About the Author
Read an Excerpt
Biomass for Sustainable Applications
Pollution Remediation and Energy
By Sarra Gaspard, Mohamed Chaker Ncibi
The Royal Society of ChemistryCopyright © 2014 The Royal Society of Chemistry
All rights reserved.
Biomass for Water Treatment: Biosorbent, Coagulants and Flocculants
SANDRO ALTENOR AND SARRA GASPARD
Department of Chemistry, COVACHIM-M2E Laboratory, University of Antilles and Guyane, Pointe a Pitre 97159, Guadeloupe, France
Pollution by organic and inorganic contaminants is an important environmental problem due to their toxic effects and possible accumulation throughout the food chain and hence in the human body. Many hazardous compounds (metals, dyes, phenolic compounds, etc.) have found widespread use in industries such as metal finishing, leather tanning, electroplating, nuclear power, textile, pesticide and pharmaceutical. Thus, water pollution by these contaminants is of considerable concern around the world.
Conventional methods (bioaccumulation, precipitation, reverse osmosis, oxidation/reduction, filtration, evaporation, ion exchange and membrane separation) used for the removal of hazardous compounds from wastewater are expensive and/or inefficient in reducing the effluent concentration to the required levels. The search for new and low-cost techniques is therefore of great importance for the removal of organic and inorganic contaminants from drinking water and wastewater. Biosorption is becoming a potential alternative to traditional treatment processes used for the removal of hazardous metals and organic compounds. Biosorption represents a biotechnological innovation as well as a cost-effective and excellent tool for sequestering hazardous compounds from aqueous solutions.
Biosorption is a term that describes the property of some biomolecules or types of biomass to remove and concentrate by passive binding, selected metallic ions or other molecules from aqueous solutions. This implies that the removal mechanism is not metabolically controlled. Biomass exhibits this property, acting just like a chemical substance, as for example, an ion exchanger of biological origin. The cell wall structure of certain algae, woody biomass, mosses, fungi and bacteria in particular are found to be responsible for this phenomenon. In addition, bacteria, fungi, seaweeds, agricultural waste and raw plants can also produce biomolecules having coagulating/flocculating activities. Indeed, the use of biological materials for the treatment of wastewaters containing organic and inorganic contaminants is growing. This relatively new technology has received considerable attention in recent years as it has many advantages over traditional methods. It uses inexpensive and abundant renewable materials with good ability for the recovery of metal pollutants. Hence studies on the use of biomass, such as agricultural wastes, mosses, fungi, bacteria or seaweeds, as a raw material for the production of sorbents is steadily increasing.
Among the many types of biosorbents (i.e. algae, fungi, bacteria, yeasts, etc.) investigated for their ability to sequester contaminants, algal biomass has proven to be highly effective as well as reliable and predictable in the removal of hazardous compounds from aqueous solutions. Marine algae are a renewable natural biomass and are very abundant in the littoral world. These biomasses have attracted the attention of many investigators as organisms to be tested and used as new supports to concentrate and adsorb hazardous compounds. This chapter is devoted to biosorption, coagulation and flocculation by various biomaterials with an emphasis on algal biomass and the fundamental parameters that come into play during biosorption.
1.2 Biosorption by Biomass
1.2.1 Biosorption by Different Types of Biomass
The removal of hazardous compounds from aqueous solution by biomaterials is an innovative and promising technology. In recent years, research on biosorption mechanisms has been intensified as various biomasses can be employed to sequester organic and inorganic pollutants from industrial effluents. The efficiency of the biomass used depends upon the capacity, affinity and specificity related to its physico-chemical properties. A large variety of biosorbents have been tested for the removal of both organic and inorganic compounds. Among the different biological substrates used for biosorption are fungi, bacteria, yeast, algae and other biowaste materials such as agricultural by-products. The biomasses are taken either in their natural form, or slightly modified by chemical and thermal treatment to increase their sorption capacities.
Bacteria (Gram-positive and Gram-negative cells; Figure 1.1 (b) and 1.1 (c) respectively) are abundant microorganisms and constitute a significant fraction of living terrestrial biomass. In the last decade, some microorganisms were found to accumulate metallic elements with high capacity. Bacteria were identified as biosorbents; they are widely used because of their small size, their ability to grow under controlled conditions, and their resistance to extreme environmental conditions.
Bacteria species such as Bacillus, Pseudomonas, Streptomyces, Escherichia and Micrococcus have been the subject of hazardous compound removal studies, for example: removal of Cd(II), Cr(VI), Fe(III) and Ni(II) from aqueous solutions by an Escherichia coli; Cr(VI) uptake by Bacillus thuringiensis and by Trichoderma species; acid dyes by Paenibacillus macerans; and lead and nickel by Pseudomonas aeruginosa. Important results of pollutant biosorption using bacterial biomasses are listed in Table 1.1, which draws on references cited by Wang and Chen and Vijayaraghavan and Yun.
