Effectiveness of beetroot seeds and H 3 PO 4 activated beetroot seeds for the removal of dyes from aqueous solutions

Raw beetroot seeds (BS) and H 3 PO 4 activated beetroot seeds (H 3 PO 4 -BS) were evaluate for their effectiveness in removing methylene blue (MB) and malachite green (MG) from aqueous solution. BS were carbonized at 500 (cid:1) C for 2 h, and then impregnated with phosphoric acid (phosphoric acid to BS ratio of 1.5 g/g). The impregnated BS were activated in a tubular vertical furnace at 450 (cid:1) C for 2 h. Batch sorption experiments were carried out under various parameters, such as solution pH, adsorbent dosage, contact time, initial dyes concentration and temperature. The experimental results show that the dye sorption was in ﬂ uenced by solution pH and it was greater in the basic range. The sorption yield increases with an increase in the adsorbent dosage. The equilibrium uptake was increased with an increase in the initial dye concentration in solution. Adsorption kinetic data conformed more to the pseudo-second-order kinetic model. The experimental isotherm data were evaluated by Langmuir, Freundlich, Toth and Dubinin – Radushkevich isotherm models. The Langmuir maximum monolayer adsorption capacities were 61.11 and 74.37 mg/g for MB, 51.31 and 213.01 mg/g for MG, respectively in the case of BS and H 3 PO 4 -BS. The thermodynamic parameters are also evaluated and discussed.


INTRODUCTION
The contamination of natural and industrial waters by textile dyes is now recognized as a major environmental concern.
These environmental problems may be due to the rapid development of various industries, such as textile, paper, leather, coating, cosmetic and plastic. Among the 7 × 10 5 tons and 10,000 different types of dyes and pigments produced globally every year, it has been estimated that 1-15% of dyes were expelled in the effluents during the dyeing process (Zollinger ). This massive influx of untreated organic chemicals into waterways not only gives rise to aesthetic concerns, but far more importantly it promotes eutrophication and adversely affects environmental conditions. Due to their persistence and their refractory carcinogenic nature, dyes represent an increasing environmental danger (Reife ). In order to protect the environment, synthetic dyes should be removed from wastewaters.
A wide variety of techniques have been used for dye removal from wastewaters including biological degradation (Santos et al. ), photodegradation (Abaamrane et al. have also been tested (Tounsadi et al. ). These materials generally have low sorption efficiencies and therefore, a large adsorbent dosage is necessary to remove a low dye concentration. In order to enhance the performance efficiency of the sorption processes, it is necessary to develop cheaper, easily available adsorbents with great sorption capacities. In particular, the use of lignocellulosic biomass is a promising alternative adsorbent due to its relative abundance, availability and low commercial value.
Beetroot seeds (BS) are a cheap, abundantly available and renewable precursor. Their high ligno-cellulosic content makes them an efficient precursor for the preparation of activated carbon. Preparation involves treatment with H 3 PO 4 which acts as a dehydrating catalyst, promoting decomposition of the cellulosic precursor at a lower heat treatment temperature. The presence of H 3 PO 4 in the interior of the precursor restricts tar formation and inhibits the shrinkage of the precursor particle by occupying a substantial volume resulting in the lower weight loss and higher yield for H 3 PO 4 impregnated carbon (Ahmad & Thyodan ).
The aim of the present work was to evaluate the ability of BS and H 3 PO 4 activated beetroot seeds (H 3 PO 4 -BS) to adsorb methylene blue (MB) and malachite green (MG) from aqueous solution. Various parameters were studied in batch adsorption including the effect of solution pH, adsorbent dosage contact time, initial dye concentration and temperature. For adsorption kinetic modelling, two kinetic models (pseudo-first-order and pseudo-second-order kinetics) were used, and four isotherm models (Langmuir, Freundlich, Toth and Dubinin-Radushkevich) were applied to fit the experimental equilibrium data. The thermodynamics of the adsorption was also analysed.

METHODS AND MATERIALS Materials
All the chemicals used in this study were of analytical grade.
The dyes MB and MG were obtained from Sigma-Aldrich and used without further purification. The chemical formulae and some other specific characteristics of these dyes are summarized in Table 1.

Preparation and characterization of the adsorbents
Locally obtained BS were repeatedly washed with distilled water to remove dirt particles and were then dried at 80 C for 24 h. The dried seeds were further powdered using a domestic mixer. Ten grams powdered BS were pyrolysed at 500 C for 2 h under a nitrogen atmosphere. The char obtained was impregnated with phosphoric acid (purity 85%, Acros Organics) at phosphoric acid to BS ratio of 1.5 for 6 h, and followed by the removal of excess solution and overnight drying at 110 C. Then, the char sample was activated at 450 C for 2 h. Subsequently, the cooled samples were repeatedly washed with hot deionized water to remove free phosphoric acid, tar, fines and leachable matter followed by overnight drying at 110 C. and measured by a sensIONþ PH31 pH metre. The temperature was controlled using a thermostatically controlled incubator.

Adsorption tests
After sorption experiments were completed, samples were withdrawn and centrifuged at 3,000 rpm for 10 min.
The residual concentrations were further determined from UV-Vis characteristics at maximum absorption wavelength of each dye using a TOMOS V-1100 UV-vis spectrophotometer.
The adsorption efficiency and adsorption yield were calculated using Equations (1) and (2): where q (mg/g) is the adsorption quantity, C 0 (mg/L) is the initial dye concentration, C (mg/L) is the dye concentration, m (g) is the mass of adsorbents and V (L) is volume of dye solution.

