Adsorption of Chromium ( VI ) from Aqueous Solutions by Coffee Polyphenol-Formaldehyde / Acetaldehyde Resins

Removal of chromium(VI) from wastewater is essential as it is toxic. Thus, removal of chromium(VI) was performed using coffee polyphenol-formaldehyde/acetaldehyde resins as adsorbents. Adsorbent resins were prepared by condensation of decaffeinated coffee powder with formaldehyde/acetaldehyde and used for the removal of Cr(VI) ions from aqueous solutions. A simple and sensitive solid phase extraction procedure was applied for the determination of chromium at trace levels by spectroscopic method using 1,5-diphenylcarbazide reagent. The adsorption of Cr(VI) on the coffee polyphenol-formaldehyde/acetaldehyde resins was monitored by FTIR and EDX analysis. The metal adsorption parameters such as contact time, pH, Cr(VI) ion concentration, and adsorbent dose were investigated. For Cr(VI), the maximum adsorption capacity of coffee polyphenol-formaldehyde resins was 98% at pH 2. The experimental results showed that Cr(VI) bound strongly with coffee polyphenol-formaldehyde/acetaldehyde resins and utilization of resins could be improved greatly by reuse.


Introduction
Metal ions such as Cd, Cr, Co, Cu, Zn, Pd, Hg, Ni, Ag, and Sr and metalloids such as Se, As, and Sb are toxic beyond trace levels.Most of these trace elements are transition metals with variable oxidation states and coordination numbers.Strong exposure to Cr(VI) causes cancer in the digestive tract and lungs.It may also cause gastric pain, nausea, vomiting, severe diarrhea, and hemorrhage [1,2].
Treatment of wastewater generated by industrial processes today is of prime concern.Various technologies have been developed over recent years and are available for the removal of toxic metal such as chromium from wastewater [3].Chromium bearing wastewater originates from a variety of industrial processes such as electroplating, dichromate and basic chrome sulphate manufacturing, tannery, anodizing, cutting tools, and chrome mining.Many of these industrial processes usually produce a large volume of wastewater with a chromium concentration ranging from less than 1 ppm to 10 ppm.
The two common oxidation states of chromium observed in natural water are Cr(III) and Cr(VI).Cr(III) is not a significant groundwater contaminant, whereas Cr(VI) is approximately 100 times more toxic than Cr(III) [4].
Conventional methods used for removing Cr(VI) ions from industrial wastewater include reduction followed by chemical precipitation, adsorption on activated carbon, solvent extraction, freeze separation, reverse osmosis, ion exchange, and electrolytic methods.These methods have found limited applications because they often involve high capital and operational costs.Adsorption is an effective and versatile method for removing chromium which solves the problem of sludge disposal and renders the system more economically viable, especially if low-cost adsorbents are used [5][6][7][8][9].The major advantage of adsorption is its high selectivity and enrichment factor.

Materials and Methods
2.1.Materials.Roasted coffee beans were procured from a local source, crushed in a grinder, and sieved through 355 m pore size filter.Sieved coffee powder was treated with chloroform to remove caffeine.The activated decaffeinated coffee polyphenol powder was further treated with formaldehyde/acetaldehyde solution in the presence of ammonia.Kinetic studies using batch experimental method were carried out for the removal of Cr(VI) from aqueous solutions.Acetaldehyde (35%), ammonia solution (25%), sulfuric acid (98%), and nitric acid (70%) were purchased from Merck, India.Formaldehyde (37% solution) was obtained from Qualigen, India.Stock solution of Cr(VI) (17.6 mg/mL) was prepared by dissolving accurately weighed potassium dichromate (K 2 Cr 2 O 7 , Loba Chemie Pvt. Ltd.Mumbai, India) in 100 mL deionized water.A solution (0.025%) of 1,5diphenylcarbazide (DPC, Sigma-Aldrich, USA) was prepared by dissolving 25 mg of DPC in 5 mL of acetone (Merck) and 10 mL of 5 M H 2 SO 4 .This mixture was diluted to 100 mL with deionised water and stored in an amber-glass bottle.Water was deionised and purified with a Milli-Q water purification system (Millipore, USA).Freshly prepared solutions were used for all experiments.All chemicals were used without purification.

