Effect of Feedstocks and Production Methods on Activated Carbon Performance on the Sorption of Pb and Cd from an Industrial Effluent

Effect of Feedstocks and Production Methods on Activated Carbon Performance on the Sorption of Pb and Cd from an Industrial Effluent

^*1Ibrahim Adeiza Ahmed, ²Oladotun Victor Ogunyemi, ³Qudus Onagun, ⁴Omowunmi Christiana Ladipo

^1,4Department of Chemical Engineering, Faculty of Engineering, University of Ilorin, Ilorin, Kwara State

^2,3 Department of Industrial Chemistry, University of Ilorin, Ilorin, Kwara State

DOI: https://doi.org/10.51244/IJRSI.2024.1110067

Received: 10 October 2024; Accepted: 21 October 2024; Published: 22 November 2024

ABSTRACT

This study features the production of activated carbon from hardwood, softwood and their hybrid for the adsorption of Pb and Cd from industrial effluent. The wood wastes (sawdust) were carbonized at 500 °C followed by activation in air atmosphere (physical activation) and H₃PO₄-impregnation prior to carbonization (chemical activation). The resulting activated carbons (AC) were characterized for surface structural and morphological properties. The activation method impacted the physiochemical properties of the ACs. For instance, the BET surface area was highest for hardwood derived AC obtained by physical activation, while it was the softwood AC for chemical activation pairs. FTIR analysis revealed that both chemically and physically ACs have the requisite functional groups (phenol, aromatics and hydroxyls) for sorption operations; however, H₃PO₄ creates more oxy-containing functional groups in the chemically ACs, which enhanced their adsorption performance. Hardwood was the feed material controlling the yield and attributes of both physically and chemically produced hybrid AC in the 1:1 mixed feed. All ACs were employed as adsorbents for the uptake of Pb and Cd at different adsorbent dosage levels (50-250 mg/l) and contact time (5-20 mins). The batch adsorption studies showed that Pb and Cd removal rate increased with increasing adsorbent dosages and contact time until equilibrium was reached at 15 mins. Only chemically ACs achieved 100% removal of Pb and Cd from the effluent at optimum dosage amount. The sorption performance of the adsorbents (based on feedstocks) for both metals uptake can be ranked as softwood (C) > hardwood (C) > hybrid (C) > hardwood (P) > softwood (P) > hybrid (P). C and P refer to the chemical and physical activation methods. The experimental data were found to fit into Langmuir adsorption model (R² = 0.999) and pseudo second-order kinetic model (R² ₌ 0.999)

Keywords: activated carbon, adsorption, heavy metals, hardwood, activation methods.

INTRODUCTION

The lumber processing and manufacturing industry has played a key role in the development of our societies. Wood is the primary product of the lumber industry anda versatile resource known to man (Adhikari&Ozarska, 2018). However, this industry is also remarkable with high waste (wood shavings, saw dust, etc.) generation typically due to low average timber recovery in the forest and wood processing sectors. Nigeria for instance, generates about 1.8 million metric tons of sawdust annually, with poor disposal or re-use strategies, as they are mostly dumped in water bodies or incinerated openly (Jacob et al.,2016). A cleaner re-use alternative is to convert these carbonaceous wood wastes into useful products like activated carbons (AC) – whose applications lie in the heart of many industrial and domestic processes (Yahya et al, 2015; Luka et al., 2018). Several agricultural residues including apricot stones, coconut shell, palm kernels and rice husks have been investigated as feedstock for AC production due to their exceptional carbon contents (Juanget al., 2000; Diaoet al., 2002 and Kennedy et al., 2004; Diaz-Diez et al., 2004). Other waste materials such as plastic wastes, sewage sludge, manure, animal litter and tyres have also been reportedly employed for AC production (Nakagawa et al., 2004; Rozadaet al., 2005; Zang et al., 2005). However, woody biomass has generated special interest over other types of materials for several reasons, including the high carbon and low ash contents, little to no variation in compositions, abundance and seasonal availability, ease of workability and high conversion efficiency (Yeganeh et al., 2006; Ramirez et al., 2017). Moreover, AC from woody biomass generally have superior properties when compared to AC from other sources. For instance, at comparative production conditions, Diaz-Diez et al [2004] obtained a BET surface area in the range of 770-812 m²/g for AC produced from different wood wastes as opposed to 102 m²/gobtained by Mohammad et al [2015] report for rice husk produced AC.

Physical and chemical activation are the two methods of producing AC from carbonaceous materials. The pyrolysis/carbonization of the carbon precursor, followed by the controlled gasification of the resulting biochar in oxidation gases like; steam, CO₂, N_2, and air mixture, is regarded as a physical activation method (Taddaet al., 2016; Bubanale and Shivashankar, 2017). On the other hand, chemical activation involves the impregnation of the carbon precursor in dehydrating chemicals such as H₃PO₄, ZnCl₂ and KOH before or after carbonization (Amirzaet al., 2017; Dehghaniet al., 2020). The use of phosphoric acid as an activator agent is becoming increasingly popular because of its lesser toxicity and simple recovery by washing with water (Yashimet al., 2016; Deng et al., 2009). Physical activation is considered a cost-effective and green method due to non-use of chemical solvents (Byamba-Ochiret al., 2016; Yahya et al., 2015; Pallareset al., 2018). Meanwhile, dehydrating chemicals agents can inhibit several undesired pyrolytic reactions, thereby lowering tar and non-condensable gases formation; thus, enhancing the yield of solid products (Kumar & Jena, 2016). The chemical activation process can produce AC with improved physicochemical and structural features because impregnation process facilitates intimate contact between the base material and the activating agent, which promotes the formation of a structure with high porosity and surface area (Taddaet al., 2016; Nowicki et al., 2006; Bubanale and Shivashanakar, 2017).Many studies have used physical and chemical activation methods to produce AC, butthe findings on product properties and application performance have not been consistent. For example, Phan et al.,[2006] found that the ACs produced via chemical activation have slightly higher surface areas (960-1300 m²/g) than that obtained from physical activation methods (912-1088 m²/g) from the same feedstock. On the contrary, Waziri et al., 2020 reported 502 m²/g for rice-husk derived AC via physical activation, whereas Saad et al., 2020 obtained a relatively lower surface area (430 m²/g) for a NaOH-activated AC from rice husk.

