Utilization of rice husk ash as novel adsorbent: A judicious recycling of the colloidal agricultural waste

Utilization of rice husk ash as novel adsorbent: A judicious recycling of the colloidal agricultural waste

Advances in Colloid and Interface Science 152 (2009) 39–47 Contents lists available at ScienceDirect Advances in Colloid and Interface Science j o u...

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Advances in Colloid and Interface Science 152 (2009) 39–47

Contents lists available at ScienceDirect

Advances in Colloid and Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c i s

Utilization of rice husk ash as novel adsorbent: A judicious recycling of the colloidal agricultural waste K.Y. Foo, B.H. Hameed ⁎ School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

a r t i c l e

i n f o

Available online 2 October 2009 Keywords: Adsorbent Biomass Renewable Rice husk Rice husk ash

a b s t r a c t Concern about environmental protection has aroused over the years from a global viewpoint. To date, the ever-increasing importance of biomass as the energy and material resources has lately been accounted by the rising prices for the crude petroleum oil. Rice husk ash, the most appropriate representative of the high ash biomass waste, is currently obtaining sufficient attraction, owning to its wide usefulness and potentiality in environmental conservation. Confirming the assertion, this paper presents a state of the art review of the rice milling industry, its background studies, fundamental properties and industrial applications. Moreover, the key advance on the preparation of novel adsorbents, its major challenges together with the future expectation has been highlighted and discussed. Conclusively, the expanding of rice husk ash in the field of adsorption science represents a viable and powerful tool, leading to the superior improvement of pollution control and environmental preservation. © 2009 Elsevier B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. The origin and critical properties of the rice crop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. The present status of the world production, consumption and world market of rice crop . . . . . . . . . . . . . . 4. Typical properties and usefulness of the rice husk, an abundantly available by-product from the rice processing industries 5. The generation of rice husk ash as a global environmental and health concern . . . . . . . . . . . . . . . . . . 6. Rice husk ash as a novel adsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Major challenges and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Water scarcity and pollution rank equal to climate change as the most urgent environmental turmoil for the 21st century [1]. Arising from the steep enrichment of globalization and metropolitan growth which has intensified numerous deteriorations on several ecosystems [2] and seriously threatens the human health and environment [3], adsorption process particularly by utilization of activated carbons has become the focus of intense research and applied to almost every field of chemistry. With the prices of the activated carbons hitting as high as US $602 per tonne in the world market and the global demand escalating to

⁎ Corresponding author. Fax: +60 45941013. E-mail address: [email protected] (B.H. Hameed). 0001-8686/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2009.09.005

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an unprecedented height every other day (800,000 tonnes per year), the quest for an alternative adsorbent which is affordable and environmentfriendly is inevitable [4]. Undoubtedly, agricultural waste biomass is presently one of the most challenging topics, which is gaining stern considerations during the past several decades [5]. In perspective, rice husk ash, an agro-based waste collected from rice mill boilers, has emerged to be an invaluable source mainly in the third-world economies, implying a generation rate of approximately 0.046 tonnes for every ton of rice produced. Over the years, a number of studies and researches have drastically been addressed and confronted for the conversion of rice husk biomass into the high value-added and useful income-generating products [6]. With the aforementioned, this bibliographic review attempts to postulate an initial platform in describing the origin, properties, developments and potential applications of the rice milling industry. The present work is

