Thermal and thermal-acid treated sewage sludge for the removal of dye reactive Red 120: Characteristics, kinetics, isotherms, thermodynamics and response surface methodology design

Accepted Manuscript Title: Thermal and Thermal-acid treated sewage sludge for the removal of dye reactive Red 120: characteristics, kinetics, isotherms, thermodynamics and Response Surface Methodology design Authors: I.C. Pereira, K.Q. Carvalho, F.H. Passig, R.C. Ferreira, R.C.P. Rizzo-Domingues, M.I. Hoppen, G. Macioski, A. Nagalli, Felipe Perretto PII: DOI: Reference:

S2213-3437(18)30669-9 https://doi.org/10.1016/j.jece.2018.10.060 JECE 2746

To appear in: Received date: Revised date: Accepted date:

6-7-2018 24-10-2018 26-10-2018

Please cite this article as: Pereira IC, Carvalho KQ, Passig FH, Ferreira RC, Rizzo-Domingues RCP, Hoppen MI, Macioski G, Nagalli A, Perretto F, Thermal and Thermal-acid treated sewage sludge for the removal of dye reactive Red 120: characteristics, kinetics, isotherms, thermodynamics and Response Surface Methodology design, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.10.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Thermal and Thermal-acid treated sewage sludge for the removal of dye reactive Red 120: characteristics, kinetics, isotherms, thermodynamics and Response Surface Methodology design

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Dominguese; M. I. Hoppenf; G. Macioskig; A. Nagallih; Felipe Perrettoi

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I. C. Pereiraa; K. Q. Carvalho2b; F. H. Passigc; R. C. Ferreirad; R. C. P. Rizzo-

The Federal University of Technology – Paraná (UTFPR) - Civil Engineering

Graduate Program. Deputado Heitor de Alencar Furtado St., 5000, Ecoville, Postal

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Code: 81280-340. Curitiba, Paraná, Brazil. Phone number: +55 (41) 3279-4500. E-

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The Federal University of Technology – Paraná (UTFPR) – Civil Construction

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2b,g,h,i

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mail: [email protected]

Academic Department. Deputado Heitor de Alencar Furtado St., 5000, Ecoville,

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Postal Code: 81280-340. Curitiba, Paraná, Brazil. Phone number: +55 (41) 32794500. E-mail: [email protected]; [email protected]; [email protected];

The Federal University of Technology – Paraná (UTFPR) – Chemistry and Biology

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c,e

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[email protected]

Academic Department. Deputado Heitor de Alencar Furtado St., 5000, Ecoville,

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Postal Code: 81280-340. Curitiba, Paraná, Brazil. Phone number: +55 (41) 32794500. E-mail: [email protected]; [email protected] d

State University of Maringá (UEM) - Department of Chemical Engineering. Colombo

Ave., 5790, Jd. Universitário, Postal Code: 87020-900. Maringá, Paraná, Brazil. Phone number: +55 (44) 3011-4778. E-mail: [email protected]

2

f

The Federal University of Technology – Paraná (UTFPR). Environmental Sciences

and Technology Graduate Program. Deputado Heitor de Alencar Furtado St., 5000, Ecoville, Postal Code: 81280-340. Curitiba, Paraná, Brazil. Phone number: +55 (41)

author: [email protected]

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2Corresponding

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3279-4500. E-mail: [email protected]

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Highlights:

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Graphical abstract

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 Mesoporous activated carbon was produced from anaerobic sewage sludge.  Surface area of pyrolyzed sludge (PS) increased with thermal-acid treatment (FS).  The optimum conditions were temperature 60 °C and pH 3.5 and 5.4 for PS and FS, respectively.

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 Predicted regression models were experimentally validated with optimal pairs pH/temperature.

Abstract: Sludge from wastewater treatment plant was used as low-cost adsorbent

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to remove the dye Reactive Red 120 (RR 120) from an aqueous solution. Adsorbents were prepared through thermal (pyrolysis) and chemical treatment (functionalization

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with HNO3) of sewage sludge. Adsorbents were characterized through physicalchemical and textural analyses. The mesoporous nature of pyrolyzed sludge (PS)

and functionalized slugde (FS) influenced the adsorption of RR 120. Moreover,

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adsorbents surface was rich mainly in carboxylic groups. Batch kinetic experiments

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were conducted on the statistical design elaborated with central composite rotational

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design under different pH (3.5 to 11.5) and temperature (30 °C to 60 °C) to achieve

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the best operational conditions. The response surface methodology (RSM) indicated optimal dye removal efficiency above 94% and 98% at pH and temperature pairs of

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3.5/60 °C and 5.4/60 °C for PS and FS, respectively. Kinetics data revealed that

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pseudo-second-order model better described the adsorption with 2.46 mg g-1 at pH 3.5 for PF and 2.61 mg g-1 at pH 5.4 for FS. Langmuir isotherm model was the best

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fitted to the adsorption equilibrium data with monolayer maximum adsorption capacities of 14.69 mg g-1 for PS and 46.81 mg g-1 for FS at optimal conditions. The positive ΔH° results indicated an endothermic process (4.73 kJ mol-1 and 8.02 kJ molThus, it was concluded that the sewage sludge can be used as an alternative low-

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cost adsorbent in the removal of RR 120.

Keywords: Adsorption; Pyrolysis; Chemical-Treatment; Kinetics; Isotherms

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Nomenclature Section initial concentration of RR 120 (mg L-1)

Cf

remaining concentration (mg L-1) at time t

Ce

concentration of adsorbate at the equilibrium (mg L-1)

DP

average pore diameter (nm)

k1

constant rate of pseudo-first order adsorption (min-1)

k2

constant rate of pseudo-second order adsorption (g mg-1 min-1)

kL

Langmuir adsorption constant (L mg-1) (adsorption strength)

kF

Freundlich adsorption constant (mg g-1) (L mg-1)1/n (adsorption capacity)

KD

equilibrium constant

m

mass of adsorbents (g)

nF

Freundlich exponent

n

dimensionless heterogeneity factor

qe

adsorption capacity of adsorbate at equilibrium (isotherm) (mg g-1)

qm

maximum adsorption capacity (mg g-1)

qt

adsorption capacity of adsorbate (mg g-1) at time t (min)

R

gas constant (8314 kJ mol-1 K-1)

R2

determination coefficient

RL

dimensionless constant separator factor or equilibrium parameter

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SBET BET surface area (m² g-1) temperature (K)

V

volume of solution (L)

VT

total pore volume (cm³ g-1)

Vα

microporous volume (cm3 g-1)

qe

normalized standard deviation (%)

∆G°

Gibbs free energy (kJ mol-1)

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T

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∆H°

enthalpy change (kJ mol-1)

∆S°

entropy change (J mol-1 K-1)

1. Introduction

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Sewage sludge is an inevitable by-product of municipal wastewater treatment plants, which is produced in large volumes and contains heavy metals, organic micro-pollutants and pathogens [1,2]. Therefore, if no proper management and treatment occurs, it represents possible risk to the environment and to human health

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[3]. In recent years, sludge production from human activities has increased significantly due to industrialization, urbanization and the necessity of complying with

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legal regulations of wastewater treatment [4].

