J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 Journal of the Nigerian Society of Physical Sciences Removal of Trimethoprim from Water using Carbonized Wood Waste as Adsorbents S. A. Adesokana, A. A. Giwaa,∗, I. A. Belloa aDepartment of Pure and Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso Abstract Daniellia—oliveri sawdust-based adsorbents were employed to remove trimethoprim (TMP) from water. The sawdust was thermally carbonized and activated in-stu with ZnCl2 and H3 PO4 separately. The adsorbents surface features were profiled using scanning electron microscopic (SEM) and pH point of zero charge (pHpzc) analyses. The prospects of the adsorbents for the removal of trimethoprim from water were verified. The adsorption processes were performed under different experimental conditions. The adsorption isotherm, the kinetics, and the thermodynamics were studied; and the data fitting output revealed that both chemisorptions and physisorption occurred. Surface and pore diffusion played active role in the adsorption of TMP by the adsorbents. The optimum conditions for adsorption of TMP by the adsorbents were pH at slightly acidic to neutral medium and temperature at room temperature. The fitting isotherm models were: Langmuir (R2 = 0.993) for the zinc-chloride-activated- carbon, Temkin (R2 = 0.962) for the phosphoric-acid-activated-carbon, and the kinetics: pseudo-second order (R2 = 0.997) for both. The maximum monolayer adsorption capacities of the adsorbents for TMP was 4.115 and 6.495 mg/g, respectively. The thermodynamic parameters determined suggested feasibility, spontaneity, and endothermicity of the adsorption processes. The results reveal that the adsorbents were good prospects for the removal of TMP from water. DOI:10.46481/jnsps.2021.320 Keywords: Carbonization, Activation, Adsorption, Daniellia-oliveri sawdust, Activated Carbon, Trimethoprim. Article History : Received: 28 July 2021 Received in revised form: 15 September 2021 Accepted for publication: 16 September 2021 Published: 29 November 2021 c©2021 Journal of the Nigerian Society of Physical Sciences. All rights reserved. Communicated by: E. Etim 1. Introduction Necessities for procurement of means of livelihood, main- tenance of quality life and attainment of luxurious status had prompted man to recreate the environment. Advancements in various frontiers of human endeavors had created lots of pollu- tants and contaminants. Sources of pollutants and contaminants in the environment include: agricultural practices, medicine/pharmaceutical indus- ∗Corresponding author tel. no: +2348035065456 Email address: giwa1010@gmail.com (A. A. Giwa ) tries, automobile/mechanical works, transportation, construc- tion, manufacturing, tourism, research, productions and so on [1]. The propensity of these hazardous substances into the litho- sphere and hydrosphere was high as a result of their proximity to man. Most of these anthropogenic toxic materials upset the delicate balance of the ecosystem. The resultant of this upset- ting was untoward environmental episodes. The advancement of man in various facets has led to new products, generation of new by-products and new wastes. Some of new wastes had been identified as emerging contaminants of concerns (ECCs). The ECCs were of different categories: 344 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 345 nano-materials [1] pesticides, herbicides, personal care prod- ucts and pharmaceuticals (PCPPs) [2]. The pharmaceuticals are designed to fight specific ailment and/or microorganisms that cause diseases. When administered through involuntary exposure, the pharmaceuticals had been appraised to induce an array of health and ecological complications [3]. Some of the identified ecological complications include and not limited to: hermaphroditic feature in fish; regeneration impairment in in- jured toads; and drug resistance in microbe populations [4, 5, 6]. The environmental levels of pharmaceuticals are generally low to cause acute health effect on human. However consider- ing chronic exposure, precautionary principle approach should be adopted to forestall incidence of