Recyclable magnetic waste tyre activated carbon-chitosan composite as an effective adsorbent rapid and simultaneous removal of methylparaben and propylparaben from aqueous solution and wastewater

Journal of Water Process Engineering 33 (2020) 101011

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Recyclable magnetic waste tyre activated carbon-chitosan composite as an effective adsorbent rapid and simultaneous removal of methylparaben and propylparaben from aqueous solution and wastewater

T

Geaneth Pertunia Mashilea, Anele Mpupaa, Azile Nqomboloa,b, K. Mogolodi Dimpea, Philiswa N. Nomngongoa,b,c,* a

Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, P.O. Box 17011, Doornfontein, 2028, South Africa1 DST/Mintek Nanotechnology Innovation Centre, University of Johannesburg, Doornfontein, 2028, South Africa c DST/NRF SARChI Chair: Nanotechnology for Water, University of Johannesburg, Doornfontein, 2028, South Africa

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Emerging contaminants Parabens Adsorption technology Regeneration Magnetic adsorbent

A recyclable magnetic waste tyre activated carbon-chitosan composite was synthesized as a suitable adsorbent material in the adsorptive parabens removal from model solutions and real wastewater samples. Characterization techniques such as, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy as well as X-ray diffraction confirmed the formation of adsorbent material. The results of Brunauer–Emmett–Teller isotherms showed that the adsorbent has a specific surface area of 1281 m2 g−1 and pore size of 4.05 nm. These results offer relatively high adsorption capacity for the parabens. Under optimised conditions, kinetics results demonstrated that the adsorption fitted the pseudo-second-kinetic model. While, the simultaneous adsorption of methylparaben and propylparaben was described better by Langmuir and Redlich-Peterson isotherm models. The maximum adsorption capacity for monolayer adsorption from the Langmuir model as 85.9 mg.g−1 and 90.0 mg.g−1 for methylparaben and propylparaben, respectively. It was observed that the composite was stable after seven cycles of adsorption-desorption with obvious loss of adsorption efficiency (> 95%). Therefore, it was concluded that as-prepared composite had considerable reusability properties which could make it a cost-effective adsorbent for the removal of parabens from various media. Furthermore, the eco-friendly and cost-effective magnetic adsorbent was used for remediation of parabens from wastewater samples and up to 100% removal was achieved.

1. Introduction Parabens are esters in an alkyl group derived from the family of phydroxybenzoic acid commonly used in cosmetic products, pharmaceuticals, food and beverages as preservatives due to their antifungal and antimicrobial properties [1]. The four widely used preservatives in daily-use products include Butylparaben (BP), ethylparaben (EP), methylparaben (MP), and propylparaben (PP),) which are either used singly or in combination [2]. They offer advantages which include as low cost, chemical and/or thermal stability with other properties such as low toxicity, odourless, tasteless and non-decolorization action [3]. However, studies have previously indicated that continued exposure to parabens even in low concentrations can cause alteration of the

endocrine system pathway of vertebrates [3]. Recently, evidence of parabens have been identified in human breast tumor tissues have been presented [4] and were also found to as cause of male infertility as a result of testis mitochondrial dysfunctions [5]. In aquatic organisms parabens are considered toxic when the lethal dose (LC50) are greater than 10 mg L−1 and less than 100 mg L−1 [6]. Thus, evidence of toxicity was provided by Lee et al. [6] where acute toxicity test conducted on Daphnids (Daphnina magna) revealed the lethal concentration (LC50) values were 11.4 mg L−1 to 73.4 mg L−1 which therefore indicated that parabens could be to harmful to aquatic organisms. Therefore, various legislations by different countries were introduced in order to control the use of parabens and reduce their potential health impacts on the environment as well as humans [7].

⁎

Corresponding author at: Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, P.O. Box 17011, Doornfontein, 2028, South Africa. E-mail address: [email protected] (P.N. Nomngongo). 1 Formerly known as Department of Applied Chemistry, University of Johannesburg, Doornfontein Campus https://doi.org/10.1016/j.jwpe.2019.101011 Received 18 July 2019; Received in revised form 11 October 2019; Accepted 15 October 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

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were subjected to four widely used models i.e. Langmuir isotherm, Freundlich isotherm, Sips and Redlich-Peterson isotherms. Additionally, the adsorption mechanisms were also assessed by means of kinetic models. Currently, no evidence known to us on the use MWTACC composite has ever been reported in the literature.

