Removal of Cd²⁺ and Cr³⁺ ions from Aqueous Solution by Modified Polypropylene Plastic Waste: Equilibrium Study

Kelvin Kibet¹*, Esther W. Nthiga², Moses A. Ollengo³

^1,2,3Chemistry Department, Dedan Kimathi University of Technology, Private Bag – 10143,

Dedan Kimathi, Nyeri – Kenya.

*Corresponding Author E-mail: kelvinkibet60@gmail.com

ABSTRACT:

Heavy metals pollution of water poses a major environmental challenge due to its antagonistic effects. They are not biodegraded and tend to accumulate in human body to toxic levels linked to some health effects. Therefore, there is need for their removal in water. Technologies that exist such as membrane filtration are limited by their high operation cost. However, adsorption is a cheap, efficient, and easy technique of removal of pollutants from wastewater. There are limited information available on value addition of plastic waste for application as adsorbents. Global production of polypropylene amount to about 56 million metric tons with a large percent of these going to waste. Application of polypropylene plastic waste was explored in this study for remediation of Cd²⁺ and Cr³⁺ ions from model solution. This was achieved by first chemically modifying the plastic by use of hydrogen peroxide and hydrochloric acid then applying it for adsorption of Cd²⁺ and Cr³⁺ ions. The chemically modified plastic waste was characterized and applied for equilibrium adsorption batch experiments. The images obtained from SEM analysis confirmed availability of pores in the adsorbent which are critical for Cd²⁺ and Cr³⁺ ions adsorption. The FTIR results confirmed the attachment of hydroxyl groups on the adsorbent which are vital for removal of heavy metals. Effect of adsorbent dosage, contact time, solution pH and initial metal ion concentration were investigated and the concentration of the resulting filtrate after adsorption determined using ICP-AES. The results from optimization experiments showed that Cd²⁺ and Cr³⁺ ions adsorption was optimal at initial metal ion concentration values between 15mg/L and 20mg/L and contact time of 45 mins and 60 mins respectively. Optimum adsorbent mass of 0.1g and optimal pH values between 4.5 and 5.5. Equilibrium experimental results showed adsorption capacities of 7.395mg/g (Cd²⁺) and 6.225mg/g (Cr³⁺) for Langmuir isotherm model with R²˃0.99 indicating chemisorption process.

KEYWORDS: Polypropylene, Adsorption, Heavy metals, Equilibrium, Adsorption capacity, Adsorption isotherms.

INTRODUCTION:

Water is a natural resource whose importance to mankind civilization cannot be substituted¹. It is relied on for industrial development and agricultural sustainability amongst many other uses. Over the last decade, an increase in urbanization and industrialization has been linked to the rising cases of water pollution². Water can be said to have been polluted if the foreign substance introduced into it reduces its quality³.

Heavy metal pollution of water is increasingly becoming a major environmental problem due to its antagonistic effects². They are non-biodegradable, accumulative, have long biological half-lives and are often transformed from one oxidation state to the other^4,5. Chromium is a transition metal whose trivalent and hexavalent oxidation states are the most stable forms that can also interconvert with each other⁶. Chromium compounds have found application in various modern industries such as tanneries in tanning leather, catalysts, textile industries and metal finishing^7,8. However, the wastewaters discharged from these industries contain large concentrations of chromium^9,10. Chromium poisoning has been reported to cause allergic dermatitis, low birth weight and also affects immune, urinary, respiratory and cardiovascular system. Cadmium occurs naturally in fossil fuels, zinc and copper ores which find way to the atmosphere during volcanic eruption. It is used in modern industries such as electric batteries, plastic and painting industries¹¹. However, it has been reported to be a potential cause of cancer in human beings^12–14. Various methods exist for water purification such as electrochemical treatment, solvent extraction, membrane filtration and adsorption amongst others^15,16. Most of these conventional methods are limited by high capital and operational cost^17,18. Adsorption however, is a highly efficient technique that is easy to operate at low cost making it a suitable method. Various adsorbents have been used for removal of heavy metals by adsorption such as fly ash¹⁹ , carbon foam²⁰, activated carbon²¹, clay mineral²², organic polymers²³, bio char and biomass^24,25.

