Equilibrium isotherms, kinetics and thermodynamics studies of phenolic compounds adsorption on palm-tree fruit stones

https://doi.org/10.1016/j.ecoenv.2012.06.019Get rights and content

Abstract

Adsorption capacity of an agricultural waste, palm-tree fruit stones (date stones), for phenolic compounds such as phenol (Ph) and p-nitro phenol (PNPh) at different temperatures was investigated. The characteristics of such waste biomass were determined and found to have a surface area and iodine number of 495.71 m2/g and 475.88 mg/g, respectively. The effects of pH (2-12), adsorbent dose (0.6–0.8 g/L) and contact time (0–150 min) on the adsorptive removal process were studied. Maximum removal percentages of 89.95% and 92.11% were achieved for Ph and PNPh, respectively. Experimental equilibrium data for adsorption of both components were analyzed by the Langmuir, Freundlich and Tempkin isotherm models. The results show that the best fit was achieved with the Langmuir isotherm equation with maximum adsorption capacities of 132.37 and 161.44 mg/g for Ph and PNPh, respectively. The kinetic data were fitted to pseudo-first order, pseudo-second order and intraparticle diffusion models, and was found to follow closely the pseudo-second order model for both components. The calculated thermodynamic parameters, namely ΔG, ΔH, and ΔS showed that adsorption of Ph and PNPh was spontaneous and endothermic under examined conditions.

Highlights

► Date stones were used to remove phenol (Ph) and p-nitro phenol (PNPh) from aqueous solutions. ► Maximum removal percentages of 89.95% and 92.11 % were achieved for Ph and PNPh, respectively. ► The isotherm data were well fitted to the Langmuir isotherm. ► Maximum adsorption capacities of 132.37 and 161.44 mg/g were obtained for Ph and PNPh, respectively. ► The kinetic data were well described by pseudo-second order model.

Introduction

Wastewaters from paint, pesticide, coal conversion, polymeric resin, gasoline, rubber proofing, steel, petroleum, and petrochemical industries contain considerable amounts of phenols. These pollutants are known to be toxic, carcinogenic, and pose serious hazards to aquatic living organisms. Phenol (Ph) and p-nitro phenol (PNPh) are the most hazardous polluting phenols to the environment. The presence of Ph even at low concentrations can cause unpleasant taste and odor of drinking water. PNPh can enter the human body through all routes and reacts easily in the blood to convert hemoglobin to methamoglobin, thereby preventing oxygen uptake. Indeed, it is necessary to remove these pollutants from wastewaters before discharge into the environment (Zhu et al., 2000).

Various processes have been employed for the removal of phenols from aqueous media including chemical oxidation (He et al., 2009), membrane filtration (Kujawski et al., 2004), biodegradation (El-Naas et al., 2009), electro coagulation (Ugurlu et al., 2008), photo degradation (Gomez et al., 2011), solvent extraction (Xu et al., 2006), and adsorption (Yang et al., 2008). Among these methods adsorption is still the most popular and widely used technique for phenol removal, because of its simple design, easy operation, and relatively simple regeneration (Nevskaia et al., 2004).

In the design and optimization of adsorption processes the adsorption isotherms and kinetics are of utmost importance to study. Adsorption isotherms are important for the description of how adsorbate will interact with adsorbent and are critical in optimizing the use of adsorbent. Thus, the correlation of experimental equilibrium data using either a theoretical or empirical equation is essential for adsorption data prediction. Adsorption kinetics involves the study of the rate at which pollutants are removed from aqueous solution onto adsorbent surface, which in turn controls the residence time of the adsorbate uptake at the solid–solution interface (Altenor et al., 2009).

Currently adsorption on activated carbon is widely used for removal of phenols, but it remains an expensive material for this purpose. Because the efficient removal of phenols required activated carbons of high micropores structure which can be obtained at specific operating conditions by the use of specific activators such as FeCl3 and KOH (Theydan and Ahmed, 2012, Kilic et al., 2011). Recently, many non-conventional and low-cost adsorbents, including agricultural waste biomass materials have been investigated intensively as adsorbents for removal of pollutants from aqueous solutions. Because these materials are cheaper, renewable and abundantly available; also constitute an environmental problem (Adiuata et al., 2007). One of such low-cost adsorbents is date stones, which can be considered as one of the best candidate among the agricultural wastes because they are cheap and quite abundant, especially in Mediterranean countries.

In 2006, world production of dates was about 7 million tons and the top 10 producing countries were Iraq, Egypt, Saudi Arabia, Iran, United Arab Emirates, Pakistan, Algeria, Sudan, Libya, and Tunisia (Bhansali, 2010). Iraq is the largest world producer of dates with more than 21 million date palm trees and an annual production of about 400,000 t of the fruit. About 14% of the fruit is waste material in the form of seeds. Such a waste stream from the palm oil and palm syrup factories has been an environmental problem. Therefore, its recycling or reutilization is useful (Hameed et al., 2009).

Many studies have been done on adsorption isotherms, kinetics and thermodynamics of phenols on various agricultural wastes and natural materials such as soil (Subramanyam and Das, 2009), chitin (Dursun and Kalayci, 2005), clinoptilolite rock (Sprynskyy et al., 2009), lignite (Polat et al., 2006), olive pomace (Stasinakis et al., 2008), deolied soya (Mittal et al., 2009), mauritonian clay (Ely et al., 2011), mansonia wood sawdust (Ofomaja, 2011), chitosan (Li et al., 2009), and alignate (Peretz and Cinteza, 2008). Date stones have been effectively employed for removal of dyes (Altaher and ElQada, 2011, Belala et al., 2011) and heavy metals (Al-Ghouti et al., 2010; Banat et al., 2002). Though, previous studies have been done on removal of phenol and 2,4-dinitrophenol using date stones (Al-Mutairi, 2010, Okasha and Ibrahim, 2010), those studies didn not contain any detailed informations about the kinetics and mechanism of adsorption process.

Therefore, the main objective of the present work is to find the efficient conditions for adsorptive removal of Ph and PNPh onto date stones and studying experimentally and theoretically the equilibrium isotherms, kinetics, and thermodynamics of adsorption process.

Section snippets

Materials

Date stones were used as adsorbents. Stones as received were first washed with water to get rid of impurities, dried at 110 °C for 24 h, crushed using disk mill, and sieved. Fraction of average particle size of 250 μm was selected for the study. The characteristics of date stones represented by bulk density, surface area, ash content, moisture content, and iodine number were determined. Bulk density was determined according to procedure followed by Ahmedna et al. (1997). Surface area was estimated

Adsorbent characteristics

The characteristics of date stones were determined and summarized in Table 1. The most important characteristics are surface area and iodine number, and the results showed that their values are 495.71 m2/g and 475.88 mg/g, respectively. This high surface area for an agricultural waste biomass enables date stones to be used successively for removal of dyes from aqueous solutions, as explained by Belala et al. (2011) and Banat et al. (2003). Also this high iodine number which is a measure of

Conclusions

Phenol and p-nitrophenol were removed successfully from aqueous solution by adsorption on date stones. In this study, maximum phenol removal percentage of 89.95% were reported at conditions of 4 pH value, 0.75 g/L adsorbent dose and 120 min contact time. On the other hand, 92.11% maximum removal percentage for p-nitrophenol at 5 pH, 0.7 g/L adsorbent dose and 120 min contact time. Equilibrium adsorption data of phenol and p-nitrophenol onto date stones at 303, 313 and 323 K were well represented by

Acknowledgment

We gratefully acknowledge university of Baghdad and chemical engineering department for assist and support of this work.

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