Preparation of Coffee Polyphenol-Formaldehyde (CFA)
Resin.The composition of reactants is illustrated in Table 1.In a 100 mL plastic container, 4 g of activated decaffeinated coffee powder and desired amount (5 to 25 mL) of 37% formaldehyde solution were added and stirred for 5 min to achieve uniform mixing.To this mixture, 10 mL ammonia (25%) solution was added and stirred for an additional 5 min.The resulting reaction mixture was kept for 15 days at room temperature.A precipitate was observed at the bottom of the plastic container.The reaction mixture was neutralized with 1 M hydrochloric acid solution and the precipitate formed was filtered using a filter paper.The precipitate was treated with 1.2 M hydrochloric acid solution to make it insoluble in both acidic and basic medium.The solution was filtered and the residue was treated with deionized water followed by drying at 80 ∘ C for 8 h so as to obtain insoluble coffee polyphenolformaldehyde resins.Analysis of particle size was in the range of 20.85 to 28.29 m.These resins were prepared in duplicate to check reproducibility and whether results obtained were within the same range.

Preparation of Coffee Polyphenol-Acetaldehyde (CAA)
Resin.The composition of reactants is presented in Table 2.
In a 100 mL plastic container, 4 g of activated decaffeinated coffee powder and desired amount (5 to 25 mL) of acetaldehyde solution (35%) were added and stirred for 5 min to achieve uniform mixing.To this mixture, 10 mL ammonia (25%) solution was added and stirred for an additional 5 min.The resulting reaction mixture was kept for 15 days at room temperature.A precipitate was observed at the bottom of the plastic container.The reaction mixture was neutralized with 1 M hydrochloric acid solution and the precipitate formed was filtered using ordinary filter paper.Precipitate obtained was treated with 1.2 M hydrochloric acid to make it insoluble in both acidic and basic medium.The solution was filtered and the residue was treated with deionized water followed by drying at 80 ∘ C for 8 h so as to obtain insoluble coffee polyphenol-acetaldehyde resins.Analysis of particle size was in the range of 16.71 to 26.23 m.These resins were prepared in duplicate to check reproducibility and whether results obtained were within the same range.

Coffee Polyphenol-Formaldehyde/Acetaldehyde Resin.
Aromatic compounds found in coffee beans are mainly phenolic compounds.Robusta coffee has slight higher phenolic content than Arabica [20,21].Decaffeinated coffee beans were crushed in a grinder and sieved through 355 m pore size filter.Sieved coffee powder was further treated with chloroform to remove alkaloids such as caffeine and trigonelline.The residue contains mostly chlorogenic acid and coffee melanoidins, useful from the resin's point of view [22,23].The structure of coffee melanoidins is presented in Figure 1.Adsorbents were prepared by condensation of decaffeinated coffee powder with formaldehyde/acetaldehyde in the presence of ammonia.Batch experiments were carried out to study the kinetics of removal of Cr(VI) from aqueous solutions.
Comparison plot of adsorption of chromium with blank coffee power, coffee-polyphenol-formaldehyde, and coffeepolyphenol-acetaldehyde resins is shown in Figure 2 and it showed that the adsorption of blank coffee powder was lower than coffee-formaldehyde resins.Our objective was to develop new resins with coffee powder for toxic metal removal which should have better thermal and mechanical performance.So that, the same resins can be used repeatedly thus increasing turn over number which would be economically beneficial.Blank coffee powder does not have enough thermal or mechanical performance, because basically it contains organic compounds, which will not serve our purpose.In this case, formaldehyde acts as a cross-linker and forms coffee polyphenol-formaldehyde/acetaldehyde resins.Coffee contains polyphenols which are linked together by forming -CH 2 -bridges.The resins prepared by condensation polymerization of coffee polyphenols with formaldehyde/acetaldehyde are thus insoluble in acidic as well as basic media and increasing reusability and simultaneously turn over number.
Mainly two cross-linkers such as formaldehyde and acetaldehyde were studied to see the effect on chromium metal adsorption.It was observed that formaldehyde (pK a , 13.3) showed higher chromium adsorption than acetaldehyde (13.57) due to more acidic nature and steric hindrance.variety of functional groups such as hydroxyl, carboxyl, carbonyl, amino and imino, which are important sites for metal sorption.The FTIR spectra of blank decaffeinated coffee powder (A), coffee polyphenol-formaldehyde resins (B), and Cr(VI) adsorbed coffee polyphenol-formaldehyde resins powder (C), respectively, are depicted in Figure 3.The absorbance bands observed at 3423 and 2934 are assigned to -OH and -CH 2 stretching, respectively.The sharp peak observed at 1744 cm −1 for decaffeinated coffee is due to the C=O stretching of carboxyl acid, ketone, or its ester.The small peak observed at 1080 cm −1 is due to the N=C stretching of caffeine.All the absorptions observed at 3423, 2934, 2884, 1744, and 1665 cm −1 are shifted to higher wavelength with adsorption of Cr(VI) ion.From these results, it is presumed that the Cr(VI) metal ion is incorporated with CFA resins through interaction with active functional groups such as -OH and -CO-.

