ELSEVIER Desalination 175 (2005) 305-316

DESALINATION

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Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal: an overview

T.G. Chuah a*, A. Jumasiah a, I. Azni a, S. Katayon b, S.Y. Thomas Choong a °Department of Chemical and Environmental Engineering, bDepartment of Civil Engineering, Faculty of Engineering,

Universiti Putra Malaysia, 43400 UPM Serdang, Selangor DE, Malaysia Tel. +60 (3) 8946-6288; Fax +60 (3) 8656-7120; email." chuah@eng.upm.edu.my

Received 31 August 2004; accepted 18 October 2004

Abstract

Rice husk as a low-value agricultural by-product can be made into sorbent materials which are used in heavy metal and dye removal. It has been investigated as a replacement for currently expensive methods of heavy metal removal from solutions. CkLrrently, the study of rice husk as a low-cost sorbent for removing heavy metals has regained attention. The heavy metals being studied are: As(V) [1], Au [2,3], Cr(IV) [4], Cu and Pb [5,6], Fe, Mn, Zn, Cu [7] and Cd(II) [8-10]. Rice husk is also being used to treat textile dyes such as like malachite green [11,12] and acid yellow 36 [13]. The treatment and preparation office husk activated carbon are of importance and became a subject of study [14-16]. In this review an extensive list ofprevions and current literature on rice husk activated carbons in removing heavy metals and dyes, their preparation or treatment and isotherms studies were complied to provide a summary of available information on rice husk and its potential as a low-cost sorbent.

Keywords: Rice husk; Heavy metals; Adsorption; Dye; Isotherms; Sorbents

I. Introduction

Heavy metal contamination exists in aqueous waste streams from many industries such as metal plating, mining, tanneries, painting, car radiator manufacturing, as well as agricultural sources where fertilizer and fungicidal spray are intens- ively used. Cu, Zn, Cr, Cd, etc., are harmful wastes produced by industry that pose a risk of

*Corresponding author.

contaminating groundwater and other water resources. Heavy metals are not biodegradable and tend to accumulate in living organisms, caus- ing various diseases and disorders. For example, chromium causes serious ailments in both animal and plant bodies [4]. Table 1 shows the Environ- mental Quality Act 1974, Environmental Quality (Sewage and Industrial Effluents) Regulations, 1979, in Malaysia with selected parameter limits of effluent o f heavy metals.

0011-9164/05/$- See front matter © 2005 Elsevier B.V. All fights reserved doi: 10.1016/j.desal.2004.10.014

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Table 1 Environmental Quality Act 1974, Environmental Quality (Sewage and Industrial Effluents) Regulations, 1979, Malaysia: Selected parameter limits of effluent of Standards A and B (Department of Environment, DOE, Malaysia)

Parameters, mg/l Standard

A a B

Mercury 0.005 0.05 Cadium 0.01 0.02 Arsenic 0.05 0.10 Lead 0.10 0.50 Chromium, Irivalent 0.20 1.0 Copper 0.20 1.0 Nickel 0.20 1.0 Zinc 1.0 1.0 Iron 1.0 5.0

IThis standard applies to the industrial and development projects which are located within catchment areas (areas upstream of surface or above sub-surface water supply intakes, for the purpose of human consumption including drinking).

Dyes are widely used in industries such as textiles, rubber, paper, plastics, cosmetics, etc., to colour their products. The dyes are invariably left as the major waste in these industries. Many of the organic dyes are hazardous and may affect aquatic life and even the food chain. For example, malachite green, a common silk and cotton dye- ing agent, has been found to be highly cytotoxic in mammalian cells and also acts as a liver tumour-enhancing agent [17]. The removal of dyes is an essential task for environmental protec- tion. In developed countries such as the UK and many EU countries, environmental policies have required that zero synthetic chemicals should be released into the marine environment [ 18]. Due to their chemical structures, dyes are resistant to fading on exposure to light, water and many

chemicals and, therefore, are difficult to be de- colourised once released into the aquatic environ- ment [ 19,20].

Treatment processes for metal-contaminated waste streams include chemical precipitation, membrane filtration, ion exchange, carbon adsorption and coprecipitation/adsorption [21] Theses processes usually require expensive facili- ties and have high maintenance costs. More economical alternative technologies or sorbents for the treatment of metal-contaminated waste streams are needed.

Various techniques have been employed for the removal of dyes from wastewaters [22-30]. Conventional physical and chemical methods are either costly, e.g., activated carbon, or produce concentrated sludge, e.g., Fenton's reagent, or may not capable of treating large volumes of effluent without the risk of clogging, e.g., mem- brane filtration [31]. Pearee et al. [32] made a comprehensive review of a method using bacterial cells to remove the dye from textile wastewater.

