Synthesis of Cost-Effective Pomelo Peel Dimethoxydiphenylsilane-Derived Materials for Pyrene Adsorption: From Surface Properties to Adsorption Mechanisms.

This study investigated the adsorption behaviors of pyrene (PYR) on a pomelo peel adsorbent (PPA), biochar (PPB), and H3PO4-modified (HPP), NaOH-activated (NPP), and dimethoxydiphenylsilane-treated (DPDMS-NPP) pomelo peel materials. SEM, FTIR, and elemental analyses of DPDMS-NPP's surface structure showed that the material was characterized by a well-developed porous structure, a large specific surface area (698.52 m2 g-1), and an abundance of phenyl functional groups. These properties enhance the PYR adsorption performance of DPDMS-NPP. Experimental results indicated that the adsorption capacity of DPDMS-NPP was significantly affected by the amount of material used and the initial concentration of PYR. Kinetic assessments suggested that PYR adsorption on PPA, NPP, and DPDMS-NPP could be accurately described by the pseudo second-order model. The adsorption process was controlled by several mechanisms, including electron donor-acceptor (EDA), electrostatic, and π-π interactions as well as film and intraparticle diffusion. The adsorption isotherm studies showed that PYR adsorption on DPDMS-NPP and PPA was well described by the Langmuir model and the maximum Langmuir adsorption capacity of DPDMS-NPP was 531.9 μg g-1. Overall, the results presented herein suggested that the use of DPDMS-NPP adsorbents constitutes an economic and environmentally friendly approach for the mitigation of PYR contamination risks.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, anthracene, phenanthrene, and pyrene (RYR) are classified as toxic organic pollutants that are widely distributed in water bodies and soil.[1−3] PYR, a hydrophobic PAH, is known for its carcinogenic, teratogenic, and mutagenic activity in humans.[4,5] As such, it has been identified by the U.S. Environmental Protection Agency as one of the precedence-controlled contaminants.[6] Knowing that toxic PYR has been detected at relatively high concentrations in water and various effluents,[7,8] it is essential to develop effective methods for the elimination of this compound in aqueous solutions.

Microbial degradation, supercritical oxidation, photooxidative degradation, and chemical oxidation degradation are some of the methods that have been used to remove PYR from water.[9,10] In general, these methods have been shown to be effective in reducing the concentrations of PYR in water; however, their efficiency could be further improved.[11] Compared to other available methods, biochar adsorption is one of the most promising techniques for PYR removal due to its accessibility, cost-effectiveness, high efficiency, and complete harmlessness to the environment.[12,13] The good adsorption performance of biochar is attributed to the presence of various functional groups that are distributed over a vast and complex surface structure.[14] According to Amstaetter et al., the PYR adsorption capacity of biochar is greater than that of coal-based activated carbon materials, mainly owing to the pure ingredients and developed pore structures of the former.[15] Chen et al. reported that biochar materials characterized by relatively large coefficients of normalized carbon distribution are more likely to react with PAH compounds that have high octanol–water partitioning coefficients than with other PAHs.[16] Moreover, the strong hydrophobicity of the biochar surface enhances its naphthalene adsorption capacity.[17] In addition to being environmentally friendly, biochar is a new type of highly effective adsorbent material that can be simply prepared by activation modification.[18] Moreover, in order to enhance the adsorption capacity of biochar, some modifications of biochar have been investigated such as steam activation, acid treatments, alkali treatments, oxidized treatment, and supercritical technology.[19−22] Knowing that biochar and graphite are equally effective in adsorbing various organic compounds, their structures are expected to be similar. Therefore, biochar, like graphite, possesses a layered structure that allows it to bind to the benzene rings of organic compounds via π–π interactions.[23] The adsorption performance of biochar can thus be enhanced simply by increasing the content of fused aromatic hydrocarbons;[24] however, more extensive research is needed to confirm and explore this hypothesis.

Recently, various biochars derived from agricultural and forestry waste (wheat straw, sawdust, and orange peel) as well as from animal excrement (cow, chicken, and pig) have been successfully applied in the treatment and remediation of soil and water environments.[25−28] In particular, pomelo peel has shown great potential for the removal of PYR from contaminated water samples. The developed pore structures of cellulose and lignin, two of the main components of pomelo peel, render this agricultural waste an effective and environmentally friendly adsorbent.[29] According to incomplete statistics, the utilization rate of agricultural and forestry waste in the world is less than 2%, and the amount of utilized pomelo peel is even less than that.[30] Domestically discarded pomelo peel constitutes a wasted resource that could alternatively be used as an advanced biomass composite.[31] However, research regarding the potential uses, including PYR adsorptivity, of pomelo peel is scarce. Consequently, more studies are needed to determine the detailed characteristics and mechanisms of PYR adsorption in activated biochar derived from pomelo peel.

