Facile synthesis of graphene wool doped with oleylamine-capped silver nanoparticles (GW-αAgNPs) for water treatment applications

The facile synthesis of graphene wool doped with oleylamine-capped silver nanoparticles (GW-αAgNP) was achieved in this study. The effect of concentration, pH, temperature and natural organic matter (NOM) on the adsorption of a human carcinogen (benzo(a)pyrene, BaP) was evaluated using the doped graphene wool adsorbent. Furthermore, the antibacterial potential of GW-αAgNP against selected drug-resistant Gram-negative and Gram-positive bacteria strains was evaluated. Isotherm data revealed that adsorption of BaP by GW-αAgNP was best described by a multilayer adsorption mechanism predicted by Freundlich model with least ERRSQ < 0.79. The doping of graphene wool with hydrophobic AgNPs coated with functional moieties significantly increased the maximum adsorption capacity of GW-αAgNP over GW based on the qmax and qm predicted by Langmuir and Sips models. π-π interactions contributed to sorbent-sorbate interaction, due to the presence of delocalized electrons. GW-αAgNP-BaP interaction is a spontaneous exothermic process (negative ΔH∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta H^\circ$$\end{document} and ΔG)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta G)$$\end{document}, with better removal efficiency in the absence of natural organic matter (NOM). While GW is more feasible with higher maximum adsorption capacity (qm) at elevated temperatures, GW-αAgNP adsorption capacity and efficiency is best at ambient temperature, in the absence of natural organic matter (NOM), and preferable in terms of energy demands and process economics. GW-αAgNP significantly inhibited the growth of Gram-negative Pseudomonas aeruginosa and Gram-positive Bacillus subtilis strains, at 1000 mg/L dosage in preliminary tests, which provides the rationale for future evaluation of this hybrid material as a smart solution to chemical and microbiological water pollution.


Introduction
Benzo(a)pyrene (BaP) is regarded as one of the most hazardous environmental pollutants exhibiting both genotoxic and carcinogenic toxicity in humans according to the International Agency for Research on Cancer (IARC) (IARC 2010;Hardonnière et al. 2016). BaP belongs to the group of ubiquitous emerging chemical pollutants (ECPs) known as polycyclic aromatic hydrocarbons (PAHs) (Adeola & Forbes 2020;Munyeza et al. 2020). BaP is persistent in the environment and poses health risk due to its recalcitrance to biodegradation (Yerushalmi et al. 2006). The maximum acceptable concentration (MAC) of PAHs in surface water is 0.01 µg/L; however, several reports suggest that BaP levels detected in South Africa are higher than the MAC value, thus posing a potential health risk (Adeniji et al. 2019).
Furthermore, the adaptive resistance of several bacteria to antibiotics, such as chloramphenicol, penicillin, etc., has led to the interesting discovery that silver nanoparticles can inhibit microbial growth and may be lethal against drugresistant bacteria (Anthony et al. 2014;McBirney et al. 2016;Huang et al. 2017). Advances in research into a hybrid approach to environmental protection and remediation have brought about the need for the development of "smart" materials/composites with multifunctional capabilities for improved efficiency and process economics (Bezza and Chirwa 2016;Miren et al. 2018;Adeola & Forbes 2021b). Several materials with antimicrobial properties have been developed for the removal of pollutants in aqueous matrices, examples of such materials are polyanilineTi(IV)arsenophosphate (Bushra et al. 2014), iron and manganese coated silica gel (Ahmad et al. 2015), chitosan doped with silver nanoparticles (Ishihara et al. 2015), nano-silver-supported activated carbon (Eltugral et al. 2016), graphene foam/TiO 2 nanosheet hybrids , iron nanoparticles (Da'na et al. 2018), silk fiber doped with tannic acid , antimicrobial polymer (Li et al. 2020) chitosan/ nitrogen-doped graphene quantum dots (Amari et al. 2021), etc. The design of composites has reportedly enhanced physicochemical properties of adsorbents such as specific surface area, stability, conductivity, tensile strength, chemical robustness, charge mobility, flexibility, thin-film thickness, and provided a basis for the growing interest in the utilization of composites for water treatment applications (Adeola & Forbes 2021b).
A comprehensive risk-based assessment of graphenebased composites is currently unavailable; however, it is assumed that the composites may not pose a significant health risk based on their composition, but their lightweight nature may pose inhalation risks (Schinwald et al. 2012). Thus, the physical structure of the graphene-based material and the fabrication method is critical. With respect to graphene wool doped with oleylamine-capped silver nanoparticles (GW-αAgNPs), the quartz wool substrate acts as a solid support, assisting with immobilization of the graphene and silver nanoparticles. Furthermore, unlike most composites generated in the form of flakes and powder, GW-αAgNPs presents a wool-like form that may be more suitable as a packing material for filters and other water polishing tools.
The overall aim of this study was to synthesize a composite of graphene wool and silver nanoparticles (GW-αAgNPs) with antibacterial activity, for the removal of a human carcinogen, namely benzo(a)pyrene, from polluted water. The influence of process variables such as pH, temperature, and initial concentration of BaP on the sorption mechanism was established for optimum efficiency of the composite. Furthermore, the antibacterial activity of the composite was tested and is discussed briefly for potential dual application toward water treatment.

