cationic and anionic dye adsorption by agricultural solid wastes: a comprehensive review by:

IOSR Journal of Applied Chemistry (IOSR-JAC)

e-ISSN: 2278-5736. Volume 5, Issue 3 (Sep. – Oct. 2013), PP 91-108 www.iosrjournals.org

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Cationic and anionic dye adsorption by agricultural solid wastes:

A comprehensive review by:

Namit Tripathi * Chemical Engg. Student ,Madhav Institute Of Technology And Science ,Gwalior 474005,Madhya

Pradesh ,India

Abstract: Dyes are an important class of pollutants, and can even be identified by the human eye. Disposal of

dyes in precious water resources must be avoided, however, and for that various treatment technologies are in

use. Among various methods adsorption occupies a prominent place in dye removal. Recently many researchers

have proved that agricultural solid wastes can be effectively used as adsorbents for the removal of many

pollutants including dyes. This review represents the effectiveness of agricultural solid wastes in the removal of

dyes, of cationic and anionic classes, description of classification of dyes and comparison among cationic and

anionic dyes adsorption by the same adsorbent, thus, possibly opening the door for a better understanding of the dye classified adsorption process. Both these classes of dyes are toxic and cause severe problems to aquatic

environment. Some agricultural solid wastes can remove both dye classes. The dye adsorption capacities of

agricultural waste adsorbents vary along with the variation in pH of solution, initial dye concentration,

adsorbent dosage and process temperature. As the pH of the solution affects the surface charge of the adsorbent

and degree of ionization of the adsorbate, it is directly related to the dye classified adsorption. This review also

contains the table representing the adsorbent and subsequent dye/dyes appropriate for a particular process.

Conclusions have been drawn from the literature reviewed, and suggestions for future research are proposed.

I. Introduction: Industrial developments in the recent years have left their impression on the environmental society.

Industries like textile industry uses dyes to color their products and thus produce waste water containing organics where in the dyeing processes the percentage of the dye lost waste water is 50% of the dye because of

the low levels of dye fiber fixation [2]. Discharge of these dyes into effluents affects the people who may use

these effluents for living purposes such as washing, bathing and drinking [3]. Therefore it is very important to

verify the water quality, especially when eve just 1.0mg/L of dye concentration in drinking water could impart a

significant color, making it unfit for human consumption [4]. Dyes can affect the aquatic plants because they

reduce sunlight transmission through water. Dyes may impart toxicity to aquatic life and may be mutagenic,

carcinogenic and may cause severe damages to human beings, such as dysfunction of kidneys, liver, brain and

central nervous system [5-7].

There are more than 100,000 commercially available dye exist and more than 7x105 tones per year

are produced annually [8]. Wastewater containing dyes is very difficult to treat, since the dyes are recalcitrant

organic molecules, resistant to aerobic digestion, and are stable to light. A synthetic dye in wastewater cannot be

efficiently decolorized by traditional methods. This is because of the high cost and disposal problems for treating dye wastewater at large scale in the textile and paper industries [9].

Removal of color from waste effluents is environmentally important because even a small quantity of

dye in water can be toxic and highly visible [10]. Since the removal of dyes from waste water is considered an

environmental challenge and government legislation requires textile waste water to be treated, therefore there is

a constant need of a process that can effectively remove these dyes [11].

In spite of the availability of many techniques to remove these pollutants from the waste water as legal

requirements, such as coagulation, chemical oxidation, membrane separation, electrochemical and aerobic and

anaerobic microbial degradation. These methods are not very successful due to many restrictions [12]. Among

all the different processes available, adsorption has been preferred due to its cheapness and the high quality of

the treated effluents, especially for well-designed sorption processes [13]. Adsorption by activated carbon is an

important way to clean up effluents and waste water [14], where, it used to polish the influent before it is discharged into the environment [15].

However adsorption by activated carbon has some restrictions such as the cost of the activated carbon,

the need for regeneration after exhausting and the loss of adsorption efficiency after regeneration [16].

Therefore adsorption by agricultural by-products used recently as an economical and realistic method

for removal of different pollutants has proved to be an efficient at removing many types of pollutants such as

heavy metals [17,18], COD [19,120], phenol [21,22], gases [23] and dyes [24-27].

Cationic and anionic dye adsorption by agricultural solid wastes:A comprehensive review by:


In order to increase the adsorption capacity of the adsorbent, researchers have followed different

activation methods and they usually used the Langmuir isotherm to indicate the effectiveness of the activation

process. Activation methods involve physical activation such as carbonization of material and chemical

activation such as using chemical activating agents.

Real textile waste water is a mixture of dyes, organic compounds, heavy metals, total dissolved solids,

surfactants, salts, chlorinated compounds, COD and BOD [28,29].

Therefore some studies tested the agricultural wastes as adsorbents for these pollutants. Ahmad and Hameed [20] studied the reduction of color and COD using bamboo activated carbon, and found that the

maximum reduction of color and COD were about 91.84% and 75.21%, respectively.

Anionic and cationic surfactants may effect on the dye adsorption depending on the dye type. The

adsorption of the basic dyes can be enhanced in the presence of anionic surfactant. On the other hand the

adsorption of anionic dyes can be enhanced in the presence of cationic surfactant. The negative ion of surfactant

may be adsorbed on the adsorbent by van der waal interaction and then the anionic dye can be adsorbed by the

anionic exchange. Although high concentrations of surfactants may cause aggregation or dye solubilization thus

decreases the dye adsorption [30,31].

