I: Carbon Nanomaterials
Biocomposite Materials Based on Carbonized Rice Husk in Biomedicine and Environmental Applications
Zulkhair A. Mansurov,a,b Jakpar Jandosov,a,b D. Chenchik, Seitkhan Azat,a,b Irina S. Savitskaya,b Aida Kistaubaeva,b Nuraly Akimbekov,b Ilya Digel,c and Azhar A. Zhubanovab
aThe Institute of Combustion Problems,
- 172 Bogenbay Batyr Str., Almaty 050012, Kazakhstan bAl-Farabi Kazakh National University,
- 71 Al-Farabi Ave., Almaty 050040, Kazakhstan cAachen University of Applied Sciences,
Heinrich-Mufimann-Str. l,Julich 52428, Germany
In this chapter, the future prospects for biomedical and environmental engineering applications of heterogeneous materials based on nanostructured carbonized rice husk are studied. The use of the nanostructured carbonized sorbents as delivery vehicles for the oral administration of probiotic microorganisms has a very big potential for improving functionality, safety and stability of probiotic preparations. The other possible mechanism of nanostructured carbonized sorbents is wound healing activity; the results demonstrated that the use of this material may offer multiple specific advantages in topical wound management. For bioremediation purposes, nanostructured carbonized sorbents can be applied as biocomposite sorbent with immobilized microbial consortium consisting of bacterial strains with high oil-oxidizing activity.
Rice husk, a by-product of rice milling industry, is a waste with the annual world production of ca. 545 million tons . In the rice producing countries, rice husk is used as a fuel. However, this product is characterized by low caloric value, 13-15 MJ/kg , and high mineral content. On the other hand, rice husk as a lignocellulose biomass is a valuable carbonaceous precursor that can be used to obtain a carbon material with special textural properties, high specific surface area and large pore volume 
The manufacture processes of activated carbons include physical or chemical methods. Both methods have previously been investigated to produce activated carbons from rice husk. Activated carbons with surface area of 273 m2/g were produced from rice husk by one-step steam activation ; activated carbons with surface areas as high as 3000 and 2500 m2/g were prepared from rice husk by KOH and NaOH chemical activation [5, 6]. Activated carbons with surface areas of 1100 m2/g in pores greater than 1 nm was prepared from rice husk by ZnCl2 activation . Activated carbons prepared from rice husk by H3P04 activation have been investigated for the removal of different pollutants [8, 9], however, limited information about the properties of these carbons was provided. Highly mesoporous carbons with relatively low surface area and total pore volume were obtained from rice husk through a two-stage process (precarbonization followed by H3PO4 activation in temperature range of 700-900°C) . Activated carbon with BET surface area and total pore volume as high as 874 m2/g and 0.713 cm3/g was also prepared from rice husk by H3PO4 activation . Previous results indicated that activated carbons with desirable surface area and pore structure can be prepared from rice husk. However, high mineral content of activated carbons prepared from rice husk by H3P04 activation, which is usually in the range of 20-70% [8, 9, 11], prohibits their application and commercial production. In view of that phosphoric acid-activated carbons have been proven to be highly effective adsorbents for the removal of heavy metal ions from aqueous solutions  because of their remarkable high cation-exchange capacities, which is due to the existence of a large number of acidic surface groups that provide exchangeable protons . Considering its plentiful and renewable supply, rice husk as a starting material for the production of activated carbon by H3PO4 activation deserves more intensive investigation . In a multitude of previous studies on H3P04 chemical activation of lignocellulosic materials it has been suggested that the optimal conditions to attain highest surface area are 0.5 to 2 h of activation time, 450-550°C and H3P04/precursor (wt/wt) impregnation ratio of about 1.5 up to 2. To elucidate the trends of how these parameters affect the yield, specific surface area and development of pore structure of carbonized rice husk (CRH), reasonably broader ranges were chosen and concentration of leaching solution to remove most of minerals was added as a variable. To achieve this objective, a two-step H3P04 activation-desilication process (acid/ base treatment) was applied to a pseudo-random selection of rice husk samples (just 16 combinations to minimize the experiment).
