Extracting phenol from wastewater using HTL biochar
Mr Xola Mgwebi
submitted in pursuit of the degree
BACHELOR OF ENGINEERING
North-West University: Potchefstroom Campus
Supervisor: Prof P van der Gryp
2.1 Adsorption theory and basic concepts
Adsorption is the phenomenon of accumulation of a gas or liquid molecular species at the surface of the liquid or solid as compared to the bulk phase (Musin, 2018). The molecular species that is absorbed on the surface is known as the adsorbate and the surface on which adsorption takes places is known as the adsorbent. Major adsorbents include activated Alumina, carbons, clay, polymers and resins, silica and zeolites (Musin, 2018). Each adsorbent has unique characteristics such as pore size, porosity and surface area. Pore size classified into 3 groups, marcopores with a diameter above 50nm, mesopores have a range between 2 and 50nm and micropores which are less than 2nm in diameter (Samal, 2014).
2.1.1 Mechanism of Adsorption
Adsorption occurs as result of unbalanced or residual forces of intermolecular interactions which contribute to surface energy of the solid or liquid phase (Krakil, 2014). These residual unbalanced forces attract and retain the molecular species with which they come in contact with at the surface. Adsorption is purely a surface phenomenon (Krakil, 2014).
When one mole of the adsorbate is adsorbed by the adsorbent heat evolved and is known as enthalpy of adsorption. Adsorption is an exothermic process and the change in enthalpy is always negative. When adsorbate molecules are adsorbed, their movement is reduced and this results in entropy reduction. At constant pressure and constant temperature adsorption is a spontaneous process (Rouquerol, 2014).
2.1.2 Types of adsorption
Depending on the types of attraction between the adsorbent and the adsorbate. The process of adsorption can be divided into two types, namely physical and chemical adsorption (Musin, 2018).
184.108.40.206 Physical adsorption of physiosorption
Physiosorption occurs when weak Van der Waal forces of attraction exist between the adsorbate and adsorbent. Physiosorption results in the formation of a multilayer of adsorbate on the adsorbent (Musin, 2018).
Physiosorption has a very low activation energy which makes it a reversible process. Physiosorption is an exothermic process with low enthalpy values as a result of the weak Van der Waal forces (Musin, 2018).
According to Le’ Chatelier’s principle physiosorption occurs at low temperatures since it is an exothermic process and decreases with an increase in temperature. For gases physiosorption increases with increase in pressure as the volume of the gas decreases (Rouquerol, 2014).
Adsorbents do not exhibit specificity for physiosorption since Van der Waal forces are universal. Physiosorption increases with increase in surface area (Musin, 2018).
220.127.116.11 Chemical adsorption or Chemisorption
Chemisorption results from chemical forces of attraction or chemical bonds that exist between the adsorbate and the adsorbent. Chemisorption is an exothermic process with high enthalpy values as result of the chemical bond formation (Krakil, 2014).
Although chemisorption is an exothermic process, the adsorption process increases with increase in temperature up to a certain limit and then it starts to decrease. This due the kinetic energy barrier. At high pressure chemisorption is favoured (Krakil, 2014).
Chemisorption has high specificity due to the that it occurs only there is chemical bonding between the adsorbate and the adsorbent. Chemisorption increase with increase in surface area of the adsorbent (Krakil, 2014).
2.2 Wastewater treatment
Wastewater contains organic and inorganic contaminants that is harmful to the environment and should be treated to remove these contaminants (Musin, 2018). The treated water must be free or have acceptable levels of the contaminants before it can be released to the environment or be used (Musin, 2018). This makes water treatment technologies and their development a priority in many industries (Musin, 2018. Water treat technologies are divided into two categories the passive and active technologies. Passive technologies use naturally available energy, these include photosynthesis, microbial active energy and topographical gradient. Active technologies require external energy and maintenance, such adsorption, filtration, ion exchange, eletrodialysis, neutralisation and solvent extraction. Passive technologies are less efficient than active technologies (Musin, 2018).
In study, adsorption will be the technology investigated, where phenol is the adsorbate and biochar and activated carbon are the adsorbents.
