India generates more than 565 million tons of agricultural and forestry residues annually [1
]. It has been noticed that in the absence of any available robust technologies, farmers/owners of farms try to burn them as a solution to clearing the fields for the next crop. The burning of these solid wastes generates greenhouse gases. The technologies available may not be applicable for the utilization of biomasses from a location, because their properties vary with sites [3
]. So, there is a great need to investigate various possible ways in which they may be utilized as valuable products. It has been reported that agricultural wastes and forestry residues suffer from two drawbacks: (i) They have a high moisture content and (ii) low bulk density [3
]. As a result of these constraints, it is desired that they may be utilized at the site of their availability.
Investigators have reported the applications of biomasses as low-cost adsorbents for various pollutants, namely phenol, dyes, acids, and heavy metals in industrial effluents [4
]. The present work focuses on the removal of phenol using biomasses in the forms of agricultural wastes and forestry residues as sources of adsorbents. Phenol is colorless and soluble in water and is characterized by its unique odor. If it comes into contact with the skin, it causes irritation and necrosis. If ingested, even a small amount damages kidneys, liver, and muscles [9
]. Effluents with an appreciable amount of phenol, when discharged in water bodies, are highly toxic to all types of aquatic life. The significant sources of phenol in the aquatic environment are wastewaters from paint, pesticide, coal conversion, polymeric resin, petroleum, and petrochemicals industries [10
]. Phenols are listed in the United States Environment Protection Agency (USEPA) and the European Union (EU) as pollutants of priority concern [11
Many findings are reported related to the removal of phenol from aqueous solutions using agricultural wastes and forestry residues. Ahmaruzzaman and Sharma reported rice husk and rice-husk char as adsorbents with adsorption capacities of 4.7 and 7.9 mg/g [12
]. Mohd Din et al. have reported the adsorption capacity of coconut shell as 205.8 mg/g. The adsorption isotherms fit Langmuir and Freundlich equations. The kinetics of adsorption is followed by the pseudo-second-order equation [13
]. Stasinakis et al. studied olive pomace. Different forms of adsorbents have been obtained by physical and chemical treatment. The removal efficiency is also improved by decreasing the particle size [14
]. Alhamed studied the use of date stones as carbonaceous adsorbent and its use for the removal of phenol from wastewaters. He used different particle sizes for packed bed studies and estimated the adsorption capacity of activated date stone as 90.4 mg/g [15
]. Tseng and Tseng reported corn cob as an adsorbent for phenol removal. Activation of the residue was done using KOH [16
]. Jadhav and Vanjara studied sawdust as a promising adsorbent [17
]. Sarker and Fakhruddin studied rice straw. They treated the residue, with a particle size less than 1 mm, by physical and thermal methods, which could remove 84% of phenol from aqueous solutions [18
In this work, 10 biomasses, namely Acacia nilotica
branches (ANB), bagasse, corn cob, cotton stalk, ground nutshell (GNS), Lantana camera
(LC), rice husk (RH), rice straw, sawdust, and wheat straw, were dried and ground. They were characterized in terms of proximate analysis (ash, fixed carbon (FC), and volatile matter (VM)) and bulk density as per ASTM standards. Based on the characterization and availability of biomasses, ANB, LC, and RH were selected for further studies as adsorbents for the removal of phenol from aqueous streams. Figure 1
indicates the pictures of the selected biomasses.
The percentage removals of phenol from 1 g/L of aqueous solution on ANB, LC, and RH were very low. The maximum removal of 35% was observed with the adsorbent ANB. The characterization results of these adsorbents showed lower Brunauer Emmett Teller (BET) surface areas and FCs and higher VM contents compared to that of commercial-grade carbons used for phenol adsorption. The total surface area of any commercial-grade carbon ranges from 450 to 1500 m2
/g. The actual surface area available for adsorption is dependent on the nature of the adsorbate, which could be considerably less than the total [19
The methods available for activation are physical methods [20
], steam activation [21
], carbon dioxide activation [22
], air activation [23
], and chemical activation [24
]. There are reports of thermo-chemical treatment (a combination of physical and chemical methods) of biomasses to make the activated carbon comparable to that of commercial-grade carbon for appreciable removal of phenol from aqueous solutions [26
]. Therefore, the selected biomasses were treated thermo-chemically for the desired changes in their characteristics. The treatment reduced VM and ash, thereby increasing the FC percentages. These procedures also helped to increase the BET surface areas and methylene blue adsorption values of the resulting adsorbents. The thermo-chemical treated forms of ANB, LC, and RH were activated ANB (ANBC), activated LC (LCC), and activated RH (RHC), respectively.
To optimize the operating parameters to generate adsorption isotherms for ANBC, LCC, and RHC, the effects of dosage, pH, time of contact, initial phenol concentration, and agitation speed were studied, individually on these adsorbents. At the optimized conditions, the generated adsorption curves were fit into Langmuir, Freundlich, and Temkin isotherms. Based on the percentage removal of phenol with time, the kinetic analysis was done using pseudo-first-order and pseudo-second-order kinetics. It was possible to recover a high percentage of phenol in a single-step regeneration of these adsorbents.
This paper explains the conversion of ordinary biomasses, ANB, LC, and RH, to its activated forms as ANBC, LCC, and RHC, respectively. They were comparable to commercial-grade carbons in terms of their properties for the removal of phenol from aqueous streams. It was possible to study the adsorption of phenol in the entire supernatant concentration range of 100 to 975 mg/L at equilibrium, using all three activated biomasses.
Based on the characterization and availability of biomasses, ANB, LC, and RH were selected for the adsorption study of phenol. As the percentage removal of phenol from aqueous solutions was low, they were thermochemically treated to obtain activated forms of adsorbents as ANBC, LCC, and RHC with BET surface areas of 450, 151, and 301 m2
/g, respectively. Adsorbents helped the removal of phenol by 97%, 90%, and 83%, from aqueous solutions. The adsorption conditions for phenol removal were optimized. Based on the optimum condition selected, the adsorption isotherm of ANBC fit into the Temkin model in the concentration range of 903 to 945 mg/L of the supernatant solution. The adsorption process with LCC followed the Langmuir model in the concentration range of 100 to 400 mg/L. LCC had a monolayer adsorption capacity (Q0
) of 16.49 mg/g. The separation factor, RL
, was 0.067, indicating that the adsorption of phenol on LCC was favorable. Kinetic data of ANBC and LCC fit into the second-order model whereas RHC indicated its fit was first-order kinetics. Activated biomasses were subjected to single-step regeneration using 1 M NaOH. It was possible to recover 90%, 87%, and 80% of phenol using ANBC, LCC, and RHC, respectively. Thus, selected biomasses, Acacia nilotica
branches, Lantana camera
, and rice husk, yielded value-added products in the forms of activated adsorbents with the appreciable capacity of phenol removal from aqueous solutions. Biomasses, ANB, LC, and RH, causing threats to the environment, can be used effectively for the removal and recycling of phenol from aqueous solutions under the concept of a circular economy [38
In the future, ANB, LC, and RH should be considered as adsorbents for the removal of other toxic chemicals in the form of dyes, organic acids, heavy metals, and oil spills from aqueous streams, located in India and other parts of the world.