When choosing adsorbents and catalyst supports, highly developed porous structure is a preferable property. A special niche in this field belongs to natural porous materials [15, 16] including rice husk [17, 18]. Usually, these materials are represented by powders or granule pieces having wide particle size dispersion and low mechanical strength. One of the promising directions of their practical application is extrusion as monoliths with a regular honeycomb structure.
The goal of this work is also to prepare honeycomb monoliths from carbonized rice husk with the emphasis on control of textural characteristics and extension of functional properties on the basis of well-known methods  and our previous experience [20, 21].
For direct extrusion of carbon monoliths, a large amount of binder should be added because of low plasticity of carbon materials. Natural clays, in particular montmorillonite Ca0.2(Al,Mg)2Si4O10 (0H)2»4H20 (Ca-M), are commonly used as a binder [20-22]. On the one hand, binder increases the plasticity of carbonaceous molding composition and mechanical properties of monoliths. On the other hand, the increased mechanical strength of products has negative effect on their porous structure since a part of pores is plugged with a binder. To enhance the porosity of composite monolith substrate the chemical treatment of finished monoliths or their initial components with KOH and Na2C03 solutions can be applied. This technique of alkaline treatment early was used for carbonized rice husk and a considerable development of the porous structure achieved [5, 6, 23,24]
The fact that microorganisms prefer to grow on liquid/solid phase surfaces rather than in the surrounding aqueous phase was noticed long time ago . Virtually any surface—animal, mineral, or vegetable—is a subject for microbial colonization and subsequent biofilm formation. It would be adequate to name just a few notorious examples on microbial colonization of contact lenses, ship hulls, petroleum pipelines, rocks in streams and all kinds of biomedical implants. The propensity of microorganisms to become surface-bound is so profound and ubiquitous that it vindicates the advantages for attached forms over their free- ranging counterparts . Indeed, from ecological and evolutionary standpoints, for many microorganisms the surface-bound state means dwelling in nutritionally favorable, non-hostile environments . Therefore, in most of natural and artificial ecosystems surface-associated microorganisms vastly outnumber organisms in suspension and often organize into complex communities with features that differ dramatically from those of free cells . Initially introduced as just an imitation of Mother Nature, artificial immobilization of cells and enzymes has now transformed itself into a valuable biotechnological instrument. Its growing practical application and development over years led to appearance of fascinating novel microbial and enzymatic technologies [29-31]. Research on the immobilized biocatalysts is currently conducted in many laboratories around the world. In Japan, USA and other countries immobilized microbial cells have been successfully applied for adsorption of heavy metals from dilute solutions [32,33], for purification of sewage  as well as for intensification of microbiological technologies (production of antibiotics, organic acids, sugar syrups, fermented drinks, etc.) . It was shown that immobilized cells allow conducting biotechnological process over extended periods of time, under strict control of the process kinetics, product quality and microbial activity . Immobilization of cells can be carried out mainly by two methods: by entrapment of the microorganisms into porous polymers or microcapsules or by binding to an organic or inorganic support matrix (adsorption methods). The latter is considered to be more suitable for retaining cell viability . Adsorption is also one of the easiest methods of immobilization of microbial cells, especially those that adhere naturally to the surfaces of materials . It should be noted here that rapid development of technology of receipt of the immobilized biocatalysts resulted in contradictory results. So, the first attempts of immobilization were related with adsorption of enzymes and cells on arboreal sawdust and coal. In these experiments, adsorption was accompanied by a considerable desorption. In this connection, regarding the simplicity and availability of adsorption immobilization it has been having a reputation like "easy come easy go.” Though never forgotten, in the past decade, adsorption methods of immobilization gained increasingly more interest caused by considerable expansion in assortment of carriers with outstanding absorption properties, by better understanding of mechanisms