In Brazil, the generation of this residue is estimated in 150 to 220 million tons per year dry mass (85% water content), considering that only 40% of the urban

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population has access to proper wastewater treatment [5]. Regardless of

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composting, incineration and use in agriculture [2], others applications have been

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explored in alternative to sludge disposal in landfills such as the preparation of

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activated carbons from the sewage sludge as a precursor in adsorption of dyes [6,7] and other hazardous materials from different effluents [8,9].

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Sewage sludge [10,11], agricultural wastes [12], sawdust [13], sugarcane

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bagasse [14], textile sludge [4,15], chitosan [16], among others [17,18] have been studied as alternative to high-cost activated carbons, which are well-known due to

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their features of good porosity with high specific surface area, high adsorption capacity and stability, and good potential of regeneration [8]. However there is a lack of studies about optimization of physical-chemical processes with sewage sludge as

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precursor, contributing to the novelty and originality features of this study. Some of these alternative adsorbents have been applied in the adsorption of

several compounds, among them, the dyes. Annually, more than 700.000 tons of dyes used in industrial processes are produced, especially in textile industries [19], which are responsible for the consumption of 67% of the produced dyes [20].

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In this industry, preparation, dyeing and finishing stages are responsible for the highest production and discharge of effluents containing a variety of complex and hazardous chemicals [21]. As a result, a significant aesthetic problem is generated due to disposal in water bodies and wastewater treatment plants; also, serious

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disturbances to aquatic fauna and flora might occur with inadequate treatment [22]. Moreover, several dyes are toxic, mutagenic or carcinogenic and can cause damage

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to human health [3,23]. The RR 120 is one of the frequently used dyes in textile

industries, with poor biodegradability [24] and resistant to natural biodegradation, due to the aromatic rings in its structure [25].

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Several physical, chemical and/or biological techniques have been applied to

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treat effluents containing dyes, including eletrocoagulation [26], biosorption [27],

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Fenton’s reagent [28,29], H2O2/UV radiation [28], ozonization [30,31], membrane

activated carbon [34,35].

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separation [31], activated sludge [31,32], UASB-type reactors [33] or adsorption with

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Although their aforementioned applications in removing dyes, some of them

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present disadvantages including high capital and operational costs, requirement of area, possibility of clogging, replacement of structures, disposal of the residues left,

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sensitivity and inhibition by influent characteristics and climatic variations, causing an inhibition by the industries in implanting these technologies [27,31]. These factors encouraged research on the adsorption process mainly with

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commercial activated carbons, the most used adsorbents, but with limited application due to their high costs [31]. Thus, the search for lower costs and similar or higher efficiencies has increased the application of alternative materials as adsorbents. The removal of RR 120 can be influenced by several factors, such as pH, temperature, contact time, among others [36]. Thus, it is important to study the

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effects of these factors in the process by using a statistical technique elaborated with central composite rotational design (CCRD) under different conditions. As a component of the CCRD, the Response Surface Methodology (RSM) tool is used to investigate the complex interaction of two or more factors to develop and optimize

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the process [34]. In this technique, fewer experimental assays are needed as compared with the

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study of one variable at a time. RSM answers the question of how to select the levels

for the applied factors to obtain the desirable, smallest or largest, a value of the response function in a reduced number of experiments [37].

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In the present study, RSM was used to verify the combination of the optimal

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pair of factors (pH and temperature) to the RR 120 dye removal efficiency and then

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validate the regression models generated by the statistical software.

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Considering the above-mentioned facts, the objective of this study was to verify and optimize the adsorption capacity of sewage sludge, thermally (PS) and

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chemically (FS) treated, as adsorbents in the removal of reactive dye Red 120

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(RR 120) from aqueous solution under different pH and temperature conditions. Subsequently, the experimental results were analyzed using kinetic, equilibrium and

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thermodynamic models.

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2. Material and methods

2.1 Materials Sewage sludge (SS) was collected from an upflow anaerobic sludge blanket (UASB-type) reactor of a wastewater treatment plant of Curitiba, Paraná State, Brazil. The analytical grade chemicals, such as HCl, NaOH and Reactive Red 120

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(RR 120) dye (CAS number 61951-82-4) were purchased from Sigma-Aldrich Co., USA. RR 120 has molecular formula C44H24Cl2N14O20S6Na6 (Figure 1) and molar weight of 1469.98 g mol-1. Figure 1 presents the molecular structure and size of

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RR 120, which was determined using Avogadro software (version 1.2.0)

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Figure 1 near here

2.2 Methods 2.2.1 Adsorbent Preparation

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Samples of the sludge were dried in an oven at 105 (5) °C for approximately

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24 h. Part of the dry sludge was subjected to thermal and chemical treatments to

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obtain the modified adsorbents whereas the untreated sludge (SS) was used as the

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control adsorbent.

Fractions with diameters of between 0.075 and 0.150 mm were selected for

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the experiments. Procedures for the thermal and chemical treatments were based on

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the methodologies described by Vasques et al. [38]. For the thermal treatment, approximately 100 g of SS were heated at 200 °C

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for 1 h and then pyrolyzed at 500 °C for 1 h under inert atmosphere of nitrogen (gas flow of 2 mL min-1) in a muffle furnace. The obtained adsorbent is henceforth mentioned as pyrolyzed sludge (PS). For the chemical treatment, 250 g of pyrolyzed

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material were selected. For that, 1 g of PS was placed in 125 mL Erlenmeyer flasks with 50 mL of a 0.1 M HNO3 (nitric acid) solution and kept in an orbital shaker set at 115 rpm for 3 h at 25 °C. However, the adsorbents were prepared fresh. The resulting product was filtered through 0.45 m membrane filters and placed in the

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oven at 105 °C until a constant weight was obtained. The obtained adsorbent is henceforth mentioned as functionalized sludge (FS). Posteriorly, the adsorbents were finely ground according to ASTM C136/C136M–14 [39] and properly stored for further use.

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2.2.2 Adsorbent characterization

The contents of moisture, volatile material and ash of the adsorbents were

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determined in triplicate according to ASTM-D2867 [40], ASTM-D5832-98 [41] and ASTM-D2866-94 [42], respectively.

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The pH of the adsorbent materials was determined according to procedures

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described in ASTM-D3838-80 [43]. The point of zero charge (pHPZC) was estimated

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through an adapted version of the batch equilibrium method described by Babić et al.

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[44]. The pHPZC is assigned to the point where ΔpH (pHfinal-pHinitial) = 0. The textural properties of the adsorbents were characterized by N 2 adsorption

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and desorption isotherms at -196,15 °C (77 K) using ASAP 2020 Micromeritics equipment with liquid N2, following the methodology described by Rodriguez-Reinoso

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et al. [45]. The BET surface area (SBET) was determined from the linear fit of the

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equation of Brunauer-Emmett-Teller (BET) in the range of relative pressure (p/p 0) from 0.010 to 0.100. The total pore volume (VT) corresponds to the maximum amount of N2 adsorbed at p/p0 = 0.95. The mesopore volume (VM) was calculated as the

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difference between the total volume (VT) and the micropore volume (Vα). The average pore diameter (DP) was determined by the ratio 4VT/SBET and pore size distribution from the Barrett-Joyner-Halenda method (BJH) [46]. The heavy metals concentrations were detected by atomic absorption spectrometer (SavantAA

AAS,

GBC Scientific equipment, Australia). Each

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experimental test was detected three times and the results were shown as mean values. The main elements content are analyzed by energy dispersive X-ray spectrometer (EDS). The morphologies of the materials were examined by scanning electron

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microscopy (SEM) using Zeiss, model EVO/MAI 15 after gold metallization. The Xray diffraction patterns of the adsorbents after being dispersed in a 200 μm sift were using

Shimadzu

diffractometer,

model

XRD-7000,

operating

with

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obtained

Cu radiation source ( = 1.540598 Å), with diffraction angle 2Ө ranging from 5 to 90°, sampling pitch of 0.02° min-1 with a step of 2° min-1. The crystallographic

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compounds were indexed according to the Crystallography Open Database (COD)

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2014 database, and a quantitative approximation was performed by the fitted profile

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of the diffractograms.

