Oxidized Renewable Materials for the Removal of Cobalt(II) and Copper(II) from Aqueous Solution Using in Batch and Fixed-Bed Column Adsorption

Rodrigues, Josilene Aparecida Vieira; Martins, Luide Rodrigo; Furtado, Laís Milagres; Xavier, Amália Luísa Pedrosa; Almeida, Francine Tatiane Rezende de; Moreira, Ana Luísa da Silva Lage; Melo, Tânia Márcia Sacramento; Gil, Laurent Frédéric; Gurgel, Leandro Vinícius Alves

doi:https://doi.org/10.1155/2020/8620431

Advances in Polymer Technology

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Research Article | Open Access

Volume 2020 | Article ID 8620431 | https://doi.org/10.1155/2020/8620431

Oxidized Renewable Materials for the Removal of Cobalt(II) and Copper(II) from Aqueous Solution Using in Batch and Fixed-Bed Column Adsorption

Josilene Aparecida Vieira Rodrigues,¹Luide Rodrigo Martins,¹Laís Milagres Furtado,¹Amália Luísa Pedrosa Xavier,¹Francine Tatiane Rezende de Almeida,¹Ana Luísa da Silva Lage Moreira,¹Tânia Márcia Sacramento Melo,¹Laurent Frédéric Gil,¹and Leandro Vinícius Alves Gurgel¹

Academic Editor: Khalid Z. Elwakeel

Received10 Jun 2019

Revised07 Aug 2019

Accepted16 Sept 2019

Published10 Jan 2020

Abstract

Batch and continuous adsorption of Co²⁺ and Cu²⁺ from aqueous solutions by oxidized sugarcane bagasse (SBox) and oxidized cellulose (Cox) were investigated. The oxidation reaction of sugarcane bagasse and cellulose was made with a mixture of H₃PO₄‒NaNO₂ to obtain SBox and Cox, with the introduction of high number of carboxylic acid functions, 4.5 and 4.8 mmol/g, respectively. The adsorption kinetics of Co²⁺ and Cu²⁺ on SBox and Cox were modeled using two models (pseudo-first-order and pseudo-second-order) and the rate-limiting step controlling the adsorption was evaluated by Boyd and intraparticle diffusion models. The Sips and Langmuir models better fitted the isotherms with values of maximum adsorption capacity of 0.68 and 0.37 mmol/g for Co²⁺ and 1.20 and 0.57 mmol/g for Cu²⁺ adsorption on Cox and SBox, respectively. The reuse of both spent adsorbents was evaluated. Adsorption of Cu²⁺ and Co²⁺ on SBox in continuous was evaluated using a 2² factorial design with spatial time and initial metal concentration as independent variables and and effective use of the bed as responses. The breakthrough curves were very well described by the Bohart–Adams original model and the values for Co²⁺ and Cu²⁺ were 0.22 and 0.55 mmol/g. SBox confirmed to be a promising biomaterial for application on a large scale.

1. Introduction

The pollution of water resources by toxic metal species is one of the main environmental problems in the world. These inorganic pollutants can cause serious negative effects on the ecosystem, because they have high toxicity to living organisms and are persistent in the environment due to their nonbiodegradability. Metal ions such as Cu²⁺and Co²⁺ are essential, but become toxic by excess exposure. Cobalt is an essential element of vitamin B12, which is necessary for the red blood cell production, but becomes toxic [1] when a high amount is ingested by humans, producing an erythropoietic effect, cardiomyopathy, hypothyroidism, and polycythemia. On the other band, the immoderate accumulation of copper in the human body can produce hepatic necrosis and Wilson’s disease, resulting in abnormalities of the nervous system, liver, kidneys, and cornea [2, 3].

Nowadays, several processes have been used to remove inorganic pollutants of different sources from water. These processes are mostly conventional processes based on physicochemical treatments (in combination or not) such as coagulation, flocculation, precipitation, and nano- and ultra-filtration [4, 5]. However, these processes have a high cost and/or a low efficiency in removing metal ions at low concentrations and, in addition, some of them can generate a large amount of waste that requires to be further disposed of [6]. Therefore, these problems have increased the number of studies in this area, aimed at the development of original processes with a higher efficiency and lower cost [7]. In this context, adsorption has been highlighted with the development of new adsorbent materials of lower cost and can be efficiently reused; these materials were obtained from designed syntheses using agroindustrial residues as natural materials [8]. During the two last decades, low-cost bioadsorbents such as lignocellulose biomass [9, 10] from agricultural crop waste or chitosan [11–13] from fishing industry waste have been studied as potential materials to be used in the treatment of effluents containing organic pollutants or toxic metal species.

The reuse of agroindustrial residues adds a greater economic value and mitigates a series of environmental problems caused by them. Agroindustrial residues mainly consist of lignocellulose biomass (LB), which can be applied in the adsorption of toxic metal species as an alternative to conventional adsorbents as activated carbons and styrene-divinylbenzene ion-exchange resins, since agroindustrial residues are cheap and efficient metal ion accumulators [14]. However, LB can be chemically modified to increase their attraction towards specific pollutants or to be applied to specific processes, enhancing their physicochemical properties, such as adsorption capacity, chemical resistance to acidic and basic media, and selectivity [15, 16].

Oxidation process is an interesting strategy to modify chemically LB. In addition, the type of oxidizing mixture used will determine the changes in the chemical groups, physical structure, and crystallinity of the LB, producing oxidized materials with different physicochemical properties [17]. Thus, materials with new interesting properties for many purposes can be obtained [18].

Most previous studies using agroindustrial residues for adsorption of toxic metal species are based on batch equilibrium and kinetic studies. Batch adsorption is useful to provide information about the efficiency of an adsorbent and its physicochemical properties; however, batch adsorption may not be the most convenient process to be applied to an industrial scale that requires the treatment of high flow rates. In this way, for a full-scale adsorption process, continuous adsorption is often preferred [19].

