Lignin Removal by Adsorption to Fly Ash in Wastewater Generated by

Article pubs.acs.org/IECR

Lignin Removal by Adsorption to Fly Ash in Wastewater Generated by Mechanical Pulping Kerstin I. Andersson,*,†,‡ Marie Eriksson,‡ and Magnus Norgren† †

Department of Natural Sciences, Engineering and Mathematics, FSCN, Mid Sweden University, Sundsvall, Sweden SCA R&D Centre, Sundsvall, Sweden

‡

ABSTRACT: Stringent discharge requirements call for advanced methods of wastewater treatment to take on where biological treatment fails to succeed. Here, the adsorption potential of fly ash, an on-site available and cheap material, was tested in batch and continuous flow fixed bed experiments using bleaching effluent from an integrated mill producing mechanical pulp. Various models were fitted to the experimental data to find the best description of the adsorption system and to obtain important model parameters: the Freundlich model yielded the highest correlation and indicated that the process was favorable. The bed depth service time model suggested that the adsorption in the column setup involved more than one rate limiting step, and the Thomas and Clark models generated similar curves which satisfactorily described adsorption at short bed depth. The fly ash showed good adsorptive properties of wood derived substances: both lignin and extractives were effectively separated from the effluent.

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INTRODUCTION Biological treatment has become the most widely applied method to reduce the discharge of wood derived substances released during processing of the raw material into the final lignocellulosic-based product. Although this has proven a cost efficient method to remove oxygen consuming substances in wastewater, stringent discharge requirements demand measures to eliminate also substances which resist biological treatment. One such compound is lignin which is released from the wood during pulping and bleaching operations. Adsorption is a potent method for separation of pollutants in industrial wastewater, well-established in the pharmaceutical and chemical industry. However, the method seems rarely adopted in other areas, such as the pulp and paper industry. In order to make adsorption a competitive alternative to other available options for advanced wastewater treatment, such as membrane filtration, advanced oxidation processes, and various physicochemical treatments, a low-cost and readily available adsorbent must be sifted out. A variety of ashes have been investigated as potential low-cost adsorbents by numerous authors and have been found to possess interesting properties for the adsorption of organic pollutants.1−4 Necessary information for large-scale adsorption design can be obtained by evaluating the breakthrough characteristics of an adsorbent bed in a column setup.1,5 Before experiments are run in continuous columns, a preliminary screening of adsorbents in batch experiments is recommended. Batch experiments and the use of adsorption isotherms provide a measure of the effectiveness of adsorption for removing specific adsorbates, as well as the maximum adsorption capacity. However, the equilibrium conditions of batch experiments do not account for hydrodynamics and mass transfer occurring in a fixed-bed continuous adsorption column: the conditions at any cross section in the fixed adsorbent bed is affected by the upstream behavior.1,5 Column operations create a continuous concentration gradient in the adsorption zone as the liquid passes through the column, whereas in batch experiments, the © 2012 American Chemical Society

concentration gradient between the solid and aqueous phases decreases with time. In theory, continuous flow over a fixed bed therefore increases the adsorbent capacity by maintaining the mass transport of solute across the liquid/solid interface. In earlier publications,6,7 we have demonstrated in batch experiments the good sorptive properties of fly ash in the removal of aqueous lignin isolated from mechanical pulping effluent. The Redlich-Peterson as well as the Langmuir and Freundlich equilibrium models were found to provide adequate descriptions of the experimental data. Kinetic studies showed that equilibrium between the adsorbate in solution and on the adsorbent surface was essentially achieved after 30 min and that pseudo-second-order rate kinetics best described the experimental data. In the present study, the practical applicability of adsorption for treating process wastewater in mechanical pulping was evaluated. Bleaching effluent was treated with fly ash and the removal of wood derived organic compounds such as lignin and extractives was investigated. Batch-experiments, similar to the previous tests with aqueous lignin samples, were performed, and the fit of sorption isotherms was evaluated to find the most suitable model. Finally, fixed bed continuous column experiments were run to obtain the breakthrough characteristics in an attempt to find the most suitable adsorption model: the bed depth service time, Thomas, and Clark models were fitted to the experimental data.

