Adsorption of Nitrate , Nitrite , and Fluoride Ions by Carbonaceous Material Produced from Coffee Grounds in a Complex Solution System

Carbonaceous materials produced from coffee ground (virgin CG, CG600, CG800, and CG1000) were prepared. Specific surface areas, mean pore diameters, pore volumes, and SEM images of the CGs were investigated. The specific surface areas were in the order CG1000 (23.5 m/g) < CG800 (31.5 m/g) < CG600 (52.6 m/g), and the mean pore diameters were in the order CG600 (77.0 Å) < CG800 (139.3 Å) < CG1000 (273.8 Å). The amounts of nitrate, nitrite, and fluoride ions adsorbed in a single solution system were greater than the amounts adsorbed in a ternary solution system; this indicated that the ions were competitively adsorbed onto the CGs in the complex solution system. Moreover, the adsorption mechanism was ion exchange with chloride ion onto the CGs in a 1:1 ratio. Adsorption isotherms were fitted to both the Freundlich equation and the Langmuir equation. The amounts adsorbed increased with increasing temperature. The adsorption affinities onto the CGs were in the order nitrate ion < nitrite ion < fluoride ion. The most suitable breakthrough curve conditions were Space velocity: 4.24 1/h and Linear velocity: 0.38 m/h. Thus, carbonaceous materials produced from coffee grounds were useful for the adsorption of nitrate, nitrite, and fluoride ions in a ternary solution system. [DOI: 10.1380/ejssnt.2012.493]


I. INTRODUCTION
Water typically contains dissolved materials, suspended solids, and dissolved gases.Some of these are physiologically necessary, but concentrations above permissible limits may adversely affect health [1].Several nitrogenous compounds, including ammonia, nitrite, and nitrate, are frequently present in drinking water and various types of agricultural, domestic, and industrial wastewaters.Nitrate and nitrite can be attributed to fertilizers, animal excretion, and industrial effluents.High concentrations of these compounds in drinking water can cause health problems, such as cyanosis among children and cancer of the alimentary canal [2].Further, in many parts of the world, fluoride concentrations in drinking water are higher than the allowed limits.Fluoride-related health hazards are a major environmental problem because fluoride can promote bone diseases (pain and tenderness of the bones) and mottled teeth.The main sources of this contaminant in drinking water are additives for strong teeth, erosion of natural deposits, and discharge from fertilizer and aluminum factories [3].
Nitrogen is traditionally removed from wastewater by a variety of methods, including conventional treatment, biological and chemical processes, physical operations, and land application [4][5][6].Moreover, many technologies have been applied to remove fluoride from water: flocculation and electrocoagulation, chemical precipitation, adsorption, ion exchange, and membrane technology.Among the various methodologies, adsorption has been found to be more appropriate and attractive [7][8][9][10][11][12].
In recent years, considerable attention has been devoted to the study of different types of low-cost materials, such as tree bark, wood charcoal, sawdust, alum sludge, red mud, and other waste materials, for the adsorption of var-ious toxic substances [13].
In 2004, 52 million tons of coffee grounds (CGs) was produced as waste in Japan, and CGs are easily obtained worldwide.Therefore, if an efficient adsorbent could be produced from CGs, this technique would constitute a sustainable development for waste recycling.Yokoyama et al. reported a new and highly efficient waste-recycling technique that involves ion exchange [14].
The aim of this study was to determine the efficacy of CGs in the removal of nitrate, nitrite, and fluoride ions in a ternary solution system, and to investigate the interaction between anions and CG surfaces during adsorption.

A. Materials
Extracted coffee grounds (CGs) were obtained from UCC Ueshima Coffee Co., Ltd.(virgin CGs).Surface modification of the CGs was performed according to the method reported by Yokoyama et al. [14].Virgin CGs (40 g) were added to 1 M calcium chloride solution (500 mL, Wako Pure Chemical Industries, Co., Ltd.) and stirred for 24 h at room temperature.The suspensions were filtered using a 0.45-µm membrane filter (Advantec MFS, Inc.) and the CG residues were dried for 5 h at 110 • C. The CGs were then carbonized in a muffle furnace by heating for 2 h at 600, 800, or 1000 • C under a nitrogen gas flow.Following carbonization, the CGs were decomposed by hydrochloric acid treatment, in which carbonized CG samples were added to 6 M hydrochloric acid solution (100 mL).The suspensions were filtered and subsequently dried for 5 h at 110 • C to obtain the samples, which are hereafter referred to as CG600, CG800, and CG1000, respectively, where the numeral indicates the carbonization temperature.The CG samples were investigated by scanning electron microscopy (SEM) using a JSM-5200 (JEOL, Japan).Specific surface areas, pore  volumes, and mean pore diameters were measured using a NOVA4200e specific surface analyzer (Yuasa Ionic, Japan).Percent yields for the CGs were calculated from the weights of the CGs before and after carbonization.

