INTRODUCTION:
Water stands as an essential necessity on our planet, serving a critical function in the existence of all living creatures. About 70% of the Earth's surface is covered by water, existing in diverse manifestations like oceans, rivers, lakes, and glaciers1.
The availability of pure and drinkable water is of paramount importance for the health and prosperity of both humans and other forms of life. Predictions indicate that by 2025, more than half of the world's population will confront challenges connected to a shortage of water resources. On a worldwide scale, the U.S. Geological Survey reported that the United States produced roughly 4.4 billion pounds of phenol and acetone in 2020, aiming to fulfill the global demand, which was estimated to be about 11 million metric tonnes in 2019. The survey also anticipated an annual growth rate of 2.6% in phenol demand from 2020 to 2027. This surge in demand is ascribed to the increasing need for phenol in diverse industries such as automotive, construction, electronics, and healthcare, as detailed in the study by2.
In 2019, the International Phenol Study Group forecasted that global phenol production would reach approximately 16.5 million tonnes. Nearly half of this production was dedicated to the manufacturing of Bisphenol A, a common component in various everyday products, including food and beverage containers, dental fillings, and thermal paper receipts. Phenol formaldehyde resins, a type of synthetic thermosetting polymer, are widely employed in numerous applications, including adhesives, coatings, and molded products.
In Africa, there is a notable scarcity of data regarding the presence of Phenolic substances in both the environment and aquatic ecosystems, despite their extensive usage in industries and agricultural practices across the continent. It's worth noting that there are no established limits for these pollutants in water, particularly in West African countries, as indicated by 3. In Kenya, water pollution represents a significant challenge, affecting approximately 40% of the population who still rely on unsanitary water sources like stagnant wells, dams, rivers, streams, and ponds for their daily needs. These water sources are afflicted by bacterial contamination and pollution stemming from heavy metals and organic contaminants, as reported by4,5. The primary sources of this pollution can be traced back to agricultural runoffs, untreated sewage, and nutrient-rich fish water, as highlighted by 5.
Organic substances in the water display toxicity, leading to skin irritations and allergies. Moreover, their carcinogenic properties pose significant health risks to various life forms, including the fish population, other aquatic organisms, and the people who depend on the lake for their livelihoods and as a source of drinking water, as described in studies by6,7.
Thermodynamic research entails the examination and interpretation of the thermodynamic characteristics of a system, which elucidate how energy and substances behave within that system. Such investigations not only offer insights into the limitations of a process but also shed light on the spontaneity and direction of chemical reactions. These characteristics encompass variables like temperature, pressure, volume, entropy, and Gibbs free energy. In this particular study, the researchers delved into the thermodynamic parameters related to adsorption, which included the thermodynamic potential (ΔG ͦ), heat content (ΔH ͦ), and the degree of randomness (ΔS ͦ). These values were determined through the application of the Van't Hoff plot 8 and the Arrhenius equation 9.
ΔH ͦ is applied to clarify and characterize the thermodynamic aspect of adsorption, whether it exhibits an endothermic or exothermic nature 9. Positive values falling within the range of 40 to 500 kJ/mol indicate chemisorption, which involves a strong electrostatic interaction between the metal ion and active sites 10. ΔG ͦ values determine the extent of favorability and the spontaneous tendency for adsorption 11. On the other hand, ΔS ͦ provides insights into the level of organization or randomness observed at the external surface of the adsorbent during its interactions with the adsorbate12. Additionally, the characterization of adsorption behavior can also involve the utilization of activation energy (Ea) and sticking probability (S*), which can be derived using the Arrhenius equation 9. in conjunction with surface coverage (Ɵ), as described in equations 2.10-2.13 13. The concept of sticking probability (S*) elucidates the likelihood of metal ions that have been adsorbed to remain on the surface of binding sites within a temperature-dependent system 14. A negative activation energy (Ea) signifies an exothermic character in the adsorption process 15, while positive values indicate an endothermic nature 16. Additionally, these factors play a significant role in influencing the type of adsorption behavior observed, whether it falls into the category of physisorption (with energy values ranging from 5 to 40 kJ/mol) or chemisorption (with energy values spanning from 40 to 800 kJ/mol) 17.
MATERIALS AND METHODS:
RESEARCH DESIGN:
The objective of the research was to investigate the thermodynamics of PNP ion adsorption from an aqueous solution. This was accomplished by utilizing activated carbon derived from macadamia nutshells and optimizing various parameters including pH, time, speed, dosage, and initial concentration and characterizing and contacting analysis on the same as described in18
A summary of the experimental procedure is presented in Figure 1.
