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Rice Husk and Its Ash as Low-Cost Adsorbents in Water and Wastewater Treatment M. Ahmaruzzaman*,† and Vinod K. Gupta‡,§ †
Department of Chemistry, National Institute of Technology Silchar, Silchar-788010, Assam Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand § Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia ‡
ABSTRACT: Rice husk, which is a relatively abundant and inexpensive material, is currently being investigated as an adsorbent for the removal of various pollutants from water and wastewaters. Various pollutants, such as dyes, phenols, organic compounds, pesticides, inorganic anions, and heavy metals can be removed very effectively with rice husk as an adsorbent. This article presents a brief review on the role of rice husk and rice husk ash in the removal of various pollutants from wastewater. Studies on the adsorption of various pollutants by rice husk materials are reviewed and the adsorption mechanism, influencing factors, favorable conditions, etc., discussed in this article. It is evident from the review that rice husk and its ash can be potentially utilized for the removal of various pollutants from water and wastewaters.
1. INTRODUCTION Environmental pollution has reached a stage in which it should be seriously examined; otherwise, civilization will be in real danger. Among the various types of pollution, water pollution has attracted the attention of various researchers and scientists around the world. The problem of removing various pollutants from water and wastewater has grown with rapid industrialization. Heavy metals, dyes, phenols (including other organic compounds), inorganic anions, and pesticides (which are toxic to many living lifeforms and organisms) are present in the wastewater streams of many industrial processes. Dyeing, printing, mining and metallurgical engineering, electroplating, nuclear power operations, semiconductor, aerospace, and battery manufacturing processes are some of the industries that are generating various types of pollutants in the wastewater effluents.1,2 All of the above-mentioned industries are faced with increasing pressure, regarding environmental and wasterelated concerns, as a result of the quantity and toxicity of the wastewaters generated. There are several methods available for the removal of dyes, organic pollutants, pesticides, heavy metals, and other industrial effluents from wastewaters (including membrane filtration, coagulation, adsorption, oxidation, ion exchange, and precipitation). However, some of them were utilized because of their low cost, high efficiency, and applicability to a wide variety of pollutants. Heavy-metal contamination exists in aqueous waste streams from many industries, such as metal plating, mining, tanneries, painting, and car radiator manufacturing, as well as agricultural sources, where fertilizer and fungicidal spray are intensively used. Metals such as copper, zinc, chromium, and cadmium are harmful wastes that are produced by industry and pose a risk of contaminating groundwater and other water resources. Heavy metals are not biodegradable and tend to accumulate in living organisms, causing various diseases and disorders. For example, chromium causes serious ailments in both animal and plant bodies.3 r 2011 American Chemical Society
Dyes are widely utilized in industries such as textiles, rubber, paper, plastics, and cosmetics, to impart color of 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, which is a common silk and cotton dyeing agent, has been found to be highly cytotoxic in mammalian cells and also acts as a liver tumor-enhancing agent. Therefore, the removal of dyes is essential for the protection of the environment.4 In developed countries such as the United Kingdom (U.K.) and many European Union (EU) countries, environmental policies have required that zero synthetic chemicals should be released into the marine environment.5 Because of their chemical structures, dyes are resistant to fading upon exposure to light, water, and many chemicals; therefore, these compounds are difficult to decolorize once they are released into the aquatic environment.6,7 Treatment processes for metal-contaminated waste streams include chemical precipitation, membrane filtration, ion exchange, carbon adsorption, and coprecipitation/ adsorption.8 These processes usually require expensive facilities and have high maintenance costs. More economical alternative technologies or sorbents for the treatment of metal-contaminated waste streams are needed. Increasing use of pesticides in agriculture, forestry, and domestic activities for controlling pests is polluting our water resources day by day. The leaching runoff from agricultural and forest lands; deposition from aerial applications and discharge of industrial wastewater are responsible for this water contamination. The pesticides form a strong class of water pollutants, because they are sometimes nonbiodegradable. Moreover, pesticides are carcinogenic in nature. Therefore, the toxicity of Received: July 11, 2011 Accepted: October 28, 2011 Revised: October 9, 2011 Published: October 28, 2011 13589