This study investigated the PYR removal efficiencies of different materials derived from pomelo peel. In addition to being directly assessed as an adsorbent (PPA), pomelo peel was used to prepare pomelo peel biochar (PPB), H3PO4-modified biochar (HPP), NaOH-activated biochar (NPP), and dimethoxydiphenylsilane-treated biochar (DPDMS-NPP). The physicochemical properties (elemental composition, specific surface area, and functional groups) and adsorption characteristics of these materials were experimentally evaluated using a variety of analytical techniques. Isotherms and kinetic models were used to elucidate the mechanisms of PYR adsorption on PPA, NPP, and DPDMS-NPP.

Results and Discussion

Characterization of PPA, PPB, HPP, NPP, and DPDMS-NPP

The surface properties and interior structures of PPA, PPB, HPP, NPP, and DPDMS-NPP were analyzed using scanning electron microscopy (SEM). As shown in Figure , PPA is characterized by a flat surface with no obvious pore structure. Comparatively, the surface of PPB is relatively uneven, and its flat structure seems to be collapsed. However, the biomass skeleton structure of PPA is mostly maintained after pyrolysis, probably due to the presence of pectin, cellulose, and hemicellulose as major components in PPA.[32] Unlike PPB, NPP exhibits an obvious porous structure, which indicates that alkali treatment promotes the development of pores and stabilizes the distribution of small particles on the NPP surface. This is attributed to the effect of NaOH in accelerating the dissolution of cellulose and hemicellulose, which leads to the etching of the biomass skeleton structure and ultimately the formation of well-developed pores.[33] Moreover, NaOH treatment promotes the oxidation of PPB carbon, resulting in the evolution of CO2 and generation of many pores.[34] As for DPDMS-NPP, its surface features are clearly similar to those of NPP as both are characterized by highly porous structures with numerous small particles distributed on the external surface of large particles (observed at higher magnification). This suggests that the hydrothermal activation treatment of NPP does not have a significant effect on the surface morphology of the adsorbent and variations in the adsorption performances of NPP and DPDMS-NPP are due to other factors. Finally, the surface of HPP is obviously rough, compared to that of PPB, and it exhibits many fractured channels. Such observations may be ascribed to the water dissolution of some hydrosoluble metallic compounds in PPB during the acid-impregnation process, which alters the flat skeletal structure by forming loose channels that offer more adsorption sites.[35] The difference between the surface structures of NPP and HPP is quite remarkable. Based on SEM images, the former has many micropores, while the latter does not. This is consistent with the results of BET surface analysis.

Figure 1

SEM micrographs of (a, b) PPA, (c, d) PPB, (e, f) NPP, (g, h) HPP, and (i, j) DPDMS-NPP.

As shown in Table , the estimated pore volumes of NPP and DPDMS-NPP (0.36 and 0.32 cm3 g–1, respectively) are much greater than those of PPB (0.04 cm3 g–1). Furthermore, the specific surface area of PPB increases from 76.24 m2 g–1 to 726.79 (NPP) and 236.35 m2 g–1 (HPP) upon alkali treatment and acid modification, respectively. As for NPP and DPDMS-NPP, their specific surface areas are three times larger than those of HPP, resulting in a greater number of adsorption sites. Therefore, it is expected that NPP and DPDMS-NPP should be more suitable for PYR adsorption than HPP.

Table 1

Elemental, BET, and Pore Parameters Analysis of PPA, PPB, HPP, NPP, and DPDMS-NPP

	elemental analysis (% mass)							BET and pore parameters analysis
sample	C	N	H	O	Si	O/C	H/C	SBET (m2 g–1)	Vtotal (cm3 g–1)	Vmicro (cm3 g–1)	pore size (nm)
PPA	46.58	1.98	5.82	41.86		0.8987	0.1249	2.26
PPB	52.16	1.06	2.92	26.25		0.5033	0.0559	76.24	0.04	0.02	6.76
NPP	76.85	2.96	1.06	13.69		0.1781	0.0138	726.79	0.36	0.19	2.23
HPP	72.62	2.68	4.52	15.63		0.2152	0.0622	236.35	0.09	0.04	2.03
DPDMS-NPP	80.21	0.98	1.48	15.36	1.62	0.1915	0.0185	698.52	0.32	0.15	1.91