Advantages and disadvantages of dye removal methods:

The table below shows the advantages and disadvantages of different techniques used for the removal of

dyes;

Technology Advantages Disadvantages

Conventional

treatment

processes

Coagulation

Flocculation

Simple, economically

feasible

High sludge production,

handling and disposal

problems

Biodegradation Economically attractive,

publicly acceptable

treatment

Slow process, necessary to

create an optimal favorable

environment, maintenance

and nutrition requirements

Adsorption on

activated carbons

The most effective

adsorbent, great,

capacity, produce a

high-quality treated

effluent

Ineffective against disperse

and vat dyes, the

regeneration is expensive

and results in loss of the

adsorbent, non-destructive

process

Established

recovery

processes

Membrane

separations

Removes all dye types,

produce a high-quality

treated effluent

High pressures, expensive,

incapable of treating large

volumes

Technology Advantages Disadvantages

Ion-exchange No loss of sorbent on

regeneration, effective

Economic constraints, not

effective for disperse dyes

Oxidation Rapid and efficient

process

High energy cost, chemicals

required

Emerging

removal

processes

Advanced

oxidation

process

No sludge production,

little or no consumption

of chemicals, efficiency

for recalcitrant dyes

Economically unfeasible,

formation of by-products,

technical constraints

Selective

bioadsorbents

Economically attractive,

regeneration is not

necessary, high

selectivity

Requires chemical

modification, nondestructive

process

Biomass Low operating cost,

good efficiency and

selectivity, no toxic

effect on

microorganisms

Slow process, performance

depends on some external

factors (pH, salts)



Classification and characteristics of dyes:

This table below shows types of dyes and their description;

Dye

Class Description Method

Fibers

Typically

Applied to

Typical

Fixation

(%)

Typical Pollutants

Associated with Various

Dyes

Acid water-soluble anionic compounds Exhaust/ Beck/

Continuous (carpet)

wool, nylon 80-93 color; organic acids; unfixed

dyes

Basic water-soluble, applied in weakly

acidic dyebaths; very bright dyes

Exhaust/ Beck acrylic, some

polyesters

97-98 N/A

Direct water-soluble, anionic

compounds;can be applied directly

to cellulosics without mordants (or

metals like chromium and copper)

Exhaust/

Beck/Continuous

cotton, rayon,

other

cellulosics

70-95 color; salt; unfixed dye;

cationic fixing agents;

surfactant; defoamer; leveling

and retarding agents; finish;

diluents

Disperse not water-soluble High temperature

exhaust Continuous

polyester,

acetate, other

synthetics

80-92 color; organic acids; carriers;

leveling agents; phosphates;

defoamers; lubricants;

dispersants; delustrants;

diluents

Reactive water-soluble, anionic compounds;

largest dye class

Exhaust/ Beck Cold

pad batch/

Continuous

cotton, other

cellulosics,

wool

60-90 color; salt; alkali; unfixed dye;

surfactants; defoamer;

diluents; finish

Sulfur organic compounds containing

sulfur or sodium sulfide

Continuous cotton, other

cellulosics

60-70 color; alkali; oxidizing agent;

reducing agent; unfixed dye

Vat oldest dyes; more chemically

complex; water-insoluble

Exhaust/Package/

Continous

cotton, other

cellulosics

80-95 color; alkali; oxidizing agents;

reducing agents

Cationic dyes:

Cationic dyes are widely used in acrylic, wool, nylon and silk dyeing [32]. These dyes include different

chemical structures based on substituted atomic groups [33]. These types of dyes are considered as toxic colorants and can cause harmful effects such as allergic dermatitis, skin irritation, mutations and cancer [34].

These dyes are also called basic dyes and depend on the positively charged ion, which are generally

hydrochloride or zinc chloride complexes [35]. Cationic dyes carry a positive charge in their molecule [36],

furthermore it is water soluble and yield colored cations in solution. Basic dyes are highly visible and have high

brilliance and intensity of colors [37]. Cationic functionality is found in cationic azo dyes and methane dyes,

also in anthraquinon, di- and tri-arylcarbenium, phthalocyanine dyes, various polycarbocyclic and solvent dyes

[38]. Cationic dyes were used intensely as a model in dye adsorption studies such as crystal violet [39],

methylene blue [40,41], basic blue 41 [42] and basic red 46 [43]. methylene blue is an important basic dye and

widely used in the textile industry. Acute exposure to methylene blue may cause increased heart rate, shock,

vomiting, cyanosis, jaundice, quadriplegia, Heinz body formation and tissue necrosis in humans [44]. Many

researchers have studied the adsorption of methylene blue dye using agricultural wastes such as peanut hull [45],

castor seed shell [46], coconut shell [47], guava leaf [48], neem leaf [49] and gulmohar plant [50], where the dye adsorption capacities were 123.5, 158, 277.9, 295, 351, 186.22 mg/g respectively. All these wastesshowed

goodadsorption capacities for methylene blue dye adsorption.

