Various physical and chemical processes are involved in the extraction of metals

from their ores. Ores generally occur in the form of compounds of metal oxides,

sulfides, carbonates, or halides.

These processes are:

Lignocellulose biomass, including wood waste, agricultural waste, household waste, etc. represents a renewable resource which has stored solar energy in its chemical bonds [120]. It has great potential for bioethanol production, when compared to ethanol produced from grain, tubers, and sugar plants, because it is a widely available cheap feedstock which does not compete with human food products.

9.7.12.1 Pretreatment

It is known that the main difficulty in converting lignocellulose biomass into second-generation ethanol consists in breaking down structural and chemical biomass complex. In the course of the breakdown process, cellulose feedstock is affected by enzymes which allow further recovery of ethanol. Biomass consists of polysaccharides-cellulose and hemicellulose, which are hydrolyzed into single sugar components, followed by further recovery of ethanol by well-known and elaborated fermentation technologies. Enzymatic activity in lignocellulose hydrolysis gives a good yield and minimum amount of by-products; it has lower energy consumption, milder operating conditions, and represents an environ­mentally friendly processing method [157, 194]. Considering that the sugars required for fermentation are bound to the lignocellulose structure, pretreatment of biomass is required in order to remove and/or modify lignin and hemicellulose matrix before enzymatic hydrolysis of polysaccharides. Unlike starch which is a crucial source of energy in plants, cellulose has mostly a structural role as it provides plant cells with mechanical durability with hemicellulose and lignin. Natural cellulose materials do not have high reactivity; therefore, fermentable saccharification requires a large cellulose surface and broken cellulose microfilm structure. Reactivity of natural substrates is also reduced by lignin. The most commonly applied methods can be classified into two groups: chemical hydrolysis (dilute and concentrated acid hydrolysis) and enzymatic hydrolysis. In addition, there are some other hydrolysis methods in which no chemicals or enzymes are applied. For instance, lignocellulose may be hydrolyzed by thermal treatment, wet — oxidation, gamma-rays or electron-beam irradiation, or microwave irradiation. However, these processes are commercially unimportant.

As indicated before, under serious conditions (high temperature, high sulfuric acid dosage and long reaction time), more degraded saccharine-derived chemicals, such as furfural, levulinic acid and HMF, can be obtained in high amounts. Furfural has a variety of applications for a broad spectrum of derivatives chemicals and polymer products [81]. In addition, furfural is a good solvent widely used in lubricant, coatings, adhesives, furan resin and so on [82]. Levulinic acid is a key platform compound for the synthesis of a series of value-added products, including chiral reagents, biologically active materials, polyhydroxyalkanoates, antifouling compounds, personal care products, lubricants adsorbents, printing inks, coatings [83], etc. HMF is also a platform compound for the synthesis of many chemicals derived from petroleum. For example, it can be converted into 2,5-furandicarb — oxylic acid, 2,5-dihydroxymethylfuran and 2,5-bis(hydroxymethyl) tetrahydrofu — ran [84]. In addition, it is a precursor in the synthesis of liquid alkanes used for diesel fuel [85].

In this section, the focus is on downstream processes for recovering vanillin from the oxidation media. The extraction and purification of vanillin from the reaction mixture have been matters of great concern, leading to the development of several technologies on chemical engineering separation methods such as solvent extraction, distillation, acidification/precipitation, bisulfitation, membrane, crys­tallization, supercritical extraction, adsorption, and ion exchange.

After the chemical alkaline oxidation of the lignin several low molecular weight aromatic compounds besides vanillin are present in the solution as water-soluble sodium phenolates. Clearly, the full composition of the complex mixture derived from the oxidation process depends on the lignin-based raw materials, operating conditions and applied chemicals. It is expected a mixture containing lignin oligomers, simple phenolics as aromatic aldehydes, and respective ketones and acids, and other secondary products in minor amount as lactones and also guaiacol and syringol [160, 161].