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Thermogravimetric analyses of the grains of the materials in their natural size were carried out in thermic analyzer, BP Engineering, model RB-3000-20, under N2

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flow of 100 mL min-1, heating rate of 12.5 °C min-1 from 25 to 1000 °C.

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The main functional groups present on the surface of the adsorbents were identified by Fourier transform infrared spectroscopy. Spectra were obtained using a

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spectrometer, FTIR Varian, model 640-IR, with a resolution of 4 cm-1 and acquisition rate of 55 scans min-1 in the range between 4000 and 400 cm−1. KBr pellets were prepared with 1.0% of sample. The acid properties of the adsorbents surfaces were

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determined by the method of Boehm [47].

2.2.3 Determination of RR 120 concentration in liquid samples A UV-vis spectrophotometer (Hach, DR 5000) was used to determine the dissolved dye RR 120 concentrations. Analytical curves were built to determine the

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final concentration in solution after the adsorption experiments, measured at the longest adsorption wavelength for this dye with λmax = 511 nm (pH 3.5; 4.7; 5.4 and 7.5), λmax = 476 nm (pH 11.5) and λmax = 477 nm (pH 10.7). UV-Vis spectrums are shown in Supplementary Material 1 (S.1).

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The verification and validation of the data experimental can be reliably obtained by UV-vis method, simpler than HPLC method. However, the HPLC plays

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an important role to evaluate and validate the performance of the results, since it is

considered a more precise and sensitive methodology. Thus, comparative investigation between these quantification methods may provide greater reliability to

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the experimental results.

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2.2.4 Statistical Experimental Planning

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The selected parameters (optimal pH and temperature) in this study were investigated using central composite rotational design method (CCRD) due to its

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suitability for quadratic surface fitting, effective parameters optimization with a minimum number of experiments, and interaction analysis between these

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parameters, as previously reported by Kaçan and Kütahyalı [48].

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This method requires factorial runs (2k), axial runs (2k) and center runs (3), where k is the number of parameters. Center runs include 11 replications, which are performed by setting all factors at their midpoints to estimate the residual error for

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each adsorbent. Therefore, the number of batch kinetics experiments required for each absorbent was 11 (N = 2k + 2k = nc= 22 + 2 x 2 + 3 = 11). Commercial software Statistica was used to carry out the modelling and planning. Table 1 presents the range and levels of the independent numerical variables in terms of actual and coded values. The variables studied were pH: 3.5-11.5 and

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temperature: 30-60 °C. The resulting data were regressed to derive a suitable equation for each response. All variable parameters and their interactions were considered in a model to obtain the greatest efficiency on the removal of dye RR 120

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for each adsorbent (Table 1).

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Table 1 near here

2.2.5 Batch adsorption studies

For the kinetics studies, a graph of dye concentration versus absorbance on

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different concentrations (2, 4, 6, 8, 10, 12, 15, 18, 20, 22 and 25 mg L-1) was

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prepared from a stock solution of 25 mg L-1 of reactive dye RR 120 in deionized

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water.

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The pH of each solution was adjusted to the desired value (described in 2.4 Statistical Experimental Planning), according to the pH values of the statistical

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planning, with HCl 0.1 mol L-1 or NaOH 0.1 mol L-1 solutions.

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Based on the statistical planning, kinetics adsorption studies were carried out in the orbital shaker at 150 rpm, where 0.2 g of each absorbent were added to 20 mL

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of RR 120 dye solution (25 mg L-1) in erlenmeyers flasks. Aliquots (10 mL) of each flask were collected at predetermined time intervals (1, 3, 5, 10, 15, 30, 45, 60, 90, 120, 180, 240, 300, 360 at 480 minutes) and filtered using 0.45 m membrane filters.

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RR 120 concentrations were determined on a UV-vis spectrophotometer (Hach, 5000). All the experiments were performed in triplicates. The adsorption capacity of RR120 at time t (qt) was calculated using Equation 1, shown in Supplementary Material 2 (S.2). The percentage of dye removed from the solution was also calculated.

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For the adsorption isotherms, a graph of dye concentration versus absorbance on different concentrations (15, 25, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900 and 1000 mg L-1) was prepared from a stock solution of 1000 mg L-1 of reactive dye RR 120 in deionized water. In these studies, 0.2 g of adsorbent were added to 20 mL

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of RR 120 solutions in the mentioned concentrations in erlenmeyers flasks for equilibrium tests.

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The samples were placed under constant stirring at 150 rpm in the orbital shaker for 3 h to guarantee the adsorption equilibrium. Different temperature and pH values were tested for each adsorbent as previously determined through the

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statistical desirability parameters. The samples were filtered using 0.45 m

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membrane filters and 10 mL were collected from the samples to quantify the

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remaining dye concentration. The adsorption capacity of dye qe (mg g-1) was

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calculated through the mass balance relation (Eq. 1).

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Non-linear equations of kinetic models of pseudo-first order (Eq. 2) and pseudo-second order (Eq. 3), and isotherm models of Langmuir (Eq. 4) and

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Freundlich (Eq. 5) were fitted to the experimental data using a commercial software Origin to assess the dynamics, and adsorption capacity. The model equations are

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shown in Supplementary Material 2 (S.2). The effect of temperature in the RR 120 adsorption onto adsorbents was

evaluated through thermodynamic studies (Eq. 6 – 9). Aliquots of 20 mL of RR 120

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solution of different concentrations (equal to the adsorption isotherms) were placed in contact with 0.2 g of each adsorbent in erlenmeyers flasks and stirred for 3 h using the orbital shaker at temperatures of 25, 45 and 55 °C, under different pH values, previously obtained through the statistical desirability parameters.

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After the equilibrium time, 10 mL of each sample were filtered in 0.45 m membrane filters, and the remaining RR 120 concentrations were determined by spectrophotometry. The adsorbed maximum amount was calculated from Eq. (1). Thermodynamic parameters such as Gibbs free energy (∆G°), enthalpy change (∆H°)

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and entropy change (∆S°) were calculated.

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3. Results and discussion

3.1. Adsorbents characterization

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The data on the physical-chemical, pH, pHPZC, heavy metal content, proximate

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analysis, method of Boehm and textural properties of SS, PS and FS are presented

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Table 2 near here

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in Table 2.

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Great amounts of inorganic content (43.5 ± 0.3% and 41.6 ± 1.4%) were obtained from FS and PS adsorbents, which influenced their pH and, consequently,

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pHPZC. The percentage of ash content from SS (24.8 ± 0.6%) increased significantly by 75% and 67% with pyrolysis at 500 °C and chemical treatment with HNO3, respectively. This may indicate that the majority of inorganic constituents were

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concentrated and retained in the adsorbents during both processes. Similar values were verified by Fonts et al. [49], who obtained from 39.9 to 52.0% of ash for samples of raw sewage sludge. The BET surface area and total pore volume were still low due to high ash content and low carbonization temperature (500 °C) used in the pyrolysis (Table 2).