Adsorption is highly efficient in removing low levels of organic toxins from dilute solutions, highly selective, minimum products of by-products, regeneration ability., low cost and simple utilisation (Musin, 2018).
Phenol is a colourless crystalline solid a sweet tarry odour, it is an aromatic compound containing a hydroxyl group and a benzene ring, it is soluble in water. Phenol is produced industrially from petroleum. Phenol is used in the synthesis of products such insulation material, illuminating gases, lacquers, adhesives, paints, rubber, ink, dyes, solvents, soaps, perfumes, explosives, pharmaceuticals disinfectants, resins and nylon (Shin, 2017). Phenol is also used in the medical field as a neurolytic agent, topical anaesthetic and in dermatology (Shin, 2017).
Phenol is classified as hazardous pollutant since it is poisonous to microorganisms, animals, and human even at low concentrations (Shin, 2017). The consumption of phenol contaminated water causes tissue erosion, paralysis of the nervous system and protein degeneration (Yousef, El-Eswed and Al-Muhtaseb, 2011). It is well demonstrated that phenol is exceedingly toxic and carcinogenic (Shin, 2017). The World Health Organization(WHO) requires that the allowable constraint for phenol concentration in consumable water is 0.002 mg/L (Shin, 2017). In some cases, the levels of phenol in water reach or surpass the directed concentration due to wastewater release or spilling accidents (Shin, 2017). It is therefore vital to investigate strategies for removal of phenol from a wastewater.
Biochar is a porous carbonaceous solid product of pyrolysis or direct liquefaction.
Pyrolysis is the thermochemical treatment, which be applied to organic. During pyrolysis the matter is exposed to high temperature in the absence of oxygen and is broken down to its constituents. Pyrolysis temperature greatly affects the characteristics of biochar in as far at its surface chemistry and its elemental composition. (Gollakota, Kishore and Gu, 2018)
Direct liquefaction or Hydrothermal liquefaction(HTL) is also known as hydrous pyrolysis although when compared to pyrolysis HTL is carried out at low temperatures and heating rates
2.2.3 Activated carbon
Activated carbon is carbonaceous, highly porous adsorptive medium that has a complex structure composed of primarily of carbon, which is activated by steam or chemical. (Gollakota, Kishore and Gu, 2018)
In steam activation the raw material is carbonized by heating in an inert atmosphere so that dehydration and devalatilization of the carbon occurs with temperatures not above 7000c.
The carbonized product is activated with steam at temperatures between 9000C and 10000C. This reaction between carbon and steam enlarges the pores.
During chemical activation the raw material is mixed an activating chemical agent. The mixture is carbonized at temperatures of 4000C to 5000C. The chemical agent hydrates the raw material resulting in armartization and charring of the carbon which creates a porous structure and an extended surface area.
3. Phenol adsorption
3.1 Effect of adsorbate initial concentration
Figure 1 shows that an increase in the initial phenol concentration results in the increase in adsorption capacity. According to (Saha, Srivastava and Chowdhury, 2013) this increase is due to the fact that increasing the concentration gradient provides an increasing driving force for which phenol can overcome the mass transfer resistance of the phenol between the liquid and solid phases. This leads to an increase equilibrium capacity until adsorbent is saturated. On the other hand, increase in the initial concentration the adsorption efficiency decreases. This can be attributed to the fact that adsorption sites saturation on the adsorbent decrease as the phenol accumulates. The total number of adsorption sites is fixed for a given dose of the adsorbent, therefore the resulting in the same amount of phenol adsorbed. As a result, the percentage of phenol adsorbed decreases as the initial concentration of phenol increases (Saha, Srivastava and Chowdhury, 2013).
Figure SEQ Figure * ARABIC 1:Effect of initial concentration on phenol adsorption adapted from (Saha, Srivastava and Chowdhury, 2013)
Equation (1) is used to calculate the adsorption capacity where qe is the amount of phenol adsorbed per unit of adsorbent, C0 is the initial concentration of phenol, Ce is the equilibrium concentration of phenol, m is the mass adsorbent and V is the volume of the solution. Equation (2) was used to calculate the percentage of phenol removed.
