Journal of Saudi Chemical Society (2017) 21, S120–S127 King Saud University Journal of Saudi Chemical Society www.ksu.edu.sa www.sciencedirect.com ORIGINAL ARTICLE Removal of hazardous Rhodamine dye from water by adsorption onto exhausted coﬀee ground Kai Shen a, M.A. Gondal b,* a College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China Laser Research Group, Physics Department and Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b Received 21 June 2013; revised 13 November 2013; accepted 17 November 2013 Available online 26 November 2013 KEYWORDS Coffee grounds; Batch adsorption; Rhodamine dye; Electrostatic; Hydrophobic Abstract Exhausted coffee ground powder (CGP) was proved to be an efﬁcient adsorbent for the removal of Rhodamine dyes (i.e. Rhodamine B and Rhodamine 6G) from aqueous solutions by batch adsorption experiments. The morphology, chemical structure as well as the surface property of the as-prepared CGP adsorbent were investigated by using SEM, FT-IR and contact angle meter analytical techniques. The adsorption kinetics and isotherm behaviors of Rhodamine molecules onto CGP were studied and compared using pseudo-1st, pseudo-2nd and Langmuir/Freundlich models, respectively. The maximum adsorption capacities of Rh B and Rh 6G were calculated at 5.255 and 17.369 lmol g1 by Langmuir model ﬁtting. The effects of temperature, ionic strength, solution volume and the co-existing anions on the sorption behavior were also investigated. Furthermore, the adsorption mechanism responsible for the efﬁcient removal of dyes is discussed in terms of adsorption process caused by electrostatic and intermolecular forces. ª 2013 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction The textile industry is one of the major sources, which discharges large amounts of industrial waste water. The discharge of such contaminated water into public streams is a great environmental challenge not only due to its treatment for reuse but * Corresponding author. Tel.: +966 38602351; fax: +966 38602293. E-mail address: email@example.com (M.A. Gondal). Peer review under responsibility of King Saud University. Production and hosting by Elsevier also its toxicity to human beings and animals by contaminating underground water reservoirs . The Rhodamine dye is one of fresh peach of synthetic dyes and it is widely used as a colorant in the manufacturing of textiles and food stuffs. It has been medically proven that drinking water contaminated with Rhodamine dyes could lead to subcutaneous tissue borne sarcoma which is highly carcinogenic . In addition, others kinds of toxicity such as reproductive and neurotoxicity have been widely and intensively investigated and proved as well by exposure to these dyes . Various natural or wasted materials have been extensively explored and investigated for the adsorption removal of different contaminants from aqueous solutions [3–6]. Coffee beans are now produced and used in signiﬁcant quantities worldwide. http://dx.doi.org/10.1016/j.jscs.2013.11.005 1319-6103 ª 2013 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground According to the data reported by United States Department of Agriculture (USDA), the annual global production capacity of coffee beans in the year 2012/2013 was estimated as exceeding 150 million of 60 kg bags and in future more production and waste of coffee ground are expected . Therefore, the efﬁcient utilization of the waste of coffee grounds has attracted considerable attention as millions of posts of coffee are brewed and millions of pounds of wet grounds are thrown every day all around the world. The carbonized form of coffee grounds has been attempted for soil remediation, adsorption removal of hazardous molecules from aqueous or gas phases or waste water desalination [8–10]. In this work, exhausted coffee ground powder (CGP) was used directly