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METHODS AND MATERIALS Effluent. Bleaching effluent was collected from alkaline peroxide bleaching of thermomechanical pulp (TMP) produced from Norway Spruce at the integrated SCA Ortviken mill in Sundsvall, Sweden. The bleaching effluent contributes to

Received: Revised: Accepted: Published: 3444

October 27, 2011 January 17, 2012 January 23, 2012 January 23, 2012 dx.doi.org/10.1021/ie202462z | Ind. Eng. Chem. Res. 2012, 51, 3444−3451

Industrial & Engineering Chemistry Research

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The content of extractives in aqueous samples was determined by solid phase extraction at pH 2 using 3 M Empore extraction disks (Scantec Lab) and derivatization followed by quantification using a gas chromatograph equipped with a flame ionization detector (Agilent 6890). The identified groups of extractives were fatty and resin acids, lignan, sterols, steryl esters, and triglycerides: quantification was made by internal standards. The carbohydrate content in aqueous samples was analyzed by gas chromatography and flame ionization detection (Hewlett and Packard 5890) after acid hydrolysis: the identified monomers were arabinose, xylose, mannose, galactose, and glucose and quantified by internal standards. Low molecular weight organic acids, including butyric acid, propanoic acid, formic acid, and acetic acid were analyzed in aqueous samples by More Research (Ö rnsköldsvik, Sweden) using high pressure liquid chromatography (Dionex UltiMate 3000). The analyses of inorganic compounds were performed on a TJA-IRIS-Advantage spectrometer (Thermo Jarrell Ash Corporation, Franklin, MA) using inductively coupled plasma optical emission spectrometry (ICP-OES). The aqueous samples were prepared by digestion with H2O2 and HNO3, evaporation, redissolution in deionized water, and finally filtration. Internal standards were used for quantification. Batch Adsorption Studies. The batch adsorption experiments were performed at room temperature with varying initial effluent concentration and adsorbent dose. The effects of temperature were not investigated here: a previous study6 has shown that an increase in temperature, to resemble mill-like conditions, did not significantly affect the amount of lignin adsorbed. The adsorbent was added to the effluent samples in 200 mL sample flasks mounted on a shaker at 200 rpm to keep the adsorbent suspended. After the set adsorption time of 6 h, the samples were filtered using Whatman GFA fibreglass filters to separate the adsorbent. The adsorption time was chosen based on a previous study7 which showed that with aqueous lignin samples, and using the same adsorbent, quasi-equilibrium was reached after 6 h. All samples were analyzed for lignin content; for a selection of samples, a more detailed characterization of chemical composition including COD, extractives, and carbohydrates was made. Fixed Bed Continuous Column Experiments. A Millipore glass column, i.d. 1.3 cm, with sealable adjuster tubes was used in the fixed bed continuous column experiments. Breakthrough curves were obtained in room temperature for different initial concentrations and varying bed depth. The column was packed using a weighed sample of fly ash, dissolved in deionized water, and treated in an ultrasonic bath to remove air bubbles, and then immediately poured into the glass column. The ash was allowed to settle by gravity and excess liquid was removed before the top adjuster tube was fitted. The final adsorbent bed depth was controlled by adjusting the top tube until a volume ratio of 0.75 g/cm3 was achieved. The effluent was pumped through the adsorbent bed at 2 mL/min using a HPLC pump (Waters 590) and collected at the outlet in 5 mL samples using a fraction collector. The amount and rate of adsorption was controlled by means of TOC, COD, and lignin analyses. The results of adsorption experiments may be misinterpreted by the formation of insoluble complexes between leached elements from the fly ash and lignin or by a changed absorptivity of formed soluble complexes. It has previously