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Despite the lower values, compared to other adsorbents reported in the literature, the BET surface area increased significantly with thermal and chemical treatment. According to Smith et al. [50], one of the reasons for the low values of BET surface area is the high level of inorganic material present in some adsorbents.

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The pH value of the raw sewage sludge was near to neutral (6.3). Pyrolysis of the sewage sludge led to alkaline PS adsorbent with pH value of 8.2, as also noted

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by Jin et al. [10] who obtained 9.7 in biochar derived from municipal sewage sludge (pH of 7.2) pyrolyzed at 500 °C.

The chemical treatment with nitric acid resulted in acid FS adsorbent with pH

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of 3.4 (Table 2), as also reported by Sonai et al. [51] who verified pH reduction from

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7.7 to 3.8 with 0.1 M sulphuric acid (H2SO4) in sludge samples from biological

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treatment of textile effluents. The results obtained indicate that pH plays an important

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role in the adsorption of RR 120 for both the PS and FS. It is possible to observe that dye adsorption is favored under more acid medium, mainly for FS (Table 2).

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The dye used in this study has sulfonate ions (R – SO3-), which may allow

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adsorption on positively-charged adsorbents according to Netpradit et al. [52]. Sonai et al. [51] stated that the adsorbent surface has positive charges at pH < pHPZC,

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favoring the adsorption of anions, while the adsorption of cations is favored at pH > pHPZC due to the negative charges on the surface of the material. The pHPZC of PS and FS were 7.4 and 4.8, respectively, indicating that, above

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these values of pHPZC, these adsorbents will be negatively charged. As a result, their capacity of adsorption can be reduced, since there will be a repulsion of charges between the adsorbent and RR 120. Sonai et al. [51] corroborated that the chemically modified adsorbent (acid) tends to increase the functional groups and decrease the pHPZC, as also verified in this research. In this study the better

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adsorption of the anionic dye at pH 5.4, higher than pHPZC 4.8, may be related to the proximity between these pH values and to the possible neutrality of the adsorbent in this pH range (4.8-5.4), which favored the adsorption. The structure of RR 120 has six sulfonic groups (R – SO3-), thus with a greater

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amount of negative charges, there is a higher ionic strength to be attracted by the positive charges of the adsorbent. Decreasing of pHPZC was noted by Sonai et al. [51]

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in samples of textile sludge adsorbents treated thermally and chemically with 0.1 M sulfuric acid (H2SO4) (4.0-4.5).

Through the method of Boehm, it was possible to quantify the presence of acid

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functional groups in the surface of the adsorbents. It was verified that the most

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carboxylic, phenolic and lactonic (Table 2).

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significant presence is linked to oxygen-containing functional groups, such as

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Similar characteristics between the adsorbents were noted with part of their surface composed also by phenolic and lactonic groups. However, FS showed higher

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total acid groups than PS probably due to the chemical treatment, which promotes

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insertion or increase of acid groups under the surface of the adsorbent. The identification of the functional groups by this method corroborated the

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results obtained in the FTIR, since the band with greater representativeness indicated the presence of groups related to bands involving carboxyl. The results also explain the increase of the ash content for the FS sample, since the weight loss

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during the test was evaluating the rich oxygen acid groups on the surface of the sludge after the treatment process. The contents of oxygen-containing functional groups herein were lower to those obtained in previous studies for sewage sludge ([53] of 2.54 mEq g-1).

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3.2 Textural and morphological properties The N2 adsorption and desorption isotherms of PS and FS adsorbents can be classified according to the International Union of Pure and Applied Chemistry (IUPAC) as Type II and hysteresis H3. The type II isotherm is the normal form

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obtained in non-porous or meso/macroporous adsorbents, and is not restricted when adsorbed in monolayer or multilayer.

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According to Barret, Joyner and Halenda (BJH) method, based on the

desorption curve [54], the distribution of pore volume and pore size in the evaluated

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adsorbent materials are demonstrated in Figure 2.

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Figure 2 near here

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The average pore diameter (DP) for PS and FS were 26 nm and 17 nm, respectively, corroborating their mesoporous characteristics in accordance to the

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IUPAC classification (2-50 nm) [55]. The SBET for SS was 4.6 ± 0.2 m² g-1, which

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under pyrolysis (thermal) followed by chemical treatment with nitric acid promoted an increase to 35.7 ± 0.3 and 81.0 ± 0.2 m² g-1 for PS and FS, respectively (Table 2).

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The values achieved in this study are in accordance with the wide range reported in literature for SBET of sewage sludge as adsorbent, non-activated and/or physically and chemically activated. This is corroborated by the studies reported by

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Jin et al. [10] and Silva et al. [4] who obtained 0.68 m² g-1 and 2.58 m² g-1 in nonactivated samples, respectively. Regarding pyrolysis, Jin et al. [10] noted 7.73 m² g-1 at 500 °C and Nielsen; Bandosz [18] obtained 63 m² g-1 (total pore volume of 0.074 cm³ g-1 and micropore volume of 0.030 cm³ g-1) at 650 °C.

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Furthermore, Dave et al. [56] verified 39.32 m² g-1 in samples chemically treated (sulphuric acid); and Alvarez et al. [11] achieved 235 m² g-1 (total pore volume of 0.36 cm³ g-1) in pyrolyzed (500 °C) and functionalized (hydrochloric acid) samples. Despite the variety of observed results, some authors indicated that values of

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SBET are directly related to the characteristics of the precursors used, as well as the selected activation procedure [4]. Moreover, these authors verified the development

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of pores and increase of the surface area by combining or not thermal and chemical treatments [4,8,57].

Scanning electron microscopy (SEM) was used to investigate the morphology

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of the adsorbents before and after adsorption of RR 120 in SS (Figure 3 a, b), PS

N

(Figure 3 c, d) and FS (Figure 3 e, f) samples, respectively. The SEM images were

A

captured at 4000x magnification to determine how the surface morphology and

M

porous structure of the adsorbents were changed after pyrolysis and chemical

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treatment.

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Figure 3 near here

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The predominant formation of mesopores and minor macropores was observed in both adsorbents. The presence of mesopores and macropores in the morphological structure of sewage sludge used as adsorbent was also identified by

A

Maderova et al. [6], Alvarez et al. [11] and Rassol; Lee [21]. When checking the photomicrographs of the SS (Figure 3 a and b) physical

changes could be observed. Before adsorption, a more linearized, compacted material could be visualized. Although this material had grooves and cavities, these

19

are presented in small amounts. After adsorption, a less compacted material was noticed and with greater structural differences, when compared to PS and FS. Physical changes in the structures of the PS and FS, before adsorption (Figure 3 c and e), did not cause any significant differences in porous structure of

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these adsorbents. Besides their porous structure, the photomicrographs indicate irregular surfaces with cavities and non-uniform grooves that possibly favored the

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adsorption process.

Despite the porosity, both adsorbents presented partial compaction after the adsorption process, more accentuated in functionalized sludge (Figure 3 f), which

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could be justified by the filling of the pores with the adsorbed dye (PS, Figure 3 d).

