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Abstract

The separation of one or a group of plastics from a plastics mix by flotation technique is an essential topic in plastic waste management. Basis of the technique refers to the selective adsorption of a depressant on the surface of the plastic. Consequently, the surface energy of plastic alters. Adsorption of the lignosulfonic acid sodium salt (SL) on the surface of the selected available plastics in the waste stream was studied at different SL concentrations and operative temperatures. Studied plastics were Polyvinylchloride (PVC), acrylonitrile-butadiene-styrene polymer (ABS), polystyrene (PS), polypropylene (PP), polyoxymethylene (POM), and polycarbonate PC. Based on the measured equilibrium adsorption capacities ( $Q_{e}^{'} s$ ), the SL adsorbed on the selected plastics surfaces with different amounts. Increasing the temperature had a positive effect on the above-mentioned parameter. Suitable conformity was observed between the experimental results with predicted values by the traditional adsorption isotherms, Freundlich and Langmuir. However, for the most studied plastics, the Freundlich model was more suitable than Langmuir. Measured thermodynamic parameters showed a spontaneous adsorption process along with endothermic (except of PC) reactions. The complete separation of PC from a plastics mix in all SL concentrations and temperatures was observed during the flotation process.

Graphical Abstract

Keywords

Adsorption plastic sodium lignosulphonate isotherm flotation

Introduction

To save natural resources, and avoid environmental impacts, waste plastics should be recycled.¹ In most waste streams, there is a mixture of different types and grades of plastics. As a preliminary step for the plastics recycling, the available plastics in a plastics mix should be separated from each other appropriately. Regardless of manual sorting, there are various developed separation techniques, including electric separation,^2–4 selective dissolution,^5–8 and density separation.^9–12 The recently developed floatation technique^13–16 is a safe, economical, and efficient separation technique for separating an individual and or a group of plastics from a plastics mix. The alteration of the surface energy of the plastic is the basis of the mentioned technique. Approaching and or distancing the surface energy of the plastic with the surface tension of the used liquid media, i.e., water, may cause sink and or float the plastic in the floatation tank, respectively.

Generally, the surface energy of the plastic changes by the adsorption of a chemical (depressant) on the surface of the plastics.

The adsorption process depends on several parameters,^17–19 including the available active sites on the plastics surface, mono or multilayer of the adsorbed depressant on the polymer surface, the enthalpy of adsorption, the Gibb’s free energy of the adsorption, The indirect and direct interaction between the adjacent molecules of the adsorbed depressant, the solubility of the depressant (for a liquid-solid system), and the pressure (for a gas-solid system) and the temperature of the system.

In general, adsorption is an exothermic process.²⁰ Therefore, it is expected that with increasing temperature, the adsorption of the depressant on the surface of the plastic decreases. It is true for gas-solid systems. However, in adsorption from solutions, as observed in the floatation technique, temperature affects not only the adsorption process but also may change the solubility of the depressant in the liquid media. Since the solubility of a depressant is a crucial parameter in the adsorption process, the solubility factor should be considered in any investigation of adsorption from solutions.

So far, numerous depressants have been introduced in literature.^{13–16,21,22} However, among them, sodium lignosulphonate or lignosulfonic acid sodium salt (SL) as a biopolymer (Scheme 1) has distinguished characteristics, i.e., non-toxic, economy, water-soluble, renewable natural resource substance, and accessible in commercial grade in the market.²³

Scheme 1.

The chemical structure of used lignosulfonic acid sodium salt.

In many industries, namely food, building, dye manufacturing, and agriculture, it is applicable as a strong surfactant. Normally, SL extracts from broad and needle-leaved trees using sodium sulfite and pulp wastes. The guaiacyl, phenylpropane, syringyl, and p-hydroxyphenyl are the primary chemical parts of the highly branched three-dimensional structure of SL.²⁴ As shown in Scheme 1, SL has hydrophobic and hydrophilic groups in chemical structure. Because it is a salt, it readily dissolves in water. Simultaneously, the hydrophobic groups of the SL facilitate the adsorption of the substance on the surface of the plastic. Considering aspects mentioned above, Sl should be a suitable depressant for the sink-float of the plastics in aqueous liquid media.

To describe the adsorption of a chemical on the surface of a solid, two traditional mathematical models are available, Freundlich and Langmuir isotherms.²⁵ The Langmuir is a theoretical based model. It has several assumptions, including a reversible process for the adsorption and desorption, and equivalent and unit occupancy for active sites on the solid surface. However, the Freundlich isotherm is an empirical based model.

