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Abstract

Separating different components of a plastic mixture is crucial in its recycling. Among the different separation techniques, flotation was selected as a cheap, non-toxic, and efficient process. Basis of the technique refers to the selective adsorption of a depressant on the plastics surface which cause the alteration in the surface energy of the plastic. Adsorption of the lignosulfonic acid sodium salt (SL) on the surface of the selected available plastics in the waste stream was studied. Plastics used in this study were Polyvinylchloride (PVC), acrylonitrile-butadiene-styrene polymer (ABS), polystyrene (PS), polypropylene (PP), polyoxymethylene (POM), and polycarbonate PC. It results showed that SL adsorbed on the surface of the selected plastics considerably. It was evidenced by the measured equilibrium adsorption capacities ( ${Q_{e}}^{'} s$ ) of the SL and also, the SEM and AFM images. SL adsorbed on the plastic surface in the sequence of $P C > P V C > A B S > P S > P O M > P P$ . Also, the parameters of the Freundlich and Langmuir adsorption models were derived. Experimental data fit with the mentioned models appropriately. However, for the most studied plastics, the Freundlich model was more suitable. As important conclusion, PP separated from plastics mix using the flotation separation technique with the aid of SL as a depressant.

Graphical Abstract

Keywords

Plastic waste management adsorption plastic sodium lignosulphonate isotherm flotation

Introduction

The annual plastics production reached more than 400 million tones.¹ Consequently, plastic recycling is essential to protect the environment and resources.² In most cases, we face a mixture of plastics in the waste stream. For re-using them, they should separate from each other efficiently. Influential separation of an individual plastic from a mixture of waste plastics is a crucial factor for the quality assurance of recycled plastic. Several techniques have been developed for the separation of the plastic(s) from plastics mix, including gravity or density separation,^3–5 selective dissolution,^6–9 triboelectric separation,^10–12 and flotation based separation.^13–15 Among techniques mentioned above, the flotation technique is a simple, cheap, and efficient method. In this technique, a chemical adsorbs on the surface of a selected plastic and alters the surface energy of the plastic. The new formed surface energy makes the plastic surface more hydrophobic, and or hydrophilic depending on the chemical structure of the adsorbed agent. Subsequently, the treated plastic introduces into a flotation vessel filled with a liquid media (water). Depending on the new surface energy of the studied plastic, it may float on the top and or sink at the bottom of the flotation vessel.

Numerous chemicals used as a depressant to alter the sink-float behavior of the selected plastics.^16,17

The sodium lignosulphonate or lignosulfonic acid sodium salt (SL) are prepared from needle-and broad-leaved trees through the treatment of sodium sulfite. It also can be prepared using pulp wastes. It appears as an odorless brown powder. It is cheap, nontoxic, accessible, and easily soluble in water and alkali materials. It is categorizes as an important renewable natural resource material.^18–21 It is a highly branched three-dimensional biopolymer composed of phenylpropane, p-hydroxyphenyl, syringyl, and guaiacyl units.²² It is used in numerous industries, including building, agriculture, dye, and food industries as an efficient dispersant.

The SL is a surfactant having hydrophilic and hydrophobic groups in chemical structure. The hydrophobic groups enable the SL to adsorb on a plastic surface efficiently. However, because it is a salt, it dissociates into several ions when immersed in water. Considering the points mentioned above, it should be a suitable volunteer as a depressant for plastic flotation in an aqueous liquid media.

The adsorption of a chemical on the polymer surface is a surface phenomenon dedicated to the accumulation of chemicals film on a solid (polymer) surface. The numerous aspects should consider during the adsorption-desorption process, including:^23–25

• The availability and the number of suitable active sites on the polymer surface

• The mono or multilayer of the adsorbed chemical on the polymer surface

• The enthalpy of adsorption

• The rate of adsorption and desorption of the chemical on and from the polymer surface

• The Gibb’s free energy of the adsorption process

• The direct and indirect interaction between the adjacent molecules of the adsorbed chemical

To understand the extent and degree of adsorption, two models, Langmuir and Freundlich isotherms are widely used.²⁶ The latter is an empirically based model. According to the Langmuir model, adsorption and desorption are reversible processes. Also, the active sites on the solid surface are equivalent and have unit occupancy.