The elemental compositions of PPA, PPB, HPP, NPP, and DPDMS-NPP are also listed in Table . Apparently, acid modification and alkali activation increase the carbon (C) content in PPB from 52.16% to 72.62 and 76.85%, respectively. Similarly, the nitrogen (N) content in HPP (2.68%) and NPP (2.96%) is significantly larger than that in PPB (1.06%). Remarkably, the amount of hydrogen (H) increases slightly after acid treatment; however, it decreases from 2.92 to 1.06% upon activation with the NaOH base. The O/C and H/C ratios, defined as the polarity coefficient and aromaticity indicator,[36,37] respectively, significantly decrease (from 0.5033 and 0.0559 to 0.1781 and 0.0138, respectively) under the effect of alkali activation. Acid modification produces a similar effect in reducing the ratios of O/C and H/C. This indicates that both acidic and basic conditions promote the carbonization and hydrophobicity of PPB, thereby enhancing its activation. Overall, the results suggest that the elemental composition of PPB is somewhat affected by acid modification and alkali activation and carbonization processes increase material hydrophobicity by reducing the content of oxygen-containing polar functional groups. Compared to NPP, DPDMS-NPP contains higher amounts of carbon and silicon but lower amounts of other elements, especially nitrogen. Thus, the aromaticity (C/O ratio) of DPDMS-NPP is greater than that of NPP, which indicates successful adherence of DPDMS groups on the biochar surface of NPP. This is further confirmed by FT-IR analysis.

The identification of functional groups in PPA, PPB, HPP, NPP, and DPDMS-NPP facilitates the elucidation of PYR adsorption mechanisms.[38] The Fourier transform infrared (FTIR) spectra presented in Figure show that all investigated materials exhibit absorption bands at approximately 2930, 2365, 1549, and 776 cm–1, corresponding to the stretching vibrations of C–H (sp3-hybridized carbon), P–H/O–H (organic phosphorous or carboxylic acid groups), C=C (aromatic ring), and C–H (sp2-hybridized carbon).[39−41] However, the peak intensities observed in PPB, HPP, NPP, and DPDMS-NPP spectra are generally weaker than those recorded for PPA. In fact, certain absorption peaks of PPA, particularly the one observed at 3316 cm–1 (stretching vibration of O–H bonds in alcohol and phenol functional groups),[42] disappear upon pyrolysis and acid/base treatment. Concurrently, new peaks appear in the spectra of PPB, HPP, NPP, and DPDMS-NPP materials, such as the one recorded at 918 cm–1 (C–C and P–O stretching).[43] These spectral differences indicate that pyrolysis activation eliminates some oxygenated functional groups while promoting the incorporation of phosphorylated groups. Acid modification and alkali activation also produce noteworthy alterations in the chemical structure of PPB. This is evident in the spectral changes observed at 3656 (symmetric stretching of N–H) and 1509 cm–1 (C=C stretching of aromatic ring).[44] Finally, unlike NPP, the FTIR spectrum of DPDMS-NPP presents characteristic absorption peaks at 1760, 1318, 1051, and 851 cm–1, corresponding to the stretching vibration of C=O in carbonyls groups, bending vibration of C–H (sp3-hybridized carbon), stretching vibrations of Si–O bonds, and bending vibration of C–H (aromatic ring), respectively.[43] This confirms the effective attachment of DPDMS onto the NPP surface by a hydrothermal reaction.

Figure 2

FTIR spectra of PPA, PPB, HPP, NPP, and DPDMS-NPP.

Adsorption Capacities of PPA, PPB, HPP, NPP, and DPDMS-NPP

As shown in Figure a,b, the adsorption capacity and removal efficiency of PPA are lower than those of PPB, HPP, NPP, and DPDMS-NPP. This is probably due to the undeveloped pore structure of the original non-treated material (PPA), as demonstrated by SEM and BET surface analyses. The effects of acid and base treatment in increasing the specific surface area and removing ash are enhanced,[45] and the PYR elimination efficiency of HPP and NPP increased by 404.05 and 468.54%, respectively. Alkali activation promotes the oxidation of carbon and accelerates the dissolution of organic matter during soaking, which significantly alters the surface functional groups and interior structure of PPB.[46] This enhances the material’s adsorption capacity to a great extent, even more so than acid treatment. Among all investigated materials, DPDMS-NPP exhibits the highest PYR adsorption capacity and removal efficiency, probably due to the fact that it has the largest content of aromatic carbon. In general, benzene rings in biochar materials provide sites for adsorption via π–π interactions with aromatic organic compounds.[47] Thus, the DPDMS present on the surface of DPDMS-NPP may easily interact with the benzene rings of PYR, resulting in more efficient adsorption.

