ORIENTAL JOURNAL OF CHEMISTRY ISSN: 0970-020 X CODEN: OJCHEG An International Open Free Access, Peer Reviewed Research Journal 2016, Vol. 32, No. (6): Pg. 2889-2900 www.orientjchem.org

Biosorption Kinetic Models on the Removal of Congo red onto Unripe Calamansi (Citrus microcarpa) peels

DERICK ERL P. SUMALAPAO1, 2,*, JERIC R. DISTOR3, IRIS D. DITAN3, NINA THERESE S. DOMINGO3, LOUIE F. DY3 and NELSON R. VILLARANTE4

1Biology Department, College of Science, De La Salle University, 2401 , , . 2Mathematics Area, School of Multidisciplinary Studies, De La Salle-College of Saint Benilde, 2544 Taft Avenue, Manila, Philippines. 3College of Medicine, University of the Philippines Manila, 547 Street, Manila, Philippines. 4Department of Physical Sciences and Mathematics, College of Arts and Sciences, University of the Philippines Manila, , Manila, Philippines. *Corresponding author E-mail: [email protected]

http://dx.doi.org/10.13005/ojc/320607

(Received: May 24, 2016; Accepted: October 03, 2016)

ABSTRACT

Unripe calamansi peels were prepared and used as a bioadsorbent in the removal of congo red from an aqueous solution using batch adsorption studies. The efficiency of adsorption was evaluated by varying adsorbent dose and contact time. The removal of congo red increased at higher adsorbent dose and longer contact time. The overall rate of adsorption processes appeared to be in accordance with the pseudo-second order reaction mechanism. Higher initial adsorption rate, extent of surface coverage, and activation energy were favored at a lower adsorbent dose, while the intraparticle diffusion was relatively faster at a higher adsorbent dose. The intraparticle diffusion, Elovich, and MacArthur-Wilson models were adequate in describing the chaotic behavior of the kinetic processes involved in the removal of congo red dye onto unripe calamansi peels.

Keywords: Biosorption kinetics, Pseudo-second order, Elovich, MacArthur-Wilson, Intraparticle diffusion, Citrus microcarpa.

