Removal of dyes from aqueous solutions with untreated coffee residues as potential low-cost adsorbents: Equilibrium, reuse and thermodynamic approach

Abstract

Industrial wastes of coffee (untreated coffee residues, UCR) were used as low-cost adsorbents for the removal of dyes (reactive and basic) from single-component aqueous solutions. The cost potential is high given for the non-further treatment of the coffee residues (just only washing with distilled water to remove dirt/color, and drying in oven). The characterization of adsorbents was carried out with SEM micrographs, titrations for point of zero charge (PZC), BET analysis for surface area, titrations (Boehm method) for surface functional groups, and energy dispersive X-ray microanalysis (EDX) for elemental analysis/composition. The optimum pH found after adsorption experiments was pH = 2 for reactive and pH = 10 for basic dyes. The equilibrium experimental data were fitted to Langmuir, Freundlich, and Langmuir–Freundlich model (best correlation: R² > 0.997). The calculated maximum adsorption capacities (Q_max) for the reactive dye at 25 °C were 179 mg/g (pH = 2) and 295 mg/g for the basic one (pH = 10). Pseudo-first, -second, and -third order kinetic equations were used to fit the kinetic data (pseudo-second order equation presented the most sufficient correlation, R² > 0.992). Some other adsorption parameters, as agitation rate, initial dye concentration and temperature (25, 45 and 65 °C) were also determined. Also, a full thermodynamic evaluation was realized, calculating the parameters of enthalpy, free energy and entropy (ΔH⁰, ΔG⁰, and ΔS⁰). The desorption was evaluated with experiments for the optimum desorption pH and desorption kinetics, while the ability of reuse was determined with 10 cycles of adsorption–desorption (the reduction in adsorption percentages from the 1st to 10th cycle was approximately 7% for both dyes). Additionally, experiments in dyeing mixtures were realized in dyeing mixtures composed of (i) separately reactive or basic dyes, and (ii) simultaneously reactive and basic dyes. After the dosage of 3 g/L of adsorbent, a very slight change was observed in equilibrium for all types of dyeing mixtures studied.

Introduction

Wastewater discharged from the dye-houses can be one of the biggest contributors to aquatic pollution. The most studied dye classes, in the dye bearing effluent treatment, are reactive and basic [1]. The dye loss from the dyeing process to the effluent is estimated 10–50% for reactive dyes and 0–5% for basic ones [2]. However, the ADMI study proved that basic dyes are generally more toxic than reactive and direct dyes [3]. Given that reactive and basic dyes could simultaneously co-exist in the equalization tank of a dye-house, it is of fundamental importance to remove/treat both of them [4]. Furthermore, wastewater containing dyes is very difficult to be treated, since the dyes are recalcitrant organic molecules, resistant to aerobic digestion, and stable to light, heat and oxidizing agents [5].

Among the numerous techniques for dyes removal, adsorption is believed to give the best results as it can be used to remove different types of coloring materials [6]. The majority of commercial systems currently use activated carbon as adsorbent to remove dyes in wastewaters because of its excellent adsorption ability. The adsorption with activated carbon been cited by US EPA as one of the best available control technologies [7]. However, although activated carbon is a preferred adsorbent [8], [9], its widespread use is restricted due to its high cost [10]. In order to decrease the cost of treatment, attempts have been made to find inexpensive alternative adsorbents.

Recently, many works have been studied for the development of both effective and low-cost adsorbents. A review publication of Crini [11] reported in details many potential low-cost and non-conventional adsorbents for dyes removal (biosorbents, natural, industrial and agricultural materials) [11], [12], [13]. Some of the reported adsorbents include clay materials (bentonite, kaolinite), zeolites, siliceous material (silica beads, alunite, perlite), agricultural wastes (bagasse pith, maize cob, rice husk, coconut shell) [11], [14], [15], industrial waste products (waste carbon slurries, metal hydroxide sludge) [11], [16], biosorbents (chitosan, peat, biomass) and others (starch, cyclodextrin, cotton) [11], [17]. However, there is a lack of literature dealing with the possible application of coffee residues as adsorbents (i.e. for metals [18], [19]), and in particular as dye adsorbents [20], [21], [22], [23]. In general, “coffee residues” are generally called the solid wastes discarded from the extraction process of instant coffee manufacturing, and the final residues originated from cafeterias.

In the last years, the instant coffee industry has experienced a constant growth as instant coffee has become one of the most popular kinds of coffee drinks by million of people around the world. As a consequence, large amounts of coffee grounds, which are the solid residues obtained during the processing of coffee powder with hot water or steam to prepare instant coffee, have been generated worldwide (in the order of 6,000,000 t/yr) [24].

In this study, a novel approach was attempted: the removal of dyes from synthetic aqueous solutions (both single-component and dyeing mixtures) with untreated coffee residues (UCR). There is a high potential in this attempt given that despite the possible limited adsorption capacity of these materials, their cost approaches zero (only the residues were dried at ambient temperature and used for the adsorption experiments). It is a great of interest a probable next step of a future research, as setting up a bed column system to treat real industrial dyeing effluents with zero cost and sufficient capacity.

In the current work, the adsorption behavior was performed in batch mode, studying the effect of: (i) pH on adsorption and desorption; (ii) agitation rate; (iii) initial dye concentration; (iv) contact time on adsorption and desorption; (v) temperature (isotherms), and (vi) reuse in sequential cycles of adsorption–desorption. Additionally, apart from single-component dye solutions, experiments in dyeing mixtures were realized both in dyeing mixtures composed of (i) separately reactive or basic dyes, and (ii) simultaneously reactive and basic dyes. The coffee wastes were also fully characterized.