Facile synthesis of GW-αAgNPs
Graphene wool was synthesized using the chemical vapor deposition method on a quartz wool substrate whereby an optimized stream of argon, hydrogen, and methane gas was temperature ramped to 1200 °C as previously described (Adeola & Forbes 2019Schoonraad et al. 2020). Lipopeptide-coated silver nanoparticles were synthesized in phenyl ether with oleylamine and oleic acid as both reducing agents and capping agents (Liu et al. 2011;Sha et al. 2011;Çınar et al. 2011).
The composite was prepared as follows: Briefly, GW (200 mg) and DI water (100 mL) were added into a sealed bottle (250 mL) and stirred gently for 1 h using a magnetic stirrer, before the addition of the dopant mixture. Ag nanoparticles (300 mg) dispersed in diphenyl ether (100 mL) were added into the GW solution and stirred for 12 h at room temperature under argon, to ensure that AgNPs coordinated with graphene wool at the water/diphenyl ether interface. The GW-αAgNP composite was rinsed with acetone and centrifuged at 6000 rpm for 10 min, three times consecutively. The obtained GW-αAgNP composites were then washed with hexane to remove residual oleylamine. The final GW-αAgNP composite was freeze-dried for 48 h. The facile synthesis is illustrated in Scheme 1.

Characterization of the synthesized adsorbent
The morphology of GW and GW-αAgNPs was examined by a combination of techniques including scanning electron microscopy (SEM), with images obtained from a Zeiss Ultra-Plus 55 field emission scanning electron microscope (FE-SEM), operated at 2.0 kV (Zeiss, Germany). High-resolution transmission electron microscopy (TEM) images of capped-AgNPs and GW-αAgNPs were taken using a JEOL JEM 2100F (JOEL Ltd, Tokyo, Japan) operated at 200 kV and equipped with an energy dispersive X-ray spectrometer (EDS) (OXFORD Link-ISIS-300 Zeiss, Germany). The specific surface area (SSA) of GW was determined using the modified Sears' method (Sears 1956;Adeola & Forbes 2019). FTIR spectra of GW, capped AgNPs and GW-αAgNPs were obtained using a Bruker Alpha-T spectrometer (Bruker Optik GmbH, Ettlingen, Germany). Elemental analysis of natural organic matter (NOM) was examined using inductively coupled plasma-optical emission spectrometry (ICP-OES, Spectro Arcos model, Thermo Fisher Scientific, South Africa). The conductivity of the background electrolyte was confirmed using an Orion Star A112 conductivity benchtop meter (Thermo Scientific, South Africa), and pH was monitored using a 780-pH meter (Metrohm Herisau, Switzerland).