as a zero-cost adsorbent for the application of adsorption removal of series of Rhodamine dyes like Rhodamine B (Rh B) and Rhodamine 6G (Rh 6G) from aqueous solutions. The crystal and chemical structures of the asprepared CGP were examined to understand the possible adsorption mechanism for removal of dyes. In addition, the adsorption kinetic, isotherm behaviors were compared and the thermodynamic parameters were calculated as well. Furthermore, the adsorption mechanism responsible for the efﬁcient adsorption removal behavior is discussed in terms of adsorption caused by electrostatic and intermolecular forces. 2. Experimental (k ¼ 0:15418 nm). The morphology of the as-prepared CGP was studied by a scanning electron microscope (SEM, FEI inspect F50). Fourier transform infrared spectra (FTIR) were recorded at room temperature using a FTIR spectrophotometer (NEXUS 670). The zeta potentials of the as-prepared adsorbent at different pHs were determined by a zeta potential instrument (Malvern, Great Britain). The water contact angle was measured by a contact angle meter (SL200B, Shanghai) at ambient conditions. 2.3. Batch adsorption experiments Batch adsorption experiments were conducted to examine the adsorption kinetics, adsorption isotherm, the effect of temperature, solution pH and the ionic strength on the adsorption process as well as desorption and regeneration. A certain amount of CGP was mixed with Rhodamine dye aqueous solutions with a known initial concentration, and the mixture was stirred in a stir machine at a constant stirring speed and temperature. The mixture was centrifuged at 4000 rpm in a centrifugation machine after batch adsorption experiments so that the absorbance of Rh B and Rh 6G can be measured at 554 and 526 nm by means of UV–vis spectrophotometer (JASCO, V-570), respectively. The concentrations of the solutions were determined using linear regression equation. 2.3.1. Kinetics  2.1. Materials The coffee powders were purchased from local market in Saudi Arabia. To obtain the ﬁnal CGP product, each 5 g of coffee powders was washed with 200 mL of boiled deionized water 3 times to get rid of any impurities. Salts of KCl and K2SO4 were purchased from BDH chemicals (Poole England) and K2HPO4 was purchased from Sigma–Aldrich (USA). The Rh B and Rh 6G dyes were purchased from Lambda Physik (USA) and Merck (Germany), and their properties are listed in Table 1. All the chemical reagents are of analytical grade and used without further puriﬁcation. 2.2. Characterization The crystal structure of the as-prepared CGP adsorbent was investigated by X-ray diffractometer (XRD, Bruker D8 ADVANCE) with 2h scope of 10–90 using Cu-Ka X-ray source Table 1 Two kinetic models were used to ﬁt the experimental data at different temperatures. The pseudo-ﬁrst-order rate expression of Lagergren model is generally expressed as follows: dq ¼ k1 ðqeq qÞ dt ð1Þ where qeq and q are the amounts of adsorbed dye onto the CGP at equilibrium and at time t, respectively. k1 is the rate constant of ﬁrst-order adsorption. The integrated form of Eq. (1) is: 1 k1 1 ¼ þ q tqeq qeq ð2Þ The plots of 1/q against 1/t for the pseudo-ﬁrst-order equation give a linear relationship and k1 and qeq values can be determined from the slope and intercept of this equation, respectively. The pseudo-second-order kinetic rate equation is expressed as: Selected properties of Rh B and Rh 6G. Dyes Rh B Rh 6G C28H31N2O3ÆCl 479.02 C27H29ClN2O3 464.98 Structure Chemical formula MW S121 S122 K. Shen, M.A. Gondal 2 dq ¼ k2 qeq q dt ð3Þ where k2 is the rate constant of second-order adsorption. After integrating, the following equation is obtained: t 1 t ¼ þ q k2 q2eq qeq ð4Þ 2.3.4. Inﬂuence of solution volume The Rhodamine dye solution volume was adjusted continuously and increased from 10 to 200 mL with the constant initial concentration of Rhodamine dye of 15 lmol L1, corresponding to the simulation of the waste water basin where the solution volume varies with time and place . 