health complications. The involuntary presence of pharmaceuticals should be prevented in public resources like drinking water and air to protect the vul- nerable groups like children and populations with compromised immune system from involuntary exposure. Pharmaceuticals were of different categories classified by their functions. One of the main categories with high dispens- ability was antibiotics e.g. trimethoprim (TMP) [7, 8, 9]. High dispensability of antibiotics was responsible for their high lev- els in the environment. Presence of antibiotics in the environ- ment was highly undesirable as they induce drug resistance in microorganisms and this is a major factor of pandemic [10, 11]. TMP is an antibacterial and anti-malarial drug. It has reported half-lives in the range 5 - 100 days. It is insignificantly photo- degraded, which suggests a relative persistence [12] in the en- vironment. TMP is mutagenic, teratogenic, embryotoxic and folate antagonism [13]. Hence TMP should be removed from water resources. The researchers are working to develop technologies [14] and strategies [15, 16, 17, 18] to remove various forms of pol- lutants and contaminants, including pharmaceuticals from wa- ter. Recently the most sustainable technology so far devel- oped was adsorption process [19, 20, 21, 22, 23, 24, 25]. The trending strategy was the use of agro-wastes as adsorbents for the removal of pollutants and contaminants. In Nigeria, wood waste like sawdust remains one of the high volumes, posing great challenge to manage. In 2010, Nigeria generated over 1,000,000 m3 (353,146.667 tonnes) [26] of sawdust and most managed by open air combustion. This combustion method led to generation of COx, NOx, SOx: radiation forcing substances [26]. Conversion of the sawdust to highly demanded product like adsorbent is both economically and environmentally sus- tainable strategy. In this work, Daniellia-oliveri sawdust, a high volume agro- processing waste in Nigeria and not yet reported for any usage, was processed and used to remove from contaminated water. 2. Materials and Methods 2.1. Materials Some of the materials used include Daniellia–oliveri saw- dust (collected from a local factory, washed, dried and segre- gated), analytical grade ZnCl2, H3 PO4, (purchased from lo- cal chemical dealer) TMP (primary standard, >98.5% purity) (supplied Bond Chemical Industries Limited, Awe, Oyo) and HCl. The equipment made used of were furnace (Carbolite AAF 1100), ultraviolet-visible (UV) (B-UV 1800PC) spectropho- tometer, pH meter (Jenway 3520), and scanning electron micro- scope (SEM) (Aspex 3020) and oven (Carbolite). 2.2. TMP The structural formula and the properties of trimethoprim are given in Figure 1 [27] and Table 1 [28] below. Figure 1: Trimethoprim 2.3. Preparation of Adsorbents The Daniellia—oliveri sawdust was washed with plenty of distilled water to remove surface impurities and then sundry. The sample was dried in an oven at 70 ◦C for 72 hours; then ground and stored as Raw Daniellia Sawdust (RDS). The method was described elsewhere [29], 3g of RDS was mixed with 3 mL of 1 M H3PO4 and 3 mL of 1 M ZnCl2 each in separate crucibles. These were subjected to the furnace at 800 ◦C (car- bonization and activation in self-generated atmosphere) for 5 minutes. The adsorbents were stored in air tight container as H3PO4 activated carbon (PACB) and ZnCl2 activated carbon (ZAC) respectively. 2.4. Characterization of Adsorbents SEM and pHpzc analyses were carried out using methods [30] and [31] respectively to study the surface morphology and pH of point of zero charge of the adsorbents. 