B. Adsorption isotherms of nitrate, nitrite, and fluoride ions
CGs (0.05 g) were added to nitrate, nitrite, and fluoride ions (50 mL) at different initial concentrations (5-50 mg/L) in a single or ternary solution system.The suspensions were shaken at 100 rpm for 24 h at 25 • C (5 and 35 • C).The suspensions were then filtered using a 0.45-µm membrane filter.The concentrations of nitrate, nitrite, and fluoride ions were measured using an ion chromatograph (Prominence HIC-NS, Shimadzu).The measurement was performed using the following: column: Shim-pack IC-A3 (Shimadzu); mobile phase: 8.0 mmol/L p-hydroxybenzoic acid, 3.2 mmol/L bis-tris, and 50 mmol/L boric acid (1:1:1); flow rate: 1.2 mL/min; temperature: 40 • C; detector: CDD-6A conductivity detector (Shimadzu); and sample volume: 50 µL.The ion concentrations were calculated using Eq. ( 1): where q is the amount adsorbed (mg/g), C 0 is the initial concentration (mg/L), C e is the equilibrium concentration (mg/L), V is the solvent volume (L), and W is the weight of the CG sample (mg).

C. Amounts of nitrate, nitrite, and fluoride ions adsorbed in a ternary solution system
CGs (0.05 g) were added to nitrate, nitrite, and fluoride ions (50 mL) at 2.0 mmol/L in a ternary solution system.The suspensions were shaken at 100 rpm for 24 h at 25 • C, and then filtered using a 0.45-µm membrane filter.The ion concentrations were measured using the method described above.

D. Breakthrough curves of nitrate, nitrite, and fluoride ions using a column in a ternary solution system
The amounts of nitrate, nitrite, or fluoride ions adsorbed on a column (diameter × height: 1.0 cm×10.0cm) were measured as follows.CG600 (particle diameter: 840-2000 µm) was first added at a level 3 or 9 cm above the base of the column.Approximate conditions for the column experiments are shown in Table I.The amounts of nitrate, nitrite, or fluoride ions adsorbed were calculated as the differences between the initial concentrations and the concentrations in the effluent from the column.

B. Adsorption isotherms
The adsorption isotherms of nitrate, nitrite, and fluoride ions onto CGs in a single or a ternary solution system are shown in Fig. 2. The amounts of the ions adsorbed in the single solution system were in the following order: CG1000 < CG800 < CG600 [16].Further, the specific surface areas were in the following order: CG1000 (23.5 m 2 /g) < CG800 (31.5 m 2 /g) < CG600 (52.6 m 2 /g), which indicated that the amounts of ions adsorbed depended on the specific surface area of the CGs.The quantities of ions adsorbed in the ternary solution system exhibited the same trend.However, in the single solution system, larger amounts were adsorbed than in the ternary system, which indicated that the ions were competitively adsorbed onto the CG surfaces.In this study, the adsorbents were prepared from coffee grounds as biomass.The reported mechanism of anion adsorption using these materials suggested that nitrate, nitrite, or fluoride ions were exchanged with chloride ion in a 1:1 ratio (Fig. 3).We investigated the adsorption mechanism of the three ions by CGs in a ternary solution system (Fig. 4).The three ions were exchanged with chloride ion on the CGs.Moreover, the amount adsorbed in the single solution system was greater than in the ternary solution system, which indicated that the ion exchange ability with chloride ion on the CGs was limited.The correlation coefficient between the amount of nitrate, nitrite, or fluoride ions on CG600 and the amount of chloride ion eluted from CG600 in the ternary solution system was 0.961.These results were also in agreement with the anion adsorption mechanism in which ions exchanged with chloride ion in a 1:1 ratio.
In the Langmuir theory, the basic assumption is that sorption takes place at specific homogeneous sites within the adsorbent.This equation can be written as follows [16,17]: where q (mg/g) is the amount adsorbed, a (L/mg) is a constant related to the affinity to the binding sites and the adsorption, W s (mg/g) is the maximum monolayer adsorption capacity, and C e (mg/L) is the equilibrium concentration.
The correlation coefficients for the nitrate, nitrite, and fluoride ions were 0.990-0.998,0.532-0.997,and, 0.982-0.993,respectively.The W s values for the nitrate, nitrite, and fluoride ions using CG600 were 6.564, 10.954, and 37.682 mg/g, respectively.These results showed that  the adsorption ability of CG600 was greater than that of CG800 or CG1000 (Table III).
The Freundlich isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution of the heat of adsorption over the surface.The Freundlich equation is expressed by the following [18]: where K and 1/n represent the Freundlich capacity factor and the Freundlich intensity parameter, respectively.The Freundlich constants were determined from the slope of a plot of log q versus log C e .The correlation coefficients of the Freundlich plots were lower relative to Langmuir plots.The correlation coefficients for the nitrate, nitrite, and fluoride ions were 0.960-0.986,0.246-0.975,and 0.940-0.970,respectively (Table III).The values of n give an indication of the favorability of adsorption.A value of n greater than unity indicates favorable adsorption [18].Moreover, when 1/n is in the range of 0.1 to 0.5, the adsorbate is easily adsorbed.On the other hand, if 1/n > 2, adsorption is considered difficult [19].Our results indicated that nitrate, nitrite, and fluoride ions were easily adsorbed onto CGs.The amounts of nitrate, nitrite, and fluoride ions adsorbed from the ternary solution system are shown in Fig. 5.The amounts adsorbed were in the following order: fluoride ion (0.94 mmol/L) < nitrite ion (0.38 mmol/L) < nitrate ion (0.20 mmol/L); this indicated that the ion exchange of fluoride with chloride on the CG surface was greater than that of nitrate ion or nitrite ion.The ionic diameter is in the order fluoride ion < nitrite ion < nitrate ion; therefore, fluoride ion was easily exchanged with chloride ion on the CG surface.

