Meso-microporous activated carbon derived from Raffia palm shells: optimization of synthesis conditions using response surface methodology.

This study investigated the optimal synthesis conditions for the production of Raffia Palm Shell Activated Carbon (RPSAC) using phosphoric acid as activation agent. The optimization of the synthesis conditions was achieved using the Central Composite Design (CDD) in Response Surface Methodology (RSM). The influences of impregnation ratio, temperature, time and concentration on the specific surface area and yield of RPSAC were evaluated. Based on the CDD, 2FI and quadratic models were developed for the two responses. Analysis of Variance (ANOVA) was utilized to determine the significant factors and factor interactions for each response. All process variables except impregnation ratio were observed to significantly influence the quality of RPSAC. The optimal synthesis conditions for RPSAC were; 523.68 °C, 76.91%, and 103.83 min for temperature, concentration, and time respectively which provided a specific surface area and yield of 1762.92 m2/g and 77.98 % respectively. The Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray (EDX) analyses proved that RPSAC had a meso-micro-porous morphology with high carbon and oxygen contents. Fourier-transform infrared spectroscopy (FTIR) revealed the abundance of hydroxyl, carbonyl and carboxylic groups on RPSAC. X-ray Powder Diffraction (XRD) analysis showed that RPSAC composed mainly of amorphous and disordered microcrystalline phases ascribed to the high quartz content of the precursor. The Brunauer-Emmett-Teller (BET) surface area, average pore diameter, total pore volume, and pHpzc of RPSAC were obtained as 456.10 m2/g, 0.25 cm3/g, 2.13 nm and 2.10 correspondingly. Thus, RSM was found to be an excellent and desirable tool for optimal synthesis of RPSAC that possess high surface area and porosity suitable for application in the adsorption of both large and small molecular sized pollutants such as dyes and fluoride in real and aqueous solution.

Introduction

Activated carbons (ACs) have emerged recently as an efficient material for diverse applications such as in environmental remediation. Prominent applications of ACs are in water, wastewater and air quality controls (Jawad et al., 2020a; Ogungbenro et al., 2017). Others are in energy storage and medicine (Eguchi et al., 2020). The demand for activated carbons has also recently upstretched due to its applications in medicine, agriculture (soil amendment) and adsorption refrigeration systems (Utsev et al., 2020; Sakar et al., 2020; Dzigbor, 2019). Notably, it has been asserted that the annual increase in the consumption of activated carbon in the last decade was 5.5% and currently stands at 8.1% since 2018 and could further increase up to 15% by 2021 due to the advancements in industrial and agricultural activities. The foregoing has obviously increased the global demand for activated carbon (Pallaras et al., 2018).

Previously, AC was primarily obtained from coal deposits. However, the cost of ACs derived from coal sources is prohibitive. This has led to a growing interest in the synthesis of ACs from renewable and cost effective biomass and industrial waste-based precursors (Ndongo et al., 2020; Pallaras et al., 2018; Demiral et al., 2016).

Numerous bio-wastes such as peanut noodles, bagasse pith, sago bark, rice husk (lignocellulose), cattle dung, pig bristles, among others have been reported to be frequently and widely available for conversion into other useful products including activated carbon (Xu et al., 2019). For instance, bio-wastes such as bone wastes have been successfully utilized for the production of nano-catalyst and biofuels for various applications (Nasrollahzadeh et al., 2020a). Recently, researchers have also valorized bio-wastes via novel technologies for the production of nanocomposites targeted at hydrogen storage and environmental remediation (Mohazzab et al., 2020; Nasrollahzadeh et al., 2020b). Thus, the growing interest in the conversion of bio-wastes into valuable products seems to be rising as more bio-waste materials emerge.

The synthesis of biomass-based ACs involves the use of the one-step or two-step activation techniques. In the one-step activation method, both carbonization and activation are performed concurrently, while in the two-step method, they are undertaken separately. The activation process could be physically achieved, requiring the use of gases (steam, nitrogen, carbon dioxide etc) or chemically done with the use of dehydrating agents or oxidants such as phosphoric acid, sodium chloride, nitric acid, sulphuric acid, sodium hydroxide and potassium hydroxide, among others (Lekene et al., 2018; Vargas et al., 2011).

The production of ACs via the chemical activation method is preferred over the physical method. The foregoing holds true when the end product is to be hydrophilic and with sufficient oxygen and carbon-bearing functional groups (Jawad et al., 2018a). Phosphoric acid is more desirable and widely exploited mong the tested chemical agents for AC production from biomass. This is due to its ability to serve as an eco-friendly, corrosion-free and a low-cost dehydrating catalyst that decomposes biomass precursors even at reduced activation temperatures (Jawad et al., 2018b, 2020b).

The production of ACs from biomass-based precursors by carbonization and activation process is influenced by several process variables. These variables include; type and nature of precursor, carbonization/activation temperatures, residence time, gas flow rate or chemical impregnation ratio, activating agent concentration and type of activating agent among others. Ultimately, in ACs production, it is desirable to firstly select a suitable precursor and activating agent depending on the availability of such precursors. The foregoing could be achieved through a chronological strategy viz: (i) an initial screening of the precursors on the basis of its proximate and ultimate compositions, (ii) selection of a suitable activating agent and method and (iii) optimizing the influence of the other process conditions towards the worth and yield of AC.