Polypropylene (PP) is a low-cost polymer with desirable properties like high heat distortion solidity and fire resistance. It makes up 16% of the whole plastic industry with growing demand for it due to increase in industrialization²⁶. However, its usage is another major contributor of environmental degradation today because most of the polypropylene plastics that are disposed in nature have very slow degradation process, some taking even hundreds to thousands of years²⁷. Environmental pollution as a result of indiscriminate plastic waste disposal is evident in several ways such as sewage system blockage in urban areas, entanglement and death of aquatic organisms and reduction in water infiltration in soil and soil aeration resulting in low productivity in such lands²⁸. Polypropylene plastic has found application in wastewater treatment because of its impressive chemical stability, thermal and mechanical properties and low cost²⁹. These properties together with surface area, functional groups and surface topography of the plastic makes it viable for use as an adsorbent³⁰.

Vegetable oil contains functional groups that can be used to react with other compounds³¹. It has unsaturated fatty acids which enables the reaction to occur at the C=C as well as at the ester groups. Vegetable oil also has the advantage of low cost and low toxicity which makes it suitable for use as a solvent in modification of adsorbents. This study focused on chemical modification of polypropylene plastic waste using hydrogen peroxide and hydrochloric acid, characterization of the adsorbent using FT-IR and SEM and removal of chromium and cadmium ions from aqueous solution. The effect of experimental conditions were investigated and equilibrium adsorption isotherm models applied to determine the metal ions adsorption capacity.

MATERIALS AND METHODS:

Chemicals and Reagents:

The purity of chemicals used was ˃98.9% and the solutions prepared using double distilled water. Chromium nitrate nanohydrate, hydrochloric acid, cadmium nitrate tetrahydrate, sodium hydroxide, hydrogen peroxide, sodium acetate and acetic acid were sourced from Sigma Aldrich (Kobian, Nairobi). Liquid vegetable oil used in this research was purchased from a supermarket in Nyeri, Kenya.

Preparation of metal ion solutions:

A 1000mg/L of Cd²⁺ and Cr³⁺ ions were prepared by dissolving 2.74g Cd (NO₃)₂.4H₂O and 7.69g of Cr (NO₃)₃.9H₂O respectively in 200 mL of distilled water then topping to a litre. The working solutions were prepared using the progressive dilution procedure. A 0.1 M NaC₂H₃O₂ buffer solution was used to maintain a constant pH while 0.1 M NaOH and 0.1 M HCl solutions were used to adjust the pH to the required value.

Adsorbent preparation:

Polypropylene plastic waste was collected randomly from dumpsites within Nyeri Municipality independent of the colour, weight or age of the plastic and transported to the laboratory where it was washed thoroughly with tap water then washed again thoroughly with distilled water to remove impurities. The clean polypropylene waste was cut into small pieces and dried to eliminate moisture. Vegetable oil was used as a solvent in this study to disperse molten polypropylene waste in the synthesis of epoxides. The small pieces of cleaned and dried polypropylene plastic waste were added to 100mL of boiling vegetable oil till saturation. A 10mL of strongly basic H₂O₂ was then added drop wise to form the epoxide. The epoxidation process was given 15 minutes for the reaction to come to completion after which the material was refluxed for 120mins, cooled and ground into fine powder³¹. The ground epoxidated plastic was activated with hydrochloric acid for 72hours at 373 K.

Instrumentation:

Scanning Electron Microscope, FEI ESEM (Tescan Vega LMH) was used to shed light on the surface morphology of the adsorbent. The mechanical shaker used in this study was lab-line mechanical reciprocating shaker. Solutions pH were monitored using pH meter (HANNA model). The concentration of Cd²⁺ and Cr³⁺ ions in the final filtrates were determined using Plasma Atomic Emission–Spectrometer (ICP 9000, Shimadzu).