Figure 3

(a) Adsorption capacities and (b) removal rate of different materials (PPA, PPB, HPP, NPP, and DPDMS-NPP).

Effects of the PYR Initial Concentration and Adsorbent Dosage on the Adsorption Capacities of PPA, NPP, and DPDMS-NPP

The amount of adsorbent used in experiments is one of the main factors affecting PYR removal efficiency.[48] The results presented in Figure a indicate that the PPA dosage has little effect on adsorption performance (the removal rate is almost constant). However, the PYR removal efficiencies and adsorption capacities of NPP and DPDMS-NPP are significantly influenced by dosage. The rate of PYR elimination by DPDMS-NPP increases sharply with increasing material concentrations between 4 and 14 g L–1. It may be suggested that the enhanced adsorption performance at higher DPDMS-NPP concentrations is due to the availability of a greater number of adsorption sites. However, the analyses indicate that when DPDMS-NPP dosage rises from 8 to 14 g L–1, the adsorption capacity of the material decreases from 410.65 to 339.55 μg g–1. Thus, it is concluded that the overcrowding of the adsorbent at higher concentrations deactivates the adsorption sites, particularly those with relatively high energies.[49] It can be seen that PPA, NPP, and DPDMS-NPP exhibit the highest PYR adsorption capacity when the dosage of adsorbents was 8 g L–1. Therefore, the appropriate dosage of adsorbents was defined with 8 g L–1 (0.2 g) during the isotherm and kinetic studies.

Figure 4

Effect of the experimental parameters on PYR adsorption by PPA, NPP, and DPDMS-NPP: (a) adsorbent dosage effect and (b) initial concentration effect.

Knowing that the design of wastewater treatment systems significantly depends on the initial concentration of the targeted contaminant,[50] the effect of this parameter on PYR adsorption capacity was also assessed. As shown in Figure b, the adsorption capacities of PPA, NPP, and DPDMS-NPP are enhanced by 38.47, 41.49, and 57.18%, respectively, upon increasing the initial concentration of PYR from 2.4 to 6.0 μg mL–1. The change in adsorption capacity may be attributed to the effect of varying initial concentrations in altering steric hindrance and electrostatic repulsion interactions.[51] The obtained results also demonstrate that increasing the PYR initial concentrations in the range of 1.6–10 μg mL–1 reduces the removal efficiency of this PAH by NPP and DPDMS-NPP materials. This may be ascribed to the space resistance created by the growing amounts of PYR molecules in solution. It should be noted that PYR initial concentration (1.6–10 μg mL–1) does not appreciably affect the removal rate by PPA. Therefore, this material is only suitable for the pretreatment of low-concentration PYR contaminated solutions.

Adsorption Kinetics

Research regarding adsorption kinetics is essential for the development of wastewater treatment systems as it provides significant information concerning the adsorption mechanism and rate-limiting step.[52] In this study, the pseudo-first-order (eq ), pseudo-second-order (eq ), intraparticle diffusion (eq ), and Weber–Morris adsorption diffusion (eq ) models were used to simulate the kinetics of PYR adsorption on PPA, NPP, and DPDMS-NPP. The equations involved in the calculation are as follows.[53,54]Qe (μg g–1) and Q (μg g–1) represent the adsorption capacity of the material at equilibrium and at time t (h), respectively, whereas k1 (h–1), k2 (μg g–1 h–1), kp (μg g–1 h1/2), and k3 (h–1) represent the rate constants of the pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Weber–Morris adsorption diffusion models, respectively. Besides, c (μg g–1) represents the thickness of the boundary layer.

Figure a,b presents the linear fits of eqs and 2, respectively. The rate constant and linear regression coefficient (R2) values obtained for each adsorbent are listed in Table . The obtained results indicate that the process of PYR adsorption on PPA, NPP, or DPDMS-NPP takes place in two steps: a relatively rapid initial adsorption step during the first 16 h followed by a much slower step that lasts until reaching equilibrium at approximately 30 h.

