INTRODUCTION:

Anthropogenic activities contribute large amounts of organic and inorganic pollutants to the environment which threaten both animal and human health¹. There is increased realization of the effect of these toxins on surface and ground water, consequently, their elimination is very vital in rendering secure water for drinking as well as culpable release of effluents to human and animal habitats². Inadequacy of drinking water is worsened by organic pollutants contamination of water reserves, one example of such contaminants are the phenols³.

Phenolic compounds contain one or more hydroxyl groups directly bonded to a benzene ring⁴, these compounds are injurious to human and animal health³. Moreover, a phenolic compounds concentration of 1ppb will impact negatively on the taste and odour of water as well as meat quality of animals consuming this polluted water⁵.

Dissolution of PNP in water yields an aqueous solution that is slightly acidic. Consequently, PNP is a central chemical compound in a host of industrial processes which include: pesticide, petrochemical, pharmaceutical, dyes and paints, oil refineries, plastics, pulp and paper mill, hence effluents generated from such industries will contain PNP⁶. The effluents containing PNP will then be deposited in soil, water, including rain water and drinking water⁷. In humans PNP gets into the body system via the lungs by way of light droplets, skin or gastrointestinal tract. Its existence in the blood leads to conversion of hemoglobin to methemoglobin hence causing anemia, liver damage, palpitations and other related symptoms⁸. In addition⁹ reported that PNP is an eye irritant while its ingestion or inhalation brings about vomiting, sleepiness, headaches and ataxia. Recently PNP was reported by US Environmental Protection Agency to be one of the most poisonous, bioaccumulative and indestructible organic compound which causes adverse effects to humans and animals even at negligible concentrations¹⁰. Therefore, it is imperative to concentrate more on research involving the complete removal of PNP or its reduction in aqueous media.

To reduce the environmental impacts of PNP various techniques have been used, including catalytic reduction¹¹, biodegradation¹², liquid membrane separation¹³, solvent extraction¹⁴, ion exchange¹⁵ and adsorption¹⁶. Amongst the techniques listed, adsorption technique is more convenient and offers more advantages because of its design simplicity, operating flexibility besides the fact that locally available materials can be used to generate more adsorbents for continued application^17-19. In spite of its great use as an adsorbent for removal of PNP, activated carbon remains an expensive material, this makes it unattractive to small scale industries and to the poor people especially in rural areas²⁰. There is need therefore to explore cheaper alternatives for water decontamination to the poor communities in developing nations by making use of locally available materials and mostly regarded as wastes hence the need for this study.

Quaternary ammonium salts have one nitrogen atom that is surrounded in its four sights by substitutes of carbon atoms (R₄N⁽⁺⁾), these salts react with phenolic compounds to yield complexes that are stable²¹. Since the cations have a permanent positive charge, they can therefore interact and bond strongly with any material that is negatively charged or neutral²². The reaction between cellulosic materials and quaternary ammonium compounds was exploited in the present study for its potential to offer a lasting remedy for removing PNP from water. Preparation of chemically modified peels of Afromomum melegueta with polyDADMAC, a quaternary ammonium salt and its use in the uptake of PNP from model aqueous solution and subsequent UV-Vis analysis of PNP is reported in this study.

MATERIALS AND METHODS:

Chemicals, reagents and solvents:

The reagents used were of analytical grade and all the solutions prepared in distilled-deionized water at chemistry laboratory of Kenyatta University in Nairobi, Kenya. The phenolic compound used was p-Nitrophenol (PNP). Other reagents included: propylene oxide, epichlorohydrin, polyDADMAC, sodium hydroxide, hydrochloric acid, ammonium acetate and methanol. Stock solutions of 1000 µg/l concentration of PNP were prepared in ammonium acetate buffer solution from which working solutions were obtained. The pH of working solutions was adjusted with either 0.1M sodium hydroxide or 0.1M hydrochloric acid.

Instrumentation:

The raw and quaternised adsorbents were characterized using a Shimadzu Fourier transform infrared (FTIR-IR tracer-200) spectrophotometer to identify the functional groups contained in the raw and modified adsorbents²³. The PNP concentration in water was obtained using a double beam UV-Vis spectrophotometer (Model Specord 200, Analytik Jena) at maximum solution wavelength of 318 nm.