In this study, sugarcane bagasse (SB) and cellulose (Cel) were chemically modified by oxidation with a mixture of sodium nitrite (NaNO₂) and orthophosphoric acid (H₃PO₄) to obtain oxidized sugarcane bagasse (SBox) and oxidized cellulose (Cox). Adsorption capacity of SBox and Cox for Co²⁺ and Cu²⁺ ions from spiked aqueous solutions was investigated. The batch adsorption was carried out as a function of the contact time, solution pH and initial Co²⁺ or Cu²⁺ concentration. The reuse of SBox and Cox was also evaluated. The adsorption of Co²⁺ and Cu²⁺ in continuous using a fixed-bed column filled with SBox was evaluated with a 2² experimental design.

2. Material and Methods

2.1. Material

Cellulose grade chromatography paper (3MM, Cat. No. 3030–861) was acquired from Whatman, England. Sugarcane bagasse was collected from the Jatiboca Ethanol and Sugar Plant, Urucânia, Minas Gerais, Brazil. HCl (37 wt.% in water), NaOH, HNO₃, H₃PO₄ (85 wt.%), and CuSO₄·5H₂O were acquired from Synth (Brazil). CoCl₂·6H₂O and NaNO₂ were acquired from Vetec (Brazil).

2.2. Cellulose and Sugarcane Bagasse Preparation

An analytical mill (Model A11, IKA) was used to mill 50 mm² paper sheets of Cel before oxidation reaction. For oxidation reaction, sugarcane bagasse (SB) was treated using the methodology described by Ramos et al. [20]. The SB fraction utilized in this study was that retained on the 0.150 mm (100 mesh) sieve.

2.3. Oxidation of Sugarcane Bagasse and Cellulose

The oxidation reaction of SB and Cel was performed using the procedure previously described by Martins et al. [21]. In a typical methodology, 5.000 g of Cel or SB was weighed into a 250 mL Erlenmeyer flask and then 80.0 mL of H₃PO₄ (85% wt.%) was added to the flask under agitation at 25°C. Afterward, it was added in this flask 4.000 g of NaNO₂ under vigorous agitation for about 10 min and the mixture was allowed to stand for 5 h with the flask opened. Then, a vacuum filtration on a sintered glass funnel (porosity 2) was used to separate the reaction mixture from the oxidized material (SBox or Cox), which was washed with distilled water until pH reached neutrality (pH ~ 7). The drying of the oxidized material was performed in an oven at 65°C for 4 h. The oxidized material was kept in a desiccator prior to use.

2.4. FTIR Spectroscopy

To prepare the FTIR KBr pellets of 13 mm, 100 mg of KBr (spectroscopy grade) and 1 mg of sample (dried powder) were mixed and pressed using a hydraulic press (Model 181-1110, Pike Technologies, Canada) at 6 ton for 30 s. The spectrum was obtained from an FTIR spectrometer (Model MB3000, ABB Bomen, Canada) from 500 to 4000 cm⁻¹ with a resolution of 4 cm⁻¹ and 32 scans per sample.

2.5. Adsorption Experiments

The adsorption of Cu²⁺ and Co²⁺ on SBox or Cox was studied as a function of the contact time, solution pH, and initial Cu²⁺ or Co²⁺ concentration. The following typical methodology was used in all adsorption experiments, which were carried out in duplicate. Samples of 20 mg of SBox or Cox and 100.0 mL of aqueous Cu²⁺ or Co²⁺ solution were added to 250 mL Erlenmeyer flasks. Then, drops of aqueous 0.1 mol/L NaOH or HCl solutions were added to adjust the pH. The flasks were placed in an orbital incubator shaker (Model TE-424, Tecnal, Brazil) at 150 rpm and 25°C. At the end of the adsorption experiments, a single filtration (filter paper JP-41) was carried out to separate the liquid and solid fractions. A flame atomic absorption spectrophotometer (FAAS) (Model SpectrAA 50B, Varian) was used to determine the concentration of Cu²⁺ or Co²⁺ in the liquid fraction ( = 324.8 nm [22], = 240.7 nm [22]). The adsorption capacity, , was calculated utilizing Equation (1):

where is the amount of Cu²⁺ or Co²⁺ adsorbed per unit weight of SBox or Cox at time or equilibrium, is the volume of Cu²⁺ or Co²⁺ solution, is the initial Cu²⁺ or Co²⁺ concentration, is the Cu²⁺ or Co²⁺ concentration at time or equilibrium and is the weight of SBox or Cox.

2.5.1. Adsorption of Cu²⁺ and Co²⁺ on SBox or Cox as a Function of Solution pH

The following conditions were employed to evaluate the effect of solution pH on the adsorption capacity of Cu²⁺ and Co²⁺ on SBox or Cox: pH from 2.0 to 5.5, contact time of 4 h and initial Cu²⁺ or Co²⁺ concentration of 50 mg/L.

2.5.2. Adsorption of Cu²⁺ or Co²⁺ on SBox or Cox as a Function of Contact Time

The following conditions were employed to evaluate the effect of contact time on the adsorption capacity of Cu²⁺ and Co²⁺ on SBox or Cox: time interval from 5 to 600 min, pH 5.5 and initial Cu²⁺ or Co²⁺ concentration of 50 mg/L.

2.5.3. Adsorption of Cu²⁺ or Co²⁺ on SBox or Cox as a Function of Cu²⁺ or Co²⁺ Concentration

The adsorption isotherms were obtained to evaluate the influence of initial Cu²⁺ or Co²⁺ concentration on metal ion uptake by SBox and Cox by varying the initial Cu²⁺ or Co²⁺ concentration. The following conditions were employed to obtain the adsorption isotherms: initial Cu²⁺ or Co²⁺ concentration range from 8 to 100 mg/L, equilibrium time of 3 h and pH 5.5.