Figure 5

Kinetics of PYR adsorption on PPA, NPP, and DPDMS-NPP by fitting (a) pseudo-first-order, (b) pseudo-second-order, (c) Weber–Moris intraparticle diffusion, and (d) Weber–Morris adsorption diffusion models.

Table 2

Parameters of the Pseudo-First-Order Kinetic Model, Pseudo-Second-Order Kinetics Model, Intraparticle Diffusion Model, Weber–Morris Adsorption Diffusion Model, and Langmuir, Freundlich, and Temkin for PYR Adsorption onto NPP, PPA, and DPDMS-NPP

models	parameters	NPP	PPA	DPDMS-NPP
pseudo-first-order model	Qe (μg g–1)	188.65	37.116	275.19
	k1 (h–1)	0.128	0.1796	0.1479
	R2	0.9619	0.9522	0.9011
pseudo-second-order model	Qe (μg g–1)	316.46	54.495	448.43
	k2 (g μg–1 h–1)	0.00113	0.00969	0.000689
	R2	0.9985	0.9994	0.9962
intraparticle diffusion model	kp (μg g–1 h1/2)	80.571	11.369	116.67
	R2	0.9295	0.8471	0.9642
Weber–Morris adsorption diffusion model	k3 (h–1)	0.12803	0.17985	0.14797
	R2	0.9619	0.9474	0.8913
Langmuir	qm (μg g–1)	285.71	77.279	531.91
	k (mL μg–1)	4.0698	0.4259	2.5066
	R2	0.9668	0.9959	0.9829
Freundlich	KF (μg 1 – 1/n mL1/n g–1)	201.79	24.059	294.48
	n	3.2544	2.1833	3.1991
	R2	0.9372	0.9359	0.7428
Temkin	bT	43.366	155.33	33.604
	KT (mL g–1)	42.854	4.5016	70.302
	R2	0.9836	0.9946	0.8696

Based on the values of the linear regression coefficient, the pseudo-second-order model (R2 = 0.9977, 0.9999, and 0.996) fits the adsorption kinetic data better than the pseudo-first-order model (R2 = 0.9619, 0.9522, and 0.9011). The similarity between the experimentally determined rate constant values and those calculated using the pseudo-second-order kinetic model further confirms the suitability of this model. This result is similar to the previous kinetic results obtained for various adsorbent–adsorbate systems.[55,56] Comparatively, the kinetic parameters estimated based on the pseudo-first-order kinetic model were found to be consistently lower than those derived experimentally (Qexp). This is consistent with the results reported by Li et al.[57]

Figure c,d presents the linear fits of eqs and 4, respectively. The plot of Q versus t1/2 is a straight line that passes through the origin when the adsorption process is controlled only by intraparticle diffusion, but if it does not pass through the origin, the adsorption process is controlled by several diffusion mechanisms.[58] It can be seen that the regression linear curves during two stages failed to cross the origin and the plots did not pass through the origin, indicating that intraparticle diffusion could not be considered as the only step to control the rate during the sorption process. Besides, the plot of ln(1 – Q/Qe) versus t should give a straight line with a slope of −k3 if the film diffusion is the rate-limiting step.[59] The results of the Weber–Morris adsorption diffusion model are shown in Table . As it can be seen in Figure d, the plot is linear, and it was concluded that film diffusion plays an important role in the adsorption process.

Overall, our results indicate that PYR adsorption on PPA, NPP, and DPDMS-NPP is controlled by several mechanisms, including hydrophobic interactions, covalent bonding, film diffusion, and intraparticle diffusion.[60] Moreover, PPA presents a relatively higher adsorption rate constant (k2) than that of NPP and DPDMS-NPP, which implies that PYR adsorbs more quickly on PPA than on the modified materials.[61,62]

Adsorption Isotherms

In general, adsorption isotherms reflect the relationship between the adsorbent and adsorbate. In this study, we used the Langmuir, Freundlich, and Temkin models (eqs –7, respectively) to describe the isotherms of PYR adsorption on PPA, NPP, and DPDMS-NPP.[63−65]Qe (μg g–1) and Qm (μg g–1) represent the equilibrium and maximum adsorption capacities, respectively; Ce is the equilibrium concentration of PYR (μg mL–1); k (mL μg–1) is the Langmuir constant; KF (μg 1 – 1/ mL1/ g–1) and 1/n are the Freundlich constants related to adsorption capacity and energy heterogeneity (intensity of the adsorption), respectively; b is the Temkin constant related to the heat of adsorption; and KT (mL g–1) is the equilibrium bond constant related to the maximum energy of bonding.

