Several authors have been studying the influence of the reaction conditions and vanillin and syringaldehyde yield as depicted in Table 12.3. The most compre­hensive work found in literature is that developed in LSRE in batch [20, 36, 116, 118, 119, 121, 133, 153] and continuous processes [119, 120, 122, 133].

Figure 12.9 shows the vanillin concentration profile during reaction of lignin in aqueous NaOH with molecular oxygen considering variation of O2 partial pressure (pO2), initial concentration of NaOH (CNaOH), concentration of lignin (CL) and initial temperature (Ti) [36].

Vanillin oxidation was also studied [149] since the degradation of the produced vanillin is a key point on the sustainability of the process. Figure 12.10 shows the main results of experimental and simulation work in this subject [121, 149], where the influence of pO2, pH, and Ti are shown.

• Reduction: The process can be done by either heating the metal oxide or chemically reducing the metal oxide using chemical reducing agents such as carbon, aluminium, sodium, or calcium.

• Electrolytic reduction: Electrolytic reduction is the process used to extract oxides (or chlorides) of highly reactive metals like sodium, magnesium, aluminium, and cal­cium. Molten oxides (or chlorides) are electrolyzed. The cathode acts as a powerful reducing agent by supplying electrons to reduce the metal ions into metal. For example: Fused alumina (molten aluminium oxide) is electolysed in a carbon lined iron box. The box itself is the cathode. The aluminium ions are reduced by the cathode.

Fig. 14.12 Magnetic separation

The use of alkaline pretreatments is effective depending on the lignin content of the biomass. Alkali pretreatments increase cellulose digestibility and they are more effective for lignin solubilization, exhibiting minor cellulose and hemicellulose solubilization than acid or hydro-thermal processes [24]. Alkali pretreatment can be performed at room temperature and times ranging from seconds to days. It is described to cause less sugar degradation than acid pretreatment and it was shown to be more effective on agricultural residues than on wood materials [100]. In alkali hydrolysis possible loss of fermentable sugars and production of inhibitory compounds must be taken into consideration to optimize the pretreatment condi­tions. Sodium, potassium, calcium, and ammonium hydroxides are suitable alka­line pretreatments. NaOH causes swelling, increasing the internal surface of cellulose and decreasing the degree of polymerization and crystallinity, which provokes lignin structure disruption from 24-55% to 20% [101, 182]. The example of alkali hydrolysis cited below by using Lime pretreatment Ca(OH)2 removes amorphous substances such as lignin, which increases the crystallinity index. Lignin removal increases enzyme effectiveness by reducing non-productive adsorption sites for enzymes and by increasing cellulose accessibility [96]. Lime also removes acetyl groups from hemicellulose reducing steric hindrance of enzymes and enhancing cellulose digestibility [126]. Lime has been proven suc­cessfully at temperatures ranging from 85 to 150°C and for 3-13 h with corn stover or poplar wood [27]. Pretreatment with lime has lower cost and less safety requirements compared to NaOH or KOH pretreatments and can be easily recovered from hydrolysate by reaction with CO2 [126].

The composition of degraded hemicelluloses liquor obtained from organic acid fraction of the raw material is very complex and various pretreatments are needed for further fermentation. Especially many components that are inhibitory to microorganism should be removed. The hemicelluloses liquor from Milox pulping of reed was pretreated with powered activated carbon aimed at lowering the content of formic acid, and the remaining sugar fraction was subjected to lactic acid fermentation by Lactobacillus pentosus. Nearly complete conversion of sugars was achieved, i. e., the product contained 33 g/l lactic acid and 17 g/l acetic acid with volumetric productivity of 0.6 g/(l h) [121] .

Xylose can be produced by hydrolysis of straw with formic acid containing hydrochloric acid. In a previous study, the straw was hydrolyzed with saturated formic acid containing 10% HCl with a liquor solid ratio of 24 in 0.5 h at 65°C, to obtain xylose with yield of 23.62% [122]. The hydrolyzate was purified with D311 and activated carbon for decoloration and then crystallized, resulting in a pure xylose fraction with a yield of 15.87%.

The isolation of vanillate from kraft lignin oxidation media by ultrafiltration (UF) has been investigated by Zabkova et al. [135]. The higher molecular weight com­pounds can be easily retained using membranes technology. During the UF process vanillin is collected in the permeate stream, whilst the lignin as a macromolecule stays in the retentate. The appropriate size of membrane cut off can significantly reduce the high molecular weight components from the lignin/vanillin mixture. Due to high physical and chemical resistance the ceramic membranes can be applied under strong pH conditions and high temperature. They observed a high flux decline at higher pH of the filtrated solution and ascribed it to the hydrophobicity membrane surface and solute. To obtain higher flux with acceptable rejection values, a scheme of staging UF membranes starting from larger cut off has been proposed. Formerly, UF was reported as process for fractionation of waste sulfite liquor to obtain a concentrated lignin-rich fraction in order to increase yields of vanillin and reduction of crust formation on reactors in the production of vanillin [169].