INTRODUCTION water1. Due to their high water solubility and ability to color vividly at low concentrations, dyes easily Dyes are colored organic compounds which contaminate the wastewater produced by industries2. are used in industries that produce textiles, paper, This affects gas solubility and transparency of and distilleries. As a result of incomplete binding with bodies of water, which have a detrimental effect on fibers during the dyeing process, dyes are found in aquatic life1. One of these dyes is congo red which highly colored wastewaters which pollute bodies of is a common, toxic, and carcinogenic dye used 2890 SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016) for coloration of paper products. Paper industries adsorption and potential limiting steps will be dispose of congo red in large amounts and are described using several equations, namely the usually found in sediments in polluted canals3. When pseudo-first order, pseudo-second order, Elovich, dyes such as congo red enter the water systems, they MacArthur-Wilson, and intraparticle diffusion can cause pollution, consequently harming humans nonlinear kinetic models. and aquatic organisms4. Nonlinear models, in general, are more As such, one way to address this problem difficult to specify, identify, and estimate than linear is through biosorption. It is a physico-chemical models, and it involves nonlinear iterative processes process which makes use of biological materials in determining the solution. Gauss-Newton, as one of to adsorb pollutants from aqueous solutions5. the iterative methods employed in classic nonlinear Currently, activated carbon is one effective yet regression analysis, was considered in estimating expensive biosorbent used in treating groundwater the parameters of nonlinear models in this study. and industrial wastes6. Agricultural wastes are being The adequacy and sufficiency of the various models considered as an alternative adsorbent compared were individually assessed. It is important to be to other materials due to its availability in large able to predict the rate at which hazardous wastes quantities, renewability, and inexpensiveness7. are removed from an aqueous solution in order to Compared to conventional physical and chemical design appropriate adsorption treatment systems. methods, biosorption studies are looking for easier Moreover, exhaustive exploration on the adsorption and less costly treatment, often making use of these kinetics involving wastewater treatment is important agricultural wastes as bioadsorbents6. in providing better understanding of the reaction pathways and possible mechanisms involved. Currently, no biosorption studies have been conducted for calamansi (Citrus microcarpa) peels, The use of commercial water treatment although there have been for orange peels8, which processes such as activated carbon is limited in the belong to the same family. Finding potential uses textile industry due to the high cost of production and for these discarded peelings may reduce waste and problems with regeneration6. Hence, this necessitates pollution. Hence, this study explores on the removal a need for cheaper and more efficient alternative of congo red from an aqueous solution by unripe methods, and one of these is biosorption. One reason calamansi peels in a single component adsorption that makes biosorption a potential alternative to the system at varying adsorbent doses and contact times current commercial methods is the abundance of using spectrophotometry. biomass9. Utility of calamansi peels as bioadsorbent would help in the reduction of agricultural waste and In addition, this study attempts to elucidate pollution. In a system design perspective, aggregate the mechanisms governing the removal of congo analysis of varying adsorption rates is a prerequisite red, and to find a suitable mathematical model for practical application and this requires examination for the kinetics of the removal involving adsorbent of the controlling mechanism of the adsorption dose and contact time. In particular, comparison process which includes the mass transfer, chemical on the removal of congo red from an aqueous reaction, and diffusion control. As such, several solution using calamansi peels under constant pH, adsorption kinetic models were tested using the dye concentration, and temperature will be made. obtained experimental data. Therefore, the study Only unripe calamansi peels as adsorbent with could provide potential alternatives to other dye- congo red as the adsorbate were used in this study. removal methods like membrane filtration, reverse Furthermore, factors such as the dye concentration, osmosis, coagulation or flocculation, and fenton pH, and temperature were kept constant at 0.1 g/L, reagents which are not economically feasible6. 4.00, and room temperature (27oC), respectively. The Considering the lack of studies exploring calamansi bioadsorptive properties of calamansi peels were peels as bioadsorbent, the study will provide baseline measured across different contact times, namely information on the adsorptive properties of calamansi 10, 20, 30, 40, 50, and 60 min under 0.25 g and peels. 0.50 g adsorbent doses. The mechanisms of SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016) 2891

MATERIALS AND METHODS of zero, and the surface of an adsorbent will be positive and will attract anions if the pH is lower than

Preparation of Adsorbents and Adsorbate pHpzc, while a negatively charged surface will favor Calamansi Peels the adsorption of cationic substances if the pH is 9 Citrus microcarpa is a fruit available in above the pHpzc . At pH below the isoelectric point, a the Philippines year-round, but is most abundant dye exists predominantly in its molecular form, while from August to October10. Due to its varied uses, above the isoelectric point, it exists with a higher the plant is cultivated both on a large and small proportion in its dissociated form. The isoelectric scale throughout the country. Unlike oranges, which point of congo red is 3, thus it would be negatively are also part of the Rutaceae family, calamansi charged at pH range of 5.0-10.018,19. Considering the fruit peelings have not been tested for adsorptive anionic nature of congo red, it can be said that setting properties. the stock solution to an acidic pH will theoretically allow better adsorption. Rice husk ash was observed Five kilograms of unripe calamansi was to exhibit a high adsorption capacity on congo red procured from Villa Colongan, Barangay Colongan, at pH 4.020. Ibaan, Batangas, Philippines and subsequently peeled after two days. The peels were washed Congo red solution with a concentration with distilled water to remove impurities and other of 0.1 g/L was prepared by dissolving 0.1 g congo particles, cut into small pieces, and dried in an oven red in 1 L acetate buffer with a pH of 4.00. A buffer at 100°C for 90 min8. The dried peels were then solution with a pH of 4.00 was prepared using 0.1 M crushed using a mortar and pestle, then grounded NaOH and 1 M acetic acid12. The flask was wrapped into granules and sifted through a 1.00 mm mesh in aluminum foil and stored inside an icebox until it in order to control particle size. Degraded cells offer was needed. a larger available surface area and intracellular component resulting to increase in the availability of Establishment of Calibration Curve more surface binding sites and modifies the functional A calibration standard for the congo groups present in the cell wall11. Subsequently, the red was prepared by mixing the dye with acetate sifted granules were washed with distilled water buffers resulting into five different concentrations and decanted until the supernatant fluid had a which range from 5 to 85 ppm of three replicates comparable absorbance to that of distilled water. The per trial. The absorbance for each calibration adsorbent was dried again in an oven for 80 min at solution was then determined using a UNICO UV-

50°C and subsequently stored in a desiccator until Vis spectrophotometer set at lmax = 500 nm with a it was used in the experiment12. uniform pathlength of 1 cm for all the trials21. Simple linear regression analysis was employed to generate Congo red and Point of Zero Charge the calibration curve.