As shown in Figure , the isotherms of PYR adsorption on PPA, NPP, and DPDMS-NPP all have the inverted “L” shape typically observed for biochar adsorbents. Beyond specific equilibrium concentration values (between 3 and 5 μg mL–1), the adsorption capacities no longer change, as evidenced by the emergence of isothermal plateaus. However, the removal rates of PYR by PPA, NPP, and DPDMS-NPP decrease continuously with increasing PYR equilibrium concentrations. The isotherms illustrated in Figure also show that DPDMS-NPP adsorbs approximately eight times more PYR than PPA, irrespective of the concentration. This suggests that the adsorption capacity of the former is greater than that of the latter.

Figure 6

(a) Adsorption isotherm experimental data. (b) Langmuir isotherm model, (c) Freundlich model, and (d) Temkin model isothermal fittings for PYR adsorption on PPA, NPP, and DPDMS-NPP.

The values listed in Table indicate that the Langmuir model fits the DPDMS-NPP adsorption data slightly better than Freundlich or Temkin models. Therefore, it may be concluded that the interactions between PYR and DPDMS-NPP are best described as monolayer adsorption on a homogeneous surface.[66] In the literature, a similar isotherm model fitting has been obtained for the adsorption isotherms of various pollutants onto different adsorbents.[67,68] The applicability of the Langmuir model may be attributed to the planar geometry of PYR, which minimizes space resistance and promotes adsorption.[69] This is consistent with the characterization and kinetic results discussed previously. The values of the Freundlich constant (1/n < 1) and the Temkin transformation energy of adsorption (33.604 < bT < 43.366) determined herein confirm that PYR adsorption on PPA, NPP, and DPDMS-NPP surfaces is favorable and exothermic.[70] Based on the parameters of the Langmuir model, the maximum PYR adsorption capacities of PPA, NPP, and DPDMS-NPP are 77.279, 285.71, and 531.91 μg g–1, respectively. Previously, it has been reported that the maximum adsorption capacities of biochar were 10.1 and 187.27 μg g–1,[71,72] which are much less than the value determined in this study.

Thermodynamic Studies

The thermodynamic parameters of DPDMS-NPP, the material with the highest PYR adsorption capacity, were calculated according to eqs and 9. These parameters include free energy change (ΔG°), entropy change (ΔS°), and enthalpy change (ΔH°).[73,74]T, R, and k0 refer to the adsorption temperature (K), gas constant (8.314 J mol–1 K–1), and thermodynamic equilibrium constant, respectively. The values of k0 can be acquired by plotting ln(Qe/Ce) versus Qe and by extrapolating Qe to zero.

The results summarized in Figure and Table show that, for temperatures between 293 and 310 K, the values of ΔG° are negative. This indicates that PYR adsorption on DPDMS-NPP is a spontaneous and favorable process. The minimal variations in the values calculated at 293, 303, and 310 K show that temperature has an insignificant effect on PYR adsorption. The values of ΔH° were also found to be negative, which means that the process of PYR adsorption on DPDMS-NPP is exothermic (similar to the Temkin model results). According to Dula, ΔH° values in the range of 0 to −20 kJ mol–1 are indicative of physical adsorption via van der Waals interactions.[75] Therefore, with a ΔH° value of −12.835 kJ mol–1, we may say that PYR is physically adsorbed on DPDMS-NPP. Finally, the positive value of ΔS° (0.06098 kJ mol–1) indicates an increase in randomness at the solid/solution interface upon PYR adsorption, meaning that the process is also entropically favored.[76]

Figure 7

(a) Plots of ln Q/Ce vs Q at various temperatures and (b) the plot of R × ln(Q/Ce) vs 1/T.

Table 3

Thermodynamic Parameters for PYR Adsorption onto DPDMS-NPP

adsorbent	T (K)	ΔG° (kJ mol–1)	ΔH° (kJ mol–1)	ΔS° (kJ mol–1)
DPDMS-NPP	293	–5.016	–12.835	0.06098
	303	–5.697
	310	–6.051

Regeneration of DPDMS-NPP

The evaluation of regeneration of adsorbents is vital for the selection in their potential applications. Thermal regeneration is rapid and one of the most effective methods to achieve desorption.[77] After adsorption stage, the DPDMS-NPP was separated from the solution by filtration with 0.45 μm membrane filters and washed with cyclohexane several times. Then, DPDMS-NPP was regenerated at 200 °C for 2 h and weighted for the next reuse cycle. Figure shows the recycling of DPDMS-NPP in the removal of PYR. It could be seen that the DPDMS-NPP possessed more than 39% adsorption capacities for PYR after the first cycle. This may be because the oxidizing gases reacted with PAHs during thermal activation, resulting in an increase in active sites on the surface of biochar.[71] After three cycles, the adsorption capacity for PYR was 106 μg g–1. The decrease in the adsorption capacity was caused by the changes in the physical properties of the biochar after repeated high-temperature treatment.