2.3.5. Inﬂuence of ionic strength 2.3.2. Thermodynamics  The Gibbs free energy change DGo, indicates the degree of spontaneity of the adsorption process. For signiﬁcant adsorption to occur, the free energy changes (DGo) of adsorption must be negative. The Gibbs free energy change of adsorption is deﬁned as: DGo ¼ RT lnK To investigate the effect on ionic strength on the adsorption capacity, the ionic strength varied from 0.001 to 0.1 M by adding different amounts of K2SO4, and the concentration of Rhodamine dye was 15 lmol L1. ð5Þ The other thermodynamic parameters, change in the enthalpy (DHo), and entropy (DSo), were determined by using following equations: K¼ CA CS DGo ¼ DHo TDSo ð6Þ ð7Þ where K is the equilibrium constant; CA, the amount of dye adsorbed on the adsorbent of the solution at equilibrium; CS, the equilibrium concentration of the dye in the solution. T is the solution temperature and R, the gas constant. 2.3.3. Isotherms  In this study, two classical adsorption models, i.e. Langmuir and Freundlich isotherms, were employed to describe Rh B and Rh 6G adsorption equilibrium. The Langmuir isotherm is valid for monolayer adsorption onto a surface with a ﬁnite number of identical sites. The Langmuir model is based on the assumption of adsorption homogeneity, such as equally available adsorption sites, monolayer surface coverage, and no interaction between adsorbed species. The Langmuir model can be expressed as: qe ¼ KL qm Ce 1 þ Ce ð8Þ where Ce is the equilibrium concentration of the adsorbates in the solution, qm the maximum adsorption capacity and KL is the adsorption equilibrium constant. The Freundlich equation is the empirical relationship whereby it is assumed that the adsorption energy of a protein binding to a site on an adsorbent depends on whether or not the adjacent sites are already occupied. One limitation of the Freundlich model is that the amount of adsorbed solute increases indeﬁnitely with the concentration of solute in the solution. This isotherm can be described as follows: qe ¼ KF ðCe Þ1=n ð9Þ where qe is the adsorption capacity at equilibrium, Ce the equilibrium concentration of adsorbates in the solution, and KF and n are the physical constants of Freundlich adsorption isotherm which indicate the adsorption capacity and adsorption intensity, respectively. Figure 1 Representative SEM images (a, b) and FTIR (c) of the as-prepared CGP adsorbent. The inset ﬁgures a and c show the particle size distributions (sample size = 168) and the image of a water droplet on the surface of CGP surface demonstrating its hydrophobic property, respectively. Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground 2.3.6. Inﬂuence of co-existing anions The effects of co-existing anions on the Rhodamine dye adsorption were studied in 15 lmol L1 Rhodamine dye solution. Three kinds of salts of KCl, K2SO4 or K2HPO4 were selected to study the effect of co-existing anions on the adsorption capacity. 2.3.7. Regeneration by photolysis and reuse The regeneration of saturated adsorbent by photolysis was carried out under a Xenon lamp irradiation with full arc (460 Watts, Oriel instrument). FTIR measurement suggested that the CGP adsorbent can be almost regenerated after irradiation for 2 h as no FTIR signals of Rhodamine dye molecules can be detected. The regenerated adsorbent was re-used in the next cycle of adsorption. 