Effect of pH.
The pH of aqueous solution is a significant parameter for the removal of metal ions by adsorption [24].Adsorption of Cr(VI) was studied in the pH range of 1-8 and the effect of pH on Cr(VI) removal using coffee-polyphenolformaldehyde and coffee-polyphenol-acetaldehyde resins is shown in Figures 4 and 5, respectively.The experimental results reveal that Cr(VI) removal efficiency increases with the decrease in pH and reached up to 99.7% at pH 2. Cr(VI) exists in aqueous phase in different anionic forms such as chromate (CrO can be concluded that the active form of Cr(VI) can be adsorbed onto HCrO 4 − .The adsorption of Cr(VI) is more effective on coffee polyphenol-formaldehyde/acetaldehyde adsorbents.Some of the functional groups such as phenolic -OH bind electrostatically with negatively charged metal complexes.The decrease in the adsorption with increase in pH may be due to the decrease in electrostatic force of attraction between sorbent and sorbate ions.At lower pH, the percentage of Cr(VI) removal is high as the surface of adsorption becomes protonated; thus, the electrostatic force of attraction becomes high, resulting in an increase in the attraction of anionic species of Cr(VI) towards adsorbent coffee resins.The decrease in adsorption above pH 4 may be due to saturation of the adsorption sites by anionic species like CrO 4 2− and Cr 2 O 7 2− which retards the adsorption of such ions further towards the adsorbent surface.In other words, as the pH increased above the zeta potential of the adsorbent there is a reduction in the electrostatic attraction between the Cr(VI) species and the adsorbent surface.

Effect of Contact Time.
Time of contact has a significant influence on the adsorption of Cr(VI).The adsorption experiments were carried out with respect to contact time ranging from 10 to 150 min and the results are presented in Figure 7.The results indicated that metal adsorption increases with contact time.For the first 10 min, the percentage removal of Cr(VI) from aqueous solution increases rapidly and reaches a value of 81%; later on adsorption rate becomes slower and reaches up to 97% at 150 min.Further increase in contact time has a negligible effect on the percentage removal of Cr(VI).The fast initial uptake may be due to the accumulation of metal ions on the surface of adsorbent which is a rapid step.
The mechanism of solute transfer to the solid includes diffusion through the fluid film around the adsorbent particle and pores to the internal adsorption sites.Initially, the concentration gradient between the film and the solid surface is high, and hence the transfer of solute onto the solid surface is faster.Thus, it takes a smaller amount of time to attain 81% percent removal of Cr(VI).As the contact time increases, intraparticle diffusion becomes predominant.Hence, after 81% adsorption, solute takes more time in the transfer of Cr(VI) from solid surface to internal adsorption sites through the pores.Therefore, the optimum time for adsorption of Cr(VI) on coffee polyphenol-formaldehyde/acetaldehyde resins is 150 min for all batch studies.

