Adsorption of dark coloured compounds from peach pulp by using powdered-activated carbon

Abstract

The potential of powdered-activated carbon (PAC) for the adsorption of dark coloured compounds from peach pulp was studied at different PAC concentrations (0.5, 1, 3, 5 kg PAC per m³ peach pulp) and for the temperature range of 20–60 °C. The isotherm studies were carried out to evaluate the effect of adsorbent dosage, contact time and initial absorbance. For equilibrium adsorption curves, the Langmuir, Freundlich, Temkin and Frumkin models were attempted by using the absorbance data at 420 nm and model parameters were obtained for different temperatures. The results clearly showed that the adsorption data of dark coloured compounds present in peach pulp onto PAC fits well to the Langmuir and Temkin models having correlation coefficients higher than 0.90 at all temperatures. Five kinetic models including the first-order equation, second-order equation, the modified Freundlich, the pore diffusion model and the Elovich equation were tested to follow the adsorption processes. Results of the kinetic studies show that the adsorption reaction was well represented by the Elovich model. The activation energy of adsorption was found to be 13.79 kJ/mol. The low activation energy indicated that the adsorption reaction was also diffusion controlled.

Introduction

Peaches are fruits of great commercial importance. Besides the raw natural fruits, peaches are utilized in the form of products such as juices, pulp and nectar. During processing of fruit juices and also during storage, detrimental changes such as loss of volatile compounds, destruction of vitamins and amino acids, hydrolysis of carbohydrates, development of undesirable odours and tastes and browning reactions can occur. One of the most common and maybe the most important of these changes is browning caused by enzymatic or non-enzymatic processes that are caused by the Maillard reaction which takes place between carbonyl and free amino groups (Carabasa et al., 1998, Toribio and Lozano, 1984). Browning in juices and fruit purees during manufacture and storage is of vital interest to the food industry. The formation of brown-colored products known as melanoidins is the result of polymerisation reactions of highly reactive intermediates formed during the Maillard reaction (Martins, Jongen, & Van Boekel, 2001). Non-enzymatic browning not only undermines the sensorial characteristics but also causes loss of nutrients and the formation of intermediate undesirable compounds, like furfural and 5-hydroxymethylfurfural (Buedo, Elustondo, & Urbicain, 2001).

Melanoidins and polyphenols have been found to be responsible for physico-chemical deterioration of some fruit juices because they are relatively small molecules that can easily pass through the membranes during ultrafiltration processing (Borneman et al., 2001, Martins et al., 2001). In order to prevent fruit juices from browning and haze formation, a reduction of melanoidins and phenolics is necessary (Borneman et al., 2001, Martins et al., 2001).

Adsorption is commonly used to purify contaminated fluids that are unacceptable in smell and taste. Typical adsorbents studied for the removal of polyphenols and brown colour from fruit juices include activated carbon, gelatine/bentonite, casein, ion-exchange waxes and polyvinylpolypyrrolidone (Giovanelli & Ravasini, 1993). Activated carbons (ACs) are widely used as adsorbents in decontamination processes because of their extended surface area, high adsorption capacity, microporous structure and special surface reactivity (Ania, Parra, & Pis, 2002). Activated carbon has been widely used to remove organic pollutions including phenols because of the existence of functional groups such as carboxyls, lactones, aldehydes, ketones, quinones, hydroquinones, anhydride and ethereal structures (Jung et al., 2001).

The adsorption isotherms are the equilibrium relationship between the concentration in the fluid phase and concentration in the adsorbent particles at a given temperature. Isotherm models are often used to describe adsorption equilibrium on a quantitative basic. The analysis of the isotherm data by fitting them to different isotherm models is an important step in finding the suitable model for the design process (Gökmen and Serpen, 2002, Kadirvelu and Namasivayam, 2003; Khan, Al-Bahri, & Al-Haddad, 1997). Equilibrium data is correlated with the Langmuir, Freundlich, Temkin and Frumkin isotherms which are the most commonly used models in the case of mono component systems if the solute is retained in only one molecular layer (Ajmal et al., 2000, Mattson and Mark, 1971).

The mechanism of adsorption from a solution consists of diffusion across the boundary layer surrounding the adsorbent particle, mass transfer within the internal surface of the adsorbent particle and adsorption at a suitable site (McKay, Blair, & Aga, 1982). In order to examine the controlling mechanism of the adsorption process such as mass transfer, diffusion control and chemical reaction, several kinetic models are used to test experimental data (Dogˇan and Alkan, 2003, Shekinah et al., 2002, Wu et al., 2002). Most of these models include surface reaction kinetics as well as various steps of mass transfer resistances (Tsai, Lai, & Hsien, 2003).

Khan, Al-Wahheab, and Al-Haddad (1996) collected experimental data for biosolute systems from the literature and tested for various proposed models. The data were analysed using Langmuir, Freundlich and empirical models for multi-component systems. Costa and Rodrigues (1982) studied the adsorption of phenol and p-nitrophenol in an aqueous solution on granular activated carbon. Literature on the adsorption of phenolic compounds onto ACs is abundant (Bercic and Pintar, 1996, Cooney and Xi, 1994, Halhouli et al., 1997, Khan et al., 1997, Kilduff and King, 1997, Mollah and Robinson, 1996). However, the studies on the adsorption of dark coloured compounds from peach pulp by PAC, to the best of our knowledge, are scarce (Carabasa et al., 1998).

The present study was designed (a) to investigate the adsorption behaviour with activated carbon of dark coloured compounds produced by browning reactions in peach pulp (b) to use the Langmuir, Freundlich, Temkin and Frumkin isotherms models to describe the adsorption process onto powdered-activated carbon of the dark coloured components in peach pulp and (c) to use the kinetic models such as first-order and second-order equations, an intraparticle diffusion model, Elovich equation and modified Freundlich equation for quantifying and predicting the adsorption behaviour.