3. Results and discussion 3.1. Characterization of CGP adsorbent The amorphous characteristic was proved by XRD measurement (not shown) as there is no apparent diffraction peak in the 2h range of 1–90. Fig. 1(a and b) depicts the SEM image of the as-prepared CGP adsorbent. The average particle size is calculated at 11.4 lm under the sample size of 168. Most of the particle sizes are ranged 1–20 lm. The enlarged image of the adsorbent is shown in Fig. 1(b), which suggests the hierarchical morphology of the as-prepared CGP adsorbent. As clear from Fig 1(b), the particles of adsorbent are of micrometer size S123 and also consisted of numerous nanoparticles of hundreds of nanometers size. FTIR spectrum of the as-prepared CGP adsorbent was measured to study the surface chemical structure. As depicted in Fig. 1(c), the hydrophobic functional group of the C–H group was centered at wavenumbers 2921 and 2841 cm1, and the C–O group at 1741 cm1. The hydrophobic property of the CGP surface was also studied. The image of a water droplet on the surface of the CPG layer was captured and shown in the inset of Fig. 1(c). The contact angle of the droplet was measured at 113, further suggesting the hydrophobic nature of the CPG surface. 3.2. Adsorption kinetics As depicted in Fig. 2, a sharp increase can be observed by raising the adsorption temperature from 278 to 292 K. After adsorption for 2 h, the adsorption–desorption equilibrium can be reached and the adsorption capacity of Rh B was calculated at 3.02 and 1.661 lmol g1 at 292 and 278 K for Rh B, respectively. However the saturated adsorption capacity of Rh 6 G was found to be 4.16 and 5.45 times that of Rh B, which is calculated at 12.7 and 7.00 lmol g1 at 292 and 278 K, respectively. The ﬁtted constants by the ﬁrst-order and second-order kinetic models for Rh B and Rh 6G adsorption are listed in Table 2. The correlation coefﬁcients for the ﬁrst-order kinetic and the second-order kinetic models were 0.99, close to 1.0 for Rh 6G at 292 and 278 K, Rh B at 292 K. Therefore, the adsorption of Rh B and Rh 6G by the adsorbent CGP can be approximated favorably both by the pseudo-ﬁrst-order model and the pseudo-second-order model. 3.3. Adsorption thermodynamics It can be noticed clearly from Fig. 2 that the adsorption capacity of Rh B and Rh 6G on CGP are increased by increasing the adsorption temperature, demonstrating the endothermic nature of the adsorption process, which is further demonstrated by the calculated positive value of DHo. As shown in Table 3, the positive value of DSo indicates the reversible characteristic of the adsorption of Rh B and Rh 6G onto CGP. DGo values were positive indicating that the adsorption process led to an increase in Gibbs free energy. Positive DGo values of Rh B indicate the nonspontaneity of the adsorption process and the negative DGo value of Rh 6G indicates the adsorption process was spontaneous. Figure 2 The variations of adsorption capacity of Rh B and Rh 6G as a function of contact time by the as-prepared CGP adsorbent at 278 and 292 K (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h; temperature = 292 K). Table 2 Dye 3.4. Adsorption isotherms Fig. 3 depicts the adsorption amount of RhB and Rh 6G on the CGP adsorbent at different equilibrium concentrations, and the Constants for the pseudo ﬁrst-order and pseudo second-order kinetics for Rh B and Rh 6G adsorption on CGP adsorbent. Temperature (K) First-order kinetics k1 (min1) qeq (lmol g1) Second-order kinetics r2 k2 (g lmol1 min1) qeq (lmol g1) r2 Rh B 292 278 37.944 14.599 4.018 1.661 0.999 0.981 6.56 41.22 4.018 1.661 0.999 0.981 Rh 6G 292 278 37.537 28.808 16.702 9.057 0.999 0.990 1.59 3.83 16.703 9.058 0.999 0.990 S124 Table 3 K. Shen, M.A. Gondal Thermodynamic parameters for the adsorption of Rh B and Rh 6G on CGP adsorbent. Dye Temperature (K) K Rh B 292 278 0.369 0.125 Rh 6G 292 278 3.923 1.327 DGo (kJ/mol) DHo (kJ/mol) DSo (J/(mol K)) 2.420 4.806 52.185 170.429 3.318 0.654 52.246 190.286 of Rh B and Rh 6G was theoretically calculated at 5.255 and 17.369 lmol g1 by Langmuir model ﬁtting, respectively. For different adsorbents, the saturated adsorption capacity for adsorbates can be compared