Figure 1: Research design
WASTEWATER SAMPLING:
Wastewater samples were obtained from the Nairobi River using a non-probability sampling method. Selection of specific sampling points was based on informed professional judgment. Random samples were taken from various locations within these sampling points, and a subsample was extracted and stored in a cooling box at 4°C. These samples were then transported to the Chemistry laboratories at Dedan Kimathi University of Technology for analysis.
ADSORBENT PREPARATION:
The fruits were gathered from Mudavadi local market in Nyeri, Kenya, and then brought to the Chemistry laboratories at Dedan Kimathi University. They were thoroughly cleaned with distilled water and opened to extract the inner contents. The samples were subsequently cut into small pieces and air-dried until they reached a constant weight over the course of one day. The unmodified sample underwent a chemical modification process, followed by placement in a muffle furnace and heating at 600°C for a duration of 2 hours, following the method detailed by 18 to produce biochar.
The pellets were subsequently subjected to analysis utilizing an FT-IR spectrophotometer (model FT/IR-4700) manufactured by JASCO in Japan. To examine the external characteristics, surface structure, and morphology, a scanning electron microscope (SEM model FEI ESEM - Tescan Vega LMH) was employed. The samples were first dried and crushed, and the resulting powder was pressed onto aluminum stubs covered with double-sided carbon tape. Subsequently, they were examined at an accelerating voltage of 20 Kv.
ADSORBATE PREPARATION:
To create stock solutions of PNP with a concentration of 1000 µg/l, distilled water was utilized. Various concentrations were achieved by gradually diluting these stock solutions. To adjust the pH, solutions containing 0.1 M NaOH and 0.1 M HCl were employed.
A. p-NITROPHENOL ANALYSIS:
In this particular scenario, the utilization of adsorption isotherm analyses served the purpose of evaluating the effectiveness of activated macadamia nutshell waste. 18 employed Freundlich and Langmuir isotherm simulations to not only examine physico-chemical interactions but also to establish the maximum adsorption capacity of the adsorbent. Furthermore, the study involved the application of various rate equations in kinetic modeling to elucidate the mechanism governing the uptake of metal ions. As highlighted by 19 in their research, kinetic modeling emerged as a valuable instrument for comprehending and optimizing intricate systems, offering extensive applications across a diverse range of fields.
All the parameters were examined with the UV-Vis spectrophotometer (Analytik Jena Model Specord 200) at a wavelength where the maximum absorption occurred, which was 400 nm, for their concentrations in the respective supernatant solutions as indicated by 20.
B. THERMODYNAMIC EXPERIMENTS:
The experiments were carried out in a batch mode, utilizing a temperature-controlled water bath shaker and 120 mL plastic bottles. For the thermodynamic investigation, 0.1 g of modified adsorbents were added to the plastic bottles, which already contained 20 mL of a concentrated solution of the phenolic compound at a concentration of 20 mg/L. The mixture was subjected to agitation in the water bath shaker for a duration of 60 minutes, with temperatures varying between 298 K, 303 K, 308 K, 313 K, 318 K, and 323 K. Following this, the substance was extracted, and the concentration of the filtered solution containing phenolate ions was determined.
The quantity of phenolate ions absorbed per unit mass of the adsorbent was calculated using equation 1, as described by 21:
(1)
Where, qe represents the adsorption capacity, V (in liters) denotes the total volume of the phenolate ion solution, co is initial adsorbate concentration and ce is adsorbate final concentration of cations at equilibrium (mg/L), and m (in grams) signifies the mass of the adsorbent.
To assess thermodynamic parameters, the experimental data underwent analysis through calculations involving the equilibrium constant (Ke), the determination of Gibbs' free energy (ΔG ͦ ), and the utilization of the thermodynamic Van't Hoff equation 22, as described by equations 2-4.
(2)
(3)
(4)
Where Ke; is the equilibrium constant (l/g); Ce is the equilibrium concentration of the phenolate ions (mg/L); qe is the PNP ion uptake per unit mass of the adsorbent (mg/g); ΔG ͦ is the standard Gibbs free energy (kJ/mol). ΔH ͦ is the standard heat of reaction (kJmol-1); ΔS ͦ is the standard entropy change (Jmol-1K). T is the absolute temperature (K) and R is the molar gas constant (8.314 jmol-1K-1). From equation 3, ΔG ͦ = ΔH ͦ - TΔS ͦ. Therefore, ΔH ͦ - TΔS ͦ = -RT ln ke. Hence, ln ke = -ΔH ͦ /RT + ΔS ͦ /R. From this equation, the slope is ΔH ͦ /R and the intercept is ΔS ͦ /R. ΔH ͦ and ΔS ͦ are then determined from the plot of ln Ke versus 1/T (k-1) while ΔG ͦ is calculated from equation 3 23.
