• Electrolytic refining: The process of electrolysis is used to obtain very highly purified metals. It is very widely used to obtain refined copper, zinc, tin, lead, chromium, nickel, silver, and gold metals. In this process, the anode is made as impure slab of metal and cathode as pure thin sheet of same metal and a salt solution of the metal is used as the electrolyte. On passing current, pure metal from the electrolyte is deposited on the cathode. The impure metal dissolves from the anode and goes into the electrolyte. The impurities collect as the anode mud below the anode (Fig. 14.13).

• Liquation process: In this process, the block of impure metal is kept on the sloping floor of a hearth and heated slowly. The pure metal liquefies (melts) and flows down the furnace. The non-volatile impurities are infusible and remain behind (Fig. 14.14).

• Distillation process: In this process, metals with low boiling point, such as zinc, calcium, and mercury are vaporized in a vessel. The pure vapor are condensed into pure metal in a different vessel. The non-volatile impurities are not vaporized and so are left behind.

• Oxidation process: In this process, the impurities are oxidized instead of the metal itself. Air is passed through the molten metal. The impurities like phos­phorus, sulfur, silicon, and manganese get oxidized and rise to the surface of the molten metal, which are then removed.

All these methods are effective but result in the generation of toxic chemical sludges and waste products.

Another approach which involves aqueous chemistry for the recovery of pure metals from ores is termed as hydrometallurgy. It is typically divided into three general areas:

• Leaching

• Solution concentration and purification

• Metal recovery

Fig. 14.13 Electrolytic refining

Fig. 14.14 Liquation process

Leaching

Leaching involves the use of aqueous solutions containing a lixiviant is brought into contact with a material containing a valuable metal. The lixiviant in solution may be acidic or basic in nature. In the leaching process, oxidation potential, temperature, and pH of the solution are important parameters, and are often manipulated to optimize dissolution of the desired metal component into the aqueous phase. The three basic leaching techniques are in situ leaching, heap leaching, and vat leaching.

After leaching, the leached solids and pregnant solution are usually separated prior to further processing.

Solution concentration and purification

After leaching, the leach liquor must normally undergo concentration of the metal ions that are to be recovered. Additionally, some undesirable metals may have also been taken into solution during the leach process. The solution is often purified to eliminate the undesirable components. The processes employed for solution concentration and purification include:

• Precipitation

• Cementation

• Solvent Extraction

• Ion Exchange

Metal Recovery

Metal recovery is the final step in a hydrometallurgical process. Metals suitable for sale as raw materials are often directly produced in the metal recovery step.

Sometimes, however, further refining is required if ultra-high purity metals are to be produced. The primary types of metal recovery processes are electrolysis, gaseous reduction, and precipitation.

Enzymatic hydrolysis has an upper edge over acid hydrolysis to produce sugars for alcohol fermentations. Enzymes are naturally occurring plant proteins that cause certain chemical reactions to occur. There are two technological developments: enzymatic and direct microbial conversion methods. The chemical pretreatment of the cellulosic biomass is necessary before enzymatic hydrolysis. The first appli­cation of enzymatic hydrolysis was used in separate hydrolysis and fermentation steps. Enzymatic hydrolysis is accomplished by cellulolytic enzymes. Different kinds of “cellulases”, i. e., endoglucanases, exoglucanases, glucosidases, and cellobiohydrolases are commonly used [75, 107] to cleave cellulose and hemi — cellulose. The endoglucanases randomly attack cellulose chains to produce poly­saccharides of shorter length, whereas exoglucanases attach to the non-reducing ends of these shorter chains and remove cellobiose moieties, glucosidases hydrolyze cellobiose, and other oligosaccharides to glucose [142]. In order to enhance the susceptibility of cellulose for enzymatic hydrolysis, the pretreatment of cellulosic material is, therefore, an essential prerequisite. Physical and chemical pretreatments like ball milling, irradiation, alkali treatment, acid treatment, hydrogen peroxide treatment are highly recommended to enhance saccharification of cellulosic material after their enzymatic hydrolysis [6, 167].

So far, cellulose has been hydrolyzed with enzyme cellulase only at pilot plant scale. The process is divided into many steps and includes two basic inputs, namely, nutrients for the fungus and cellulosic material to be hydrolyzed. The nutrients supply include nitrogen and other supplements required for the growth of celluloytic microorganisms and is given in the form of sterilized nutrient medium. Cellulosic materials are pretreated. The celluloytic microorganism is grown and subsequently the enzyme is produced. The microorganism (such as fungus) is propagated as a submerged culture in a fermentation unit equipped for mixing and aerating the growth medium.