approximately half of the chemical oxygen demand (COD) load and two-thirds of the lignin load in the mill’s total mixed effluent. For the batch experiments, the effluent sample was filtered through a Whatman GFA fibreglass filter to remove any suspended solid material. For the column experiments, the effluent was further treated by ultrafiltration using a Millipore laboratory Pellicon cassette system with a 300 000 NMWL PTMK000C5 filter to remove colloidal material, which otherwise could clog the adsorbent bed. Bleaching effluent samples were characterized before and after ultrafiltration by means of total organic carbon (TOC), COD, and chemical composition. The sample stability was investigated by subjecting reference samples, without added adsorbent, to stirring and filtration using the same procedure as in the batch adsorption experiments described below. Adsorbent. Fly ash from the mill’s steam producing boilers was used as adsorbent in both batch and column experiments. A previous characterization6 has shown that oxygen, calcium, and carbon dominated the ash composition with 37, 31, and 9.2 wt %, respectively. Other inorganic compounds present in the ash were potassium, sulfur, manganese, magnesium, phosphorus, aluminum, and iron, together amounting to 22 wt %. The previously measured6 mean particle diameter (D[43]) and surface area were 23 μm and 28.6 m2/g, respectively. Analyses. The TOC and COD of the samples before and after treatment were measured using Lange cuvette tests analyzed on a spectrophotometer DR2800 (Hach Lange, Sköndal, Sweden). The relative analytical precision values of TOC and COD in the relevant range reported as a standard deviation are 2 and 1.5%, respectively. Lignin in aqueous samples is normally evaluated by UV absorption and using an absorptivity coefficient to estimate the concentration. Measurements at both 205 and 280 nm are reported in the literature. The choice of wavelength is justified by (1) the occurrence of a distinct peak at the selected wavelength and (2) by attempting to minimize the contribution of adsorption by compounds, other than lignin, present in the sample. The samples investigated here exhibited a plateau at 205 nm and a distinct peak at approximately 280 nm, enabling a precise reading at this wavelength. Degradation products of carbohydrates, such as furfural, are known to also absorb at 280 nm; however, the conditions of mechanical pulping are not considered to cause degradation of carbohydrates to an extent where the degradation products may interfere with the measured UV absorption. Rådeström and Sjöström8 estimated the maximum disturbance of such compounds of the estimated lignin concentration in process water from the production of TMP to maximum 2.7% at 280 nm. The impact on UV absorption and light scattering of dissolved and colloidal extractives and suspended fiber fragments is reduced by filtration of the sample. Rådeström and Sjöström8 estimated the impact of dissolved extractives to 4% of the total absorption at 280 nm in TMP process water. The impact of pH on UV analysis was investigated by adding either diluted HCl or NaOH to samples of bleaching effluent and recording the maximum absorption as well as peak wavelength. A Cary 100 UV−vis spectrophotometer (Varian, Palo Alto, CA) and the absorptivity coefficient 0.0112 dm3/g cm, obtained from previous investigations,9 was used here to estimate the lignin concentration of aqueous samples either after adjusting pH to 7 or by calculating a change in absorption compared to a reference sample measured at pH 13. 3445

dx.doi.org/10.1021/ie202462z | Ind. Eng. Chem. Res. 2012, 51, 3444−3451


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been shown6 that inorganic compounds were leached from the adsorbent material in batch experiments. Here, the extent of leaching was investigated in a fixed bed setup by pumping deionized water at 2 mL/min through the column. Samples were collected at the outlet in 10 mL fractions: the first fraction, which during column preparation, had a very long contact time with the adsorbent was discarded. The fractions of deionized water collected at 15, 55, and 110 min were mixed with bleaching effluent and any change in UV absorption was recorded. Adsorption Modeling. The capacity of the adsorbent needs to be predicted in any large scale adsorption design: the relationship between the equilibrium concentration of the adsorbate in the aqueous and solid phases can be described by isotherm models. In a previous study7 of lignin adsorption onto fly ash, various equilibrium models are described and it was found that the Temkin (eq 1), Langmuir (eq 2), and Freundlich (eq 3) models all provided reasonable descriptions of the experimental data. Here, the same models were used to find the best fit of the experimental data of adsorption of lignin in bleaching effluent. Q eq =

Q eq =

RT ln(KTCeq) bT

The time of breakthrough is defined as the time when the outlet adsorbate concentration equals a certain fraction of the original adsorbate concentration at the inlet of the column: e.g., 50% breakthrough occurs when the outlet concentration is 50% of the inlet concentration, i.e. C/C0 = 0.5 and t = t0.5. The bed depth service time model (BDST), based on the Bohart−Adams model, assumes that the rate of sorption is controlled by the residual capacity of the adsorbent and the concentration of adsorbate.15,16 The model proposes a relationship between the bed depth and the time taken for breakthrough to occur, called service time.17,18 ⎡ ⎛ ⎤ ⎛C ⎞ Z⎞ ln⎜ 0 − 1⎟ = ln⎢exp⎜kN0 ⎟ − 1⎥ − kC0t ⎝C ⎠ ⎣ ⎝ ⎦ U⎠

where C0 (mg/dm3) is the initial sorbate concentration, C (mg/ dm3) is the aqueous adsorbate concentration at t = t, Z (cm) is the bed depth, U (cm/min) is the linear velocity, N0 (mg/dm3) is the adsorbent bed capacity, and k [dm3/(min mg)] is a rate constant. Equation 5 can be expressed linearly as t=