Based on the knowledge of the authors, there is no systematic study on the adsorption of SL on a group of traditional plastics (polymers) at different temperatures. 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 at different temperatures. The studied polymers were acrylonitrile-butadiene-styrene polymer (ABS), Polyvinylchloride (PVC), polypropylene (PP), polystyrene (PS), polyoxymethylene (POM), and polycarbonate PC. Also, the separation of the studied plastics from each other in their mix was studied using the novel floatation technique with the SL and water as floatation chemical and liquid media, respectively.

Experimental

Materials

The 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 represents the specifications of the studied plastics. All used plastics were commercial grades and supplied from local market.

Table 1.

The specifications of the studied plastics.

Plastic	POM	PS (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, $\frac{m^{2}}{g}$ )	3.00	3.06	7.29	1.20	0.80	3.66

Testing procedure, characteristic techniques, and equipment

All plastics were washed with de-ionized water and dried in a desiccator at ambient temperature before 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). Two grams of a specified plastic immersed in 10 mL of a known SL concentration (1–4 × 10⁻⁵ mol/L) glass baker at selected different temperatures 30, 50, and 70°C. 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.²⁶ 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.^13–16 For flotation experiments, 20 granules of plastic were introduced into the flotation tank. The flotation tank media (tap water) pH was 6.5 at the above-mentioned selected temperatures. The dimensions of the glass-made flotation tank were 20 cm (diameter) by 80 cm (height). The flotation tank equipped with an air-blowing system (13 cm circular air distributor diameter) with 4 L/minute as air bubble flow rate. The residence time of the granules in the flotation tank was kept at 1 min. All experiments were repeated three times, and the median values were reported.

Results and discussion

Table 1 shows the highest and the lowest special surface area values belonging to ABS and PC with the values of 7.29 and 0.80

\frac{m^{2}}{g}

, respectively. The observed low values were due to the negligible porosity of the used plastics. 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 (mol/L), the volume of the solution (L), initial plastic weight (2 g), and the special plastic surface area (

\frac{m^{2}}{g}

), respectively. The unit of

Q_{e}

\frac{m o l}{m^{2}}

. By multiplying the M.W of the SL (52000

\frac{g}{m o l}

), it converts to

\frac{g}{m^{2}}

. Tables 2–4 show the initial and equilibrium SL solution concentrations, and the equilibrium adsorption capacity for the studied plastics at selected temperatures 30, 50, and 70°C, respectively. As observed, although the PC had the lowest special surface area (S) among of the studied plastics (0.80

\frac{m^{2}}{g}

-Table 1), but the corresponding

Q_{e}

values for studied temperatures were extremely higher when compared with the rest plastics. The

Q_{e}

values ranged between 4.3

\times 10^{- 9}

to 30.0

\times 10^{- 9}

\frac{m o l}{m^{2}}

, 6.1

\times 10^{- 9}

to 49.2

\times 10^{- 9}

\frac{m o l}{m^{2}}

, and 2.2

\times 10^{- 9}

to 22.5

\times 10^{- 9}

\frac{m o l}{m^{2}}

(Tables 2–4) for studied temperatures 30, 50, and 70°C, respectively, when the initial SL concentration in the solution increased from

1 \times 10^{- 5}

4 \times 10^{- 5}

mol/L. In general, increasing the SL concentration and also, operative temperature had positive effects on the equilibrium adsorption capacity (

Q_{e}

). As an illustration for the effect of temperature, the

Q_{e}

values for SL concentration of

4 \times 10^{- 5}

mol/L increased from 15.3

\times 10^{- 9}

40.7 \times 10^{- 9}

\frac{m o l}{m^{2}}

, 2.9

\times 10^{- 9} t o 5.8 \times 10^{- 9} \frac{m o l}{m^{2}}

, 5.5

\times 10^{- 9}

to 10.1

\times 10^{- 9}

\frac{m o l}{m^{2}}

, 4.1

\times 10^{- 9}

to 6.8

\times 10^{- 9}

\frac{m o l}{m^{2}}

, 7.6

\times 10^{- 9}

to 20.6

\times 10^{- 9}

\frac{m o l}{m^{2}}

, and 22.5

\times 10^{- 9}

to 49.2

\times 10^{- 9}

\frac{m o l}{m^{2}}

for PVC, ABS, PS, PP, POM, and PC, respectively, when temperature rose from 30 to 70°C.