The extensive use of parabens contributes largely to their distribution into the environment where the main pathway is via the domestic and industrial wastewater [8]. Despite the efficiency of treatment plant in removal of parabens some are said to be pseudo-persistent under wastewater treatment processes as they have been detected in effluent wastewater at levels of sub-ng mL−1 level concentration, thus posing a risk to environmental waters [9]. Moreover, conventional removal methods such as ozonation [10], chlorine dioxide treatment [11] and photosensitised degradation [12] are effective in removal of parabens, but lead to production of disinfection-by products [12]. Thus, adsorption methods can provide a much safer and environmentally friendly alternative for the removal of parabens in wastewater [13].Where a well-designed adsorption process can results in high efficiency for high quality effluents after treatment which can be recycled [14,15]. The efficiency of this technique is also dependent on the choice of adsorbent material which should be readily available, cost- effective, should have no economic value and feasibility in regeneration [16].Generally, a wide variety of adsorbent material like carbon nanotube, mesoporous silica, biopolymers, fly ash, magnetic nanoparticles [17], activated carbon [18] and chitosan amongst others, has been applied in the adsorption of different pollutants from aqueous solutions [19]. Recently a great interest has been to discover cost effective adsorbent material with adsorption capacities that are high such as biopolymers, activated carbon and natural molecules particularly as adsorbents for organic pollutants like parabens [13,20]. Amongst, the low cost sorbent chitosan has been used to fabricate polymeric sorbent for water decontamination [21]. Due to it being an eco-friendly biopolymer easily obtained by alkaline deacetylation of chitin [22]. In addition it has properties which include, biodegradable, bio-compatibility, non-toxic and wide uses [23]. Moreover, the existence of primary eOH groups, secondary eOH groups and free –NH2 groups, on chitosan makes it a suitable support for chromatographic use [23]. Thus, the use of chitosan in fabrication of several polymeric sorbent in water decontamination [24]. However, chitosan on its own presents some drawbacks based on its weak mechanical strength and dissolution in acidic solution [25].Thus, emphasis is placed on improving its performance by modifications to the physical and chemical properties where various common substances have been used as support for immobilizing chitosan and also forming sorbent were less quantities of chitosan are used [26]. Different types of material have been applied in order to form composites combined with chitosan such as polyanaline [16], perlite [27], magnetite [28], waste material from shoe material [18,29], waste tyre [30,31] Activated carbon [21,25,32–34] amongst others. Activated carbon (AC) has been one very useful adsorbent in the removal of wide variety range of either organic or inorganic pollutants is [35,36]. As such activated carbon from waste tyre offered attractive capabilities due to its attractive adsorption capacity, surface area, pore volume, and pore size distribution. [37]. Therefore, this in turn allows effective distribution of contaminants on its large internal surface, thus allowing for easy accessibility to reactants. In this work, the aim was to synthesize a magnetic waste tyre activated carbon-chitosan (MWTACC) composite with magnetic properties by easier one step co-precipitation method. Characterised of the adsorbent was done using instruments which include: Brunauer–Emmett–Teller (BET), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Adsorption properties for MWTACC material were in analysed in the removal of simultaneous methyl and propylparaben. The adsorption properties of MWTACC composite were investigated for the simultaneous removal of methyl and propylparaben [32]. Batch systems were used to carry out adsorption experiments and the experiments variables including; initial concentration, contact time, mass of adsorbent and sample pH, were optimised by means of response surface methodology (RSM). There equilibrium data for adsorption

