1. Introduction
Polycyclic aromatic hydrocarbons (PAHs) are considered highly toxic, as well as persistent, environmental contaminants. This group of compounds contains multiple groups of aromatic rings infused together, which have abilities to metabolize and yield some harmful derivatives [
1,
2]. Polycyclic aromatic hydrocarbons (PAHs) are volatile organic compounds. Consequently, these derivatives, when released into the environment without careful monitoring, can react with DNA to stimulate carcinogenic and mutagenic responses in living organisms [
1,
2]. Due to their persistent nature, high toxicity and easy mobility, the European Environmental Protection (EEA) Agency, as well as the World Health Organization, classified it as a priority pollutant that needs to be monitored strictly in any industrial effluents and recommended a safe, tolerable limit in water for public consumption [
3,
4]. In spite of these strict regulations, a lot of relevant studies have illustrated that the inadequate combustion of fossil fuels (mainly coke or coal) with the incineration of waste are the two most leading sources for emissions of PAHs in the air, soil and overall aquatic environment [
5,
6]. It was observed that around 20–30% of PAHs are emitted from power plants burning coal for energy [
7,
8]. It was estimated that coal burning per ton can emit 1.56 mg of PAHs into the environment, which is quite alarming [
7,
8]. Furnaces used for pulverizing coal can generate 8.8–16.3 μg·m
3 [
9]. The application of activated carbon with requisite properties for the adsorption of PAHs from aqueous effluents is considered one of the most feasible techniques for its cost effectiveness, simple operational features with regeneration efficiencies [
3,
10]. In this research, naphthalene (C
10H
8) was chosen as an adsorbate that is a representative of PAHs and is widely found in waste aqueous stream due to careless industrial discharge.
The imminent shortage of nonrenewable fossil fuels has been levied to search substitute materials to produce nano and microporous carbons suitable for the adsorption of persistent organic, especially nonpolar, volatile aromatic compounds from waste streams. Lignocellulosic biomass residues (LSM) are ecofriendly, renewable—thus, cost-effective—sustainable, greener resources to generate high-quality carbon, dependent on the appropriate selection of process parameters for production [
11,
12,
13,
14]. Hence, LSM residues have been widely used to produce activated carbon for environmental remediation for the last few decades. However, the complete mechanism of volatile aromatic compound (PAH) adsorption onto the surface of the lignocellulosic substrate (LSM) itself or the activated carbon derived from it is, to some extent, yet uncertain [
3,
11]. Inspite of the growing trend for exploiting LSM residues to yield valuable solid, liquid and gaseous chemicals, this impending resource is sometimes not utilized properly or even dumped as solid waste to decompose and burnt in certain developing countries [
10]. The thermochemical conversion or pyrolysis of LSM wastes can yield activated carbon with effective physiochemical properties for versatile applications [
11]. To date, a lot of research has been carried out to produce carbon adsorbent from LSM substrates. Earlier, date stone was activated to obtain activated carbon by the steam activation technique where the surface area of the prepared carbon reached up to 635 m
2/g [
12]. Some research has demonstrated that the activation process using one-step CO
2 gas can produce carbon with micropores, while, in two-step activation processes, using CO
2 can produce carbon having a mixture of micro and mesopores [
10,
13,
14,
15,
16].