Sorption isotherm experiments
Batch adsorption experiments of BaP onto GW and GW-αAgNPs were carried out in 40 mL PTFE screw cap amber vials (Stargate Scientific, South Africa) at 25 ± 1 °C in a thermostated shaking water bath (Wisebath, Celsius Scientific, South Africa). Background electrolyte (pH = 7.0) contained 0.01 mol/L CaCl 2 (ACE, South Africa) in DI water with 200 mg/L of sodium azide (Sigma-Aldrich, Germany) as a biocide. The isotherm experiment was conducted with initial concentrations of the BaP solutions ranging from 100 µg/L to 500 µg/L. The BaP desorption isotherm was examined by the addition of 5 mL fresh electrolyte with equilibration for 24 h, after decanting the adsorption supernatant as previously described (Wang et al. 2008;Adeola & Forbes 2021a). Adsorption isotherms of BaP were also performed at varying temperatures of 25, 35, and 45 °C using a thermostated shaking water bath (Wisebath, Celsius Scientific, South Africa) to determine adsorption thermodynamics. The role of solution pH was evaluated by pH adjustment with 0.1 M HCl (Merck, South Africa) or NaOH (ACE, South Africa) over the pH range from 2 to 12, to elucidate the pH effect on the removal of BaP from aqueous solution.

Quantification
After equilibration, centrifugation of the vials was performed at 6000 rpm for 10 min to recover a clear supernatant. BaP concentrations were analyzed in triplicate (n = 3) by fluorescence spectroscopy (Horiba Jobin Yvon Fluoromax-4 spectrofluorometer). For all fluorescence measurements, the excitation and quantification emission wavelengths were at 330 and 464 nm, while the excitation and emission slit widths were set at 5 nm. The regression coefficient (R 2 ) of the calibration curve was obtained from working solutions in the range of 100 µg/L to 500 µg/L of BaP and blanks were included for both calibration and sorption experiments. The equilibrium concentration (C e , µg/L) was deduced from the calibration equation. The amount of solute adsorbed (q e , µg/g) was extrapolated using a mass-balance equation (Eq. 1) and removal efficiency was estimated using Eq. 2: where C o (µg/L) is the initial concentration, C e (µg/L) is the equilibrium solute concentration, V o is the initial volume (L) and S m is the mass (g) of the adsorbent.

Antibacterial test of GW-αAgNPs
Sterilization of all glassware and media was carried out in an autoclave at 121 °C for 15 min. A facile test was conducted of bacteria inhibition of GW-αAgNP against model Gramnegative and Gram-positive bacteria strains (Pseudomonas aeruginosa CB1 and Bacillus subtilis CN2) previously isolated in our laboratory (Bezza et al., 2020). The inocula of P. aeruginosa and B. subtilis were cultured overnight in Tryptic Soybean Broth (TSB) under aerobic conditions at 37 °C. The inhibitory concentration of the composite against visible growth of P. aeruginosa and B. subtilis after 24 h of incubation at 37 °C was investigated. Concentrations ranging from 0-1000 mg/L of GW-αAgNP were prepared in sterilized conical flasks containing 100 mL TSB. Thereafter each flask was inoculated with 10 µL of the cultured inoculum. Optical density measurements were taken after the incubation period. Experiments were conducted in duplicate and controls containing nutrient broth inoculated with inoculum without the inclusion of GW-αAgNPs. Bacteria concentration was estimated in relation to absorbance/optical density at 600 nm (OD 600 ) using a UV/Vis spectrophotometer (Shimadzu UV-1800, Labotec, South Africa) and corrected by subtracting the background absorbance of the control (Anthony et al. 2014;Bezza et al. 2020).

Adsorbent characterization
The morphology of the synthesized composite was examined using SEM and TEM ( Fig. 1a and b). The high-resolution images revealed a heterogeneous surface structure with extensive coverage of GW with AgNPs. The oleylaminecapped AgNPs were analyzed with TEM prior to conjugation with GW, and spherical particles with a mean diameter of 12.67 ± 3.9 nm were estimated via particle size analysis using the ImageJ software ( Fig. 1b and d). Qualitative analysis of GW-αAgNP using EDS confirmed the presence and relative abundance of silver and carbon (Fig. 1c). FTIR ( Fig. 1e and f) revealed two prominent peaks associated with the sp 2 hybridized C=C backbone of graphene and a broad peak of Si-O-C of functionalized quartz wool (SiO 2 ) coated with graphene at 775 and 1059 cm −1 , respectively (Adeola & Forbes 2020). Bands at 2921, 2856, 1631, 1450 cm −1 regions arising from C-H, C=O, C-N stretching vibrations were observed in GW-αAgNP, respectively (Fig. 1e).