A

N

PS and FS adsorbents did not show remarkable structural differences between them.

M

3.3 X-ray diffraction

Figure 4 shows the X-ray diffraction patterns for the samples of SS, PS and

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FS before (a) and after (b) adsorption, respectively. The reference code of the

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indexed peaks and the profile fitting statistics are presented in Supplementary Material 3 (S.3).

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The XRD analyses were performed to clarify the microstructural modifications that occurred in the treated sludge samples, in order to comprehend how these new-

A

formed structures affected the adsorption process.

Figure 4 near here

The presence of an amorphous halo between 20° and 35º (2θ) in all diffractograms should be highlighted, which indicates low crystalline structure and

20

possible high reactivity since the compounds are not organized in low energy crystal structure. Generally, the XRD patterns of the samples show the presence of common clay minerals, such as muscovite (M), quartz (Q), dolomite (D) and annite (A) (Figure 4). According to Eberlei et al. [58] and Silva et al. [4], the muscovite is a stable

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mineral at high temperature and pressure, commonly found on Earth’s crust. Furthermore, dolomite is a carbonate sedimentary rock primarily composed of

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calcium carbonate and magnesium [59]. Silva et al. [4] also verified the presence of

minerals such as muscovite, dolomite and quartz in laundry sewage sludge, corroborating similarity with the minerals noticed in this study.

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From the profile fitting quantification, no substantial differences were observed

N

between the crystalline composition of SS before and after the adsorption, presenting

A

46% of muscovite, 34% of quartz, 13% of annite and 7% dolomite (Figure 4a). Before

M

the adsorption, PS showed precipitation of quartz, reduction of annite to muscovite content, and complete consumption of the dolomite phase. A partial decarboxylation

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of the dolomite is possible at 500 °C, as previously observed by [60]. The pyrolysis

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allowed the formation of voids/pores in the microstructures of the sludge left from the CO2 compound, increasing the adsorption properties of the material.

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FS behaved oppositely, reducing quartz and annite content and increasing the muscovite. FS did not also present a dolomite phase as noted in its previous form as PS. However the reduction of quartz crystalline structure can be attributed to the

A

HNO3 treatment, creating amorphous content, lifting voids and helping with the dye adsorption. After the adsorption, both samples remained without the formation of dolomite crystals, even though low intensities peaks are shown in the diffractograms. Therefore, the reaction kinetics of the sludge was affected by the treatment used on the sludge and can be identified by the XRD quantitative analysis.

21

3.4 Thermogravimetric analysis (TG) and Derivative Termogravimetric (DTG) Figure 5 shows the thermograms of SS, PS and FS, before and after

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adsorption.

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Figure 5 near here

Before adsorption, total weight losses of approximately 53.23%, 24.13% and 40.60% were observed in SS, PS and FS under temperatures ranging 22 °C to

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1000 °C, respectively. The behavior of the thermogravimetric curves is similar to the

N

samples before adsorption. After adsorption, these total losses were approximately

A

67.09%, 27.87% and 50.54% in SS, PS and FS, respectively, to the same range of

M

temperature.

The thermograms of the adsorbents have two distinct regions of weight loss,

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which was also reported by Silva et al. [4] and Zhang et al. [61] who indicated that

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the first weight loss until 100 °C corresponds to the moisture present in the material. Second weight loss was verified in the range between 115 °C and 475 °C, as

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also noted by Ogawa et al. [62], who attributed the weight loss until 900 °C to the loss of organic, hydrogen and oxygen surface groups. The total weight losses of PS and FS samples compared to sewage sludge

A

were lower, 54% and 23% before adsorption and 58% and 24% after adsorption, respectively. This indicates the degradation of organic material during the treatment, especially those relating to hidroxyl (-OH) groups with reduction of the weight loss rate (DTG) between 200 °C and 500 °C.

22

3.5 FT-IR analysis FT-IR spectra of RR 120, SS, PS and FS are shown in Figure 6, before adsorption. The FTIR was performed to identify the main functional groups present in

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the structures of the adsorbents.

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Figure 6 near here

No different groups were added to the surface of the adsorbents after the adsorption of dye RR 120 even with the thermal and thermal-acid treatments.

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In all cases, it is possible to observe a band broad in the region of 3200 to

N

3600 cm-1 (at around 3417 cm-1) which corresponds to the asymmetric stretching

A

vibrations of O – H and N – H. O – H group may indicate the presence of hydroxyl

M

groups, carboxylic groups, alcohols and phenols [4,63]. N – H group is related to amine and amide groups from the proteins in the sewage sludge [18]. Oxygen

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functional groups are generally be related to the acidity of the material [64].

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The two bands verified around 2925 cm-1 and 2852 cm-1 in SS samples could be due to the symmetric and asymmetric stretching vibration absorption peaks of C –

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H, which may indicate the existence of aliphatic chain [65]. According to Jindarom et al. [66] and Kacan [15], the bands observed in the

region of 1650 cm-1 to 1850 cm-1 (at around 1621 cm-1) correspond to C = O

A

stretching of carboxylate groups and the band around 1384 cm-1 indicates the presence of alkenes, as verified in the adsorbents of this study. The bands at 1200 cm-1 and 1044 cm-1 corresponded to the vibrations of symmetric and asymmetric of the O-S-(O2) group, respectively [67]. These bands almost disappeared in the spectra of PS and FS or were very weak after adsorption.

23

These results suggest the participation of single and double sulfur bonds (S - O and S = O) on the interaction with the sludge surface, indicating the adsorption of dye RR 120 molecules by electrostatic attraction via sulfonic groups. Bands around 1081 cm-1 observed in all spectrograms were attributed to the

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angular deformation of C – O groups, such as carboxylic acids, ethers, lactones and phenols according to Méndez et al. [68]. The band observed around 603 cm-1 refers

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to the angular deformation outside the plane of N – H bonding [66].

Similar results were obtained by Maderova et al. [6], who observed bands values of 3077 cm-1, 1622 cm-1, 1033 cm-1 in samples of magnetically modified

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sewage sludge, which indicates the presence of C = O and O – H. Sonai et al. [51]

N

noted bands of 3394, 1620 and 1094 cm-1 in samples of textile sewage sludge and

A

identified them as bands corresponding to O – H stretching of hydroxyl groups and to

M

N – H stretching of amine and amide groups.

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3.6. Adsorption studies of RR 120 on pyrolyzed and functionalized adsorbents

3.6.1 Adsorption kinetics

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The studies of adsorption kinetics were performed for PS and FS in all corresponding pairs (pH and temperature), according to the CCRD (Table 1). However, in this study only the best results of RR 120 removal for PS and FS

A

are addressed. The optimal pairs were verified at pH 4.7 and 55 °C for PS and pH 7.5 and 60 °C for FS (Table 1). Graphs and fittings obtained by the pseudo-first and pseudo-second order equations are shown in Figure 7 and Table 3. The adsorption kinetics studies were performed to investigate the adsorption dynamics of RR 120 onto PS and FS at pH and temperature pairs of 4.7/55 °C and

24

7.5/60 °C for PS and FS, respectively; besides, the mechanisms involved in the adsorption processes were also explored.

