### Chemicals

The phenanthrene (C_{14}H_{10}) standard was purchased from Supelco, Bellefonte, PA, USA. Separations (South Africa) supplied the GHP acrodisc syringe filters and amber glass vials with Teflon lined screw caps. Phosphoric acid (H_{3}PO_{4}), zinc chloride (ZnCl_{2}), acetonitrile and all other chemicals were obtained from Sigma Aldrich (South Africa). Milli-Q water from Merck Millipore (USA) Milli-Q synthesis system was used for all analytical preparations.

### Collection and analysis of *V. vinifera* leaf litter

Dried *V. vinifera* leaf litter, an agricultural waste was collected at Stellenbosch, Western Cape, South Africa, into a precleaned sack. It was transported to the laboratory and stored in a cool dry place until further processing. The suitability of *V. vinifera* leaf litter as precursor for activated carbons was assessed by the determination of moisture, ash and crude fibre contents. The elemental composition of the milled *V. vinifera* leaf litter was obtained through Energy-dispersive X-ray spectroscopy (EDS).

### Carbonisation

The carbonisation method was based on that reported by Sudaryanto et al. [23]. The leaf litter was milled and sieved with a standard mesh to obtain a particle size of ≤ 25 mesh (≤ 707 μm). The sieved milled leaf litter was impregnated with the activating agent (H_{3}PO_{4} and ZnCl_{2}) at 5:2, 5:1 and 10:1 biomass to activating agent ratios, respectively. The slurry after impregnation was sonicated at 50 °C for 3 h before drying at 110 °C overnight. 20 g of the impregnated biomass was carbonised at 600 °C for 1 h and N_{2} flow of 150 mL/min. The carboniser was switched off and allowed to cool to 200 °C with the nitrogen gas still flowing. The gas supply was cut off at 200 °C, the activated carbon obtained was placed in a desiccator to cool. The charred biomass was subsequently weighed and percentage yield (Eq. 1), burn off (Eq. 2) and attrition (Eq. 3) calculated. The carbonised biomass was washed with hot 1 M HCl, followed by milli-Q water until all acid was removed, and further dried at 50 °C for 5 h.

$$\% Yield{\text{ }} = ~~\frac{{{M_1}}}{{{M_2}}}~\times~100\%$$

(1)

$$\% Burn{\text{ }}off{\text{ }} = {\text{ }}\left( {100{\text{ }} - {\text{ }}\% yield} \right)$$

(2)

$$\% Attrition{\text{ }} = \frac{{A - B}}{A}~ \times ~100\%$$

(3)

where M_{1} is the mass of charred biomass, M_{2} is the mass of uncharred biomass, A is the Initial weight of charred biomass before washing with hot 1 M HCl and milli-Q water and B is the Final weight of charred biomass after washing with hot 1 M HCl and milli-Q water.

### Characterisation of activated carbon

#### Fourier transform infrared spectrometry

The Fourier transform infrared (FTIR) spectra of the activated carbons and raw *V. vinifera* leaf litter (recorded over 4000–400 cm^{−1} range) were obtained on a Universal Attenuated Total Reflectance (UATR) Infrared spectrometer Perkin Elmer Spectrum 2 (UK). The crystal area of the instrument was cleaned prior to analysis and background correction made. The samples were placed directly on the crystal area of the universal diamond attenuated total reflectance (ATR) top-plate. The pressure arm was positioned over the crystal-sample area, then locked into a precise position above the diamond crystal and force applied to the sample, pushing it onto the diamond surface. The sample was scanned to obtain the spectrum.

#### Scanning electron microscopic analysis

The surface morphologies of the adsorbents were obtained, using scanning electron microscope (Nova Nano SEM 230, USA). A gold sputtering device (JOEL, JFC-1600) was utilised in coating the samples with a fine layer of gold for clarity to obtain the surface morphology. The elemental contents of activated carbons were also obtained by EDS [24].