SEM and EDX Analysis.
The morphological analysis of coffee resins was performed by SEM as shown in Figure 8.Many small pores and particles >5 m diameter are observed on the surface of coffee polyphenol-formaldehyde/acetaldehyde resins.EDX spectrum (Figure 9) also showed a peak at 5.4 keV, which confirmed that Cr(VI) was adsorbed on coffee polyphenol-formaldehyde/acetaldehyde resins.It supports that the reaction of metal ion and phenolic -OH group on coffee polyphenol-formaldehyde/acetaldehyde resins surface may be partly ion exchange or complexation.The results of analysis of particle size and particle size distribution are reported in Table 3.A plot of particle size distribution of coffee polyphenol-formaldehyde/acetaldehyde resins (CAA 05) is depicted in Figure 10.CFA resins showed higher particle sizes than CAA resins.

Effect of Chromium(VI) Concentration. Adsorption of
Cr(VI) is significantly influenced by the concentration of Cr(VI) in aqueous solutions.The adsorption of Cr(VI) was examined with respect to the amount of Cr(VI) in solution.The Cr(VI) concentration varied from 5 to 20 ppm.The effect of initial concentration on percentage removal of Cr(VI) is depicted in Figure 11.The percentage removal of Cr(VI) decreases with the increase in initial Cr(VI) concentration.It may be due to an increase in the number of Cr(VI) ions for a fixed amount of adsorbent.The rate of percent Cr(VI) removal is higher at the beginning due to larger surface area of coffee resins being available for the adsorption.Once saturation point is reached the capacity of the adsorbent gets exhausted and uptake rate is controlled by the sorbate, transported from the exterior to the interior sites of the adsorbent particles.The results indicated that the initial Cr(VI) ion concentration determines the equilibrium concentration and also determines the rate of uptake of Cr(VI) ion and its kinetic character.

Effect of Adsorbent Dose.
The various doses consisting of the adsorbents (100 mg to 300 mg) were mixed with metal is held by chemisorption [26,27].Various eluents such as hydrochloric acid, nitric acid, and acetic acid are used.The effect of various eluents used for desorption studies indicates that hydrochloric acid is a better eluent for desorption, because results showed that the removal of adsorbed chromium ions was more than 75%.The reversibility of adsorbed Cr(VI) in mineral acid or base is in agreement with the pH dependent results obtained [28,29].The reusability of the resin is important to reduce the process cost.Desorption was performed by using 1 M hydrochloric acid.The metal desorbed resin was reused for three times and it shows good results.Figure 13 represents the adsorption-desorption cycle of CFA 01 resin.There was no significant change in adsorption capacity after three cycles.Adsorption capacity of resin after the third cycle was reduced by 8.68% compared to the first cycle.Also, it was observed that desorption capacities were 75, 72, and 62% for the first, second, and third cycle, respectively.These results show that coffee resins are effective and efficient adsorbent for Cr(VI) recovery.