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Figure 7 near here

High adsorption rate was achieved in the first 30 minutes. Posteriorly, the

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adsorbed amount increased until reaching the equilibrium between 60 min and 90 minutes for both adsorbents (Figure 7). Adsorption occurring satisfactorily in the early stages of the experiment indicates high affinity of the adsorbents for the adsorbate

N

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[69].

M

A

Table 3 near here

The pseudo-second order model best fitted the experimental data for both

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adsorbents, presenting the highest values of R² (0.98 and 0.99), the lowest values of

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Δqe (4.62 and 0.79%) and adsorbed quantity of 2.46 and 2.61 mg g-1 at pH 4.7 and 7.5, for PS and FS, respectively (Table 3).

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This fitting model is associated with chemical bonds, representing the process of chemisorption in which valence forces from the sharing of electrons between RR 120 molecules and the adsorbent surfaces are involved [70]. The same was

A

observed in adsorption of dyes Basic Red 2 [6], and Reactive Red 2 [51] onto sewage sludge chemically modified and textile sludge pyrolyzed, respectively. However, the pseudo-first order model fitting was substantial, since the values obtained for the determination coefficients (R²) and adsorbed quantities were similar to pseudo-second order model, thus indicating the influence of physisorption. For this

25

reason, it is important to characterize and investigate the adsorption capacity of a given adsorbate in different adsorbents before proposing a definitive method. Coded and decoded experimental data are used in CCRD, and predicted and actual responses for both adsorbents are given in Table 1. The adsorption efficiency

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values obtained from 22 experiments determined at the CCRD conditions (pH and temperature) were compared with the calculated adsorption efficiency values from

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the applied models for the optimal results previously verified in the desirability test.

The optimal conditions were found by desirability test at pH 3.5 and 5.4 (-1.414 and 0.738 - coded values) and 60 °C (1.414 - coded value) for PS and FS, respectively,

N

U

for both desirability responses for RR 120 removal (Figure 8 a, b).

M

A

Figure 8 near here

The effects, standard errors, t values, p values, errors of coefficients and

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regression coefficients according to the investigated parameters (pH and temperature) for PS and FS are shown in Supplementary Material 4 (S.4). The main

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effect of each factor and the interaction effects are statistically significant when p ≤

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0.05. The data obtained by the effects were visualized in the form of a Pareto chart (Figure 8 c, d for PS and FS, respectively). To indicate the minimum statistically

A

significant effect for p ≤ 0.05, a vertical line was drawn. For PS, significant effects were observed for linear (L) and quadratic (Q) pH,

and for quadratic (Q) temperature, since they presented p values ≤ 0.05 (5% significance level,  = 0.05). For FS, all factors, except the interaction between pH and temperature (pH x temperature), had significant effects on the dye removal.

26

To generate the regression models, the non-significant factors (p value > 0.05) were not considered. The coefficients of the model equation and their statistical significance were evaluated using Statistica®. The quadratic regression model for RR 120 removal efficiency with PS and FS as adsorbents in terms of coded factors

YPS = 93.20 – 3.15 X1 – 1.24 X12 + 1.26 X22

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(R² 0.86)

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are given by Equation 10 and Equation 11, respectively.

(11)

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YFS = 96.53 – 1.80 X1 – 1.22 X12 + 0.89 X2 + 0.31 X22 (R² 0.88)

(10)

N

where YPS and YFS is the percentage of RR 120 removal (%); X1 and X2 are

A

the coded values of pH and temperature, respectively. X1X2 is the interaction

M

between pH and temperature and X12 and X22 are the square terms of each main factor, respectively.

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The response surfaces (3D) were constructed based on the regression models previously presented for PS and FS to obtain the optimal condition (pH and

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temperature) of the dye removal. Results of dye removal greater than 95% for PS in

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the pH range from 3.5 to 7.0 and temperature from 30 °C to 34.4 °C and from 55.6 °C to 60 °C; and greater than 97% for FS in the pH between 3.5 and 7.5 with temperatures superior to 50 °C were noted.

A

Experiments * and ** (Table 1) represent the results of the optimal condition

for PS and FS, respectively, being 97.69% and 98.64% the calculated predicted values for the models (Eq. 10 and 11) and 94.01% and 98.06% the experimental values, respectively.

27

The results obtained indicate the validation of the models thus allowing the use of CCRD to estimate the performance of experimental conditions and adjust the response surfaces with quadratic terms. As a result, it is possible to optimize more than one response at the same time, in addition to establishing the desired

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conditions. Therefore, it was concluded in the statistical analysis and desirability that dye removal was optimized under the specific conditions obtained for each

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adsorbent.

3.6.2. Adsorption isotherms

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Langmuir and Freundlich models were fitted to the adsorption data with

N

various initial RR 120 concentrations (Supplementary Material, S.2). So, the

A

corresponding constants related to the strength and capacity of adsorption (KL and

M

KF, respectively) and the determination coefficients (R²) were obtained (Table 3). Figure 9 demonstrates the plots of the experimental data and predicted

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isotherm models for adsorption of RR 120 onto PS (pH 3.5) and FS (pH 5.4) at 60 °C

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and contact time of 180 minutes (equilibrium time obtained by the kinetic assays). Adsorption of RR 120 of 15.39 mg g-1 at pH 3.5 and 45.38 mg g-1 at pH 5.4

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were verified in the PS and FS, i.e., values similar to qm obtained by Langmuir fitting (14.69 mg g-1 and 46.81 mg g-1) in which was also verified the highest R² and the

A

lowest Δqe (Table 3).

Figure 9 near here

According to Leng et al. [53], the better fitting with Langmuir isotherm model indicates that active sites on the adsorbent are homogeneously distributed, and the

28

dye adsorption onto adsorbents is assumed monolayer. Moreover, all sites possess an affinity with the dye [71]. The adsorption here in investigated could have been favored, considering the mesoporosity characteristics of adsorbents (DP of 26 nm and 17 nm to PS and FS,

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respectively) and the dimensions of RR 120 molecule of 3.74 nm and 1.11 nm. Some fittings have been performed with Langmuir isotherm model using

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different adsorbents and different types of sludge on the adsorption of RR 120. Among these studies, relevant maximum adsorption capacity (qm) include 19.6 mg g-1 (R² of 0.994) obtained by Dehghani et al. [72] with natural zeolite, while Mubarak et

U

al. [16] reported 114.9, 123.5 and 129.9 mg g-1 at 30 °C, 40 °C and 50 °C (R² of

N

0.9989, 0.9955 and 0.9941), respectively, with chitosan beads.

A

As stated by Wu [73], the shape of an isotherm is indicated by the

M

dimensionless constant separator factor or equilibrium parameter (RL). In this study, RL resulted in 0.48 and 0.63 for PS and FS, respectively, i.e., values between 0 and

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1.0, indicating that the adsorption was favorable with both adsorbents (Table 3).

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Despite the better fitting of Langmuir isotherm model obtained in this study, Freundlich isotherm model also presented significant values and can be applied in

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the fitting of the experimental data for PS (R² of 0.93) and FS (R² of 0.96). Maderova et al. [6]; Leng et al. [53] and Hadi et al. [35] stated that values of heterogeneity factor (n) in the range of 2-10 represent favorable adsorption. This can be observed in

A

Table 3 in which n was 3.64 ± 0.44 and 2.69 ± 0.26 for PS and FS, respectively. Moreover these authors indicated that higher values of KF represent higher

adsorption capacity, as also noted in this study in which KF was higher for FS (4.78 ± 0.97 mg g-1 (L mg-1)1/n) than for PS (2.56 ± 0.52 mg g-1 (L mg-1)1/n).