ACs have found versatile applications as adsorbent, thanks to theirpolymodal structure, large surface area, micro-porosity and abundant surface chemical functional groups (Moreno-Pirajan, 2008), AC is widely used as adsorbents in wastewater treatment, gas cleaning, resource uptake in liquid effluents, pollutants removal and sugar decolourization, (Yahyaet al., 2015; Al-Doury& Ali, 2015). A common application of AC is in heavy metals sorption from wastewater or industrial effluents. Heavy metals are significant contributors to water pollution and due to their known harmful effects on many life forms, they are at the center of many wastewater research works (Mahmud et al., 2014). Lead, cadmium, copper, chromium, mercury, and nickel are popular heavy metal pollutants that enter water bodies (Yarkandi, 2014; Amarasinghe& Williams, 2007). The removal of these heavy metals from aqueous solutions using AC has been largely demonstrated (Table S1).

The numerous studies on the adsorption of heavy metals from aqueous media (simulated wastewater) offer researchers many opportunities to tweak adsorption parameters to enhance a comprehensive understanding of the adsorption mechanism. However, such studies might not give a real-time picture of how adsorbents interact with solutes or adsorbate molecules in typical industrial effluent. The performance of many AC adsorbents may be compromised due to the adsorbent preferentialselectivity for specific solutes in a solution that is present in uniform concentrations in the simulated aqueous solvent.

There are no comprehensive studies on the combined effects of wood material types and production methods for AC synthesis and the subsequent use of the AC as adsorbents in removing heavy metals contaminants in a typical industrial effluent. This studyemployedAC produced from two types of wood wastes (hardwood and softwood) and their hybrid via the two activation methods (physical and chemical) for the sorption of Pb²⁺ and Cd²⁺ from real industrial wastewater. The specific objectives of this study were: i) investigate the effects of activation methods and precursor materials on the yield and properties of activated carbon, ii) study the effects of contact time and adsorbent dosage on the removal of Pb²⁺ and Cd²⁺ from a practical industrial effluent, iii) determine Pb²⁺ and Cd²⁺ adsorption isotherms and kinetics of the sorption process and iv) compare the overall adsorption performance of AC from hardwood, softwood and their hybrid.

MATERIALS AND METHODS

Sample collection and preparation

Wood wastes from pine tree (softwood) and Mahogany tree (hardwood) are sourced from Surulere sawmill plant, located in Ilorin, Nigeria. The wood waste samples were initially air-dried then oven-dried at 110 °C for 24 hours to remove free moisture. The dried samples were grounded using a laboratory mill, and the grains were graded with sieves to obtain 100-micron particle sizes. Chemicals used are of analytical grades and include, H₃PO₄ and HCl.

Activated carbon synthesis

Physical and chemical activation methods were used to produce activated carbon from hardwood (HW), softwood (SW) and their hybrid (HW+SW). The procedure for each of the methods are described as follows:

Chemical activation: 50 g of each wood samples was impregnated with 1.5 M phosphoric acid at a solvent to biomass ratio of 2.5 v/w for 24 hours without agitation. Thermogravimetric analyses of both impregnated and raw samples were conducted using a Thermogravimetric Analyzer (TGA 4000, PerkinElmer) to discern the samples’ thermal decomposition behavior. Carbonization was carried out in a muffle furnace at 500 °C for 40 min at the pyrolysis temperature in a nitrogen atmosphere flowing at 200 ml/min. The carbonized samples were collected and cooled in a desiccatorto ambient temperature. The samples were thoroughly washed with distilled water until neutral pH. This procedure was adapted from Shariaty-Niassaret al., (2013).

Physical activation: 50 g of each wood samples is carbonized at 500 °C for 30 mins at pyrolysis temperature under N₂ atmosphere to create an inert environment that prevents oxidation. The obtained char was activated in air atmosphere at 800 °C for 40 mins. After gasification, samples were collected and allowed to cool in a desiccator. No washing was carried out.

The same physical and chemical activation procedures produced hybrid AC with a 1:1 (w/w) (hardwood: softwood) feed. The produced activated carbons were denoted as shown in Table 1.

Table 1. Samples notation of synthesized adsorbents and their mode of activation

Adsorbent Name	Adsorbent feedstock	Mode of activation
AC-HW	Hardwood	Physical
AC-SW	Softwood	Physical
AC-(HW+SW)	Hardwood-softwood (hybrid)	Physical
AC-HW*	Hardwood	Chemical
AC-SW*	Softwood	Chemical
AC-(HW+SW)*	Hardwood-softwood (hybrid)	Chemical

Characterization

Ultimate and proximate properties of the ACs were determined using a CHNS instrument (Perkin Elmer 2400 Series II) and a TGA 400 (Perkin Elmer) respectively. Microscopic images of the samples were obtained using Scanning Electron Microscopy (SEM, Aspex Model 3025). The surfacefunctional groups in the synthesized adsorbents were analyzed using Fourier-Transform Infrared (FTIR) Spectroscopy (Shimadzu, Perkin Elmer). The FTIR instrument was operated at a resolution of 4 cm^-1, scanning time of 15 min and the scanning mode was in transmittance. The physical properties (surface area, pore width and volume) of the synthesized AC samples were determined by the Brunauer, Emmett, Teller (BET) theory using a surface area analyzer Quantachrome Nova 4200e (USA) instrument from nitrogen adsorption-desorption isotherms obtained at different pressures and temperature.