Natural materials that are available in large quantities, or certain waste products from indus- trial or agricultural operations, may have poten- tial as inexpensive sorbents. Due to their low cost, after these materials have been expended, they can be disposed of without expensive regeneration. The abundance and availability of agricultural by-products make them good sources of raw materials for activated carbons. Many car- bonaceous materials such as bark, coal, lignite, coconut shells, wood, dead biomass, seaweed, pecan shell and peat are used in the production of commercial activated carbons [21,33]. However, the review presented here concerns the use office husk for the removal of heavy metals and of water-soluble dyes present in wastewater.

Rice husk, an agricultural waste, contains about 20% silica, and has been reported as a good sorbent of many metals and basic dyes [34-36]. According to the statistics compiled by the Malaysian Ministry of Agriculture, there are

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408,000 t of rice husk produced in Malaysia annually [5]. These rice husks, as the commodity waste, can be made into activated carbons which are used as adsorbents in water purification or the treatment of industrial wastewater. It would add value to these agricultural commodities, help reduce the cost of waste disposal, and provide a potentially cheap alternative to existing commer- cial carbons.

The removal of heavy metals by agricultural waste or by-products has been extensively reviewed by Bailey et al. [21]. However, they only reported little information on rice husk being a low cost sorbent. Robinson et al. [31 ] has inves- tigated and reported on several available treat- ment technologies on dye removal. Removal of dye by agricultural by-products and other low- cost sorbents has also been investigated intens- ively by Banat [37], Garg et al. [38], Guo et al. [11,12], Low and Lee [39], Malik [14], McKay et al. [40], Nawar and Doma [41 ], Nigam et al. [42], and Robinson et al. [43-45].

An overview of some of the literature on rice husk as a sorbent of both heavy metals and dyes is presented as follows. Reported adsorption capacities are noted when possible to give some idea ofsorbent effectiveness and carbon prepara- tion. Sorption depends heavily on individual experimental conditions such as pH, metal con- centration, temperature, contact time, competing ions and particle size. It is advised that the reported sorption capacities should be taken as an example of values that can be achieved under a specific set of conditions and not as maximum sorption capacities. The authors strongly recom- mend that the reader should refer to the original articles for information on experimental condi- tions. The report will also review those effects on the adsorption using rice husk and mechanisms. It also covers the methodology on modification of rice husk in order to enhance sorption capacities for metal ion and dyes.

2. Properties of rice husk

The abundance of rice husk has been well documented by Grist [46]. From the statistical data of the Food and Agriculture Organisation, 1995, there is an estimated annual rice production of 500 million tonnes in developing countries; approximately 100 million tonnes of rice husk is available annually for utilization in these coun- tries alone. However, the amount of rice husk

Table 2 Typical composition office husk [52-54]

Composition Percent

Cellulose 32.24 Hemicellulose 21.34 Lignin 21.44 Extraetives 1.82 Water 8.11 Mineral ash 15.05

Chemical composition in mineral ash: SiO~ 96.34 K20 2.31 MgO 0.45 Fe20 a 0.2 Al203 0.41 CaO 0.41 K~o 0.08

Table 3 Reported physicochemical characteristics of dee husk [13]

Characteristics Values 1

Bulk density (g/ml) 0.73 Solid density (g/ml) 1.5 Moisture content (%) 6.62 Ash content (%) 45.97 Particle size (mesh) 200-16 Surface area (m2/g) 272.5 Surface acidity (meq/gm) 0.1 Surface basicity (meq/gm) 0.45

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available is far in excess of any local uses and, thus, has posed disposal problems [ 15]. Rice husk accounts for about 20% of the whole rice and contains about 20% silica. Rice husk possesses a granular structure, is insoluble in water, has chemical stability and high mechanical strength. A previous study reported that rice husk has a low calorific value of 3585 kcal kg-1 and high ash content [47]. Due to its high ash content, a proper method of disposal and utilization of rice husk has yet to be developed [48]. However, studies have also shown that rice husk is suitable for the production of activated carbon [16,49-51]. Tables 2 and 3 show the typical composition and chemical compositions office husk and reported values on their physicochemical characteristics.

3. Current technologies available for treatment methods of heavy metals and dyes

Heavy metals are not biodegradable and tend to accumulate in living organisms, causing vari-

ous diseases and disorders. Treatment processes for metal-contaminated waste streams include chemical precipitation, membrane filtration, ion exchange, and carbon adsorption. Currently, the maj or methods of industrial wastewater treatment involve physical and/or chemical processes. Some examples are summarised in Table 4.