Fungi are another ubiquitous biomass in the natural environment that are important in industrial processes. Fungal cell walls consist mainly of poly-saccharide with proteins, lipids, polyphosphates and inorganic ions, with a chitin network, making up the wall-cementing matrix. Chitin is a common constituent of fungal cell walls (Figure 1.1(d)). The knowledge that metallic ions are very important to fungi metabolism created interest in relating the behaviour of fungi to the presence of metallic ions, particularly heavy metals.
There is much published work on the treatment of water polluted by hazardous compounds such as: the removal of Cr(VI) by Aspergillus tubingensis; textile dyes by Aspergillus niger, Aspergillus japonica, Rhizopus nigricans and Rhizopus arrhizus; Cr(VI) by Aspergillus niger; phenol and chlorophenol by Schizophyllum commune; and nickel by Aspergillus niger. Indeed, according to some references cited by Wang and Chen, Aksu and Kumar and Min, like bacterial biomass, fungal biomass can also concentrate from aqueous solutions considerable quantities of organic pollutants (such as dyes, phenol and its derivatives, pesticides) by adsorption even in the absence of physiological activity. Both, living and dead fungal cells have a remarkable ability to remove hazardous compounds. Table 1.2 lists some types of fungal biomass used for the uptake of organic and inorganic compounds.
Algae are eukaryotic organisms containing chlorophyll that allows them to carry out the photosynthesis process. The algal cell wall structure is similar to that of the fungal cell wall, being composed of a multi-layered microfibrillar framework generally consisting of cellulose and interspersed with amorphous material in which is embedded different polysaccharides (fucoidan matrix, al-ginate, protein) (Figure 1.1(a)). Algae abound everywhere in nature but particularly in aquatic habitats, freshwater, marine waters and moist soil.
Among the different biomaterials studied for the biosorption of hazardous compounds, seaweeds have received much attention due to their low cost and low sensitivity to environmental and impurity factors. They are often used for metal ions uptake. Algae achieve generally higher metal uptake than bacteria and fungi. Among the three groups of algae (red, green, brown), brown algae have received the most attention as they have greater pollutant uptake capability than green and red algae. This is due to the presence of alginate, which is present in a gel form in brown algae cell walls. Their macroscopic structure also offers a convenient basis for the production of biosorbent particles suitable for sorption process applications.
In addition, brown algae are cheap and readily available materials. They are commonly used as nutritional supplements, animal feed and fertilizers, and as a source of thickeners such as alginate. Brown algae are the subject of numerous biosorption studies because of their high metal uptake, for example: biosorption of copper by Ulva fasciata and Sargassum sp.; chromium by Ulva lactuca and Sargassum sp.; cadmium, zinc and lead by Sargassum filipendula, Laminaria hyperborea, Bifurcaria bifurcata, Sargassum muticum and Fucus spiralis; lead and nickel by Sargassum sp. acid dyes by Azollafiliculoides; phenol and chlorophenol by Sargassum muticum; methylene blue by Turbinaria turbinata (Figure 1.2), to name a few. The results of most of these studies are very promising. Table 1.3 lists some species of seaweeds biomass used for removal of hazardous contaminants and their performance.
184.108.40.206 Other Types of Biomass
Other biomass such as agricultural by-products can also be used for the removal of hazardous contaminants. Numerous other types of biomass are mentioned in the literature as being used as biosorbent. We list a few, particularly some agricultural by-products in Table 1.4. Examples include: removal of Cd(II) and Pb(II) by rice husk residues; uptake of Pb(II), Cu(II), Cd(II), Zn(II) and Ni(II) by tobacco dust; Pb(II) and Cd(II) by cork waste biomass;removal of Pb(II) by Gossypium hirsutum (cotton) waste biomass.
1.2.2 Characterisation of Biosorbents Surface
The ability of these biomaterials to sequester organic and inorganic pollutants is related to the structure and chemical composition of their cell wall which is composed of a fibrous structure and an amorphous matrix, in which is embedded different polysaccharides (Figure 1.1). The main mechanisms of bio-sorption include electrostatic attraction, ion exchange and complex formation, but these can differ depending on the type of biomass, origin and treatment to which the biomass has been submitted. To understand how contaminants bind to the biomass, it is essential to identify the functional groups responsible for their binding. Most of the functional groups involved in the binding process are found in cell walls.