by calculating the amount of adsorbates adsorbed on the adsorbents through adsorption isotherms. Some of the natural (waste) adsorbent materials and their adsorption capacities are given in Table 5. As shown in Table 5, the low cost natural (waste) adsorbent, such as kaolinite, wasted biological sludge had been used to remove RhB or Rh 6G, and the corresponding adsorption capacity can be reached to as high as 46 and 16.3 mg g1, respectively. Some natural adsorbents, such as natural zeolite and Na+-Montmorillonite are not inexpensive, but nevertheless their adsorption capacities for Rhodamine molecules are low and the reuse of such exhausted coffee grounds is appealing in terms of resource utilization. Figure 3 Adsorption isotherms of Rh B and Rh 6G on CGP adsorbent at 292 K (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h; temperature: 292 K). 3.5. Inﬂuence of solution volume corresponding ﬁtted results are listed in Table 4. It can be seen that the adsorption isotherms of Rh B and Rh 6G on the CGP adsorbent can be ﬁtted better by the Langmuir model than the Freundlich model, indicating the homogeneous (monolayer) adsorption characteristic. The monolayer adsorption capacity Table 4 Langmuir and Freundlich isotherm constants for adsorption of Rh B and Rh 6G on CGP adsorbent. Dye Langmuir isotherm model Rh B Rh 6G Table 5 Fig. 4 depicts the effect of solution volume on the adsorption capacity of Rh B and Rh 6G over the as-prepared CGP adsorbent. It can be found that the adsorption capacity of Rh 6G and Rh B decreases linearly from 15.2 to 9.3 lmol g1, and 5.3 to 1.6 lmol g1 with rising solution volume from 10 to Freundlich isotherm model qm (lmol g1) KL (L lmol1) r2 KF (lmol11/n L1/n g1) n r2 5.255 17.369 4.364 1.053 0.958 0.882 1.190 8.161 2.062 2.952 0.884 0.749 Comparison of adsorption capacity of a few natural (waste) adsorbents for Rh B and Rh 6G removal. Adsorbents Adsorption capacity (mg g1) Refs. Kaolinite Bentonite Luﬀa cylindrical Na+-Montmorillonite Australian natural zeolite Coal ash Cellulosic waste orange peel Anaerobic sludge Cellulose-based wastes Biological sludge Trichoderma harzianum mycelial waste Na+-Montmorillonite Coﬀee ground powder 46.1 (Rh B) 7.7 (Rh B) 9.9 (Rh B) 38.27 (Rh B) 2.12 (Rh B) 2.86 (Rh B) 3.23 (Rh B) 19.52 (Rh B) 20.6 (Rh B) 16.3 (Rh 6G) 3.40 (Rh 6G) 0.4 (Rh 6G) 5.26 (Rh B) 17.37 (Rh 6G)             This study Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground S125 ionic Rhodamine molecules to the adsorbent surface. It is also interesting to ﬁnd that there is almost no apparent effect of ionic strength on the adsorption capacity of Rh B, indicating the less important role of electrostatic force in the adsorption process of Rh B. 3.7. Inﬂuence of co-existing anions Figure 4 Effect of solution volume on the Rh B and Rh 6G by CGP adsorbent (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h; temperature: 292 K). 100 mL, respectively. The adsorption capacity decrease might be due to the overlapping of adsorption sites as a result of overcrowding of adsorbent particles . This procedure may correspond to simulation of the adsorption removal that takes place in a lake or river where the rate of solute and solvent are relatively constant and the solution volume varies with time and place. 