4. Rice husk as a sorbent for heavy metal and dye removal

Rice husk used as a sorbent in removing heavy metals has been intensively studied and reported. Among the metal ions studied are: As(V), Au, Cd, Co, Cr (III), Cr(VI), Cu, Hg, Ni, Pb and Zn. The maximum ofsorption capacity for each metal ion from previous works are summarized in Table 5.

McKay [19,59] and his co-workers [40] first reported their works on colour removal using rice husk and other low-cost sorbents. Many subse- quent works studied basic dyes. McKay [59]

Table 4 Current treatment technologies for heavy metals and dyes removal involving physical and/or chemical processes

Physical and/or chemical methods Advantages Disadvantages

Oxidation

Ion exchange

Membrane filtration technologies

Coagulation/flocculation

Electrochemical treatment

Ozonation

Photochemical Irradiation Electrokinetic coagulation Fentons reagents

Biological treatment

Rapid process for dye removal

Good removal of a wide range of heavy metals and dyes Good removes of heavy metals and dye

Economically feasible

Rapid process and effective for certain metal ions Applied in gaseous state: alteration of volume No sludge production Effective at lab scale Economically feasible Effective decolourisation of both soluble and insoluble dyes Feasible in removing some metals and dyes

High energy costs and formation of by-products Absorbent requires regeneration or disposal Concentrated sludge production, expensive High sludge production and formation of large particles High energy costs and formation of by-products Short half life

Formation of by-products Required a lot of dissolved 0 2 High sludge production Sludge generation

Technology yet to be established and commercialized

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Table 5 Reported maximum heavy metals adsorption capacities (mg/g) for rich husk

Heavy metal Researchers [ref.] Capacities

As(V)

Au

Cd(I1)

Co (II) Cr (III) Cr (VI)

Cu(II)

Hg

Ni (II) Pb

Zn

Roy et al., 1993 [55] Lee et al., 1999 [1] Nakbanpote et al., 2000 [2] Nakbanpote et al., 2002 [3]

Roy et al., 1993 [55] Munafand Zein, 1997 [56] Tarley et al., 2004 [10]

Marshall et al., 1993 [35] Marshall et al., 1993 [35] Roy et al., 1993 [55] Munafand Zein, 1997 [56] Guo et al., 2002 [4] Marshall et al., 1993 [35] Munafand Zein, 1997 [56] Wong et al., 2003 [5]

Wong et al., 2003 [6]

Tiwari et al., 1995 [57]

Marshall et al., 1993 [35] Roy et al., 1993 [55] Wong et al., 2003 [5]

Wong et al., 2003 [6]

Tarley et al., 2004 [10]

Marshall et al., 1993 [35] Mishra et al., 1997 [58]

Mtmafand Zein, 1997 [56]

615.11 18.98 21.12 39.84 (20°C) 50.50 (30°C) 64.1o (4o0c) 21.36 0.16 7 (modified with NaOH solution) 4 (unmodified) 0.32 1.90

164.31 4.02 14.2-31.5 1.21 0.2 29 (27°C) 22 (50°C) 18 (70°C) 31.85 (single metal solution) 15.58 (bi-metal solution) 66.66 (dp= 37-50 #m) 55.55 (alp = 63-130 #m) 40.0 (dp= 130--600 #m) 0.23 11.40 108 (27°C) 105 (50°C) 96 (70°C) 129.48 (single metal solution) 48.31 (bi-metal solution) 21.55 (modified with NaOH solution) 45 (unmodified) 0.75 26.94 (20°C) 28.25 (30°C) 29.69 (40°C) 30.80 (50°C) 0.173

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Table 6 Reported dye adsorption capacities (mg/g) for rich husk

Type of dye Researchers [ref.] Capacities

Safranine McKay, 1986 760 Sandolan rhodine [59] 0 Congo red 0 Foron brill red 30 Methylene blue 280 Sandolan blue MeKay et al., 12 Solar blue 1987 a [40] 38 Foron blue 6 Sandocryl orange Nawar and Dorna, 27 Lanasyn black 1989 [41] 9 Acid yellow 36 Malik, 2003 [13] 86.9

sium hydroxide and sodium hydroxide [7,10-12], quatemized method [1,36,39] and tartaric acid [5,61.