Fourier transform infrared (FTIR) spectroscopy is the most widely used technique used to characterize biomaterials. The type of binding groups present in the surface of the adsorbents can easily be identified by this method. For all types of biomass (fungi, bacteria, yeast, algae, biowaste materials) used for biosorption, the same functional groups are generally involved in the removal of pollutants. Several chemical groups such as hydroxyl, carboxyl, sulfonate, alcohol, amino and phosphate groups have been proposed as being responsible for the biosorption of hazardous compounds through binding by biomaterials. FTIR spectroscopy allows the determination of different functionalities and offers excellent information on the nature of the bonds between the pollutant and the biomass surface. The level of contaminant uptake depends on factors such as the number of binding sites, their chemical state, accessibility and affinity for the contaminants.
As we can see in the following examples for different biomass such as agricultural waste biomass Gossypium hirsutum (Figure 1.3), fungal biomass Aspergillus niger, bacterium biomass Escherichia coli (Figure 1.4), or seaweed biomass Lessonia nigrescens and Macrocystis integrifolia, FTIR spec-troscopic analysis revealed in all cases that the main functional groups in the biomass surface are carboxyl, carbonyl, hydroxyl, sulfonic, amino, phosphate and alcoholic groups.
X-ray photoelectron spectroscopy (XPS) is another widely used method for the biomaterials surface analysis. XPS analysis has the ability to determine the elemental composition on the surface of materials. XPS data give an idea of the local oxidation states and chemical bonding environment of the bio-material. XPS can be applied to determine the interactions between the organic functional groups on the biomass surface and the contaminant adsorbed. Figure 1.5 shows the XPS spectrum of Spirogyra sp. before and after sorption of fluoride in aqueous solution. In virgin biosorbent, the observed C1 peak can be convoluted into three peaks at 284.6, 286.6 and 287.9 eV, which can be attributed to the presence of C-C/C-H and a carboxylic (-O-C-O) group respectively. After fluoride sorption the C1s XPS high resolution narrow scan showed peaks at 284.6, 286.5, 287.4and 288.9 eV. The peak observed at 288.9 eV was attributed to -CH2-CF2-bond formation. Thus, like the FTIR analysis method, the compounds adsorbed on biomass surface could be also analysed by XPS analysis. Therefore, FTIR and XPS are two analytical techniques that can be used to get information on contaminant binding mechanisms on biomass.
The physical properties of biomaterials such as surface area, pore volume and pore size distribution are also studied because physical biosorption can be influenced by these characteristics of the biomaterial. In this case, the Brunauer–Emmett–Teller (BET) method based on nitrogen adsorption at 77 K is often used. However, in the most cases, the results obtained indicate non-porous or macroporous materials. For example, the BET surface area and pore volume obtained by N2 adsorption/desorption isotherms at 77 K are 2.86 m2 g-1 and 0.003 cm3 g-1 respectively for Sargassum muticum seaweed, similarly to what has been published (Table 1.3) for Schizophyllum commune fungus biomass for which the BET surface area and pore volume are 3.95 m2 g-1 and 0.0041 cm3 g-1 respectively.
Scanning electron microscopy (SEM) is an analysis method that can clearly reveal the surface texture and morphology of the biosorbent before and/or after biosorption (Figure 1.6). As can be seen when comparing SEM images of cork waste before the biosorption process (Figure 1.6(a)) and after its use (Figure 1.6(b)), there are no changes in the cork waste morphology, confirming that the morphological structure of the cork has not been affected by the metal biosorption experiments.
To select the optimum pH range for contaminant sorption, it is useful to determine the pH at zero point of charge (pHzpc) of the selected biosorbent, i.e. the pH value at which the sorbent surface is globally neutral. Indeed, when pH < pHzpc, the biosorbent surface becomes positively charged and metal sorption is inhibited due to electrostatic repulsion between metal ions and functional groups. Conversely when pH > pHzpc, the number of negatively charged sites on the biosorbent increases, and metal sorption is mostly favoured.
1.2.3 Mechanism of Biosorption
The mechanism of the biosorption of hazardous compounds is a complicated process. In order to understand how pollutants bind to the biomass, it is essential to identify the functional groups responsible for pollutant binding. Most of the functional groups involved in the binding process are found in cell walls. Biosorption is generally based on physico-chemical interactions between pollutant and functional groups present on the cell surface, such as electrostatic interactions, ions exchange, and metal ion chelation and complexation.
Biomaterial surfaces can be regarded as a mosaic of different functional groups which are responsible for binding of organic or inorganic ions, including amide (-NH2), carboxylate (-COO-), thiols (-SH), phosphate (PO3-4) and hydroxide (-OH). Therefore, the identification of functional groups is very important for understanding the mechanisms responsible for the binding of certain compounds. In the case of metal uptake, the status of biomass (living or non-living), types of biomaterials, properties of metal solution chemistry, ambient/environmental conditions, etc. also influence the mechanism of metal biosorption.
Excerpted from Biomass for Sustainable Applications by Sarra Gaspard, Mohamed Chaker Ncibi. Copyright © 2014 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.