and approaches aimed on firm attachment of biocatalyst to a carrier and by development of new methods of surface conditioning . The adhesion of microbial cells to surfaces is rendered mainly by Van der Waals forces, ionic and covalent interactions, with considerable contribution of various microbial exopolymers . Traditionally, adsorption immobilization is regarded as consisting of several relatively distinct stages, including (a) adsorption of dissolved macromolecules on the surface; (b) diffusion and concentration of cells from the bulk phase to the surface; (c) reversible attachment of cells; (d) biosynthesis of anchoring polymers by the cells which leads to an irreversible attachment stabilized by covalent bonds and entropy-driven interactions. Selection of an appropriate adsorbent, especially for industrial process is based on several criteria. Most important among them are: (a) material's costs and availability in large amounts; (b) simplicity and efficacy of the immobilization process; (c) preservation of cell viability; (d) adsorbent's specific surface (capacity). There is no an ideal material so far but many these requirements are met by inorganic (sand particles, ceramics, metallic hydroxides and porous glass) and organic (charcoal, wood shavings and cellulose, polyurethanes) carriers.
For example, porous glass-based fixed-bed reactors are successfully used for of the aerobic  and anaerobic  biotechnological transformations. The immobilization process can be characterized by several parameters: initial biomass loading, retainment of biomass, strength of the adhesion, retainment of the activity of the biocatalyst, effectiveness of mass transfer, engineering realization and general operational stability. When microorganisms are immobilized by adsorption, the initial cell loading of the immobilization matrix is one of the limiting factors . The cell loading on the adsorbent is influenced by the physical and chemical properties of the adsorption material, of the microorganism to be immobilized and by the composition and parameters of the surrounding medium. Another critical point for a system with the cells immobilized by adsorption is the retainment of the biomass on the surface. The retainment is generally ruled by the adhesion strength, which can be described in kinetic and in thermodynamic terms. Concerning the biocatalyst viability/activity retaining, the immobilization by adsorption is probably the gentlest existing method . Because the adsorptive fixation occurs under "standard” conditions, no changes of the cultivation parameters are necessary to produce the immobilized biocatalysts. Compared to cell entrapment in organic polymers it can generally be assumed that during adsorption also the enzymatic activity can be preserved at a high level. Very often the activity of only one enzyme is responsible for the catalytic process of interest. In such a process the stability is characterized by the half-life of the enzyme. Enzymatic "half-lives” up to two years have been reported . Though adsorbed biocatalyst systems are easy to run and used for many years, there is still enough space for optimization . Development and probation of new types of heterogeneous composite materials, possessing advanced properties for biological catalysts, as carrier systems, as filters, etc., on the basis of attached enzymes or whole microbial cells is of great importance for biotechnological processes. These and other tasks are addressed by engineering enzymology—a scientific and technical discipline combining principles, theoretical approaches and practical methods of chemical and enzymatic catalysis, microbiology, chemical technology and biochemistry. Recent efforts in engineering enzymology are focused (among others] on the following directions:
- • development and optimization of immobilization methods leading to novel biotechnological and biomedical applications;
- • search of materials satisfying strict requirements of biotechnological processes (such as non-toxicity, mechanical stability, etc.);
- • construction of biocomposite materials based on individual enzymes, multi-enzyme complexes and whole cells, targeted on realization of specific industrial processes;
- • development of methods for modification of surface properties aimed on fine tuning and better control of the "biocatalyst-carrier” interface.
In the light of these challenges, nanostructured carbonized materials appear as an attractive substrate for designing and production of cost effective high-performance biocomposite materials.
-  Carbon Nanomaterials in Biomedicine and the Environment Edited by Zulkhair A. Mansurov Copyright © 2020 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4800-27-3 (Hardcover), 978-0-429-42864-7 (eBook)www.jennystanford.com