2.6. Desorption and Reuse of SBox and Cox

Desorption of SBox and Cox was made to evaluate the possible reuse of these adsorbent materials. The loading of SBox or Cox (50 mg) with Cu²⁺ or Co²⁺ was performed at pH 5.5 using 100.0 mL of 125 mg/L of Cu²⁺ or Co²⁺ solution and a time of 3 h. At the end of adsorption experiments, a single filtration was made to recover the loaded adsorbent, which was washed with distilled water to eliminate any nonadsorbed Cu²⁺ or Co²⁺. The loaded adsorbent was dried at 80°C in an oven for 1 h. Then, 20 mg of Cox or SBox loaded with Cu²⁺ or Co²⁺ was weighed into Erlenmeyer flasks (125 mL) and then 20.0 mL of aqueous 0.5 mol/L HNO₃ solution was added. Desorption experiments were performed in an orbital incubator shaker (Model TE‒424, Tecnal, Brazil) at 150 rpm and 25°C for time intervals of 10 and 15 min for SBox and Cox, respectively. The concentration of Cu²⁺ or Co²⁺ in the desorption solution was determined by FAAS (Section 2.5). The desorption efficiency, , was calculated utilizing Equation (2), [17]:

where is the equilibrium Cu²⁺ or Co²⁺ concentration in aqueous HNO₃ solution, is the volume of the aqueous HNO₃ solution, is the maximum adsorption capacity obtained from loading adsorbent with Cu²⁺ or Co²⁺ before the desorption experiment and is the weight of SBox or Cox adsorbent contained in SBox or Cox loaded with Cu²⁺ or Co²⁺ [22]. The calculation of is given in Equation (3):

All reuse studies were made using the following procedure: 20 mg of SBox or Cox recovered from Cu²⁺ or Co²⁺ desorption was added to Erlenmeyer flasks (250 mL) with 100.0 mL of an aqueous 50 mg/L Cu²⁺ or Co²⁺ solution (pH 5.5). The Erlenmeyer flasks were placed in an orbital incubator shaker (Model TE‒424, Tecnal) at 150 rpm and 25°C for 3 h. Then, a single filtration (filter paper JP-41) was used to separate the liquid and solid fractions and the equilibrium concentration of Cu²⁺ or Co²⁺ in the liquid fraction was determined by FAAS (Section 2.5). The re-adsorption efficiency, , for one adsorption/desorption/re-adsorption cycle was calculated utilizing Equation (4).

where is the maximum re-adsorption capacity obtained after one adsorption/desorption/adsorption cycle [22]. is calculated utilizing Equation (5).

2.7. Fixed-Bed Column Adsorption Experiments

The adsorption experiments in continuous were carried out in a jacket glass column (10.0 cm height × 1.0 cm inner diameter) filled with 0.400 g of SBox adsorbent (dry-weight basis). The SBox adsorbent was packed between two layers containing glass beads (10 mm of diameter) and glass wool to give a uniform upflow of the Cu²⁺ or Co²⁺ solution into the column. For all experiments, the bed height was fixed at 3.80 cm. The inlet upflow rate was controlled by a peristaltic pump (Model BP600/2, Milan, Brazil). The temperature of the inlet metal solution and the column was kept at 25 ± 1°C by a thermostatic bath with forced water circulation (Model MA470, Marconi, Brazil). The Cu²⁺ or Co²⁺ solutions were prepared using a buffer of pH 5.5 (CH₃COOH/CH₃COONa). Effluent samples were sampled at the top of the column at predetermined time intervals to obtain the breakthrough curve. Metal concentration in the effluent samples was determined by FAAS (Section 2.5). To ensure that metal concentration was in the range of the calibration curve, appropriate dilutions were made. The saturation time and breakthrough point were fixed at 95% and 5% of influent metal concentration, respectively [23].

2.7.1. Design of Experiments

The continuous monocomponent adsorption of Cu²⁺ and Co²⁺ on SBox was optimized employing a 2² experimental design to evaluate the influence of the initial Cu²⁺ or Co²⁺ concentration (, mmol/L) and spatial time (, min) on the responses. The value of was calculated using the empty volume of the column (mL) and the flow rate (mL/min). The dependent variables (responses) evaluated were the maximum adsorption capacity (, mmol/g) of the bed and the effective use of the bed (, cm).

Table 1 presents the independent variables (IVs) and their levels used in the continuous adsorption of Cu²⁺ and Co²⁺ on SBox adsorbent. They were defined considering the batch adsorption studies and the lowest flow rate (1.4 mL/min) of the pump. Then, 1 min was defined as the center point of spatial time; and the center point of initial Cu²⁺ and Co²⁺ concentration was calculated based on the values of obtained from the batch adsorption isotherms.

Statistica 10.0 (StatSoft, Inc.) routines were used to evaluate the obtained results for analyses of variance (ANOVA), regression coefficients and graphical analysis. To analyze the experimental error, pure error was used and a confidence level equal to 95% was used in the statistical analyses. An IV was statistically significant when its -value <0.05.

2.8. Modeling the Experimental Data

2.8.1. Batch Kinetic Data

Pseudo-first-order (PFO) and pseudo-second-order (PSO) models were used to investigate the adsorption kinetics of Cu²⁺ and Co²⁺ on SBox and Cox.

The PFO kinetic model of Lagergren [24] is expressed by Equation (6).

where is the pseudo-first-order rate constant.

The PSO kinetic model of Ho and McKay [25] is expressed by Equation (7).

where is the pseudo-second-order rate constant.

In addition, to elucidate the adsorption mechanism of Cu²⁺ and Co²⁺ on Cox and SBox, two models were used: intraparticle diffusion (IPD) and Boyd.

The IPD model of Weber and Morris [26] is expressed by Equation (8).

where is the intercept and is the intraparticle diffusion rate constant. The constant may be correlated to the thickness of the boundary layer.

The model of Boyd et al. [27] is expressed by Equation (9):

where is the fractional surface coverage as a function of time and is a function of . Reichenberg [28] provided solutions for Equation (9) using Fourier transform, which are expressed by Equations (10) and (11), as follows:

For , Equation (10) was utilized, whereas for , Equation (11) was utilized to obtain the values of .

The effective diffusion coefficient, , was determined by Equation (12), as follows:

where is the radius of the Cox ( = 0.250 × 10⁻³ m) or SBox ( = 0.150 × 10⁻³ m) particles, assuming a spherical shape.

2.8.2. Batch Equilibrium Data

Adsorption isotherms were fitted by the Freundlich, Langmuir, and Sips isotherm models.

The Langmuir [29] isotherm model is expressed by Equation (13).

where is the maximum adsorption capacity of SBox or Cox for a metal ion and is the Langmuir binding constant.

The Freundlich [30] isotherm model is expressed by Equation (14).

where and are the Freundlich model constants. The parameter is associated to the adsorption intensity.