2. Experimental 2.1. Reagents and standards Ultra-pure water (Direct- Q® 3UV-R purifier system) was used in all experiments. The analytes methylparaben and propylparaben were reagent grade purchased from Sigma-Aldrich (South Africa) Ltd. Methanol (MeOH (99,9%) and Acetonitrile (ACN (99,9%)) were HPLC grade and purchased from Sigma-Aldrich (St Louis, MO, USA). Stock solutions of parabens (10 mg L-1) prepared and working standards were prepared by subsequent dilution of stocks with Ultra- pure water. 2.2. Sample and sample collection Both raw (influent) and treated (effluent) wastewater samples were used in this study. Sewage wastewater samples were collected in different points in the Daspoort wastewater treatment plant (Pretoria, Gauteng, South Africa). The samples were collected in pre-cleaned 500 mL glass bottles. After sampling the water samples were stored at 4 C for a maximum of 1 week until being analysed. 2.3. Instrumentation Characterization for the morphological properties of the sorbent material were analysed with the use of, scanning electron microscopy (SEM, TESCAN VEGA 3 XMU, LMH instrument (Czech Republic) coupled with energy dispersive x-ray spectroscopy (EDS) for elemental composition analysis at an accelerating voltage of 20 kV and a 120 kV accelerating voltage transmission electron microscope (TEM JOEL JEM2100, Japan). Samples for TEM analysis were prepared by dispersing the composite in methanol ultrasonically for 15 min. The samples were then placed into copper grid. While the amorphous structure of MCAC was acquired using X-ray diffraction (XRD). The adsorbent pore size distribution the surface area (SBET) were determined by N2 adsorptiondesorption isotherm using BET multipoint method using Surface Area and Porosity Analyzer (ASAP2020 V3. 00H, Micromeritics Instrument Corporation, Norcross, USA). Pore volumes were calculated using the Barrerr-Joyner-Halenda method. FT-IR spectra were recorded on a Perkin-Elmer Spectrum 100 spectrometer (Perkin-Elmer, USA) at room temperature by the KBr pellet technique, in the region 400–4000 cm−1.Chromatographic analysis was carried out by an Agilent HPLC 1200 infinity series, equipped with photodiode array detector (Agilent technologies, Waldbronn Germany). The chromatograms were recorded at 250 nm and 260 nm. An Agilent Zorbax Eclipse Plus C18 column (3.5 μm × 150 mm × 4.6 mm) (Agilent Newport, CA, USA) was at an oven temperature of 25 °C. The mobile phase consisted of water: methanol mixture, 30% water (Mobile phase A) and 70% methanol (mobile phase B). Flow rate of 0.1.00 mL min−1 was used for the entire analysis. An OHAUS starter 2100 pH meter (Pine Brook, NJ, USA) for pH adjustments of reagents and pH of samples. 2.4. Synthesis of sorbent material 2.4.1. Preparation of waste tyre based activated carbon Waste tyre activated carbon (WTAC) was synthesized using a method described by our research group [37]. Briefly, a pyrolysis process was used to prepare carbonaceous material, where waste tyre was cut into small pieces from its sides in order to prevent the steel wires. This pieces were then washed and put into quartz tube and placed in electric tubular furnace at 900 °C under N2 purging (2 mL 2

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min−1). Activation of obtained carbonaceous material was done by chemical activation method, where hydrogen peroxide (H2O2) was the activating agent using a microwave system for heating in the synthesis process. This was carried out in Terflon vessels in the microwave. Two separate carbon products were synthesized at differing power intensities 300 W and 600 W, and irradiation time was kept constant at 15 min. The desired ratio for chemical impregnation of material was set at 1:10 (m/m) carbon black/H2O2. Final products for activation process from the microwave were washed to a neutral pH and then stored for further studies.

qe =

3. Results and discussions 3.1. Characterization of adsorbent Morphological properties for chitosan, magnetic activated carbon and prepared MWTACC composite were studied by SEM and the images shown in Fig. 2A–C. As presented, Fig. 2A shows smooth surface of chitosan with small pores whereas magnetic waste tyre activated carbon (MWTAC) exhibited more porous structure. Fig. 2C shows bead like porous structure. These observations revealed that the introduction of chitosan resulted in the formation of bead like structures. The elemental analysis done by energy dispersive X-ray spectroscopy (EDS) (Fig. 2D) revealed C, O, N and Fe confirming the existence of chitosan, active carbon and Fe3O4. Fig. 3 Shows XRD patterns of waste tyre activated carbon, chitosan and MWTACC. XRD patterns for MWTACC indicated the presence of Fe3O4 with six distinct peaks with 2-Theta values at 30.1°, 35.4°, 43.1°, 53.4°, 56.9°, 62.5°. Pure chitosan comparison showed qualitative changes observed at 2θ = 10° and 20° confirming that the incorporation of chitosan into MWTAC changed the crystalline structure of chitosan. Moreover, the binding process of chitosan did not cause a phase change of Fe3O4. The TEM images (Fig. 4) of MWTACC particles indicated different contrast of sorbent were the darker areas present the Fe3O4 while the bright regions are connected to activated carbon/chitosan beads. The TEM images of the composite confirmed that the incorporation of chitosan resulted in the formation of beads. Fig. 5 shows FTIR spectra WTAC, MWTAC, MWTACC material were mixed with Potassium Bromide (KBr) at ratio 1:15 prior to characterization. The FTIR spectra for synthesized MWTACC sorbent showed vibration bands at 632.1, 580,9 and 438.9 cm−1 to indicate characteristic peaks for magnetite [38].Where the intense band at 580.9 cm−1 could be assigned to vibration of Fe+2-O functional groups and the splitting-up peak at 632 cm−1 was a result of octahedral B sites symmetry degeneration. This is a feature not present in the WTAC. WTAC shows 3 peaks 3386.9, 1579 and 1125 cm-1 which are assigned to −OH, C]O and C–O stretch vibration. While for MWTAC has peaks at 3386.9 and 1579 cm−1 corresponds with -O-H and C]O vibrations respectively. The broad peak 1135cm-1 may be as a result of coating magnetite on the surface of activated carbon(AC). In the case of MWTACC the broad wide band at 3386.9 cm−1 corresponds to NeH and OeH stretch vibrations and also intramolecular hydrogen bonds as this indicates the interaction between amine groups in Chitosan. The BET surface area together with pore size and pore volume for MWTAC composite, chitosan, MWTACC composite were measured using nitrogen adsorption/desorption technique (Table 2) The MWTACC surface area slightly increased with addition of magnetic nanoparticles. Furthermore, according to Table 1, MWTACC composite BET surface area decreased after incorporation of chitosan. Slight changes in the surface characteristics of composite might be due to chitosan filling in the pores of WTACC composite. Point of zero charge (pHpzc) defined as the colloidal particle’s sliding plane of and associated with the particle surface charge. [39]. Zeta potentials values for MWTACC are plotted in Fig. 6. Obviously the pHpzc for the MWTACC composite fell at pH 8. These results implied that the adsorbent surface was positively charged below the pHpzc value while negative above the pHpzc.