There are several types of microorganisms that can produce hydrogen by dark fermentation. Every organism has different requirements such as; substrate pref­erence, pH and temperature. These parameters can greatly influence the hydrogen yield by affecting microbial metabolism. Fermentative hydrogen production by pure microorganisms has been studied by many researchers. Pure cultures have some advantages. In particular, they can be easily and reliably manipulated to determine the optimal growth conditions. However, there are some clear disad­vantages to using pure cultures since they can easily be affected by contamination and therefore their use requires aseptic conditions which could greatly increase overall system costs [7]. Generally pure cultures used in dark fermentation can be divided into two general groups:

1. Anaerobic bacteria (e. g. Rumen bacteria, Clostridium and other Firmicutes)

2. Facultative anaerobic bacteria (e. g. Escherichia coli, Enterobacter, Citro- bacter) [10].

Clostridium and E. coli are the two most widely used bacteria for two-stage hydrogen production. Various Clostridium species can produce hydrogen and Clostridium butyricum, a mesophilic and strict anaerobic bacteria, is perhaps the most commonly employed. As early as the 1960s, Clostridium butyricum and

Clostiridium welchii were reported to produce fermentative hydrogen in a 10 L fermenter by Magna Corporation [11]. As discussed above, the theoretical yield from glucose by Clostridium butyricum is 4 mol H2/mol glucose. A few studies have reported yields of more than 2 mol H2/mol glucose [12, 13], with the max­imum yield of 3.26 mol H2/mol glucose [14]. Hydrogen production from glucose using C. acetobutylicum [15], C. butyricum [16], C. paraputrificum [17] C. bei — jirincki AM21B [10], C. cellobioparum [18], C. pasteurianum [10] resulted between 0.42 and 2.73 mol H2/mol glucose. Clostridium species can sporulate under the proper conditions and generally produce acetate and butyrate as by-products. It is important to adjust system operation conditions to avoid spor — ulation [19]. Although they are generally known as mesophilic microorganisms, some thermophilic species have been isolated and these are generally capable of higher hydrogen yields [20]. C. thermolacticum can use lactose to produce hydrogen with a yield of 1.5 mol H2/mol lactose. C. thermoalcaliphilum [21], C. thermobutyricum [22] C. thermohydrosulphuricum [23], C. thermosaccharo — lyticum [24], C. thermosuccinogenes [25] are other thermophilic Clostridial species that have been used.

Enteric bacteria are generally not capable of metabolizing complex carbohy­drates, but the necessary genes can be introduced [26]. E. coli is one of the most studied facultative anaerobes for hydrogen production, and has been subject to a variety of genetic engineering including mutagenesis and the introduction of for­eign genes. Organisms have been genetically modified to consume pentoses, or to increase lactate and succinate activities [7]. E. coli has long been known to pro­duce hydrogen under anaerobic conditions, but with low yields if there is a large amount of residual formate [27].

Enterobacter species are gram negative motile facultative anaerobes. Hydrogen production by this organism is influenced by many process parameters such as initial substrate concentration, initial medium pH, temperature and iron concen­tration. Enterobacter aerogenes HU-101 [28], immobilized Enterobacter cloacae IITBT08 [29], immobilized Enterobacter cloacae DM11 [30] gave 1.17, 2.3 and

3.8 mol H2/mol glucose, respectively. Molasses has been used as carbon source to produce hydrogen by Enterobacter aerogenes with hydrogen yields between 0.52 and 2.2 mol H2/mol sucrose [31, 32].

The general aim of the pulping process is to delignify the wood matrix by chemically degrading and/or sulfonating the lignin to water-soluble fragments to liberate the cellulose. The origin of the lignin (wood species), the delignification type, and the recovery process from pulping liquors have remarkable influence on the structure of this biopolymer in both pulp and dissolved lignin [1, 10, 13-18]. The different structural characteristics of lignin have influence on its performance toward further processing [19, 20] and determines its suitability for different proposes.

This section deals with the most important lignin types available from the pulp and paper industry. There will be reference to lignin from emerging processes of lignocellulosic biomass conversion to ethanol and saccharide-derived chemicals. The major differences between lignins are derived from distinct delignification processes. In general, the characteristics of the lignins obtained are dependent of
the balance between the two main groups of reactions having an opposite effect: (i) cleavage reactions and introduction/liberation of hydrophilic groups, leading to dissolution of lignin fragments and (ii) condensation reactions which increase the molecular weight of lignin.