2.5. Batch Adsorption Studies The triplicate optimized batch adsorption processes were carried out. The adsorption bottles containing adsorbent-adsorbate mixtures were shake on a mechanical shaker operating at 160 rpm. The effects of contact times (0 - 240 min), adsorbent dosages (0.05 - 0.5 g), pHs (2, 5, 8), initial adsorbate concentra- tions (15 - 50 mg/L), and temperatures (29 - 60 ◦C) were deter- mined. After the processes reached equilibrium, the mixtures were filtered using filter papers and the filtrates analyzed at 282 345 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 346 Table 1: Properties of Trimethoprim Common Names: Trimethoprim, Trimethoprime, trimethoprimum, trimetoprima, trimpex IUPAC Names 5-[(3,45-trimethoxyphenyl)methyl]-2,4-pyrimidinediamine Molecular formula C14 H18 N4O3 Molecular weight 290.32 g/mol UN Number 2811 Max λ 282 nm Solubility 400 mg/L (25◦C) Toxicity Vomiting, headache, mental depression, confusion, bone marrow depression pKa 7.12 Description Antibacterial, antimalaria Interaction sites π−π, n −π, -NH2, ≡ N: nm with the UV spectrometer to determine the absorbances of trimethoprim in solutions after adsorption. The absorbances were converted to corresponding concentrations using standard curve equation. Each experiment was performed in triplicate. Equations (1) and (2) were used to determine amount of adsor- bate adsorbed per unit mass of adsorbent and percentage adsor- bate removal respectively: qe = (Ci − Ce)V m (1) % adsorbate removal = (Ci − Ce)V Ci × 100 (2) where qe, Ci, Ce, m and V are the amount of trimethoprim adsorbed by the adsorbent at equilibrium (mg/g); the initial trimethoprim concentration (mg/L); the trimethoprim concen- tration at equilibrium (mg/L); the mass of the adsorbent (g); and the volume of the solution (L) respectively. 2.6. Adsorption Isotherms Table 2 contains selected isotherm models used to process experimental data and then described the adsorption processes. The Langmuir Model: A plot of Ce/qe versus Ce gives a straight line with intercept: 1 /K · q0; and slope: 1/ q0. Lang- muir isotherm equilibrium parameter, RL: RL = 1 1+KLCi [39]. Ci = highest initial adsorbate concentration (mg/L); RL>1 = unfavourable; RL=1=linear; favorable 01); n = 1=linear; n > 1 = physical pro- cess and n < 1= chemical process. The Temkin Model: KT =Temkin isotherm equilibrium binding constant (L/g); bT = Table 2: Isotherm Equations Model Linearised Formulae Reference Halsey Harkin-Jura Langmuir lnqe= 1 n ln K− 1 n lnCe 1 q2 = B A − 1 A logCe Ce qe = 1K·q0 + Ce q0 [32] [33] [34] Freundlich ln qe = ln KF + 1 n ln Ce [35] Temkin qe = B ln KT + B ln Ce [36] Redlich-Peterson log Ceqe = log KR + βRT log Ce [37] Dubinin– Radushkevich lnqe = ln(qs ) − (Kadε2) [38] Temkin isotherm constant; R= universal gas constant (8.314 J/mol/K); T= Temperature at 298 K; B = Constant related to heat of sorption (J/mol). The Redlich-Peterson isotherm: Ce = equilibrium concentration; qe = adsorption capacity of the ad- sorbent; β = desorption constant and KR = R-P isotherm con- stant (g/L). The Dubinin-Radushkevich Model: ε = RT ln[1 + 1Ce ] ; E = [ 1√ 2BDR ]; Kad = BDR; E<8 kJ/mol (physical adsorption); E = 20–40 kJ/mol (chemical adsorption) [38]; E = 8–16 kJ/mol (ion-exchange) [40]; qe = amount of adsorbate on the adsorbent at equilibrium(mg/g); qs = theoretical isotherm saturation ca- pacity (mg/g); Kad = Dubinin-Radushkevich isotherm constant (mol2/kJ2). 346 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 347 2.7. The Kinetics Samples were taken from adsorption bottles at time inter- vals and the amounts of adsorbates were measured. The amount of adsorbed adsorbate at time t, qt (mg/g), was calculated using Equation 3 and kinetic parameters were calculated using kinetic equations presented in Table 3. qt = (C0 − Ct) V m (3) The Pseudo-First Order: qe and qt are the amounts of the dye adsorbed (mg/g) at equilibrium and at time t (min), respec- tively, and k1 is the rate constant adsorption (min−1). A plot of log(qe − qt) versus t gives a slope of ( − k1 2.303 ) and intercept of logqe . The Pseudo-Second Order: h = k2 . q2e ; qe is the amount of the solute adsorbed at equilibrium per unit mass of adsorbent (mg/g) , qt is the amount of solute adsorbed (mg/g) at any given time, t (min) and k2 is the rate constant for pseudo- second-order adsorption (g/mg/min) and h, known as the initial sorption rate. The values of qe and k2 can be obtained from the slope and intercept of the plot of t/qt versus t respectively. If the sorption follows pseudo-second order, h, is described as the initial rate constant as t approaches zero. The Elovich: α is the initial adsorption rate (mg/g. min), β is the desorption con- stant (g/mg) and qt is the amount of solute adsorbed (mg/g) at any given time, t (min). A plot of qt versus ln t gives a straight line with intercept 1 β ln(αβ) and slope ( 1 β ) . Intra-Particle Diffu- sion: qt (mg/g) = quantity of adsorbate adsorbed at time, t and kid (mg/gh0.5) = intra-particle diffusion constant. A graph of qt versus t0.5 gives a straight line with slope, kid , and intercept, x. x defines the thickness of the boundary layer. The Boyd Kinetic: B = π 2 Di r2 ; B is a constant; Di (m 2/s) is diffusion coefficient; r is radius of adsorbent particle; F is the fraction of adsorbate adsorbed at given time, t, and Bt is a function of F. If graph of [–0.4977 – ln (1 − F)] vs time, t, is linear and intercept = 0, then the determining step of the adsorption process is intra- particle diffusion; but if intercept , 0, the determining step may be film diffusion. 2.8. Thermodynamic Study Experiments were performed at 29, 40, 50, 60 and 70 ◦C to establish the effect of temperature on the adsorption capacities of ZAC and PACB for the trimethoprim. The thermodynamic Table 3: Kinetic Equations Kinetic Equation Reference Pseudo-First Order log(qe − qt) = logqe − k1 2.303 .t [41] Pseudo- Second Order t qt = 1h + t qe [42] Elovich qt = 1 β ln(αβ) + ( 1 β ) lnt [43, 44] Intra-Particle Diffusion qt = kid t0.5 + x [45] Boyd −0.4977−ln(1 − F) =Bt [46] parameters of the adsorption were determined using the equa- tions (4) and (5). ∆G0 = −2.303RT log K (4) log ( CAe Ce ) = ∆S 0 2.303R − ∆H0 2.303RT (5) K = ( CAe Ce ) ; CAe = (Ci – Ce) (mg/L) = amount of trimethoprim adsorbed on adsorbent at equilibrium; Ce (mg/L) = amount of trimethoprim in solution at equilibrium; R = gas constant (8.314 J/mol/K); T = temperature (K). The enthalpy change (∆H), the entropy change (∆H) and change in satndard free en- ergy (∆G) were calculated. The spontaneity of the processes was thereby determined. 3. Results and Discussion 3.1. Morphology of ZAC and PACB Figure 2 showed characteristic surface morphology of the adsorbents. Carbonization and activation processes impacted the adsorbents’ surfaces with characteristic morphology and chem- ical profile. Using Gwyddion 2.23-1 application for SEM im- age processing, the average pore diameters respectively of ZAC and PACB were approximately 30 and 55 nm. High tempera- ture of carbonization removed volatile compounds in the matrix of raw sawdust thereby created pores. High temperature could also break down cellulosic contents of the precursor sawdust and aided aromatization of carbon chains. High temperature chemical activation effected dehydration and decarboxylation of sawdust. It could be deduced from the changes in the surface morphology after adsorption that pores were active in adsorbing trimethoprim molecules. 3.2. Determination of Equilibrium Time and Effect of Contact Time on Trimethoprim Adsorption The amount of TMP adsorbed per unit mass (q) of each of ZAC and PACB increased rapidly within first 40 min. The q thereafter increased slowly and insignificantly over next two hours for both systems. After 60 min, almost all the available adsorption sites had been occupied for both systems. The q at time t (qt) for the TMP-ZAC system was 1.123 mg/g within first 60 min while it was 0.272 mg/g for the 180 min afterwards. The qe for the TMP-PACB system was 0.996 mg/g in first 60 min while 0.204 mg/g for the 180 min afterwards (Figure 3a). These increments for the next 180 min after first 60 min were economically insignificant. The economic time of adsorption for both systems was 60 min. 3.3. Effect of Adsorbent Dose on Trimethiprim (TMP) Adsorp- tion The increase in mass of each of ZAC and PACB led to decreased amount of TMP adsorbed per unit mass during ad- sorption processes. The increased mass of the adsorbents pro- vided more sites of adsorption for constant amount of TMP molecules. ZAC and PACB adsorbed equal amounts of the 347 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 348 (a) (b) (c) (d) Figure 2: (a) PACB (before adsorption); (b) PACB (after adsorption); (c) ZAC (before adsorption); (d) ZAC (after adsorption) at magnification × 500. TMP per unit mass at 0.15g each (at the point of intersection) (Figure 3b). 