Ferrous sulphate and Ferric chloride solutions were dissolved at the ratio 1:2 Fe2+/Fe3+ and with 5 min stirring. Then 4 g of activated carbon was added with vigorous stirring into the solution and heated at 70 °C. Subsequent addition sodium hydroxide (NaOH) (100 mL) into the mixture was performed with further heating at 70 °C for 30 min. Once cool a magnet was used to remove the formed black precipitate during colour change of the mixture. The formed mixture was then washed by ethanol/water (50%) solution for removal of any impurities. Lastly, the obtained magnetic sorbent was dried in an oven at 80 °C for 2 h. 2.6. Preparation of magnetic waste tyre activated carbon coated with Chitosan (MWTACC) The synthesis of magnetic waste tyre coated activated carbon (MWTACC) was performed based on the methodology developed by Shariffard and colleagues [25]. Briefly, 17.5 g MWTAC (Magnetic waste tyre based activated carbon) synthesized above was poured in 0.2 M Oxalic Acid (C2H2O4) for 2 h and washed using deionised water, filtered and dried at 70 °C in an oven for 12 h. Chitosan (10 g) was then added into 1 L of solution C2H2O4 with stirring continuously at 40–50 °C to form viscous gel. Thereafter, 10 g acid treated MWAC was added to the gel and stirred further at 45–50 °C for 12 h. Coating of MWTAC with chitosan beads was formed by adding MWAC gel mixture drop wise to a NaOH (0.7 M) precipitation bath. 2.7. Batch adsorption experiments Batch adsorption experiments was performed on effects of significant parameters which included; pH, contact time, adsorbent dosage, and temperature on the adsorptive removal of two parabens onto the synthesized MWTACC. This was done on a series of capped polypropylene bottles at room temperature (25 ± 1 ℃). The preparation of standard solutions was by dilution of stock solution (50 mg L−1) of parabens mixture (1:1 of MP:PP). The adjustment of the pH of solutions to the required values (4.0–8.0) was performed by adding small amounts acetic acid 1 mol L−1 and ammonium hydroxide solution. Suitable quantities of the adsorbent (20–70 mg) were added into clean polypropylene bottles and then 50 mL of parabens mixture with concentration ranging between 5 to 15 mg L−1 were mixed with synthesized sorbent. This mixture was then sonicated for about 20–35 min. The supernatant was then separated from sorbent in the presence of an external magnet (Fig. 1) and filtered by means of a syringe filtered with 0.22 μm PVDF filters. The initial concentration and equilibrium concentration of MP and PP were measured using HPLC-DAD. The optimization of adsorption process was carried using experimental approach and Table 1 summarizes the factors and their level. Calculations of the removal efficiency for MWTACC sorbent were done using Eq. (1)

Co − Ce × 100 Co

(2)

Where M (g): adsorbent mass and V (L): is the volume of the liquid sample

2.5. Preparation of magnetic waste tyre based activated carbon (MWTAC)

%RE =

(Co − Ci) V m

(1)

While calculation of the adsorption capacity (qe) was done using Eq. (2) 3

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Fig. 1. A) Magnetic adsorbent dispersed in sample solution during the adsorption process and B) Separation of sorbent using external magnet.

confidence level red line (p-value = 0.05), it means that the factor or interactions is statistical significance. According to Pareto chart (Fig. 7) for exploring the significant effect of assigned factors on the process of adsorption, such are the sample pH, initial concentration of sample (InConc) and also mass of adsorbent (MA) proved significant at the 95% confidence level. Contact time (CT) and interaction between the factors were insignificant at the 95% confidence level. The three-dimensional 3-D response surface plots shown in Fig. 8 are to demonstrate the simultaneous effect of a pair of variables in the removal of parabens while other factors were fixed at zero level (central point). As seen in this Fig. 8, several curvatures observed were ascribed to interaction between the investigated variables. According to the 3-D plots the removal of MP and PP increases when the adsorbent mass was above 60 mg. This results from the increasing surface area and also availability of binding sites on adsorbent. This was also carried out under different pH ranges in which maximum recoveries were obtained at sample pH below 8. This is because at pH ≥ pKa (pKa values for MP = 8.17 and PP = 8.5) the ionised forms of MP and PP are