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aimed at providing a concise and up to date picture of the present status of rice husk waste enhancing sustainable and renewable energy. The prospects towards utilization of rice husk ash as renewable sources (especially in accessing an ideal adsorption system) together with its comprehensive literature has been highlighted and outlined, to familiarize the knowledge deficiencies regarding rice milling industry. 2. The origin and critical properties of the rice crop Rice is one of the major crops grown throughout the world [7], sharing equal importance with wheat as the principal staple food and a provider of nourishment for the world's population [8]. Covering 1% of the earth's surface, rice is now a way of life, being grown on every continent and is deeply embedded in cultures, rituals and myths [9]. In Asia alone, rice constitutes as much as 60 to 70% of the total calorie uptake on average for more than 2000 million people. In Africa and Latin America, the production and consumption of rice is expanding, as the second most consumed cereal grain for food security in lowincome and food-deficit countries [10]. Generally, rice is a cereal foodstuff grown as a monocarpic annual plant, can survive as a perennial, producing a ratoon crop up to 20 years [11]. Domesticated rice rice can grow up to 1 to 1.8 m tall, occasionally depending on the variety and soil fertility, where its grass has long, slender leaves with 50 to 100 cm long and 2 to 2.5 cm broad. Meanwhile, the small wind-pollinated flowers are produced in a branched arching to pendulous inflorescence with 30 to 50 cm long and the seed is a 5 to 12 mm long and 2 to 3 mm thick edible grain (caryopsis) [12]. Practically, domesticated rice comprises two species of food crops in the Oryza genus of the Poaceae family, namely Oryza sativa (Asian rice), which contains two major subspecies: the sticky, shortgrained japonica or sinica variety, and the non-sticky, long-grained indica variety, and Oryza glaberrima (African rice) that is native to the West Africa [13]. Ample ancient historical documents, the cultivated rice varieties have been discovered in Yangtze valley, China dating as early as 8500 B.C. (before century), which signifies the late Pleistocene era. In less than 2000 years, O. sativa (Asian rice) which forms the basis of the most popular rice varieties today was domesticated in southern China. Likewise, in the wet western regions of African continent a local variety, O. glaberrima (African rice) is believed to be found 3000 years later. In South Asia, remains of rice have been recognized in Lothal (2300 B.C.) and Rangpur (2000 to 1800 B.C.), both interestingly dry areas in the modern Indian state of Gujerat. By 300 B.C., rice cultivation has widespread to peninsular India and by 100 B.C., Chinese traders had brought rice to early cultivations in the Philippines, leading to the creation of the vast terraced rice farms along the Philippine Cordilleras [10]. 3. The present status of the world production, consumption and world market of rice crop According to the statistical data of Food and Agriculture Organization (FAO), the global annual paddy production was reported at 582 million tones, cultivated in 171 million hectares of land [14]. Given its history, in 1960, the world rice generation was recorded at 200 million tones, which in the year of 2006 and 2007, the figure has steadily risen, designated as 488 and 498 million tonnes respectively [15]. By November 2008, the annual production is forecasted at 504 million tonnes [16], dominated by the Asian region, of which China appears to be the largest producer, following by India and Indonesia, denoting approximately 90% of the total world production [17]. Simultaneously, worldwide consumption of milled rice has increased sharply over the last 30 years. From merely a total rice use of 61.5 kg per capita in the 1970s, today the world is exhibiting a necessity of 85.9 kg per capita, of which in some countries, per capita rice consumption is declining amidst the rising incomes [10]. In 2006,