Studies have shown that treated rice husk possessed a better adsorption capacity on metal ions. Suemitsu et al. [34] and Marshall et al. [35] reported that better adsorption capacity on metal ions was achieved with treated rice husk. Marshall et al. [35] also reported that different treatment methods would yield different adsorp- tion capacities. Tarley et al. [10] also reported that modified rice husks have a higher sorption capacity on Pb(II) and Cr(II) compared with the unmodified rice husks (Table 5). 2

"Adsorption capacity is measured in mg/l.

[59] reported that untreated rice husk does not work for Congo red, Sandolan blue, Sandolan rhodine, Solar blue and Foron blue, and has a low adsorption capacity for Foron brill red. However, his findings show that the complicated chemical nature of dyestuffs and adsorbents is involved in the adsorption phenomenon rather than on the simple dependence on the type of ions being adsorbed. His findings also show that no treat- ment is not a good adsorbent for several basic dyes. Few works have been reported on removal reactive dyes using rice husk. However, Low and Lee [39] have reported their studies on quater- nized rice husk as a sorbent for reactive dyes. A summary of reported dye adsorption capacities using rich husk is listed in Table 6.

The rice husk is in the either untreated, treated or modified form using different agents. Adsorp- tion studies using untreated or high-temperature carbonized rice husks were carded out by McKay [40,59], Roy et al. [55], Tiwari et al. [57], Mishra et al. [58], Malik [ 13], Nakbanpote et al. [2,3] and Tarley and Arruda [9]. Chemical treatment was commonly used in the treatment of rice husk, including sulphuric acid [40], sodium carbonate [41 ], hexane [35], hydrochloride acid [5 6], potas-

5. Sorption isotherms

The adsorption data from previous studies are either fitted to a Langmuir or Freundlich model. The Langmuir isotherm presents here in a general linearized form:

1 1 1

N, C.N'b N*

where Ce is the equilibrium concentration of the solute (mg/1), Ne is the amount of solute adsorbed per unit weight of adsorbent (mg/g), N" is the maximum capacity of adsorbent of the system (mg/g) and b is an adsorption equilibrium con- stant related to the energy of the sorption (1/mg). The Freundlich isotherm presented here is in the following form:

- K C l l~ a e - - - e

where ae and C e are the amounts (mol/g) adsorbed at equilibrium and equilibrium bulk concentration (mol/dm3), respectively; 1/n and K are the Freund- lich constants which correspond to adsorption intensity and adsorption capacity, respectively.

Tables 7 and 8 show the reported Langmuir and Freundlich isotherms parameters from

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Table 7 Reported Langmuir isotherms from previous batch studies using rice husk as adsorbent

311

Metal ion/dyes Temperature Maximum sorption capacity Constant Researchers [ref.] (°C) at equilibrium, (rag/g) (l/rag)

Au 20 39.84 0.059 Nakbanpote et al., 2002 30 50.50 0.050 [3] 40 64.10 0.031

Co 20 0.32 1.62 Marshall et al., 1993 [35]

Cr(III) 20 1.90 0.48 Marshall et al., 1993 [35]

Cu 20 1.21 0.90 Marshall et al., 1993 [35] 27 29 0.10 Wong et al., 2003 [5] 50 22 0.05 70 18 0.03

Hg(II) 29-31 66.66 (d e -- 37-50 #m) 0.0037 Tiwari et al., 1995 [57] 55.55 (d e = 63-130 #m) 0.003 40.00 (d e = 130-600 #m) 0.0025

15 9.3200 0.0115 Feng et al., 2004 [64] 30 6.7200 0.0158

Ni(II) 20 0.23 1.62 Marshall et al., 1993 [35]

Pb 27 108 0.09 Wong et al., 2003 [5] 50 105 0.02 70 96 0.01 15 12.3460 0.3250 Feng et al., 2004 [64] 30 12.6100 0.1630

Zn(II) 20 0.75 1.94 Marshall et al., 1993 [35]

Acid yellow 36 dye 30 86.9 0.002 Malik, 2003 [13]

Acid blue dye 25 50.0 0.1 Mohamed, 2004 [67]

Table 8 Reported Freundlich isotherms from previous batch studies using rice husk as adsorbent

Metal ion/dyes Temperature (°C) Adsorption Adsorption capacity, Researchers [re£] intensity, 1/n K (tool/g)

Cr (VI) 0.6387 56.6 Sumathi et al., 2004 [65]

Hg (II) 15 0.4932 0.4179 Feng et al., 2004 [64] 30 0.4691 0.5363

Pb 15 0.2475 4.7097 Feng et al., 2004 [64] 30 0.3094 3.4986

Zn (II) 20 0.909 2.238 Mishra et al,, 1997 [58] Acid yellow 36 30 0.435 2.1 Malik, 2003 [13]

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previous studies. Most of the reported heavy metal systems suggest the applicability Langmuir, which assumes there is monolayer coverage and constant sorption energy on the sorbent surface. Mishra et al. [58], however, reported that the data of Zn ion adsorption are well fitted to the Freund- lich isotherm rather than the Langmuir isotherm. This may be due to the adsorbent's heterogeneity of the surface, and is further confirmed with the fractional value of 1/n of 0.909 (0< 1/n<l) [60].