For industrial applications, hydrogen production rates and yields need to be maximized, and at the same time, energy inputs need to be kept low. Optimizing reactor configuration is therefore an important consideration for developing a fermentative hydrogen production process. For practical dark fermentative hydrogen production, it is likely that large-scale reactors would be required, however it can be hard to adequately control all the operational parameters at this scale. Therefore, before proceeding to large-scale applications, it would be important to define the minimum requirements of the system at lab scale followed by application at pilot scale before proceeding to full scale-up.

12.2.2.1 Origin and Isolation

Lignosulfonate is the resulting lignin from acid sulfite pulping of wood, which was the dominant process for cellulose production until it was surpassed by the kraft process in the 1940s. Sulfite pulps account now for less than 10% of the total chemical pulp production. In this process, sulfites (SO32-), or bisulfites (HSO3-) are the pulping agents depending on the pH [1, 18]. The counter ion can be sodium, calcium, potassium, magnesium, or ammonium, which could change the behavior of the lignin product. Rather than splitting of b-O-4 structures and lib­eration of hydroxyl phenolic groups, the main reaction during sulfite pulping is the introduction of sulfonic groups in Ca and Cy of C3-alquil lateral chain of ppus

a Values are reported to oven-dry weight of lignin material b Values corrected for ash and sugar content c Values reported to the non-condensed moiety of lignin

d Two distinct fractions of lignosulfonate [63]; the phenolic hydroxyl groups (OHph) per ppu were calculated based on data of the Ref. [63]

LKWest kraft lignin supplied by MeadWestvaco Corp. Indulin AT kraft lignin supplied by MeadWestvaco Corp., Сити 27 IIP kraft lignin supplied by Borregaard Lignoteck, LKBoostS Kraft lignin from softwood isolated by Lignoboost process, supplied by Innventia AB, LSBor Lignotech DP257 (high molecular fraction of a calcium softwood lignosulfonate supplied by Borregaard Lignotech, Norway, LOrgsB organosolv beech wood lignin supplied by Fraunhofer, Germany, Alcell Organosolv lignin from mixed hardwoods (maple, birch, and poplar) produced by Repap Enterprises, Inc, LKEg Eucalyptus globulus kraft lignin obtained from laboratorial kraft pulping (at industrial operating conditions) and isolated by acidification, LKBoostH kraft lignin isolated by Lignoboost process supplied by Innventia AB, LSEg Magnesium lignosulfonate of Eucalyptus globulus’, sulfite liquor provided by Caima, S. A. Portugal and isolated as described in literature [63], LOrgsMxG organosolv lignin from Miscanthus x Giganteus (perennial grass), Sarkanda nonwood lignin obtained from a soda pulping-precipitation process supplied by Granit SA

(Fig. 12.3), the sulfonation, and the cleavage of a-aryl ether linkages (a-O-4) [1, 18]. Sulfonic groups increase the hydrophilicity of the lignin fragments, con­ferring them water solubility, allowing their removal from the polysaccharide matrix. In phenolic b-aryl ether structures, the initial sulfonation in the a-position may be followed by sulfidolytic cleavage of the b-aryl ether bond, but the extension of the reaction is lower than in kraft pulping [18].

Different methods have been developed for the separation of lignosulfonates from the sulfite liquor (namely from the dissolved carbohydrates) and for sepa­rating various molecular weight fractions. The traditional industrial process for recovery lignosulfonates is the Howard process, where lignosulfonate is precipi­tated in different stages from sulfite liquor by addition of lime [23]. Fermentation and yeast growth have been used to remove main sugars, allowing further utili­zation of lignosulfonates for several proposes. For lignin separation and frac­tionation, ultrafiltration [43-46], chromatographic processes [47] as adsorption in chitin [48], extraction with amines [49], liquid membranes [50] and combination of processes [51, 52] have been attempted.

Contamination of soils with toxic metals has often resulted from human activities, especially those related to mining, industrial emissions, disposal or leakage of industrial wastes, application of sewage sludge to agricultural soils, manure, fer­tilizer, and pesticide use. Excessive metal concentration in soil poses significant hazard to human, animal and plant health, and to the environment. The aim of phytoextraction is to reduce the concentration of metals in contaminated soils to regulatory levels within a reasonable time frame. This extraction process depends on the ability of selected plants to grow and accumulate metals under the specific climatic and soil conditions of the site being remediated.