0.2, 0.3 and 0.4 g of ZAC adsorbed 68 %, 82 % and 87 %; and PACB adsorbed 65 %, 76% and 75% of TMP respec- tively at equilibrium time. The equilibrium dose was reached at 0.3 g for both adsorbents. The economic dose for ZAC was 0.3 g while 0.2 for PACB. 3.4. Effect of pH on Trimethoprim Adsorption The pHpzc is the pH of solution at which electrical charge of a solid surface in the solution is neutral. The pKa of trimetho- prim and the pHpzc of the adsorbents are parameters whose elec- trical charge changes with pH of solution. The maximum at- traction between the adsorbate and the adsorbents would take place when electrical charges on them are opposite. TMP is a basic compound with pKa 7.12. At pH above 7.12, TMP was neutral while at pH below 7.12 TMP was positively charged (the hypothesized interactions were presented in Equations 6 to 10). At pH below 7.12, the oxygen and nitrogen atoms on TMP donated lone pairs of electrons to the protons in solution and TMP became positively charged. At pH above 7.12, the lone pairs of electrons on TMP oxygen and nitrogen atoms re- pelled lone pairs of electrons on hydroxyl groups in solution and TMP remained neutral. At about pH 7, TMP was at the threshold of changing from positively charged (+) to neutral (0), ZAC (pHpzc 7.6) positively charged (+) while the PACB (pHpzc 6.6) negatively charged (-). At pH below 6, the adsor- bents and trimethoprim were all positively charged and the re- pulsive effect de-enhanced adsorption. At pH above 8, lone pairs of electrons on neutral TMP repelled negatively charged adsorbents’ surface and chemisorption was de-enhanced. The maximum interactions occurred at slightly acidic to neutral pH (pH 6) where electrical charges on the adsorbents and trimetho- prim were opposite (Figure 3c). At pH 2.5, ZAC adsorbed 1.262 mg/g of TMP; at pH 5, 1.266 mg/g and at pH 8, 1.25 mg/g. And for PACB, at pH 2.5, 1.164 mg/g of TMP was ad- sorbed, at pH 5, 1.215 mg/g and at pH 8, 1.138 mg/g. TMP (pKa) pH>7.12 −→ TMP(O) (6) MeÖ ≡ N:(T MP) + OH− → MeÖ (7) ≡ N:(T MP) ↔ OH− (neutralT MP) TMP (pKa) pH<7.12 −→ TMP(+) (8) MeO ≡ N:(TMP) + 2H+ → MeOH+ (9) ≡ NH+ ( positively charged TMP ) 3.5. Effect of Temperature on Trimethoprim Adsorption TMP is a biphenyl compound with functional group branches. The bulky structure of TMP may constitute stearic hindrance to 348 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 349 pore accessibility. At elevated temperatures, TMP molecules acquired more kinetic energy which led to frequent knocking with the adsorbents’ pore openings. The impact of the knocking might ‘mend and tend’ the molecules or wear the pore open- ings and eventually forced the molecules into the pores. The higher the temperature, the higher was the kinetic energy and the contact between molecules and pores. At equilibrium, 1.6 mg/g (64 %) and 1.904 mg/g (76 %) of TMP were adsorbed by ZAC at 29 ◦C (room temperature) and 60 ◦C respectively. Also 1.196 mg/g (72 %) and 1.326 mg/g (80 %) adsorbed re- spectively at 29 ◦C and 60 ◦C by the PACB (Figure 3d). The enhanced adsorption of TMP on ZAC and PACB as a result of increase in temperature was economically unfavourable. From economic perspective, room temperature would be chosen con- sidering costs of heating and cooling the system after adsorp- tion before discharge, or environmental impact of discharged warm/hot wastewater. It would be less expensive to subject the TMP treated water to secondary adsorption process. 3.6. Effect of initial TMP Concentrations The qe (mg/g) of the adsorption of TMP onto ZAC and PACB increased as the initial TMP concentrations increased. This was due to the more TMP molecules in the solutions which were in contact with constant active adsorption sites on the ad- sorbents. For the TMP-ZAC system, at 15, 20, 25, 30, 35, 40 and 50 mg/L, 82.6 %, 83.5 %, 82.8 %, 81 %, 79.5 % 75 % and 70 % of TMP were adsorbed respectively. And for the TMP- PACB system, at 15, 20, 25, 30, 35, 40 and 50 mg/L, 75 %, 77.9 %, 81.7 %, 79.9 %, 78.8 %, 80 % and 75 % were adsorbed respectively (Figure 3e). A unit mass of ZAC had highest re- moval efficiency for TMP at 20 mg/L of solution while PACB at 25 mg/L of solution. 