Congo red (C32H22N6Na2O6S2; 696.68 g/mol) is an anionic direct diazo dye that is Batch Adsorption Study mainly used for paper coloration in the paper From the stock solution of 0.1 g/L congo industry3, and about 15% of congo red ends up in red, 70 mL was transferred to a 250-mL beaker, wastewater13. It is the sodium salt of benzidinediazo- and then 0.50 g of adsorbent was added to the bis-1-naphtylamine-4-sulfonic acid and is known to solution which was mixed using a magnetic stirrer degrade into benzidine, which is a carcinogen14. The operating at 500 rpm. A volume of 8 mL sample was treatment of congo red contaminated wastewater can taken from the mixture using a syringe at 10-min be implicated due to its complex aromatic structure15, intervals for 60 min. The samples were then filtered providing the dye physico-chemical, thermal and using Whatman® nitrocellulose membrane filters optical stability, and resistance to biodegradation of pore size 0.45 µm to separate the adsorbent and photodegradation16, 17. from the solution12, the absorbance of each sample was subsequently measured using a UNICO 8 The point of zero charge (pHpzc) is the pH at UV-Vis spectrophotometer set at lmax = 500 nm . The which the surface of an adsorbent has a net charge same procedure was done for the 0.25 g adsorbent 2892 SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016)

-1 dose. Two replicates for each adsorbent dose were denoted as Ci and Cf in (mg L ), respectively, the prepared. percent removal can be computed using (2)14.

Percent Removal of Dye The Bioadsorption Kinetic Models For each absorbance measurement, The functional relationship between contact the corresponding molar concentration was then time and the amount of congo red adsorbed, with calculated using (1). According to Beer’s law, the a levelling of the increase of the amount of dye product of molar absorptivity and pathlength is adsorbed at some contact time value, depicts a replaced by the slope obtained in the calibration chaotic nonlinear interacting relationship. A nonlinear curve. model was required to capture this phenomenon.

The Pseudo-First Order Kinetic Model The pseudo-first order kinetic model is ...(1) defined as ...(3) In order to determine the magnitude of the effects of the adsorbent on congo red, the percent where (mg g-1) is the amount of dye removal for each trial with respect to the initial mean adsorbed at time t, (mg g-1) is the adsorption molar concentration can be derived from the non- capacity at equilibrium, (min-1) is the pseudo-first treated pre-filtration using (2). order rate constant, and t (min) is the contact time. The general solution to this pseudo-first order model is ...(2) ...(4) Using the calibration curve, the final molar concentration was calculated and subsequently The Pseudo-Second Order Kinetic Model the percent removal of the dye was computed. For The pseudo-second order kinetic model22 the given initial and final concentrations of the dye, is given as

Table 1: Linearized form of the different kinetic models for initial parameter estimation procedure

Model Linearized form

Pseudo-first order

Pseudo-second order

Elovich

MacArthur-Wilson

Intraparticle Diffusion SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016) 2893

...(5) ...(9) -1 -1 where at (mg g ) is the amount of dye where ati (mg g ) is the amount of dye -1 -1 adsorbed at time t, ae (mg g ) is the adsorption adsorbed at time ti, bo (mg g ) is a parameter defined -1 -1 -1 capacity at equilibrium, c2 (g mg min ) is the as the asymptote or potential maximum, b1 (min ) is pseudo-second order rate constant, and t (min) is defined as the rate at which the response variable the contact time. The general solution to this second approaches its potential maximum, t (min) is the -1 order differential equation is defined as contact time, and ei (mg g ) is the random error term. The linearized form of (9) is defined as

...(6) ...(10)