Anionic dyes:

Anionic dyes depend on the negatively charged ions [35]. Anionic dyes include many compounds from

the most varied dye classes having characteristic differences in structure (e.g., azoic, anthraquinone,

triphenylmethane and nitro dyes) but posses as a common feature, water-solubilizing, ionic substituents. The

anionic dyes also include direct dyes, and from the chemical standpoint the group of anionic azo dyes includes a

large proportion of reactive dyes [38]. Most of the reactive dyes interact with cotton, wool, etc., to form covalent

forms. The release of the reactive dyes into the environment is undesirable, because they have a low degree of

fixation due to the hydrolysis of reactive groups in the water phase [51]. Acid dyes are hydrophilic and used with silk, wool, polyamide, modified acrylic and polypropylene fibres. Acid dyes are harmful for humans since

they are organic sulphonic acids [52].



Dye removal adsorbents:

Many adsorbents have been used for the removal of the dyes from the waste water. Adsorption of dyes

depends on the properties of the dye and the surface chemistry of the adsorbent [53].

The adsorption process is one of the effective methods for removal dyes from the waste effluent. The

process of adsorption has an edge over the other methods due to its sludge free clean operation and completely

removed dyes, even from the diluted solution. Activated carbon (powdered or granular) is the most widely used

adsorbents because it has excellent adsorption efficiency for the organic compound. Nevertheless, commercially available activated carbon is very expensive. Furthermore, regeneration using solution produced small

additional effluent while regeneration by refractory technique results in a 10-15% loss of adsorbents and its

uptake capacity [54]. The sorption data have been correlated with adsorption isotherm to determine the

efficiency of adsorption process. Numerous researchers worked earlier on variety of adsorpents as mentioned

below. Wool Fiber and Cotton Fiber [55], Banana pith [56,57], Biogas residual slurry [56], Carbonized coir pith

[58], Coir pith [59], Chitosan [60], Hardwood [61], Mahogany sawdust, rice husk [62], Parthenium

hysterophorus [63], Neem (Azadirachta Indica) husk [64], Rice husk [65], Rice husk [66],

Silk cotton hull, coconut tree sawdust [67], Gypsum [68], Tuberose Sticks [69], Tamarind Fruit Shell[70],

Some of the adsorbents are peanut hull, sugar beet pulp, rice husk ash, coir pith, tea waste, almond shells,

lemon peel, bagasse fly ash, neem sawdust, guava seed carbon, etc., .The most widely used adsorbent for the

dye removal is activated carbon. Coal, charcoal and sawdust can be the raw material for the production of the commercial activated carbon where the activation includes partial oxidation and pore structure develops. Two

types of activated carbon can be produced which are H-type and L-type. H-type is positive charge upon water

and hydrophobic while L-type assumes a negative charge in water and hydrophilic [71-73].

Activated carbon can be available in granular form (granular activated carbon (GAC)). GAC can be

prepared from hard materials that used to remove water pollutants because its adaptability for continuous

contacting and because there is no need to separate the intraparticular diffusion in GAC is a problem

encountered in the application of adsorption processes to water treatment.

Activated carbon can also be available in powdered form (powdered activated carbon (PAC)). PAC can

be obtained when small particles compose the raw materials and normally mixed with the liquid to be treated

and then disposed off; therefore the use of PAC requires the separating of carbon from fluid after use. Yet the

PAC used for waste water treatment because of low cost and less contact time, where it presents a large external

surface and a small diffusion distance [74,75].

Agricultural Solid Wastes:

There have been many attempts to find inexpensive and easily available adsorbents to remove the

pollutants such as agricultural solid wastes where according to their physic-chemical characteristics and low

cost they may be good potential adsorbents [76].

Agricultural productions are available in large quantities around the world; thus big amount of waste

rejected [77]. Agricultural wastes are lignocellulosic materials that consist of three main structural components

which are lignin, cellulose and hemicelluloses. These components contribute mass and have high molecular

weights. Lignocellulosic materials also contain extractive structural components which have a smaller molecular

size [78].

Different adsorbents derived from agricultural solid wastes have been used for dye removal from waste water and many studies of dye adsorption by agricultural solid wastes have been published. Some of the

agricultural solid wastes like sugarcane bagasse [79], sugarcane bagasse ash [80], rice husk [81], fly ash [82],

activated carbon from coir pith [83], pineapple stem waste [84], orange peel [85], mesoporous carbon [86],

hardwood sawdust [87], clay, wall nut shell [88], coconut husk [89], coal fly ash [90], cow dung [91], wheat

dust [91], activated carbon prepared from mosambi peel [92]etc. are used as adsorbents for the removal of dyes

from the waste water.

Agricultural wastes are renewable, available in large amounts and less expensive as compared to other

materials used as adsorbents. Agricultural wastes are better than other adsorbents because the agricultural wastes

are usually used without or with a minimum of processing (washing, drying, grinding) and thus reduce

production costs by using a cheap raw material and eliminating energy costs associated with thermal treatment

[93]. Oil palm has been recently utilized in many industrial fields, therefore a large amount of waste is

generated from these industries and many studies make use of these by-products as dye adsorbents, such as palm

kernel fiber [94, 95], palm shell [96] and palm kernel shell [97].