Fourier transform infrared spectroscopy
Fourier transform infrared (FTIR) spectra of BS and H 3 PO 4 -BS are given in Figure 1. The broad peaks in the 3,600-2,900, 1,700-1,400 and 1,300-900 cm À1 ranges indicative of existence of various functional groups that can participate in sorption process. FTIR spectrum of BS shows a strong band between 3,600 and 3,200 cm À1 due to overlapping of

Effect of pH on dyes adsorption
The pH of the solution is an important parameter affecting the surface charge of the adsorbents as well as the degree of ionization of different pollutants. This subsequently leads to a shift in reaction equilibrium characteristics of adsorption process. Figure 2 shows the effect of pH from 2 to 10 on the adsorption quantity (q e ) of MB and MG onto BS and H 3 PO 4 -BS. It can be seen that the adsorption is weak in acidic medium. As the pH increases, the adsorption capacities increase. This result may be due to alterations in the adsorbent's surface charge or in the charge state of dyes.
The pH values of zero charge (pH PZC ) of the adsorbents were found to be 6.2 and 4.7, respectively, for BS and H 3 PO 4 -BS. At pH values above this point, the high sorption of dyes may be explained as occurring on the negative sites, and the ionic state of functional groups such as carboxyl, phosphoryl, sulfhydryl, hydroxyl and amino will be such as to promote reaction with MB and MG as cationic dyes. The optimal values of pH used in the further experiment were identified as pH ¼ 5.00 for MG and pH ¼ 6.05 for MB.

Effect of adsorbent dosage
Adsorbent dosage is a highly influential parameter in sorption processes. It determines the capacity of an adsorbent for a given initial concentration of dye molecules. Data obtained from the experiments with varying adsorbent dosage or mass ratio (R) are presented in Figure 3. It shows that the sorption yield significantly increased with   Figure 3 shows that activation of BS strongly enhances adsorption potential for the selected dyes. Further, the optimal value of adsorbent dosage used in the following experiment was 0.5 g/L for both adsorbents in the removal of the both dyes.

Adsorption kinetics
The plot of MB and MG adsorption versus contact time is shown in Figure 4. The uptake of MB and MG increased quickly in the first period of the process and then the rate of biosorption slowed and stagnated with the increase in contact time. The equilibrium time was 60 min for the adsorption of BM by both adsorbents and was 120 min in the case of MG. In order to describe the kinetics involved in MB and MG sorption, two commonly used kinetic modelspseudo-first order and pseudo-second order rate equationswere applied to fit the kinetic data. This analysis is based on the regression coefficient (r 2 ) and the amount of dye adsorbed per unit weight of the adsorbent.
The first-order rate expression of Lagergren based on solid capacity is generally reported as in Equation (3) (Lagergren ): where q e and q (both in mg/g) are respectively the amounts of dye adsorbed at equilibrium and at any time t, and k 1 (1/min) is the rate constant of adsorption.
The pseudo-second-order model proposed by Ho & McKay () was used to characterize the sorption kinetics. This model is based on the assumption that the adsorption follows second-order chemisorption (Blanchard et al.

).
The pseudo-second-order model can be represented as: where k 2 (g/mg·min) is the rate constant of pseudo-secondorder adsorption.
Parameters of the pseudo-first-order and pseudo-secondorder models were evaluated by non-linear regression. The data and the correlation coefficients, r 2 , are summarized in Table 2: the correlation coefficients for the pseudo-secondorder kinetic model are closer to 1 than that of the

Langmuir model
The Langmuir () isotherm model assumes that the adsorption occurred in a monolayer with uniformly energetic adsorption sites and no lateral interaction between adsorbed molecules. Therefore, at equilibrium, a saturation point is reached where no further adsorption can occur. A basic assumption is that sorption takes place at specific homogeneous sites within the adsorbent. The expression formula of the Langmuir isotherm is given by Equation (5): where q e (mg/g) is the adsorbed amount at equilibrium, C e is the equilibrium dye concentration (mg/L), K L is Langmuir equilibrium constant (L/mg) and q m the maximum adsorption capacity (mg/g).   Freundlich equation is expressed by Equation (6). This equation can also fit multilayer sorption.
where K F is the Freundlich constant and n is the heterogeneity factor.
The K F value is related to the adsorption capacity; while 1/n value is dependent to the adsorption intensity.

Toth model
To ). The Toth correlation is given as: where q e is the adsorbed amount at equilibrium (mg/g), C e the equilibrium concentration of the adsorbate (mg/L), q m the Toth maximum adsorption capacity (mg/g), K T the Toth equilibrium constant and t is the Toth model exponent. If the exponent t is equal to unity, the model of Toth can be reduced to the Langmuir model.

Dubinin-Radushkevich model
The Dubinin-Radushkevich isotherm is a more general model than Langmuir model, although, it does not assume a homogenous surface or constant sorption potential. The

Dubinin-Radushkevich isotherm equation is given by
Equations (8) and (9) (Dubinin & Radushkevich ): where B is a constant related to the adsorption energy, q m is the theoretical saturation capacity and ϵ is the Polanyi potential.

Analysis of adsorption isotherms
The constant parameters of the isotherm models were evaluated by nonlinear regression analysis of the experimental adsorption isotherms obtained and the adsorption isotherm models. After analysis, the data with correlation coefficients (r 2 ) are shown in From these results, it can be concluded that the activation enhances the adsorption capacity of BS more strongly for MG than for MB.

Effect of temperature
The effect of temperature on the adsorption of MB and MG was tested, bearing in mind the specific For such equilibrium reactions, K D , the distribution constant, can be expressed as: The Gibbs free energy is: where R is the universal gas constant (8.314 J mol/K) and T is solution temperature in K.
The enthalpy (ΔH ) and entropy (ΔS ) of adsorption were estimated from the slope and intercept of the plot of ln K D versus 1/T yields, respectively.
As shown in Table 4, because the values of ΔG are negative at different temperatures, the adsorption of dyes