Table 1 Experimental levels and ranges of independent variables. Parameters

Lower level (−)

Central point (0)

Higher level (+)

Adsorbent Mass (mg) Initial Concentration (mg L−1) Contact time (min) pH

20 5 20 4.0

45 10 35 6.0

70 15 50 8.0

3.2. Optimization strategy Effects of influential parameters were valuated i.e.; mass of adsorbent, sample pH, for the removal of MP and PP from aqueous solution was using response surface methodology (RSM) obtainable via central composite design (CCD). Pareto chart from the analysis of variance (ANOVA) was applied to view the significance of the interactive as well as individual effects (Fig. 7). If the bar length passes the 95%

Fig. 2. Images of SEM indicating; A) chitosan, B) magnetic waste tyre activated carbon (MWTAC) composite, C) magnetic waste tyre activated carbon-chitosan (MWTACC) composite and, D) EDS spectrum of (MWTACC) composite. SEM magnification = 8.15 kx; SEM HV=20 kV. 4

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Fig. 3. XRD of chitosan, magnetic waste tyre activated carbon (MWTAC) composite, magnetic waste tyre activated carbon-chitosan (MWTACC) composite.

predominant; while at pH ≤ pKa, the molecular forms will be the dominant in the solution [40]. Consequently, as pH increases from 4 to values slightly below 8 (point of zero charge), the adsorbent surface is positive and ionised forms lead to an enhanced electrostatic attraction. Sample pH values above 8 led to decrease in %RE due electrostatic repulsion. Furthermore, the adsorption of MP and PP on the adsorbent was conducted at different InConc of samples ranging from 2 to18 mg.L−1 (Fig. 5). It was observed from the response surface graphs that MP and PP adsorption efficiency was enhanced with InConc ranging between 8 and 10 mg.L−1 with contact time between 20–60 min. The reasoning behind this is that the adsorbent with a large amount of activated sites could provide sufficient adsorption sites for the two parabens. The desirability function (DF) for optimizing investigated variables at the same time. Applying the method of desired function (Fig. 9), optimum conditions were: mass of adsorbent at 64 mg, initial concentration at 63 mg.L−1, sample pH value 6 and contact time of 57 min with an overall desirability value of 1.00. Contact time of 57 min was

found to rather too high and it was reduced to 35 min. The optimal conditions were validated experimentally and %RE of 99.3% was obtained. These results agreed with the predicted values at 95% confidence level. 3.3. Adsorption models Adsorption isotherms models are crucial in describing interaction behaviour of analytes and sorbent material [41]. Thus to optimise the design for the absorption process, the establishment of the most suitable correlation for the equilibrium curve was of vital importance [42]. The equilibrium data for the removal analysis of MP and PP was carried using different isotherms model such as Langmuir model, Freundlich, Sips and Redlich-Peterson isotherms. The linear expressions for each isotherm are presented in (Eqs. (3) and (4)) 3.3.1. Langmuir isotherm model This equation is applicable in homogeneous adsorption, where

Fig. 4. TEM of magnetic waste tyre coated activated carbon-chitosan (MWTACC) under different magnifications. 5

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adsorption of the individual absorbate molecules has equal sorption energy on the surface [43] Linear expression of the model is expressed as:

1 1 1 = Ce + q qmax KL qmax

(3) −1

Where: qm theoretical monolayer saturation capacity (mg. g ), Ce described as unadsorbed adsorbate concentration of a solution at equilibrium (mg.L−1), qe: amount of adsorbed adsorbate per unit weight of adsorbent (mg. g−1), KL Langmuir equilibrium constant (L mg−1). In addition, the crucial properties of the Langmuir isotherm are presented in terms of a dimensionless constant called separation factor defined by Eq. (4)

RL =

1 1 + KLC0

(4) −1

Where Co: highest initial concentration of absorbate (mg.L ) and RL values denotes the shape of isotherms as either favourable (0 < RL < 1), unfavourable (RL < 1), irreversible (RL = 0) or linear (RL = 1). The equilibrium isotherm data for the adsorption for both parabens fitted Langmuir isotherms with higher correlation coefficient R2 (Table S1-2). The Langmuir isotherm proved the sorbent to be homogenous therefore, thus assuming monolayer adsorption as all the sorption sites were uniform. Further observation indicated that values of qmax and KL were higher for adsorption of PP than MP. Moreover, the separation factor (RL) lied between 0 and 1 (0 < RL < 1) for both parabens showing that it was favourable adsorption process.