Malik [13] reported that both Langmuir and Freundlich models could be used to represent the trends of adsorption on Acid yellow 36 dye. His finding indicates that Acid yellow 36 is favour- ably adsorbed by the activated carbon prepared from rice husk.

It should be noted that some studies have shown data well fitted in the Freundlich isotherm; however, the values of 1/n and K were not in their works [ 11,12,41 ]. Guo et al. [ 11,12], however, emphasized that the Freundlich model in no way is meant to imply that the model assumptions are valid for their works. Moreover, the model used is only to provide a simple representation of the trends they observed.

6. Effect of contact time and effect of initial concentration

Tiwari et al. [57] found that the initial concen- tration of Hg(II) has little effect on the contact time to reach equilibrium. However, adsorption capacity is increased with the increment of initial concentration. A similar situation was observed for Zn(II) adsorption by rice husk [58]. Initial concentration of Zn(II) has little effect on the contact time to reach equilibrium. The adsorption capacity is also increased with the increment of initial concentration of Zn(II).It should be noted that Marshall et al. [35] and Munaf and Zein [56] did not report on the effect of contact time and effect initial concentration of Zn(II) by rice husk.

Lee et al. [ 1 ] have reported that the amount of uptake of As (V) by rice husk follows the trend of

a greater amount of uptake with decreasing initial concentration. However, sorption of As(V) was very rapid, taking 20 min to achieve equilibrium irrespective of the initial concentration. It also attains equilibrium faster than the adsorption such as fly ash and chrome waste, which attained equi- librium after 72 min and 2 h, respectively [61,62].

Guo et al. [4] reported their findings in ad- sorption of Cr(VI); equilibrium was achieved in about 2 h and it was found that the equilibrium time was much faster than any other adsorbents such as cow dung carbon, coconut shell carbon and peach stones. The same equilibrium time was achieved and observed by them on the adsorption of malachite green using the same adsorbent dose of 0.8 g/1 of rice husk [11]. Both experimental results indicate the rapid uptake of the treated rice husk on heavy metals and dyes.

Nawar and Doma [41] studied the effect of contact time on basic dye (Sandocryl orange B-3RLE) and acid dye (Sandoz lanasyn black BRL ABK). The contact time required to reach equilibrium for the basic dye is short using rice husk - - 15 min. This agreed with the findings reported by Tinh et al. [63] on saturation of mate- dais with a cellulose-type structure that was achieved after only 15 min for low concentrations of a basic dye. However, for acid dyes, lower uptake is observed at equilibrium.

7. Effect of pH

It is known that the sorption of heavy metal ions by solid substracts depends on the pH of the solution. Mishra et al. [58] suggested that the contents of amino acids, the active functional groups and metal oxides in rice husk, may have an effect on the uptake process. Adsorption of Zn(II) increases with the increase of pH values. Rice husk could exhibit a hydration shell at lower pH positively charged by the hydronium ion, H +, in the solution. This restricted the uptake of Zn(II) ions. Adsorption capacity increases with increasing pH values. At moderate to high pH

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values, zinc ions exist as Zn 2+, Zn(OH) ÷ and Zn(OH)2 , which are favourable species for adsorption of trace zinc ions. A similar observa- tion was found in the adsorption ofCr(IV), Cu(II) and Cd(II) by rice husk [56]. This is further supported by the work of Wong et al. [5].

The effect of pH on Hg(II) adsorbed by rice husk was studied by Tiwari et al. [57]. They found that a maximum adsorption efficiency at pH 6 seems to be related to the dominant species of Hg(OH)2. Chloride species, such as HgC12, dominate at pH <6. Disappearance of hydroxyl- mercury group species at pH <6 caused a de- crease in adsorption. Dissolution of metal oxides office husk may also increase the pH value of the solution. This is further supported by the findings observed by Feng et al. [64]. By using solutions with an initial pH of 5.60-5.80, they obtained the highest absorption capacity values on Hg.