Based on the knowledge of the authors, there is no systematic study on the adsorption of SL on a group of traditional plastics (polymers). In this study, the parameters of the models mentioned above were derived using the equilibrium concentration of the SL on the surface of the selected polymers. The studied polymers were Polyvinylchloride (PVC), acrylonitrile-butadiene-styrene polymer (ABS), polystyrene (PS), polypropylene (PP), polyoxymethylene (POM), and polycarbonate PC. The results were supported by conventional characteristic techniques, including contact angle ( $θ$ ) measurement, scanning electron microscopy (SEM), and atomic force microscopy (AFM).

Also, an efficient method based on flotation technique with the SL and water as flotation chemical and liquid media, respectively was introduced for separating PP from a plastics mix.

Experimental

Materials

Table 1 represents the specifications of the studied plastics. All used plastics were commercial grades and supplied from local market. The used depressant was lignosulfonic acid sodium salt (SL, CAS number 8061-51-6, TITRACHEM, Iran) with a chemical formula of C₂₀H₂₄Na₂O₁₀S₂. The average molecular weight of SL was 535 g/mol. It was a light tan powder (Scheme 1).

Table 1.

The specifications of the studied plastics.

Plastic	POM	PS (general purpose polystyrene,GPPS)	ABS	PVC	PC	PP
Density ( $g / {c m}^{3})$	1.41	1.04	1.07	1.35	1.20	0.90
Surface tension (mN/m)	44.6	40.7	42.7	41.5	42.9	30.8
Granule shape	Sphere	Sphere	Sphere	Sphere	Sphere	Sphere
Diameter (mm)	4	4	4	4	4	4
Supplier	Korea	Iran	Iran	Iran	Korea	Iran
Specific surface area (N2-BET, $m^{2} / g$ )	3.00	3.06	7.29	1.20	0.80	3.66

Scheme 1.

The chemical structure of lignosulfonic acid sodium salt.

Equipment, testing procedure, and characteristic techniques

All plastics were washed with de-ionized water and dried in a desiccator at ambient temperature for further use. The Brunauer-Emmett-Teller (N2-BET, Belsorp mini II, Microtrac Bel corp, Japan) adsorption method was used to determine the specific surface area of the used plastics (Table 1). 2 g of a specified plastic was immersed in 10 mL of a known SL concentration glass baker. After passing an hour to reach the equilibrium, the equilibrium SL concentration was measured by a UV-visible spectrophotometer (CECIL 8000 series, UK) at 204 nm using the Beer-Lambert law.²⁷ The measured equilibrium concentrations were used to determine the parameters of the Langmuir and Freundlich isotherms subsequently. The flotation experiments were performed as described earlier.^13–15 For flotation experiments, 20 granules of plastic were introduced into the flotation tank. Subsequently, the number of floated granules was used for calculation of the flotation % (20 minus the number of floated granules divided by 20 and multiply by 100). The flotation tank media (tap water) had a pH of 6.5 at ambient temperature.

For flotation experiments, 20 granules of plastic were introduced into a glass-made flotation tank with dimensions, 20 cm (diameter) by 80 cm (height) and kept for 1 min. The flotation tank equipped with an air-blowing system (13 cm circular air distributor diameter) with 4 L/min as air bubble flow rate. All experiments were repeated three times, and the median values were reported. The contact angle ( $θ$ ) between a water droplet (50 µL) and the plastic surface was measured by a stereomicroscope (SZH10) equipped with a camera (Olympus DVP1) with 25X magnitude. The morphology of the plastics surfaces was examined by an MES OEL-PV 1450 field emission gun scanning electron microscope (SEM) with 20 kV and a resolution of 2 nm with a 12 mm working distance. The specifications of the surface of the plastic were evaluated by atomic force microscopy (AFM, full plus series 0101, ARA, Iran). A cleaned glass disc was selected as substrate. The samples were glued using PLL solution on a substrate surface. Subsequently, they were rinsed with de-ionized water and dried with nitrogen before visualization. The mean difference between the highest peaks and lowest valleys ( $R_{z})$ was determined and reported.

Equation (1) introduces a useful adsorption parameter known as the equilibrium adsorption capacity ( $Q_{e}$ ) as below:

Q_{e} = \frac{V (C_{0} - C_{e})}{W S}

(1)

Where

C_{0}, C_{e}

, V, W, and S were the concentrations of the initial and equilibrium SL solution (

m o l . L^{- 1}

), the volume of the solution (L), initial plastic weight (2 g), and the special plastic surface area (

m^{2} . g^{- 1}

), respectively. The unit of

Q_{e}

o l . m^{- 2}

. By multiplying the M.W of the SL (52,000

g . {m o l}^{- 1}

), it converts to

g . m^{- 2}

Results and discussion

Table 1 shows the highest and the lowest special surface area values belonged to ABS and PC with the values of 7.29 and 0.80 $m^{2} / g$ , respectively. The observed low values were due to the negligible porosity of the used plastics.