The Langmuir–Freundlich isotherm model, developed by Sips [31], is a hybrid model of the Freundlich and Langmuir isotherm models, which is expressed by Equation (15).

where is a parameter describing the heterogeneity of the system.

2.8.3. Analysis of the Breakthrough Curve

The area under the breakthrough curves gives the amount of Cu²⁺ or Co²⁺ adsorbed in the bed [32]. Then, the value of of the bed can be obtained from Equation (16), until the saturation time is reached, as follows:

where is the inlet flow rate (mL/min), is the time, and are the influent and effluent Cu²⁺ or Co²⁺ concentrations (mmol/L) and is the weight of SBox (g).

The value of is defined as the height of the bed that was effectively utilized in the continuous adsorption process [33]. The value of was obtained from Equation (17), as follows:

where is the bed height (), is the amount of Cu²⁺ or Co²⁺ adsorbed on the adsorbent until the breakthrough time () and is the amount of Cu²⁺ or Co²⁺ adsorbed on the adsorbent until the saturation time (). The values of and were determined by the integration of the area under the breakthrough curves until and , respectively.

(i) Modeling the Breakthrough Curves.The development of mathematical models to predict the breakthrough curve, the mechanism involved in the process and even the evaluation of the effect of different variables on adsorption process is an important demand. However, this is a difficult task for such systems since the concentration profiles in both solid and liquid phases vary with both and [32]. Thus, analytical models based on modeling the experimental data by means of nonlinear or linear regression analyses with physical meaning are available [34–38]. The original model of Bohart and Adams [38] is successfully employed when the adsorption isotherm is favorable. This model supposes that the rate of adsorption is proportional to the remaining adsorption capacity of the adsorbent and the concentration of the solute, neglecting axial dispersion. Equations (18)–(20) describe the relationship between and for this model.

where (cm/min) is the interstitial velocity, (mmol/L) is the liquid phase Cu²⁺ or Co²⁺ concentration initially and at time , (cm) is the column height, (min) is the time, is the column void fraction, (g/mL) is the bulk density and (L/mmol min) is the Bohart–Adams kinetic rate constant.

The relation between the adsorbent density and bulk density was used to determine the porosity of the bed, as given in Equation (21).

2.8.4. Calculation of Variation in Standard Free Energy of Adsorption

The variation in standard free energy of adsorption for batch adsorption was determined by Equation (22) [39].

where is the absolute temperature, is the thermodynamic equilibrium constant (dimensionless) and is the gas constant (8.3144 J/K mol).

Liu [39] suggested that (Langmuir constant) can be used to calculate the value of , as given in Equation (23).

where is the activity coefficient (dimensionless) at equilibrium at 25°C.

The value of can be calculated utilizing the extended Debye Hückel law (Equation (24)), which is applied to ionic strengths up to 0.1 mol/L.

where (mol/L) is the ionic strength, is the charge of the Cu²⁺ or Co²⁺ and (pm) is the hydrated ion size of Cu²⁺ or Co²⁺ (600 pm) [40]. The values of were calculated as described by Ramos et al. [20].

2.8.5. Regression Analysis and Error Evaluation

(i) Batch Adsorption Data. The experimental batch equilibrium and kinetic data were modeled with the PFO and PSO and Freundlich, Langmuir, and Sips models by nonlinear regression (NLR) analysis using Microcal OriginPro®2015. The modeling of the kinetic data with IPD and Boyd models was made by linear regression (LR) analysis of the experimental data. The software was set to use the weight method named “statistical” and the Levenberg–Marquardt algorithm. The reduced chi‒square was employed to determine which model best described the experimental adsorption data [41].

(ii) Continuous Adsorption Data. The breakthrough curves obtained were fitted using the set of Equations (18)–(20). The algorithm genetic function (ga) of the MATLAB®2010a software (Mathworks Inc.) was used for adjusting the parameters of the Bohart-Adams model to the continuous experimental data by minimization of the objective function. The objective function used was the root-mean-square error (RMSE) [18], as given in Equation (25).

where is the number of experimental points and is the value of experimental point is the predicted value by the model for the experimental point .

3. Results and Discussion

3.1. Synthesis and Characterization of the Oxidized Materials

The oxidation reaction of SB and Cel was performed using a mixture of NaNO₂ and H₃PO₄ to obtain the adsorbent materials SBox and Cox with high amounts of carboxylic acid groups according to the optimized procedure reported in a previous study by our research group [21]. The use of this oxidation system (H₃PO₄‒NaNO₂) provides a cheaper methodology to prepare materials containing carboxyl functionality, thus adding value to these materials. The reaction mainly produced the oxidation of the primary R-CH₂OH groups of SB and Cel into R-COOH groups. The reaction conditions were optimized by response surface methodology (RSM) and design of experiments (DOE). Through the optimization of the synthesis conditions, SBox and Cox were obtained with a number of R-COOH functions () of 4.5 and 4.8 mmol/g, respectively, which were determined as described by Martins et al. [21]. Cel oxidation was accomplished with a weight gain equal to 7.7%. This can be explained by the fact that the primary alcohol groups were converted into carboxylic acid groups, increasing the molar mass of the biopolymer. A weight loss equal to 18.4% was reported for SB oxidation, because of the presence of oxidant species and the acidic conditions, which degraded lignin and hemicelluloses fractions, increasing the solubility of these fractions in an aqueous medium.

SBox and Cox were characterized by FTIR spectroscopy and solid‒state ¹³C NMR spectroscopy, as described in our previous study by Martins et al. [21]. The ¹³C NMR spectrum of Cox indicated that the main product has carboxylic acid groups but that starting material (nonoxidized cellulose) is also still present in much smaller amounts.

The main advantage of these oxidized materials (SBox and Cox) comparing to other carboxylated materials prepared by our research group [14, 15, 20, 22, 42–45] is that carboxylic acid functions in SBox and Cox are mainly from the oxidation of primary hydroxyl groups (R-CH₂OH) of cellulose (in SB or Cel). On the contrary, when carboxylic acid groups are introduced in the Cel or SB through esterification reaction with cyclic carboxylic acid anhydrides, the application of esterified Cel or SB is limited to a pH range where the ester groups may not be easily hydrolyzed in the aqueous phase, e.g., 2–9 [46].