The Elovich Model Intraparticle Diffusion Model The Elovich model23 is defined as Another functional relationship identified to some adsorption processes is that the uptake varies almost proportionally with t1/2 rather than with ...(7) the contact time, t. The Weber-Morris plot examines

The solution to this differential equation the possible relationship between at and t½ using the describes the activated chemisorptions24 and is equation defined by

...(11) -1 ...(8) where at (mg g ) is the amount of dye -1 where at (mg g ) is the amount of dye adsorbed at time t, kid is the intraparticle diffusion adsorbed at time t, a (mg g-1 min-1) is defined as rate constant. The intercept, C, represents the the initial adsorption rate, b (g mg-1) is the extent thickness of boundary layer, a larger intercept of surface coverage and activation energy for suggests a greater boundary layer effect because of chemisorption, and t (min) is the contact time. the instantaneous utilization of the readily available adsorbing sites on the adsorbent surfaces26. The MacArthur-Wilson Model The MacArthur-Wilson model is a two- The Gauss-Newton Method parameter negative exponential model25 which takes The proc nlin (nonlinear regression the form procedure) of SAS (Statistical Analysis System) was

Fig. 1: The effect of contact time and adsorbent dose on the removal of congo red 2894 SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016)

Fig. 2: Linear plots using (a) pseudo-first order, (b) pseudo-second order, (c) Elovich, (d) MacArthur-Wilson, and (e) intraparticle diffusion kinetic model

Obs- observed experimental values, MAW-MacArthur-Wilson, IPD-intraparticle diffusion Fig. 3: Comparison of the best -fit kinetic models on the effect of contact time on the adsorption of congo red onto (a) 0.5 g and (b) 0.25 g adsorbent dose of calamansi peels SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016) 2895 used to fit the models and estimate the parameters. on the specified initial values. Poor starting points In software computing algorithms such as SAS, in can result the procedure to move in an undesired finding the least squares estimator, in a nonlinear direction, and so convergence is never met. Hence, model, one of the most often methods used is the transformation of a nonlinear model to a linear form Gauss-Newton procedure. This method is iterative in estimating the parameters by linear regression is and requires starting value estimates for the a possible solution and was employed in this study, parameters25. Inference on the coefficients using and subsequent values obtained were taken as initial this method is based on the asymptotic variance- estimates. Table 1 summarizes the linearized form covariance matrix of the regression coefficients. of the different nonlinear kinetic models employed Convergence of the parameter estimates depends in this study.

Table 2: Parameter estimates of the adsorption kinetic models at varying adsorbent doses

Model Parameter Adsorbent Dose Levels 0.5 g 0.25 g

-1 Pseudo-first order ae obs (mg g ) 2.800 2.370 -1 ae pred (mg g ) 5.029 0.119 -1 c1 (min ) 0.110 0.044 R2 (%) 85.88 82.25 -1 Pseudo-second order ae obs (mg g ) 2.800 2.370 -1 ae pred (mg g ) 2.818 2.385 -1 -1 c2 (g mg min ) 0.093 0.714 R2 (%) 99.63 99.99 Elovich a (mg g-1 min-1) 10.69 2.17x1014 b (g mg-1) 2.758 16.837 R2 (%) 88.00 93.39 -1 MacArthur-Wilson B0 (mg g ) 2.562 2.347 -1 B1 (min ) 0.163 0.320 R2 (%) 99.37 99.99 -1 ½ Intraparticle Diffusion kid (mg g min- ) 0.148 0.022 C (mg g-1) 1.613 2.205 R2 (%) 92.37 88.65

Table 3: Error analysis of the different adsorption kinetic models at varying adsorbent doses

Model SSE SAE ARE 0.5 g 0.25 g 0.5 g 0.25 g 0.5 g 0.25 g

Pseudo-first order 2.472 0.602 4.035 2.214 60.721 12.012 Pseudo-second order 7.286 0.089 8.351 0.782 6.676 0.412 Elovich 0.168 0.002 1.352 0.132 4.594 0.470 MacArthur-Wilson 0.464 0.003 2.277 0.139 7.667 0.472 Intraparticle Diffusion 0.109 0.004 1.008 0.181 0.411 0.078

SSE - Sum of Squares of the Errors; SAE - Sum of the Absolute Errors; ARE - Average Relative Error 2896 SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016)