Batch adsorption experiments

The adsorption experiments were carried out at room temperature (23±2 °C) using an industrial effluent, with predetermined levels of lead and cadmium concentrations (1.2 ppm Pb²⁺ and 1.0 ppm Cd²⁺). The pH of the wastewater was adjusted to neutrality using HCl to optimize metal binding and prevent precipitation. The adsorption experiments were performed in 250 ml conical flasks. Adsorbent dosage range (50-250 mg/L) was systematically selected based on preliminary studies and practical considerations. Starting with a minimum dosage of 50 mg/L to establish baseline performance, the concentration was incrementally increased by 50 mg/L up to 250 mg/L to capture complete adsorption behavior. For each experiment, specific adsorbent amounts (0.005, 0.010, 0.015, 0.020, and 0.025 g) were added to 100 mL of the wastewater, corresponding to concentrations of 50, 100, 150, 200, and 250 mg/L respectively.The resultant mixture was stirred continuously on a mechanically controlled orbital shaker and after 15 min contact time, the mixtures were filtered using Whatman filter papers (11 μm, Grade 1). The effect of contact time on the adsorption process was investigated within the range 5 – 20 minutesat 5 minute intervals. After each adsorption experiment, the Pb²⁺ and Cd²⁺ concentrations remaining in the wastewater samples were determined using Atomic Adsorption Spectrophotometer. At least three replicates were performed for each experiment and results averaged for enhanced accuracy and precision.

The adsorption capacity at equilibrium (ԛ_ein mg/g) and removal efficiency (%Re) were calculated using equations 1 and 2:

Where Co and Ce are initial and equilibrium concentrations of metals (mg/mL) respectively. V is the volume (ml) of wastewater solution containing Pb²⁺ and Cd²⁺ and W is the adsorbent mass (g).

Adsorption Isotherm

Adsorption isotherms are used to understand how the molecules of the adsorbents and adsorbates interact or distribute themselves between liquid and solid phase media during the adsorption process. Two adsorption isotherm models (Langmuir and Freundlich) were used to fit the experimental data using (equations 3 and 4). Langmuir isotherm model considers adsorption to be a chemical phenomenon with an assumption that adsorption occurs uniformly on the active sites of the adsorbent surface forming a monolayer. On the other hand, Freundlich model suggests a heterogeneous surface with an exponential distribution of active sites and their corresponding energies (Aly et al., 2013).

The expression of the Langmuir model is given as follows;

Where Ce is the metal concentration (mg/L) at equilibrium, Qe is the amount (mg/g) of adsorbed Pb²⁺/Cd²⁺ at equilibrium, while Qm (mg/g) and K_L are Langmuir constants that are related to adsorption capacity and adsorption rates respectively (Yu et al., 2020).

The following expression represent the Freundlich isotherm model.

Where K_F(mg/g) relates to adsorption energy, represents the intensity of adsorption and, Qe and Ce correspond to equilibrium adsorption capacity per unit weight of the adsorbent and equilibrium concentration of the solution (mg/L) respectively (Yu et al., 2020).

Determination of adsorption kinetic parameters

The mechanism controlling thesorption of Pb and Cd cations on the various wood-derived adsorbents was investigated, and kinetic parameters were determined by fitting experimental data to equation 5 and 6. Two kinetics models – pseudofirst-order and pseudosecond-order model were employed to describe the adsorption process. The linearized expression for the pseudofirst-order and pseudosecond-order equations are outlined as equation 5 and 6 respectively (Aly & Hanley, 2014);

Where Qt and Qe (mg/g) are the amounts (mg) of Pb and Cd adsorbed per mass of the adsorbents (g) at time t (mins) and at equilibrium respectively. K₁ and K₂ are the rate constants corresponding to pseudo-first order and pseudo-second order adsorption models respectively.

RESULTS AND DISCUSSION

Feedstock decomposition behaviour and AC yield.

The TGA profile of the HW and SW samples are shown in Figure 1. From the TGA curve, the first stage of degradation was attributed to the loss of moisturein the wood waste sample (5% and 10% for HW and SW respectively). The second stage was attributed to the decomposition of cellulose and hemicellulose in the wood samples and had a mass loss of 75% for HW and 76% for SW. The residual masses of the HW and SW samples were 16% and 10% respectively, indicating that SW had overall higher weight loss during the thermal decomposition.

Figure 1: TGA profile of the hardwood and softwood feedstock

After the physical and chemical activation of the wood materials, the solid product yield is shown in Table 2.

The yields of chemically derived ACs were2-3-fold higher than the yield from the physical process, with softwood having the lowest product yield in both activation methods. Consistent with literature findings, chemical activation produces higher char than the physical method due to the drastic mass loss during air gasification involved in the physical process. Also, chemical impregnation converts the ash-forming elements (particularly the alkali and alkaline earth metals) into thermally stable salts. The normal catalytic functions of the metals are passivated in the char cracking phase during pyrolysis leading to higher yield (Nzihouet al., 2019). Also, the residual H₃PO₄ in the impregnated feed could participate in catalytic activitiesduring the pyrolysis process, particularly if the impregnation is carried out at excess solvents to biomass ratio. Zhou et al., (2013) found that the pyrolysis of sulphuric acid impregnation of Douglas Fir and hybrid poplar produced higher char yield which increases with increasing sulphuric acid concentration in the feed. The authors speculated that the residual acid in the biomass facilitated hydrolysis, dehydration and cross-linking reactions responsible for the formation of additional char. Therefore, the absence of air gasification and poor char cracking due to the chemical passivation of inherent metals during pyrolysis contributes to the higher AC yield obtained in the chemical activation method than the physical method. Feedstocks affected AC yield, with HW having higher char conversion than SW. The higher AC yield observed in HW can be attributed to its higher lignin and ash contents relative to SW since lignin have the highest thermal decomposition stability among the three biochemical components (hemicellulose, cellulose and lignin) in woody biomass (Yang et al., 2007). Finally, we observed that the thermal decomposition behavior of HW largely controlled the AC yield in the hybrid feed sample (HW+SW) despite equal weights of HW and SW in the mixed feed. The yield in HW+SW is almost the same as HW irrespective of activation methods.