3.7. Adsorption Isotherms for TMP Adsorption The isotherms applied to describe the adsorption of TMP onto ZAC and PACB were Langmuir, Freudlich, Temkin, Redlich- Peterson, Dubinin-Radushkevich, Harkin-Jura and Halsey. The isotherms that best fit were Langmuir (R2=0.993) and Temkin (R2=0.962) for TMP-ZAC adsorption and TMP-PACB respec- tively (Table 4). These suggested that TMP-ZAC adsorption system was monolayer and TMP-PACB multilayer. The Lang- muir isotherm equilibrium parameter, RL values: 0.108 and 0.206 for the TMP-ZAC and the TMP-PACB meant that both processes were favorable. The adsorption affinity (KL) for ZAC- TMP process was higher, which implied that it was more fa- vorable compare to PACB-TMP process. The qmax for ZAC was 4.115 mg/g and PACB 6.495 mg/g (Table 6). The multi- layer coverage exhibited by the TMP-PACB process explained its higher qmax. Also wider pores exhibited by PACB (Fig- ure 2) might be more accessible to TMP molecules than for ZAC. Comparison of qmax of some adsorbents reported in lit- eratures indicated that ZAC and PACB were better adsorbents for TMP adsorption from aqueous solution. The 1/n as de- termined from the Freundlich Isotherm for the TMP-ZAC and the TMP-PACB processes were 0.472 g/L and 0.636 g/L re- spectively. These further confirmed the processes to be favor- able. Both processes were physical because n greater than 1 for Table 4: Isotherm parameters for TMP adsorption onto the ZAC and the PACB Isotherm ZAC PACB Langmuir q0 (mg/g) 4.115 6.495 kL (L/mg) 0.165 0.077 RL 0.108 0.206 R2 0.993 0.913 Freundlich KF (mg/g) 0.849 0.653 n (L/g) 2.120 1.572 1/n 0.472 0.636 R2 0.959 0.955 Temkin KT (mg/L) 1.306 1.673 bT (KJ/mol/g.L) 2.531 0.664 β (L/g) 0.992 1.500 R2 0.988 0.962 Redlich-Peterson KR (g/L) 1.178 1.528 βR (g/mg.K) 0.0017 0.0013 R2 0.966 0.879 Dubinin-Radushkevich qs (mg/g) 2.821 3.320 Kad (JL/mol.mg) 2×10−6 1×10−8 E (KJL/mol.mg) 0.50 2.5 R2 0.966 0.921 Harkin-Jura qe (mg/g) 1.287 1.220 A (mg3/g3) 1.783 1.745 R2 0.840 0.908 Halsey n (g/L) -2.105 -1.572 K (mg/L) 1.415 0.668 R2 0.959 0.957 349 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 350 (a) (b) (c) (d) (e) Figure 3: Effect of (a) contact time, (b) adsorbent dose, (c) solution pH, (d) temperature and (e) initial adsorbate concentration on the adsorption of TMP onto ZAC and PACB each (Table 4). Adsorption intensity (n) was higher for ZAC- TMP process which in agreement with explanation given under Langmuir isotherm. 1/n, a function of surface heterogeneity, was higher for PACB-TMP system. Positive values of bond- ing energy (bT ) obtained from Temkin isotherm for both pro- cesses indicated endothermic processes. The adsorption energy, E (kJ/mol), derived from Dubinin-Radushkevich isotherm for the TMP-ZAC and the TMP-PACB processes were 0.50 and 2.5 respectively. These values described both processes as physical. Halsey isotherm described a heterogeneous adsorbent surface and multilayer adsorption system. Relatively close values of adsorption coefficients of both Halsey and Temkin isotherms further established PACB-TMP system as multilayer adsorp- tion and PACB had heterogeneous surface.As could be deduced from the coefficient values of the isotherm and kinetic (section 3.9 below) models, the adsorption of TMP by ZAC and PACB involved both chemisorption and physisorption. Chemisorption involves reaction among functional groups on both the adsor- bents (ZAC and PACB) and the adsorbates (TMP). Hydroxyl group would like form hydrogen bond with adsorbents’ N and 350 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 351 (a) (b) Figure 4: van’t Hoff plot of log K vs 1/T adsorption of TMP onto (a) ZAC and (b) PACB . Table 5: Kinetic parameters for TMP adsorption onto the ZAC and the PACB Kinetic ZAC PACB qe, ex p (mg/g) Pseudo-first order 1.395 1.200 qe (mg/g) 0.670 0.581 K1 (min−1) 0.014 0.014 R2 0.964 0.920 Pseudo-second order qe (mg/g) 1.425 1.230 h (min−1) 0.107 0.097 K2 0.053 0.064 R2 0.997 0.997 Elovich A 0.535 0.445 β (min.g/mg) 4.40 5.05 R2 0.939 0.969 Intraparticle diffusion x (mg/g) 0.594 0.518 Kid (mg/g.min1/2) 0.061 0.053 R2 0.877 0.888 Boyd X 1.475 1.617 B (min.g/mg) -0.004 0.003 R2 0.211 0.017 Table 6: Comparison of maximum monolayer adsorption capacities of different adsorbents for TMP Adsorbent Adsorbent capacity (mg/g) Reference GAC 0.51 [47] PAC 0.51 [47] Clay 47 [48] BHB 0.508 [49] LZB 0.294 [49] ZAC 4.115 This work PACB 6.495 This work GAC – granular activated carbon PAC – powdered activated carbon sediment BHB; sediment LZB O bearing functional groups, and vice versa: NH2 − TMP − NH2+HO − ZAC/PACB→H2N − TMP (10) − HN· · ·HO − ZAC/PACB Multilayer adsorption of TMP onto PACB was as a result of reaction between electron withdrawing and electron donating functional groups on already adsorbed TMP and free adsorbate molecules in solution: H2N − T MP − HN· · ·HO − PACB+H2N − TMP→ (11) →TMP−H2N· · ·H2N − TMP−H2N· · ·OH − PACB π-π and n-π are electron density centers capable of facilitat- ing electrostatic interactions between TMP molecules and the adsorbents through resonance of conjugated double bonds and aromaticity of benzene rings: TMP − N−−C =N+H2+O −C6H6−PACB/ZAC→ (12) TMP−N+· · ·O−C6H6−PACB/ZAC 3.8. Kinetic Studies of Adsorption of TMP onto ZAC and PACB The data of adsorption of TMP on ZAC and PACB were tested with0 five different kinetic models: Pseudo-first order, 351 S. A. Adesokan et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 344–353 352 Table 7: Thermodynamic parameters of the adsorption of TMP onto the ZAC and PACB ∆G0 (kJ/mol/K) ∆S0 (J/mol/K) ∆H0 (kJ/mol/K) 302 313 323 333 ZAC -1.444 -2.39 -2.64 -3.22 +54.60 +14.99 PACB -2.342 -2.74 -2.90 -3.76 +42.16 +10.46 Pseudo-second order, Elovich, Intraparticle and Boyd. Both TMP-ZAC and the TMP-PACB processes data fitted pseudo- second order equation with R2 = 0.997 (Table 5). The rate of adsorption in both processes depended on the states of both the adsorbate and the adsorbents. The initial rate constants for ZAC and PACB, h (mg/g/min), were 0.107 and 0.097 respec- tively, implied that rate at which TMP adsorbed on to the ZAC was higher initially. However, overall rate constant of PACB- TMP process was higher. High adsorption coefficient (R2) for Elovich model suggested a level of chemisorptions for both sys- tems. Higher Elovich R2 for PACB-TMP process also indi- cated considerable multilayer coverage compare to ZAC-TMP process. However, earlier it was explained from the isotherm data fitting that adsorption of TMP by the adsorbents were both chemisorptions and physisorption. High coefficient values for pseudo first order kinetic and relatively high R2 for intraparti- cle diffusion model were pointers to physical adsorption. Sur- face and pore diffusions played important role in the adsorp- tion of TMP by ZAC and PACB. PACB having wider average pore diameter than ZAC could easily accommodate bulky TMP molecules and hence higher qmax. Diffusion of TMP molecules at the surface of the adsorbents occurred rapidly while diffu- sion down the adsorbents’ pores took place steadily until equi- librium is attained. 3.9. Thermodynamic Study of the TMP Adsorption The functions of temperature, the standard enthalpy (∆H0), the standard entropy (∆S0) and the standard Gibb’s free energy (∆G0) of the adsorption processes were investigated for spon- taneity and feasibility. The van’t Hoff plot of graph of log K ver- sus 1/T gave the slope and the intercept as ∆H0 and ∆S0 respec- tively (Table 7, Figure 4). The negative values for ∆G0 at all investigated temperatures indicated that the adsorption of TMP onto ZAC and PACB were spontaneous. The positive value of ∆S0 showed high degree of randomness of the processes while positive value of ∆H0 indicated endothermic processes. 4. Conclusion This study verified the prospects of the processed Daniella— oliveri sawdust as adsorbents for adsorption of TMP from aque- ous solution. The economic time of adsorption of TMP by both adsorbents was 60 minutes while economic doses of ad- sorbents were 0.3 and 0.2 g for ZAC and PACB respectively at 29 ◦C. 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