Rakesh Kumar Sharma, Alok Adholeya, Aditi Puri and Manab Das

The interfacial face of bioextraction arises as it uses the technologies in which plants clean up the contaminated sites by immobilizing the contaminants in the soil. This technique is mostly applied to heavy metals in soil sediments and sludges. These metals are either trapped within the root system or taken up to the tissues by selected fast growing plant species. These species are grown under normal farming conditions until they reach their maximum size. Throughout the growth period, amendments are added to soil to increase availability of metals to plants. When the plants are mature, metal specific chelating agents are applied to the harvested biomass for the recovery of accumulated metals [1]. So, selection of plant materials is an important factor for this technique. Therefore, two main strategies are proposed to clean up toxic metals from soil. The first approach is the use of metal hyper-accumulator species for cleaning up of soil, as they can take up significant amount of metals from contam­inated soils, but their low annual biomass production tends to limit its ability. This problem can be overcome by using high biomass plants that can be easily cultivated. So, the efficiency of this technique is determined by two key factors: metal hyper — accumulating capacity and biomass production [2].

Plant-based environmental remediation technology has been widely pursued in recent years as greener cost effective strategy to trap metals and radionuclide contaminants that are in mobile chemical forms which are most threatening to human and environmental health. Once the removal of contaminants is complete the soil generated from this process is fertile and is able to support the growth of plants [1].

R. K. Sharma (H) • A. Puri

Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007, India e-mail: rksharmagreenchem@hotmail. com

A. Adholeya • M. Das

Biotechnology and Management of Bioresources Division, The Energy and Resources Institute, New Delhi 110003, India

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_14, © Springer-Verlag Berlin Heidelberg 2012

This technology has been applied at a number of sites all over the world. Examples include Magic Marker site in New Jersey and a Daimler Chrysler site in Detroit, Michigan (induced accumulation of lead in soil); Argonne National Laboratory-West (mercury, silver, chromium in soil/sediment removed by whole plant harvesting). This technique can also be merged with other techniques, like it can be used in conjunction with electrochemical technology to remove contami­nants (such as cesium-137), which bioextraction technology alone cannot remove and many more permutations and combinations can be tried [1].

Florescent lamps, halogen lamps, optical fibers, neon tubes, light emitting diodes all giving photosynthetically active radiation (PAR) are possible artificial light sources for photofermentation. One of the main important parameters deciding system productivity is the light conversion efficiency (g):

[33.61 + pH x VH21 g = H H2 x 100

I x A x t

where VH2 is the volume of the produced H2 in l, qH2 is the density of the produced hydrogen gas in g/l, I is the light intensity in W/m2, A is the irradiated area in m2 and t is the duration of hydrogen production in hours [94]. The light conversion efficiencies depend on the strain and the substrate used. Generally varying between 1 and 6% but a 9.23% light conversion efficiency was achieved by Rhodobacter sphaeroides using lactate as carbon source and a tungsten lamp with a light intensity of 200 W/m2 [123]. The photobioreactor can use the sunlight alone or it can be combined with artificial lights. Light intensity may be measured by either W/m2 or lux. The conversion between these two units depends on the wavelength but it can be assumed as 1 W/m2 is equal to 30-100 lux [5]. For photofermentation experiments it is important to decide the best light intensity value before starting the large-scale experiments. Especially in large-scale application the shading effect of organisms could affect the system performance. At this point internal illumi­nation could be a good option. Different kinds of light sources were combined in terms of increasing the hydrogen productivity and the photobioreactor was illu­minated by combining light sources, including an internal illumination with optical fiber excited by solar energy (OF(sunlight)) as well as external irradiation of tungsten filament lamp (TL). A 138 and 136% increase in cumulative hydrogen production achieved by combination of OF (sunlight)/TL was found to be more effective than TL/TL combination [14].