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Table 1. Permissible Limits and Health Effects of Various Toxic Heavy Metalsa Permissible Limits for Industrial Effluent Discharge (mg/L) Permissible Limits for Indian Standard inland metal
public
WHO
marine
inland
surface water sewers coastal areas surface water
nickel
3.0
3.0
5.0
zinc
5.0
15.0
15.0
copper
3.0
3.0
3.0
Potable Water (mg/L) Indian standard IS 10500
WHO USEPA EU Standard
0.02
0.02
0.1
5.015.0
5.0
3.0
5.0
0.051.5
1.5
2.0
1.3
0.02
health hazards causes chronic bronchitis, reduced lung function, cancer of the lungs causes short-term illness called “metal fume fever” and restlessness
2.0
long-term exposure causes stomach ache, irritation of nose, mouth, and eyes, headaches
cadmium
2.0
1.0
2.0
0.1
0.01
0.003 0.005
0.005
carcinogenic, causes lung fibrosis, dyspnoea
lead
0.10
1.0
2.0
0.1
0.05
0.01
0.01
suspected carcinogen, anemia,
0.015
muscle and joint pains, kidney problems, and high blood pressure total chromium
2.0
2.0
2.0
0.05
0.05
0.1
0.05
suspected human carcinogen,
arsenic
0.2
0.2
0.2
0.01
0.01
0.01
0.01
carcinogenic, producing liver tumors, and gastrointestinal effects
mercury
0.01
0.01
0.01
0.001
0.001 0.002
0.001
excess dose may cause headache, abdominal pain,
producing lung tumors
and diarrhea, paralysis, and gum inflammation, loosening of teeth, loss of appetite, etc. iron
3.0
3.0
3.0
0.11.0
0.3
0.2
0.3
0.2
excess amounts cause rapid pulse rates, congestion
manganese
2.0
2.0
2.0
0.050.5
0.1
0.5
0.05
0.05
excess amounts toxic, and causes growth
of blood vessels, hypertension retardation, fever, sexual impotence, muscles fatigue, eye blindness vanadium a
0.2
0.2
0.2
1.4
very toxic, and may cause paralysis
Data taken from ref 11.
pesticides and their degradation products makes these chemical substances a potential hazard by contaminating our environment. Phenols are generally considered as one of the important organic pollutants discharged into the environment; such compounds give an unpleasant taste and odor to drinking water. The major sources of phenol pollution in the aquatic environment are wastewaters from paint, pesticide, coal conversion, polymeric resin, and the petroleum and petrochemicals industries. Introducing phenolic compounds into the environment or degradation of these substances means the appearance of phenol and its derivatives into the environment. The chlorination of natural waters for disinfection produces chlorinated phenols. Phenols and other phenolic compounds are considered as priority pollutants, because they are harmful to organisms at low concentrations.9,10 Adsorption with activated carbon was highly efficient for the removal of various impurities/pollutants from wastewater; however, the high cost of activated carbon inhibits its large-scale application as an adsorbent. Therefore, there is a need for the effective removal of various pollutants from wastewaters and resulted in a search for various low-cost adsorbents utilized for the removal of various impurities, such as heavy metals, dyes, pesticides, and other organic pollutants.14 Table 1 shows the permissible limits and health effects of various toxic heavy metals.11 The permissible limits and health effects of various inorganic
anions, pesticides and other organic compounds are also shown in Tables 2 and 3.1214 Natural materials that are available in large quantities, or certain waste products from industrial or agricultural operations, may have potential as inexpensive adsorbents. The abundance, availability, and low cost of agricultural byproducts make them good adsorbents for the removal of various pollutants from wastewaters. Agricultural waste biomass currently is gaining importance. In this perspective rice husk, which is an agro-based waste, has emerged as an invaluable source for the utilization in the wastewater treatment. Rice husk contains ∼20% silica, and it has been reported as a good adsorbent for the removal of heavy metals, phenols, pesticides, and dyes. Rice husk is extensively used in rural India, because of its widespread availability and relatively low cost. The annual generation of rice husk in India is ∼1822 million tons.15 These rice husks, as a commodity waste, can also be made into activated carbon, which is used as an adsorbent in the wastewater treatment. It would add value to these agricultural commodities, help to reduce the cost of waste disposal, and provide a potentially inexpensive alternative to the existing commercially available activated carbons. Rice husks are accounting for approximately one-fifth of the annual gross of rice in the world (545 million metric tons).16 Because of the growing concern with environment pollution, and the need to conserve 13590
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Table 2. Permissible Limits and Health Effects of Various Organic Compounds, Pesticides, and Inorganic Anionsa Permissible Limits for Industrial Effluent Discharge (mg/L) Indian Standard compound phenols
inland surface water
public sewers
marine coastal areas
health hazards
1.0
5.0
5.0
headache; fainting; vertigo; mental disturbances; ochronosis; vomiting;
10.0
20.0
10.0
difficulty swallowing; ptyalism; diarrhea; and anorexia oil and grease pesticides
0.010
0.010
0.010
highly poisonous to humans and animals; destruction of nerve
benzene hexachloride
0.010
0.010
0.010
carcinogenic; nausea; eye irritation; headache;