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 at 30 $℃$ .

Plastic	C₀ (mol/l)	Ce (_mol/l) × 10⁻⁵	C_e (g/l)	Q_{e (mol/m}²_)×10⁻⁹	Q_{e (g/m}²_)×10⁻⁴
PVC	1 × 10⁻⁵	0.9752	0.5071	1.033	0.537
ABS		0.9938	0.5167	0.042	0.022
PS		0.9785	0.5088	0.351	0.183
PP		0.9847	0.5120	0.209	0.108
POM		0.9701	0.5044	0.498	0.260
PC		0.9643	0.5014	2.231	1.162
PVC	2 × 10⁻⁵	1.9365	1.0069	2.645	1.375
ABS		1.8343	0.9538	1.136	0.591
PS		1.9075	0.9919	1.511	0.785
PP		1.8962	0.9860	1.418	0.737
POM		1.8791	0.9771	2.015	1.047
PC		1.8336	0.9534	10.400	5.408
PVC	3 × 10⁻⁵	2.9050	1.5106	3.958	2.058
ABS		2.7974	1.4546	1.389	0.722
PS		2.8032	1.4576	3.215	1.672
PP		2.8308	1.4720	2.311	1.201
POM		2.7809	1.4460	3.651	1.898
PC		2.7102	1.4093	18.112	9.478
PVC	4 × 10⁻⁵	3.6339	1.8896	15.254	7.932
ABS		3.5735	1.8582	2.925	1.521
PS		3.6605	1.9034	5.547	2.884
PP		3.7009	1.9244	4.0860	2.125
POM		3.5442	1.8429	7.596	3.950
PC		3.6408	1.8932	22.450	11.674

Table 3.

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 at 50 $℃$ .

Plastic	C₀ (mol/l)	C_e (mol/l) _×10⁻⁵	C_{e (g/l)}	Q_{e (mol/m}²_)×10⁻⁹	Q_{e (g/m}²_)×10⁻⁴
PVC	1 × 10⁻⁵	0.9522	0.4951	1.991	1.035
ABS		0.9636	0.5010	0.249	0.130
PS		0.9602	0.4993	0.650	0.338
PP		0.9708	0.5048	0.398	0.207
POM		0.9603	0.4993	0.661	0.345
PC		0.9318	0.4845	4.262	2.221
PVC	2 × 10⁻⁵	1.8827	0.9790	4.887	2.541
ABS		1.8047	0.9384	1.339	0.696
PS		1.8075	0.9399	3.145	1.635
PP		1.8247	0.9488	2.394	1.252
POM		1.7181	0.8934	4.698	2.443
PC		1.7181	0.8934	17.618	9.161
PVC	3 × 10⁻⁵	2.6350	1.3702	15.208	7.908
ABS		2.6483	1.3772	2.412	1.253
PS		2.7242	1.4165	4.506	2.343
PP		2.6757	1.3913	4.430	2.303
POM		2.5214	1.3111	7.976	4.147
PC		2.6356	1.3705	22.775	11.843
PVC	4 × 10⁻⁵	3.4084	1.7723	24.657	12.820
ABS		3.3416	1.7363	4.515	2.347
PS		3.5002	1.8201	8.166	4.246
PP		3.6096	1.8769	5.333	2.773
POM		3.3915	1.7635	10.141	5.282
PC		3.5198	1.8303	30.012	15.606

Table 4.

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 at 70 $℃$ .

Plastic	C₀ (mol/l)	C_{e (mol/l))×10}⁻⁵_{) Ce}	C_{e (g/ l)}	Q_{e (mol/m}²_)×10⁻⁹	Q_{e (g/m}²_)×10⁻⁴
PVC	1 × 10⁻⁵	0.9102	0.4733	3.741	1.94
ABS		0.9531	0.4956	0.321	0.167
PS		0.9436	0.4906	0.921	0.479
PP		0.9690	0.5038	0.423	0.220
POM		0.9341	0.4857	1.098	0.571
PC		0.9022	0.4691	6.112	3.178
PVC	2 × 10⁻⁵	1.7572	0.9137	10. 116	5.26
ABS		1.6623	0.8643	2.316	1.204
PS		1.7237	0.8963	4.514	2.348
PP		1.8220	0.9474	2.431	1.264
POM		1.6588	0.8625	5.686	2.967
PC		1.700	0.884	10.875	9.750
PVC	3 × 10⁻⁵	2.56	1.3312	18.333	9.533
ABS		2.2143	1.1514	5.388	2.802
PS		2.5101	1.3052	8.004	4.162
PP		2.6553	1.3807	4.709	2.448
POM		2.324	1.2084	11.266	5.858
PC		2.5925	1.3481	25.468	13.243
PVC	4 × 10⁻⁵	3.0218	1.5713	40.758	21.194
ABS		3.1491	1.6375	5.836	3.034
PS		3.3802	1.7577	10.127	5.266
PP		3.5028	1.8214	6.7923	3.532
POM		2.7623	1.4363	20.628	10.726
PC		3.2124	1.6704	49.225	25.600