Preparation of the quartenised adsorbents:

The thorn melon peels were collected locally and were transported to the laboratories, washed thoroughly with deionized water to purify them, then chopped into pieces before they were oven dried at 105ºC to eliminate moisture. The adsorbents were ground into powder and sieved. The resulting solids were labeled and kept in desiccators until used. The ground adsorbents were then chemically modified using method described by⁴. The dry powder of thorn melon peels was activated for 12 hrs; 280 g of this sample was transferred into a two litre three necked flask after which 420 cm³ of 0.625 M NaOH was added. A solution made by dissolving 16.8 cm³ of epichlorohydrin in 140 cm³ of propylene oxide was introduced into the flask contents. Thorough stirring of the mixture was done until all the liquid had dried and there was no more enlargement of the material. This reaction was carried out at a temperature of 25ºC and gradually raising it to 50ºC for 1 hour in a thermostated bath water. Epoxidation procedure followed is coherent with the one used by²⁴. A mixture of 40 g solid sample of hydroxypropylated thorn melon peels and the epoxidated polyDADMAC at 4ºC were placed in a 500 cm³ three neck flask after which 2.8g of sodium hydroxide was added and the mixture agitated intensively for 30 mins. The agitation continued for further 6 hours at 60ºC while maintaining the pH above 8²⁴. Finally, the mixture was filtered by vacuum filtration to obtain the residue which was washed using distilled water to lower the pH to 7. The material was air dried and later characterized using FTIR.

Optimization of adsorption parameters:

Effect of parameters such as pH, contact time, sorbent dosage, initial PNP concentration and temperature on removal efficiency of the raw and quaternised adsorbents was optimized by keeping other parameters constant while varying the one under investigation as described by²⁵. The process parameters were varied as follows; solution pH (3 - 9), contact time (1 - 150 min), sorbent dose (0.01 - 0.06g), initial phenolate ion concentration (5 - 60mg/L) and temperature (15-90˚ C) in 20mL aqueous solution.

RESULTS AND DISCUSSIONS:

FTIR characterisation:

From figure 1, the FT-IR spectrum of the raw and quaternised thorn melon peels indicated the existence of many functional groups that are important for uptake of PNP. In RTM peels, the broad peak at 3402.43 cm^-1 can be assigned to normal polymeric –OH stretch or –NH groups²⁶ whereas the band at 2924.09 cm^-1 are attributed to CH₃CH₂ with -CH symmetrical and antisymetrical stretch and hydroxyl group in carboxylic acid. The band at 1651.07 cm^-1 was assigned to a –CO functional group²⁶ while the band at 1751.36 cm^-1 was allocated to an ester. The band at 1527.62 cm^-1 can be attributed to an aromatic ring of lignin in RTM peels. In the QTM peels, the band at 3402.43 cm^-1 dissipates and a different peak at 3363.66 cm^-1 emerges and this is linked to amine NH stretching frequencies after modification while the band at 1651.07 cm^-1 was assigned to a conjugated ketone and the band at 1458 cm^-1 allocated to --OH stretch in carboxylic acids whereas the band at 1072. 42 cm^-1 was allocated to –CN stretch in a primary amine. The band at 1527.62 cm^-1 disappeared in the QTM peels, this is linked to a depolymerization reaction of aromatic ring of lignin that could have led to elimination of lignin²⁷. The signal at 1751.36 cm^-1 was relocated to 1157 cm^-1 due to deformation of amines, the signal was assigned to –CN stretch in tertiary amine.

Impact of pH on uptake of PNP using raw and quaternised adsorbents:

Influence of pH on uptake was studied at optimum condition identified and the findings were recorded in Figure 2. Phenolic compounds undergo ionization in solution to form the phenolate ions and hydrogen ions before they can attach themselves on the surface of the adsorbent as shown in Eqn 1

Eqn 1

Increase in protons makes the equilibrium to shift to the left and hence this ionization is affected by changes in pH. The results in figure 2 show the highest uptake percentage of 64.04% for RTP and 92.13% for QTP at pH 3 and pH 5 respectively. It was established that uptake of PNP was favorably done at pH 3 using RTM whereas pH 5 was the best for adsorption of PNP onto QTP. It was noted that the percentage uptake of PNP was higher in acidic than in basic medium. The concentration of OH ions increases at high pH leading to increased hindrance of uptake of negatively charged PNP ions.

