Application of Agricultural Wastes Activated Carbon for Dye Removal – An Overview

Dyes are an important class of pollutants and can even be identified by the human eyes. Disposal of dyes in precious water resources must be avoided especially those that are not easily biodegradable, however, and for that various treatment technologies are in use. Among various methods adsorption occupies a prominent place in dye removal. The growing demand for efficient and low-cost treatment methods and the importance of adsorption has given rise to agricultural waste. This review highlights and provides an overview of these activated carbons prepared by non-woody and woody materials and their application for dyes removal. From a comprehensive literature review, it was found that many researchers used non-woody material as activated carbons to removal dye contaminants.


Introduction
Saving water to rescue the planet from water polluted situation and to make the future of mankind being safe is what we need now. With the growth of mankind, society, science and technology our world is reaching to new high horizons but the cost which we are paying or will pay in near future is surely going to be too high. Among the consequences of this rapid growth is environmental disorder with a big pollution problem. Besides other needs the demand for water consumption ("Water for People Water for Life" United Nations World Water Development Report UNESCO) has increased tremendously with agricultural, industrial and domestic sectors consuming 70, 22 and 8% of the available fresh water, respectively and this has resulted in the generation of large amounts of wastewater [1] containing a number of 'pollutants'.
Apparently, activated carbons are prepared by physical and chemical activation processes [15]. In physical activation also known as pyrolysis process, the precursor is being carbonized at high temperature and activated by passing CO 2 of steam under pressure to increase the porosity and surface area [8,28]. Meanwhile, in chemical activation, both activation and carbonization process take place simultaneously in which the raw precursor is impregnated with activating agents and heated at desired temperature [5,8,25,28,48]. Chemical activation leads to both physical and chemical modifications on produce activated carbon. The agricultural wastes provide both woody and non-woody materials for adsorption methods replaced with commercial activated carbon. For woody materials composed lignocellulose only compared with non-woody materials composed sugar, starch, lignocellulose and oils [49]. Many authors used non-woody materials due to the composition of materials such as starch and lignocellulose which have high pore structure compared with woody materials.
Based on the review of Table 1 and Table 2 for materials woody and non-woody, it can be concluded that specific surface area of activated carbon produced depends significantly on the type of materials and activating agents used in the adsorption processes. Specific surface area was important parameters used in evaluating many capabilities of powders and porous materials such as activity, adsorptive performance and catalytic performance [50]. The factors affecting on the measured value of specific surface area of activated carbon such as degassing temperature, degassing time and adsorption point number.
Certain agricultural waste products have been tested and proposed for dye removal. Which agricultural waste adsorbent is better to being used? There is no direct answer to this question due to their each agricultural waste has its specific physical and chemical characteristics such as porosity, surface area and physical strength, as well as inherent advantages and disadvantages in wastewater treatment. However, it is clear from the present literature survey that agricultural waste adsorbents may have potential as readily available, inexpensive and effective sorbents. They also possess several other advantages that make them excellent materials for environmental purposes such as high surface area (Table 1 and 2). Table 1. Non-woody materials of agricultural waste activated carbon.

Raw precursor
Chemical used in impregnation method

Pollutants in dye removal
Specific surface area analysis (BET) (m 2 /g)