DDT
0.010
0.010
0.010
shallow breathing; endocrine disrupter affects stomach, intestines, heart, blood vessels, kidneys, and
cells in certain regions of brain, resulting in a loss of dopamine
nervous system; kidney and liver dysfunction; breast cancer; Hodgkin’s disease; skin, lung, and liver cancer; general weakening of the immune system, lymphatic system, and reproductive system endosulfun
0.010
0.010
0.010
itching; burning; tingling of the skin; headache; dizziness; nausea; vomiting; lack of coordination; tremor, mental confusion seizures, respiratory depression, coma
malathion
0.010
0.010
0.010
headache; excessive salivation and tearing; muscle twitching; nausea; diarrhea; respiratory depression; seizures; loss
methyl malathion
0.010
0.010
0.010
headache; excessive salivation and tearing, muscle twitching; nausea;
of consciousness; pinpoint pupils diarrhea; respiratory depression; seizures; loss of consciousness; pinpoint pupils paraquat
2.30
2.30
2.30
burning in mouth, throat, chest, and upper abdomen; diarrhea; giddiness; headache; fever, lethargy; dry,
cyanides
a
0.20
2.00
0.20
fluorides
2.00
15.00
15.00
boron
2.00
2.00
2.00
phosphate
5.0
cracked hands; ulceration of the skin very poisonous; affects brain and heart and leads to coma and death dental fluorosis; bone cancer; affects CNS irritation of eye, upper respiratory tract, and nasopharynx
Data taken from ref 12.
energy and resources, efforts have been made to burn rice husks under controlled temperature and atmosphere as supplementary cementing material. However, the amount of rice husk available is far in excess of any local uses and, thus, has posed disposal problems. Rice husk was chosen to be applied as a precursor material, because of its granular structure, insolubility in water, chemical stability, high mechanical strength, and its local availability at almost no cost. The advantage in the application of rice husk and its ash as adsorbents is that there is no need to regenerate them, because of their low production costs. The utilization of rice husk would solve both a disposal problem and also access to less-expensive material in the wastewater treatment. It has the potential to be used as an adsorbent, because of the presence of carbon and silica. When rice husk is burnt, ∼20 wt % of the ash is generated. The rice husk ash (RHA) has more than 95 wt % of silica with high porosity and large surface area, because it retains a cellular structure skeleton. An overview of rice husk and RHA as low-cost adsorbents is presented in this paper, and their removal performance is compared. The main goal of this review is to provide a summary of information concerning the utilization of rice husk and RHA as adsorbents for the removal of heavy metals, dyes, organic
compounds, surfactants, pesticides, and inorganic anions from wastewater. It also highlights that chemically modified rice husk has enhanced adsorption capacities for heavy metals, phenols, pesticides, dyes, and other organic compounds from wastewaters.
2. PROPERTIES OF RICE HUSK AND RICE HUSK ASH Rice husk possesses a granular structure, is insoluble in water, and has chemical stability and high mechanical strength, making it a good adsorbent material for treating various wastes from water and wastewater. Tables 4 and 5 show the physicochemical properties and chemical compositions of rice husk, respectively,1726 whereas the properties of rice husk ash (RHA) are shown in Tables 6 and 7.21,23,2730 The chemical components of RHA are found to be SiO2, H2O, Al2O3, Fe2O3, K2O, Na2O, CaO, and MgO (Table 6), fluctuating upon the varieties of paddy, proportion of irrigated area, geographical conditions, fertilizer used, climatic variation, soil chemistry, and timeliness of crop production operations and agronomic practices in the paddy growth process.31,32 From chemical analysis via energydispersive X-ray spectroscopy (EDX), it is indicated that the most abundant component in rice husk is silicon. The morphology of 13591
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Table 3. Permissible Limits and Health Effects of Various Organic Compounds and Inorganic Anionsa Permissible Limits for Potable Water (mg/L) species
Indian Standard
phenols
0.001
polychlorinated biphenyls
0.0005
WHO (μg/L)
USEPA
EU Standard
0.0005
health hazards
skin changes; thymus gland problems; immune deficiencies; reproductive or nervous system difficulties; increased risk of cancer
pentachlorophenols
0.009
0.001
0.0001
liver or kidney problems; increased cancer risk
benzene
0.01
0.005
0.001
carcinogen; dizziness; sleepiness; convulsions; and death
toluene
0.7
1.0
xylenes
0.5
ethyl benzenes
0.3
styrene
carcinogen; liver and kidney damage; memory loss; and death
10
carcinogen; dizziness; headache; affects stomach,
0.7
CNS, liver, and kidneys affects eyes and respiratory tract; irritation of throat, eyes, and nose
0.1
possible carcinogen; affects CNS; irritations
0.7
0.0002
carcinogen; affects gastrointestinal, renal, and pulmonary system
0.3
0.1
irritation; vomiting; diarrhea; nausea; headaches; sleepiness;
200 0.007
0.07 0.04
20
of respiratory tract, skin, and eyes polynuclear aromatic
0.0001
hydrocarbons (PAH) monochlorobenzene dichlorobenzene
also affects central nervous system, respiratory tract.