Adsorption Isotherm. Adsorption isotherms of Cr(VI)
were investigated with respect to biomaterials using an electrolyte solution at the optimum pH 2. The adsorption data were fitted by least square method to linearly transformed Freundlich and Langmuir adsorption isotherms.The Freundlich adsorption isotherm was applied for the adsorption of Cr(VI) onto coffee polyphenol-formaldehyde resins.The linear Freundlich equation is mentioned below: where   is the amount of Cr(VI) adsorbed per unit mass of adsorbent (mg/g),   is the residual concentration of Cr(VI) in solution (mg/L),   is a constant which is a measure of adsorption capacity, and 1/ is a measure of adsorption intensity.The linear Freundlich adsorption isotherm is shown in Figure 14.
The Langmuir adsorption was applied to the present study to estimate the adsorption capacity of coffee polyphenol-formaldehyde resins.The linear form of the Langmuir adsorption isotherm is presented below: where   is the equilibrium concentration (mg/L) and   is the amount of Cr(VI) adsorbed per gram at equilibrium   15.The high value of correlation coefficient ( 2 = 0.948) indicates a good agreement with the parameters and confirms the monolayer adsorption of Cr(VI) onto the adsorbent surface.The isotherm data is linear using the Freundlich and Langmuir equations.The high value of correlation coefficient ( 2 = 0.989) was found to be in good agreement with Freundlich equation.
These results indicated that adsorption of Cr(VI) followed both adsorption isotherm models.Freundlich isotherm model is fitted better than Langmuir, because the correlation between calculated and experimental (found) values as well as regression factors is in good agreement with Freundlich isotherm rather than Langmuir.
Langmuir constants were determined and are presented in Table 4.
Freundlich constants were determined and are depicted in Table 5.
To evaluate sorption dynamics, it requires consideration of two important physicochemical parameters such as kinetics and equilibrium of adsorption.Kinetics describe the solute uptake rate which governs the contact time.The study of equilibrium is determining the distribution of solute between solid-liquid phases and determining the feasibility and capacity of the sorbent for adsorption.Several kinetic models, currently in use to explain the mechanism of adsorption progress, are most simple and widely used is pseudo-first order equation of Lagergren [30]: where   is the mass of metal adsorbed at equilibrium (mg/g),   is mass of metal adsorbed at time  (mg/g), and   is the first order reaction constant (L/min).The pseudo-first order considers that the rate of occupation of adsorption sites is proportional to the number of unoccupied sites.A straight line of log(  −   ) versus  indicates the application of the first order kinetic model (Figure 16).On the other hand, equilibrium capacity may be expressed by pseudo-second order equation as follows: where  2 is the second order reaction rate equilibrium constant (g/mg⋅min).A plot of /  versus  gives a linear relationship for the applicability of the second order kinetic model (Figure 17).The adsorption capacity of coffee polyphenol resins compared to other materials is reported in the literature [31][32][33][34][35][36][37][38][39][40][41][42][43] and is given in Table 6.The sorption capacity of CFA and CAA resins are 175.44 and 143.32 mg/g, respectively, which are higher than adsorbents reported in the literature such as industrial as well as low-cost natural materials.And the second advantage is that resins can be reused number of times by adsorption-desorption method due to better thermal and mechanical performance than without cross-linked natural materials, ultimately reducing the cost.
Most of the low-cost adsorbents have the limitations such as low adsorption capacity and also generate more solid waste causing disposal problems.Hence, there is a need to explore low-cost adsorbents with high adsorption capacity.In our study, we have prepared particulate systems where average particle size ranges are 20.85 to 28.89 m for CFA resins and 16.71 to 26.23 m for CAA resins.

Conclusion
The use of bioadsorbent prepared from coffee powder, capable of adsorption of Cr(VI) from aqueous stream, is cost effective and efficient.The maximum adsorption of Cr(VI) took place at pH 2. The active form of Cr(VI) adsorbed onto coffee polyphenol-formaldehyde/acetaldehyde resins at lower pH is found to be HCrO 4 − .The percentage of the removal of Cr(VI) decreases with the increase in pH.The adsorption process of Cr(VI) is described by Freundlich and Langmuir isotherm models.However, Freundlich isotherm model shows a better agreement with the equilibrium data.The kinetic studies indicate that Cr(VI) adsorption on coffee polyphenol-formaldehyde/acetaldehyde resins is very fast (within the first 150 minutes).Percentage removal of Cr(VI) increases and adsorption capacity decreases with the increase in adsorbent amount, because of the availability of more adsorption sites.These results demonstrate the great potential of coffee polyphenol-formaldehyde/acetaldehyde resins which provide a convenient and low-cost adsorbent for heavy metal removal.Heavy metal ion Cr(VI) removal technique using such biomaterials would be an effective method for economic treatment of wastewater.

Table 3 :
Particle size analysis of coffee resins.

Table 4 𝑄
plot was obtained for   /  versus   and is depicted in Figure

Table 6 :
Comparison of adsorption of Cr(VI) with naturally occurring adsorbents.