(1)

Q eq = KFCeq1/ n

(2) (3)

t0.5 =

Qeq is the amount of adsorbate adsorbed on the adsorbent at equilibrium, and Ceq is the aqueous adsorbate concentration at equilibrium; T is the temperature, and R is the universal gas constant. The other isotherm parameters are obtained by converting the two-parameter equations into linear expressions: qm (mg/g) signifies adsorption capacity; bT (J/mol) is related to the heat of adsorption; 1/n (dimensionless) indicates both the heterogeneity of the adsorbent and the affinity of the adsorbate; KF (L/g), KT (L/g), and KL (L/mg) are isotherm constants. The Langmuir model is further described by a dimensionless equilibrium parameter RL (eq 4), indicating the favorability of the process. More detailed descriptions of the isotherm models can be found elsewhere.10−14 RL =

1 1 + KLC0

⎞ N0 1 ⎛ C0 Z− ln⎜ − 1⎟ ⎝ ⎠ C0U kC0 C

(6)

At 50% breakthrough, i.e. C/C0 = 0.5, the last expression in eq 6 above equals zero: the plot of t at 50% breakthrough (t0.5) against Z must therefore pass through the origin and N0 can be obtained from the slope (eq 7). Finally N0 is used to solve k from the plot of t, at any breakthrough, against Z.

qmKLCeq 1 + KLCeq

(5)

N0 Z−0 C0U

(7)

The Thomas model assumes plug-flow through the column, second-order reversible reaction kinetics, and a good fit of the Langmuir model for equilibrium conditions:1,17 C 1 = C0 1 + exp[kT(q0m − C0Veff )/Q ]

(8)

where C and C0 have the same meanings as above, kT [dm3/ (min mg)] is the rate constant, q0 (mg/g) represents the maximum solid-phase concentration of the adsorbate, Q (dm3/ min) is the volumetric flow, Veff (dm3) is the throughput volume, and m (g) is the mass of adsorbent in the column. Equation 8 can be expressed linearly as kTq0m ⎛C ⎞ ln⎜ 0 − 1⎟ = − kTC0t ⎝C ⎠ Q

(4)

In large scale adsorption applications, the continuous flow fixed bed setup is a well-established design. The purpose is to reduce the concentration in the effluent so that it does not exceed a breakthrough value which is defined to appropriately fit the objectives of the treatment. The breakthrough curve, or the shape of the sorption wavefront, determines the bed depth and the operating lifespan and is affected by operating variables as well as hydrodynamics and mass transfer in the adsorbent bed Studying the adsorption behavior and breakthrough characteristics is thus a prerequisite in large scale process design and optimization. A number of models have been developed to describe the breakthrough behavior of fixed bed continuous flow adsorption columns. Here, the bed depth service time, Thomas, and Clark models were used to find the best description of experimental data and to obtain values of adsorbent capacity and sorption rate.

(9)

The model constants kT and q0 can be obtained from the slope and intercept of the plot of ln((C0/C) − 1) versus t. The Clark model uses a mass-transfer coefficient in combination with the constant n, obtained from the Freundlich isotherm, to define a breakthrough curve as1,17 ⎞1/(n − 1) ⎛ C 1 =⎜ ⎟ C0 ⎝ 1 + A exp − rt ⎠

(10)

where C and C0 have the same meanings as above. Equation 10 can be expressed linearly as ⎡⎛ C ⎞n − 1 ⎤ ln⎢⎜ 0 ⎟ − 1⎥ = − rt + ln A ⎢⎣⎝ C ⎠ ⎥⎦ 3446

(11)



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where the equation constants r and A can be obtained from the slope and intercept.

ting to more than 60% of the total carbohydrates content. The most abundant organic acid was acetic acid representing more than 95% of the total amount. Sample Stability. The lignin concentration was reduced by less than 3% in reference samples mounted on a shaker for 6 h and filtered through a glass fiber filter. Treatment with fly ash increased the sample pH, and it is therefore not likely that precipitation influenced the aqueous lignin concentration: lignin precipitates at low pH, below 2. It thus seemed as if neither stirring followed by filtration nor precipitation significantly contributed to the removal of lignin in effluent samples. Batch Experiments. The evaluation of equilibrium batch adsorption experiments showed that high removal of lignin was possible. Figure 2 shows increased lignin removal with