Table 2.    Yields of carbon products from woody biomass

Feedstock	Mode of Activation	AC yield (feed wt. %)
HW	Physical	17
SW	Physical	12
(HW+SW)	Physical	18
HW*	Chemical	41
SW*	Chemical	38
(HW+SW)*	Chemical	41

Characterization of adsorbents

The proximate and ultimate analysis of the produced ACs are presented in Table 3. Fixed carbon content and volatile matter were analyzed as key indicators of porosity development and carbonization extent. Our chemically derived ACs exhibited higher fixed carbon and lower volatile matter content compared to physically activated pairs, demonstrating the effectiveness of the dehydrating agent in inhibiting devolatilization reactions. Among all samples, AC-HW showed the highest fixed carbon content, while AC-HW* had the lowest volatile matter. Notably, in the hybrid ACs, the fixed carbon and volatile matter contents were predominantly controlled by the HW and HW* properties.

Ultimate analysis revealed that physically activated carbons contained higher HNO elemental compositions compared to their chemically activated counterparts. The O content was notably higher in HW, SW, and HW+SW due to exposure to gaseous oxygen during activation, increasing their hydrophobic character.

Table 3. Proximate and Ultimate Analyses of adsorbents

Properties                                                                                   Activated carbons
Proximate Analysis (wt. %)	AC-HW	AC-SW	AC-(HW+SW)	AC-HW*	AC – SW*	AC-(HW+SW)*
Fixed Carbon	22.62	16.23	20.32	48.75	38.01	46.34
Volatiles	63.31	66.41	63.99	40.25	46.13	41.21
Ash	0.40	0.60	0.49	0.13	0.21	0.15
Moisture content	13.67	16.76	15.19	10.79	15.63	12.21
Ultimate Analysis (wt. %)
Carbon	49.05	40.10	46.23	70.34	50.45	68.76
Hydrogen	4.34	6.10	5.01	1.150	3.20	1.623
Nitrogen	0.477	0.780	0.701	0.350	0.512	0.400
Oxygen	46.100	53.02	48.025	28.10	45.82	29.20

SEM analysis revealed distinct surface morphologies between physically and chemically activated adsorbents (Figure 2). Examination of HW and HW* showed similar pore widths, but AC-HW* demonstrated higher surface area and somewhat larger pore volumes, evidenced by greater pore concentrations in Figure 2(D). These observations aligned with our BET surface measurements, where AC-HW* showed superior properties (surface area: 574.2 m²/g vs 553.23 m²/g; pore volume: 0.28 cc/g vs 0.27 cc/g) compared to AC-HW.A particularly interesting finding emerged with softwood samples – while SW showed the lowest BET surface area among physically activated carbons, SW* achieved the highest surface area in chemically activated samples. This dramatic improvement suggests that softwood material is especially responsive to acid treatment during activation.

Figure 2: SEM images of the adsorbents (A) AC- HW (B) AC- SW (C) AC- (HW+SW) (D) AC- HW* (E) AC- SW* (F) AC- (HW+SW)*

Table 4. BET data of synthesized adsorbents

Adsorbent	Surface area (m2/g)	Pore width (nm)	Pore volume (cc/g)
AC- HW	553.23	2.14	0.27
AC- SW	426.15	2.13	0.21
AC- (HW+SW)	513.19	2.13	0.25
AC- HW*	574.2	2.12	0.28
AC- SW*	637.66	2.12	0.31
AC- (HW+SW)*	554.57	2.05	0.31

Figure 3: FTIR Spectra of adsorbents produced by (A) physical activation (B) chemical activation

FTIR analysis (Figure 3) revealed that both activation methods produced ACs with essential functional groups for sorption operations. All samples showed characteristic bands at 3700-3400 cm⁻¹ (OH stretching) and 3000-3500 cm⁻¹ (carboxylic acid groups). However, chemically activated samples displayed additional features: stronger carbonyl group presence (1670-1870 cm⁻¹) and distinctive phosphorous-oxy-containing functional groups (1300-900 cm⁻¹). These additional functional groups in chemically activated samples enhanced their adsorption capabilities through increased opportunities for complexation and hydrogen binding interactions.

The BET data (Table 4) confirmed that all six synthesized adsorbents exhibited mesoporous characteristics, with pore widths ranging from 2.05-2.14 nm. These mesopores function as transport channels that facilitate adsorbate movement to active sites, contributing to the overall effectiveness of our activated carbons as metal adsorbents.

The characterization results basically demonstrate that chemical activation consistently produced ACs with enhanced properties – higher fixed carbon content, greater surface area, improved pore development, and additional functional groups – compared to physical activation. The most striking improvement was observed in softwood-derived samples, where chemical activation transformed the material from having the lowest to the highest BET surface area among all samples.

Adsorption studies

Effect of adsorbent dosages on the removal Pb²⁺ and Cd²⁺

The effect of adsorbent amount on the uptake of Cd²⁺ and Pb²⁺ metals were studied. The initial concentration of Pb²⁺ and Cd²⁺ metals present in the wastewater were determined to be 1.2 ppm and 1.0 ppm respectively, which are unacceptably higher than the safe level (0.1 ppm for Pb²⁺ and 0.01 ppm for Cd²⁺) specified by the Standard Organization of Nigeria (SON). To be within the specified standard safety margin, the required percentage reduction of Pb²⁺ and Cd²⁺ ions from the effluent should be at least 92% and 99% respectively.

For physically activated carbons (Figure 4A), Pb²⁺ removal showed a sharp increase with increasing adsorbent dosage up to 150 mg/L, after which the removal efficiency plateaued. At low dosages (50 mg/L), removal efficiencies were poor, ranging from 29-42%, with AC-HW showing the highest initial performance. All three adsorbents achieved similar maximum removal efficiencies (~91-92%) at 150 mg/L, slightly below the required 92% threshold. Further increases in dosage produced minimal improvement in removal efficiency.

In contrast, chemically activated carbons (Figure 4B) demonstrated superior Pb²⁺ removal performance. AC-SW* exhibited exceptional performance, exceeding the 92% threshold even at the lowest dosage (50 mg/L) and achieving complete removal (100%) at 250 mg/L. AC-HW* and AC-(HW+SW)* showed steady improvement with increasing dosage, reaching >99% removal at 250 mg/L. Notably, all chemically activated carbons surpassed the required 92% removal at dosages ≥150 mg/L.