The characterization of adsorption behavior can also involve the utilization of activation energy (Ea) and sticking probability (S*). These parameters were determined through the application of the Arrhenius equation, as described by 9, and their connection to surface coverage (Ɵ), as outlined in equations 5-8, as detailed by 13.
(5)
(6)
(7)
(8)
Adsorbed metal ions to remain on the surface of binding sites within a temperature-dependent system, as elucidated by 14. It is typically expected to fall within the range of 0 < S* < 1. Specifically, if S* > 1, it implies no adsorption; if S* = 1, it indicates a combination of physisorption and chemisorption; if S*=0, chemisorption dominates; and if 0<S* < 1, physisorption dominates, as outlined by 24.
On the other hand, the activation energy (Ea) with a negative value signifies an exothermic nature of adsorption, as suggested by 15, Conversely, positive (Ea) values indicate an endothermic process, as explained by 16. These (Ea) values also have a significant impact on the type of adsorption behavior, with (Ea) values falling within the range of 5-40 kJ/mol associated with physisorption and values in the range of 40-800 kJ/mol linked to chemisorption, as highlighted by17. Both (S*) and (Ea) values are derived by plotting (1-Ɵ) against 1/T.
RESULTS AND DISCUSSION:
FTIR results
The figures presented by 18 illustrated the FTIR spectra of both inactive and active adsorbents, revealing the presence of functional groups with the ability to bind to PNP ions. In the spectrum before adsorption occurred, there were minor peaks observed at 3782.69 cm-1 and 3709.41 cm-1 in the region associated with single bonds, potentially indicating the presence of amide functional groups, typically characterized by narrow peaks between 3600-3645 cm-125.
A broader peak at 3405.67 cm-1 was attributed to an O-H stretch, possibly originating from secondary or tertiary alcohols and phenols. The peak at 2915.84 cm-1 corresponded to C-H symmetric and anti-symmetric vibrations, which decreased in intensity after modification due to significant deprotonation26. A prominent peak at 1586.16 cm-1 was assigned to N-H vibrations arising from the amides within the aromatic part of the nutshell material26. Another smaller peak at 1324.86 cm-1 indicated the presence of -CH2 groups in the MMNS material, which weakened and decreased in size due to the loss of cellulose and hemicellulose during modification26. A broad peak at 1004.73 cm-1 was associated with C-O-C stretch vibrations, falling within the range of 1050-1155 cm-1 in the spectrum, potentially originating from alcohols or alkyl-substituted ethers. Following the process of adsorption, certain functional groups participated in hydrogen bonding π-π interactions between aromatic rings and electron donor mechanism, leading to the broadening of the peaks, especially the OH group at approximately 3400 cm-1. Additionally, there was a reduction in the intensity of the N-H peak due to its involvement in the adsorption process.
PNP Removal from Wastewater by UMNS anf MMNS:
The potential of employing macadamia nutshell biomass for wastewater treatment was assessed using actual wastewater sourced from the Nairobi River. The experimentation was carried out at room temperature under optimal conditions, including a pH of 4.0, a contact time of 30 minutes for both UMNS and MMNS, and a dosage of 0.1g. The outcomes of this study are illustrated in Figure 2.
Figure 2: Adsorption capacity of PNP ions removed from the wastewater
In Figure 2, it is evident that the UMNS and MMNS had recorded PNP ion concentrations of 1.781 mg/g and 2.434 mg/g in the real wastewater. The findings indicate that the MMNS adsorbent effectively captured a substantial amount of PNP ions, although the concentration was somewhat lower compared to that observed in aqueous solutions with 1.956 mg/g and 2.333 mg/g for the MMNS and UMNS respectively. This discrepancy may be attributed to the presence of numerous competing cations and ligands commonly found in natural wastewater.
Thermodynamic Analysis:
The outcomes for ΔG ͦ, ΔH ͦ, ΔS ͦ, S* and EA were determined using the Van't Hoff and Arrhenius equations and have been recorded in Table 1&2 for the UMNS and MMNS.