3.6. Inﬂuence of ionic strength It is well known that the ionic strength could affect the electrostatic interactions between adsorbents and adsorbates to a certain extent, meanwhile the electrolytes can affect the adsorption behavior by competing with adsorbate ions on the surface of adsorbent as well. The effect of ionic strength on the uptake of Rhodamine molecules by the as-prepared CGP is shown in Fig. 5. A negative correlation between adsorption capacity of Rh 6G molecules and ionic strength can be found. The reason could be possibly the fact that the increased concentration of K2SO4 could neutralize the surface negative charge, and an increase in the electrostatic repulsion between the negatively charged CGP adsorbent surface and the cationic Rh 6G molecules, inhibiting the adsorption of cat- Figure 5 Effect of ionic strength on the Rh B and Rh 6G by CGP adsorbent (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h; temperature: 292 K). The anions of chloride, sulfate and hydrogen phosphate which are considered to exist commonly in actual groundwater, were adopted to investigate the inﬂuence of anions on the adsorption capacity of Rhodamine dyes onto the as-prepared CGP adsorbent. As depicted in Fig. 6, it can be found that anions of hydrogen phosphate lead to the greatest decrease of the adsorption capacity of Rh B, but no apparent negative effect of chloride anions on the adsorption capacity was found for both Rh B and Rh 6G. The decrease of adsorption capacity could possibly be caused by the competition adsorption of anions with target model compounds (Rhodamine dye molecules) on the surface of adsorbent. Besides, an electrostatic ﬁeld might form around the surface of adsorbent as well, which also can promote the electrostatic repulsion. 3.8. Regeneration by photolysis and reuse The saturated CGP adsorbent can be regenerated easily upon UV light exposure. In our study, a simulated sunlight irradiated from Xenon lamp was used as light source for adsorbent regeneration. FTIR measurements (not shown here) clearly show that almost no Rh 6G and Rh B signals can be detected after 2 h of exposure, suggesting that the saturated CGP adsorbent was regenerated almost completely. The adsorption capacities of Rh B and Rh 6G in the successive ﬁve cycles are shown in Fig. 7. After the ﬁrst cycle of reuse, the adsorption capacity of Rh B and Rh 6G decreases from 3.4 and 12.5 lmol g1 to 2.1 and 9.3 lmol g1, respectively. After ﬁve cycles of reuse, their capacities decrease to 0.9 and 5.5 lmol g1, respectively. The decrease in adsorption capacity Figure 6 Effect of co-existing anions on the adsorption of Rh B (left) and Rh 6G (right) on the CGP adsorbent (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h; temperature: 292 K). S126 Figure 7 Changes of adsorption capacity of Rhodamine molecules onto adsorbent in ﬁve successive adsorption–desorption cycles. after regeneration could be attributed to the unsuccessful regeneration sites on the as-prepared CGP adsorbent. 4. Possible mechanism To understand the possible adsorption mechanism, the effect of pH condition on the adsorption capacity of Rh B and Rh 6G was investigated and results are depicted in Fig. 8(a). It can be found that the adsorption capacity is highly dependent on the pH of the solution which affects the degree of ionization of adsorbate. The maximum adsorption capacities of Rh 6G and Rh B were achieved at 25.1 and 7.18 lmol g1 under conditions of pH = 2. However, only adsorption capacities K. Shen, M.A. Gondal Figure 9 The possible adsorption process trend on CGP adsorbent surface. of 9.1 and 1.7 lmol g1 were obtained for the adsorption of Rh 6G and Rh B in aqueous solutions of pH = 10, respectively, indicating that the static electric absorption mechanism occurred during the adsorption process. The zeta potentials of the CGP adsorbent surface under different pH conditions were measured. As shown in Fig. 8(b), it can be noticed that the zero point of zeta-potential (isoelectrical point, IEP) of the as-prepared CGP adsorbent was examined at 3.5, indicating the positive surface charge of samples at pH < 3.5. The initial pH value for Rh B and Rh 6G aqueous solutions with a concentration of 15 lmol L1 is around 6.0, therefore the negative charge of the CGP adsorbent surface can be expected. The electrostatic interaction might have occurred between the negatively charged surface of CGP and the