trichlorobenzene carbofuran lindane alachlor
0.02
benzo [α] pyrene atrazine
0.002
0.0001
itching, pain, and eye damage, etc. affects blood, reproductive, and nervous systems
0.0002
0.0002
0.002
0.002
affects nervous system, liver and kidneys, and may be a carcinogen
0.0007
0.0002
0.00001
carcinogen; toxicity in bone marrow and reproductive system
0.002
0.003
0.0001
irritating to skin, eyes, and respiratory tract;
anemia, possible carcinogen, and also affects liver, kidneys, and eyes
abdominal pain; diarrhea; vomiting paraquat
0.0001
fluoride nitrate boron
1.0 45
1.5 11
1
0.5
bromate cyanide a
0.05
4.0 10
1.5 11
methemoglobinemia; stomach acidity
1
0.01
0.01
0.01
0.07
0.2
0.05
nausea; vomiting; diarrhea and abdominal pain; affects kidney, CNS
Data taken from refs 13 and 14.
Table 4. Proximate and Ultimate Analyses of Rice Huska Proximate Analysis (wt %) combustibles
moisture
72.87
10.00
ash
a
voltaile matter
fixed carbon
64.3
15.9
17.13 19.8
7.9
Ultimate Analysis (wt %) C
H
O
N
S
Cl
38.92
5.55
37.94
0.35
0.02
0.09
37.00
5.10
36.00
0.40
17.1
59.9
44.6
5.6
49.3
20.00
66.40
13.60
37.8
5.20
39.0
0.39
17.9
72.8
9.3
48.9
6.2
44.1
0.8
0.05
Data taken from refs 1722.
rice husk may facilitate the adsorption of metals and other pollutants, because of the irregular surface of rice, thus making possible the adsorption of metals in different parts of this material. The physical characterizations of rice husk and RHA have pointed out some properties such as the presence of functional groups (carboxyl, silanol, etc.) that make adsorption processes possible. The chemical structure of the rice husk is of vital importance in understanding the adsorption process. The Fourier transform infrared (FTIR) technique is an important tool to identify the characteristic functional groups, which are instrumental in the adsorption of various pollutants from water and wastewaters.
To get better insight into the surface functional groups present on the surface of rice husk, FTIR spectra of rice husk, RHA, and metal-loaded RHA are presented in Figures 1 and 2, respectively.34 Table 8 also highlights the major stretching frequencies of the surface functional groups present in rice husk and its ash. The spectra shows that silanol groups may be present in the form of a silicon dioxide structure (SiOSiOH) and is similar to the silanol groups of silicic acid.38 The presence of polar groups on the surface is likely to give considerable cation exchange capacity to the adsorbents. 39 With the loading of metal ions, the shifting of the peaks is found from ∼3430, 2920, 13592
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Table 5. Chemical Analysis of the Rice Huska composition
(%)
cellulose
32.24
34.4
29.20
32.24
33.47
hemicellulose
21.34
29.3
20.10
21.34
21.03
lignin
21.44
19.2
30.70
21.44
26.70
extractives
1.82
water
8.11
mineral ash a
1.82
15.05
8.11 17.1
Figure 1. Fourier transform infrared (FTIR) spectra of rice husk. (Adopted with permission from ref 21. Copyright 2003, Elsevier.)
15.05
Data taken from refs 18, 19, and 2326.
