Current reviews on biosorption are related to general approaches90-93 to diverse types of biomass such as microbial biomass, plant wastes, and agro-based waste materials, or to a specific metal.494-98 However, a review on metal biosorption using macrophytes biomass is not available. In this chapter, a review on the current knowledge of biosorption using preferentially nonliving biomass from aquatic plants is presented.
Biosorption is a property of both living and dead organisms and may be simply defined as the removal of substances from solution by biological material. Such substances can be organic and inorganic, and in gaseous, soluble, or insoluble forms.93 Biosorption has shown to be a very promising biotechnology for metal removal from effluents.90 Its major advantages are its low cost, high metal-binding capacity, high efficiency for reducing quickly the pollutant concentration in an environmentally friendly manner, simplicity, and availability of biomass.93,99,100
Ahluwalia and Goyal94 have pointed out some disadvantages of biosorption such as the early saturation of biomass, which can be a problem since metal desorption is necessary prior to further use, irrespective of the metal value. Furthermore, the potential for biological process improvement is limited because cells are not performing an active metabolism.
Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4,90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101
Additionally, studies about chemical modification of the biomass surface to improve biosorption have been widely reported.102,103 Equilibrium91 and kinetics104 biosorption have been described using different models.
Recent reports on biosorbents based on diverse types of macrophytes are found widely in the literature. Free-floating aquatic plants from the genera Salvinia, Azolla, Eichhornia, Lemna, and Pistia have been described the most. S. natans biomass was able to uptake As(V) at low initial concentrations from 0.25 to 2 mg/L (74.8% and 54%, respectively). The experimental data fitted well to both Langmuir and Freundlich isotherms. The effect of pH and biomass quantities on sorption rate has also been investigated along with some metabolic parameters.105
Recently, nonliving biomass of S. cucullata has been described as a low-cost absorbent of Cr(VI).106 Optimum conditions for the Cr(VI) adsorption by acid-treated S. cucullata were found out using a full factorial design. The Cr(VI) removal efficiency of the adsorbent was found to increase with the increase in time, temperature, adsorbate concentration, and stirring speed, and to decrease with increase in pH and adsorbent dose. The Fourier transform infrared spectroscopy (FT-IR) analysis revealed that in addition to electrostatic force, the adsorption may be due to
formation of complex with the ligands (Lewis bases) available in the adsorbents. The vital role played by chelation was also shown. Authors suggest that the formation of chelates could have been favored at lower pHs and therefore the adsorption efficiency was higher. Column studies were carried out at the optimum operating conditions and the maximum uptake obtained was 98.75 mg/g. Adsorption data from the column studies fitted well to the Bohart-Adams model. Furthermore, Cr(VI) removal studies were carried out by using activated carbon obtained from S. cucullata. Results showed that the initial part of the adsorption process confined only to surface adsorption and the slower kinetics could be due to intraparticle diffusion. The FT-IR showed the anionic binding with the chelate forming part of the adsorbent. Column studies were also carried out to evaluate the suitability of the adsorbent in treating Cr(VI)-contaminated water and the maximum uptake of Cr(VI) observed was 156 mg/g at an initial concentration of 100 mg/L.107
A. filiculoides biomass has been evaluated for Pb(II), Cd(II), Ni(II), and Zn(II) adsorption in different stages. Nonliving biomass of A. filiculoides was activated by NaOH and then CaCl2/ MgCl2/NaCl. This process can occur due to the increase of ion-exchange agents such as (-COO)2Ca and (-COO)2Mg bindings and/or -COONa2OOC- groups. These binding sites can be formed from demethylation of cell wall pectin in the alkali solution and then contacting with ternary chloride salts solution. Such an activation resulted in a higher qmax obtained for the alkali-treated biomass, especially for Pb(II) and Cd(II) at the highest temperature (313°K) (1.272 versus 0.977 and 1.35 versus 0.931 for Pb(II) and Cd(II), respectively). This biomass had also faster adsorption kinetics in comparison to the nonactivated Azolla biomass.108