Guo et al. [4] observed the maximum adsorp- tion of Cr(VI) was found in the range below 5, and the adsorbed amount was found to be negli- gible in the pH value over 8. These findings are further supported by the work of Sumathi et al. [65]. They found that the solution pH had a marked influence on Cr(VI) adsorption. Gene- rally, adsorption of Cr(VI) was higher at lower pH and decreased with increasing pH. This could be due to the redox reaction between the sorbent surface groups and sorbate. Higher H + ion con- centration could strengthen the redox reaction and enable the carbon to adsorb more Cr(VI). Vasan- thy and Lakshmanaperumalsamy [66] also sug- gested that the high adsorption of Cr(VI) at low pH was due to the fact at low pH a large number of hydrogen ions can neutralize the oppositely charged surface and thus enhance the electrostatic attraction between the adsorbent and adsorbate overcoming electrostatic repulsion between them.

The effect of different pH values on dye adsorption capacity of rice husk was studied by Nawar and Doma [41 ]. They observed that there is a general trend for the adsorption capacity to be constant at pH values between 3and 9 for a basic

dye (Sandocryl orange) and an acid dye (Sandoz lanasyn black).

The adsorption of malachite green [11] was increased with pH when the same rice husk was used. This may be due to the increase of negative charges on the carbon surface; hence, the electro- static force, like van de Waals, between the car- bon surface and dye ion, were increased.

8. Effect of particle size

The adsorption capacity of rice husk very much depends on the surface activities-- in other words, specific surface area available for solute- surface interaction, which is accessible to the solute. It is expected that adsorption capacity will be increased with a larger surface area. In other words, smaller particle size increases the adsorp- tion capacity. Munaf and Zein [56] reported that when the size of rice husk particles increases, metal ions decrease. Similar trends have been reported by Wong et al. [5]. Adsorption being a surface phenomenon, the smaller adsorption sizes will offer comparatively larger surface areas and higher adsorption will occur at equilibrium.

Tiwafi et al. [57] found that the adsorption capacity of Hg(II) on rice husk decreased with increasing particle size. Higher sorption for smaller particle size seems due to larger surface area per unit mass of rice husk. His findings also indicated that, in the initial stage of adsorption, Hg(II) adsorption was dominated by diffusional transport.

9. Effect of agitation rate

Wong et al. [5] show that the uptakes of Pb and Cu increased with increasing agitation rates. An increasing agitation rate may reduce the film boundary layer surrounding the sorbent particles, thus increasing the external film diffusion rate and the uptake rate. However, Lee et al. [1] showed that varying the agitation rate does not

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have any effect on the uptake rate of As(V) on rice husks. This is due to the As(V) by quatenized rice husk, is a very rapid process. It can be con- cluded that due to the nature of sorbents and sorbates, the effect of agitation is varied.

10. Effect of competitive ions

Different reasons have been given regarding the sorption affinity of biosorbent such as rice husk. The amount of sorbed ions depends on the equilibrium between sorption competition from all the cations, ionic size, stability of bonds between metal ions and biosorbent [68]. Wong et al. [5] investigated the effect of competitive ions between Pb and Cu. The finding shows that the affinity of a modified rice husk (MRH) for metal ions is Pb>Cu in mixed metal solutions. The stability and enthalpy of formation are greater for Pb-MRH than for Cu-MRH.

12. Conclusions

Rice husk as an agricultural by-product has been made into a potentially low-cost biosorbent material used in heavy metal and dye removal. Previous studies have focused on untreated rice husk. However, a study of treated rice husk on removing heavy metals has been of interest by many researchers. This paper summarizes the studies that have been carded out by many other workers on the adsorption of heavy metals and dyes. It was found that rice husk possesses different characteristics due to the method of treatment. Treatment methods on rice husk have a strong infiuenee on adsorption efficiency. Opti- mization of biosorption parameters using rice husks can be achieved by manipulating the ope- rating parameters and sorbent characteristics.

References

11. Effect of activation conditions

The effect of activation conditions on rice husk was studied intensively by Guo et al. [15]. A characterization of porous rice husk carbon pre- pared with different precalcination times was made. Precalcination time has a great effect on the porosity of the porous carbons prepared by KOH but not significant for the carbon prepared by NaOH. Lower precalcination time can produce carbons exhibiting microporosity and meso- porosity. Microporous carbon can be produced with higher precalcination times. It was found that there is no increase of pore volume in the presence of alkali chlorides, which restrict the activating agents from entering into the interior of the carbon structure. It can be concluded that the activation mechanism of rice husk carbon is complicated.

[1] C.K. Lee, K.S. Low, S.C. Liew and C.S. Choo, Environ. Technol., 20 (1999) 971-978.