Figure 8

Reusability of DPDMS-NPP for PYR adsorption.

Possible Adsorption Mechanisms

The adsorption of PYR on pomelo peel-derived materials is a complicated process involving numerous interactions, such as the EDA interaction between the carbonyl groups (electron donors) of DPDMS-NPP and the aromatic system of PYR (electron acceptor).[72] The carbonyl groups identified on the DPDMS-NPP surface using FTIR spectroscopy constitute active sites for PYR adsorption via EDA interactions. Based on the thermodynamic results reported herein, physical interactions, such as van der Waals dispersion, dipole/induced dipole, quadrapole, and π–π interactions, are also implicated in the adsorption of PYR on DPDMS-NPP. The π–π physisorption mechanism is particularly important in that it controls the packing or assembly of compounds in the material.[78] In the absence of polar interactions, the aromatic system of planar PYR molecules will inevitably exhibit strong π–π interactions with the benzene rings of DPDMS. Such interactions usually occur in face-to-face, offset stacking, and/or edge-to-face stacking ring arrangements.[79] Moreover, kinetic assessments show that the mechanism of PYR adsorption involves hydrophobic interactions, film diffusion, and intraparticle diffusion.

Conclusions

This study proposes a new method for the preparation of biochar-based adsorbent materials derived from pomelo peel biowaste (PPA, PPB, NPP, HPP, and DPDMS-NPP). The prepared materials are characterized by low costs and high removal efficiency of PYR from water systems. To the best of our knowledge, this study is the first to report the PYR adsorption properties of PPA, PPB, NPP, HPP, and DPDMS-NPP. Moreover, unlike previous studies, the methods of biochar modification used herein do not just rely on acid modification and alkali treatment. Batch adsorption experiments indicate that, among all investigated materials, DPDMS-treated NPP exhibits the best adsorption performance and PYR removal efficiency. The Langmuir maximum adsorption capacity of DPDMS-NPP was estimated to be 531.91 μg g–1; it was higher than of previous reports (10.1 and 187.27 μg g–1).[71,72] Kinetic assessments indicate that the pseudo-second-order kinetic model fits the PYR adsorption data well and the process of adsorption on DPDMS-NPP is controlled by several mechanisms—mainly hydrophobic, EDA, electrostatic, and π–π interactions—as well as film and intraparticle diffusion. Overall, our results indicate that DPDMS-NPP has great potential for use as an alternative adsorbent of PYR contaminants owing to its high adsorption capacity, ease of preparation, extensive pore structure, and large specific surface area as well as abundance of phenyl functional groups.

Materials and Methods

Reagents

Dimethoxydiphenylsilane and pyrene were of analytical grade and purchased from Macleans Reagent Website (Shanghai, China). Cyclohexane, methanol, sodium hydroxide, hydrochloric acid, and phosphoric acid were purchased from Tianjin Chemical Reagent Factory (Tianjin, China). Pomelo peel was obtained from the local supermarket (Shaanxi, China).

Preparation of PPA, PPB, HPP, NPP, and DPDMS-NPP

Pomelo peel samples were cleaned with deionized water and dried in an oven at 80 °C for 24 h. The dried samples were subsequently chopped using a mechanical mill, crushed, then sieved through a 1 mm mesh. The sieved powder, designated as PPA, was pyrolyzed in a muffle furnace whose temperature was set to increase to 450 °C at the rate of 6 °C min–1. The pyrolysis process was carried out in an inert atmosphere (50 mL min–1 N2 flow), and it lasted for 2 h.

The pyrolyzed sample, labeled PPB, was thereafter used to prepare the HPP and NPP materials via H3PO4 and NaOH activation, respectively. For this purpose, 20 g of PPB was soaked in either 200 mL of 4 M H3PO4 or 200 mL of 2 M NaOH then incubated at room temperature for 24 h. Subsequently, the dried samples were thermally activated in a muffle furnace with the temperature set to increase to 450 °C at the rate of 3 °C min–1. The activation process was performed in a nitrogen atmosphere, and it lasted for 2 h. The activated HPP and NPP mixtures were repeatedly washed with 0.1 M NaOH and 0.1 M HCl solutions, respectively, until pH neutrality of the filtrates was achieved.