positive charge of two kinds of Rhodamine molecules. It is worth noticing that CGP adsorbent exhibits a much higher adsorption capacity of Rh 6G than Rh B and threefold improvement of capacity can be found by the batch adsorption mentioned above. The reason should be possibly due to the repulsion forces that existed between the dissociated carboxylic (–COOH) group (pKa = 3.1 ) and the negatively charged surface of CGP adsorbent, which might inhibit the adsorption process through electrostatic forces. However, the adsorption can still be found under basic conditions, even though the adsorption capacity is much lower, indicating that the inter-molecular interaction might possibly be involved. On the other hand one can expect, the hydrophobic interaction between the surface of CPG (which has been proved above) and the ester group (–COOCH3) in Rh 6G molecules might took place in the adsorption process as well, which facilitates the efﬁcient uptake on the surface of CGP adsorbent. Therefore, on the basis of the electrostatic and inter-molecular interactions, it can be preliminarily concluded that the much more higher adsorption capacity of Rh 6G should be caused by the repulsion forces from the dissociated carboxylic (– COOH) group in Rh B molecules, and the hydrophobic interaction forces from the ester group (–COOCH3) in Rh 6G molecules (as illustrated in Fig. 9). 5. Conclusion Figure 8 Changes of adsorption capacity of Rhodamine molecules (a) and zeta potential (b) on the surface of the as-prepared CGP adsorbent as a function of pH conditions. In this work, exhausted coffee ground powder has been proved to be an efﬁcient adsorbent for the removal of Rhodamine dyes of Rh B and Rh 6G from aqueous solutions. The pseu- Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground do-2nd order adsorption kinetic model can describe best the adsorption process of both Rh B and Rh 6G on CGP adsorbent. Temperature-dependent adsorption experiments suggest that the adsorption process is of exothermic and endothermic nature. The maximum adsorption capacities toward Rh B and Rh 6G were estimated at 5.255 and 17.369 lmol g1 according to the Langmuir model. Anions of hydrogen phosphate were found to signiﬁcantly interfere with Rh B and Rh 6G adsorption. Both the increase of ionic strength and solution volume could not be related to the improved uptake capacity. The saturated CGP can be regenerated easily by the photolysis method. The higher adsorption capacity of Rh 6G onto CGP adsorbent may be caused by the repulsion forces from the dissociated carboxylic (–COOH) group in Rh B molecules and through the hydrophobic interaction forces from the ester group (–COOCH3) in Rh 6G molecules. Acknowledgments The support under project # R15-CW-11 (MIT11109 & MIT11110) and RG1011-1 by KFUPM is highly appreciated. References  E. Errais, J. Duplay, F. Darragi, Textile dye removal by natural clay-case study of Fouchana Tunisian clay, Environ. Technol. 31 (2010) 373–380.  R. Jain, M. Mathur, S. Sikarwar, A. Mittal, Removal of the hazardous dye rhodamine B through photocatalytic and adsorption treatments, J. Environ. Manage. 85 (2007) 956–964.  V.K. Gupta, A. Mittal, A. Malviya, J. Mittal, Adsorption of carmoisine A from wastewater using waste materials–bottom ash and deoiled soya, J. Colloid Interf. Sci. 335 (2009) 24–33.  A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, Adsorption studies on the removal of coloring agent phenol red from wastewater using waste materials as adsorbents, J. Colloid Interf. Sci. 337 (2009) 345–354.  A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, Adsorption of hazardous dye crystal violet from wastewater by waste materials, J. Colloid Interf. Sci. 343 (2010) 463–473.  A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, Removal and recovery of Chrysoidine Y from aqueous solutions by waste materials, J. Colloid Interf. Sci. 344 (2010) 497–507.  