Table 6. Chemical Composition of Rice Husk Asha constituent
(%)
SiO2 Al2O3
94.50
92.00 0.29
94.64
88.47
81.09 0.05
92.40 0.30
Fe2O3
75%) up to 15 min; thereafter, the removal of Pi is very sluggish and the initial sorption rate is rapid, because of the availability of more adsorption sites.41 Imyim122 investigated the effect of contact time on the removal of HA onto RHA and RHA-NH2 and the contact times required to reach adsorption equilibrium were 60 and 30 min for RHA and RHA-NH2, respectively. The adsorption of MB and MG onto oxalic acid-modified rice husk increased with increasing contact time at different initial dye concentrations.107 A similar observation is reported for the removal of Methylene Blue on RHA.27 The effect of contact time on basic dye and acid dye removal was investigated, and the contact time required to reach equilibrium for the basic dye is short using RH.105 Mbui et al.118 studied the effect of contact time on the removal of phenol and resorcinol and reported that the percentage adsorption was increased with time and found to be 72%, 52%, and 48%, for phenol, 2-chlorophenol, and resorcinol, respectively, after 60 min. Their findings also showed that RHA has a higher affinity for phenol than resorcinol. 13.2. Effect of pH. The pH of the aqueous solution is one of the most important factors affecting the adsorption process. It influences not only the surface charge of the adsorbent, the degree of ionization of the material present in the solution, and the dissociation of functional groups on the active sites of the adsorbent, but also the solution solute chemistry. Choudhury et al.106 studied the effect of pH on the removal efficiency of MG and reported that the adsorption of MG was strongly pHdependent. The equilibrium uptake of dye increased notably as the pH increased from 2.0 to 4.0; above that, the adsorption capacity did not change significantly, up to pH 9.0. At low pH values, the protonation of the functional groups present on the adsorbent surface easily takes place, and thereby restrict the approach of positively charged dye cations on the surface of the
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adsorbent resulting in low adsorption of dye in acidic solution. With decreasing acidity of the solution, the functional groups on the adsorbent surface become deprotonated, resulting in an increase in the negative charge density on the adsorbent surface and facilitate the binding of dye cations. The increase in dye removal capacity at higher pH also may be explained by the reduction of H+ ions, which compete with dye cations at lower pH for appropriate sites on the adsorbent surface. However, with increasing pH, this competition weakens and dye cations replace H+ ions bound to the adsorbent surface, resulting in increased uptake of dye. The effect of the initial solution pH on the amount of MB and MG ions adsorbed under equilibrium conditions was reported.107 The values of adsorption capacity for RH and MRH were the smallest at the initial pH of pH0 = 2.0. The lower uptake capacities of dyes at low pH are probably due to the presence of excess H+ ions holding the sorption sites on RH or MRH. For MB or MG basic dye, it will be positively charged in solution. As the pH of the dye solution becomes higher, the association of dye cations on the solid will occur more easily, resulting in an increase in adsorption. For RH, its main functional group is the hydroxyl group. Along with an increase of the pH value, the concentration of H+ ions that compete with the dye cations for sorption sites decreased. The uptake of dyes adsorbed on RH gradually increased as the initial pH was increased. For MRH, its practical functional group is the carboxyl group. When the pH was low, the presence of excess H+ ions could restrain the ionization of the carboxyl group, so the nonionic form of the carboxyl group (COOH) was present. The adsorption capacity of the dyes was small, because of the absence of electrostatic interaction. When the pH was high, the carboxyl group is changed to COO, and the adsorption capacity of the dye increased. The values of adsorption capacity increased as the initial pH increased from pH 1 to pH 7; insignificant changes were observed beyond pH 7. The uptake of MB and MG by MRH was much higher, compared to RH. The adsorption capacity of HA on rice husk ash (RHA) increased as the initial pH decreased from pH0 5 to pH0 3.122 The maximum adsorption of HA on RHA was reported at pH 34, while the constant adsorption capacity was over pH 5. In the higher pH range (pH > pHZPC, where pHZPC is the pH value of the zeta potential charge), the silanol groups contribute a negative charge, and HAs contributes negative charges because of the dissociation of carboxylic groups.123 Consequently, the uptake of HA would be reduced. For RHA-NH2, the capacity of HA adsorption was much higher than that of unmodified RHA. Similar to RHA, the maximum adsorption was found at pH 34. At this pH range, which is below pHZPC, the amine groups present on the surface of RHA-NH2 takes up H+ ions from solution and, hence, the presence of primary ammonium ions or SiRNH3+. On the other hand, HA are predominantly negatively charged due to an abundance of dissociated carboxylic groups or HAO, and it has been noted that HAO will tend to be associated with an equivalent number of SiRNH3+ on the RHA surface. The effect of pH on the uptake of dodecylbenzenesulfonate (DBS) by NR and QRH was investigated and reported that QRH exhibited much higher capability to adsorb DBS than RH in the pH range of 210.127 Feng et al.70 reported that the adsorption capacity of lead and mercury was low at lower pH and increased as the initial pH increased, with a more pronounced effect being observed on lead. The maximum adsorption capacity was reported with an initial pH of 5.605.80. The adsorption of phenol from aqueous solution is also reported to be dependent on the pH of the solution.119 13607