It uses plants to remove metals from soils and to transport and concentrate them in above-ground biomass [4]. In this process, plant roots sorb the contaminants along with other nutrients and water. The contaminant mass is not destroyed but ends up in the plant’s shoots and leaves. This method is used primarily for wastes containing metals where water-soluble metals are taken up by plant species selected for their ability to take up large quantities of metals. The metals stored in the plant’s aerial shoots are harvested and smelted for potential metal recycling/ recovery which were earlier disposed off as a hazardous waste. As a general rule, readily bio-available metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium, and copper. Moderately bio-available metals are cobalt, manganese, and iron. They can be made much more bio-available by the addition of chelating agents to soils.

Phytoextraction has been growing rapidly in popularity worldwide for the last 20 years or so. In general, this process has been tried more often for extracting heavy metals than for organics. The plants absorb contaminants through the root system and store them in the root biomass and/or transport them up into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. After the process, the cleaned soil can support other vegetation.

There are two main categories of plants to clean up toxic metals from soil:

• Metal hyper-accumulator plants

• High biomass plants

Organosolv fractionation of lignocelluloses has a long history, which undergoes a change from structure study to pulping, and currently to energy usage. The earliest study applying organic solvents to treat lignocellulosic material was back in 1893, when Klason [9, 10] used ethanol and hydrochloric acid to separate wood into its components to study the structure of lignin and carbohydrates. After that, Pauly et al. [11, 12] applied formic and acetic acids to delignify wood for the purpose of characterization of the main components of wood in 1918. Subsequently, a wide variety of other organic solvents, e. g., various alcohols, phenol, acetone, propionic acid, dioxane, various amines, esters, formaldehyde, chloroethanol, whether pure or in aqueous solutions, and in the presence or absence of acids, bases or salts as catalysts, were used to delignify lignocellulosic materials [3]. Since 1980s, a number of pulping processes involving the aforementioned solvents have been investigated as alternatives to the classic pulping process in the field of pulp and paper industry [13]. The main advantage of the so-called organosolv pulping process was a higher efficient use of the raw materials in an environmentally friendly way, as compared to the drawbacks of the classic pulping process (e. g., odors, low yields, high pollution, poor bleachability of pulp and high investment cost). In 1992, two organosolv pulping processes based on methanol— Orgnaocell and alkaline sulfite-antraquinone-methanol (ASAM) were first oper­ated at a full scale. Meanwhile, the organic acid pulping processes, Acetosolv (based on acetic acid) and Milox (based on formic acid with the addition of hydrogen peroxide) were at a pilot scale [14]. Many such processes were employed to obtain multiple products, i. e., hydrolyzable cellulose, sugars and high quality lignin other than pulp, aimed at exploiting the full potential of the feedstocks. More recently, ethanol pulping process is modified from a pulping process to a pretreatment process integrated with biofuel production, mainly aimed at obtaining hydrolyzable cellulose fraction for the production of ethanol [15].

The effect of pO2 in the range 2.1-6.6 bar (continuous supply) was tested for lignin (softwood kraft lignin supplied by MeadWestvaco Corp.) at initial con­centration 60 g/l and initial temperature 393 K with a total pressure (N2, O2 plus water vapor) of about 9.3 bar [36]. The results demonstrate that the main effect of pO2 was on the rate of vanillin formation, shortening the time to maximum, with no influence on yield [118], as depicted in Fig. 12.9a. Other authors [110, 125] have studied the effect of O2 using two approaches: (1) continuous supply of

oxygen to the reactor, maintaining the initial pO2 in the course of reaction; (2) O2 introduced at the beginning and immediately interrupted. In the second case, it was observed a rapid decrease of pressure due to the O2 consumption in reaction. While Xiang and Lee [125] reported lower yields on vanillin and syringaldehyde for the first approach, Wu et al. [110] demonstrated that the yields of products did not change, but the rate of vanillin and syringaldehyde formation was higher for the reaction with continuous supply of O2, in accordance with Mathias’s results [36, 118]. Furthermore, after the maximum, an accentuated decrease of aldehydes concentration was noticed for constant supply of O2. This observation is in accordance with the higher rate of vanillin decline after the maximum depicted in Fig. 12.9a and in Fig. 12.10a for higher initial pO2. In the oxidation of a hardwood lignin [110], this effect was more evident for syringaldehyde probably due to its higher reactivity than guaiacyl counterparts in alkaline systems [156] and under conditions of O2 oxidation in alkaline medium [157] leading to its faster degradation.

The origin of lignin and, consequently, its structure and reactivity have influ­ence on kinetic parameters. The reaction order of vanillin production with respect to the oxygen concentration was 1.75 [116] for Westvaco Co. kraft lignin and 1.00
for an Eucalytus lignosulfonate (for both vanillin and syringaldehyde), showing that the oxidation of first lignin has a higher dependence of oxygen concentration in reaction medium.