Figure 2: Impact of pH on uptake of PNP on raw (RTP) and quaternised thorn melon peels (QTP)

Further, The adsorption of PNP can therefore be said to be favorable at low pH since in its molecular form it has its pH being lower than pKa hence the increased uptake of PNP could be as a result of the bonded –NO₂ which reduces the electron density in the aromatic ring since it’s an electron withdrawing group. The results are coherent with those reported elsewhere^28-30,44who established higher adsorption of PNP in aqueous solutions within pH range of 3-5.

Impact of contact time on uptake of PNP:

The influence of this parameter was investigated by varying contact time from 1-150 mins. Uptake of PNP was influenced by adsorbate residence time as shown in figure 3, the rate of uptake was rapid during the first 30 minutes in RTM and QTM peels. Thereafter, the rate of uptake of PNP decreased and almost approached a steady state, indicating attainment of equilibrium. At the beginning, the accelerated adsorption rate in the first 20-30 min can be attributed to physical adsorption occurring at the adsorbent surface while the decreased uptake is as a result of other processes such as complexation³¹. It was established that to realize maximum uptake of PNP, contact time of 30 minutes was adequate. Increase in contact time didn’t show any variation in concentration. Results reported are coherent with those reported by^28-31,44

Figure 3: Influence of contact time on the adsorption of PNP on raw and quaternised thorn melon peels (RTP and QTP)

Effect of sorbent dose on adsorption of 4-Nitrophenol:

The influence of the amount of adsorbent on uptake of PNP was evaluated by changing sorbent dose from 0.01 to 0.06g at fixed volume of 20mL containing 20mg/L. Results from figure 4 shows that uptake of PNP was influenced by adsorbent dosage. The uptake percentage of PNP by RTM increased exponentially from 11.83 to 81.27 % at the dose of 0.01g and 0.03g respectively, thereafter the uptake decreases and then stabilizes. Increase in the amount of QTM peels from 0.01g to 0.03g leads to a slight increase of the PNP adsorbed from 93.21% to 93.42%. The findings indicate that adsorption of PNP ions rises as the sorbent amount increases this can be attributed to the expanded surface area and hence sites of adsorption, this is in agreement with findings reported elsewhere^30,33,34, Further increase in sorbent dose led to deceased uptake; since the sites of adsorption are laden with the sorbate besides there is solid aggregation as a result of inter particle interaction^35-36. Modification improved the uptake of PNP by the adsorbent; this can be attributed to improved surface texture and micro pore structure after modification as shown in figure 5 fast-tracking the entrance of the PNP ions into sites of adsorption³⁶. Since there is no increase in the adsorption beyond 0.03g for both RTM and QTM, these amounts were considered as the optimum sorbent doses which were used in subsequent studies.

Figure 4: Influence of sorbent dose on the adsorption of PNP on raw and quaternised thorn melon peels (RTP and QTP)

Impact of initial concentration on uptake of PNP:

The impact of starting concentration C₀ (5-60 mg/L) and sorbent mass of 0.01 and 0.03g of RAM and QAM respectively on the removal of PNP at t=30 minutes was investigated and results are shown in Figure 5. Adsorption capacity of the quaternised adsorbent was obtained by plotting the collected data as a function of PNP ion concentration adsorbed versus the initial PNP ion concentration. It was established that the amount of PNP removed rises in a linear manner and then stabilizes beyond a concentration of 30 mg/L due to the fact that the fixed sorbent mass adsorbs a fixed amount of PNP at constant volume of adsorbent. The adsorbent is at this instance said to be saturated with the sorbate³⁸. It was established that the uptake of PNP increases from 79.5 % to 93.7 % after modification. The percentage uptake of PNP using RTM was lower than that of QTM. Sorption of PNP from water is thus greatly favored by modification of the raw sorbents since it makes the surface of the adsorbent to have a micro rough texture and porous. At low initial PNP concentration almost all the phenolate ions have been adsorbed from aqueous solution. As the initial PNP concentration is increased the efficiency of adsorption does not change due to the fact that the active sites available become limited²⁵. The findings compare with those reported in the study of activated carbons in removal of PNP²⁸, adsorption of Ni (II), Cu (II) and Fe (III) on kammoni leaf powder³⁹, adsorption of phenolic compounds from water by quaternised treated maize tassels⁴ and application of acid treated biosorbents derived from lemon, sweet yellow passion, banana, watermelon peels and avocado seeds to adsorb heavy metals⁴⁰, uptake of PNP from water using raw and quaternised Afromomum melegueta peels⁴⁴.