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The uptake of Zn(II), as a function of hydrogen ion concentration, was examined over a pH range of 311, and maximum removal was obtained in the pH range of 57.59 For RHA with an initial concentration of 25 mg/L of Zn(II), the maximum removal (96.8%) was obtained at pH 5. The effect of pH can be explained by considering the surface charge on the adsorbent material. At low pH, because of the high positive charge density due to protons on the surface sites, electrostatic repulsion will be high during the uptake of metal ions, resulting in lower removal efficiency. With increasing pH, electrostatic repulsion decreases, because of the reduction of positive charge density on the sorption sites, thus resulting in an enhancement of metal adsorption. A similar observation was reported in the adsorption of Cr(IV), Cu(II), and Cd(II) by RH.57 Tiwari et al.75 also reported that the adsorption of Hg(II) on RHA was dependent on the solution pH. The maximum adsorption of Cr(VI) was found in the pH range below 5, and the adsorbed amount was negligible in pH values over 8.75 These findings are supported by the work reported by Sumathi et al.48 13.3. Effect of Temperature. The temperature of the system plays an important role in adsorption capacity. Thermodynamic parameters such as standard free-energy change (ΔG0), enthalpy change (ΔH0), and entropy change (ΔS0) were determined using the following equation: ln Kc ¼
ΔG0 ΔS0 ΔH 0 ¼ RT R RT
ð1Þ
where Kc is the equilibrium constant that is resulted from the ratio of the equilibrium concentrations of the adsorbate on the adsorbent and in the solution, respectively. The linear property of ln Kc against 1/T has been proven in many studies that have been reported in the literature. The values of ΔG0 or ΔS0 and ΔH0 were determined from a plot of ln Kc versus 1/T. A negative ΔG0 value indicates the process to be feasible and suggests the spontaneous nature of adsorption. A positive ΔH0 value indicates the endothermic nature of adsorption, and ΔS0 can be used to describe the randomness at the solidsolution interface during the adsorption. In most of the cases, it has been found that the adsorption capacity increases as the temperature increases. Table 14 showed the reported values of ΔG0 or ΔS0 and ΔH0 for the removal of metals, dyes, phenols, and other organic compounds from aqueous solution onto RH and its ash. Choudhury et al.106 reported that temperature remarkably influenced the equilibrium dye uptake. The adsorption of dye increased as the temperature increased, indicating that a high temperature favored MG removal by adsorption onto chemically modified rice husk. The enhancement in adsorption, with the increase in temperature, may be attributed to the increase in the number of active surface sites available for adsorption, the increase in the porosity of the adsorbent, and increases in the pore volume of the adsorbent. An increase in temperature increases the rate of diffusion of the adsorbate molecules across the external boundary layer and within the internal pores of the adsorbent particle, because of a decrease in the viscosity of the solution. The enhancement in adsorption also may be a result of an increase in the mobility of the dye molecules with an increase in their kinetic energy. The removal of Pi onto RHA was also found to increase with increasing temperature.41 The increase in adsorption capacity is probably because of the creation of some new active sites on the surface of adsorbents. The increase in the adsorption capacity of
the adsorbents with an increasing temperature indicates that the adsorption is an endothermic process.41 Lee et al.127 showed that an increase in temperature from 4 °C to 50 °C increased the percentage uptake of dodecylbenzenesulfonate onto RH; however, further increases in temperature did not show any increase in uptake. The adsorption of phenolic compounds on RHA was investigated and reported no significant effect on the percent adsorption of the phenolic compounds.118
14. COLUMN STUDIES Column-type continuous flow operations possess distinct advantages over batch-type process. The adsorbents are continuously in contact with a fresh solution in the column operation, and, therefore, the concentration of the solution in contact with a given layer of adsorbent in a column is relatively constant. For batch treatment, the concentration of solute in contact with a specific quantity of adsorbent steadily decreases as adsorption proceeds, hence decreasing the effectiveness of the adsorbent for the removal of solute from their aqueous solution. The column operation is also important from the practical point of view for the treatment of real wastewater. Sumathi et al.48 utilized rice husk (RH) for their effectiveness in removing chromium from tannery effluent through column experiments. The adsorption capacity of RH was compared to that of sawdust, coir pith, and vermiculite. The sawdust exhibited a higher adsorption capacity, followed by coir pith. About 94% removal of chromium was achieved by a column of coir pith, and equally (93%) by a column containing a mixture of coir pith and vermiculite. Amin et al.54 investigated