Sorption isotherm experiments
Adsorption isotherm models are used to investigate the nature of sorbent-sorbate interactions of adsorption Zhang et al. 2019;Adeola & Forbes 2021a). Linear regression and nonlinear isotherm models such as Linear (Eq. 3), Freundlich (Eq. 4), Langmuir (Eq. 5), and Sips model (Eq. 6) were used to fit adsorption experimental data. The Error Sum of Squares (ERRSQ) (Eq. 7) was used to test all models used in this study.
where K F (mg/g) (L/mg) N ) and N (dimensionless) is the Freundlich constant and intensity parameter, an indicator of site energy heterogeneity; q max (mg/g) and K L (L/mg) are the Langmuir maximum adsorption capacity and Langmuir constant associated with solute-surface interaction energy, respectively; K s (L/mg) and q max (mg/g) are Sips isotherm model constants and maximum adsorption capacity and ms is Sips isotherm exponent; q e is the solid-phase (q e,cal − q e, exp ) 2 i concentration (mg/g), C e is the liquid phase equilibrium concentration (mg/L), and K d (L/g) is the sorption distribution coefficient (Ololade et al. 2018;Adeola & Forbes 2019).
The isotherm regression parameters for Freundlich, Langmuir, Linear, and Sips model are presented in Table 1 and Fig. 2. The hysteresis index (H) which is a measure of the irreversibility of the sorption process was calculated for doped graphene wool and pristine graphene wool (Table 1). Isotherm data for GW-αAgNP adsorption of BaP was best described by a multilayer adsorption mechanism predicted by the Freundlich model with least ERRSQ < 0.79, while BaP adsorption onto GW was best fitted to the Sips model  (Langmuir-Freundlich hybrid) with ERRSQ < 1.88, respectively (Table 1). These findings are consistent with previous results obtained from the adsorption of phenanthrene and pyrene onto pristine graphene wool (Adeola & Forbes 2019). The doping of graphene wool with hydrophobic AgNPs coated with organic functional moieties significantly increased the maximum adsorption capacity of GW-αAgNP over GW based on the q max & q m predicted by Langmuir and Sips models, respectively (Table 1). BaP is a hydrophobic PAH with a high octanol-water partition coefficient logK ow of 6.13 (Adeola & Forbes 2020), and several reports suggest a strong affinity between PAHs and hydrophobic surfaces of adsorbents (Khan et al. 2007;Lamichhane et al. 2016;Yakout & Daifullah 2013;Yuan et al. 2018). π-π interactions between the graphene wool composite and the aromatic structure of BaP, due to the presence of delocalized electrons, also contributes to the adsorption process (Zhao et al. 2011;Zhang et al. 2013;Yang et al. 2015;Adeola & Forbes 2019). Furthermore, oleylamine and oleic acid used as capping agents as well as the linker between GW and AgNP are large hydrophobic organic molecules that may have improved the surface hydrophobicity of the composite. Thus, this may in turn enhance partitioning (mass transfer) of hydrophobic BaP onto the surface of the composite, leading to enhanced adsorption capacity (K d and q max ). The doping of graphene with oleylamine-capped AgNPs accounts for the comparatively high surface and adsorption heterogeneity (N & m s ) index (Table 1). It is evident that adsorption-desorption interactions between sorbate and sorbents displayed a significant degree of hysteresis, as calculated H-index values for both sorbates were greater than 1 (N ads > > > N des ) (Table 1) (Ololade et al. 2018;Adeola & Forbes 2021a). However, irreversible entrapment and/or slow rate of desorption of sorbed BaP was three-fold higher in GW-αAgNP than pristine GW, further confirming higher binding strength with BaP. Pore deformation and alteration of the surface structure of sorbents via build-up in unrelaxed pore volume also cause hysteretic behavior in sorption processes (Nguyen et al. 2004;Cornelissen et al. 2005). Therefore, the high hysteretic behavior of GW-αAgNP, which exemplifies better retention of BaP against recontamination of treated water, maybe due to entrapment of solutes by the collapse of the GW-αAgNP composite structure due to the adsorption process conditions and agitation. Table 2 reveals that graphene wool (GW) and doped graphene wool (GW-αAgNP) competes favorably with other adsorbents reported in the literature for the removal of benzo(a)pyrene from aqueous solutions, with efficiency ˃94%. The maximum adsorption capacity deduced from the Langmuir isotherm model (q max ) for GW is lower than some of the adsorbents, however, the adsorption capacity of GW-αAgNPs is higher than activated carbon (AC), biochar and granular activated carbon (GAC) for BaP adsorption based on available literature. The higher adsorption capacity of GW-αAgNP may be due to surface modification associated with the doping experiment; creation of binding sites/ pores and enhanced hydrophobic sorbate-sorbent interactions. Oleylamine and oleic acid used as capping agents as well as the linker between GW and AgNP are large hydrophobic organic molecules that may have improved the surface hydrophobicity of the composite.