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RESULTS AND DISCUSSION Effects of pH on UV-absorption. pH influenced the UV absorptivity as well as the wavelength of maximum absorption. At pH between 6 and 9, the absorption was stable: above pH 10, the absorptivity increased and the peak was shifted toward longer wavelength (Figure 1). The absorptivity was affected

Figure 1. Wavelength of maximum absorption of UV radiation of bleaching effluent at different pH.

also at low pH, and the analysis was furthermore disturbed by precipitation of lignin at pH below 2. In this investigation, the sample pH was increased by the treatment; the pH of treated samples was either adjusted to 7, or the results were reported as a change in UV absorption compared to a reference sample with similar pH. Chemical Composition of Bleaching Effluent and Effects of Ultrafiltration. The effects of ultrafiltration on the chemical composition of the bleaching effluent was investigated by analyzing TOC and COD as well as the permeate and concentrate content of lignin, extractives, carbohydrates, and low molecular weight organic acids (Table 1). Ultrafiltration reduced the permeate content of lignin and

Figure 2. Lignin adsorption to fly ash with increasing adsorbent dose in miligrams per gram added adsorbent and in percent of the initial aqueous lignin concentration. Experimental setup: initial lignin concentration 0.6 g/L and adsorbent dose 0−300 g/L.

increasing adsorbent dose: 97% lignin removal was achieved at 300 g/L fly ash. The adsorbent efficiency was also increased by increasing initial lignin concentration reaching 18 mg/g under the specific experiment conditions (Figure 3).

Table 1. Chemical Composition of Bleaching Effluent and Permeate from Ultrafiltration with 300 kDa NMWLa parameter

bleaching effluent

permeate

TOC,b g/L COD,c g/L lignin, g/L extractives, g/L carbohydrates, g/L organic acids, g/L

2.7 7.8 2.0 0.16 0.5 1.6

2.5 6.6 1.5 0.03 0.5 1.6

a

Extractives: fatty- and resin acids, lignan, sterols, steryl esters, and triglycerides. Carbohydrates: arabinose, xylose, mannose, galactose, and glucose. Organic acids: butyric acid, propanoic acid, formic acid, and acetic acid. bStandard deviation: 2%. cStandard deviation: 1.5%.

Figure 3. Lignin adsorption to fly ash with increasing equilibrium aqueous lignin concentration in miligrams per gram added adsorbent and in percent of the initial aqueous lignin concentration. Experimental setup: initial lignin concentration 0.6−1.9 mg/L and adsorbent dose 50 g/L.

extractives by 25 and 79%, respectively. The dominating extractives group in the bleaching effluent and permeate was fatty- and resin acids contributing to 30 and 80%, respectively, of the total extractives amount before and after ultrafiltration: dehydroabietic acid, isopimaric acid, and linoleic acid were the main substances. Ultrafiltration did not seem to separate neither carbohydrates nor organic acids. The dominating carbohydrate monomers were mannose and glucose contribu-

As the chemical characterization of effluent samples revealed, the system under study is a complex mixture of wood derived substances. In a multicomponent system, all composing compounds compete for the available adsorption sites and will be more or less strongly bound to the adsorbent. The 3447



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Table 2. Removal of COD, Lignin, and Extractives in Batch Experiments with Varying Adsorbent Dose and Initial Concentration (C0) sample 1 sample 2 sample 3 a

adsorbent dose, g/L

C0, g/L CODa

C0, g/L lignin

C0, mg/L extractives

removal, % COD

removal, % lignin

removal, % extractives

20 50 50

2.5 2.5 5.3

0.6 0.6 1.4

48 48 90

40 52 40

47 63 50

78 85 82

Standard deviation: 1.5%.