T. Halstead, Coffee: World Markets and Trade, Foreign Agricultural Service/U.S. Department of Agriculture, 2012. Available from: <http://www.fas.usda.gov/psdonline/circulars/ coffee.pdf>.  M. Hirata, N. Kawasaki, T. Nakamura, K. Matsumoto, M. Kabayama, T. Tamura, S. Tanada, Adsorption of dyes onto carbonaceous materials produced from coffee grounds by microwave treatment, J. Colloid. Interf. Sci. 254 (2002) 17–22.  M.G. Plaza, A.S. Gonzalez, C. Pevida, J.J. Pis, F. Rubiera, Valorisation of spent coffee grounds as CO2 adsorbents for postcombustion capture applications, Appl. Energy 99 (2012) 272–279.  K. Benrachedi, K. Bensouali, H. Houchati, Coupling ultraﬁltration with adsorption on activated coffee for use as a reverse osmosis pretreatment, Desalination 239 (2009) 122–129. S127  B.H. Hameed, A.A. Ahmad, N. Aziz, Isotherms, kinetics and thermodynamics of acid dye adsorption on activated palm ash, Chem. Eng. J. 133 (2007) 195–203.  A. Özcan, E.M. Öncü, A.S. Özcan, Kinetics, isotherm and thermodynamic studies of adsorption of Acid Blue 193 from aqueous solutions onto natural sepiolite, Colloid Surf. A 277 (2006) 90–97.  R. Dhankhar, A. Hooda, Fungal biosorption-an alternative to meet the challenges of heavy metal pollution in aqueous solutions, Environ. Technol. 32 (2011) 467–491.  Q. Zhou, G. Pan, J. Zhang, Effective sorption of perﬂuorooctane sulfonate (PFOS) on hexadecyltrimethylammonium bromide immobilized mesoporous SiO2 hollow sphere, Chemosphere 90 (2013) 2461–2466.  T.A. Khan, S. Dahiya, I. Ali, Use of kaolinite as adsorbent: Equilibrium, dynamics and thermodynamic studies on the adsorption of Rhodamine B from aqueous solution, Appl. Clay Sci. 69 (2012) 58–66.  S.S. Tahir, N. Rauf, Removal of cationic dye from aqueous solutions by adsorption onto bentonite clay, Chemosphere 63 (2006) 1842–1848.  A. Altinisik, E. Gür, Y. Seki, A natural sorbent, Luffa cylindrica for the removal of a model basic dye, J. Hazard. Mater. 179 (2010) 658–664.  P.P. Selvam, S. Preethi, P. Basakaralingam, N. Thinakaran, A. Sivasamy, S. Sivanesan, Removal of rhodamine B from aqueous solution by adsorption onto sodium montmorillonite, J. Hazard. Mater. 155 (2008) 39–44.  S. Wang, Z.H. Zhu, Characterization and environmental application of an Australian natural zeolite for basic dye removal from solution, J. Hazard. Mater. 136 (2006) 946–952.  S. Wang, M. Soudi, L. Li, Coal ash conversion into effective adsorbent for removal of heavy metals and dyes from waste water, J. Hazard. Mater. 133 (2006) 243–251.  C. Namasivayam, N. Muniasamy, K. Gayathri, Removal of dyes from aqueous solution by cellulosic waste orange peel, Bioresour. Technol. 57 (1996) 37–43.  Y. Wang, Y. Mu, Q.B. Zhao, Isotherms, kinetics and thermodynamics of dye biosorption by anaerobic sludge, Sep. Purif. Technol. 50 (2006) 1–7.  G. Annadurai, R.S. Juang, D.J. Lee, Use of cellulose-based wastes for adsorption of dyes from aqueous solutions, J. Hazard. Mater. 92 (2002) 263–274.  G. Annadurai, R.S. Juang, P.S. Yen, D.J. Lee, Use of thermally treated waste biological sludge as dye absorbent, Adv. Environ. Res. 7 (2003) 739–744.  S. Sadhasivam, S. Savitha, K. Swaminathan, Exploitation of Trichoderma harzianum mycelial waste for the removal of rhodamine 6G from aqueous solution, J. Environ. Manage. 85 (2007) 155–161.  A.H. Gemeay, Adsorption characteristics and the kinetics of the cation exchange of Rhodamine-6G with Na+-Montmorillonite, J. Colloid. Interf. Sci. 251 (2002) 235–241.  P.S. Kumar, K. Kirthika, Equilibrium and kinetic study of adsorption of nickel from aqueous solution onto. bael tree leaf powder, J. Eng. Sci. Technol. 4 (2009) 351–363.  J.L. Pittman, K.F. Schrum, S.D. Gilman, On-line monitoring of electroosmotic ﬂow for capillary electrophoretic separations, Analyst 126 (2001) 1240–1247.