Fig. 5. FTIR spectra of WTAC, MWTAC AND MWTACC. Table 2 Surface characteristics of waste tyre activated carbon (WTAC), chitosan, magnetic waste tyre activated carbon (MWTAC) composite and magnetic waste tyre activated carbon-chitosan (MWTACC) composite. Parameter 2

−1

BET surface area (m g ) Pore Volume (cm3 g−1) Pore size (nm)

Chitosan

TWAC

MTWAC

MTWACC

2.54 0.046 108

1104 0.67 4.51

1387 0.41 3.66

1281 0.53 4.05

3.3.2. Freundlich isotherm models The most crucial multilayer adsorption isotherm to describe heterogeneous surfaces. Its linear form is expressed by Eq. (5) [44]

ln qe = ln KF + ln Ce

(5)

Where qe(mg/g): amount of MP and PP adsorbed, Ce : equilibrium concentration of MP and PP in (mg L−1), KF : Freudlich constant (L g−1) and n: Freundlich exponent (g L−1). The Freudlich isotherms were tested in both parabens for sorbent MWTACC (Table 3 Figs. S3 and S4). The resultswhowed that the Freundlich exponent (n) remained constant for both MP and PP adsorption while Freundlich constant (KF) was found to be higher for PP than MP. In addition, the value of n was between 1 and 10 also proving that the process was favourable. 3.3.3. Sips adsorption isotherms This is a combination of two models, Langmuir and Freundlich isotherms derived due to their limiting behaviour. Valid for confined adsorption without adsorbate-adsorbate interaction [45,46]. Sips linear equation is expressed as Eq. (6):

Fig. 6. Point of zero charge of MWTACC for different solution of pH.

Fig. 7. Pareto chart for standardized effects on variables for the adsorptive removal of A) methylparaben and B) propylparaben. 6

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Fig. 8. 3-D plots for the interaction of optimum parameters for adsorption. 1

n 1 1 ⎛1⎞ + 1 = qe Qmax Ks ⎝ Ce ⎠ Qmax ⎜

n = 1, the equation is reduced to Langmuir and implies a homogenous adsorption process. Adsorption data for MP and PP were tested in Sips model and results are depicted in Table 4 (Figs. S5 and S6). The Sips isotherm constant (Ks) increases slightly for PP as compared to MP. While, ns equals unity (n = 1) and was constant for both parabens indicating homogeity on the adsorbent surface.

⎟

(6)

Where KS:denotes Sips equilibrium constant (1/mg), Qmax: maximum adsorption capacity (mg. g−1) and n describes surface heterogeneity. Therefore, when n is between 0 and 1 it depicts heterogeneity but, if

Fig. 9. Desirability function for optimization of independent variables. 7

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Table 3 Adsorption isotherms of MP and PP on MWTACC. Model paramaters Langmuir isotherms qmax (mg g−1) KL (L mg−1) R2 RL Freundlich isotherms KF (L/g) n R2

Table 5 Adsorption Kinetics.

Methylparaben

Propylparaben

85.9 2.54 0.9775 0.07–0.2

90.0 4.83 0.9814 0.04–0.12

66.3405 2 0.9371

89.2730 2.10837 0.9771

Table 4 Sips and Redlich-Peterson isotherms. Parameters Sips Isotherms Ks(L mg−1) Qmax(mg g−1) nS R2 Redlich-Peterson KR (L mg−1) αR βR R2

Metylparaben

Propylparaben

0.0212 85.5 1 0.9828

0.0619 90.0 1 0.9655

5.0 1.9577 1.2421 0.9981

5.0 0.6303 1.0848 0.9521

Methylparaben

Propylparaben

Pseudo-first order K1 (min−1) qe (mg. g−1) R2

0.0289 3.89 0.7831

0.0239 4.08 0.8607

Pseudo-second order K2 (g mg−1 min) qe (mg g−1) R2

0.0112 8.47 0.9829

0.0114 7.49 0.9896

Intraparticle diffusion Kidi Kid2 C1 C2 R2 R2

16.4 1.26 12.5 63.4 0.9849 0.8236

11.8 0.205 0.841 69.3 0.9984 0.7082

Boyde model R2

0.7676

0.8672

Elovich α β R2

1.65 0.0481 0.9235

1.97 0.0611 0.9461

3.4.1. Pseudo-first order Pseudo-first order model proposed by Lagersen for adsorption analysis [47]. The linear form is expressed by equation:

3.3.4. Redlich-peterson adsorption isotherms The model combines features of Langmuir and Freundlich isotherms which can be expressed as Eq. (7). Where Redlich-Peterson constants are KRP (L. mg−1) and αRP. R-P isotherms also use similar parameters to that of Sips isotherms. Equilibrium data for indicating the adsorption of both parabens was also fitted onto the Redlich-Peterson isotherms and results given in Table 4 (Figs. S7 and S8)

ln (KR − 1) = bR ln Ce + ln α

Kinetic model parameters

ln(qe − qt ) = ln qe − k1 t

(8)

Where; K1 (min−1) denotes pseudo-first order adsorption kinetics parameter; qt: amount adsorbed at time t (min) while qe: amount adsorbed at equilibrium (mg. g−1). The results for first order kinetics are shown in Table 5. The amount of MP sorbed onto MWTACC at different times decreased with an increase in time, similarly with PP adsorption. Moreover, the values of the K1 were slightly higher for the adsorption of MP than PP. A large deviation was also detected between experimental qe (exp) and calculated qe (cal) values, this might have been caused by the poor fit of the data into pseudo first order kinetic model.

(7)

The parameters αR: R-P isotherm constant and βR: exponent that lies between 0 and 1. If βR = 1, then R-P isotherm equation converts to Langmuir equation and if βR = 0 it is leans towards Freundlich equation. According to the αR data for R-P isotherms in Table 4 both MP and PP equilibrium data depicted the Langmuir monolayer characteristic. Testing of both isotherm models, it can be inferred that value of separation factor (RL) was between 0 and 1 for both parabens thus, proving that the conducted absorption process were favourable. Thus, the adsorption processes favoured Langmuir isotherm. This was due to their R2 values closer proximity. Moreover, Sips and Redlich-Peterson isotherms constants (ns and βR) also proved that the prepared adsorbent surface nature was homogenous

3.4.2. Pseudo-second order Pseudo-second order model used to describe sorption kinetics, where their linear form is expressed as Eq. (8) [42]

1 1 1 = + qt qe k2 qe2

(9)

3.4. Adsorption kinetics

Where k2 (g. mg−1 min−1) denotes pseudo second order rate constant for adsorption. If pseudo-second order kinetic model is applicable, a plot of 1 against t provides a linear relationship and from its slope and

Investigate mechanisms for MP and PP onto the MWTACC composite, where the pseudo-first-order and pseudo-second-order models were studied. Results obtained showed (Table 5, S9-S16), the kinetic interaction of two parabens with the composite were best explained using pseudo-second-order and Elovich models confirmed by high R2 values. Furthermore, the calculated equilibrium adsorption amount (qecal) for pseudo-second-order model with that of experimental (qeexp) were in agreement. Pseudo-second-order undertakes that, rate-limiting factor for the adoption of parabens by MWTACC was determined by chemi-sorption hence removal of MP and PP onto the composite was chemisorption more than physical sorption. Mechanisms of the adsorption process were studied by plots of intraparticle diffusion and Boyde plots. Where the parameters for both models are shown in Table 5.

intercept K2 and qe can be deduced. The plot of Eq. (8) gave excellent linearity R2 < 0.98 for the two parabens. Pseudo-first order and pseudo-second order kinetic parameters are determined by considering their correlation coefficient R2. Thus, based on Table 5, the adsorption kinetics of MP and PP onto MWTACC was explained better by with pseudo-second model either that pseudo-first order based on the higher R2 value of pseudo-second order in both parabens. Moreover, qcal values estimated from the pseudo-second order model were in close proximity to the experimental values qexp. Therefore, this indicated that the adsorption of MP and PP onto the MWTACC was controlled by chemical processes. Adsorption process passes through various stages which involves the transport of absorbate from aqueous phase to the surface of adsorbent and also diffusion of the adsorbate to the interior of the adsorbent pores, characterised a slow process [48].

qt

8

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3.4.3. Elovich model This model is mostly applicable for chemisorption processes where the model is presented as Eq. (13):

qt =

1 1 ln(αβ ) + ln t β β

(13) −1

Where; α denotes initial adsorption rate (mg g min) while β relates to the extent at which the surface is covered and the activation energy is described as chemisorption (g mg−1). This model was analysed so to recognize the rate determining step. Linear plot for the Elovich model gave high correlation coefficients (Table 5) which suggested that adsorption process is governed by chemisorptions which takes into account the electrostatic interaction between adsorbent and parabens. Fig. 10. Regeneration studies of MWTACC composite. Conditions for experimental: mass of adsorbent =60 mg; pH = 6.5; extraction time =35 min; initial concentration =2 mg L−1; sample volume = 50 mL; eluent = methanol; eluent 5 mL; n = 3.