the global rice consumption was achieved at 417 million tones, which in 2007, it has consistently risen to 424 million tonnes, underlying the exponential growth rate of population and social civilization, and development of industry and technology as its key drivers [16]. By November, 2008 and year of 2040, the figure is predicted to be further strengthened, individually estimated an annual consumption of 428 and 556 million tonnes [10]. Nevertheless, the global trade market is somewhere contradictory, which only about 5 to 6% of rice produced is traded internationally, with Thailand (26%), Vietnam (15%) and United States (11%) being the main exporters, while Indonesia (14%), Bangladesh (4%), and Brazil (3%) are among the top importing countries [18]. In step with the rapidly changing technologies and population growth, the heavy rice-producing countries are currently tightening and restricting the exports [19], preventing rice from escaping outwards to the international market, with Vietnam, Egypt and Guyana have issued the export bans, while India is essentially following suit. China on the other hand has switched its export subsidy to an export tariff, a cautionary measure in an unstable environment [20]. In view of the above matter, the daily bowl of rice is presently becoming more and more costly, which in the late of May 2008, the rice price has hit 24 cents a pound, twice the price that it was seven months ago [21]. 4. Typical properties and usefulness of the rice husk, an abundantly available by-product from the rice processing industries Concomitant with the rigorous development of the rice milling industries, rice husks (or rice hulls), an abundantly available byproduct, the fibrous hard outermost covering the grain of rice, is generated at 120 million tonnes per year, accounting about one-fifth of the annual gross rice production throughout the world [22]. In nature, rice husk is tough, insoluble in water, woody and characterized by its abrasive inherent resistance behaviour and silica-cellulose structural arrangement [23,24]. Its major constituents comprising of cellulose, hemicellulose, lignin, hydrated silica and ash content [25], which the exterior of rice husks are composed of dentate rectangular elements, of mostly silica coated with a thick cuticle and surface hairs, while the mid region and inner epidermis are usually containing a small amount of silica [26]. The chemical components of rice husks are found to be SiO2, H2O, Al2O3, Fe2O3, K2O, Na2O, CaO and MgO [22], fluctuating upon the varieties of paddy sown, proportion of irrigated area, geographical conditions, fertilizer used, climatic variation, soil chemistry, timeliness of crop production operations and agronomic practices in the paddy growth process [27,28]. Traditionally, rice hulls have been disposed in landfills [29], thereby resulting in a dramatic source of esthetic pollution, of eutrophication and perturbations in the aquatic life [30]. With the growing emphasis on environment-friendly industries, the departure of the concept of generating energy from rice husk has received great supports and encourages [31], particularly in those countries that are primarily dependant on imported oil as their energy needs. Rice husk, one of the largest readily available but most underutilized biomass resource, has often been linked with its low moisture content (in the range of 8 to 10%) and a calorific value of 12.1 to 15.2 MJ/kg, in heat generation for parboiling of paddy [32]. In fact, as early as the 1960s, the research and development (R&D) activities regarding rice hull power generation technology has been adopted, leading to the invention of gasification and power generation system [33]. In Indonesia, most rice husk and diesel power plants are built next to the rice milling factories, supplying a power electricity of 1600 MW to the local community and the national grid [8]. During the past few years, the utilization of rice husks has been significantly widened, serving as an ideal source of pet food fiber, building and insulating materials for reinforcing the tensile strength [34], as fertilizers through vermin-composting techniques, as microbial

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nutrients for single-cell protein production [35], for reducing sugar production and as raw products in the manufacturing of ethanol and furfural [36]. In spite of various exploitations and extensive researches have been proliferated, such efforts are handicapped by the limitations of its low nutritive properties, degradation resistance, abrasiveness, low apparent density and high ash content, attributed to another form of pollution phenomenon—rice husk ash [37,38]. Increasingly, most of the rice husk utilization plans projected by the rice-producing countries: India, Pakistan, Bangladesh, Sri Lanka, Australia, Thailand, Indonesia and United States in the 1970s and 1980s, have currently been terminated and closing down, without establishing the real actions and concrete programs, diverting towards the global attention and critical environmental considerations [39].

developed countries, the urgency of transforming the residue into a more valuable end product has been promulgated [9]. Today, the wide usefulness of rice husk ash has been witnessed, particularly as pozzolanic materials in the manufacturing of high strength concrete and refractory bricks [39], as feedstock for silicon chips (Si, SiC, SiCl4 or liquid gas) processing industries [46], as high quality steel, vinegar-tar, insulation powder, release agents in ceramic production, rubber vulcanizing and reinforcing extenders, catalysts supports, fire extinguishing powder, beer clarifier, filler in polymer production and even as composites for improving of compressive, tear, hardness and abrasion resistances (Table 1) [9].

5. The generation of rice husk ash as a global environmental and health concern

Over the past several decades, intensive wide spread contamination of atmosphere and surface water related to extensive industrial operations has inspired a great global attention for many environmentalists [47]. Accordingly, the developing research by the invention of a large variety of treatment technologies (precipitation, coagulation–flocculation, sedimentation, flotation, filtration, membrane processes, electrochemical techniques, biological process, chemical reactions, adsorption and ion exchange) with varying levels of success has attracted a growing interest in the scientific community [48–55]. Adsorption is recognized as an efficient, promising and most widely used technique in the wastewater treatment processes, especially in developing countries which the issue is highly intense and touching, mainly hinges on its simplicity, ease of handling, economically viable, technically feasible and socially acceptable [56]. For this purpose, activated carbon (AC), an adsorbent with its large porous surface area, controllable pore structure, thermo-stability and low acid/base reactivity [57], has been proven to be effective for removal of a wide variety of organic and inorganic pollutants dissolved in aqueous media, or from gaseous environment [58]. Despite its prolific use in adsorption processes, the biggest barrier of its application by the industries is the cost-prohibitive adsorbent and difficulties associated with regeneration [59]. This has exerted a growing exploitation to investigate the feasibility and suitability of natural, renewable and low-cost materials as alternative adsorbents in the water pollution control, remediation and decontamination processes (bamboo dust, peat, chitosan, lignite, fungi, moss, bark husk, chitin, coir pith, maize cob, pinewood sawdust, rice husk, sugar cane bagasse, tea leaves, and sago waste) [60–62].