Generally, efforts towards optimizing the quality of ACs derived from biomass precursors requires several experimental runs when adopting the traditional method of keeping one variable constant and changing the others. The foregoing could be time-demanding, responsible for high costs in AC production, and produce inaccurate results. Therefore, researchers have adopted the Response Surface Methodology (RSM) as a mathematical approach for predicting the response variables as a function of few or several independent variables. This is highly advantageous as it saves time, cost, ensures a minimum number of experimental runs and the accuracy of results. Previous studies have reported some of the biomass-based precursors employed for the synthesis of qualitative ACs using the RSM approach. These includes mango wastes (Dzigbo, 2019; Kwaghger and Ibrahim, 2013), egusi seed shells (Lekene et al., 2018), palm kernel shells (Hesas et al., 2013; Kundu et al., 2014), jathropha hull (Duan et al., 2011), kenaf fibre (Chowdhury et al., 2011).

Recently, studies have reported the use of RSM to simultaneously enhance activated carbon quality and its performance in the removal of pollutants in water and wastewater. These researchers have in most cases posited that RSM was a suitable tool for adsorption product and process optimization (Jawad et al., 2020c; Jawad and Abdulhameed, 2020a). In order to address the increasing demand for low-cost and efficient activated carbons, researchers have also intensified efforts towards the investigation and exploration of various potential biomass precursors that are readily available. Precursors that constitute environmental nuisance such as agricultural residues and industrial processing by-products are mostly preferred in this regard.

In the reflection of such interest, the goal of the current study is to optimally synthesis a novel AC obtained from Raffia palm (Raphia hookeri) shells precursor (an abundant agricultural residue in Nigeria which has promising proximate compositions for the synthesis of activated carbon). The specific objectives of the study include: (1) to determine the proximate composition of Raffia palm shells as precursor for AC syhthesis, (2) to model and optimize the influence of production variables (activation temperature, activation time, impregnation ratio and activating agent concentration) on the quality and yield of AC derived from Raffia palm shells using RSM as a tool and H3PO4 as an activating agent, (3) to determine the physical, chemical, morphological, textural and functional characteristics of the optimally synthesized Raffia Palm Shell Activated Carbon (RPSAC). To the best of the authors’ knowledge, this is the first study that has reported the optimal synthesis of AC derived from Raffia palm shells for consequent applications in environmental remediation.

Materials and methods

Resources for production and characterization of RPSAC

The list of materials and instruments that were used for the production and characterization of RPSAC and the reagents, suppliers, and quantity used in the current study are provided in Table 1. All reagents utilized in this work were of analytical grade.

Table 1

Materials, chemicals and instruments for activated carbon synthesis/characterization.

S/No	Materials/Instruments	Description	Quantity
1	pH meter	Elico pH meter-Model L1 – 120	1
2	Crucibles	250ml capacity	10
3	Adhesive	-	2
4	Measuring Beaker	250ml capacity	2
5	Measuring cylinder	30ml capacity	5
6	Sieves	Sizes 6mm and 3mm, made by ENDECOTTS Ltd. 667924, London, England	1
7	Activating agent	Phosphoric acid MERK, Germany	4litres
8	Digital Weighing balance	25kg capacity, Mettle Telode-AB204	1
9	Raffia Palm Shells	Freshly collected from local markets in Benue State	20 kg
10	Furnace	Carbolite- Model GPC 12/81 + 103, Max Temp. 12000 °C, Sheffield, England	1
11	Oven	Gallenkamp Oven, Made in Germany	1
12	Scanning Electron Microscope	Phenom ProX	1
13	Infrared spectrometer	Agilent Cary 630 FTIR	1
14	Surface Area and Porosity Analyzer	Quantachrome Novawin Version 11.0	1
15	X-Ray Powder Diffractometer	Rigaku Mini Flex	1
17	Vacuum Filter	Model QF 120C	1
18	Whatman Filter Paper	No 42	1 Pack
19	Stop-Watch		1
20	Distilled water	Loba Chemicals	10 L

Sample collection, preparation and proximate analysis

The dried Raffia palm fruits and shells were obtained from Buruku and Ugbema markets in Benue State, where the fruit mesocarp is widely consumed as food and the shells discarded as waste in large quantities. They were rinsed to get rid of dirt and other foreign items, and then dehydrated for seven days under sun to further lower the wetness, and subsequently pounded to small-scale particle sizes with a test site mortar and crusher. The resultant aggregates were then dried for four (4) hours to eliminate the remaining wetness and finally sieved to size 6 mm for further experimentation (Briton et al., 2020).

The proximate analysis was carried out on the prepared precursor (Raffia palm shells) to obtain the composition in accordance with standard methods (Alkali, 2016). Briefly, for moisture content, a known quantity of the raw shells was weighed, and recorded as W1 (g) and then heated in an oven to a temperature of 105 °C for 60 min and subsequently allowed to cool. It was further weighed to ascertain the final weight, W2 (g). The moisture content was determined from Eq. (1).

For the determination of ash content, 5.0 g weight of sample was placed on porcelain crucible. The content was transferred into the electric furnace set at 600 °C for 1 h to minimize volatilization. The sample was later withdrawn from the furnace and cooled down in a desiccator. The ash content was noted, weighed, and calculated using Eq. (2). For volatile matter determination, a sample of the shells was weighed and placed in a partially closed crucible and transferred to a furnace set at 600 °C for 30 minutes. The crucible with its content was retrieved and cooled in desiccators. The sample was reweighed to determine the weight difference due to the escaped volatile content, and the volatile matter was determined using Eq. (3). Finally, the determination of the fixed carbon content of the precursor was calculated from the relationship shown Eq. (4).where; Wash is the weight of the ash, Xg is the weight of the activated carbon introduced into furnace before heating. W1 is the weight of crucible and sample, W2 is the weight of crucible and the ash sample, Wi is the initial weight of shells (precursor) and crucible and Wj is the final weight of shells and crucible after heating.