The DPDMS-NPP material was directly prepared from NPP. Briefly, 2.5 g of NPP was mixed and soaked in a solution containing 1 mL of DPDMS and 70 mL of methanol. Two hours later, the mixture was transferred to a 100 mL polytetrafluoroethylene inner steel autoclave and heated at 150 °C for 24 h. Thereafter, the autoclave was rapidly cooled to room temperature, and the obtained product (DPDMS-NPP) was vacuum-filtered and repeatedly washed with methanol. All activated biochars prepared in this study were oven dried at 80 °C for 24 h then stored for later use.

Characterization

Elemental analyzer (FLASH 2000 NC Analyzer) was used to analyze the total carbon (C), nitrogen (N), hydrogen (H), and silicon (Si) content in the PPA, PPB, HPP, NPP, and DPDMS-NPP. After the samples were adhered to the conductive adhesive and sprayed for gold coating, the surface morphology was observed using a scanning microscope (Hitachi S-4800). The functional groups were confirmed using a Fourier transform infrared spectrometer (Nicolet 5700) in the range of 400–4000 cm–1 via a KBr pellet. The specific surface area and pore volumes of PPA, PPB, HPP, NPP, and DPDMS-NPP were measured by a N2 adsorption–desorption isotherm at 77.4 K and 737.6 mmHg using a NOVA 2000 specific surface area pore analyzer, respectively. The specific surface areas were calculated by the BET method (0.1–0.35 P/P0).

Batch Adsorption Experiments: Effects of the Initial PYR Concentration and Adsorbent Dosage: Isotherm and Kinetic Studies

Experiments were conducted to evaluate the efficiency of PYR removal from synthetic solutions by PPA, PPB, HPP, NPP, and DPDMS-NPP. In total, six PYR standard solutions of varying concentrations (1.6–10 μg mL–1) were prepared by dissolving different amounts of PYR in cyclohexane. The batch adsorption experiments were performed in 25 mL volumetric flasks containing 25 mL of 6.0 μg mL–1 PYR solution. An amount of 0.2 g of PPA, PPB, HPP, NPP, or DPDMS-NPP adsorbents was added to each flask, and the mixtures were shaken for 48 h in a temperature oscillator (TS-100C) set at 30 °C and 200 rpm in order to reach equilibrium. Later, the mixtures were filtered through a 0.45 μm syringe filter, and the concentration of PYR in the supernatant was determined via a UV–visible spectrophotometer (UV-752) conducted at 320 nm. The absorptivity coefficient of PYR is 0.10938 (mL μg –1 cm–1).

To investigate the effect of the initial concentration on PYR adsorption, the experiments were repeated using different initial concentrations of PYR solution (in the range of 1.6–10.0 μg mL–1). The effect of the adsorbent dosage on adsorption efficiency was also tested by varying the amount of the adsorbing material between 4 and 14 g L–1. The quantity of PYR adsorbed at equilibrium Q (μg g–1) and the PYR removal rate R (%) were calculated according to eqs and 11, respectively,where C0 (μg mL–1) and C (μg mL–1) are the initial and equilibrium concentrations of PYR, respectively; V is the volume of the PYR solution (mL); and W is the mass of the adsorbent (g).

To analyze the kinetics of PYR adsorption on PPA, NPP, and DPDMS-NPP, 0.2 g of the adsorbent was added to 25 mL of 6.0 μg mL–1 PYR solution, and the mixtures were shaken for 48 h in a temperature oscillator set at 30 °C and 200 rpm. Samples of these mixtures were collected during shaking at predetermined time intervals then they were filtered through a 0.45 μm syringe filter and analyzed by absorption spectrophotometry.

The adsorption isotherms of the prepared materials were also recorded. For this purpose, 25 mL solutions containing the adsorbent (8 g L–1) and PYR (0.8–10 μg mL–1) were agitated for 48 h in a temperature oscillator. Upon reaching adsorption equilibrium, the mixtures were filtered, and the isotherm data of the different adsorbents were simulated using three isothermal models. To evaluate the effect of temperature on PYR adsorption, the experiments were repeated at 293, 303, and 310 K for PYR solutions of initial concentrations in the range of 0.8–10 μg mL–1. To minimize error, all experiments detailed herein were performed in triplicate.