3.2. Adsorption Studies

3.2.1. Effect of Solution pH on the Removal of Cu²⁺ and Co²⁺ by SBox and Cox

The batch removal of Cu²⁺ and Co²⁺ by adsorption on SBox and Cox from spiked aqueous solutions depends on the solution pH, since both adsorbent materials contain ionizable carboxylic acid groups. The net surface charge of the adsorbents in aqueous solution is directly related to pH of point of zero charge (pH_PZC). The values of pH_PZC for SBox and Cox are 2.72 and 2.70, respectively, as reported by Martins et al. [21]. Thus, at pH values greater than pH_PZC both surfaces of SBox and Cox became predominantly negative due to conversion of carboxylic acid groups into carboxylate anion. Therefore, when pH values are higher than 2.7, SBox and Cox have a net negative surface charge, thereby promoting the adsorption of the cationic pollutants, such as Cu²⁺and Co²⁺.

Graphs of q_e against initial solution pH for adsorption of Cu²⁺ and Co²⁺ on Cox and SBox are shown in Figures 1(a) and 1(b), respectively. As expected, the adsorption of both metals increased when the solution pH was higher, reaching a maximum at pH value of 5.5 for all adsorption systems. Interestingly, Cox exhibited a higher value than SBox for both metals studied, with both adsorbent materials even having a similar value for . Then, the next studies of adsorption of Cu²⁺ and Co²⁺ on both materials as a function of the contact time (kinetics) and initial Cu²⁺and Co²⁺ concentration (isotherm) were carried out at a pH value of 5.5.

(a)

(b)

A similar tendency was reported by Gurgel et al. [14, 43] and Ramos et al. [20] for the adsorption of Cu²⁺ on succinylated cellulose (Cell 6) [14], succinylated sugarcane bagasse (SCB 2) [43] and sugarcane bagasse modified with trimellitic anhydride (STA) [20]. SCB 2 has a higher amount of carboxylic acid groups (6.0 mmol/g), but a higher pH_PZC value (5.26) [47] than SBox [21]. As a result, SCB 2 only exhibited an adsorptive capacity for Cu²⁺ ions from a pH value of 3.0. The pH_PZC value of STA is 3.16 because this material possesses carboxylic acid groups bonded to an aromatic ring and showed more significant adsorptive capacity for Cu²⁺ and Co²⁺ ions from a pH value of 3.0. In the present study, because of the structural properties of SBox and Cox, i.e. a carboxyl group directly bonded to the glycosyl ring, both SBox and Cox are able to adsorb Cu²⁺ ions from a pH of 2.0 due to their lower pH_PZC values. This represents a great advantage of SBox and Cox in comparison to other carboxylated materials prepared from lignocellulose biomass that have been reported in the literature.

3.2.2. Effect of Contact Time on the Removal of Cu²⁺ and Co²⁺ by SBox and Cox

To design a wastewater treatment plant operating in batch or continuous mode, it is fundamental to study and understand the adsorption kinetics. Thus, experiments with each metal and SBox and Cox adsorbents were performed to define the values of and equilibrium time as well as the adsorption rate constant. Other important factors to explain the adsorption mechanism are the physicochemical and textural properties of the Cox and SBox adsorbents as well as the mass transport processes.

The curves generated from the NLR analyses of the experimental data with the PFO and PSO are presented in Figures 2(a) and 2(b). Table 2 presents kinetic parameters estimated by modeling of the experimental data. Comparing the values of and presented in Table 2, it was possible to confirm that the PSO model better described the Cu²⁺ adsorption by both adsorbent materials. When comparing the values of of Cu²⁺ with those of , it can be observed for both adsorbent materials that the values of estimated by the PSO model were closer to those of .

(a)

(b)

The values of indicated a higher kinetic rate constant for Cu²⁺ adsorption on SBox than Cox. As can be seen in Figures 2(a) and 2(b), the equilibrium time for Cu²⁺ adsorption was achieved in 45 and 120 min for SBox and Cox, respectively.

For Co²⁺ adsorption on both SBox and Cox, the model that better described the experimental data was the PFO model with the values of for this model being closer to those values of . The values of k₁ showed a higher kinetic rate constant for Co²⁺ adsorption on Cox than SBox. As can be seen in Figures 2(a) and 2(b), the Co²⁺ adsorption equilibrium time was achieved in 180 and 120 min for SBox and Cox, respectively.

The parameters of IPD and Boyd models estimated by LR analyses of the experimental data are presented in Table 2. The adsorption process of Cu²⁺ and Co²⁺ on SBox and Cox exhibited three stages (Figures not shown). These plots suggest that the adsorption of Cu²⁺ and Co²⁺ on SBox and Cox was initially governed by the diffusion of Cu²⁺ or Co²⁺ through the thin solvent film surrounding both Cox and SBox particles and then changed to intraparticle diffusion. Afterward, equilibrium was reached. Boyd’s plots (Figures not shown) were only linear for the initial stage of Cu²⁺ or Co²⁺ adsorption and did not intersect the origin, suggesting that external mass transfer can be the rate-limiting step governing the initial stage of Cu²⁺ or Co²⁺ adsorption, which then changed to intraparticle diffusion, corroborating the IPD plots [48]. In addition, for both adsorbent materials, the values of were higher for Cu²⁺ adsorption than Co²⁺ adsorption, which will have a great influence in the behavior of Cu²⁺ and Co²⁺ adsorption on SBox in a fixed-bed column, as presented in Section 3.4.

3.2.3. Adsorption Isotherms of Cu²⁺ and Co²⁺ on SBox and Cox

Adsorption isotherms can be employed to describe the different affinities of various solutes for the adsorption sites of an adsorbent. To evaluate the equilibrium data, Freundlich, Langmuir, and Sips models, three of various isotherm models available in the literature, were chosen. The isotherm model parameters estimated by fitting the models to the experimental data are presented in Table 3. The curves fitted to the experimental data of Cu²⁺ and Co²⁺ adsorption on SBox and Cox are presented in Figures 3(a) and 3(b).