As an illustration for the effect of SL concentration on the equilibrium adsorption capacity, the $Q_{e}$ values at 50°C increased from 2.0 $\times 10^{- 9}$ to $24.7 \times 10^{- 9}$ $\frac{m o l}{m^{2}}$ , 0.2 $\times 10^{- 9} t o 4.5 \times 10^{- 9} \frac{m o l}{m^{2}}$ , 0.7 $\times 10^{- 9}$ to 8.2 $\times 10^{- 9}$ $\frac{m o l}{m^{2}}$ , 0.4 $\times 10^{- 9}$ to 5.3 $\times 10^{- 9}$ $\frac{m o l}{m^{2}}$ , 0.7 $\times 10^{- 9}$ to 10.1 $\times 10^{- 9}$ $\frac{m o l}{m^{2}}$ , and 4.3 $\times 10^{- 9}$ to 30.0 $\times 10^{- 9}$ $\frac{m o l}{m^{2}}$ for PVC, ABS, PS, PP, POM, and PC, respectively, when SL concentration rose from $1 \times 10^{- 5}$ to $4 \times 10^{- 5}$ mol/L. Interestingly, the ABS with the highest S value (7.3 $\frac{m^{2}}{g}$ -Table 1) had the lowest $Q_{e}$ values in all studied SL concentrations. The $Q_{e}$ values lay in the order of $P C > P V C > P O M > P S > P P > A B S$ .

It concluded that, unlike the special surface area of the plastic, the initial concentration of the SL and operation temperature were effective parameters on the adsorption of SL on the surface of the plastic. It was in conformity with the earlier report.¹⁹ The adsorption of a chemical on a polymer surface depends not several parameters, including the special surface area of the polymer and the chemical structure of the involved materials. It seemed, the latter parameter was more effective than the former in studied systems.

The Langmuir isotherm is known as a simple physical adsorption model. The Langmuir theory assumes a kinetic principle, that is, the rate of adsorption on a surface is equal to the rate of desorption from a surface. It assumes monolayer coverage, no interaction between adsorbed molecules, and constant binding energy between adsorbate and adsorbent surface. The Langmuir isotherm may write as²⁵

\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 5 shows 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. Figures 1–3 illustrate the fitted line based on the measured experimental data for the PVC in the Langmuir’s model for three operative temperatures 30, 50, and 70

℃

, respectively. The high values of

R^{2}

represent suitable data fitting with the driven line equations. The Freundlich model is an empirical isotherm. However, it has a theoretical base. It assumes the solid surface is heterogeneous and the adsorption energy distributed on the surface. From the viewpoint of this model, the solid surface contains several patches. The sites with the same adsorption energy were grouped into a patch. It relates the adsorbed amount of material at equilibrium with the initial concentration of the material as a power law dependence²⁷

{Q_{e} = K}_{F} {C_{e}}^{\frac{1}{n}} o r \log Q_{e} = \log K_{F} + \frac{1}{n} \log C_{e}

(3)

Table 5.