Preparation of activating agent

Since the focus of this study was to explore the suitability of Raffia palm shells as activated carbon source for water/wastewater treatment, the one-step chemical activation method was therefore employed. Initially, phosphoric acid (activation agent) was prepared in varying concentrations of 20, 40, 60, 80 and 100% using distilled water and then applied as the acidic oxidizing/activating agent. In achieving this, 20, 40, 60 and 80 mL of phosphoric acid was added to 80, 60, 40 and 20 mL of distilled water to attain 20, 40, 60 and 80 % by volume concentration of activating agent. For the 100% volume concentration, the phosphoric acid was used without dilution with distilled water.

Synthesis of Raffia palm shell activated carbon (RPSAC)

The prepared precursor was later incorporated with the various concentrations of phosphoric acid (20, 40, 60, 80 and 100%). Briefly, 50 g of the samples were quantified and augmented with 50, 100, 150, 200 and 250 mL of the activating agent to obtain activation ratios of 1:1, 1:2, 1:3, 1:4 and 1:5. However, for clarity, these ratios were designated as 1, 2, 3, 4 and 5 mL/g in the current study.

The modified samples were exposed to ambient temperature overnight. Afterward, the superfluous solutions were clarified, and the materials were oven dried for 72 h at ambient temperature. Lastly, the resultant samples were then carbonized in a furnace at 200, 400, 600, 800 and 1000 °C for the period of 30–150 min. The carbonized materials were then cleaned with distilled water repeatedly until the pH was almost neutral. They were then oven dried at 105 °C for 60 min and then crushed to obtain particle sizes of 0.85–2.0 mm to obtain granular activated carbons. The sieved activated carbons were later kept in an airtight vessel for further analysis.

Determination of the physico-chemical characteristics of RPSAC

The synthesized RPSAC was characterized at different stages of the optimization schedule for moisture content, ash content, bulk density, carbon yield and specific surface area in accordance with the methods described in previous studies (Atemkeng et al., 2020; Alkali, 2016; Mohammed et al., 2015).

The ash content of RPSAC was determined following ASTM standard method D2866-94 (ASTM, 2004). The bulk density was determined using a weighed and RPSAC-loaded gauging cylinder. The loaded cylinder was moderately compressed by tampering up to a time when the level of the sample in it remained unchanged. The working volume of the packed sample was noted as Vs. The weight of the sample Ws was calculated using Eq. (5), while the bulk density was determined using Eq. (6) (Atemkeng et al., 2020). The carbon yield was determined in accordance with the method of Ayyalusamy and Mishra (2018). The dried weight of precursor, Wp (g) was determined and the carbon yield calculated using Eq. (7).

The specific surface area of RPSAC was determined using the Sear's approximation method (Mohammed et al., 2015). It should be noted that the sear's method has the ability to approximate the effects of process variables on the specific surface area of activated carbons. Therefore, it was adopted during the modelling and optimization experiments to minimize the research cost, however, the BET-surface area of the optimally synthesized RPSAC was later determined to reflect the true surface area of RPSAC.

Briefly, in accordance with the Sear's method, 1.5 g of RPSAC sample was acidified with diluted hydrochloric acid (HCl) in a drop-wise manner, until a pH of 3–3.5 was attained. A 30.0 g sodium chloride (NaCl) was added to the acidified samples with constant stirring; then distilled water was used to raise the capacity to 150 mL. The solution was titrated with 0.1 M sodium hydroxide (NaOH) solution till a pH of 9 was achieved. The volume of sodium hydroxide used was noted, while the specific surface area was determined using Sear's method as shown in Eq. (8).where; Wf is the dry weight of RPSAC, Wp is the dry weight of precursor, Wc is the weight of blank cylinder, W is the weight of the cylinder and sample, Vs is the volume of cylinder, V is the volume of sodium hydroxide used to alter the pH of the solution to 9.

Modeling and optimization of preparation conditions for RPSAC

RSM is a statistical instrument deployed to establish the interrelationships among process factors (variables). It has emerged as a time and cost-efficient approach in process/product optimization as a result of its ability to lower the number of experimental runs and yet provide for accurate results and inferences. In this work, Design-Expert Version 10 software (Stat-Ease Inc., Minneapolis, USA) was utilized for modeling and optimization. It is appropriate for fitting quadratic, cubic and two factor interaction (2FI) models. It is also useful for optimizing the working parameters, using a lowest number of experimental runs to evaluate the interactions among influencing variables (Zhou et al., 2019).

In the present study, quadratic and 2FI models were developed using actual experimental values by applying the Central Composite Design (CCD) with 30 experimental runs (comprising 16 factor, 8 axial and 6 central points) in the synthesis of RPSAC and in accordance with Eq. (9) below. The levels of all factors considered and the experimental design matrix in this study can be found in Table 2.where, “N” is the entire quantity of experimental runs and “n” is the number of independent factors. In this study, the independent variables optimized were; activation temperature (°C), activation time (hours), activating agent concentration (%) and chemical impregnation ratio (g/ml). The responses were carbon yield (%) and specific surface area (m2/g).