#### Brunauer–Emmett–Teller

An automatic adsorption instrument (Quanta chrome Corp. Nova-1000 g gas sorption, USA) was utilised in obtaining the textural properties of the activated carbons. The Brunauer–Emmett–Teller (BET) surface area, total pore volume, micropore area, micropore volume and pore size were obtained. Degassing of samples was carried out at 170 °C for 13 h, before the adsorption and desorption of liquid N_{2} at 77 K. MicroActive 4.00 software (TriStar II 3020 version 2.00) was utilised to generate the BET surface area and the BJH (Barrett, Joyner and Helenda) pore distribution of the activated carbons.

#### Adsorption studies

Adsorption studies were carried out using phenanthrene as the adsorbate. Phenanthrene is one of the most abundant PAHs and an acceptable representative of semi volatile organic compounds [25, 26]. Due to the poor solubility of phenanthrene in water, milli-Q water containing 30% acetonitrile was utilised for this study. Parameters such as contact time, adsorbent load, pH and initial concentration were investigated to establish the optimum values for adsorbents efficiencies. The obtained data was fitted into adsorption kinetic models and isotherms to evaluate the quality of the adsorbents and to describe the mechanism of the sorption process.

#### Effect of pH

To investigate the effect of pH, 25 mL of 1 mg/L phenanthrene solutions with varying pH values (3–12) and 0.1 g activated carbon was utilised. The desired pH of the 0.1 mg/L phenanthrene solution was prepared from 10 mg/L solution with the addition of either 0.1 M HCl or 0.1 M NaOH to adjust the pH as required for obtaining the pH values of 3, 6, 9 and 12 investigated. A 25 mL phenanthrene solution that has been adjusted to the required pH was added to 0.1 g activated carbon in a 50-mL amber bottle and covered with Teflon-lined lid. This was thereafter placed on an orbital shaker at 298 K and allowed for 180 min at 100 revolutions per minute (rpm). After which 1 mL was filtered through a GHP acrodisc syringe filter (0.2 µm, 13 mm), prior to GC-FID analysis for the quantification of the residual phenanthrene.

#### Effect of adsorbent dosage

The effect of adsorbent dosage was studied using 25 mL of 1 mg/L phenanthrene solution at pH 3 with varying weights (0.01–0.1 g) of adsorbents. The investigated weights (0.01, 0.025, 0.050, 0.075 and 0.1 g) of adsorbents were carefully weighed into 50-mL amber bottles and 25 mL of phenanthrene solution added and covered with Teflon-lined lids. The bottles and their contents were then placed on the orbital shaker at 298 K and allowed for 180 min at 100 rpm. Thereafter, 1 mL solution from each of the bottles was filtered through GHP filter and the filtrates analysed with GC-FID for the residual phenanthrene.

#### Effect of initial phenanthrene concentration

The effect of initial concentration was studied using 25 mL of phenanthrene solution of varying concentrations. The concentrations investigated were 1, 2, 3, 4 and 5 mg/L that have been adjusted to pH 3. Aliquot 25 mL of the phenanthrene solutions were measured into 50-mL amber bottles containing 0.1 g of adsorbents. The experiments were carried out on an orbital shaker at 298 K, 100 rpm for 180 min and the GHP filtrates analysed.

#### Effect of contact time

The effect of contact time on the adsorption of phenanthrene was investigated by varying the time of mixing the adsorbent and the phenanthrene solution on an orbital shaker at 298 K. Phenanthrene concentration of 1 mg/L (25 mL) that have been adjusted to pH 3 and 0.1 g of adsorbents were utilised. The time intervals investigated were 10, 20, 40, 60, 80, 120, and 180 min. Amber bottles with Teflon-lined lids were used and the GHP filtrates analysed.

Gas chromatography–flame ionisation detection (GC-FID) instrument was utilised for the determination of phenanthrene in aqueous solutions before and after adsorption experiment. The percentage of phenanthrene removal and the equilibrium adsorption capacity (\(q_{e}\)) were estimated using Eqs. 4 and 5, respectively.

$$\% ~Adsorbed~ = ~\frac{{{C_0}~ - ~{C_t}}}{{{C_0}}}~ \times ~100$$

(4)

$${{q}}_{{{e}}} \user2{ } = { }\frac{{{V }({{C}}_{0} { } - { C}_{{{e}}} )}}{{{m}}}$$

(5)

where \(C_{0}\) (mg/L), \(C_{e}\) (mg/L) and \(C_{t}\) (mg/L) are initial, equilibrium and after time t concentration of phenanthrene solution, respectively, V (L) is the volume of phenanthrene solution, m (g) is the mass of adsorbent and \(q_{e}\) (mg/g) is the equilibrium adsorption capacity of adsorbent [26].