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

Plastic	${F i t t e d l i n e e q u a t i o n}^{*}$	$R^{2}$	Temperature (C°)	$Q_{m_{a x}} (\frac{g}{m^{2}})$	$k_{L} (\frac{L}{g})$
PVC	Y = 11426x-3868.7	0.987	30	2.59 × $10^{- 4}$	0.321
	Y = 6230x-2841.9	0.992	50	3.52 × $10^{- 4}$	0.456
	Y = 3110x-1428.5	0.998	70	7.00 × $10^{- 4}$	0.459
ABS	Y = 333713x-228212	0.894	30	0.44 × $10^{- 4}$	0.683
	Y = 52666x-31571	0.958	50	0.32 × $10^{- 4}$	0.599
	Y = 42965x-31227	0.913	70	0.32× $10^{- 4}$	0.726
PS	Y = 36703x-19193	0.977	30	0.52 × $10^{- 4}$	0.522
	Y = 19113x-10071	0.952	50	0.99 × $10^{- 4}$	0.526
	Y = 13512x-7804.6	0.941	70	1.28 × $10^{- 4}$	0.577
PP	Y = 63595x-36441	0.943	30	0.27 × $10^{- 4}$	0.573
	Y = 32456x-18720	0.936	50	0.53 × $10^{- 4}$	0.576
	Y = 31009x-18370	0.948	70	0.54 × $10^{- 4}$	0.592
POM	Y = 25385x-12956	0.980	30	0.77 × $10^{- 4}$	0.510
	Y = 19744x-12628	0.914	50	0.79 × $10^{- 4}$	0.639
	Y = 12502x-8941	0.963	70	1.12 × $10^{- 4}$	0.715
PC	Y = 5529x-2826	0.951	30	3.54 × $10^{- 4}$	0.511
	Y = 2615x-1146	0.933	50	8.73 × $10^{- 4}$	0.438
	Y = 1772x-709	0.978	70	14.10 × $10^{- 4}$	0.400

Figure 1.

An illustration of the fitted line based on the measured experimental data for the PVC in the Langmuir’s model at 30 $℃$ .

Figure 2.

An illustration of the fitted line based on the measured experimental data for the PVC in the Langmuir’s model at 50 $℃$ .

Figure 3.

An illustration of the fitted line based on the measured experimental data for the PVC in the Langmuir’s model at 70 $℃$ .

Table 6 represents the Freundlich’s parameters,

n

and

K_{F}

, coefficient of determination (

R^{2}

), and the equations of the fitted lines for the studied plastics at studied temperatures 30, 50, and 70

℃

. It concluded that the high

R^{2}

values show proper data fitting for the studied Freundlich and Langmuir models. Figures 4–6 compare the predicted

Q_{e}

values by the Langmuir and Freundlich models with the experimental values for the selected plastics, PVC, PP, and PC at studied temperatures 30, 50, and 70

℃

, respectively. As shown, the predicted values by studied models had not a considerable deviation from experimental values for the PP at low

C_{e}

,’s, i.e., less than 1 g/L at 30

℃

. However, with increasing the

C_{e}

, only the Freundlich model represented close values with the experimental values. It was due to higher interaction between SL molecules at high concentrations, just unlike the assumptions for the derived equation by the Langmuir model. Also, the predicted values for both, the Freundlich and Langmuir models were not close to experimental values for the studied

C_{e}

,’s for PC and PVC. Here, the polar surface of the mentioned plastics was responsible for the deviation between predicted and experimental values with emphasis for the Langmuir model. Figure 5 shows with increasing the temperature to 50

℃

, the same trend was observed for the PP. However, for low

C_{e}

,’s, PVC showed good conformity between experimental and predicted values by both models. As observed, increasing the SL concentration deteriorated the eligibility of the Langmuir model to predict the actual

Q_{e}

values. Also, the observation for PC showed that none of the studied models were suitable to predict the actual

Q_{e}

values in studied

C_{e}

,’s. Increasing the temperature to 70

℃

had a positive effect on the prediction of the

Q_{e}

values at low SL concentrations (Figure 6). The predicted

C_{e}

,’s were close to the experimental values for all studied plastics, PP, PVC, and PC. However, with increasing the SL concentration, the Langmuir model showed, the predicted

Q_{e}

values were different from the actual values remarkably.

Table 6.