In the cellulose hydrolysis or saccharification step, the enzyme produced in the previous step comes into contact with the pretreated cellulosic materials. The enzyme solution hydrolyzes the solid cellulose to the glucose units. The product stream is continuously withdrawn from the unit. Finally, the glucose solution is separated from unhydrolyzed cellulose by filtration. The glucose solution can be used for fermentation to ethanol.

The rate and extent of enzymatic hydrolysis is affected by the pretreatment method, substrate concentration and accessibility, enzyme activity, and reaction conditions such as pH, temperature and mixing [121, 181]. Different strategies for enzymatic hydrolysis and ethanolic fermentation have been developed to address specific process engineering issues (Table 9.14).

In addition to the aforementioned fractionation process, other organic solvents, such as methanol, ethylene glycol, ethanolamine, acetone and dimethyl formam- ide, also attract much attention in fractionation lignocellulosic material in recent years. The typical processes are listed in Table 11.3 [134-145].

11.5.1 Methanol

Methanol fractionation of lignocellulosic material can be carried out without or with the addition of catalysts. In non-catalyzed (auto-catalyzed) methanol frac­tionation of lignocellulosic material, the cooking liquor becomes acidified due to the acetic acid released from the feedstock. In catalyzed processes, the liquor can be acidic, neutral or alkaline depending on the nature of the additives employed. During acid ethanol fractionation process, lignin is mainly dissolved by cleavage of a-aryl ether and arylglycerol-b-aryl ether bonds in the lignin macro­molecule [146]. Whereas the cleavage of b-aryl ether bonds occurs to a lower extent [147]. The cleavage of ether bonds gives rise to new phenolic hydroxyl groups in lignin.

Some lignocellulosic materials, such as wheat straw [148], Eucalyptus globulus [134, 135] and poplar [149] can be delignified by methanol/water without the addition of catalyst. The optimum conditions result in pulps with a high yield and a low kappa number and an acceptable viscosity. By the addition of sulfuric acid as a catalyst, black cottonwood can be delignified in 70% methanol at temperatures ranging from 130 to 210°C. In a typical reaction, pulp with a high yield of 47% and a low kappa number of 8 was obtained in 3 h. The recovered lignin had a high molecular weight, indicating it was a potential chemical feedstock [150]. Aspen (Populus tremuloides) and black cottonwood (Populus trichocarpa) have been fractionated in 30-70% methanol catalyzed with H2SO4 and H3PO4 at pH lower than 3 [151]. After the pretreatment, glucomannan and arablnogalactan were dissolved into liquor and were easily digested by enzymes. The total yields of hydrolysis residues ranged from 40 to 60%, which generated 70-88% of the original six-carbon sugars contained in the wood by further enzyme hydrolysis.

Miscanthus x giganteus was pulped in the alkaline-methanol-anthraquinone process to prepare pulp for thermoplastic composite reinforcement [136]. Under the optimum conditions, methanol concentration 10% (v/v), alkaline concentration 15%, pulping time 25 min, pulping temperature 170°C, the produced pulp had a high thermally stable temperature of 255°C and an aspect ratio of 40, a straightness of 95% and high tensile strength of 890 MPa. The obtained pulp with good strength and thermal properties was an attractive low-weight and low-cost sub­stitute for short glass fiber.

Klemola and Tuovinen [170] have developed the technology of supercritical extraction applied to the vanillin production process in order to replace extraction with organic solvents and reextraction to aqueous solution. After the air oxidation of lignin under alkaline conditions, the resulting solution is submitted to a supercritical carbon dioxide flow in the range of operation 75-400 bars and 303-373 K extracting vanillin and other chemically related compounds. The vanillin dissolved in CO2 can be recovered by passing the gas flow into a receiver with suitable pressure and temperature conditions. The supercritical extraction can also be associated to the bisulfite treatment for vanillin recovery [171]. These solutions are treated with supercritical CO2 and then the gas flow passes through an aqueous bisulfite solution that dissolves vanillin and liberates the CO2 for reuse. Subsequently, the aqueous solution containing vanillin-bisulfite adducts is acidi­fied with sulfuric acid and heated up to 90°C. After the breakage of adducts by acidification, the aqueous solution is cooled off and the vanillin crystallizes reaching to an appreciable purity.