Removal of Hg(II) from aqueous solutions has been also possible using A. filiculoides nonliving biomass. Diverse techniques such as scanning electron microscopy (SEM), determination of nitrogen and amino acid content, BET (Brunauer, Emmett, Teller) surface area by N2 adsorption at 77 K, acid/ base titration, ion-exchange capacity, and electrophoretic measurements were used in an attempt to elucidate the mechanisms involved in mercury sequestration. Samples were also characterized by energy dispersive spectroscopy and x-ray diffraction after contact and equilibration with mercury solution. The reduction of Hg(II) to Hg(I), that is, soluble mercuric to insoluble mercurous chloride (Hg2Cl2) on the adsorbent surface, was found to be a controlling reaction mechanism. Although mercury sorption was studied at elevated concentrations similar to those encountered in industrial effluents, it is suggested that the findings would also apply to final stage water treatment.109
Less complex techniques have been reported to be useful to study the acidic and alkaline treatment processes of biosorbents and the role of carboxyl and carboxylate groups in metal adsorption. Rakhshaee and coworkers101 used potentiometric titration curves to assess the content of such groups in L. minor biomass treated with NaOH and HCl. The results showed an increase (up to 25%) in the adsorption of Hg(II), Cr(III), Cr(VI), and Cu(II) with NaOH-treated biomass as a consequence of an increase of -COO- groups (0.92-2.42 mmol/g). On the contrary, the -COOH groups increase observed (1.50-2.41 mmol/g) due to the acidic treatment led to a decrease in the metal ions uptake (up to 33%) despite activation by the chloride salts.
Recent reports pointed out that water hyacinth (E. crassipes) nonliving biomass is suitable for development of an efficient biosorbent for the removal of chromium from wastewater of chemical and allied process industries. Gude and Das110 found that the adsorption rate of Cr(VI) from aqueous solutions was rapid following the first-order kinetic model and the equilibrium exhibited a Langmuir-type behavior. The maximum Cr(VI) adsorption was found to be 7.5 mg/g of dry weight, whereas the calculated activation energy was around 54.6 kJ/mol. Based on these results, the authors mentioned that about 500 mg of dry biomass could remove Cr(VI) successfully from 100 mL of chromite mine water containing 2.8 mg/L Cr(VI). On the contrary, it has been reported111 that although the Largreen first-order model was applicable to some of the data of Cr(VI) adsorption by water hyacinth biomass, the pseudo-second-order reaction model was applicable to all data. Furthermore, the Freundlich isotherm was found to represent the measured sorption data well. The FT-IR showed that the hydroxyl group was the chromium-binding site within a pH range from 1 to 5, where chromium did not precipitate.
Spirodela intermedia, L. minor, and P. stratiotes were able to remove Pb(II), Cd(II), Ni(II), Cu(II), and Zn(II), although the two former ions were removed more efficiently. Data fitted the Langmuir model only for Ni and Cd, but the Freundlich isotherm for all metals tested. The adsorption capacity values (KF) showed that Pb was the metal more efficiently removed from water solution (166.49 and 447.95 mg/g for S. intermedia and L. minor, respectively). The adsorption process for the three species studied followed first-order kinetics. The mechanism involved in biosorption resulted in an ion-exchange process between monovalent metals as counterions present in the mac-rophytes biomass and heavy metal ions and protons taken up from water.112
The effect of activation treatments has been also evaluated in multimetal (Cu(II), Cd(II), and Ni(II)) systems using untreated, acid pretreated (H2SO4), and alkali pretreated (NaOH) biomass of L. minor. The results revealed that the adsorption capacities of the biomass in multimetal systems were lower than those obtained in a single-metal system, that is, Cd(II) adsorption decreased by almost 60% in the untreated biomass. The ionic charge, ionic radii, and electrode potential affect metal ions adsorption in the multimetal systems. On the other hand, the maximum adsorption capacities were higher with alkali pretreated biomass (83, 69, and 59 mg/g for the Cd(II), Cu(II), and Ni(II) ions, respectively). The FT-IR results showed that dried biomass have different functional groups for heavy metal ions binding, such as carboxyl, phosphate, amide, thiol, and hydroxide groups.113
Nonliving biomass of E. crassipes, Valisneria spiralis, and P. stratiotes were examined in terms of their heavy metal (Cd(II), Ni(II), Zn(II), Cu(II), Cr(II), and Pb(II)) sorption capacity, from individual-metal and multimetal aqueous solutions. Surprisingly, V. spiralis was the most efficient plant and E. crassipes was the least efficient one for removal of all the metals. Cd(II) was removed up to 98% by V. spiralis. Sorption data for Cr(II), Ni(II), and Cd(II) fitted better to the Langmuir isotherm equation, while the sorption data for Pb(II), Zn(II), and Cu(II) fitted better to the Freundlich isotherm equation. In general, the presence of other metal ions did not influence significantly the targeted metal sorption capacity of the test plant biomass. Ion exchange was proven to be the main mechanism involved in biosorption and there was a strong ionic balance between adsorbed (H+ and M2+) and the released ions (Na+ and K+) to and from the biomass.114