Coconut is grown in more than 80 countries of the world and its products are applied in food industries,

and as a result many wastes are generated from these industries and used intensely for dye adsorption studies

such as empty coconut bunch [98], coconut-husk [99], coconut coir dust [100] and coconut tree sawdust [101].



On the other hand, there are abundant agricultural wastes which have a good adsorption capacity for dye

adsorption but are little used as adsorbents such as pomelo peel (Citrus grandis) [102], castor seed shell [103],

shells of bittim (Pistacia khinjuk stocks) [104] and jatropha husk [105].

Adsorbent Dyes References

Sugar beet pulp German turquoise blue-G [106]

Powdered peanut hull

Sunset yellow, Amaranth and Fast

green

[107]

Rice husk ash Indigo Carmine [108]

Chemically modified peanut hull

Methylene blue, Brilliant cresyl blue,

Neutral red, sunset yellow and fast

green.

[109]

Peanut hull

Methylene blue, brilliant cresyl blue,

neutral red.

[110]

Coir pith activated carbon

Reactive orange 12, reactive red 2,

reactive blue 4 and Congo red.

[111,112]

Coir pith carbon Methylene blue [113]

ZnCl2 activated coir pith carbon

Acid brilliant blue, Acid violet,

methylene blue and Rhodamine B.

[114]

Coir pith Acid violet [115]

Rice husk activated carbon

Malachite green

[116]

Rice husk based porous carbon

Malachite green

[117]

Rice husk

Congo red

[118]

Tea waste

Methylene blue

[119]

Coniferous pinus bark powder

Crystal violet

[120]

Orange peel activated carbon

Direct N Blue-106

[121]

Neem Sawdust

Malachite green

[122]

Guava seed carbon

Acid Blue 80

[123]

Peanut hull

Reactive Black 5

[124]

Loofa activated carbon

Reactive orange

[125]

Apricot stone activated carbon

Astrazon Yellow (7GL)

[126]

Almond shells

Direct red 80

[127]

Lemon peel

Malachite green

[128]

Bagasse fly ash

Methyl violet

[129]

Polygonum orientale Linn activated

carbon

Malachite green

[130]

Effect of adsorption factors on dye uptake:

II. Effect of solution pH: pH is a measure of acidity or basicity of an aqueous solution. The pH factor is very important in the

adsorption process especially for dye adsorption. The pH of a medium will control the magnitude of electrostatic

charges which are imparted by the ionized dye molecules. As a result the rate of adsorption will vary with the

pH of an aqueous medium [131]. The effect of pH solution on the adsorption process can be studied by prepare

adsorbent–adsorbate solution with fixed adsorbent dose and dye concentration but with different pH by adding

NaOH (1 M) or HCl (1 M) solutions and then shaken until equilibrium. Generally, at low pH solution, the



percentage of dye removal will decrease for cationic dye adsorption, while for anionic dyes the percentage of

dye removal will increase. In contrast, at a high pH solution the percentage of dye removal will increase for

cationic dye adsorption and decrease for anionic dye adsorption.

At high pH solution, the positive charge at the solution interface decreases and the adsorbent surface

appears negatively charged [132]. As a result, the cationic dye adsorption increases and anionic dye adsorption

shows a decrease. In contrast, at a low pH solution, the positive charge on the solution interface will increase

and the adsorbent surface appears positive charged, which results in an increase in anionic dye adsorption and a decrease in cationic dye adsorption. Osma et al. [133] studied the effect of solution pH on the adsorption of

Reactive black 5 dye by sunflower seed shells and they noticed that at a pH range from 2 to 4, the dye removal

ratio was minimal at a pH 4. Aksu and Isoglu [106] studied the effect of solution pH on the adsorption of

Gemazol turquoise blue-G as a reactive dye using sugar beet pulp and they noticed that the adsorption was at

maximum at pH 2 where the adsorption capacity was 83.7 mg/g and then decreased with a further increase in

pH and reached zero at pH 6. Hameed et al. [134] studied the adsorption of Methylene blue (MB) dye as a

cationic dye by banana stalk and they noticed that the adsorption of MB was at minimum at pH 2 and maximum

at pH 4.

The results shown in several descriptions, except for the removal of Rhodamine-B dye using orange

peel waste, where the percentage of dye removal decrease with increasing pH value. Rhodamine B dye (RhB)

(C28H31N2O3Cl) is basic, red colored and has two molecular forms (Cationic and Zwitterionic form) [135]. Its chemical structure is shown in Fig. 1 Rhodamine B dye is used in textile and food industries, where it has a high

solubility in water and it is a water tracer for biological stains and fluorescents. A few publications have the

same result of the pH effect on Rhodamine-B dye adsorption, where Gad et al. [136] studied the adsorption of

Rhodamine B dye by bagasse pith activated carbon and they concluded that at a high pH the zwitterionic form

of RhB is responsible for the aggregation increase, where according to Guo et al. [137], the increase of

aggregation of Rhodamine-B dye may form a bigger molecular form and become unable to enter into the

adsorbent pore.

Effect of pH and different dosage of orange peel under alkaline conditions are depicted in Figures given below;

Figure 1: Effect of pH under alkaline conditions at regular time interval [85].

Figure 2: Effect of pH under alkaline conditions at various adsorbent dosages [85].