The oxidation of lignin to produce vanillin has been demonstrated at high pH (almost 14), and high temperatures (higher than 100°C) with molecular oxygen (oxygen pressure equal or higher than 3 bar). The main advantages of this oxidant are its environmental friendliness, the high efficiency per weight of oxidant, and comparatively low price (for example, air can often be used). The limitation in this process is the low solubility of oxygen in the reaction medium of NaOH (and lignin) in the high operational temperatures [147, 148]. Nevertheless, the oxygen partial pressure should be controlled to avoid further oxidation of vanillin [149, 150]. The high pH is required for the total ionization of phenolic groups and conversion to reactive quinonemethide as presented in Fig. 12.7 (initial step I-II). The pKa values of lignin-related phenolics are in the range of 10-11.5 at 25 °C [25] decreasing as the temperature raise [151]. However, it is expected that the phenolic groups are even less acidic in the lignin macromolecule with the concomitant higher pKa than the reported. A minimum of 2 M in NaOH is referred in some

Fig. 12.7 Proposed mechanism for lignin oxidation by Tarabanko et al. [140, 141] here represented for a typical guaiacyl type unit patents [148, 152]. Therefore, the initial pH value in the range 13-14 has been used in several works to maintain a high alkalinity in the entire reaction time. One other side, in the proposed mechanism by Tarabanko et al. [140, 141] (Fig. 12.7) the strong alkaline medium is required also for the proton detachment (step III-IV) and nucleophilic addition of OH — to the intermediary quinonemethide (step V-VI) and also for the final retroaldol cleavage (final step in Fig. 12.7). In fact, this step is the main difference between the mechanism proposed by Tarabanko for vanillin production and the mechanism via dioxetane formation established by Gierer et al. [146].

The energy barrier for electron transfer from the organic substrate to the oxidant is usually high. Considering this, the temperature should be one of the most significant factors to consider in lignin oxidation. Catalyst has also been consid­ered to improve the yields and selectivity.

Lignin oxidation with O2 in alkaline medium has been intensively studied in Laboratory of Engineering of Separation and Reaction (LSRE, Porto) [20, 36, 116-122, 153, 154]. The batch experiments have been performed in a jacketed reactor Buchi with a capacity to 1 l with control and register temperature, pressure and gas flow. The reaction mixture composed by NaOH and lignin is kept at high stirring and the reactor is purged and pressurized with N2. At steady state tem­perature, the oxygen is introduced at controlled pressure and the reaction is con­sidered to start at this point. During the reaction, the total pressure is maintained constant thought feed of O2. Samples are collected at controlled time intervals and, after acidification, the compounds are extracted with organic solvent and GC-FID analysis or recovered by solid-phase extraction and analyzed by HPLC-UV using external calibration [154].

Fig. 12.8 Products concentration and temperature evolution during the reaction time for lignin oxidation with O2 in alkaline medium (Tj = 393 K, pO2 = 3 bar, Pt = 9.7 bar, CL = 60 g/l, CNaOH = 80 g/l) for two different lignins [20]. a Kraft lignin from softwood isolated by Lignoboost process (supplied by Innventia AB) referred as LKBoostS in Table 12.2 and Table 12.3. b Organosolv beech wood lignin (supplied by Fraunhofer, Germany) referred as LOrgsB in Table 12.2

• Hydraulic washing: This process separates the heavier ore particles from the lighter gangue particles. This is done by washing them in a stream (jet) of water over a vibrating, sloped table with grooves. Denser ore particles settle in grooves. Lighter gangue particles are washed away (Fig. 14.10).

• Froth floatation: In this process, separation of the ore and gangue particles is done by preferential wetting. This process is generally used for sulfide ores of copper, lead, and zinc. The finely powdered ore is mixed with water and suitable oil in a large tank. A current of compressed air agitates the mixture. The ore particles are wetted by oil and form froth at the top, which is removed. The gangue particles wetted by water settle down. Ore preferentially wetted by oil is removed as froth. Gangue wetted by water is removed after it settles down (Fig. 14.11).

• Magnetic separation: This process is used in the extraction of metals which exhibit magnetic properties. For example, in the extraction of iron, crushed magnetite ore (iron) particles are separated using their magnetic property. The pulverized ore is moved on a conveyor belt. Electromagnetic wheel of the conveyor attracts only the magnetic particles into a separate heap. Only the magnetic particles are attracted by the magnetic wheel. These particles fall separately into a different heap (Fig. 14.12).

• Chemical separation: This process utilizes the difference in some chemical properties of the metal and gangue particles for their separation. For example, in the Bayer’s process of aluminium extraction, the bauxite ore is treated with hot sodium hydroxide solution. Water-soluble sodium aluminate formed is filtered to separate the undissolved gangue particles. Sodium aluminate (NaAlO2) is further processed to get aluminium oxide (Al2O3).