In general, rice husk ash is a term describing the waste collected from the particulate collection equipment attached upstream to the stacks of rice husk-fired boilers [40], containing 80 to 95% of silica in the crystalline form and minor amounts of metallic elements [25]. Practically, the type of ash varies considerably according to the burning technique, depending on the temperature regime and gasification structures. During the combustion at 550 °C to 800 °C, amorphous silica tends to be formed which at greater temperatures, crystalline silica is generated, both having different properties and specification for particular end use [9]. Whereas, controlled burning in air may lead to the production of white rice husk ash consisting almost pure silica (95%) in a hydrated amorphous form, with high porosity and reactivity, which controlled pyrolysis in nitrogen atmosphere will direct towards the generation of black rice husk ash which contains different amounts of carbon and silica content [41,42]. Hitherto, the potential global rice husk ash production is estimated at 21 million tonnes per year [43], which in Malaysia alone, a total amount of 78 thousand tonnes is produced annually [44]. Within recent decades, the emission of rice husk ash into the ecosystem has attracted huge criticisms and complaints, mainly associated with its persistent, carcinogenic and bio-accumulative effects, resulting in silicosis syndrome, fatigue, shortness of breath, loss of appetite (respiratory failure) and even death [45]. With the price of the ash disposal cost (either in landfills or ash ponds) hitting as high as $5/tonne in developing countries and $50/tonne in

6. Rice husk ash as a novel adsorbent

Table 1 Opportunity matrix of uses and potential markets the rice husk ash [9]. Application

Current state of development

Current demand

Potential demand

Geographical use

Purchase price

Suitability as a market

Flat steel production Concrete manufacture

Market in existence Market in existence, and ongoing research

Medium Low to medium

Decreasing High

World wide Worldwide

Medium Low

Silica fume replacement

Market in existence and ongoing research Market in existence and ongoing research Market in existence Research

Low

High

Worldwide

High

Not expanding Expanding and CER (Closer Economic Relations) potential Expanding and CER potential

Low

Low to medium

Worldwide

Low

Low Low

Decreasing High

Worldwide Worldwide

Medium Low

Research and anecdotal Research

Low Low

Low High

Asia Worldwide

Low High

Currently localized and potential in future Small and not expanding No ready market and limited potential Low demand, local use Potentially large market

Research Research

– –

High –

Worldwide –

– –

No ready market No ready market

Anecdotal

Low

Low

Asia

Low

Little evidence

Anecdotal use Market in existence and ongoing research

Low Low

Low Medium

Asia Currently USA

Low Medium

Low value local use Potential for marketing as a new product

Lightweight construction materials Refractory bricks Manufacture of silicon chips Insect control Activated carbon in water purification Vulcanizing process Extraction of gold, and other chemical uses Household ceramic products (tiles, glazes) Soil ameliorant Oil absorbent

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Table 2 Previous researches in the utilization of risk husk ash as novel adsorbents for different applications. Adsorbate

Adsorption capacity (mg/g)

Phase

Reference

Phenol 1,3-Dihydroxybenzene 2-Chlorophenol Lead (II) ions Mercury (II) ions Brilliant green dyes Lead (II) ions Zinc (II) ions Gold Indigo carmine dyes Picoline Nickel (II) ions Cadmium (II) ions Nickel (II) ions Cadmium (II) ions Zinc (II) ions Chromium (VI) ions Sulphur dioxide Pyridine Chromium (III) ions Lead (II) ions Cadmium (II) ions Nickel (II) ions Zinc (II) ions Methylene blue dye Sulphur dioxide Phenol Cadmium (II) ions Nickel (II) ions Zinc (II) ions Free fatty acids Congo red Pump oil Free fatty acid Carotenoid Mercury (II) ions Lauric acids Myristic acids Steric acids Carotene Phospholipid Cadmium (II) ions Zinc (II) ions