Figure 3: Chemical structure of Rhodamine B dye: (a) Cationic form and (b) Zwitterionic Form.

The isoelectric point (pI) or point of zero charge is an important factor that determines the linear range of pH sensitivity and then indicates the type of surface active centers and the adsorption ability of the surface [138].

Many researchers studied the isoelectric point (pI) of adsorbents that prepared from agricultural solid

wastes in order to better understand of adsorption mechanism. Cationic dye adsorption is favored at pH>pI, due

to presence of functional groups such as OH−, COO− groups. Anionic dye adsorption is favored at pH<pI where

the surface becomes positively charged [139,140].

In order to determine the pI, dye solutions with different range of pH should prepare and consider as

pHinitial then fix amount of adsorbent should be added to the solutions.

These solutions should be shaken until equilibrium where the pH at equilibrium considers pHfinal, then

plot the pH(final) values against pH(initial) where pI is the point when pH(initial) = pH(final) [141].

Karagöz et al. [142] studied the adsorption of Methylene blue (MB) onto sunflower oil cake activated

carbon and they found that the zero point of charge (pI) for the activated carbon lies between pH 2.5 and 5.5, while the maximum adsorption capacity of Methylene Blue (MB) was at pH 6, in other word pH>pI.

Vieira et al. [143] studied the adsorption of Blue Remazol (R160) onto babassu coconut mesocarp and

they found that the zero point of charge (pI) for the babassu coconut mesocarp was 6.7, while the maximum

adsorption capacity of Blue Remazol R160 was at pH 1, in other word pH<pI

Figure 4: Effect of solution pH on the adsorption of Methylene blue on banana stalk waste.



III. Effect of initial dye concentration: The effect of initial dye concentration can be carried out by prepare adsorbent–adsorbate solution with

fixed adsorbent dose and different initial dye concentration for different time intervals and shaken until

equilibrium. The percentage removal of dye is highly dependent on the initial amount of dye concentration. The

effect of the initial of dye concentration factor depends on the immediate relation between the concentration of

the dye and the available binding sites on an adsorbent surface. Generally the percentage of dye removal

decreases with an increase in the initial dye concentration, which may be due to the saturation of adsorption sites

on the adsorbent surface [144]. At a low concentration there will be unoccupied active sites on the adsorbent

surface, and when the initial dye concentration increases, the active sites required for adsorption of the dye

molecules will lack [46]. On the other hand the increase in initial dye concentration will cause an increase in the

loading capacity of the adsorbent and this may be due to the high driving force for mass transfer at a high initial

dye concentration [145]. Garg et al. [146] studied the adsorption of Methylene blue by sulphuric acid treated

sawdust (SDC) at an adsorbent dose of (0.4 g/100 mL), at a temperature of (26±1 °C) and at pH (7.0) and they

found that the unit adsorption for SDC increased from 12.49 mg/g to 51.4 mg/g as the Methylene blue

concentration was increased from 50 mg/L to 250 mg/L, while the percentage of dye removal decreased from 99.9% to 82.2% as the Methylene blue concentration was increased from 50 mg/L to 250 mg/L. Table 6 shows

previous studies on the effects of initial dye concentration on the percentage of dye removal according to the

dye class, and it is obvious that the percentage removal of both dyes (cationic and anionic) decreases with

increasing initial dye concentration.

Real textile wastewaters includes high concentration of dyes, which highest than the concentrations

that used in literatures, therefore researchers used empirical design procedures based on adsorption equilibrium

conditions in order to predict the adsorber size and performance [Fig. 5].

Figure 5: Single stage batch adsorber design

The design objective is to reduce the dye solution of volume V (L) from an initial concentration of Cï to C1

(mg/L). The amount of adsorbent is M (g) and the solute loading changes from qï to q1 (mg/g). At time t=0,

qï=0 and as time proceeds the mass balance equates the dye removed from the liquid to that picked up by the

solid. The mass balance equation for the sorption system in can be written as:

V (Cο−C1) = M(qo – q1) = Mq1

Vadivelan and Kumar [147] studied the adsorption design of Methylene blue removal using rice husk and they

found that the amount of rice husk required removing 90% of Methylene blue solution of concentration 100

mg/L was 3.828, 7.655, 11.482 and15.309g for dye solution volumes of 1, 2, 3 and 4 L, respectively.

Adsorption process design model has been developed for the design of two-stage batch adsorber [Fig.

4] and can save adsorbent to meet the needs for higher dye removal efficiency and minimize capital investment costs [148]. Özacar et al. [149] studied the adsorption design of Metal complex yellow dye removal using pine

sawdust and they found that single stage process needs more time from two-stage process, where the required

time for 75–90% dye removal in single stage increased 4–15 min.



Figure 6: Multi-stage batch adsorption process for dye removal.

IV. Effect of adsorbent dosage: The effect of adsorbent dosage on the adsorption process can be carried out by prepare adsorbent–

adsorbate solution with different amount of adsorbents added to fixed initial dye concentration then shaken

together until equilibrium time.