Figure 5: Influence of initial concentration on the adsorption of PNP on raw and quaternised thorn melon peels (RTP and QTP)

Impact of temperature on adsorption of PNP:

The uptake of PNP from solution by RTM and QTM peels at temperatures (15-90°C) is shown in Figure 6. Increase in temperature leads to decreased uptake of PNP. This is expected for physical adsorption process which in most cases is usually exothermic⁴¹. It was indicated that the efficiency of adsorption was higher at 25°C, further increase in temperature lowered the amount of PNP removed Therefore adsorption isotherms were determined at 25°C. The findings compare with those reported in the study of removal of PNP using activated carbons derived from sewage sludge²⁸.

Figure 6: Impact of temperature on the adsorption of PNP on raw and quaternised thorn melon peels (RTP and QTP)

Adsorption isotherms:

The data gotten for PNP against initial concentration for RAM and QAM was analysed using Freundlich and Langmuir equation. The values of qmax and b (qmax is the adsorption capacity in mg/g, b is a Langmuir constant) were calculated from the slope and intercept of the Langmuir equation plot of versus Ce, (Ce is the PNP concentration at equilibrium) which gives a straight line with as the intercept and as the slope, from which qmax and b can be determined.

For Freundlich equation, a plot of logqe, versus logce yields a straight line from which K_f and can be determined from the intercept and slope respectively; Kf and n are constants representing adsorption capacity and intensity respectively. The constants for the adsorption isotherms used in this study are presented in Table 1

The adsorption of PNP onto RTM and QTM gave R² of 0.96 and 0.93 respectively indicating the data fitted well in Langmuir model. The values of b of 0.2577 and 0.4136 indicate favorable adsorption process for RTM and QTM peels, respectively. This model prescribes a single-layered adsorption and it indicates a chemisorptions process²⁵. Adsorption capacities (Qmax) recorded by RTM peels were lower than those recorded by QTM peels. This clearly shows that quarternisation improved the sorption capacities of these sorbents. These results compare with those reported in study of quaternised maize tussles to remove chlorophenols⁴, sewage sludge based activated carbons to remove PNP²⁸. In the case of Freundlich model that prescribes a multilayer adsorption which is a descriptive of both chemisorptions and physical sorption of PNP as a result of weak Vander Waal forces²⁵. Adsorption capacity and intensity are represented by constants Kf and n respectively⁴². From Table.1 the value of n observed indicates cooperative adsorption of PNP by both RTM and QTM peels since the value of is greater than 1 in either case^43-44. Further based on Freundlich model, the values for Kf for QTM peels are higher than those of RTM peels as shown in table 1, thus indicating the efficiency of the quaternisation process.

Table 1: Langmuir and Freundlich constants for PNP adsorption using RTM and QTM peels

Ads	Langmuir isotherm			Freundlich isotherm
	*Qmax* (mg/g)	b (L/mg)	R²	**K_f(mg/g)**	n	R²	Best model
RTM	12.79	0.2577	0.9636	3.65	0.45	0.7989	Langmuir
QTM	16.31	0.4136	0.928	6.15	0.53	0.7176	Langmuir

CONCLUSIONS:

In this study, adsorption efficiency of raw and quartenised thorn melon peels towards removal of PNP from water was evaluated. Removal efficiency of PNP is high at the first 30 mins of contact and at sorbent dosage of 0.03g. Quantity of PNP removed increases as the initial concentration rises however, adsorption decreases after a concentration exceeding 30mg/L. The ideal pH for uptake of PNP is 3 and 5 for RTM and QTM respectively and at a temperature of 25˚C. Adsorption of PNP onto RTM and QTM peels follows the Langmuir isotherm model. Raw and quartenised adsorbents showed different levels of efficiency in removing the PNP; quartenisation was shown to greatly improve the adsorption of PNP. A follow-up study is ongoing on the adsorption kinetic studies as well as adsorption efficiency of the thorn melon peels for other phenolic compounds and microbes from environmental water samples.

ACKNOWLEDGEMENT:

The authors are grateful to African Development Bank (AfDB) for funding the project and the assistance accorded to us by the chemistry technical staff at Kenyatta University and the support of family, friends and colleagues.

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Received on 05.08.2020 Modified on 16.09.2020

Asian J. Research Chem. 2021; 14(1):1-6.

DOI: 10.5958/0974-4150.2021.00001.8