 83.3 [118]
Cr(IV) Pseudomonas sp. 95.0 [116]
Cr(IV) Staphylococcus xylosus 143.0 [116]
Cr(VI) Actinomycete sp. 32.63 [119]
Cr(VI) Chrococcus 21.36 [120]
Cr(VI) N. calicola 12.23 [120]
Ni(II) Bacillus thuringiensis 45.9 [121]
Ni(II) Actinomycete sp. 36.55 [119]
Biomass : Fungi
Hg(II) Tolypocladium sp. (residue from fermentation industry) 161.1 [122]
Cd(II) Saccharomyces cerevisiae (waste brewer’s yeast) 15.4 [123]
Cd(II) Baker’s yeast (lab cultured) 11.63 [124]
Cd(II) Phomopsis sp. (lab cultured) 29 [125]
Pb(II) Saccharomyces cerevisiae (waste brewer’s yeast) 85.6 [123]
Pb(II) Penicillium chrysogenum (lab cultured) 204 [126]
Pb(II) Penicillium oxalicum (residue from fermentation industry) 47.4 [122]
Ni(II) Saccharomyces cerevisiae (waste brewer’s yeast) 6.34 [123]
Ni(II) Penicillium chrysogenum (lab cultured) 55 [126]
Ni(II) Penicillium chrysogenum (raw) 13.2 [127]
Ni(II) Penicillium chrysogenum (alkaline pre-treatment) 19.2 [127]
Cr(VI) Mucor hiemalis 53.5 [128]
Cu(II) Aspergillus niger 26.0 [129]
Cr(III) Penicillium chrysogenum (raw) 18.6 [127]
Cr(III) Penicillium chrysogenum (alkaline pre-treatment) 27.2 [127]
Cr(III) Saccharomyces cerevisiae (waste brewer’s yeast) 12.8 [123]
Zn(II) Phomopsis sp. (lab cultured) 10.3 [125]
Zn(II) Penicillium chrysogenum (raw) 6.8 [127]
Zn(II) Penicillium chrysogenum (alkaline pre-treatment) 25.5 [127]
Biomass : Algae
Hg(II) Cystoseira baccata 329 [130]
Cd(II) Ulva onoi 61.9 [131]
Cd(II) Ulva onoi (NaOH pre-treatment) 90.7 [131]
Cd(II) Gelidium sesquipedale 18.0 [132]
Cd(II) Parthenium hysterophorous 27 [133]
Cd(II) Spirodela polyrhiza (L.) Schleiden biomass 36 [134]
Cd(II) Spirulina 357 [135]
Cd(II) Fucusceranoides 90 [136]
Cd(II) Oedogonium h. 88.90 [137]
Pb(II) Sargassum sp. 303 [138]
Pb(II) Sargassum sp. 266 [139]
Pb(II) Spirodela polyrhiza (L.) Schleiden biomass 137 [134]
Pb(II) Oedogonium h. 145.00 [140]
Pb(II) Spirogyra 140.00 [141]
Pb(II) Nostoc 93.50 [140]
Zn(II) Ulva onoi 74.6 [131]
Cu(II) Sargassum sp. 87.1 [138]
Cu(II) Spirogyra 133.30 [142]
Cr(VI) U. lactuca (dry) 10.61 [143]
Cr(VI) U. lactuca (activated) 112.36 [143]
Cr(VI) Oedogonium h. (raw) 31 [144]
Cr(VI) Oedogonium h. (acid activated) 35.2 [144]
Cr(VI) Nostoc 22.92 [145]
Cr(VI) Spirogyra 14.70 [146]
Ni(II) Sargassum sp. 71.6 [147]
Ni(II) Sargassum (acid treated) 250.00 [148]
Ni(II) Sargassum (raw) 181.00 [148]
Ni(II) Oedogonium h. (acid treated) 44.20 [149]
Ni(II) Oedogonium h. (raw) 40.90 [149]
Table 3
Desorption of Metal Ions and Regeneration of the Biosorbents
Metal ions Desorption medium Biosorbent Source of biomass Cycle Metal recovery (%) Reference
Cd(II) 0.1 M HCl Rice husk Cellulose based 1 98.6 [35]
Cd(II) 0.1 M HCl Baker’s yeast Microbial based 6 95.0 [125]
Cd(II) 0.1 M HCl Oedogonium Sp. Microbial based 5 84.8 [137]
Hg(II) 0.2 M HCl Coir pith Cellulose based 4 98.3 [48]
Hg(II) 0.2 M HCl Coconut button Cellulose based 3 96.5 [222]
Ni(II) 0.1 M NaOH Oedogonium Sp. Microbial based 4 70.0 [148]
Cr(VI) 0.1 M NaOH Oedogonium Sp. Microbial based 4 75.0 [144]
Cr(VI) 0.1 M NaOH Mucor hiemalis Microbial based [128]
Cr(VI) NaOH Wheat bran Cellulose based 1 100 [60]
Cr(VI) 0.1 M EDTA Nostoc muscorum Microbial based 5 90.0 [145]
Pb(II) 0.1 mol/L HCl Nostoc Microbial based 5 90.0 [140]
Pb(II) 0.1 mol/L HCl Oedogonium Microbial based 5 90.0 [140]
Pb(II) 0.1 M HCl Baker’s yeast Microbial based 6 95.0 [125]
Pb(II) Na2EDTA Sargassum Microbial based - 95.0 [139]
Pb(II) 0.2 mol/L HCl Coconut button Cellulose based 3 94.3 [219]
Cu(II) 0.2 mol/L HCl Coconut button Cellulose based 3 97.4 [219]
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