For Cd²⁺ removal, physically activated carbons (Figure 4C) showed similar trends to Pb²⁺ removal but with lower overall efficiencies. Starting from very low removal rates (20-35%) at 50 mg/L, the adsorbents reached maximum efficiencies of only 90-91% at 250 mg/L, significantly below the required 99% threshold. AC-HW again showed better performance at lower dosages, but all three adsorbents converged to similar efficiencies at higher dosages.

Chemical activation dramatically improved Cd²⁺ removal capabilities (Figure 4D). AC-SW* demonstrated outstanding performance, achieving >93% removal at 50 mg/L and reaching the required 99% threshold at 200 mg/L. AC-HW* and AC-(HW+SW)* required higher dosages but still achieved >99% removal at 250 mg/L. The superior performance of chemically activated carbons, particularly AC-SW*, can be attributed to their enhanced surface properties and functional groups created during H₃PO₄ activation.

These results indicate that chemical activation significantly improves the metal removal capacity of the adsorbents, with AC-SW* consistently showing the best performance for both metals. An optimal dosage of 200 mg/L for chemically activated carbons is sufficient to meet regulatory requirements for both Pb²⁺ and Cd²⁺ removal, while physically activated carbons failed to achieve the required removal efficiencies even at the highest dosage tested.

Figure 4: Effect of adsorbent dosages on the adsorption of Pb²⁺ and Cd²⁺. Contact time = 15 min

Effect of contact time on the removal of Pb and Cd

The effects of contact time on Pb²⁺ and Cd²⁺ removal are illustrated in Figure 5. For physically activated adsorbents and Pb²⁺ removal (Figure 5A), a distinct two-stage adsorption process was observed. The first stage (5-10 min) showed rapid uptake, particularly for AC-HW which achieved approximately 91.7% removal at 10 min. The second stage (10-15 min) exhibited slower adsorption rates until equilibrium was reached at 15 min, with AC-HW achieving the target 92% removal efficiency. AC-SW and AC-(HW+SW) showed similar trends but with slightly lower removal efficiencies (91.9% and 91.8% respectively) at equilibrium.

In contrast, chemically activated adsorbents (Figure 5B) demonstrated superior Pb²⁺ removal performance starting from the earliest contact time. AC-SW* showed the highest initial removal (99.6%) and achieved complete removal (100%) at 15 min. AC-HW* and AC-(HW+SW)* also exhibited excellent performance, reaching >99% removal efficiency at equilibrium, well above the required 92% threshold.

For Cd²⁺ removal, physically activated carbons (Figure 5C) showed relatively lower performance. Starting from initial removal efficiencies of 88.5-89%, all three adsorbents gradually improved to 90-90.5% at 20 min, but failed to reach the required 99% removal threshold. The adsorption curves showed continuous improvement even at 20 min, suggesting that longer contact times might be needed for optimal performance.

Chemical activation dramatically improved Cd²⁺ removal efficiency (Figure 5D). AC-SW* again demonstrated the best performance, achieving 99.5% removal at 5 min and reaching 100% removal at 15 min. AC-HW* and AC-(HW+SW)* followed similar trends, exceeding the 99% threshold at 15 min contact time. Notably, all chemically activated adsorbents maintained stable removal efficiencies after reaching equilibrium at 15 min, indicating strong metal binding and minimal desorption.

The superior performance of chemically activated carbons can be attributed to their enhanced surface properties and functional groups created during H₃PO₄ activation, as confirmed by FTIR analysis. The consistent 15-minute equilibrium time across all systems suggests this as the optimal contact time for practical applications, with no significant benefits observed from extended treatment periods.

Figure 5: Effect contact time on the adsorption of Pb²⁺ and Cd²⁺ metals. Adsorbent dosage = 200 mg/l

Adsorption Isotherms

Constants Qm, K_L, and correlation coefficient R² for Pb²⁺ and Cd²⁺ were evaluated from the graph and summarized in Table 5. The Table showed that AC-SW* has the greatest adsorption capacity, Qm and adsorption rate, K_L for Pb²⁺and Cd²⁺, which is indicative of the highest adsorbent to adsorbates interactions.

Table 5. Isotherm parameters of Langmuir model

Langmuir
Adsorbents	Pb2+			Cd2+
	Qm (mg/g)	KL (L/g)	R2	Qm (mg/g)	KL (L/g)	R2
AC- HW	4.16	30.03	0.9999	3.59	45.67	0.9995
AC- SW	3.22	11.63	1	3.45	34.94	1
AC- HW+SW	3.76	19.98	0.9995	3.60	43.42	1
AC- HW*	5.00	111.11	1	4.15	126.89	0.9994
AC- SW*	5.21	174.36	1	4.24	157.07	1
AC- HW+SW*	4.98	105.63	1	4.06	102.71	1

Parameters from the plots are shown in Table 6. The Freundlich isotherm is characterized by a heterogeneity factor 1/n, and for a good adsorption process, 1/n is expected to be within 0.1 < 1/n < 1.0 (Mouniet al., 2011). The values of 1/n obtained in this study for both Pb²⁺ and Cd²⁺ were outside the range. Hence, the experimental data cannot be sufficiently described by the Freundlich model.  In addition, comparing the magnitude of the coefficients of determination (R²) for the two metals across the two models, it can be concluded that the Langmuir model provided a better a fit for our experimental data than the Freundlich model and thus, the Langmuir theory well described by the adsorption experiment.

Table 6. Isotherm parameters of Freundlich model

Freundlich
Adsorbents	Pb			Cd
	KF (mg/g)	N	R2	KF (mg/g)	N	R2
AC- HW	4.32	-12.99	0.9548	3.04	-5.74	0.9932
AC- SW	3.41	-7.00	0.9176	2.89	-4.90	0.9999
AC- HW+SW	2.25	-0.82	0.8853	3.07	-5.85	0.9999
AC- HW*	4.87	-20.45	0.8751	3.62	-10.52	0.9987
AC- SW*	4.82	-17.15	0.9822	3.71	-11.71	0.9998
AC-HW+SW*	4.00	-7.43	0.9937	3.49	-9.10	0.9997

Adsorption Kinetics

The rate-controlling mechanism of Pb²⁺ and Cd²⁺ adsorption was investigated by analyzing the experimental data using pseudo-first-order and pseudo-second-order kinetic models. The kinetic parameters, correlation coefficients (R²), and the comparison between experimental and calculated equilibrium adsorption capacities (Qe_expand Qe_cal) revealed distinct differences between the two models’ applicability.