Table 3. Correlation Coefficients (R2) and Equation Constants for the Tested Equilibrium Models

results of the investigated adsorption system with regards to lignin are shown in Figures 2 and 3. A more detailed chemical analysis of selected samples was made to investigate the extent of adsorption of substances other than lignin. All compounds which can be oxidized contribute to the total sample COD: conversion factors 1.9 and 2.7 were used to calculate the corresponding COD equivalents of lignin and extractives, respectively.19 Table 2 shows the removal of lignin and extractives in three different samples with varying initial concentration and adsorbent dose. The corresponding contribution to the overall COD removal by lignin and extractives amounted to 53−63 and 8−10%, respectively. This implied that compounds other than lignin and extractives, e.g. carbohydrates and organic acids, were not strongly adsorbed to the fly ash and only to a small extent contributed to the removal of COD. The main extractives group remaining after adsorption was fatty- and resin acids: the dominating compound was dehydroabietic resin acid. Triglycerides and steryl esters were almost completely eliminated in treated samples. Adsorption Isotherms. To describe the equilibrium adsorption behavior of lignin, the linear equations of the Langmuir, Freundlich, and Temkin models were tested against experimental data. In Figure 4, the obtained equation constants were used to express Qeq as a function of experimental Ceq.

Langmuir

R2 = 0.9691 qm = 28 (mg/g)

KL = 0.0017 (L/mg) RL = 0.55

Freundlich

R2 = 0.997 1/n = 0.53

KF = 0.5 (mg/g)(mg/L)

Temkin

R2 = 0.9711 bT = 379 (kJ/mol)

KT = 0.015 (L/mg)

obtained from a previous study6 of aqueous lignin adsorption onto fly ash. The heat of adsorption, here 379 kJ/mol and reported as the bT parameter, was lower than 1107 kJ/mol, as reported in the previous adsorption study of aqueous lignin and fly ash. Column Experiments. Breakthrough Curves. The breakthrough characteristics from column experiments give important information about adsorption behavior and adsorbent capacity. A comparison of breakthrough curves of lignin, COD, and TOC are shown in Figures 5 and 6. Both

Figure 5. Breakthrough of COD and lignin in column experiments using bleaching effluent and fly ash. Experimental setup: bleaching effluent treated by ultrafiltration with an initial COD concentration of 5.1 g/L and an initial lignin concentration of 1.0 g/L, bed depth 3.7 cm, and volumetric flow rate 2 mL/min.

Figure 4. Equilibrium amount of lignin adsorbed on the adsorbent (Qeq) at varying equilibrium lignin concentrations (Ceq) expressed by the Langmuir, Freundlich, and Temkin isotherm models. Experimental setup: initial lignin concentration 0.6−1.9 mg/L and adsorbent dose 50 g/L.

COD and TOC showed a faster breakthrough than did lignin. This was likely due to the content of carbohydrates in the sample: carbohydrates were only weakly, if at all, adsorbed to the fly ash and followed the bulk fluid in the profile. Any references to concentration hereafter refer to lignin concentration if not otherwise indicated. Figure 7 and 8 show the breakthrough curves for lignin with varying initial aqueous concentration and bed depth. As discussed previously, pH affects the absorptivity of UV radiation as well as the wavelength of maximum absorption. In Figure 7, all analyses were performed at pH 7 whereas in Figure 8, a reference sample with the same pH as treated

Although the Freundlich model provided the highest correlation coefficient (Table 3) and thus the best fit to experimental data, all three models provided acceptable correlation to experimental data. It is therefore assumed that all equation parameters may be used to describe the process favorability and adsorption capacity. The obtained RL value between 0 and 1 indicated that the sorption process was favorable, supported by the 1/n value of less than 1. The obtained qm value, signifying maximum adsorption capacity, of 28 mg/g was somewhat higher than the qm value of 13 mg/g 3448