Comparison of adsorbents reported for benzo(a)pyrene removal
Several factors influence the choice of adsorbent for water treatment applications, these factors include efficiency, non-toxicity, availability of material, flexibility, reusability, etc. (Adeola et al. 2021). However, the wool-like form and porosity of GW-αAgNP, in addition to the potential antibacterial activity (discussed in Sect. 3.6), are advantages to the use of GW-αAgNP as a packing material for water treatment applications.

Effect of initial pH on BaP adsorption
The mineral, organic and biotic composition of surface waters depends on the source and geographic location, which in turn affects the water pH and influences the adsorption of chemical pollutants (Kulthanan et al. 2013). Solution pH affects the net charge of the adsorbent and adsorbate, and the alterations are more impactful in compounds and materials with protonated moieties (-OH, -COOH, -NH 2 group, etc.) because they tend to form deprotonated groups/ complexes under variable pH conditions (Ahmed & Gasser 2012). In principle, at pH < point of zero charge (PZC), the surface of the adsorbent is positively charged and at pH > PZC, sorbents become negatively charged (Liikanen et al. 2006;Ololade et al. 2018). The results obtained in this study revealed that the optimum adsorption of BaP by GW-αAgNP occurred under basic pH conditions. This is in contrast to GW adsorption of PAHs that was slightly favored under acidic pH (Adeola & Forbes 2019). Figure 3 reveals that the adsorption is favored to the right side of the pH scale due to the nature of the oleylaminecapped AgNP-GW complex, the surface modification, and the abundance of hydroxide ions in basic pH that potentially facilitates hydrogen bonding as discussed in Sect. 3.2. Furthermore, excess hydroxide ions at pH > 7 could potentially lead to the formation of silver hydroxide, which is hydrophobic and thus enhances the more hydrophobic interactions with BaP, which often governs the adsorption and partitioning of hydrophobic organic compounds (HOC) in water (Vasileva et al. 2009;Apul et al. 2013;Bai et al. 2017;Adeola & Forbes 2020, 2021b).

Influence of NOM on sorbent-sorbate interaction
Natural organic matter (NOM) is a complex matrix of organic materials which are present in aquatic environments, including drinking water, due to the interconnectivity between the hydrologic cycle, biosphere, and geosphere within the ecosystem (Sillanpää et al. 2018). The composition of NOM is influenced by biogeochemical processes that have occurred within the environment (Myneni 2019). In this study, NOM was isolated from stream sediment collected from the University of Pretoria sports campus, South Africa (latitude E28° 14′ 46′′ and longitude S25° 45′ 10′′) using established procedures (Ran et al. 2007;Ololade et al. 2018;Adeola & Forbes 2021a). The mineral phase was removed from bulk samples via treatment with 1 N HCl for 45 min at ambient temperature, followed by three consecutive treatments with 1 N HCl and 10% HNO 3 for 12 h  (Gelinas et al. 2001). The residue was washed each time with DI water, centrifuged at 6000 rpm for 10 min, decanted, and freeze-dried at − 4 • C for 24 h prior to use. Morphological and basic characterization of the NOM was carried out as presented in Fig. 4 (see Adeola and Forbes 2021b for more details). The NOM isolate had an irregular, spherical grain structure with heterogeneous and porous surface morphology (Fig. 4). The effect of NOM on the adsorption of BaP by GW-αAgNP was evaluated using a preloading batch experiment (Adeola & Forbes 2021a;Ersan et al. 2016). Figure 5 suggests that significant competitive interactions took place between the NOM, BaP molecules, and the adsorbent leading to the comparative decline in removal efficiency, Freundlich adsorption capacity (K f ), partition coefficient (K d ), and maximum adsorption capacity (q max ). NOM has been reported to cause fouling of membranes, and retention of hydrophobic compounds and metals in solution, thus limiting the efficiency of conventional water treatment plants (Mehta et al. 2017;Kurwadkar et al. 2019;Adeola & Forbes 2021b). Essentially, NOM often alters the solution's chemistry such as pH, ionic strength, and the presence of leachable trace and heavy metals (Fig. 4), providing a plausible explanation for the inhibitory role of NOM (Ersan et al. 2016;Lamichhane et al. 2016;Adeola & Forbes 2021a).