Regarding submerged plants, sorption of Cu(II) by Myriophyllum spicatum L. (Eurasian water milfoil) has been shown to be fast and fits isotherm models such as Langmuir, Temkin, and Redlich-Peterson. The maximum sorption capacity (qmax) of copper onto M. spicatum L. was 10.80 mg/g, while the overall sorption process was best described by the pseudo-second-order equation.115 Likewise, Hydrilla verticillata has been described as an excellent biosorbent for Cd(II). In batch conditions, the qmax calculated was 15.0 mg/g. Additionally, H. verticillata biomass was capable of decreasing Cd(II) concentration from 10 to a value below the detection limit of 0.02 mg/L in continuous flow studies (fixed-bed column). It was also found that the Zn ions affected Cd(II) biosorption.116
Sorption mechanisms of Hg(II) by the nonliving biomass of Potamogeton natans was also elucidated using chemical and instrumental analyses including atomic absorption, electron microscopy, and x-ray energy dispersion analyses. The results showed a high maximum adsorption of Hg(II) (180 mg/g), which took place over the entire biomass surface. Nevertheless, there were spots on the surface where apparent multilayer sorption of Hg(II) occurred. The minimum concentration of Hg(II) in solution that can be removed appears to be about 4-5 mg/L.117
Other aquatic weeds such as reed mat, mangrove (leaves), and water lily (Nymphaceae family plants) have been found to be promising biosorbents for chromium removal. The highest Cr(III) adsorption capacity was exhibited by reed mat (7.18 mg/g), whereas for Cr(VI), mangrove leaves showed maximum removal capacity (8.87 mg/g) followed by water lily (8.44 mg/g). It is interesting to mention that Cr(VI) was reduced to Cr(III), with the help of tannin, phenolic compounds, and other functional groups on the biosorbent, and subsequently adsorbed. Unlike the results discussed previously for the use of acidic treatments, in this case, such treatments significantly increased the Cr(VI) removal capacity of the biosorbents, whereas the alkali treatment reduced it.118
Lichen biomass from Parmelina and Cladonia genera have resulted good biosorbents of Pb(II), Cr(III), and Ni(II) ions. The Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models were applied to describe the biosorption isotherm of Pb(II) and Cr(III) ions by Parmelina tiliaceae biomass. The monolayer biosorption capacity of the lichen found for Pb(II) and Cr(III) ions was 75.8 and 52.1 mg/g, respectively. The D-R isotherm model indicated that the biosorption was carried out by a chemical ion-exchange mechanism, since the mean free energy calculated was 12.7 and 10.5 kJ/mol for Pb(II) and Cr(III) biosorption, respectively. The calculated thermodynamic parameters such as the change of free energy (AG0), enthalpy (AH0), and entropy (AS0) showed that the biosorption of Pb(II) and Cr(III) ions onto P. tiliaceae biomass was feasible, spontaneous, and exothermic under the examined conditions.119 The equilibrium, thermodynamic, and kinetic models mentioned before were also used to describe the biosorption of Pb(II) and Ni(II) ions from aqueous solution using Cladonia furcata biomass. The monolayer biosorption capacity of the biomass was found to be 12.3 and 7.9 mg/g for Pb(II) and Ni(II) ions, respectively. From the D-R model, the mean free energy calculated was 9.1 kJ/mol for Pb(II) biosorption and 9.8 kJ/mol for Ni(II) biosorption, indicating that the biosorption of both metal ions was carried out by a chemical ion-exchange mechanism. Thermodynamic parameters related to the biosorption capacity indicated the occurrence of a feasible, spontaneous, and exothermic process. Experimental data were also tested in terms of kinetic characteristics and it was found that biosorption processes of both metal ions followed well pseudo-second-order kinetics.120
Finally, Chojnacka121 investigated the biosorption characteristics of Riccia fluitans and its potential to adsorb Cr(III) from aqueous solutions. The results showed that the biomass was rich in protein (27-31%) and possessed a high cation-exchange capacity (14.5 mequiv/g). The carboxyl groups were found in a higher quantity (6.08 mequiv/g). Additionally, it was found that in multi-ion systems (Cu, Mn, and Zn), the Cr(III) biosorption capacity was significantly affected (3.91 versus 6.10 mequiv/g) since metal ions competed for metal-binding sites. The results also showed that bound metal ions were exchanged with alkaline earth metals, confirming that the dominating mechanism of metal binding by R. fluitans was ion exchange.