The final product with up to 85-90% of vanillin can be further purified by successive crystallization and dissolution steps in methanol:water [172], fractional precipitation using magnesium or zinc salts [164] successive liquid-liquid (co-) extractions in alkaline solutions and n-butanol and vacuum distillation with or without an inert [173]. The final purification represents a difficult task because the phenolic impurities have very similar chemical and physical properties to vanillin, such that conventional fractionation techniques are inadequate and only multistage crystallization could lead to a final product of the desired high purity [173]. The main impurities of vanillin obtained from lignin processes mainly consist of vanillin-related species as o-vanillin, 5-formyl vanillin, vanillin acid, and aceto — vanillone. Apart from multiple water-methanol crystallization process, the purified vanillin can be obtained also by one or more crystallizations from water, using charcoal to adsorb last traces of impurities [172, 174]. Ibrahim et al. [136] reported the separation of vanillin from soda lignin, (from the black liquor of oil palm empty fruit bunches) by crystallization based on the solubility of vanillin in ace­tone. Afterwards, they developed the molecular imprinting polymer technique that allowed removing additional impurities in the sample.

A practical source of mixed cultures can easily be obtained from industrial wastewater treatment plants since they are non-sterile. However, this provides a very complex microbial community which includes together hydrogen producing and hydrogen consuming bacteria. Therefore, it is important to select a proper pretreatment method that will adequately select for the hydrogen producers and reduce or eliminate the hydrogen consumers. It is not usually possible to produce hydrogen by using typical anaerobic sludge because in anaerobic digesters hydrogen serves merely as a transitory intermediate, being almost immediately consumed in a CH4 producing reaction. By using the physiological differences between the cultures using pretreatment can eliminate competing organisms from the system. The successful application of various pretreatment methods; heat shock, chemical, acid, alkaline, ultrasonic, etc. has been reported. Each method has its own efficiency according to its composition. Combinations of different methods can also be used. Therefore, since the mixed culture does not have the same nature before using it for dark fermentative hydrogen production it is important to select the best suitable pre-treatment according to process efficiency and economical considerations [87].

The weight-average molecular weight (Mw) reported for softwood lignosulfonates are in the range of 10-60 kDa with high polidispersity [23, 60, 61]. These lignins present, in general, higher molecular weight and have fewer hydroxylphenolic (OHph) groups than kraft lignin [20, 23] (as depicted in Table 12.2) which is in accordance with low rate of cleavage of ether linkages of lignin reported for sulfite pulping. The condensation reactions in sulfite pulping and kraft pulping follow the same pattern, leading mainly to Ca-aryl linkages (diphenylmethane structures) [18]. The increase of condensed lignin in spent liquor results from these reactions, and also from the dissolution of condensed lignin moieties present in wood lignin. The frequency of sulfonation is about 0.4-0.5/ppu [62].

Published studies concern mostly softwood lignosulfonates. However, more recently, hardwood lignosulfonates have been the subject of increasing research and interest [55, 61, 63-65] and substantial differences on characteristics, partic­ularly concerning Mw, have been found. Recent results concerning Eucalyptus globulus lignosulfonates from Caima, Industria de Celulose S. A., in Portugal showed an high content of partially sulfonated fragments with rather low molec­ular weight (around 1 kDa) formed via cleavage of b-O-4 bonds [63]. The Mw of Eucalyptus globulus lignosulfonates (1-2.4 kDa) is even lower than the already low value reported for lignosulfonates from other hardwoods (5.7-12 kDa) as compared to typical Mw of softwood lignosulfonates [60, 61]. Some of lignins characteristics are summarized in Table 12.2.

They take up significant amounts of metal from contaminated soil but their low biomass production tends to limit their phytoextraction ability. As these plants have natural ability to extract metal ions, so it is known as natural phytoex­traction. Hyper-accumulating plants have natural ability to extract high amounts of metals from soil, have efficient mechanism to translocate metals from roots to shoots, and can accumulate and tolerate high metal concentrations due to inherent mechanisms to detoxify metals in the tissues. Metal hyper-accumulators have the extraordinary capacity to accumulate high concentrations of heavy metals in the above-ground biomass. By virtue of this remarkable characteristic, phytoextraction is economically viable alternative to the extreme expense of conventional reme­diation methods [5].

Example: Alyssum lesbiacum as Ni hyper-accumulator, Thlaspi caerulescens/ Alpine pennycress as Zn/Cd hyper-accumulator (Figs. 14.1, 14.2).

Ethanol has been used to split lignocelluloses into their components to study the structure of lignin, and used as a pulping agent in organosolv pulping. Recently, ethanol fractionation is becoming a major fractionation process among the organosolv fractionation processes for pretreatment lignocellulosic material to produce bioethanol. Generally, ethanol fractionation process is carried out under elevated temperatures without or with the addition of acidic or alkaline catalyst, and some organosolv fraction processes with ethanol are illustrated in Table 11.1 [16-26].