Table 2

Levels of process factors employed in CCD.

Factor	Levels
	-α	α	0	α	+α
Impregnation Ratio (g/ml)	1	2	3	4	5
Activation Temperature (0C)	200	400	600	800	1000
Activation Time (minutes)	30	60	90	120	150
Activating Agent concentration (%)	20	40	60	80	100

Evidence-based models that correlate the responses to the four factors were developed using a second-order polynomial equation (Eq. (10)). The suitability of the model equations was determined using ANOVA, coefficient of determination (R2), P-value, F-value and residuals (Hassani et al., 2014; Roy et al., 2014; De Lima et al., 2011). Also, 3D plots were employed to analyze the combined effect of factors on the responses, while 2D graphs were used to collate the predicted versus actual value of the response variables. The optimization was achieved using the desirability function in Design Expert software (Version 10).Where Y is the predicted response, bo is the constant coefficient, bi is the linear coefficient, bij is the interaction coefficient, bii is the quadratic coefficient, Xi and Xj, are the coded values for the factors (Mohammad et al., 2014).

Model validation

The validation of the model was undertaken by producing RPSAC under the various optimum operating conditions. The investigational values gotten were compared with the model projected values by means of percentage error analysis.

Characterization of the optimally synthesized RPSAC

The optimally produced RPSAC was subjected to Scanning Electron Microscopy (SEM), BET surface area and porosity analyses, Fourier Transform Infrared spectroscopy (FTIR), X-Ray powder diffraction (XRD) and particle size analyses in other to further understand the pore structure of the carbon as well as the functional groups present on the surface. These properties are necessary for providing useful information on the possible applications of such synthesized AC.

The IR spectrum of RPSAC was recorded using a Nicolet Avatar FT-IR in the range of 4000 cm−1 and 400 cm−1, with KBr disc. Morphological composition of RPSAC was perceived using Phenom Pro X (Phenom World) Scanning electron microscope at 10000, 12000 and 15000 magnifications, 200 μm resolution, 15 kV bright field and pressure of 70 Pa. The samples were coated with gold prior to SEM analysis in accordance with the method of Giwa et al. (2013).

The BET surface area, total pore volume and average pore diameter of the optimally synthesized RPSAC was verified using nitrogen adsorption at 77 K using the BET surface area and porosity analyzer (Quantachrome Novawin Version 11.03) according to the procedure given by Kang et al. (2013).

The phase/crystallographic composition of the adsorbent was determined by X-ray powder diffraction (XRD) analysis by means of a Rigaku Mini Flex X-ray Diffractometer following the methods of Kang et al. (2013).

Determination of surface charge of RPSAC

The pHpzc of the adsorbent (RPSAC) was determined using the salt addition technique. A 50 mL of 0.01 molL-1 NaCl solution was measured into several Erlenmeyer flasks. The pH of the solution in the flask was altered to values of 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11 by adding 0.1 molL-1 of HCL or NaOH solution. Then 0.5 g of the adsorbent was added and stirred on a vibrating shaker for 1 h and then enabled to settle for 48 h to attain equilibrium at the prevailing temperature. The pH of the solution was then determined for the adsorbent. A graph of initial pH (pHo) versus the differences amongst the first and last pH values (ΔpH) was plotted. The pHpzc was adopted as the point where ΔpH = 0 (Jawad et al., 2020a).

Results and discussion

Proximate structure of Raffia palm shells precursor

The proximate analysis of Raffia palm shells precursor was conducted to assess the amount of moisture content, fixed carbon, volatile matter, and ash content present in the raw sample. It was noticed that the moisture content and ash content of RPSAC were low; volatile content was moderate while the fixed carbon content was high and desirable.

The result of the proximate analysis of Raffia palm shells is presented in Table 3. The proximate analysis revealed that the sample had promising potentials to be utilized as activated carbon for various applications. This was evidenced from the high percentage of fixed carbon (68.83%), low ash content (1.37%) and moisture content (7.7%). The findings of the current work are in tandem with that of Akpen et al. (2018) who reported similar proximate composition for Raffia palm shells.

Table 3

Proximate composition of the precursor (Raffia palm shells).

Characteristics	Value, %
Moisture Content (MC)	7.77 ± 1.82
Ash Content (AC)	1.37 ± 0.55
Volatile Content (VC)	22.03 ± 0.82
Fixed carbon Content (FCC)	68.83 ± 0.58

Modeling of experimental factors of phosphoric acid modified RPSAC

The corresponding results with respect to the experimental design matrix earlier presented in this work for the specific surface area and the percentage carbon yield of RPSAC is as shown in Table 4. Tables 5 and 6 are the analyses of variance (ANOVA) for carbon yield and specific surface area respectively. Also 2FI models were established to analyze factor interactions and detect the important factors making-up the models for predicting specific surface area and carbon yield.

Table 4

Results of CCD matrix for the production of RPSAC.