FT-IR Characterization:

FTIR characterization was done to confirm the presence of the functional groups responsible for binding of the metal ions from waste water. Analysis of polypropylene, vegetable oil, epoxidated adsorbent, activated adsorbent and heavy metal loaded adsorbent was done using FT-IR. (FT/IR-4700, JASCO made in Japan). FT-IR analysis of the vegetable oil involved putting a drop of the sample on the ATR crystal and taking the measurement. The powdered samples were placed on an ATR crystal and pressed down to ensure total contact between the crystal and sample using swivel press and the measurement taken. All the FT-IR spectra were plotted at a mid IR range of 4000-500 cm^-1 at a room temperature.

SEM Analysis:

The surface morphologies of modified polypropylene plastic waste (before adsorption and after adsorption) were determined using Scanning Electron Microscope, FEI ESEM (Tescan Vega LMH). The dry SEM samples in powder form were placed on the sample holder, placed in the scanning electron microscope and analysed at 20KV.

Optimization experiments:

These were done in 100mL elernmeyer flasks at 25⁰C using a reciprocating mechanical shaker (160rpm). The experimental conditions that were optimized include pH, contact time, adsorbent dosage and initial metal ion concentration³². The effect of these experimental conditions was optimized by keeping other parameters constant while varying the one under consideration. A 0.1g of the adsorbent was weighed into an elernmeyer flask containing 50mL of the desired metal ion concentration then agitated for 60mins (Cr³⁺) and 45 mins (Cd²⁺) using a mechanical shaker. Effect of contact time (0-120 minutes), pH (3.0-8.0), initial metal ion concentration (5-80mg/L) and adsorbent mass (0.1-2g) were investigated and each of the agitated mixture was filtered through Whatman filter paper No. 42. The metal ions concentration in the resulting filtrate were determined by ICP-AES.

Data analysis:

The percentage of Cr³⁺ and Cd²⁺ ions removed in the solution were calculated from equation 1 ³³.

Where R% is the percentage removal of the metal ions, C_O is the concentration at equilibrium and C_i is the initial concentration.

The amount of Cr³⁺ and Cd²⁺ ions adsorbed per unit mass of the adsorbent were calculated from equation 2 ³⁴.

Where Ci is the initial concentration, M is the amount of adsorbent in grams, V is the volume of reaction mixture in liters, Ce is the equilibrium concentration and qe is the amount adsorbed per gram of adsorbent.

Adsorption capacity:

Experimental data was fitted into Langmuir and Freundlich isotherm models to shed light on the adsorption capacity. This was done in 100mL volumetric flasks containing 50mL Cr³⁺ and Cd²⁺ ions solutions of varying concentrations between 5-80mg/L at 25±0.5 ⁰C and optimum conditions of: 0.1g adsorbent dosage; 60 mins (Cr³⁺) and 45mins (Cd²⁺) contact time and solution pH of 4.5(Cr³⁺) and 5.5(Cd²⁺). The mixtures were shaken and filtered after attaining the equilibrium time. The filtrates concentration were analysed using ICP-AES³⁵.

Adsorption isotherms:

Langmuir and Freundlich isotherm models were used to quantify the adsorption of Cd²⁺ and Cr³⁺ ions at equilibrium. This is because the study of isotherms gives information on the interaction between the adsorbent and the adsorbate as well as monolayer or multilayer formation (surface coverage)³⁶. Langmuir isotherm is based on monolayer adsorption which assumes homogenous adsorption. Linearized Langmuir equation can be represented as shown in equation 3.

Where C_e is the metal ions concentration at equilibrium (mg/L); qe is the amount of metal ions (mg/g) adsorbed at equilibrium time; b and Q_max are constants related to energy of adsorption (L/mg) and maximum adsorption capacity (mg/g) respectively³⁷. Langmuir constants (b and Q_max) are calculated from the intercept and slope respectively of linear plot of C_e/q_e against C_e.