3.4.4. Intraparticle diffusion This equation is given by Eq. (10) [49] 1

qt = kid t 2 + C

(10) −1

Where; qt (mg g ) amount of solute on sorbent surface at a time t, kid (mg g−1 m1/2): intra-particle diffusion rate constant. This model is used for describing adsorption that is competitive. The linear portion for extensive range of contact time of the plot between adsorbent and adsorbate did not pass through the origin or near saturation could be due to the variation of mass transfer in initial and final stage adsorption [48]. Thus, indicating that the sorptive removal for both parabens consisted of the first-boundary layer then intra-particle diffusion. Constant C value (intercept value that provides an indication of boundary layer thickness) [50] was increased from 12.5 to 63.4 for MP and 0.84 to 69.3 for PP, where the large value indicates a larger influence of the boundary layer. Therefore, it was confirmed that adsorption of parabens was controlled by the intra-particle and also liquid film diffusion model.

coated with chitosan composite has the potential applicability in water treatment. 3.6. Applications in real samples Applicability of MWTACC was analysed by carrying out adsorptive removal of MP and PP from influent wastewater samples. The samples were first filtered to eliminate particulates and then analysed under optimum conditions. The initial concentrations of MP and PP in influent wastewater were determined by solid phase extraction using the prepared adsorbent and the results were verified by using commercial SPE cartridge (Table 6). The removal efficiency were 100% for both methylparaben and propylparaben (Table 6). Therefore, the results indicated that MWTACC can be applicable for adsorptive removal of MP and PP in real samples with complex matrix.

3.4.5. Boyd model To determine whether the adsorption process continued through an external or intraparticle diffusion mechanism, the kinetics data was subjected to Boyd kinetics model (Eq. (11)) [51]

Bt = −0.4977 − ln(1 − F )

3.7. Comparison of MWTACC with other adsorbents The adsorption capacity of MWTACC for this study was compared with other adsorbent reported in literature (Table 7). It was observed that the as-prepared adsorbent showed much higher adsorption capacity for MP and PP as compared the reported adsorbents in literature (Table7).

(11)

Where Bt: represents the mathematical function F where F: is the fraction of the solute adsorbed at any given time, t (min), calculated by Eq. (12).

F=

qt q0

4. Conclusion

(12)

Where; q0; amount of adsorbates at infinite time (mg. g−1) and qt : is the amount of analytes adsorbed a given time t (min). Bt values at various contact time are plotted against time t. Boyd plots (Table 5) revealed that the adsorption mechanism had an element external mass transport were particle diffusion because the linear plots which did not pass through the origin.

The isotherm and kinetics of magnetic water tyre carbon-chitosan based sorbet (MCAC) on the uptake of parabens from water system was investigated. The equilibrium data were fitted into Langmuir, Freundlich, Sips and Redlich-Peterson isotherms models and according to results obtained, the Langmuir isotherm model made for the best fit. This was indicated through the maximum monolayer adsorption capacity of 85.87 and 90.0 (g/g) for MP and PP respectively. Moreover, the kinetics models proved to follow a pseudo-second order kinetic model than first-order, and this is evident on their R2 values, meanwhile intra-particle diffusion indicated that the boundary layer had less significant effects on the diffusion mechanism of the sorbate. In addition, the Elovich model indicated that intra-diffusion was the rate

3.5. Stability and regeneration Adsorption-desorption regeneration experiments were done in order to investigate the reuse of MWTACC composite as this is a necessary feature in treatment processes for water. This study, regeneration of the adsorbent was carried according to previous studies reported elsewhere [52]. The analytes were desorbed using methanol and the adsorbent was washed three times using deionised water before use. Recoveries of each analyte using the regenerated and recycled magnetic adsorbent are presented in Fig. 10. The results obtained demonstrated that recovery and adsorption for MP and PP were not affected for up to seven adsorption/desorption cycles. Therefore, this demonstrated the great reusability of the synthesized composite and also indicated its excellent regeneration properties. Thus, magnetic waste tyre activated carbon

Table 6 Initial concentration in ng L−1 of Methylparaben (MP) and Propylparaben (PP) from influent water samples.

9

Samples

MP initial concentration

PP initial concentration

Influent 1 Influent 1 Influent 1

947 ± 3 889 ± 13 1293 ± 20

108 ± 1 1988 ± 9 2113 ± 15

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Table 7 Comparison for the amount of parabens removed by different adsorbents. Adsorbent

Adsorption Capacity (mg g−1)

References

β- CD-HMDI β-CD-TDI Il-MNP-βCD-TDI Activated carbon based on coconut Fe3O4@SiO2-NH2 MWTACC

MP = 0.0305; PP = 0.1854; MP = 0.1019; PP = 0.2551 PP = 18.48 BP = 7.52 MP = 75 MP = 85.9; PP = 90.0

[12] [12] [53] [54] [40] This work

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