(a)

(b)

The isotherm curves obtained presented a downward-concave profile, characterizing a favorable adsorption [35]. The steepest initial slopes of adsorption isotherms of Cu²⁺ on SBox and Cox in comparison to Co²⁺ adsorption isotherms on SBox and Cox indicate that Cu²⁺ ions had more affinity for the adsorption sites of SBox and Cox than Co²⁺ ions. This is corroborated by the higher values of for Cu²⁺ adsorption than for Co²⁺ adsorption (Table 3). In addition, for both Cu²⁺ and Co²⁺, even at low metal concentrations, the adsorbed amount was relatively high until reaching a plateau at which the adsorbed amount remained constant. In agreement with the classification of isotherms proposed by Giles et al. [49], the profiles of the isotherm curves for all adsorption systems studied corresponded to group L, subgroup 2.

The adsorption isotherms (Figures 3(a) and 3(b)) with initial curvature (type L) suggest that the available active sites are more difficult to find when the surface coverage increases, indicating that the Cu²⁺ and Co²⁺ may have been adsorbed onto a monolayer.

The evaluation of the values of and the comparison between the values of and were used to select the best isotherm model in order to characterize the adsorption systems. Analyzing the values of and presented in Table 3, it can be seen that for Cu²⁺ or Co²⁺ adsorption on both adsorbents, both the Sips and Langmuir isotherm models can describe very well the experimental data. The Sips model is a hybrid of the Freundlich and Langmuir isotherms. The parameter of the Sips model can be employed to evaluate the degree of heterogeneity of an adsorption system. Thus, if the value of is closer to unity, this suggests a more homogeneous adsorption; and if the value of is equal to unity, the Sips isotherm is reduced to the Langmuir isotherm. Contrarily, if the value of is far from unity, this indicates a more heterogeneous adsorption system, approaching to the Freundlich model.

From the values of reported in Table 3 for the Sips model, it is concluded that the adsorption systems involving the adsorption of Co²⁺ on SBox and Cox are more homogeneous than the adsorption systems involving the adsorption of Cu²⁺ on SBox and Cox. It is also noted that the adsorption of Cu²⁺ on Cox is significantly more heterogeneous than other adsorption systems studied.

Parameter of the Langmuir model is associated to the solute affinity for the adsorbent sites [50]. Comparing the values of of the Langmuir model (Table 3), it is concluded that Cu²⁺ has a greater affinity for both Cox and SBox adsorption sites than Co²⁺, as was previously concluded by the initial slopes of Cu²⁺ adsorption isotherms in comparison to Co²⁺ adsorption isotherms. The solute affinity for the adsorbent sites of the materials increased in the order: Co²⁺-SBox < Cu²⁺-SBox < Co²⁺-Cox < Cu²⁺-Cox.

The values of variation in standard free energy of adsorption were negative for all adsorption systems studied, indicating the spontaneous character of the adsorption in the standard conditions. Adsorption experiments of Cu²⁺ on raw cellulose (Cel) and raw sugarcane bagasse (SB) were only performed for comparison purposes. The values of for Cu²⁺ adsorption on SB and Cel were 0.23 mmol/g (13.3 mg/g) and 0.21 mmol/g (13.2 mg/g), respectively, which showed that SBox and Cox presented a much higher than SB and Cel. For, example, the values of for Cu²⁺ adsorption on SBox and Cox were 2.7 times and 5.8 times higher than SB and Cel, respectively, which shows the efficiency of the functionalization process of SB and Cel to obtain SBox and Cox.

The values of the coordination numbers (CN) (Table 3) [14] for Cu²⁺ and Co²⁺ adsorption on SBox and Cox suggested that not all carboxylic acid groups were available to adsorb metals. The values of CN for adsorption of Cu²⁺ and Co²⁺ on Cox were lower than those for adsorption on SBox, indicating that the taking part on the adsorption of Cu²⁺ and Co²⁺ by SBox and Cox was different. Probably, this was responsible for a decrease in the value of for SBox in comparison to Cox, as both Cox (4.8 mmol/g) and SBox (4.5 mmol/g) have similar values for .

3.3. Desorption and Reuse of SBox and Cox

Desorption and re-adsorption were evaluated for both SBox and Cox adsorbents with Cu²⁺ and Co²⁺, respectively. The desorption efficiencies and re-adsorption efficiencies obtained for both SBox and Cox are presented in Table 4.

It was observed that the desorption of the metals was fast, with a maximum value of achieved in a short time, i.e., 10 min for SBox and 15 min for Cox. The best result for was obtained for the SBox-Cu system; however, the other systems showed values of between 72.3% and 87.7%. Even though SBox and Cox were not completely desorbed, this should not impair their reuse as they still present a large amount of free adsorption sites.

The desorption experiments performed at acidic aqueous medium showed that both metals were fast desorbed, which indicated the ion-exchange was the mechanism controlling both adsorption and desorption of both metals. Xavier et al. [23], who investigated the adsorption/desorption/re-adsorption of Co²⁺, Cu²⁺ and Ni²⁺ on trimellitated sugarcane bagasse reported a similar behavior in comparison to that observed in the present study.

The infrared spectra of Cox and SBox before and after desorption of Cu²⁺ (Cox-D-Cu²⁺, SBox-D-Cu²⁺) and Co²⁺ (Cox-D-Co²⁺, SBox-D-Co²⁺), respectively, are presented in Figures 4(a) and 4(b). As can be seen in Figures 4(a) and 4(b), the band at 1730 cm⁻¹ related to C=O stretching in carboxylic acid groups is still present in both spectra of Cox and SBox after desorption of Cu²⁺ and Co²⁺, showing that both adsorbent materials were not degraded after desorption.

(a)

(b)

Re-adsorption was performed to evaluate the alternative of reusing both adsorbents. The desorbed SBox and Cox were used in a new adsorption cycle to evaluate their performances as they were not completely desorbed. The obtained results are shown in Table 4. The lower value of was for Co²⁺-SBox system; however, the other systems showed values of higher than >98%. Therefore, SBox and Cox still exhibited excellent values of in comparison to their values of showing that these materials are interesting adsorbents from both point of view of chemical resistance to degradation and reuse.

3.4. Monocomponent Adsorption of Cu²⁺and Co²⁺ on SBox Adsorbent in a Fixed-Bed Column

The choice of SBox to investigate continuous adsorption of Cu²⁺ and Co²⁺ in a fixed-bed column is due to its lower cost in comparison to Cox, although Cox had higher values of in comparison to SBox.