3.2 Effect of temperature
Figure 2 below shows the adsorption of phenol at different temperatures. The adsorption capacity decrease with the increase in temperature. The results according to (Saha, Srivastava and Chowdhury, 2013) indicates that adsorption of the phenol is kinetically controlled by an exothermic process. The increase in temperature weakens the bonds between the adsorbate and the binding site of the adsorbent from the liquid interface at higher temperature (Saha, Srivastava and Chowdhury, 2013).
Figure SEQ Figure * ARABIC 2: Effect of temperature on phenol adsorption adapted from (Saha, Srivastava and Chowdhury, 2013)
3.3 Effect of adsorbent dosage
Figure 3 shows an increase in the removal of phenol with the increase in the adsorbent dose from 0.5 g to 5.0 g. This is because the increase in surface area increases the binding sites (Saha, Srivastava and Chowdhury, 2013). The equilibrium adsorption capacity decreases with the decreasing total surface area in the adsorbent dose. This could be the result of the decrease total surface area available to the phenol molecules resulting from the aggregation and overlapping of adsorption sites according to (Saha, Srivastava and Chowdhury, 2013). This causes the equilibrium value to decrease. Maximum adsorption occurs at 2.0 g and a further increase in the amount of adsorbent concentration does not result in change in the amount of phenol adsorbed. This is a result of the fact that equilibrium has been established between the adsorbent and adsorbate. (Shin, 2017)
Figure SEQ Figure * ARABIC 3:Effect of adsorbent dose in the adsorption of phenol adapted from (Saha, Srivastava and Chowdhury, 2013)
3.4 Phenol adsorption kinetics
Figure 4 shows the amount of adsorbate adsorbed over a period of time. From the figure it can been seen that the adsorption of phenol increases until equilibrium is reached. According to literature the adsorption of phenol by biochar takes about 180 min to reach equilibrium. Table 1 shows parameters for pseudo – first and second order models obtain by Shin W using biochar obtained H. fusiformis plant. These results indicate that the pseudo- second order equation to the phenol adsorption on the H. fusiformis biochar is applicable.
Figure SEQ Figure * ARABIC 4:Phenol adsorption profile on biochar adapted from (Shin, 2017)
Figure 5 shows that 98-99% phenol removal occurred within the first hour of contact with activated carbon. This is a result of high number of available vacant sites in the activated carbon and high affinity for organic compounds. A high degree of partition exists between solid and liquid phases (Mukherjee et al., 2007).
Figure SEQ Figure * ARABIC 5: Phenol adsorption profile on activated carbon adapted from (Mukherjee et al., 2007)
Pseudo-first order Pseudo-second order
q(mg/g) k1(1/min) R^2 q(mg/g) k(1/g.mg/min) R^2
33.47 0.182 0.985 34.68 0.011 0.996
Pseudo- first order dqdt=k1qe-qt (3)Pseudo-second order dqdt=k(qe-qt)2 (4)k and k1 are rate constants, qe adsorption capacity at equilibrium and qt adsorption capacity at time t
3.5 Phenol adsorption Isotherms Langmuir and Freundlich isotherms models are used to determine models that the data fit. The constants of Langmuir and Freundlich models for adsorption of phenol in are obtained from plots (figure 6) and the values are presented in table 2. Phenol is best described the Langmuir isotherm in both activated carbon and biochar. Langmuir are based on the assumptions that the surface of the adsorbent is uniform i.e. all the adsorption sites are equivalent, adsorption molecules do not interact, all adsorption occurs through the same mechanism, and at the maximum adsorption only a monolayer is formed i.e. molecules adsorbed do not deposit on other already adsorbed molecules (Shin, 2017)
Figure SEQ Figure * ARABIC 6:Comparison of Langmuir and Freundlich with experimental data
Langmuir model qe=QmbC1+bC (5) Freundlich model qe=KC1n (6)Qm is the amount adsorption amount, b is the Langmuir constant, C equilibrium concentration of the adsorbate, K is the distribution constant and n is the Freundlich constant.
Table 2: Langmuir and Freundlich parameters adopted from (Shin, 2017)
Langmuir model Freundlich model
Q(mg/g) b(L/mg) R^2 K 1/n R^2
30.039 49.480 0.961 3.803 0.528 0.956
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