Run	A:Concentration	B:Impregnation Ratio	C:Activation Temperature	D:Activation Time	Carbon Yield	Specific Surface Area
	%	g/ml	°C	Minutes	%	m2/g
1	60	3	600	90	79.05	1786
2	40	2	800	60	71.01	1801
3	60	3	600	90	79.55	1750
4	80	2	400	120	76.3	1742
5	40	4	400	120	72.82	1684
6	40	4	400	60	74.29	1714
7	80	2	400	60	76.66	1667
8	40	4	800	60	72.8	1796
9	80	4	800	60	68.1	1795
10	60	3	600	90	70.83	1769
11	40	2	800	120	67.18	1788
12	60	3	600	90	77.25	1767
13	40	2	400	120	72.7	1687
14	80	4	400	60	75.47	1746
15	80	4	400	120	78.94	1739
16	60	3	600	90	76.06	1766
17	80	2	800	60	66.05	1835
18	40	2	400	60	73.25	1725
19	60	3	600	150	70.39	1753
20	60	5	600	90	79.82	1788
21	60	1	600	90	69.5	1776
22	80	2	800	120	63.57	1795
23	100	3	600	90	73.57	1753
24	60	3	200	90	77.55	1394
25	80	4	800	120	69.11	1810
26	60	3	1000	90	67.43	1852
27	20	3	600	90	70.05	1750
28	60	3	600	30	68.56	1759
29	40	4	800	120	72.83	1818
30	60	3	600	90	71.2	1780

Table 5

ANOVA for carbon yield of phosphoric acid modified RPSAC.

Source	Sum ofSquares	Df	MeanSquare	FValue	p-valueProb > F
Model	441.23	14	31.52	4.76	0.0024	significant
A-Concentration	0.79	1	0.79	0.12	0.7342
B-Impregnation Ratio	61.06	1	61.06	9.22	0.0083
C-Activation Temperature	204.28	1	204.28	30.86	<0.0001
D-Activation Time	0.011	1	0.011	1.702E-003	0.9676
AB	0.012	1	0.012	1.828E-003	0.9665
AC	61.23	1	61.23	9.25	0.0082
AD	3.48	1	3.48	0.53	0.4797
BC	9.64	1	9.64	1.46	0.2462
BD	6.58	1	6.58	0.99	0.3346
CD	2.53	1	2.53	0.38	0.5458
A2	26.85	1	26.85	4.06	0.0623
B2	2.10	1	2.10	0.32	0.5814
C2	18.41	1	18.41	2.78	0.1161
D2	67.88	1	67.88	10.25	0.0059
Residual	99.29	15	6.62
Lack of Fit	26.76	10	2.68	0.18	0.9886	not significant
Pure Error	72.53	5	14.51
Cor Total	540.52	29

Values of “Prob > F” <0.05 will implies that the model terms are significant.

Table 6

ANOVA for specific surface area of phosphoric acid modified RPSAC.

Source	Sum ofSquares	df	MeanSquare	FValue	p-valueProb > F
Model	1.483E+005	14	10590.68	4.30	0.0041	significant
A-Concentration	620.17	1	620.17	0.25	0.6231
B-Impregnation Ratio	308.17	1	308.17	0.13	0.7285
C-Activation Temperature	1.134E+005	1	1.134E+005	46.05	<0.0001
D-Activation Time	32.67	1	32.67	0.013	0.9098
AB	100.00	1	100.00	0.041	0.8430
AC	169.00	1	169.00	0.069	0.7969
AD	650.25	1	650.25	0.26	0.6149
BC	240.25	1	240.25	0.098	0.7591
BD	16.00	1	16.00	6.496E-003	0.9368
CD	16.00	1	16.00	6.496E-003	0.9368
A2	9.33	1	9.33	3.789E-003	0.9517
B2	1848.05	1	1848.05	0.75	0.4000
C2	27288.05	1	27288.05	11.08	0.0046
D2	80.05	1	80.05	0.032	0.8594
Residual	36946.58	15	2463.11
Lack of Fit	36165.25	10	3616.53	23.14	0.0014	significant
Pure Error	781.33	5	156.27
Cor Total	1.852E+005	29

Values of “Prob > F” <0.05 will implies that the model terms are significant.

Effects of synthesis condition on the carbon yield of RPSAC

The ANOVA for carbon yield presented in Table 5 implies that the model was significant with p-value of 0.0024 which is less than 0.05. It was noticed that the impregnation ratio, activation temperature, the interaction of concentration and temperature as well as the square of activation time were also significant factors in the process of producing RPSAC using phosphoric acid as stimulating agent. The factors had p-values of 0.0083, 0.0001, 0.0082 and 0.0059 respectively. Figure 1A presents the effect of activation temperature and impregnation ratio on carbon yield of phosphoric acid modified RPSAC at constant activation time (90 min) and activating agent concentration (60 %). Figure 1B elucidates the influence of activation time and activating agent concentration on carbon yield at constant levels of the temperature (600 °C) and impregnation ratio (3 g/mL).

Figure 1

(A) Effects of activation temperature and impregnation ratio on carbon yield at constant activation time (90 min) and activating agent concentration (60 %) (B) Effects of activation time and activating agent concentration on carbon yield at constant activation temperature (600°C) and impregnation ratio (3 g/mL), (C) Effects of activation temperature and impregnation ratio on Specific surface area at constant activation time (90 min) and activating agent concentration (60 %) (D) Effects of activation time and temperature on Specific surface area at constant concentration (60 %) and impregnation ratio (3 g.mL).

It was observed that carbon yield was lowered as the activation temperature and time were raised but did not change significantly as activating agent concentration or impregnation ratio was increased. The significant interaction between the concentration and temperature was more pronounced with higher activation temperature which resulted to a greater likelihood that the carbon was exposed to the vapor environment. This ultimately offered better prospects for the activation reaction to occur. It is expected that during activation, superior temperatures and prolonged reaction time provided room for more samples to react and more gasification to take place such that there was a corresponding increase in the ash content. However, it is pertinent to note that with further reactions taking place at higher temperatures and longer activation times, it was noticeable that more carbon (the desired product) would have been consumed and resulted to a decreased carbon yield.

