Factors Affecting Adsorption of Dye

Many physico-chemical factors influence the adsorption process and these include; adsorbate/adsorbent interaction, adsorbent surface area and pore structure, chemistry of the adsorbent surface, nature of the adsorbate, presence of other ions, particle size, pH, temperature, contact time, etc. [21, 31, 58]. Thus, the effects of these parameters are to be taken into account. Optimization of such conditions will greatly help in the development of industrial-scale dye removal treatment technology. In this section, some of the leading factors affecting adsorption of dyes are therefore discussed as follows:

Influence of Solution pH on Dye Adsorption

In wastewater treatment, the initial pH of dye solution is one of the most important factors that plays a vital role particularly on the adsorption capacity by influencing the chemistry of dye molecule and adsorbent in aqueous solution [59]. Initial pH of aqueous solution influences not only site dissociation, but also the solution chemistry of the dyes: hydrolysis, complexation by organic and/or inorganic ligands, redox reactions and precipitation are strongly influenced by pH [60]. The effect of solution pH on the adsorption can be determined by preparing adsorbent-adsorbate solution with fixed dye concentration and adsorbent dose but with different pH by adding NaOH (1 M) or HCl (1 M) solutions [27, 28].

Pavan et al. [61] studied the effect of solution pH on the adsorption of Crystal Violet (CV) by Formosa papaya seed powder and they noticed that at a pH range from 2 to 8, the dye removal percentage was maximum at pH 8. Argun et al. [62] studied the effect of solution pH for the removal of Reactive Blue 114 dye from aqueous solutions by using Pomelo (Citrus grandis) peel and noticed that the maximum adsorption was found at pH of 2.0. Tilaki et al. [63] studied the adsorption of Acid Orange 7 dyes by rice stem biomass and they found that Acid Orange 7 gives 98% removal efficiency at pH of 3 and removal percentage started to decrease below 50% as the pH increased. Zhao et al. [64] reported that adsorption of anionic dye Light Green (LG) onto surfactant modified peanut husk biomass was decreased with increase in solution pH. The compilation of different studies on the effect of solution pH on dye adsorption by agricultural by-product solid adsorbents is presented in Table 4.

The pH at which the surface charge is zero is called the point of zero charge (pHpzc). The point of zero charge (pzc) is relating to the adsorption phenomenon and it describes the condition when the electrical charge density on a surface is zero. Therefore the adsorption ability of the surface and the type of surface active centres are indicated by the point of zero charge (pHpzc) [65] for systems where H+/OH are the potential-determining ions. Due to presence of functional groups such as OH group on adsorbent surface, cationic dye adsorption is favoured at pH>pHpzc, whereas, anionic dye adsorption is favoured at pH< pHpzc [66]. In order to understand the adsorption mechanism, the point of zero charge (pHpzc) of various adsorbents prepared from agricultural solid wastes was investigated by many researchers [61, 67, 68]. Han et al. [69] studied the adsorption of Methylene blue (MB) onto poplar leaf and they found that the zero point of charge (pHpzc) for the poplar leaf was around 5.6, while the maximum adsorption capacity of MB was at pH 9, in other word pH>pHpzc. The adsorption of Blue Remazol onto babassu coconut mesocarp has been studied by Vieira et al. [70] and they found that the maximum adsorption capacity of Blue Remazol was at pH 1, while the point of zero charge (pHpzc) for the babassu coconut mesocarp was 6.7.

Table 4. Influence of solution pH on dye adsorptive behaviour of different agricultural

by-product adsorbents

Adsorbents

Dye

pH range

Percentage (%) of removal range

References

Coffee waste

Toluidine blue

2.0-10.5

50-95

[71]

Formosa papaya seeds

Crystal violet

2.0-8.0

41.3-97.3

[61]

Coffee husk

Fast green

2-5

Decrease

[60]

Parsley stalks

Methylene blue

2-10

20.72-82.84

[72]

Cucumber peels

Methylene blue

2-10

13.44-77.70

[72]

Cinnamomum camphora sawdust

Malachite green

2.46-7.18

52.0-90.8

[73]

Potato peel waste

Acid blue 1

2-11

Decrease

[74]

Garlic peel

Methylene blue

4-10

Increase

[75]

Activated- Rice husk

Acid yellow 36

2-9

80-45

[76]

Bagasse fly ash

Orange G

3-12

Decrease

[77]

Pine cone

Congo red

3.55-10.95

60.5-5.75

[78]

Spent coffee grounds

Methylene blue

3-11

81-95

[79]

Modified sawdust

Methylene blue

2-11

Increase

[80]

Pine leaves

Methylene blue

2-11

20-80

[13]

 
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