The breakthrough curves plotted as effluent concentration-time profiles provide information about the dynamic behavior of a determined fixed-bed column adsorption from the mass transfer zone (MTZ). MTZ is the area of the bed where the adsorption of Cu²⁺ and Co²⁺ occurs. MTZ varies with , which is the time when the value of in the effluent reaches 5% of , to , which is the time when the value of in the effluent reaches 95% of . Figures 5(a) and 5(b) present the breakthrough curves obtained for adsorption of Cu²⁺ and Co²⁺ on SBox in a fixed-bed column. Table 5 presents the values of and .

(a)

(b)

The results presented in Table 5 show that the higher value of (0.554 mmol/g) for Cu²⁺ adsorption was obtained in experiment 3, while the lower value of (0.237 mmol/g) was obtained in experiment 2. For Co²⁺ adsorption, the higher and lower values of (0.224 and 0.088 mmol/g) were obtained in experiments 3 and 2, respectively. These results suggest that higher levels of initial metal concentration and lower levels of spatial time favored higher values of . Furthermore, lower levels of initial metal concentration and higher levels of spatial time disfavored higher values of . In addition, the value of for Cu²⁺ in continuous adsorption was 2.9% lower than in batch adsorption, while the value of for Co²⁺ in continuous adsorption was 65.2% lower than in batch adsorption.

The rate-limiting steps of an adsorption process are: (1) the mass transfer of the Cu²⁺ or Co²⁺ from the bulk phase to the SBox surface; (2) the diffusion of the Cu²⁺ or Co²⁺ into the SBox porous structure; and (3) the adsorption of Cu²⁺ or Co²⁺ onto the SBox surface [51]. In a mass transfer process, the mass flow is proportional to the concentration gradient difference. This is the driving force for the adsorption process. Moreover, the resistance at the liquid film interface (step 1) decreases with increasing flow rate. Therefore, it is expected that higher levels of inlet Cu²⁺ or Co²⁺ concentration and lower levels of spatial time provide a higher concentration gradient difference between the adsorbed Cu²⁺ or Co²⁺ at the interface and the Cu²⁺ or Co²⁺ in the bulk solution, as well as a smaller resistance at the liquid film, thereby increasing the value of of the adsorbent.

Higher values of for adsorption of Cu²⁺ and Co²⁺ on SBox were obtained at the center point for both metal ions, with 1.67 (experiment 7) and 2.12 cm (experiment 5), respectively. The lower values of , 0.40 cm for Cu²⁺ and 0.23 cm for Co²⁺, were obtained in experiment 1. The characteristics of the breakthrough curve are related with the equilibrium adsorption, mass transfer of Cu²⁺ or Co²⁺ from the bulk solution to the SBox surface and porous structure of the SBox as well as with fluid flow properties during continuous adsorption into the column [52]. The instantaneous equilibrium produces, under ideal conditions, a shock-like wave which moves as a sharp concentration step through the bed [53], where the entire length of bed is utilized. This suggests that, at the center point, the overall resistance to the adsorption of Cu²⁺ or Co²⁺ on SBox is reduced, favoring the adsorption. As a consequence, the application of lower levels of and lower levels of τ increased the overall resistance to the adsorption.

The 2² experimental design was employed to evaluate the effect of and on the adsorption of Cu²⁺ and Co²⁺ on SBox in continuous. The analysis of variance (ANOVA) of the experimental data was carried out. The results are shown in Table 6. It is necessary to have a significant regression for a well-adjusted model, i.e., and together with a non-significant lack of fit, i.e., and . The linear model used to fit the response yielded coefficients of determination of 0.963 for Cu²⁺ and 0.857 for Co²⁺. However, the values of for for adsorption of Cu²⁺ (0.579) and Co²⁺ (0.358) were not high as for . In addition, the models used to fit the DVs for Co²⁺and for Cu²⁺ exhibited a lack of fit, indicating that these models cannot predict the behavior of these dependent variables very well.

As can be seen in Table 6, both independent variables were statistically significant for response for Cu²⁺ with presenting a negative effect and a positive effect on Cu²⁺ adsorption. This means that the use of in a higher level and in a lower level favored a higher value of . The DV for Co²⁺ adsorption was only influenced by with this variable presenting a statistically significant positive effect. For the DV , both variables were not statistically significant for Co²⁺ adsorption, and only was statistically significant, presenting a positive effect on Cu²⁺ adsorption. The interactions between the independent variables had no statistically significant effects on any DVs ( or ) evaluated for the adsorption of both metals on SBox.

When comparing these results of adsorption in continuous with the profiles of the kinetic curves obtained for batch adsorption of Cu²⁺ and Co²⁺ on SBox (Figures 2(a) and 2(b)), it can be seen that the spatial time only influenced Cu²⁺ adsorption, which presented kinetic curves with steeper initial slopes than those for Co²⁺ adsorption. This suggests that the mass transfer of Cu²⁺ or Co²⁺ from the bulk phase to the SBox surface may have contributed more to the rate-limiting step for Cu²⁺ adsorption on SBox while Co²⁺ adsorption on SBox may be influenced by both diffusional and adsorption rates.

Table 7 presents the results when Bohart-Adams original model was used to model the experimental breakthrough curves. The Bohart-Adams original model better fitted the experimental breakthrough curves for Co²⁺ adsorption than those for Cu²⁺ adsorption, presenting higher values and smaller RMSE values. The values of estimated by the Bohart-Adams original model and those obtained experimentally presented low values of mean absolute error (MAE), between 4.2% and 1.9% for Cu²⁺ and Co²⁺ adsorption, respectively, showing that this model can predict the experimental data very well.