As was mentioned earlier in this chapter, most of the biosorption reviews deal with nonliving biomass. However, it is also relevant to understand the presence of various biosorption mechanisms when metabolism is active, using living biomass. S. minima has proven to be an excellent Pb biosor-bent. The biosorption process was found to follow a pseudo-second-order kinetics and to be dependent on the initial metal concentration (from 0.8 to 28.40 mg Pb/L). Data fitted well both the Langmuir and the Freundlich models . Very high qmax were obtained for both, synthetic wastewater and deionized water (58 and 44 mg/g, respectively). Such a high capacity to adsorb Pb was most likely due to its exceptional physicochemical characteristics such as a very high surface area (264 m2/g) and a good content of carboxylic groups (0.95 mmol H+/g dry weight).77
In natural conditions, Ceratophyllum demersum and Potamogeton pectinatus L. have been found to be effective adsorbents of Cd(II), Cu(II), and Pb(II). The adsorption percentage of the metals onto plant surfaces followed the pattern: Pb(II) > Cu(II) > Cd(II). P. pectinatus biomass adsorbed a higher content of heavy metals than C. demersum. According to the results, both species are of interest in the phytoremediation and biomonitoring studies of polluted waters.122
In controlled conditions, Lesage and collaborators123 assessed the sorption/desorption properties of Co, Cu, Ni, and Zn by living Myriophyllum spicatum biomass. The sorption process was well described by the Langmuir model for Co, Ni, and Zn, whereas sorption of Cu was better described by the Freundlich isotherm. The biomass showed the highest affinity for Cu being the maximum sorption capacity (113 mg/g), 49-, 38-, and 17-fold that of Co, Ni, and Zn (2.3, 3.0, and 6.8 mg/g, respectively). At the highest initial concentration of 100 mg/L, a maximum of 29 mg/g of Cu was sorbed onto the surface of the biomass. The potential regeneration of the biomass and the recovery of heavy metals were also evaluated using HCl (0.1 M). However, the acid wash did not fully recover the metals sorbed onto the surface and evidence of leaching within the biomass was observed. Therefore, this procedure was not suggested as a viable strategy. On the other hand, Keskinkan and collaborators124 found a lower qmax for Cu(II) (10.37 mg/g) and Zn(II) (15.59 mg/g) removal using the living biomass of M. spicatum. On the contrary, both M. spicatum and C. demersum showed to be excellent Pb(II) adsorbents (qmax = 46.49 and 44.8 mg/g, respectively). The thermodynamic parameters, specifically, the Gibbs free energy with negative values, indicated the spontaneity of the adsorption process between metals and plants. The lowest value was obtained for copper and the C. demersum system (-0.45), while the highest value of this parameter was achieved for lead and the M. spicatum system (-10.83).
Phytofiltration, defined as the use of plants to remove pollutants from wastewaters, has proven to be an efficient and environmentally friendly biotechnology. All phytofiltration systems, such as rhizo-filtration, CWs and lagoons, and biosorbents-based systems, are very efficient in metal removal. However, the selection of the appropriate plant species and/or a specific system of phytofiltration is critical for a successful application at field scale. Aquatic plants, especially the free-floating and submerged plants, have shown a great potential in this area. Finally, even though most of the removal mechanisms of such pollutants have been studied, a better understanding of them within a particular system will be necessary for increasing the cases of successful applications of phytofiltration.
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