Analogous inclinations were previously reported by (Chan et al., 2006) using HCL as activating agent and waste tires as precursor. Adel et al. (2010) also observed a similar trend while utilizing beans husk and rice husk together with HCL as the activation agent for activated carbon production. Similarly, Kwaghger and Ibrahim (2013) in their study also reported a similar trend while using mango nut as precursor and HCL as activating agent.

The observations depicted in Figure 1A suggests that activation temperature played a vital part in the production of RPSAC. The rise in temperature could discharge large levels of volatiles as a result of intensified dehydration and degradation reaction by H3PO4 as well as an increased C–H3PO4 reaction rates. The foregoing ultimately resulted to a decrease in carbon yield (Adinata et al., 2007; Lua and Yang, 2004). Furthermore, an increase in the reaction time and temperature accelerated the gasification reactions of carbon and consequently stroke the amorphous constituent which blocked the pores and initiated a decline in carbon yield.

In a systematic approach, the findings of the present study with respect to the influence of the studied process variables for carbon yield could be ascribed to the reaction of lignocelluloses with H3PO4, which occured instantaneously during the mixing of constituents. Initially, the acid attacks hemicelluloses and lignin owing to higher resistance of cellulose to acid hydrolysis. The acid then hydrolyzes the glycosidic links in the lignocellulosic materials and discontinues aryl ether bond in lignin. These reactions could occur along with further chemical conversions including condensation, reduction and dehydration (Sanni et al., 2017). A similar finding was described by Adel et al. (2010).

The predicted values versus Actual values plot (Figure 2A) shows nearly a straight line, which is an indication that the model has the ability to predict the response (carbon yield). The errors in the model were found to be insignificant, since the plot shows that the predicted values coincides with the actual values on the model line; therefore, the errors in the model were negligible.

Figure 2

Predicted and actual values plot for (A): Carbon yield, (B): Specific surface area.

The CCD in Response Surface Methodology for the current study also sort to relate activation temperature, activation time, activating agent concentration and chemical impregnation ratio and their interactions to develop second order regression model (equation) that can be employed to predict the carbon yield as a response as shown in Eq. (11). Therefore, Eq. (11) could be adopted in future studies to predict the yield of activated carbon obtained from raffia palm shells or other precursors of similar proximate and ultimate composition.

Effects of synthesis condition on the specific surface area of RPSAC

The ANOVA for the specific surface area in Table 6 implies that the model was important (p = 0.0041). Table 6 also reveals that the activation temperature and the square of activation temperature were highly substantial with P- values of 0.0001 and 0.0046 correspondingly. The increase in activation temperature to a certain threshold value enhances formation of micro-pores and higher surface area. On the other hand, an excessive increase in temperature leads to expansion of the existing micro-pores to form meso and macro pores on the surface of the activated carbon. This consequently led to a lower surface area of the AC (Ayyalusamy and Mishra, 2018).

The effects of the process factors on the specific surface area of RPSAC are elucidated in Figures 1C and 1D above. From Figures 1C – D, it was observed that the specific surface area enlarged with rise in temperature, impregnation ratio and activation time within the intervals considered in the current work. This was probably due to the acute increase of the carbon material when these variables were increased. This led to the formation of huge surface area on the adsorbent surface.

The findings from the present study could be attributed to the depolymerization of cellulose in the reactions that ensued as temperature increased. The foregoing was also responsible for tar development and deposition that would have normally blocked the pore spaces on the activated carbon surface and retarded surface area and pore volume. However, the deposited tar was consequently oxidized/dehydrated by the action of phosphoric acid (activating agent): an action that further enhanced the specific surface area and pore structure.

Activation temperature has substantial influence on the formation of permeable links in a carbon and capable of eliminating volatile and moisture constituents in the precursor to trigger the development of the pores. As evolution of volatiles during temperature-dependent activation processes in AC production indicates the buildup of the primary pore network, it was pertinent to restrict the activation temperature up to the intrinsic position. As stated earlier, this was because; elevated temperatures could promote the expansion of pores and undermine the specific surface area. For instance, Diao et al. (2002) reported a similar trend when sorghum-based biomass was used as a starting material for the production of AC at a temperature of 500 °C. They posited that micro-porous carbons with decreased surface area were obtained. They however reported high surface area of activated carbon at temperature of 600 °C. This finding was also corroborated by a recent study from Feng et al. (2020) who also reported enhanced surface area of the AC at elevated temperatures when phosphoric acid was used to transform corncob-based activated carbon. This shows that the studied temperature range in the current work were within the threshold values for which higher specific surface area will be obtained as activation temperatures were heightened.

Predicted versus actual value plot for specific surface area provide nearly a straight line as illustrated in Figure 2B above. This signifies that the developed model has the ability to predict the response (specific surface area) with negligible errors. The plot shows that the predicted values coincide with the actual values on the model line and indicates adequacy of the model.

Central Compost Design in the Response Surface Methodology relates activation temperatures, activation time, activating agent concentration and chemical impregnation ratio as well as their interactions to develop the second order regression model (equation) that can be used to calculate the specific surface area as a response (equation 12).