Although the pseudo-first-order model (Table 7) showed relatively high correlation coefficients (R² = 0.89-0.99), it failed to accurately predict the equilibrium adsorption capacities. The calculated Qe values (0.06-0.25 mg/g) were substantially lower than the experimental values (3.62-4.80 mg/g) for both metals across all adsorbents, indicating that the pseudo-first-order kinetics cannot adequately describe the adsorption mechanism despite the reasonable R² values.

The pseudo-second-order model (Table 8), however, demonstrated exceptional agreement with the experimental data, achieving perfect correlation coefficients (R² = 1.000) for all adsorbents and showing close agreement between calculated and experimental Qe values. Notably, the pseudo-second-order rate constants (K₂) were significantly higher for chemically activated carbons compared to their physically activated counterparts. For Pb²⁺ adsorption, chemically activated carbons showed K₂ values ranging from 2.595 to 5.541 g/mg·min, while physically activated carbons exhibited lower values between 1.726 and 1.875 g/mg·min. Similarly, for Cd²⁺ adsorption, chemical activation resulted in higher K₂ values (3.074-5.478 g/mg·min) compared to physical activation (1.262-2.213 g/mg·min). AC-SW* demonstrated the highest adsorption rate for Pb²⁺ (K₂ = 5.541 g/mg·min), while AC-HW* showed superior kinetics for Cd²⁺ (K₂ = 5.478 g/mg·min).

The excellent fit of the pseudo-second-order model suggests that chemisorption is the rate-limiting step in the adsorption process, involving valence forces through sharing or exchange of electrons between the adsorbent surface and metal ions. This finding aligns with the FTIR analysis results, which showed that chemical activation enhanced the presence of functional groups capable of metal binding through chemical interactions. The superior kinetic behavior of chemically activated carbons can be attributed to their enhanced surface functionality from H₃PO₄ treatment, larger available surface area, improved pore structure facilitating rapid mass transfer, and higher concentration of active sites for chemical bonding.

Table 7. Pseudo-first order model

Adsorbent	Pseudo-first order model
	Pb2+				Cd2+
	K1 (min-1)	R2	Qeexp (mg/g)	Qecal (mg/g)	K1 (min-1)	R2	Qeexp(mg/g)	Qecal (mg/g)
HW	1.498	0.9614	4.420	0.22	1.736	0.9202	3.624	0.18
SW	1.479	0.9758	4.416	0.23	1.368	0.9505	3.664	0.25
HW+SW	1.721	0.9687	4.412	0.18	1.541	0.9765	3.624	0.21
HW*	2.867	0.9934	4.796	0.06	2.883	0.8898	3.996	0.06
SW*	6.348	0.7716	4.796	0.00	6.092	0.3456	3.996	0.00
HW*+SW*	2.152	0.9480	4.780	0.12	2.152	0.9829	3.992	0.12

Table 8. Pseudo-second order model

Adsorbent	Pseudo-second order model
	Pb2+				Cd2+
	K2 (g/mg. min)	R2	Qeexp (mg/g)	Qecal (mg/g)	K2 (g/mg. min)	R2	Qeexp (mg/g)	Qecal (mg/g)
AC-HW	1.726	1	4.420	4.45	2.213	1	3.624	3.64
AC-SW	1.875	1	4.416	4.44	1.262	1	3.664	3.69
AC-HW+SW	1.873	1	4.412	4.43	1.611	1	3.624	3.65
AC-HW*	4.935	1	4.796	4.80	5.478	1	3.996	4.00
AC-SW*	5.541	1	4.796	4.81	4.511	1	3.996	4.01
AC-HW*+SW*	2.595	1	4.780	4.79	3.074	1	3.992	4.00

CONCLUSION

This study demonstrated the successful synthesis and application of activated carbons (ACs) from hardwood, softwood, and their hybrid using physical and chemical activation methods. The sorption performance of the adsorbents followed a clear hierarchical order, with chemically activated softwood showing the highest removal efficiency, followed by chemically activated hardwood and hybrid. The physically activated carbons consistently showed lower performance, ranking as hardwood, softwood, and hybrid in descending order. This performance hierarchy was consistent for both Pb²⁺ and Cd²⁺ removal, highlighting the crucial role of activation method in determining adsorption efficiency.

Chemical activation proved superior to physical activation across all metrics, producing higher yields (38-41% versus 12-18%) and enhanced adsorptive properties. The H3PO4 activation process created additional oxygen-containing functional groups that significantly improved metal adsorption through complexation and hydrogen binding. Particularly noteworthy was the transformation of softwood through chemical activation – while it showed the lowest surface area (426.15 m²/g) under physical activation, it achieved the highest BET-surface area (637.66 m²/g) when chemically activated, demonstrating the material’s exceptional response to chemical treatment.

The chemically activated adsorbents achieved complete removal of both Pb²⁺ and Cd²⁺ at 200 mg/L dosage, whereas physically activated carbons could not achieve 100% removal even at higher dosages. The optimal contact time was established at 15 minutes for all adsorbents, with no significant improvement in removal efficiency beyond this duration. The adsorption process followed the Langmuir model and pseudo-second-order kinetics, indicating uniform monolayer adsorption controlled by chemisorption mechanisms.

These findings demonstrate the superiority of chemical activation for producing high-performance activated carbons from wood waste, with softwood emerging as the most promising feedstock when chemically activated. The clear performance hierarchy established in this study provides valuable guidance for the selection of both feedstock and activation method in the practical application of activated carbons for heavy metal removal from industrial effluents.