14.40 8.89 0.21 12.61 9.32 24.20 91.74 14.30 21.12 65.91 15.46 4.71 3.04 2.62 2.30 3.08 26.31 24.02 11.72 240.22 207.50 25.27 25.33 26.10 690.00 17.20 25.00 11.79 13.89 17.84 45.00 171.00 31.76 168.00 252.00 46.14 35.70 43.50 36.00 0.43 7.00 3.04 5.88

Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Gas Liquid Liquid Liquid Liquid Liquid Liquid Liquid Gas Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid

[7] [7] [7] [22] [22] [30] [40] [47] [64] [65] [66] [67] [67] [68] [68] [68] [69] [70] [71] [72] [73] [74] [74] [74] [75] [76] [77] [78] [78] [78] [79] [80] [80] [81] [81] [82] [83] [83] [83] [84] [85] [86] [86]

In light with the above challenges, recently, the potentiality of rice husk ash, an abundantly available throw-away waste from the rice husk fired-boiler furnaces, has shown to be prominent in wastewater treatment processes, for removing organic or inorganic compounds [63], as ideal adsorbents in refining gold–thiourea complex [64] and as air purifier in cleaning of atmosphere contaminants [19]. Table 2 lists previous researches in the utilization of risk husk ash as novel adsorbents for different applications. The findings will provide a twofold advantage with respect to environmental management. First, large volume of rice husk waste could be partly reduced, converted to useful, value-added adsorbents, and second, the low-cost adsorbent, if developed, may overcome the wastewaters and air pollution at a reasonable cost, solving part of the global agricultural wastes and wastewater treatment problem. Tables 3–5 illustrate adsorption isotherms, kinetics and thermodynamics of different pollutants onto the rice husk ash. In most studies, rice husk ash adsorption systems are featured well by the Freundlich, Langmuir and Redlich–Peterson isotherm models, and follow pseudosecond-order kinetic model. In this respect, negative Gibbs free energy change (ΔG) indicates the feasibility (favorability) and spontaneous nature of adsorption with high preference of adsorbates onto rice husk ash (higher driving force and thereby resulting in higher adsorption capacity), while positive enthalpy change (ΔH) ranges from 4 to 40 kJ/mol

Table 3 Adsorption isotherm studies of different pollutants onto rice husk ash. Adsorbate

Applicable isotherm models

Reference

Phenol, 1,3-Dihydroxybenzene, 2-Chlorophenol Lead (II), Mercury (II) ions Brilliant green dyes Lead (II) ions Zinc (II) ions Indigo Carmine dyes Picoline

Freundlich, Langmuir

[7]

Freundlich, Langmuir Langmuir, Redlich–Peterson Langmuir Freundlich, Langmuir Freundlich, Redlich–Peterson Radke–Prausnitz, Toth, Redlich– Peterson Freundlich, Redlich–Peterson Freundlich, Redlich–Peterson

[22] [30] [40] [47] [65] [66]

Freundlich, Langmuir Redlich–Peterson Toth

[69] [71] [74]

Freundlich, Freundlich, Freundlich Freundlich Langmuir Langmuir Freundlich, Freundlich Freundlich,

[75] [77] [79] [81] [82] [83] [86] [87] [88]

Nickel (II), Cadmium (II) ions Nickel (II), Cadmium (II), Zinc (II) ions Chromium (VI) ions Pyridine Zinc (II), Cadmium (II), Nickel (II) ions Methylene blue dye Phenol Free fatty acids Carotenoid, Free fatty acid Mercury (II) ions Lauric, myristic, steric acids Cadmium (II), Zinc (II) ions Lutein Cadmium (II), Lead (II), Copper (II), Zinc (II) ions