To date, different types of LSM waste residues, such as corn cob [
17], apricot stone [
18,
19], date pits [
20], peach stone [
21], corn stover [
22], coconut shell [
23], grape seed [
24], cherry stone [
25], rice husk [
26], etc., have been converted to carbon, having suitable porous textures. However, the application of different types of chemicals as an activating agent can significantly change the overall qualities of carbon [
27,
28]. The organic complex present in LSM waste can produce tar, which can cause blockage of the pores during the activation process. The application of acidic or basic types of agents can promote pore formations, leading to enlarged surface areas. This will help in dehydration reactions and initiate the burning of tars to yield carbon with desirable properties [
29,
30]. PAHs have high molecular weights with larger size molecular dimensions [
31]. Therefore, it can be easily understood that activated carbon with an adequate porous texture would be useful for the adsorptive removal of PAHs from an aqueous stream. LSM precursors having high cellulosic contents usually give activated carbon with a microporous texture, which is suitable for the adsorption of gaseous molecules, as well as heavy metals, having smaller ionic sizes. However, the starting materials having higher lignin contents will produce activated carbon containing mesopores inside them [
32,
33,
34]. Thus, process variables like the pyrolysis temperature, gas flow rate, time, ratio and chemical activation agent for impregnation should be carefully monitored. In this research, preliminary studies were performed to optimize the temperature, time and impregnation ratio using zinc acetate to ensure the maximum adsorption uptake of the adsorbate naphthalene (C
10H
8) molecules. In this research, date stone-based activated carbon was prepared using zinc acetate as an activating agent, which is comparatively less harsh, a weak Lewis acid, than other inorganic strong Lewis acids of H
3PO
4, H
2SO
4 or concentrated Lewis base solutions like KOH or NaOH. The ratio of date seed powder with zinc acetate salt was kept to a minimum at 2:1 to avoid corrosion inside the pyrolysis equipment. Batch adsorption for the removal of naphthalene (C
10H
8) was carried out using both LSM wastes of date seed powder (DSP) and its activated carbon (CDSP). The reaction kinetics in terms of the pseudo-first and second orders were evaluated. The mechanism of adsorption was determined using the intraparticle diffusion model. The equilibrium isotherm model parameters were determined. The physiochemical characterizations of both the DSP and CDSP samples were carried out in terms of a Fourier-transform emission scanning electron microscope (FESEM), ultimate analysis, BET surface area and FTIR analysis. The findings of this study showed that weak Lewis acid-based chemical activation using zinc acetate dehydrate salt (Zn(CH
3CO
2)
2·2H
2O]) is suitable to develop porous carbon, which can entrap naphthalene (C
10H
8) molecules from wastewater.
4. Conclusions
In this research, date seeds (DSP) were used to prepare activated carbon (CDSP) using the one-step chemical activation method. The activation process used here did not use conventional harsh Lewis acids like H2SO4 or H3PO4 or a strong Lewis Base like NaOH, LiOH or KOH. The activation process was carried out using weak Lewis acids of zinc acetate dihydrate salt (Zn(CH3CO2)2·2H2O) in the presence of nitrogen gas. The surface area enhanced drastically after the chemical activation process using Zn(CH3CO2)2·2H2O. Based on the BET isotherm shape, it was concluded that the activated sample is mainly microporous in texture, with a certain proportion of mesopores. Due to the activating effect of weak Lewis acids of zinc acetate dihydrate salt (Zn(CH3CO2)2·2H2O) at higher temperatures, some micropore walls may disintegrate, leading to the formation of mesopores. The sorption performance of both the adsorbent (raw DSP and CDSP) materials was compared for the removal of naphthalene (C10H8) from synthetic wastewater. The prepared activated sample (CDSP) showed a higher sorption performance, rather than the DSP sample, for the removal of naphthalene (C10H8) from a synthetic aqueous solution. At room temperature (30 °C), the equilibrium isotherm data was fitted with linear regression equations of the Langmuir, Freundlich and Temkin isotherm models. A comparatively higher correlation coefficient, R2, was obtained for the Langmuir isotherm model for the adsorbate (C10H8) for adsorption onto the DSP and CDSP samples, which indicated homogeneous surface sites with monolayer sorption processes. The exponent, n, obtained from the Freundlich process was below 1 for adsorption onto the DSP and CDSP samples, indicating a favorable sorption process. The equilibrium data followed the pseudo-second order and Elovich kinetic models. This indicated that the chemisorption process took place for naphthalene (C10H8) onto the surfaces of the DSP and CDSP samples. The research findings indicated that the unactivated raw DSP and activated CDSP have great potential for the removal of a preselected poly aromatic hydrocarbon (naphthalene, C10H8) from a polluted aqueous stream.