REFERENCES

1. Abdulaziz , A. A., Abdel-Basit, A., Waseem, S. S., Abdullahi, A., Fahad, A. A., & Taieb, A. (2019). Efficient adsorption of lead(II) from aqueous phase solutions using polypyrrole-based activated carbon. Materials, 12, 1-16.
2. Adie, D. B., Okuofu, C. A., & Osakwe, C. (2012). Isothermal and batch adsorption studies of the use of Borassus Aethiopum and Cocos Nucifera for wastewater treatment. American International Journal of Contemporary Research, 2(7), 119-130.
3. Ahumada, E., Lizama , H., Orellana, F., Suarez, C., Huidobro, A., Sepulveda-Escribano, A., & Rodriguez-Reinoso, F. (2002). Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen. Effect of the surface oxidation degree in the catalytic activity. Carbon, 40, 2827-2834.
4. Ajay, K. M., Kadirvelu, K., Mishra, G. K., Rajagopal, C., & Nagar, P. N. (2008). Adsorptive removal of heavy metals from aqueous solution by treated sawdust (Acacia arabica). Journal of Hazardous Materials, 150(3), 604-611.
5. Al-Doury, M. M., & Ali, S. S. (2015). Removal of phenol and parachlorophenol from synthetic waste water using prepared activated carbon from agricultural wastes. International Journal of Sustainable and Green Energy, 4(3), 92-101.
6. Aly, Z., & Hanley, T. (2014). Removal of aluminium from aqueous solutions using PAN-based adsorbents: characterisation, kinetics, equilibrium and thermodynamic studies. Environmental Science and Pollution Research, 21, 3972-3986.

