Continuous flow adsorption for phenol removal using environmentally friendly naturally derived bed

Phenol is one of the common contaminants observed in many wastewater industries. It is toxic, hazardous, carcinogenic and listed as a priority pollutant by the Environmental Protection Agency (EPA). This research aimed to establish the efficacy of activated carbon derived from walnut shell in extracting phenol from aqueous solutions using a fixed-green bed column adsorption system. The effect of parameters affecting the adsorption process including the initial concentration of phenol, bed ratio, and flow rate, was studied. The results showed that the highest percentage of phenol removal by the activated carbon is 86.2% at pH 7, initial concentration of phenol 0.001M, bed ratio 1:3 sand/activated carbon, and flow rate 10 ml/min. The breakthrough behavior of the fixed-bed adsorption process was studied. It was observed that the adsorption process equilibrium is practically reached after 105 minutes. The adsorption column dynamic behavior was investigated using three numerical models. The results confirmed that Thomas and Yoon-Nelson models are found more fitted to the adsorption experimental results. Moreover, modeling and interpretation of the column adsorption isotherms predicted that the Freundlich isotherm model is better than Langmuir isotherm model to describe the column adsorption data indicating that the phenol adsorbed molecules are not restricted to monolayer formation and the mechanism of adsorption is chemo-sorption. Briefly, the results of this study pointed out that the activated carbon derived from the walnut shell is not only a low-cost green adsorbent but also has high performance in the removal of phenol from aqueous solutions.


Introduction
2nd International Scientific Conference of Al-Ayen University (ISCAU-2020) IOP Conf. Series: Materials Science and Engineering 928 (2020) 022050 IOP Publishing doi: 10.1088/1757-899X/928/2/022050 2 The growth of world world's population, the huge increase in environmental pollution and the decrease in the sources of clean water have enforced the pollution prevention and environmental protection stakeholders to work intensively for reducing and regulating pollution.

Restricted instructions and rules are implemented by the Environmental Management
Authority and the governments around the world, the rules are geared towards enforcing the leaders in industrial and municipal facilities to control and minimize the formation of waste products as well as hindering the disposal of sewage and wastewater in land and water sources without treatment [1].
Wastewater may contain large levels of pollutants including organic [2, 3, 4 and 5] and inorganic pollutants such as toxic chemicals [6,7] that may discharge from domestic and industrial facilities. These contaminants cause serious pollution of the aquatic environment and have negative impact on water quality making the water resources unusable suppliers for drinking water. Therefore, primary, secondary and tertiary treatments technique are applied, however, the final treatment methods may be some times not selective for specific contaminants besides they are not cost-effective. Accordingly, water treatment leaders have evolved considerably to imply new and cost-effective water treatment technologies. Among the technologies used is the adsorption technique, which becomes more popular purification process in recent years owing to its versatility selectivity and low cost. The term adsorption could be defined as the change in the concentration of a molecule in the surface layer of a solid material in comparison with the bulk phase with respect to unit surface area. The adsorption process involves the selective interaction of one or more compounds from the liquid phase on a solid called adsorbent. Separation by adsorption is highly effective, simple design, cheap, easy to adopt and scale-up compared to the other conventional treatment in water pollution control [8]. In addition to removing pollutants, the adsorbent can act as a particulate trap as well as a means of removing taste and odor.
Adsorption processes could be carried in different modes, nevertheless, adsorption is commonly held in column continuous processes witch are more efficient than batch processes, it allows a more efficient use of the adsorbent, since the dynamic system enables an evaluation of the material saturation in relation to the time, space and length of the adsorption column [9].
New adsorbents have superior tendency to remove undesirable chemicals from hydrological systems are synthesized and employed , however the high cost and relative inability to regenerate of some of them hinders their use at larger scales, hence, the main benefit of adsorption has been the use of low-cost materials, to reduce the cost of the procedure. Different types of adsorbents are usable including natural and synthetic clays like zeolites [10,11]. Other types are the adsorbents could be derived from industrial and biogenic wastes like activated carbon and ashes [12,13].
The utilizing of renewable, agricultural, and industrial materials and by-products as adsorbents is one of the tools for applying the green sustainability concepts in pollution remediation. Green adsorbents are eco-friendly materials, popular, and available in large quantities in most countries, they have been confirmed to act as cheap filters having high potential, strength, and selectivity to interact with contaminants such as dyes, heavy metals, phenols, pesticides and pharmaceuticals [14].
On another hand, Phenolic compounds exist in water bodies due to the discharge of polluted wastewater from industrial, agricultural and domestic activities. Phenol compounds are known to be toxic and inflict both severe and long-lasting effects on both humans and animals. They act as carcinogens and cause damage to the red blood cells and the liver, even at low concentrations.
The interaction of these compounds with microorganisms, inorganic and other organic compounds in water can produce substituted compounds or other moieties, which may be as toxic as the original phenolic compounds [15].
Although numerous numbers of studies dealt with studying phenol adsorption by different adsorbents from aqueous mixtures, only a few studies focused on the fixed-green bed column adsorption processes. In the current work, activated carbon derived from walnut shell is used as adsorbents for phenol removal from aqueous solution using fixed bed down flow process.