29

3.6.3. Adsorption thermodynamics Thermodynamic parameters (associated with RR 120 adsorption process) were determined from adsorption data at 25, 45 and 55 °C under different acid pH values (3.5 and 5.4) previously obtained through the statistical desirability

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parameters. Gibbs free energy (∆G°), enthalpy change (∆H°) and entropy change (∆S°) were calculated for the adsorption of the RR 120 on PS and FS using the van’t

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Hoff equation (Supplementary Material, S.2). The thermodynamic data and the values of thermodynamic parameters are summarized in Table 3.

Enthalpy change (∆H°) and entropy change (∆S°) were calculated from the

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linear and angular coefficient of the linear regression of ln KD versus (1/T). The plot

A

that calculated ∆H° and ∆S° were convenient.

N

ln KD versus (1/T) straight lines (R² > 0.99) for the adsorbents and R² values indicated

M

The positive values of enthalpy (ΔH°) (4.73 and 8.02 kJ mol-1) indicate favorable endothermic nature of adsorption process and possible strong bonding

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between dye and the adsorbents, as also reported by Mahjoub; Brahim [7].

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The positive values for the entropy change (ΔS°) (46.57 and 51.06 J mol-1 K-1) suggest affinity between the dye and the adsorbents, as indicated by Malekbala et al.

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[74]. Moreover, Angin [75] stated that positive ΔS° increases randomness at the solid-liquid interface with the loading of dye molecules on the surface of the

A

adsorbents.

According to Dural et al. [76], negative values for the free energy of Gibbs

(ΔG°) corroborate the spontaneous adsorption process of the adsorbate with the adsorbents and the spontaneity increases as a function of the temperature increase. In this study, the adsorption process was found to be more spontaneous (i.e., higher

30

negative ΔG° values) by increasing the temperature from 25 to 55 °C, confirming the endothermic nature of this process. Silva et al. [4] and Zeng et al. [77] reported that the removal of adsorbates may be considered primarily characteristic of physisorption since ΔG° values were

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between -20 and 0 kJ mol-1, as noted in this study for both adsorbents.

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4. Conclusions

Adsorption of RR 120 from aqueous solution onto sewage sludge, thermal and chemically activated, was investigated. These treatments changed the chemical and

U

textural characteristics of the adsorbents. The temperature rise promoted an increase

N

of BET surface area (SBET) (4.6, 35.7 and 81 m²g-1 for SS, PS and FS, respectively)

A

and average pore diameter (Dp) (23 – 26 nm from SS to PS).

M

The characterization results revealed that the adsorbents presented mesoporous predominant structure, rich in mainly carboxylic groups. Moreover,

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heterogeneous and irregular surfaces and porous sludge structure were observed.

PT

The optimum conditions for adsorption were found at pH and temperature pairs of 3.5/60 °C and 5.4/60 °C for PS and FS, respectively. 3D response surfaces

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plots showed that the major significant effect for maximizing responses of the adsorbents was the pH. This optimization, combined with thermal and thermal

A

treatments, reinforces the characteristics of novelty and originality of this study. Kinetic studies revealed that pseudo-second order model better described the

adsorption of RR 120 whereas Langmuir isotherm model was the best fitted to the adsorption equilibrium data for PS and FS. The thermodynamic data showed that RR 120 adsorption occurred via endothermic processes and spontaneous nature.

31

Lastly, the application of thermal and chemical treatments was suitable for obtaining alternative low-cost adsorbents compared to commercial-grade activated carbon. Therefore, the valorization perspectives of sewage sludge and its

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applicability are confirmed.

Acknowledgments

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The authors thank the support of the Coordination for the Improvement of

Higher Education Personnel (CAPES); MSc. Joziane Gimenes Meneguin (on behalf of LATI-UEM) for providing the textural analyses; Alexandre José Gonçalves (on

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behalf of CMCM-UTFPR) for providing the SEM and X-ray diffraction analyses; Dr.

N

Rúbia Camila Ronqui Bottini (on behalf of LAMAQ-UTFPR) for providing the FT-IR

A

analysis; and Prof. Dr. José Alberto Cerri (on behalf of NPT-UTFPR) for providing the

M

TG and DTG analyses.

[1]

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Title of Figures

Figure 1 - Chemical structure of the dye Reactive Red 120 (RR 120) Figure 2 - N2 adsorption and desorption isotherms and pore distribution calculated by BJH method for adsorbents: (a) pyrolyzed sludge (PS) and (b) functionalized sludge

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(FS)

Figure 3 - Images of scanning electron microscopy (SEM) before and after

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adsorption of SS (a, b), PS (c, d) and FS (e ,f), respectively, at 4000x magnification

Figure 4 - XRD patterns of sewage sludge (SS), pyrolyzed sludge (PS) and functionalized sludge (FS), before (a) and after adsorption (b)

U

Figure 5 - Thermograms (TG and DTG) of SS, PS and FS before and after

N

adsorption

A

Figure 6 - FT-IR spectra of RR 120, sewage sludge (SS), pyrolyzed sludge (PS) and

M

functionalized sludge (FS), before adsorption

Figure 7 - Adsorption kinetics for dye RR 120 using a) PS at pH 4.7 and 55 °C and b)

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FS at pH 7,5, 60 °C, in initial concentration of 25 mg L-1; experimental results () and their fitting to the pseudo-first order (---) and pseudo-second order (---) models

PT

Figure 8 - Desirability test of the statistical planning to obtain the optimum adsorption condition of RR 120; and Standardized main effect Pareto chart for PS (a, c) and FS

CC E

(b, d), respectively.

Figure 9 - Equilibrium isotherms for RR 120 adsorption at 60 °C with 10 g L-1 onto PS adsorbent (a) pH 3.5 and FS adsorbent (b) pH 5.4; experimental results ( ) and their

A

fitting to the non-linear adjustments of Langmuir (---) and Freundlich models (---)

U

SC R

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40

A

CC E

PT

ED

M

A

N

Figure 1 - Chemical structure of the dye Reactive Red 120 (RR 120)

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41

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PT

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M

A

N

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(a) PS

(b) FS

Figure 2 - N2 adsorption and desorption isotherms and pore distribution calculated by BJH method for adsorbents: (a) pyrolyzed sludge (PS) and (b) functionalized sludge

A

(FS)

42

After Adsorption

a)

b)

N

U

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Before Adsorption

d)

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M

A

c)

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e) f) Figure 3 - Images of scanning electron microscopy (SEM) before and after

A

adsorption of SS (a, b), PS (c, d) and FS (e ,f), respectively, at 4000x magnification

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43

b)

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M

A

N

U

a)

Figure 4 - XRD patterns of sewage sludge (SS), pyrolyzed sludge (PS) and functionalized sludge (FS), before (a) and after adsorption (b)

A

Note: X – Mineral name (chemical formula of the basic structural unit; COD 2014 Reference code) A - Annite (Si4.56Al5.04Fe4.62K1.84O24; 96-900-2315); D – Dolomite (Ca3Mg3C6O18; 96900-3523); M – Muscovite (Al11.52Fe0.08Mn0.08Si12.32K3.56Na0.40Rb0.04H16O48; 96-9004477); Q – Quartz (Si3O6; 96-900-9667).