7. Amarasinghe, B., & Williams, R. (2007). Tea waste as a low cost adsorbent for the removal of copper and lead from wastewater. Chemical Engineering Journal, 132, 299-309.
8. Amirza, M. A., Adib, M. M., & Hamdan, R. (2017). Application of agricultural wastes activated carbon for dye removal – An overview. MATEC Web of Conferences(103), 1-12.
9. Bubanale, S., & Shivashankar, M. (2017). History, method of production, structure and applications of activated carbon. International Journal of Engineering Research and Technology, 6(6), 495-497.
10. Byamba-Ochir, N., Shim, W. G., Balathanigaimani, M., & Moon , H. (2016). Highly porous activated carbons prepared from carbon rich Mongolian anthracite by direct NaOH activation. Applied Surface Science, 379, 331-337.
11. Diao, Y., Walawender, W. P., & Fan, L. T. (2002). Activated carbons prepared from phosphoric acid activation of grain sorghum. Bioresource Technology, 81, 45-52.
12. Diaz-Diez, M. A., Gomez-Serrano, V., Gonzalez, C. F., Cuerda-Correa, E. M., & Macias-Garcia, A. (2004). Porous texture of activated carbons prepared by phosphoric acid activation of woods. Applied Surface Science, 238, 309-313.
13. Eletta, A. O., Adeniyi, G. A., Magaji, M. M., & Ighalo, J. O. (2019). A mini-review on the application of alumina nanoparticles for water treatment. FUOYE International Science Conference, 4-8.
14. Food and Agriculture Organization of the United Nations. (2015). Energy Conservation in the Mechanical Forest Industries. Food and Agricultural organization of the United Nations, 1990.
15. Guo, J., Song, Y., Ji, X., Ji, L., Cai, L., Wang, Y., . . . Song , W. (2019). Preparation and characterization of nanoporous activated carbon derived from prawn shell and its application for removal of heavy metal ions. Materials, 12, 241.
16. Idris , S., Alotaibi, K. M., Peshkur, T. A., Anderson , P., Morris , M., & Gibson , L. T. (2013). Adsorption kinetics: effect of adsorbent pore size distribution on the rate of Cr(VI) uptake. Mesoporous Materials, 165, 99-105.
17. Ighalo, J. O., Adedeji , A. A., & Adewale, A. G. (2020). Thermochemical conversion of oil palm Fiber-LDPE hybrid waste into biochar. Biofuels, Bioproducts and Biorefining, 14(6), 1313-1323.
18. Jacob, O. M., Habeeb, Z. O., & Isa, E. O. (2016). Sustainable wood waste management in Nigeria. Environmental & Socio-economic Studies, 4(3), 1-9.
19. Jeminat, O. A., Jose, H. S., Zahangir, M. A., Aminul, H. M., & Chan, C. M. (2016). Adsorption of methylene blue from aqueous solution using untreated and treated (Metroxylon spp.) waste adsorbent: Equilibrum and kinetics studies. International Journal of Industrial Chemistry, 7, 333-345.
20. Juang, R. S., Wu, F. C., & Tseng, R. L. (2000). Mechanism of adsorption of dyes and phenols from water using activated carbons prepared from plum kernels. Journal of Colloid and Interface Science, 227(2), 437-444.
21. Kennedy, J. L., & Sekaran, G. (2004). Integrated biological and catalytic oxidation of organics/inorganics in tannery wastewater by rice husk based mesoporous activated carbon––Bacillus sp. Carbon, 42(12-13), 2399-2407.
22. Kubier, A., Richard, T. W., & Pichler, T. (2019). Cadmium in soils and groundwater: A Review. Applied Geochemistry, 108, 1-16.
23. Kumar, A., & Jena, H. M. (2016). Removal of methylene blue and phenol onto prepared activated carbon from Fox nutshell by chemical activation in batch and fixed-bed column. Journal of Clean Production, 137, 1246-1259.
24. Leandro, O. S., & Adriana, F. S. (2012). Conventional and non-conventioanl thermal processing for the production of activated carbons from agro-industrial wastes. New York: Nova Science Publishers.
25. Luka, Y., Highina, K., & Zubairu, A. (2018). Precursors for Development of Activated Carbon: Agricultural Waste Materials – A Review. International Journal of Advances in Scientific Research and Engineering, 4(2), 45-59.
26. Mahmud, H. N., Huq, A. O., & Binti, Y. R. (2016). The removal of heavy metal ions from wastewater/aqueous solutions using polypyrrole-based adsorbents: A Review. Royal Society of Chemistry Advances, 6, 14778-14791.
27. Mahmud, H., Hosseini, S., & Yahya, R. (2014). Polymer adsorbent for the removal of lead ions from aqueous solution. International Journal of Technology and Research Application, 11, 04-08.
28. Mall, I. D., Srivastava, V. C., & Agarwal, N. K. (2006). Removal of orange-G and methyl violet dyes by adsorption onto bagasse fly ash-kinetic study and equilibrum isotherm analyses. Dyes Pigment, 69, 210-223.
29. Mdoe, J. E. (2014). Agricultural waste as raw materials for the production of activated carbon? Can Tanzania venture into this business. Journal of the Open university of Tanzania, 16, 89-103.
30. Moreno-Pirajan, J. C., & Giraldo, L. (2010). Study of activated carbons by pyrolysis of cassava peel in the presence of chloride zinc. Journal of Analytical and Applied Pyrolysis, 87(2), 288-290.
31. Mouni, L., Merabet, D., Bouzaza, A., & Belkhiri, L. (2011). Adsorption of Pb (II) from aqueous solutions using activated carbon developed from Apricot stone. Desalination, 276, 148-153.
32. Nabais, J. M., Pedro, N., Peter , J. M., Ribeiro Carrott, M., Macias Garcia, A., & Diaz-Diez, M. A. (2008). Production of activated carbons from coffee endocarp by CO2 and steam activation. Fuel Processing Technology, 89(3), 262-268.
33. Nakagawa, K., Namba, A., Mukai, S. R., Tamon, H., Ariyadejwanich, P., & Tanthapanichakoon, W. (2004). Activated carbon form municipal waste. Water Research, 38, 1791-1798.
34. Pallares, J., Gonzalez-Cencerrado, A., & Arauzo, I. (2018). Production and characterization of activated carbon from barley straw by physical activation with carbon dioxide and steam. Biomass Bioenergy, 115, 64-73.
35. Ramirez, A. P., Giraldo, S., Ulloa, M., Florez, E., & Acelas, N. Y. (2017). Production and characterization of activated carbon from wood wastes. Journal of Physics, 935(1), 1-6.
36. Ranjeet, K. M., & Kaustubha, M. (2019). Thermal and catalytic pyrolysis of pine sawdust (Pinus ponderosa) and Gulmohar seed (Delonix regia) towards production of fuel and chemicals. Materials Science for Energy Technologies, 2(2), 139-149.
37. Rozada, M., Otero, J. B., Parra, A. M., & Garcia, A. I. (2005). Producing adsorbents from sewage sludge and discarded tyres: characterization and utilization for the removal of pollutants from water. Chemical Engineering Journal, 114, 161-169.
38. Rozada, M., Otero, J. B., Parra, A. M., & Garcia, A. I. (2005). Producing adsorbents from sewage sludge and discarded tyres: characterization and utilization for the removal of pollutants from water. Chemical Engineering Journal, 114, 161-169.
39. Seema, T., Indra, P. T., & HI, T. (2013). Effects of lead on environment. International Journal of Emerging Research in Management & Technology, 2(6), 1-5.
40. Shariaty-Niassar, M., Hafizi-Atabak, H. R., Ghanbari-Tuedeshki, H., Shafaroudi, A., Akbari, M., & Safaei-Ghomi, J. (2013). Production of activated carbon from cellulose wastes. Journal of Chemical and Petroleum Engineering, 13-25.
41. Singh, C., Sahu, J., Mahalik, K., Mohanty, C., & Mohan, B. R. (2008). Studies on the removal of Pb (II) from wastewater by activated carbon developed from Tamarind wood activated with sulphuric acid. Journal of Hazard and Materials, 153, 221-228.
42. Sutrisno, B., Rizka, N. A., Hidayat, A. S., & Hidayat, A. (2016). Preparation and characterization of activated carbon from sugarcane bagasse by physical activation with CO2 gas. IOP Conference Series: Materials Science and Engineering, 105(012027), 1-8.
43. Tadda, M. A., Shittu, A., Ahsan, A., & Elsergany, M. (2016). A review on activated carbon: process, application and prospects. Journal of Advanced Civil Engineering Practice and Research, 7-13.
44. Thue, S. P., Adebayo, M. A., Lima, E. C., Sieliechi, J. M., Machado, F. M., Dotto, G. L., . . . Dias, S. L. (2016). Preparation, characterization and application of microwave-assisted activated carbons from wood chips for removal of phenol from aqueous solution. Journal of Molecular Liquids, 223, 1067-1080.
45. Yahya, M. A., Al-Qodah, Z., & Ngah, C. Z. (2015). Agricultural Bio-waste Materials as Potential Sustainable Precursors used for activated carbon production: A review. Renewable and Sustainable Energy Reviews, 46, 218-235.
46. Yarkandi, N. (2014). Removal of lead (II) from wastewater by adsorption. International Journal of Current Microbiology and Applied Sciences, 3, 207-228.
47. Yasemin, B., & Zeki, T. (2007). Removal of heavy metals from aqueous solution by sawdust adsorption. Journal of Environmental Sciences, 19(2), 160-166.
48. Yu, K., Xiaoping, Z., & Shaoqi, Z. (2020). Adsorption of methylene blue in water onto activated carbon by surfactant modification. Water, 12(587), 1-19.
49. Zaharaddeen, G. N., Abdulkadir, T., & Zakariyya, Z. U. (2019). Borassus Aethiopum shell-based activated carbon as efficient adsorbent for carbofuran. Bulletin of the Chemical Society of Ethopia, 33(3), 425-436.
50. Zaynab, A., Adrien , G., Nicholas, S., & Tracey, H. (2013). Removal of aluminium from aqueous solutions using PAN-based adsorbents: Characterization, Kinetics, equilibrum and thermodynamic studies. Environmental Science and Pollution Research, 3972-3986.
51. Zhang, F. S., Nriagu, J. O., & Itoh, H. (2005). Mercury removal from water using activated carbons derived from organic sewage sludge. Water Research, 39, 389-395.