Effect of temperature and thermodynamic studies
Several physicochemical and biological processes are influenced by temperature. Therefore, the role of temperature on the adsorption of BaP by pristine GW and doped GW-αAgNP was studied at 35, 45, and 55 °C, respectively. The adsorption data were fit to a linear and Sips isotherm models (Eq. 3 and 6), and it was observed that the maximum adsorption capacity (q m ) significantly reduced for GW-αAgNP with an increase in temperature, while the reverse was observed for GW (Table 3, Fig. 6). The free energy change (ΔG˚), enthalpy (ΔH˚) and entropy (ΔS˚) were calculated using the Van't Hoff plot and equations   where ΔG is the change in the Gibbs free energy (cal/mol); ΔH is the change in enthalpy (cal/mol), and ΔS is the change in entropy (kJ/mol.K), R = gas constant (8.314 J/mol.K), T = thermodynamic temperature (K) and K d is adsorption capacity determined from the linear isotherm model. Table 3 revealed positive values of adsorption enthalpy ( ΔH • ) and entropy (ΔS • ), and a negative value of ΔG for GW-BaP interaction, which indicates a spontaneous endothermic process as the temperature increased (Ahmed & Gasser 2012). This result is in agreement with a previous study on the adsorption of phenanthrene and pyrene by GW (Adeola & Forbes 2019). In contrast, GW-αAgNP-BaP interaction is a spontaneous exothermic process (negative ΔH • and ΔG), with a reduction in system chaos (negative ΔS • ) and adsorption capacity as the temperature is increased. While GW is more efficient at elevated temperatures, doping GW with oleylamine-capped AgNPs improved the adsorption capacity and ensured optimum removal efficiency at ambient temperature, which is better in terms of energy economics and industrial application.