Table 2 shows the initial and equilibrium SL solution concentrations, and the equilibrium adsorption capacity for the studied plastics. The experiments carried out at 25

℃

. As observed, although the PC had the lowest special surface area (S) among the studied plastics, but the corresponding

Q_{e}

values were the highest. The

Q_{e}

values ranged between 3.06

\times 10^{- 9}

to 31.47

\times 10^{- 9}

m o l / m^{2}

when the initial SL concentration in the solution arose from

1 \times 10^{- 5}

5 \times 10^{- 5}

m o l . L^{- 1}

. Interestingly, the ABS with the highest S value had the lowest

Q_{e}

values in all studied

{C_{0}}^{'} s

. The

Q_{e}

values lay in the order of

P C > P V C > P S > P O M > P P > A B S

. As expected, with increasing the

C_{0}

, the

Q_{e}

values increased. As an illustration, the

Q_{e}

increased from 0.973

\times 10^{- 9}

to 5.809

\times 10^{- 9}

m o l / m^{2}

and 1.100

\times 10^{- 9}

to 5.846

\times 10^{- 9}

m o l / m^{2}

for the POM and PS, respectively, when the

C_{0}

rose from

1 \times 10^{- 5}

5 \times 10^{- 5}

m o l . L^{- 1}

. It concluded, unlike the initial concentration of the SL, the special surface area of the plastic was not influential on the SL adsorption on the surface of the plastic directly. The founds conformed with an earlier report.²⁴ The adsorption of a chemical on a polymer surface depends not only on the special surface area of the polymer but also on the chemical structure of the involved materials. It seemed that the latter parameter was more obvious in the studied systems.

Table 2.

The initial and equilibrium SL concentrations ( $C_{0} a n d C_{e})$ , and the equilibrium adsorption capacity ( $Q_{e}$ , equation (1)) for the studied plastics. The experiments carried out at 25 $℃$ .

Plastic	$C_{0}$ ( $m o l / L)$	$C_{e}$ ( $m o l / L)$ $\times 10^{- 5}$	$C_{e}$ ( $g / L)$	$Q_{e} (m o l / m^{2})$ $\times 10^{- 9}$	$Q_{e} (g / m^{2})$ $\times 10^{- 4}$
PVC	$1 \times 10^{- 5}$	0.935	0.4864	2.68	1.41
ABS		0.963	0.5008	0.25	0.13
PS		0.932	0.4848	1.10	0.57
PP		0.959	0.4986	0.56	0.29
POM		0.942	0.4896	0.97	0.51
PC		0.951	0.4944	3.06	1.59
PVC	$2 \times 10^{- 5}$	1.874	0.9747	5.08	2.64
ABS		1.905	0.9906	0.65	0.34
PS		1.866	0.9702	2.19	1.13
PP		1.904	0.9902	1.26	0.66
POM		1.903	0.9896	1.62	0.81
PC		1.876	0.9755	7.68	3.99
PVC	$3 \times 10^{- 5}$	2.770	1.4403	9.60	4.99
ABS		2.792	1.4519	1.42	0.74
PS		2.814	1.4634	3.03	1.58
PP		2.792	1.4519	2.83	1.48
POM		2.870	1.4921	2.18	1.13
PC		2.677	1.3922	20.05	10.44
PVC	$4 \times 10^{- 5}$	3.615	1.8798	14.52	7.57
ABS		3.643	1.8945	2.44	1.27
PS		3.619	1.8820	6.21	3.24
PP		3.636	1.8906	4.97	2.59
POM		3.689	1.9184	5.17	2.70
PC		3.573	1.8584	26.46	13.79
PVC	$5 \times 10^{- 5}$	4.611	2.3978	16.18	8.45
ABS		4.672	2.4293	2.24	1.17
PS		4.642	2.4137	5.85	3.04
PP		4.453	2.3155	7.45	3.88
POM		4.652	2.4190	5.81	3.02
PC		4.494	2.3368	31.47	16.39

The Langmuir isotherm is a simple physical adsorption model. The Langmuir theory is bases on a kinetic principle.