[2] W. Nakbanpote, P. Thiravavetyan and C. Kalam- baheti, Mineral Eng., 13 (2000) 391-400.

[3] W. Nakbanpote, P. Thiravavetyan and C. Kalam- bahefi, Mineral Eng., 15 (2002) 549-552.

[4] Y. Guo, J. Qi, S. Yang, K. Yu, Z. Wang and H. Xu, Mat. Chem. Physics, 78 (2002) 132-137.

[5] K.K. Wong, C.K. Lee, K.S. Low and M.J. Haron, Chemosphere, 50 (2003) 23-28.

[6] K.K.Wong, C.K. Lee, K.S. Low and M.J. Haron, Process Bioehern., 39 (2003) 437--445.

[7] A.A.M. Daifulla, B.S. Girgis and H.M.H. Gad, Mat. Lett., 57(2003) 1723-1731.

[8] M. Ajmal, R.A.K. Ran, S. Anwar, J. Ahamad and R. Abroad, Bioresource Technol., 86 (2003) 147- 149.

[9] C.ILT. Tarley and M.A.Z. Arruda, Chemosphere, 54 (2004) 987-995.

[10] C.R.T. Tarley, S.L.C. Ferreira and M.A.Z. Arruda, Microchem. J., 77 (2004) 163-175.

T.G. Chuah et al. / Desalination 175 (2005) 305-316 315

[11] Y. Guo, S. Yang, W. Fu, J. Qi, R. Li, Z. Wang and H. Xu, Dyes Pigments, 56 (2003) 219-229.

[12] Y. Guo, H. Zhang, N. Tao, Y. Liu, J. Qi, Z. Wang and H. Xu, Mat. Chem. Physics, 82 (2003) 107-115.

[13] P.K. Malik, Dyes Pigments, 56 (2003) 239-249. [14] N. Yalcin and V. Sevinc, Carbon, 38 (2000) 1943-

1945. [15] Y. Guo, K. Yu, Z. Wang and H. Xu, Carbon, 41

(2000) 1645-1687. [16] Y. Guo, J. Qi, S. Yang, K. Yu, J. Zhao, Z. Wang and

H. Xu, Mat. Chem. Physics, 74 (2002) 320-323. [17] K.V.K. Ran, Toxicol. Lea., 81 (1995) 107-113. [18] C. O'Neill, F.R. Hawkes, N.D. Lourenco, H.M.

Pinbeiro andW. Deice, J. Chem Technol. Bio- teehnol., 74 (1999) 1009-1018.

[19] G. McKay, Am. Dyest. Rep., 68 (1979) 29-36. [20] V.J.P. Poots and G. McKay, Water Res., 10 (1976)

1061-1066. [21 ] S.E. Bailey, T.J. Olin, R.M. Bricka and D.D. Adrian,

Water Res., 33 (1999) 2469-2479. [22] K.K.H. Choy, G. MeKay and J.F. Porter, Resour.

Conserv. Recy., 27 (1999) 57-71. [23] M.P. Gupta and P.K. Bhattaeharrya, J. Chem.

Teehnol. Bioteehnol,m 35B (1985) 23. [24] S.D. Khattri and M.K. Singh, Water Air Soil Poll.,

120 (2000) 283-294. [25] L. Kun-She, L. Chnoong-Kheng and T. Bee-Foong,

App. Biochem. Bioteetmol., 87 (2000) 233-245. [26] R.M. Liversidge, G.J. Lloyd, D.A.J. Wase and

C.F. Forster, Proc. Biochem., 32 (1997) 473-477. [27] G. McKay, M.S. Otterburn and A.G. Sweeney,

Water Air Soil Poll., 24 (1985) 147-161. [28] A.K. Mittal and S.K. Gupta, Water Sci. Technol.,

34 (1996) 81-87. [29] F. Perineau, J. Molinier and A. Gaset, J. ChertL

Technol. Biotechnol., 32 (1982) 749-758. [30] V.N. Singh, G. Mishra and K.K. Panday, Ind. J.

Teehnol., 22 (1984) 70-71. [31] T. Robinson, G. McMuUan, R. Marchant and P.

Nigam, Bioresource Teehnol., 77 (2001) 247-255. [32] C.I. Pearce, J.R. Lloyd and J.T. Guthrie, Dyes

Pigments, 58 (2003) 179-196. [33] R.R. Bansode, J.N. Losso. W.E. Marshall, R.M. Rao

and R.J. Portier, Bioresource Technol., 89 (2003) 115-119.