The Freundlich isotherm describes binding sites with different energy based on multilayer adsorption on a heterogeneous surface. The model in its linear form is given as equation 4.

Where q_e is the amount of metal ions absorbed at equilibrium time (mg/g); C_e is the concentration of metal ions at equilibrium (mg/L); n and K_F are Freundlich constants related to adsorption intensity and adsorption capacity respectively. The Freundlich constants (n and K_F) are determined from the slope and intercept of a linear plot of lnq_e against lnC_e³⁸.

RESULTS AND DISCUSSION:

FTIR Results:

The results in figure 1 represents the combined FT-IR spectra of polypropylene, vegetable oil, epoxidated adsorbent, activated and heavy metal ions treated adsorbent.

Figure 1: Polypropylene (A), vegetable oil (B), epoxidated adsorbent (C), activated adsorbent (D) and metal ions treated adsorbent (E) FT-IR spectra

The spectrum for polypropylene (A) shows a strong peak between 2970.77 and 2860.01 cm^-1 attributed to asymmetric -CH₃ stretching vibration from aliphatic groups in polypropylene plastic waste³⁹. The signal at 976.77 cm^-1 corresponds to polypropylene plastic aliphatic C-C stretching vibration⁴⁰. The two signals at 1457.92 and 1370.18 cm^-1 were attributed to -CH₂ deformation and symmetric in-plane -CH (CH₃ deformation) of polypropylene³¹.

The spectrum for vegetable oil (B) shows a vibration at 3009.37 cm^-1 which represented –CH(-HC=CH-) stretching of alkene⁴⁰. The two signals at 2922.59 and 2856.06 cm^-1 were indicative of asymmetric -CH₃ vibration from the fatty acids groups in vegetable oil³⁹. The small peak seen at 1638.23 cm^-1 wavenumber was attributed to alkene stretching⁴¹. The two peaks at 3009.37 and 1638.23 cm^-1 are clear indication of C=C stretching vibration in fatty acids found in vegetable oil. The C=C plays a vital role in formation of epoxide ring. A strong signal seen at 1743.33 cm^-1 was attributed to carbonyl stretch of the ester groups in the fatty acids³⁷. The stretching vibration indicated by the signals between 1300.00 and 1050.00 cm^-1 was attributed to C-O-C from the fatty acids^42,43. The -CH₂ oscillating vibration from the C=C in fatty acids was shown by the signal at 720.28 cm^{-1 41}.

The spectrum for epoxidated adsorbent (C) shows a strong signal at 1743.33 cm^-1 which corresponded to carbonyl stretch of an ester from the fatty acids in the vegetable oil used⁴². The signal that emerged at 1014.37 cm^-1 was attributed to cyclic ethers such as epoxide and the peak that disappeared at 3009.37 cm^-1 indicated that the material was successfully epoxidated.^42,43.

The spectrum for activated adsorbent (D) shows an emergence of a broad peak between 3550 and 3200 cm^-1. attributed to overlap of -OH stretch of carboxylic acid and alcohol resulting from opening of the epoxide ring and hydrolysis of the ester group in fatty acids^42,40. The signal between 1300 and 1000 cm^-1 decreased in intensity confirming the opening of the epoxide ring to form hydroxyl groups of alcohols and hydrolysis of the ester groups into carboxylic acids which are responsible for binding of metal ions⁴⁴.

The spectrum for metal ions loaded adsorbent (E) shows disappearance in the peak between 3550 cm^-1 and 3200 cm^-1 indicating that the hydroxyl was used in binding of the metal ions. A decrease in the intensity of other peaks can also be seen such as carbonyl which is also crucial as adsorption sites for Cd²⁺ and Cr³⁺ ions⁴⁵. Similar results have been reported using water hyacinth powder⁴⁶ whereby a reduction and disappearance of some peaks was observed after adsorption.

SEM Characterization:

Scanning electron microscope (SEM) was used to shed light on the surface morphology of the adsorbent. The images obtained have been presented in figure 2.

Figure 2: SEM images for (A) modified polypropylene plastic waste before adsorption and (B) metal ion loaded adsorbent.