Antibacterial activity of GW-αAgNP
Silver nanoparticles and composites have attracted scientific attention due to the continuous upsurge of drug-resistant bacteria (Bezza et al. 2020;Cobos et al. 2020). Bacterial strains are classified as Gram-positive (G + ve) or Gramnegative (G-ve) based on film assemblage with layers of peptidoglycan (PG) (Proft and Baker 2009). G − ve microbes have a thin PG (1-5 nm) between the cytoplasmic film and external layer, while G + ve microbes have a thicker PG layer (~ 30 nm) without an external film (Kim et al. 2007). The mechanism of action of silver-containing materials is described in Fig. 7. Silver damages the cytoplasmic membrane of microbes, creates oxidative stress with the cells, damages DNA, denatures cell proteins and has a lethal effect on microorganisms, including drug-resistant bacteria (Ahmad et al. 2020).  Ahmad et al. Copyright 2020, Elsevier) Several studies have established the antibacterial action of AgNPs and composites, with different minimum inhibitory concentrations against some drug-resistant microbes ( Table 4). The minimum inhibitory concentration (MIC) is regarded as the lowest concentration of an antimicrobial agent that inhibits the growth of microbes, recorded in mg/L or μg/mL (Cobos et al. 2020). The literature suggests that AgNPs and bimetallic nanocomposites (such as Au-Ag, Fe-Ag) are more effective against microorganisms based on MIC values; however, the potential adverse effects of AgNPs on human health have been a major concern, hence research into stabilizing AgNPs using capping agents and entrapment within a bulky substrate to limit its release potential/ contamination in aqueous media has gained vast attention (Liu et al. 2011;Loan Khanh et al. 2019;Bezza et al. 2020).
The antibacterial activity of GW-αAgNPs and pristine AgNPs was tested against the Gram-negative Pseudomonas aeruginosa CB1 and Gram-positive Bacillus subtilis CN2 strains by the standard micro-dilution method. Earlier reports suggest that P. aeruginosa and B. subtilis strains are capable of adaptive resistance to antibiotics such as penicillin and tetracycline (Araya et al. 2019;Pang et al. 2019). Figure 8 reveals a dose-dependent reduction in the concentration of Gram-negative Pseudomonas aeruginosa CB1 and Gram-positive Bacillus subtilis CN2 strains after 24 h incubation period as a decrease in turbidity was recorded spectrophotometrically at an optical density of 600 nm. The decline in turbidity is a function of bacteriostatic/bactericidal activity of the composite, relative to control experiments. The reduction is most significant at 1000 mg/L (GW-αAgNP) and was more visible in doped GW than pristine GW. Factors such as particle size, stabilizing agent, composition of culture media and bacteria type, inoculum size, and leachability of silver ions from the composite, play a critical role in determining the MIC values. Optical density measurement is the most common technique for determining bacteria concentration and efficacy of antibacterial agents (McBirney et al. 2016;Huang et al. 2017). The agar disk diffusion test is another method often used for nanoparticles that are dispersible in water to form a uniform solution, however, due to the macroscopic and fibrous nature of GW, it was impossible to obtain a uniform dispersion of the material in the solution. When such experiments were attempted, the release and mobility of silver ions from the macrostructure through the semi-solid agar were significantly limited, thus constricting the inhibition zone to the site of deposition on the plate (Fig. 9), a similar finding was reported for large-sized AgNPs (Xiu et al. 2012;Bezza et al. 2020). However, the texture, fibrous nature, antibacterial and adsorptive properties of GW-αAgNP suggest that the material may be suitable for the fabrication of adsorbent layer(s) in water filtration or purification devices.

Conclusion
Facile synthesis of GW doped with oleylamine-capped AgNPs was achieved in this study. The effect of concentration, pH, and temperature on the adsorption of benzo(a) pyrene, a human carcinogen, was evaluated using the doped graphene wool. Isotherm data suggest that GW-αAgNP-BaP interaction is a spontaneous exothermic process (negative ΔH • and ΔG), characterized by a decline in system chaos (negative ΔS • ) and adsorption capacity as temperature increases. This study suggests that the adsorption capacity of GW improved and will be more efficient at ambient temperature ( ΔG and q max ), when doped with AgNPs as a result of improved surface hydrophobicity and heterogeneity, leading to the creation of more binding sites for BaP to adhere to. Results revealed a high degree of desorption hysteresis and irreversible sorption, suggesting a strong binding strength between the doped GW and pollutant, thus limiting the ease of recontamination. Results also show that the presence of NOM in the aqueous matrix is undesirable for the application of the synthesized adsorbent due to competitive adsorption. Several studies have established the cytoplasmic toxicity and antimicrobial properties of silver-containing nanomaterials and in this study, preliminary results also revealed that there was a dose-dependent reduction in the concentration of Gram-negative Pseudomonas aeruginosa CB1 and Gram-positive Bacillus subtilis CN2 strains tested in the presence of GW-αAgNP. Furthermore, unlike most composites that are generated in the form of flakes or powder, GW-αAgNP provides a wool-like form that may be more suitable as a packing material for filters and other water polishing tools due to its lightweight and porous nature. The synthesis of graphene wool is facile and eco-friendly without extensive use of chemicals. Therefore, under appropriate operating conditions, the graphene-based composite can potentially be utilized as a water polishing tool for the removal of emerging organic chemical pollutants. With further studies and suitable fabrication, this hybrid material has the potential to serve as a smart solution to chemical and microbial pollution in water.
Funding This study is supported by University of Pretoria Commonwealth Doctoral Scholarship funding (AA) and the Rand Water Professorial Chair program (PF and EC).

Conflict of interest
The authors declare that there is no conflict of interest.
Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Fig. 9 Agar disk diffusion test a Before incubation-100 mg of GW-αAgNP was placed on agar disk with growth medium and Bacillus subtilis culture spread over uniformly, b After 24 h incubation-visible growth of the bacteria around GW-αAgNP with inhibition zone limited to the site of deposition