Usually the percentage of dye removal increases with increasing adsorbent dosage, where the number of

sorption sites at the adsorbent surface will increase by increasing the dose of the adsorbent [150], and as a result increase the percentage of dye removal from the solution. Study of the effect of adsorbent dosage gives an idea

of the effectiveness of an adsorbent and the ability of a dye to be adsorbed with a minimum dosage, so as to

identify the ability of a dye from an economical point of view. Sonawane and Shrivastava [151] studied the

effect of adsorbent dose on the removal of Malachite green by maize cob and they concluded that at 20 mg/L of

dye, pH of 8 and a contact time of 25 min, the increase of percentage of dye removal from 90.0% to 98.5%

when the adsorbent dose increased from 0.5 to 12 g/L. Table 7 shows previous studies of the effect of adsorbent

dosage on the percentage of dye removal according to the dye class, and it is obvious that the percentage of both

dyes (cationic and anionic) increase with increasing the adsorbent dosage. Some experimental data for orange

peel, neem leaves, banana peel and activated carbon is shown below in graph;

Figure 7: Comparative results of various Adsorbents on to Effect of Adsorbent Dosage.

Effect of temperature:

The effect of temperature on the adsorption process can be carried out by prepare adsorbent–adsorbate

solution with different initial dye concentration then shaken together until equilibrium time at 30, 40 and 50 °C.

Temperature is an indicator for the adsorption nature whether it is an exothermic or endothermic process. If the adsorption capacity increases with increasing temperature then the adsorption is an endothermic process.

This may be due to increasing the mobility of the dye molecules and an increase in the number of active sites for

the adsorption with increasing temperature [152]. This effect depends mainly on the movement of dye

molecules of each dye class. The decrease of adsorption capacity with increasing temperature indicates that the

adsorption is an exothermic process [153]. Increasing temperature may decrease the adsorptive forces between

the dye species and the active sites on the adsorbent surface as a result of decreasing adsorption capacity [94].

Senthilkumaar et al. [112] studied the adsorption of Crystal violet (CV) on phosphoric and sulphuric

acid activated carbons (PAAC and SAAC), prepared from male flowers coconut tree. They conclude that the

adsorption capacities increased with temperature increasing [Fig. 5]. Önal [154] studied the adsorption of

Methylene blue (MB), Malachite green (MG) and Crystal violet (CV) by carbon prepared from waste apricot

and he concluded that the adsorption rate of the three dyes may be enhanced by increasing the adsorption temperature. Hameed and Ahmad [155] studied the adsorption of Methylene blue (MB) by garlic peel and they



found that the adsorption capacity increased from 82.64 to 142.86 mg/g when the temperature increased from

30 °C to 50 °C indicating that the adsorption is endothermic. Previous studies on the effect of temperature on the

nature of the adsorption process according to the dye class shows that the adsorption of anionic and cationic

dyes by each adsorbent increases with increasing temperature, indicating the adsorption is an endothermic

process, except for the adsorption of (4Bromoanilineazo-1,8-di-hydronaphthalene-3,6-di-sodiumsulphate(BDH))

by palm kernel fiber, since the adsorption of cationic dye is endothermic, while the adsorption of anionic dye is

exothermic, where according to Ofomaja and Ho the decrease in the adsorption capacity with increasing temperature is due to the weakening of the sorptive forces between the active sites on the sorbent and the dye

species, and also between adjacent dye molecules on the adsorbed phase.

Figure 8: Effect of temperature on the removal of Crystal violet on PAAC and SAAC

V. Effect of time: As a result of many experiments performed by different scientists is being observed that the dye

removal efficiency on an adsorbent varies with the time. Variation may be of both nature either positive or negative that means dye removing efficiency of an adsorbent may increase or decrease with course of time.

Some graphical data from the experiment performed on orange peel, neem leaves, banana peel, activated carbon

and papita saha(tamarind fruit shell,2010) is given below;

Figure 9: Comparative results of various adsorbents on to Effect of time.[85]

Adsorption isotherm:

The adsorption isotherm is important for the description of how the adsorbate will interact with the adsorbent and give an idea of the adsorption capacity of the adsorbent. The surface phase may be considered as

a monolayer or multilayer. Langmuirian kinetic is based on the ideal monolayer adsorbed model [156]. The



Langmuir isotherm is the most popular isotherm model and it is used to describe the adsorption process where

the occupancy occurs at on one adsorption site at an energetically homogeneous range of adsorption sites [157].

The expression of the Langmuir isotherm equation is represented

by the following equation

q(e) = q(max.)K(l)C(e)/1+K(l)C(e)

q(e)— Amount of adsorbate adsorbed at equilibrium (mg/g)

q(max) - Maximum monolayer adsorption capacity of the adsorbent

(mg/g)

C(e) — Equilibrium concentration of adsorbate (mg/L)

K(l)— Langmuir adsorption constant related to the free energy

adsorption (L/mg)

Studies of the Langmuir isotherm for anionic and cationic dye adsorption by various agricultural

adsorbents generally shows, the adsorption capacity for cationic dye adsorption is higher than anionic dye

adsorption on the same adsorbent except for the pinewood. Since the carboxyl group is one of the major

functional groups in agricultural wastes, it will have an effect on the adsorption capacity according to the dye class. The carboxyl group bears a negative charge, and therefore it is the major functional group in the

adsorption of cationic dyes. On other hand it will inhibit the adsorption of anionic dyes [158]. Namane et al.