Langmuir Langmuir

Redlich–Peterson Langmuir

[67] [68]

represents endothermic nature of an adsorption and possibility of physical adsorption. Accordingly, positive entropy change (ΔS), a measure of the “saddle point of energy” over the reactant molecules onto activated complexes denotes the affinity of rice husk ash and increasing randomness at solid–solution (or solid–gas) interface during the fixation of both gas or liquid adsorbates on active sites of the adsorbent. The morphological studies of the raw rice husk and rice husk ash are individually presented in Figs. 1–3. Typically, rice husk has a globular structure in nature, of which its main components are in the lemma or palea form, tightly interlock with another [91]. The corrugate structural outer epidermis is highly ridged, containing papillae and hairs of varying sizes and well organized in a linear profile (linear ridges and furrows), while its ridges are punctuated with the prominent globular protrusions [39]. Whereas, the biomass are assembled around the stable Si–O carcass, concentrated in the protuberances and hairs (trichomes) on the outer and inner epidermis, adjacent to the rice kernel [92]. Many cavities having varying particle sizes were indicated distributing within the ash samples, evidenced of the interconnected porous net work and large internal specific surface area [75]. Fig. 4 exhibits the FTIR images for adsorption of rice husk ash of different pollutants. In most cases, there are nine adsorption main regions at 1050, 1200, 1300, 1380, 1470, 1600, 1800, 2940 and 3100 cm− 1. The region between 3100 and 3700 cm− 1 is related to the free hydroxyl

Table 4 Kinetics studies of different pollutants onto rice husk ash. Adsorbate

Applicable kinetic models

Reference

Brilliant green dyes Lead (II) ions Zinc (II) ions Indigo carmine dyes Chromium (VI) ions Chromium (III) ions Lead (II) ions Methylene blue dye Cadmium (II), Nickel (II), Zinc (II) ions

Pseudo-second-order Pseudo-second-order Pseudo-first-order Pseudo-second-order Pseudo-second-order Pseudo-second-order Pseudo-second-order Pseudo-second-order Pseudo-second-order

[30] [40] [47] [65] [69] [72] [73] [75] [78]

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Table 5 Thermodynamics studies of different pollutants onto rice husk ash. Adsorbate

−ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (kJ/K mol)

Reference

Brilliant green dyes Lead (II) ions Zinc (II) ions Indigo Carmine dyes Picoline Chromium (VI) ions Pyridine Cadmium (II) ions Nickel (II) ions Zinc (II) ions Lauric acids Myristic acids Steric acids

46.51 13.70 9.71 26.91 0.02 9.85 17.33 14.19 14.82 17.57 27.80 26.08 27.17

28.30 28.92 – 7.78 0.02 – 8.03 40.21 37.93 33.35 – – –

0.23 0.13 – 107.41 1.27 × 10− 4 – 0.08 166.89 162.83 157.87 – – –

[30] [40] [47] [65] [66] [69] [71] [74] [74] [74] [83] [83] [83]

groups on the adsorbent surface and an intensive peak at 1080 cm− 1 as well as at band 1200 cm− 1, are assigned to siloxane groups, in the form of silicon dioxide (–Si–O–Si–OH) [93]. Similarly, the stretching of OH groups bound to methyl radicals shows a signal between 2940 and 2820 cm− 1 and peaks around 1380 and 1470 cm− 1 are indicative of the –CH2 and – CH3 groups [74]. Meanwhile, the IR spectra indicated weak and broad peaks about 1600 cm− 1 corresponding to the –C = O and –C–OH groups stretching from aldehydes and ketones, and the transmittance in the 1050 to 1300 cm− 1 region is ascribed to the vibration of the CO group in

Fig. 2. Scanning electron micrographs (SEM) of rice husk ash (a) (1000×) [71] and its morphological details (b) (1000×) [70], (c) (650×) [25].

Fig. 1. Scanning electron micrographs (SEM) of fibrous rice husk (20×) (a), and rice husk showing protuberance, outer epidermis and silica content (500×) (b) [39].

lactones [94]. A peak at 1300 cm− 1 band may attributed to the aromatic CH and carboxyl-carbonate structures [95], which the presence of polar groups on the surface is likely to provide a considerable ion exchange capacity to the adsorbents [96]. Table 6 summarizes chemical composition of the rice husk ash from different origins where Fig. 5 shows its X-ray diffraction (XRD) analysis, with a broad hump around 2θ = 23° was observed, indicating the presence of amorphous silica (disordered cristobalite). Whereas, the maxima of the diffused peaks were found vary from 2θ = 23° for raw rice husks to 2θ = 20.7° for white ash, attributed to a gradual change in the bonding of silicon with organic material in raw rice husks to silica-silica bonding in white ash [19].