Table 4: Differential forms of the second-order derivatives of the kinetic models ...(14) Model Second-order derivative RESULTS AND DISCUSSION

Elovich The uptake of congo red dye continues to increase until it reaches the equilibrium, for which such phenomenon is typical from the presence of MacArthur-Wilson several surface sorption sites, which gradually get saturated with the dye at increasing contact time. The mean removal of congo red revealed a relatively Intraparticle diffusion slow initial rate during the first 10- and 20-min intervals (p>0.05), and does not differ between the two adsorbent doses (p>0.05). This is in contrast to the rapid initial adsorption of congo red taking place Error Analysis within the first 20-min interval using chitosan, after Since the parameter estimation procedure which the adsorption slowed down and then almost involves nonlinear models, three different error reached equilibrium at 120 min29. Beyond the 20-min functions of nonlinear regression were utilized to interval, the removal rate gradually increases, and evaluate the adequacy of the best-fit bioadsorption eventually remains constant for the 0.25 g adsorbent kinetic model to the experimental data. dose as illustrated in Figure 1. However, for the 0.5 g adsorbent dose, removal rate continues to rise until Sum of Squares of the Errors (SSE) the 50-min interval, and eventually starts to gradually The most commonly employed error remain constant. The mean amount of dye adsorbed function is SSE27 and is defined as during the 50- and 60-min intervals significantly differ between the two adsorbent doses (p <0.001).

Figure 1 describes the uptake of congo red ...(12) as it increases up to a certain threshold and then remains almost unchanged. The possible presence where and are the predicted of repulsive forces between adsorbate molecules and the observed values (mg g-1) at equilibrium, on the solid and bulk phases might contribute to respectively, and n is the number of observations. the observed moderate rates of adsorption as it approaches the equilibrium. The equilibrium Sum of the Absolute Errors (SAE) contact time for 0.5 g and 0.25 g adsorbent doses Parameter estimates employing the SAE was observed at 50 min and 30 min, respectively describe a better fit as the value of the errors (p<0.01). Adsorption curves are single, smooth, increases as it favors the fit towards the high- and continuous leading to saturation and indicate concentration observations27. The expression for the possible monolayer coverage on the surface of SAE is defined as adsorbents by the dye molecules30. Due to the rapid adsorption of all available sites and a relatively small