[159] studied the adsorption of Acid blue dye as an anionic dye and Basic yellow dye as a cationic dye by coffee

grounds, and they concluded that the basic yellow dye is adsorbed faster and has a better uptake than the acid

blue dye.

Kumar et al. [160] studied the adsorption of Bismark brown dye by activated carbons prepared from

rubber wood sawdust using different activation methods. They studied the chemical activation carbon

(impregnated with phosphoric acid and activated in a fixed bed at 400 °C for 1 h), steam-activation (activated in

a fluidized bed reactor at 750 °C for 1 h with a steam flow rate of 4 mL min−1) and chemical activation

followed by steam carbon (the char was impregnated with phosphoric acid (0.45) IR and activated in a fluidized

bed at 800 °C for 1 h with a steam flow rate of 5 mL min−1). The concluded that the steam-activated carbon was

a better choice to remove the dye as it had higher adsorption capacity (2000 mg/g). Tseng [161], studied the removal of Acid blue 74 (AB74), Basic brown 1(BB1), Methylene blue (MB)

and Phenol by activated carbon prepared from plum kernels. The activation of plum kernels char was done by

NaOH activation at six different NaOH/char ratios. NaOH/char ratio is defined as PKN0, PKN0.5, PKN1,

PKN2, PKN3, and PKN4, where trailing numeric value represents the weight of NaOH/char ratio. The surface

area obtained per unit activation agent (SP/agent ratio) of PKN2 was the highest. Although the adsorption

capacity of PKN4 was the highest to remove all the four adsorbates but the value of KLCi, was suggested by the

researcher to evaluate a favorable level, where Ci is the highest initial adsorbate concentration (mg/L). Tseng

[113]121 found that the values of KLCi for the adsorption of MB, AB74, BB1 and phenol by PKN2 were

392,65, 49 and 20 respectively. He concluded that the KLCi value of adsorption of MB is in the hundreds and

therefore extremely favorable; adsorption of AB74, BB1, and phenol are in the tens, and therefore is highly

favorable; while if KLCi value is a single digit, then it is favorable and if it is a decimal then it is weakly favorable.

Wu et al. [162] studied the adsorption of AB74, BB1, MB and 4-Chloropenol (4-CP) by fir wood

activated carbon by NaOH (soaked in a concentrated NaOH solution, oven-dried and activated with different

NaOH/char weight ratio (2, 3 and 4)). They concluded that the maximum adsorption capacity values for the

adsorption of AB74 and BB1 by FWNa4 were the highest and those for the adsorption of MB and 4-CP by

FWNa3 were the highest.

Altenor et al. [163] studied the adsorption of Methylene blue and Phenol by vetiver roots activated

physically (carbonization) and chemically (different impregnation ratios of phosphoric acid (gH3PO4/g

precursor): (0.5:1); (1:1) and (1.5:1)). They used four isothermmodels to study the adsorption isotherm:

Freundlich, Langmuir, Redlich– Peterson and Brouers–Sotolongo equations. The data was a best fit with the

Brouers–Sotolongo equation and with the Redlich–Peterson equation. They concluded that the chemically activated samples had a higher adsorption capacity than the physically activated samples for the removal of MB

dye and the adsorption capacities of the adsorbent with an activation ratio of (1.5:1)) were 423 mg/g and 444

mg/g for the Langmuir and Brouers–Sotolongo models.



Adsorption kinetics:

The dynamics of the adsorption can be studied by the kinetics of adsorption in terms of the order of the

rate constant [164]. The adsorption rate is an important factor for a better choice of material to be used as an

adsorbent; where the adsorbent should have a large adsorption capacity and a fast adsorption rate. Most of

adsorption studies used Pseudo-first-order and Pseudo-second-order models to study the adsorption kinetics. For

the Pseudo-first-order model, the adsorption rate was expected to be proportional to the first power of

concentration, where the adsorption was characterized by diffusion through a boundary. The Pseudo-first-order model sometimes does not fit well for the whole range of contact time when it failed theoretically to predict the

amount of dye adsorbed and thus deviated from the theory. In that case, the Pseudo-second-order equation used

was based on the sorption capacity of the solid phase, where the Pseudo-second-order model assumes that

chemisorption may be the rate-controlling step in the adsorption processes [165,166].

For the Pseudo-first-order model (Lagergren model) under initial and end boundary conditions t=0to t=t and

qt=0to qt=qt, a linear equation is obtained:

Log[q(e) – q(t)] = log[q(e)] – K(1).t/2.303

For the Pseudo-second-order model under the initial and end boundary conditions t=0 to t = t and qt = 0 to qt

= qt, a linear equation is obtained:

t/q(t) = 1/K(2).q(e).q(e) + t/q(e)

q(t) — The amount of adsorbate adsorbed at time t (mg/g)

q(e) — The amount of adsorbate adsorbed at equilibrium (mg/g)

k(1) — The pseudo-first-order rate constant of adsorption (1/h) or

(1/min)

K(2) — The pseudo-second-order rate constant of adsorption

(g/mg h) or (g/mg min)

Usually the best-fit model can be selected based on the linear regression correlation coefficient R2

values. Generally the kinetic adsorption is better represented by Pseudo-second-order model for anionic and cationic dye adsorption. Lakshmi et al. [167] evaluated the adsorption of Indigo carmine dye by rice husk ash.