#### Adsorption isotherms

Adsorption isotherm models were used to describe adsorption behaviour of analytes onto the surface of adsorbents at equilibrium. The amount of phenanthrene adsorbed, and removal efficiency of adsorbents could be deduced from the adsorption isotherm models.

#### Langmuir isotherm

The sorption of phenanthrene onto single layer of selected activated carbon surface was studied with Langmuir isotherm model. Langmuir isotherm model postulates that, there is no transmigration of adsorbate in the plane of adsorbent surface for single layer adsorption onto a surface with a finite number of identical sites and uniform energies of adsorption [27]. A linearised equation for Langmuir isotherm model is given in Eq. 6.

$$\frac{1}{{{{q}}_{{{e}}} }} = \frac{1}{{{{q}}_{{{m}}} }} + \frac{1}{{{{q}}_{{{m}}} { K }_{{{L}}} { C}_{{{e}}} }}$$

(6)

where *q*_{e} is the amount of phenanthrene adsorbed per gram of the adsorbent at equilibrium (mg/g), *q*_{m} represent the maximum monolayer coverage capacity (mg/g), *K*_{L} is Langmuir isotherm constant (L/mg) and C_{e} is the equilibrium concentration of phenanthrene (mg/L).

The separation factor or equilibrium parameter (*R*_{L}) which is the crucial feature of the Langmuir isotherm model [28] is presented as Eq. 7:

$${{R}}_{{{L}}} = \frac{1}{{1 + \left( {1 + {{K}}_{{{L}}} {{C}}_{{{o}}} } \right)}}$$

(7)

where C_{o} is the initial phenanthrene concentration (mg/L).

The nature of adsorption can be adjudged from the value of *R*_{L}; *R*_{L} > 1 unfavourable, *R*_{L} = 1 linear, 0 < *R*_{L} < 1 favourable and *R*_{L} = 0 irreversible [29].

#### Freundlich isotherm

The adsorption characteristic of phenanthrene onto heterogeneous surfaces of the produced activated carbons was investigated by Freundlich adsorption isotherm. This isotherm model, assumes that the adsorbent has a heterogenous surface with adsorption sites that have different energies of adsorption that are not always available [30]. The linearised Freundlich adsorption isotherm equation is presented as Eq. 8:

$$\log {{q}}_{{{e}}} = \log {{K}}_{{{f}}} - \frac{1}{{{n}}}\log {{C}}_{{{e}}}$$

(8)

where \(q_{e}\) is the amount of phenanthrene adsorbed per gram of adsorbent at equilibrium (mg/g), *K*_{f} is the Freundlich isotherm constant (mg/g), *n* is adsorption intensity, and *C*_{e} is the concentration of phenanthrene at equilibrium (mg/L).

The Freundlich constant *n* gives an indication of adsorption intensity, while *K*_{f} gives an indication of adsorption capacity. The extent of non-linearity between solution concentration and adsorption depends on n. Linear adsorption, chemical adsorption and favourable physical adsorption processes are indicated by n = 1, n < 1, and n > 1, respectively [31].

#### Temkin isotherm

Experimental data obtained were assessed with Temkin isotherm model. Indirect adsorbent/adsorbate interactions influence on adsorption isotherms could be evaluated [32]. The model ignores the extremely low and large adsorbate concentration values and assumes that the heat of adsorption of all of the molecules in the layer would decrease linearly instead of logarithmically with coverage due to adsorbate/adsorbent interactions [31]. The linear equation is expressed as Eq. 9:

$${{q}}_{{{e}}} = {{B}}\ln {{K}}_{{{T}}} + {{B}}\ln {{C}}_{{{e}}}$$

(9)

where \(B\) (J/mol) and *K*_{T} are Temkin constants related to heat of sorption and maximum binding energy, respectively, R is the gas constant (8.31 J/mol K), and T (K) is the absolute temperature.