It declares that the rate of adsorption on a surface is equal to the rate of desorption from a surface. It assumes that there is no interaction among adsorbed molecules, monolayer coverage, and constant binding energy between the adsorbent surface and the adsorbate. The Langmuir isotherm can be express as:²⁴

\frac{1}{Q_{e}} = \frac{1}{Q_{\max}} + \frac{1}{Q_{\max} . K_{L}} . \frac{1}{C_{e}}

(2)

Where

Q_{\max}

(g/

m^{2}

) and

K_{L}

(L/g) are the maximum adsorption capacity and Langmuir isotherm constant, respectively. Table 3 represents the Langmuir’s parameters,

Q_{\max}

and

K_{L}

, coefficient of determination (

R^{2}

), and the equations of the fitted lines drawn by ORIGIN 2019 software for the studied plastics. Figure 1 illustrates the fitted line based on the measured experimental data for the POM in the Langmuir’s model. The Freundlich model is an empirical isotherm. However, it has a theoretical base. It assumes that the solid surface is heterogeneous, and the adsorption energy was distributed on the surface. This model suggests that the solid surface contains several patches which contain sites with the same adsorption energy. It relates the adsorbed amount of material at equilibrium with the initial concentration of the material as a power law dependence as described by following equations:²⁸

{Q_{e} = K}_{F} {C_{e}}^{\frac{1}{n}}

(3)

Log Q_{e} = Log K_{F} + \frac{1}{n} Log C_{e}

Table 3.

The Langmuir’s parameters, $Q_{\max}$ and $K_{L}$ , coefficient of determination ( $R^{2}$ ), and the fitted lines equation for the studied plastics.

Plastic	$F i t t e d l i n e e q u a t i o n$ ^a	$R^{2}$	$Q_{\max} (g / m^{2})$	$K_{L} (L / g)$
PVC	y = 3731.6x − 443.2	0.990	22.5612 $\times 10^{- 4}$	0.1188
ABS	y = 44666x − 14277	0.990	0.7004 $\times 10^{- 4}$	0.3196
PS	y = 8796.1x − 496.4	0.986	20.1464 $\times 10^{- 4}$	0.0564
PP	y = 20573x − 6623.1	0.997	1.5098 $\times 10^{- 4}$	0.3219
POM	y = 9906.9x + 276.3	0.940	36.1946 $\times 10^{- 4}$	0.0279
PC	y = 3703.4x − 1289.9	0.988	7.7534 $\times 10^{- 4}$	0.3483

^a $y = 1 / Q_{e} a n d x = 1 / C_{e}$ .

Figure 1.

An illustration of the fitted line based on the measured experimental data for the POM in the Langmuir’s model.

Table 4 depicts the Freundlich’s parameters,

n

and

K_{F}

, coefficient of determination (

R^{2}

), and the equations of the fitted lines for the studied plastics. As observed, the high

R^{2}

values represent suitable data fitting for both, the Langmuir and Freundlich models. Figure 2 compares the experimental and predicted

Q_{e}

values by the Langmuir and Freundlich models for the selected plastics, PVC, PP, and PC. The predicted values have not considerable deviation from experimental values for the PP at low

C_{e}

’s. However, unlike the PP, a massive deviation was observed for the PC because the higher interaction between the polar groups of the SL molecules with the polar surface of the PC. For the other plastic, the experimental data fit with the Freundlich model better than the Langmuir model. This result can be explained by the fact that the Langmuir model’s assumption, as described earlier, were not valid for the SL-PC system. Our results conformed with those reported, by Gregg and Sing, for PVC.²⁵ Figure 3 shows the SEM images of the PP and PC surfaces. The PP and PC were selected because they had the lowest and highest

Q_{e}

values. For treated plastics, the SL concentration was maximum (

C_{0}

5 \times 10^{- 5} m o l / L)

. As observed, the surfaces of the un-treated plastics were relatively smooth and homogeneous. However, after surface treatment with the SL, they changed to a rough, coarse, and heterogeneous surface. From AFM images (Figure 4), the

R_{z}

values of the untreated PP and PC were 334.3 and 142.3 nm, respectively, which were reduced to 233.9 and 8 nm for the surface-treated plastics with the SL. As expected, the SL adsorption on the plastic surface reduced the

R_{z}

values. However, it was more evident for the PC.

Table 4.

The Freundlich’s parameters, $n$ and $K_{F}$ , coefficient of determination ( $R^{2}$ ), and the equations of the fitted lines for the studied plastics.