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Fig. 3. Scanning electron micrographs (SEM) of black rice husk ash (a) (50×) [75], white rice husk ash (b) (5000×) [39], and field emission scanning electron microscopy (FESEM) images of rice husk ash (c) (1000×) [89], (d) (20,000×) [90].

Table 7 and Fig. 6 demonstrate physiochemical properties (BET specific surface area, total pore volume and mean radius) of the rice husk ash in conjunction with its pore size distribution. Generally the BET specific surface area lies between 20.26 and 126.00 m2/g, while the total pore volume and mean radius (d) are in the average ranges from 1.370 × 10− 7 to 0.208 cm3/g and 0.92 to 18.00 nm respectively. Hereby, the BJH adsorption/desorption pore surface area is 27.45/ 22.18 m2/g, where the single point (d < 2178 Å) and cumulative adsorption/desorption (17 Å < d < 3000 Å) pore volume are determined to be 0.0388 and 0.03858/0.03523 cm3/g, accounting a total pore area of 20% micropores ( d < 20 Å), 78% mesopores (20 Å < d < 500 Å) and 2% macropores (d > 500 Å). Relatively, desorption mesopores pore distribution is found predominantly at 99%, with its BJH adsorption/desorption average pore diameter is calculated as 56.222/63.3535 Å (Fig. 6). Comparative nitrogen adsorption–desorption curves for the raw rice husk, black rice husk ash and white rice husk ash are shown in Fig. 6(a), (b) and (c) respectively. According to the Brunauer–Deming–Deming– Teller classification, raw rice husk obeys type III adsorption isotherm, with no obvious hysteresis loop convex over the entire range, suggesting relatively low specific surface area, pore volume, weak forces and low adsorption capacity within the whole range of studied pressures. With the same classification, white and black rice husk ashes are characterized by the type IV adsorption isotherm, resulting of the surface coverage of mesopore walls followed by pore filling associated with various hysteresis loops. At relatively low Pi/P0 values, the isotherms shape is similar to type II isotherm but at Pi/P0 values above 0.4, pore condensation will be taking place, illustrating porous structure of the rice husk ash. Hysteresis loops

(associated with capillary condensation) found in both samples are the narrow type H1 hysteresis loop (IUPAC classification), with almost vertical and parallel branches (opened ended cylindrical channel with uniform size and shape), associated with delayed condensation and little percolation hold-up [19,100–103]. 7. Major challenges and future prospects The world is currently facing the worst energy crisis in its entire history. For the past two eras, the enormous potentialities of converting biomass into renewable and environmental friendly alternative energy are receiving esthetic momentum and intensive attention abroad the nations. Presently, biomass energy has contributed approximately 9 to 13% of the global energy supply, accomplishing a total energy of 45 EJ per year [104]. With the expanding of the food demand [105] and elevating prices of the exhaustible fossil fuels, renewable energy is set to play a key role in satisfying the rising requirement and environmental remediation in a sustainable manner [31]. Along with the notable trend of the growing rice consumption, the anxiety of huge waste production and resource preservation has focused greatest attention towards the recovery of input resources, offering new opportunities for diversification of the rice husk ash [17,72,73]. Lately, environmental consideration and public concern are increasingly becoming more important, striving towards the quality and environmental preservation through sustainable development and cleaner technology approach [106]. In line towards achieving the status of green energy policy, a supportive environment to incorporate energy conservation and energy efficiency measures has been

K.Y. Foo, B.H. Hameed / Advances in Colloid and Interface Science 152 (2009) 39–47

45

Fig. 4. Fourier Transformed Infra-red (FTIR) images for rice husk ash prepared at different temperature (a) [64], and FTIR before and after the adsorption of dyes (b) [65]. Fig. 5. X-ray diffraction (XRD) spectra for rice husk ash (a) [70], (b) [69].