The image of the adsorbent before adsorption (A) is porous, uneven and has large surface area. This is favourable in heavy metal adsorption because it exposes many functional groups to participate in adsorption⁴⁷. After adsorption it can be seen that the pores that were there initially in (A) have disappeared and some reduce in size. This is because of attachment of heavy metals on the active sites in the adsorbent⁴⁴. These results are consistent with the results from the infrared analysis. Similar findings have been reported using Microcystis aeruginosa⁴⁷.

Optimization Experiments:

Solution pH effect on Cr³⁺ and Cd²⁺ ions removal:

The results on how solution pH influences removal of Cr³⁺ and Cd²⁺ ions onto modified polypropylene plastic waste are presented in figure 3.

Results in figure 3 shows an increase in removal capacity as the solution pH increases to a maximum of 4.5(9.63mg/g) for Cr³⁺ and 5.5(7.41mg/g) for Cd²⁺ beyond which a decrease in removal capacity is seen. Therefore, adsorption of the two metal ions was optimum between 4.5 and 5.5 pH values which was used in the rest of the experiment. There are excess H⁺ ions at lower pH values, hence the competition for binding sites between the metal ions and the H⁺ ions³⁸. At higher pH (beyond pH 6), the metal ions are inaccessible for adsorption because precipitation occurs which halts the metal ions from moving around⁴⁴. Similar trend have been reported using Artocarpus heterophyllus L. seeds⁴⁵.

Adsorbent mass effect on Cr³⁺ and Cd²⁺ ions removal:

Figure 4 is a representation of how adsorbent mass influences removal of Cr³⁺ and Cd²⁺ ions onto modified polypropylene plastic waste.

Figure 3: Solution pH effect on Cr³⁺ and Cd²⁺ ions adsorption onto modified polypropylene plastic waste: 60 mins (Cr³⁺) and 45 mins (Cd²⁺); 20 mg/L (Cr³⁺) and 15 mg/L (Cd²⁺); 0.1 g; 25⁰C

Figure 4: Adsorbent mass effect on sorption of Cr³⁺ and Cd²⁺ ions onto modified polypropylene plastic waste: pH 4.5 (Cr³⁺) and 5.5 (Cd²⁺); 60 mins (Cr³⁺) and 45 mins (Cd²⁺); 20 mg/L (Cr³⁺) and 15 mg/L (Cd²⁺); 25⁰C

Figure 4 shows that percentage removal increased proportionally with the adsorbent mass till an optimum dosage of 0.1g was attained beyond which there were minimal increase in % removal for both ions. At low adsorbent mass, there are few binding sites available for adsorption but as the mass increases, the % removal increases to a maximum point where there are no more available ions for adsorption⁴⁸. The close to plateau level beyond optimum point is due to overlapping and aggregation of the adsorbent binding sites⁴⁹. Previous studies have reported similar trend using magnetite nanoparticles, agricultural residuals and recycled brick waste respectively^50,49,51.

Contact time effect on Cr³⁺ and Cd²⁺ ions removal:

Figure 5 is a representation of how contact time affects Cd²⁺ and Cr³⁺ ions removal using modified polypropylene plastic waste.

Figure 5 shows an optimum removal capacity of 8.29 mg/g attained within the first 60 mins for Cr³⁺ ions and 7.30mg/g within the first 45mins for Cd²⁺ ions beyond which a plateau was obtained. When the amount of ions being adsorbed and desorbed are at an equilibrium, the process is said to have attained equilibrium⁵². There were many vacant active sites available early in the adsorption process which caused rapid uptake and the levelling of the curve after equilibrium time was due to exhaustion of the adsorption sites³⁷. The decrease in the uptake at later stages before equilibrium time has been obtained was due to continuous reduction in the active sites ⁴⁸. Similar trend has been reported using activated carbon ^53,54.