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generated a more typically S-shaped breakthrough curve. With regards to initial concentration, Figure 8 shows that the higher the inlet concentration, the steeper the slope and the earlier the breakthrough point. This can be explained by the rapid exhaustion of available sorption sites at higher aqueous adsorbate concentration. Also, a higher surface coverage, resulting from the higher initial concentration of sorbate, likely increased the activation energy, thereby making it more difficult for the remaining molecules to adsorb onto the surface. High initial lignin concentration yielded an S-shaped curve, whereas, at low initial concentration, the shape of the breakthrough curve indicated that the column was more slowly exhausted. An S-shaped curve is considered optimal when the aim of adsorption is to maintain the outlet concentration at very low levels, i.e. the adsorbent material is quickly and efficiently exhausted. A flat curve means that the adsorbent capacity is only slowly exhausted and a larger effluent volume can be treated, albeit at higher outlet concentration. The column thus needs to be designed in such a way that the treatment corresponds to adsorbent availability and outlet concentration requirements. In a multicomponent system, such as bleaching effluent, the available adsorption sites on the adsorbent will initially be occupied by both strongly and weakly bound substances. With increasing time, substances which strongly adsorb will replace weakly adsorbed substances which in turn move ahead with the bulk fluid and occupy active sites further along in the fixed adsorbent bed. This results in an increased local concentration of the weakly adsorbed compounds within the fixed bed. As the adsorption zone is moved forward, a temporary increase in outlet concentration of solutes can occur which may exceed the inlet concentration. The bleaching effluent used in these experiments was a multicomponent system in terms of compounds present as well as the complexity of these substances: there are also variations within the different compound groups. For example, lignin was defined as UV absorbing substances and quantified by means of a UV absorptivity coefficient. However, lignin is not a uniform material but in fact a mixture of substances with varying chemical characteristics, UV absorptivity, and likely also somewhat varying propensity to adhere to the adsorption sites. Furthermore, by exposure to the adsorbent material, the chemical characteristics and absorptivity of the lignin substances may be changed. It is thus possible that the outlet lignin composition, and thereby the absorptivity, is different from the inlet. Nevertheless, all curves exhibited the typical characteristics of a breakthrough curve, were similar to the COD and TOC breakthrough curves, and all approached a concentration ratio C/C0 = 1. Models. For the evaluation of breakthrough models, the experimental data presented in Figure 7 was used. The times for 50 and 10% breakthrough, t0.5 and t0.1, at varying bed depth were used in the BDST model which was found to provide a reasonable fit to the experimental data with R2 values above 0.998 (Figure 9). However, the plot of t0.5 did not pass through the origin, indicating that the transport of adsorbate from the aqueous solution onto the adsorbent was complex and involved more than one rate limiting step. The lignin adsorption capacity, N0, was obtained from the slope of the t0.5 plot according to eq 7 and amounted to 7266 mg/dm3 bed volume, corresponding to adsorption of 9.6 mg/g fly ash. The value of k, obtained from the intercept of the 10% breakthrough curve according to eq 6, amounted to 0.00054 dm3/(min mg).

Figure 6. Breakthrough of TOC and lignin in column experiments using bleaching effluent and fly ash. Experimental setup: bleaching effluent treated by ultrafiltration with an initial TOC concentration of 2.5 g/L and an initial lignin concentration of 1.5 g/L, bed depth 4.5 cm, and volumetric flow rate 2 mL/min.

Figure 7. Breakthrough curve for UV-absorption in column experiments with bleaching effluent and fly ash. Experimental setup: bleaching effluent treated by ultrafiltration with an initial lignin concentration of 1.5 g/L; bed depth 3, 4, and 7 cm; volumetric flow rate 2 mL/min; and all samples adjusted to pH 7 before analysis.

Figure 8. Breakthrough curve for UV-absorption in column experiments with fly ash and bleaching effluent. Experimental setup: bleaching effluent treated by ultrafiltration with an initial lignin concentration of 1.5, 0.9, and 0.5 g/L; bed depth 3 cm; volumetric flow rate 2 mL/min; and reference sample at pH 13.

samples, was used to calculate the concentration ratio. As can be seen from the breakthrough curves, the outlet concentration increased rapidly as the sorption zone reached the top of the adsorbent bed. The time required to reach breakthrough increased with increasing bed depth, and also, the shape of the curves were different. At higher bed depth, the breakthrough curve tended to be more gradual whereas the shorter beds 3449