amount of absorbent used, removal efficiency of ...(13) congo red decreased; an increase in the amount of Average Relative Error (ARE) adsorbent may therefore reverse adsorption trend31. The average relative error function attempts The percentage removal of congo red remarkably to minimize the fractional error distribution across increases at an increasing dose. An increase in the the entire range of observations28, and is computed dose from 0.25 g to 0.5 g resulted to 33-40% increase using the expression in the percent removal. This is possibly explained by SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016) 2897 the increase in the surface area for dye bindings and model suggest that higher initial adsorption rate, availability of additional sorption sites29, 31, 32. extent of surface coverage, and activation energy are favored at a lower adsorbent dose. The values Parameter Estimates and Analysis of these parameters are reflected in Table 2. The Initial parameter estimates for the varying agreement of the results with Elovich model with nonlinear kinetic models employed in this study the observed values is subsequently explained. The were obtained using the respective linearized form adequacy of Elovich model to fit experimental data as presented in Table 1. In the pseudo-first order to heterogeneous catalyst surfaces helps to explain kinetic model, the linear plots of against the fit in predicting the adsorption of congo red onto is reflected in Figure 2(a). From these plots, the calamansi peels. This form of kinetic law involves a value of in the pseudo-first order kinetic model variation of the energetics of chemisorption with the was determined, and is given in Table 2. Due to active sites resulting to different activation energies the inherent disadvantage of correctly estimating for chemisorption since the peels of calamansi the equilibrium amount adsorbed as obtained from have been found to contain hydroxycinnamic acids the linearity of the curve, the relationship does not and phenolic acids33. Since calamansi and orange necessarily reflect the pseudo-first order mechanism. belong to the same family, and both peels contain Further, the values obtained from the linear plots various functional groups; electrostatic attraction, are different from the observed values, the pseudo- organic properties, and structure of dye molecules first order adsorption kinetic model is inadequate can influence the adsorption process. Dried orange to describe the given rate processes as justified by peel with pH 4 has higher tendency of electrostatic the coefficients of determination2 (R ) significantly attraction since congo red has an anionic functional deviating from unity. group, a high electrostatic attraction exists between the positively charged surface of the absorbent and Figure 2(b) describes the linear plots of the anionic dye32. against in the pseudo-second order kinetic model. The estimate of the constant, , was obtained The initial parameter estimates for the from these linear plots, and is given in Table 2. MacArthur-Wilson model were obtained from the It is important to note that the observed and linear plots given in Figure 2(d). The observed and predicted values obtained in the pseudo-second predicted values obtained using the MacArthur- order biosorption kinetic model are almost the Wilson model are very close to each other as justified same (R2=0.99) which suggests that the adsorption by the coefficients of determination which are almost phenomenon can be modelled approximately unity as shown in Table 2. This suggests that the using the pseudo-second order kinetic equation adsorption phenomenon can be approximately for the different adsorbent doses. As such, the described using the MacArthur-Wilson model for the equilibrium system capacity for the pseudo-second different adsorbent doses. However, the rate (b1) at order is relatively more reasonable compared to which the uptake approaches its potential maximum the pseudo-first order when predicted results were is relatively faster at a lower adsorbent dose. compared with experimental measurements. The pseudo-second order model provides the best Estimates for the diffusion rate constant correlation in the system processes indicating that and thickness of boundary layer of the intraparticle the overall rate of congo red adsorption processes diffusion model are also reflected in Table 1. The appear to be governed by the chemical process in intraparticle diffusion model offers an adequate fit accordance with the pseudo-second order reaction for the experimental data involving higher adsorbent mechanism. dose. The intraparticle diffusion is noted to be relatively faster at a higher adsorbent dose as a The pseudo-second order equation result of the instantaneous utilization of the most provides the best correlation for the kinetic process readily available adsorbing sites on the adsorbent while the Elovich model adequately fits the surface, while the boundary layer effect appeared to experimental measurements well at lower adsorbent be larger at the lower adsorbent dose as described dose as depicted in Figure 2(c). Results of Elovich by the linear plots in Figure 2(e). 2898 SUMALAPAO et al., Orient. J. Chem., Vol. 32(6), 2889-2900 (2016)

Elovich model provides the best fit in the CONCLUSION uptake of the dye in any adsorbent dose with error values noted to be smaller in the case of lower The removal of congo red reflects a adsorbent dose as reflected in Table 3. Intraparticle relatively slow initial and final increase rates against diffusion model offers a better fit involving higher contact time. The removal of congo red increases at adsorbent dose as justified by small error measures higher adsorbent dose and longer contact time. The and high coefficients of determination. Identifying the kinetic processes involved behave under a pseudo- best-fit kinetic model is important in predicting the second order reaction, based on the assumptions mechanisms involving chemical reaction or mass that the rate limiting step, may be chemical transport process that control the rate of the pollutant adsorption or chemisorption involving valency forces removal since this information is important in the through sharing or exchange of electrons between design of appropriate sorption treatment plants31. congo red and calamansi peels. Elovich model suggests that higher initial adsorption rate, extent of Moreover, analysis of the differential forms surface coverage, and activation energy are favored and the second-order derivatives of the Elovich, at a lower adsorbent dose, while the intraparticle MacArthur-Wilson, and intraparticle diffusion diffusion model offers an adequate fit for the models point out that these are negative over the experimental data involving higher adsorbent dose. entire range of contact time. These second-order The intraparticle diffusion is noted to be relatively derivatives are presented in Table 4. As such, these faster at a higher adsorbent dose as a result of the models justify the concave downward appearance instantaneous utilization of the most readily available of the curve when the experimental values on the adsorbing sites on the adsorbent surface. amount of dye adsorbed from an aqueous solution onto calamansi peels versus contact time are Given the results of the study, the removal of fitted. The graphs of these models are displayed in congo red from an aqueous solution onto calamansi Figure 3. peels has been found possible. With this information, it is possible to create a design that will optimize a treatment procedure employing calamansi peels as a bioadsorbent.

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