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	${F i t t e d l i n e e q u a t i o n}^{*}$	$R^{2}$	Temperature (C°)	$K_{F}$	$n$
PVC	Y = 1.84x-3.80	0.869	30	4.14 × $10^{- 4}$	0.554
	Y = 2.01x-3.43	0.960	50	3.72 × $10^{- 4}$	0.497
	Y = 1.85x-3.15	0.952	70	7.06 × $10^{- 4}$	0.541
ABS	Y = 3.17x-4.56	0.892	30	0.28 × $10^{- 4}$	0.315
	Y = 2.28x-4.17	0.990	50	0.67 × $10^{- 4}$	0.439
	Y = 2.55x-3.88	0.910	70	1.31 × $10^{- 4}$	0.392
PS	Y = 2.09x-4.12	0.999	30	0.77 × $10^{- 4}$	0.479
	Y = 1.89x-3.85	0.970	50	1.40 × $10^{- 4}$	0.530
	Y = 1.92x-3.65	0.954	70	2.22 × $10^{- 4}$	0.521
PP	Y = 2.21x-4.26	0.972	30	0.56 × $10^{- 4}$	0.452
	Y = 2.03x-3.99	0.943	50	1.01 × $10^{- 4}$	0.492
	Y = 2.19x-3.95	0.977	70	1.12 × $10^{- 4}$	0.475
POM	Y = 2.03x-3.98	0.992	30	1.04 × $10^{- 4}$	0.492
	Y = 2.19x-3.69	0.923	50	2.03 × $10^{- 4}$	0.457
	Y = 2.64x-3.40	0.995	70	3.97 × $10^{- 4}$	0.378
PC	Y = 1.98x-3.34	0.958	30	4.61 × $10^{- 4}$	0.562
	Y = 1.44x-3.12	0.921	50	7.71 × $10^{- 4}$	0.693
	Y = 1.53x-2.98	0.967	70	10.40 × $10^{- 4}$	0.653

Figure 4.

The comparison between the experimental and predicted $Q_{e}$ values by the Langmuir and Freundlich models for the selected plastics, PVC, PP, and PC at 30 $℃$ .

Figure 5.

The comparison between the experimental and predicted $Q_{e}$ values by the Langmuir and Freundlich models for the selected plastics, PVC, PP, and PC at 50 $℃$ .

Figure 6.

The comparison between the experimental and predicted $Q_{e}$ values by the Langmuir and Freundlich models for the selected plastics, PVC, PP, and PC at 70 $℃$ .

The relationship between the Langmuir isotherm constant ( $K_{L}$ , L/mol) with Gibbs free energy ( $∆ G$ ), enthalpy ( $∆ H$ ), and entropy ( $∆ S$ ) change of adsorption may express by the Equations (4)–(6) as below²⁸

∆ G = - R T l n K_{L}

(4)

∆ G = ∆ H - T ∆ S

(5)

Ln K_{L} = \frac{∆ S}{R} - \frac{∆ H}{R T}

(6)

Where R and T represent gas constant (

0.008314 k j . k^{- 1} . {m o l}^{- 1}

) and absolute temperature (K), respectively. By drawing the

\ln K_{L}

against 1/T, the values of

∆ S

(vertical intercept) and

∆ H

(slope of the drawn line) obtain for adsorption of a substance (adsorbate) on the surface of adsorbent. Figure 7 illustrates the

\ln K_{L}

against 1/T for PC. As observed, the high

R^{2}

value (0.9796) shows suitable data fitting with the driven line equation. Table 7 summarizes the thermodynamic parameters, Gibbs free energy (

∆ G

), enthalpy (

∆ H

), and entropy change of the SL adsorption (

∆ S

) on the surface of the studied plastics. As expected, the adsorption of SL on the selected plastics was a spontaneous process because the values of

∆ G

were negative. Also, the numerical values of

∆ G

ranged between −17.00 to −12.97 kJ/mol, which were conformed with the physical adsorption with the

∆ G

range of −20 to 0 kJ/mol.²⁹ The reader may compare the above-mentioned values with those values for the chemisorption with the

∆ G

values range of −400 to 80 kJ/mol.¹⁹ With increasing the temperature, the

∆ G

values reduced. It means, increasing the temperature facilitates the adsorption process because, in most cases, the adsorption process was endothermic with positive

∆ H

values (Table 7). As an illustration, the

∆ G

values for POM reduced from −14.13 to −16.97 kJ/mol when temperature increased from 30 to 70

℃

, respectively. Furthermore, except for PC, the

∆ H

values were positive and small, ranging between +0.73 for PP to +7.44 for PVC. As observed, it was a sign of an endothermic process and also, the lack of chemical bonds between SL molecules and the studied plastics surfaces. For a chemical bond, the

∆ H

values are between 80 to 200 kJ/mol, normally.³⁰ Among the studied plastics, the adsorption process for the PC was exothermic with the

∆ H

value of −4.99 kJ/mol. It may be due to the higher hydrophilic properties of the PC surface when compared with the other studied plastics.¹⁴ The measured adsorption

∆ S

values were very small and positive, ranging between +0.030 for PC to +0.075 kJ/mol for PP. This fact shows a small increase in irregularity (randomness) at the solid-liquid interface during the adsorption process.³¹

Figure 7.