[34] R. Suemitsu, R. Uenishi, I. Akashi and M. Nakano, J. Appl. Polym. Sci, 1 (1986) 75-83.

[35] W.E. Marshall, E.T. Champagne and W.J. Evans, J. Environ. Sei. Health, A28 (1993) 1977-1992.

[36] C.K. Lee, K.S. Low and S.J. Mah, Adv. Environ. Res., 2 (1998) 351-359.

[37] F. Banat, S. A1-Asheh and L. AI Makhadmeh, Proc. Biochem., 39 (2003) 193-202.

[38] V.K. Garg, R. Gupta, A.B. Yadav and R. Kumar, Bioresouree Technol., 89 (2003) 121-124.

[39] K.S. Low and C.K. Lee, Bioresource Technol., 61 (1997) 121-125.

[40] G. McKay, G. Ramprasad and P. Mowli, Water Res., 21 (1987) 375-377.

[41] S.S. Nawar and H.S. Doma, Sci. Total Environ., 79 (1989) 271-279.

[42] P. Nigam, G. Armour, I.M. Banat, D. Singh and R. Marchant, Bioresource Technol., 72 (2000) 219- 226.

[43] T. Robinson, B. Chandran and P. Nigam, Bioresource Technol., 85 (2002) 119-124.

[44] T. Robinson, B. Chandran and P. Nigarn, Environ. Internat., 28 (2002) 29-33.

[45] T. Robinson, B. Chandran and P. Nigam, Water Res., 36 (2002) 2824-2830.

[46] D.H. Grist, Rice, 5th ed, Longman, London, 1975. [47] H. Olmez, Utilization of agriculture and industrial

wastes in production of the cement, Samsun: Ondokuz Mayis University, Publ. No. 47, 1988.

[48] K.G. Mansaraly and A.E. Gbaly, Energy Sources, 20 (1998) 653-663.

[49] U. Sanigok, Inorganic Industrial Chemistry, Istanbul University Press, Istanbul., 1987.

[50] T.H. Usmani, T.W. Ahmad, S.Z. Ahmed and A.H.K. Yousufzai, Pakistan J. Sci. Indus., 36 (1993) 236-- 239.

[51 ] T.H. Usmani, T.W. Ahrnad, S.Z. Ahmed and A.H.K. Yousufzai, Pakistan J. So. Indus., 36 (1993) 240- 243.

[52] I.A. Rahman and J. Ismail, J. Mater. Chem., 3 (1993) 931-934.

[53] I.A. Rahman, J. Ismail and H. Osman, J. Mater. Chem., 7 (1997) 1505-1509.

[54] S.A. Darnel, Rice hull ash as a piazzolanic material, Masters Thesis, Asian Institute of Technology, 1976.

316 T.G. Chuah et al. /Desalination 175 (2005) 305-316

[55] D. Roy, P.N. C_rreenlaw and B.S. Shane, J. Environ. Sci. Health, A28 (1993) 37-50.

[56] E. Munafand R. Zein, Environ. Teehnol., 18 (1997) 359-362.

[57] D.P. Tiwari, D.K. Singh and D.N. Saksena, J. Env. Eng., 121(1995) 479-481.

[58] S.P. Mishra, D. Tiwari and R.S. Dubey, Appl. Radiat. Isot., 48 (1997) 877-882.

[59] G. McKay, Water Air Soil Poll., 29 (1986) 273-283. [60] P. Benes, Tmee Chemistry of Aqueous Solutions,

Elsevier, Amsterdam, 1980. [61] E. Diamadopoulos, S. Ioanidis and G.P. Sakellarou-

polos, Water Res., 27 (1993) 1773-1777.

[62] M. Sadiq, T.H. Zaida and A.A. Mian, Water Air Soil Poll., 26 (1983) 369-377.

[63] V.Q. Tinh, R. Leblane, J.M. Janssens and M. Ruel, Can. Mining Metall. Bull., 64 (1971) 99.

[64] Q. Feng, Q. Lin, F. Gong, S. Sugita and M. Shoya, J. Coll. Interf. Sei., 278 (2004) 1-8.

[65] K.M.S. Sumathi, S. Mahirnairaja and R. Naidu, Bioresource Technol., in press.

[66] M. Vasanthy and P. Lakshmanaperumalsamy, Eco- toxieol. Environ. Monit., 3 (1995)47-50.

[67] M.M. Mohamed, J. Coll. Interf. Sci., 272 (2004) 28-34.

[68] K.S. Low, C.K. Lee and A.C. Lee, Bioresource Technol., 51 (1995) 227-231.