Coded factor-based model for specific surface area RPSAC

Optimization of synthesis conditions for production of RPSAC and validation

Generally, superior quality products are expected with respect to large surface area and carbon yield for effective adsorption capacity and economic viability in the production of commercial activated carbons (Kwaghger and Ibrahim, 2013). Hence, this study investigated the optimal process conditions at which the synthesized RPSAC provided a superior carbon yield and better specific surface area. It is noteworthy to mention that, it is a challenging task to improve these responses under similar experimental condition due to the diverse interest regions of the factors considered in this study. Succinctly, higher specific surface area and yield are both desirable at optimum conditions to offer ACs with superior adsorptive features.

In an attempt to balance the tradeoffs among the two dependent variables, the desirability function in Design-Expert software was applied to assess the best combination of the process factors that would give the best result for the responses (carbon yield and specific surface area). In this context, responses were set to be maximized as the objective function, while the values of the factors were pegged within the spans considered in this study. The experimental parameters with the superior desirability were chosen and utilized to synthesize the optimum phosphoric acid modified RPSAC.

According to the desirability function in Design Expert software, version 10, the optimum RPSAC was predicted to be obtained at 76.91%, 4 g/ml (1:4), 523.68 °C, 103.83 min for chemical concentration, impregnation ratio, activation temperature and activation time respectively. Predicted optimum RPSAC had a yield of 77.98% and specific surface area of 1762.93 m2/g as shown in Figure 3.

Figure 3

(A); Optimization of RPSAC Yield in terms of impregnation ratio and concentration at constant temperature (523.68°C) and activation time (103.83 min) (B); Optimization of Specific surface area of RPSAC in terms of activation temperature and time at constant impregnation ratio (4 g/mL) and activating agent concentration (76.91 %).

Through process modeling and optimization, raffia palm shells were demonstrated to be a suitable and efficient starting material for synthesis of RPSAC with large surface area and superior carbon yield as required. The predicted ideal conditions for the production of RPSAC were validated with experimentations at the suggested optimal factor levels. It was observed that the experimental RPSAC had a specific surface area of 1815.62 m2/g and a yield of 79.44%, which were very close to the predicted values as errors in prediction were found to be 2.90 and 1.84 % for specific surface area and yield respectively (Table 7).

Table 7

Predicted and actual optimum conditions for production of RPSAC.

Parameter	Predicted	Observed	Variation (%)
Temperature (°C)	523.68	523.68	0.00
Concentration (%)	76.91	76.91	0.00
Impregnation ratio (g/mL)	4.00	4.00	0.00
Activation Time (min)	103.83	103.83	0.00
Carbon Yield (%)	77.98	79.44	1.84
Specific Surface Area (m2/g)	1762.93	1815.62	2.90

Note: desirability = 0.845.

Characteristics of optimally produced adsorbent (RPSAC)

The optimally made RPSAC was further branded based on its BET-surface area, XRD, EDX, FTIR, pHpzc, SEM, bulk density, pH, moisture content, and porosity. Table 8 shows some of the characteristics of the novel adsorbent (RPSAC). The BET-surface area of RPSAC was found to be 456.10 m2/g. This was an indication that it had sufficient morphological composition for its utilization in environmental remediation. The BET-surface area obtained in the current study was comparable and, in most cases, higher than the values reported in other studies (Sarkar et al., 2020; Ndongo et al., 2020; Pallaras et al., 2018) for biomass-based activated carbons.

Table 8

Characteristics of the optimally synthesized RPSAC.

S/No	Parameter	RPSAC
1	Bulk Density (g/cm3)	0.45
2	pH	2.00
3	pHpzc	2.10
4	Moisture Content (%)	18.50
5	Multi-point BET Surface Area (m2/g)	456.10
6	DR Micro-pore Area (m2/g)	491.30
7	BJH Total Pore Volume (cc/g)	0.25
8	BJH Average Pore Diameter (nm)	2.13
9	Yield (%)	79.44
10	Pore size Distribution (μm)	100–300

Dampness level of RPSAC was found to be as high as 18.50%. The high moisture content was a confirmation of the amorphous and staggered micro-crystallinity of RPSAC which renders the adsorbent more hygroscopic (high likelihood to captivate dampness from the atmosphere). Notably this property of most activated carbons does not hinder the performances in the various systems where they are utilized (Bello et al., 2021; Inyinbor et al., 2016).

In the same vein, the bulk density, pH, pHpzc and particle size distribution of RPSAC was observed to be 0.45 g/cm3, 2.00, 2.10 and 0.1–0.3 mm respectively. The adsorbed nitrogen volume was noticed to rise with a surge in comparative stress (p/p0), signifying a broader pore spread of the material. The pH of point of zero charge (pHpzc) implies that RPSAC would be positively charged in a mixture of an acidic solution. This could enhance the uptake of negatively charged ion such as anionic dyes and fluoride in solution by electrostatic attraction mechanisms.

Other characteristics of RPSAC were found to be 2.13 nm, 0.25 cc/g and 79.44 % in terms of average pore diameter, total pore volume and yield respectively. The foregoing is a reflection of the meso/micro-porous composition and enhanced carbon levels of RPSAC: The high average pore diameter of RPSAC implies that both meso and micro pores were found on its surface. This portends the potential suitability of RPSAC as an effective adsorbent for the removal of small and large sized pollutant molecule in water.