Plastic	$F i t t e d l i n e e q u a t i o n$ ^a	$R^{2}$	$K_{F}$ ( $g / m^{2}) {(L / g)}^{1 / n}$	$n$
PVC	y = 1.20x – 3.5	0.981	3.147 $\times 10^{- 4}$	0.836
ABS	y = 1.52 − 4.4	0.965	0.3797 $\times 10^{- 4}$	0.659
PS	y = 1.12x − 3.9	0.948	1.2294 $\times 10^{- 4}$	0.895
PP	y = 1.71x – 4.1	0.981	0.8327 $\times 10^{- 4}$	0.585
POM	y = 1.16x – 4.0	0.901	0.9928 $\times 10^{- 4}$	0.865
PC	y = 1.59x – 3.3	0.973	4.8685 $\times 10^{- 4}$	0.629

^a $y = Log Q_{e} a n d x = Log C_{e}$ .

Figure 2.

The comparison between the experimental and predicted $Q_{e}$ values by the Langmuir and Freundlich models for the selected plastics, PVC, PP, and PC.

Figure 3.

The SEM images of the PP and PC surfaces. The plastic’s surface un-treated and treated with the maximum SL concentration ( $C_{0}$ = $5 \times 10^{- 5} m o l / L)$ before photography.

Figure 4.

The AFM images of the PP and PC surfaces. The plastic’s surface un-treated and treated with the maximum SL concentration ( $C_{0}$ = $5 \times 10^{- 5} m o l / L)$ before photography.

The PP, with a contact angle of 122.8°, was more hydrophobic than the PC with the contact angle of 98.1°. It was considering the high hydrophilic property of the SL results in higher SL adsorption on the PC surface. The SEM and AFM images for the treated plastics confirmed the above-mentioned facts.

Table 5 represents the flotation % of the studied plastics conditioned with different SL’s concentrations at ambient temperature. The liquid media was tap water with a pH of 6.5. As observed, initially, 90, 80, 100, 100, 70, and 75% of the un-conditioned PVC, ABS, PS, PP, POM, and PC, respectively, floated on the tap water. Interestingly, although the densities of the PVC, POM, and PC were remarkably higher than the used tap water (Table 1), only a part of them sunk in the flotation tank. The reason refers to the high surface energy (tension) of the water, which prevents the above-mentioned plastics to sink in the flotation tank. However, immediately after conditioning the plastic surface with different concentration of SL, they precipitated at the bottom of the flotation tank, except PP. It concluded that the SL had not efficient adsorption on the surface of the PP because they have an inherent differences in chemical structure. The comparison between the

Q_{e}

values (Table 2) for the studied plastics confirmed the mentioned conclusion.

Table 5.

The flotation % of the studied plastics vs different SL concentrations at ambient temperature. The liquid media was tap water with a pH of 6.5.

Flotation (%)
Plastic	SL ( $m o l . L^{- 1}$ )
Plastic	None	$1 \times 10^{- 5}$	$2 \times 10^{- 5}$	$3 \times 10^{- 5}$	4 $\times 10^{- 5}$	$5 \times 10^{- 5}$
PVC	90	0	0	0	0	0
ABS	80	0	0	0	0	0
PS	100	0	0	0	0	0
PP	100	100	100	100	100	100
POM	70	0	0	0	0	0
PC	75	0	0	0	0	0

Concluding remarks

From this study, we can conclude that the SL can be adsorbed on the selected plastics surface considerably. It was evidenced by the measured equilibrium adsorption capacities ( ${Q_{e}}^{'} s$ ) and the SEM and AFM images. The SL adsorbed on the plastic surface in the sequence of $P C > P V C > A B S > P S > P O M > P P$ . The experimental data fitted with the studied models. However, for the most studied plastics, the Freundlich model was more suitable. As an important conclusion, flotation technique combined with SL, as a depressant agent, could be used to separate PP from a plastic mixture.

Footnotes

Author contributions

Fateme Shariatikia, Nadia Ostad Movahed and Saeed Ostad Movahed designed the experiments. Dr. Saeed Ostad Movahed prepared the manuscript with contributions from all co-authors. The authors applied the SDC approach for the sequence of authors.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors sincerely thank the staffs of the polymer chemistry laboratory located at faculty of science, Ferdowsi University of Mashhad for their sincere cooperation. Approval no. 3/53876.

ORCID iD

Saeed Ostad Movahed

Data availability statement

The generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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