Ref.
Pomegranate (1632 m 2 /g), apple peel (1552 m 2 /g), hazelnut bagasse and grape waste with the surface area of (1489 m 2 /g) and (1455 m 2 /g). As usual, the solution pH is an important parameter in the adsorption process due to the interaction effect of the surface functional groups of the adsorbate and the adsorbent. Besides that, the properties of the activated carbon and of the dyes may indicate whether adsorption process is favorable in acid or base solutions. [23] stated that at initial concentration of 500 mg/L, the maximum removal percentages of activated carbon from flamboyant pods were 99.88%, 99.87% and 98.97% for acid yellow 6, acid yellow 13 and acid red 18 dyes, respectively. Activated carbons prepared by Jerusalem artichoke stalk based mesoporous (MAC) were studied as adsorbents [33] for adsorption removal of anionic (methyl orange, MO) and cationic dyes (methylene blue, MB). It was observed that the amount of dyes adsorbed, q t (mg/g) values increased from 170.2 mg/g to 223.7 mg/g with the pH value increased from 3.44 to 10.6 at initial 60 min for MO dye, but different condition for anionic dye (MO) stated that the q t value decreased from 238.1 mg/g to 179.3 mg/g when pH was increased from 3.15 to 10.7 at initial 60 min. For instance, the MAC becomes negatively charged, which may produce a considerable high electrostatic attraction to simulate the MB moving toward the MAC surface, but when pH value was lower than pH pzc (3.70) condition, the surface of MAC is positively charged by adsorbing H + ions which is attractive to the anionic dye due to electrostatic repulsion between MO and negative charges on the MAC surface. Therefore, the authors suggested that the electrostatic interaction attraction may enhance the adsorption rate but is not the primary adsorption mechanism between the cationic dye and the partially negative charge MAC surface.
Grape processing waste (GW) was studied by Saygili et al., (2015) for the adsorption of methylene blue (MB) and metanil yellow (MY) dyes. The authors investigated the impregnation ratio of ZnCl 2 increased from 1:1 to 1:6 (GW/ZnCl 2 ), the surface area values also increased from 911 m 2 /g to 1361 m 2 /g, respectively. It is obvious that impregnated with chemical as an activating agent is very efficient in order to produced ACs with high surface area and porosity. Due to the chemical activation, the equilibrium data for both dye adsorptions onto grape processing waste activated carbon (GWAC), showing the maximum monolayer adsorption capacity of 417 mg/g for MB and 386 mg/g for MY. This may attributed to the fact that ZnCl 2 selectively extracted H and O away from the GW rather than ACs. Hence, this causes eventually to an increase in the surface area and porosity [51].
Due to their low cost and local availability, agricultural wastes such as Fox nutshell and Holm oak acorn are classified as woody materials (Table 2) and can produced activated carbon used as adsorbents for dye removal [10,21]. Recently, [21] studied the removal of methylene blue and phenol onto activated carbon from Fox nutshell (FNAC) by chemical activation (ZnCl 2 ). The maximum % removal of MB was observed at pH 11. Similar results are reported for the adsorption of methylene blue on Jute fiber and wheat shells [52][53]. When the pH value decreased at pH 3, the authors found that the low adsorption rate of MB occurred on FNAC due to the positive charged on the surface, causing H + ions to compete effectively with MB cations. The equilibrium adsorption of MB was increased from 249.88 tom968.74 mg/g which indicates that the MB adsorption process onto FNAC was exothermic in nature.
[10] studied that the highest micropore percentages was obtained using KOH as an activating agent for activated carbon from Holk oak acorn. Further, in order to know the effect of chemical treatment and to improve its efficiency the authors also tested the potential of the adsorbent by treating it with H 3 PO 4 and using ZnCl 2 , found that activated carbons obtained by H 3 PO 4 had a mesopore structure while those obtained from ZnCl 2 had a heterogeneous pore distribution consisting of micropores and mesopores. Similar to the findings of [54] preparation of activated carbons from sewage sludge via chemical activation using H 3 PO 4 may contribute large mesoporous and some microporous being observed for all activated carbons. The increase in surface area of ACs due to a phosphoric acid reagent has been explained in the reviews as H 3 PO 4 aiding the thermal decomposition of the lignocelluloses material.

CONCLUSIONS
The majority of the reviewed studies concluded that a wide range of activated carbons prepared from agricultural waste as adsorbents of dye removal. It is worthwhile noting that the removal of dyes can be done by various materials such as non-woody and woody waste. However, there exists no such methodology of which activated carbons can successfully remove all types of dyes at low cost adsorbents. The agricultural waste is converted into value-added materials as activated carbons, which are generated in huge amounts annually. The modification of the surface chemistry of carbons is also an important influenced, although depending on the raw materials of activated carbon, both increases and decreases in surface area have been reported. It has been generally shown that many researchers using agricultural waste from non-woody materials compared to woody materials in term of removal dyes in water polluted. As a reason, non-woody materials is ease to be conducted and chemical composition of these agricultural waste have high ash content and fiber such as hemicellulose, cellulose and lignin which may reduce the mechanical strength and adsorption capacity of activated carbon.
As environmental regulations become stricter, the effectiveness and cost of treatment processes for dye becomes more significant. Instead, the dye removal data available in literature suggests that removal of dyes is possible by agricultural waste to a certain extent since some promising results are obtained in some of the studies. Therefore, it is understood that impregnation ratio of chemical activating agents significantly affect to porosity. With low impregnation ratio, the formation of tar is inhibited and the release of volatiles is promoted, producing more mesopores. Rather at higher impregnation ratio, the more swelling impregnated precursor materials and stronger release of volatiles in the activation process will be lead to the widening of pores: micropores formed are subsequently converted to mesopores [55]. The application of activated carbon in adsorption process was mainly depends on the surface chemistry and pore structure or porous carbons. If possible, develop modification of surface chemical activation method on activated carbon prepared by non-woody material should be provided. Various methods should be produced such as acidic treatment and alkaline treatment.