U

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44

N

Figure 5 - Thermograms (TG and DTG) of SS, PS and FS before and after

A

adsorption

M

Note: SS (TG) Before adsorption (—); SS (TG) After adsorption (—); SS (DTG)

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Before adsorption (---); SS (DTG) After adsorption (---); PS (TG) Before adsorption (—); PS (TG) After adsorption (—); PS (DTG) Before adsorption (---); PS (DTG) After

PT

adsorption (---); FS (TG) Before adsorption (—); FS (TG) After adsorption (—); FS

A

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(DTG) Before adsorption (---) and FS (DTG) After adsorption (---)

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M

A

N

U

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45

Figure 6 - FT-IR spectra of RR 120, sewage sludge (SS), pyrolyzed sludge (PS) and

A

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functionalized sludge (FS), before adsorption

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46

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M

A

N

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(a)

(b)

Figure 7 - Adsorption kinetics for dye RR 120 using a) PS at pH 4.7 and 55 °C and b) FS at pH 7,5, 60 °C, in initial concentration of 25 mg L-1; experimental results () and

A

their fitting to the pseudo-first order (---) and pseudo-second order (---) models

b)

M

A

N

U

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a)

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47

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c)

d)

Figure 8 - Desirability test of the statistical planning to obtain the optimum adsorption

PT

condition of RR 120; and Standardized main effect Pareto chart for PS (a, c) and FS

A

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(b, d), respectively

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48

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PT

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M

A

N

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(a)

(b)

Figure 9 - Equilibrium isotherms for RR 120 adsorption at 60 °C with 10 g L-1 onto PS

A

adsorbent (a) pH 3.5 and FS adsorbent (b) pH 5.4; experimental results () and their fitting to the non-linear adjustments of Langmuir (---) and Freundlich models (---)

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49

Table 1 - Central composite design model for RR 120 dye adsorption by pyrolyzed sludge (PS) and functionalized sludge (FS). Coded (Decoded) variables

-1 (35)

PS 93.85 [91.99]

FS 93.84 [96.92]

-1 (35)

87.55 [88.75]

90.24 [92.42]

X2, Temperature (°C)

-1 (4.7)

2

+1 (10.3)

3

-1 (4.7)

+1 (55)

93.85 [93.46]

95.02 [96.89]

4

+1 (10.3)

+1 (55)

87.55 [91.69]

91.42 [92.00]

5

-1.414 (3.5)

95.17 [91.30]

93.64 [96.32]

+1.414 (11.5)

0 (45)

86.27 [74.66]

88.55 [96.48]

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ED

1

0 (45)

7

0 (7.5)

-1.414 (30)

90.68 [83.37]

93.33 [96.59]

8

0 (7.5)

+1.414 (60)

90.68 [86.26]

94.99 [97.78]

9

0 (7.5)

0 (45)

93.20 [86.78]

93.53 [96.62]

10

0 (7.5)

0 (45)

93.20 [85.49]

93.53 [96.57]

11

0 (7.5)

0 (45)

93.20 [85.69]

93.53 [96.49]

*

-1.414 (3.5)

1.414 (60)

97.69 [94.01]

-

**

-0.738 (5.4)

1.414 (60)

-

98.64 [98.06]

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6

A

Removal efficiency (%) Predicted [Actual]

A

X1, pH

Removal efficiency (%) Predicted [Actual]

M

Experiment

Response value % efficiency (Y)

Note: α = 1,414 = (2N)1/4; *Optimum condition found by desirability for pyrolyzed sludge (PS); **Optimum condition found by desirability for functionalized sludge (FS); [Experimental value obtained (%)].

50

Table 2 - Results of physical-chemical characterization and textural properties of the adsorbents SS

PS

FS

Moisture (%)

3.8 ± 0.2

2.6 ± 0.4

6.4 ± 0.4

Volatile materials (%)

52.7 ± 0.3

24.1 ± 0.5

52.0 ± 0.6

Ash (%)

24.8 ± 0.6

43.5 ± 0.3

41.6 ± 1.4

pH

6.3

8.2

pHPZC

7.5

7.4

SBET (m² g-1)

4.6 ± 0.2

35.7 ± 0.3

81.0 ± 0.2

VT - Total pore volume (cm³ g-1)

0.03

0.12

0.10

Vα - Microporous volume (cm3 g-1)

-

0.003

0.015

DP - Average pore diameter (nm)

23

26

17

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Analysis/Properties

3.4

U

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4.8

0.89

Lactonic

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Total

Cd

0.09

0.10

0.06

0.13

0.31

0.30

1.11

1.32

1.35

Heavy metal element - Content (mg L-1) -

-

-

5.09

6.62

2.77

Cu

3.69

7.75

1.61

Ni

0.35

0.36

2.11

1.74

3.87

0.089

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Cr

Pb

A

0.99

M

Phenolic

0.91

A

Carboxylic

N

Functional groups (mEq g-1)

Proximate analysis - Content (wt. %)

C

39.85

29.43

34.00

O

35.92

43.23

44.75

S

3.20

1.65

1.63

Al

4.58

6.15

3.70

Fe

3.46

3.71

3.17

P

1.65

2.29

0.79

51

Note: Number of samples - 3; *The values correspond to the mean ± standard

A

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M

A

N

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deviation; - Not detected.

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Table 3 – Kinetic, Isotherms and Thermodynamic parameters and statistical data for dye RR 120 adsorption with PS and FS adsorbents, in different pH and temperature pH

PS FS

qm (mg g-1)

4.7

55

2.42 ± 0.025

7.5

60

2.60 ± 0.004

3.5

ED

5.4

60

R²

Δqe (%)

qm (mg g-1)

k2 (g mg-1 min-1)

R²

Δqe (%)

2.12 ± 0.34

0.97

5.44

2.46 ± 0.018

2.10 ± 0.39

0.98

4.62

0.99

0.92

2.61 ± 0.003

12.03 ± 1.57

0.99

0.79

3.69 ± 0.26

Langmuir

Freundlich

qm (mg g-1)

kL (L mg-1)

RL

R²

qe (%)

kF (mg g-1) (L mg-1)1/n

n

R²

qe (%)

14.69 ± 0.47

0.043 ± 0.010

0.48

0.96

5.89

2.56 ± 0.52

3.64 ± 0.44

0.93

8.22

46.81 ± 2.01

0.023 ± 0.005

0.63

0.98

6.48

4.78 ± 0.97

2.69 ± 0.26

0.96

6.69

PT

FS

60

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3.5

Pseudo-second order

k1 (min-1)

M

(°C)

PS

PS

Pseudo-first order

Temp.

A

Sample

ΔH° (kJ

mol-1)

4.73

ΔS° (J

mol-1 K-1)

46.57

R² 0.99

ΔG° (kJ mol-1) 25 °C

45 °C

55 °C

-9.15

-10.08

-10.55

A

FS 5.4 8.02 51.06 0.99 -7.21 -8.23 -8.74 Note: qe - normalized standard deviation (%) calculated according to Silva et al. (2016), where qe,exp - experimental adsorption capacitiy (mg g-1); qe,cal - calculated adsorption capacity (mg g-1); N - number of samples.

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M

N U SC R

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I