D. Thermodynamic parameters
The adsorption isotherms of the three ions onto the CGs at different temperatures are shown in Fig. 6.The adsorption capacity of the CGs increased with the increase in temperature.This suggests that the adsorption process was endothermic when the temperature increased from 5 to 35   might be due to an increase in the number of active sites for adsorption with increasing temperature.This may also be a result of an increase in the mobility of the ions with the rise in temperature.The thermodynamic parameters were also analyzed on the basis of the adsorption isotherm plots (Fig. 6).The distribution coefficient (K d ) can be expressed as follows: where C e and q e are the concentrations of anions at equilibrium (mmol/L) and the amount adsorbed (mmol/g) at equilibrium, respectively.The enthalpy (∆H) and entropy (∆S) values were calculated from the slopes and intercepts, respectively, of the linear variations of plots of ln K d versus the reciprocal of the temperature, 1/T , using the following relation: ln where K d is the distribution coefficient (L/g), ∆S is the standard entropy (J/mol/K), ∆H is the standard enthalpy (kJ/mol), T is the temperature, and R is the gas constant (kJ/mol/K).The free energy ∆G of specific adsorption was calculated using the van't Hoff equation: The thermodynamic parameters are presented in Table IV.The positive values of ∆H indicate an endothermic process.The positive values of the entropy (∆S) imply the good affinity of the anions toward the adsorbent with increasing randomness at the solid-solution interface during the adsorption process [20].The values of the Gibbs energy change (∆G) decreased with the increase in temperature, which indicated the spontaneous nature of the adsorption.

E. Breakthrough curves of nitrate, nitrite, and fluoride ions using a column in a ternary solution system
The breakthrough curves of the nitrate, nitrite, and fluoride ions are shown in Fig. 7.It can be seen that, under condition 1, ions were detected in the column effluent immediately after starting the adsorption process.On the other hand, under condition 2, fluoride ion was released 155 h, and nitrate and nitrite ions were released 140 h, after starting the adsorption process.These results indicated that S.V. and L.V. in condition 2 were greater than those in condition 1, which suggested that condition 2 was most useful in practice for the column adsorption of nitrate, nitrite, and fluoride ions.Moreover, CG600 was useful for removal of nitrate, nitrite, and fluoride ions in a ternary solution system.

IV. CONCLUSIONS
Carbonaceous materials were prepared from coffee grounds.The specific surface areas were in the following order: CG1000 (23.5 m 2 /g) < CG800 (31.5 m 2 /g) < CG600 (52.6 m 2 /g).The amounts of nitrate, nitrite, and fluoride ions adsorbed were in the order CG1000 < CG800 < CG600.Moreover, the amount adsorbed increased with increasing temperature.The positive entropy values (∆S) implied the good affinity of the anions toward the adsorbent, with increasing randomness at the solid-solution interface during the adsorption process.The values of the Gibbs energy change (∆G) decreased with increasing temperature, which indicated the spontaneous nature of the adsorption.These results showed that the adsorption mechanism for the nitrate, nitrite, and fluoride ions was ion exchange with chloride ion on the CG surface with a 1:1 ratio.The adsorption isotherm on CG600 was fitted to both the Freundlich equation and the Langmuir equation.The adsorption affinity on CGs was in the order nitrate ion < nitrite ion < fluoride ion.CG600 was useful for the adsorption of nitrate, nitrite, and fluoride ions using a column in a ternary solution system.

FIG. 4 :
FIG. 4: Relationship between amount of F − , NO − 2 , and NO − 3 adsorbed onto CG600 and amount of Cl − eluted from CG600 in a ternary solution system.

TABLE I :
Conditions of flow experimentation.

TABLE III :
Freundlich and Langmuir constants of adsorption isotherms of F − , NO − 2 , and NO − 3 onto CGs in a ternary solution system.

TABLE IV :
Thermodynamic parameters for the adsorption of F − , NO − 2 and NO − 3 onto CG600 in a ternary solution system.