developed. Increasingly, there are recognition worldwide technological advances and innovations in natural adsorbents production of the necessity to reconcile agriculture practices with the need for environmental conservation, for insuring long-term agricultural operations and sustainability of the cropping systems [107]. Consequently, numerous investigations and development efforts have been undertaken to utilize rice husk ask contemplated mainly on the production of porous adsorbent, for accruing worldwide environmental benefit and shaping the national economy. Although there has been some successful industrial-scale production of renewable resources from rice husk ash, generally the industry

is still facing various challenges, the availability of economically viable technology, sophisticated and sustainable natural resources management, and proper market strategies under competitive energy markets [9]. Amidst these challenges, the development and implementation of suitable policies by the local policy-makers is still the single and most important factor which determines a successful utilization of renewable resources. During the last few years, the enforcement of environmental rules and regulations concerning various contaminants from agricultural waste streams by regulatory agencies are becoming more stringent and restrictive, inevitably affect

Table 6 Chemical composition analysis of the rice husk ash from different origins. Chemical constituents

Insoluble Loss on ignition Fe2O3 Al2O3 CaO MgO MnO SiO2 Na2O K2O P2O5 SO3 Cl2O Cl Others

Weight (%) [22]

[69]

[70]

[71]

[74]

[75]

[76]

[78]

[97]

[98]

[99]

– – 0.21 0.10 0.21 <0.10 0.02 98.61 – 0.16 0.18 – – 0.01 –

– 12.2 0.6 0.3 1.4 0.5 – 84.3 0.4 0.2 – – – – –

– 4.20 – 0.21 0.68 0.86 – 89.0 – 2.60 1.50 0.40 0.26 – 0.29

65.17 – 3.38 – 17.40 0.96 – 12.60 – – – – – – 0.49

75.17 – 3.38 – 17.40 0.96 – 2.60 – – – – – – 0.49

– – 0.23 – 1.89 0.96 – 94.64 0.39 0.58 – – – – –

– 21.0 0.25 0.35 0.62 0.89 – 73.00 – 2.20 1.40 – – – 0.29

– – 0.20 0.41 0.41 0.45 – 96.34 – 2.31 – – – – –

– – 0.51 0.63 1.96 0.77 – 93.26 0.01 2.85 – 0.01 – – –

– – 2.00 1.00 0.65 0.70 0.04 87.50 1.45 2.10 0.06 – – – 4.50

– 3.66 0.07 0.13 1.23 0.25 0.33 93.20 0.08 0.78 0.15 – – – 0.01

46

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References

Table 7 Physiochemical properties of the rice husk ash from different origins. Ash type

BET specific surface area (m2/g)

Pore volume (cm3/g)

Mean radius (nm)

Reference

White Black White Black White White White White White

126.00 96.00 40.93 20.26 36.44 65.36 47.47 101.29 88.00

0.208 0.185 0.155 0.042 0.039 0.039 1.370 × 10− 7 0.047 0.113

8.90 3.60 15.16 4.20 4.26 3.47 9.22 0.92 18.00

[19] [19] [64] [64] [65,67,74] [66,71] [73] [75] [86]

[1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11]

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] Fig. 6. Nitrogen adsorption–desorption curves for raw rice husk (a), black rice husk ash (b) and white rice husk ash [19].

the waste disposal practice of the rice milling industry. Ultimately, full co-operation and joint venture between different parties (nations, states, local government, private sector and communities) from upstream till the bottom line with compatible technologies is a promising sign for the race to the end line.

8. Conclusion Today, the world's accessibility oil reserves are gradually depleting, riding towards the overwhelming researches dealing with agricultural biomass waste utilization. The past ten years has seen a developing interest in the preparation of low-cost adsorbents as alternatives to activated carbons in water and wastewater treatment processes. Lately, the limited success of adsorbents in field applications has raised apprehensions over the use of rice husk ash in the preparation of novel adsorbents as a measure to the environmental pollution control. The evolution has turned from an interesting alternative approach into a powerful standard technique by offering a numbers of advantages: better performance in terms of ulterior adsorption capacity, rate of adsorption, solving wastewaters pollution at a cost effective way and overcome part of the agricultural wastes problem around the world. Despite various drawbacks and challenges has been identified and clarified, a widespread and great progress of in this area can be expected in the future.

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Acknowledgements The authors acknowledge the research grant provided by the Universiti Sains Malaysia under the Research University (RU) Scheme (Project No. 1001/PJKIMIA/814005).

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