#### Dubinin–Radushkevich isotherm

Experimental data obtained were also fitted into the Dubinin–Radushkevich isotherm model, which had been widely utilised to describe adsorption onto microporous materials of carbonaceous origin [33,34,35]. The linear equation is expressed as Eq. 10:

$$\ln {{q}}_{{{e}}} = { }\ln \left( {{{q}}_{{{{DRB}}}} } \right) - \left( {{{K}}_{{{{ad}}}} {{\varepsilon}}^{2} } \right)$$

(10)

where \({{q}}_{{{e}}}\) is the amount of phenanthrene adsorbed per gram of adsorbent at equilibrium (mg/g), \(q_{DRB}\) is the theoretical isotherm saturation capacity (mg/g), \(K_{ad}\) is the Dubinin–Radushkevich isotherm constant (mol^{2}/kJ^{2}) and \(\varepsilon\) is the Dubinin-Radushkevich isotherm constant [34].

#### Kinetic studies

To gain valuable information into the reaction pathways, the rate of adsorption and the mechanism of adsorption, adsorption kinetics study were carried out. These insights were needed to establish the efficiency of adsorbents and to determine the optimum operating conditions for the adsorption process [36]. Experimental data obtained were subjected to four kinetic models (pseudo-first order kinetic, pseudo-second order kinetic, Elovich and intra-particle diffusion models).

#### Pseudo-first order kinetic model

The pseudo-first order kinetic model, also known as Lagergren kinetic, had its adsorption rate equation (for a liquid–solid system) derived, based on the adsorbent adsorption capacity. This adsorption rate equation is commonly used for solute adsorption from liquid matrix [37].

The simplified pseudo first order kinetic equation can be expressed as Eq. 11:

$${\text{In}}\left( {{q_e} - {q_t}} \right) = {\text{In}}{{\text{q}}_{\text{e}}} - {k_{\text{1}}}t$$

(11)

where \(q_{e}\) and \(q_{t}\) are the amount of solute adsorbed per unit mass of adsorbent (mg/g) at equilibrium and at time t respectively and \(k_{1}\) is the rate constant.

#### Pseudo-second order kinetic model

The pseudo-second order model is based on the sorption capacity of the solid phase which is associated with the number of available active sites. The linearised form of the kinetic model is expressed as Eq. 12 [38].

$$\frac{{{t}}}{{{{q}}_{{{t}}} }} = \frac{1}{{{{k}}_{2} {{q}}_{{{e}}}^{2} }} + \frac{1}{{{{q}}_{{{e}}} }}\left( {{t}} \right)$$

(12)

k_{2} is the pseudo-second order rate constant. This model is advantageous as it eliminates the problem of assigning q_{e.} The kinetics is presumed to proceed via chemisorption, being the rate determining step [38].

#### Elovich model

The Elovich model (Eq. 13) has been widely utilised to describe chemical adsorption processes and it is applicable for systems with heterogenous adsorbing surfaces [39].

$${q_t} = \left( {\frac{1}{\beta }} \right)ln\left( {\alpha \cdot \beta } \right) + \left( {\frac{1}{\beta }} \right)ln\left( t \right)$$

(13)

where \(q_{t}\) is the sorption capacity at time t (mg/g), α is the initial sorption rate (mg/g min), β is the desorption constant (g/mg) during any one experiment [38].

A plot of q_{t} versus ln(t) gives a straight-line graph with a slope of (1/β) and intercept of (1/β) ln(αβ).

#### Intraparticle diffusion model

To describe the adsorption of phenanthrene onto activated carbon from a mechanistic point of view, the Weber Morris intraparticle diffusion model was used. This model is based on the hypothesis that the overall adsorption process maybe controlled either by one or combinations of more than one factors. These include film or external diffusion, pore diffusion, surface diffusion and adsorption onto the adsorbent pore surface [40, 41]. The expression for the model is expressed as Eq. 14:

$$q_t={K_{id}} \cdot {t^{\frac{1}{2}}}+C$$

(14)

where \({{q}}_{{{t}}}\) is the amount of phenanthrene adsorbed at time t (mg/g), \({{K}}_{{{{id}}}}\) is the intraparticle diffusion rate constant (mg/g min^{1/2}) and C (mg/g) is the constant related to the thickness of the boundary layer (the higher the value of C, the greater the boundary layer effect) [40].