Figures 6(a) and 6(b) show the breakthrough curves obtained for the experiment 7 for adsorption of Cu²⁺ and Co²⁺ on SBox in continuous modeled by the Bohart-Adams original model. The values of Bohart-Adams rate constants for the metals studied were smaller for higher levels of and/or , whereas higher values of were observed for lower levels of and/or . The is a lumped parameter [51], which includes all effects of internal and external diffusion, adsorption kinetics as well as any dispersion in the bed. Then, the adsorption in continuous at the conditions evaluated in this study is influenced by all of these effects. However, for Cu²⁺ adsorption, the value at center point was closer to experiment 1, while for Co²⁺ adsorption in the experiment 4 had a value of closer to that obtained at the center point. As the levels of and in experiments 1 and 4 were opposite (Table 5), this indicates that the adsorption of these metals on SBox, in fact, presented different rate-limiting steps.

(a)

(b)

3.5. Comparison with Literature Adsorption Data for Cu² and Co²⁺ on Different Adsorbents

Table 8 presents some adsorbents reported in the literature for the removal of Co²⁺ and Cu²⁺ from spiked aqueous solutions in continuous. It is possible to compare the performance of different adsorbent materials in continuous from the values of , , and . Both the values of and affect the value of of an adsorbent material. It is possible to have a high value of using a high value of combined with a low value of or a low value of combined with a high value of . Comparing the performance of SBox adsorbent in continuous with those reported in the literature data, it should be noted that SBox showed a very good performance, mainly for Cu²⁺ removal, because it presented a medium value of , which was obtained from a lower value of combined with a medium value of . However, SBox presented a lower value of for Co²⁺ removal.

4. Conclusions

The oxidized sugarcane bagasse (SBox) and cellulose (Cox) were successfully synthesized by a cheap chemical modification method and shown to be efficient for the removal of Cu²⁺ and Co²⁺ from spiked single aqueous solutions in both batch and continuous. The adsorption of Cu²⁺ on SBox and Cox was better modeled by the pseudo-second-order kinetic model, while the adsorption of Co²⁺ on SBox and Cox was better modeled by the pseudo-first-order kinetic model. The results of equilibrium adsorption were modeled by three isotherm models and the Sips and Langmuir models closely fitted the experimental data with maximum adsorption capacities of 1.20 and 0.57 mmol/g for Cu²⁺ and 0.68 and 0.37 mmol/g for Co²⁺ adsorption on Cox and SBox, respectively. In addition, Cox and SBox exhibited excellent desorption efficiencies and re-adsorption efficiencies , allowing their reuse in further adsorption cycles. SBox was further used in fixed-bed column adsorption experiments. The Bohart–Adams original model fitted continuous adsorption data very well. The value of for Cu²⁺ (0.55 mmol/g) was closer to batch adsorption, while for Co²⁺ (0.22 mmol/g) was lower than that obtained in batch adsorption. Thus, the main results obtained in this study show that both adsorbents are interesting materials for a real application in a wastewater treatment plant, mainly SBox, which is a cheaper adsorbent in comparison to Cox.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors are grateful to Universidade Federal de Ouro Preto (UFOP), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq grant number 448346/2014‒1 and CNPq PVE grant number 400739/2014-3) and Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG grant numbers CEX‒APQ‒01764/14 and CEX‒APQ‒01287-15) for funding this research. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior ‒ Brasil (CAPES) ‒ Finance Code 001. The authors are also grateful to UFOP (for a master scholarship awarded to J. A. V. Rodrigues).

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Copyright

Copyright © 2020 Josilene Aparecida Vieira Rodrigues et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Advances in Polymer Technology

Oxidized Renewable Materials for the Removal of Cobalt(II) and Copper(II) from Aqueous Solution Using in Batch and Fixed-Bed Column Adsorption

Abstract

1. Introduction

2. Material and Methods

2.1. Material

2.2. Cellulose and Sugarcane Bagasse Preparation

2.3. Oxidation of Sugarcane Bagasse and Cellulose

2.4. FTIR Spectroscopy

2.5. Adsorption Experiments

2.5.1. Adsorption of Cu2+ and Co2+ on SBox or Cox as a Function of Solution pH

2.5.2. Adsorption of Cu2+ or Co2+ on SBox or Cox as a Function of Contact Time

2.5.3. Adsorption of Cu2+ or Co2+ on SBox or Cox as a Function of Cu2+ or Co2+ Concentration

2.6. Desorption and Reuse of SBox and Cox

2.7. Fixed-Bed Column Adsorption Experiments

2.7.1. Design of Experiments

2.8. Modeling the Experimental Data

2.8.1. Batch Kinetic Data

2.8.2. Batch Equilibrium Data

2.8.3. Analysis of the Breakthrough Curve

2.8.4. Calculation of Variation in Standard Free Energy of Adsorption

2.8.5. Regression Analysis and Error Evaluation

3. Results and Discussion

3.1. Synthesis and Characterization of the Oxidized Materials

3.2. Adsorption Studies

3.2.1. Effect of Solution pH on the Removal of Cu2+ and Co2+ by SBox and Cox

3.2.2. Effect of Contact Time on the Removal of Cu2+ and Co2+ by SBox and Cox

3.2.3. Adsorption Isotherms of Cu2+ and Co2+ on SBox and Cox

3.3. Desorption and Reuse of SBox and Cox

3.4. Monocomponent Adsorption of Cu2+and Co2+ on SBox Adsorbent in a Fixed-Bed Column

3.5. Comparison with Literature Adsorption Data for Cu2 and Co2+ on Different Adsorbents

4. Conclusions

Data Availability

Conflicts of Interest

Acknowledgments

References

Copyright

2.5.1. Adsorption of Cu²⁺ and Co²⁺ on SBox or Cox as a Function of Solution pH

2.5.2. Adsorption of Cu²⁺ or Co²⁺ on SBox or Cox as a Function of Contact Time

2.5.3. Adsorption of Cu²⁺ or Co²⁺ on SBox or Cox as a Function of Cu²⁺ or Co²⁺ Concentration

3.2.1. Effect of Solution pH on the Removal of Cu²⁺ and Co²⁺ by SBox and Cox

3.2.2. Effect of Contact Time on the Removal of Cu²⁺ and Co²⁺ by SBox and Cox

3.2.3. Adsorption Isotherms of Cu²⁺ and Co²⁺ on SBox and Cox

3.4. Monocomponent Adsorption of Cu²⁺and Co²⁺ on SBox Adsorbent in a Fixed-Bed Column

3.5. Comparison with Literature Adsorption Data for Cu² and Co²⁺ on Different Adsorbents