The SEM image for RPSAC with elemental constituents is illustrated in Figure 4, and the EDX is illustrated in Figure 5. From Figure 4, it was found that the RPSAC composed mainly of meso/micro-pores. The EDX spectrum in elemental context showed that RPSAC comprised majorly of silicon, carbon, phosphorus, and oxygen with some traces of macro/micro elements such as potassium, sodium, calcium, iron, nitrogen, sulphur, aluminium etc (Figures 4 and 5).

Figure 4

SEM image of RPSAC with elemental composition.

Figure 5

EDX spectrum of RPSAC

The high phosphorous concentrations in RPSAC may be attributed to the residual activating agent (phosphoric acid) which may also be responsible for the moderately high surface area and lower carbon content of RPSAC. The high phosphorus content in RPSAC was however suspected to be beneficial as it aided in the formation of apatite and hydroxyapatite on the surface of the RPSAC (Figure 6). These compounds have previously been reported to be present in bone char and other natural adsorbents and are useful for the adsorption of fluoride and other anions from solution via ion exchange and precipitation mechanisms (Nijhawan et al., 2020; Nayak et al., 2017; Mourabet et al., 2015; Nie et al., 2012).

Figure 6

Crystallographic composition of RPSAC.

Results of FTIR analysis for RPSAC

The FTIR spectrum of RPSAC is presented in Figure 7A. It was noticed that there were over five (5) peaks in the FTIR band, which indicated that the surface of RPSAC contained multifaceted compounds (Nandiyanto et al., 2019). The strong peaks found on the surface of RPSAC at 3213 cm−1 was ascribed to stretching vibrations of O–H groups. The peaks at 2344–2117 cm−1 were due to C–H stretching vibrations. The FT IR band at 1990 cm−1 is designated to transition metal carbonyl stretch. The peak at 1625 cm−1 is allotted to stretching vibrations of the C=O bonds of esters, and acid. The band at 1121 cm−1 is assigned to stretching vibrations of the C–O bonds of alcohols and phenols. The peak at 969 cm−1 is due to aliphatic phosphates (P–O–C) stretch, while the moderate peak at 879 cm−1 is designated to the aromatic phosphate stretch.

Figure 7

(A): FTIR spectrum and (B): XRD Profile of RPSAC.

The observed peaks in the present study were in tandem with previous studies (Surip et al., 2020; Coates, 2000). For instance, Olugbenga et al. (2017) reported such peaks in their work on the removal of Rhodamine B dye from solution utilizing raw and phosphoric acid modified Moringa oliefera seed pod as adsorbents. Similarly, Okoye et al. (2018) also stated such peaks on salt activated Raphia hookeri seeds as bio-sorbent for Erythrosine B dye removal from aqueous solution. Jawad and Abdulhameed (2020b) also reported similar peaks on the surface of KOH modified bamboo activated carbon for methylene blue adsorption.

Results of XRD analysis for RPSAC

The X-ray powder diffraction (XRD) provides information about the chemical constituents, crystallographic structure, physical characteristics, and molecular/atomic compositions of materials and typically applied to provide insights on the crystal and phase structure of resources. The result of the XRD pattern for RPSAC is illustrated in Figure 7B above.

It was observed from Figure 7B, that RPSAC was amorphous in nature owing to a staggered degree of micro crystallinity which was attributed to the high quartz content in the base material as explained previously from the results of the EDX investigations. Three broad peaks were identified in the XRD configurations of RPSAC. These included the peaks at 2ɵ = 15.22, 23.27 and 26.73o. The peaks at around 23 and 26o match to the graphite framework and the one at 15o was credited to the multi-layer stacking of graphite-like micro-crystalline structures which was responsible for the micro-porous and micro-crystalline disposition of RPSAC (Iwar et al., 2021; Mopoung et al., 2015). The mineral composition of the RPSAC as a fallout of the XRD analysis was earlier shown (Figure 6). It can be seen that RPSAC was largely composed of apatite (46%) and hydroxyapatite which are proven compounds with high affinity towards fluoride ions in solution.

Conclusions

This study has shown that RSM is a suitable tool for modeling and optimization of process conditions for the production of RPSAC. The RSM showed that activation temperature, activation time and activating agent concentration and their interactions were the most influential factors affecting the specific surface area and yield of RPSAC. The optimal conditions for the production of RPSAC were predicted and confirmed to be obtained at 76.91%, 4.0 g/ml (1:4), 523.68 °C, 103.83 min of chemical concentration, impregnation ratio, activation temperature and activation time respectively, These factor levels were responsible for the production of a mesoporous/microporous RPSAC with high yield (77.98%), high BET-surface area (456.10 m2/g) and suitable functional groups for application in the removal of both large and small molecular sized compounds such as dyes and fluoride in water and wastewater. In the future, RPSAC could also be explored for the removal of small molecular sized pollutants such as fluoride or heavy metals in water and wastewater due to the presence of micro-pores and hydroxyl groups on its surface. Additionally, RPSAC could be further functionalized to produce composites for targeting specific pollutants in the environment. Such functionalization could involve coating the surface of RPSAC with metal (Al, Cu, Fe, Mn etc) oxides or nano materials.

Declarations

Author contribution statement

Raphael Terungwa Iwar: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Kola’ Ogedengbe: Analyzed and interpreted the data; Wrote the paper.

Kamil Kayode Katibi: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Linus Esekwe Oshido: Performed the experiments; Analyzed and interpreted the data.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data included in article/supplementary material/referenced in article.

Declaration of interests statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.