Preparation and characterization of the green bed
Walnut shell was collected from household waste of Erbil Province gardens and mainly from Erbil city. After screening the shells and separation from impurities, in order to softening Walnut shells, walnut shells were placed for 24 hours in water rinsed and then dried for 5 hours at 105 o C in thermostatic oven. The dry shells were milled using electric mill and then were screened with 2nd International Scientific Conference of Al-Ayen University (ISCAU-2020) IOP Conf. Series: Materials Science and Engineering 928 (2020) 022050 IOP Publishing doi:10.1088/1757-899X/928/2/022050 4 standard laboratory sieves. Particles with an average particle size of 0.38 mm were separated and immersed for two hours in acid solution (phosphoric acid H 3 PO 4 ) for the purpose of chemical activation. The activated shells were then carbonized in an ash furnace at 500 o C for 60 min in the presence of carbon dioxide gas. In the final stage, the activated material was washed various times with distilled water at 100ºC to remove the excess reagent used in the chemical activation until the pH of the water had reached around 7.0. Finally, the activated carbon was dried (in an oven at 110ºC) for 24 hours, cooled and then used as the green adsorbent bed. Figure 1 shows the steps of preparation the green bed. The prepared bed was characterized; the characteristic of the adsorbent is illustrated in Table 1.  The adsorbent shows high specific surface area and pore volume which indicated that it has promising adsorption efficiency.

Preparation of the adsorbate (phenol solution) & absorbance determination
A stock solution of phenol 0.001M was prepared by dissolving predetermined amounts of phenol in distilled water. This solution was diluted as required to obtain standard solutions. The  were organized to make graphics, in order to better observe the adsorption behavior.

Numerical modeling for studying the column adsorption dynamics
In this study, the rate of adsorption was studied by employing three models. The models are Bohart-Adams, Thomas, and Yoon-Nelson. The models are used to predict the dynamic behavior of the fixed-bed column [16,17]. where k YN (min−1) is the rate velocity constant, and τ (min) is the time required for 50% adsorbate breakthrough (i.e. C t /C 0 ≈ 0.5). From the linear dependence of ln[C t /(C 0 − C t )] versus t, the model parameters can be determined.

Modeling of the adsorption isotherms
Two isotherm models have been used for the equilibrium modeling of the adsorption system.
Where q e is the amount of adsorbate at equilibrium per gram of adsorbent (mg/g), Ce the equilibrium concentration of sorbate in the solution (mg/L), Q 0 and K are Langmuir constants related to sorption capacity and sorption energy, respectively. K f , and 1/n are the Freundlich model constants.

Results and discussion
Adsorption processes for decontamination of wastewaters can be carried out either discontinuously in batch reactors or continuously in fixed-bed reactors or columns. Fixed-bed systems have an important advantage because adsorption depends on the concentration of the solute in the solution being treated [20]. The adsorbent is continuously in contact with fresh solution; hence the concentration in the solution in contact with a given layer of adsorbent in a column is relatively constant. Other advantages are higher residence times and better heat and mass transfer characteristics compared to batch processes [21].
The effect of some parameters, namely, volumetric flow rate, ratio of the green packing material    The results obtained concerning the breakthrough behaviors of fixed-green bed column adsorption process on % removal of phenol are represented Figure 5. It is obvious to note that, breakthrough curves exhibit a characteristic 'S' shape but with a specific degree of steepness.
The equilibrium is practically reached at 105 minutes. The results obtained show that the plot has two steps; the first is rapid and the second is a slow one. This relates to the great availability of the free active sites of the adsorbent at the beginning of the experiment and which becomes less available as time proceeds.   The results regarding the numerical modeling for column adsorption dynamics are illustrated in Figures 6,7 and 8. The plot of Bohart-Adams model is shown in Figure 6. The Yoon-Nelson model plot is illustrated in Figure 7, while Thomas model plot is displayed in Figure 8.  The equilibrium modeling of the adsorption system in the current work is figured out using  The two equilibrium adsorption models parameters estimated from the plots are listed in Table 2.  [19] . The numerical value of 1/n >1 indicates that adsorption capacity is only slightly suppressed at higher equilibrium concentrations.

Conclusions
The fixed green bed down flow adsorption process for phenol removal from aqueous solutions is feasible and effective by using a walnut shell activated carbon as green adsorbent derived from natural waste. The bed is cost-effective and environment friendly as it derived from biomass; a natural waste that is considered as a source of pollution. The natural waste has been reused in the current work to produce the green adsorbent instead of disposing it in landfills. The adsorption process could be scaled up to industrial level simply and used for removal of phenol compounds from wastewater. The initial concentration of phenol solution, its volumetric flow rate and the mass of the green bed seemed playing vital role on the adsorption capacity of the bed. The dynamic of the column adsorption and the adsorption isotherms could be studied using numerical models developed for such purposes. The adsorption of phenol on the green bed using the down flaw mode seemed to be multilayer kept on via chemo-sorption mechanism. A comprehensive study on the green bed regeneration as well as the adsorption kinetics will be the items of a future study.