The $\ln K_{L}$ against 1/T for PC.

Table 7.

The thermodynamic parameters Gibbs free energy ( $∆ G$ ), enthalpy ( $∆ H$ ), and entropy change of SL adsorption ( $∆ S$ ) on the surface of the studied plastics.

Plastic	T(K)	$K_{L} (\frac{L}{m o l})$	ΔH $° (k j . {m o l}^{- 1})$	ΔS $° (k j . {m o l}^{- 1} {. k}^{- 1})$	$∆ G ° (k j . {m o l}^{- 1})$
PVC	303 323 343	171.72 245.96 245.57	+7.44	+0.07	−12.97 −14.74 −15.68
ABS	303 323 343	365.41 320.47 388.41	+1.25	+0.05	−14.86 −15.49 −17.00
PS	303 323 343	279.27 281.41 308.70	+2.08	+0.05	−14.18 −15.15 −16.34
PP	303 323 343	306.56 308.16 316.72	+0.73	+0.08	−14.42 −15.40 −16.43
POM	303 323 343	272.85 341.87 382.53	+7.07	+0.07	−14.13 −15.66 −16.97
PC	303 323 343	273.39 234.33 214.00	−4.99	+0.03	−14.13 −14.66 −15.31

Table 8 depicts the flotation % of the studied plastics conditioned with different SL’s concentrations at studied temperatures 30, 50, and 70

℃

. The liquid media was tap water with a pH of 6.5. As observed, all studied plastics were on the surface of the flotation media (tap water) in the flotation tank without surface conditioning with SL, regardless of considering the plastics’ densities (Table 1). The high surface tension of the tap water prevents the sinking of those plastics with higher densities than water. Table 8 shows that for all studied SL concentrations and temperatures, PP and PC floated on and sunk at the bottom of the flotation tank, respectively. The reader may attend to this point that PP and PC had the most hydrophobic and hydrophilic properties among the studied plastics, respectively. The comparison between the equilibrium adsorption capacity (

Q_{e}

) values of the above-mentioned plastics in different SL concentrations at studied temperatures (Tables 2–4) conformed with the above floatation observations. However, conditioning the surface of the plastic with different SL concentrations adversely affected on the flotation of the rest plastics, ABS, PS, and POM with different values. For mentioned plastics, increasing the temperature favored the flotation for some SL concentrations, but no regular trend observed. Regardless of considering the PC, POM had the least flotation %’s among the studied plastics at selected SL concentrations and temperatures. For the PVC, the flotation was independent of the SL concentration at 30

℃

and also, for higher SL concentrations at 50

℃

. Interestingly, the trend changed remarkably at 70

℃

and flotation became SL concentration dependent at this temperature.

Table 8.

The flotation % of the studied plastics versus different SL concentrations at studied temperatures 30, 50, and 70 $℃$ . The liquid media was tap water with a pH of 6.5.

Concluding remarks

Based on the measured equilibrium adsorption capacities ( ${Q_{e}}^{'} s$ ), the SL adsorbed on the selected plastics surfaces with different amounts. Increasing the temperature had positive effects on above mentioned parameter. The SL adsorbed on the plastic surface in the sequence of

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

Suitable conformity was observed between the experimental results with predicted values by the traditional adsorption isotherms, Freundlich and Langmuir for studied temperatures. However, for the most studied plastics, the Freundlich model was more suitable than Langmuir. The measurement of thermodynamic parameters revealed spontaneous adsorption processes along with endothermic (except for PC) reactions. The relatively low thermodynamic values were proper evidence for the physical (not chemical) adsorption of SL on the surface of the selected plastics. The complete separation of PC from a plastics mix in all SL concentrations and temperatures was observed during the flotation process.

Footnotes

Acknowledgements

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. 3/56351.

Author contributions

Atefeh Salari 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) received no financial support for the research, authorship, and/or publication of this article.

Data availability

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

ORCID iD

Saeed Ostad Movahed

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The effect of temperature on the adsorption of the sodium lignosulphonate (SL) on the surface of the selected plastics as a potential solution for the plastics waste management

Abstract

Keywords

Introduction

Experimental

Materials

Testing procedure, characteristic techniques, and equipment

Results and discussion

Concluding remarks

Footnotes

Acknowledgements

Author contributions

Declaration of conflicting interests

Funding

Data availability

ORCID iD

References