Figure 5: Contact time effect on sorption of Cr³⁺ and Cd²⁺ ions onto modified polypropylene plastic waste: pH 4.5 (Cr³⁺) and 5.5 (Cd²⁺); 20 mg/L (Cr³⁺) and 15 mg/L (Cd²⁺); 0.1 g; 25⁰C

Figure 6: Initial metal ion concentration effect on sorption of Cr³⁺ and Cd²⁺ ions onto modified polypropylene plastic waste: pH 4.5 (Cr³⁺) and 5.5 (Cd²⁺); 60 mins (Cr³⁺) and 45 mins (Cd²⁺); 0.1 g; 25⁰C

Table 1: Cr³⁺ and Cd²⁺ ions adsorption onto modified polypropylene plastic waste isotherm constants

Metal ion		Langmuir			Freundlich			Best Model
	qmax, exp (mg/g)	qmax, calc (mg/g)	B (L/mg)	R^2	1/n	Kf (mg/g)	R^2
Cr^3+	6.225	6.357	0.7131	0.9921	0.3204	1.662	0.6990	Langmuir
Cd^2+	7.395	7.788	0.8332	0.9908	0.1105	1.426	0.1395	Langmuir

Initial metal ion concentration effect on removal of Cr³⁺ and Cd²⁺ ions:

Figure 6 is a representation of how initial metal ion concentration affects removal of Cr³⁺ and Cd²⁺ ions using modified polypropylene plastic waste.

Results in figure 6 shows that the removal capacity increases proportionally with initial metal ion concentration up to 15mg/L for Cd²⁺ ions and 20mg/L for Cr³⁺ ions beyond which a plateau was obtained. Concentration is the driving force for adsorption of heavy metals⁵². This can be used to explain the low adsorption capacity at the initial stages as seen in the results. The levelling of the curve beyond optimum concentration is due to exhaustion of the active sites. This is because of the higher chromium and cadmium ions concentration compared to the available binding sites⁵⁵. Similar trend has been reported using Artocarpus heterophyllus L. seeds⁴⁵.

Adsorption capacity:

Adsorption isotherms were used to determine the efficiency of modified polypropylene plastic waste. Freundlich and Langmuir isotherm models were used to provide physico-chemical interaction as well as to determine the maximum removal capacity of the adsorbent. This was done in batches using optimum conditions obtained from the optimization experiments and the results presented in table 1.

The results in table 1 shows R²˃0.9900 indicating that adsorption of Cd²⁺ and Cr³⁺ ions onto modified polypropylene plastic waste fitted Langmuir isotherm model which describes chemisorption process and a monolayer adsorption⁴⁸. The experimental values of q_max being closer to the calculated values confirmed the suitability of this model. The values of b in both the adsorption processes being less than 1 is a clear indication of the high affinity of the adsorbent for Cd²⁺ and Cr³⁺ ions³⁸. Cd²⁺ ions (7.395mg/g) showed higher adsorption capacity compared to Cr³⁺ (6.225mg/g). This was consistent with Pauling electronegativity scale; Cd²⁺ ions (1.69) > Cr³⁺ ions (1.66). Similar findings have been reported using chitosan and husk biomass respectively^56,57.

CONCLUSION:

Polypropylene plastic waste adsorbent used in this study was successfully modified as confirmed by the results from FTIR and SEM. The experimental conditions of contact time, pH, initial metal ion concentration and adsorbent mass greatly influenced the adsorption process. Langmuir isotherm model best described the adsorption process indicating chemisorption process with removal capacity of Cr³⁺ (6.225mg/g) and Cd²⁺ (7.395mg/g).

CONFLICT OF INTEREST:

There is no conflict of interest among the authors in this research.

ACKNOWLEDGEMENT:

The authors would like to thank Dedan Kimathi University of Technology (DeKUT) and United States International University (USIU) for their contribution towards this study.

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Received on 18.04.2023                    Modified on 24.07.2023

Accepted on 20.09.2023                   ©AJRC All right reserved

Asian J. Research Chem. 2023; 16(5):349-357.

DOI: 10.52711/0974-4150.2023.00056