They found that the values of the Pseudo-first-order rate constant increases from 0.0087 to 0.0122 min-1 with an

increasing initial dye concentration from 50 to 500 mg/L, which indicates that the adsorption rate increases with

an increase in initial dye concentration while the R2 values were closer to unity for the Pseudo-second-order

model than that for the Pseudo-first-order model. Bulut and Aydin [145] investigated the adsorption of

Methylene blue using wheat shells and they found that the values of the constants for the Pseudo-first and

Pseudo-second order models were increased with increasing temperature and the R2 values for second order

model were greater than 0.999 indicating the second-order nature of the adsorption process. Ponnusami et al.

[48] studied the use of guava leaf powder for adsorption of Methylene blue. They found that the values of R2 of

the Pseudo-first-order model were between 0.70 and 0.85, while the values of R2 for the second order model

were 0.999, indicating the conformity of second order model. Pavan et al. [168] studied the adsorption of Methylene blue by yellow passion fruit waste and they found that the Pseudo-first-order model was better fitted

than the Pseudo-second-order kinetic, where the lower standard deviation of the residues for the Pseudo-first-

order model were (b0.49 mg g-1) and the R2 value was 0.9906. Table 9 shows previous kinetic studies of

cationic and anionic dyes adsorption by various agricultural adsorbents and it shows that the kinetic studies

followed the Pseudo-second-order model.

Desorption study:

In order to regenerate the adsorbent and recovery the adsorbed compounds, desorption process

necessary to be study were also desorption study help to explain the adsorptionmechanism.Desorption rate is

proportional to the driving force and desorption kinetics is very important for the contaminant transport

modeling [169-171]. Desorption process usually done by mixing a suitable solvent with the dye-saturated substrate and

shaken together for fixed time, until the dye extract on the solvent and then using filtration to separate the

adsorbent. The dye–solvent mixture dried at high temperature to evaporate the solvent. The desorbed dye then

determine in spectrophotometer [172].

Robinson et al. [172] studied the desorption of Cibarcon red from corncob using mixture of methanol,

chloroform and water. They found that the maximum value of desorption was 93%.



María et al. [173] studied the desorption of three classes of textile dyes from maize waste and they found

that the adsorption efficiencywas minor for reactive dyes at the same time the desorption was

higher in comparison with the basic dye. They found that the reactive blue 235 can be recovered up to 40% by

using warm water as solvent.

Won et al. [174] studied the desorption of Reactive black 5 from Corynebacterium,

Glutamicum waste biomass and they found that the desorption efficiency was lower at almost 80% compared to

100% of adsorption efficiency. Kumar and Ahmad [175] studied the desorption of Crystal violet dye from ginger waste and they found

that NaOH and H2O did not showed any desorption while acetic acid desorbed about 35–50% of dye.

Mahmoodi et al. [176] studied the desorption of three textile dyes from pinecone and they concluded that

the maximum desorption for Acid black 26, Acid green 25 and Acid blue 7 was 93.16%, 26.97% and 98%,

respectively.

VI. Conclusion: The literature reviewed revealed the fact that there has been a high increase in production and

utilization of dyes in last few decades resulting in a big threat of pollution. It is worthwhile noting that the removal of dyes can be done by various techniques; however, there exists no such methodology which can

successfully remove all types of dyes at low cost. The literature survey results and the methods discussed above

lead us to the conclusion that for removal of dyes adsorption can give fruitful results. During the last few years

many articles concerning the adsorption of dyes by agricultural solid wastes have been published, therefore a lot

of assumptions and results exist. This paper is an attempt to highlight the effect of the dye class on the

adsorption process and review some of the studies of dye adsorption on agricultural wastes according to the dye

class. This review makes a simple comparison between the cationic and anionic dye removal using agricultural

byproducts. It can be concluded that the agricultural wastes are effective adsorbents for cationic and anionic

dyes, but in their natural form, the agricultural wastes are better for adsorbing the cationic dyes rather than the

anionic dyes. In most cases these adsorbents require a treatment process to enhance their capability for anionic

dye removal. The most important factor that affects dye-classified adsorption is the pH factor,where a high pHvalue

is preferred for cationic dye adsorption while a low pH value is preferred for anionic dye adsorption. Previous

studies showed that cationic dye adsorption was favored at pHNpHpzc, while, anionic dye adsorption was

favored at pHbpHpzc. It was also noticed that the effect of adsorbent dose with respect to the concentration of

dye is effective for cationic and anionic dye removal. The Langmuirmodel is usually used to evaluate the

adsorption capacity of the agricultural wastes as adsorbents and most of studies of dye adsorption by agricultural

solid wastes showed a higher adsorption capacity for cationic dyes than an adsorption capacity for anionic dyes.

The kinetic data of adsorption of cationic and anionic dyes onto agricultural solid wastes usually follows the

Pseudo-second-order model. Cost comparison between the cationic and anionic dye adsorptionby agricultural

solidwastes is animportant need in order to evaluate the dye-classified adsorption process from the economic

point of view.

The literature review shows that there is a need for more detailed systematic studies on dye removal process and also some technical improvements in preparing and utilizing adsorbent. As regards to future work,

the following recommendations are suggested.

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