11.3.1 Effect of Treatment on the Structure of Lignocellulosic Material

11.3.1.1 Severity Parameter

Under given conditions in ethanol fractionation (auto — and acid — catalyzed frac­tionation processes), reaction temperature, reaction time and the concentration of H+ are the major contributed parameters to the severity of fractionation. A pro­posed parameter to describe the severity for ethanol fractionation is defined as a severity parameter:

where t is the reaction time (min), and T is the reaction temperature (°C), and [H+] represents the pH of the cooking liquor at 20°C for the solutions.

The effects of severity parameter on the removal of lignin and hemicelluloses are different. Cooking liquor rich in ethanol acts as an effective solubilizer of lignin, but the elution of hemicelluloses is minor. It has been reported that under the highest severity value, about 80% of the original lignin was dissolved into the solution as compared with a low value of around 30% for hemicelluloses [22].

Figure 12.9b reveals the importance of the OH — on the yield of the process of lignin oxidation at the conditions of this study: a considerable increment of vanillin is achieved by increasing NaOH concentration. The reasons for this impact on yield were already stated in Sect. 4.4.1 (regarding postulate chemical mechanism of oxidation). However, other considerations should be noted: the concentration of vanillin at each moment is the result of formation and degra­dation in the reaction medium. Fargues et al. [149] studied the kinetics of vanillin oxidation and concluded that at pH <11.5 the vanillin oxidation become more significant being of second order in vanillin concentration and zero order in O2 concentration. At pH >11.5, the reaction rate of vanillin oxidation is first order for both vanillin and O2 concentration. Therefore, at least 2 M in NaOH is required to achieve the favorable condition to preserve the produced vanillin. Figure 12.10b demonstrates clearly the initial pH effect on vanillin oxidation. It should be pointed out that the pH is the parameter in discussion, since the same alkali concentration could lead to different values for aqueous solutions of different lignins, depending of the raw material composition. Considering this, the pH of the final solution should be measured at each case, confirming the required value. The operational problems (incrustation in the reactor) related with the solution NaOH 4 M had led Mathias et al. to avoid to such concen­tration and to adopt the 2 M.

For lignosulfonate (from E. globulus), the reaction order relative to OH — concentration is 1.9 for vanillin and 1.4 for syringaldehyde. The reason suggested for these high values was the step involving the desulfonation reaction in the case of lignosulfonates (carrying SO3- groups at Ca and Cy see Fig. 12.3) [113]. The removal of sulfonic groups leads to the formation of units with double bonds in propane lateral chain (as intermediate IIa in Fig. 12.7), species that are naturally more reactive with O2 than the saturated counterpart.

There are number of ligands capable of binding metal ions through multiple sites, usually because they have lone pairs on more than one atom. Ligands that bind via more than one atom are often termed chelating ligands. The organic moiety that can trap or encapsulate the metal ion, forming coordinate bond through two or more atoms, to form a chelate is known as chelating agent/ligand. So, ‘‘chelate’’ denotes a complex between a metal and a chelating agent. A chelating agent can be chemically anchored on various inorganic polymeric solid supports to form ‘‘chelating resin’’. The ligand/agent attached to chelating resin makes it specific and selective for extraction of a particular metal ion (Fig. 14.15).

Various solid supports that are used for scavenging of metal ion are: Chelamine, Silica gel, Amberlite, XAD, Polyurethane foam, Polyacrylonitrile, and Activated Carbon.

The tremendous amount of biomass which is produced after phytoextraction is rich source of heavy metals drawn from the soil which are otherwise the major environmental concern. This biomass is digested and a particular metal specific chelating resin, which possesses high selectivity to the targeted metal ion in a par­ticular pH — range, is used for separation of metal ion. An assortment of novel metal specific chelating resin has been designed which can be easily recovered and reused several times making the process environmentally benign and green (Table 14.2).

Extraction of metal ions from biomass using specifically designed chelating resin has numerous advantages [26]:

• Selective extraction of metal ions is possible by using a chelating resin having multidentate ligand as it possesses high selectivity to the targeted metal ion.

• The chelating sorbent method is an economical method since it uses only a small amount of resin and is free from difficult phase separation and extraction solvent.

• As the target ion specific chelating agent is enriched on solid phase, even ppb level concentrations can also be extracted.

• The chelating resin can be recycled and reused several times as they can be easily recovered merely by filtration and have high physical and chemical stability.