the possibility of the utilization of RH without any pretreatment in the removal of arsenic from aqueous media. Adsorption column methods showed the complete removal of both As(III) and As(V) under the following conditions: initial As concentration, 100 μg/L; RH amount, 6 g; average particle size, 780 and 510 μm; treatment flow rate, 6.7 and 1.7 mL/min; and pH, 6.5 and 6.0, respectively. The desorption efficiencies with 1 M of KOH after the treatment of groundwater were in the range of 71%96%, and the method is effective for a wide range of concentrations (i.e., 50500 μg/L). The adsorption potential of rice-husk-based activated carbon (RHAC) to remove Cu(II) from aqueous solution was investigated using a fixed-bed adsorption column.145 The highest bed capacity of 34.56 mg/g was obtained using a 10 mg/L inlet Cu(II) concentration, 80 mm bed height, and 10 mL/min flow rate. Column experiments indicated that the adsorbed amount of Hg(II) decreased with increasing flow rate and decreasing bed height.75 Studies using glass columns filled with RH were carried out at room temperature, employing 100 mL of synthetic solutions containing Cd(II) and Pb(II) at 100 mg/L in order to see the effects of pH, flow rate, and particle size on Cd(II) and Pb(II) adsorption.146 The potential of tartaric acid-modified rice husk (TARH) for the removal of copper and lead from semiconductor electroplating wastewater was also reported.147 Application of the Langmuir isotherm indicated that there was no difference in the adsorption capacity of TARH for copper and lead in synthetic solution and wastewater. Increases in column bed depth yielded longer service time, while increases in influent concentration and flow rate resulted in faster breakthrough. Theoretical breakthrough curves at different bed heights and flow rates generated using a twoparameter model agreed closely with experimental values in the treatment of semiconductor wastewater. The metals, copper and 13608
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Industrial & Engineering Chemistry Research lead, could be recovered almost quantitatively by eluting the column with 0.1 M HCl and the column could be used repeatedly for at least five cycles. The ability of RH to adsorb Methylene Blue (MB) from aqueous solution was tested in a fixed-bed column.148 The effects of important parameters, such as the value of initial pH, ionic strength the flow rate, the influent concentration of MB, and bed depth, were studied. When the flow rate was 8.2 mL/min and the influent concentration of MB was 50 mg/L, the equilibrium adsorption reached 4.41 mg/g, according to the Thomas model. They extended their study for the removal of Congo Red (CR) from aqueous solution, using a continuous bed of RH.149 The effects of important factors, such as the value of pH, ionic strength the flow rate, the influent concentration of CR, and bed depth, were investigated. RHA obtained by acid washing and calcination at 600 °C was mixed with kaolin and starch to make pellet adsorbents and utilized for the removal of CR in a packed column.150 Both ash and pellet samples showed good adsorption capacities toward the organic substances in wastewater. Furthermore, the surface nature of the white ash and pellet adsorbent could be modified through either hydration or esterification reactions. Corresponding changes in silanol concentrations were successfully correlated to the changes in adsorption capacity toward CR.
15. DESORPTION STUDIES The disposal or regeneration of spent adsorbent is one of the important factors in assessing the feasibility of an adsorption system151162 therefore, it is suggested that, as the purchase costs of the rice husk (RH) is negligible and it is primarily carbonaceous and cellulosic, the preferred disposal method is by dewatering, drying, and burning. It was also suggested that the heat of combustion may be recovered as waste heat and utilized for drying of the adsorbent and steam generation. Desorption of the rice-husk-loaded adsorbate (metals, dyes, phenols, organic compounds, and other inorganic ions) enables reutilization of RH, as well as the recovery of adsorbed materials. In some cases, desorption treatments may improve further adsorption capacities, although in other cases, there may be a loss of efficiency of the RH. For the operation of continuous flow systems, columns in parallel arrangements may allow adsorption and desorption processes to occur without significant interruption. A variety of substances have been used as metal/radionuclide desorbents, including acids, alkalis, and complexing agents, depending on the substance adsorbed, the process requirements, and economic considerations. In addition, there may be a means of selective desorption (e.g., for certain metals). Dye-loaded rice husk can be eluted and regenerated by some organic solvents, such as methanol, ethanol, surfactants, and NaOH. Distilled deionized water, CaCl2, and NaOH have been used to desorb phenolic compounds and pesticides. 16. CONCLUSIONS AND FUTURE RECOMMENDATIONS This review article summarizes the investigations carried out by various workers on the adsorption of various pollutants from wastewater onto rice husk (RH) and rice husk ash (RHA). The optimization of adsorption parameters utilizing RH and its ash could be achieved by correlating operating parameters and adsorbent characteristics. It was demonstrated from the literature that RH has been used as a potential low-cost adsorbent material for the removal of various pollutants from wastewater. Because of