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increasing bed depth. However, when using only the experimental data up to C/C0 = 0.7, a much better fit to the 7 cm bed depth column data was achieved (not shown here) and the obtained value of q0 indicated a lesser decrease in adsorbent capacity at 11.3 mg/g. The adsorbent capacity obtained here by adaptation of the BDST and Thomas models (Table 4) were of the same magnitude as those of previous investigations. Srivastava et al.1 studied phenol adsorption onto bagasse fly ash in a fixed bed system with a bed depth between 40 and 90 cm and an initial concentration of 100 mg/L: the calculated N0 from t0.5 amounted to 2686 mg/dm3, and q0 increased with increasing bed depth and varied between 0.52 and 3.21 mg/g. Singh et al.17 studied the fixed bed adsorptive removal of furfural by activated carbon and N0 calculated from the 50% breakthrough curve is reported as 7657 mg/g and q0 of 1.09−1.77 mg/g for experiments with an initial concentration of 100 mg/L and bed depth between 15 and 60 cm. Leaching. Leached inorganic elements were analyzed in fractions of deionized water pumped through the column. The most abundant compound was calcium with a concentration of 1400 mg/L in the first fraction and decreasing to 360 mg/L after 105 min corresponding to 210 mL throughput volume. The sulfur concentration was quickly reduced from 370 to 1.5 mg/L after 65 min. The concentration profile of potassium ranged from 30 to below 10 mg/L after 50 min. The measured concentrations of barium, sodium, zinc, and silicon were all below 10 mg/L and manganese, magnesium, and boron below 1 mg/L. The fractions collected at 15, 55, and 110 min were mixed with bleaching effluent; the UV absorption was measured and compared to a sample mixed with deionized water. The leachate fractions increased the measured UV absorption by 0.7, 3, and 17%, respectively, possibly due to the formation of insoluble complexes which (1) increased the light scattering or (2) changed the UV absorptivity. Because the results showed an increased UV absorption and thus an overestimated lignin concentration, it was assumed that any type of reaction between leached elements and lignin did not contribute to an overestimation of the potential use of adsorption in this particular application.

Figure 9. Times for 50 and 10% breakthrough (t0.5 and t0.1) at varying bed depth. Experimental setup: bleaching effluent treated by ultrafiltration with an initial lignin concentration of 1.5 g/L; bed depth 3, 4, and 7 cm; volumetric flow rate 2 mL/min; and all samples adjusted to pH 7 before analysis.

The Thomas and Clark equations provided almost identical curves when applied to experimental data and generated acceptable models for the 3 cm bed depth column data (Figure 10). However, both models deviated from experimental data at increasing bed depth. As can be seen in Figure 7, the second half of the breakthrough curves differ slightly: the 3 cm column

Figure 10. Experimental and calculated values of C/C0 using the Thomas and Clark models for fixed bed adsorption of lignin in bleaching effluent. Experimental setup: bleaching effluent treated by ultrafiltration with initial lignin concentration 1.5 g/L; bed depth 3, 4, and 7 cm; volumetric flow rate 0.002 dm3/min; and all samples adjusted to pH 7 before analysis by UV absorption.

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CONCLUSION Adsorption using fly ash is an effective way to remove organic substances in wastewater generated by bleaching of mechanical pulp. Lignin and extractives were strongly adsorbed whereas carbohydrates seemed only weakly bound to the adsorbent material. The Freundlich, Langmuir, and Temkin isotherms all provided acceptable models of the experimental data. The reaction was, as indicated by the Freundlich and Langmuir models, favorable and more than 90% removal of lignin was achieved in batch experiments. The maximum equilibrium adsorbent capacity, qm, was estimated to 28 mg/g.

curve coheres to the typical S-shape whereas the rate of adsorption in the 7 cm column curve drops slightly after C/C0 = 0.7, generating a less steep curve. The shape of the curve and the poor fit of the Thomas model at increasing bed depth of course influenced the obtained values of q0, signifying adsorbent capacity. As such, it seemed as if the adsorption capacity of the column decreases from 13.3 to 9.8 mg/g with

Table 4. Correlation Coefficients (R2) and Equation Constants for the Thomas and Clark Modelsa experimental setup 3

a

Thomas model 3

2

C0 (g/dm )

Z (cm)

Q (dm /min)

R

1.3 1.3 1.3

3 5 7

0.002 0.002 0.002

0.9079 0.9458 0.9383

3

Clark model 2

kT (dm /min mg)

q0 (mg/g)

R

0.0002 0.00009 0.00004

13.3 12.9 9.8

0.9075 0.9417 0.9405

R

A

0.26 0.13 0.05

45 21 3.2

Notations: initial lignin concentration (C0), bed depth (Z), volumetric flow (Q). 3450



Article

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The bed depth service time (BDST), Thomas, and Clark models were tested against breakthrough curves generated by column experiments and were all found to provide acceptable descriptions of the adsorption at short bed depth. The BDST and Thomas models provided estimates of the adsorbent capacity, with regards to lignin, of between 9.6 and 13 mg/g.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Lignin Removal by Adsorption to Fly Ash in Wastewater Generated by

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