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the presence of silica, RH exhibited high performance in the adsorption of various pollutants from wastewater. It was found that adsorption capacity of RH increased after chemical and thermal treatment. However, this increase in adsorption capacity depends on the method and conditions of treatment and also varies from researcher to researcher. Modification of RH using some heavy metals can also make them applicable for the adsorption of inorganic anions via surface precipitation. The findings will provide a 2-fold advantage, namely (i) a large volume of RH could be partly reduced, converted to useful, value-added adsorbents, and (ii) developed low-cost adsorbent may overcome the wastewaters pollution at a reasonable cost. Detailed mechanism of contaminant/modification reagent/RH interactions is required for a better design of adsorption experiments with modified rice husk. In addition, regeneration of spent RH and disposal of contaminants-loaded rice husk using simple methods/techniques should also be a focus in future research. There exists some relationship between adsorption properties and the structure of the adsorbate or the surface properties and groups of the adsorbent, besides the porosity/surface area properties. Although much research has been done on the adsorption process, there are still many amphibolous matters for further research, such as the quantificational relationship between pore size distribution of the adsorbent, size of adsorbate molecules, functional groups present on the surface of adsorbent, batch or column conditions, particle size of adsorbent, and the adsorption capacity, which will help in predicting and selecting adsorbents in the application of real wastewater treatment. Some researchers confirmed that the batch technique contains various sources of uncertainty and brings relevant uncertainty to determine adsorption parameters. It has been found that experimental procedure represents important source of uncertainties, and a great portion of the final distribution coefficient uncertainty comes from this source. For performance assessment studies in the future, understanding the variation in distribution coefficient uncertainty sources is important to weigh the influence of experimental setup on final adsorption data. Besides the studies of single-component adsorption of metals, dyes, phenols, and other organic compounds, multicomponent adsorption should be addressed in future research. In the literature, most studies utilizing RH were based on batch mode, highlighting their applicability and selectivity. However, once conditions of batch experiment, adsorptive capacity, and mechanism are established, research work should be conducted to design and carry out pilotplant studies to verify their viability at an industrial scale. Moreover, real wastewater should be tested, instead of synthetically prepared wastewater. In the wastewater treatment, the interaction among the different adsobates in solution will play an important role in the adsorption efficiency of different adsorbates. The adsorbents with high adsorption capacity, easy separation from aqueous solution, low cost, and recycling use are promising materials in the future. The high selectivity for the interested adsorbate is most important for the materials in the removal of mostly interested adsorbates from large volumes of aqueous solutions.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: md_a2002@rediffmail.com.
’ ABBREVIATIONS RH = rice husk 13609
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Industrial & Engineering Chemistry Research RHA = rice husk ash LOI = loss on ignition RSM = response surface methodology FT-IR = Fourier transform infrared spectroscopy SEM = scanning electron microscopy EDAX = energy-dispersive advanced X-ray analysis XRD = X-ray diffraction ATR-IR = attenuated total reflectioninfrared spectroscopy EM = electrophoretic mobility EXAFS = extended X-ray absorption fine structure spectroscopy TEM = transmission electron microscopy BFA = bagasse fly ash ARH = activated rice husk MRH = rice husk modified with oxalic acid EDTA = ethylenediamine tetraacetic acid ERH = EDTA-modified rice husk NTA = nitrilotriacetic acid IC = Indigo Carmine MB = Methylene Blue CR = Congo Red BG = Brilliant Green NR = Neutral Red CMC = carboxymethyl cellulose DDT = dichlorodiphenyl trichloroethane BET = BrunauerEmmettTeller BJH = BarrettJoynerHalenda TAP = triazophosphate MAA = methacrylic acid MP = methyl parathion pesticide DBS = dodecylbenzene sulfonate PAH = polynuclear aromatic hydrocarbons Al-RHA = aluminum-doped rice husk ash-derived silica DO = dissolved oxygen SRS = SheindorfRebuhnSheintuch MPSD = Marquardt’s percent standard deviation M = moisture VM = volatile matter FC = fixed carbon RB = Rhodamine B MG = Malachite Green RO16 = Reactive Orange-16 RHZ = zinc chloride-activated rice husk RHS = sulfuric acid-activated rice husk TARH = tartaric acid-modified rice husk RHT = rice husk, thermally treated ACC = activated carbon commercial GAC = granular activated carbon DCP = 2,4-dichlorophenol RHUT = rice husk, untreated RHCT = rice husk, chemically treated QRH = quaternized rice husk HA = humic acids RHA-NH2 = 3-aminopropyltriethoxysilane-modified rice husk ash Pi = α-picoline RHA-Al = silica-incorporated aluminum-modified rice husk ash LABS = linear alkyl-benzene sulfonates AS = alkyl sulfates AES = alkyl ether sulfates AE = alkyl ethoxylates APE = alkyl phenol ethoxylates COD = chemical oxygen demand PVA = poly(vinyl alcohol)
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